Hydrogen bond types, binding energies, and H-1 NMR chemical shifts

Ab initio calculations at the MBPT(2)/(qzp,qz2p) level of theory were carri ed out using the gauge-invariant atomic orbital method to evaluate H-1 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-HT O, N-H-O, and N-H-N hydrogen bonds. In both series a correlation exists bet ween increasing hydrogen bond strength and increasing proton chemical shift relative to the corresponding neutral proton donor molecule. However, whil e this correlation does not hold between the two series, complexes with pro ton-shared hydrogen bonds have similar chemical shifts of about 20 ppm for the hydrogen-bonded proton in both series, independent of the binding energ y of the complex. H-1 NMR chemical shifts computed along the proton-transfe r coordinate for ClH:NH3 also approach 20 ppm for a proton-shared hydrogen bond.