Weak hydrogen bonding. Part 1. Neutron diffraction data of amino acid C α –H suggest lengthening of the covalent C–H bond in C–H ⋯ O interactions

In C–H ⋯ O contacts observed in 16 high-precision neutron crystal-structure determinations of α-amino acids, there is good evidence for a correlation between the covalent bond length of Cα–H and the Hα⋯ O separation. For the shortest H ⋯ O contacts, ∼2.3 Å, the lengthening of C–H is 0.008 Å compared with the longest contacts of ∼2.7 Å. This supports the view that C–H ⋯ O interactions have the nature of hydrogen bonds. The experimental results are in excellent agreement with calculations from the valence model of the hydrogen bond.