Charles A. McDowell

Photoelectron Spectra of the Halogens and the Hydrogen Halides

A spherical‐grid photoelectron spectrometer has been used to measure the ionization potentials of HF, HCl, HBr, HI, F2, Cl2, Br2, and I2. The relative transition probabilities to several of the ionic states are given. There is evidence that the 2Σ+ ionic states in HI+, Cl2+, Br2+, and I2+ are unstable.

One‐ and two‐dimensional 31P cross‐polarization magic‐angle‐spinning nuclear magnetic resonance studies on two‐spin systems with homonuclear dipolar coupling and J coupling

The effect of dipolar interactions on the solid state 31P cross‐polarization magic‐angle‐spinning (CP‐MAS) nuclear magnetic resonance (NMR) line shapes for the coupled two‐spin systems, sodium pyrophosphate decahydrate, Na4P2O7⋅10H2O, and tetraphenyl diphosphine‐1‐oxide, (C6H5)2PP(O)(C6H5)2, has been investigated. The one‐dimensional (1D)CP‐MAS spectra of Na4P2O7⋅10H2O shows spinning frequency dependent sideband splittings. A theory was developed to permit the calculation of the MAS NMR line shapes of the dipolar and J‐coupled two‐spin systems. An exact solution of the periodic Hamiltonian was obtained by the use of Floquet Hamiltonian theory. The experimental spectra of Na4P2O7⋅10H2O are well reproduced by the theoretical ones calculated from the present theory, in which the homonuclear dipolar interaction between the two 31P nuclei in the P2O74− group was taken into consideration. Our Hamiltonian also leads to calculated spectra which are in good agreement with the experimental observations even at low ...

Application of Floquet theory to the nuclear magnetic resonance spectra of homonuclear two‐spin systems in rotating solids

The theory for the nuclear magnetic resonance (NMR) spectra of homonuclear two‐spin systems under sample spinning has been developed. The propagator describing the time‐evolution of the systems, which is driven by the homogeneous Hamiltonian composed of the chemical‐shift difference and the flip–flop parts of the dipolar and J couplings, is treated using Floquet theory; the Floquet eigenvalues and eigenvector components determine the resonance frequencies and intensities, respectively. Nondegenerate Rayleigh–Schrodinger perturbation theory is used to solve the Floquet secular equation. Recurrence equations for the perturbation corrections have been derived, allowing us to evaluate efficiently very high‐order terms, such as 20th‐order terms. Analytical expressions for the resonance frequency obtained in the low‐order approximations provide an intuitive understanding of the main spectral features; the second‐order equations can describe the conspicuous solid‐state effects on the magic‐angle spinning (MAS) s...

Spectral spin diffusion in polycrystalline solids under magic-angle spinning

Spectral spin diffusion in 13C NMR of double 13C-labelled sodium acetate trihydrate (SAC), and in 31P NMR of zinc(II) bis(O,O′-diethyldithiophosphate)(ZNP) has been studied under magic-angle spinning conditions. Spin-diffusion time constants, TSD, were determined from the intensities of the spinning sidebands in experiments using rotation-synchronized DANTE pulse sequences, at several different spinning frequencies. The theory of Suter and Ernst, developed for spectral spin diffusion in single crystals, was extended to the case of polycrystalline samples rotating under magic-angle spinning conditions. We considered two mechanisms for the spin diffusion, i.e. dipolar interaction and J-coupling. The spin-diffusion time constants, TSD, were related to the zero-quantum lineshape functions in a manner similar to the theory of Suter and Ernst. The zero-quantum lineshape functions were estimated from the observed single-quantum lineshape functions. In the present studies the dependence of the experimental values for TSD on the rotational frequency vr are in good agreement with those calculated from the theory based on the dipolar interaction mechanism. The values of TSD for SAC showed a deep minimum at Δω≈ 2ωr, and a shallow minimum at Δω≈ 3ωr. This phenomenon is rotational relaxation resonance.

