QCISD Methods Research Articles

Theoretical study on the insertion reaction of the stannylenoid H2SnLiF with X-H bonds (X = N, O, F).

The insertion reactions of p-complex (RP) and three-membered ring configuration (RS) of stannylenoid H2SnLiF with NH3, H2O and HF have been studied theoretically by quantum chemical calculation. The structures of reactants, precursors, transition states, intermediates and products have been fully optimized at the M06-2X/def2-TZVP level. The single point energy of all fixed points were calculated using the QCISD method. The calculation results show that the three-membered ring configuration is easier to conduct the insertion reaction. Comparing the reaction energy barriers of RP, RS to NH3, H2O and HF, we found that the difficulty of the insertion reaction is NH3 > H2O > HF. The solvent corrected calculation results show that in THF, the reaction energy barrier of RP is lower than that in vacuum, while the reaction energy barrier of RS is higher. This work provides theoretical support for the reaction properties of stannylenoid.

Structure, dissociation pathways and thermochemistry of some nitrogen-containing astrophysical molecules: a theoretical study

The structure, relative energies, dissociation pathways and thermochemistry of acetonitrile (CH3CN or methyl cyanide), acrylonitrile (CH2CHCN or vinyl cyanide), propionitrile (CH3CH2CN or ethyl cyanide) and their stable isomers have been studied in detail using density functional B3LYP, B3PW91 and ab initio MP2, QCISD and CCSD(T) methods. Several space missions have already confirmed the abundance of these molecules in different interstellar mediums (ISMs). The existence of related radicals, atoms and molecules are also confirmed using various IR-spectrometric techniques and that may be evolved in solar irradiation in the cold dark core TMC-1 or massive star-forming regions such as Sagittarius B2 or Oion-KL. Possible ways of fragmentation of ground state CH3NC, CH2CHNC and CH3CH2NC through different dissociation channels and their link to other isomers have been considered to justify the presence of such observed radicals in the interstellar cloud. Cyanide(-C≡N), iso-cyanide (-N = C) and imine (-C = N-H) isomeric forms have been considered here and out of these, cyanide isomers are more stable species compared to its iso-nitrile and imine form. Favourable product molecules, such as CH2 radical, C2H2, C2H4 and HCN/HNC, are considered in the dissociation channels.

A Simulation of Hydrazine Molecule’s Potential Energy Surface using Variational Quantum Eigensolver Algorithm

Quantum computing is a technology that utilizes the principles of quantum mechanics to perform complex computational processes. In this work, we use Qiskit Module from IBM to do quantum computational calculation using Variational Quantum Eigensolver (VQE) algorithm. VQE is a hybrid quantum-classical method that combines a quantum computer to measure energies and a classical computer to process the measurement results and update the parameters of the quantum computer. The purpose of VQE is to find the ground state energy of a chemical system. In the previous study, many of the VQE calculations have been done on simple molecules. So, in this study, we would like to use Hydrazine molecule as our object of VQE calculation. Furthermore, these results will be compared with the results from the classical calculation (MP2, CCSD(T), QCISD(T), and CASSCF) methods for testing the effectiveness of VQE using Unitary Coupled-Cluster Single and Double excitations (UCCSD) Ansatz. The quantum algorithm based on the UCCSD Ansatz led to a simplification of the algorithm by reducing the circuit depth. Then, the possibility to use active space approximation, can be used to reduce the quantum gates while trying to keep a good level of accuracy. In this study, we chose (2,2) and (4,4) active spaces. Based on the results, as we increase the size of the active space during the evaluation of the single-point energy, the estimated ground states obtained from the VQE algorithm yield nearly identical values. Conversely, in CASSCF calculations, expanding the active space introduces more energy corrections, thus making it more sensitive. Additionally, when examining potential energy surfaces, VQE demonstrates results that gradually align with CCSD(T) and QCISD(T) methods.

Theoretical investigation on isomerization and decomposition reactions of pentanol radicals-part II: linear pentanol isomers.

