Experimental, DFT dimeric modeling and AIM study of H-bond-mediated composite vibrational structure of Chelidonic acid.

The composite vibrational structure near 3650-3200 and 3000-2400 cm-1 in the observed IR absorption spectrum of Chelidonic acid has been explained in terms of intra- and inter-molecular -O-H∙∙∙O H-bonding attributed to monomer and dimer species computed at B3LYP/6-311++G(d,p) level. Three of the six dimer species derived out of ten monomeric components have shown both intra- and inter-molecular H-bonding. Vibrational modes of the monomer and dimer species are satisfactorily identified with the observed IR and Raman bands including frequency shifts associated with the H-bondings. The H-bond interactions in the monomer and dimer species have been characterized in terms of electron density, ρ(r), its Laplacian, ∇2ρ(r) and potential energy density at the O∙∙∙H bond critical points (BCPs) based on the Atoms in Molecules (AIM) theory. The attractive (van der Waals, H-bonds) and repulsive steric clash (SC) interactions are explained using computed reduced density gradient values from the noncovalent interaction (NCI) method. The AIM analysis confirms the presence of the intra- and inter-molecular H-bondings in the monomer/dimer species. The natural bond orbital (NBO) analysis of the natural charges and stabilization energy of the H-bonds for the dimer species further points to the stronger inter-than intra-molecular H-bonding.

Introduction

The intra- and inter-molecular H-bonding in carboxylic acid (CBA) dimeric units are of unwavering interest and are commonly characterized by IR spectroscopic studies of −O−H stretching vibrations in the region 4000–400 cm−1 [1-6]. The most common form of H-bonding is the dimerization between the two carboxylic groups often cyclically bound by a local center of inversion, resulting in a larger down-shift of the −O−H stretching frequency of the dimeric carboxylic acids (3000–2500 cm−1) [1]. Diverse complexes such as these dimeric species governed by the structure and strength of the H-bonds have been largely the subject of intense experimental and theoretical studies with wider implications in multidisciplinary subjects [7, 8, 9, 10, 11]. In addition to their biological interest and anti-carcinogenic drug properties, the study of homo-/hetero-complexes of CBAs in gaseous, liquid, and solid states have provided insights into the nature of noncovalent interactions including −O−H∙∙∙O=C interactions [12, 13, 14, 15]. In the recent past, studies on simple CBA systems, for example, the formic acid dimer species participating in strong to weak inter-molecular interactions have been reported [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. However, there have been apparently fewer similar investigations of the H-bond induced composite vibrational structure of the benzenedicarboxylic acids (BDAs) [16, 17, 18, 19, 20, 21]. Recently, Karabacak et al have assigned IR and Raman modes of the solid Isophthalic acid on the basis of computed monomers and O−H∙∙∙O bound dimer species [19]. Ab initio predictions of the effects of temperature and solvation on the dimerization of benzoic acid have been reported by Taylor et al [22]. They conclude that benzoic acids form strong H-bonds with each other in different complexes but a cyclic dimer among them is the most stable configuration. Martsinovich et al have reported modeling of the self-assembly of BDAs using Monte-Carlo and molecular dynamics simulations (MD) [23]. The structural studies by R. Alcala et al, J.L. Derissen and by M. He et al on Isophthalic acid and 5-Fluoroisophthalic acid also support the ordered cyclic structures stabilized by double O−H∙∙∙O bonds between two carboxylic groups [24, 25, 26].

In the present work, we report extensive study of the structural, vibrational and electronic properties of the conformational and dimerization aspects of H-bonded species of Chelidonic acid (CA for short; it is also called 4-Oxo-4H-pyran-2,6-dicarboxylic acid, C7H4O6, see Fig. 1). The CA is a pyran skeleton with dicarboxylic acid group and is a suitable intermediate in certain synthesis work but more importantly its pharmacological effects and therapeutic potential have been demonstrated [27, 28, 29]. From structural and spectroscopic view-points, the CA is an attractive candidate for the investigation of H-bonding in the formation of dimer species. This has been borne out by the observation of broad IR absorption bands near 3650–3200 cm−1 with a series of bands in 3000–2400 cm−1 (Fig. 2(a), see also Fig. S1, Supplementary Material). This vibrational structure extending from 3650 to 2400 cm−1 including two IR and Raman bands near 1648 and 1723 cm−1 is the apparent manifestation of intra- in monomer and both intra- and inter-molecular O−H∙∙∙O bonding in the dimer species of CA (see Fig. 2). We have computed all the possible dimer species at B3LYP and BP86 methods combined with the 6-31G(d) and 6–311++G(d,p) basis sets, yielding six dimer species of which three are inter-molecularly H-bonded and three are mixed inter- and intra-molecularly H-bonded species. In the proposed dimer models, the two carboxylic motifs simultaneously act as H-bond donor and acceptor. The inter-H-bonds are strong since computed interaction energy per bond is 9 kcal/mol. As a consequence, the −O−H and −C=O stretching modes have shown large red-shifts in frequencies with broadening of the bands. We have shown that the simulated IR bands spectra derived from six dimer species of CA are in agreement with the measured spectrum in the 3700–400 cm−1. Furthermore, we have characterized the inter- and intra-H-bonding in the dimer species as noncovalent interactions by the electron density topological analysis by employing the quantum theory of Atoms in Molecules (AIM) and noncovalent interaction (NCI) methods. The charge transfer in the monomer and dimer species due to the H-bonding interactions is explained using the natural bond orbital (NBO) analysis.

Fig. 1

Molecular structure of Chelidonic acid.

Fig. 2

Experimental FT-IR (a) and simulated IR spectra for D dimer (b) and M monomer (c) species of Chelidonic acid. Experimental FT-Raman spectrum (d) is also shown.

Methods

Experimental methods

The sample of Chelidonic acid (Sigma-Aldrich, part of Merck; assay, 98.0%; mp 265 °C) was used as received. The IR absorption spectrum of the sample was measured at room temperature using standard KBr pellet technique. The spectrum was measured at 2 cm−1 resolution with 100 scans in the range 4000–400 cm−1 on a Thermo-Fisher Nicolet 6700 FT-IR spectrometer equipped with a KBr beam splitter, a ETC EverGlo mid-IR radiation source, and a deuterated L-alanine doped triglycine sulfate (DLaTGS) detector. The Raman spectrum was measured from 3500 – 100 cm-1 at a spectral resolution of 2 cm−1 with a Thermo-Fisher NXR 6700 FT-Raman module spectrometer, equipped with a diode-pumped air cooled Nd:YVO4 laser source (1064 nm excitation line) and a LN2 cooled high-performance Ge detector. The spectra were averaged over 500 scans. Both the spectrometers were purged with nitrogen to remove the interference due to the atmospheric CO2 and moisture. The solid-phase near-IR (NIR) spectrum of Chelidonic acid was measured in the 4000–8000 cm−1 region and for the sake of brevity, the analysis has been presented in the section S10 (see also Fig. S10a).

