The structure of carbodiimide, HNCNH

An experimentally determined r(s)-type structure of HNCNH is reported: r(NH ) = 1.0074 Angstrom, r(CN) = 1.2242 Angstrom, angle HNC = 118.63 degrees, a ngle NCN = 170.63 degrees, angle HN ... NH = 88.99 degrees. The number of d igits quoted allow for errors with two significant figures. In order to obt ain these values we recorded rotational-torsional spectra of (HNCNH)-C-13, (HNCNH)-N-15-N-15 and DNCND, by using isotopically enriched cyanamide. A ch emical equilibrium exists between carbodiimide, HNCNH, and the more stable isomer cyanamide. H2NCN, which strongly favours cyanamide (approximately 1: 115 at 110 degrees C). The expensive C- and N-substituted isotopomers could only be investigated in the millimetre wave region, while for DNCND the fa r infrared spectrum between 10-350cm(-1) was also recorded. Rotational cons tants of the three isotopomers, as well as of the parent species, were dete rmined by fitting the assigned spectral transitions to the Watson Hamiltoni an in S reduction. Using fitting programs written by Schwendeman and Rudolp h: r(0), r(s), and r(m)(rho) structures of HNCNH were derived. The experime ntally determined structural parameters are compared with an ab initio r(e) structure.