Ethyl substitution enhances field-induced polarization in selenols and lowers the C-Se bond dissociation threshold.

CONTEXT: Alkyl substitution may alter the polarization and bond-selective dissociation of selenols in oriented external electric fields. We compare methaneselenol (CH3SeH) and ethaneselenol (C2H5SeH) to determine how replacing methyl with ethyl affects their structural, electronic, spectroscopic, and dissociation responses. Increasing the field strength elongates the C-Se bond, increases the dipole moment, and generally narrows the frontier-orbital energy gap in both molecules. Changes in C-Se-related infrared bands, red shifts of ultraviolet-visible absorption, and lower excitation energies indicate field-induced electronic redistribution and bond weakening. These responses are more pronounced for C2H5SeH. Potential-energy scans show preferential promotion of C-Se cleavage, whereas Se-H cleavage is affected less strongly. The magnitude of the fitted slope of the C-Se scan barrier versus field strength is approximately 21.5% greater for C2H5SeH. Linear extrapolation to a zero scan barrier gives critical fields of approximately 26.8 and 20.5 V nm-1 for CH3SeH and C2H5SeH, respectively, a reduction of approximately 23.5%. These extrapolated values suggest that ethyl substitution enhances field-induced polarization and lowers the C-Se dissociation threshold. METHODS: Geometry optimizations, vibrational analyses, and TD-DFT calculations were performed using Gaussian 16 at the B3PW91/6-311+G(d,p) level. Structural, electronic, and spectroscopic responses were examined over 0-15.43 V nm-1. Single-point potential-energy scans at zero and finite fields were combined with estimates of tunneling ionization rates and mean ionization times.