MEASUREMENTS OF THE ROTATIONAL AND VIBRATIONAL ZEEMAN-EFFECT OF METHANE BY MAGNETIC VIBRATIONAL CIRCULAR-DICHROISM

A magnetic vibrational circular dichroism (MVCD) study of the two IR allowed transitions of CH4 and CD4 is presented. From analysis of the MVCD and absorption spectra of the nu(3), C-H(D), stretching mode and the nu(4), C-H(D), deformation mode, respectively, the molecular Zeeman g-values, which include contributions from both molecular rotation and vibration, are found to be +0.29 and +0.44 for CH4 and +0.17 and +0.20 and for CD4. For CH4, the nu(3) transition yields a g-value that is the same, within our experimental error (<10%), as the vibrational ground-state g-value. Thus no vibrational Zeeman effect is detectable for this transition. Since the nu(4) transition has an similar to 40% higher g-value (well beyond possible error) than the ground and nu(3) transition g-values in CH4, MVCD provides the first clear experimental evidence of a significant vibrational contribution to the molecular Zeeman effect in the nu(4) mode. A similar trend of g-values is also observed for CD4, however, the experimental difference between the nu(3) and nu(4) g-values of CD4 is much smaller. These results are consistent with expectations based on the relative magnitude Coriolis coupling seen for each of these T-2 symmetry modes and with the predicted effect of isotopic substitution. A self consistent field (SCF) quantum mechanical evaluation of the effect of vibrational distortion on the methane g-values is consistent with the experimentally determined dominance by the Coriolis contribution.