Abstract
The infrared spectra (3600–100
cm
−1) of gaseous and solid cyclobutylamine, c-C
4H
7NH
2 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 NH
2 moiety of the equatorial conformer has been determined with the following potential coefficients (cm
−1):
V
1
=
−176
±
20,
V
2
=
−199
±
25,
V
3
=
721
±
5 and
V
4
=
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,
r
0 structural parameters have been obtained for this conformer. The determined
r
0 heavy atom distances in Å are: (C
1N)
=
1.453(5); (C
1C
4)
=
1.560(5); (C
1C
5)
=
1.547(5); (C
4C
6)
=
1.557(5); (C
5C
6)
=
1.555(5) and the angles in degrees ∠NC
1C
4
=
122.8(5)°; ∠NC
1C
5
=
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.