Zero-point Vibrational Energy Corrections Research Articles

Quantum chemical pathways for the formation of 2,3,7,8-tetrachloro dibenzo-p-dioxin (TCDD) from 2,4,5-trichlorophenol: a mechanistic and thermo-kinetic study.

Dioxins, specifically 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD), are highly toxic dioxins known for their severe health impacts and persistent environmental pollutants. This study focuses on understanding the formation pathways of TCDD from its precursor molecule 2,4,5-trichlorophenol (2,4,5-TCP). In our exploration of reaction pathways from 2,4,5-trichlorophenol (TCP), we delve into three reaction mechanisms: free-radical, direct condensation, and anionic. Our findings highlight the significance of the radical mechanism, particularly propagated by H radicals, with a notable increase in dioxin formation around 900K. These results are consistent with experimental observations indicating an increase in the conversion of trichlorophenol from 600 to 900K in the non-catalytic gas phase reaction. Thermodynamic parameters (∆H, ∆S, and ∆G), reaction barriers, and rate constants (k) were calculated across a temperature range of 300-1200K to support the findings and provide insights into the optimal temperature range for controlling dioxins during the incineration process. In this study, quantum chemical calculations were conducted using density functional theory (DFT) with the B3LYP functional and the 6-311 + + G(d,p) basis set in Gaussian 16 software. Stationary points, including transition states (TS), were confirmed with frequency calculations. Intrinsic reaction coordinate (IRC) calculations ensured minimum energy paths between TS and products, visualized in GaussView 6.0 Program. Single-point energy calculations utilized a more precise basis set, 6-311 + + G(3df,2p), for enhanced energy accuracy, incorporating zero-point vibrational energy (ZPE) and other energy corrections. These calculations were repeated over a temperature range of 298.15-1200K at 1atm pressure. Finally, rate constant (k) expressions associated with TCDD formation were determined using transition state theory (TST).

Anharmonic rotational and vibrational spectroscopic constants of NH2CH2OH

The recent experimental identification of aminomethanol (NH2CH2OH) and its synthesis under simulated interstellar conditions suggests that it may be formed, and thus observable, in the interstellar medium. To aid in its possible detection, this work presents high-level theoretical, anharmonic vibrational and rotational spectroscopic data for the three lowest-energy conformers of NH2CH2OH. All three conformers fall within 0.80 kcal mol−1 of each other at the CCSD(T)-F12b/cc-pCVTZ-F12 level, with an anharmonic zero-point vibrational energy correction. Additionally, all three conformers are shown to have multiple vibrational frequencies with intensities greater than 100 km mol−1. The most notable of these are the symmetric O-C-N stretch of the lower-energy C1 conformer at 976.4 cm−1, the C-O stretch of the Cs conformer at 964.9 cm−1, and the antisymmetric O-C-N stretch of the higher-energy C1 conformer at 1099.0 cm−1. While much of the vibrational spectra of the three overlap, these features should help to separate them. Finally, all three conformers have non-zero dipole moments on the order of 1 D, suggesting that they may be observable by rotational spectroscopy, as well. In all cases, the computational data provided herein will help to facilitate future experimental and observational studies on this key molecule in the formation of extraterrestrial amino acids.

Study on the Mechanism of Lipid Peroxidation Induced by Carbonate Radicals.

Based on the reported research, hydroxyl radicals can be rapidly transformed into carbonate radicals in the carbonate-bicarbonate buffering system in vivo. Many of the processes considered to be initiated by hydroxyl radicals may be caused by carbonate radicals, which indicates that lipid peroxidation initiated by hydroxyl radicals can also be caused by carbonate radicals. To date, theoretical research on reactions of hydrogen abstraction from and radical addition to polyunsaturated fatty acids (PUFAs) of carbonate radicals has not been carried out systematically. This paper employs (3Z,6Z)-nona-3,6-diene (NDE) as a model for polyunsaturated fatty acids (PUFAs). Density functional theory (DFT) with the CAM-B3LYP method at the 6-311+g(d,p) level was used to calculate the differences in reactivity of carbonate radicals abstracting hydrogen from different positions of NDE and their addition to the double bonds of NDE under lipid solvent conditions with a dielectric constant of 4.0 (CPCM model). Grimme's empirical dispersion correction was taken into account through the D3 scheme. The energy barrier, reaction rate constants, internal energy, enthalpy and Gibbs free energy changes in these reactions were calculated With zero-point vibrational energy (ZPVE) corrections. The results indicated that carbonate radicals initiate lipid peroxidation primarily through hydrogen abstraction from diallyl carbon atoms. The reaction of hydrogen abstraction from diallyl carbon atoms exhibits the highest reaction rate, with a reaction rate constant approximately 43-fold greater than the second-ranked hydrogen abstraction from allyl carbon atoms. This process has the lowest energy barrier, internal energy, enthalpy, and Gibbs free energy changes, indicating that it is also the most spontaneous process.

