Complete, Theoretical Rovibronic Spectral Characterization of the Carbon Monoxide, Water, and Formaldehyde Cations.

New high-level ab initio quartic force field (QFF) methods are explored which provide spectroscopic data for the electronically excited states of the carbon monoxide, water, and formaldehyde cations, sentinel species for expanded, recent cometary spectral analysis. QFFs based on equation-of-motion ionization potential (EOM-IP) with a complete basis set extrapolation and core correlation corrections provide assignment for the fundamental vibrational frequencies of the A˜2B1 and B˜2A1 states of the formaldehyde cation; only three of these frequencies have experimental assignment available. Rotational constants corresponding to these vibrational excitations are also provided for the first time for all electronically excited states of both of these molecules.

Ames-2021 CO Dipole Moment Surface and IR Line Lists: Toward 0.1% Uncertainty for CO IR Intensities.

A highly accurate CO dipole moment surface (DMS), Ames-2021, is reported along with CO infrared (IR) intensity comparisons approaching a 1-4‰ level of agreement and uncertainty. The Ames-2021 DMS was accurately fitted from CCSD(T) finite-field dipoles computed with the aug-cc-pVXZ (X = T, Q, 5) basis for C atom and the d-aug-cc-pVXZ (X = T, Q, 5) basis for O atoms, and extrapolated to the one particle basis set limit. Fitting σ is 3.

What It Takes to Compute Highly Accurate Rovibrational Line Lists for Use in Astrochemistry.

ConspectusWe review the Best Theory + Reliable High-Resolution Experiment (BTRHE) strategy for obtaining highly accurate molecular rovibrational line lists with InfraRed (IR) intensities. The need for highly accurate molecular rovibrational line lists is twofold: (a) assignment of the many rovibrational lines for common stable molecules especially those that exhibit a large amplitude motion, such as NH, or have a high density of states such as SO; and (b) characterization of the atmospheres of exoplanets, which will be one of the main areas of research in astronomy in the coming decades. The first motivation arises due to the need to eliminate lines due to common molecules in an astronomical observation in order to identify lines from new molecules, while the second motivation arises due to the need to obtain accurate molecular opacities in order to characterize the atmosphere of an exoplanet.

Highly Accurate Quartic Force Field and Rovibrational Spectroscopic Constants for the Azirinyl Cation (c-CNH) and Its Isomers.

The azirinyl cation is an aromatic cyclic molecule that is isoelectronic with cyclopropenylidene, c-CH, and c-CH. Cyclopropenylidene has been shown to be ubiquitous, existing in many different astrophysical environments. Given the similar chemistry between C and N, and the relative abundances between C and N in astrophysical environments, it is expected that there should be aromatic ringed molecules that incorporate N in the ring, but as yet, no such molecule has been identified.

The anharmonic quartic force field infrared spectra of hydrogenated and methylated PAHs.

Polycyclic aromatic hydrocarbons (PAHs) have been shown to be ubiquitous in a large variety of distinct astrophysical environments and are therefore of great interest to astronomers. The majority of these findings are based on theoretically predicted spectra, which make use of scaled DFT harmonic frequencies for band positions and the double harmonic approximation for intensities. However, these approximations have been shown to fail at predicting high-resolution gas-phase infrared spectra accurately, especially in the CH-stretching region (2950-3150 cm, 3 μm).

Towards completing the cyclopropenylidene cycle: rovibrational analysis of cyclic N, CNN, HCNN, and CNC.

The simple aromatic hydrocarbon, cyclopropenylidene (c-CH), is a known, naturally-occurring molecule. The question remains as to whether its isoelectronic, cyclic, fellow aromatics of c-N, c-CNN, HCNN, and c-CNC are as well. Each of these are exciting objects for observation of Titan, and the rotational constants and vibrational frequencies produced here will allow for remote sensing of Titan's atmosphere or other astrophysical or terrestrial sources.

The anharmonic quartic force field infrared spectra of five non-linear polycyclic aromatic hydrocarbons: Benz[a]anthracene, chrysene, phenanthrene, pyrene, and triphenylene.

The study of interstellar polycyclic aromatic hydrocarbons (PAHs) relies heavily on theoretically predicted infrared spectra. Most earlier studies use scaled harmonic frequencies for band positions and the double harmonic approximation for intensities. However, recent high-resolution gas-phase experimental spectroscopic studies have shown that the harmonic approximation is not sufficient to reproduce experimental results.

The anharmonic quartic force field infrared spectra of three polycyclic aromatic hydrocarbons: Naphthalene, anthracene, and tetracene.

