Rovibrational Data Research Articles

Far-infrared laboratory spectroscopy of aminoacetonitrile and first interstellar detection of its vibrationally excited transitions

Context. Aminoacetonitrile, a molecule detected in the interstellar medium only toward the star-forming region Sagittarius B2 (Sgr B2), is considered an important prebiotic species; in particular, it is a possible precursor of the simplest amino acid glycine. To date, observations have been limited to ground state emission lines, whereas transitions from within vibrationally excited states remained undetected. Aims. We wanted to accurately determine the energies of the low-lying vibrational states of aminoacetonitrile, which are expected to be populated in Sgr B2(N1), the main hot core of Sgr B2(N). This step is fundamental in order to properly evaluate the vibration-rotation partition function of aminoacetonitrile as well as the line strengths of the rotational transitions of its vibrationally excited states. This is necessary to derive accurate column densities and secure the identification of these transitions in astronomical spectra. Methods. The far-infrared ro-vibrational spectrum of aminoacetonitrile has been recorded in absorption against a synchrotron source of continuum emission. Three bands, corresponding to the lowest vibrational modes of aminoacetonitrile, were observed in the frequency region below 500 cm−1. The combined analysis of ro-vibrational and pure rotational data allowed us to prepare new spectral line catalogs for all the states under investigation. We used the imaging spectral line survey ReMoCA performed with ALMA to search for vibrationally excited aminoacetonitrile toward Sgr B2(N1). The astronomical spectra were analyzed under the local thermodynamic equilibrium (LTE) approximation. Results. Almost 11 000 lines have been assigned during the analysis of the laboratory spectrum of aminoacetonitrile, thanks to which the vibrational energies of the v11 = 1, v18 = 1, and v17 = 1 states have been determined. The whole dataset, which includes high J and Ka transitions, is well reproduced within the experimental accuracy. Reliable spectral predictions of pure rotational lines can now be produced up to the THz region. On the basis of these spectroscopic predictions, we report the interstellar detection of aminoacetonitrile in its v11 = 1 and v18 = 1 vibrational states toward Sgr B2(N1) in addition to emission from its vibrational ground state. The intensities of the identified v11 = 1 and v18 = 1 lines are consistent with the detected v = 0 lines under LTE at a temperature of 200 K for an aminoacetonitrile column density of 1.1 × 1017 cm−2. Conclusions. This work shows the strong interplay between laboratory spectroscopy exploiting (sub)millimeter and synchrotron-based far-infrared techniques, and observational spectral surveys to detect complex organic molecules in space and quantify their abundances.

Spectral characterization for small clusters of silicon and oxygen: SiO2, SiO3, Si2O3, & Si2O4

Rocky bodies are made of minerals comprised largely of silicon and oxygen. How these minerals are formed from their constituent atoms is not fully known. The rovibrational IR spectral data produced in this work may help to observe small molecules containing silicon and oxygen so that these potential molecular intermediates can be observed. These molecules have strong absorption features between 7.0 ​μm and 8.0 ​μm, most notably, and are fully characterized in the IR from the present quantum chemical data. The antisymmetric Si−O stretches of small silicon oxide clusters also fall in this range and have large intensities. Hence, this quantum chemical analysis provides spectral data for such molecules that may be of significance for astrochemical classification and could play a role in the formation or degradation of mineral nanocrystals from or into their constituent atoms. Both explicitly computed anharmonic fundamental vibrational frequencies and those determined from scaled harmonic frequencies agree well with known experimental data, and spectroscopic constants are provided herein such that astronomical rotational spectral characterization may also be possible for the C2v SiO3 and Si2O3 molecules.

Morphing radial molecular property functions of hydroxyl

Effective (mass-dependent) potential energy curves of the X2Π ground electronic states of isotopomers of the hydroxyl radical OH are constructed by morphing literature RKR and ab initio potential energy curves within the framework of the reduced potential energy curve (RPC) approach of Jenč. The morphing is performed by fitting the RPC parameters to available experimental ro-vibrational data. In turn, the resulting potential energy curves are used to determine the radial property functions describing the spin-rotation and fine and hyperfine energy patterns of the spin-ro-vibrational energy levels by morphing their RKR and ab initio approximants. The resulting functions enable reliable prediction for untested spectral regions on the one hand and quantitative testing of the impacts of external fields on the observed spectral transitions on the other.

