Lon B. Knight

An experimental procedure for ESR studies of rare gas matrix isolated molecular cation radicals: 12CO+, 13CO+, 14NH3+, and 15NH3+

A generally applicable technique is described that enables the ESR study of molecular cation radicals isolated in neon matrices at 4 K. Preliminary evidence indicating the possible use of the method for the study of ion‐neutral reactions is briefly discussed. The ’’gaslike’’ nature of the trapped matrix ions is demonstrated by a comparison with gas phase results for 13CO+, one of the few radical cations studied by high resolution microwave spectroscopy. Application of the method to the following radical cations is presented in this report: 12CO+, 13CO+, 14NH3+, and 15NH3+. The ions are produced during neon matrix deposition by photoionization with an open tube neon discharge source (16.8 eV).

ESR investigations of H2O+, HDO+, D2O+, and H2 17O+ isolated in neon matrices at 4 K

The H2 16O+, HD 16O+, D2 16O+, and H2 17O+ ion radicals have been generated by photoionization, trapped in neon matrices at 4 K, and investigated by ESR (electron spin resonance) spectroscopy. The magnetic parameters are giso=2.0093(3), Aiso(H)=73.7(6) MHz, and Aiso(17O)=83.5(6) MHz. A weak electronic absorption feature at 6036(3) A was observed which is tentatively assigned to a known gas phase band. The H2O+ radical appears to be rotating in neon and exhibits an unusual intensity increase as the matrix temperature is lowered from 9 to 4 K. Comparisons between H2O+ and the isoelectronic radicals NH2, NH+3, and CH3 are made.

ESR investigation of matrix isolated B 16O and B 17O radicals: Comparison of nuclear hyperfine structure with ab initio calculations

An extensive ESR investigation of the 2Σ B16O and B17O radicals in rare gas matrices has been conducted. Several different trapping sites in neon matrices have been found and investigated as a function of temperature over the range 3–10 K both during and after deposition. Motional averaging of the nuclear hyperfine tensor seems to be occurring for two neon sites and the argon matrix. The large isotropic boron hyperfine coupling of these sites and the 17O hfs show excellent agreement with ab initio theoretical results. BO radicals trapped in a third neon site do not appear to be undergoing motional behavior. Isotropic and dipolar hyperfine parameters for this site show reasonable agreement with theoretical results. BO radicals in one neon site which is extremely sensitive to temperature effects exhibit nearly perfect ’’single‐crystal‐like’’ preferential orientation which has not been previously observed in rare gas matrix ESR studies. The results of this BO investigation help to correct erroneous conclusio...

An electron bombardment procedure for generating cation and neutral radicals in solid neon matrices at 4 K: ESR study of 14N2+ and 15N2+

A new generation technique for matrix isolated cations and neutrals involving the electron bombardment of the matrix sample during deposition is presented. Use of the method for the study of ion‐neutral reactions and other types of matrix experiments is discussed. Results for the gas phase electron bombardment of (CH3)2O are compared with electron bombardment of (CH3)2O conducted during neon deposition. The relative amounts of N and N2+ radicals trapped in neon matrices have been measured as a function of electron energies. An ESR analysis of 14N2+ and 15N2+ generated by electron bombardment in neon matrices at 4 K yields: giso=2.0004(2); Aiso(14N)=104.1(6) MHz and Aiso(15N)=146.1(6) MHz. The ESR results are used to estimate the extent of s‐p hybridization of N2+ and a comparison with theoretical calculations and several other 13 electron diatomic radicals is made.

Theoretical and neon matrix electron spin resonance studies of the methanol cation: CH3OH+, CH3OD+, CH2DOH+, and 13CH3OH+

Four isotopes of the methanol cation radical (CH3OH+) have been generated by three independent methods and isolated in neon matrices at 4 K for a detailed electron spin resonance (ESR) investigation. The ion generation methods employed were X irradiation, photoionization, and electron ionization. The nuclear hyperfine (A tensors) measurements were compared with those obtained from ab initio extended basis set multireference configuration interaction (CI) wave functions. The relationships between geometry and electronic structures were fully explored. The trend in the large isotropic methyl hydrogen A values for the isoelectronic series CH3F+, CH3OH+, and CH3NH2+ was found to follow the trend in dissociation energies for these radical cations. The neon magnetic parameters for CH3OH+ are gx=2.0036(4) and gz=2.010(1); Aiso (methyl hydrogens)=229(1) MHz, ‖Ax‖=54(2) and ‖Az‖=80(3) MHz for the hydroxy hydrogen; ‖Ax‖=40(2) and ‖Az‖=29(4) MHz for 13C. The observed magnetic parameters for CH2DOH+ indicate an unusu...

