The combined fit of both anti and gauche conformers has been described in
(1) A. Belloche, R. T. Garrod, H. S. P. Müller, K. M. Menten, C. Comito, and P. Schilke, 2009, Astron. Astrophys., 499 215.
The transition frequencies were taken from
(2) J. Demaison and H. Dreizler, 1982, Z. Naturforsch. A, 37, 199;
(3) K. Vormann and H. Dreizler, 1988, Z. Naturforsch. A, 43, 338;
and from
(4) G. Wlodarczak, L. Martinache, J. Demaison, K.-M. Marstokk, and H. Møllendal, 1988, J. Mol. Spectrosc., 127, 178;
Erratum, 1991, J. Mol. Spectrosc., 146, 224.
In addition, supplementary material to (3) was also used in the fits.
With respect to first entry from Apr. 2009, we corrected the energy difference.
(5) J. R. Durig, B. R. Drew, A. Komer, and S. Bell, 2001, Phys. Chem. Chem. Phys., 3 766,
reported the gauche conformer to be lower than the anti conformer by 0.48 ± 0.04 kJ/mol ≡ 40.1 ± 3.3 cm^–1 ≡ 57.7 ± 4.8 K. A reverse energy ordering was determined in (4). Since this may be caused by an underestimation of the a-dipole moment component of the anti conformer, that value was modified as indicated below.
¹⁴N hyperfine splitting as well as CH₃ internal rotation splitting has been resolved for some transitions at MW frequencies. Both types of splitting are unlikely to be resolved in astronomical observations.
The predictions may be sufficient for astronomical observations, but should be viewed with caution if the uncertainties are larger than 0.3 MHz.
Note: the statistical weight of the anti and gauche conformers is 1 : 2 because of symmetry. With respect to the second entry from May 2014, we have estimated vibrational corrections in the harmonic oscillator approximation using fundamental vibrations of the gauche-conformer from (5) as commented in
(6) H. S. P. Müller, A. Walters, N. Wehres, A. Belloche, O. H. Wilkins, D. Liu, R. Vicente, R. T. Garrod, K. M. Menten, F. Lewen, and S. Schlemmer, 2016, Astron. Astrophys., 595 Art. No. A87.
Additional information on vibrational states is also available.
Most of the dipole moment components were taken from (4). The a-dipole moment component of the anti conformer was estimated based on experimental and quantum-chemical values published in
(7) H. S. P. Müller, A. Coutens, A. Walters, J.-U. Grabow, and S. Schlemmer, 2011, J. Mol. Spectrosc., 267, 100.
The small c-dipole moment component of the gauche conformer was estimated by quantum-chemical means by
(8) H. S. P. Müller, 2014, unpublished..