Effect of protonation state and interposed connector groups on bond dissociation enthalpies of alcohols and related systems.

High-level quantum chemical procedures have been used to study how the C-H bond dissociation enthalpies (BDEs) of alcohols and related systems are affected by changes to their protonation state. The high-level procedures used have been determined from a benchmark of 25 neutral, protonated, and deprotonated substituted methanes. The benchmark calculations suggest that the experimental C-H BDEs for CH3NH2 and CH3SH should be reassessed. We confirm previous findings that protonation increases the BDEs of alcohols, while deprotonation decreases the BDEs. For the prototypical alcohol, methanol, reducing the strength of the proton donor or acceptor leads to a smaller change in the BDE, and a smooth variation of C-H bond strength with the extent of protonation or deprotonation is observed. Changes in the BDE with protonation state are reduced for alcohols with a connector group separating the oxygen center and the site of C-H bond scission. These changes are rationalized through introduction of three new quantities, termed the effect of protonation state on dissociation energies, the alcohol radical connector energy, and the alcohol molecule connector energy. Gas-phase acidities and proton affinities for all relevant alcohols have been computed and compared with experiment. The agreement between theory and experiment is generally reasonable, with just one notable outlier (the proton affinity of CH3CH2CH2OH). In this case, we suggest that the experimental value should be reevaluated.