The J-dependent rotational Hamiltonian method for analyzing rovibrational spectra: Application to HO2, H2O, and O3

A J-dependent rotational Hamiltonian method is presented for analyzing rovibrational spectra. The method is designed to: assign/verify (J,Ka,Kc) and v quantum numbers to rovibrational energy levels; fit flexible rotor rotational constants to experimental/theoretical rovibrational energy level data; interpolate/extrapolate missing energy level data; assess the quality/consistency of the rovibrational data. The method resembles the standard “effective Hamiltonian” approach (Watson, 1967, 1968) except that the rotational constants themselves depend on J, which provides a number of advantages. The method is applied here to three molecules: HO2, O3, and H2O.

First Laboratory Detection of Vibration-Rotation Transitions of 12CH+ and 13CH+ and Improved Measurement of their Rotational Transition Frequencies.

The long-searched C-H stretches of the fundamental ions CH+ and 13CH+ have been observed for the first time in the laboratory. For this, the state-dependent attachment of He atoms to these ions at cryogenic temperatures has been exploited to obtain high-resolution rovibrational data. In addition, the lowest rotational transitions of CH+, 13CH+ and CD+ have been revisited and their rest frequency values improved substantially.

Construction of a coarse-grain quasi-classical trajectory method. I. Theory and application to N2-N2 system.

This work aims to construct a reduced order model for energy transfer and dissociation in non-equilibrium nitrogen mixtures. The objective is twofold: to present the Coarse-Grain Quasi-Classical Trajectory (CG-QCT) method, a novel framework for constructing a reduced order model for diatom-diatom systems; and to analyze the physics of non-equilibrium relaxation of the nitrogen molecules undergoing dissociation in an ideal chemical reactor. The CG-QCT method couples the construction of the reduced order model under the coarse-grain model framework with the quasi-classical trajectory calculations to directly construct the reduced model without the need for computing the individual rovibrational specific kinetic data. In the coarse-grain model, the energy states are lumped together into groups containing states with similar properties, and the distribution of states within each of these groups is prescribed by a Boltzmann distribution at the local translational temperature. The required grouped kinetic properties are obtained directly by the QCT calculations. Two grouping strategies are considered: energy-based grouping, in which states of similar internal energy are lumped together, and vibrational grouping, in which states with the same vibrational quantum number are grouped together. A zero-dimensional chemical reactor simulation, in which the molecules are instantaneously heated, forcing the system into strong non-equilibrium, is used to study the differences between the two grouping strategies. The comparison of the numerical results against available experimental data demonstrates that the energy-based grouping is more suitable to capture dissociation, while the energy transfer process is better described with a vibrational grouping scheme. The dissociation process is found to be strongly dependent on the behavior of the high energy states, which contribute up to 50% of the dissociating molecules. Furthermore, up to 40% of the energy required to dissociate the molecules comes from the rotational mode, underscoring the importance of accounting for this mode when constructing non-equilibrium kinetic models. In contrast, the relaxation process is governed primarily by low energy states, which exhibit significantly slower transitions in the vibrational binning model due to the prevalence of mode separation in these states.

Towards a quantum chemical protocol for the prediction of rovibrational spectroscopic data for transition metal molecules: Exploration of CuCN, CuOH, and CuCCH.

High accuracy electronic structure computations for small transition metal-containing molecules have been a long term challenge. Due to coupling between electronic and nuclear wave functions, even experimental/theoretical identification of the ground electronic state requires tremendous efforts. Quartic force fields (QFFs) are effective ab initio tools for obtaining reliable anharmonic spectroscopic properties. However, the method that employs complete basis set limit extrapolation ("C"), consideration of core electron correlation ("cC"), and inclusion of scalar relativity ("R") to produce the energy points on the QFF, the composite CcCR methodology, has not yet been utilized to study inorganic spectroscopy. This work takes the CcCR methodology and adapts it to test whether such an approach is conducive for the closed-shell, copper-containing molecules CuCN, CuOH, and CuCCH. Gas phase rovibrational data are provided for all three species in their ground electronic states. Equilibrium geometries and many higher-order rovibrational properties show good agreement with earlier studies. However, there are notable differences, especially in computation of fundamental vibrational frequencies. Even with further additive corrections for the inner core electron correlation and coupled cluster with full single, double, and triple substitutions (CCSDT), the differences are still larger than expected indicating that more work should follow for predicting rovibrational properties of transition metal molecules.

