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 singly excited methyl torsion, v₂₃ = 1, is the second lowest excited vibrational state. It is perturbed at higher J or K_a levels by v₁₅ = 1 (264.0 cm^–1) or by v₂₄ = 2 (269.3 cm^–1). The methyl torsional splitting is considerably larger than in the ground state with A being higher than E in the rotationless level.
The 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) S. S. Butcher and E. B. Wilson, jr., 1964, J. Chem. Phys. 40, 1671.
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 for J + 2K_a > 50 or if the predicted uncertainties are larger than 0.3 MHz.
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 assumed to agree with that of the ground vibrational state which was determined in (2).