Synthesis and experimental versus theoretical research on spectroscopic and electronic properties of 3-methyl-4-nitroisothiazole

Abstract

The alternative way of synthesis of valuable 5-amino-3-methylisothiazole and 3-methyl-4-nitroisothiazole has been presented and described. It appeared that new, undescribed 3,3′-dimethyl-4,4′-dinitro-5,5′-bisisothiazole has been produced as a side-product during synthesis. Both experimental and calculated spectral and electronic properties of 3-methyl-4-nitroisothiazole have been extensively researched and discussed. The complete vibrational assignments were made on the basis of potential energy distributions PED. The fully anharmonic infrared and Raman spectra with calculated anharmonic intensities for fundamental bands, overtones as well as combination bands are presented. Vibrational spectra predicted by anharmonic approximation agree very well with experimental data. The stability of molecules of 3-methyl-4-nitroisothiazole arising from hyper conjugative interaction has been estimated using natural bond orbital (NBO 6.0) analysis. The combined frontier molecular orbital and NBO analysis of canonical molecular orbitals was used to predict the most probable sites of 3-methyl-4-nitroisothiazole molecule where single electron can be accepted to and detached from. Measured reduction potential of 3-methyl-4-nitroisothiazole and calculated electron affinity point out that this compound is susceptible to reduction at the similar level of 1-methyl-3-nitropyrazole and this susceptibility is much more than respective 1-methyl-4-nitropyrazole. UV-Vis spectra analysis reveals a nature of valence electron excitation and electron transition of 3-methyl-4-nitroisothiazole. Besides, unambiguous assignment of NMR signal shifts of carbon atoms of isothiazole ring of 3-methyl-4-nitroisothiazole, 5-amino-3-methylisothiazole and intermediate and side products of synthesis was conducted thanks to full detailed analysis of 1H, 13C NMR spectra and their two dimensional (2D) variants. Additionally, molar enthalpy of vaporization (ΔHvap) of 3-methyl-4-nitroisothiazole has been estimated (ΔHvap = 52.2 kJ/mol).