Dicyanomethane : microwave spectrum, quadrupole coupling and structure

Abstract

The microwave spectra of seven isotopic species of CH₂(CN)₂ have been studied in the ground vibrational state. The zero-point average structure has been determined to be: r(C—C)= 1.459(2)Å, r(CN)= 1.160(2)Å and r(C—H)= 1.109(2)Å with ∠ CCN = 180 – 1.4(3)°, ∠ CCC/2 = 56.3(2)° and ∠ HCH/2 = 53.5(2)°. A near-equilibrium structure has also been estimated using an r^ρ_m method tested by a similar calculation for formaldehyde. Microwave Fourier-transform spectroscopy has been used to investigate the main species and single-nitrogen-15 species under high resolution. For CH₂(CN)C¹⁵N accurate ¹⁴N quadrupole coupling constants have been determined as χ_aa=–2.364(9) MHz, χ_bb= 0.313(6) MHz and χ_cc= 2.051(6) MHz. These results have been combined with those previously obtained for the main species to show that the quadrupole tensor is almost symmetrical about the CN bond direction. There is some disagreement between the constants for the two isotopic molecules close to experimental error, possibly indicating zero-point vibrational effects on the coupling tensor. The quadrupole tensor is of especial interest in CH₂(CN)₂ in view of the bent CCN moieties.