EPR of methylene. Temperature dependence in a xenon matrix

Abstract

The EPR spectra of CH2 and CD2 were examined between 2.18 and 9.2 °K in a xenon matrix. The zero-field splitting and linewidths of the randomly oriented triplet CD2 were temperature independent below 4.2 °K which is consistent with the currently accepted model in which the trapped methylene occupies its lowest discrete tortional level in the matrix. Increasing the temperature above 4.2 °K produces a decrease in the zero-field splitting of CD2 and an increase in linewidth. The zero-field splitting for CH2 exhibited a temperature dependence between 4.2 and 2.18 °K. The variation of the zero-field splitting parameters could be described by a thermal activation energy of about 30 cm−1 for CD2 and approximately 14 cm−1 for CH2. These relative magnitudes are likely a consequence of the fact that CH2 has a larger zero-point energy than CD2 and therefore requires a smaller energy of activation to jump or tunnel to a random orientation. The temperature dependence of the homogeneous broadening of the z1 and z2 EPR spectral lines of CD2 could be described by a thermal activation energy between 36 and 42 cm−1. The mechanism for the line broadening is probably the same for the decrease in D, and the difference in experimental activation energies probably arises from uncertainties in the deconvolution of the spectral linewidth into a homogeneous and an inhomogeneous part. It was also found that methylene in a xenon matrix could be warmed to 50 °K before significant diffusion with subsequent reactive recombination takes place.