No abstract

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); D(zfs)=1701(2); 11B: ‖A∥‖ =100(1); ‖A⊥‖ =79(1); 13C: ‖A∥‖ =5(2) and ‖A⊥‖ =15(1). Based on comparison with the theoretical results, the most likely choice of signs is that all A values are positive.

The 14N+4 and 15N+4 molecular cation radicals have been generated by the ion–neutral reaction N+2 +N2 and isolated in solid neon matrices at 4 K for detailed ESR (electron spin resonance) investigation. Both photonionization at 16.8 eV and electron bombardment (50 eV) were used in conjunction with the neon matrix trapping technique to produce the N+4 dimer cation. The ESR results clearly show that N+4 is linear and has a 2Σμ ground electronic state. The magnetic parameters in neon are: g∥=2.0016(4) and g⊥=1.9998(2); A∥(14N)=311(1) MHz and A⊥(14N)=264(1) MHz for the central atoms and ‖A∥‖=10.4(5) MHz and ‖A⊥‖=20.4(1) MHz for the outer or terminal 14N atoms. Electronic structure information for N+4 was obtained from the ESR results and compared with ab initio CI calculations. The unpaired electron resides primarily on the inner nitrogen atoms with significant 2pσ and 2s character. Orbital characters obtained from the commonly applied free atom comparison method (FACM) were compared with the results of a Mulliken type spin population analysis conducted on the calculated wave function. The calculated nuclear hyperfine parameters (A tensors) showed reasonable agreement with experiment except for the very small Aiso parameter for the outer nitrogen atoms. Benchmark calculations employing large basis sets were conducted for the free nitrogen atom; these efforts demonstrate the difficulty in computing the Aiso parameter when inner shell effects are important.

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.

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.