Correlated Molecular Orbital Theory Predictions of Hydrogen-Containing Halomethane Thermochemistry: Heats of Formation, C–H Bond Dissociation Energies, and p <i>K</i> _a Values

Heats of formation, bond dissociation energies, proton affinities, gas phase acidities, and p<i>K</i>_a values in water, dimethyl sulfoxide, acetonitrile, and tetrahydrofuran were calculated for all hydrogen-containing halomethanes and methane using composite correlated molecular orbital theory at the G3(MP2) and Feller-Peterson-Dixon (FPD) levels. Notably, the G3(MP2) method was extended to include iodine-containing compounds. The calculated gas phase acidities generally agree with available experimental data within experimental error limits, often within ±4 kJ/mol; however, CH₂F₂ is a significant exception where theory and experiment differ by nearly 40 kJ/mol for the acidity Δ<i>G</i>. Aqueous p<i>K</i>_a values range from 53.6 for CH₃F to 28.0 for CHF₂I. The latter's unexpectedly high acidity results from the CF₂I^- anion resembling a CF₂ carbene interacting with an iodide anion. These computed values rationalize literature base choices for anion generation: trihalomethanes (p<i>K</i>_a 28.0-34.2) are deprotonated by nonorganometallic bases (KOH, DBU, KOtBu), whereas less acidic dihalomethanes (p<i>K</i>_a ≳ 38), particularly fluorodihalomethanes (p<i>K</i>_a 42-49), require strong metal amides (e.g., LTMP, LDA), with LHMDS proving inadequate. An experimental CHBrCl₂ case study corroborates these predictions, showing clean deprotonation with lithium amides compared to diminished efficiency with weaker bases due to competitive hydroxide addition. This work provides the most comprehensive high-accuracy thermochemical data set for the complete set of hydrogen-containing halomethanes.