Synthesis of Diaminopyrimidine Sulfonate Derivatives and Exploration of Their Structural and Quantum Chemical Insights via SC-XRD and the DFT Approach.

Two heterocyclic compounds named 2,6-diaminopyrimidin-4-ylnaphthalene-2-sulfonate (A) and 2,6-diaminopyrimidin-4-yl4-methylbenzene sulfonate (B) were synthesized. The structures of heterocyclic molecules were established by the X-ray crystallographic technique, which showed several noncovalent interactions as N···H···N, N···H···O, and C-H···O bonding and parallel offset stacking interaction. Hydrogen-bonding interactions were further explored by the Hirshfeld surface (HS) analysis. Nonlinear optical (NLO) and natural bond orbital (NBO) properties were calculated utilizing the B3LYP/6-311G(d,p) level. Frontier molecular orbitals (FMOs) and molecular electrostatic potential (MEP) were calculated utilizing the time-dependent density functional theory (TD-DFT) at the same level. The NBO analysis showed that the molecular stabilities of compounds A and B were attributed to their large stabilization energy values. The second hyperpolarizability (γtot) values for A and B were obtained as 3.7 × 104 and 2.7 × 104 au, respectively. The experimental X-ray crystallographic and theoretical structural parameters of A and B were found to be in close correspondence. Both the molecules reveal substantial NLO responses that can be significant for their utilization in advanced applications.

Introduction

Pyrimidines, also called nitrogen bases, are aromatic systems found widely in nature or also synthesized in the laboratory. In nature, the pyrimidine cores can be found in many remarkable chemical skeletons like antibiotics, caffeine, hormones, etc.[1] Besides, it is also part of valuable chemical architectures frequently available in nature, such as vitamin B1 (thiamine) and nucleic acids like thymine, uracil, and cytosine.[2] Moreover, there are a number of available protocols for the synthesis of these pyrimidine cores. For example, the pyrimidine core can be furnished by the condensation reaction of acetamidine and ethylacetoacetate.[3] Urea is one of the most frequently utilized substrates for the synthesis of these conjugated heterocycles. Kim et al. used the derivatized urea in a basic medium for the synthesis of trisubstituted pyrimidine by the 1,3-dielectrophilic approach.[4] The nonsubstituted pyrimidine skeleton could be accomplished using the nonderivatized urea as a substrate via decarboxylation followed by intramolecular cyclization.[3] Besides these, there are a number of other protocols available for the synthesis of pyrimidine building blocks.[5] Whatever the origin of pyrimidine occurrence, whether natural or synthesized, it has proved to be privileged for therapeutics as remarkable pharmacophores. Some of the fused heterocyclic ring systems are potential antibacterial (5-amino-thiazolo[4,5-d]pyrimidine),[6] antifungal (pyrrolo[2,3-d]pyrimidines),[7] antiviral (2,4-diaminopyrimidinederivative),[8] anticancer (pyrazolo-pyrimidine),[9] anti-inflammatory and analgesic (N-(4-hydroxy-6-tosyl-5,6,7,8-tetrahydropyrido[4,3-d]pyrimidin-2-yl)isonicotinamide),[10] and antimalarial (sulfadiazine) agents, as shown in Figure . These heterocyclic compounds are highly recognized chemical building blocks with significant medicinal applications. Therefore, it will be of enormous interest to explore these classes of compounds in the field of optoelectronics, optical fibers, and telecommunication by investigating their nonlinear optical (NLO) properties.

Figure 1

Functionalized pyrimidine compounds with their pharmacological potential.

