Interpretation of Nitroindolinospirobenzothiopyran Vibrational Spectra

Abstract

The structures of four possible stereoisomers of the closed form of photochromic nitroindolinospirobenzothiopyran (NISTP) {1′,3′-dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzothiopyran-2,2′-(2H)-indoline]} were determined by the DFT method. The geometry of the most stable isomer was defined. Nitro-substitution changes mainly the lengths of bonds formed by S and N with spiro-atom Cs. According to the calculations, the CsS bond changes most and lengthens by 0.019 A. It is shown that the S atom has large displacement amplitudes in normal modes assigned to Raman lines at 230, 285, 360, and 575 cm–1 and weak IR bands at 467 and 577 cm–1. Oscillations involving the nitro group are very active in Raman and IR spectra. Their frequencies are slightly lower than similar frequencies of nitrobenzene and nitroindolinospirobenzopyran, indicating a higher degree of vibrational coupling of the NO2 group with the NISTP molecular skeleton.