Current efforts to characterize and study interstellar polycyclic aromatic hydrocarbons (PAHs) rely heavily on theoretically predicted infrared (IR) spectra. Generally, such studies use the scaled harmonic frequencies for band positions and double harmonic approximation for intensities of species, and then compare these calculated spectra with experimental spectra obtained under matrix isolation conditions. High-resolution gas-phase experimental spectroscopic studies have recently revealed that the double harmonic approximation is not sufficient for reliable spectra prediction.

Linear transformation of anharmonic molecular force constants between normal and Cartesian coordinates.

A full derivation of the analytic transformation of the quadratic, cubic, and quartic force constants from normal coordinates to Cartesian coordinates is given. Previous attempts at this transformation have resulted in non-linear transformations; however, for the first time, a simple linear transformation is presented here. Two different approaches have been formulated and implemented, one of which does not require prior knowledge of the translation-rotation eigenvectors from diagonalization of the Hessian matrix.

Quartic force field rovibrational analysis of protonated acetylene, C2H3(+), and its isotopologues.

Protonated acetylene, C2H3(+), is among the simplest carbocations. Comprehensive experimental or highly accurate computational spectroscopic data is lacking for this system due to its inherent complexities. Utilizing state-of-the-art quartic force fields (QFFs), the spectroscopic constants and fundamental vibrational frequencies are provided in this work for the nonclassical, bridged, cyclic global minimum.

Highly accurate potential energy surface, dipole moment surface, rovibrational energy levels, and infrared line list for ³²S¹⁶O₂ up to 8000 cm⁻¹.

A purely ab initio potential energy surface (PES) was refined with selected (32)S(16)O2 HITRAN data. Compared to HITRAN, the root-mean-squares error (σ(RMS)) for all J = 0-80 rovibrational energy levels computed on the refined PES (denoted Ames-1) is 0.013 cm(-1).

Fundamental vibrational frequencies and spectroscopic constants of cis- and trans-HOCS, HSCO, and isotopologues via quartic force fields.

Highly accurate, coupled-cluster-based quartic force fields (QFFs) have been employed recently to provide spectroscopic reference for a myriad of molecules. Here, we are extending the same approach to provide vibrational and rotational spectroscopic reference data for the sulfur analogues of HOCO, HSCO, and HOCS, in both the cis and trans conformations as well as the D and (34)S isotopologues of each system. The resulting energies corroborate previous computations showing that trans-HSCO is the lowest-energy isomer for this system.

Anharmonic rovibrational calculations of singlet cyclic C4 using a new ab initio potential and a quartic force field.

We report a CCSD(T)/cc-pCV5Z quartic force field (QFF) and a semi-global CCSD(T)-F12b/aug-cc-pVTZ potential energy surface (PES) for singlet, cyclic C4. Vibrational fundamentals, combinations, and overtones are obtained using vibrational second-order perturbation theory (VPT2) and the vibrational configuration-interaction (VCI) approach. Agreement is within 10 cm(-1) between the VCI calculated fundamentals on the QFF and PES using the MULTIMODE (MM) program, and VPT2 and VCI results agree for the fundamentals.

Protonated nitrous oxide, NNOH+: fundamental vibrational frequencies and spectroscopic constants from quartic force fields.

The interstellar presence of protonated nitrous oxide has been suspected for some time. Using established high-accuracy quantum chemical techniques, spectroscopic constants and fundamental vibrational frequencies are provided for the lower energy O-protonated isomer of this cation and its deuterated isotopologue. The vibrationally-averaged B0 and C0 rotational constants are within 6 MHz of their experimental values and the D(J) quartic distortion constants agree with experiment to within 3%.

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.

Dipole surface and infrared intensities for the cis- and trans-HOCO and DOCO radicals.

The vibrational spectra for the HOCO radical in both of its conformers and deuterated isotopologues are shown here for the first time. Building on previous work with coupled cluster quartic force fields (QFFs) in the computation of the fundamental vibrational frequencies for both cis- and trans-HOCO, coupled cluster dipole surfaces are now provided for both HOCO conformers and their corresponding deuterated isotopologues. These surfaces and subsequent vibrational configuration interaction (VCI) computations produce the intensities of transitions into vibrational states including the fundamentals, overtones, and first few combination bands of less than 4000 cm(-1), slightly beyond the O-H stretch.

The 1 3A' HCN and 1 3A' HCO+ vibrational frequencies and spectroscopic constants from quartic force fields.

Building on previous studies involving coupled cluster quartic force fields for the description of spectroscopic constants and vibrational frequencies of astronomically relevant molecules, this work applies the same techniques to the elucidation of such properties for the bent 1 (3)A' state of HCN and the isoelectronic 1 (3)A' HCO(+). Core correlation is treated both by explicit means and as a correction. Each approach gives closely comparable spectroscopic constants and vibrational frequencies once more, indicating that the composite method is a viable and less costly alternative.