Computational methods

All the electronic structure calculations were performed using the Gaussian 09W, GaussView 5, Multiwfn, VMD and Chemcraft suite of programs [30, 31, 32, 33, 34, 35]. Molecular structure of CA is presented in Fig. 1. Initially, the conformational search was performed by a relaxed potential energy surface scan (PES) at B3LYP/6-31G(d) level by rotating the two –COOH groups from 0° to 350° with step of 10° (see Fig. S11). The initial PES scan at B3LYP/6-31G(d) level reduced the computational cost. However, this level for H-bonded structures is known to produce less accurate geometrical and energetic parameters when compared to the experimental values. Moreover, in addition to the inclusion of the electron correlation effects on geometry optimization as defined in the B3LYP method, the use of flexible basis set with polarization and diffuse functions on hydrogen atom orbitals is necessary for the accurate description of H-bond interactions. These basis set requirements are usually met by using the flexible and extended basis set 6–311++G(d,p). Therefore, in the present study, we used 6–311++G(d,p) basis set for the calculations. We found ten minima on the PES which were again optimized at the B3LYP/6–311++G(d,p) level. Moreover, the harmonic frequency analysis at the same level confirmed all the ten minima as true stationary points, yielding a total of ten monomers (Fig. 3). The observed nature and intensity of the IR and Raman stretching modes of −O−H and −C=O groups near 3650–2400 cm−1 and 1800–1600 cm−1 region strongly suggest that the modes would arise in H-bonded dimer species from the monomers. As a next logical step, we constructed a total of six dimer species, D (i = 1 to 6) that are shown in Fig. 4. The dimer species D, D and D are inter-molecularly bonded via −O−H∙∙∙O=C and D, D and D are both intra- and inter-molecularly bonded, with the intra-molecular H-bond referring to −O−H∙∙∙O(pyran ring). The optimized geometries of the most stable monomer and dimer species of CA with the numbering scheme are presented in Fig. 5. Calculations at BP86/6-31G(d) level were also performed, yielding slightly better frequency values than B3LYP/6–311++G(d,p) values. However, for the reasons already stated above and the B3LYP/6–311++G(d,p) level is known to predict fairly accurate results for AIM analysis, all the results obtained from this level alone will be discussed, though occasionally reference is made to BP86/6-31G(d) results. We computed the strength of the H-bonds as the energy difference between dimer and monomer components (ΔEint, this energy is a measure of strength of the H-bond) and these resultant energies are corrected for the basis-set-superposition-error (BSSE) using the counterpoise (CP) method of Boys and Bernardi [36]. The H-bond interactions in the dimer species have been characterized in terms of electron density, ρ(r), its Laplacian ∇2ρ(r) and potential energy density at bond critical points (BCP) using the Multiwfn program based on the quantum theory of Atoms in Molecules (AIM) [32, 33] and reduced density gradient, s(r), by the noncovalent interaction (NCI) methods using the Multiwfn and VMD programs [37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48]. The nature of electronic charge delocalization accompanied by H-bond formation has been investigated by employing NBO analysis at the B3LYP/6–311++G(d,p) level [49].

Fig. 3

Optimized structures of ten monomers of Chelidonic acid.

Fig. 4

Optimized structures of six dimer species of Chelidonic acid. Inter-H-bonding is in D, D, D; both intra- and inter-bonding is in D, D and D.

Fig. 5

Optimized structures of most stable monomer (a), and dimer (b) of Chelidonic acid.

Results and discussion

Vibrational and H-bonding analysis

The carboxylic acids show extensive inter-molecular H-bonding in the formation of dimer species and as a result, the IR frequency of the −O−H stretching mode shifts toward lower frequency region [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Flett and others reported that, whereas the −O−H stretching frequency of monomer species lies near 3600 cm−1, it gives rise to a broad absorption with many sub-bands near 2500–3000 cm−1 due to dimeric species [50]. These assignments have been used as reference for the assignments of −O−H stretching mode and other modes of CA. The IR and Raman bands of CA are shown in Fig. 2 (see also Fig. S1, Supplementary Material). To satisfactorily account for the vibrational structure of CA apparently induced by intra- and inter-molecular H-bonding, we assume dimerization of its monomer components. Accordingly, as a first step, we computed ten possible monomer species (i.e. conformers), yielding, say, M, M,…, M, that are shown in Fig. 3. The M, M, M and M species belong to C symmetry; M, M, M and M belong to C symmetry; M and M belong to C symmetry. The M, M, M do not show intra-molecular H-bonding, whereas, M shows it (see Fig. S2). Except for the Boltzmann populations of M, M, M and M, the populations of other monomer species are too small to be useful for discussion. Therefore, in the second step we computed ten dimer species all only from M, M, M and M species. Fig. 4 presents D, D, D, D, D and D dimer structures; the remaining four dimer species called D, D, D and D are identical and are separately presented in Fig. S3. The analysis of all the dimer species indicated that the computed geometrical, vibrational, AIM, NCI and NBO properties of the D−D dimers are identical to those for the D−D species. That is to say, D−D, dimers are not distinct and they map identically to the D−D species. Therefore, only D−D dimer species were chosen for the discussion. The M, M, M and M have twisted −COOH structure and exhibit intra-molecular −O−H∙∙∙O−C bonding with the oxygen from the pyran ring (see Fig. S2). The M is found to be the most stable and energies of other monomers relative to M's are shown in Table 1. The dimer energetics is collected in Table 2. The geometrical parameters of M, M and D species are presented in Table 3. All the dimer species exhibit proton transfer reaction at the inter-molecular bonding site. Of the six dimer species, predicted IR bands of D, D and D are identified with the observed −O−H stretching bands because they have shown both intra- and inter-bonding. For the sake of brevity, only calculated and observed frequencies of the main bands of −O−H and −C=O groups are presented in Table 4 (see also Table S4) [51, 52].

Table 1

Computed Gibbs free energies and relative energies for the ten monomers of Chelidonic acid. The energies are calculated at B3LYP/6–311++G(d,p) level.

Monomers	Gibbs free energy, ΔG (hartree)	Gibbs free energy, ΔG (kcal/mol)	Relative Gibbs free energy, ΔG (kcal/mol)
M1	-720.638165	-452207.29460007	0.00
M2	-720.637978	-452207.17725579	-0.12
M3	-720.637411	-452206.82145791	-0.47
M4	-720.636174	-452206.04522865	-1.25
M5	-720.635501	-452205.62291476	-1.67
M6	-720.627271	-452200.45851158	-6.84
M7	-720.626495	-452199.97156420	-7.32
M8	-720.625840	-452199.56054548	-7.73
M9	-720.625313	-452199.22984797	-8.07
M10	-720.615112	-452192.82862356	-14.47

Table 2

Computed Gibbs free energies, relative energies and interaction energies for the six dimers of Chelidonic acid. The energies are calculated at B3LYP/6–311++G(d,p) level. The interaction energy (ΔEint) is obtained as the difference between the energy of the complex (i.e. a dimer) and that of the monomer components. The ΔEint values are corrected for basis set superposition error (BSSE).

