Molecular g-Value Tensor, the Molecular Susceptibility Tensor, and the Sign of the Electric Dipole Moment in Formaldehyde

The diagonal elements in the molecular g-value and magnetic-susceptibility tensors in the principal inertial axis in formaldehyde have been determined by observing the molecular rotational Zeeman effect. A small J dependence was observed in the molecular g values where the absolute values increased with rotational excitation. The g values averaged over the three rotational states studied for each isotopic species are gaa = − 2.899 ± 0.002, gbb = − 0.2256 ± 0.0008, and gcc = − 0.1004 ± 0.0007 for H2CO, and gaa = − 1.445 ± 0.002, gbb = − 0.1917 ± 0.0005, and gcc = − 0.0788 ± 0.0004 for D2CO. The diagonal elements in the molecular magnetic-susceptibility tensor are χaa = − 6.2 ± 0.5, χbb = − 15.8 ± 0.4, and χcc = − 23.0 ± 0.4, all in units of 10−6erg/G2·mole. The sign of the electric dipole moment along the C-O bond in formaldehyde was determined to have +C-O− polarity from the isotope dependence in the above molecular g values. From the g values and the known structure of formaldehyde, the paramagnetic-susceptibility tensor elements can be calculated to be χaap = 29.5 ± 0.1, χbbp = 46.1 ± 0.5, and χccp = 47.7 ± 0.5, all in units of 10−6erg/G2·mole. These data, combined with our above experimental magnetic-susceptibility diagonal tensor elements give the experimental ground-state average values of the sums of squared Cartesian center-of-mass electronic coordinates. The results are 〈0 | Σixi2 | 0〉 = (3.2 ± 0.3) Å2, 〈0 | Σiyi2 | 0 = (5.2 ± 0.3) Å2, and 〈0 | Σizi2 | 0〉 = (11.4 ± 0.3) Å2. The z axis is along the O-C bond and the y axis is in the molecular plane. These new data are used to interpret the ground- and excited-state electronic structure of the formaldehyde molecule. The experimental information is also sufficient to determine the diagonal elements in the molecular quadrupole moment tensor.