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 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 MO Optimizations of Osmiumtetracarbonyldihydride and Metallacyclophanes with two Osmium Atoms and their Molecular Complexes with Different Guests

1998 ◽  
Vol 53 (10) ◽  
pp. 1223-1235
Author(s):  
Inge Warttmann ◽  
Günter Häfelinger
Keyword(s):  
Ab Initio ◽  
Density Functional ◽  
Cyano Group ◽  
Single Point ◽  
Molecular Complexes ◽  
Basis Set ◽  
Basis Sets ◽  
Basis Set Superposition Error ◽  
Hartree Fock ◽  
Surprising Effect

AbstractAb initio Hartree-Fock (HF) and density functional (DFT) optimizations on the test m olecule osmiumtetracarbonyldihydride (13) with various basis sets show that the lanl2mb pseudopotential basis set for osmium leads in the HF approximation to more reliable molecular geometries than the DFT calculations. This HF procedure was used for the optimizations of molecular geometries of three isomeric 4,4,4,4,17,17,17,17-octacarbonyl-4,17-diosma[7.7]ortho-, meta- and paracyclophanes 1 to 3, of which 3 was found to be predestined for formation of various host-guest complexes with possible guests benzene (4), fluorobenzene (5), 1,3,5- trifluorobenzene (6), 1,2,4,5-tetrafluorobenzene (7), hexafluorobenzene (8), fluoroanil (9), tetrafluoroethene (10), tetracyanoethene (11) and aniline (12). Results of optimized hostguest geometries are presented graphically for inclusions and associations of guest 4 to 12 with 3. Calculated lanl2mb interaction energies, after correction for basis set superposition error (BSSE), remain favourable only for inclusion of 5 and associations of 5, 10, 11 and 12. Additionally lanl2dz single point calculations for inclusion, which may not need BSSE correction because of the improved basis set, are favourable for 6 and 12. According to lanl2mb HOMO and LUMO energies, 3 may as well easily donate or accept electrons. This may be an interpretation to the surprising effect, that Mulliken total charges are positive on the electron accepting guest molecules 4 to 11. There are geometrical peculiarities in the optimized host-guest complexes for inclusion and association. Fluorine atoms of 5 to 10 and nitrogen atoms of a cyano group of 11 and the amino group of 12 like to come close to one or two carbonyl groups. Similar distances of 2.70 Å to 3.57 Å between the O atom of the carbonyl group and the F atom or N atom appear in all optimizations of inclusion and association of 5 to 12 except in the case of association of tetrafluoroethene (10).

An ab initio Study of the Equilibrium Structures of Prop-2-ynamine (Propargylamine) and the Transition Structures Connecting Them

1987 ◽  
Vol 40 (3) ◽  
pp. 435 ◽  
Author(s):  
NV Riggs
Keyword(s):  
Ab Initio ◽  
Internal Rotation ◽  
Vibrational Energy ◽  
Vibrational Analysis ◽  
Zero Point ◽  
Basis Set ◽  
Basis Sets ◽  
Experimental Result ◽  
Transition Structure ◽  
Equilibrium Structures

Optimization with the 3-21G and 3-21G(N*) basis sets finds, in agreement with previous ab initio studies and the experimental result, the anti conformation of prop-2-ynamine to be of lowest energy and, after zero-point vibrational -energy ( Ezpv ) corrections, the gauche form to lie 11 kJ mol-l higher; by vibrational analysis, both are confirmed as equilibrium structures. The synform was not able to be optimized with the 3.21G basis set but, with the 3-21G(N*) basis set, is found to lie 8.6 kJ mol-1 (after Ezpv corrections) above the gauche form, and is shown by vibrational analysis to be a transition structure connecting enantiomeric gauche forms by internal rotation about the N-C bond. The transition structure connecting gauche and anti forms by internal rotation lies 6.3 kJ mol-1 (after Ezpv corrections) above the gauche form, and the transition structure for inversion at the nitrogen atom lies 31.1 kJ mol- l (after Ezpvcorrections) above the anti form.

scholarly journals Acceleration vs Accuracy: Influence of Basis Set Quality on the Mechanism and Dynamics Predicted by Ab Initio Molecular Dynamics