Dimer	Monomer combinations	Gibbs free energy, ∆G (hartree)	Gibbs free energy, ∆G (kcal/mol)	Relative Gibbs free energy, ΔG (kcal/mol)	Interaction Energy, ΔEint (kcal/mol)
D1	M1 with M1	-1441.281299	-904417.70729484	0.00	-17.77
D2	M1 with M2	-1441.280977	-904417.50523678	-0.20	-17.95
D3	M2 with M2	-1441.280708	-904417.33643673	-0.37	-18.10
D4	M1 with M4	-1441.278555	-904415.98540877	-1.72	-17.63
D5	M2 with M4	-1441.278286	-904415.81660872	-1.89	-17.90
D6	M4 with M4	-1441.275438	-904414.02946166	-3.68	-17.22

Table 3

Computed geometrical parameters for monomer M, M and dimer D of Chelidonic acid. The geometrical parameters are calculated at B3LYP/6–311++G(d,p) level. For comparison of the geometrical parameters in the bonded and non-bonded case, monomers M (free), M (intra-molecularly bonded) and the dimer D (both intra-and inter-molecularly bonded) species were chosen, Ref [56].

Parameter	Monomer		Dimer	a Experimental
	M1	M4	D4	Meconic acid
Bond lengths (Å)
C4−C3	1.468	1.468	1.467	1.431
C4−C5	1.468	1.473	1.470	1.463
C4 = O12	1.223	1.221	1.222	1.245
C3 = C2	1.347	1.345	1.348	1.345
C3−H11	1.082	1.081	1.082	−
C2−O1	1.357	1.363	1.356	1.349
C2−C7	1.497	1.507	1.499	1.501
O1−C6	1.357	1.360	1.355	1.362
C6 = C5	1.347	1.345	1.348	1.363
C6−C14	1.497	1.495	1.496	1.475
C5−H13	1.082	1.081	1.081	−
C7 = O8	1.204	1.199	1.204	1.219
C7−O9	1.343	1.343	1.342	1.309
O9−H10	0.970	0.970	0.970	−
C14 = O15	1.204	1.203	1.317	1.235
C14−O16	1.343	1.345	1.221	1.302
O16−H17	0.970	0.969	−	−
O15−H27	−	−	0.997	−
O26∙∙∙H27	−	−	1.687	−
O16∙∙∙H17	−	−	1.654	−
Bond angles (°)
C3−C4−C5	113.1	113.4	113.1	114.7
C3−C4 = O12	123.5	123.7	123.7	124.1
C5−C4 = O12	123.5	122.9	123.2	121.2
C4−C3 = C2	120.9	120.7	120.8	120.8
C4−C3−H11	119.7	120.0	119.8	−
C2 = C3−H11	119.4	119.3	119.3	−
C3 = C2−O1	123.5	123.2	123.6	123.8
C3 = C2−C7	122.1	124.3	122.0	124.4
O1−C2−C7	114.4	112.5	114.4	111.8
C2−O1−C6	118.2	119.0	118.3	118.0
O1−C6 = C5	123.5	122.7	123.5	122.9
O1−C6−C14	114.4	110.7	111.7	114.9
C5 = C6−C14	122.1	126.6	124.9	122.1
C4−C5 = C6	120.9	121.1	120.9	119.7
C4−C5−H13	119.7	118.9	119.2	−
C6 = C5−H13	119.4	120.0	120.0	−
C2−C7 = O8	122.8	121.7	122.7	111.9
C2−C7−O9	112.7	115.6	112.7	122.0
O8 = C7−O9	124.5	122.7	124.6	126.1
C7−O9−H10	107.3	111.0	107.4	−
C6−C14 = O15	122.8	123.8	112.9	119.2
C6−C14−O16	112.7	111.6	121.8	116.0
O15 = C14−O16	124.5	124.7	125.3	124.8
C14−O16−H17	107.3	107.8	−	−
C14−O15−H27	−	−	110.2	−
O15−H27∙∙∙O26	−	−	176.7	−
O16∙∙∙H17−O18	−	−	177.5	−

Table 4

Computed scaled harmonic frequencies (cm−1) for −O−H and −C=O modes in monomers and dimers of Chelidonic acid compared with experimental values. The frequencies are calculated at B3LYP/6–311++G(d,p) level. All the frequencies (in cm−1) are scaled with the single scaling factor of 0.9688 [51,52].

Experimental		M1	M4	D1	D2	D3	D4	D5	D6	Assignments
IR	Raman
2962	−	−	−	3100	3098	3101	3111	3111	3089	O−H stretching (inter-bonded)
2922	−	−	−	3009	3007	3010	2982	2982	2998	O−H stretching (inter-bonded)
3468	−	−	3593	−	−	−	3601	3601	3604	O−H stretching (medium strong inter-and intra-bonded)
3629	−	3615	3620	3614	3623	3624	−	3623	−	O−H stretching (free)
3601	−	3614	−	3614	3614	3624	3614	−	−	O−H stretching (free)
1648	1646	−	−	1702	1703	1703	1697	1698	1691	C=O (carboxyl) stretching (inter-bonded)
1723	1736	−	−	1660	1661	1661	1650	1650	1643	C=O (carboxyl) stretching (inter-bonded)
1830	−	1738	1768	1737	1754	1755	1769	1769	1770	C=O (carboxyl) stretching (free)
1452	1444	−	−	1423	1422	1421	1427	1426	1427	O−H in-plane bending (inter-bonded)
1423	−	−	−	1404	1404	1403	1406	1406	1410	O−H in-plane bending (inter-bonded)
−	−	−	1162	−	−	−	1158	1158	1158	O−H in-plane bending (intra-bonded)
1129	1134	1130	1127	1141	1141	1132	1142	1132	−	O−H in-plane bending (free)
969	965	−	−	920	921	920	939	939	932	O−H out-of-plane bending (inter-bonded)
−	−	−	−	851	852	851	849	849	861	O−H out-of-plane bending (inter-bonded)
603	−	604	564	598	597	565	601	565	−	O−H out-of-plane bending (free)

