Spectroscopic Constants and Vibrational Frequencies for l-C3H+ and Isotopologues from Highly-Accurate Quartic Force Fields: The Detection of l-C3H+ in the Horsehead Nebula PDR Questioned

Very recently, molecular rotational transitions observed in the photon-dominated region of the Horsehead nebula have been attributed to l-C3H+. In an effort to corroborate this finding, we employed state-of-the art and proven high-accuracy quantum chemical techniques to compute spectroscopic constants for this cation and its isotopologues. Even though the B rotational constant from the fit of the observed spectrum and our computations agree to within 20 MHz, a typical level of accuracy, the D rotational constant differs by more than 40%, while the H rotational constant differs by three orders of magnitude. With the likely errors in the rotational transition energies resulting from this difference in D on the order of 1 MHz for the lowest observed transition (J = 4 <- 3) and growing as J increases, the assignment of the observed rotational lines from the Horsehead nebula to l-C3H+ is questionable

RELIABLE IR LINE LISTS FOR SO2 AND CO2 ISOTOPOLOGUES COMPUTED FOR ATMOSPHERIC MODELING ON VENUS AND EXOPLANETS

For SO$_{2}$ atmospheric characterization in Venus and other Exoplanetary environments, recently we presented Ames-296K line lists for 626 (upgraded) and other 4 symmetric isotopologues: 636, 646, 666 and 828. For CO$_{2}$, we reported Ames-296K (1E-42 cm/molecule) and Ames-1000K (1E-36 cm/molecule) IR line lists up to E'=18000 cm$^{-1}$ for 13 CO$_{2}$ isotopologues, including symmetric species 626, 636, 646, 727, 737, 828, 838, and asymmetric species 627, 628, 637, 638, 728, 738. CO$_{2}$ line shape parameters were also determined for four different temperature ranges: Mars, Earth, Venus, and higher temperatures. General line position prediction accuracy up to 5000 cm$^{-1}$ (SO$_{2}$) or 13000 cm$^{-1}$ (CO$_{2}$) is 0.01 � 0.02 cm$^{-1}$. Most transition intensity deviations are less than 5-10%, when compare to experimentally measured quantities. With such prediction accuracy, these SO$_{2}$ and CO$_{2}$ isotopologue lists are the best available alternative for those wide spectra region missing from spectroscopic databases such as HITRAN and CDMS. For example, only very limited data exist for SO$_{2}$ 646/636 and no data at all for other minor isotopologues. They should greatly facilitate spectroscopic analyses in future laboratory or astronomical observations. Our line list work are based on "Best Theory + Reliable High-Resolution Experiment" strategy, i.e. using an ab initio potential energy surface refined with selected reliable high resolution experimental data, and high quality CCSD(T)/aug-cc-pVQ(or Q+d)Z dipole moment surfaces. Note that we have solved a convergence defect on SO$_{2}$ Ames-1 PES and further improved the quality and completeness of the Ames-296K SO$_{2}$ list by including most recent experimental data into the refinement. We will compare the Ames-296K SO$_{2}$ and CO$_{2}$ lists to latest experiments and HITRAN/CDMS models. We expect more interactions between experimental and theoretical efforts. Currently the Ames-296K lists are available at http://huang.seti.org/