6.2 μm spectrum and 6-dimensional morphed potentials of OC-H2O

Rovibrational transitions associated with tunneling states in the ν5 (water bending) vibration of the OC-H2O complex have been recorded using a supersonic jet mode-hop free quantum cascade laser spectrometer at 6.2 μm. Analysis of the resulting spectra is facilitated by incorporating fits of previously recorded microwave and submillimeter data accounting for Coriolis coupling. The theoretical basis of morphing a 5-D frozen monomers potential was initially developed and then extended to two 6-D morphed potentials. A combination of these spectroscopic results and previous rovibrational data for the ν5 vibration in OC-D2O are then used to generate a 6-D morphed potential surface for the intermolecular and the water bending vibrations. An alternative 6-D morphed potential of the intermolecular and the ν3 (CO stretching) vibrations was also generated. These determined morphed potentials then formed the basis for modeling the dynamics of the complex and prediction of accurate intermolecular rovibrational frequencies of the complex.

High resolution infrared spectra of the 16O16O17O and the 16O17O16O ozone isotopic species. The 5 and 10 micron spectral ranges revisited

The spectra of ozone, enriched by 17O oxygen, were recorded and reinvestigated in the 5 μm and 10 μm regions using the Reims Fourier Transform Spectrometer. Here we focus on the analysis of the ν1, ν3 and ν1+ν3 bands of the 16O17O16O isotopologue, which were modeled simultaneously to improve the parameters of the ground state energy level. The analysis of the ν1+ν3 bands of 16O16O17O is also extended. We report here the new system of effective Hamiltonian parameters, give some examples of comparison between observed and simulated spectra, and of the improvement for the ro-vibrational transition data available in spectroscopic databases.

Rovibrational Characterization and Interstellar Implications of the Proton-Bound, Noble Gas Complexes: ArHAr+, NeHNe+, and ArHNe+

The detection of the noble gas molecule ArH+ initially in the Crab nebula and elsewhere since has reinvigorated the search for naturally occurring, noble gas compounds. Additionally, proton-bound complexes are known to possess exceptionally bright vibrational modes corresponding to the shifting of the proton between the two, more massive moieties. As such, smaller column densities are required for unique detection of molecules with such intense transitions. In light of this, vibrational, rotational, and rovibrational spectroscopic data are provided here for ArHAr+, NeHNe+, and ArHNe+ to assist in laboratory characterization or even astronomical detection of these noble gas molecular cations. NeHNe+ is shown to be a surprisingly well-bound system, while the neon atom is not nearly as tightly bound to ArHNe+ in comparison. Furthermore, the reaction of the astronomically detected argonium cation with the strongly supported, potential interstellar ArH3+ cation will produce ArHAr+ with favorable energetics. Th...

Characterising molecules for fundamental physics: an accurate spectroscopic model of methyltrioxorhenium derived from new infrared and millimetre-wave measurements.

Precise spectroscopic analysis of polyatomic molecules enables many striking advances in physical chemistry and fundamental physics. We use several new high-resolution spectroscopic devices to improve our understanding of the rotational and rovibrational structure of methyltrioxorhenium (MTO), the achiral parent of a family of large oxorhenium compounds that are ideal candidate species for a planned measurement of parity violation in chiral molecules. Using millimetre-wave and infrared spectroscopy in a pulsed supersonic jet, a cryogenic buffer gas cell, and room temperature absorption cells, we probe the ground state and the Re[double bond, length as m-dash]O antisymmetric and symmetric stretching excited states of both CH3187ReO3 and CH3185ReO3 isotopologues in the gas phase with unprecedented precision. By extending the rotational spectra to the 150-300 GHz range, we characterize the ground state rotational and hyperfine structure up to J = 43 and K = 41, resulting in refinements to the rotational, quartic and hyperfine parameters, and the determination of sextic parameters and a centrifugal distortion correction to the quadrupolar hyperfine constant. We obtain rovibrational data for temperatures between 6 and 300 K in the 970-1015 cm-1 range, at resolutions down to 8 MHz and accuracies of 30 MHz. We use these data to determine more precise excited-state rotational, Coriolis and quartic parameters, as well as the ground-state centrifugal distortion parameter DK of the 187Re isotopologue. We also account for hyperfine structure in the rovibrational transitions and hence determine the upper state rhenium atom quadrupole coupling constant eQq'.