The geometrical criterion for the formation of an −O−H∙∙∙O bond is the O∙∙∙H contact distance required to be less than the sum of van der Waals (vdW) radii of O and H, namely, 2.720 Å [53]. In Fig. 4, each dimer species has a double inter-bonded O∙∙∙O bonds involving two −COOH groups. In all the dimer species, the two O−H∙∙∙O bond angles are 177° (Table S5). Additionally, the D4 and D are stabilized by a single intra-bond and D is stabilized by two intra-bonds. The D is the most stable with the ΔEint = -17.77 kcal/mol (Table 2). The two O∙∙∙H and O∙∙∙O contact distances are calculated at 1.674, 1.672 and 2.672, 2.670 Å respectively (Table S5). The D has ΔEint = -17.95 kcal/mol. In this dimer the two O15∙∙∙O26 and O16∙∙∙O18 bond lengths are nearly close, 2.670 and 2.669 Å. The O26∙∙∙H27 and O16∙∙∙H17 distances are calculated at 1.673 and 1.671 Å respectively. The D with ΔEint = -18.10 kcal/mol is stabilized by two H-bonds with O∙∙∙H distance at 1.673 Å and the two O∙∙∙O distances being equal (2.671 Å). In D (ΔEint = -17.63 kcal/mol) and D (ΔEint = -17.90 kcal/mol), the O30−H31 group interacts with the pyran ring's oxygen atom, O21, for which the contact distances, O21∙∙∙H31, are calculated at 2.051 and 2.053 Å respectively (these are not shown in Table S5). The inter-molecular O∙∙∙H and O∙∙∙O distances are computed at 1.687, 1.654 and 2.682, 2.655 Å for the dimer D and 1.688, 1.654 and 2.683, 2.655 Å for the dimer D. With ΔEint = -17.22 kcal/mol, the D species, in addition to two inter-molecular O26∙∙∙H27 and O16∙∙∙H17 distances with 1.671 Å, has shown two intra-molecular bonds with O1∙∙∙H10 and O21∙∙∙H31 at 2.055 Å (these are not shown in Table S5) and the two inter-molecular O∙∙∙O distances are computed at 2.669 and 2.670 Å. It may be noted that, although the relative energies of the six dimer species are not negligible, the differences in the ΔEint are rather very small and this may be attributed to the differences in the energies of the monomer components and geometries of the −COOH groups involved in the H-bonding. The ΔEint values are comparable with the experimental values of the dimerization energy of Benzoic acid, -16.20 kcal/mol, derived from the measurements of its monomer and dimer IR spectra [54, 55]. Table 3 shows the geometrical parameters of M, M and D species around the two −COOH groups (see also Fig. 5 and Fig. S6). To the best of our knowledge, there appears to be no crystal structure data for CA in the literature; therefore, it is not untenable to compare the main geometrical parameters of the two −COOH groups in the monomer and dimer species (see Table 3 and Table S5) with the crystal structure data of Isophthalic acid, 5-Fluoroisophthalic acid and Meconic acid [24, 25, 26, 56]. From Table S5, one can see that the bond lengths and angles of the two −COOH groups have shown attendant changes due to the H-bonding.

As shown in Fig. S1 (Supplementary Material), the absorptions in the 3650–2400 cm−1 are characterized by a broad band structure with a series of closely-lying bands at 3629, 3601, 3557, 3468, 3418 cm−1 and 2962, 2922, 2854, 2814, 2756, 2736 cm−1 and 2597, 2564, 2489, 2474 cm−1. We tentatively assign the bands near 3630–3550 cm−1 to a free −O−H stretching mode. The bands near 3470–3410 cm−1 are assigned to both medium strong inter- and intra-molecularly bound −O−H stretching modes and those near 2970–2470 cm−1 to strong inter-molecularly bound −O−H stretching modes. Two strong to medium bands at 3127 and 3055 cm−1 are readily assigned to −C−H stretching modes. Further, the strong bands near 1800–1600 cm−1 in the IR and Raman spectra being characteristic bands of −C=O stretching modes of −COOH groups are rather broad and red-shifted, indicating the participation of −C=O groups in H-bonding. This is consistent with the assignments for formic acids and benzenedicarboxylic acids that show similar modified vibrational structure upon H-bonding [16, 19, 50]. Computed harmonic frequencies of some of the main bands of M, M, D, D, D, D, D and D species in comparison with the experimental values are presented in Table 4. The computed IR spectra of all the dimer species are shown in Figs. 6a and 6b. It is apparent from Fig. 4 that the H-bond forms a local cyclic −O−H∙∙∙O=C configuration influencing the −O−H and −C=O stretching modes of the −COOH group with their frequencies showing red-shifts and intensity enhancements (Figs. 6a and 6b). The observed IR bands at 3629 and 3601 cm−1 are assigned to the free −O−H stretching vibrations. The corresponding predicted frequencies for the M are 3615 and 3614 cm−1. In going from the monomers to dimers, except for the D, D and D, these two frequencies show no substantial shift. In D, D and D, the O30−H31 group is intra-molecularly bonded to the pyran ring oxygen atom O21. This caused a small red-shift in the −O−H stretching frequency predicted at 3601 cm−1 for D and D and is tentatively correlated to a broad absorption at 3468 cm−1, the difference in deviation being ∼4%. It is also possible that in the solid phase of CA, the monomeric structures stabilized by intra-molecular H-bonding are not favored and therefore, in the absence of strong experimental evidence, we tentatively assign the bands observed near 3400 cm−1 to both medium strong inter- and intra-molecularly bound −O−H stretching modes. We have already noted the two band systems with sub-maxima at 2962, 2922, 2854, 2814, 2756, 2736 and 2597, 2564, 2489, 2474 cm−1 in the 3000–2400 cm−1 absorption region (see Fig. S1). We assume all these bands to have originated from the strong inter-molecular H-bonding of the −O−H groups in the dimer species. The large red-shifts, 775 and 1065 cm−1 of the bands at 2854 and 2564 cm−1 with respect to the free −O−H band at 3629 cm−1 demonstrates the strong interaction between the −O−H group of one conformer component and the −C=O group of another conformer component. For D, the bands are calculated at 3100 and 3009 cm−1 (Table 4) and are correlated to the observed bands at 2962 and 2922 cm−1. The D and D species show small shifts in these frequencies and are calculated at 3098, 3007 and 3101, 3010 cm−1 respectively. For D, the frequencies are predicted at 3111 and 2982 cm−1. The two frequencies are comparable with the observed modes at 2962 and 2922 cm−1 respectively. For D and D the bands are 3111, 2982 and 3089, 2998 cm−1 respectively and are also correlated to the observed bands at 2962 and 2922 cm−1. Clearly, the predicted frequencies at B3LYP/6–311++G(d,p) higher level is not much of gain in so far as agreement with the experimental values is concerned. However, we preferred this higher level to a lower level of calculation for reasons of predicting accurate geometries and energies of CA. Further, we tested this premise by performing calculation at BP86/6-31G(d) level as well, yielding satisfactory frequency values for the dimer species (see Table S4). Notwithstanding, the advantage of B3LYP/6–311++G(d,p) level will be evident in AIM analysis to be discussed later where computed values of ρ(r) and its Laplacian ∇2ρ(r) are more closer to the results reported in the literature [57, 58].

