Search

To facilitate atmospheric and spectroscopic studies of carbon disulfide, or CS _2 , in both planetary and exoplanetary atmospheres, we adopt the “Best Theory + Reliable High-resolution Experiment” algorithm to generate semiempirical IR line lists for the 20 most abundant CS _2 isotopologues, denoted as Ames-296K. The IR lists are computed using the Ames-1 potential energy surface, refined using the experimental transition set and an ab initio dipole moment surface fitted from CCSD(T)/aug-cc-pV(T/Q/5+d)Z dipoles extrapolated to a one-particle basis set limit. The IR lists cover the range of 0–10,000 cm ^−1 , with an S _296K cutoff at 10 ^−31 cm ^−1 /molecule·cm ^−2 (abundance included). A “natural” IR line list at 296 K includes about 10 million lines of the 20 isotopologues, with their intensities scaled by the corresponding abundances. The zero-point energy, partition functions, and abundances are reported for each isotopologue. The energy levels in the global effective Hamiltonian model for ^12 C ^32 S _2 are adopted to improve the line position accuracy. This new IR list for the main isotopologue is denoted as A+I.296K. Reliable HITRAN2020 line positions are also utilized to improve the accuracy of the ^32 S ^12 C ^34 S, ^32 S ^12 C ^33 S, and ^32 S ^13 C ^32 S isotopologue line lists. The final composite line list is validated against Pacific Northwest National Laboratory experimental cross sections, showing excellent agreement. The agreement supports the quality of the composite line list and the power of synergy between experiment and theory. The new data are proposed for use in updating and expanding the CS _2 data in HITRAN and other high-resolution IR databases. Supplementary files are available in Zenodo and AHED.

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. EOM-IP-CCSDT/CcC computations support tentative re-assignment of the ν1 and ν3 frequencies of the B˜2B2 state of the water cation to approximately 2409.3 cm−1 and 1785.7 cm−1, respectively, due to significant disagreement between experimental assignment and all levels of theory computed herein, as well as work by previous authors. The EOM-IP-CCSDT/CcC QFF achieves agreement to within 12 cm−1 for the fundamental vibrational frequencies of the electronic ground state of the water cation compared to experimental values and to the high-level theoretical benchmarks for variationally-accessible states. Less costly EOM-IP based approaches are also explored using approximate triples coupled cluster methods, as well as electronically excited state QFFs based on EOM-CC3 and the previous (T)+EOM approach. The novel data, including vibrationally corrected rotational constants for all states studied herein, provided by these computations should be useful in clarifying comet evolution or other remote sensing applications in addition to fundamental spectroscopy.

We present the first high spectral resolution mid-infrared survey in the Orion BN/KL region, covering 7.2–28.3 μ m. With SOFIA/EXES, we target the enigmatic source Orion IRc2. While this is in the most prolifically studied massive star-forming region, longer wavelengths and molecular emission lines dominated previous spectral surveys. The mid-infrared observations in this work access different components and molecular species in unprecedented detail. We unambiguously identify two new kinematic components, both chemically rich with multiple molecular absorption lines. The “blue clump” has v _LSR = −7.1 ± 0.7 km s ^−1 , and the “red clump” has 1.4 ± 0.5 km s ^−1 . While the blue and red clumps have similar temperatures and line widths, molecular species in the blue clump have higher column densities. They are both likely linked to pure rotational H _2 emission also covered by this survey. This work provides evidence for the scenario that the blue and red clumps are distinct components unrelated to the classic components in the Orion BN/KL region. Comparison to spectroscopic surveys toward other infrared targets in the region show that the blue clump is clearly extended. We analyze, compare, and present in-depth findings on the physical conditions of C _2 H _2 , ^13 CCH _2 , CH _4 , CS, H _2 O, HCN, ${{\rm{H}}}^{13}\mathrm{CN}$ , HNC, NH _3 , and SO _2 absorption lines and an H _2 emission line associated with the blue and red clumps. We also provide limited analysis of H _2 O and SiO molecular emission lines toward Orion IRc2 and the atomic forbidden transitions [Fe ii ], [S i ], [S iii ], and [Ne ii ].

