Experimental and theoretical vibrational study of isatin, its 5-(NO₂, F, Cl, Br, I, CH₃) analogues and the isatinato anion

Effects of 5-R substitution (R = NO₂, F, Cl, Br, I, CH₃) and N-deprotonation on the 4000-400 cm^-1 region of the low temperature FT IR spectrum and the molecular structure of solid isatin are investigated. Harmonic IR spectra and molecular geometries of the 5-R isatins (except for Br and I analogues) are calculated at the HF/6-31G(d, p) level and compared with the experimental solid-state data. In general, substitution has small effect on the molecular structure and the IR spectrum of isatin. The ν(CO) triplet in the IR spectra of isatin and its 5-substituted analogues is resulted by vibrational splitting of the out-of-phase CO stretching, ν_op[(CO)₂]. While the frequency of the ν_op[(CO)₂] mode is relatively less affected by 5-substitution and mainly depends on the substituent mass, the frequency of the in-phase stretching, ν_ip[(CO)₂], is strongly sensitive to both mass and electronic properties of the substituent. Substitution at C5 has relatively greater influence on the electron density and the force constant of the amide than on the ketone carbonyl group. Strong electron-donors shorten and stabilize the unusually long α-dicarbonyl CC bond, while electron-accepting groups tend to stretch this bond further. N-Deprotonation brings to elongation of the five membered-ring along the N-C_CO(ketone) vector and expansion of the bonds within the α-dicarbonyl part. Theoretical ν(CO) frequency of isatin is lowered for about 180 cm^-1 upon conversion into isatinato ion. Harmonic vibrational analysis reveals that only the highest-frequency ν(CO) mode of the isolated isatinato anion can be considered good group vibration for empirical assignments in spectra of solid isatinates. Owing to the solid-state influences on the ν[(CO)₂] modes, no reliable spectra-structure correlations could be established from the present experimental spectroscopic data.