CCSD Method Research Articles

Theoretical study of H-atom abstraction reactions from CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 by NO3 radical & subsequent degradation

The nocturnal reactions of CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 initiated by NO3 radicals are important sources of alkyl radicals and nitric acids. In this paper, the thermodynamics and kinetics of CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 induced by NO3 radical in gas phase are studied in detail by BHandHLYP method combined with 6-311G(d,p) basis set, and the single point correction is calculated by relatively accurate CCSD(T) method. In the temperature range of 200–400 K, the rate constants of title reactions are fitted to the three-parameter Arrhenius formula:k1 = 1.13 × 10−40T9.24exp(1675.99/T)k2 = 2.23 × 10−23T2.81exp(-4476.24/T)k3 = 5.63 × 1043T−19.20exp(-9344.12/T)All the rate constants calculated by the canonical variational transition state theory and the small curvature tunneling are basically consistent with the limited experimental data. By comparing the reaction rate constants of ethyl ether and its isomer methyl propyl ether with NO3 radical at 293 ± 2 K, the higher the symmetry is, the faster the reaction rate of ether is.Thermodynamic calculations and kinetic data of the title reactions indicate that the H-abstraction reactions at the –OCH2- sites are the main reaction pathways. The thermodynamic and kinetic data of the reaction CH3CH2OCH2CH3, CHF2CF2OCH2CF3 with NO3 radical, showing that the reaction activity could be reduced due to the addition of fluorine atoms, which is further verified by the enthalpies, Gibbs free energies of the title reactions and C–H bond dissociation energies of the CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 molecules. The reaction thermodynamics and kinetics are determined, and the formation mechanisms of the products are proposed, which are crucial to determine the influence of CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 on air quality, as well as its atmospheric lifetime and durability.The atmospheric lifetimes of CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 are evaluated in the NO3-concentration range of 5 × 108-2 × 109 molecule cm−3 to fully consider the effects of different regions on their nocturnal migration. The radiation efficiency and global warming potentials (GWPs) have been reported. The products of title reaction CH3CH2OCHCH3, CF3CHOCF2CHF2 and CF3CHOCH3 are further oxidized into organic nitrates in the presence of O2 and NO. Organic nitrites can be isomerized into organic nitrates or degraded to form CH3C(O•)HOCH2CH3, CF3C(O•)HOCF2CHF2 and CH3C(O•)HOCF3 alkoxy radicals and NO2. This work provides deep insight into the night migration and transformation mechanism of the three ethers.

Reactivity of HCO with CH3 and NH2 on Water Ice Surfaces. A Comprehensive Accurate Quantum Chemistry Study

Interstellar complex organic molecules (iCOMs) can be loosely defined as chemical compounds with at least six atoms in which at least one is carbon. The observations of iCOMs in star-forming regions have shown that they contain an important fraction of carbon in a molecular form, which can be used to synthesize more complex, even biotic molecules. Hence, iCOMs are major actors in the increasing molecular complexity in space and they might have played a role in the origin of terrestrial life. Understand-ing how iCOMs are formed is relevant for predicting the ultimate organic chemistry reached in the interstellar medium. One possibility is that they are synthesized on the interstellar grain icy surfaces, via recombination of previously formed radicals. The present work focuses on the reactivity of HCO with CH3/NH2 on the grain icy sur-faces, investigated by means of quantum chemical simulations. The goal is to carry outa systematic study using different computational approaches and models for the icy surfaces. Specifically, DFT computations have been bench-marked with CASPT2 and CCSD(T) methods, and the ice mantles have been mimicked with cluster models of 1, 2, 18 and 33 H2O molecules, in which different reaction sites have been considered. Our results indicate that the HCO + CH3/NH2 reactions, if they actually occur, have two major competitive channels: the formation of iCOMs CH3CHO/NH2CHO, or the formation of CO + CH4/NH3. These two channels are either barrierless or presentrelatively low ($\leq$ 10 kJ/mol equal to about 1200 K) energy barriers. Finally, we briefly discuss the astrophysical implications of these findings.

