Effect of CH···O hydrogen bond length on the geometric and spectroscopic features of the peptide unit of proteins

A formamide molecule is allowed to form a CH···O H‐bond with a glycine dipeptide in both its C5 and C7 internal conformations. As this intermolecular H‐bond is elongated, the contraction of the CH covalent bond is monitored, as is its vibrational stretching frequency and intensity, along with NMR chemical shifts of the atoms involved in the H‐bond. The degree of shortening of the CH bond becomes progressively smaller as the proton acceptor is moved further away, as does the blue shift of the CH stretching frequency, and the chemical shift of the bridging proton. These changes are monotonic and regular enough that experimental evaluation of CH bond length or blue shift or proton NMR chemical shift may be used to accurately predict the energetics of the H‐bond for any particular geometry.