High-level ab initio calculations are conducted for studying the kinetics of three linear pentanol radicals generated through H-atom abstraction reactions. The species involved are optimized using the M06-2X/6-311++G(d,p) level of theory, while a relaxed scan at the M06-2X/6-31g level of theory with 10° increments is used for the hindrance potential for low-frequency torsional modes. Single-point energies for all stationary points are obtained through the QCISD(T) and MP2 methods in combination with cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets, which can be extrapolated to the complete basis set (CBS) limit. The rate constants and branching ratios for isomerization and decomposition reactions are computed over a temperature range of 250-2000 K and a pressure range of 0.01-100 atm. Isomerization reactions are dominant at low temperatures, while decomposition reactions are more dominant at high temperatures. The branching ratio of the isomerization reaction exhibits a slight decrease with increasing pressure, while the trend for decomposition reactions depends on the type of the breaking bond. Based on the calculations for five branched pentanol radicals in part I, kinetics of linear and branched pentanol radicals are compared in this work and the results reveal that, for the same kind of β-scission reaction at similar positions of linear and branched pentanol radicals, the rate constants of branched ones are faster than those of linear ones at low temperatures. The hydroxyl group adjacent to the breaking bond can increase the β-scission reaction rate constants, while the effect can be ignored when the hydroxyl group is not adjacent to the breaking bond. Moreover, compared to when the hydroxyl group is located in the middle of the carbon chain, its positioning at the chain's end yields a more noticeable impact on the products and rate constants of C-O bond and O-H bond β-scission reactions. Besides, when incorporating calculated rate constants into the CRECK model, the updated mechanism shows a better performance for ignition delay times of 1-pentanol in the NTC range but exhibits lower reactivity at higher temperatures. The simulation of speciation profiles also shows better agreement with the experimental data obtained using a flow reactor.

Better performance of Hartree–Fock over DFT: a quantum mechanical investigation on pyridinium benzimidazolate types of zwitterions in the light of localization/delocalization issues

ContextWith the advent of fast computing facilities, combined with rapid emerges of many new and intricate quantum mechanical functionals, computations with pure Hartree–Fock (HF) theory are now-a-days regarded as trivial or obsolete, or even considered as not reliable by many researchers. Consequently, current trends in computational chemistry show extensive use of post-HF theories for smaller molecular systems and various DFT methods for organic and inorganic chemistry related problems (larger molecules/systems). In this contribution, I have tried to show that sometimes, HF might be more suitable over DFT methodologies in addressing structure–property correlations. Molecules studied here were previously synthesized by Boyd in 1966 and important experimental data were produced by Alcalde and co-workers in 1987. Comparison of computed and experimental results clearly shows that HF method was more effective in reproducing the experimental data compared to especially the DFT methodologies. Reliability of HF method was further assured from the very similar results shown by the CCSD, CASSCF, CISD and QCISD methods. Current study also indicates that the localization issue associated with HF proved to be advantageous over delocalization issue of DFT based methodologies, in correctly describing the structure–property correlation for zwitterion systems.MethodsAll computations were performed with Gaussian 09. A wide-range of quantum mechanical methodologies, HF, B3LYP, CAM-B3LYP, BMK, B3PW91, TPSSh, LC-ωPBE, M06-2X, M06-HF, ωB97xD, MP2, CASSCF, CCSD, QCISD, CISD and semi-empirical methods like, Huckel, CNDO, AM1, PM3MM and PM6, were used for investigations.Graphical abstract

Reaction pathways for palladium(I) reduction in laser-induced particle formation of Pd: An ab initio molecular orbital study

Previously, we have theoretically shown that the UV irradiation can decompose the PdCl42- anion into PdCl2- + Cl + Cl-, in which palladium is reduced from Pd(II) to Pd(I). In this work we perform ab initio calculations to investigate reactions that can immediately follow this reaction in aqueous alcohol solution. Geometry optimizations and harmonic vibrational analyses are done for the species pertinent to the Pd(I) reduction with the MP2(full) method. The employed basis sets are aug-cc-pVTZ-PP for Pd and aug-cc-pVTZ for other elements. Single-point energies at higher-levels of theory are obtained with the MP4(SDTQ) and QCISD(T) methods with the same basis sets. Solvent effects are also included in all calculations using the polarizable continuum model and water is used as the solvent. Based on the energetics of the relevant reactions, we find likely pathways leading to the formation of Pd(0) species.

Spectroscopic properties and bond dissociation energies of PbX, PbX±, PbX2 and [formula omitted] (X ​= ​F, Cl, Br, I)

Ab initio calculations have been performed to investigate some of the spectroscopic properties, like geometry, frequency, electron affinity, ionization potential and finally adiabatic bond dissociation energies (BDEs) of lead monohalides, lead dihalides and their ions viz. PbX, PbX±, PbX2, PbX2± (X ​= ​F, Cl, Br, I) in their ground state at the QCISD(T)//MP2 level of theory using correlation consistent basis sets. For the validation of MP2 optimized geometry and frequency, we further obtained geometry and frequency of all the neutral and ionic systems using QCISD(T) method with the same basis sets. The BDEs of PbX2 molecules are calculated using the BDEs of PbX2± ions and taking ionization potential and electron affinity of various systems. The calculated values are found in good agreement with the available data. Most of the data for ionic systems are reported first time in literature.

