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, n ∆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 ∆C-(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 R OH in going from 1 to 3 and a large corresponding decrease in the CdO‚‚‚HsO distance. R O‚‚‚O , A OH‚‚‚O , R OH‚‚‚O , as well as R OH and R CdO correlate linearly as do 2 ∆C(OD) and R O‚‚‚O . The nuclear shielding 1 and the first derivative of the 13 C nuclear shielding with respect to O-H bond stretching, (dσ/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 R OH distance upon deuteriation (∆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 dσ/dR OH and ∆R OH(D) . The variations in the calculated 2 ∆C(OD) are dominated by ∆R OH(D) . The calculated n ∆C(OD) correlate well with experimental isotope effects. Three parameters, 2 ∆C(OD), ∆R OH(D) , and R O‚‚‚O all show promise as gauges of hydrogen bond strength. Calculated OH and 1 H chemical shifts in general show good agreement with experimental values (RMSD ) 0.40 ppm) as do the 13 C chemical shifts (RMSD ) 1.9 ppm). The large experimental 2 ∆C(OD) values can be understood in terms of a steric effect caused by the neighboring CH 3 CO group leading to shorter OH‚‚‚O and O‚‚‚O distances and consequently stronger hydrogen bonds.

Variable-temperature NMR study of the enol forms of benzoylacetones

“…13 We suggest a reduction in twist angle to be the driving force leading to more steric compression 25,26 and a shorter OÐ Ð ÐO distance that ultimately leads to a longer O-H distance and to a more symmetrical structure.…”

nJ(13C, O1H) coupling constants of intramolecularly hydrogen-bonded compounds

“…For compounds such as 3, theoretical calculations on the non-methoxy derivative, 1,3-diacetyl-2,4-dihydroxybenzene, show that the OÈH bond length is considerably increased compared with 2-hydroxyacetophenone and the CxOÉ É ÉHÈO distance decreased. 27 The large coupling observed for 3b can therefore be ascribed partly to orbital overlap. The very small value observed for 5 can be related to twisting of the carbonyl group, thereby reducing the overlap between the CxO group and the OH group and simultaneously decreasing the conjugation.…”

Deuterium isotope effects on 13C chemical shifts of nitromalonamide

“…2 C(OD) isotope effects have been shown to correlate to p H(OD) [18] and also to δOH. [19,20] In the latter case, a linear relationship is found in the range δOH 12-17.2 ppm. The 2 C(OD) varied from 0.2 to 0.76 ppm.…”