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.
