Ab initio MO studies on the photodissociation of C 2H 2 from the S 1 ( 1A u) state. Non-adiabatic effects and S-T interaction

An ab initio MO study on the hydrolysis of methyl chloride

An ab initio MO study on the hydrolysis of methyl chloride with explicit consideration of 13 water molecules

Ab initio MO calculations were carried out, at the MP2/6-311G(d,p)//MP2/6-31G(d) level, to investigate the conformational Gibbs energy of alkyl 1-phenylethyl ketones, C6H5CHCH3COR, 1 (R = CH3, C2H5, i-C3H7, t-C4H9). Rotamers a and a′ whereby R is synclinal to C6H5 and the benzylic methyl group is nearly eclipsed to CO have been shown to be the most stable in every case. Rotamer a is stabilized by a 5-member CH/π hydrogen bond and is more abundant than a′, which is stabilized by a less effective 6-member CH/π bond. The diastereomeric ratio of the product secondary alcohols in the nucleophilic addition to 1 was estimated on the basis of the ground-state rotamer distribution. The Gibbs energy of the diastereomeric transition states was also calculated for a model reaction (C6H5CHCH3COR + LiH) at the same level of approximation. The transition-state geometries leading to the predominant product are similar to those of the ground-state conformation. In geometries leading to the minor product, the relevant torsion angles are twisted to avoid unfavourable steric interactions. The short CH/π and CH/O distances suggest that these weak hydrogen bonds are operating in stabilizing the transition structures. The above two methods gave results consistent with each other. The mechanism of 1,2-asymmetric induction can thus be explained on the basis of the simple premise that the geometry of the transition state resembles the ground-state conformation of the substrate and the nucleophilic reagent approaches from the less hindered side of the carbonyl π-face.

Transition structures for H2 elimination from XH4 hypervalent species (X = S, Se and Te). Ab initio MO study

Ab initio MO study on the difference of solvent effect between exo and endo stereoisomers of aflatoxin B1 8,9-oxide for SN2 type nucleophilic ring opening

The structure and spectroscopic characteristics of various 4‐silamethylenecyclopropene (4‐silatriafulvene) derivatives were investigated using ab initio MO calculations. The minimum energy geometries of all of the derivatives studied here were trans‐bent around the SiC double bond, and the planar structures were found to be the transition states for the inversion motion between the two equivalent bent structures. The barriers were very low to allow facile inversion at room temperature. It was found that silyl substitution at the unsaturated silicon lowered the barrier height of the inversion motion. Significant differences were found in the calculated absorption maxima and 29Si NMR chemical shifts between the planar and trans‐bent 4‐silatriafulvene derivatives, which suggested significant temperature dependence in the spectra. All of these features were in good agreement with the experimental results for a 4‐silatriafulvene derivative, 1,2‐di‐tert‐butyl‐4,4‐bis(tert‐butyldimethylsilyl)‐4‐silamethylenecyclopropene, that we recently isolated. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 198–207, 2001

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTransition structures for conjugate and carbonyl additions of cyanide to s-cis- and s-trans-acrolein. An ab initio MO studyStephen S. Wong, Michael N. Paddon-Row, Yi Li, and K. N. HoukCite this: J. Am. Chem. Soc. 1990, 112, 24, 8679–8686Publication Date (Print):November 1, 1990Publication History Published online1 May 2002Published inissue 1 November 1990https://pubs.acs.org/doi/10.1021/ja00180a007https://doi.org/10.1021/ja00180a007research-articleACS PublicationsRequest reuse permissionsArticle Views239Altmetric-Citations24LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

C–H⋯π interactions involving acetylene: an ab initio MO study

N–H stretching frequencies and the conformation of substituted ureas: an ab initio MO study

Dissociative ionization of 4,5-bis(methylthio)-1,2-dithiole-3-thione (1) has allowed the detection of a series of carbon disulfide radical cations, SCnS.+ (n = 1-3), and their S-methylated forms, CH3SCnS+, in the gas phase of a mass spectrometer. The connectivities and stabilities of these ions and the corresponding neutrals have been probed by collisional activation (CA) and neutralization-reionization (NR) mass spectrometry. Moreover, the sites of protonation and cationic methylation of the polycarbon disulfides have been investigated by a combination of flash vacuum pyrolysis, chemical ionization, and CAMS (FVP/CI/MS/MS). The experimental results are supported by ab initio MO calculations at the QCISD(T)/6-311+G(2d,p) + ZPVE level. Both experiments and calculations indicate that the most favorable site of protonation and methylation is the carbon atom for C2S2 and C3S2 but a sulfur atom for CS2. The proton affinities (298 K) of C2S2 and C3S2 are predicted to be 842 and 818 kJ mol(-1), respectively.

