pbqff: Push-Button Quartic Force Fields.

pbqff is an open-source program for fully automating the production of quartic force fields (QFFs) and their corresponding anharmonic spectroscopic data. Rather than being a monolithic piece of code, it consists of several key modules including a generic interface to quantum chemistry codes and, notably, queuing systems; a molecular point group symmetry library; an internal-to-Cartesian coordinate conversion module; a module for the ordinary least-squares fitting of potential energy surfaces; and an improved second-order rotational and vibrational perturbation theory package for asymmetric and symmetric tops that handles type-1 and -2 Fermi resonances, Fermi resonance polyads, and Coriolis resonances. All of these pieces are written in Rust, a modern, safe, and performant programming language that has much to offer for scientific programming. This work introduces pbqff and its surrounding ecosystem, in addition to reporting new anharmonic vibrational data for c-(C)C3H2 and describing how the components of pbqff can be leveraged in other projects.

Red-Shifting the Excitation Energy of Carbonic Acid Clusters Via Nonminimum Structures.

Nonminimum carbonic acid clusters provide excitation energies and oscillator strengths in line with observed ice-phase UV absorptions better than traditional optimized minima. This equation-of-motion coupled cluster quantum chemical analysis on carbonic acid monomers and dimers shows that shifts to the dihedral angle for the internal heavy atoms in the monomer produce UV electronic excitations close to 200 nm with oscillator strengths that would produce observable features. This τ(OCOO) dihedral is actually a relatively floppy motion unlike what is often expected for sp2 carbons and can be distorted by 30° away from equilibrium for an energy cost of only 11 kcal/mol. As this dihedral decreases beyond 30°, the excitation energies decrease further. The oscillator strengths do, as well, but only to a point. Hence, the lower-energy distortions of τ(OCOO) are sufficient to produce structures that exhibit excitation energies and oscillator strengths that would red-shift observed spectra of carbonic acid ices away from the highest UV absorption feature at 139 nm. Such data imply that colder temperatures (20 K) in the experimental treatment of carbonic acid ices are freezing these structures out after annealing, whereas the warmer temperature experiments (80 K) are not.

Polycyclic aliphatic hydrocarbons: is tetrahedrane present in UIR spectra?

The smallest Platonic hydrocarbon, tetrahedrane, has been subject to frequent theoretical and experimental study for 50 years, but its infrared spectrum and synthetic pathway remain a mystery. The recent partial attribution of the ultraviolet extinction bump observed in the interstellar medium (ISM) of the Milky Way galaxy to hydrogenated T-carbon, a larger tetrahedral cluster formed from tetrahedrane and C4 monomers, has brought renewed interest to the molecule. Similarly, as a polycyclic hydrocarbon, tetrahedrane is similar in structure to the molecules proposed to be responsible for the so-called unidentified infrared bands (UIRs) observed in all kinds of astronomical environments. Furthermore, tetrahedrane's ν2 and ν7 fundamental vibrational frequencies, with values of 3210.6 cm-1 (3.11 µm) and 752.5 cm-1 (13.29 µm) as computed in the present quantum chemical study, have substantial intensities of 59 and 183 km mol-1, respectively. These come tantalizingly close to, but potentially distinct from, the 3.3 and 13.2 µm regions of the infrared spectrum typically included in the UIRs. As such, tetrahedrane or related clusters of these polycyclic aliphatic hydrocarbons may have a role to play in both of these sets of observations and could even help to explain the relation between them. Regardless, if tetrahedrane is present in the ISM, the highly-accurate theoretical data reported herein should help to aid in its identification and may assist in guiding future synthetic experiments as well.

Benzvalyne: Real or imaginary?

