Scott T. Cobranchi

Electron spin resonance investigations of 11B12C, 11B13C, and 10B12C in neon, argon, and krypton matrices at 4 K: Comparison with theoretical results

The first spectroscopic study of the diatomic radical BC is reported which confirms previous theoretical predictions of a 4∑− electronic ground state. The nuclear hyperfine interactions (A tensors) obtained for 11B, 10B, and 13C from the electron spin resonance (ESR) measurements are compared with extensive ab initio CI calculations. The BC molecule is one of the first examples of a small high spin radical for such an in‐depth experimental–theoretical comparison. The electronic structure of BC obtained from an analysis of the nuclear hyperfine interaction (hfi) is compared to that obtained from a Mulliken‐type population analysis conducted on a CI wave function which yields Aiso and Adip results in good agreement with the observed values. The BC radical was generated by the laser vaporization of a boron–carbon mixture and trapped in neon, argon, and krypton matrices at 4 K for a complete ESR characterization. The magnetic parameters (MHz) obtained for 11B13C in solid neon are: g∥ =2.0015(3); g⊥ =2.0020(3)...

Neon matrix ESR investigation of 12C+2 and 13C+2 : Confirmation of a 4Σ−g ground electronic state

The 12C+2 and 13C+2 cation radicals have been isolated in neon matrices by the photoionization of C2(g) generated by the laser vaporization of thin carbon disks. A new procedure of supporting and thermally outgassing such targets for laser vaporization ESR studies in rare gas matrices is described. The ESR results confirm the 4Σ−g ground electronic state assignment for C+2 and provide a valence orbital electronic structure characterization based on the 13C hyperfine structure (hfs). The ESR lines are extremely narrow for a high‐spin powder sample and an intense off‐angle or ‘‘extra’’ absorption feature exibits unusual hfs. Normally forbidden (ΔMS>1) transitions were detected at low magnetic fields. The magnetic parameters for 13C+2 in a neon matrix at 4 K are: D=3668(3) MHz; g⊥ =2.0018(5); g∥ =2.0023(5); ‖A∥‖ =99.4(8) MHz; and ‖A⊥‖ =95.2(5) MHz.

Laser vaporization generation of Al12C, Al13C, Al12C2, and Al13C2 for rare gas matrix electron spin resonance studies: Experimental–theoretical comparisons

The metal carbide radicals AlC and AlC2 have been generated by the laser vaporization of aluminum carbide and trapped in neon and argon matrices at 4 K for electron spin resonance (ESR) characterization. These results provide the first experimental evidence showing that AlC has a 4Σ ground electronic state and that AlC2 is X 2A1. Ab initio theoretical calculations were conducted for the geometries and various nuclear hyperfine parameters in both radicals which yielded A values in reasonable agreement with the observed. In AlC, the three unpaired electrons reside primarily on carbon with the following neon matrix magnetic parameters (MHz): g∥=2.000(1); g⊥=2.0010(5); ‖A⊥(Al)‖=33.2(5); ‖A∥(Al)‖=3(3); A⊥(13C)=52.1(5); A∥(13C)=52(2); and D(zero field splitting)=374(1). For AlC2, the spin density resides predominantly in an aluminum 3pz/3s hybrid directed away from C2. The neon magnetic parameters (MHz) are: g∥=2.0005(5); g⊥=1.9965(3); A⊥(Al)=941.5(5); A∥(Al)=1067(1); ‖A∥(13C)‖=59(1); and ‖A⊥(13C)‖=52(1).

Reinvestigation of the aluminum hydride (AlH+ and AlD+) cation radicals by ESR in argon matrices at 4 K: Generation by reactive laser sputtering