Ab initio potential data at the CCSD(T) level of theory with an aug-cc-pVTZ basis was used for an accurate determination of potential energy surface. The resulting potential function is valid for all twelve vibrational degrees of freedom and a torsional reaction path was derived for this potential and then low lying vibrational levels of methanol were calculated by MULTIMODE.

A full-dimensional ab initio potential energy surface and dipole moment surface for water dimer are presented. The surfaces can dissociate into two water molecules correctly, with equilibrium dissociation energy D(sub e) = 1665.74 cm(super -1).

Argon predissociation spectroscopy is used to observe the first vibrational spectra of the OH(super -).H2O and Cl(super -).H2O complexes in the 1000--1900 cm(super -1) energy region. The extension to lower energy allows the exploration of the spectral signatures of the intramolecular bending fundamentals (v2) of the bound water molecule, as well as, in the strongly bond systems such as OH(super -).H2O, the motion of the shared proton.

An effort is made to obtain vibrational energies of H5O2(super +1) in full dimension, in this study. For this purpose, the diffusion quantum Monte Carlo method and the vibrational configuration interaction methods are used.

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. In our previous work, we presented the anharmonic theoretical spectra of three linear PAHs, showing the importance of including anharmonicities into the theoretical calculations. In this paper, we continue this work by extending the study to include five non-linear PAHs (benz[a]anthracene, chrysene, phenanthrene, pyrene, and triphenylene), thereby allowing us to make a full assessment of how edge structure, symmetry, and size influence the effects of anharmonicities. The theoretical anharmonic spectra are compared to spectra obtained under matrix isolation low-temperature conditions, low-resolution, high-temperature gas-phase conditions, and high-resolution, low-temperature gas-phase conditions. Overall, excellent agreement is observed between the theoretical and experimental spectra although the experimental spectra show subtle but significant differences. [ABSTRACT FROM AUTHOR]

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. In this paper, we present the anharmonic theoretical spectra of three PAHs: naphthalene, anthracene, and tetracene, computed with a locally modified version of the SPECTRO program using Cartesian derivatives transformed from Gaussian 09 normal coordinate force constants. Proper treatments of Fermi resonances lead to an impressive improvement on the agreement between the observed and theoretical spectra, especially in the C-H stretching region. All major IR absorption features in the full-scale matrix-isolated spectra, the high-temperature gas-phase spectra, and the most recent high-resolution gas-phase spectra obtained under supersonically cooled molecular beam conditions in the CHstretching region are assigned. [ABSTRACT FROM AUTHOR]

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. The validity of this method is tested using two molecules H2O and c-C3H2D+. [ABSTRACT FROM AUTHOR]

A purely ab initio potential energy surface (PES) was refined with selected 32S16O2 HITRAN data. Compared to HITRAN, the root-mean-squares error (σRMS) for all J = 0-80 rovibrational energy levels computed on the refined PES (denoted Ames-1) is 0.013 cm-1. Combined with a CCSD(T)/aug-cc-pV(Q+d)Z dipole moment surface (DMS), an infrared (IR) line list (denoted Ames-296K) has been computed at 296 K and covers up to 8000 cm-1. Compared to the HITRAN and CDMS databases, the intensity agreement for most vibrational bands is better than 85%- 90%. Our predictions for 34S16O2 band origins, higher energy 32S16O2 band origins and missing 32S16O2 IR bands have been verified by most recent experiments and available HITRAN data. We conclude that the Ames-1 PES is able to predict 32/34S16O2 band origins below 5500 cm-1 with 0.01-0.03 cm-1 uncertainties, and the Ames-296K line list provides continuous, reliable and accurate IR simulations. The Ka-dependence of both line position and line intensity errors is discussed. The line list will greatly facilitate SO2 IR spectral experimental analysis, as well as elimination of SO2 lines in high-resolution astronomical observations. [ABSTRACT FROM AUTHOR]

