Microwave spectrum, conformational equilibrium and quantum chemical calculations of urethane (ethyl carbamate)

摘要

The microwave spectrum of urethane (H2NCO2CH2CH3) has been investigated in the 16.5-56.0 GHz spectral region at room temperature. Two rotamers denoted conformer I and conformer II were assigned. The C=O and -O-CH2 bonds are oriented in the syn conformation in both these rotamers. The H2NCO2CC atoms are co-planar in conformer I with the methyl group anti to the C-carbonyl-O bond. The ethyl group is related 98 degrees in conformer II from the position it has in I. Conformer I is found to be 0.5(5) kJ mol(-1) more stable than II by relative intensity measurements. The ground vibrational state was assigned for I, whereas the ground and one vibrationally excited state were assigned for II. The microwave work has been assisted by ab initio computations at the MP2/cc-pVTZ level of theory, as well as density theory calculations at the B3LYP/6-31G* level. Both these methods predict similar geometrical structures for the two conformers and relational constants that are close to the experimental ones. Both theoretical procedures predict a shallow pyramid around the nitrogen nucleus. [References: 26]