High-resolution ESR spectroscopy and structure of the acetaldehyde radical cation (CHCHO) in neon matrices at 4 K: Comparison with results in freon matrices

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Journal of the American Chemical Society, ISSN: 1520-5126, Vol: 110, Issue: 2, Page: 327-336

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Knight, Lon B.; Gregory, Brian W.; Cobranchi, Scott T.; Williams, Ffrancon; Qin, Xue Zhi
American Chemical Society (ACS)
Chemical Engineering; Chemistry; Biochemistry, Genetics and Molecular Biology
article description
Detailed ESR studies are reported for the acetaldehyde radical cation (CHCHO) trapped in neon matrices, The cation was generated by photoionization, electron bombardment, or laser multiphoton ionization of dilute acetaldehyde neon mixtures (10to 10dilution factor) during codeposition on a flat target at 4 K. Its ESR spectrum was well resolved and showed strong preferential orientation effects, two sets of strong features being observed for field directions in the target plane, while another set of weaker line components became strongly enhanced when the field was applied normal to this plane. These results indicate that the cations are aligned with their CCHO planes parallel to the deposition surface. Accordingly, the largest principal value of the g tensor is found to be perpendicular to the CCHO plane, as previously determined for the formaldehyde cation (HCO). The measured values of the g and aldehydicH hyperfine tensors are consistent with the assignment of the unpaired electron to the nonbonding oxygen-centered 10a′ molecular orbital which is, however, about 50% delocalized onto the aldehydic hydrogen (1s) and methyl carbon (2p) orbitals. The g and aldehydicH A tensor components are the following: g= 2.0069 (3), A-355 (1) MHz; g2.0019 (6), A= 374 (3) MHz, g= 2.0026 (3), A= 353 (1) MHz. The much smaller hyperfine interaction (4.3 MHz) with the hydrogens of the methyl group is exceedingly well resolved into a 1:1:1:1 quartet at 4 K with the field normal to the CCHO plane, and these intensity ratios indicate some form of restricted rotation of the methyl group, possibly via tunneling. In contrast to the neon studies, the ESR spectra of the acetaldehyde radical cation in Freon matrices show much poorer resolution and are complicated by the appearance of a highly structured anisotropicCl hyperfine interaction below 120 K. As expected for the formation of a σ* complex between the in-plane oxygen 2p orbital from the SOMO of CHCHOand a filled chlorine 3p orbital from a solvent (CFCl) ligand, the largestCl coupling is associated with the smallest g component along the σ* bond direction. Despite this chlorine interaction, the average or isotropic aldehydicH hf. coupling appears to be about 5-10% larger in CFClthan in neon, indicating that spin delocalization to this β hydrogen in CHCHOis remarkably insensitive to σ* complex formation. © 1988, American Chemical Society. All rights reserved.