QCISD Levels Of Theory Research Articles

Exploring the gas-phase reaction of methylenecyclopropane anion with N2O from theoretical viewpoint

A detailed investigation on the reaction mechanism of methylenecyclopropane anion with nitrous oxide has been performed theoretically by using the MP2 and QCISD levels of theory with the same basis set (6-31++G(d,p)). As a result, our theoretical studies strongly suggest that the main pathway is the reaction of methylenecyclopropane anion with terminal nitrogen of N2O expressed by Channel 1 which includes two alternative competitive pathways, 1 and 2. The primary ion product was CN−, synchronously, a little amount of 2-nitrosoallyl anion should also be obtained. Theoretical results are consistent with the experimental observation.

Comparative Studies on the Reactions of Acetyl and Thioacetyl Halides with NH3 in the Gas Phase and in Aqueous Solution: A Theoretical Study

The reactions of acetyl halides, CH3C(═ O)X and corresponding sulfur analogues, thioacetyl halides, CH3C(=S)X, where X = F and Cl, with NH3 nucleophile were studied theoretically, at the QCISD level of theory, in the gas phase and in aqueous solution. All reactions occurred via the tetrahedral species, and reactions through neutral intermediates both in the gas phase and in aqueous solution could be ruled out, except for the case of the gas-phase reaction of acetyl fluoride. The tetrahedral structure was a transition state (TS) in the reactions of acetyl chloride, while it was a stable intermediate in reactions of thioacetyl halides. These differences could be caused by the π-bond strength of C ═ O and C ═ S. In the case of acetyl fluoride, the T(±)-type species was neither a saddle point nor an energy minimum in the gas phase, but existed as a stable intermediate in aqueous solution due to solvation. Moreover, in reactions of thioacetyl chloride, the rate-limiting step changed from the first step in the gas phase to the second step in aqueous solution, since the zwitterionic intermediates become more stabilized in aqueous solution. However, lower activation energies (ΔG(‡)) in aqueous solution were not caused by the solvent effects, but smaller deformation effects, in going from reactants through the TS.

Theoretical Study of CH4−CH4, CHF3−CH4, CH4−H2O, and CHF3−H2O Dimers

We have studied systems with typical hydrogen bonding and others with interaction involving hydrogen. CH(4)-CH(4), CH(4)-H(2)O, CHF(3)-CH(4), and CHF(3)-H(2)O dimers were studied using MPWB1K, PBE1PBE, MP2, and QCISD levels of theory with a large number of basis functions. The Pople 6-31+G(2d), 6-311++G(2d,2p), and 6-311++G(3df,3pd) as well as Dunning augmented aug-cc-pVDZ and aug-cc-pVTZ basis sets were used. The dimer geometries were fully optimized. An optimal basis set was determined for these systems to achieve a suitable compromise between accuracy and computational feasibility. A proper strategy was found for the electronic property calculations of dimers studied: the use of aug-cc-pVDZ as the optimal basis set at MP2 level. Dipole moments, polarizabilities, BSSE effects, and DeltaZPE were also analyzed for these dimers.

Experimental and theoretical study of Ne·CO cluster

The photoionization of Ne·CO complex in the energy region of 13.8–14.2 eV was investigated with photoionization mass spectrometry using a synchrotron radiation source in Hefei. The ionization energy (IE) of Ne·CO is determined to be 13.965 eV from its photoionization efficiency curve whose characterization is affected by the photoabsorption of CO molecule. The equilibrium geometry and harmonic vibrational frequencies of Ne·CO and Ne·CO + are calculated using cc-pVTZ basis sets at the QCISD level of theory. The results indicate that they are of a skewed T-shaped structure and show the configuration changes accompanying the ionization of Ne·CO complex. The parameters of thermo-chemistry of Ne·CO and Ne·CO + are calculated using Gaussian-2 method, which provide information concerning the reduction of the ionization potentials of CO in the Ne·CO complex, relative to that of the bare molecule.

Theoretical study of the properties of sulfone and sulfoxide functional groups

An ab initio study on the properties of the sulfone and sulfoxide functional groups is proposed. Structural, energetic, and charge distribution of the radical, positive, and negative species are studied at DFT, MP2, MP4, and QCISD level of theory. Global properties of these functional groups, such as electronegativity and hardness, can be extracted by considering these extreme situations. Studying the properties of these latter in a molecular environment completes the analysis on these sulfur containing functional groups.

High level ab initio study of the structure and vibrational spectra of HO(2)NO(2).

