Gauge-including atomic orbital proton chemical shifts of strong hydrogen bonds: The importance of electron correlation

The effects of electron correlation are often of little importance in theoretical H-1 NMR chemical shift calculations. Indeed, the differences between uncorrelated and correlated values are typically ca. 0.2 ppm or less for organic compounds. Here we demonstrate a very important case where this assumption breaks down; protons involved in strong hydrogen bonds. We found that the isotropic shifts calculated with the gauge-including atomic orbital (GIAO) approach at the RHF level overestimate the corresponding MP2 values by well over 1 ppm and commonly by 2 ppm. This is true for minimum energy geometries as well as for the transition states for proton transfer. In contrast, electron correlation effects are an order of magnitude smaller for the non-hydrogen-bonding protons in the structures we studied. The systems treated theoretically were FHF-, N2H7+, H3O2-, the enol of 2,4-pentanedione, the monoanion of cis-maleic acid, and the monoanion of dimethylmalonic acid. Geometries were calculated at either the MP2/6-311++G** or MP2/aug-cc-pVTZ Level of theory. The donor-acceptor distances and other geometric parameters for most structures satisfy the standard criteria for "strong" hydrogen bonds, A notable exception is the minimum energy structure for the enol of 2,4-pentanedione, which is better classified as a "moderate" hydrogen bond on the basis of both geometric and chemical shift criteria. The effect of electron correlation on the H-1 chemical shift in the latter case was the smallest of any structure we considered.