Fig. 6

(a) Computed IR spectra of monomers (M and M) and dimers (D, D, D, D, D and D) of Chelidonic acid in the 3700–3000 cm−1 region showing dimerization-induced characteristic band shifts and intensity enhancements. Positions of the free −OH stretching bands are marked with ν. The positions of the intra- and inter-molecular H-bonded −OH stretching bands are marked with ν′ and ν" respectively. In each of the overlaid spectra in the 3700–3500 cm−1, for ease of comparison, the intensities of the free (ν) and H-bonded bands (ν′ and ν") are scaled by a suitable factor. (b) Computed IR spectra of monomers (M and M) and dimers (D, D, D, D, D and D) of Chelidonic acid in the 1800–100 cm−1 region showing dimerization-induced characteristic band shifts and intensity enhancements. In each of the overlaid spectra in the 1000–100 cm−1, for ease of comparison, the intensities of the bands are scaled by a suitable factor.

In carboxylic acid dimers, the symmetric and anti-symmetric −C=O stretching bands show large splitting, as for example, up to 74 cm−1 in the formic acid dimer [15, 59]. Of the two possible −C=O stretching modes, the in-phase carbonyl mode is Raman active and the out-of-phase mode is IR active [15, 59]. In CA, a very strong Raman band appearing at 1646 cm−1 and a strong absorption at 1648 cm−1 are identified with the H-bonded −C=O symmetric stretching mode; the anti-symmetric mode is assigned to a strong absorption at 1723 cm−1 with a corresponding shoulder Raman band at 1736 cm−1 (Table 5). Another strong Raman band at 1750 cm−1 is readily assigned to the oxo group −C=O stretching mode. For the two carboxylic groups in M, the free −C=O stretching modes are predicted at 1738 and 1734 cm−1 and for the oxo group it is computed at 1655 cm−1 (Table 5). Upon association of the two −COOH groups of two different conformer components into an H-bonded dimer, the red-shifts of the carbonyl stretching frequencies are in agreement with the prediction. The symmetric and anti-symmetric carbonyl stretching frequencies for D are predicted at 1650 and 1697 cm−1 respectively. The frequencies calculated at 1427, 1406, 1158, 1142 cm−1 show contribution from −O−H in-plane bending mode. The monomeric band for M at 1130 cm−1 show blue-shift in the dimer D and the frequency is computed at 1427 cm−1. The corresponding IR band is observed at 1452 cm−1 and the Raman band at 1444 cm−1. The intra-molecularly bonded −O−H in-plane bending mode shows a small blue-shift and is predicted at 1162 cm−1 for M. The out-of-phase −O−H bending mode for M is calculated at 604 cm−1 and is observed at 603 cm−1 as an IR band. In D, the two frequencies are calculated at 939 and 849 cm−1; the band at 939 cm−1 is observed at 969 cm−1 in IR and at 965 cm−1 in the Raman spectrum. To conclude, the characteristic blue-shifts in the frequencies of −O−H deformation vibrations are indicative of the −O−H group's involvement in the dimerization. As for the modes of other groups, namely, −CH, −CC and −CO below 1400 cm−1, they are all satisfactorily assigned and for the sake of brevity, are summarized in Table 5 (also see Section S7).

Table 5

Experimental IR, Raman and computed scaled harmonic frequencies for monomers (M and M) and dimer (D) of Chelidonic acid with assignments. sym-symmetric, vs-very strong, s-strong, ms-medium strong, w-weak, vw-very weak, sh-shoulder, oxo-oxo (−C=O) group, carboxyl-carboxyl (−COOH) group. For comparison of the −OH group vibrational modes in the bonded and non-bonded case, monomers M (free), M (intra-molecularly bonded) and the dimer D (both intra- and inter-molecularly bonded) species were chosen. All the frequencies (in cm−1) are scaled with the single scaling factor of 0.9688 [51,52].

Observed frequencies		Computed frequencies			Assignments [Relative weight (%)]
IR	Raman	Monomer		Dimer
		M1	M4	D4
3629 w	−	3615	3620	3614	OH stretch (free) [93]
3601 w	−	3614	−	−	OH stretch (free) [93]
3468 s	−	−	3593	3601	OH stretch (medium strong inter- and intra-bonded) [96]
2962 ms	−	−	−	3111	OH stretch (inter-bonded) [89]
2922 ms	−	−	−	2982	OH stretch (inter-bonded) [92]
3127 s	3110 ms	3101	3100	3107, 3101	CH stretch [97]
3055 ms	3055 w	3101	3105	3100, 3100	CH stretch [98]
1830 w	−	1738	1768	1769	C=O (carboxyl) stretch (free) [93]
−	−	1734	1743	1737	C=O (carboxyl) stretch (free) [94]
−	1750 s	1655	1658	1661, 1657	C=O (oxo) stretch (free) [94]
1723 s	1736 sh	−	−	1697	C=O (carboxyl) stretch (bonded) [82]
1648 vs	1646 vs	−	−	1650	C=O (carboxyl) stretch (bonded) [84]
1603 sh	1592 s	1604	1610	1612, 1599	CC stretch (ring) [79], CO (oxo) stretch [6]
1579 vs	1547 s	1571	1575	1580, 1567	CC stretch [78], CH deformation [12]
1452 w	1444 ms	−	−	1427, 1406	COH deformation (bonded) [76], C−COOH stretch [9]
1423 ms	−	1358	1352	1358	CC stretch [60], COH deformation [15], CO (ring) stretch [8]
−	−	1355	1325	1330	CC stretch [72], CO (ring) stretch [13], COH deformation [9]
1370 vw	−	−	−	1339	CC stretch (ring + C−COOH) [53], COH deformation (bonded) [22], CH wag [12]
1315 vw	−	1298	1281	1300	COH deformation [57], CC stretch (ring CC + C−COOH) [22], CH wag [14]
1257 s	1269 ms	1275	1280	1280	CH deformation [65], CC stretch [12], COH deformation [9], CO stretch (ring) [8]
1241 s	−	1181	1179	1183, 1177	CH deformation [83], CC stretch [13]
1204 sh	−	1153	1162	1158	COH deformation [82], C−COOH stretch [11]
1129 s	1134 s	1130	1127	1142	COH deformation [69], CC stretch (ring + C−COOH) [11], CO (ring) stretch [7]
−	1118 s	1065	1077	1091, 1075	CH wag [69], CO stretch (carboxyl) [14], COC scissoring [10]
−	−	1054	1047	1060, 1048	CH wag [53], CO stretch (ring + carboxyl) [24], CC stretch [13]
969 s	965 ms	−	−	939, 849	OH out-of-plane bending (inter-bonded) [87]
921 s	931 w	927	930	931, 928	COC (ring)+CCC deformation [81], CH wag [12]
−	906 ms	886	883	890, 888	CH out-of-plane bending [91]
796 ms	796 vw	882	878	883, 883	CH out-of-plane bending [92]
−	790 vw	871	866	880, 878	CCC sym stretch [70], CCH deformation [13], C−COOH stretch [8]
781 w	781 vw	866	863	870, 865	C−COOH stretch [72], ring deformation [16], CO stretch (carboxyl) [8]
743 ms	748 sh	768	765	776, 768	C−COOH torsion [79], ring deformation [14]
−	732 s	757	744	764, 745	C−COOH torsion [79], CH wag [12]
694 s	682 w	687	686	688, 685	CCCO + CCOC torsion [63], CH wag [33]
−	−	664	677	711, 694	COO (carboxyl) bend [69], CC stretch [17], COC (ring) bend [6]
−	−	620	638	656, 634	COO (carboxyl) bend [70], CCC bend [24]
603 w	622 vw	604	564	601	OH out-of-plane bending [94]
−	−	584	547	539	OH out-of-plane bending [93]
535 w	550 sh	581	577	593, 584	C−COOH bend [66], OCC (oxo) in-plane-bending [29]
531 vw	540 ms	506	506	512, 505	CCC bend [64], COC (ring) bend [17], C−CO bend (oxo) [12]
486 wv	487 w	478	477	498, 493	C−COH twist [53], CCOC (ring)+CCCO (oxo) torsion [37]
468 w	467 ms	436	436	451, 450	CCOC torsion [59], CCCO torsion [23], COOH torsion [15]
425	−	434	435	441, 434	OCC (oxo)+CCO (carboxyl) bending [73], CCC (ring) rock [24]
413	387 s	420	427	459, 432	C−COH bending [93]
−	365 ms	338	332	370, 337	CCC + COC (ring) bend [53], ring−COOH torsion [44]
−	261 w	320	331	345, 328	CCO (oxo) bend [50], C−COOH bend [45]
−	201 ms	205	208	239, 229	CCC + CCO (oxo) rock [61], C−COOH rock [29]
−	186 ms	160	163	173, 165	CCCC + CCCO (ring) torsion [82], CCCH torsion [14]
−	150 ms	149	142	149, 144	CCOC (ring)+CCCO (oxo) torsion [90]
−	128 ms	120	118	159, 152	C−COOH in-plane-bending [92]
−	104 vw	109	101	125, 112	CCO (oxo) torsion [51], ring−COOH torsion [45]
−	−	48	63	63, 87	Ring−COOH torsion [98]
−	−	45	52	50, 42	Ring−COOH torsion [94]

