Estimation of spectroscopic constants of isotopologues: Dependence of the scaling factors on isotopic mass

Scaling factors have been applied to the estimation of the spectroscopic constants of pure isotopologues of two polyatomic molecules: ammonia (NH3) and formaldehyde (HCHO). Vibrational frequencies and rotational constants of four isotopologues of each molecule have been calculated using the resolution-of-the-identity second-order coupled cluster (RICC2) and coupled cluster singles and doubles (CCSD) methods with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. It has been observed that the scaling factors for the vibrational frequencies and rotational constants of the isotopologues are dependent on the isotopic mass. With the given scaling factors calculated at the CCSD/aug-cc-pVTZ level of theory, the experimental spectroscopic data are reproduced with an error range of 0.6% for the D13CD16O isotopologue. In addition, the spectrum of H13CH16O is accurately predicted with the scaling factors calculated from the other isotopologues. The estimated spectrum is in good agreement with the experimental spectrum.