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.
The vibrational structure of the title compound (TTP) was studied
by experimental and theoretical methods.
IR absorption spectra were recorded in argon matrix and in
stretched polyethylene at 12 K. The linear dichroism
(LD) observed in the latter solvent provided experimental symmetry
assignments of the observed vibrational
states. Molecular geometries and harmonic force fields were
calculated ab initio with the 6-311G** basis
set using three different procedures: restricted Hartree−Fock
theory (HF), second-order Møller−Plesset
perturbation theory (MP2), and density functional theory (DFT). In
the latter, Becke's gradient-corrected
exchange functional was combined with Perdew and Wang's correlation
functional (BPW91), leading to
excellent agreement with observed IR transitions. The combined
experimental and theoretical evidence enabled
an essentially complete assignment of the fundamental vibrations.
Of particular importance is the assignment
for the first time of the long-sought “bell-clapper” mode
associated with the unique S−S−S structural element
of TTP, giving rise to an intense, long-axis polarized transition in
the far-IR (153 cm-1).
The and 13C NMR spectra of the soluble square-planar o.o'-dihydroxyazoarene-platinum(II) complexes [Pt-(L)(tba)] (L = 5,5'-dichloro-2,2'-diftydroxyazoi>enzenate (dhab), (5-chloro-2-/iydroxyphenylazo)-3-oxo-A/-phenylfcutanamidate (hpab), or (5-chloro-2-/zydroxyphenylazo)-2-naphtholate (hpan) and tba = ni¿mty lamine) and the protonated ligands have been assigned by use of homoand heteronuclear 2D correlated NMR spectroscopy. The assignment of spectra of the analogous complexes [M(L)(py)] (M = nickel(II), palladium(II), and platinumill) and py = pyridine) has been achieved by comparison of the chemical shifts. 13C chemical shifts of corresponding carbon signals of the 5-chloro-2-hydroxyphenyl group, common to all the eight complexes with hpab and hpan, are very similar to each other, and in turn these shifts are very similar to the one set of signals from the 5-chloro-2-hydroxyphenyl groups in the four complexes of the symmetrically substituted ligand dhab.These chemical shifts exhibit the same periodic change when the ligand is coordinated to different group 10 metal ions, and the signals are assumed to originate from 5-chloro-2-hydroxyphenyl groups with equal coordination environment in all the reported complexes; thus 13C NMR spectroscopy is believed to constitute an excellent way of establishing isostructurality. On the basis of the crystal structures of complexes with the unsymmetrical ligands hpab and hpan, all the reported complexes with these ligands are Na-isomers coordinated to the metal at the azo nitrogen attached to the 5-chloro-2-hydroxyphenyl substituent. The changes in 13C chemical shift with coordination can be used to distinguish Na-from N^-isomers. Deuterium isotope effects on 13C chemical shifts in the hpab complexes with one exchangeable amine proton and in the free unsymmetrically substituted ligands (hpab)H2 and (hpan)H2 themselves reveal intramolecular hydrogen bonds and predominant tautomeric forms, these being possible indicators for predicting the identity of the ligating nitrogen. nJt*sfl_i3c couplings in the [Pt(L)(tba)] complexes have been measured at low field, and the magnitudes of these couplings are analyzed in terms of coupling paths in the two annulated fiveand six-membered rings formed by the ligand and the central platinum-(II) ion.
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