Accurate ab initio quartic force fields for NH2− and CCH− and rovibrational spectroscopic constants for their isotopologs

Abstract

A series of high-quality, purely ab initio, quartic force fields (QFFs), computed using a procedure we recently proposed, is reported for NH2− and CCH−. The singles and doubles coupled-cluster method with a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), was used with TZ, QZ, and 5Z quality basis sets and was combined with extrapolation to the one-particle basis-set limit, core-correlation effects, scalar relativistic effects, and higher-order correlation effects to yield accurate QFFs. A “best-guess” reference geometry was determined at the CCSD(T)/5Z level of theory. Analytical transformation removes nonzero gradients to facilitate a second-order perturbation theory spectroscopic analysis. The QFF is transformed into Morse/cosine coordinates in order to perform exact vibrational configuration interaction computations. Equilibrium structures, vibrational frequencies, rotational constants, and selected spectroscopic constants are reported in comparison with experimental values and previous theoretical studies. Higher-order correlation effects are found comparable to core-correlation effects in magnitude, e.g., ∼10 cm−1 for fundamentals, but are of opposite sign. For CCH−, a thorough discussion is presented on effective rotational constants B0. It is concluded that the “best” QFF should incorporate all the small corrections mentioned above. Correspondingly, the best vibrational fundamentals of CCH− are estimated at 502.0 cm−1 (ν2), 1800.9 cm−1 (ν3), and 3204.3 cm−1 (ν1), while the best vibrational fundamentals of NH2− are at 3118.5 cm−1 (ν1), 1447.8 cm−1 (ν2), and 3186.5 cm−1 (ν3). Excellent agreement with high-resolution experiments has been obtained for fundamentals—e.g., 1–3 cm−1 deviation for the symmetric and antisymmetric stretches of NH2−, 3121.93 cm−1 (ν1) and 3190.29 cm−1 (ν3), respectively. Isotopic effects are studied and presented to aid future experimental analyses. The present study should facilitate future characterizations of NH2− and CCH− from astronomical observations or other high-resolution laboratory studies.