Exploring the Tl H potential energy surface: A comparative analysis with group 13 systems and experiment.

Thallium chemistry is experiencing unprecedented importance. Therefore, it is valuable to characterize some of the simplest thallium compounds. Stationary points along the singlet and triplet Tl H potential energy surface have been characterized. Stationary point geometries were optimized with the CCSD(T)/aug-cc-pwCVQZ-PP method. Harmonic vibrational frequencies were computed at the same level of theory while anharmonic vibrational frequencies were computed at the CCSD(T)/aug-cc-pwCVTZ-PP level of theory. Final energetics were obtained with the CCSDT(Q) method. Basis sets up to augmented quintuple-zeta cardinality (aug-cc-pwCV5Z-PP) were employed to obtain energetics in order to extrapolate to the complete basis set limits using the focal point approach. Zero-point vibrational energy corrections were appended to the extrapolated energies in order to determine relative energies at 0 K. It was found that the planar dibridged isomer lies lowest in energy while the linear structure lies highest in energy. The results were compared to other group 13 M H (M = B, Al, Ga, In, and Tl) theoretical studies and some interesting variations are found. With respect to experiment, incompatibilities exist.

Electron-Phonon Interaction Contribution to the Total Energy of Group IV Semiconductor Polymorphs: Evaluation and Implications.

In density functional theory (DFT)-based total energy studies, the van der Waals (vdW) and zero-point vibrational energy (ZPVE) correction terms are included to obtain energy differences between polymorphs. We propose and compute a new correction term to the total energy, due to electron-phonon interactions (EPI). We rely on Allen's general formalism, which goes beyond the quasi-harmonic approximation (QHA), to include the free energy contributions due to quasiparticle interactions. We show that, for semiconductors and insulators, the EPI contributions to the free energies of electrons and phonons are the corresponding zero-point energy contributions. Using an approximate version of Allen's formalism in combination with the Allen-Heine theory for EPI corrections, we calculate the zero-point EPI corrections to the total energy for cubic and hexagonal polytypes of carbon, silicon and silicon carbide. The EPI corrections alter the energy differences between polytypes. In SiC polytypes, the EPI correction term is more sensitive to crystal structure than the vdW and ZPVE terms and is thus essential in determining their energy differences. It clearly establishes that the cubic SiC-3C is metastable and hexagonal SiC-4H is the stable polytype. Our results are consistent with the experimental results of Kleykamp. Our study enables the inclusion of EPI corrections as a separate term in the free energy expression. This opens the way to go beyond the QHA by including the contribution of EPI on all thermodynamic properties.

Solvent Effects in the Regioselective N-Functionalization of Tautomerizable Heterocycles Catalyzed by Methyl Trifluoromethanesulfonate: A Density Functional Theory Study with Implicit Solvent Model

