Trends and anomalies in H–AH_n and CH₃–AH_n bond strengths (AH_n = CH₃, NH₂, OH, F)

Along the series of H–AH_n bonds, with AH_n = CH₃, NH₂, OH, and F, the bond dissociation energies show a steady increase as can be expected from the increasing difference in electronegativity along this series. However, in the same series for CH₃–AH_n the bond strength first decreases from CH₃–CH₃ to CH₃–NH₂ and only thereafter increases again along CH₃–NH₂, CH₃–OH and CH₃–F. To understand the origin of the apparent anomaly occurring for the trend in C–A bond strengths, we have analyzed the bonding mechanism in H–AH_n, CH₃–AH_n and other model systems, using density functional theory at BLYP/TZ2P. We recover that increasing electronegativity difference across a bond causes an increasing stability. But we also find that the nature of the bond changes qualitatively for AH_n = CH₃ due to the saturation of A with hydrogens. The need of the methyl group to adopt an umbrella shaped geometry plays a key role in the difference between bonds with CH₃˙ and other second period AH_n˙ radicals.