The chain-like normal isomer of butyl cyanide, n-C₄H₉CN, may exist in five different conformations. Low-resolution infrared spectroscopy and molecular mechanics calculations were reported by
(1) G. A. Crowder, 1989, J. Mol. Struct. (Theochem), 200, 235.
The anti-anti- (or AA-) conformer was found to be lowest in energy. The heavy atoms form a planar zig-zag structure in this conformer. The gauche-anti- (or GA-) conformer, with the cyano group rotated by about 120^o, is only slightly higher in energy, by about 65 K. The AG-conformer, with the methyl group rotated by about 120^o, is about 440 K higher in energy, and the GG'-conformer, with both group rotated about 120^o away from each other, is about 440 K higher in energy. The GG-conformer, with both groups rotated toward each other, is much higher higher in energy than the AA-conformer, around 1000 K. This conformer was not considered or seen in (1). Note: While the AA-conformer is unique, all others are conformationally doubly degenerate because the rotations can be performed clockwise as well as counter-clockwise.
Transition frequencies < 22 GHz were taken from
(2) R. K. Bohn, J. L. Pardus, J. August, T. Brupbacher, and W. Jäger, 1997, J. Mol. Struct., 413–414, 293;
¹⁴N hyperfine structure was resolved for several transitions.
Extensive additional transitions were reported by
(3) M. H. Ordu, H. S. P. Müller, A. Walters, M. Nuñez, F. Lewen, A. Belloche, K. M. Menten, and S. Schlemmer, 2012, Astron. Astrophys., 541, Art. No. A121.
Predictions with uncertainties much larger than 0.5 MHz should be viewed with caution. This should not be any limitation for astronomical observations.
¹⁴N hyperfine structure is unlikely to be resolved in astronomical observations.
Note: The main entry has been calculated assuming the AG-conformer to be 443 K higher than AA and taking into consideration the conformational degeneracy. Since the energy difference is very uncertain, an alternative entry is provided with energy origin at zero and the factor of 2 in the degeneracy removed. The corresponding energy file and partition function values are available. Note also that the partition function does not include contributions from other vibrations or conformers.
The dipole moment components were estimated in (2). Deviations from the estimates are probably small, but not necessarily negligible. No c-type have been predicted because the transitions frequencies are rather uncertain.