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. Tunneling between the two equivalent minima causes a splitting into a symmetric and an antisymmetric tunneling state. a- and b-type transitions occur within the tunneling states, c-types between them. Splitting caused by the methyl internal rotation is very small and can be neglected.
The present fit 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) H. M. Pickett and D. G. Scroggin, 1974, J. Chem. Phys. 61, 3954;
and by
(3) J. Randell, A. P. Cox, and H. Dreizler, 1987, Z. Naturforsch. 42a, 957.
State number 0 indicates the symmetric tunneling state, 1 the antisymmetric one.
The predictions should be accurate enough for all astronomical observations. They should be viewed with caution for K_a > 10 because of difficulties to model the experimental transition frequencies at higher J, see (1) for details. Caution is also advised for b- or c-type with K_a > 2 or if the predicted uncertainties are larger than 0.2 MHz
The partition function includes only the ground vibrational state of the syn-conformer. Vibrational and conformational correction factors should be available in the near future.
The dipole moment was determined in (2).