A theoretical study of silyl anions and radicals. Change of mechanism from vertex inversion to edge inversion with increasing substitution by halogens

1990 ◽  
Vol 68 (8) ◽  
pp. 1309-1316 ◽  
Author(s):  
A. C. Hopkinson ◽  
C. F. Rodriquez ◽  
M. H. Lien
Keyword(s):  
Theoretical Study ◽  
Single Point ◽  
Lone Pair ◽  
Basis Set ◽  
Transition Structure ◽  
Silyl Radicals ◽  
Inversion Mechanism

Structures for trivalent silyl anions [Formula: see text] and [Formula: see text], where n takes values from 0 to 3, have been optimized at the HF/6-31 + +G* level and single point calculations made at the MP2/6-31 + +G* level (core included). SiH3− and ions containing one halogen invert by the vertex mechanism in which the lone-pair has π-symmetry, and the monosubstituted ions have high barriers (SiH2F− 45.2 kcal/mol and SiH2Cl− 44.0 kcal/mol). Further substitution by halogens results in a change to the edge inversion mechanism involving a T-shaped transition structure with the lone-pair coplanar with the ligands. Barriers (kcal/mol) at the MP2/6-31 + +G* level including ZPE are lower than for the monosubstituted ions and are SiHF2− 35.0, SiF3− 35.9, SiHCl2− 28.4, and SiCl3−32.5. In SiLi3− edge inversion is preferred, but the surface is much flatter and the barrier is low (9.8 kcal/mol). Trivalent silyl radicals SiHnF(3−n), SiHnCl(3−n) and SiFnCl(3−n) (with n having values 0 to 3) have also been examined with the 6-31 + +G* basis set, with optimization at the UHF level and single point calculations at the UMP2 level. Radicals SiH3, SiH2F, SiH2Cl, and SiHCl2 all invert by the vertex mechanism. Increased halogenation results in a change of mechanism and SiF3, SiCl3, SiF2Cl, and SiFCl2 invert by the edge mechanism. For radical SiHF2 the calculated barriers for the two mechanisms are almost identical with the higher level of theory slightly favouring edge inversion. Keywords: inversion mechanism, halogenated silyl radicals and anions.

Ab initio studies on the reaction SN+ + SNS+ → S3N22+

1993 ◽  
Vol 71 (3) ◽  
pp. 335-340 ◽  
Author(s):  
F. Grein
Keyword(s):  
Ab Initio ◽  
Single Point ◽  
Relative Orientation ◽  
Basis Set ◽  
Frontier Orbital ◽  
Electrostatic Forces ◽  
Transition Structure ◽  
Electronic Frontier

The reaction SN+ + SNS+ → S3N22+ has been studied by ab initio SCF methods. Geometry optimizations with the 3-21G(*) basis set were followed by 6-31G* single-point calculations. The barrier of S3N22+ to dissociation is 10.9 kcal/mol with 3-21G(*), and 19.7 kcal/mol with the (single-point) 6-31G* basis set. The transition structure is cyclic (Cs), and is similar to the optimized C2v structure of S3N22+. The relative orientation of the two approaching cations is governed by electrostatic forces rather than electronic (frontier orbital) effects. The product lies 99.3 (3-21G(*)) and 95.2 (6-3IG*) kcal/mol higher than the reactants.

A theoretical study of the CSH4 and CPH5 hypersurfaces. Geometries, tautomerization, and dissociation of sulfonium and phosphonium ylides

1981 ◽  
Vol 59 (23) ◽  
pp. 3280-3292 ◽  
Author(s):  
David John Mitchell ◽  
Saul Wolfe ◽  
H. Bernhard Schlegel
Keyword(s):  
Theoretical Study ◽  
Lone Pair ◽  
Basis Set ◽  
Basis Sets ◽  
Proton Affinities ◽  
Model Calculations ◽  
Bond Stretching ◽  
Geometrical Constraints ◽  
True Energy

