The measurements were taken from
(1) H. Habara, S. Yamamoto, and T. Amano, 2002, J. Chem. Phys. 116, 9232.
Only K_a = 0 transitions have been observed in the laboratory so far. Therefore, the rotational constants are known only approximately. Only (B + C)/2 is well determined as 20145.6 MHz.
Transitions involving K_a = 1 cannot be predicted reliably predominantly because ε_aa is not known. Moreover, since the barrier to linearity may be small it is conceivable that knowledge of the K_a = 1 energies will not help much in locating higher energy levels. Therefore, only transitions involving K_a = 0 are provided. Even some of these transitions around 1 THz may be poorly predicted because of electron spin-rotation interaction between K_a = 0 and 1 around N = 24.
The partition function takes into account higher K_a values as far as possible at present.
The total dipole moment (1.02) is from an ab initio calculation from
(2) C. Ochsenfeld, R. I. Kaiser, Y. T. Lee, and M. Head-Gordon, 1999, J. Chem. Phys. 110, 9982.
An a dipole component of 0.4 D was communicated to the authors of a HSC article
(3) H. Habara and S. Yamamoto, 2000, J. Chem. Phys. 112, 10905.
Therefore, there should be a b dipole component of about 0.9 D, but b-type transitions cannot be reliably predicted at present.