Currently, computer-assisted calculation performs a crucial role in the comprehension of reaction mechanisms, structural stability, electronic properties, reacting sites, etc. In this scenario, nonlinear optical (NLO) properties have found significant consideration because of their applications in the areas of information technology (IT) and electronic communication. NLO defines light behavior in nonlinear media due to its ability to respond to the electromagnetic field. Nowadays, organic compounds with excellent NLO properties have become attractive materials for researchers.[11] NLO-associated organic materials are broadly used in the fields of optoelectronics, optical fibers, and telecommunication due to their instantaneous and sharp electronic responses, minor dielectric constants, large electronic delocalization, and intramolecular charge-transfer (ICT) properties.[12] The literature review shows that optical nonlinearities of many organic materials including liquid crystals, polymers, crystals, amorphous materials, semiconductors, and plasmas are well-explored.[13] NLO properties generally appeared due to variation in symmetry, alignment of atoms, and structural design of a molecule.[14] Recently, our research group published reports on the quantum chemical designing and modifications of π-linkers and donor and acceptor groups of chemical structures of organic compounds.[15] Also, very recently our research group has reported that O-benzenesulfonylated pyrimidines possess promising NLO properties.[16] Herein, our results on the synthesis, structural exploration with the help of single-crystal X-ray diffraction (SC-XRD) analysis, and comprehensive computational analysis of crystalline organic compounds, i.e., 2,6-diaminopyrimidin-4-yl naphthalene-2-sulfonate (A) and 2,6-diaminopyrimidin-4-yl 4-methylbenzenesulfonate (B), are given.

Materials and Techniques

Analytical-scale purified reagents and simply distilled solvents were used for the synthesis. For thin-layer chromatography, the Merck-made aluminum sheet having precoated silica gel with specifications of a 0.25 mm thick layer and an F254 fluorescent indicator was used for monitoring the progress of the reaction.

General Procedure: Synthesis of 2,6-Diaminopyrimidin-4-yl Naphthalene-2-sulfonate (A)

For the synthesis of compound A, 1.0 mmol (1 equiv) of 2-amino-6-methylpyrimidin-4-ol was added to a 50 mL round-bottom reaction flask with the addition of K2CO3 (3 equiv) in 10 mL of acetone. After stirring the reaction material mixture at ambient temperature, it was refluxed for 4 h after the addition of naphthalene-2-sulfonyl chloride (1.3 mmol with 1.3 equiv). After completion (monitored by thin-layer chromatography (TLC)), the crude product was obtained upon filtration. Column chromatography was employed for the final purification, and recrystallization was achieved using analytical-grade ethanol (Figure ).

Figure 2

Synthetic pathway of O-aryalsulfonylation of 2,6-diaminopyrimidin-4-ol.

Synthesis of 2,6-Diaminopyrimidin-4-yl 4-Methylbenzenesulfonate (B)

Here, the above-mentioned procedure was adopted exactly, but p-toluenesulfonyl chloride was used as the starting material instead of naphthalene-2-sulfonyl chloride to obtain the targeted 2,6-diaminopyrimidin-4-yl 4-methylbenzenesulfonate (Figure ).

Single-Crystal Analysis

The diffractometer, Kappa APEX-II (Bruker made), possessing an X-ray tube with targeted molybdenum and a monochromator was used for the single-crystal analysis, and CCD of the graphite base was used for intensity recording of the peaks. Additionally, the correction and reduction of data were done using APEX-II and SAINT, respectively.[17] Moreover, the structure solution was achieved using SHELXS97 software[18] and improvement was completed using SHELXL2018/3 to minimize the structural errors.[19] Asymmetric units present in the compounds were represented graphically using the ORTEP technique, while for hydrogen bonding, PLATON was used.[20] The crystallographic details are tabulated in Table .