Theoretical investigation on the addition reaction of the germylenoid H2GeLiCl with acetone.

In this work, theoretical calculations were performed on the addition reaction of the germylenoid H2GeLiCl with acetone. The DFT M06-2X method was used to optimize the geometries of the whole stationary points on the potential energy surfaces and the QCISD method to calculate the single-point energy. The results reveal that the addition reaction of H2GeLiCl with acetone firstly generates an oxagermacyclopropane c-H2GeOC(CH3)2 and then c-H2GeOC(CH3)2 further reacts with acetone along two possible pathways, pathway I and pathway II, in which the 2,4-dioxagermolane is formed at the end of pathway I and 2,5-dioxagermolane is formed at the end of pathway II, respectively. According to the calculated barrier heights, we can deduce that the pathway I is more favorable than pathway II. The computational results suggest that this reaction model can provide new inspiration for the synthesis of heterocyclic germanium compounds.

Theoretical investigation on the reaction mechanism of ozone with chlorine, bromine and iodine atoms

Theoretical investigation on the reaction mechanism of X + O3 → XO + O2 (X = Cl, Br and I) reactions have been performed at the ab initio level using correlation consistent and effective core potential basis sets. The geometry and frequency of the reactants, products, equilibrium geometries and transition states are obtained using MP2 level of theory and energetics are calculated at the CCSD(T) and QCISD(T) methods. Heat of reactions are found to be consistent with the experimental data. Possible reaction pathways are discussed. In case of reactions of ozone with chlorine and brome, it starts producing a nonplanar trioxide (OOOX) transition state with acute dihedral angle. It then proceeds via nonplanar equilibrium geometry and transition state. Finally dissociates to XO + O2 from a planar O2·OX equilibrium geometry. For the reaction with iodine, initial transition state and equilibrium geometry with acute dihedral angle are disappeared. The reaction proceeds through planar intermediates and finally dissociated to IO + O2 via O2·OI isomer. There exists a flat plateau on the potential energy surface in the intermediate region for all the cases. Finding of O2·OX equilibrium geometry strengthens the suggestion of previous investigations and for the possibility of direct reaction mechanism. High level theoretical investigation including reaction dynamics on these reactions can reveal the actual reaction mechanism.

Theoretical study of the reactions of hydrogen atom with methyl and ethyl hydroperoxides

An ab initio dynamic study of the reactions of H atom with CH3OOH and CH3CH2OOH was performed over the temperature range of 250–1500 K. The geometries of all stationary points were optimised at the MP2, B3LYP and M06-2X theories. The final electronic energies for all species were refined by the CCSD(T) and QCISD(T) methods. The rate coefficients were calculated using canonical variational transition state theory (CVT) with small-curvature tunnelling (SCT) correction or the multichannel Rice−Ramsperger−Kassel−Marcus (RRKM) and CVT methods. The results showed that the channel of H-abstraction from OH group is dominant channel only at temperature less than 350 K. With the temperature higher than 350 K, the OH-transfer channel becomes dominant channel. For the CH3OOH + H reaction, the channel to produce H2 + CH2O + OH becomes the dominant channel at temperature higher than 800 K. For the CH3CH2OOH + H reaction, the channel to produce H2 + CH3CH2O + OH is important at temperature less than 600 K and has a maximum branching ratio of 0.3 around 350 K.

A Spiro-Ge-Heterocyclic Compound Formation via Germylenoid and Formaldehyde: A Theoretical Study

Addition reaction of the germylenoid H2GeLiF (R1) and formaldehyde HCHO (R2) leading to the formation of a spiro-Ge-heterocyclic compound was investigated theoretically by using the M062X and QCISD methods for the first time. The geometries of the stationary points along the potential energy surfaces of the addition reaction were optimized at the M062X/6-311+G(d,p) level at first, and then the single-point energies were calculated at QCISD/6-311++G(d,p) level. The addition reaction of H2GeLiF and HCHO firstly generated an oxagermacyclopropane (c-H2GeOCH2, P1) and then P1 can further react with HCHO along two possible pathways (paths II and III). At the end of path II, one spiro-Ge-heterocyclic product (P2) forms, which is the 2,4-dioxagermolane. At the end of path III, the other spiro-Ge-heterocyclic product (P3) forms, which is the 2,5-dioxagermolane. According to the calculated barrier heights we can conclude that the path II is more favorable than path III. Therefore the dominant reaction pathway of the addition reaction between H2GeLiF and HCHO is that firstly R1 + R2 → P1 and then P1 + R2 → P2. The computational studies suggest that this reaction model can provide new ideas for the synthesis of heterocyclic germanium compounds.

