Search

Determination of equilibrium molecular structures is an essential ingredient in predicting spectroscopic parameters that help in identifying molecular carriers of microwave transitions. Here, the performance of two different ab initio composite approaches for obtaining equilibrium structures, "energy scheme" and "geometry scheme," is explored and compared to semi-experimental equilibrium structures. This study is performed for a set of 11 molecules which includes diatomics, linear triatomics, and a few non-linear molecules. The ab initio calculations were performed using three tiers of composite chemical recipes. The current results show that as the overall rigor of calculation is increased, the semi-experimental and the ab initio numbers agree to within 0.0003 Å for all molecules in the test set. The composite approach based on correcting the potential energy surface (energy scheme) and the one based on correcting the geometry directly (geometry scheme) show excellent agreement with each other. This work represents a step toward development of efficient and highly accurate procedures for computing ab initio equilibrium structures., (© 2024 Wiley Periodicals LLC.)

Unimolecular decay of the formaldehyde oxide (CH 2 OO) Criegee intermediate proceeds via a 1,3 ring-closure pathway to dioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected. Vibrational activation of jet-cooled CH 2 OO with two quanta of CH stretch (17-18 kcal mol -1 ) leads to unimolecular decay at an energy significantly below the transition state barrier of 19.46 ± 0.25 kcal mol -1 , refined utilizing a high-level electronic structure method HEAT-345(Q) Λ . The observed unimolecular decay rate of 1.6 ± 0.4 × 10 6 s -1 is 2 orders of magnitude slower than that predicted by statistical unimolecular reaction theory using several different models for quantum mechanical tunneling. The nonstatistical behavior originates from excitation of a CH stretch vibration that is orthogonal to the heavy atom motions along the reaction coordinate and slow intramolecular vibrational energy redistribution due to the sparse density of states.

Delocalization of the unpaired electron in π-conjugated radicals has profound implications for their chemistry, but direct and quantitative characterization of this electronic structure in isolated molecules remains challenging. We apply hyperfine-resolved microwave rotational spectroscopy to rigorously probe π-delocalization in propargyl, CH 2 CCH, a prototypical resonance-stabilized radical and key reactive intermediate. Using the spectroscopic constants derived from the high-resolution cavity Fourier transform microwave measurements of an exhaustive set of 13 C- and 2 H-substituted isotopologues, together with high-level ab initio calculations of zero-point vibrational effects, we derive its precise semiexperimental equilibrium geometry and quantitatively characterize the spatial distribution of its unpaired electron. Our results highlight the importance of considering both spin-polarization and orbital-following contributions when interpreting the isotropic hyperfine coupling constants of π radicals. These physical insights are strengthened by a parallel analysis of the isoelectronic species cyanomethyl, CH 2 CN, using new 13 C measurements also reported in this work. A detailed comparison of the structure and electronic properties of propargyl, cyanomethyl, and other closely related species allows us to correlate trends in their chemical bonding and electronic structure with critical changes in their reactivity and thermochemistry.

Semi-experimental structures (reSE) are derived from experimental ground state rotational constants combined with theoretical vibrational corrections. They permit a meaningful comparison with equilibrium structures based on high-level ab initio calculations. Typically, the vibrational corrections are evaluated with second-order vibrational perturbation theory (VPT2). The amount of error introduced by this approximation is generally thought to be small; however, it has not been thoroughly quantified. Herein, we assess the accuracy of theoretical vibrational corrections by extending the treatment to fourth order (VPT4) for a series of small linear molecules. Typical corrections to bond distances are on the order of 10-5 Å. Larger corrections, nearly 0.0002 Å, are obtained to the bond lengths of NCCN and CNCN. A borderline case is CCCO, which will likely require variational computations for a satisfactory answer. Treatment of vibrational effects beyond VPT2 will thus be important when one wishes to know bond distances confidently to four decimal places (10-4 Å). Certain molecules with shallow bending potentials, e.g., HOC+, are not amenable to a VPT2 description and are not improved by VPT4., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Atmospheric formic acid is severely underpredicted by models. A recent study proposed that this discrepancy can be resolved by abundant formic acid production from the reaction (1) between hydroxyl radical and methanediol derived from in-cloud formaldehyde processing and provided a chamber-experiment-derived rate constant, k 1 = 7.5 × 10 -12 cm 3 s -1 . High-level accuracy coupled cluster calculations in combination with E,J -resolved two-dimensional master equation analyses yield k 1 = (2.4 ± 0.5) × 10 -12 cm 3 s -1 for relevant atmospheric conditions ( T = 260-310 K and P = 0-1 atm). We attribute this significant discrepancy to HCOOH formation from other molecules in the chamber experiments. More importantly, we show that reversible aqueous processes result indirectly in the equilibration on a 10 min. time scale of the gas-phase reaction [Formula: see text] (2) with a HOCH 2 OH to HCHO ratio of only ca . 2%. Although HOCH 2 OH outgassing upon cloud evaporation typically increases this ratio by a factor of 1.5-5, as determined by numerical simulations, its in-cloud reprocessing is shown using a global model to strongly limit the gas-phase sink and the resulting production of formic acid. Based on the combined findings in this work, we derive a range of 1.2-8.5 Tg/y for the global HCOOH production from cloud-derived HOCH 2 OH reacting with OH. The best estimate, 3.3 Tg/y, is about 30 times less than recently reported. The theoretical equilibrium constant K eq (2) determined in this work also allows us to estimate the Henry's law constant of methanediol (8.1 × 10 5 M atm -1 at 280 K)., Competing Interests: Competing interests statement:The authors declare no competing interest.