CH3D photomixing spectroscopy up to 2.5 THz: New set of rotational and dipole parameters, first THz self-broadening measurements

Several previously unmeasured transitions of 12CH3D have been recorded by a terahertz photomixing continuous-wave spectrometer up to QR(10) branch at 2.5THz. An improved set of rotational constants has been obtained utilizing a THz frequency metrology based on a frequency comb that achieved an averaged frequency position better than 150kHz on more than fifty ground-state transitions. A detailed analysis of the measured line intensities was undertaken using the multispectrum fitting program and has resulted in a determination of new dipole moment parameters. Measurements at different pressures of the QR(7) transitions provide the first determination of self-broadening coefficients from pure rotational CH3D lines. The THz rotational measurements are consistent with IR rovibrational data but no significant vibrational dependence of self-broadening coefficient may be observed by comparison.

Morphing ab initio potential energy curve of beryllium monohydride

Effective (mass-dependent) potential energy curves of the ground electronic states of 9BeH, 9BeD, and 9BeT are constructed by morphing a very accurate MR-ACPF ab initio potential of Koput (2011) within the framework of the reduced potential energy curve approach of Jenč (1983). The morphing is performed by fitting the RPC parameters to available experimental ro-vibrational data. The resulting potential energy curves provide a fairly quantitative reproduction of the fitted data. This allows for a reliable prediction of the so-far unobserved molecular states in terms of only a small number of fitting parameters.

In Process Citation

A novel model for the analysis of the rovibrational structure of a molecule based on anharmonic ladder operators associated with the vibrational degrees of freedom is presented. This is devised as an alternative method for the global spectral analysis of rovibrational data considering vibrational anharmonicities from the outset. The present method is thought up with an effective rovibrational Hamiltonian written in terms of angular momentum components and anharmonic Morse ladder operators, associated with rotational and vibrational degrees of freedom, respectively. The resulting Hamiltonian is diagonalized in a symmetry-adapted basis set expressed as a product of rotational states and individual 1D-Morse wave functions for each local vibrational degree of freedom. This approach has been successfully applied to the study of the vibrational structure (up to polyad 14) and the rovibrational structure (up to polyad 2 and J(max) = 20) of hydrogen sulfide. It was shown that this new global analysis formalism is able to reduce considerably the number of fitted parameters with respect to the spectral analysis carried out for separate polyads.

Energetics, structure, and rovibrational spectroscopic properties of the sulfurous anions SNO− and OSN−

The SNO(-) and OSN(-) anions are shown in this work to be very stable negatively charged species in line with other recent work [T. Trabelsi et al., J. Chem. Phys. 143, 164301 (2015)]. Utilizing established quartic force field techniques, the structural and rovibrational data for these anions are produced. The SNO(-) anion is less linear and has weaker bonds than the corresponding neutral radical giving much smaller rotational constants. OSN(-) is largely unchanged in these regards with inclusion of the additional electron. The S-N bond is actually stronger, and the rotational constants of OSN(-) versus OSN are similar. The vibrational frequencies of SNO(-) are red-shifted from the radical while those in OSN(-) are mixed. OSN(-) has mixing of the stretching modes while the S-N and N-S stretches of SNO(-) are largely independent of one another. The ω3 stretches are much brighter in these anions than they are in the radicals, but the ω1 stretches are still the brightest.

Optimal cloud use of quartic force fields: The first purely commercial cloud computing based study for rovibrational analysis of SiCH−

Commercial cloud computing (CCC) has the promise of an untold number of computing nodes available for the researcher as long as he or she has the financial means to absorb these costs and the administrative skills necessary to effectively utilize the resources. The key is finding how to maximize parallelization for a minimum of monetary and management costs. Previous work has shown that CCC resources are viable for use on large numbers of small-to-medium sized quantum chemical computations. Composite energy quartic force fields (QFFs) are a highly-attractive platform for subsequent testing of CCC resources to find the proper balance between time savings of the cloud versus monetary expenditure. Use of this type of potential energy surface has lead to highly-accurate rovibrational data in earlier work. QFFs use large numbers of stand-alone energies that have to be computed for various molecular geometries. At each geometry, different methods and/or basis sets are used to efficiently generate accurate representations of the nuclear potential. For this initial study, the small molecular anion, SiCH− of interest in astrochemistry, is chosen for analysis as it can be done cheaply on the cloud while still providing insight into the nature of CCC usage. Additionally, no rovibrational data exists for this molecule making it the first molecule quantum chemically computed purely via CCC tools. © 2015 Wiley Periodicals, Inc.

