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

TL;DR

This paper refines the spectroscopic parameters of sodium cyanide (NaCN) by addressing previous fitting difficulties, resulting in more accurate predictions of its rotational spectrum and isotopic variants.

Contribution

It introduces an improved Hamiltonian diagonalization approach with fewer parameters, enhancing the accuracy of spectral predictions for NaCN and its isotopologues.

Findings

Transition frequencies fit within experimental uncertainties

Predictions for $a$-type $R$-branch transitions are reliable up to 570 GHz

Modified parameters for 13C isotopic species are provided

Abstract

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.