Can continuum-solvated DFT resolve the bisulfite protonation-isomer equilibrium? A quantitative benchmark of S- and O-protonated HSO3
Year: 2026
Type: preprint
Abstract: The aqueous bisulfite anion supports experimentally established S- and O-protonated isomers whose relative populations vary with temperature, making the pair a compact test of solution-phase electronic-structure methods. We carried out 46 independent Q-Chem 6.2 calculations spanning conformer generation, B3LYP, PBE0 and ωB97X-D, diffuse double-, triple- and quadruple-ζ basis sets, SMD and PCM water, gas-phase controls, frequency calculations and temperature-dependent thermochemistry. Four S-protonated starts collapsed to one minimum within 0.035 kcal mol-1, whereas four O-protonated starts formed a 0.412 kcal mol-1 manifold. At B3LYP/def2-SVPD/SMD, the O-protonated minimum was 12.11 kcal mol-1 lower in electronic energy, but a def2-TZVPD single-point comparison on the selected minima inverted the ordering to +0.641 kcal mol-1, favoring the S-protonated isomer. Energy decomposition localized the 12.75 kcal mol-1 basis shift almost entirely to the intrinsic electronic term; the differential SMD polarization remained near −5.46 kcal mol-1. def2-QZVPD changed the O−S gap by only 0.008 kcal mol-1 for B3LYP, 0.016 for PBE0 and −0.073 for ωB97X-D, establishing triple-ζ convergence. Combining the basis-converged B3LYP electronic energy with validated qRRHO corrections gave ΔG298 = −0.473 kcal mol-1 and Qi = 2.22, close to the experimental Qi = 2.67. However, experiment decreases from Qi = 3.31 at 278 K to 1.63 at 358 K, whereas the calculation increases from 2.03 to 2.54. The associated effective enthalpy changes sign, from −1.746 kcal mol-1 experimentally to +0.550 kcal mol-1 computationally. Continuum-solvated DFT can therefore reproduce the room-temperature equilibrium fortuitously while failing the more stringent temperature-dependent benchmark.
Source: ChemRxiv
Author: Aris Tsai
Institution: Stanford University
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7204958614