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The infrared (IR) emission of polycyclic aromatic hydrocarbons (PAHs) permeates our universe; astronomers have detected the IR signatures of PAHs around many interstellar objects. The IR emission of interstellar PAHs differs from their emission as seen under conditions on Earth as they emit through a collisionless cascade down through their excited vibrational states from high internal energies. The difficulty in reproducing interstellar conditions in the laboratory results in a reliance on theoretical techniques. However, the size and complexity of PAHs require careful consideration when producing the theoretical spectra. In this work, we outline the theoretical methods necessary to lead to fully theoretical IR cascade spectra of PAHs including: an anharmonic second order vibrational perturbation theory treatment, the inclusion of Fermi resonances through polyads, and the calculation of anharmonic temperature band shifts and broadenings (including resonances) through a Wang–Landau approach. We also suggest a simplified scheme to calculate vibrational emission spectra that retain the essential characteristics of the full IR cascade treatment and can directly transform low temperature absorption spectra in IR cascade spectra. Additionally we show that past astronomical models were in error in assuming a 15 cm−1 correction was needed to account for anharmonic emission effects. [ABSTRACT FROM AUTHOR]

While the issue of properly describing the out-of-plane bends (OPBs) in sp2 hybridized carbon atoms has reappeared for c-SiC2H2, the present quantum chemical study provides a new characterization of this molecule in order to aid in its potential detection in astrophysically relevant studies. Combining the previous, high-level approach with MP2-F12/aug-cc-pVDZ gives exceptionally accurate results for the comparison of experimental rotational constants and seemingly reliable vibrational frequencies. Most notably, the brightest fundamental vibrational frequency in c-SiC2H2, the b1 OPB, is predicted to lie at 673.4 cm−1, within 4.0 cm−1 of the previous matrix isolation experiment. As with c-C3H2, CCSD(T)-F12/aug-cc-pVTZ appears to be quite susceptible to over estimating the OPB anharmonic correction in c-SiC2H2 and may also do such for in-plane bends, as well. MP2-F12/aug-cc-pVDZ is less susceptible to these errors, and increasing the step size reduces this positive anharmonicity issue in both the cases. The OPB underestimation, however, likely still remains. Finally, estimates for some anharmonic vibrational frequencies are provided for the methylated form, c-SiC2HCH3, which is likely also a product of gas phase reactions of ·SiH with various alkynes. [ABSTRACT FROM AUTHOR]

Carbon-carbon multiply bonded systems are improperly described with standard,wave function-based correlation methods and Gaussian one-particle basis sets implying that thermochemical, spectroscopic, and potential energy surface computations are consistently erroneous. For computations of vibrational modes, the out-of-plane bends can be reported as imaginary at worst or simply too low at best. Utilizing the simplest of aromatic structures (cyclopropenylidene) and various levels of theory, this work diagnoses this known behavior as a combined one-particle and n-particle basis set effect for the first time. In essence, standard carbon basis sets do not describe equally well sp, sp², and sp³ hybridized orbitals, and this effect is exacerbated post-Hartree-Fock by correlation methods. The latter allow for occupation of the π and π* orbitals in the expanded wave function that combine with the hydrogen s orbitals. As a result, the improperly described space is non-physically stabilized by post-Hartree-Fock correlation. This represents a fundamental problem in wavefunction theory for describing carbon. [ABSTRACT FROM AUTHOR]

Only one fundamental vibrational frequency of protonated carbon dioxide (HOCO+) has been experimentally observed in the gas phase: the ν1 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 ν1 is within less than 1 cm-1 of the experimental value. Our computations also include predictions of the gas phase fundamental vibrational frequencies of the deuterated form of the cation, DOCO+. Additionally, other spectroscopic constants for both systems are reported as part of this study, and a search for a cis-HOCO+ minimum found no such stationary point on the potential surface indicating that only the trans isomer is stable. [ABSTRACT FROM AUTHOR]