Table 1

Experimental Details of Entitled Compounds

compounds	A	B
CCDC	1995364	1995365
chemical formula	C14H12N4O3S	C11H12N4O3S
molecular weight (g/cm3)	316.34	280.31
crystal system	monoclinic	monoclinic
space group	P21/c	P21/c
T	296 K	296 K
a, b, c (Å)	13.979 (3), 7.7541 (13), 13.345 (2)	12.573 (2), 7.7930 (9), 13.395 (3)
α, β, γ (deg)	90, 102.289 (8), 90	90, 104.610 (6), 90
V  (Å3)	1413.4 (5)	1270.0 (4)
Z	4	4
density (calculated)	1.487 Mg/m3	1.466 Mg/m3
F(000)	656	584
type of radiation used	Mo Kα	Mo Kα
wavelength (λ)	0.71073 Å	0.71073 Å
μ (1/mm)	0.25	0.27
size of crystal (mm)	0.42 × 0.36 × 0.20	0.38 × 0.30 × 0.16
data collection
diffractometer	Bruker APEX-II CCD	diffractometer
absorption correction	multiscan (SADABS, Bruker, 2007)
no. of measured, independent, and observed [I > 2σ(I)] reflections	8949,  3062,  1999	8086, 2751, 1912
Rint	0.061	0.065
data collection range of θ (deg)	2.983–26.990	3.050–26.990
index ranges	–16 ≤ h ≤17, −8 ≤ k ≤ 9, −17 ≤ l ≤ 16	–16 ≤ h ≤ 16, −8 ≤ k ≤ 9, −16 ≤ l ≤ 17
(sin θ/λ)max (Å–1)	0.639	0.639
refinement
R[F2> 2σ(F2)], wR(F2), S	0.055, 0.139, 1.02	0.055, 0.147, 1.05
no. of reflections	3062	2751
no. of parameters	199	173
treatment of H-atoms	H-atom parameters constrained	H-atom parameters constrained
Δρmax, Δρmin (e Å–3)	0.30, −0.40	0.25, −0.33

Computational Procedure

The quantum chemical computations such as optimized geometries, vibrational frequencies, natural bond orbital (NBO), and NLO properties of the entitled compounds were executed with the help of Gaussian 09 software[21] using “DFT/B3LYP/6-311G(d,p)”.[22] Frontier molecular orbital (FMO) investigation was completed by the method of the “time-dependent density functional theory (TD-DFT) with B3LYP/6-311G(d,p)” level.[23] The hyperconjugative interactions that are liable for the compound’s stability were evaluated by means of natural bond orbital (NBO) analysis.[24] All data of A and B was well-arranged by applying software like Gauss View 5.0,[25] Chem Craft,[26] and Avogadro.[27]

Results and Discussion

Compounds A and B are accomplished according to the procedure mentioned above. After recrystallization from ethanol, the crystals are analyzed via single-crystal XRD analysis. The structural details are given as follows. In A (Figure and Table ), the 2,6-diaminopyrimidin-4-ol (C1–C4/N1–N4/O3) and naphthalene ring (C5–C14) moieties exist in the planar form, having root-mean-square (RMS) divergence values of 0.0037 and 0.0204 Å, respectively. In B (Figure and Table ), the 2,6-diaminopyrimidin-4-ol (C1–C4/N1–N4/O3) and toluene ring (C5–C11) moieties exist in the planar form, with RMS discrepancy values of 0.0264 and 0. 0166 Å, respectively, and have a dihedral angle of 79.02 (7)°. In both entitled compounds, the molecular conformation is described by the C5–S1–O3–C1 torsional angle having values of −81.53(2) and −81.16(2)° for compounds A and B, respectively, and these values are almost identical, which showed that the molecular conformation of both entitled compounds is almost the same. This conformational analysis also suggested that there would be similarity in the H-bonding interaction for both entitled compounds.In both entitled compounds, the molecules are at first connected with each other through strong N4···H4A···N2 bonding in the form of a dimer to form an R22(8) loop as shown in Figure and given in Table . This dimer is directly linked with six of its neighboring molecules via N···H···N and N···H···O bonding to form four R33(10) loops, where the acceptor N-atom is from the pyrimidine ring, which is not involved in the formation of the R22(8) loop, and the O-atom is from the 2,6-diaminopyrimidin-4-ol moiety. The R22(8) loop is also directly connected with four of its other neighboring molecules via N···H···O and comparatively weak C···H···O bonding to make four R21(6) loops, where CH is from the pyrimidine ring and the acceptor O-atom is from sulfonyl group (6) chains formed by N4···H4B···O3 and N3···H3A···N1 bonding, respectively. These C (6) chains run parallel to the crystallographic c-axis in a zigzag manner. In addition to C (6) chains, the C (8) chain is formed by N3···H3B···O1 bonding. These chains connect the R22(8) loop formed by dimerization with its neighboring molecules. The naphthalene ring of A and the toluene group of B are not involved in any H-bonding and are omitted from the packing diagram for clarity. Thus, the molecules are interlinked by strong N···H···N and N···H···O and somewhat weak C···H···O bonding in such a way so as to create a two-dimensional hydrogen-bonded network in the crystallographic plane (100) of base vectors [010] and [001]. The same type of H-bonding and resulting patterns are found in both entitled compounds, but the difference is found with respect to the angle between the atoms involved in H-bonding, separation between the donor and acceptor, and separation between the hydrogen and acceptor, as specified in Table .