A purely ab initio potential energy surface (PES) was refined with selected 32S16O2 HITRAN data. Compared to HITRAN, the root-mean-squares error (σRMS) for all J = 0-80 rovibrational energy levels computed on the refined PES (denoted Ames-1) is 0.013 cm-1. Combined with a CCSD(T)/aug-cc-pV(Q+d)Z dipole moment surface (DMS), an infrared (IR) line list (denoted Ames-296K) has been computed at 296 K and covers up to 8000 cm-1. Compared to the HITRAN and CDMS databases, the intensity agreement for most vibrational bands is better than 85%- 90%. Our predictions for 34S16O2 band origins, higher energy 32S16O2 band origins and missing 32S16O2 IR bands have been verified by most recent experiments and available HITRAN data. We conclude that the Ames-1 PES is able to predict 32/34S16O2 band origins below 5500 cm-1 with 0.01-0.03 cm-1 uncertainties, and the Ames-296K line list provides continuous, reliable and accurate IR simulations. The Ka-dependence of both line position and line intensity errors is discussed. The line list will greatly facilitate SO2 IR spectral experimental analysis, as well as elimination of SO2 lines in high-resolution astronomical observations. [ABSTRACT FROM AUTHOR]

We report the properties of two novel transition states of the bimolecular hydrogen exchange reaction in the water dimer, based on an ab initio water dimer potential [A. Shank et al., J. Chem. Phys. 130, 144314 (2009)]. The realism of the two transition states is assessed by comparing structures, energies, and harmonic frequencies obtained from the potential energy surface and new high-level ab initio calculations. The rate constant for the exchange is obtained using conventional transition state theory with a tunneling correction. We employ a one-dimensional approach for the tunneling calculations using a relaxed potential from the full-dimensional potential in the imaginary-frequency normal mode of the saddle point, Qim. The accuracy of this one-dimensional approach has been shown for the ground-state tunneling splittings for H and D-transfer in malonaldehyde and for the D+H2 reaction [Y. Wang and J. M. Bowman, J. Chem. Phys. 129, 121103 (2008)]. This approach is applied to calculate the rate constant for the H2O+H2O exchange and also for H2O+D2O→2HOD. The local zero-point energy is also obtained using diffusion Monte Carlo calculations in the space of real-frequency-saddle-point normal modes, as a function of Qim. [ABSTRACT FROM AUTHOR]

A series of high-quality, purely ab initio, quartic force fields (QFFs), computed using a procedure we recently proposed, is reported for NH2- and CCH-. The singles and doubles coupled-cluster method with a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), was used with TZ, QZ, and 5Z quality basis sets and was combined with extrapolation to the one-particle basis-set limit, core-correlation effects, scalar relativistic effects, and higher-order correlation effects to yield accurate QFFs. A “best-guess” reference geometry was determined at the CCSD(T)/5Z level of theory. Analytical transformation removes nonzero gradients to facilitate a second-order perturbation theory spectroscopic analysis. The QFF is transformed into Morse/cosine coordinates in order to perform exact vibrational configuration interaction computations. Equilibrium structures, vibrational frequencies, rotational constants, and selected spectroscopic constants 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-1 for fundamentals, but are of opposite sign. For CCH-, a thorough discussion is presented on effective rotational constants B0. It is concluded that the “best” QFF should incorporate all the small corrections mentioned above. Correspondingly, the best vibrational fundamentals of CCH- are estimated at 502.0 cm-1 (ν2), 1800.9 cm-1 (ν3), and 3204.3 cm-1 (ν1), while the best vibrational fundamentals of NH2- are at 3118.5 cm-1 (ν1), 1447.8 cm-1 (ν2), and 3186.5 cm-1 (ν3). Excellent agreement with high-resolution experiments has been obtained for fundamentals—e.g., 1–3 cm-1 deviation for the symmetric and antisymmetric stretches of NH2-, 3121.93 cm-1 (ν1) and 3190.29 cm-1 (ν3), respectively. Isotopic effects are studied and presented to aid future experimental analyses. The present study should facilitate future characterizations of NH2- and CCH- from astronomical observations or other high-resolution laboratory studies. [ABSTRACT FROM AUTHOR]

