Abstract
<title>Abstract</title> <p>Context: Oriented external electric fields can reshape molecular potential-energy surfaces, but controlled comparisons of how simple alkyl substitution changes competing bond-cleavage pathways in selenols remain scarce. Methaneselenol and ethaneselenol were therefore compared at the same level of theory and with the same field orientation. Field-free single-point potential-energy scans showed that replacing methyl with ethyl reduced the C-Se dissociation energy from 4.046 to 3.739 eV, whereas the Se-H pathway changed by only 0.020 eV. With increasing field strength, ethaneselenol exhibited greater C-Se elongation, dipole growth, negative-charge accumulation at Se, and HOMO-LUMO gap narrowing; the magnitude of its barrier-field slope was 21.6% larger than that of methaneselenol. Linear fits to the single-point scan barriers reduced the extrapolated critical field from 26.84 to 20.50 V nm⁻¹, while geometry optimizations placed the corresponding critical-field intervals at 23.91–24.17 and 17.74-18.00 V nm⁻¹. Low-frequency IR/Raman softening, the long-wavelength shift of the UV-Vis response, and the higher boundary vibrational wavenumber supported the same trend. Ethyl substitution therefore selectively lowers the C-Se dissociation threshold through cooperative local bond weakening and enhanced field-induced polarization. Methods Calculations were performed with Gaussian 16 at the B3PW91/6-311 + G(d,p) level. Geometry optimizations, harmonic frequencies, and TD-DFT spectra were combined with field-free and field-dependent single-point potential-energy scans, geometry optimizations near the critical field, and estimates of the tunneling-ionization rate and mean tunneling-ionization time over oriented fields of 0-15.43 V nm⁻¹.</p>