Calculations have been performed at four basis set levels (STO-3G, STO-3G*, 4-31G, 4-31G*) on the model ylides methylenesulfurane (CH2SH2) and methylenephosphorane (CH2PH3), their stable tautomers (CH3SH and CH3PH2), their dissociation products (SH2, PH3 and CH2), and the protonated species CH3SH2+ and CH3PH3+. At each basis set level all geometries have been optimized fully, using the FORCE method. The conformational behaviour of the ylides as a function of C—X bond-stretching, C—X torsion, and CH2 (or SH2) bending has been examined in some detail. The experimental properties (i.e., geometries, relative stabilities, proton affinities, rotation–inversion behaviour) of sulfonium and phosphonium ylides are reproduced well by the model calculations with the 4-31G* basis set, which contains d-type functions on both carbon and sulfur (or phosphorus). All other basis sets are deficient in different ways and for different reasons, which are discussed in detail. The principal result of this work is the conclusion that d-type functions are essential for a proper description of the hypervalent species CH2SH2 and CH2PH3, but not for the normal-valent species SH2, PH3, CH3SH, and CH3PH2. The conclusion concerning hypervalent species reverses our earlier views. The role of the d-type functions is to concentrate charge into the C—X region of the ylides, and thus to stabilize the system. Evidence for this effect has been obtained from quantitative perturbational molecular orbital (PMO) analyses of interactions associated with the carbon lone pair, as well as comparisons of electron density plots with and without the d AO's. A second conclusion is that the imposition of various geometrical constraints such as assumed C—X, C—H, or X—H bond lengths, and HCH or XHn bond angles, as was necessary for computational reasons in all previous work on such systems, has major and previously unrecognized consequences. For example, the assumption that the CH2 moiety is planar in CH2SH2 leads to very similar geometries with and without d AO's, although only in the latter case does such a geometry at carbon correspond to the true energy minimum; in the absence of d AO's the C—S bond length is maintained by a symmetry-enforced barrier to dissociation. These and other consequences of geometrical constraints at carbon, sulfur, or phosphorus are analyzed in detail.

Comparison of an intramolecular Michael-type addition with its intermolecular counterpart: an abinitio theoretical study

1989 ◽  
Vol 67 (12) ◽  
pp. 2173-2177 ◽  
Author(s):  
C. I. Bayly ◽  
F. Grein
Keyword(s):  
Transition State ◽  
Michael Addition ◽  
State Energy ◽  
Theoretical Study ◽  
Theoretical Calculations ◽  
Single Point ◽  
Basis Set ◽  
Energy Change ◽  
Transition State Geometry ◽  
Michael Type Addition

Abinitio theoretical calculations were carried out on the intramolecular addition of 3-ketopent-4-en-1-olate 1 to form 1-oxacyclohex-3-en-4-olate 2 (reaction I; Scheme 1) and the corresponding intermolecular Michael addition of methanolate to butenone forming 4-methoxybut-2-en-2-olate 4 (reaction II; Scheme 1). The calculations were carried out at the RHF level using an augmented 4-31G basis set; single point energies were then taken using the 6-31 + G* basis set. The overall energy change of the reaction in going from reactants to products is very similar for both reactions, with products being about 8.3 kcal/mol more stable than reactants; the major difference energetically between the two reactions is at the level of the transition state: the transition state energy for reaction I (intramolecular) is 6 kcal/mol higher than that for reaction II (intermolecular). The transition state geometry of reaction II conforms quite closely to the stereoelectronic requirements for a Michael addition, but that of reaction I is very distorted, reflecting significant compromises in the stereoelectronic requirements. Keywords: abinitio theoretical study, Michael-type addition, intramolecular, transition state.