Decomposition of 3-methylcyclohexene radicals: Beginning of its mechanism development

Cyclohexenes are important oxidation intermediates for cyclohexanes and precursors of aromatic hydrocarbons. 3-methylcyclohexene is one of important intermediate products of methylcyclohexane oxidation. In order to improve the understanding of oxidation for 3-methylcyclohexene, the decomposition pathways of 3-methylcyclohexene radical isomers were investigated using ab initio theoretical method. The molecular geometric structures of stationary points on the potential energy surfaces were optimized at B3LYP/6–311++G(d,p) level and then the single point energies were extrapolated using complete basis set (CBS) based on the results obtained using MP2 and QCISD(T) methods with basis sets of cc-pVDZ and cc-pVTZ. The reactivity of reaction channels are discussed with the energy barriers on potential energy surfaces. It is observed the decompositions of allylic radicals have clearly high barrier heights. The products of decomposition of 3-methylcyclohexene radicals are alkyl and alkenyl chain radicals via ring opening reactions, cyclic diolfins plus H atom via C–H bond fission, and cyclohexadienes plus methyl via side chain dissociation. Then it is shown that energies delivered with various quantum chemistry methods are of great similarity. Rate constants of major channels for all radicals were computed with information of stable and saddle points collected on the PESs. The high pressure limits of ring opening and dehydrogenation reactions are comparable for all alkyl radicals, while the other two allylic radicals possess much distinguished values. Finally, the pressure-dependent rate constants are computed based on RRKM/ME theory. The rate constants of ring opening reactions are compared with literature data for similar molecules.

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.

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.

C/N/O centred metal clusters: super valence bonding and magic structure with 26 valence electrons

By using genetic algorithm combined with B3LYP and QCISD methods, this paper investigates the stabilities and electronic structures of Al6OM m (M = Na, K; m = 2,4,6) and a few other Al n XNa m (X = C, N, O) clusters. The results show that the nonmetal doped metal clusters with 26 valence electrons have enhanced stabilities and large energy gaps. This paper extends the Jellium model for the application to the nonmetal doped metal clusters and explains the electronic origin of this strong magic structure. The nonmetal X atom is situated in the centre of the magic clusters. The 2s/2p orbitals of the central atom interact strongly with the superatomic 1S/1P orbitals and form bonding and antibonding orbitals. The bonding orbitals make the C/N/O atoms form s2p6 shell closure, and the antibonding orbitals make the metal moieties form closed 2S22P6 shells. The 26 valence electrons form closed s2p6S2P6D10 shells, and this electronic configuration can be taken as the combination of the octet rule and 18-electron rule. The octahedral Al6O2− core is a superatomic anion with great stability, and it can be used as building blocks to assemble Zintl phase materials by interaction with alkali metals.

Cyclic dimers of formamidine with its N-halogenated formamidine analogues: Structure, energetics, and proton-halonium transfer

Ab initio calculations were carried out to investigate cyclic dimers of formamidine with its N-halogenated analogues, HN = CHNHX (X = Cl, Br, or I). Geometry optimizations and frequency calculations were completed with the MP2 method and both the aug-cc-pVDZ and the aug-cc-pVTZ basis sets. BSSE-corrected interaction energies were calculated at the MP2, QCISD, and CCSD(T) methods. Electron density at relevant critical points, and charge transfers were examined through AIM and NBO analyses respectively. Both the electron density at each of the critical points and the amount of charge transfer correlate with the strength of the dimer interactions. The N-X stretching frequencies and the 15N NMR chemical shifts were also found to correlate with the dimer interaction strengths. Proton-halonium transfer for each of the dimers was also investigated. For any given X, the potential energy profile along the intrinsic reaction coordinate demonstrates that the proton-halonium transfer occurs in one step, without any intermediates. Similarly, the profile of the corresponding force constant (the second derivative of the potential energy) in the transition region indicates that the proton-halonium transfer is a synchronous process. In all cases, the energy barrier for proton-halonium transfer was found to decrease with increasing size of the halogen. Solvent effects were examined using the SMD model with two solvents of differing polarity: tetrahydrofuran and water. Although the proton-halonium transfer remains concerted and synchronous, the energy barrier to proton-halonium transfer increases in solution to the extent that the barrier becomes higher for X = I than it is for X = Br.

