Noncoincidence Effects of Dimethyl Carbonate in Binary Mixtures Probed by Raman Spectroscopy: Experimental and DFT Calculations.

The components of isotropic Raman and anisotropic Raman for dimethyl carbonate (DMC) dispersed in cyclohexane and acetone at different volume fractions were recorded separately. The noncoincidence effects (NCE) of the ν7(C=O) stretching mode were calculated accordingly. The NCE values (ΔνNCE) of the ν7(C=O) versus DMC volume fractions in the DMC/C6H12 mixtures exhibits a convex (upward) curvature pattern, while the ΔνNCE vs concentration in the DMC/CH3COCH3 mixtures exhibits a concave (downward) curvature. These different NCE behaviors in the different binary mixtures may arise from the solvent-induced aggregation character. Thus, monomer and dimer structures of DMC were optimized and the vibration spectra were obtained using density functional theory (DFT) calculations. An aggregation model was suggested to expound the DMC's characteristic NCE behavior and concentration effect. We found that the theoretical spectra from DFT/polarizable continuum model calculation based on the aggregation model is in accordance with our experimental data. Solvent-dependent experiments show the ΔνNCE values increase with the decrease of the solvent dielectric constant under the identical volume fractions.

Introduction

Most of the reactions take place in the liquid phase in the form of mixtures, and various molecular interactions like solute–solute, solute–solvent, and solvent–solvent interactions are present in the solution.[1−3] The competition between these interactions plays a crucial role in determining ground-state and transition-state structures,[4−6] reaction pathway and kinetics, activation energy, etc..[5−7] For a binary solute–solvent solution, the dilution process of the solution results in three effects, i.e., breaking of the solute aggregation structure,[8,9] changes of concentration fluctuation applicable solution theory, and modification of the local chemical composition.[10] These effects can be efficiently traced by the polarized Raman spectra (i.e., isotropic and anisotropic Raman spectra) of the specific vibrational mode.[9,11−13] The phenomenon of the noncoincidence frequency of the same vibrational mode between isotropic and anisotropic Raman spectra was named as the noncoincidence effect (NCE).[13,14] The analysis of aggregation complexes in the presence of the solvent interactions has been discussed by the dilution process of solution; changes take place in the aggregation behavior of the solute, i.e., it may either make or break, according to the Raman NCE behavior.[13,14] By Raman NCE behavior measurements, the molecular interactions in pure liquids (solute–solute interaction), solutions (solute–solvent interaction), and mixtures (solute–another solute interaction) have been studied.[15−19] It is a useful and reliable tool to study the properties of solutions of ketone,[9,14,17,20,21] amines,[17] alcohols[22−24] etc.

Vibration spectra are a useful index to intermolecular interactions.[25] Concentration effects and solvent effects mainly reflect in frequency shifts,[22−24] changes in absolute Raman cross section,[11] vibrational line full width at half-maximum,[3] and the value of noncoincidence effects.[13,14] Of course, solvents can also cause more profound effects, such as solubility, equilibrium geometry structure, and aggregation behavior in their ground state.[26] In this paper, we report the intermolecular interaction between solute–solute C=O vibration coupling and solute–solvent interaction of dimethyl carbonate (DMC) and their competition behavior in different concentrations or in various solvents by Raman spectroscopy. Based on the density functional theory (DFT) calculations and the isotropic and anisotropic Raman spectra assignment, we know that the interactions that are taking place between dimethyl carbonate and the solvent mainly come from the C=O group of dimethyl carbonate. Experiments show that the NCE of C=O stretch mode is a particularly meaningful index to intermolecular interactions, including transition dipole–transition dipole coupling (TD–TD) and van der Waals force, and demonstrate that isotropic Raman and anisotropic Raman spectroscopy techniques provide dynamic molecular pictures of the competition between solute–solute and solute–solvent interactions with different concentrations and different solvents.