Figure 3

ORTEP diagrams of A and B drawn at a probability level of 50%. Hydrogen atoms are indicated by small circles of arbitrary radii.

Figure 4

Packing illustration of molecules A and B. The H-atoms not involved in H-bonding are omitted for clarity.

Table 2

Hydrogen-Bond Geometric Parameters (Å, deg) for A and Ba

	D–H···A	D–H	H···A	D···A	<(D–H···A)°
A	N3···H3A···N1i	0.86	2.46	3.172 (3)	141
	N3···H3B···O1ii	0.86	2.20	3.022 (3)	159
	N4···H4A···N2iii	0.86	2.23	3.054 (3)	161
	N4···H4B···O3iv	0.86	2.56	3.333 (3)	150
	C4···H4···O1ii	0.93	2.65	3.374 (3)	135
B	N3···H3A···N1v	0.86	2.50	3.208 (3)	140
	N3···H3B···O1vi	0.86	2.23	3.043 (3)	157
	N4···H4B···N2vii	0.86	2.24	3.071 (3)	161
	N4···H4A···O3viii	0.86	2.56	3.329 (3)	149
	C4···H4···O1vi	0.93	2.66	3.379 (3)	135

Symmetry codes: (i) x, −y – 1/2, z – 1/2; (ii) x, −y + 1/2, z – 1/2; (iii) −x + 1, −y – 1, −z + 1; (iv) −x + 1, y – 1/2, −z + 3/2; (v) x, −y – 1/2, z + 1/2; (vi) x, −y + 1/2, z + 1/2; (vii) −x, y – 1/2, −z – 1/2; (viii) −x, −y – 1, −z.

During the intermolecular interaction study, parallel stacking interactions[28,29] are found that assist in further stabilization of crystal packing in both entitled compounds. The parallel offset stacking interaction between rings for A and B is shown in Figure . In both compounds, Cg (1) represents the centroid of the pyrimidine (C1–C4/N1/N2), whereas Cg (2) is the centroid of the naphthalene (C5–C14) and benzene ring (C5–C10) in A and B, respectively. In A, the parallel offset stacking interaction is found between pyrimidine rings having an intercentroid distance of 3.623 Å and a ring offset of 1.367 Å with one pyrimidine ring at the general position and the other pyrimidine ring at (1 – x, −y, 1 – z). In B, the inversion-related benzene rings are in contact by the parallel offset stacking communication with an intercentroid distance of 3.962 Å, and a ring offset of 1.593 Å.

Figure 5

Parallel offset stacking interaction of A and B where the distances are presented in angstrom. The H-atoms are not shown for clarity.

For the deep inspection and interpretation of intermolecular interactions, a Hirshfeld surface investigation was performed within software called Crystal Explorer version 3.1.[30] The Hirshfeld surface calculation was carried out by assuming the electron density around the nuclei of all atoms as spherical in symmetry and summing all of the spherical electron densities of all of the atoms involved. Hirshfeld surfaces provided a pictorial way of presenting different noncovalent interactions and finding the strength of interactions.[31−37] In Figure S1a,b, HS was mapped over dnorm in the ranges −0.423 to 1.439 au and −0.408 to 1.343 au for A and B, respectively, in which different colors represent the strength of interactions. Red, white, and blue colors are used for the representation of strong, intermediate, and weak interactions, respectively. Figure S1a,b shows that in both A and B, all of the amino groups and nitrogen atoms of pyrimidine and the O-atom of 2,6-diaminopyrimidin-4-ol were involved in strong interactions along with one O-atom of the sulfonyl group. In both A and B, the white color of HS near the pyrimidine and benzene rings indicated the presence of parallel offset stacking interactions. In Figure S1c,d, HS was mapped over de in the ranges 0.849–2.761 and 0.859–2.769 au for A and B, respectively, with de representing the closest space of an atom contained by the HS to the atom within its surrounding.[38,39] The smallest, intermediate, and long distances were denoted by red, green, and blue colors, respectively. In Figure S1e,f, HS was mapped throughout the shape index in the range −1 to 1 au for both molecules. The occurrence of triangular red regions around the rings indicated the presence of stacking interactions. The occurrence of two triangular regions around rings indicated that there exist parallel offset stacking interactions between similar rings as well as between dissimilar rings. In Figure S1g,h, HS is mapped over curvedness in the −4 to 0.4 au range for both compounds. The regular green sections near the rings indicated the presence of parallel offset stacking interactions. In Figure S1i,j, HS was mapped over the electrostatic potential in the −0.109 to 0.117 au range for A and −0.108 to 0.113 au range for B, in which donors and acceptors are represented by blue and red colors, respectively.[40,41] In both compounds, amino groups were donors, whereas nitrogen atoms of pyrimidine, the O-atom of the 2,6-diaminopyrimidin-4-ol part, and the sulfonyl group oxygen were acceptors.

