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

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.

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.