Molecular g-values, magnetic susceptibility anisotropies, second moment of the charge distribution, and molecular quadrupole moments in acetaldehyde

Abstract

The high-field molecular Zeeman effect for acetaldehyde has been observed in both the A and E internal rotation states. The values of the molecular g-values and magnetic susceptibility anisotropies are g_aa=–(0.3609 ± 0.0021), g_bb=–(0.0731 ± 0.0003), and g_cc=–(0.0245 ± 0.0006), 2χ_aa–χ_bb–χ_cc=(8.1 ± 2.5)× 10^–6 erg/G² mole, and 2χ_bb–χ_aa–χ_cc=(9.6 ± 1.4)× 10^–6 erg/G² mole. Only the relative signs of the g-values can be determined. However, we show by other arguments that the signs are all negative. The a axis is off the CO bond by about 23° and the b axis is also in the CO plane. The molecular quadrupole moments are Q_aa=–(1.2 ± 1.5)× 10^–26, Q_bb=+(1.0 ± 0.9)× 10^–26, and Q_cc=–(0.2 ± 1.8)× 10^–26 all in units of e.s.u. cm². The values of the individual diagonal elements in the magnetic susceptibility tensor are χ_aa=–(20.0 ± 0.9)× 10^–6, χ_bb=–(19.5 ± 0.5)× 10^–6, and χ_cc=–(28.6 ± 1.4)× 10^–6 all in units of erg/G² mole.

The molecular structure is combined with the above data to yield the diagonal elements in the paramagnetic and diamagnetic susceptibility tensors and also the second moment of the electronic charge distribution. The magnetic susceptibilities are χ^p_aa=–44.1 ± 0.6, χ^p_bb= 140.9 ± 0.6, and χ^p_cc= 148.8 ± 0.6, χ^d_aa=–64.1 ± 1.4, χ^d_bb=–160.4 ± 1.0, and χ^d_cc=–177.4 ± 1.8 all in units of 10^–6 erg/G² mole. The values for the second moment of the electronic charge distribution are 〈a²〉= 32.3 ± 0.5, 〈b²〉= 9.6 ± 0.4, and 〈c²〉= 5.6 ± 0.5 all in units of 10^–16 cm². These numbers are all in the centre of mass principal inertial axis system. Some features of the Zeeman effect in molecules exhibiting internal rotation are discussed.