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 theor etical H-1 NMR chemical shift calculations. Indeed, the differences between uncorrelated and correlated values are typically ca. 0.2 ppm or less for o rganic compounds. Here we demonstrate a very important case where this assu mption breaks down; protons involved in strong hydrogen bonds. We found tha t the isotropic shifts calculated with the gauge-including atomic orbital ( GIAO) approach at the RHF level overestimate the corresponding MP2 values b y well over 1 ppm and commonly by 2 ppm. This is true for minimum energy ge ometries as well as for the transition states for proton transfer. In contr ast, electron correlation effects are an order of magnitude smaller for the non-hydrogen-bonding protons in the structures we studied. The systems tre ated theoretically were FHF-, N2H7+, H3O2-, the enol of 2,4-pentanedione, t he 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 paramete rs 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 e lectron correlation on the H-1 chemical shift in the latter case was the sm allest of any structure we considered.