Through-Space Transmission of 19F-13C Coupling Via an Intermediate Bond. An Experimental and Theoretical Study

1987 ◽  
Vol 40 (12) ◽  
pp. 1923 ◽  
Author(s):  
ID Rae ◽  
ID Rae ◽  
A Staffa ◽  
A Staffa ◽  
AC Diz ◽  
...  
Keyword(s):  
Theoretical Study ◽  
Lone Pair ◽  
Electron Excitation ◽  
Carbonyl Groups ◽  
Polarization Propagator ◽  
Close Proximity ◽  
Fermi Contact ◽  
Intramolecular Hydrogen ◽  
Intermediate Bond ◽  
Insight Into

In order to obtain a deeper insight into the title effect, several compounds with an F atom very close to a C-H of a nearby functional group were synthesized and the relevant couplings measured. The most conspicuous case was that of 8-fluoro-2-hydroxynaphthalene-1-carbaldehyde where a close proximity between the F and H atoms is the result of fluorine-oxygen repulsion and the formation of an intramolecular hydrogen bond between the hydroxyl and carbonyl groups. The experimental four-bond J(F,CHO) coupling is 26.2 Hz. A compound very similar to this one, but without the OH group, was chosen on which to perform a polarization propagator analysis of the through-space (TS) coupling pathways, at the RPA-INDO level. The expression for the TS coupling in terms of the projected polarization propagator and perturbators was numerically analysed. It is found that this coupling is completely dominated by a TS component of the Fermi contact (FC) term, the main features of which are: ( i ) It decays exponentially with the F-H distance; (ii) Its main contribution comes from an electron excitation involving the F lone-pair, the C-H bond of the CHO moiety and its corresponding antibonding orbital;(iii) The π-type lone-pair does not contribute to the TS coupling pathway of the FC term.

Liquid Crystalline Properties of 4,4and#39;-methylenebis(N-(4-Alkanoxybenzylidene)aniline): Synthesis, Characterization, and Theoretical study

2019 ◽  
Vol 41 (6) ◽  
pp. 1107-1107
Author(s):  
Mohammed Taha Yaseen and Abdullah Hussein Kshash Mohammed Taha Yaseen and Abdullah Hussein Kshash
Keyword(s):  
Liquid Crystalline ◽  
Theoretical Study ◽  
Basis Set ◽  
Spectroscopic Techniques ◽  
Benzene Nucleus ◽  
Crystal Properties ◽  
Ft Ir ◽  
Alkoxy Groups ◽  
Gaussian Program ◽  
Mesomorphic Properties

The paper presents six homologues series of Schiff bases ether compounds distinguished by the length of terminal alkoxy groups which substituted on a side benzene nucleus. The above structures were demonstrated through the use of spectroscopic techniques, like FT- IR and 1H-NMR. Polarized hot stage optical microscopy was used to study both mesomorphic properties and phase transitions. The results showed that out of the six compounds only three (B2, B3 and B4) were pure (marble) nematic mesophase, while no liquid crystal properties for (B5, B6 and B7) compounds. The theoretical study for the electronic structures was intended to study the effects of alkyl chain length on the electronic structure by using Gaussian program, DFT and 6-31G as basis set. The theoretical results indicate that there is no effect to the terminal substituted alkoxy groups on the HOMO energies but there is an effect on LUMO energies through decreasing energy for the prepared compounds.

Theoretical study on NO3-initiated oxidation of acenaphthene in the atmosphere

2008 ◽  
Vol 86 (2) ◽  
pp. 129-137 ◽  
Author(s):  
Xiaohui Qu ◽  
Qingzhu Zhang ◽  
Wenxing Wang
Keyword(s):  
Product Information ◽  
Environmental Concern ◽  
Single Point ◽  
Basis Set ◽  
Orbital Theory ◽  
Night Time ◽  
Point Energy ◽  
Secondary Reactions ◽  
Favorable Reaction ◽  
High Level

Acenaphthene is widespread and toxic, and thus of substantial environmental concern. The reaction with NO3 radicals is an important atmospheric loss process of acenaphthene at night time. In this work, the mechanism for the NO3-initiated atmospheric oxidation reaction of acenaphthene has been studied using high level molecular orbital theory. Geometries of all the related species have been optimized at the MPWB1K level with the 6–31G(d,p) basis set. The single-point energy calculations have been carried out at the MPWB1K/6–311+G(3df,2p) level. The possible secondary reactions were also studied. Several energetically favorable reaction pathways were revealed for the first time.Key words: acenaphthene, NO3 radicals, reaction mechanism, product information, oxidation degradation.