For a quantitative inspection of intermolecular interaction, two-dimensional (2D) fingerprint plots were drawn, which gave the % of interaction of an atom with other atoms.[42] Two-dimensional fingerprint plots for A and B are represented by Figures S2a–d and S3a–d, respectively, and are given in Figure S4a and b, respectively. H···H contacts were noticed to be the most important contributor between aromatic parts, and their influence was 32.3 and 35.8% for A and B, respectively. In both compounds, the sulfur atom does not interact with any atom. The other strong interatomic contacts for A were O···H, Cs and finding the strength of interaction of H, and Ns and finding the strength of interaction of H with % involvement rates of 25.8, 18, and 14.1%, respectively, whereas the other strong interatomic contacts for B were Os and finding the strength of interaction of H, Ns and finding the strength of interaction of H, and Cs and finding the strength of interaction of H with % contributions of 28.8, 15.3, and 12.4%, respectively. The atoms existing inside the Hirshfeld surface interact powerfully with H-atoms of the molecules situated within the HS environment, and the % contribution values of this interaction were found to be 63.9 and 66.6% for A and B, respectively, as given in Figure S5a and b, respectively. The H-atoms present inside the Hirshfeld surface interact sharply with atoms of the molecules appearing in the surrounding of the HS, and the % contribution values of this interaction were found to be 57.6 and 60.8% for A and B, respectively, as given in Figure S5c and d, respectively.

Molecular Geometric Structures

The bond lengths, as well as their angles of A and B as determined by the utilization of DFT and single-crystal X-ray diffraction, are listed in Tables S1 and S2. The slight difference in structural parameters of entitled compounds was obtained because the DFT computations were carried out in the gaseous phase, whereas the experimental data was performed in the solid state.

Natural Bond Orbital (NBO) Analysis

NBO was used to probe the HB (both inter- and intramolecular), hyperconjugative interactions, and charge transmission between the electron donor (i) and acceptor (j).[43]Equation defines the stabilization energy E(2) regarding the second-order perturbation theory.[44]where the stabilization energy is represented by E(2), the donor orbital occupancy is represented by q, and F(i,j) is the diagonal and offdiagonal NBO Frock matrix elements.[45]

For A and B, the interactions such as LP → σ*, π → π*, and σ → σ* were observed with promising stabilization energies as shown in Tables S3 and S4. Henceforth, the stronger hyperconjugative interactions were perceived due to ICT, which can be responsible for the stabilization of A and B systems.

Natural Population Analysis (NPA)

The atomic charges for A and B were computed by the utilization of NBO analysis. The charge distributions of A and B are presented in Figure S6. A charge distribution demonstrated that all oxygen atoms coupled with a sulfur atom (S1) were negatively charged, but sulfur itself carried a high positive charge (1.119007e). Moreover, all nitrogen atoms and carbon atoms, i.e., N5, N6, N7, and N10 and C13, C16, C17, C19, C21, C23, C25, C26, C28, and C31, carried negative charges, while some carbon atoms like C14, C30, C33, and C34 were positively charged. Similarly, all hydrogen atoms contained a positive charge, as shown in Figure S6. In B, all atoms of oxygen (2, 3, and 4) bonded with sulfur (1) and nitrogen (5, 6, 7, and 10) atoms were negatively charged. Furthermore, sulfur, carbon C27, C28, and C29 and all -H atoms were positively charged, whereas carbon atoms such as C13, C14, C16, C18, C19, C21, C23, and C30 were negatively charged, as shown in Figure S6.