Results and Discussion

The vibrational spectra have been calculated for DMC using DFT to assign and describe the molecular spectra obtained by Fourier transform infrared (FT-IR) and FT-Raman. Figure shows the comparison among the FT-IR, FT-Raman, and micro-Raman spectra. Table displays the DFT calculated vibrational wavenumbers with B3LYP-D3/6-311G(d,p) basis set, corresponding to the experimental vibrational frequencies obtained by FT-Raman and FT-IR accordingly. The corresponding data between the DFT calculated vibrational frequencies and the experimental vibrational frequencies is in good agreement. The dashed lines in Figure illustrate the correlation of the vibrational frequencies in FT-IR to the corresponding frequencies in FT-Raman and micro-Raman, respectively. The noticeable frequency difference (17 cm–1 frequency difference) of the C=O stretch mode obtained from the FT-Raman spectrum and FT-IR measurements is one key character of the noncoincidence effect. The C=O vibration TD–TD interaction may account for this noncoincidence effect. The TD–TD interaction organized the DMC in a short- or long-range orientation order; we proposed one aggregated dimer model, and the corresponding DFT calculations based on the DMC monomer and the proposed dimer structure had been carried out to manifest the rationality of the model. The comparison between these data and the isotropic and anisotropic Raman spectra at different concentrations will be discussed later.

Figure 1

Comparison of the micro-Raman and FT-Raman with FT-IR spectra of DMC.

Table 1

B3LYP-D3/6-311G(d,p) Computed Frequency, Depolarization Ratio, ZPE-Corrected Total Energy of the DMC Monomer and Dimera

	computed/cm–1				experimental/cm–1
	monomer		dimer		Raman	IR
modes	freq.	D. ratio	freq.	D. ratio			descriptions
ν1	3159	0.68	3166/3166	0.68/0.68			H–C–H asymmetric stretch
ν2	3153	0.62	3160/3160	0.68/0.68			H–C–H asymmetric stretch
ν3	3126	0.75	3144/3144	0.72/0.72			H–C–H asymmetric stretch
ν4	3122	0.75	3143/3143	0.74/0.74			H–C–H asymmetric stretch
ν5	3050	0.01	3061/3061	0.01/0.01			H–C–H symmetric stretch
ν6	3046	0.01	3059/3059	0.05/0.04			H–C–H symmetric stretch
ν7	1825	0.23	1815/1793	0.30/0.12	1753	1736	C=O stretch
ν8	1500	0.73	1507/1506	0.73/0.74			H–C–H scissor
ν9	1494	0.75	1503/1450	0.73/0.74			H–C–H scissor
ν10	1492	0.73	1497/1492	0.75/0.73			H–C–H scissor
ν11	1487	0.75	1488/1486	0.67/0.72			H–C–H twist
ν12	1480	0.75	1486/1481	0.72/0.73			H–C–H twist
ν13	1469	0.74	1468/1464	0.68/0.69	1460	1458	H–C–H wag
ν14	1275	0.48	1296/1272	0.62/0.55			O–C–O stretch
ν15	1213	0.43	1224/1221	0.46/0.50			O–CH3 twist
ν16	1193	0.75	1203/1199	0.75/0.75			CH3 twist
ν17	1178	0.75	1185/1185	0.72/0.72			CH3 wag
ν18	1174	0.75	1176/1175	0.73/0.73			CH3 wag
ν19	1110	0.55	1114/1114	0.53/0.53			O–CH3 stretch
ν20	1038	0.65	1040/1038	0.70/0.70			O–CH3 stretch
ν21	869	0.14	878/876	0.11/0.11	916	924	C–O stretch
ν22	789	0.75	800/783	0.60/0.20			C=O out-of-plane bend
ν23	641	0.47	644/642	0.44/0.43			O–CH3 wag
ν24	580	0.58	582/581	0.58/0.58			O–CH3 wag
ν25	353	0.13	352/351	0.12/0.12			O–CH3 out-of-plane bend
ν26	244	0.71	254/250	0.75/0.74			O–CH3 rocking
ν27	216	0.75	216/216	0.75/0.75			CH3 rocking
ν28	161	0.75	173/170	0.72/0.72			CH3 rocking
ν29	141	0.75	161/155	0.75/0.74			H–C–H rocking
ν30	110	0.75	135/133	0.65/0.68			H–C–H rocking
ν31			98/91	0.69/0.68			C–H rocking
ν32			78/63	0.72/0.71			relative translation
ν33			43/28	0.73/0.75			relative translation
ZPE-corrected total energy (kJ/mol)			monomer		HF = −1037149.32
			dimer	HF = −2074374.10	ΔE = HF(dimer) – 2HF(monomer) = −75.46

Remarks: In the dimer, there are synchronous and asynchronous vibrational modes and the synchronous vibrational frequency is less than the asynchronous vibrational frequency.

