An ESR investigation of the formaldehyde cation radicals (H2 12CO+ and H2 13CO+) in neon matrices at 4 K

The H2 12CO+ and H2 13CO+ radicals have been trapped in neon matrices at 4 K for detailed ESR (electron spin resonance) study. Both photoionization (17 eV) from an open tube neon discharge source and electron bombardment, recently developed techniques for generating matrix isolated cations, were employed in this investigation. The ESR spectra were sufficiently resolved to allow a complete characterization of the nuclear A and g tensors. The magnetic parameters are: gx=2.0069(2), gy=2.0015(2), gz=2.0025(2), Ax(H)=129.4(2), Ay=(H)=134.5(2), Az(H)=134.3(2) g, ‖Ax(13C)‖ =43.9(2), ‖Ay(13C)‖=37.3, and ‖Az(13C)‖ =35.1(2) g. Of particular interest is the conclusion that the direction of the largest g component (x) lies perpendicular to the radical plane rather than along the in-plane C2 symmetry axis. These H2CO+ results were compared with MINDO and ab initio CI type theoretical calculations and to previous experimental results for the isoelectronic radicals H2CN, H2BO, H2C2−, and the acetaldehyde cation CH3HCO+. Theoretical values for the isotropic nuclear hyperfine interaction for this series of H2XY radicals are consistently below the experimental results, although good agreement is obtained for the H and 13C dipolar interactions in H2 13CO+.