2019 ◽  
Author(s):  
Sharma Yamijala ◽  
Zulfikhar A. Ali ◽  
Bryan Wong
Keyword(s):  
Molecular Dynamics ◽  
Ab Initio ◽  
Density Functional ◽  
Single Point ◽  
Basis Set ◽  
Ab Initio Molecular Dynamics ◽  
Energy Calculation ◽  
Shift Reaction ◽  
The Impact

<p>Ab initio molecular dynamics (AIMD) is an indispensable tool for understanding the mechanistic details of externally-energy mediated chemical reactions. In this work, we show that the predicted thermodynamic and catalytic properties of certain reactions using AIMD simulations critically depend on the quality of the employed basis set. To this end, we have examined the reactants and products of the water-gas shift reaction (viz., CO, CO<sub>2</sub>, H<sub>2</sub>, and H<sub>2</sub>O) and studied their interaction with the ZnO(101̄0) surface using density functional theory (DFT) and Born Oppenheimer Molecular Dynamics (BOMD) simulations. By merely increasing the quality of the basis, from double zeta (commonly used in most calculations of these systems) to triple zeta, we surprisingly find that the reaction outcome of an H<sub>2</sub>O molecule colliding with a ZnO surface pre-covered with carbon monoxide gives qualitatively different results. These surprising results are shown to be robust with similar trends that are also obtained with other software packages. Furthermore, we show that the calculated adsorption energies can vary by as much as 380 meV (which is an order of magnitude larger than room temperature) by simply changing the basis set. Using electron density difference maps, we present mechanistic insight into the origin of these changes. Finally, we propose a simple diagnostic test that uses a single-point binding energy calculation to estimate the impact of basis-set quality, which can be used before carrying out a computationally-expensive BOMD simulation.</p>

scholarly journals Acceleration vs Accuracy: Influence of Basis Set Quality on the Mechanism and Dynamics Predicted by Ab Initio Molecular Dynamics

2019 ◽  
Author(s):  
Sharma Yamijala ◽  
Zulfikhar A. Ali ◽  
Bryan Wong
Keyword(s):  
Molecular Dynamics ◽  
Ab Initio ◽  
Density Functional ◽  
Single Point ◽  
Basis Set ◽  
Ab Initio Molecular Dynamics ◽  
Energy Calculation ◽  
Shift Reaction ◽  
The Impact

<p>Ab initio molecular dynamics (AIMD) is an indispensable tool for understanding the mechanistic details of externally-energy mediated chemical reactions. In this work, we show that the predicted thermodynamic and catalytic properties of certain reactions using AIMD simulations critically depend on the quality of the employed basis set. To this end, we have examined the reactants and products of the water-gas shift reaction (viz., CO, CO<sub>2</sub>, H<sub>2</sub>, and H<sub>2</sub>O) and studied their interaction with the ZnO(101̄0) surface using density functional theory (DFT) and Born Oppenheimer Molecular Dynamics (BOMD) simulations. By merely increasing the quality of the basis, from double zeta (commonly used in most calculations of these systems) to triple zeta, we surprisingly find that the reaction outcome of an H<sub>2</sub>O molecule colliding with a ZnO surface pre-covered with carbon monoxide gives qualitatively different results. These surprising results are shown to be robust with similar trends that are also obtained with other software packages. Furthermore, we show that the calculated adsorption energies can vary by as much as 380 meV (which is an order of magnitude larger than room temperature) by simply changing the basis set. Using electron density difference maps, we present mechanistic insight into the origin of these changes. Finally, we propose a simple diagnostic test that uses a single-point binding energy calculation to estimate the impact of basis-set quality, which can be used before carrying out a computationally-expensive BOMD simulation.</p>

A Systematic DFT Study of Two Elementary Steps Involving Hydrogen Peroxide and the Hydroxyl Radical in the Fenton Reaction

2018 ◽  
Author(s):  
Danilo Carmona ◽  
David Contreras ◽  
Oscar A. Douglas-Gallardo ◽  
Stefan Vogt-Geisse ◽  
Pablo Jaque ◽  
...  
Keyword(s):  
Hydrogen Peroxide ◽  
Hydroxyl Radical ◽  
Ab Initio ◽  
Fenton Reaction ◽  
Basis Set ◽  
Basis Sets ◽  
Dft Methods ◽  
Elementary Steps ◽  
Oxygen Oxygen ◽  
Complete Basis Set

