Abstract
<jats:p>Selenenic acids (RSeOH) are reactive intermediates of atmospheric, biological, and environmental importance, but their thermochemistry has been experimentally inaccessible and computationally under-characterized. We report the first systematic thermochemistry of five selenenic acids (HSeOH, CH3SeOH, C2H3SeOH, C2HSeOH, C6H5SeOH) using three independent composite methods, G4, mChS (a modified jun-ChS scheme, applied here for the first time to selenium), and DLPNO-CCSD(T)/CBS, by way of isodesmic S/Se exchange reactions anchored to the sulfenic acid values of Ventura et al. (J. Phys. Chem. A 2022, 126, 6091); atomization energies were employed for the purpose of diagnostic comparisons rather than as a primary method. Recommended enthalpies of formation (kcal mol-1, 298.15 K) are HSeOH = −20.6 ± 0.8, CH3SeOH = −28.7 ± 0.8, C2H3SeOH = −3.1 ± 1.0, C2HSeOH = +40.1 ± 0.9, and C6H5SeOH = +4.4 ± 1.1. Across all five substituents the Se/S shift is nearly constant at +(7–8) kcal mol-1, giving a simple scaling rule (selenenic acids are ~7.2 kcal mol-1 less stable than their sulfenic analogs). Eleven X–H and X–OH bond dissociation energies are reported using the three methods (the sample standard deviations across the available methods average 1.7 kcal mol-1 and range from 0.1 to 2.4 kcal mol-1), including CH3SeO–H = 80.9 ± 1.1 and both phenyl bonds at the three-method consensus (C6H5Se–OH = 67.4 ± 1.4, C6H5SeO–H = 80.2 ± 2.2). The pipeline is supported by external bond dissociation energy (BDE) checks: CH3S–H within experimental uncertainty and HSO–H within ~2 kcal mol-1 of the high-level sulfenic benchmark (with CH3SO–H as a qualitative third comparison), while CASSCF/NEVPT2 analyses confirm single-reference character for the CH3SeO• radical but reveal pronounced multireference character for the parent HSeO• radical, whose O–H BDE is therefore reported from SC-NEVPT2 corroborating G4 (84.6 ± 1.1 kcal mol-1). These results provide the first systematic isodesmic thermochemical dataset for the selenenic acid family and the first composite-method BDE table beyond the sulfenic benchmark.</jats:p>