Hydrogen Bond Types, Binding Energies, and ¹H NMR Chemical Shifts

Abstract

Ab initio calculations at the MBPT(2)/(qzp,qz2p) level of theory were carried out using the gauge-invariant atomic orbital method to evaluate 1H NMR chemical shifts for the hydrogen-bonded proton in two series of complexes, the first containing Cl−H−N and Cl−H−Cl hydrogen bonds, and the second O−H−O, N−H−O, and N−H−N hydrogen bonds. In both series a correlation exists between increasing hydrogen bond strength and increasing proton chemical shift relative to the corresponding neutral proton donor molecule. However, while this correlation does not hold between the two series, complexes with proton-shared hydrogen bonds have similar chemical shifts of about 20 ppm for the hydrogen-bonded proton in both series, independent of the binding energy of the complex. 1H NMR chemical shifts computed along the proton-transfer coordinate for ClH:NH3 also approach 20 ppm for a proton-shared hydrogen bond.