Electron spin resonance studies of the methane radical cations (12,13CH+4, 12,13CDH+3, 12CD2H+2, 12CD3H+, 12CD+4) in solid neon matrices between 2.5 and 11 K: Analysis of tunneling

The radical cation of methane isolated in neon matrices exhibits highly unusual electron spin resonance (ESR) spectral features between 2.5 and 11 K. The anomaly has been clarified by invoking large amplitude tunneling motions of the hydrogens among several symmetrically equivalent Jahn–Teller distorted structures. The effect of the tunneling motions upon the ESR spectrum was investigated by an analysis scheme based upon permutation–inversion group theory. All the deuterium substituted cations, i.e., CDH+3, CD2H+2, CD3H+, and CD+4 were also studied. The hyperfine coupling constant of 13C was obtained from the study of 13CDH+3 and 13CH+4. Several independent generation methods were employed during the course of these methane cation studies, including photoionization, electron bombardment, x‐irradiation, and a pulsed laser surface ionization technique.

Electron spin resonance studies of 45Sc17O, 89Y17O, and 139La17O in rare gas matrices: Comparison with ab initio electronic structure and nuclear hyperfine calculations

The first nuclear hyperfine measurements of 17O (I=5/2) have been made for Sc17O, Y17O and La17O in their X 2Σ ground electronic states. These metal oxide radicals were generated by the pulsed-laser vaporization of the metals in the presence of 16O2/17O2 and trapped in neon and argon matrices for electron spin resonance investigations. The fully resolved A tensors of the metal and 17O were compared with ab initio theoretical calculations—a comparison previously reported only for the ScO radical. The computational methods employed were unrestricted Hartree–Fock, density functional theory (DFT), and restricted open-shell Hartree–Fock. Having the metal and 17O hyperfine interactions available has permitted a more thorough description of the electronic structure and charge distribution in these metal oxide molecules. An electronic structure comparison with the AlO, GaO, and InO radicals has also been made. Reasonably good agreement between the observed and calculated values of Aiso and Adip were achieved with...

Laser vaporization generation of the SiB and SiAl radicals for matrix isolation electron spin resonance studies; comparison with theoretical calculations and assignment of their electronic ground states as X 4Σ

The first experimental spectroscopic study of the SiB and SiAl diatomic radicals is reported. Electron spin resonance results indicate that both molecules have X 4Σ ground electronic states, in agreement with earlier theoretical calculations. The SiB and SiAl radicals were generated in neon matrices at 4 K by trapping the products produced from the pulsed laser vaporization of their alloys. Electronic structure information for these radicals is especially interesting given the utilization of silicon doped materials in semiconductor applications. The observed nuclear hyperfine interactions (A tensors) for 10B, 11B, and 27Al in these molecular radicals were compared with the results of ab initio configuration‐interaction theoretical calculations which were conducted as part of this experimental study. The neon matrix magnetic parameters (MHz) for Si 11B are D=800(2), g∥=2.0014(8), g⊥=2.0005(4), A⊥=92.4(5), and A∥=111(2). For Si 27Al the results (MHz) are D=9710(2), g∥=1.9994(8), and g⊥=1.9978(4), ‖A⊥‖=10.3(...

Electron spin resonance investigation of Sc+2 in neon matrices and assignment of its ground electronic state as X 4Σ−. Comparison with theoretical calculations

The discandium radical cation, Sc+2, has been isolated in neon matrices at 4 K and studied by electron spin resonance (ESR) spectroscopy and theoretical methods. It was produced by the x‐irradiation of neon matrix samples containing neutral Sc2 which was formed by trapping the products generated from the pulsed laser vaporization of scandium metal. The experimental and theoretical findings indicate that Sc+2 has a 4Σ−g electronic ground state compared to an X 5Σ−u state for the neutral discandium radical. The large decrease in the 45Sc hyperfine interaction (A tensor) going from Sc2 to Sc+2 provides direct experimental information concerning the types of valence molecular orbitals that are involved in these diatomic radicals. The neon matrix magnetic parameters for 45Sc+2 are g∥≊2.00, g⊥=1.960(1), ‖A∥‖=28(6), and ‖A⊥‖=26.1(3) MHz; the D value (zero field splitting) was 15 381(3) MHz. Ab initio configuration interaction (CI) calculations of the nuclear hyperfine interactions yielded results in reasonable a...

Theoretical and electron spin resonance studies of the H⋯H, H⋯D, and D⋯D spin-pair radicals in rare gas matrices: A case of extreme singlet–triplet mixing

The H⋯H, H⋯D, and D⋯D spin-pair radicals have been thoroughly investigated in neon, argon, krypton, and xenon matrices near 4 K by electron spin resonance (ESR). A theoretical model has been developed that treats these spin-pairs as weakly interacting atoms. The model includes the effects of 3Σ/1Σ mixing in the analysis of the observed ESR spectral results and yields a consistent set of magnetic parameters for these three isotopomers in all four rare gas hosts. The consideration of H atoms interacting with other H atoms over a distribution of internuclear distances in the rare gas lattice is included in the theoretical and experimental analyses. Application of the model to earlier ESR results for H⋯CH3 reveals a value for its Heisenberg exchange interaction (J) which is found to be considerably larger than that for the H⋯H spin-pair. The effects of methane and neon on the J value are calculated for these spin-pairs. The H⋯H case is unusual in that the nuclear hyperfine interaction (A) is considerably larg...