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.
Quantum mechanical calculations are presented that predict that one-bond deuterium isotope effects on the 15 N chemical shift of backbone amides of proteins, 1 D 15 N(D), are sensitive to backbone conformation and hydrogen bonding. A quantitative empirical model for 1 D 15 N(D) including the backbone dihedral angles, U and W, and the hydrogen bonding geometry is presented for glycine and amino acid residues with aliphatic side chains. The effect of hydrogen bonding is rationalized in part as an electric-field effect on the first derivative of the nuclear shielding with respect to N-H bond length. Another contributing factor is the effect of increased anharmonicity of the N-H stretching vibrational state upon hydrogen bonding, which results in an altered N-H/N-D equilibrium bond length ratio. The N-H stretching anharmonicity contribution falls off with the cosine of the N-HÁÁÁO bond angle. For residues with uncharged side chains a very good prediction of isotope effects can be made. Thus, for proteins with known secondary structures, 1 D 15 N(D) can provide insights into hydrogen bonding geometries.
Results of an experimental and theoretical study of cyclopenta-2,4-dienylideneketene (3), a highly unstable reactive intermediate, are reported. The ketene was prepared, under matrix isolation conditions at 4.2 or 10 K, by laser photocarbonylation of 1,2-didehydrobenzene (1) photogenerated earlier from phthalic anhydride (2). FTIR polarization measurements performed on partially photooriented samples of 3 immobilized in solid neon or argon provide infrared transition moment directions for most of the observed vibrations. Experimental results confirm that the ketene is bent, as predicted by ab initio calculations. Utilizing two isotopically modified 3, 3b and 3c, on the basis of the infrared absorption spectrum alone, we have analyzed and assigned its vibrations in a way, which leaves no doubt about the bent ketene structure. This work was motivated by a long standing confusion surrounding the assignments of the vibrations in 1,2-didehydrobenzene (1), especially of its "triple" bond stretch.
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