Conformational and Structural Studies of Aminomethyl Cyclopropane from Temperature Dependent FT-IR Spectra of Rare Gas Solutions and ab Initio Calculations

Infrared (3500−50 cm-1) spectra of gaseous and Raman (3500−50 cm-1) spectra of liquid aminomethyl cyclopropane (cyclopropyl methylamine), c−C3H5CH2NH2 have been recorded. Additional variable temperature (−55 to −100 °C) studies of the mid-infrared (3500−400 cm-1) spectra of the sample dissolved in liquid xenon as well as variable temperature (−79 to −112 °C) studies of the far-infrared spectra (600−50 cm-1) of krypton solutions have been obtained. From these data the enthalpy difference has been determined to be 109 ± 11 cm-1 (1.30 ± 0.13 kJ/mol) between the most stable gauche−gauche-1 (Gg-1) conformer (the first gauche designation, capital G, for the heavy atom conformation along the C−C bond, and the second gauche designation, lower case g, for the amino torsion along C−N bond) and the second most stable conformation, gauche-trans (Gt). The third most stable conformer is the cis-gauche (Cg) form with an enthalpy difference of 267 ± 28 cm-1 (3.19 ± 0.33 kJ/mol) to the most stable conformer. Larger enthalpy values of 400 ± 40 cm-1 and 480 ± 48 cm-1 were obtained for the Gg-2 and Ct conformers, respectively. From these data, the following conformer percentages are estimated at ambient temperature: 49% Gg-1, 29% Gt, 13% Cg, 7% Gg-2, and 2% Ct. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311G(2df,2pd) as well as with diffuse functions to determine the conformational stability. Without diffuse functions, the Gt conformer is predicted as the most stable conformer, whereas with diffuse functions, the Gg-1 form is predicted to be the most stable rotamer, and the density functional calculations by the B3LYP method with the same corresponding basis sets all predict the Gt form as the most stable conformer. Additionally, force constants, infrared intensities, Raman activities, depolarization ratios, and scaled vibrational frequencies have been determined from MP2/6-31G(d) calculations. Vibrational assignments are provided for most of the fundamentals for the Gg-1 and Gt conformers. Adjusted r 0 structural parameters have been obtained by combining ab initio MP2/6-311+G(d,p) predicted values and previously reported microwave data for the Gg-1 and Gt forms. Many of the determined results are compared to the corresponding parameters for some other similar organoamines.