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

Deuterium isotope effects on chemical shifts, (n)Delta C(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). (2)Delta C(OD) increase as the number of neighboring hydrogen-bonded moieties increase. The calculated molecular ab initio geometries with Density Functional Theory(BPW91/6-31 G(d(p)) (5D) with p functions on the chelate protons only) show a large increase in R-OH in going from 1 to 3 and a large corresponding decrease in the C=O ... H-O distance. R-O ... O, A(OH ... O), R-OH ... O, as well as ROH and R-C=O correlate linearly as do (2)Delta C(OD) and R-O ... O. The nuclear shielding(1) and the first derivative of the C-13 nuclear shielding with respect to O-H bond stretching, (d sigma/dR(OH)), 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 deuteriation (Delta R-OH(D),) was obtained from a potential scan of OH bond stretching and analyzing the data with a fitted Morse function. Isotope effects are calculated as the product of d sigma/dR(OH) and Delta R-OH(D)(D) The variations in the calculated (2)Delta C(OD) are dominated by Delta R-OH(D.) The calculated (n)Delta C(OD) correlate well with experimental isotope effects. Three parameters, (2)Delta C(OD), Delta R-OH(D), and R-O ... O all show promise as gauges of hydrogen bond strength. Calculated OH and H-1 chemical shifts in general show good agreement with experimental values (RMSD = 0.40 ppm) as do the C-13 chemical shifts (RMSD = 1.9 ppm). The large experimental (2)Delta C(OD) values can be understood in terms of a steric effect caused by the neighboring CH3CO group leading to shorter OH ... O and O ... O distances and consequently stronger hydrogen bonds.