The symmetric (ν s ) and antisymmetric (ν as ) O-C-O stretching modes of CO 2 -containing compounds encode structural information that can be difficult to decipher, due to the sensitivity of these spectral features to small shifts in charge distribution and structure, as well as the anharmonicities of these two vibrational modes. In this work, we discuss the relation between the frequency of these modes and the geometry of the O-C-O group, showing that the splitting between ν s and ν as (Δν as-s = ν as - ν s ) can be predicted based only on the O-C-O bond angle obtained from quantum chemical calculations with reasonable accuracy (±46 cm -1 , R 2 = 0.994). The relationship is shown to hold for the infrared spectra of a variety of CO 2 -containing molecules measured in vacuo. The origins of this model are discussed in the framework of elementary mode analysis.

Recent advances in gas-phase structure determination afford outstanding agreement between the CCSD(T)/cc-pCVTZ-corrected semi-experimental ( r e SE ) equilibrium structures and their corresponding best theoretical estimates (BTEs) of the equilibrium structures ( r e ) based upon corrections to the CCSD(T)/cc-pCV5Z geometries for the aromatic heterocycles pyrimidine and pyridazine. Herein, that same analysis is extended to the fundamental aromatic molecule benzene, using published experimental spectroscopic data for a total of 11 available isotopologues. The incorporation of rotational constants from all of these isotopologues and CCSD(T) corrections to address the impacts of both the vibration-rotation interaction and electron-mass distribution results in a highly precise and accurate r e SE structure. The CCSD(T)/cc-pCV5Z optimized geometry has been further corrected to address a finite basis set, untreated electron correlation, relativistic effects, and a breakdown of the Born-Oppenheimer approximation. This analysis achieves outstanding agreement between the r e (BTE) and r e SE structural parameters of benzene to a highly satisfying level (0.0001 Å), an agreement that surpasses our recently published structures of the aforementioned nitrogen-substituted benzene analogues. The D 6 h geometry of benzene is now known to an unprecedented precision: R C-C = 1.3913 (1) Å and R C-H = 1.0809 (1) Å. The mutual agreement between theory and experiment presented in this work validates both, substantially resolving all discrepancies between the r e SE and theoretical r e structures available in the literature.

Computational studies of small beryllium clusters (Be N ) predict dramatic, nonmonotonic changes in the bonding mechanisms and per-atom cohesion energies with increasing N . To date, experimental tests of these quantum chemistry models are lacking for all but the Be 2 molecule. In the present study, we report spectroscopic data for Be 3 and Be 4 obtained via anion photodetachment spectroscopy. The trimer is predicted to have D 3 h symmetric equilibrium structures for both the neutral molecule and the anion. Photodetachment spectra reveal transitions that originate from the X 2 A 2 ″ ground state and the 1 2 A 1 ' electronically excited state. The state symmetries were assigned on the basis of anisotropic photoelectron angular distributions. The neutral and anionic forms of Be 4 are predicted to be tetrahedral. Franck-Condon diagonal photodetachment was observed with a photoelectron angular distribution consistent with the expected Be 4 - X 2 A 1 → Be 4 X 1 A 1 transition. The electron affinities of Be 3 and Be 4 were determined to be 11363 ± 60 and 13052 ± 50 cm -1 , respectively.

The bond dissociation energy of methylidyne, D 0 (CH), is studied using an improved version of the High-Accuracy Extrapolated ab initio Thermochemistry (HEAT) approach as well as the Feller-Peterson-Dixon (FPD) model chemistry. These calculations, which include basis sets up to nonuple (aug-cc-pCV9Z) quality, are expected to be capable of providing results substantially more accurate than the ca. 1 kJ mol -1 level that is characteristic of standard high-accuracy protocols for computational thermochemistry. The calculated 0 K CH bond energy (27 954 ± 15 cm -1 for HEAT and 27 956 ± 15 cm -1 for FPD), along with equivalent treatments of the CH ionization energy and the CH + dissociation energy (85 829 ± 15 cm -1 and 32 946 ± 15 cm -1 , respectively), were compared to the existing benchmarks from Active Thermochemical Tables (ATcT), uncovering an unexpected difference for D 0 (CH). This has prompted a detailed reexamination of the provenance of the corresponding ATcT benchmark, allowing the discovery and subsequent correction of a systematic error present in several published high-level calculations, ultimately yielding an amended ATcT benchmark for D 0 (CH). Finally, the current theoretical results were added to the ATcT Thermochemical Network, producing refined ATcT estimates of 27 957.3 ± 6.0 cm -1 for D 0 (CH), 32 946.7 ± 0.6 cm -1 for D 0 (CH + ), and 85 831.0 ± 6.0 cm -1 for IE(CH).

The millimeter-wave rotational spectrum of ketene (H2C=C=O) has been collected and analyzed from 130 to 750 GHz, providing highly precise spectroscopic constants from a sextic, S-reduced Hamiltonian in the Ir representation. The chemical synthesis of deuteriated samples allowed spectroscopic measurements of five previously unstudied ketene isotopologues. Combined with previous work, these data provide a new, highly precise, and accurate semi-experimental (reSE) structure for ketene from 32 independent moments of inertia. This reSE structure was determined with the experimental rotational constants of each available isotopologue, together with computed vibration-rotation interaction and electron-mass distribution corrections from coupled-cluster calculations with single, double, and perturbative triple excitations [CCSD(T)/cc-pCVTZ]. The 2σ uncertainties of the reSE parameters are ≤0.0007 Å and 0.014° for the bond distances and angle, respectively. Only S-reduced spectroscopic constants were used in the structure determination due to a breakdown in the A-reduction of the Hamiltonian for the highly prolate ketene species. All four reSE structural parameters agree with the "best theoretical estimate" (BTE) values, which are derived from a high-level computed re structure [CCSD(T)/cc-pCV6Z] with corrections for the use of a finite basis set, the incomplete treatment of electron correlation, relativistic effects, and the diagonal Born-Oppenheimer breakdown. In each case, the computed value of the geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The discrepancies between the BTE structure and the reSE structure are 0.0003, 0.0000, and 0.0004 Å for rC-C, rC-H, and rC-O, respectively, and 0.009° for θC-C-H., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