Benzvalyne (C6H4) is a bicyclic structural isomer of o-benzyne that some typically trusted levels of theory do not report as a minimum on the potential energy surface (PES). The structure was found to be a C2v minimum at the MCSCF, MP2, coupled-cluster single double, coupled-cluster single double triple (CCSDT)-1b, and CCSDT-2 levels of theory. Density functionals at the B3LYP-D3, B2PLYP-D3, and M06-D3 levels also produced a minimum structure. On the other hand, the CCSD(T), CCSD(T)-F12, and CCSDT-1a methods produced a single imaginary frequency for benzvalyne. However, the increase in the correlation for the CCSDT-1b and CCSDT-2 methods implies that benzvalyne is, in fact, a true, if highly strained, minimum on the PES. The C-C≡C bond angle was found to be only 108°; this angle is 180° for an unstrained C-C≡C triple bond moiety. As a result, the strain energy is notably high at 159 kcal mol-1. Comparing the strain energy of the rest of the molecule gives a strain energy of 92 kcal mol-1 for this triple bond region alone. The computed harmonic frequencies contain normal modes consisting of two hindered rotations of the C≡C diatomic part of the molecule, suggesting that the dissociation of this diatomic from the bicylobutane moiety may be important in the chemistry of this molecule. Because the putative C2v minimum is predicted to have a significant dipole moment (2.6 D), benzvalyne may be detectable in TMC-1, where the related o-benzyne molecule has recently been observed by radio astronomy.

Anharmonic Vibrational Frequencies of Water Borane and Associated Molecules.

Water borane (BH3OH2) and borinic acid (BH2OH) have been proposed as intermediates along the pathway of hydrogen generation from simple reactants: water and borane. However, the vibrational spectra for neither water borane nor borinic acid has been investigaged experimentally due to the difficulty of isolating them in the gas phase, making accurate quantum chemical predictions for such properties the most viable means of their determination. This work presents theoretical predictions of the full rotational and fundamental vibrational spectra of these two potentially application-rich molecules using quartic force fields at the CCSD(T)-F12b/cc-pCVTZ-F12 level with additional corrections included for the effects of scalar relativity. This computational scheme is further benchmarked against the available gas-phase experimental data for the related borane and HBO molecules. The differences are found to be within 3 cm-1 for the fundamental vibrational frequencies and as close as 15 MHz in the B0 and C0 principal rotational constants. Both BH2OH and BH3OH2 have multiple vibrational modes with intensities greater than 100 km mol-1, namely ν2 and ν4 in BH2OH, and ν1, ν3, ν4, ν9, and ν13 in BH3OH2. Finally, BH3OH2 has a large dipole moment of 4.24 D, which should enable it to be observable by rotational spectroscopy, as well.

F12-TZ-cCR: A Methodology for Faster and Still Highly Accurate Quartic Force Fields.

The F12-TZ-cCR quartic force field (QFF) methodology, defined here as CCSD(T)-F12b/cc-pCVTZ-F12 with further corrections for relativity, is introduced as a cheaper and even more accurate alternative to more costly composite QFF methods like those containing complete basis set extrapolations within canonical coupled cluster theory. F12-TZ-cCR QFFs produce B0 and C0 vibrationally averaged principal rotational constants within 7.5 MHz of gas-phase experimental values for tetraatomic and larger molecules, offering higher accuracy in these constants than the previous composite methods. In addition, F12-TZ-cCR offers an order of magnitude decrease in the computational cost of highly accurate QFF methodologies accompanying this increase in accuracy. An additional order of magnitude in cost reduction is achieved in the F12-DZ-cCR method, while also matching the accuracy of the traditional composite method's B0 and C0 constants. Finally, F12-DZ and F12-TZ are benchmarked on the same test set, revealing that both methods can provide anharmonic vibrational frequencies that are comparable in accuracy to all three of the more expensive methodologies, although their rotational constants lag behind. Hence, the present work demonstrates that highly accurate theoretical rovibrational spectral data can be obtained for a fraction of the cost of conventional QFF methodologies, extending the applicability of QFFs to larger molecules.