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. Correspondence between vibrational perturbation theory and variational vibrational configuration interaction for prediction of the frequencies of each mode is strong, better than 5 cm-1, except for the torsional motion, similar to what has been previously identified in the trans-HOCO radical. Among other considerations, our results are immediately applicable to dissociative photodetachment experiments which initially draw on the cis-HOCO anion since it is the most stable conformer of the anion and is used to gain insight into the portion of the OH + CO potential surface where the HOCO radical is believed to form, and we are also providing highly accurate electron binding energies relevant to these experiments. [ABSTRACT FROM AUTHOR]

Several aspects of ammonia rovibrational spectra have been investigated using the new HSL-2 potential energy surface that includes an approximate correction for nonadiabatic effects. The unprecedented accuracy of rovibrational energy levels and transition energies computed using HSL-2 was demonstrated in Part I of this study. For 14NH3, new assignments for a few ν3 + ν4 band transitions and energy levels are suggested, and discrepancies between computed and HITRAN energy levels in the 2ν4 band are analyzed (2ν4 is the most difficult band below 5000 cm-1). New assignments are suggested for existing or missing 2ν4 levels. Several new vibrational bands are identified from existing, unassigned HITRAN data, including 2ν2 + ν4, (ν3 + ν4) -A′/A″, ν1 + 2ν2, and 2ν2 + 2ν4. The strong mixing between the 2ν4 and 2ν2 + ν4 bands is carefully examined and found to be the source of the difficulties in the experimental modeling of 2ν4. Discussion is presented for preliminary J = 10 results, where the overall root-mean-square error is estimated to be less than 0.039 cm-1. The analysis of the 4ν2 band demonstrates both the reliability and the accuracy of predictions from HSL-2. The full list of computed J = 0 band origins (with assignments) and the inversion splittings up to 7000-8000 cm-1 above the zero-point energy are presented. J = 0-2 levels are reported for those bands below 5100 cm-1 that are missing from the HITRAN database. For 15NH3, excellent agreement is found for the available ν2 and ν3 + ν4(E) transition energies, but significant deficiencies are shown for HITRAN levels and several corrections are suggested. The 15N isotopic effects are presented for the J = 0-6 levels of 13 HITRAN bands. For 14ND3, we reproduce the pure rotational inversion spectra line frequencies with an accuracy similar to that for 14NH3. However, it is not possible to reproduce simultaneously all four pairs of inversion-split vibrational fundamentals to better than 0.05 cm-1 uncertainty. It is suggested that a reanalysis of some suspicious 14ND3 fundamental bands is required. The analyses presented here and in Part I show that rovibrational energy levels and transition frequencies computed with HSL-2 (with nonadiabatic corrections) remain highly accurate well beyond the experimental data used in the refinement procedure. Calculations using HSL-2 are capable of revealing many deficiencies in experimental analyses of ammonia spectra and provide reliable predictions with similar accuracy. It is expected that the results of this study will be useful in the future interpretation of high-resolution spectra from laboratory experiments or from astronomical observations. The present work represents a very significant advance in the state of our knowledge of the spectroscopy of ammonia and its isotopologues. [ABSTRACT FROM AUTHOR]

In this work, we build upon our previous work on the theoretical spectroscopy of ammonia, NH3. Compared to our 2008 study, we include more physics in our rovibrational calculations and more experimental data in the refinement procedure, and these enable us to produce a potential energy surface (PES) of unprecedented accuracy. We call this the HSL-2 PES. The additional physics we include is a second-order correction for the breakdown of the Born-Oppenheimer approximation, and we find it to be critical for improved results. By including experimental data for higher rotational levels in the refinement procedure, we were able to greatly reduce our systematic errors for the rotational dependence of our predictions. These additions together lead to a significantly improved total angular momentum (J) dependence in our computed rovibrational energies. The root-mean-square error between our predictions using the HSL-2 PES and the reliable energy levels from the HITRAN database for J = 0-6 and J = 7/8 for 14NH3 is only 0.015 cm-1 and 0.020/0.023 cm-1, respectively. The root-mean-square errors for the characteristic inversion splittings are approximately 1/3 smaller than those for energy levels. The root-mean-square error for the 6002 J = 0-8 transition energies is 0.020 cm-1. Overall, for J = 0-8, the spectroscopic data computed with HSL-2 is roughly an order of magnitude more accurate relative to our previous best ammonia PES (denoted HSL-1). These impressive numbers are eclipsed only by the root-mean-square error between our predictions for purely rotational transition energies of 15NH3 and the highly accurate Cologne database (CDMS): 0.00034 cm-1 (10 MHz), in other words, 2 orders of magnitude smaller. In addition, we identify a deficiency in the 15NH3 energy levels determined from a model of the experimental data . [ABSTRACT FROM AUTHOR]

