Accurate ab initio quartic force fields for NH[sub 2]−] and CCH[sup −] and rovibrational spectroscopic constants for their isotopologs

Vibrational frequencies and spectroscopic constants of three, stable noble gas molecules: NeCCH+, ArCCH+, and ArCN+

The search for possible, natural, noble gas molecules has led to quantum chemical, spectroscopic analysis of NeCCH⁺, ArCCH⁺, and ArCN⁺.

Quartic force fields for excited electronic states: rovibronic reference data for the 1 (2)A' and 1 (2)A″ states of the isoformyl radical, HOC

Quartic force fields (QFFs) have been shown to be an effective, accurate, and relatively compact means of computing rovibrational spectroscopic data for numerous molecules with numerous applications. However, excited states have been nearly excluded from the this approach since most accurate QFFs are based on the "gold standard" coupled cluster singles, doubles, and perturbative triples [CCSD(T)] method which is not readily extended to excited states. In this work, rovibronic spectroscopic data is provided for the isoformyl radical, a molecule of significance in combustion and astrochemistry, both through the traditional means of variational access to excited states with CCSD(T) and in the novel extension of QFFs routinely to treat electronically excited states through the standard coupled cluster excited state approach, equation of motion (EOM) CCSD. It is shown here that the new EOM-based QFF provides structural parameters and rotational constants that are quite close to those from a related CCSD(T)-based QFF for the 1 (2)A(″) excited state of HOC. The anharmonic vibrational frequency percent differences between the two QFFs are less than 0.4% for the O-H stretch, less than 1.9% for the C-O stretch, and around 3.0% for the bend. Even so, the pure excited state EOM-QFF anharmonic frequencies are still very good abinitio representations that may be applied to systems where electronically excited states are not variationally accessible. Additionally, rovibrational spectroscopic data is provided for the 1 (2)A(') ground state of HOC and for both the ground and excited state of DOC.

Limited rotational and rovibrational line lists computed with highly accurate quartic force fields and ab initio dipole surfaces

In this work, computational procedures are employed to compute the rotational and rovibrational spectra and line lists for H2O, CO2, and SO2. Building on the established use of quartic force fields, MP2 and CCSD(T) Dipole Moment Surfaces (DMSs) are computed for each system of study in order to produce line intensities as well as the transition energies. The computed results exhibit a clear correlation to reference data available in the HITRAN database. Additionally, even though CCSD(T) DMSs produce more accurate intensities as compared to experiment, the use of MP2 DMSs results in reliable line lists that are still comparable to experiment. The use of the less computationally costly MP2 method is beneficial in the study of larger systems where use of CCSD(T) would be more costly.

Vibrational frequencies and spectroscopic constants for 1 3A' HNC and 1 3A' HOC+ from high-accuracy quartic force fields

The spectroscopic constants and vibrational frequencies for the 1 (3)A' states of HNC, DNC, HOC(+), and DOC(+) are computed and discussed in this work. The reliable CcCR quartic force field based on high-level coupled cluster ab initio quantum chemical computations is exclusively utilized to provide the anharmonic potential. Then, second-order vibrational perturbation theory and vibrational configuration interaction methods are employed to treat the nuclear Schrödinger equation. Second-order perturbation theory is also employed to provide spectroscopic data for all molecules examined. The relationship between these molecules and the corresponding 1 (3)A' HCN and HCO(+) isomers is further developed here. These data are applicable to laboratory studies involving formation of HNC and HOC(+) as well as astronomical observations of chemically active astrophysical environments.

Theoretical characterization of the SiC3H- anion

Highly correlated ab initio methods are used to predict the equilibrium structures and spectroscopic parameters of the SiC(3)H(-) anion. The total energies and physical properties are reported using CASSCF/MRCI, RCCSD(T), and RCCSD(T)-F12 approaches and extended basis sets. The search of stable geometries leads to a total of 12 isomers (4 linear and 8 cyclic), for which electronic ground states have close-shell configurations. The stability of the linear form, l-SiC(3)H(-), is prominent. For the most stable linear isomer, the B(e) equilibrium rotational constant has been calculated with RCCSD(T) and a complete basis set. Core-correlation and vibrational effects have been taken into account to predict a B(0) of 2621.68 MHz for l-SiC(3)H(-) and 2460.48 MHz for l-SiC(3)D(-). The dipole moment of l-SiC(3)H(-) was found to be 2.9707 D with CASSCF/aug-cc-pV5Z and the electron affinity to be 2.7 eV with RCCSD(T)-F12A/aug-cc-pVTZ. Anharmonic spectroscopic parameters are derived from a quadratic, cubic, and quartic RCCSD(T)-F12A force field and second order perturbation theory. CASSCF/MRCI vertical excitations supply three metastable electronic states, (1)Σ(+) (3)Σ(+) and (3)Δ. Electron affinities calculated for a series of chains type SiC(n)H and SiC(n) (n=1-5) allow us to discuss the anion formation probabilities.