The molecular Zeeman effect in CH3Cl, CH3Br and CH3I

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Abstract
A microwave rotational Zeeman study is performed on various halogen isotopes of CH3Cl, CH3Br and CH3I. The rotational spectra of these molecules is complicated by nuclear quadrupolar interactions. A method of data analysis is outlined which avoids the complication of performing a complete matrix diagonalization and allows one to extract information about the molecular g values, the anisotropy in the magnetic susceptibility, as well as chemical shift anisotropies using first-order perturbation theory. This method utilizes data from transitions defined by F = I + J → F′ = I + J + 1 and |MF | = F → |MF′| = F′. These transitions were heavily relied upon in the data analysis and the following information was obtained: for CH3 127I, and For CH3 79Br, and For CH3 35Cl, and (1 - σ)gI is the shielded nuclear g value, g ⊥, and g ∥ are the molecular g values perpendicular and parallel to the molecular symmetry axis; x ⊥, x ∥, σ⊥ and σ∥ are similarly defined elements of the magnetic susceptibility and the chemical shift tensors. Values of the molecular quadrupole moments, all the elements of the paramagnetic, diamagnetic and total magnetic susceptibility tensors and second moments of the charge distributions are calculated using these results and previously determined values of the total susceptibility. Trends in the methyl halide series are found and discussed with respect to the molecular quadrupole moments, with respect to the excited state summation and with respect to c 2, the ground state expectation value of the square of the electronic coordinates perpendicular to the symmetry axis. The constant <c 2> suggests that the radius of the halogen nucleus in the methyl halides is very close to its neutral atom radius and quite far from its negative ion radius. The expected sign for the electric dipole moment of +CH3Cl- is found by Zeeman studies of the 35Cl and 37Cl isotopic species.