One-particle basis set extrapolation is compared with one of the new R12 methods for computing highly accurate quartic force fields (QFFs) and spectroscopic data, including molecular structures, rotational constants, and vibrational frequencies for the H2O, N2H+, NO2+, and C2H2 molecules. In general, agreement between the spectroscopic data computed from the best R12 and basis set extrapolation methods is very good with the exception of a few parameters for N2H+ where it is concluded that basis set extrapolation is still preferred. The differences for H2O and NO2+ are small and it is concluded that the QFFs from both approaches are more or less equivalent in accuracy. For C2H2, however, a known one-particle basis set deficiency for C-C multiple bonds significantly degrades the quality of results obtained from basis set extrapolation and in this case the R12 approach is clearly preferred over one-particle basis set extrapolation. The R12 approach used in the present study was modified in order to obtain high precision electronic energies, which are needed when computing a QFF. We also investigated including core-correlation explicitly in the R12 calculations, but conclude that current approaches are lacking. Hence core-correlation is computed as a correction using conventional methods. Considering the results for all four molecules, it is concluded that R12 methods will soon replace basis set extrapolation approaches for high accuracy electronic structure applications such as computing QFFs and spectroscopic data for comparison to high-resolution laboratory or astronomical observations, provided one uses a robust R12 method as we have done here. The specific R12 method used in the present study, CCSD(T)R12, incorporated a reformulation of one intermediate matrix in order to attain machine precision in the electronic energies. Final QFFs for N2H+ and NO2+ were computed, including basis set extrapolation, core-correlation, scalar relativity, and higher-order correlation and then used to compute highly accurate spectroscopic data for all isotopologues. Agreement with high-resolution experiment for 14N2H+ and 14N2D+ was excellent, but for 14N16O2+ agreement for the two stretching fundamentals is outside the expected residual uncertainty in the theoretical values, and it is concluded that there is an error in the experimental quantities. It is hoped that the highly accurate spectroscopic data presented for the minor isotopologues of N2H+ and NO2+ will be useful in the interpretation of future laboratory or astronomical observations. [ABSTRACT FROM AUTHOR]

Harmonic and anharmonic vibrational frequencies, as well as infrared and Raman intensities, are calculated for MgSO[sub 4]·nH[sub 2]O (n = 1-3). Electronic structure theory at the second order MollerPlesset perturbation theory level with a triple-ζ+polarization basis set is used to determine the geometry, energetics, charge distributions, and vibrational spectra of pure and hydrated MgSO[sub 4] salts. The direct vibrational self-consistent field method and its correlation corrected extension are used to determine anharmonic corrections to vibrational frequencies and infrared intensities for the pure MgSO[sub 4] and its complex with one water molecule. Very significant differences are found between vibrational spectra of water molecules in complexes with MgSO[sub 4] and pure water. Some of the O-H stretching frequencies are shifted to the red very significantly (by up to 2000 cm[sup -1]) upon complexation with magnesium sulfate. They should be observed between 1700 and 3000 cm[sup -1], in a region very different from the corresponding O-H stretch frequency region of pure water (3700-3800 cm[sup -1]). In addition, the SO[sub 2] stretching vibrations are found at lower frequency regions than the water vibrations. They can serve as unique identifiers for the presence of sulfate salts. The predicted infrared and Raman spectra should be of valuable help in the design of future missions and analysis of observed data from the ice surface of Jupiter's moon Europa that possibly contains hydrated MgSO[sub 4] salts. [ABSTRACT FROM AUTHOR]

