AB Initio (DFT) and Vibrational Studies of the Synthesized Heterocyclic Compound 2-6-oxo-2-Thioxotetrahydropyrimidin-41H-Ylidene Hydrazine Carbothioamide

About 6g 0.25mol of sodium metal is dissolved in 200ml of ethanol. To this solution 15g 0.25mol of thiourea and 40ml of diethyl malonate is added. The reaction mixture is refluxed for 6hrs in an oil bath and then vacuum distillated to recover ethanol. The clear solution thus obtained is filtered, cooled in an ice bath overnight and the resulting solution is acidified with HCl. The crude product obtained is collected, washed with 50ml water and dried in oven at 105-110⁰C for nearly 4 hrs. The obtained white coloured precipitate Thiobarbituric acid-TBA is purified by recrystallization with ethanol. M.P - 243C, yield-80%.

An equimolar mixture of thiobarbituric acid 2.2g 0.01mol, thiosemicarbazide 2.5g 0.01mol are dissolved in ethanol, the content is condensed on an oil bath for 4 hours, then the reaction mixture is standing overnight and the precipitate obtained is filtered washed with ethanol and dried to obtain the Dark yellow colour powdered product. Yield - 68%; M.P. - 174⁰c. The scheme and of the reaction are represented in Figure 1.

Figure 1.Scheme of The synthetic reaction of formation of 2 - 6 – Oxo - 2- Thioxotetrahydropyrimidin – 4 1H - Ylidene Hydrazine Carbothioamide.

The absorption value of λ max at 385.5 nm and 256 nm preferably favors n-π* and n-* transition due to lone pair of atoms present in the compound. The NH frequency value is absorbed experimentally at 3720 cm-¹ and 3255 Cm-¹ in IR and Raman and theoretically they are assigned at 3570 Cm-¹ and 3377 Cm-¹. The functional group C=N in the compound confirms the formation of hydrazone and by using DFT/6-31G (d, p) they are computed and the values are probably noted at 1633 Cm-1 which coincides well with experimental IR and FT-Raman values 1639 Cm-¹ and 1604 Cm-¹. The NH2 and C-N vibrational frequencies are illustrated at 1537 Cm-¹ and 1249 Cm-¹ in IR and at 1517 Cm-¹ and 1301 Cm-¹ in Raman which slightly varies from the calculated scaled value 1583 Cm-¹ for NH2 functional group and 1308 Cm-¹ for C-N group. The lone pair atoms oxygen and sulphur with carbon C=O and C=S shows the vibrations at 1834 Cm-¹ and 1165 Cm-¹ in IR and in Raman C=S shows a medium peak at 1049 Cm-¹ which agrees well with examined values 1763 Cm-¹ for C=O and 1127 Cm-¹ for C=S. The torsional C=S vibration of the substituted group is absorbed at 644 Cm-¹ at IR and 647 Cm-¹ at Raman which correlates well with scaled value 612 Cm-¹. The frequency output table is shown in Table S1 and the respective IR, Raman theoretical and experimental spectra are shown in Figure 2 and the above spectral values assigned are found to be in good agreement with literature ¹¹.

Figure 2.Experimental and Theoretical IR and Raman spectra of OTTHPYHCT

The correlation graph between the calculated and observed values is drawn by the RMS value obtained by using the below eq. (1) correlation coefficient of the study compound is determined and it is represented in Figure 3.

Figure 3.Correlation Graph of OTTHPYHCT between Experimental and Theoretical Value

RMS= — 1

The RMS error of the observed IR and Raman bands are found to be 23.83, 12.05 for DFT method. The small differences in observed and calculated vibrational modes are due to the performance of calculation in the gaseous phase.

The proton NMR shows NH _Hydrazide peak at 7.908 and NH _RING peak at 10.218, 11.923. The active methylene hydrogen CH₂ methylene is confirmed by the presence of a sharp peak at 4.64. The amine NH₂ proton noted at the substituted chain gives a peak at the range 9.48 respectively.