Methyl trifluoromethanesulfonate was found to catalyze the reaction of the nucleophilic substitution of the hydroxyl group of alcohols by N-heterocycles followed by X- to N- alkyl group migration (X = O, S) to obtain N-functionalized benzoxazolone, benzothiazolethione, indoline, benzoimidazolethione and pyridinone derivatives. A high degree of solvent dependency on the yield of the products was observed during optimization of the reaction parameters. The yield of the product was found to be 0%, 48% and 70% in acetonitrile, 1,2-dichloroethane and chloroform, respectively. The mechanism of the reaction was established through experiments as well as DFT calculations. The functional B3LYP and 6-311++G(d) basis function sets were used to optimize the molecular geometries. D3 Grimme empiric dispersion with Becke–Johnson dumping was employed, and harmonic vibrational frequencies were calculated to characterize the stationary points on the potential energy surface. To ensure that all the stationary points were smoothly connected to each other, intrinsic reaction coordinate (IRC) analyses were performed. The influence of solvents was considered using the solvation model based on density (SMD). The free energy profiles of the mechanisms were obtained with vibrational unscaled zero-point vibrational energy (ZPE), thermal, enthalpy, entropic and solvent corrections.

Choice of functional for iron porphyrin density functional theory studies: Geometry, spin-state, and binding energy analysis

Simultaneously predicting the geometries, spin states and binding energies of iron porphyrins using DFT methods is challenging, as one method and basis set could lead to the accurate prediction of one aspect but imprecisions on the others. In this work, six functionals and three basis sets were tested based on their description of the geometries, spin states and binding energies of an heme model. An iron porphyrin ring with an axial imidazole (FePIm) was used as the model molecule. Optimized geometries of FePIm and dioxygen-coordinated FePIm-O2 were obtained in their singlet, triplet, and quintet spin states. Energy calculations with zero-point vibrational energy (ZPVE) corrections were performed on the optimized geometries, at the same level of theory used on the geometry optimization. For the oxygen-bonded FePIm-O2, the singlet spin states were calculated to be closed-shell for all the methods, even if an unrestricted wavefunction and broken spin symmetry were imposed on the system. Consequently, the open-shell singlets were obtained using the wavefunction of their corresponding triplet states as the initial guess for the geometry optimizations and energy calculations. The functional B97D with the basis sets A (def2-TZVP on Fe, def2-SVP on other atoms) and C (6–311+G(2df) on Fe atom; 6–311+G(d) on N and O atoms; 6–31G on C and H atoms) was able to produce an accurate geometry, the correct spin states for FePIm (Quintet) and FePIm-O2 (open-shell singlet), and a binding energies of 13.51–13.07 kcal·mol−1, in good agreement to the experimental values. The basis set B resulted in good FePIm geometrical parameters with all the studied functionals, but led to significant errors in the FePIm-O2 geometries and binding energies. For the study of FePIm and similar molecules were the change of spin play a key role, the selection of constraints and initial guess are very important, as the methods could yield multiplicites which do not agree with experimental observations. This study provides valuable information for the selection of a suitable method for DFT studies on small iron porphyrins and their interaction with oxygen, based on the required accuracy for geometry, spin, or binding energies.

Threshold photoelectron spectroscopy of 9-methyladenine: theory and experiment.

We present a combined experimental and theoretical study of single-photon ionization of 9-methyladenine (9MA) in the gas phase. In addition to tautomerism, several rotamers due to the rotation of the methyl group may exist. Computations show, however, that solely one rotamer contributes because of low population in the molecular beam and/or unfavorable Franck-Condon factors upon ionization. Experimentally, we used VUV radiation available at the DESIRS beamline of the synchrotron radiation facility SOLEIL to record the threshold photoelectron spectrum of this molecule between 8 and 11 eV. This spectrum consists of a well-resolved band assigned mainly to vibronic levels of the D0 cationic state, plus a contribution from the D1 state, and two large bands corresponding to the D1, D2 and D3 electronically excited states. The adiabatic ionization energy of 9MA is measured at 8.097 ± 0.005 eV in close agreement with the computed value using the explicitly correlated coupled cluster approach including core valence, scalar relativistic and zero-point vibrational energy corrections. This work sheds light on the complex pattern of the lowest doublet electronic states of 9MA+. The comparison to canonical adenine reveals that methylation induces further electronic structure complication that may be important to understand the effects of ionizing radiation and the charge distribution in these biological entities at different time scales.

