Accurate ab initio quartic force fields for NH_2-and CCH- and rovibrationalspectroscopic constants for their isotopologs

摘要

A series of high-quality, purely ab initio, quartic force fields (QFFs), computed using a procedurewe recently proposed, is reported for NH_2andThe singles and doubles coupled-clustermethod with a perturbational estimate of the effects of connected triple excitations, denotedCCSD(T), was used with TZ, QZ, and 5Z quality basis sets and was combined with extrapolationto the one-particle basis-set limit, core-correlation effects, scalar relativistic effects, and higher-ordercorrelation effects to yield accurate QFFs. A "best-guess" reference geometry was determined at theCCSD(T)/5Z level of theory. Analytical transformation removes nonzero gradients to facilitate asecond-order perturbation theory spectroscopic analysis. The QFF is transformed into Morse/cosinecoordinates in order to perform exact vibrational configuration interaction computations.Equilibrium structures, vibrational frequencies, rotational constants, and selected spectroscopicconstants 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-I for fundamentals, but are of opposite sign. For CCH-, a thorough discussion is presentedon effective rotational constants Bo. It is concluded that the "best" QFF should incorporate all thesmall corrections mentioned above. Correspondingly, the best vibrational fundamentals of CCH- areestimated at 502.0 cm-I ( v2), 1800.9 cm-I ( v3), and 3204.3 cm-I (v1), while the best vibrationalfundamentals of NH2 are at 3118.5 cm-I (v1), 1447.8 cm-1 ( v2), and 3186.5 cm-1 ( v3). Excellentagreement with high-resolution experiments has been obtained for fundamentals—e.g., 1-3 cm-Ideviation for the symmetric and antisymmetric stretches of NH2, 3121.93 cm-I ( v1) and3190.29 cm-1 ( v3), respectively. Isotopic effects are studied and presented to aid futureexperimental analyses. The present study should facilitate future characterizations of NH2 andCCH- from astronomical observations or other high-resolution laboratory studies.