The spectroscopic parameters of sodium cyanide, NaCN (X 1A'), revisited

The study of the rotational spectrum of NaCN (X $^1$A') has recently been extended in frequency and in quantum numbers. Difficulties have been encountered in fitting the transition frequencies within experimental uncertainties. Various trial fits traced the difficulties to the incomplete diagonalization of the Hamiltonian. Employing fewer spectroscopic parameters than before, the transition frequencies could be reproduced within experimental uncertainties on average. Predictions of $a$-type $R$-branch transitions with $K_a \le 7$ up to 570 GHz should be reliable to better than 1 MHz. In addition, modified spectroscopic parameters have been derived for the 13C isotopic species of NaCN.