Frank J. Adrian

Matrix Effects on the Electron Spin Resonance Spectra of Trapped Hydrogen Atoms

A study is made of the matrix effects on the electron spin resonance (ESR) spectra of hydrogen atoms stabilized in nonpolar matrices. It is assumed that the perturbing effect of the matrix consists of van der Waals interactions and the overlap or Pauli exclusion forces. These two effects are treated separately and the results added to get the net result. The van der Waals effect, which is treated by perturbation theory, leads to a reduction in the hfs splitting. The overlap effect, which is treated by requiring that the hydrogen atom wave function be orthogonal to the wave functions of the matrix particles, leads to an increase in the hfs splitting. In addition, the exclusion effect tends to introduce a small amount of the unpaired electron charge density onto the matrix particles. This can lead to a change in the electronic g factor, and also to hyperfine interactions with the nuclei of the matrix particles. The theory gives a good qualitative picture of the various matrix effects and their dependence on...

ESR Study of Ethynyl and Vinyl Free Radicals

ESR spectra have been observed for ethynyl (C≡CH) and vinyl (HC=CH2) radicals trapped in solid argon at liquid‐helium temperatures. Ethynyl radical was obtained by the photolytic decomposition of acetylene. Its ESR spectrum consisted of two narrow lines corresponding to a proton hyperfine splitting of 16.1 Oe. The assignment of this spectrum to ethynyl was confirmed by a study of the photolysis of deuteroacetylene. Hydrogen atoms were not observed in the acetylene—argon photolytic system, suggesting that this photolysis is a second‐order process involving two molecules of acetylene. Vinyl radical was obtained by the addition of an H atom, obtained from the photolysis of HI, to acetylene. The ESR spectrum of the vinyl radical consisted of eight broad overlapping lines resulting from the interaction of the unpaired electron with three nonequivalent protons. Addition of an H atom to deuteroacetylene at 4.2°K gave only one of the two possible structural isomers of the 1,2 dideuterovinyl radical. The ESR spect...

Wave Functions for Electron-Excess Color Centers in Alkali Halide Crystals1

Publisher Summary The chapter describes wave functions for electron-excess color centers in alkali halides of the NaCl structure. The importance of color centers in the alkali halides derives not only from their inherent significance, but also from the fact that they are among the simplest defects in some of the simplest solids. Thus, they provide an important testing ground for various theories of deep traps. Methods of calculation may be worked out for these simple centers, and they can be checked by comparison with a wide variety of experimental data. The material discussed in this chapter indicates that the effective mass approximation cannot be trusted in deep trap problems. Nor can the MO-LCAO method be employed, except where the orbitals used in the expansions do not overlap to any appreciable extent. Methods entailing the use of orthogonalized orbitals, localized in the vacancy (or vacancies), are far more accurate, and are likely to assume increasing importance in the study of other deep trap problems.

ESR Spectrum and Structure of the Formyl Radical

The electron spin resonance (ESR) spectra of the formyl radical (HCO) and the deuterated radical (DCO) have been observed in solid carbon monoxide over the temperature range 4.2° to 30°K. The observed line shapes were temperature dependent, but the cause of the temperature dependence was not definitely determined. The radical was produced by the reaction of CO with hydrogen atoms produced by the photolytic decomposition of HI. The radical was also produced by the photolytic decomposition of formaldehyde in solid argon, and it was found to be one of the products of the photolysis of methyl alcohol in solid argon. The most striking feature of the ESR spectrum of HCO is the very large proton hyperfine splitting (137 oe). Also the lines were broad and unsymmetrical with the linewidth varying from one hyperfine component to the next. It was shown that the line shapes were due to the combination of a rather pronounced anisotropy in the electronic g factor with a smaller anisotropy in the proton hyperfine intera...

Theory of Anomalous Electron Spin Resonance Spectra of Free Radicals in Solution. Role of Diffusion‐Controlled Separation and Reencounter of Radical Pairs

It is proposed that the anomalous spin state populations observed in electron spin resonance spectra of transient radicals in liquids are produced by the following sequence of encounters between two radicals: The radicals either collide or are produced together by a chemical reaction and, instead of recombining, diffuse apart and subsequently undergo a second nonreactive collision. During the interval between the first and second encounters the radical pair wavefunction changes under the influence of singlet–triplet mixing by the magnetic interactions within the radicals. At the second encounter the singlet–triplet splitting associated with the valence bonding–antibonding interactions between the radicals produces further changes in the radical pair wavefunction. The net effect of these changes in the radical pair wavefunction is to increase somewhat the probability of finding the finally separated radicals in those spin states which correlate separationwise with the initial state of the radical pair. The...

