Molecular g Values, Magnetic Susceptibility Anisotropies, Diamagnetic and Paramagnetic Susceptibilities, Second Moment of the Charge Distribution, and Molecular Quadrupole Moments of H3CCN and H3CNC

Abstract

The high-field, linear and quadratic Zeeman effect has been observed in methyl cyanide (H3CCN) and methyl isocyanide (H3CNC). Both spectra are complicated by the presence of the 14N nuclear quadrupole coupling. The H3CNC system leads to uncoupled spectra in high fields, and the H3CCN system does not exhibit fully uncoupled spectra even at fields of 25 000–30 000 G. In the numbers given below, c = b is perpendicular to the C3 or a axis. The perpendicular g values in H3CC14N and D3CC14N are g⊥ = − 0.0338 ± 0.0008 and g⊥ = − 0.0315 ± 0.0008, respectively. The sign of the electric dipole moment is +H3CCN–. The average magnetic susceptibility anisotropy from the above isotopic molecules is χ⊥ − χ‖ = (10.2 ± 1.0) × 10−6erg/G2·mole. Assuming g‖ = 0.310 from other similar systems gives the molecular quadrupole moment of Q‖ = −(1.8 ± 1.2) × 10−26esu·cm2. Using the known molecular structure and above g values allows a determination of the paramagnetic susceptibilities which are χ⊥p = (145.3 ± 1.0) × 10−6erg/G2·mole and χ‖p = (9.6 ± 0.5) × 10−6erg/G2·mole. Combining the above experimental magnetic susceptibility anisotropy with the known bulk magnetic susceptibility gives the total and diamagnetic terms, χ⊥ = −(24.1 ± 0.4), χ⊥d = −(169.4 ± 1.1), χ‖ = −(34.6 ± 0.5), and χ‖d = −(44.2 ± 1.0), all in units of 10−6erg/G2·mole. The individual elements in the tensor describing the second moment of the electronic charge distribution are 〈c2〉 = 5.2 ± 0.3 and 〈a2〉 = 34.7 ± 0.4 in units of 10−16 cm2. The Zeeman spectra of H3CC15N was also observed to confirm the above results with the 14N isotope. The molecular g value in H3CC15N is g⊥ = − 0.0317 ± 0.0003 and (χ⊥ − χ‖) = (10.5 ± 0.5) × 10−6erg/G2·mole. The perpendicular g value and magnetic susceptibility anisotropy of H3C14NC are g⊥ = − 0.0546 ± 0.0015 and χ⊥ − χ‖ = (13.5 ± 0.7) × 10−6erg/G2·mole. The sign of g⊥ was not determined experimentally; however, an analysis of the molecular quadrupole moment indicates the proper choice of sign. Assuming g‖ = 0.310 from similar systems gives the molecular quadrupole moment of Q‖ = −(2.7 ± 1.6) × 10−26esu·cm2. Using techniques as outlined above give the paramagnetic susceptibilities of χ⊥p = 137.8 ± 0.9 and χ‖p = 9.6 ± 0.6 in units of 10−6 erg/G2·mole. The total and diamagnetic susceptibilities are also estimated. The anisotropy in the second moment of the electronic charge distribution is 〈a2〉 − 〈c2〉 = (27.0 ± 0.5) × 10−16cm2.