Synthesis, Crystal Structural Investigations, and DFT Calculations of Novel Thiosemicarbazones

The crystal and molecular structures of three new thiosemicarbazones, 2-[1-(2-hydroxy-5-methoxyphenyl)ethylidene]-N-methyl-hydrazinecarbothioamide monohydrate (1), 2-[1-(2-hydroxy-5-methoxyphenyl)ethylidene]-N-ethyl-hydrazinecarbothioamide (2) and 2-[1-(2-hydroxy-4-methoxyphenyl)ethylidene]-N-ethyl-hydrazinecarbothioamide acetonitrile solvate (3), are reported and confirmed by single crystal X-ray diffraction, NMR and UV-vis spectroscopic data. Compound (1), C11H15N3O2S ̈H2O, crystallizes in the monoclinic with space group P21/c, with cell parameters a = 8.2304(3) Å, b = 16.2787(6) Å, c = 9.9708(4) Å, and β = 103.355(4) ̋. Compound (2), C12H17N3O2S, crystallizes in the C2/c space group with cell parameters a = 23.3083(6) Å, b = 8.2956(2) Å, c = 13.5312(3) Å, β = 91.077(2) ̋. Compound (3), C11H15N3O2S ̈C2H3N, crystallizes in the triclinic P-1 space group with cell constants a = 8.9384(7) Å, b = 9.5167(8) Å, c = 10.0574(8) Å, α = 110.773(7) ̋, β = 92.413(6) ̋, and γ = 90.654(7) ̋. DFT B3LYP/6-31(G) geometry optimized molecular orbital calculations were also performed and frontier molecular orbitals of each compound are displayed. The correlations between the calculated molecular orbital energies (eV) for the surfaces of the frontier molecular orbitals to the electronic excitation transitions from the absorption spectra of each compound have been proposed. Additionally, similar correlations observed among three closely related compounds, (4), 2-[1-(2-hydroxy-4-methoxyphenyl)ethylidene]-N-methyl-hydrazinecarbothioamide, (5), 2-[1-(2-hydroxy-6-methoxyphenyl)ethylidene]-N-methyl-hydrazinecarbothioamide acetonitrile monosolvate and (6), 2-[1-(2-hydroxy-6-methoxyphenyl)ethylidene]-N-ethyl-hydrazinecarbothioamide, examining structural differences from the substitution of the methoxy group from the phenyl ring (4, 5, or 6 position) and the substitution of the terminal amine (methyl or ethyl) to their frontier molecular orbital surfaces and from their Density Functional Theory (DFT) molecular orbital energies provide further support for the suggested assignments of the title compounds.