Electron Capture Processes in the Hydrogen Halides

The formation of negative ions by resonance capture processes of the type XY+e=X+Y—, has been studied for the hydrogen halides. A monoenergetic electron source has been used and it has been possible to determine the true shape of the resonance capture peaks in these cases. It has been found that the negative ions appear at energies which are in agreement with recent values for the dissociation energies of the H—X bonds, and the electron affinities of the halogen atoms.

The high resolution photoelectron spectrum of CS

Abstract The high resolution photoelectron spectrum of CS has been obtained. Ionization potentials are found at 11.34 (X 2 Σ), 12.78 (A 2 Π), 15.83 (B 2 Σ) and 18.03 (C 2 Σ) eV, and a spin-orbit splitting of 278 cm −1 is observed on the second band.

Electron spin resonance of an X-ray irradiated single crystal of potassium dihydrogen arsenate, KH2AsO4

The electron spin resonance of an x-ray irradiated single crystal of potassium dihydrogen arsenate has been examined and a radical which is thought to be AsO4 4- identified. Analysis of the spectra for various orientations of the crystal with respect to the external magnetic field leads to an axially symmetric hyperfine interaction tensor which is in agreement with what would be expected for the AsO4 4- radical. Semi-empirical LCAO molecular orbital calculations for the AsO4 4- radical with symmetry D 2d trapped in the tetragonal KH2AsO4 crystals leads to the conclusion that the odd electron enters a molecular orbital with A 1 symmetry. The isotropic part of the hyperfine interaction tensor is calculated from this theory to be 2098 Mc/s which must be compared with the experimental value of 3038·6 Mc/s, The anisotropic part of the hyperfine interaction calculated from the theory, however, is much too low when compared with the value estimated from the experimental data. It is possible that part of this dif...

Nuclear magnetic resonance studies of urea and thiourea adducts

The ‘wide-line’ proton magnetic resonance spectra of n-tridecane and n-hexadecane and their urea adducts have been examined from 77°K to 298°K. The results show that the enclosed component is capable of a considerable degree of molecular motion. Estimates of the energy barriers involved support the idea that the interaction between the urea cage and the adducted hydrocarbon is only slight. The thiourea adducts of some cyclic hydrocarbons have also been examined.

J coupling in chemically equivalent spin pairs as studied by solid‐state nuclear magnetic resonance

The presence of J coupling was observed in the solid‐state nuclear magnetic resonance (NMR) spectrum of the 31P spin pair in polycrystalline 1,2‐bis (2,4,6‐tri‐tert‐butylphenyl)‐diphosphene (TBPDP) using the magic‐angle spinning (MAS) technique. This represents the first example of J coupling in a spin‐pair system with nuclei having identical isotropic chemical shifts. Average Hamiltonian theory is used to derive the time‐independent eigenstates for the system. It is shown that a second‐order perturbation term, which is dependent on the difference between the chemical‐shift tensor orientations in addition to the strength of the dipolar coupling, allows the recoupling of the J interaction for the two chemically equivalent 31P nuclei. In the absence of the perturbation term, the system is reduced to an A2 case in solution‐state NMR for which, of course, no J coupling may be observed. The magnitude of the J coupling, namely, 580±20 Hz obtained from two‐dimensional (2D) J‐resolved experiments was found to be ...

Photoelectron kinetic energy analysis in gases by means of a spherical analyser

A spherical grid analyser has been used to measure the kinetic energy distribution of photo-electrons produced by the interaction of 584 Å radiation with argon, krypton, xenon, hydrogen, HD, deuterium, oxygen and nitrogen. The relative transition probabilities to the 2P3/2 and 2P½ levels of the rare gas ions and to various vibronic states of the others are given. A detailed description of the apparatus is included. It is demonstrated that the ionic energy levels measured agree extremely well with those found by optical spectroscopy, and that it will be possible, in favourable cases, to determine the bonding or antibonding nature of an ionized orbital from the shape of the photoelectron stopping curve.