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 CAH 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 CAH bond becomes progressively smaller as the proton acceptor is moved further away, as does the blue shift of the CAH stretching frequency, and the chemical shift of the bridging proton. These changes are monotonic and regular enough that experimental evaluation of CAH 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. VC 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110: 2775–2783, 2010