Frontier Molecular Orbital Analysis

These include the “highest occupied molecular orbital (HOMO)” along with the “lowest unoccupied molecular orbital (LUMO)”.[46] The TD-DFT/B3LYP/6-311G(d,p) level was utilized for the calculation of FMO energies of A and B as shown in Table .

Table 3

Calculated Energies (E) via DFT for Compounds A and B

	A		B
MO(s)	E (eV)	ΔE (eV)	E (eV)	ΔE (eV)
HOMO	–6.174	4.198	–6.004	4.667
LUMO	–1.976		–1.337
HOMO – 1	–6.693	5.693	–6.900	6.045
LUMO + 1	–1.347		–0.855
HOMO – 2	–7.077	6.420	–7.032	6.937
LUMO + 2	–0.657		–0.095

E, energy; ΔE (eV) = ELUMO – EHOMO; MO, molecular orbital.

Energy gaps for HOMO/LUMO in A and B were observed as 4.198/4.667 eV. Moreover, for A and B, the HOMO – 1 → LUMO + 1 and HOMO – 2 → LUMO + 2 energy gaps were found at 5.693, 6. 045 eV and 6.420, 6. 937 eV, respectively. The decreasing trend of their energy gap was found to be B > A, which is the same as observed in HOMO/LUMO.

Figure displays the charge density distribution on the molecular orbitals: HOMO, LUMO, HOMO – 1, LUMO + 1, HOMO – 2, and LUMO + 2 of investigated compounds A and B. The charge densities for HOMO are located over the pyrimidine-2,4-diamine largely and slightly over the oxygen atoms of sulfonate. For LUMO, it is present at the methyl naphthalene-2-sulfonate group. Similarly, in compound B, for HOMO, maximal charge is concentrated on the toluene part and minimally over the oxygen atoms of the sulfonate part, and for LUMO, it exists at the 6-methoxyprimidine-2,4-diamine group.

Figure 6

Frontier molecular orbitals of A and B, in electronvolts.

Usually, energy gap describes the properties of a molecule like reactivity, hardness, and softness stability. A high energy gap represents more hardness, nonreactivity, and high stability of studied compounds. Energy values of FMOs are utilized to compute the global reactivity parameters (GRPs).[47,48] To find the values of the electron affinity (A) and the ionization potential (I), eq and eq are employed, respectively.[49]Equation is employed for electronegativity (X), while eq is used for hardness (η) values of A and B.Electrophilicity (ω) can be computed using eq .[50]Equation was used for global softness (σ).The results achieved from eqs –7 for A and B are shown in Table S5.

In A and B, the calculated ionization potentials were found as 6.174 and 6.004 eV, respectively. Similarly, A showed a high electron affinity (1.976 eV), while B afforded a low electron affinity (1.337 eV).

The global hardness value in A appeared higher than B. The electronegativity amplitudes of A and B were found to be 4.075 and 3.670 eV, respectively. Chemical potential amplitudes were used to demonstrate the reactivity and stability of compounds. It has been seen that molecules with less chemical potential were considered additionally reactive, less stable, and vice versa. In A and B, the chemical potential (μ) values were measured at −4.075 and −3.670 eV, respectively. Furthermore, the global softness (σ) values of A and B were seen at 0.238 and 0. 214 eV, respectively. Similarly, global electrophilicity (ω) values were found at 3.955 and 2.886 eV for A and B, respectively. From the above discussion, it could be concluded that ionization potential (IP) values were observed to be much higher than the computed electron affinity. These results strongly support the electron-donating ability of title compounds. These findings showed that all entitled compounds were chemically hard with better kinetic stability as well as electron-donating ability.