AIM and NCI analysis of H-bonding

The geometrical criterion satisfied by the dimer species derived from DFT calculations alone is not sufficient to characterize the −O−H∙∙∙O=C bonding because computed H-bonded dimer populations for vapor phase do not adequately correspond to possible inter-molecular linkages in the solid phase of the sample of CA. The analysis of the topological properties of ρ(r) by Bader's quantum theory of Atoms in Molecule (AIM) is an alternative tool to investigate inter-atomic interactions such as H-bonding [37, 38, 41]. The presence of bond critical point (BCP), where the gradient of electron density, ∇ρ(r), vanishes between two interacting atoms, is a good measure for characterizing the H-bond [37, 38, 39, 40, 41, 42]. Experimentally, the crystal structure determinations by the low-temperature X-ray diffraction (XRD) measurements provide reasonably accurate values of the ρ(r) and ∇2ρ(r) [44, 45, 48]. Recently, the inter- and intra-molecular interactions in crystalline Phthalic acid have been measured from the topology of the electron density distributions obtained from a low temperature (20 K) X-ray diffraction analysis and the values are in good agreement with theoretically-fit values [48]. In the present study, the geometries of the selected CA monomers and dimers showing BCP are presented in Fig. 7a–7d. The AIM analysis has provided BCP for each of the covalently bonded C−C, C−H, C−O, C=O and O−H groups. The existence of an intra-bonding between −O−H group and pyran ring oxygen in M is evidenced by a BCP between hydrogen and oxygen atom (see Fig. 7b). Each dimer species exhibits two −O−H∙∙∙O inter-molecular H-bonds. The presence of these two bonds is confirmed by two BCPs between the interacting carboxylic groups (see Figs. 7c and 7d). Furthermore, for D, a BCP for −O−H∙∙∙O intra-molecular bonding has also been determined. The topological parameters calculated at the BCP values along the O∙∙∙H contacts for the monomer and dimer species of CA are presented in Tables Table 6a, Table 6ba and 6b (see also Table S8a – S8c). On comparing with the values of ρ(r) and ∇2ρ(r) computed at the same B3LYP/6–311++G(d,p) level for the same H-bonding in 2-pyrrolidine and pyrrole-2-carboxylic acid, we find good agreement with the values computed for CA [57, 58]. However, as for the agreement with the XRD values of Phthalic acid, it is not satisfactory with respect to the ∇2ρ(r) values because these are known to vary rapidly at bond curvatures depending on dimer species [39, 40, 41]. It is to be noted that the ρ(r) values, 0.0446–0.0487 a.u. for the inter-molecular H-bonds are evidently higher than the values, 0.0217–0.0219 a.u. for the intra-molecular bonding (see Tables Table 6a, Table 6ba and 6b).

Fig. 7

Molecular structures of monomers and dimers of Chelidonic acid (Fig. 7a – 7d) showing bond critical points (BCP). Topological properties of the electron density at BCP for the O∙∙∙H contacts computed at B3LYP/6–311++G(d,p) level are presented in Tables Table 6a, Table 6ba and 6b.

Table 6a

Topological parameters computed at the O1∙∙∙H10/O21∙∙∙H31 bond critical point (BCP) in the monomer and dimer species of Chelidonic acid. All the properties are computed at B3LYP/6–311++G(d,p) level.*Except interaction energy, EHB (kcal/mol), all magnitudes are in a.u. The values of the EHB listed in the above table are computed from potential energy density V(r), by using the expression EHB = 1/2(V(r)) [42].

Species	ρ(r)	∇2ρ(r)	V(r)	∗Interaction Energy, EHB	s*sign(λ2)ρ(r)
M4	0.0219	0.0980	-0.0185	-5.81	-0.0219
D4	0.0219	0.0975	-0.0185	-5.80	-0.0219
D5	0.0218	0.0972	-0.0184	-5.77	-0.0218
D6	0.0217	0.0968	-0.0183	-5.75	-0.0217

Table 6b

Topological parameters computed at the O26∙∙∙H27 and O16∙∙∙H17 bond critical points (BCPs) in the dimer species of Chelidonic acid. All the properties are computed at B3LYP/6–311++G(d,p) level.*Except interaction energy, EHB (kcal/mol), all magnitudes are in a.u. The values of the EHB listed in the above table are computed from potential energy density V(r), by using the expression EHB = 1/2(V(r)) [42].

