Conformational analysis of (aminomethyl)cyclopropane hydrochloride using vibrational spectroscopy and ab initio calculations

Abstract

Raman spectra (from 100 to 3500 cm −1) of the salt (aminomethyl)cyclopropane hydrochloride and its N- d 3 isotopomer have been recorded in the polycrystalline solid phase and in H 2O and D 2O solutions respectively. In addition, the infrared spectrum of a KBr pellet of the light compound has also been obtained from 4000 to 600 cm −1. Supporting the experimental data, ab initio calculations at the RHF/6-31G* and MP2 levels have been carried out for the cyclopropylmethyl ammonium cation present in both of these molecules. Unlike the structural results obtained from similar studies of the isoelectronic molecules, ethyloxirane and ethylcyclopropane, the cyclopropylmethyl ammonium cation does not exhibit any experimental evidence for the existence of a cis conformer of this species. All of the observed spectral peaks—in both the Raman and infrared spectra—can be assigned as arising from the gauche rotameric form of this ion. This result is comparable with those found for ethyloxirane and ethylcyclopropane, in that the gauche conformer (or conformers, in the case of ethyloxirane) is the predominant form. However, for both of the ethyl-substituted three-membered ring compounds, small amounts of the cis conformer were also detected. We suspect that the positive charge of the cyclopropylmethyl ammonium cation, and the presence of the chloride counter-ion, tend to destabilize the cis structure of the cyclopropylmethyl ammonium ion. The computational results support the structural conclusions from the experimental study. At both the RHF/6-31G* and MP2 levels, the gauche rotamer of the cyclopropylmethyl ammonium ion is predicted to be the more stable form of this species, with the cis conformer also stable, but either 4.8 (6-31G*) or 4.7 kcal mol −1 (MP2) higher in energy and, therefore, not populated at ambient conditions. The Cartesian force constants obtained from the ab initio calculations were transformed into internal coordinate force constants, and a normal coordinate calculation was performed, yielding simulated infrared and Raman spectra of the title compound and its isotopomer. The simulated spectra of the gauche conformer in each case agree very well with those observed experimentally, thereby lending additional credence to our structural assignments.