Vernon A. Bowers

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...

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...

Multiple Trapping Sites for Hydrogen Atoms in Rare Gas Matrices

Hydrogen atoms have been stabilized in nonequivalent lattice sites in matrices of the rare gases at liquid helium temperature. Electron spin resonance spectra of H atoms in argon, krypton, and xenon show that at least two trapping sites are involved in each case. In a neon matrix, H atoms have been stabilized in only one site. Attainability of the various trapping sites apparently depends on the initial energy of the H atom, a simple doublet spectrum being obtained when the atoms are deposited from the gas phase, while multiple trapping spectra are obtained when the atoms are produced by photolysis in the solid.The hyperfine coupling contants and the electronic g factors for H atoms trapped in the various matrix sites have been determined. The deviation of the hyperfine coupling constant from the free‐state value is positive in some cases and negative in others. The experimental results are in good agreement with theoretical predictions. A complex multicomponent H atom spectrum was obtained by photolysis ...

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 ...

ESR spectra and structures of NaSO2 and NaO2

We have observed the electron spin resonance (ESR) spectra of NaSO2 and NaO2, trapped in argon at 4 °K, following their production by codeposition of a beam of sodium atoms with SO2 and O2, respectively, in a large excess of argon. Each spectrum is a triplet of quartets, the triplet structure arising from an orthorhombic g tensor (g), and the quartet structure resulting from the Na hyperfine (hfs) interaction. For NaSO2, the components of g are g1=2.0024, g2=2.0060, and g3=2.0101 where axis 1 is perpendicular to the SO2 plane and axis 2 is the SO2 symmetry axis. In this axis system, the Na hfs tensor of NaSO2 is A11=−1.49, A22=−1.22, A33=−1.85, A12=0.56, and A13=A23=0.00 Oe. For NaO2, g1=2.0029, g2=2.0063, g3=2.1112, and A1=−3.5, A2=−1.8, and A3=−3.4 Oe, where axis 1 is perpendicular to the NaO2 plane and axis 3 is the O–O bond. The magnitudes of the Na hfs splittings showed that each molecule is a bound ion pair. Comparison of the observed anisotropies in the Na hfs splittings with calculated anisotropic...

Electron Spin Resonance Spectrum of FCO

The electron spin resonance spectrum of the fluoroformyl radical (FCO) has been observed at 4.2°K in solid carbon monoxide. The radical was prepared by reaction between the CO matrix and fluorine atoms produced by in situ photolytic dissociation of fluorine oxide (OF2). The fluorine hyperfine splitting of FCO is very large and approximately axially symmetric: A1/h=1437.5, A2/h=708.2, and A3/h=662.0 Mc/sec. The large hyperfine splitting results from spin polarization of an exceptionally weak CF bond, and is consistent with the theory for the proton hyperfine interaction in HCO. This implies that FCO, like HCO, is a sharply bent molecule in which the unpaired electron occupies an in‐plane or σ orbital, a conclusion which is in agreement with theoretical predictions.

Electron Spin Resonance Spectra of ClCO and ClOO

We have observed the electron spin resonance (ESR) spectra of ClCO and ClOO trapped in polycrystalline CO and Ar, respectively, at 4°K. The radicals were prepared by reaction of a Cl atom, produced by photolytic decomposition of Cl2, with CO and O2. The ESR spectra of these radicals are complicated by, among other things, the Cl nuclear quadrupole interaction. For ClCO the g factor tensor is gz = 1.9980, gx = 2.0061, gy = 2.0003; the hyperfine splitting (hfs) tensor is Az = 227, Ax = 58, Ay = 50 MHz; and the nuclear quadrupole coupling constants are e2Qq = 76 MHz and η = 0.25. For ClOO these spin Hamiltonian parameters are: gz = 2.0100, gx = 1.9987, gy = 1.9915  ; | Az | = 50, | Ax | = 9, | Ay | = 16 MHz; and e2Qq = 105 MHz. The z principal axis is along the Cl bond, and the x principal axis is perpendicular to the molecular plane. The signs of the hfs tensor components of ClCO were assigned in accordance with previous results for FCO. This assignment led to a value of 0.42 for the unpaired electron densi...

g-tensor and spin doubling constant in the 2Σ molecules CN and C2H☆

Abstract Free-molecule perpendicular g factor shifts (Δ g ⊥) of the CN and C 2 H radicals, determined by analysis of high resolution ESR spectra of the radicals in Ar and Kr matrices at 4K, support the predicted relation γ = 2 B O Δ g ⊥, where γ and B O are the spin doubling and rotational constants. The analysis also shows that CN in Ar at 4 K undergoes a complex motion resulting in a nonaxial spin hamiltonian.