While Coupled-Cluster methods have been proven to provide an accurate description of excited electronic states, the scaling of the computational costs with the system size limits the degree for which these methods can be applied. In this work different aspects of fragment-based approaches are studied on noncovalently bound molecular complexes with interacting chromophores of the fragments, such as π-stacked nucleobases. The interaction of the fragments is considered at two distinct steps. First, the states localized on the fragments are described in the presence of the other fragment(s); for this we test two approaches. One method is founded on QM/MM principles, only including the electrostatic interaction between the fragments in the electronic structure calculation with Pauli repulsion and dispersion effects added separately. The other model, a Projection-based Embedding (PbE) using the Huzinaga equation, includes both electrostatic and Pauli repulsion and only needs to be augmented by dispersion interactions. In both schemes the extended Effective Fragment Potential (EFP2) method of Gordon et al. was found to provide an adequate correction for the missing terms. In the second step, the interaction of the localized chromophores is modeled for a proper description of the excitonic coupling. Here the inclusion of purely electrostatic contributions appears to be sufficient: it is found that the Coulomb part of the coupling provides accurate splitting of the energies of interacting chromophores that are separated by more than 4 Å.

Absorption spectroscopy has long been known as a technique for making molecular concentration measurements and has received enhanced visibility in recent years with the advent of new techniques, like cavity ring-down spectroscopy, that have increased its sensitivity. To apply the method, it is necessary to have a known molecular absorption cross section for the species of interest, which typically is obtained by measurements of a standard sample of known concentration. However, this method fails if the species is highly reactive, and indirect means for attaining the cross section must be employed. The HO 2 and alkyl peroxy radicals are examples of reactive species for which absorption cross sections have been reported. This work explores and describes for these peroxy radicals the details of an alternative approach for obtaining these cross sections using quantum chemistry methods for the calculation of the transition dipole moment upon whose square the cross section depends. Likewise, details are given for obtaining the transition moment from the experimentally measured cross sections of individual rovibronic lines in the near-IR Ã-X̃ electronic spectrum of HO 2 and the peaks of the rotational contours in the corresponding electronic transitions for the alkyl (methyl, ethyl, and acetyl) peroxy radicals. In the case of the alkyl peroxy radicals, good agreement for the transition moments, ≈20%, is found between the two methods. However, rather surprisingly, the agreement is significantly poorer, ≈40%, for the HO 2 radical. Possible reasons for this disagreement are discussed.

Geminal diols are known to be important intermediates in atmospheric ozonolysis and the aerosol cycle. Recently, the simplest member of this class, methanediol, was interrogated in the gas phase with infrared spectroscopy. To aid in future spectroscopic investigations of methanediol, including in the interstellar medium, we report fundamental frequencies and rovibrational constants for the two rotamers of this molecule using ab initio composite methods along with vibrational perturbation theory. Sensitivity of the predictions to the level of theory and the treatment of anharmonic resonances are carefully assessed. The OH stretching harmonic frequencies of both rotamers are particularly sensitive to the level of theory. The CH stretches of the C s rotamer are sensitive to the treatment of anharmonic resonances with VPT2-based effective Hamiltonian models. Equilibrium bond distances and harmonic frequencies are converged conservatively to within 0.0005 Å and 3 cm -1 , respectively. The effect of tunneling on the rotational constants is investigated with a 2D variational calculation, based on a relaxed hydroxyl torsional potential energy surface. Tunneling is found to be negligible in the lower energy C 2 rotamer but should modify the rotational constants of the C s rotamer on the order of MHz, giving rise to rotational line splittings of the same order. The rovibrational constants of the C s rotamer are dominated by hydroxyl torsional effects, and here we see evidence for the breakdown of vibrational perturbation theory.

In this work, we present the spectral analysis of 1H- and 2H-1,2,3-triazole vibrationally excited states alongside provisional and practical computational predictions of the excited-state quartic centrifugal distortion constants. The low-energy fundamental vibrational states of 1H-1,2,3-triazole and five of its deuteriated isotopologues ([1- 2 H]-, [4- 2 H]-, [5- 2 H]-, [4,5- 2 H]-, and [1,4,5- 2 H]-1H-1,2,3-triazole), as well as those of 2H-1,2,3-triazole and five of its deuteriated isotopologues ([2- 2 H]-, [4- 2 H]-, [2,4- 2 H]-, [4,5- 2 H]-, and [2,4,5- 2 H]-2H-1,2,3-triazole), are studied using millimeter-wave spectroscopy in the 130-375 GHz frequency region. The normal and [2- 2 H]-isotopologues of 2H-1,2,3-triazole are also analyzed using high-resolution infrared spectroscopy, determining the precise energies of three of their low-energy fundamental states. The resulting spectroscopic constants for each of the vibrationally excited states are reported for the first time. Coupled-cluster vibration-rotation interaction constants are compared with each of their experimentally determined values, often showing agreement within 500 kHz. Newly available coupled-cluster predictions of the excited-state quartic centrifugal distortion constants based on fourth-order vibrational perturbation theory are benchmarked using a large number of the 1,2,3-triazole tautomer isotopologues and vibrationally excited states studied.

Rotational spectroscopy is the technique of choice for investigating molecular structures in the gas phase. Indeed, rotational constants are strongly connected to the geometry of the molecular system under consideration. Therefore, they are powerful tools for assessing the accuracy that quantum chemical approaches can reach in structural determinations. In this review article, it is shown how it is possible to measure the accuracy of a computed equilibrium geometry based on the comparison of rotational constants. But, it is also addressed what accuracy is required by computations for providing molecular structures and thus rotational constants that are useful to experiment. Quantum chemical methodologies for obtaining the "0.1% accuracy" for rotational constants are reviewed for systems ranging in size from small molecules to small polycyclic aromatic hydrocarbons. This accuracy for systems containing two dozen or so atoms opens the way towards future applications such as the accurate characterization of non-covalent interactions, which play a key role in several biological and technological processes.