Structural and Vibrational Properties of Amino Acids from Composite Schemes and Double-Hybrid DFT: Hydrogen Bonding in Serine as a Test Case

The structures, relative stabilities, and vibrational wavenumbers of the two most stable conformers of serine, stabilized by the O-H···N, O-H···O═C and N-H···O-H intramolecular hydrogen bonds, have been evaluated by means of state-of-the-art composite schemes based on coupled-cluster (CC) theory. The so-called "cheap" composite approach (CCSD(T)/(CBS+CV)MP2) allowed determination of accurate equilibrium structures and harmonic vibrational wavenumbers, also pointing out significant corrections beyond the CCSD(T)/cc-pVTZ level. These accurate results stand as a reference for benchmarking selected hybrid and double-hybrid, dispersion-corrected DFT functionals. B2PLYP-D3 and DSDPBEP86 in conjunction with a triple-ζ basis set have been confirmed as effective methodologies for structural and spectroscopic studies of medium-sized flexible biomolecules, also showing intramolecular hydrogen bonding. These best performing double-hybrid functionals have been employed to simulate IR spectra by means of vibrational perturbation theory, also considering hybrid CC/DFT schemes. The best overall agreement with experiment, with mean absolute error of 8 cm-1, has been obtained by combining CCSD(T)/(CBS+CV)MP2 harmonic wavenumbers with B2PLYP-D3/maug-cc-pVTZ anharmonic corrections. Finally, a composite scheme entirely based on CCSD(T) calculations (CCSD(T)/CBS+CV) has been employed for energetics, further confirming that serine II is the most stable conformer, also when zero-point vibrational energy corrections are included.

Lowest Triplet and Singlet States in N-Methylacridone and N,N'-Dimethylquinacridone: Theory and Experiment.

In this work, radical anion photodetachment photoelectron (PD-PE) spectra of N-methylacridone (NM-AC) and N,N'-dimethyl-trans-quinacridone (NNM-QAC) are presented, from which we derived electron affinities and transition energies from S0 to the lowest excited triplet and singlet states (T1, T2, and S1). Because in molecules with extended π systems and heteroatoms the state density even in the energy range of the lowest excited electronic states is already high, assignment of most of the spectral structures in the PD-PE spectra was possible only on the basis of theoretical calculations. To this end, adiabatic transition energies including zero-point vibrational energy corrections were determined using a combination of density functional theory, time-dependent density functional theory, and multireference configuration interaction methods. Calculated Franck-Condon spectra proved to be particularly valuable for the assignment of the spectra. Surprisingly, the density of electronically excited states in the low-energy regime is smaller for NNM-QAC than for NM-AC. This is due to the fact that the nπ* energies remain nearly the same in the two molecules whereas the lowest ππ* excited singlet and triplet transitions are strongly red-shifted in going from NM-AC to NNM-QAC.

Transformation of an Embedded Five-Membered Ring in Polycyclic Aromatic Hydrocarbons via the Hydrogen-Abstraction-Acetylene-Addition Mechanism: A Theoretical Study.

Five-membered rings are constituents of many polycyclic aromatic hydrocarbons (PAHs), and their presence on the edges of large PAHs has been repeatedly observed experimentally. However, modern kinetic combustion models often do not consider the growth of PAHs through the transformation of the five-membered rings. In connection with the above, we carried out a theoretical study of the mechanism of hydrogen-abstraction-acetylene-addition (HACA) transformation of an embedded five-membered ring on the armchair PAH edge to a six-membered ring, considering cyclopenta[d,e,f]phenanthrene (4,5-methylenephenanthrene) as a prototype system for this process. The potential energy surface for the reactions of cyclopenta[d,e,f]phenanthrenyl radicals produced by direct H abstractions from cyclopenta[d,e,f]phenanthrene with acetylene has been compiled at the G3(MP2,CC)//B3LYP/6-311G(d,p) level of theory including zero-point vibrational energy corrections. The computed energies and molecular parameters were then used to solve the Rice-Ramsperger-Kassel-Marcus master equation in order to calculate the reaction rate at various pressures and temperatures, which were fitted to the modified Arrhenius equation for further kinetic modeling. The results show that the HACA transformation of the embedded five-membered ring to a six-membered ring is possible, albeit slow. The most viable reaction mechanism involves the R2 + C2H2 reaction, where the acetylene molecules add to a σ-radical in the six-membered ring adjacent to the five-membered ring via a low entrance barrier. The predominant product of R2 + C2H2 is predicted to be 3-ethynyl-4H-cyclopenta[def]phenanthrene Pr5 via immediate H elimination from the initial addition complex. Next, Pr5 undergoes H-assisted isomerization to 4aH-pentaleno[4,3,2,1-cdef]phenanthrene Pr4, and the latter adds a H-atom eventually forming the 1-pyrenylmethyl radical Pr3: R2 + C2H2 ⇆ 3-ethynyl-4H-cyclopenta[def]phenanthrene (Pr5) + H or 4aH-pentaleno[4,3,2,1-cdef]phenanthrene (Pr4) + H; Pr5 + H ⇆ Pr4 + H; Pr4 + H → 1-pyrenylmethyl (Pr3). This HACA sequence may be competitive with the methyl radical addition to the R1 radical formed by H abstraction from the CH2 group in the five-membered ring of cyclopenta[d,e,f]phenanthrene, which provides a pathway to pyrene following two H-atom losses. Relative contributions of the two mechanisms of the five- to six-membered ring transformation would strongly depend on the branching ratios of the R1 and R2 radicals produced by the H abstractions and the available concentration of C2H2 versus CH3 and hence differ in different flames.