Nonlinear Optical (NLO) Properties

Organic compounds with excellent NLO properties have gained significant recognition in various fields of life.[51−53] In the current study, we determined NLO properties of synthesized organic crystals. eq was utilized for the evaluation of average polarizability ⟨α⟩.[54]The second hyperpolarizability (γtot) was worked out using eq .[55]where

For A and B, NLO properties were inspected at the B3LYP/6-311G(d,p) functional. The simulated computations of polarizabilities, hyperpolarizabilities, and dipole moments of A and B are shown in Tables S6 and S7. In A and B, the calculated dipole moment values were found as 7.30 and 4.62 D, respectively. Generally, the decreasing dipole moment trend was observed as A > B (Table ).

Table 4

Dipole Polarizability and Second Hyperpolarizabilities with Their Contributing Tensor (au) of A and B

dipole polarizability			second hyperpolarizability
	A	B		A × 104	B × 104
αxx	267.146	246.650	γX	25.49	19.90
αyy	206.579	161.732	γY	8.151	4.981
αzz	157.776	128.105	γZ	3.418	1.799
⟨α⟩	210.5	178.8	γtot	3.7	2.7

A and B contained the first-order polarizability tensors along the x-, y-, and z-axes, which were computed as 267.132, 206.574, 157.777 and 246.643, 161.736, 128.103 au, respectively, whereas the resulted ⟨α⟩ value was computed as 210.49 and 178.83 au, respectively. In the case of A, the calculated ⟨α⟩ values were observed to be greater than B. In A and B, the γtot (second hyperpolarizability) values were observed as 3.7 × 104 and 2.7 × 104 au, respectively. The results depicted that A has higher γtot values than B.

Molecular Electrostatic Potential (MEP)

The physical and chemical interactions of a compound of any kind could be investigated via the MEP plot.[56] To find the suitable regions for electrophilic/nucleophilic attack on the entitled molecule, the MEP plot can be utilized.[57] Orange, green, red, yellow, and blue colors on the MEP diagram indicate the possible points that favor electrophilic/nucleophilic attack on molecules. The extent of the electrostatic potential in the decreasing order is observed as blue > green > yellow > orange > red. Characterization of MEP can be performed using eq .In the above-mentioned equation, ZA represents the nuclear charge at RA and ρ(r′) expresses the electron density.

In Figure , the red regions on MEP indicate the presence of oxygen, nitrogen, and sulfur atoms in chemical structures of A and B. The blue and green colors represent carbon and hydrogen atoms in A and B. Therefore, oxygen, nitrogen, and sulfur atoms have more electron density, which could be favorable positions for electrophilic attacks. The carbon and hydrogen atoms with blue and green colors are electron-deficient zones, which could attract nucleophilic moieties.

Figure 7

MEPs and the color schemes of A and B.

Conclusions

The syntheses of 2,6-diaminopyrimidin-4-yl naphthalene-2-sulfonate (A) and 2,6-diaminopyrimidin-4-yl 4-methylbenzenesulfonate (B) are successfully accomplished. The structures of the synthesized molecules were confirmed by the SC-XRD technique. The SC-XRD analysis uncovered that both A and B have a monoclinic crystal system with the space group 21/. Additionally, the Hirshfeld surface analysis was also performed for the exploration of noncovalent interactions in terms of crystal structure stability of the titled compounds. Furthermore, NBO analysis showed that the titled molecules are involved in the intramolecular charge transfer and hyperconjugation that is involved in greater molecular stability. MEP and NPA analyses also suggested noncovalent interactions in both compounds, which endorsed the findings of SC-XRD and Hirshfield surface analyses. The chemical reactivity and charge transfer of the title compounds are described by the FMOs. The global reactivity parameters were calculated from energies of FMOs. Herein, the global hardness (η = 2.333 eV) of B is found to be higher than A (η = 2.099 eV), whereas the global softness values of A and B are determined as 0.238 and 0.214 eV, respectively. The global ionization potential values for A and B were found to be 6.174 and 6.004 eV, respectively. The global reactivity data revealed that compound B is more stable than compound A. Moreover, A contains greater average polarizability ⟨α⟩ and dipole moment values than B and the same trend was observed in terms of the second hyperpolarizability (γtot) responses. Nonlinear optical (NLO) properties of the synthesized compounds may have important benefits for NLO-based technological applications.