The ESR spectra previously assigned to the AlH+ radical ion (X 2Σ) [J. Chem. Phys. 71, 3991 (1979)] actually belong to the divalent neutral aluminum radical AlHOH whose charge distribution and electronic structure can be described as AlH+OH− with 90% of the unpaired electron on the AlH+ part of the molecule. Reactive laser sputtering and photoionization of AlH(g) were used to generate the AlH+ and AlD+ cation radicals, whose ESR spectra have been observed for the first time. A detailed analysis of the ESR results for argon matrices at 4 K reveal unusually large Al hyperfine interaction (hfi) with Aiso and Adip values of 1586(2) and 49(1) MHz, respectively. The H hfi is essentially isotropic with Aiso=442(2) MHz. The observed nuclear hyperfine A tensors for Al and H show excellent agreement with ab initio CI theoretical calculations. The results for AlH+ are compared with the isoelectronic neutral radical MgH, and the similar AlF+ cation radical. The Al hfi is slightly larger in AlD+ relative to AlH+. This interesting isotopic effect is qualitatively explained on the basis of electronic structure dependence on small changes in the bond distance for the two isotopic radicals.The ESR spectra previously assigned to the AlH+ radical ion (X 2Σ) [J. Chem. Phys. 71, 3991 (1979)] actually belong to the divalent neutral aluminum radical AlHOH whose charge distribution and electronic structure can be described as AlH+OH− with 90% of the unpaired electron on the AlH+ part of the molecule. Reactive laser sputtering and photoionization of AlH(g) were used to generate the AlH+ and AlD+ cation radicals, whose ESR spectra have been observed for the first time. A detailed analysis of the ESR results for argon matrices at 4 K reveal unusually large Al hyperfine interaction (hfi) with Aiso and Adip values of 1586(2) and 49(1) MHz, respectively. The H hfi is essentially isotropic with Aiso=442(2) MHz. The observed nuclear hyperfine A tensors for Al and H show excellent agreement with ab initio CI theoretical calculations. The results for AlH+ are compared with the isoelectronic neutral radical MgH, and the similar AlF+ cation radical. The Al hfi is slightly larger in AlD+ relative to AlH+. This...

Laser vaporization/rare gas matrix-isolation electron spin resonance studies of PdH + 2 , PdD + 2 , PdH, and PdD: Theoretical investigation of PdH + 2

The reactive laser vaporization of palladiummetal in the presence of H2(g) has been used to generate and trap neutral and charged palladium hydride radicals in neon and argon matrices for electron spin resonance investigations. The radicals identified are: 0PdH+ 2, 1 0 5PdH+ 2, 0PdD+ 2, 1 0 5PdD+ 2, 0PdH, 0PdD, 1 0 5PdH, and 1 0 5PdD. The electron spin resonance results prove that the H atoms are equivalent in PdH+ 2 but it cannot be determined for certain whether a highly bent 2 A 1ground state complex or a linear 2Σ state is being observed, although the evidence seems to favor the 2 A 1 state. The observed nuclear hyperfineproperties (A and g tensors) are best accounted for by assigning the unpaired electron predominantly to 4d 2 z /5s orbitals on the metal in PdH+ 2 with only about 3% of the spin density on each H atom. Electron spin resonance results for PdH constitute the first observations for a diatomic metal hydride radical trapped in a neon matrix. The magnetic parameters (MHz) for 1 0 5PdH+ 2 in a neon matrix at 4 K are: g ⊥=2.568(2); g ∥≂1.92; A ⊥(1 0 5Pd)=−713(2); A ∥(1 0 5Pd)=−688(15); A ⊥(H)=45(1). Multireference SCF (MCSCF) and configuration interaction (CI) calculations were conducted for PdH+ 2 to calculate the energies and gross population distributions in the lowest‐lying electronic states. The lowest of these, the 2 A 1 state, has a calculated stabilization energy of 9.3 kcal/mol relative to Pd+(4d 9)+H2.

Neon matrix ESR and CI theoretical investigation of 10BF+ and 11BF+: Photoionization of BF from reactive laser sputtering and high temperature sources

The 11BF+ and 10BF+ radical cations have been generated and trapped in neon matrices at 4 K using the combined techniques of pulsed reactive laser sputtering and photoionization at 16.8 eV. An independent high temperature source of BF(g) in conjunction with photoionization was also employed and a comparison between these two different generation methods reveals no significant differences in their ESR spectra. The magnetic parameters for 11BF+ measured in neon matrices are g∥=2.0012(3), g⊥=2.0004(3), (11B) A∥=1784(1), and A⊥=1727(1) MHz, (19F)A∥=410(1), and A⊥=152(1) MHz. Extensive ab initio theoretical calculations have been conducted for BF+ and the nuclear hyperfine properties obtained from various types of CI wave functions show excellent agreement with the experimental measurements. Valence orbital occupancies obtained from a Mulliken‐type population analysis performed on the CI wave functions are compared with the conventional free atom comparison method for obtaining electronic structure information...