The millimeter/submillimeter spectrum of 1 H -1,2,4-triazole is reported from 70 to 700 GHz, providing spectral frequencies directly comparable to radio telescopes and enabling an astronomical search. Using four deuteriated samples of 1,2,4-triazole, we measured, assigned, and least-squares fit transitions for 26 isotopologues to sextic A- and S-reduced Hamiltonians. An accurate and precise semi-experimental ( r e SE ) structure from 50 independent moments of inertia has been obtained. Structural parameters are provided with 2σ uncertainties within 0.0009 Å for bond distances and 0.09° for bond angles. The structural parameters are in quite good agreement with the best theoretical estimate (BTE) obtained using CCSD(T)/cc-pCV5Z, where an agreement within the 2σ uncertainty is observed for all but one case. Despite the large number of isotopologues already included in this structure, more may be useful. One isotopologue, [1,3- 2 H]-1 H -1,2,4-triazole, is observed to closely approach the oblate asymmetric-top limit, resulting in a clear breakdown of the A-reduction Hamiltonian. The highly accurate r e SE structure and subsequent analysis demonstrates that the S-reduction is also unable to adequately model the spectrum of this isotopologue.

High-level coupled cluster theory, in conjunction with Active Thermochemical Tables (ATcT) and E , J -resolved master equation calculations, was used in a study of the title reactions, which play an important role in the combustion of hydrocarbons. In the set of radical/radical reactions leading to soot formation in flames, the addition of H-atoms to alkenes is likely a common reaction, triggering the isomerization of complex hydrocarbons to aromatics. The heats of formation of C 2 H 3 , C 2 H 4 , and C 2 H 5 are established to be 301.26 ± 0.30 at 0 K (297.22 ± 0.30 at 298 K), 60.89 ± 0.11 (52.38 ± 0.11), and 131.38 ± 0.22 (120.63 ± 0.22) kJ mol -1 , respectively. The calculated rate constants from first principles agree well with experiments where they are available. Under conditions typical of high temperature combustion - where experimental work is very challenging with a consequent dearth of accurate data - we provide high-level theoretical results for kinetic modeling.

We present a thorough pure rotational investigation of the CH 2 CN radical in its ground vibrational state. Our measurements cover the millimeter and sub-millimeter wave spectral regions (79-860 GHz) using a W-band chirped-pulse instrument and a frequency multiplication chain-based spectrometer. The radical was produced in a flow cell at room temperature by H abstraction from acetonitrile using atomic fluorine. The newly recorded transitions of CH 2 CN (involving N ″ and K a ″ up to 42 and 8, respectively) were combined with the literature data, leading to a refinement of the spectroscopic parameters of the species using a Watson S-reduced Hamiltonian. In particular, the A rotational constant and K -dependent parameters are significantly better determined than in previous studies. The present model, which reproduces all experimental transitions to their experimental accuracy, allows for confident searches for the radical in cold to warm environments of the interstellar medium.

We provide compelling experimental and theoretical evidence for the transition state nature of the cyclopropyl cation. Synchrotron photoionization spectroscopy employing coincidence techniques together with a novel simulation based on high-accuracy ab initio calculations reveal that the cation is unstable via its allowed disrotatory ring-opening path. The ring strains of the cation and the radical are similar, but both ring opening paths for the radical are forbidden when the full electronic symmetries are considered. These findings are discussed in light of the early predictions by Longuet-Higgins alongside Woodward and Hoffman; we also propose a simple phase space explanation for the appearance of the cyclopropyl photoionization spectrum. The results of this work allow the refinement of the cyclopropane C-H bond dissociation energy, in addition to the cyclopropyl radical and cation cyclization energies, via the Active Thermochemical Tables approach.

Isotropic hyperfine properties have been obtained for the second row elements Al-Cl using a systematic composite approach consisting of a sequence of core/valence correlation consistent basis sets, up through aug-cc-pCV7Z, along with configuration interaction and coupled cluster methods. The best nonrelativistic final values for the atomic ground states (in MHz) are -1.80 27 Al ( 2 P o 1/2 ), -24.31 29 Si ( 3 P 0 ), 63.70 31 P ( 4 S o 3/2 ), 20.77 33 S ( 3 P 2 ), and 35.42 35 Cl ( 2 P o 3/2 ). We find a large K shell contribution to the spin density at the nucleus that is almost canceled by the L and M shell contributions. The spin density in atomic units is approximately linear with respect to the atomic number.

The 1H- and 2H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130-375 GHz frequency range, we measured the spectroscopic constants for thirteen 1H-1,2,3-triazole and sixteen 2H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium (r e SE ) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration-rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a "best theoretical estimate" (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1H tautomer are determined to <0.0008 Å and bond angles to <0.2°. For the 2H tautomer, bond angles are also determined to <0.2°, but bond distances are less precise (2σ ≤ 0.0015). Agreement between BTE and r e SE values is discussed.

High-resolution velocity-map imaged photoelectron spectra of the ethynyl anions C 2 H - and C 2 D - are measured at photon wavelengths between 355 and 266 nm to investigate the complex interactions between the closely lying X̃ 2 Σ + and à 2 Π electronic states. An indicative kinetic energy resolution of 0.4%, together with the full angular dependence of the fast electrons, provides a detailed description of the vibronically coupled structure. It is demonstrated that a modest quadratic vibronic coupling model, parameterized by the quasidiabatic ansatz, is sufficient to accurately recreate all the observed vibronic interactions. Simulated spectra are shown to be in excellent agreement with the experimental data, verifying the proposed model and providing a framework that may be used to accurately simulate spectra of larger C 2n H monohydride carbon chains. New spectral assignments are supported by experimental electron anisotropy measurements and Dyson orbital calculations.

