Microwave spectrum of dibromomethane. Part 2.—Nuclear quadrupole coupling tensor

Abstract

The nuclear quadrupole hyperfine structure in the spectra of four isotopic species of dibromomethane, CH₂⁷⁹Br₂,CH₂⁸¹Br₂,CD₂⁷⁹Br₂ and CD₂⁸¹Br₂, has been analyzed to give the quadrupole coupling tensor directly, using second-order perturbation theory. Coupling constants (MHz) in the principal inertial axes system are as follows, CH₂⁷⁹Br₂: χ_aa= 334.9 ± 12, χ_bb=–25.2 ∓ 6, χ_cc=–309.7 ∓ 6, χ_ab= 395.9 ± 13; CH₂⁸¹Br₂: χ_aa= 280.6 ± 6, χ_bb=–21.4 ∓3, χ_cc=–259.2 ∓ 3, χ_ab= 332.5 ± 10. Consistent values are reported for CD₂⁷⁹Br₂ and CD₂⁸¹Br₂. Diagonalization of the quadrupole coupling tensor yields, CH₂⁷⁹Br₂: χ_zz= 591 ± 20, χ_xx=–281∓14, χ_yy=–310 ∓ 6 and CH₂⁸¹Br₂: χ_zz= 495 ± 12, χ_xx=–236∓9, χ_yy=–259.2∓3. It is concluded that the Br-bond projection coincides with the internuclear axis within the experimental error of 1°; if bent bonding is involved in the C—Br bond the bend cannot be more than the experimental error of 2°. The analysis of the nuclear quadrupole hyperfine structure of a dibromine molecule is outlined and a comprehensive list of assignments presented in an appendix.