Theoretical study of the internal rotational barriers of fluorine, chlorine, bromine, and iodine-substituted ethanes

Physical and chemical characteristics of flexible compounds are extremely dependent of internal rotation barriers. On the other hand, halides of organic compounds are extremely common, and the nature of their rotation barriers are still poorly understood. As a simple example, the experimental height of the internal rotational barrier decreases with the number of fluorine-substituted in ethane, C2H5F, C2H4F2, and C2H3F3. For chlorine-substituted ethane C2H5Cl, C2H4Cl2 and C2H3Cl3 the internal rotational barrier converges in an opposite manner. No experimental results are available for the bromine- and iodine-substituted compounds C2H5Br, C2H4Br2, C2H3Br3, C2H5I, C2H4I2, and C2H3I3. In light of lack of both an adequate explanation for this phenomenon and experimental results for Br and I, the present work studied these compounds using different levels of theory: MP2, MP3, MP4, QCISD(T) and CCSD(T) methods, and the G3 and G3CEP composite theories. The results showed that the G3 and G3CEP theories were the most accurate calculations for the F- and Cl-substituted compounds as a result of the additive contributions of the energy values. From Natural Bond Orbitals (NBO), Quantum Theory of Atoms in Molecules (QTAIM) and Energy Decomposition Analysis (EDA) it was verified that the internal rotational barriers decrease with the addition of Fluorine atoms for fluorine-containing ethanes and can be explained by: π character in the CF bond and delocalization indexes (DI(A)) by QTAIM analysis; the interaction of the ligand and anti-ligand orbitals by NBO analysis and for the descriptors by EDA. On the other hand, the enlargement of the barriers with the addition of Cl, Br and I atoms for substituted ethanes, are dominated by the large electronic cloud of these halogens, which generates significant steric repulsion.

Ground and Low-Lying Excited States of NbC3–/0 Clusters: Assignment of the Anion Photoelectron Spectra from Multiconfigurational Calculations

The BP86 DFT, CCSD(T), and CASPT2 methods were employed to study the low-lying states of NbC3-/0 clusters. The anionic and neutral ground states were determined to be the 1A1 and 12A1 of the cyclic-NbC3-/0 isomers. All bands in the photoelectron spectra of the NbC3- cluster were interpreted based on electron detachment processes from the 1A1 of the cyclic-NbC3- isomer. The X, A, B, and C bands in the spectra are, respectively, ascribed to the transitions from 1A1 to the 12A1, 2B1, 22A1, and 2B2 states. The wave function of the initial 1A1 anionic state exhibits a pronounced multireference character that plays to some extent a role in the 1A1 → 12A1 and the 1A1 → 22A1 ionizations. Only the 1A1 → 2B2 transition appears as a clear one-electron detachment process. The Franck-Condon factor simulations for the 1A1 → 12A1 and 1A1 → 2B1 transitions are consistent with the observed vibrational progressions of the X and A bands in the spectra.

Accuracy of Spin-Component-Scaled CC2 Excitation Energies and Potential Energy Surfaces.

Benchmark calculations with the Spin-Component-Scaled CC2 variants SCS-CC2 and SOS-CC2 are presented for the electronically excited valence and Rydberg states of small- and medium-sized molecules. Besides the vertical excitation energies and excited state gradients, the potential energy surfaces are also investigated via scans following the forces that act in the Franck-Condon region. The results are compared to the regular CC2 ones, as well as higher level methods CCSD, CCSD(T)(a)*, and CCSDT. The results indicate that a large fraction of the flaws of CC2 revealed by earlier studies disappears if spin-component scaling is employed. This makes these variants attractive alternatives of their unscaled counterparts, offering competitive accuracy of vertical excitation energies of both valence and Rydberg type states and reliable potential energy surfaces, while also maintaining a low-power-scaling computational cost with the system size.

Calculation of Vibrational Parameters of an Electride-Like Molecule Li4C4H2N2 and the Pyridazine Molecule C4H4N2

The frequencies and intensities are calculated for fundamental transitions between vibrational states of an electride-like molecule Li4C4H2N2 that can be obtained from the pyridazine molecule C4H4N2 by replacing two hydrogen atoms by lithium atoms and adding two other lithium atoms to nitrogen atoms. Spectral parameters of Li4C4H2N2 are calculated in the harmonic and anharmonic approximations using the MP2, CCSD, and QCISD methods with the sets of atomic functions aug-cc-pVDZ and aug-cc-pVTZ. For comparison the absorption spectrum of a pyridazine molecule was calculated in the same approximations. The calculations showed that, upon introducing lithium atoms into the pyridazine molecule, new intense bands appear and the spectral parameters of the bands present in the pyridazine spectrum are significantly changed. The results obtained may be useful for spectroscopic observation and identification of the new c-ompound.

The mechanism of tert-butylthiol formation via hydrosulfurization of isobutene catalyzed by superacids (HBF4, HAsF6, and HSbF6)

The mechanism of the (CH3)2CCH2 + H2S → (CH3)3CSH reaction catalyzed by various superacids (HBF4, HAsF6, and HSbF6) was investigated theoretically using the ab initio CCSD(T) and MP2 methods and the aug-cc-pVDZ/LANL2DZ basis sets. The effects of surrounding solvent molecules were approximated by employing the PCM solvation model. The most important findings include the observation that each superacid considered is capable of effectively catalyzing isobutene hydrosulfurization as it reduces the activation barrier by ca. 95% in comparison to the uncatalyzed process.

