Correlated Molecular Orbital Theory Predictions of Hydrogen-Containing Halomethane Thermochemistry: Heats of Formation, C–H Bond Dissociation Energies, and pKa Values

Heats of formation, bond dissociation energies, proton affinities, gas phase acidities, and pKa 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 ΔG. Aqueous pKa 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 (pKa 28.0–34.2) are deprotonated by nonorganometallic bases (KOH, DBU, KOtBu), whereas less acidic dihalomethanes (pKa ≳ 38), particularly fluorodihalomethanes (pKa 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.