scholarly journals Experimental and Theoretical Study of the Enthalpy of Formation of 3,6-Diphenyl-1,2,4,5-Tetroxane Molecule

2002 ◽  
Vol 2 ◽  
pp. 455-460 ◽  
Author(s):  
N.L. Jorge ◽  
L.C.A. Leiva ◽  
M.G. Castellanos ◽  
M.E. Gomez Vara ◽  
L.F.R. Cafferata ◽  
...  
Keyword(s):  
Vapor Pressure ◽  
Enthalpy Of Formation ◽  
Vapor Phase ◽  
Theoretical Study ◽  
Theoretical Calculations ◽  
Basis Set ◽  
Sublimation Enthalpy ◽  
Combustion Heat ◽  
Different Temperatures ◽  
The Enthalpy Of Formation

We report the results obtained for the experimental determination and the theoretical calculation of the enthalpy of formation of 3,6-diphenyl-1,2,4,5-tetroxane molecule. The experimental work was performed using a macrocalorimeter to measure the combustion heat, and the sublimation enthalpy was determined via the measurement of the vapor pressure at equilibrium with the vapor phase at different temperatures resorting to the Clapeyron-Claussius equation. Theoretical calculations were performed using semiempirical AM1 and PM3 methods as well asab initiotechniques at the 3-21, 6-31G(d,p), and 6-311G(d,p) basis set levels.

scholarly journals Theoretical Study of Solvent Effects on the Electronic and Thermodynamic Properties of Tetrathiafulvalene (TTF) Molecule Based on DFT

2021 ◽  
pp. 42-54
Author(s):  
Rabiu Nuhu Muhammad ◽  
N. M. Mahraz ◽  
A. S Gidado ◽  
A. Musa
Keyword(s):  
Thermodynamic Properties ◽  
Bond Length ◽  
Density Functional ◽  
Theoretical Study ◽  
Energy Gap ◽  
Basis Set ◽  
Basis Sets ◽  
Frontier Molecular Orbital ◽  
Orbital Energy ◽  
Chemical Hardness

Tetrathiafulvalene () is an organosulfur compound used in the production of molecular devices such as switches, sensors, nonlinear optical devices and rectifiers. In this work, a theoretical study on the effects of solvent on TTF molecule was investigated and reported based on Density Functional Theory (DFT) as implemented in Gaussian 03 package using B3LYP/6-31++G(d,p) basis set. Different solvents were introduced as a bridge to investigate their effects on the electronic structure. The HUMO, LUMO, energy gap, global chemical index, thermodynamic properties, NLO and DOS analysis of the TTF molecule in order to determine the reactivity and stability of the molecule were obtained. The results obtained showed that the solvents have effects on the electronic and non-linear-optical properties of the molecule. The optimized bond length revealed that the molecule has strong bond in gas phase with smallest bond length of about 1.0834Å than in the rest of the solvents. It was observed that the molecule is more stable in acetonitrile with HOMO-LUMO gap and chemical hardness of 3.6373eV and 1.8187eV respectively. This indicates that the energy gap and chemical hardness of TTF molecule increases with the increase in polarity and dielectric constant of the solvents. The computed results agreed with the results in the literature. The thermodynamics and NLO properties calculation also indicated that TTF molecule has highest value of specific heat capacity (Cv), total dipole moment () and first order hyperpolarizability () in acetonitrile, while acetone has the highest value of entropy and toluene has a slightly higher value of zero point vibrational energy (ZPVE) than the rest of the solvents. The results show that careful selection of the solvents and basis sets can tune the frontier molecular orbital energy gap of the molecule and can be used for molecular device applications.

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