Implementation of analytic gradients for CCSD and EOM-CCSD using Cholesky decomposition of the electron-repulsion integrals and their derivatives: Theory and benchmarks.

We present a general formulation of analytic nuclear gradients for the coupled-cluster with single and double substitution (CCSD) and equation-of-motion (EOM) CCSD energies computed using Cholesky decomposition (CD) representations of the electron repulsion integrals. By rewriting the correlated energy and response equations such that the storage of the largest four-index intermediates is eliminated, CD leads to a significant reduction in disk storage requirements, reduced I/O penalties, and an improved parallel performance. CD thus extends the scope of the systems that can be treated by (EOM-)CCSD methods, although analytic gradients in the framework of CD are needed to extend the applicability of (EOM-)CCSD methods in the context of geometry optimizations. This paper presents a formulation of analytic (EOM-)CCSD gradient within the CD framework and reports on the salient details of the corresponding implementation. The accuracy and the capabilities of analytic CD-based (EOM-)CCSD gradients are illustrated by benchmark calculations and several illustrative examples.

The mechanisms of isobutene hydration yielding tert-butanol catalyzed by a strong mineral acid (H2SO4) and Lewis-Brønsted superacid (HF/SbF5)

The mechanism of the (CH3)2CCH2+H2O→(CH3)3COH reaction catalyzed by two strong acids (H2SO4 and HSbF6) was investigated theoretically using the ab initio MP2 and CCSD(T) methods and the aug-cc-pVDZ/LANL2DZ and aug-cc-pVTZ/LANL2DZ basis sets. The effects of surrounding solvent molecules were approximated by employing the polarized continuum solvation model. The most important findings include the observation that both acids are capable of catalyzing isobutene hydration but the reaction is predicted to proceed faster when the HSbF6 superacid plays the catalyst role.

Atmospheric reaction pathways of methanimine and nitroxyl: a theoretical study

The kinetics and mechanisms of atmospheric reaction of methanimine (H2CNH) with nitroxyl on the singlet surface were studied by employing DFT, MP2, and CCSD(T) methods along with the 6-311++G(3df, 3pd) and aug-cc-pVTZ basis sets. Also, thermodynamic parameters and rate constants have been computed in the temperature range of 300–3000 K using conventional transition state theory and RRKM theories at MP2/6-311++G(3df,3pd) level. The results show that seven adducts can be produced. Two of them are highly thermodynamically stable (CH4+N2O and HNCNH+H2O). In kinetic point of view, HCN + HNOH and H2CN + H2NO adducts (due to passing one corresponding low-level transition state) are favorable pathways of title reaction.

Chemical Kinetics Approves the Occurrence of C (3Pj) Reaction with H2O.

Although both atomic carbon and water are omnipresent in human life, there is a debate about the possibility of carbon reaction with water. Some low-temperature spectroscopic investigations have rejected the reaction, whereas some room-temperature experiments and theoretical studies have accepted the possibility of the reaction by reporting rate coefficients ranging from 105 to 109 L mol-1 s-1. This study provides new lines of evidence about the reaction through exploration of the reaction mechanism using the CCSD(T) method and solving the corresponding master equation by following two main approaches. According to the results, the rate coefficient of the reaction is significantly influenced by the tunneling and hindered rotation effects, in addition to the selected total angular momentum (J). Furthermore, the total rate coefficient of the reaction increases dramatically (from 107 to 1011 L mol-1 s-1) with the rise of temperature from 100 to 4000 K, while the total rate coefficient is insensitive to pressure (0.1-10 atm). Despite some differences between the results of the two approaches, the rate coefficients of both methods are consistent with the previously reported rate coefficients. Also, in agreement with the previous studies, the major products are 2HOC + 2H and 2HCO + 2H. In general, the findings approve the occurrence of the title reaction and indicate that the mentioned conflict is due to the sensitivity of the reaction to the investigated temperature and J level. The sensitivity does not permit low-temperature spectroscopic studies to detect any products and varies the measured and calculated rate coefficients.