The Lennard-Jones Potential Revisited: Analytical Expressions for Vibrational Effects in Cubic and Hexagonal Close-Packed Lattices.

Analytical formulas are derived for the zero-point vibrational energy and anharmonicity corrections of the cohesive energy and the mode Grüneisen parameter within the Einstein model for the cubic lattices (sc, bcc, and fcc) and for the hexagonal close-packed structure. This extends the work done by Lennard-Jones and Ingham in 1924, Corner in 1939, and Wallace in 1965. The formulas are based on the description of two-body energy contributions by an inverse power expansion (extended Lennard-Jones potential). These make use of three-dimensional lattice sums, which can be transformed to fast converging series and accurately determined by various expansion techniques. We apply these new lattice sum expressions to the rare gas solids and discuss associated critical points. The derived formulas give qualitative but nevertheless deep insight into vibrational effects in solids from the lightest (helium) to the heaviest rare gas element (oganesson), both presenting special cases because of strong quantum effects for the former and strong relativistic effects for the latter.

DFT Investigation of Hydrogen Atom Abstraction from NHC-Boranes by Methyl, Ethyl and Cyanomethyl Radicals-Composition and Correlation Analysis of Kinetic Barriers.

Understanding the hydrogen atom abstraction (HAA) reactions of N-heterocyclic carbene (NHC)-boranes is essential for extending the practical applications of boron chemistry. In this study, density functional theory (DFT) computations were performed for the HAA reactions of a series of NHC-boranes attacked by •CH2CN, Me• and Et• radicals. Using the computed data, we investigated the correlations of the activation and free energy barriers with their components, including the intrinsic barrier, the thermal contribution of the thermodynamic reaction energy to the kinetic barriers, the activation Gibbs free energy correction and the activation zero-point vibrational energy correction. Furthermore, to describe the dependence of the activation and free energy barriers on the thermodynamic reaction energy or reaction Gibbs free energy, we used a three-variable linear model, which was demonstrated to be more precise than the two-variable Evans–Polanyi linear free energy model and more succinct than the three-variable Marcus-theory-based nonlinear HAA model. The present work provides not only a more thorough understanding of the compositions of the barriers to the HAA reactions of NHC-boranes and the HAA reactivities of the substrates but also fresh insights into the suitability of various models for describing the relationships between the kinetic and thermodynamic physical quantities.

