The conformer I of α-alanine is the one lowest in energy. It has an intermolecular H bond between the H atoms of the amino group and the O atom of the carboxyl group. The microwave frequencies have been reported by
(1) S. Blanco, A. Lesarri, J.C. Lòpez, and J.L. Alonso, 2004, J. Am. Chem. Soc. 126, 11675.
The millimeter wave frequencies have been reported by
(2) P. D. Godfrey, S. Firth, L. D. Hatherley, R. D. Brown and A. P. Pierlot, 1993, J. Am. Chem. Soc. 115, 9687;
and by
(3) Y. Hirata, S. Kubota, S. Watanabe, T. Momose, and K. Kawaguchi, 2008, J. Mol. Spectrosc. 251, 314.
Extrapolations are assumed to be reliable as long as the predicted uncertainties do not exceed 0.3 MHz. Transitions with J > 40 and K_a > 30 should be viewed with some caution even if the predicted uncertainties are small.
The ¹⁴N hyperfine splitting was resolved in (1). It may be resolved in cold sources or at lower frequencies. Therefore, separate predictions with hyperfine splitting are provided for J up to 10. The partition function takes into account the spin multiplicity g_I = 3 of the ¹⁴N nucleus !
No vibrational state and no other conformer were considered in the calculation of the partition function.
The dipole moment was reported in (2). Note: The b-dipole moment component in (2) is 1.60 D with a large uncertainty of 0.46 D; ab initio calculations in (1) predict a value of 1.19 D. Since these calculations are in reasonable, but not very good agreement with the experimental values with smaller uncertainties, a recommended average value of 1.3 D has been used in the present calculations.