¹³C-NMR spectrum gives peaks at 183.3, 175.39, 166.45, 154.81 and 82.49 for respective carbon atoms present in the functional group of the molecule. The visual presentation of ¹HNMR and ¹³CNMR are shown in Figure 4 and Figure 5.

Figure 4.1H-NMR spectra of OTTHPYHCT

Figure 5.13C-NMR of OTTHPYHCT

The optimized geometric parameters of OTTHPYHCT calculated by DFT/B3LYP/6-31G (d, p) are represented in Table S2 shown with atom numbering in Figure 6. The molecule under investigation consists of one ring and a substituent chain is attached. The ring and substituent chain are attached by C=N hydrazone moiety. The introduction of the new substituent in the ring results in distortion of the ring from the planar position and thereby the bond angle, bond length of the compound after substitution varies from the parent compound ¹².

Figure 6.Optimized structure of OTTHPYHCT

The bond length of C6-N13 of hydrazone linkage is about 1.2855 A whereas the C-N bonds values are nearly about 1.3 A . The reason may be hybridization where C=N is Sp² hybridized and C-N SP³ hybridized. Lone pair atoms show the difference in bond length i.e. the bond length of C=O are shorter than C=S mainly due to polarizing nature. N-H and C-H bond length are calculated to be nearby 1 A . The dihedral angles of N2-C6-N13, C5-C6-N13, C6-N13-N14 are 127.2 A, 116.9 A , 118.8 A which is in agreement with the literature ¹³. All the bond length and bond angles values of functional groups present in the compound are tabulated in Table S2. A small difference is observed between computed value and experimental literature value as computed deals with gaseous phase and the other deals with the solid phase.

FMO analysis using computational methods is widely employed to explain the electronic as well as the optical properties of organic compounds ¹⁴. According to the frontier molecular orbital theory FMO, the shapes and symmetries of the highest occupied and lowest unoccupied molecular orbitals HOMO and LUMO are crucial in predicting the reactivity of a species and the stereo chemical and regiochemical outcome of the chemical reaction. The electronic reconfiguration and electronic excitations helps in studying the electric and optical properties of the organic molecules. Overlapping of molecular orbitals decides the stabilization of the bonding orbital and destabilization of the antibonding molecular orbital. Here, irrespective of HOMO and LUMO, the s and p orbital lobs were interlinked and the electron clouds were shared their spatial orbital’s and also spent their time in the place of link. During that time, the analogous physical and chemical properties mutually shared between the atoms. The charge levels were fluctuated due to the electron cloud sharing. A typical electron density distribution of HOMO and LUMO for studied compounds is shown in Figure 7. The HOMO of is OTTHPYHCT mainly located around the substituted thiosemicarbazide linkage, LUMO is located over the C=O, C=S and thiobarbituric ring. From the Figure 7, it could be seen that HOMO of OTTHPYHCT is mainly located over C=S moiety, LUMO of this molecule is in the C=O, C=S moiety and C=N bonded to ring. The energy gap obtained is about 4.1868 eV. And the HOMO-LUMO orbital values are represented in Table S3.

Figure 7.HOMO-LUMO diagram of OTTHPYHCT

To compute the reactivity of the synthesized thiosemicarbazone OTTHPYHCT we also evaluate various global reactivity indices, like the chemical potential, chemical hardness, electrophilicity and nucleophilicity ^{15, 16, 17, 18, 19, 20, 21, 22}.

The vertical ionization potentials IPs and electron affinities EAs were obtained from the HOMO-LUMO values, which can be used to determine the global electrophilicity value ω. It is a measure of the energy stabilization of the system. The global electrophilicity value was calculated as μ2/2η ²³, where μ is the chemical potential approximated as - IP +EA/2, and η is the chemical hardness approximated as IP - EA. Chemical hardness measures the resistance to change in the electron distribution in a collection of nuclei and electrons ²⁴. The calculated values of the chemical potential, chemical hardness and global electrophilicity are 4.12 eV, 2.09 eV and 4.05 eV, respectively. The value of η for thiosemicarbazone OTTHPYHCT is small, which indicates that it is relatively soft on the scale of hardness.