Hydrazoic acid (HN 3 ) is used as a case study for investigating the accuracy and precision by which a molecular structure-specifically, a semi-experimental equilibrium structure (r e SE )-may be determined using current state-of-the-art methodology. The influence of the theoretical corrections for effects of vibration-rotation coupling and electron-mass distribution that are employed in the analysis is explored in detail. The small size of HN 3 allowed us to deploy considerable computational resources to probe the basis-set dependence of these corrections using a series of coupled-cluster single, double, perturbative triple [CCSD(T)] calculations with cc-pCVXZ (X = D, T, Q, 5) basis sets. We extrapolated the resulting corrections to the complete basis set (CBS) limit to obtain CCSD(T)/CBS corrections, which were used in a subsequent r e SE structure determination. The r e SE parameters obtained using the CCSD(T)/cc-pCV5Z corrections are nearly identical to those obtained using the CCSD(T)/CBS corrections, with uncertainties in the bond distances and angles of less than 0.0006 Å and 0.08°, respectively. The previously obtained r e SE structure using CCSD(T)/ANO2 agrees with that using CCSD(T)/cc-pCV5Z to within 0.000 08 Å and 0.016° for bond distances and angles, respectively, and with only 25% larger uncertainties, validating the idea that r e SE structure determinations can be carried out with significantly smaller basis sets than those needed for similarly accurate, strictly ab initio determinations. Although the purely computational r e structural parameters [CCSD(T)/cc-pCV6Z] fall outside of the statistical uncertainties (2σ) of the corresponding r e SE structural parameters, the discrepancy is rectified by applying corrections to address the theoretical limitations of the CCSD(T)/cc-pCV6Z geometry with respect to basis set, electron correlation, relativity, and the Born-Oppenheimer approximation, thereby supporting the contention that the semi-experimental approach is both an accurate and vastly more efficient method for structure determinations than is brute-force computation.

Magnesium tricarbide isomers are studied herein with coupled cluster theory and multireference configuration interaction to support their possible detection in astrochemical environments such as the circumstellar envelope surrounding the star IRC +10216 or in terrestrial laboratories. Magnesium-bearing species may abound in the interstellar medium (ISM), but only eight (MgNC, MgCN, HMgNC, MgC 2 H, MgC 3 N, MgC 4 H, MgC 5 N, and MgC 6 H) have been directly identified thus far. Several possible isomers for the related MgC 3 system are explored in their singlet and triplet spin multiplicities. Overall, this work offers quantum chemical insight of rovibrational spectroscopic data for MgC 3 using quartic force fields (QFFs) based on the CCSD(T) and CCSD(T)-F12 levels of theory at the complete basis set (CBS) limit. Additional corrections with small basis set CCSDT(Q) and scalar relativistic effects are also included in the analysis. Salient multireference character is found in the singlet diamond electronic state, which makes a definitive assignment of the ground state challenging. Nevertheless, coupled cluster-based composite energies and multireference configuration interaction both predict that the 1 A 1 diamond isomer is 1.6-2.2 kcal mol -1 lower in energy than the 3 A 1 diamond isomer. Furthermore, highly accurate binding energies of various isomers MgC 3 are provided for comparison to photodetachment experiments. Dipole moments along with harmonic infrared intensities will guide efforts for astronomical and spectroscopic characterization.

Benchmark quality isotropic hyperfine properties have been obtained for first row elements (B-F) using a systematic composite approach consisting of a sequence of core/valence correlation consistent basis sets, up through aug-cc-pCV8Z, along with configuration interaction and coupled cluster theory methods. The best nonrelativistic final values (in MHz) are 10.64 (B), 20.22 (C), 10.59 (N), -31.74 (O), and 318.30 (F) and are in very good agreement with available experimental values for these difficult-to-describe properties. Agreement is especially close in the case of N, which has the most accurate experimental value. The spin densities derived from the best composite level of theory were found to closely follow a simple quadratic scaling with the atomic number, Z. Observed convergence rates in the 1-particle and n-particle expansions obtained here may be useful in judging likely accuracy that can be expected in studies of molecular systems.

Polycyclic aromatic hydrocarbons (PAHs) are intermediates in the formation of soot particles and interstellar grains. However, their formation mechanisms in combustion and interstellar environments are not fully understood. The production of tricyclic PAHs and, in particular, the conversion of a PAH containing a five-membered ring to one with a six-membered ring are of interest to explain PAH abundances in combustion processes. In the present work, resonant ionization mass spectrometry in conjunction with isotopic labeling is used to investigate the formation of the phenalenyl radical from acenaphthylene and methane in an electrical discharge. We show that in this environment the CH cycloaddition mechanism converts a five-membered ring to a six-membered ring. This mechanism can occur in tandem with other PAH formation mechanisms such as hydrogen abstraction/acetylene addition (HACA) to produce larger PAHs in flames and the interstellar medium.

Transition state dynamics of bimolecular reactions can be probed by photodetachment of a precursor anion when the Franck-Condon region of the corresponding neutral potential energy surface is near a saddle point. In this study, photodetachment of anions at m / z = 49 enabled investigation of the exit channel of the OH + CH 3 OH → H 2 O + CH 3 O reaction using photoelectron-photofragment coincidence spectroscopy. High-level coupled-cluster calculations of the stationary points on the anion surface show that the methoxide-water cluster CH 3 O - (H 2 O) is the stable minimum on the anion surface. Photodetachment at a 3.20 eV photon energy leads to long-lived H 2 O(CH 3 O) complexes and H 2 O + CH 3 O products consistent with both direct dissociative photodetachment and resonance mediated processes on the neutral surface. The partitioning of total kinetic energy in the system indicates that water stretch and bend excitation is induced in dissociative photodetachment and evidence for long-lived complexes consistent with vibrational Feshbach resonances is reported.

