CONFORMATIONAL-ANALYSIS OF (AMINOMETHYL)CYCLOPROPANE HYDROCHLORIDE USING VIBRATIONAL SPECTROSCOPY AND AB-INITIO CALCULATIONS

Raman spectra (from 100 to 3500 cm(-1)) of the salt (aminomethyl)cyclo propane hydrochloride and its N-d(3) isotopomer have been recorded in the polycrystalline solid phase and in H2O and D2O solutions respectiv ely. In addition, the infrared spectrum of a KBr pellet of the light c ompound has also been obtained from 4000 to 600 cm(-1). Supporting the experimental data, ab initio calculations at the RHF/6-31G* and MP2 l evels have been carried out for the cyclopropylmethyl ammonium cation present in both of these molecules. Unlike the structural results obta ined 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 conforme r of this species. All of the observed spectral peaks-in both the Rama n and infrared spectra-can be assigned as arising from the gauche rota meric 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. Howe ver, 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 cia str ucture of the cyclopropylmethyl ammonium ion. The computational result s support the structural conclusions from the experimental study. At b oth the RHF/6-31G* and MP2 levels, the gauche rotamer of the cycloprop ylmethyl 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*) o r 4.7 kcal mol(-1) (MP2) higher in energy and, therefore, not populate d at ambient conditions. The Cartesian force constants obtained from t he ab initio calculations were transformed into internal coordinate fo rce constants, and a normal coordinate calculation was performed, yiel ding simulated infrared and Raman spectra of the title compound and it s isotopomer. The simulated spectra of the gauche conformer in each ca se agree very well with those observed experimentally, thereby lending additional credence to our structural assignments. (C) 1998 Elsevier Science B.V.