An electron paramagnetic resonance study of methylene

Publisher Website
Google Scholar
Abstract
The present paper is a detailed account of the study of the methylene molecule trapped in matrices at 4.2 °K by electron paramagnetic resonance spectroscopy. The methylene is generated by photolysis of its diazirine precursor dissolved in the matrix material. Besides verifying that the methylene ground electronic state was a spin triplet, the first observation of the EPR spectrum revealed that the molecule was nonlinear. Through assumptions concerning the amounts of spectral averaging owing to molecular motion and the relationship of the zero‐field splitting parameters D and E with each other through an s–p hybrid model of the electronic structure, other workers showed the bond angle to be substantially bent. This was later confirmed by ourselves and others with a measurement of the isotropic carbon‐13 hyperfine interaction. Using 90.5% carbon‐13 enrichment in the present work, a more precise determination of the isotropic carbon‐13 hyperfine interaction was found to be 239 MHz. The increased resolution also permits a measurement of the anisotropic carbon‐13 hyperfine interaction from which a bond angle of 134° can be derived. All three methods of bond angle determination are in substantial agreement. Chemical trapping of the products of the photolysis with a 100% mass balance accounting for all substances supplement the EPR of the isotopically substituted species to show conclusively that the trapped paramagnetic species is methylene. Experiments in a nonnuclear magnetic matrix show that the linewidths are not due to hyperfine interactions with matrix nuclei. In addition to xenon matrices, experiments in krypton and xenon–krypton mixtures are reported where the observed zero‐field splitting parameters are modified by molecular motion and interaction with the trapping matrix. The present work also verifies that the heat of formation of methylene is 91.9±1.0 kcal/mole.

This publication has 55 references indexed in Scilit: