Moeller-Plesset Perturbation Theory Research Articles

Is N8 cubane stable?

The question of N{sub 8} cubane's stability is addressed via ab initio calculations. An O{sub h} symmetry stationary point is found on the energy hypersurface for all three levels of theory used; these were restricted Hartree-Fock (RHF) self-consistent-field theory using STO-3G, 4-31G, and 4-31G* basis sets. Vibrational frequency calculations, at the same three levels of theory, all indicate that the cubic structure is stable. The effect of correlation on the stable structure energies is examined with Moeller-Plesset perturbation theory up to fourth order; these post-Hartree-Fock calculations were performed at the RHF optimized geometries. The calculations indicate that, if it could be synthesized, N{sub 8} cubane would be a highly energetic material, metastable to dissociation to four N{sub 2} molecules; the energy release for this reaction is predicted to be 530 {plus minus} 50 kcal/mol. The energy barrier to dissociation is estimated via reaction coordinate calculations.

Fifth order Moeller-Plesset perturbation theory: comparison of existing correlation methods and implementation of new methods correct to fifth order

Full fifth-order MOller-Plesset perturbation theory of electron correlation is presented in algebraic form and used to compare the behavior of other approximate methods that are size-consistent and exact for two electrons. Considering only single and double substitutions, quadratic configuration interaction (QCISD), coupled cluster (CCSD), and Brueckner doubles (BD) theories are shown to deviate from each other in fifth order. The BD method contains the most parts of the fifth-order energy in a correct manner. The corresponding methods with noniterative triples corrections QCISD(T), CCSD(T), and BD(T) are also analyzed. These methods are all correct in those parts of the fifth-order energy that are linear in the higher (triple, quadruple) substitutions

Ab initio study of the addition reaction of the methyl radical to ethylene and formaldehyde

A detailed study of the potential energy surfaces for methyl radical plus ethylene and methyl radical plus formaldehyde has been carried out with the 3-21G and 6-31G* basis sets at the Hartree-Fock level. Heats of reaction and barrier heights have been computed with the Moeller-Plesset perturbation theory up to the fourth order with and without annihilation of spin contamination. The results of the calculations indicate the formation of an early transition state with reactant-like structure for both reactions. In the case of methyl radical plus ethylene, spin annihilation lowers the barrier height by 7 kcal/mol, while in the reaction between methyl radical and formaldehyde the barrier is lowered by 6 kcal/mol when spin annihilation is considered. A comparison between the calculated and experimental values of the barrier height for the methyl addition to ethylene (6.8 kcal/mol vs 7.9 kcal/mol) and formaldehyde (6.3 kcal/mol vs. 6.8 kcal/mol) indicates very good agreement between theory and experiment.

Quadrupole polarizabilities and hyperpolarizabilities of Kr and Xe from fourth-order many-body perturbation theory calculations

Fourth-order Moeller–Plesset perturbation theory calculations on Kr and Xe atoms surrounded by strategically placed point charges are performed to predict the dipole (α1) and quadrupole (α2) polarizabilities, and the dipole–dipole–quadrupole (B) hyperpolarizabilities of Kr and Xe. A semiempirical value is thought to provide the best estimate for α1(Kr). Our recommended values for the other quantities are α1(Xe)=(27.7±0.3) e2 a20 E−1h, α2(Kr)=(99±2) e2 a40 E−1h, α2(Xe)=(210±6) e2 a40 E−1h, B(Kr)=−(343±10) e3 a40 E−2h, B(Xe)=−(812±24) e3 a40 E−2h.

Evaluation of semiempirical quantum-chemical methods in solid-state applications. I. Molecular-cluster calculations of defects in silicon.

Three typical semiempirical molecular-orbital methods developed by chemists; the spectroscopic version of the complete neglect of differential overlap (CNDO/S), the modified intermediate neglect of differential overlap (MINDO/3), and the modified neglect of diatomic overlap (MNDO), are evaluated in molecular-cluster model applications. The ionization energies, wave-function localizations, and equilibrium geometries of the clusters X${\mathrm{Si}}_{4}$${\mathrm{H}}_{12}$, where X is a silicon, sulfur, or oxygen atom or a vacancy, are compared with results obtained from ab initio Hartree-Fock calculations using an extended Gaussian-orbital basis set with third-order Moeller-Plesset perturbation theory. The importance of using an absolute ionization-energy scale in locating levels relative to the crystalline band gap is discussed. It is concluded that the MINDO/3 is superior among the three methods in representing defect properties using a molecular-cluster model of defects in silicon.

An ab initio study of the reaction pathways for OH + C2H4 .fwdarw. HOCH2CH2 .fwdarw. products

The energetically favorable reaction paths for the unimolecular decomposition of the primary addition product of OH + C/sub 2/H/sub 4/ have been studied with ab initio techniques. Equilibrium geometries and transition structures were fully optimized with 3-21G and 6-31G* basis sets at the Hartree-Fock level. Heats of reaction and barrier heights have been computed with Moeller-Plesset perturbation theory up to fourth order, with and without annihilation of spin contamination. At the MP4 level barrier heights are lowered by 2-7 kcal/mol when the largest spin contaminant is removed. After the addition of OH + C/sub 2/H/sub 4/ to form the 2-hydroxyethyl radical, the most favorable reaction path (other than decomposition to reactants) is the (1,3)-hydrogen shift to form ethoxy radical followed by a dissociation into CH/sub 3/ + CH/sub 2/O. Other slightly higher energy paths include dissociation of ethoxy into H + CH/sub 3/CHO and decomposition of the 2-hydroxyethyl radical into H + HOCHCH/sub 2/.

Quadrupole and hexadecapole moments for molecular nitrogen

Complete fourth-order Moeller-Plesset perturbation theory (MPPT) calculations for the quadrupole (Q2) and hexadecapole (Q4) moments of N2 are reported. The authors' fourth-order MPPT values are Q2=-1.171 ea20 and Q4=-5.8 ea40 at a bond length of R=2.073 42 a0. Taking basis-set truncation and vibrational corrections into account, these values are in good agreement with experiments. Preliminary values for the longitudinal components of the quadrupole polarisability, mixed dipole-quadrupole hyperpolarisability and dipole hyperpolarisability are Czz,zz=33.64 e2a40E-1h, Bzz,zz=-215 e3a404 E-2h and gamma zzzz=1173 e4a40E-3h, respectively.

Calculated barrier heights for OH + C2H2 and OH + C2H4 using unrestricted Moeller-Plesset perturbation theory with spin annihilation

The reactions of OH radical with C2H2 and CzH4 have been studied with ab initio molecular orbital techniques. Reactants, loose clusters, transition structures, and products were optimized at UHF/3-21G and UHF/6-3 1G*. The barrier heights have been computed by using unrestricted Hartree-Fock and Mdler-Plesset perturbation theory up to fourth order, including single, double, and quadruple excitations. Spin contamination in the UHF wave function has been corrected by annihilating the largest spin contaminant. The vibrational frequencies were computed by using analytical derivative methods at the UHF/3-21G level. The barrier heights for both reactions are overestimated by 7-15 kcal/mol at the UMPZ, UMP3, and UMP4 levels. Annihilation of the largest spin contaminant lowers the barrier heights by 7-15 kcal/mol. Calculations at the PMP4/6-3 lG* level are in good agreement with the estimated experiment barrier heights. The reactions of OH with acetylene and ethylene are known to be important in hydrocarbon combustion as well as atmospheric chemistry.'-5 Experimental studies on OH + C2H21-12 and OH + c2H4'-43'1-25 have shown that near room temperature the predominant mechanism involves the electrophilic addition of the OH radical to the 7r bond, forming an activated complex which can be collisionally stabilized. OH + C2H2, - C2H2,0H* (n = 1, 2) (1) (2) C2H2,0H* + M --* C2H2,OH + M In early experimental work on OH + C2H2, no pressure de- pendence of the rate constant was found at low temperature^.^^^ However, recent studies covering a wider range of temperature and pressure, and using flash photolysis/resonance fluorescence and laser pyrolysis/laser fluorescence techniques,8-10 indicate that the rate constant does depend on the pressure, consistent with eq 1 and 2. Analysis of the temperature and pressure dependence yields an activation energy of 1.3 =! 0.1 kcal/m~l,~.'~ based on an estimated heat of reaction of -36 f 6 kcal/mol for OH + C2H2 - C2H20H. At temperatures higher than 1000 K, the addition channel becomes less important because of competition from hydrogen abstraction (estimated activation energy 6-8 kcal/ The OH + C2H4 system has also received considerable atten- ti~n.-~~ In accord with eq 1 and 2, the rate for OH + C2H4 is found to be dependent on the total pressure. However, unlike addition to acetylene, a small negative activation energy has been found (-0.9 * 0.2,'3*14 -0.7 f 0.3,15 -0.6 f 0.316 kcal/mol). Several explanations have been given for this, including the for- mation of a weakly bound complex.26 Estimates of the heat of reaction for the addition process range from -29 to -32 kcal/ m0I.~3'~-'* Mass spectral methods have been used to observe directly the primary adduct, C2H40H, and to study its decom- position into CH, + CH20 and H + CH3CH0.18,'9 At higher temperatures, hydrogen abstraction becomes the dominant pathway (estimated activation energy ca. 3 kcal/m~l).',~~~~ ~ In an earlier theoretical study, Melius, Binkley, and Koszy- kowskiZ7 examined the reactions of OH with C2H2, C2H4, and HCN. Geometries were optimized at the HF/6-31G* level and energies were calculated by using fourth order unrestricted Mdler-Plesset perturbation theory (UMP4/6-3 lG**). The heats of reaction and barrier heights were estimated by applying bond additivity corrections to the UMP4 calculations (BAC-MP4). These corrections were determined by a least-squares fit to ca. 50 molecules with well-established heats of formation. For m01).~J~