Charge transfer effects in surface‐enhanced Raman scatteringa)

Surface‐enhanced Raman scattering (SERS) due to charge‐transfer interactions between the adsorbed molecule and the metal surface is analyzed using the semiempirical Wolfsberg–Helmholz method to relate the molecule–surface interactions and the resulting charge‐transfer states to the overlap integrals between the metal conduction‐band orbitals and an acceptor or donor molecular orbital of the molecule. Calculations for the model system of ethylene adsorbed on silver, with charge‐transfer excitation of an electron from the metal to the antibonding etylene π orbital, show that charge‐transfer Raman enhancements of the order of 10 to 1000 are possible if the charge‐transfer band is partially resonant with the exciting radiation. Symmetric vibrations usually will be enhanced substantially more than nonsymmetric ones by this mechanism because the vibrational coupling is primarily Franck–Condon rather than Herzberg–Teller. The presence of overtone and combination bands in charge‐transfer‐enhanced Raman spectra is also possible.Surface‐enhanced Raman scattering (SERS) due to charge‐transfer interactions between the adsorbed molecule and the metal surface is analyzed using the semiempirical Wolfsberg–Helmholz method to relate the molecule–surface interactions and the resulting charge‐transfer states to the overlap integrals between the metal conduction‐band orbitals and an acceptor or donor molecular orbital of the molecule. Calculations for the model system of ethylene adsorbed on silver, with charge‐transfer excitation of an electron from the metal to the antibonding etylene π orbital, show that charge‐transfer Raman enhancements of the order of 10 to 1000 are possible if the charge‐transfer band is partially resonant with the exciting radiation. Symmetric vibrations usually will be enhanced substantially more than nonsymmetric ones by this mechanism because the vibrational coupling is primarily Franck–Condon rather than Herzberg–Teller. The presence of overtone and combination bands in charge‐transfer‐enhanced Raman spectra is...

Role of Diffusion‐Controlled Reaction in Chemically Induced Nuclear‐Spin Polarization II. General Theory and Comparison with Experiment

A more detailed study is made of an earlier proposal that the nuclear‐spin polarizations observed in nuclear magnetic resonance studies of the products of free radical reactions are produced when a pair of radicals fail to react the first time they are together but instead diffuse apart and subsequently come together again. In this model the magnetic interactions within the radicals have ample time to produce a nuclear‐spin dependent growth or decay of the singlet character of the radical pair during the interval between separation and return. Since the singlet character of the radical pair determines its recombination probability the products formed at the second encounter will be nuclear‐spin polarized. This process is analyzed for the cases where the initial state of the radical pair is: (1) a triplet; (2) a singlet; and (3) uncorrelated electron spins. The predictions that sign of the nuclear‐spin polarization is the same for the cases of a triplet and uncorrelated spins but is reversed for the single...

Anisotropic Hyperfine Interactions in the ESR Spectra of Alkyl Radicals

We have studied the effect of anisotropic hyperfine interactions on the electron spin resonance (ESR) spectra of alkyl radicals trapped in polycrystalline matrices. The anisotropy broadens some or all of the hfs components, thus complicating the spectra. In alkyl radicals one has both isotropic and anisotropic hfs interactions with the α protons, but only an isotropic interaction with the β protons. Computed and experimental line shapes for the ethyl and propyl radicals are in qualitative agreement, provided that the hfs interaction is averaged over the various equilibrium orientations of the —CH2· group. This implies a rapid reorientation of the —CH2· group. It is noteworthy that when the two α protons are antiparallel the hyperfine anisotropy cancels and sharp intense hfs components result, while parallel orientations of the α protons give broad weak hfs lines. In particular, if the broad weak lines associated with the α proton hfs interactions are overlooked, the propyl radical spectrum appears to be a...

13C Hyperfine Splittings in the Electron Spin Resonance Spectra of HCO and FCO

The radicals H13CO and F13CO have been prepared in an argon matrix at 4°K by the addition of an H atom and an F atom, respectively, to CO enriched to 52% in 13C. The electron spin resonance spectra of these radicals contain large 13C hyperfine splittings in addition to the previously observed proton and fluorine hyperfine splittings. In HCO the principal components of the 13C hyperfine splitting tensor are: Ax(C)/h=365.7, Ay(C)/h=427.9, and Az(C)/h=338.9 Mc/sec. These results give an approximate value of 125 deg for the HCO bond angle and a value of 0.45 for the unpaired electron density on the carbon atom. The isotropic part of the 13C hyperfine splitting in FCO (some unusual complications made it impossible to determine the anisotropic terms) is A(C)/h=802.5 Mc/sec, where A(C) has the same sign as the fluorine hyperfine splitting constant. This result indicates that FCO is more sharply bent than HCO, the bond angle having an approximate value of 110 deg.

ESR Detection of the Cyanogen and Methylene Imino Free Radicals

The electron spin resonance (ESR) spectra of the cyanogen (CN) and methylene imino (H2CN) free radicals trapped in argon have been observed over the temperature range 4.2° to 37°K. The CN radical was produced by the vacuum uv photolysis of HCN in argon. The CN spectrum was a triplet whose outer lines varied with temperature from broad and weak at 4.2°K to narrow and strong at 37°K. This behavior was attributed to the broadening effects of an anisotropic nitrogen hyperfine interaction which was averaged out by the increased motional freedom of the radical at higher temperatures. The isotropic part of the nitrogen hyperfine splitting in CN was estimated to be 4.6 oe. The HCN photolysis also gave some weak ESR lines which were believed to be due to a radical formed by the addition of H atoms to HCN. This was confirmed when this weak spectrum was obtained with much greater intensity by photolyzing HI in the presence of HCN. From an analysis of the ESR spectrum it was shown that the addition of H atoms to HCN ...