The Fenton reaction plays a central role in many chemical and biological processes and has various applications as e.g. water remediation. The reaction consists of the iron-catalyzed homolytic cleavage of the oxygen-oxygen bond in the hydrogen peroxide molecule and the reduction of the hydroxyl radical. Here, we study these two elementary steps with high-level ab-initio calculations at the complete basis set limit and address the performance of different DFT methods following a specific classification based on the Jacob´s ladder in combination with various Pople's basis sets. Ab-initio calculations at the complete basis set limit are in agreement to experimental reference data and identified a significant contribution of the electron correlation energy to the bond dissociation energy (BDE) of the oxygen-oxygen bond in hydrogen peroxide and the electron affinity (EA) of the hydroxyl radical. The studied DFT methods were able to reproduce the ab-initio reference values, although no functional was particularly better for both reactions. The inclusion of HF exchange in the DFT functionals lead in most cases to larger deviations, which might be related to the poor description of the two reactions by the HF method. Considering the computational cost, DFT methods provide better BDE and EA values than HF and post--HF methods with an almost MP2 or CCSD level of accuracy. However, no systematic general prediction of the error based on the employed functional could be established and no systematic improvement with increasing the size in the Pople's basis set was found, although for BDE values certain systematic basis set dependence was observed. Moreover, the quality of the hydrogen peroxide, hydroxyl radical and hydroxyl anion structures obtained from these functionals was compared to experimental reference data. In general, bond lengths were well reproduced and the error in the angles were between one and two degrees with some systematic trend with the basis sets. From our results we conclude that DFT methods present a computationally less expensive alternative to describe the two elementary steps of the Fenton reaction. However, choice of approximated functionals and basis sets must be carefully done and the provided benchmark allows a systematic validation of the electronic structure method to be employed

Rotational de-excitations of C3H+ (1Σ+) by collision with He: new ab initio potential energy surface and scattering calculations

2020 ◽  
Vol 494 (4) ◽  
pp. 5675-5681 ◽  
Author(s):  
Sanchit Chhabra ◽  
T J Dhilip Kumar
Keyword(s):  
Potential Energy ◽  
Potential Energy Surface ◽  
Ab Initio ◽  
Cross Sections ◽  
Energy Surface ◽  
Molecular Ions ◽  
Basis Set ◽  
Rate Coefficients ◽  
Close Coupling ◽  
Collisional Rate Coefficients

ABSTRACT Molecular ions play an important role in the astrochemistry of interstellar and circumstellar media. C3H+ has been identified in the interstellar medium recently. A new potential energy surface of the C3H+–He van der Waals complex is computed using the ab initio explicitly correlated coupled cluster with the single, double and perturbative triple excitation [CCSD(T)-F12] method and the augmented correlation consistent polarized valence triple zeta (aug-cc-pVTZ) basis set. The potential presents a well of 174.6 cm−1 in linear geometry towards the H end. Calculations of pure rotational excitation cross-sections of C3H+ by He are carried out using the exact quantum mechanical close-coupling approach. Cross-sections for transitions among the rotational levels of C3H+ are computed for energies up to 600 cm−1. The cross-sections are used to obtain the collisional rate coefficients for temperatures T ≤ 100 K. Along with laboratory experiments, the results obtained in this work may be very useful for astrophysical applications to understand hydrocarbon chemistry.

scholarly journals Density Functional Theory and Complete Basis Set Ab Initio Computational Study of Five-, Six-, Seven- and Eight-hydrogen Coordinated Carbon Cations

1999 ◽  
Vol 23 (8) ◽  
pp. 502-503
Author(s):  
Branko S. Jursic
Keyword(s):  
Density Functional Theory ◽  
Ab Initio ◽  
Density Functional ◽  
Computational Study ◽  
Basis Set ◽  
Functional Theory ◽  
Complete Basis ◽  
Complete Basis Set ◽  
High Level

High level ab initio and density functional theory studies are performed on highly protonated methane species.

Vicinal proton—proton coupling constants. Basis set dependence in SCF ab initio calculations

1993 ◽  
Vol 206 (1-4) ◽  
pp. 253-259 ◽  
Author(s):  
Jesús San-Fabián ◽  
Joaquín Guilleme ◽  
Ernesto Díez ◽  
Paolo Lazzeretti ◽  
Massimo Malagoli ◽  
...  
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Coupling Constants ◽  
Basis Set ◽  
Proton Proton ◽  
Proton Coupling ◽  
Vicinal Proton
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