The dicarbon molecule (C 2 ) is found in flames, comets, stars, and the diffuse interstellar medium. In comets, it is responsible for the green color of the coma, but it is not found in the tail. It has long been held to photodissociate in sunlight with a lifetime precluding observation in the tail, but the mechanism was not known. Here we directly observe photodissociation of C 2 From the speed of the recoiling carbon atoms, a bond dissociation energy of 602.804(29) kJ·mol[Formula: see text] is determined, with an uncertainty comparable to its more experimentally accessible N 2 and O 2 counterparts. The value is within 0.03 kJ·mol -1 of high-level quantum theory. This work shows that, to break the quadruple bond of C 2 using sunlight, the molecule must absorb two photons and undergo two "forbidden" transitions., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Community efforts in the computational molecular sciences (CMS) are evolving toward modular, open, and interoperable interfaces that work with existing community codes to provide more functionality and composability than could be achieved with a single program. The Quantum Chemistry Common Driver and Databases (QCDB) project provides such capability through an application programming interface (API) that facilitates interoperability across multiple quantum chemistry software packages. In tandem with the Molecular Sciences Software Institute and their Quantum Chemistry Archive ecosystem, the unique functionalities of several CMS programs are integrated, including CFOUR, GAMESS, NWChem, OpenMM, Psi4, Qcore, TeraChem, and Turbomole, to provide common computational functions, i.e., energy, gradient, and Hessian computations as well as molecular properties such as atomic charges and vibrational frequency analysis. Both standard users and power users benefit from adopting these APIs as they lower the language barrier of input styles and enable a standard layout of variables and data. These designs allow end-to-end interoperable programming of complex computations and provide best practices options by default.

Empirical, highly accurate non-relativistic electronic total atomization energies (eTAEs) are established by combining experimental or computationally converged treatments of the nuclear motion and relativistic contributions with the total atomization energies of HF, CO, N 2 , and H 2 O obtained from the Active Thermochemical Tables. These eTAEs, which have estimated (2σ) uncertainties of less than 10 cm -1 (0.12 kJ mol -1 ), form the basis for an analysis of high-level ab initio quantum chemical calculations that aim at reproducing these eTAEs for the title molecules. The results are then employed to analyze the performance of the high-accuracy extrapolated ab initio thermochemistry, or High-Accuracy Extrapolated Ab Initio Thermochemistry (HEAT), family of theoretical methods. The method known as HEAT-345(Q), in particular, is found to benefit from fortuitous error cancellation between its treatment of the zero-point energy, extrapolation errors in the Hartree-Fock and coupled cluster contributions, neglect of post-(T) core-correlation, and the basis-set error involved in higher-level correlation corrections. In addition to shedding light on a longstanding curiosity of the HEAT protocol-where the cheapest HEAT-345(Q) performs comparably to the theoretically more complete HEAT-456QP procedure-this study lays the foundation for extended HEAT variants that offer substantial improvements in accuracy relative to the established approaches.

A semi-experimental equilibrium structure ( r e SE ) of pyridazine ( o -C 4 H 4 N 2 ) has been determined using the rotational spectra of 18 isotopologues. Spectroscopic constants of four isotopologues are reported for the first time (measured from 235 to 360 GHz), while spectroscopic constants for previously reported isotopologues are improved by extending the frequency coverage (measured from 130 to 375 GHz). The experimental values of the ground-state rotational constants ( A 0 , B 0 , and C 0 ) from each isotopologue were converted to determinable constants ( A 0 ″ , B 0 ″ , and C 0 ″ ), which were then corrected for the effects of vibration-rotation interactions and electron-mass distributions using CCSD(T)/cc-pCVTZ calculations. The resultant r e SE for pyridazine determines bond distances to within 0.001 Å and bond angles within 0.04°, a reduction in the statistical uncertainties by at least a factor of two relative to the previously reported r e SE . The improvement in precision appears to be largely due to the use of higher-level theoretical calculations of the vibration-rotation and electron-mass effects, though the incorporation of the newly measured isotopologues ([4- 2 H, 4- 13 C]-, [4- 2 H, 5- 13 C]-, [4- 2 H, 6- 13 C]-, and [4,5- 2 H, 4- 13 C]-pyridazine) is partially responsible for the improved determination of the hydrogen-containing bond angles. The computed equilibrium structure ( r e ) (CCSD(T)/cc-pCV5Z) and a "best theoretical estimate" of the equilibrium structure ( r e ) both agree with the updated r e SE structure within the statistical experimental uncertainty (2σ) of each structural parameter.

The marked anharmonicity of the CC stretching mode in the ethynyl radical (C 2 H) is investigated in terms of a vibronic coupling model. It is demonstrated that the large difference between the harmonic frequency and the fundamental level (about 5-10 times that for triple-bond stretches in the related species HCCH, HCN, HNC and CN) can be attributed to the well-known vibronic interaction between the X̃ 2 Σ + and à 2 Π states of CCH. Although the mode has σ symmetry and it is the perturbations of π symmetry that mix the two electronic states, a combination of large intrinsic coupling strength, modest energy gap, and-most importantly-the strong tuning of the gap energy by the CC stretch mode leads to a profound vibronic influence on parts of the anharmonic force field that sample the CC stretch. Finally, calculations of the force field for the X̃ 2 Σ + state with different flavors of coupled-cluster (CC) theory provide insight and underscore an intrinsic advantage of equation-of-motion CC (EOM-CC) methods.

The thermal unimolecular decomposition of a methoxy radical (CH 3 O), a key intermediate in the combustion of methane, methanol, and other hydrocarbons, was studied using high-level coupled-cluster calculations, followed by E , J -resolved master equation analyses. The experimental results available for a wide range of temperature and pressure are in striking agreement with the calculations. In line with a previous theoretical study that used a one-dimensional master equation, the tunneling correction is found to exhibit a marked pressure dependence, being the largest at low pressure. This curious effect on the tunneling enhancement also affects the calculated kinetic isotope effect, which falls initially with pressure but is predicted to rise again at high pressures. These findings serve to reconcile a set of conflicting results regarding the importance of tunneling in this prototype unimolecular reaction and also motivate further experimental investigation. This study also exemplifies how changes in the energy redistribution due to collisions manifest in the tunneling rates.