An accurate ab initio quartic force field for HNO has been determined using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), in conjunction with the correlation consistent polarized valence triple zeta (cc-pVTZ) basis set. Improved harmonic frequencies were determined with the cc-pVQZ basis set. Fundamental vibrational frequencies were determined using a second-order perturbation theory analysis and also using variational calculations. The N–O stretch and bending fundamentals are determined well from both vibrational analyses. The H–N stretch, however, is shown to have an unusually large anharmonic correction, and is not well determined using second-order perturbation theory. The H–N fundamental is well determined from the variational calculations, demonstrating the quality of the ab initio quartic force field. The zero-point energy of HNO that should be used in isodesmic reactions is also discussed. [ABSTRACT FROM AUTHOR]

The three lowest-lying isomers of Cl2O2 have been investigated using state-of-the-art ab initio quantum-mechanical methods. Electron correlation methods that have been used include second-order Mo\ller–Plesset perturbation theory, singles and doubles coupled-cluster (CCSD) theory, and the CCSD(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. Accurate relative energies have been obtained using the CCSD(T) method in conjunction with large atomic natural orbital basis sets that include up to g-type functions. Our best estimate is that the ClClO2 and ClOClO isomers lie 0.9±2.0 and 10.1±4.0 kcal/mol higher in energy (0 K), respectively, than the more stable ClOOCl peroxide form. In order to obtain accurate equilibrium geometries it is necessary to include f-type functions in the one-particle basis set. The vibrational spectra (including IR intensities) of all three isomers are computed and compared with experimental data for ClOOCl and ClClO2. The theoretical and experimental vibrational frequencies agree very well except for the symmetric combination of Cl–O stretches in ClOOCl, where it is concluded that the experimental band is most likely due to the antisymmetric Cl–O stretch. The heat of formation of ClOOCl is computed using an isodesmic reaction involving Cl2O, H2O, and HOOH, and determined to be 34.2 kcal/mol (0 K). The largest uncertainty in this value is due to potential errors in the experimental heat of formation of Cl2O. Using the experimental heat of formation of ClO, the dissociation energy of ClOOCl relative to 2 ClO is computed to be 14.9 kcal/mol at 298 K. The equilibrium structures and vibrational spectra of Cl2O, H2O, and HOOH from our highest-level calculations are found to be in excellent agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

The vibrational frequencies and IR intensities of C2H+3 and C2D+3 have been predicted using the methods of ab initio molecular quantum mechanics. Specifically, configuration interaction (CI) wave functions have been constructed from double zeta plus polarization (DZ+P) basis sets. In addition, a much larger basis set, triple zeta plus diffuse plus double polarization, has been used at the self-consistent-field (SCF) level of theory. The results are qualitatively similar to those (at much lower levels of theory) reported earlier by Raine and Schaefer. At the highest level of theory, CI including all single, double, and triple excitations (213 780 configurations for the classical structure), the nonclassical structure is the lower by only 0.7 kcal/mol. [ABSTRACT FROM AUTHOR]

Examines the equilibrium dipole moment, harmonic vibrational frequencies and infrared intensities of bromine nitrate. Importance of the reaction of ozone with halogen compounds; Implications for the ozone layer depletion; Inquiry on the gas-phase vibrational spectrum of bromine nitrate.

Sulphur dioxide is well-known in the atmospheres of planets and satellites, where its presence is often associated with volcanism, and in circumstellar envelopes of young and evolved stars as well as the interstellar medium. This work presents a line list of 1.3 billion 32S16O2 vibration-rotation transitions computed using an empirically adjusted potential energy surface and an ab initio dipole moment surface. The list gives complete coverage up to 8000 cm-1 (wavelengths longer than 1.25 µm) for temperatures below 2000 K. Infrared absorption crosssections are recorded at 300 and 500 C are used to validated the resulting ExoAmes line list. The line list is made available in electronic form as supplementary data to this article and at www.exomol.com. [ABSTRACT FROM AUTHOR]

