Microwave spectrum, conformation, dipole moment, and quantum chemical calculations of 1-amino-1-ethenylcyclopropane

The microwave spectrum of 1-amino-1-ethenylcyclopropane has been investigated in the 13-40 GHz spectral region at about -45 degrees C. The prevailing (ap)-rotamer (sp with respect to the cyclopropyl and amino, np with respect to cyclopropyl and the vinyl group) was assigned. This rotamer is virtually a hybrid of the most stable conformer of unsubstituted ethenylcyclopropane and unsubstituted cyclopropylamine, each in its preferred orientation; it has a symmetry plane (C-s symmetry) with the C-N bond and the C=C double bond syn to each another. In this conformation, both of the hydrogen atoms of the amino group may be involved in weak intramolecular hydrogen bonds with the pi electrons of the double bond. The dipole moment components and the total dipole moment are (in units of 10(-30) C m): mu(a) = 2.00(1), mu(b) = 0.89(14), mu(c) = 0.0 (for symmetry reasons), and mu(rot) = 2.19(9). Four vibrationally excited states belonging to three different normal modes were assigned and their frequencies determined by relative intensity measurements. The microwave work has been assisted by quantum chemical computations at the elaborate MP2/cc-pVTZ, as well as at the B3LYP/6-31G* levels of theory.