Quartic force field-derived vibrational frequencies and spectroscopic constants for the isomeric pair SNO and OSN and isotopologues.

The SNO and OSN radical isomers are likely to be of significance in atmospheric and astrochemistry, but very little is known about their gas phase spectroscopic properties. State-of-the-art ab initio composite quartic force fields are employed to analyze the rovibrational features for both systems. Comparison to condensed-phase experimental data for SNO has shown that the 1566.4 cm(-1) ν1 N-O stretch is indeed exceptionally bright and likely located in this vicinity for subsequent gas phase experimental analysis. The OSN ν1 at 1209.4 cm(-1) is better described as the antisymmetric stretch in this molecule and is also quite bright. The full vibrational, rotational, and rovibrational data are provided for SNO and OSN and their single (15)N, (18)O, and (34)S isotopic substitutions in order to give a more complete picture as to the chemical physics of these molecules.

Theoretical Rovibronic Treatment of the X̃ (2)Σ(+) and à (2)Π States of C2H and the X̃ (1)Σ(+) State of C2H(-) from Quartic Force Fields.

Quartic force fields (QFFs) have been shown to be an effective ab initio tool for the generation of highly accurate anharmonic spectroscopic data but have only recently been extended to the description of electronically excited states using equation-of-motion (EOM) coupled cluster theory. In this study, rovibrational data are provided for the X̃ (2)Σ(+) ground state of the ethynyl radical as well as the X̃ (1)Σ(+) ground state of the acetylide anion using traditional QFF methods in conjunction with the new EOM-based QFF method to describe the ethynyl radical's à (2)Π excited state. These data sets include fundamental vibrational frequencies, rotational constants, and vibrationally averaged structures in addition to other spectroscopic constants. An anticipated theoretical correction (ATC) is shown to be an effective way of improving the accuracy of the fundamental modes of the ethynyl radical's à (2)Π electronically excited state, lowering the frequencies from 2 to 9%. The anharmonic fundamental vibrational frequencies of à (2)Π C2H are reported to be ν2 = 538.0 cm(-1) (bend), ν3 = 1832.2 cm(-1) (C-C stretch), and ν1 = 3008.1 cm(-1) (C-H stretch). These à (2)Π frequencies match closely to predicted transitions from previous theoretical work as well as observed transitions from matrix isolation results. Electron binding energies for the X̃ (2)Σ(+) C2H ← X̃ (1)Σ(+) C2H(-) and à (2)Π C2H ← X̃ (1)Σ(+) C2H(-) transitions are found to be 2.975 and 3.4 eV, respectively. Spectroscopic data are also provided for the (13)C and deuterated isotopologues of X̃ (2)Σ(+) C2H, à (2)Π C2H, and X̃ (1)Σ(+) C2H(-).

Rovibrational and energetic analysis of the hydroxyethynyl anion (CCOH−)

For the first time, high-level structural and rovibrational data are provided for the hyroxyethynyl anion, CCOH−. CCOH− is a promising molecule for interstellar detection even though no new anions have been observed in the interstellar medium for the past half-decade. The large dipole moment of the corresponding neutral radical may be key for its creation as has been hypothesised and supported for other anions known to exist in various astronomical environments. Highly accurate quartic force fields are employed where previous benchmarks have produced spectroscopic constants and anharmonic vibrational frequencies within 20 MHz and 1 cm−1, respectively, of experiment. This same approach is applied here for CCOH− and its deuterated isotopologue with the goal of assisting laboratory experiments and/or astronomical observers in the potential detection of this anion.

QUANTUM CHEMICAL ROVIBRATIONAL DATA FOR THE INTERSTELLAR DETECTION OFc-C3H–

The anion chemistry of the interstellar medium (ISM) has almost exclusively been limited to linear hydrocarbons and cyanocarbons. Of the hydrocarbons, only the even n C n H– chains have been detected in the ISM, and lines hypothesized to originate with b-C3H– have been conclusively linked to the corresponding cation, as originally claimed. However, no reason has yet been provided as to why other anions cannot form, and the cyclic form of C3H– is actually the lowest-energy isomer on the anion's potential energy surface. As such, this work provides the necessary rovibrational reference data for the potential detection of this anion in the ISM or related laboratory experiments. Improvements over previously calculated rovibrational spectroscopic constants are contained herein along with graphical depictions of the pure rotational spectra at 100 K, 40 K, 20 K, and 2.7 K.