The pure rotational spectrum of thiazole (c-C 3 H 3 NS, C s ) has been studied in the millimeter-wave region from 130 to 375 GHz. Nearly 4800 newly measured rotational transitions for the ground vibrational state of the main isotopologue were combined with previously reported measurements and least-squares fit to a complete sextic Hamiltonian. Transitions for six singly substituted heavy-atom isotopologues ( 13 C, 15 N, 33 S, 34 S) were observed at natural abundance and likewise fit. Several deuterium-enriched samples were prepared, which gave access to the rotational spectra of 16 additional isotopologues, 14 of which had not been previously studied. The rotational spectra of each isotopologue were fit to A- and S-reduced distorted-rotor Hamiltonians in the I r representation. The experimental values of the ground-state rotational constants (A 0 , B 0 , and C 0 ) from each isotopologue were converted to determinable constants (A 0 ″, B 0 ″, and C 0 ″), which were corrected for effects of vibration-rotation interactions and electron-mass distributions using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The moments of inertia from the resulting constants (A e , B e , and C e ) of 24 isotopologues were used to determine the precise semi-experimental equilibrium structure (r e SE ) of thiazole. As a basis for comparison, a purely theoretical equilibrium structure was estimated by an electronic structure calculation [CCSD(T)/cc-pCV5Z] that was subsequently corrected for extrapolation to the complete basis set, electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The precise r e SE structure is compared to the resulting "best theoretical estimate" structure. Some, but not all, of the best theoretical r e structural parameters fall within the narrow statistical limits (2σ) of the r e SE results. The possible origin of the discrepancies between the best theoretical estimate r e and semi-empirical r e SE structures is discussed.

The rotational spectrum of thiophene (c-C 4 H 4 S) has been collected between 8 and 360 GHz. Samples of varying deuterium-enrichment were synthesized to yield all possible deuterium-substituted isotopologues of thiophene. A total of 26 isotopologues have been measured and least-squares fit using A- and S-reduced distorted-rotor Hamiltonians in the I r representation. The resultant rotational constants (A 0 , B 0 , and C 0 ) from each reduction were converted to determinable constants (A″, B″, and C″) to remove the impact of centrifugal distortion. The computed vibrational and electron mass corrections [CCSD(T)/cc-pCVTZ] were applied to the determinable constants to obtain semi-experimental equilibrium rotational constants (A e , B e , and C e ) for 24 isotopologues. A precise semi-experimental equilibrium (r e SE ) structure has been achieved from a least-squares fit of the equilibrium moments of inertia. The combination of the expanded isotopologue rotational data with high-level computational work establishes a precise r e SE structure for this sulfur-containing heterocycle. The CCSD(T)/cc-pCV5Z structure has been obtained and corrected for the extrapolation to the complete basis set, electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The precise r e SE structure is compared to the resulting "best theoretical estimate" structure. Several of the best theoretical r e structural parameters fall within the narrow statistical limits (2σ) of the r e SE results. The possible origin of the discrepancies for the computed parameters that fall outside the statistical uncertainties is discussed.

Investigations into bimolecular reaction kinetics probe the details of the underlying potential energy surface (PES), which can help to validate high-level quantum chemical calculations. We utilize a combined linear Paul ion trap with a time-of-flight mass spectrometer to study isotopologue reactions between acetylene cations (C 2 H 2 + ) and two isomers of C 3 H 4 : propyne (HC 3 H 3 ) and allene (H 2 C 3 H 2 ). In a previous study [Schmid et al., Phys. Chem. Chem. Phys. 22, 20303 (2020)], 1 we showed that the two isomers of C 3 H 4 have fundamentally different reaction mechanisms. Here, we further explore the calculated PES by isotope substitution. While isotopic substitution of reactants is a standard experimental tool in the investigation of molecular reaction kinetics, the controlled environment of co-trapped, laser-cooled Ca + ions allows the different isotopic reaction pathways to be followed in greater detail. We report branching ratios for all of the primary products of the different isotopic species. The results validate the previously proposed mechanism: propyne forms a bound reaction complex with C 2 H 2 + , while allene and C 2 H 2 + perform long-range charge exchange only.

We report a production-level implementation of the equation-of-motion (EOM) coupled-cluster (CC) method with double electron-attaching (DEA) EOM operators of 2p and 3p1h types, EOM-DEA-CCSD. This ansatz, suitable for treating electronic structure patterns that can be described as two-electrons-in-many orbitals, represents a useful addition to the EOM-CC family of methods. We analyze the performance of EOM-DEA-CCSD for energy differences and molecular properties. By considering reduced quantities, such as state and transition one-particle density matrices, we compare EOM-DEA-CCSD wave functions with wave functions computed by other EOM-CCSD methods. The benchmarks illustrate that EOM-DEA-CCSD is capable of treating diradicals, bond-breaking, and some types of conical intersections.

This article primarily discusses the utility of vibrational perturbation theory for the prediction of X-H stretching vibrations with particular focus on the specific variant, second-order vibrational perturbation theory with resonances (VPT2+K). It is written as a tutorial, reprinting most important formulas and providing numerous simple examples. It discusses the philosophy and practical considerations behind vibrational simulations with VPT2+K, including but not limited to computational method selection, cost-saving approximations, approaches to evaluating intensity, resonance identification, and effective Hamiltonian structure. Particular attention is given to resonance treatments, beginning with simple Fermi dyads and gradually progressing to arbitrarily large polyads that describe both Fermi and Darling-Dennison resonances. VPT2+K combined with large effective Hamiltonians is shown to be a reliable framework for modeling the complicated CH stretching spectra of alkenes. An error is also corrected in the published analytic formula for the VPT2 transition moment between the vibrational ground state and triply excited states.