Fundamental Vibrational Frequencies and Spectroscopic Constants of Substituted Cyclopropenylidene (c-C3HX, X = F, Cl, CN).

The recent detection of ethynyl-functionalized cyclopropenylidene (c-C3HC2H) has initiated the search for other functional forms of cyclopropenylidene (c-C3H2) in space. There is existing gas-phase rotational spectroscopic data for cyano-cyclopropenylidene (c-C3HCN), but the present work provides the first anharmonic vibrational spectral data for that molecule, as well as the first full set of both rotational and vibrational spectroscopic data for fluoro- and chloro-cyclopropenylidenes (c-C3HF and c-C3HCl). All three molecules have fundamental vibrational frequencies with substantial infrared intensities. Namely, c-C3HCN has a moderately intense fundamental frequency at 1244.4 cm-1, while c-C3HF has two large intensity modes at 1765.4 and 1125.3 cm-1 and c-C3HCl again has two large intensity modes at 1692.0 and 1062.5 cm-1. All of these frequencies are well within the spectral range covered by the high-resolution EXES instrument on NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA). Further, all three molecules have dipole moments of around 3.0 D in line with c-C3H2, enabling them to be observed by pure rotational spectroscopy, as well. Thus, the rovibrational spectral data presented herein should assist with future laboratory studies of functionalized cyclopropenylidenes and may lead to their interstellar or circumstellar detection.

Anharmonic vibrational frequencies of ammonia borane (BH3NH3).

The fundamental vibrational frequency of the B-N stretch in BH3NH3 has eluded gas-phase experimental observation for decades. This work offers a theoretical anharmonic prediction of this mode to be 644 cm-1, using a Cartesian quartic force field at the CCSD(T)-F12/cc-pVTZ-F12 level of theory. The other fundamental frequencies reported herein have a mean absolute error of only 5 cm-1 from the seven available gas-phase experimental frequencies, making the anharmonic vibrational frequencies and rotational constants the most accurate computational data available for BH3NH3 to date. The inclusion of Fermi, Coriolis, and Darling-Dennison resonances is a major source of this accuracy, with the non-resonance-corrected frequencies having a mean absolute error of 10 cm-1. In particular, the inclusion of the 2ν6 = ν5 type 1 Fermi resonance increases the B-N stretching frequency by 14 cm-1 compared to previous work. Ammonia borane also represents one of the largest molecules ever studied by quartic force fields, making this work an important step in extending the breadth of application for these theoretical rovibrational techniques.

Highly-accurate quartic force fields for the prediction of anharmonic rotational constants and fundamental vibrational frequencies.

The CcCR quartic force field (QFF) methodology is capable of computing B0 and C0 rotational constants to within 35 MHz (0.14%) of experiment for triatomic and larger molecules with at least two heavy atoms. Additionally, the same constants for molecules with four or more atoms agree to within 20 MHz (0.12%) of experiment for the current test set. This work also supports previous claims that the same QFF methodology can produce fundamental vibrational frequencies with a deviation less than 5.7 cm-1 from experiment. Consequently, this approach of augmenting complete basis set extrapolated energies with treatments of core electron correlation and scalar relativity produces some of the most accurate rovibrational spectroscopic data available.

Overcoming the out-of-plane bending issue in an aromatic hydrocarbon: the anharmonic vibrational frequencies of c-(CH)C3H2.