Full conformational optimization for the DMC was performed to establish the most stable molecule structure as the initial geometry for further calculations. The polarizable continuum model (PCM) solvent model at the B3LYP-D3/6-311G(d,p) level of theory was applied to calculate the vibrational spectra of the DMC monomer and dimer dispersed in C6H12. Figure demonstrates the calculated most stable structures of the monomer and dimer of DMC. The dimer geometry adopted a face-to-face, head-to-tail antiparallel pattern with intermolecular weak interaction. The as-calculated theoretical vibrational frequencies, their corresponding depolarization ratios, the assignment of experimental Raman and IR spectra, and the ZPE-corrected total energy of the monomer and dimer are given in Table . TD–TD interactions drive molecules to reorientate their alignment, reduce the molecular potential energy, and gain the most stable conformation. From Table and Figure , we observe that the TD–TD coupling of two neighbor C=O stretching modes from the dimer differentiates the interaction in two ways. One interaction is synchronous and the second is asynchronous, which leads to a discrepancy in the C=O vibrational frequency and depolarization ratio. The synchronous C=O stretching frequency lies below the asynchronous one.

Figure 2

B3LYP-D3/6-311G(d,p) computed geometry of DMC and its aggregates.

The DMC dimer has 24 atoms and produces 66 normal modes of vibration, in which 60 normal modes arise from the synchronous and asynchronous coupling between these two neighbor molecules and 6 normal modes come from the relative translation and rotation of these two neighbor molecules. The assignment and description of the experimental Raman and IR frequencies for monomer and dimer structures are listed in Table . Synchronous and asynchronous vibration patterns produce differences in vibrational wavenumbers; the value of these differences depend on the strength of interaction between neighbor molecules. The intermolecular interactions between different parts of the neighbor molecules have different strengths. The computed frequencies in Table show that the intermolecular coupling in the dimer structure breaks the degeneracy of the individual vibrational level; however, only few interaction pairs split distinctly. These splits may still beyond the Raman resolution limits. Thanks to the prominent difference in the depolarization ratio of coupled C=O stretching pairs, we can take advantage of these differences to preferentially collect parallel and perpendicular polarized Raman spectra separately. With the parallel and perpendicular polarized Raman spectra, we can get the isotropic and anisotropic components of Raman spectra using the equation.[27]where IVV(v) and IVH(v) are experimentally collected Raman intensities of the polarized and depolarized Raman components, respectively, and v is the frequency in cm–1. For the detailed meaning of VV and VH, please see the Experimental and Computational Methods section.

Due to the significant difference in the depolarization ratio (ρ), this collected parallel or perpendicular polarized spectra preferentially get the relative small ρ component or large ρ of the C=O stretching pairs, respectively; these two ρ components for the C=O stretching pairs have different frequencies and, thus, present the noncoincidence effects of these two components, that is the so-called NCE phenomenon. In other words, only the interaction pairs with a distinct difference simultaneously in the vibrational frequency and depolarization ratio present the NCE phenomenon. By screening the DFT computed data in Table , only C=O vibration mode satisfies these terms. Figure shows the isotropic and anisotropic components of Raman spectra of DMC for ν7(C=O) vibration. The isotropic component frequency at 1750.2 cm–1 was ascribed to the computed frequency at 1793 cm–1. The anisotropic component at 1754.5 cm–1 was assigned to the calculated frequency at 1815 cm–1. Their corresponding depolarization ratios are 0.12 and 0.30. The frequencies calculated using the dimer model are consistent with the experimentally observed isotropic Raman and anisotropic Raman spectra.

Figure 3

ν7(C=O) vibration isotropic and anisotropic parts of the Raman spectra in the region 1710–1830 cm–1 for DMC and six other volume fractions of DMC, 0.900, 0.800, 0.700, 0.600, 0.500, and 0.400 in the binary mixture (DMC + C6H12).

