Charles L. Perrin

Vibronic Coupling. VI. Vibronic Borrowing in Cyclic Polyenes and Porphyrin

Vibronic borrowing in closed‐shell aromatics is investigated by means of the cyclic‐polyene model with the π electrons treated according to simple Huckel theory and the σ electrons treated as outlined by Longuet‐Higgins and Salem. The basis set for the vibronic wavefunctions consists of single‐product Born–Oppenheimer functions whose electronic factors are LCAO cyclic‐polyene molecular orbitals. The coupling between these Born–Oppenheimer functions is treated by means of perturbation theory. The dependence of the vibronic coupling on the size of the aromatic is given. Intensities of the vibronic transitions in benzene, coronene, triphenylene, 18‐annulene, and porphyrin are calculated. Porphyrin is approximated by a 16‐membered cyclic polyene with 18 electrons, and the absorption spectra, fluorescence polarization, and Zeeman spectra are interpreted in terms of vibronic interactions. The Jahn–Teller distortion vanishes in these cases, a result shown to be a general phenomenon for one‐electron terms in E st...

Rotation and solvation of ammonium ion

From nitrogen-15 spin-lattice relaxation times and nuclear Overhauser enhancements, the rotational correlation time &T;c for 15NH4+ was determined in a series of solvents. Values of &T;c range from 0.46 to 20 picoseconds. The solvent dependence of &T;c cannot be explained in terms of solvent polarity, molecular dipole moment, solvent basicity, solvent dielectric relaxation, or solvent viscosity. The rapid rotation and the variation with solvent can be accounted for by a model that involves hydrogen bonding of an NH proton to more than one solvent molecule in a disordered solvation environment.

Reactions of hydroperoxides with iron(III) porphyrins: Heterolytic cleavage followed by hydroperoxide oxidation

Abstract Reaction of α,α-dimethylphenethyl hydroperoxide with iron(III) tetramesitylporphyrin in protic solvent gives alkoxy-radical products and little epoxide. Such observations are usually interpreted as evidence for homolysis of the OO bond in the catalytic process. Yet the same products are now obtained under similar conditions from the reaction of this hydroperoxide with the high-valent oxene (Fe+O), generated unambiguously. Therefore such products are not necessarily evidence for homolysis but are consistent with heterolysis. Nevertheless, the solvent can affect the nature of O-O cleavage.

Saturation-transfer studies of three-site exchange kinetics

Abstract The application of the saturation-transfer method with Fourier transform NMR to multisite spin systems is discussed. Three-site exchange is considered in detail: exact equations for the calculation of rate constants are given, and approximate equations are presented which may be used in certain simplifying situations. The method is illustrated with several examples of three-site proton exchange: acetamide in fuming sulfuric acid, acrylamide in ethylene glycol, and acetamidinium ion in ethylene glycol. In each case, all six rate constants have been measured and are nonzero.

Is the isomer with the largest dipole moment always the least stable

Abstract Two sets of calculated counterexamples are presented to the proposition that the isomer with the largest dipole moment is the least stable.

Position-Specific Secondary Deuterium Isotope Effects on Basicity of Pyridine

Secondary isotope effects (IEs) on basicities of various deuterated pyridine isotopologues and of 2,6-lutidine-2,6-(CD(3))(2) have been accurately measured in aqueous solution by an NMR titration method applicable to a mixture. Deuteration at any position of pyridine increases the basicity, but the IE per deuterium is largest for substitution at the 3-position and smallest for the 2-position, which is closest to the site of N-protonation, smaller even than that for 2-CD(3) substitution. Computations at the B3LYP/cc-pVTZ level overestimate the magnitude of the measured IEs but largely reproduce the variability with isotopic position. Because the calculated IEs are based on changes in vibrational frequencies on N-protonation, the correspondence between calculated and experimental IEs implies that they arise from zero-point energies, rather than from inductive effects.

Variable-Temperature Study of Hydrogen-Bond Symmetry in Cyclohexene-1,2-dicarboxylate Monoanion in Chloroform-d

The symmetry of the hydrogen bond in hydrogen cyclohexene-1,2-dicarboxylate monoanion was determined in chloroform using the NMR method of isotopic perturbation. As the temperature decreases, the (18)O-induced (13)C chemical-shift separations increase not only at carboxyl carbons but also at ipso (alkene) carbons. The magnitude of the ipso increase is consistent with an (18)O isotope effect on carboxylic acid acidity. Therefore it is concluded that this monoanion is a mixture of tautomers in rapid equilibrium, rather than a single symmetric structure in which a chemical-shift separation arises from coupling between a desymmetrizing vibration and anharmonic isotope-dependent vibrations, which is expected to show the opposite temperature dependence.

Basicities of cycloalkylamines: Baeyer strain theory revisited

Abstract A new, accurate NMR titration method was utilized to determine relative basicities in four solvents of the two norbornylamines 2 and a series of cycloalkylamines 3–15 , as well as some sec -alkyl amines. Cyclopentylamine ( 5 ) and cyclohexylamine ( 6 ) are substantially more basic than the small rings 3–4 and slightly less basic than the medium rings 7–11 , with cyclooctylamine ( 8 ) the most basic. The large rings 12–15 are significantly less basic than 6 . The basicities reflect CCC angles, hybridization, and also solvation. These results and their relationship to Baeyer strain theory are discussed.

Direct observation of β-chloride elimination from an isolable β-chloroalkyl complex of square-planar nickel.

Reported here are the isolation, structural characterization, and decomposition kinetics of the four-coordinate pentachloroethyl nickel complex, NiCl(CCl2CCl3)(CNAr(Mes2))2 (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3). This complex is a unique example of a kinetically persistent β-chloroalkyl in a system relevant to coordination-insertion polymerization of polar olefins. Kinetic analysis of NiCl(CCl2CCl3)(CNAr(Mes2))2 decomposition indicates that β-chloride (β-Cl) elimination proceeds by a unimolecular mechanism that does not require initial dissociation of a CNAr(Mes2) ligand. The results suggest that a direct β-Cl elimination pathway is available to four-coordinate, Group 10 metal vinyl chloride polymerization systems.

Hydrogen-bond symmetry in difluoromaleate monoanion.

The symmetry of the hydrogen bond in hydrogen difluoromaleate monoanion is probed by X-ray crystallography and by the NMR method of isotopic perturbation in water, in two aprotic organic solvents, and in an isotropic liquid crystal. The X-ray crystal structure of potassium hydrogen difluoromaleate shows a remarkably short O-O distance of 2.41 Å and equal O-H distances of 1.206 Å, consistent with a strong and symmetric hydrogen bond. Incorporation of (18)O into one carboxyl group allows investigation of the symmetry of the H-bond in solution by the method of isotopic perturbation. The (19)F NMR spectra of the mono-(18)O-substituted monoanion in water, CD(2)Cl(2), and CD(3)CN show an AB spin system, corresponding to fluorines in different environments. The difference is attributed to the perturbation of the acidity of a carboxylic acid by (18)O, not to the mere presence of the (18)O, because the mono-(18)O dianion shows equivalent fluorines. Therefore, it is concluded that the monoanion exists as an equilibrating pair of interconverting tautomers and not as a single symmetric structure not only in water but also in organic solvents. However, in the isotropic liquid crystal phase of 4-cyanophenyl 4-heptylbenzoate, tetrabutylammonium hydrogen difluoromaleate-(18)O shows equivalent fluorines, consistent with a single symmetric structure. These results support earlier studies, which suggested that the symmetry of hydrogen bonds can be determined by the local environment.