A global potential energy surface (PES) that includes short and long range terms has been determined for the NH3 molecule. The singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations and the internally contracted averaged coupled-pair functional electronic structure methods have been used in conjunction with very large correlation-consistent basis sets, including diffuse functions. Extrapolation to the one-particle basis set limit was performed and core correlation and scalar relativistic contributions were included directly, while the diagonal Born–Oppenheimer correction was added. Our best purely ab initio PES, denoted “mixed,” is constructed from two PESs which differ in whether the ic-ACPF higher-order correlation correction was added or not. Rovibrational transition energies computed from the mixed PES agree well with experiment and the best previous theoretical studies, but most importantly the quality does not deteriorate even up to 10 300 cm-1 above the zero-point energy (ZPE). The mixed PES was improved further by empirical refinement using the most reliable J=0–2 rovibrational transitions in the HITRAN 2004 database. Agreement between high-resolution experiment and rovibrational transition energies computed from our refined PES for J=0–6 is excellent. Indeed, the root mean square (rms) error for 13 HITRAN 2004 bands for J=0–2 is 0.023 cm-1 and that for each band is always ≤=0.06 cm-1. For J=3–5 the rms error is always ≤=0.15 cm-1. This agreement means that transition energies computed with our refined PES should be useful in the assignment of new high-resolution NH3 spectra and in correcting mistakes in previous assignments. Ideas for further improvements to our refined PES and for extension to other isotopolog are discussed. [ABSTRACT FROM AUTHOR]

A global potential energy surface (PES) that includes short and long range terms has been determined for the NH3 molecule. The singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations and the internally contracted averaged coupled-pair functional electronic structure methods have been used in conjunction with very large correlation-consistent basis sets, including diffuse functions. Extrapolation to the one-particle basis set limit was performed and core correlation and scalar relativistic contributions were included directly, while the diagonal Born–Oppenheimer correction was added. Our best purely ab initio PES, denoted “mixed,” is constructed from two PESs which differ in whether the ic-ACPF higher-order correlation correction was added or not. Rovibrational transition energies computed from the mixed PES agree well with experiment and the best previous theoretical studies, but most importantly the quality does not deteriorate even up to 10 300 cm-1 above the zero-point energy (ZPE). The mixed PES was improved further by empirical refinement using the most reliable J=0–2 rovibrational transitions in the HITRAN 2004 database. Agreement between high-resolution experiment and rovibrational transition energies computed from our refined PES for J=0–6 is excellent. Indeed, the root mean square (rms) error for 13 HITRAN 2004 bands for J=0–2 is 0.023 cm-1 and that for each band is always ≤=0.06 cm-1. For J=3–5 the rms error is always ≤=0.15 cm-1. This agreement means that transition energies computed with our refined PES should be useful in the assignment of new high-resolution NH3 spectra and in correcting mistakes in previous assignments. Ideas for further improvements to our refined PES and for extension to other isotopolog are discussed. [ABSTRACT FROM AUTHOR]

A procedure for the calculation of molecular quartic force fields (QFFs) is proposed and investigated. The goal is to generate highly accurate ab initio QFFs that include many of the so-called “small” effects that are necessary to achieve high accuracy. The small effects investigated in the present study include correlation of the core electrons (core correlation), extrapolation to the one-particle basis set limit, correction for scalar relativistic contributions, correction for higher-order correlation effects, and inclusion of diffuse functions in the one-particle basis set. The procedure is flexible enough to allow for some effects to be computed directly, while others may be added as corrections. A single grid of points is used and is centered about an initial reference geometry that is designed to be as close as possible to the final ab initio equilibrium structure (with all effects included). It is shown that the least-squares fit of the QFF is not compromised by the added corrections, and the balance between elimination of contamination from higher-order force constants while retaining energy differences large enough to yield meaningful quartic force constants is essentially unchanged from the standard procedures we have used for many years. The initial QFF determined from the least-squares fit is transformed to the exact minimum in order to eliminate gradient terms and allow for the use of second-order perturbation theory for evaluation of spectroscopic constants. It is shown that this step has essentially no effect on the quality of the QFF largely because the initial reference structure is, by design, very close to the final ab initio equilibrium structure. The procedure is used to compute an accurate, purely ab initio QFF for the H2O molecule, which is used as a benchmark test case. The procedure is then applied to the ground and first excited electronic states of the HO2+ molecular cation. Fundamental vibrational frequencies and spectroscopic constants from these highly accurate QFFs are compared and contrasted with previous experiment and theory. It is concluded that the spectroscopic constants determined for the X3A″ and A1A′ states of HO2+ are the most accurately available. [ABSTRACT FROM AUTHOR]