The generation of 12C31P and 13C31P by reactive laser vaporization for rare gas matrix electron spin resonance studies: Comparison with abinitio theoretical calculations

The 12C31P and 13C31P diatomic radicals have been generated by the laser vaporization reaction between carbon and phosphorus which were pressed into a pellet to form the laser target. This method is applicable to a wide range of nonmetallic systems for generating new reactive intermediates which cannot be done with more conventional approaches. The radicals were isolated in neon and argon matrices at 4 K for detailed ESR investigations. The magnetic parameters (MHz) for neon were: g∥=2.0009(3); g⊥=1.9902(3); A∥(31P)=145.0(3); A⊥(31P)=−269.0(2); A∥(13C)=580.0(3); and A⊥(13C)=422.0(3). The argon A and g tensors were virtually indistinguishable from these neon results. Extensive ab initio theoretical calculations were conducted for CP which yielded nuclear hyperfine A values in close agreement with the experimental results. Valence orbital spin populations extracted from the calculated CI wave functions are compared with those determined directly from the ESR hyperfine parameters for 13C and 31P. The observe...

Electronic ground state assignment for O+4: Neon matrix electron‐spin resonance investigation

The O+4 radical cation has been generated by two different methods and trapped as an isolated ion in neon matrices at 4 K for electron‐spin resonance (ESR) investigation. The O+4 species is a potentially important atmospheric ion for which little experimental or theoretical information is available. These ESR results indicate that the electronic ground state of O+4 is a nonlinear quartet (S=3/2) with the three unpaired electrons occupying primarily 2p(π) orbitals. The fine structure‐parameters obtained from an analysis of the ESR spectrum are D=1.875(5) and E=0.0350(5) cm−1 . The isotopic ions16,17 O+4 were also generated but the 17 O hfs could not be fully analyzed because of the large number of overlapping hyperfine lines.

Rare gas matrix ESR investigations of 12CH3 63,65CuF, 13CH3 63,65CuF, H63,65CuF, and D63,65CuF generated by reactive laser vaporization

A new experimental procedure for conducting reactive laser vaporizations is described and applied to the generation of the previously unreported copper radicals, 13CH3CuF, 12CH3CuF, HCuF, and DCuF, for rare gas matrix ESR investigations. Laser vaporization was conducted on a thin film of reactant condensed on the copper metal surface. The g and A tensors for these are compared with other small copper radicals in order to determine electronic structure and bonding trends. The neon magnetic parameters (MHz) for 13CH363CuF are: g∥ =1.965(1), g⊥ =2.3626(3); A∥(63Cu) =3076(2), A⊥(63Cu) =2993(1); A∥(19F) =94(1), A⊥(19F) =68(1); A∥(13C) =19(1), A⊥(13C) <3. The H quartet hfs for CH3CuF was resolved on the perpendicular transitions, having an A value of 9.3(3) MHz. For H63CuF, the neon results (MHz) are: g∥ =1.975(1); g⊥ =2.4075(4); A∥(63Cu) =2826(3), A⊥(63Cu) =2717(1); A∥(19F) =127(2), A⊥(19F) =94(1); A∥(H) =61(3), and A⊥(H) =29(1) MHz.

An electron spin resonance investigation of the 12C11B12C, 12C11B13C, and 13C11B13C radicals in neon, argon, and krypton matrices: Comparison with ab initio calculations

The 11 electron CBC radical has been generated by the pulsed laser vaporization of elemental carbon–boron mixtures and trapped in neon, argon and krypton matrices for detailed electron spin resonance (ESR) studies. Extensive comparisons of the experimental nuclear hyperfine A tenors for carbon and boron were made with a variety of ab initio computational results that involved different levels of theory and basis sets. These new ESR results agree with recent vibrational studies of CBC that show it to have a nonlinear symmetric geometry with a 2A1 electronic ground state. These ESR results provide a description of the singly occupied molecular orbital. The spin density resides primarily on boron in 2s and 2pz orbitals, however a complete resolution of the 13C hyperfine structure does show that approximately 20% resides on the carbon atoms.