Noncoincidence effects always come with the concentration effect. Figure illustrates the polarized Raman spectra (isotropic and anisotropic parts) of DMC at various volume fractions in the DMC/C6H12 mixture. It demonstrates that when the DMC concentration decreases, both of the polarized Raman frequencies of C=O stretch, including the isotropic and anisotropic Raman frequencies, increase. The NCE value (that is, ΔνNCE = νaniso – νiso) is 4.3 cm–1 for DMC, while it reduces to 1.9 cm–1 for DMC at ΦA = 0.4 in the DMC/C6H12 mixture. To explore the relationship between the changes in the frequency and the volume fraction, both the C=O stretching frequencies of the isotropic and anisotropic Raman spectra in six volume fractions, 0.900, 0.800, 0.700, 0.600, 0.500, 0.400 in the binary mixture (DMC + C6H12), as well as in pure DMC are abstracted from Figure , which were drawn as a function of concentration, as shown in Figure . The ΔνNCE will eventually becomes 0 at an extreme concentration ΦA. The Raman frequencies of both components increase with the decrease of solute concentrations. Especially, the isotropic ν7(C=O) frequencies for neat DMC and ΦA (DMC in C6H12) of 0.400 are 1750.2 and 1754.0 cm–1, respectively. That is to say, the wavenumbers of isotropic C=O stretching blue-shifted by 3.8 cm–1 from the highest to the lowest concentration of DMC, while other vibrational bands remain the same. The NCE value, that is, ΔνNCE = νaniso – νiso, can be calculated from Figure and is plotted with volume fractions in C6H12, as shown in Figure . The ΔνNCE for the C=O stretching mode decreases upon dilution with C6H12 from 4.30 cm–1 in neat DMC to 1.90 cm–1 at ΦA (DMC in C6H12) of 0.400. The change of ΔνNCE with dilution is due to the decrease in the TD–TD interaction. For the variation of isotropic and anisotropic Raman peak frequencies of C=O stretching mode with volume fractions from 0.1 to 1, see the Supporting Information, S1.

Figure 4

Variation of isotropic and anisotropic Raman peak frequencies of the C=O stretching mode of DMC as a function of solute volume fractions (DMC + C6H12).

Figure 5

Variation of NCE of C=O stretching mode of DMC as a function of solute volume fractions (DMC + C6H12).

The noncoincidence phenomenon between isotropic and anisotropic Raman spectra of the C=O mode in DMC implies that there may exist C=O coupling that degenerated the C=O vibration frequency by dipole–dipole interactions. Dipole–dipole coupling inclines to array the molecules to lower its total energy. During the dilution process, the emergence of a large number of solvent molecules isolate the reference molecules and weaken the dipole–dipole interaction, till the C=O coupling between the solute molecules breaks. In consideration of the low energy barrier (75.46 kJ/mol) between the DMC monomer and dimer from DFT calculations, it is easy to transform from dimer to monomer, and vice versa. During the dilution process, the DMC-aggregated dimer structure gradually separates and breaks into monomers; accordingly, the pattern of Raman spectra gradually switches from the dimer feature to the monomer feature. By scrutinizing the calculated frequencies for dimers in Table , only ν7(C=O) coupling pairs present distinct vibrational frequency difference; moreover, their vibration frequencies (1815/1793 cm–1) are all lower than that of the monomer (1825 cm–1). These computed results are in accordance with the experimental data that we observed in Figure , that is, the frequency of C=O stretching blue-shifted upon the dilution process (dimer transforms to monomers) and simultaneously the peak sharpens and become symmetric. The value of ΔνNCE gets smaller and smaller upon dilution.

Until now, all positive and negative ΔνNCE cases have been reported, which depend on the orientation of the dipoles by TD–TD interactions. For the case of DMC, the ΔνNCE is positive and it takes on the antiparallel side-by-side interaction of the intermolecular C=O dipoles, which is shown in Figure . It is well known that the NCE phenomenon is a spectroscopic manifestation of the existence of the resonant intermolecular dipole–dipole coupling by the TD–TD interaction.[28,29] This coupling results in the degeneration in energy of resonant dipole oscillators by assuming a short-range orientational order.[21,28,30] The property and value of ΔνNCE depend on the pattern of short-range orientation and the strength of the TD–TD interaction. The Born–Oppenheimer approximation and all quantum chemistry state that the electric dipole moment remains constant at different concentrations. However, the fluctuation of the concentration of the reference molecules changes the relative alignment of the coupling dipoles. The decrease of the concentration leads to a weakening of the coupling between resonant dipole oscillators and results in the decrease of the value of ΔνNCE.

To investigate the dependence of NCE behavior on the solvents properties, especially on polarity of the solvents, the NCE measurements were extended to polar solvents such as CH3COCH3 and a thorough comparison was made between these two different polarity solvent data.