Through established, highly accurate ab initio quartic force fields, a complete set of fundamental vibrational frequencies, rotational constants, and rovibrational coupling and centrifugal distortion constants have been determined for both the cyclic 1 1A′ and bent 2 1A′ DCCN, H13CCN, HC13CN, and HCC15N isotopologues of HCCN. Spectroscopic constants are computed for all isotopologues using second-order vibrational perturbation theory (VPT2), and the fundamental vibrational frequencies are computed with VPT2 and vibrational configuration interaction (VCI) theory. Agreement between VPT2 and VCI results is quite good, with the fundamental vibrational frequencies of the bent isomer isotopologues in accord to within a 0.1-3.2 cm–1 range. Similar accuracies are present for the cyclic isomer isotopologues. The data generated here serve as a reference for astronomical observations of these closed-shell, highly dipolar molecules using new, high-resolution telescopes and as reference for laboratory studies where isotopic labeling may lead to elucidation of the formation mechanism for the known interstellar molecule: X3A′ HCCN. [ABSTRACT FROM AUTHOR]

syn-HOSO is the most stable form of hydrogenated sulfur dioxide, a possible intermediate in the creation of sulfuric acid. Recent work has characterized the structure and rotational constants, but the present work provides new insights into the vibrational spectra of this molecule. The central S-O bond is weaker than the terminal S-O bond, which allows for near-free rotation of the hydrogen atom. The stretches and the torsional modes are relatively bright infrared emitters/absorbers. The torsional mode is very low in frequency making it a potential target for terahertz observation. The other vibrational frequencies for HOSO are reduced relative to the corresponding frequencies in SO2, H2SO4, and the HOS radical making the infrared features of syn-HOSO likely red-shifted in mixed spectral observation, where oxygen, hydrogen, and sulfur are all found. [ABSTRACT FROM AUTHOR]

A new 626-based Ames-2 PES refinement and Ames-2016 line lists for 13 CO 2 isotopologues are reported. A consistent σ RMS = ±0.02 cm −1 is established for hundreds of isotopologue band origins using the Ames-2 PES. Ames-2016 line lists are computed at 296 K, 1000 K and 4000 K using the Ames-2 PES and the same DMS-N2 dipole surface used previously, with J up to 150, E ′ up to 24,000 cm −1 or 18,000 cm −1 and appropriate intensity cutoffs. The lists are compared to the CDSD-296, CDSD-4000 databases, UCL line lists, and a few recent highly accurate CO 2 intensity measurements. Both agreements and discrepancies are discussed. Compared to the old Ames CO 2 lists, the Ames-2016 line lists have line position deviations reduced by 50% or more, which consequently leads to more reliable intensities. The line shape parameters in the Ames-2016 line lists are predicted using the newly assigned conventional vibrational polyad quantum numbers for rovibrational levels below 12,000 cm −1 so the quality of the line shape parameters is similar to that of CDSD or HITRAN. This study further proves that a semi-empirically refined PES (Ames-1 and Ames-2) coupled with a high quality ab initio DMS (DMS-N2 and UCL) may generate IR predictions with consistent accuracy and is thus helpful in the analysis of laboratory spectra and simulations of various isotopologues. The Ames-2016 lists based on DMS-N2 have reached the ∼1% intensity prediction accuracy level for the recent 626 30013-00001 and 20013-00001 bands, but further quantification and improvements require sub-percent or sub-half-percent accurate experimental intensities. The inter-isotopologue consistency of the intensity prediction accuracies should have reached better than 1–3% for regular bands not affected by resonances. Since the Effective Dipole Models (EDM) in CDSD and HITRAN have 1–20% or even larger uncertainties, we show that the Ames lists can provide better alternative IR data for many hard-to-determine isotopologue bands. Comparison at 4000 K suggests that the Ames-4000 K 12 C 16 O 2 line list is reliable and consistent within the current cutoffs of J  ≤ 150 and E ′ ≤ 24,000 cm −1 , but intensity contributions involving higher energy levels should not be omitted and future computations need to be converged up to at least 32,000 cm −1 or higher. The remaining issues are discussed regarding the source of energy level discrepancies, intensity underestimations by ∼50% for some weak bands, etc. and also future work. [ABSTRACT FROM AUTHOR]

