Rotational dependence of the dipole moment of 13CH3F

When a gas sample of 13CH3F is prepared with a population of spin isomers far from the equilibrium given by nuclear spin statistics, it relaxes towards this equilibrium with an exponential decay rate. This rate is increased by submitting the molecules to a static electric field, emphasizing the role of the magnetic intramolecular interactions and of the collisions in the “quantum relaxation” process (P. L. Chapovsky, Phys. Rev. A, 1991, 43, 3624–3630; P. Cacciani, J. Cosleou, F. Herlemont, M. Khelkhal and J. Legrand, Eur. Phys. J. D., 2003, 22, 199–207). We scanned the conversion spectrum, i.e. the conversion rate in presence of a static electric field versus the strength of the field, for electric fields up to 15 kV cm−1. The quantitative reproduction of this conversion spectrum required to take into account the rotational dependence of the dipole moment of 13CH3F, which is expanded as μ = μ0 + μJ(J + 1) + μKK2. The newly determined μJ and μK parameters are compared to the values expected using the Watson's centrifugal distortion dipole coefficients, which can be derived from the geometry, the force field, and the derivatives of the dipole moment with respect to either symmetry or normal coordinates of the molecule (J. K. G. Watson, J. Mol. Spectrosc., 1971, 40, 536–544). Constraining the μ0 and μK values to 1.8579 D and −3.7 × 10−5 D respectively, we determined a μJ value between 1.1 × 10−5 D and 4.4 × 10−5 D, in agreement with the theoretical value μJ = 1.50 × 10−5 D.