Similarly, we carried out the concentration dependence experiments also. The measured νiso and νaniso components of the C=O stretching mode vs volume fractions in DMC/CH3COCH3 mixtures are plotted and shown in Figure . From this, we learn that the νiso of C=O stretching (isotropic Raman frequency) increases with the dilution of DMC in DMC/CH3COCH3 mixtures, whereas the νaniso component decreases with the dilution of DMC. The fitted curve of DMC in DMC/CH3COCH3 mixtures displays a downward (concave) curvature feature, contrary to the upward (convex) fitted curve obtained for nonpolar solvents, as shown in Figure .

Figure 6

Concentration dependence of the isotropic and anisotropic Raman frequencies for the ν7(C=O) stretching mode of DMC in the binary mixture (DMC + CH3COCH3).

By drawing the values of νiso and νaniso components of C=O stretching, shown in Figure , we can calculate the ΔνNCE values, which are shown in Figure . Figure clearly shows that the fitted curve of ΔνNCE vs volume fractions in DMC/CH3COCH3 mixtures presents a downward (concave) curvature feature. On the contrary, as shown in Figure , the upward (convex) curvature was obtained for the relationship of ΔνNCE vs volume fractions in DMC/C6H12 mixtures. These results are the same as the rule found for acetone.[31] Moreover, this rule has been manifested by further MD simulations. They ascribed the phenomenon to the decrease (or the increase) of the pair alignment in the acetone/DMSO (or acetone/CCl4) mixtures. When εsolute > εsolvent, a convex curvature of ΔνNCE with respect to the solute volume fraction was observed, whereas a concave curvature was observed for the case of εsolute < εsolvent. Similarly in our study, because εDMC > εC, a upward (convex) curvature was obtained for the ΔνNCE in the DMC/C6H12 mixtures, whereas a concave curvature was obtained for the ΔνNCE in the DMC/CH3COCH3 mixtures. Thus, a similar conclusion can be drawn that the decrease (or the increase) of the dimer structure of DMC in the DMC/CH3COCH3 (or DMC/C6H12) mixture may account for the shift of peak frequencies.

Figure 7

Concentration dependence of the NCE of C=O stretching mode of DMC in the binary mixture (DMC + CH3COCH3).

Figures and 7 show that the character of ΔνNCE behavior largely depends on the relative dipole moment of the solute to the solvent. To further know the effect of dipole moment on ΔνNCE, a series of solvents with different static dielectric constants were chosen to prepare the same concentration of DMC (ΦA = 0.5) and their isotropic and anisotropic Raman spectra of DMC (ΦA = 0.5) were collected, as shown in Figure . The ΔυNCE vs ΦA is illustrated in Figure , which shows that under the same volume fraction the ΔυNCE increases with the decrease of the solvent dielectric constant, which is in good agreement with Logan’s theory.[32] Within this theory, the character shown in Figure could be expected, that is, a downward (convex) curvature for a lower-polarity solvent and an upward (concave) curvature for a higher-polarity solvent.

Figure 8

Isotropic and anisotropic parts of the ν7(C=O) vibration Raman spectra of DMC in the binary mixture with different solvents (ΦA = 0.500).

Figure 9

Variation of NCE of C=O stretching mode of DMC as a function of solvent dielectric constant.

Figure 10

Scheme for calculated rule and the expected curves of NCE vs concentration.

To get further evidence on this property rule and strengthen the rationality of the dimer model, DFT calculations in association with the polarizable continuum model (PCM) at the hybrid B3LYP-D3 levels of theory with the 6-311G(d,p) basis set have been carried out to obtain the DMC dimer structure using the Gaussian 09 program. The results are shown in Table , wherein the C=O vibrational frequencies, the corresponding depolarization ratios, and the ΔυNCE of DMC in a variety of solvents are listed in detail. The calculations show that with the decrease of the solvent dielectric constant the ΔυNCE increases. This result is based on our proposed dimer model and is consistent with the conclusion drawn from Figure , which further manifests the rationality of our proposed dimer model.

Table 2

DFT/PCM Calculated C=O Vibrational Frequencies, Depolarization Ratios, Intermolecular Distance (Rd/Å), NCE, and ΔE of the DMC Dimer in a Variety of Solventsa

			dimer
solvents	dielectric constant (ε)	dipole moments (μ)/D	freq.	D. ratio	NCE/cm–1
CH3COCH3	20.70	2.91	1789/1773	0.11/0.13	16
CHCl3	4.81	*	1799/1781	0.74/0.12	18
CCl4	2.24	0	1811/1789	0.21/0.12	22
C6H12	2.02	0	1812/1790	0.24/0.12	22
dimethyl carbonate	3.09	0.91	1827/1802	0.46/0.12	25

Asterisk (*) indicates undetermined values.