Species	O∙∙∙H	ρ(r)	∇2ρ(r)	V(r)	*Interaction Energy, EHB	s*sign(λ2)ρ(r)
D1	O26∙∙∙H27	0.0463	0.1366	-0.0435	-13.65	-0.0463
	O16∙∙∙H17	0.0465	0.1372	-0.0439	-13.76	-0.0465
D2	O26∙∙∙H27	0.0464	0.1370	-0.0437	-13.70	-0.0464
	O16∙∙∙H17	0.0466	0.1372	-0.0439	-13.78	-0.0466
D3	O26∙∙∙H27	0.0463	0.1369	-0.0436	-13.69	-0.0463
	O16∙∙∙H17	0.0463	0.1369	-0.0436	-13.69	-0.0463
D4	O26∙∙∙H27	0.0447	0.1350	-0.0416	-13.05	-0.0447
	O16∙∙∙H17	0.0487	0.1396	-0.0467	-14.64	-0.0487
D5	O26∙∙∙H27	0.0446	0.1347	-0.0415	-13.01	-0.0446
	O16∙∙∙H17	0.0487	0.1398	-0.0467	-14.65	-0.0487
D6	O26∙∙∙H27	0.0466	0.1374	-0.0440	-13.81	-0.0466
	O16∙∙∙H17	0.0466	0.1374	-0.0440	-13.81	-0.0466

Noncovalent interactions (NCI) index developed by Johnson et al [43] investigates the van der Waals interactions (vdW), steric clashes (SC), and H-bonds in a molecule based on ρ(r), and their reduced density gradient, s(r), defined as,

An NCI plot of s(r) versus ρ(r) essentially identifies bonding or nonbonding situations by producing the change in s(r) between the interacting units. Further, the value of ρ(r) is a measure of the interaction strength. The attractive and repulsive interactions are discriminated by multiplying the ρ(r) with the sign of second eigenvalue (λ2) of the Hessian (a matrix of second-order partial derivatives of ρ(r)) [43]. The sign of λ2 may either be negative or positive depending on the accumulation or depletion of the electron density between the two interacting atoms. The former situation corresponds to bonding (vdW and H-bonds) where λ2 < 0, while the latter for nonbonding interactions (SC) with λ2 > 0. We show in Fig. 8a–8e plots of the s(r) against sign(λ2)ρ(r) for the monomers and dimers of CA. The plots display the features of the attractive and repulsive NCI in terms of two or more spikes in the low-density and low-gradient regions, while the weak vdW interactions are at near zero density values. Fig. 8b for M shows low-density and low-gradient spikes lying around ±0.01 a.u, being indicative of weak vdW attractions. The spike for SC interactions has appeared at higher positive value. As discussed in the preceding Section 4.1, the M geometry has one intra-molecular H-bond. In Fig. 8c, the low-gradient spike appeared at about sign(λ2)ρ(r) = -0.0219 a.u is attributed to intra-molecular H-bond in M. The sign(λ2)ρ(r) of -0.0219 a.u for this bond as compared to the vdW interactions (±0.01 a.u) suggests that the intra-molecular H-bonding is relatively strong. The s(r) versus sign(λ2)ρ(r) plots for the dimer species are shown in Fig. 8d (for D without intra-bonding) and Fig. 8e (for D with intra-bonding) respectively. In Fig. 8d, the low-gradient but high-density spike appeared at sign(λ2)ρ(r) = -0.0463 a.u, indicating the presence of two strong inter-molecular H-bonds. For D, the spikes have appeared at sign(λ2)ρ(r) = -0.0219 a.u for intra-bond, and -0.0447 and -0.0487 a.u for two inter-bonds. Furthermore, the two inter-molecular H-bonds of D are discriminated as two separate spikes as compared to the one in D due to the differences in the two O∙∙∙H contact distances (0.033 Å and 0.002 Å respectively, Table S5). Owing to the large ρ(r) values, the H-bonds due to inter-molecular association are stronger than intra-molecular association. This is consistent with the changes in the geometrical parameters and frequencies of the monomer and dimer species discussed in the preceding section 3.1 (see Table 4, Tables S4 and S5). Fig. 8b–8e also show the NCI isosurface plots of the monomer and dimer species calculated at the B3LYP/6–311++G(d,p) level. The blue-green-red (BGR) color scheme is used in which the blue region represents strong attractive interactions (O−H∙∙∙O bonds) and the green and red regions represent the weak van der Waals (and C−H∙∙∙O bonds) and strong repulsive steric interactions respectively. The NCI isosurface with a disc shaped volume (red) is characteristic of a strong repulsive steric interaction between the oxygen and carbon atoms of the pyran ring and oxygen of carboxylic groups. The isosurface (green) between the −C−H and −C=O/−C−O groups corresponds to the various weak −C−H∙∙∙O interactions. The presence of blue-green isosurface between the pyran-ring oxygen and −OH of carboxyl group in the M (Fig. 8c) and D (Fig. 8e) species indicate the presence of the intra-molecular H-bond between the −OH and −CO groups. The blue NCI isosurface between the two carboxylic groups in the D and D species is characteristic of the strong inter-molecular O−H∙∙∙O bonding. The section S9 (see also Fig. S9a–S9f) presents the correlation of H-bond −OH group stretching frequencies of Chelidonic acid species with the s*sign(λ2)ρ(r) and interaction energy.

Fig. 8

Plots of the reduced density gradient, s, versus the electron density ρ(r), multiplied by the sign of second eigenvalue (λ2) of the electron-density Hessian matrix, sign(λ2)ρ(r) (left) and the corresponding isosurfaces with isovalue = 0.5 a.u (right) for Chelidonic acid structures. Isosurfaces are colored according to a BGR color scheme (Fig. 8a). Blue indicates strong attraction regime (intra-/inter- H-bonding), green indicates weak interaction regime (such as −C−H∙∙∙O and van der Waals attraction), and red indicates strong repulsion regime (steric clashes). The 2D NCI plots are shown for M (Fig. 8b) and M (Fig. 8c) monomers and for D (Fig. 8d) and D (Fig. 8e) dimer structures. In each of the plots, the low-gradient spikes at about sign(λ2)ρ(r) = ±0.01 a.u represents the weak van der Waals interactions. The spikes at about sign(λ2)ρ(r) = -0.0219 a.u and sign(λ2)ρ(r) = -0.0487 a.u represents the intra- and inter-molecular H-bonds respectively.