Accurate ab initio Quartic Force Fields of Cyclic and Bent HC2N Isomers

Highly correlated ab initio quartic force field (QFFs) are used to calculate the equilibrium structures and predict the spectroscopic parameters of three HC2N 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 CCSD(T). Dunning s correlation-consistent basis sets cc-pVXZ, X=3,4,5, were used, and a three-point formula for extrapolation to the one-particle basis set limit was used. Core-correlation and scalar relativistic corrections were also included to yield highly accurate QFFs. The QFFs were used together with second-order perturbation theory (with proper treatment of Fermi resonances) and variational methods to solve the nuclear Schr dinger equation. The quasilinear nature of the triplet isomer is problematic, and it is concluded that a QFF is not adequate to describe properly all of the fundamental vibrational frequencies and spectroscopic constants (though some constants not dependent on the bending motion are well reproduced by perturbation theory). On the other hand, this procedure (a QFF together with either perturbation theory or variational methods) leads to highly accurate fundamental vibrational frequencies and spectroscopic constants for the cyclic and bent singlet isomers of HC2N. All three isomers possess significant dipole moments, 3.05D, 3.06D, and 1.71D, for the quasilinear triplet, the cyclic singlet, and the bent singlet isomers, respectively. It is concluded that the spectroscopic constants determined for the cyclic and bent singlet isomers are the most accurate available, and it is hoped that these will be useful in the interpretation of high-resolution astronomical observations or laboratory experiments

ACCURATE 14NH3 ROVIBRATIONAL IR ANALYSIS AT 6000 CM−1

Ammonia is an important “weed” molecule in interstellar medium, planetary and exoplanetary atmosphere studies. In last ten years, new experimental IR analysis have been reported in the extended region between 5000 \wn (or 2 $\mu$m) and 10,000 \wn (or 1 $\mu$m). But reliable line list is still missing for the 6000 \wn (or 1.63 $\mu$m) region. Combining the JPL experimental measurements with the line positions predicted on our Ames-Pre3 potential energy surface and the 296K intensity predicted by the UCL-C2018 line list, we have been able to successfully assign more than 1300 transitions in that range. The transitions belong to following bands: \nub{2}+\nub{3}+\nub{4} (0111), \nub{1}+\nub{2}+\nub{4} (1101), 3\nub{2}+\nub{3} (0310), \nub{1}+3\nub{2} (1300), 6\nub{2} (0600s),and a "hot" band 2\nub{2}+\nub{3}+\nub{4} (0211) -- \nub{2} (0100). The combination difference for the determined experimental energy levels are about 1E-3 \wn, close to the resolution of lab measurements. Our Ames-Pre3 predictions for most J=0-10 transitions are found to be accurate within $\pm$ ~0.05 \wn, better than the C2018 line positions. Newly determined band origins and rovibrational levels will be presented along with band-by-band simulations comparing to the observed spectra. More complete Effective Hamiltonian model analysis is the target due for future work

IMPROVE THE PREDICTION ACCURACY OF ISOTOPOLOGUE MICROWAVE SPECTRA BY COMBINING AMES-296K SO2 IR LISTS WITH EXPERIMENTAL MODELS: A BENCHMARK STUDY

Theoretical rovibrational IR line lists computed on the empirically refined potential energy surfaces (PES) have excellent isotopologue consistency and reliablity to push the ongoing pursuit of “Best Theory + reliable High-resolution Experiment” (BTRHE) strategy to a higher level of prediction accuracy. The SO$_2$ benchmark uses experimental (Expt) data based Effective Hamiltonian (EH) models of a few SO$_2$ isotopologues and Ames-296K IR line lists of 30 SO$_2$ isotopologues. For microwave (MW) intensity, the Einstein A$_{21}$ coefficients demostrate isotopologue consistency better than 99.9\%, which can help identify errors and inconsistencies in existing effective dipole moment (EDM) models or lab spectra analysis. For MW line position, the study goes from simple trial to systematic investigations on the convergence, uncertainties, higher order term effects, fixing EH parameters, mass coordinates, and other prediction scheme, etc. We confirm the feasibility of a two-orders-of-magnitude accuracy improvement over the original Ames IR line lists. By refining the rotational constants and quartic centrifugal distortion constatnts using the linear or quadratic extrapolations on their differences between the EH(Expt) and EH(Ames) IR list based parameter values, A$_0$ / B$_0$ / C$_0$ deviations can be as small as 0.01-0.02 MHz, and line position deviations can be reduced to 0-5 MHz for J$<$30, K$_a$$<$10-15 transitions. We report a microwave line set consisting of 644,636 transitions with reliable 296K IR intensity and Einstein A$_{21}$ coefficient for all 30 isotopologues of SO$_2$. The line position predictions are the best available, which will facilitate both the astronomical identification and lab MW analysis of those unobserved minor isotopologues. The procedure can be easily extended onto rovibrational bands and other molecular systems, while data precision higher than 0.003-0.03 MHz, or 1E-6 - 1E-7 \wn, is preferred

