Trends and anomalies in H-AH(n) and CH(3)-AH(n) bond strengths (AH(n) = CH3, NH2, OH, F).

Along the series of H-AH(n) bonds, with AH(n) = CH(3), NH(2), 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 CH(3)-AH(n) the bond strength first decreases from CH(3)-CH(3) to CH(3)-NH(2) and only thereafter increases again along CH(3)-NH(2), CH(3)-OH and CH(3)-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), CH(3)-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) = CH(3) 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 CH(3)* and other second period AH(n)* radicals.