One of the fundamental goals of chemistry is to determine how molecular structure influences interactions and leads to different reaction products. Studies of isomer-selected and resolved chemical reactions can shed light directly on how form leads to function. In the following, we present the results of gas-phase reactions between acetylene cations (C2D2+) with two different isomers of C3H4: propyne (DC3D3) and allene (H2C3H2). Our highly controlled, trapped-ion environment allows for precise determination of reaction products and kinetics. From these results, we can infer details of the underlying reaction dynamics of C2H2+ + C3H4. Through the synergy of experimental results and high-level quantum chemical potential energy surface calculations, we are able to identify distinct reaction mechanisms for the two isomers. We find long-range charge exchange with no complex formation is favored for allene, whereas charge exchange leads to an intermediate reaction complex for propyne and thus, different products. Therefore, this reaction displays a pronounced isomer-selective bi-molecular reactive process.

Using chirped and cavity microwave spectroscopies, automated double resonance, new high-speed fitting and deep learning algorithms, and large databases of computed structures, the discharge products of benzene alone, or in combination with molecular oxygen or nitrogen, have been exhaustively characterized between 6.5 and 26 GHz. In total, more than 3300 spectral features were observed; 89% of these, accounting for 97% of the total intensity, have now been assigned to 152 distinct chemical species and 60 of their variants (i.e., isotopic species and vibrationally excited states). Roughly 50 of the products are entirely new or poorly characterized at high resolution, including many heavier by mass than the precursor benzene. These findings provide direct evidence for a rich architecture of two- and three-dimensional carbon and indicate that benzene growth, particularly the formation of ring-chain molecules, occurs facilely under our experimental conditions. The present analysis also illustrates the utility of microwave spectroscopy as a precision tool for complex mixture analysis, irrespective of whether the rotational spectrum of a product species is known a priori or not. From this large quantity of data, for example, it is possible to determine with confidence the relative abundances of different product masses, but more importantly the relative abundances of different isomers with the same mass. The complementary nature of this type of analysis to traditional mass spectrometry is discussed.

Exchange coupling constants (J) are fundamental to the understanding of spin spectra of magnetic systems. Here, we investigate the broken-symmetry (BS) approaches of Noodleman and Yamaguchi in conjunction with coupled cluster (CC) methods to obtain exchange couplings. J values calculated from CC in this fashion converge smoothly toward the full configuration interaction result with increasing level of CC excitation. We compare this BS-CC scheme to the complementary equation-of-motion CC approach on a selection of bridged molecular cases and give results from a few other methodologies for context.

The 3-ethynylcyclopentenyl radical (3ecpr) has been identified as the carrier of an electronic spectrum with origin at 21792 cm -1 using resonant ionization and laser-induced fluorescence spectroscopies. The radical was first detected in a toluene discharge and is most efficiently produced from 1,6-heptadiyne. Overwhelming spectroscopic and chemical evidence support our diagnosis: (1) the observed (6.93 eV) and calculated (CCSD(T)/pVQZ) adiabatic ionization energies are the same; (2) the origin band rotational contour can be well simulated with calculated rotational constants; (3) convincing vibrational assignments can be made using computed frequencies; and (4) the same spectrum was observed in a discharge of 1-ethynylcyclopentanol, which contains the 3ecpr carbon framework. The π-chromophore is essentially that of trans -1-vinylpropargyl, a highly resonance-stabilized C 5 H 5 radical that persists in conditions relevant to both combustion and circumstellar atmospheres. We suggest that 3ecpr may be a similarly important radical warranting inclusion in models of C 7 H 7 chemistry. It is the second C 7 H 7 isomer with a five-membered ring yet to be detected, the other being vinylcyclopentadienyl, a species crucially involved in a recently proposed mechanism of soot formation ( Science , 2018, 361, 6406, 997-1000). We argue that 3ecpr should be a significant product of H addition to ethynylcyclopentadiene (C 7 H 6 ), a known product of benzyl decomposition. Further, it is plausible that 3ecpr is the unidentified C 7 H 7 product of sequential addition of acetylene to propargyl ( J. Phys. Chem. Lett. , 2015, 6, 20, 4153-4158) in which 1-vinylpropargyl is an intermediate. As such, the n C 2 H 2  + C 3 H 3 cascade could represent a facile synthesis of a substituted five-membered ring in flames and stellar outflows.

An up-to-date overview of the CFOUR program system is given. After providing a brief outline of the evolution of the program since its inception in 1989, a comprehensive presentation is given of its well-known capabilities for high-level coupled-cluster theory and its application to molecular properties. Subsequent to this generally well-known background information, much of the remaining content focuses on lesser-known capabilities of CFOUR, most of which have become available to the public only recently or will become available in the near future. Each of these new features is illustrated by a representative example, with additional discussion targeted to educating users as to classes of applications that are now enabled by these capabilities. Finally, some speculation about future directions is given, and the mode of distribution and support for CFOUR are outlined.

Despite decades of investigation, mechanistic details of aqueous permanganate photo-decomposition remain unclear. Here we follow photoinduced dynamics of aqueous permanganate with femtosecond spectroscopy. Photoexcitation of KMnO 4 (aq) in the visible unleashes a sub-picosecond cascade of non-radiative transitions, leading to a distinct species which relaxes to S 0 with a lifetime of 16 ps. Tuning excitation to the UV shows increasing formation of a metastable intermediate, which outlives our ∼1 ns window of detection. Guided by electronic structure calculations and observations from three pulse excitation experiments, we assign the 16 ps species as the lowest Jahn-Teller component of the 3 T 1 triplet state and suggest a plausible sequence of radiationless transitions, which rapidly populate it. In conjunction with photodecomposition quantum yields obtained from the literature, these results demonstrate that aqueous permanganate photo-decomposition proceeds through a long-lived intermediate which is formed in parallel to the triplet in less than one ps upon UV absorption. The possibility that this is the postulated highly oxidative peroxo species, a fraction of which leads to the stable (MnO 2 - + O 2 ) fragments, is discussed. Finally, periodic modulations detected in the pump-probe signal are assigned to ground-state vibrational coherences excited by impulsive Raman. Their wavelength-dependent absolute phases outline the borders between adjacent electronic transitions in the linear spectrum of permanganate.