Abstract
The νC=O regions of the gas phase infrared spectra of six carbonyl isocyanates XCONCO with X=F, OMe, OEt, SMe, NCO and NMe2 have been analyzed. For all compounds except X=NMe2 two absorptions were found in the νC=O region with a separation of at least 30 cm−1. The clearly defined rotational band contours allowed assignments to the cisoid and transoid conformers of these molecules derived from rotation around the CN bond. For X=OMe, OEt and SMe the interpretation of the νCO regions suggest the alkyl group adopt the Z-arrangement. The relative intensity changes in the νCO regions via variable-temperature measurements were used to determine the relative conformational stabilities. For all compounds, the cisoid conformer was found to be more abundant in the gas phase. The results are explained in terms of varying contributions of mesomeric structures as supported by semiempirical MO calculations.
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