Расчет колебательных параметров молекулы с электридными свойствами Li-=SUB=-4-=/SUB=-C-=SUB=-4-=/SUB=-H-=SUB=-2-=/SUB=-N-=SUB=-2-=/SUB=- и молекулы пиридазина C-=SUB=-4-=/SUB=-H-=SUB=-4-=/SUB=-N-=SUB=-2-=/SUB=-

The frequencies and intensities for fundamental transitions between vibrational states of an electride-like molecule Li4C4H2N2 that can be obtained from the pyridazine molecule C4H4N2 by substituting lithium atoms for two hydrogen atoms and adding two other lithium atoms to nitrogen atoms are calculated. Spectral parameters of Li4C4H2N2 are calculated in the harmonic and anharmonic approximations using the MP2, CCSD, and QCISD methods with the atomic function sets 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 new intense bands appear upon entering lithium atoms into the pyridazine molecule and the spectral parameters of bands present in the pyridazine spectrum are significantly changed. The results obtained may be useful for spectroscopic observation and identification of the new compound.

Hetero‐Functional Polymers with Alternating Hydroxyl and Epoxy Groups Synthesized by Thiol‐yne Click (co)Polymerization

AbstractA new family of hetero‐functional polythioethers with alternating hydroxyl and epoxy groups are facilely synthesized via photo‐initiated thiol‐yne click (co)polymerization under mild reaction conditions. Their chemical structures and hetero‐functional group density can be finely tuned by adjusting the feeding mole ratios. As the evidence of nuclear magnetic resonance and gel permeation chromatography tests shows, either the molecular weight or the hetero‐functional group density depend greatly on the SH bond cleavage energy of dithiols which can be predicted by theoretical calculation, using the density functional theory (DFT) with B3LYP hybrid functional and the QCISD methods. This paper provides access to a wide range of new materials with a specific structure and plentiful alternating hetero‐functional groups. Moreover, it unveils and highlights the role of SH bond cleavage energy of dithiols during the process of step‐growth thiol‐yne click (co)polymerization.

Quantum-Chemical Ab Initio Calculations on Inda- and Thallabenzene (C5H5In and C5H5Tl) and their Structural Isomers η5-C5H5In and η5-C5H5Tl

Quantum-chemical ab initio, time-independent, as well as time-dependent density functional theory (TD-DFT) calculations were performed on the so far elusive heterocycles inda- and thallabenzene (C5H5In and C5H5Tl), employing several different methods (MP2, CISD, CCSD, CCSD(T), BD, BD(T), QCISD, QCISD(T), CASSCF, DFT/B3LYP), effective core potentials, and different basis sets. While calculations on the MP2 level predict the ground states of the title compounds to be singlets with the first triplet states between 13 and 15 kcal mol−1 higher in energy, single point calculations with the QCISD(T), CCSD(T), and BD(T) methods at CCSD-optimized structures result in energy differences between the singlet and the triplet states in the range between 0.3 and 2.1 kcal mol−1 in favour of the triplet states. According to a CASSCF(8,8) calculation the triplets are also more stable by about 2.5–2.9 kcal mol−1. Calculations were also performed for the C5v-symmetric η5 structural isomers (cyclopentadienylindium, CpIn, and cyclopentadienylthallium, CpTl, Cp = C5H5) of the title compounds. At the highest level of theory employed in this study, C5H5In is between 79 and 88 kcal mol−1 higher in energy than CpIn, while this energy difference is even larger for thallabenzene where C5H5Tl is energetically between 94 and 102 kcal mol−1 above CpTl. In addition we report on the UV/vis spectra calculated with a TD-DFT method as well as on the spectra of the normal modes of C5H5In and C5H5Tl. Both types of spectra might facilitate identification of the title compounds eventually formed in photolysis or pyrolysis experiments.

On the possibility of the existence of molecular nitrogen allotropes

The possibility of the existence of nitrogen molecules with an even number of atoms of composition N4, N6, N8, and N10 has been discussed with the use of the QCISD and G3 quantum-chemical methods. From these data, a conclusion has been made that three new nitrogen allotropes with an even number of atoms in a molecule can exist, namely, N4 shaped as a rectangle and regular tetrahedron and N6 with a shape remotely resembling an “open book.” The bond lengths and bond and torsion angles in each of these molecules have been reported.