Natural bond orbital (NBO) analysis

The nature of electronic charge delocalization accompanied by H-bond formation has been investigated by employing NBO analysis [49]. The charge delocalization, in many cases, influences the conformational behavior, H-bond strengths and IR modes. The natural charges (NC) of the atoms and stabilization energy defined as the strength of the charge transfer between oxygen lone pair orbitals and −OH anti-bonding orbital have been computed to identify and confirm the single/double H-bond structures of CA. In all the D, D, D, D, D and D species, the lone pair orbitals of the carbonyl oxygen and anti-bonding orbital of the hydroxyl group have been identified as the donor and acceptor sites. The natural charges for some selected atoms collected from natural population analysis are presented in Table 7a. As can be seen from Table 7a, on account of the association between the two −COOH groups exchanging natural charges, their simultaneous H-bond donor-acceptor behavior is confirmed. As compared to the monomers, in D, the atoms O16 and O26 become positive as they H-bond with O18−H17 and O15−H27 respectively. On the other hand, O18 and O15 in the hydroxyl groups gain some charges through inter-molecular O−H∙∙∙O dimerization. The bridging atoms, H17 and H27, become slightly more positive. The stabilization energies obtained from the NBO analysis are presented in Table 7b. All the dimers exhibit two inter-molecular donor-acceptor interactions or hyperconjugations defined as n(O16)→σ(O18−H17) and n(O26)→σ(O15−H27). The D and D are characterized by the net energy of 55.87 and 56.59 kcal/mol and D and D species are stabilized by 56.32 and 56.94 kcal/mol respectively. The analysis of Table 7b shows that the two inter-molecular H-bonds in the dimer D and D are characterized by the net stabilization energy of 57.47 kcal/mol. In addition to two inter-molecular interactions, D and D exhibit n(O21)→σ(O30−H31) intra-molecular interaction with the stabilization energies of 1.50 and 1.70 kcal/mol. The two intra-molecular bonding in D (see Fig. 4) are confirmed by n(O1)→σ(O9−H10) and n(O21)→σ(O30−H31) hyperconjugations with stabilization energy at 1.62 and 1.59 kcal/mol respectively. The estimation of the natural charges and stabilization energy of the H-bonds by NBO analysis for the dimer species of the CA confirms the presence of intra- and inter-molecular H-bonding interactions with the latter H-bonding stronger than the former. Consequently, the hydroxyl groups participating in intra-bonding show less red-shift compared to the inter-bonding, and we correlate this to two separate observed IR bands at 3468 and 2962 cm−1 respectively (see Fig. 2 and Fig. S1). The electron donor (−C=O) and acceptor (−OH) sites participated in the hyperconjugation interactions show weakening of bond strengths due to the charge transfer to −OH anti-bonding orbital. Therefore, the increase in the bond lengths and red-shift in the stretching vibrations of the carbonyl and hydroxyl groups are the consequences of the electronic charge delocalization.

Table 7a

Natural charges (NC) of atoms participating in H-bondings as computed by natural bond orbital (NBO) method. The NC are calculated at B3LYP/6–311++G(d,p) level.

Atom	M1	M4	D1	D2	D3	D4	D5	D6
O16	-0.6559	-0.6705	-0.6266	-0.6271	-0.6233	-0.6277	-0.6237	-0.6431
H17	0.4906	0.4956	0.5196	0.5173	0.52010	0.5171	0.5173	0.5173
O18	−	−	-0.6698	-0.6696	-0.6708	-0.6610	-0.6624	-0.6593
O26	−	−	-0.6269	-0.6229	-0.6282	-0.6417	-0.6409	-0.6404
H27	−	−	0.5198	0.5201	0.5177	0.5194	0.5170	0.5172
O15	-0.5676	-0.5563	-0.6698	-0.6687	-0.6684	-0.6679	-0.6674	-0.6591
O1/O21	-0.4496	-0.4938	-0.4465	-0.4466	-0.4413	-0.4969	-0.4979	-0.4990
H10/H31	0.4906	0.4964	0.4912	0.4911	0.4937	0.4943	0.4946	0.4938
O9/O30	-0.6559	-0.6530	-0.5668	-0.6538	-0.6777	-0.6531	-0.6526	-0.6523

Table 7b

Strength of the H-bonded structures (dimers) of Chelidonic acid as computed by natural bond orbital (NBO) analysis at B3LYP/6–311++G(d,p) level. All the numbers are in kcal/mol. n-refers to oxygen atom lone-pair orbital, σ-refers to −OH anti-bonding orbital.

Delocalization	D1	D2	D3	D4	D5	D6
n(O16)→σ*(O18−H17)	28.08	28.71	27.95	30.89	30.93	28.78
n(O26)→σ*(O15−H27)	27.79	27.88	28.37	26.05	26.54	28.69
n(O1)→σ*(O9−H10)	−	−	−	−	−	1.62
n(O21)→σ*(O30−H31)	−	−	−	1.50	1.70	1.59

Conclusions

The −O−H∙∙∙O H-bond mediated composite vibrational structure observed in the IR and Raman spectra (4000–100 cm−1) of Chelidonic acid has been characterized by the structures, interaction energies and harmonic frequencies for the ten monomer and six dimer species computed at B3LYP/6–311++G(d,p) and BP86/6-31G(d) levels. The AIM analysis has provided additional support to the H-bond characterization in terms of BCP values for both intra- and inter-molecular −O−H∙∙∙O H-bonding. The existence of the both type of H-bonding has been demonstrated by BCPs for each of the O∙∙∙H contacts. The electron density plots have provided insights into the nature of the H-bonds. The weak van der Waals and H-bonding interactions exhibited low-gradient spikes at BCPs having different magnitudes of electron densities. The calculation of the natural charges and stabilization energy of the H-bonds for the dimer species by NBO analysis further points to the presence of the stronger inter-molecular H-bonding interactions than the intra-H-bonding. It has been noted that while the geometries and interaction energies are accurately predicted at B3LYP/6–311++G(d,p), the BP86/6-31G(d) level is shown to produce slightly more accurate frequencies. However, the benefits of the B3LYP/6–311++G(d,p) level outweighs when it produced better results for the AIM and NCI calculations. The proposed dimer model is to be treated as a basic unit in oligomers that may make up Chelidonic acid in its solid phase.

Declarations

Author contribution statement

Jagdish Tonannavar: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Shivanand S. Malaganvi: Performed the experiments; Wrote the paper.

Jayashree Yenagi: Analyzed and interpreted the data.

Funding statement

Shivanand S. Malaganvi was supported by the University Grants Commission (UGC), New-Delhi, for the award of Project Fellowship under Major Research Project (MRP) grant No. 41–874/2012 (SR).

Competing interest statement

The authors declare no conflict of interest.

Additional information

Supplementary data

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Supplementary Material_version2

Supplementary content related to this article has been published online at https://doi.org/10.1016/j.heliyon.2019.e01586.