ChemInform Abstract: Ab initio Study of Hypervalent Sulfur Hydrides as Model Intermediates in the Interconversion Reactions of Compounds Containing Sulfur‐Sulfur Bonds.

ChemInformVolume 18, Issue 19 Physical Inorganic Chemistry ChemInform Abstract: Ab initio Study of Hypervalent Sulfur Hydrides as Model Intermediates in the Interconversion Reactions of Compounds Containing Sulfur-Sulfur Bonds. R. S. LAITINEN, R. S. LAITINEN Dep. Chem., Helsinki Univ. Technol., SF-02150 Espoo, Finl.Search for more papers by this authorT. A. PAKKANEN, T. A. PAKKANEN Dep. Chem., Helsinki Univ. Technol., SF-02150 Espoo, Finl.Search for more papers by this authorR. STEUDEL, R. STEUDEL Dep. Chem., Helsinki Univ. Technol., SF-02150 Espoo, Finl.Search for more papers by this author R. S. LAITINEN, R. S. LAITINEN Dep. Chem., Helsinki Univ. Technol., SF-02150 Espoo, Finl.Search for more papers by this authorT. A. PAKKANEN, T. A. PAKKANEN Dep. Chem., Helsinki Univ. Technol., SF-02150 Espoo, Finl.Search for more papers by this authorR. STEUDEL, R. STEUDEL Dep. Chem., Helsinki Univ. Technol., SF-02150 Espoo, Finl.Search for more papers by this author First published: May 12, 1987 https://doi.org/10.1002/chin.198719026Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume18, Issue19May 12, 1987 RelatedInformation

Ab initio study of hypervalent sulfur hydrides as model intermediates in the interconversion reactions of compounds containing sulfur-sulfur bonds

Ab initio MO calculations involving the 4-31G* basis set have been used to predict the equilibrium geometries of the hypervalent sulfur hydrides H/sub 2/SS, (HS)/sub 2/SS, H/sub 2/S(SH)/sub 2/, H/sub 2/S(SSH)/sub 2/, and the cyclic H/sub 4/S/sub 4/. The energy changes in their formation from appropriate sulfanes H/sub 2/S/sub n/ (n = 1-4) have been studied with the 6-31G* basis set including the correction for the electron correlation by the second- and third-order Moeller-Plesset perturbation theory. The results are used to discuss the possible pathways in the interconversion reactions between various sulfur compounds containing cumulated SS bonds, for example, the formation of S/sub 7/ from S/sub 8/ for which hypervalent intermediates have been proposed recently. Comparison with experimental evidence is made whenever possible.

ChemInform Abstract: Ab initio Quantum‐Chemical Study of the Unimolecular Pyrolysis Mechanisms of Formic Acid.

The mechanisms of the two unimolecular competing reactions occurring during the pyrolysis of formic acid have been investigated by ab initio methods by using a variety of basis sets. The effects of valence electron correlation were included by Moeller-Plesset (MP) perturbation theory to the fourth order. To explain the apparent inconsistency of the kinetic results, i.e., the lowest activation energy value for the decarboxylation process vs. the dehydration one and a CO/CO/sub 2/ ratio of product yield of 10, a new reaction mechanism has been proposed. That is, if the dehydration constitutes the main channel of formic acid pyrolysis with a calculated transition barrier of 67.1 kcal mol/sup -1/, then some of the produced water molecules could serve as catalysts for the decarboxylation reaction, the computed activation energy of which amounts to 48.7 kcal mol/sup -1/. The observed product ratio can furthermore be explained by the dependence of the decarboxylation process on the dehydration reaction. 23 references, 3 figures, 5 tables.