The vibrations of H3O2- and D3O2- are investigated using diffusion Monte Carlo (DMC) and vibrational configuration-interaction approaches, as implemented in the program MULTIMODE. These studies use the potential surface recently developed by Huang et al. [ J. Am. Chem. Soc. 126, 5042 (2004)]. The focus of this work is on the vibrational ground state and fundamentals which occur between 100 and 3700 cm-1. In most cases, excellent agreement is obtained between the fundamental frequencies calculated by the two approaches. This serves to demonstrate the power of both methods for treating this very anharmonic system. Based on the results of the MULTIMODE and DMC treatments, the extent and nature of the couplings in H3O2- and D3O2- are investigated. [ABSTRACT FROM AUTHOR]

The vibrations of H3O2- and D3O2- are investigated using diffusion Monte Carlo (DMC) and vibrational configuration-interaction approaches, as implemented in the program MULTIMODE. These studies use the potential surface recently developed by Huang et al. [ J. Am. Chem. Soc. 126, 5042 (2004)]. The focus of this work is on the vibrational ground state and fundamentals which occur between 100 and 3700 cm-1. In most cases, excellent agreement is obtained between the fundamental frequencies calculated by the two approaches. This serves to demonstrate the power of both methods for treating this very anharmonic system. Based on the results of the MULTIMODE and DMC treatments, the extent and nature of the couplings in H3O2- and D3O2- are investigated. [ABSTRACT FROM AUTHOR]

Predissociation spectra of the H5O2+·RGn(RG=Ar,Ne) cluster ions are reported in energy regions corresponding to both the OH stretching (3350–3850 cm-1) and shared proton (850–1950 cm-1) vibrations. The two free OH stretching bands displayed by the Ne complex are quite close to the band origins identified earlier in bare H5O2+ [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)], indicating that the symmetrical H5O2+ “Zundel” ion remains largely intact in H5O2+·Ne. The low-energy spectrum of the Ne complex is simpler than that observed previously for H5O2+·Ar, and is dominated by two sharp transitions at 928 and 1047 cm-1, with a weaker feature at 1763 cm-1. The H5O2+·Arn,n=1–5 spectra generally exhibit complex band structures reflecting solvent-induced symmetry breaking of the Zundel core ion. The extent of solvent perturbation is evaluated with electronic structure calculations, which predict that the rare gas atoms should attach to the spectator OH groups of H5O2+ rather than to the shared proton. In the asymmetric complexes, the shared proton resides closer to the more heavily solvated water molecule, leading to redshifts in the rare gas atom-solvated OH stretches and to blueshifts in the shared proton vibrations. The experimental spectra are compared with recent full-dimensional vibrational calculations (diffusion Monte Carlo and multimode/vibrational configuration interaction) on H5O2+. These results are consistent with assignment of the strong low-energy bands in the H5O2+·Ne spectrum to the vibration of the shared proton mostly along the O–O axis, with the 1763 cm-1 band traced primarily to the out-of-phase, intramolecular bending vibrations of the two water molecules. [ABSTRACT FROM AUTHOR]