The deformation of the carbon skeleton of the benzene ring under substituent impact has been analyzed from the structures of 74 monosubstituted derivatives, as determined by ab initio MO calculations. The geometry of the substituted ring is shown to contain valuable information on the electronegativity, resonance, and steric effects of the substituent, and also on other, more subtle effects, affecting primarily the length of the Cipso−Cortho bonds. The results obtained substantially augment previous knowledge from the analysis of experimental geometries (Domenicano, A.; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. B 1983, 39, 457). Varying the electronegativity of the substituent causes a concerted change of the ring angles at the ipso, ortho, and para positions, coupled with a change in the Cipso−Cortho bond length. The values of the ipso angle span a remarkably wide range, 113−126°. Enhancing the resonance interaction between a substituent and the ring causes a complex pattern of angular distortions, arising from the superposition of two separate effects. The first originates from the decreased length of the C−X bond, and consists primarily in a concerted change of the ipso and ortho angles. It occurs irrespective of whether the substituent is a π donor or a π acceptor. The second effect is associated with π-charge alternation on the ring carbons. It involves all the internal ring angles, and depends on the substituent being a π donor or a π acceptor. These angular changes are generally accompanied by changes in all C−C bond lengths, as expected from an enhanced contribution of polar canonical forms to the electronic structure of the molecule. By using symmetry coordinates, we have derived two orthogonal linear combinations of the internal ring angles, SE and SR, measuring the electronegativity and resonance effects of a substituent, respectively, as seen from their impact on the ring geometry. SE and SR values are affected in a typical way by steric effects.

Ab initio MO and density functional studies on the vibrational spectra of 1,4-benzoquinone, and its anion and dianion

Reactions of methyl(vinyl)iodonium ion and the β-substituted derivatives as well as divinyliodonium ion with chloride ion were examined theoretically by ab initio MO (MP2) calculations at the double-zeta (DZ) + d level. Interaction of the iodonium ion 1 with Cl- leads to chloro-λ3-iodane 2. Transition states for the SN2, ligand-coupling substitution (LC), and β-elimination (βE) are found for reactions at the vinyl group. The barrier to LC is usually the lowest in the gas phase, but relative barriers to SN2 and to βE change with the substituents. Effects of solvent were evaluated by a dielectric continuum model and found to be large on SN2 but small on LC. The experimental observations in solution that the SN2 is the most facile for the β-methyl-substituted 1 while it is retarded by the β-tert-butyl and β-chloro substitutions are reproduced by the calculations that take medium effects into account.Key words: vinyliodonium salt, polyvalent iodine, vinylic SN2 reaction, ligand coupling, ab initio MO calculation.

Reactions of methyl(vinyl)iodonium ion and the β-substituted derivatives as well as divinyliodonium ion with chloride ion were examined theoretically by ab initio MO (MP2) calculations at the double-zeta (DZ) + d level. Interaction of the iodonium ion 1 with Cl- leads to chloro-λ3-iodane 2. Transition states for the SN2, ligand-coupling substitution (LC), and β-elimination (βE) are found for reactions at the vinyl group. The barrier to LC is usually the lowest in the gas phase, but relative barriers to SN2 and to βE change with the substituents. Effects of solvent were evaluated by a dielectric continuum model and found to be large on SN2 but small on LC. The experimental observations in solution that the SN2 is the most facile for the β-methyl-substituted 1 while it is retarded by the β-tert-butyl and β-chloro substitutions are reproduced by the calculations that take medium effects into account.Key words: vinyliodonium salt, polyvalent iodine, vinylic SN2 reaction, ligand coupling, ab initio MO calculation.

The unimolecular loss of hydrogen fluoride from protonated fluorobenzene was studied by chemical ionization mass spectrometry and MP2/6-31G * ab initio MO calculations. Kinetic energy release measurements point to of three distinct pathways. In addifion to direct HF loss from the fluorine-protonated molecule, the calculations reveal the coexistence of 1,1- and 1,2-elimination process, both derived from ring-protonated isomers. The structures and energetics of the relevant minima and transifion structures have been determined, and the combined experimental/theoretical findings point to a uniquesituation in which protonation of the same precursor molecule by different BrOnsfed acids Affords identical products via three different reaction channels