Computational spectroscopy of diatomic tungsten carbide, WC

The bond length, electronic term symbols, bond dissociation energies (BDEs), and spectroscopic parameters of numerous low-lying electronic states of diatomic tungsten carbide (WC) have been calculated at a high level of theory. The calculations were done by using the Internally-Contracted multi-reference wave function IC-MRCI/aug-cc-pV5Z(-PP) approach. Davidson corrections were also included using the MRCI + Q method. Zero-point vibrational energy (ZPE) corrections, spin–orbit interactions, and relativistic effects are also included. The bond dissociation energy, equilibrium internuclear distance and the harmonic vibrational frequency of the ground electronic state WC(X3Δ) are computed to be D0 = 5.26 eV, Re = 1.714 Å and ωe = 979 cm−1, respectively. The energies of the lowest vibrational levels (v = 0, 1, 2 and 3) of WC(X3 Δ) are computed to be (0.00, 979.7, 1948.9, 2903.6 cm−1); their rotational constants, Bv, are also computed as (0.50717, 0.50966, 0.49990 and 0.49626 cm−1), respectively. Spectroscopic parameters (ωe, ωexe, Be and αe) and equilibrium bond lengths (Re) for several Λ-S and Ω bound states of WC are also provided in this work. These results are in excellent agreement with the available experimental data.

Intersystem crossing in tunneling regime: T1 → S0 relaxation in thiophosgene.

The T1 excited state relaxation in thiophosgene has attracted much attention as a relatively simple model for the intersystem crossing (ISC) transitions in polyatomic molecules. The very short (20-40 ps) T1 lifetime predicted in several theoretical studies strongly disagrees with the experimental values (∼20 ns) indicating that the kinetics of T1 → S0 ISC is not well understood. We use the nonadiabatic transition state theory (NA-TST) with the Zhu-Nakamura transition probability and the multireference perturbation theory (CASPT2) to show that the T1 → S0 ISC occurs in the quantum tunneling regime. We also introduce a new zero-point vibrational energy correction scheme that improves the accuracy of the predicted ISC rate constants at low internal energies. The predicted lifetimes of the T1 vibrational states are between one and two orders of magnitude larger than the experimental values. This overestimation is attributed to the multidimensional nature of quantum tunneling that facilitates ISC transitions along the non-minimum energy path and is not accounted for in the one-dimensional NA-TST.

High-Level Ab Initio Predictions for the Ionization Energy, Bond Dissociation Energies, and Heats of Formation of Vanadium Methylidyne Radical and Its Cation (VCH/VCH+).

The ionization energy (IE) of VCH, the 0 K V-CH/VC-H bond dissociation energies (D0s), and the heats of formation at 0 K (ΔHf0°) and 298 K (ΔHf298°) for VCH/VCH+ are predicted by the wave function-based CCSDTQ/CBS approach. This composite-coupled cluster method includes full quadruple excitations in conjunction with the approximation to the complete basis set (CBS) limit. The contributions of zero-point vibrational energy, core-valence (CV) correlation, spin-orbit coupling, and scalar relativistic corrections are taken into account. The present calculations show that adiabatic IE(VCH) = 6.785 eV and demonstrate excellent agreement with an IE value of 6.774 7 ± 0.000 1 eV measured with two-color laser-pulsed field ionization-photoelectron spectroscopy. The CCSDT and MRCI+Q methods which include CV correlations give the best predictions of harmonic frequencies: ω2 (ω2+) (bending) = 534 (650) and 564 (641) cm-1 and the V-CH stretching ω3 (ω3+) = 835 (827) and 856 (857) cm-1 compared with the experimental values. In this work, we offer a streamlined CCSDTQ/CBS approach which shows an error limit (≤20 meV) matching with previous benchmarking efforts for reliable IE and D0 predictions for VCH/VCH+. The CCSDTQ/CBS D0(V+-CH) - D0(V-CH) = -0.012 eV and D0(VC+-H) - D0(VC-H) = 0.345 eV are in good accord with the experimentally derived values of -0.028 4 ± 0.000 1 and 0.355 9 ± 0.000 1 eV, respectively. The present study has demonstrated that the CCSDTQ/CBS protocol can be readily extended to investigate triatomic molecules containing 3d-metals.

Semiempirical Quantum-Chemical Methods with Orthogonalization and Dispersion Corrections.