The vibrational mode coupling in the protonated water dimer is investigated by performing two types of quantum calculations of the vibrational levels of H[sub 5]O[sub 2][sup +] and D[sub 5]O[sub 2][sup +], utilizing the OSS3(p) potential energy surface by Ojamaë et al. [L. Ojamaë, I. Shavitt, and S. J. Singer, J. Chem. Phys. 109, 5547 (1998)]. One is four-dimensional (4D), treating only the central O...H(D)[sup +]...O moiety. Three of the four modes considered, the asymmetric stretch and the two bends, are largely the vibrations of the central proton, while the fourth mode is essentially the O...O stretching vibration. The vibrational levels of O...H(D)[sup +]...O are calculated rigorously, as fully coupled (FC), and also in an adiabatic (3+1)D approximation, where the proton asymmetric stretch is treated as adiabatically separated from the other three degrees of freedom. The second set of calculations, designated VCI, is full-dimensional, 15D; it is performed by the code MULTIMODE, which does configuration interaction (CI) calculations using a basis determined from a vibrational self-consistent field Hamiltonian. The FC 4D and 15D VCI calculations give very similar fundamental frequencies of the two bending modes of the central proton, as well as the O...O stretch. They differ substantially only for the fundamental of the proton asymmetric stretch, the VCI value being about 25% lower than the FC 4D result. This shows that the asymmetric stretch is strongly coupled to the vibrations outside the O...H(D)[sup +]...O fragment, in contrast to the two proton bending modes and the O...O stretching vibration. The FC 4D and 15D VCI calculations predict the same frequency ordering of the four vibrational modes of the O...H(D)[sup +]...O moiety, and are in excellent agreement with respect to the H–D shift of the fundamentals of the shared proton modes. The adiabatic (3+1)D treatment is not quantitatively accurate, yielding fundamental frequencies of the proton vibrational modes which are considerably different from the FC 4D results. Our results have potentially significant implications for the assignment of the bands associated with shared proton vibrations in the recently reported infrared multiphoton photodissociation spectrum of the protonated water dimer. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The correction of the statement about localization of the C[H.sub.5.sup.], as described in the 'Nature of the Chemical Bond in Protonated Methane', is illustrated along with a new assignment of the infrared spectrum. The analysis has concluded that the [H.sub.2.sup.+] stretch vibration cannot contribute to the observed intensity in the 2400 or 2700 [cm.sup.-1] region of the spectrum.

Two different methods are used to calculate the spectra of deuterated analogues to CH(sub 5)(super +) at various stationary points on the potential surface. The predicted spectra for CH4D(super +) and CHD4 contain features associated with the single D or H atom being preferentially located in one of the bonding sites.

An ab initio calculation of the potential surface, quantum structures, and zero-point energies of CH5(super +) and CH2D3(super +) in full dimensionality is presented. The diffusion Monte Carlo (DMC) calculations of the fully anharmonic zero-point energies and ground-state wave functions of CH5(super +) and CH2D3 (super +) uses the potential that obeys the permutational symmetry of five hydrogen atoms.

Realistic, full-dimensional calculations of vibrational energies of H5O2 are undertaken and the disappearance of the high frequency OH band for the Ar2 complex is examined. The findings indicate the high energy OH-vibrational band disappeares with two Ar atoms complexed to H3O2- instead of a single Ar atom.

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]

It has been shown that rotational lines observed in the Horsehead nebula photodissociation region (PDR) are probably not caused by l-C3H+, as was originally suggested. In the search for viable alternative candidate carriers, quartic force fields are employed here to provide highly accurate rotational constants, as well as fundamental vibrational frequencies, for another candidate carrier: 1 1 A' C3H-. The ab initio computed spectroscopic constants provided in this work are, compared to those necessary to define the observed lines, as accurate as the computed spectroscopic constants for many of the known interstellar anions. Additionally, the computed Deff for C3H- is three times closer to the D deduced from the observed Horsehead nebula lines relative to l-C3H+. As a result, 1 1 A' C3H- is a more viable candidate for these observed rotational transitions. It has been previously proposed that at least C6H- may be present in the Horsehead nebular PDR formed by way of radiative attachment through its dipole-bound excited state. C3H- could form in a similar way through its dipole-bound state, but its valence excited state increases the number of relaxation pathways possible to reach the ground electronic state. In turn, the rate of formation for C3H- could be greater than the rate of its destruction. C3H- would be the seventh confirmed interstellar anion detected within the past decade and the first CnH- molecular anion with an odd n. [ABSTRACT FROM AUTHOR]