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(subJ) quartic distortion constants agree with experiment to within 3%. The known gas phase OH stretch of NNOH(+) is 3330.91 cm(exp1), and the vibrational configuration interaction computed result is 3330.9 cm(exp1). Other spectroscopic constants are also provided, as are the rest of the fundamental vibrational frequencies for NNOH(+) and its deuterated isotopologue. This high-accuracy data should serve to better inform future observational or experimental studies of the rovibrational bands of protonated nitrous oxide in the ISM and the laboratory

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 H + molecular cation, referred to as c-C H + and I-C H +. Specifically the 33 3333 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 using both vibrational 2nd-order perturbation theory and variational methods to solve the nuclear Schroedinger equation. Agreement between our best computed fundamental vibrational frequencies and recent infrared photodissociation experiments is reasonable for most bands, but there are a few exceptions. Possible sources for the discrepancies are discussed. We determine the energy difference between the cyclic and linear forms of C H +, 33 obtaining 27.9 kcal/mol at 0 K, which should be the most reliable available. It is expected that the fundamental vibrational frequencies and spectroscopic constants presented here for c-C H + 33 and I-C H + are the most reliable available for the free gas-phase species and it is hoped that 33 these will be useful in the assignment of future high-resolution laboratory experiments or astronomical observations

Computing Highly Accurate Spectroscopic Line Lists for Characterization of Planetary Atmospheres: CO2 and SO2 Line Lists Needed for Modeling Venus

Over the last decade, it has become apparent that the most effective approach for determining highly accurate rotational and rovibrational line lists for molecules of interest in planetary atmospheres and other astrophysical environments is through a combination of highresolution laboratory experiments coupled with state-of-the art ab initio quantum chemistry methods. The approach involves computing the most accurate potential energy surface (PES) possible using state-of-the art electronic structure methods, followed by computing rotational and rovibrational energy levels using an exact variational method to solve the nuclear Schrdinger equation. Then, reliable experimental data from high-resolution experiments is used to refine the ab initio PES in order to improve the accuracy of the computed energy levels and transition energies. From the refinement step, we have been able to achieve an accuracy of approximately 0.015 cm-1 for rovibrational transition energies, and even better for purely rotational transitions. This combined "experiment / theory" approach allows for determination of essentially a complete line list, with hundreds of millions of transitions, and having the transition energies and intensities be highly accurate. Our group has successfully applied this approach to determine highly accurate line lists for NH3, CO2 and isotopologues, and SO2 and isotopologues. Here I will report our latest results for CO2 and SO2 including all isotopologues. Comparisons to the available data in HITRAN2012 and other available databases will be shown, though we note that our line lists for SO2 are significantly more complete than any other databases. Since it is important to span a large temperature range in order to model the spectral signature of Venus as well as exoplanets, we will demonstrate how the spectra change on going from low temperatures (100 K) to higher temperatures (500 K to 1500 K)

Anharmonic Rovibrational Calculations of Singlet Cyclic C4 Using a New Ab Initio Potential and a Quartic Force

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 configurationinteraction (VCI) approach. Agreement is within 10 cm(exp 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. The agreement between VPT2- QFF and MM-QFF results is also good for the C4 combinations and overtones. The J = 1 and J = 2 rovibrational energies are reported from both VCI (MM) on the PES and VPT2 on the QFF calculations. The spectroscopic constants of (12)C4 and two C(sub 2v)-symmetry, single (13)C-substituted isotopologues are presented, which may help identification of cyclic C4 in future experimental analyses or astronomical observations