Fundamental vibrational frequencies and spectroscopic constants of HOCS+, HSCO+, and isotopologues via quartic force fields.

Besides the ν(1) O-H stretching mode at 3435 cm(-1) for HOCS(+), the fundamental vibrational frequencies for this cation and its HSCO(+) isomer have not been determined experimentally. Because these systems are analogues to HOCO(+), a detected interstellar molecule, and are believed to play an important role in reactions of OCS, which has also been detected in the interstellar medium, these cations are of importance to interstellar chemistry and reaction surface studies. This work provides the fundamental vibrational frequencies and spectroscopic constants computed with vibrational perturbation theory (VPT) at second order and the vibrational configuration interaction (VCI) method conjoined with the most accurate quartic force field (QFF) applied to date for these systems.

Quartic force field predictions of the fundamental vibrational frequencies and spectroscopic constants of the cations HOCO+ and DOCO+.

Only one fundamental vibrational frequency of protonated carbon dioxide (HOCO(+)) has been experimentally observed in the gas phase: the ν(1) O-H stretch. Utilizing quartic force fields defined from CCSD(T)/aug-cc-pVXZ (X = T,Q,5) complete basis set limit extrapolated energies modified to include corrections for core correlation and scalar relativistic effects coupled to vibrational perturbation theory and vibrational configuration interaction computations, we are predicting the full set of gas phase fundamental vibrational frequencies of HOCO(+). Our prediction of ν(1) is within less than 1 cm(-1) of the experimental value.

An isotopic-independent highly accurate potential energy surface for CO2 isotopologues and an initial (12)C(16)O2 infrared line list.

An isotopic-independent, highly accurate potential energy surface (PES) has been determined for CO(2) by refining a purely ab initio PES with selected, purely experimentally determined rovibrational energy levels. The purely ab initio PES is denoted Ames-0, while the refined PES is denoted Ames-1. Detailed tests are performed to demonstrate the spectroscopic accuracy of the Ames-1 PES.

Accurate ab initio quartic force fields of cyclic and bent HC2N isomers.

Highly correlated ab initio quartic force fields (QFFs) are used to calculate the equilibrium structures and predict the spectroscopic parameters of three HC(2)N isomers. Specifically, the ground state quasilinear triplet and the lowest cyclic and bent singlet isomers are included in the present study. Extensive treatment of correlation effects were included using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted as CCSD(T).

Vibrational frequencies and spectroscopic constants from quartic force fields for cis-HOCO: the radical and the anion.

The use of accurate quartic force fields together with vibrational configuration interaction recently predicted gas phase fundamental vibrational frequencies of the trans-HOCO radical to within 4 cm(-1) of experimental results for the two highest frequency modes. Utilizing the same approach, we are providing a full list of fundamental vibrational frequencies and spectroscopic constants for the cis-HOCO system in both radical and anionic forms. Our predicted geometrical parameters of the cis-HOCO radical match experiment and previous computation to better than 1% deviation, and previous theoretical work agrees equally well for the anion.

The trans-HOCO radical: quartic force fields, vibrational frequencies, and spectroscopic constants.

In the search for a full mechanism creating CO(2) from OH + CO, it has been suggested that creation of the hydroxyformyl or HOCO radical may be a necessary step. This reaction and its transient intermediate may also be responsible for the regeneration of CO(2) in such high quantities in the atmosphere of Mars. Past spectroscopic observations of this radical have been limited and a full gas phase set of the fundamental vibrational frequencies of the HOCO radical has not been reported.

Highly accurate quartic force fields, vibrational frequencies, and spectroscopic constants for cyclic and linear C3H3(+).

High levels of theory have been used to compute quartic force fields (QFFs) for the cyclic and linear forms of the C(3)H(3)(+) molecular cation, referred to as c-C(3)H(3)(+) and l-C(3)H(3)(+). Specifically, the singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations, CCSD(T), has been used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation have been included. The QFFs have been used to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants by use of both vibrational second-order perturbation theory and variational methods to solve the nuclear Schrödinger equation.

Flexible, ab initio potential, and dipole moment surfaces for water. I. Tests and applications for clusters up to the 22-mer.

We report full-dimensional, ab initio potential energy and dipole moment surfaces, denoted PES and DMS, respectively, for arbitrary numbers of water monomers. The PES is a sum of 1-, 2-, and 3-body potentials which can also be augmented by semiempirical long-range higher-body interactions. The 1-body potential is a spectroscopically accurate monomer potential, and the 2- and 3-body potentials are permutationally invariant fits to tens of thousands of CCSD(T)/aug-cc-pVTZ and MP2/aug-cc-pVTZ electronic energies, respectively.