Recently, we reported ab initio quartic force fields (QFFs) for the cyclic and linear forms of the Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed molecular cation, referred to as Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. Specifically the singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations, CCSD(T), was used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation were included. In the present study, we use these QFFs to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants for the Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. isotoplogues of Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., and the H2CCCD+, HDCCCH+, H213CCCH+, H2C 13CCH+, and H2CC13CH+ isotopologues of Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. Improvements in ab intitio methods have now made it possible to identify small molecules in an astronomical observation without the aid of high-resolution experimental data. We also report dipole moment values and show that the above-mentioned cyclic isotopologues have values of 0.094, 0.225, and 0.312 D, respectively, while the Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. isotopologues have values that range between 0.325 and 0.811 D. Thus, it is hoped that the highly accurate spectroscopic constants and data provided herein for the 13C and deuterium isotopologues of the cyclic and linear forms of Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. will enable their identification in astronomical observations from the Herschel Space Observatory the Stratospheric Observatory for Infrared Astronomy, the Atacama Large Millimeter Array, and in the future, the James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

The effect of approximating the three- and four-virtual molecular orbital integrals in single and double coupled-cluster theory including a perturbational correction for connected triple excitations [CCSD(T)] is investigated for the calculation of higher-order properties, specifically the calculation of a molecular quartic force field and spectroscopic constants. The approach was proposed previously, but investigated for only second- and lower-order properties. It is shown that the conclusions reached previously are essentially unchanged on moving to higher-order properties. That is, approximating the selected integrals has essentially no effect on the accuracy of CCSD(T) calculations, and the error due to approximating integrals is much smaller than the residual error due to one-particle basis set deficiencies. The advantage of this approach is that it significantly reduces the amount of data needed to perform CCSD(T) calculations, thereby reducing computational requirements associated with input/output operations and message passing in massively parallel, distributed memory algorithms. These savings are particularly important for large basis set calculations where the reduction in data can be as high as three orders of magnitude for ∼1000 unoccupied molecular orbitals. The approach was tested by computing the quartic force field, vibrational frequencies, and spectroscopic constants of cyclopropenylidene and isotopologues. Comparison of our best results with available experimental data shows excellent agreement between theory and experiment. It is hoped that the theoretical spectroscopic data presented herein for cyclopropenylidene and isotopologues is useful in the interpretation of future laboratory experiments and astronomical observations. [ABSTRACT FROM AUTHOR]

An accurate potential energy surface has been determined for methanol from ab initio potential data at the CCSD(T) level of theory with an aug-cc-pVTZ basis. The resulting potential function is valid over all twelve vibrational degrees of freedom for all near-equilibrium and torsional configurations. A torsional reaction path has been derived for this potential, from which the low-lying vibrational levels of methanol have been calculated by the reaction path version of MULTIMODE. Comparisons with experiment and other calculations are made. [ABSTRACT FROM AUTHOR]

This review focuses on the calculation of rovibrational energies of polyatomic molecules using the code MULTIMODE. This code, which uses normal coordinates and a hierarchical n -mode representation of the potential, aims to be applicable to a wide class of molecules and molecular complexes. The theoretical and computational methods used in this code are described, followed by a review of selected applications. These applications illustrate various features of the code and also point out some limitations of the current version of the code. The review concludes with some ideas about possible future directions in this area of research. [ABSTRACT FROM AUTHOR]

A correction to the article "Ab Initio Potential Energy and Dipole Moment Surfaces for H5O+2" which appeared in the 2005 issue is presented.

We present high spectral resolution observations from 12.96 to 13.33 microns toward Orion IRc2 using the mid-infrared spectrograph, Echelon-Cross-Echelle Spectrograph (EXES), at Stratospheric Observatory for Infrared Astronomy (SOFIA). These observations probe the physical and chemical conditions of the Orion hot core, which is sampled by a bright, compact, mid-infrared background continuum source in the region, IRc2. All 10 of the rovibrational C2H2 transitions expected in our spectral coverage are detected with high signal-to-noise ratios (S/Ns), yielding continuous coverage of the R-branch lines from J = 9–8 to J = 18–17, including both ortho and para species. Eight of these rovibrational transitions are newly reported detections. The isotopologue, 13CCH2, is clearly detected with a high S/N. This enabled a direct measurement of the 12C/13C isotopic ratio for the Orion hot core of 14 ± 1 and an estimated maximum value of 21. We also detected several HCN rovibrational lines. The ortho and para C2H2 ladders are clearly separate, and tracing two different temperatures, 226 K and 164 K, respectively, with a non-equilibrium ortho to para ratio (OPR) of 1.7 ± 0.1. Additionally, the ortho and para VLSR values differ by about 1.8 ± 0.2 km s−1, while the mean line widths differ by 0.7 ± 0.2 km s−1, suggesting that these species are not uniformly mixed along the line of sight to IRc2. We propose that the abnormally low C2H2 OPR could be a remnant from an earlier, colder phase, before the density enhancement (now the hot core) was impacted by shocks generated from an explosive event 500 years ago. [ABSTRACT FROM AUTHOR]