MARVEL analysis of the measured high-resolution spectra of [formula omitted]NH

Rovibronic energy levels are determined for four low-lying electronic states (X3Σ-, A3Π, a1Δ, and c1Π) of the imidogen free radical (14NH) using the Marvel (Measured Active Rotational-Vibrational Energy Levels) technique. Compilation of transitions from both laboratory measurements and solar spectra, found in 18 publications, yields a dataset of 3002 rovibronic transitions forming elements of a measured spectroscopic network (SN). At the end of the MARVEL procedure, the majority of the transitions form a single, self-consistent SN component of 2954 rovibronic transitions and 1058 energy levels, 542, 403, and 58 for the X3Σ-, A3Π, and c1Π electronic states, respectively. The a1Δ electronic state is characterized by 55 Λ-doublet levels, counting each level only once. Electronic structure computations show that unusually the CCSD(T) method does not accurately predict the a1Δ excitation energy even at the complete basis set limit.

A Benchmark of Density Functional Approximations For Thermochemistry and Kinetics of Hydride Reductions of Cyclohexanones.

The performance of density functionals and wavefunction methods for describing the thermodynamics and kinetics of hydride reductions of 2‐substituted cyclohexanones has been evaluated for the first time. A variety of exchange correlation functionals ranging from generalized gradient approximations to double hybrids have been tested and their performance to describe the facial selectivity of hydride reductions of cyclohexanones has been carefully assessed relative to the CCSD(T) method. Among the tested methods, an approach in which single‐point energy calculations using the double hybrid B2PLYP−D3 functional on ωB97X−D optimized geometries provides the most accurate transition state energies for these kinetically‐controlled reactions. Moreover, the role of torsional strain, temperature, solvation, noncovalent interactions on the stereoselectivity of these reductions was elucidated. Our results indicate a prominent role of the substituent on the cis/trans ratios driven by the delicate interplay between torsional strain and dispersion interactions.

Does (H5O2)+− X (X = Ar, Ne, He) complex with C2 symmetry really exist?

Different configurations of (H5O2)+− Ar, (H5O2)+− Ne and (H5O2)+− He complexes were calculated using DFT methods and series of Dunning and Pople basis sets. The possibility of the existence of an equilibrium configuration with C2 symmetry for complexes analyzed was in focus of the studies carried out. DFT methods that do not take into account dispersion interactions, as a rule, define (H5O2)+− X (X = Ar, Ne, He) configurations with C2 symmetry as transitional states. At the same time, such DFT methods as B1B95, B98, and WB97X make it possible to determine geometric parameters of these complexes with C2 symmetry in equilibrium configurations. Taking into account dispersion interactions in the framework of D3 Grimme approach and using density functionals, including dispersion interactions (such as M06-2X, wB97XD, etc.), leads to the situation, when configurations of the complexes such as (H5O2)+− X (X = Ar, Ne, He) with C2 symmetry calculated using most of the basis sets turn out to be equilibrium. However, calculations within the framework of MP2, MP4, and CCSD (T) methods determine configurations of complexes with C2 symmetry as transition states, while the experimental data on the IR spectra of (H5O2)+− Ar complex probably point out to the existence of an equilibrium configuration of this complex with C2 symmetry. In addition to structural analysis for equilibrium configurations found, IR spectra were calculated in harmonic and in some cases also in anharmonic approximations. Calculated IR spectra were compared to protonated water dimer IR spectra calculated at the same level of theory in equilibrium configuration and in the transition state.

Topological analysis of electron localisation function: Unlocking the nature of B[sbnd]C chemical bond. Possible existence of multiple bonds B[dbnd]C and B[tbnd

Topological analysis of electron localisation function (ELF) has been applied to study the nature of the BC, BC and BC bonds (1.278–1.645 Å) in 19 molecules using the DFT(M062x) method and 18 molecules using the CCSD method. The boron–carbon bond has a covalent-polarised character and is represented by the bonding disynaptic attractor and the basin, V(B,C). Population of the V(B,C) varies between 1.22 and 5.46e (M062x) or 1.93e and 5.45e (CCSD//CCSD(T)), respectively. The existence of single, double and triple BC bonds is confirmed by the topological results.

Static second-hyperpolarizability of diffuse electron cyclic compounds M2A2 (M = Be, Mg, Ca; A = Li, Na, K): Effect of basis set and electron correlation

In the present work, a number of cyclic planar molecules M2A2 (M = Be, Mg, Ca; A = Li, Na, K) are considered to calculate second-hyperpolarizability using HF, MP2, CCSD, CCSD(T) and DFT methods and 6-311++G(d,p), Sadlej-pVTZ, aug-pc-2 and aug-cc-pVTZ basis sets. The molecules are thermodynamically stable. The diffuse electrons on alkali metals are more efficient on enhancing the magnitude of second-hyperpolarizability. The two-state model can explain the origin of second-hyperpolarizability. The rather highest magnitude of second-hyperpolarizability (107 au) of the Ca2K2 complex arises from the intense lower energy transition. The vibrational contribution to second-hyperpolarizability has been found to be significant.