A high-level ab initio study has been performed on the conformational structure and vibrational spectra of HO(2)NO(2). Calculations carried out with coupled-cluster methods using a series of Pople and Dunning basis sets reveal that there is a significant basis set dependence on the predicted ab initio structure. Higher angular momentum basis sets are shown to be necessary in order to bring the calculated structure into agreement with experimental rotational constants. Harmonic vibrational frequencies of HO(2)NO(2) are computed at the CCSD(T)/aug-cc-pVTZ level of theory while the corresponding vibrational anharmonicities are calculated at the MP2/cc-pVTZ level. In addition, the absorption cross sections of OH stretching overtones in HO(2)NO(2) are calculated using a dipole function computed at the QCISD level of theory and found to be in good agreement with the available experimental data.

Planar vs non-planar structure of gaseous catechol in the S0 state: an ab initio study

The structure of neutral gaseous catechol in its electronic ground state is investigated by means of ab initio molecular orbital theory calculations at the HF, MP2, and QCISD levels of theory, and with basis sets ranging from 6-31G(d) to 6-311++G(3d,3p). It is found that while the HF level consistently yields a planar geometry as the minimum energy structure for catechol regardless of the basis set employed, the MP2 level with the vast majority of basis set employed in this study finds the planar structure to be a saddle point of first and even second order. For these basis sets, the MP2 minimum energy structure of catechol has the free O–H bond, the one not involved in intramolecular hydrogen bonding, rotated out of the plane of the aromatic ring by as much as 24°. The electronic energy difference between the planar and non-planar structure is small, and becomes negligible with the largest basis sets used. The results obtained at the even higher level of theory QCISD with three different basis sets mirror those obtained at the MP2 level.

An ab initio study on the mechanism of the F + O 3 → FO + O 2 reaction: comparative reactivity study along the isoelectronic NH 2, OH and F radicals series

The title reaction has been theoretically studied by using MP2, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. At single-reference MP2 and QCISD levels of theory, the potential energy hypersurface (HPES) shows a two-step reaction mechanism through two transition states (TS1 and TS2) and an intermediate (Int). However, the multiconfigurational CASSCF HPES shows a one-step reaction mechanism, as was previously found for the NH 2 + O 3 and OH + O 3 reactions. The results show that the reactivity of the isoelectronic series of NH 2, OH, and F radicals increases from the amino radical to the fluorine atom.

Direct ab Initio Dynamics Study of the Unimolecular Reaction of CH2FO

A direct ab initio dynamics study is carried out for three channels of the unimolecular reaction of fluoromethoxy, CH2FO. The minimum energy paths (MEPs) of the three channels are computed with the MPW1K/6-31+G(d,p) method. The geometries, harmonic vibrational frequencies, energies, and enthalpies of all stationary points are calculated at the MPW1K and QCISD levels of theory in conjunction with the 6-31+G(d,p) basis set. The energies of stationary points and the points along MEP for each channel are further refined by using single-point multilevel energy calculations. The rate constants are evaluated by means of the conventional transition state theory (TST), the canonical varitional transition state theory (CVT), and the canonical varitional transition state theory with small-curvature tunneling correction (CVT/SCT) in the temperature range of 200−2500 K. The fitted Arrhenius expressions of the calculated CVT/SCT rate constants are k1CVT/SCT(T) = (1.26 × 106)T2.19e-7150.6/T s-1, k2CVT/SCT(T) = (3.68 × 102)T3.23e-12327.7/T s-1, k3CVT/SCT(T) = (9.97 × 10-26)T10.90e-3873.3/T s-1 for the H elimination, HF elimination, and isomerization channels, respectively.

Transition-state-theory calculations for reactions of O(3P) with halogenated olefins

The potential energy surfaces for the reactions of atomic oxygen in its ground electronic state, O(3P), with the olefins: CF2CCl2 and CF2CF–CF3, have been characterized using ab initio molecular orbital calculations. Geometry optimization and vibrational frequency calculations were performed for reactants, transition states and products at the MP2 and QCISD levels of theory using the 6-31G(d) basis set. This database was then used to calculate the rate constants by means of Transition-State-Theory. To obtain a better reference and to test the reliability of the activation barriers we have also carried out computations using the CCSD(T)(fc)/6-311G†, MP4(SDQ)(fc)/CBSB4 and MP2(fc)/CBSB3 single point energy calculations at both of the above levels of theory, as well as with the composite CBS-RAD procedure (P. M. Mayer, C. J. Parkinson, D. M. Smith and L. Radom, J. Chem. Phys., 1998, 108, 604) and a modification of this approach, called: CBS-RAD(MP2,MP2). It was found that the kinetic parameters obtained in this work particularly with the CBS-RAD (MP2, MP2) procedure are in reasonable agreement with the experimental values. For both reactions it is found that the channels leading to the olefin double-bond addition predominates with respect to any other reaction pathway. However, on account of the different substituents in the alkenes we have located, at all levels of theory, two transition states for each reaction. Moreover, we have found that, for the reactions studied, a correlation exists between the activation energies and the electronic structure of the transition states which can explain the influence of the substituent effect on the reactivity of the halo-olefins.