In this work we report on high-resolution IR absorption studies that provide a detailed view on how the peripheral structure of irregular polycyclic aromatic hydrocarbons (PAHs) affects the shape and position of their 3 μm absorption band. For this purpose, we present mass-selected, high-resolution absorption spectra of cold and isolated phenanthrene, pyrene, benz[a]antracene, chrysene, triphenylene, and perylene molecules in the 2950–3150 cm−1 range. The experimental spectra are compared with standard harmonic calculations and anharmonic calculations using a modified version of the SPECTRO program that incorporates a Fermi resonance treatment utilizing intensity redistribution. We show that the 3 μm region is dominated by the effects of anharmonicity, resulting in many more bands than would have been expected in a purely harmonic approximation. Importantly, we find that anharmonic spectra as calculated by SPECTRO are in good agreement with the experimental spectra. Together with previously reported high-resolution spectra of linear acenes, the present spectra provide us with an extensive data set of spectra of PAHs with a varying number of aromatic rings, with geometries that range from open to highly condensed structures, and featuring CH groups in all possible edge configurations. We discuss the astrophysical implications of the comparison of these spectra on the interpretation of the appearance of the aromatic infrared 3 μm band, and on features such as the two-component emission character of this band and the 3 μm emission plateau. [ABSTRACT FROM AUTHOR]

A correction to the article "High-Resolution IR Absorption Spectroscopy of Polycyclic Aromatic Hydrocarbons: The Realm of Anharmonicity" that was published in the previous issue, is presented.

We report on an experimental and theoretical investigation of the importance of anharmonicity in the 3-μm CH stretching region of polycyclic aromatic hydrocarbon (PAH) molecules. We present mass-resolved, high-resolution spectra of the gas-phase cold (∼4 K) linear PAH molecules naphthalene, anthracene, and tetracene. The measured IR spectra show a surprisingly high number of strong vibrational bands. For naphthalene, the observed bands are well separated and limited by the rotational contour, revealing the band symmetries. Comparisons are made to the harmonic and anharmonic approaches of the widely used Gaussian software. We also present calculated spectra of these acenes using the computational program SPECTRO, providing anharmonic predictions with a Fermi-resonance treatment that utilizes intensity redistribution. We demonstrate that the anharmonicity of the investigated acenes is strong, dominated by Fermi resonances between the fundamental and double combination modes, with triple combination bands as possible candidates to resolve remaining discrepancies. The anharmonic spectra as calculated with SPECTRO lead to predictions of the main bands that fall within 0.5% of the experimental frequencies. The implications for the aromatic infrared bands, specifically the 3-μm band, are discussed. [ABSTRACT FROM AUTHOR]

The article examines the effect of deuteration on the structure of infrared spectrum of CH5+. Two approaches are used to calculate the spectra of deuterated analogs to CH5+ at various stationary points on the potential surface. The predicted spectra for CH4D+ and CHD4+ both contain features associated with the single D or H atom.

The article presents the quantum calculations of vibrational energies of H<3O2- on an ab lnitio Potential. In this article, H<3O2- in full dimensionality is considered and focuses on the experiments of M.A. Johnson and co-workers. This group reported four vibrational bands of H<3O2-, using the messenger technique. The assignment of these bands was based on the results of MP2 ab initio calculations of harmonic normal-mode frequencies. The calculations also determined a slightly asymmetric, double minimum equilibrium structure with the bridging hydrogen (BH) slightly closer to one O atom. More recent ab initio studies of the motion of the BH concluded that it is delocalized over the two minima.

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]