In an extremely diluted solution, the ΔυNCE will vanish; this is independent from the solvent. In sum, our solvent-dependent experiment shows that, in a given concentration, the value of NCE increases with the decrease of the solvent dielectric constant. By extending this rule to the whole concentration, a upward (convex) fitted curve for a lower-polarity solvent and a downward (concave) fitted curve for a higher-polarity solvent could be expected, which conform to the conclusion reported by many scientists.[31,33−35]

All our experimental spectra and dimer model-based computational calculations show a satisfactory relationship between the NCE character and the resonant dipolar coupling as well as the solute to solvent polarity in mixtures at molecular resolution.

Conclusions

Experimental Raman and IR spectroscopy techniques in association with DFT/PCM calculations have been applied to investigate the NCE phenomenon of the ν(C=O) band of DMC in the liquid mixture. The ΔνNCE of the νC=O stretching vs volume fraction exhibits a downward (concave) fitted curve and an upward (convex) fitted curve in the DMC/CH3COCH3 and DMC/C6H12 mixtures, respectively. The order of the dielectric constant accounts for this phenomenon, εCH > εDMC > εC, a solvent having a larger dielectric constant can easily destroy the aggregate structure (short-range dimer structure) of DMC in the mixtures. To explain this phenomenon, a dimer model was proposed and the vibrational frequencies have been computed for both the monomer and dimer; NCE concentration effects can be explained by the transformation between dimer and monomer forms. During the dilution process, the DMC gradually transformed from a dimer form to a monomer form, corresponding to the blue shift of the ν7(C=O) vibrational frequencies. The solvent-dependent properties of the NCE phenomenon can also been explained using the dimer model. The experiment shows that the ΔνNCE increases with the decrease of the solvent dielectric constant. The DFT calculations based on the dimer model give a consistent picture with the experimental results. Our dimer model provides a satisfactory explanation of the NCE phenomenon, concentration dependence properties, and solvent dependence properties.

Experimental and Computational Methods

The experimental setup of Raman spectroscopy has been reported in the literature with modifications.[9,36] Briefly, the experimental apparatus consists of a triple monochromator (TriVista TR557, Princeton Instruments) equipped with an argon ion laser (Coherent, CVI MELLES GRIOT) as a source of excitation light at 488 nm (75 mW output) and with a liquid nitrogen-cooled CCD array (Princeton Instruments Inc.) allowing a wavenumber coverage of 1089 cm–1 and a spectral resolution (the instrumental apparatus function, FWHM) of 2.0 cm–1. The accuracy in the measurement (the physical matrix pixel of the CCD camera) of the band positions was 0.45 cm–1. Raman spectra were collected for DMC at the concentration ranging from 40 to 100% in the DMC/C6H12 mixture. A 488 nm laser was used to produce the Raman scattering signals, and a backscattering geometry was applied to collect the Raman-scattered light. The polarized Raman measurements were carried out using a polarizer and an analyzer in the VV and VH polarization configurations. First, using a polarizer by vertically (V) polarizing the exciting laser and then using an analyzer by alternatively selecting the vertically (V) or horizontally (H) to the polarizer to collect the VV or VH Raman-scattered signal. The experiments are carried out with identical environment conditions at room temperature (298 K) and atmospheric pressure. The obtained Raman frequencies were plotted and fitted with the polynomial equation y = intercept + ax + bx2.

The FT-IR spectra were obtained with 2 cm–1 resolution using an FT-IR spectrometer (Thermo Nicolet avatar 370, Thermo Fisher Nicolet). The FT-Raman spectra were obtained with an FT-Raman spectrometer at 1064 nm excitation (Thermo Nicolet 960, Thermo Fisher Nicolet).

Computational density functional theory (DFT) helps to better understand the characteristic thermally stable structure of the molecule. Herein, DFT calculations at the B3LYP-D3/6-311G(d,p) level of theory were carried out to optimize the structure and for the calculation of vibrational frequencies. DFT calculations are based on a Gaussian program.[37]