John I. Brauman

Depolarized Rayleigh scattering and orientational relaxation of molecules in solution. I. Benzene, toluene, and para‐xylene

Measurements of orientational relaxation rates of benzene, toluene, and para‐xylene in a variety of solvents have been made by depolarized light scattering. A plot of reorientational relaxation time of each solute versus solution viscosity was found to fit a straight line with nonzero intercept. The slopes of the lines are compared with those predicted by the Stokes‐Einstein relation.

Cross sections for photodetachment of electrons from negative ions near threshold

Threshold laws for the cross section for photodetachment of electrons from polyatomic negative ions are derived. The general form for the limiting behavior at threshold is derived using group theory and symmetry considerations. In addition, for energies within a few eV of threshold, a formulation using the Born approximation with partial orthogonalization is developed. The energy dependence of the photodetachment cross section for molecular negative ions is computed and shown to agree well with experimentally determined cross sections.

Spectroscopy and dynamics of the dipole-bound state of acetaldehyde enolate

Ultrahigh‐resolution photodetachment spectroscopy of acetaldehyde enolate negative ion has revealed ∼50 narrow resonances near threshold, corresponding to excitation to a diffuse state in which the electron is weakly bound by the field of the molecular dipole. A complete analysis of rotational transitions between the ground valence state and the excited dipole‐bound state has been carried out, yielding spectroscopic constants and geometries for both states. In analogy to Rydberg states, the structure of the ‘‘neutral core’’ of the dipole‐bound state is like that of the neutral radical. The dependence of autodetachment lifetimes upon the rotational quantum numbers of the dipole‐bound state has been measured. Bound levels of the dipole‐bound anion state are readily electric‐field detached. The selection rules and dynamics of autodetachment from the dipole‐bound state are discussed.

The nature of the chromium trioxide intercalation in graphite

Abstract X-ray diffraction, thermal, and chemical techniques have been used to demonstrate that chromium trioxide will intercalate graphite in the presence of a glacial acetic acid solvent. The product is a third stage ionic compound of graphite with an identity period Ic of 14·92 A, which may be hydrolyzed to give a molecular intercalate. Contrary to previous reports, it appears that chromium trioxide and graphite do not react directly to form an analogous intercalation product, yielding instead a mixture of lower oxides of chromium and unreacted graphite.

Steric and Solvation Effects in Ionic SN2 Reactions

This paper explores the contribution of solvation to the overall steric effects of S(N)2 reactions observed in solution. The reactions of chloride ion with a series of alkyl chloronitriles, RCH(CN)Cl (R = methyl, ethyl, isopropyl, tert-butyl) were investigated both experimentally and theoretically. These reactions serve as a model system for the parent reactions, Cl(-) + RCH(2)Cl, which are too slow to measure. Each additional substitution at the beta-carbon lowers the reactivity, clearly demonstrating a steric hindrance effect. The magnitude of the steric effect, however, is not significantly different in the gas phase and in solution. We conclude that the solvation energies of the corresponding S(N)2 transition states must be similar regardless of size of the substituent. This lack of size dependence in the current system is in sharp contrast with many other ionic systems such as ionization of simple alkyl alcohols, where solvation depends strongly on size. We propose that the weak size dependence results from the compensation between a direct shielding effect of the substituent and an indirect ionic solvation effect, which arises from the geometric perturbations introduced by the substitution. The conclusion is further supported by calculations using polarizable continuum models and QM/MM simulations.

Gas-Phase Ion Chemistry

Progress has been made in the understanding of potential energy surfaces for unimolecular ion dissociations and ion-molecule reactions. With recent advances in instrumentation, many new techniques have been developed to generate and study ions, ion-molecule complexes, and large ionic clusters. Developments in ion spectroscopy have enabled considerable advances to be made in the determination of ion structures.

Photodetachment of electrons from phosphide ion - The electron affinity of PH2.

The relative cross section for photodetachment of electrons from PH2− has been measured in the wavelength region 725–1020 nm (1.71–1.22 eV). An ion cyclotron resonance spectrometer has been used to generate, trap, and detect the negative ions, while two light sources have been employed to study photo‐detachment: a 1000‐W xenon arc lamp with grating monochromator and a continuously tunable laser. A single sharp threshold in the cross‐section curve has been observed, and a detailed analysis has yielded a value for the electron affinity, E.A. (PH 2·) = 1.25 ± 0.03 eV.

Photodetachment of electrons from Group IVa binary hydride anions: The electron affinities of the SiH3 and GeH3 radicals

Relative photodetachment cross sections have been determined for the silyl and germyl anions (SiH3−, GeH3−) in the wavelength ranges 400.0–870.0 nm (3.10–1.43 eV) and 400.0–720.0 nm (3.10–1.72 eV), respectively. An ion cyclotron resonance spectrometer was used to generate, trap, and detect the negative ions, and a 1000 W xenon arc lamp with a grating monochromator was employed as the light source. Both ions exhibited cross sections behaving sigmoidally over the wavelength ranges studied. These experiments afford the following electron affinities: E.A. (SiH3·) ≤1.44±0.03 eV and E.A. (GeH3·) ≤ 1.74±0.04 eV. From estimates of proton transfer equilibrium constants, bond energies are approximated for silane and germane.

Depolarized Rayleigh scattering and orientational relaxation of molecules in solution. IV. Mixtures of hexafluorobenzene with benzene and with mesitylene

Measurements of the orientational relaxation times of the components of mixtures of hexafluorobenzene with benzene and mesitylene have been made by depolarized Rayleigh scattering. In both mixtures, there were strong ’’static’’ and ’’dynamic’’ cross correlation terms which affected the individual reorientation times. There was no significant contribution to the Rayleigh spectra from a long‐lived complex in these mixtures.

Electron photodetachment of thiomethoxyl and deuterothiomethoxyl anions: Electron affinities, vibrational frequencies, and spin–orbit splitting in CH3S⋅̄ and CD3S⋅̄

The cross section for electron photodetachment has been measured for the thiomethoxyl anion (CH3S−) and the deuterothiomethoxyl anion (CD3S−) using an ion cyclotron resonance spectrometer in conjunction with a tunable dye laser. After accounting for the effective rotational broadening, these spectra yield the electron affinities, EA(CH3S) =1.861±0.004 eV and EA(CD3S) =1.858±0.006 eV. A doubling of features is observed in the photodetachment spectra which arises from transitions to the 2E3/2 and 2E1/2 spin–orbit states of the final‐state radicals. From these spacings we obtain a spin–orbit coupling constant (A) in CH3S of −280±50 cm−1 and in CD3S of −260±50 cm−1. The spectra also contain transitions to excited vibrational states of the radicals for which we obtain vibrational frequencies of 770±50 cm−1 for the symmetric C–S stretching motion in CH3S (660±60 cm−1 in CD3S) and 1360±70 cm−1 for the methyl umbrella motion (1100±50 cm−1 in CD3S).