According to the definition, the electrophilicity index is a measure of the propensity of chemical species to accept electrons. The reactivity of the nucleophile is characterized by a lower value of μ, ω, and conversely a good electrophile is characterized by a high value of μ, ω. This new reactivity index measures the stabilization in energy when the system acquires an additional electronic charge ΔN max from the environment ²⁵.

— 2

The maximum charge transfer ΔN max towards the electrophile was evaluated using Eq. (2). Thus, while the quantity defined by S= 1/ɳ describes the softness of the molecule i.e. Eq. (2) describes the charge capacity of the molecule. Very recently, Ayers and co-workers ^{26, 27} have proposed two new reactivity indices to quantify nucleophilic and electrophilic capabilities of a leaving group, nucleofugality ΔEn and electrofugality ΔEe, defined as follows,

— 3

— 4

From the above mathematical formulation the electronic charge ΔN _max, reactivity indicesnucleofugality ΔEn and electrofugality ΔEe are found to be 1.97, 9.22 and 0.98.

NBO analysis is an efficient tool for investigation of charge transfer in a molecular system ²⁸. The lone pair atoms nitrogen, oxygen and sulphur present in the molecule results in hyper conjugative interaction which helps in stabilization of the molecule. From NBO Table S4 the energies of n-π* transition N1→C3-S7, N1→C4-O12, N2→C3-S7, N2→C6-N13, N14→C16-S20 are larger and are found to be 269.37, 207.32, 292.42, 174.64, 213.76 KJ/mol probably when compared with n-* transition which shows lower energy values for N15→C16-S20, O12→N1-C4, O12→C4-C5, N13→N2-C6, S7→N1-C3, S7→N2-C3, C5, S20→N14-C16, S20→N15-C16, C16→S20→C16-S20 donor to acceptor and the energy values are as follows 195.77, 121.5, 88.74, 68.16, 49.37, 48.37, 43.14, 31.71 KJ/mol. The above interaction in the molecule results in weakening of the respective bond and makes the entire molecule highly polarized which helps in NLO study of the considered molecule. Further from the above electron transition from donor to acceptor clearly indicates the flow of electron from parent thiobarbituric ring to the thiosemicarbazide substituent attached to the ring.

Materials having high nonlinear optics NLO responses are very useful in optoelectronic. Devices and non-linear optics have great applications in information technology, and in other Industries. NLO materials also have wide range applications in photonic communication, digital memory devices, national defense and in pharmaceutical industry²⁹.

Compounds having electron donor and electron acceptor groups along with a conjugated system can be considered as strong candidates for such applications. In order to investigate the relation between molecular structure and NLO properties, the first hyperpolarizibility value was simulated at the B3LYP/6-31G d, p level of theory along with the additional keyword POLAR and mathematically calculated using the following equation: The value of the first hyperpolarizibility is determined in a.u. and then converted to esu using the conversion factor 1 a.u. = 8.6393X10^-33 esu. The calculated first hyperpolarizability parameters of the compound aregiven in Table S5. As reflected from the data, the first hyperpolarizability values show the same trend according to the electron donating and withdrawing capacity, and extended conjugation pattern.

On the basis of vibrational analysis at B3LYP/6-31G d, p level, the standard statistical thermodynamic functions: heat capacityC⁰ p, m, entropy S⁰ m and enthalpy changesH⁰ m for the title compound were obtained from the theoretical harmonic frequencies listed in Table S6. It can be observed that there thermodynamic functions are increasing with Temperature ranging from 100-1000K due to the fact that the molecular vibrational intensities increase with temperature ³⁰ and it is graphically represented in the Figure 8.

Figure 8.Thermodynamic Plot of Otthpyhct