Conformational stability, r0 structural parameters, barriers to internal rotation and vibrational assignment of cyclobutylamine

The infrared spectra (3600–100 cm−1) of gaseous and solid cyclobutylamine, c-C4H7NH2 have been recorded, as well as, the infrared (3600–60 cm−1) spectra in xenon or krypton solutions at variable temperatures. From the latter studies, the enthalpy differences have been determined to be 225 ± 26 cm−1 (2.69 ± 0.31 kJ/mol) between the most stable equatorial-gauche (Eq-g) conformer and the equatorial-trans (Eq-t) form and 417 ± 42 cm−1 (4.99 ± 0.50 kJ/mol) with the axial-trans (Ax-t) conformer. At ambient temperature there is only 13 ± 1% of the Eq-t form present and 5 ± 1% of the Ax-t form. The potential function governing the conformational interchange of the NH2 moiety of the equatorial conformer has been determined with the following potential coefficients (cm−1): V1 = −176 ± 20, V2 = −199 ± 25, V3 = 721 ± 5 and V4 = 65 ± 10 with barriers of 784 cm−1 for gauche to gauche and 834 cm−1 for gauche to trans form. By utilizing previously reported microwave rotational constants for the Eq-g conformer along with the ab initio predicted structural parameters from the MP2(full)/6-311 + G(d,p) calculations, r0 structural parameters have been obtained for this conformer. The determined r0 heavy atom distances in Å are: (C1N) = 1.453(5); (C1C4) = 1.560(5); (C1C5) = 1.547(5); (C4C6) = 1.557(5); (C5C6) = 1.555(5) and the angles in degrees ∠NC1C4 = 122.8(5)°; ∠NC1C5 = 116.8(5)° with the four ∠CCC ranging from 87.5° to 88.1°. A complete vibrational assignment is proposed for the (Eq-g) conformer based on the infrared band contours, infrared intensities, Raman activities and group frequencies which is supported by normal coordinate calculations utilizing the force constants from ab initio MP2(full)/6-31G(d) calculations. Proposed assignments are also made for several fundamentals of the (Eq-t) form and a few for the Ax-t conformer. The results are discussed and compared to the corresponding properties of some similar molecules.