The challenges associated with the out-of-plane bending problem in multiply-bonded hydrocarbon molecules can be mitigated in quartic force field analyses by varying the step size in the out-of-plane coordinates. Carbon is a highly prevalent element in astronomical and terrestrial environments, but this major piece of its spectra has eluded theoretical examinations for decades. Earlier explanations for this problem focused on method and basis set issues, while this work seeks to corroborate the recent diagnosis as a numerical instability problem related to the generation of the potential energy surface. Explicit anharmonic frequencies for c-(CH)C3H2+ are computed using a quartic force field and the CCSD(T)-F12b method with cc-pVDZ-F12, cc-pVTZ-F12, and aug-cc-pVTZ basis sets. The first of these is shown to offer accuracy comparable to that of the latter two with a substantial reduction in computational time. Additionally, c-(CH)C3H2+ is shown to have two fundamental frequencies at the onset of the interstellar unidentified infrared bands, at 5.134 and 6.088 µm or 1947.9 and 1642.6 cm-1, respectively. This suggests that the results in the present study should assist in the attribution of parts of these aromatic bands, as well as provide data in support of the laboratory or astronomical detection of c-(CH)C3H2+.

Anharmonic Frequencies of (MO)2 and Related Hydrides for M = Mg, Al, Si, P, S, Ca, and Ti and Heuristics for Predicting Anharmonic Corrections of Inorganic Oxides.

The low-frequency vibrational fundamentals of D2h inorganic oxides are readily modeled by heuristic scaling factors at fractions of the computational cost compared to explicit anharmonic frequency computations. Oxygen and the other elements in the present study are abundant in geochemical environments and have the potential to aggregate into minerals in planet-forming regions or in the remnants of supernovae. Explicit quartic force field computations at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory generate scaling factors that accurately predict the anharmonic frequencies with an average error of less than 1.0 cm-1 for both the metal-oxygen stretching frequencies and the torsion and antisymmetric stretching frequencies. Inclusion of hydrogen motions is less absolutely accurate but is similarly relatively predictive. The fundamental vibrational frequencies for the seven tetra-atomic inorganic oxides examined presently fall below 876 cm-1 and most of the hydrogenated species do as well. Additionally, ν6 for the SiO dimer is shown to have an intensity of 562 km mol-1, with each of the other molecules having one or more frequencies with intensities greater than 80 km mol-1, again with most in the low-frequency infrared range. These intensities and the frequencies computed in the present study should assist in laboratory characterization and potential interstellar or circumstellar observation.

Molecular oxygen generation from the reaction of water cations with oxygen atoms.

The oxywater cation (H2OO+), previously shown to form barrierlessly in the gas phase from water cations and atomic oxygen, is proposed here potentially to possess a 2A″ ←4A″ excitation leading to the H2⋯O2 + complex. This complex could then easily decompose into molecular hydrogen and the molecular oxygen cation. The present quantum chemical study shows that the necessary electronic transition takes place in the range of 1.92 eV (645 nm), in the orange-red range of the visible and solar spectrum, and dissociation of the complex only requires 5.8 kcal/mol (0.25 eV). Such a process for the abiotic, gas phase formation of O2 would only need to be photocatalyzed by visible wavelength photons. Hence, such a process could produce O2 at the mesosphere/stratosphere boundary as climate change is driving more water into the upper atmosphere, in the comet 67P/Churyumov-Gerasimenko where surprisingly high levels of O2 have been observed, or at gas-surface (ice) interfaces.

Binding of the atomic cations hydrogen through argon to water and hydrogen sulfide.

Water and hydrogen sulfide will bind with every atomic cation from the first three rows of the periodic table. While some atoms bind more tightly than others, explicitly correlated coupled cluster theory computations show that energy is required to be put into the system in order to dissociate these bonds even for noble gas atoms. The most promising systems have shallow entrance potential energy surfaces (PESs) that lie above deeper wells of a different spin. These wells are shown explicitly for H2OO+, H2SS+, and H2OS+ where relaxed PESs of the heavy atom bond lengths indicate that quartet states will cross more deeply-bound doublet states allowing for relatively easy association but much more difficult dissociation. In astrophysical regions that are cold and diffuse, such associations could lead to the formation of novel molecules utilizing water (or H2S) as the building blocks of more rich subsequent chemistry. Recent work has hypothesized that oxywater (H2OO) may be an intermediate in the formation of molecular oxygen in comets, and this work supports such a conclusion at least from a molecular cation perspective.