As of yet, unexamined aluminum bearing molecules may help elucidate aluminum chemistry and associated refractory atom reactions in the interstellar medium. The flexibility of modern quantum chemistry in the construction and analysis of novel molecules makes it perfectly suited to analyze molecules of astrochemical significance. In this paper, high level ab initio electronic structure calculations using the coupled cluster CCSD(T) and explicitly correlated coupled cluster CCSD(T)-F12 methods with large basis sets extrapolated to the complete basis set limit have been performed on the various [Al,N,C,O] isomers. The anharmonic rotational and vibrational spectroscopic parameters for all isomers are produced with these same levels of theory via quartic force fields and vibrational perturbation theory in order to aid in their potential laboratory or even astrophysical identification. The most stable isomer is determined here to be the aluminum isocyanate radical with linear equilibrium geometry AlNCO (X1Σ+). The NCO antisymmetric stretch of AlNCO has an intensity of 1500 km/mol, which should greatly aid in its infrared detection in the region around 2305 cm−1. Additionally, the AlOCN isomer is relatively low lying, possesses a 5.12 D dipole moment, and has a notable kinetic stability, making it a viable candidate for astronomical observation. All isomers are characterized by small frequencies, which indicates that these are floppy molecules. Isomers with a terminal aluminum atom are especially floppy, with bending modes less than 100 cm−1.
Metrics & MoreArticle Recommendations * sı Supporting InformationABSTRACT: (T)+EOM quartic force fields (QFFs) are proposed for ab initio rovibrational properties of electronically excited states of small molecules. The (T)+EOM method is a simple treatment of the potential surface of the excited state using a composite energy from the CCSD(T) energy for the ground-state configuration and the EOM-CCSD excitation energy for the target state. The method is benchmarked with two open-shell species, HOO and HNF, and two closed-shell species, HNO and HCF. A (T)+EOM QFF with a complete basis set extrapolation (C) and corrections for core correlation (cC) and scalar relativity (R), dubbed (T)+EOM/CcCR, achieves a mean absolute error (MAE) as low as 1.6 cm −1 for the A ̃2A′ state of HOO versus an established benchmark QFF with CCSD(T)-F12b/cc-pVTZ-F12 (F12-TZ) for this variationally accessible electronically excited state. The MAE for anharmonic frequencies for (T)+EOM/CcCR versus F12-TZ for HNF is 7.5 cm −1 . The closed-shell species are compared directly with the experiment, where a simpler (T)+EOM QFF using the aug-cc-pVTZ basis set compares more favorably than the more costly (T)+EOM/CcCR, suggesting a possible influence of decreasing accuracy with basis set size. Scans along internal coordinates are also provided which show reasonable modeling of the potential surface by (T)+EOM compared to benchmark QFFs computed for variationally accessible electronic states. The agreement between (T)+EOM/CcCR with F12-TZ and CcCR benchmarks is also shown to be quite accurate for rotational constants and geometries, with an MAE of 0.008 MHz for the rotational constants of (T)+EOM/CcCR versus CcCR for A ̃2A′ HOO and agreement within 0.003 Å for bond lengths.
Small, inorganic hydrides are likely hiding in plain sight, waiting to be detected toward various astronomical objects. AlH2OH can form in the gas phase via a downhill pathway, and the present, high-level quantum chemical study shows that this molecule exhibits bright infrared features for anharmonic fundamentals in regions above and below that associated with polycyclic aromatic hydrocarbons. AlH2OH along with HMgOH, HMgNH2, and AlH2NH2 are also polar with AlH2OH having a 1.22 D dipole moment. AlH2OH and likely HMgOH have nearly unhindered motion of the hydroxyl group but are still strongly bonded. This could assist in gas phase synthesis, where aluminum oxide and magnesium oxide minerals likely begin their formation stages with AlH2OH and HMgOH. This work provides the spectral data necessary to classify these molecules such that observations as to the buildup of nanoclusters from small molecules can possibly be confirmed.
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 à 2B1 and B̃ 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 B̃ 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.