Direct ab Initio Dynamics Study of the Reaction of the Hydrogen Atom with Formyl Chloride

A direct ab initio dynamics study is presented on two channels of the gas-phase reaction of the hydrogen atom with formyl chloride, HCOCl. The geometries, harmonic vibrational frequencies, energies, and enthalpies of all of the stationary points are calculated at the BHandHLYP, MP2, and QCISD levels of theory with the cc-pVDZ basis set. The minimum energy paths (MEPs) of both channels of the title reaction are also computed with the same methods and basis set. The energies of stationary points and the points along the MEP for each channel are refined by means of some single-point multilevel energy calculations (HL). The rate constants are evaluated with the conventional transition state theory (TST), the improved canonical varitional transition state theory (ICVT), and the improved canonical varitional transition state theory with small-curvature tunneling correction (ICVT/SCT) in the temperature range of 300−2500 K. The fitted Arrhenius expressions of the calculated ICVT/SCT rate constants at the HL//BHa...

Prediction of a Metastable Compound: HKrOH.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A computational study of the LiH dimer.

A theoretical study of the properties of the linear LiH dimer was undertaken. In this dimer, an unusual type of hydrogen bonding (termed "inverse" hydrogen bonding by some authors), which involves the hydrogen bonded molecule acting as an electron donor (rather than as a proton donor), is exhibited. The optimized geometry, dipole moment, interaction energy, atomic charges, harmonic vibrational frequencies, and frequency shifts for the dimer are computed at the SCF, MP2, and QCISD levels of theory using mainly a 6-31++G(d,p) basis set. We also examined the relative stability of the mono-deuterated isotopomers of linear (LiH)(2), i.e., Li-H...Li-D and Li-D...Li-H. Analysis of the normal vibrational modes, changes in the partial atomic charges, and changes in the vibrational frequencies of LiH on complexation were used to gain insight into the bonding and properties of the linear LiH dimer and its isotopomers.

Prediction of a metastable compound: HKrOH

Ab initio calculations at the B3LYP, MP2 and QCISD levels of theory were perfomed on the rare-gas containing compound HKrOH. The calculations reveal that the molecule is metastable, with the dissociation energy to the lowest energy fragmentation products Kr + H2O computed to be −598 kJ mol−1 at the QCISD level. A possible transition state was also computed for the fragmentation reaction with a zero-point energy corrected barrier height of 29 kJ mol−1, which raises the possibility that this novel molecule could be prepared and characterised experimentally.

Isotope effects in BeH 2⋯HCN

An ab initio study of the linear dihydrogen-bonded complex BeH 2⋯HCN (with BeH 2 acting as a proton acceptor) was undertaken. Equilibrium geometries, interaction energies and harmonic vibrational frequencies were computed at the SCF, MP2 and QCISD levels of theory using a 6-31++G(d) basis set. The relative stabilities of the H-bonded and D-bonded isotopomers of BeH 2⋯HCN, namely, H–Be–D⋯HCN and D–Be–H⋯HCN, were determined by differences in zero-point vibrational frequencies and it was found that H–Be–D⋯HCN was slightly favoured over with a D-Be-H⋯HCN by about 5 cm −1 at the QCISD level. The relative stabilities of the H-bonded and D-bonded species indicates that the D-bond is stronger than the H-bond, in accordance with the Buckingham–Liu theory.

A theoretical study of linear germacyanogen isomers

The structures of all possible linear isomers of Ge-containing cyanogens were optimized at MP2, B3LYP, and QCISD levels of theory. At all levels of theory, the most stable isomer was found to be GeNCN, while for cyanogen itself, the most stable isomer is well known to be NCCN. Our theoretical results predict that the order of stability of the acyclic germacyanogens is GeNCN>GeNNC>NGeCN>NGeNC. The difference between the order of stability among germacyanogen and cyanogen isomers is rationalized, taking into account the position of the unpaired electron on nitrogen in the GeN radical, and that on carbon in the CN radical.