We present two new semiempirical quantum-chemical methods with orthogonalization and dispersion corrections: ODM2 and ODM3 (ODMx). They employ the same electronic structure model as the OM2 and OM3 (OMx) methods, respectively. In addition, they include Grimme’s dispersion correction D3 with Becke–Johnson damping and three-body corrections EABC for Axilrod–Teller–Muto dispersion interactions as integral parts. Heats of formation are determined by adding explicitly computed zero-point vibrational energy and thermal corrections, in contrast to standard MNDO-type and OMx methods. We report ODMx parameters for hydrogen, carbon, nitrogen, oxygen, and fluorine that are optimized with regard to a wide range of carefully chosen state-of-the-art reference data. Extensive benchmarks show that the ODMx methods generally perform better than the available MNDO-type and OMx methods for ground-state and excited-state properties, while they describe noncovalent interactions with similar accuracy as OMx methods with a posteriori dispersion corrections.

CHF3…H2O complex revisited: a matrix isolation and ab initio study

The structural and spectroscopic features of the CHF3…H2O complex have been investigated using high-level ab initio calculations and IR matrix isolation spectroscopy. In contrast to previous findings, the computations at the CCSD(T)/L3a_3 and MP2/L3a_3 levels of theory revealed only one structure of the complex stabilized by two C-H…O and O-H…F hydrogen bonds. The interaction energy extrapolated to a complete basis set with ZPVE and BSSE corrections is 2.73 kcal/mol at the CCSD(T) level, taking into account the zero-point vibrational energy (ZPVE) and basis set superposition error (BSSE) corrections. According to Bader’s analysis, complex exhibits a cyclic structure, which confirms the existence of ring critical point as well as bond critical points in the system. The calculated complexation-induced shifts of the fluoroform fundamentals are in good agreement with the matrix isolation results. In addition to previously reported IR absorptions of the complex, the feature corresponding to the C-F bending mode of CHF3…H2O was first observed, in good correlation with the computational predictions.

TUNNEX: An easy-to-use wentzel-kramers-brillouin (WKB) implementation to compute tunneling half-lives.

Tunneling in experiments (TUNNEX) is a free open-source program with an easy-to-use graphical user interface to simplify the process of Wentzel-Kramers-Brillouin (WKB) computations. TUNNEX aims at experimental chemists with basic knowledge of computational chemistry, and it offers the computation of tunneling half-lives, visualization of data, and exporting of graphs. It also provides a helper tool for executing the zero-point vibrational energy correction along the path. The program also enables computing high-level single points along the intrinsic reaction path. TUNNEX is available at https://github.com/prs-group/TUNNEX. As the WKB approximation usually overestimates tunneling half-lives, it can be used to screen tunneling processes before proceeding with elaborate kinetic experiments or higher-level tunneling computations such as instanton theory and small curvature tunneling approaches. © 2018 Wiley Periodicals, Inc.

The quest for the carbene bent-pentadiynylidene isomer of C5H2

The equilibrium geometry of the singlet ground electronic state of the bent isomer of C5H2, bent-pentadiynylidene (4; X∼1A1;C2v), has been theoretically investigated by means of the highly accurate W3-F12 thermochemical protocol. Five isomers of C5H2, namely linear-pentadiynylidene (1; X∼3Σg-;D∞h), ethynylcyclopropenylidene (2; X∼1A′;Cs), pentatetraenylidene (3; X∼1A1;C2v), ethynylpropadienylidene (5; X∼1A′;Cs), and 3-(didehydrovinylidene)cyclopropene (6; X∼1A1;C2v) had already been identified in the laboratory. With respect to 1, the relative energy difference calculated at the CCSDT(Q)/CBS level of theory including zero-point vibrational energy corrections are: 0.66 (2), 13.53 (3), 14.12 (4), 15.40 (5), and 20.01 (6) kcal mol−1, respectively. Isomers 2–6 are associated with a non-zero dipole moment (μ≠0), however, except 4, all the other four isomers were identified by Fourier Transform Microwave spectroscopy, including 5 and 6 which lie higher in energy. Isomer 4 remains elusive to date. We believe that the theoretical data such as, optimal geometry, dipole moment, rotational and centrifugal distortion constants, harmonic vibrational frequencies, infra-red intensities, and isotopic shifts (12C–13C) in harmonic vibrational frequencies presented here would assist experimentalists in the identification of this elusive molecule (4).