Hydrolysis-induced electronic redistribution and spectral differentiation of isocycloseram and its hydrolysis products: a spectroscopy-supported DFT/TD-DFT study.

Isocycloseram (ISM) and its hydrolysis products TP1 and TP2 were investigated by combining experimental FTIR, Raman, and UV-Vis spectroscopy with DFT/TD-DFT calculations. B3LYP/6-311G(d,p) was used for geometry optimization, vibrational assignments, and comparative ground-state descriptors, while CAM-B3LYP, ωB97X-D, and diffuse-augmented TD-CAM-B3LYP/6-311+G(d,p)/SMD(acetonitrile) calculations were used to evaluate UV-Vis assignments. TP1 showed a carbonyl stretching redshift accompanied by a longer calculated C=O bond, a lower normal-mode force constant, and altered local charge separation, whereas its Mayer bond order did not support a simple single-descriptor bond-weakening explanation. Hydrolysis increased the Kohn-Sham HOMO-LUMO gap from 4.53 eV for ISM to 4.75 and 4.67 eV for TP1 and TP2, respectively, while the decrease in absolute polarizability largely followed molecular-volume reduction. Transition density matrix (TDM) analysis was combined with quantitative exciton descriptors, including the hole-electron centroid distance (D), spatial overlap (Sr), and separation index (t), as well as excited-state charge-density-difference (CDD) maps and interfragment charge-transfer (IFCT) analysis; together, these analyses indicated predominantly local excitation for ISM and TP1. TP2 State 3 and State 4 were assigned to the experimental 253 and 206 nm bands, respectively, and showed modest state-specific electron-hole redistribution without dominant charge-transfer character. These results provide a spectroscopy-supported framework for interpreting hydrolysis-induced electronic redistribution and spectral differentiation of ISM transformation products.