The abnormally long and weak methylidyne C–H bond

The C–H bond of the methylidyne radical, CH•, is abnormally long and weak, even longer weaker than that methane, CH4. This remarkable given fact has been shown to contract as number substituents around pertinent carbon atom decreases (e.g., from ethane ethene ethyne) because accompanying reduction in steric congestion. To elucidate origin this anomaly, we have analyzed bonding mechanism quartet CH••• doublet CH• compared with sterically more encumbered triplet CH2••, CH3•, singlet CH4, using quantitative (Kohn-Sham) molecular orbital theory. Our analyses reveal that, depending on effective electronic configuration its relatively weak originates from: (i) position at which maximum electron-pair overlap achieved (quartet CH•••); (ii) destabilizing Pauli repulsion between valence orbitals interacting fragments (doublet CH•). Key points radical remarkably methane molecule. determines CH•••. expands further, due 2s22p2 configuration, yielding a significant fragments.