Sulphur dioxide is well-known in the atmospheres of planets and satellites, where its presence is often associated with volcanism, and in circumstellar envelopes of young and evolved stars as well as the interstellar medium. This work presents a line list of 1.3 billion 32S16O2 vibration-rotation transitions computed using an empirically adjusted potential energy surface and an ab initio dipole moment surface. The list gives complete coverage up to 8000 cm-1 (wavelengths longer than 1.25 µm) for temperatures below 2000 K. Infrared absorption crosssections are recorded at 300 and 500 C are used to validated the resulting ExoAmes line list. The line list is made available in electronic form as supplementary data to this article and at www.exomol.com. [ABSTRACT FROM AUTHOR]

Highlyaccurate, coupled-cluster-based quartic force fields (QFFs) have beenemployed recently to provide spectroscopic reference for a myriadof molecules. Here, we are extending the same approach to providevibrational and rotational spectroscopic reference data for the sulfuranalogues of HOCO, HSCO, and HOCS, in both the cis and trans conformationsas well as the D and 34S isotopologues of each system.The resulting energies corroborate previous computations showing that trans-HSCO is the lowest-energy isomer for this system.The vibrational frequencies are computed with both second-order vibrationalperturbation theory (VPT2) and vibrational configuration interaction(VCI) methods. The VPT2 and VCI QFF frequencies largely agree withone another to better than 5.0 cm–1(often betterthan 1.0 cm–1) and are also consistent with thetype of behavior exhibited in previous studies. As such, the referencedata provided here should assist in analysis of environments in whichthese sulfur systems may be found, including the interstellar medium,combustion flames, or laboratory simulations of either. [ABSTRACT FROM AUTHOR]

The vibrational spectra for the HOCO radical in bothof its conformersand deuterated isotopologues are shown here for the first time. Buildingon previous work with coupled cluster quartic force fields (QFFs)in the computation of the fundamental vibrational frequencies forboth cis- and trans-HOCO, coupledcluster dipole surfaces are now provided for both HOCO conformersand their corresponding deuterated isotopologues. These surfaces andsubsequent vibrational configuration interaction (VCI) computationsproduce the intensities of transitions into vibrational states includingthe fundamentals, overtones, and first few combination bands of lessthan 4000 cm–1, slightly beyond the O–H stretch.Simulated spectra with an artificial full width at half-maximum broadeningof 10 cm–1are also provided in order to aid inthe characterization of HOCO’s vibrational frequencies andto assist detection in various laboratory or astronomical observations. [ABSTRACT FROM AUTHOR]

Highlights: [•] Quartic force fields (QFFs) yield highly accurate vibrational spectra using variational methods. [•] The QFF needs to be transformed into a coordinate system with correct limiting behavior. [•] For stretches, a Morse coordinate is defined and shown to perform very well. [•] For bends and other motions, cosine or sine coordinates are most often used. [•] QFFs can maintain exact symmetry for non-Abelian groups, yielding exactly degenerate vibrations. [Copyright &y& Elsevier]

Abstract: A molecule’s absorption in the atmospheric infrared (IR) window (IRW) is an indicator of its efficiency as a greenhouse gas. A model for estimating the absorption of a fluorinated molecule within the IRW was developed to assess its radiative impact. This model will be useful in comparing different hydrofluorocarbons and hydrofluoroethers contribution to global warming. The absorption of radiation by greenhouse gases, in particular hydrofluoroethers and hydrofluorocarbons, was investigated using ab initio quantum mechanical methods. Least squares regression techniques were used to create a model based on this data. The placement and number of fluorines in the molecule were found to affect the absorption in the IR window and were incorporated into the model. Several group increment models are discussed. An additive model based on one-carbon groups is found to work satisfactorily in predicting the ab initio calculated vibrational intensities. [Copyright &y& Elsevier]