Dipole-bound anions can be theoretically characterized at three fundamentally different levels. The highest are ab initio calculations, which themselves range from fairly approximate, say, Koopmans’s Theorem (KT) or second-order Møller-Plesset perturbation theory, to highly sophisticated, say, the electron affinity equation-of-motion couple-cluster with single, double, and perturbative triple substitutions, which rivals experiments in reliability. The next level down is represented by one-electron model Hamiltonians. Again, one-electron model Hamiltonians can be fairly approximate, especially if the molecular system is modeled by a simple point-dipole and point-polarizable site; however, very reliable models have been developed for specific systems, for example, water clusters. At the lowest level, one can qualitatively explain trends in classes of dipole-bound anions in terms of the dipole moment, μ, the polarizability, α, and the so-called excluded volume, Vx. This project aims at the qualitative level. While the dipole moment and the polarizability possess clear-cut definitions, the excluded volume must—similar to all molecular volumes—remain a rather vaguely defined term, and so far, we are unaware of any quantitative definition in the literature. Here, we introduce and investigate three descriptors for Vx. To this end, we first establish a dataset with consistent ab initio results for 25 amine N-oxides structures. Then, we demonstrate that the descriptors are indeed able to explain trends for sets of isomers and conformers and investigate to what extent the descriptors are able to predict electron binding energy of dipole-bound states using simple quantitative structure-property relationship-like models. It turns out that μ and Vx provide a reasonably accurate prediction of the electrostatic part of the electron bind energy (the KT value) and that the polarizability α provides an acceptable prediction of the electron correlation contribution.
High–level rovibrational characterization of methanediol, the simplest geminal diol, using state-of- the-art, purely ab initio techniques unequivocally confirms previously reported gas phase preparation of this simplest geminal diol in its...
Quartic force fields (QFFs) constructed using a sum of ground-state CCSD(T)-F12b energies with EOM-CCSD excitation energies are proposed for computation of spectroscopic properties of electronically excited states. This is dubbed the F12+EOM approach and is shown to provide similar accuracy to previous methodologies at lower computational cost. Using explicitly correlated F12 approaches instead of canonical CCSD(T), as in the corresponding (T)+EOM approach, allows for 70-fold improvement in computational time. The mean percent difference between the two methods for anharmonic vibrational frequencies is only 0.10%. A similar approach is also developed herein which accounts for core correlation and scalar relativistic effects, named F12cCR+EOM. The F12+EOM and F12cCR+EOM approaches both match to within 2.5% mean absolute error of experimental fundamental frequencies. These new methods should help in clarifying astronomical spectra by assigning features to vibronic and vibrational transitions of small astromolecules when such data are not available experimentally.
Octafluorooxalane, CFO, has recently attracted attention as a possible replacement of SF in high voltage insulation, and its reactivity with respect to free-electron attachment was investigated by mass spectrometry. The most intense signal peaks at 0.9 eV and corresponds to the parent anion, CFO; fragments stemming from complex breakup reactions are detected starting above ∼1.6 eV. Since parent anions in free-electron attachment are normally associated with threshold attachment or an embedding environment allowing excess energy deposition, this observation is highly unusual. Based on density functional calculations, it was nevertheless interpreted as attachment followed by intermolecular-vibrational-relaxation. Here, electron-attachment to octafluorooxalane is studied computationally. First, the electron affinity (EA) is characterized using density functionals and methods. Moreover, the negative vertical EA is estimated by extrapolating electron binding energies computed in the vicinity of CFO to the geometry of neutral octafluorooxalane. Then, alternative explanations for the 0.9 eV peak are considered. Specifically, a ring-opening reaction that yields a distonic isomer of CFO is identified. Our analysis reveals that the chain isomer possesses many conformers, all of which are considerably more stable than the ring isomer, and that the time scale for the unimolecular ring opening reaction is significantly faster than 1 s. Thus, at the experimental energy, the ring isomer of CFO is predicted to convert practically completely into the chain isomer, and we argue that the long lifetime and the peak position are effectively determined by the properties of the ring-opening transition state.
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