There are two conformers in propanal caused by the internal rotation of the aldehyde group -CHO. The syn-conformer is the lowest energy conformer. It has all the heavy atoms in the ab-plane; for this matter, it is sometimes called cis. The two equivalent gauche-conformers are approximately 420 ± 27 cm^–1 higher in energy as derived from relative intensity measurements. Other measurements largely support this value, even though they show some scatter. The first entry from Dec. 2008 has been revised considerably. Even though internal rotation of the methyl group has been resolved in the vibrational ground state only in part in the laboratory, it does matter in some sources with narrow lines.
The present fit was performed with ERHAM and is described in
(1) O. Zingsheim, H. S. P. Müller, F. Lewen, J. K. Jørgensen, and S. Schlemmer, 2017, J. Mol. Spectrosc. 342, 125.
By far the most data come from that study. Additional microwave frequencies have been reported by
(2) J. A. Hardy, A. P. Cox, E. Fliege, and H. Dreizler, 1982, Z. Naturforsch. 37a, 1035.
The millimeter wave data by
(3) J. Demaison, H. Maes, B. P. van Eijck, G. Wlodarczak, and M. C. Lasne, 1987, J. Mol. Spectrosc. 125, 214;
were helpful for the initial assignments, but were largely remeasured. The remainder of the lines were omitted.
State number 0 indicates methyl rotor states with A symmetry, 1 those with E symmetry. Please note that the K_c labeling may be incorrect for some prolate paired transition, i.e., transitions with high K_a with no or possibly even unresolved asymmetry splitting. As an additional consequence, a small number of experimental lines have not been merged in the default catalog file.
The predictions should be accurate enough for all astronomical observations. They should be viewed with caution if the predicted uncertainties are larger than 0.3 MHz and for b-type transitions with K_a much larger than 25.
The partition function includes only the ground vibrational state. Vibrational and conformational correction factors should be available in the near future.
The dipole moment was reported in
(4) S. S. Butcher and E. B. Wilson, jr., 1964, J. Chem. Phys. 40, 1671.