Unraveling the Electronic and Vibrational Contributions to Deuterium Isotope Effects on 13C Chemical Shifts Using ab Initio Model Calculations. Analysis of the Observed Isotope Effects on Sterically Perturbed Intramolecular Hydrogen-Bonded o-Hydroxy Acyl Aromatics

Abstract

Deuterium isotope effects on chemical shifts, nΔC(OD),have been measured in a series of o-hydroxy acyl aromatics of the type 2-hydroxyacetophenone (1) and 1,3,5-triacetyl-2,4,6-trihydroxybenzene (3). 2ΔC(OD) increase as the number of neighboring hydrogen-bonded moieties increase. The calculated molecular ab initio geometries with Density Functional Theory(BPW91/6-31G(d(p)) (5D) with p functions on the chelate protons only) show a large increase in ROH in going from 1 to 3 and a large corresponding decrease in the CO···HO distance. RO···O, AOH···O, ROH···O, as well as ROH and RCO correlate linearly as do 2ΔC(OD) and RO···O. The nuclear shielding1 and the first derivative of the 13C nuclear shielding with respect to O-H bond stretching, (dσ/dROH), has been calculated with the 6-31G(d) (6D) basis set using the GIAO/B(PW91) method (exchange term only). (Chemical shift and nuclear shielding are used intermittently. It should be remembered that they lead to different signs.) The change in the ROH distance upon ...