Acetylene ⋯ sulfur dioxide van der Waals complexes: a theoretical study

Ab initio calculations at the MP2 and QCISD levels of theory using Pople's 6-31G(d,p) and Dunning's aug-cc-pVDZ basis sets were carried out to explore the potential energy surfaces corresponding to the C 2H 2⋯SO 2 complex. A C s stacked structure was found to be the most stable one in agreement with microwave experiments. The structural information on other dimers and trimers located may be useful to fit a number of unassigned transitions. The lack of tunneling splittings in the microwave spectrum is explained in terms of the high barrier computed for the internal rotation of the C 2H 2 subunit.

Microwave spectrum, molecular structure, conformational equilibrium, vibrational frequencies and quantum chemical calculations for methyl vinyl sulfide

The microwave (MW) spectrum of methyl vinyl sulfide (CH2CHSCH3) has been investigated in the 20.0–60.5GHz spectral region at dry ice temperature. The most stable rotamer is planar with the methyl and vinyl groups in the syn conformation. A less stable skew rotamer has been assigned for the first time. Its CC–S–C torsional angle is approximately 154° from syn. Isotopic species of all the heavy atoms of the skeleton of the syn conformation have been assigned. This allowed the following structure parameters to be calculated as: C1–C2=133.15 (18) pm, C1–S3=174.11 (9) pm, S3–C4=179.70 (9) pm, C2–C1–S3=130.23 (11)° and C1–S3–C4=101.99 (9)°. The uncertainties are one standard deviation. Ten vibrationally excited states of the syn conformer were assigned, while the ground state and two excited states of the skew conformer were assigned. The frequencies of several of these excited states have been determined by relative intensity measurements. The syn form was found to be 5.0 (3)kJmol−1 more stable than the skew conformer by relative intensity measurements. The uncertainty is one standard deviation. Quantum chemical calculations at HF, MP2 (full), DFT and QCISD levels of theory using the 6-311++G∗∗, cc-pVTZ and AUG-cc-pVTZ basis sets have been made to assist the experimental work.

How Well Can Hybrid Density Functional Methods Predict Transition State Geometries and Barrier Heights?

We compare hybrid Hartree−Fock density-functional theory to ab initio approaches for locating saddle point geometries and calculating barrier heights on a Born−Oppenhiemer potential energy surface. We located reactant, product, and saddle point stationary points for 22 reactions by the MP2 and QCISD ab initio methods and the B3LYP, BH&HLYP, mPW1PW91, and MPW1K hybrid Hartree−Fock DFT methods. We examined all of these methods with two basis sets, 6-31+G(d,p) and MG3. By comparison to calculations on five systems where the saddle point has been optimized at a high level of theory, we determined that the best saddle point geometries were obtained using the MPW1K and QCISD levels of theory. Of the methods tested, mPW1PW91 and B3LYP are the least effective for determining saddle point geometries and have mean unsigned error in barrier heights of 3.4−4.2 kcal/mol, depending on the basis set. In contrast, the MPW1K level of theory predicts the most accurate saddle point geometries and has a mean unsigned error of only 1.5 kcal/mol for either basis set. For even better accuracy, the combination of MPW1K/6-31+G(d,p) geometry calculations with QCISD(T)/MG3 or CCSD(T)/MG3 single-point energy calculations is shown to have an excellent performance-to-cost ratio. As a side product of this work, we report optimized scale factors for computing zero point energies by MPW1K.

Isotope effects in X–H···π type hydrogen-bonded complexes (X = F, Cl or Br)

Ab initio calculations were performed at the B3LYP, MP2 and QCISD levels of theory using the 6-311++G(3df,2pd) basis set to obtain optimized geometries, dipole moments, binding energies and harmonic vibrational frequencies for the symmetrical T-shaped structures of XH···π bonded complexes involving acetylene (as the proton acceptor) and the halides HF, HCl and HBr (as the proton donors). The relative stabilities of the D-containing isotopomers C2H2···DX and C2HD···HX were also determined from their zero-point energies; vibrational frequency shifts of HX and acetylene on bonding were also calculated and used to gain insight into these relative stabilities. It was found that the D-bonded isotopomer (C2H2···DX) was more thermodynamically stable than the H-bonded isotopomer (C2HD···HX) for HF-, HCl- and HBr-containing complexes; previous studies on the weakly bound linear complexes CO–acetylene and N2–acetylene also found that the D-bonded species was more stable than the H-bonded species.