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

## TL;DR

This paper calculates accurate thermochemical properties of halomethanes, explaining acidity and reactivity patterns using advanced molecular orbital theory.

## Contribution

The study extends the G3(MP2) method to iodine-containing compounds and provides the most comprehensive high-accuracy dataset for hydrogen-containing halomethanes.

## Key findings

- Calculated gas phase acidities agree with experiments within ±4 kJ/mol, except for CH2F2.
- Aqueous pK a values for halomethanes range from 28.0 to 53.6, with CHF2I showing unexpectedly high acidity.
- Strong metal amides are required to deprotonate less acidic dihalomethanes, as confirmed by an experimental case study.

## Abstract

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