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The Structure of Butyl Carbamate and of Its Water Complex in the Gas Phase.

The structure of butyl carbamate and of its complex with water generated in a supersonic expansion has been characterized by Fourier transform microwave spectroscopy. Up to 13 low-energy conformations of the monomer have been predicted which differ in the relative orientation of the butyl chain and the amide group. However, only three conformations have been observed experimentally. The remaining low energy conformers are expected to interconvert into the observed rotamers through collisional relaxation processes in the supersonic jet. The values of the C-O-Cα-Cβ dihedral angle observed for the two most stable conformers of butyl carbamate, with extended configurations, can be directly correlated with the values of this angle in the two experimentally observed conformers of the shorter-chain molecule, ethyl carbamate. The less stable form shows a weak C-H···O=C intramolecular hydrogen bond from the terminal methyl group to the carbamate C=O group, stabilizing a folded configuration. For the most stable butyl carbamate monomer the complex with one molecule of water has been observed. In that complex the water molecule attaches to the amide group in a cyclic arrangement using two hydrogen bonds. The results indicate that water does not substantially alter the conformational behavior of butyl carbamate.