CONFORMATIONAL PROPERTIES OF CARBONYL ISOCYANATES - STEREOELECTRONIC EFFECTS FAVORING THE CISOID CONFORMATION

The nuC=O regions of the gas phase infrared spectra of six carbonyl is ocyanates X-CO-NCO with X = F, OMe, OEt, SMe, NCO and NMe2 have been a nalyzed. For all compounds except X = NMe2 two absorptions were found in the nuC=O region with a separation of at least 30 cm-1. The clearly defined rotational band contours allowed assignments to the cisoid an d transoid conformers of these molecules derived from rotation around the C-N bond. For X = OMe, OEt and SMe the interpretation of the nuC-N regions suggests the alkyl groups adopt the Z-arrangement. The relati ve intensity changes in the nuC=O regions via variable-temperature mea surements were used to determine the relative conformational stabiliti es. For all compounds the cisoid conformer was found to be more abunda nt in the gas phase. The results are explained in terms of varying con tributions of mesomeric structures as supported by semiempirical MO ca lculations.