Multicomponent equation-of-motion coupled cluster singles and doubles: Theory and calculation of excitation energies for positronium hydride.

The calculation of excited states in multicomponent systems, in which more than one type of particle is described quantum mechanically, is important for a wide range of applications in chemistry and physics. The nuclear-electronic orbital (NEO) approach has been used to treat all electrons and key protons, or the positron for positronic systems, quantum mechanically on the same level with density functional theory or wavefunction-based methods. The NEO coupled cluster singles and doubles (NEO-CCSD) method has been shown to provide accurate densities, energies, and optimized geometries for multicomponent systems. Herein, the multicomponent equation-of-motion CCSD (NEO-EOM-CCSD) method is developed for the calculation of excitation energies in multicomponent systems. The working equations are derived and implemented, and the programmable equations are provided to enable others to implement this method. This approach is validated by the comparison of the ground state and first three excited state energies of positronium hydride computed with the NEO-EOM-CCSD method to the values calculated with the NEO full configuration interaction and full coupled cluster methods. The development of the NEO-EOM-CCSD method paves the way for a wide range of applications in excited state multicomponent quantum chemistry.

The Lowest-Energy Isomer of C2Si2H4 Is a Bridged Ring: Reinterpretation of the Spectroscopic Data Based on DFT and Coupled-Cluster Calculations

The lowest-energy isomer of C 2 Si 2 H 4 is determined by high-accuracy ab initio calculations to be the bridged four-membered ring 1,2-didehydro-1,3-disilabicyclo[1.1.0]butane (1), contrary to prior theoretical and experimental studies favoring the three-member ring silylsilacyclopropenylidene (2). These and eight other low-lying minima on the potential energy surface are characterized and ordered by energy using the CCSD(T) method with complete basis set extrapolation, and the resulting benchmark-quality set of relative isomer energies is used to evaluate the performance of several comparatively inexpensive approaches based on many-body perturbation theory and density functional theory (DFT). Double-hybrid DFT methods are found to provide an exceptional balance of accuracy and efficiency for energy-ordering isomers. Free energy profiles are developed to reason the relatively large abundance of isomer 2 observed in previous measurements. Infrared spectra and photolysis reaction mechanisms are modeled for isomers 1 and 2, providing additional insight about previously reported spectra and photoisomerization channels.

Stability and bonding in rare gas inserted interhalogens FRgXFn (X = Br and I, n = 0 and 2)

AbstractStructures, stabilities, and bonding nature of the Rg inserted interhalogens FRgXFn (Rg = Kr, Xe, and Rn, X = Br and I, n = 0 and 2) have been studied using PBE0, MP2, and CCSD(T) methods. In these compounds, the dissociation energy for FRg bonds ranging from 160 kcal mol−1 to 180 kcal mol−1 could be best described as ionic bonds. RgX bonds have some covalent character with lower interaction energies within the range 20‐50 kcal mol−1. NBO, ELF, and EDA calculations were used to further confirm the nature of the Rg inserted compounds and present a clear description of Rg bonding properties.

High-level theoretical study of the hydrogen abstraction reaction H2S + O2 = SH + HO2 and prediction of the rate constants

Hydrogen abstraction reaction of hydrogen sulfide (H2S) by molecule oxygen (O2) forming hydroperoxyl radical (HO2) and hydrosulfide radical (SH) is very important for both atmospheric and combustion chemistry. This work reports a systematic theoretical study on this reaction using a combination of high-level quantum chemistry methods and chemical kinetic theory. Geometry optimizations for all species are performed at QCISD/cc-pVQZ level of theory. The energies for minima and transition state are computed using both single- and multi-reference ab initio methods. It is found that the transition state of this reaction exhibits significant multi-reference nature based on 〈S2〉 and T1 diagnostic during CCSD(T) calculations. The predicted reaction barrier from high-level multi-reference CASPT2(20e,13o)/aug-cc-pVTZ method is higher than those from CCSD(T) and W1BD methods by 6–7 kcal mol−1. Reaction energy barriers from a series of DFT methods also show large deviations compared with both single- and multi-reference methods. Thermal rate constants are calculated over a temperature range from 300 to 2500 K by employing canonical transition state theory with Eckart tunneling correction and the treatment of torsional mode. This work provides accurate rate constants for this reaction and fundamental benchmark results for H/O/S reaction systems, which is valuable for theoretical chemistry and kinetic modeling of sulfur chemistry.