The reaction of aluminum atoms with methane

1990 ◽  
Vol 68 (4) ◽  
pp. 633-639 ◽  
Author(s):  
Hengtai Yu ◽  
John D. Goddard
Keyword(s):  
Transition State ◽  
Energy Barrier ◽  
Energy Requirement ◽  
Basis Sets ◽  
Insertion Reaction ◽  
Reaction Coordinates ◽  
Abstraction Reaction ◽  
Level Transition ◽  
Split Valence ◽  
Good Agreement

Abinitio SCF and CISD calculations with split valence and larger basis sets have been carried out for several possible reactions of ground state (2P) aluminum atoms with methane. In particular, the barriers to insertion of Al into a CH bond to give a CH3AlH intermediate and the abstraction of a hydrogen by Al to give AlH and CH3 have been studied. At the SCF level, transition state structures have been located and verified by vibrational analyses. In addition, intrinsic reaction coordinates (IRC) have been followed from the transition state geometries down to reactants and to products. Calculated vibrational frequencies and lowest excitation energy for the CH3AlH intermediate are in good agreement with matrix isolation experimental results. As anticipated on the basis of MO natural correlation symmetry arguments, the energy barrier to the insertion reaction is high (~50 kcal mol−1 at the CI level). The energy requirement for the abstraction reaction is similar.[Formula: see text]Keywords: Al, CH4, HAlCH3, insertion and abstraction reactions.

Nitrosyl cyanide and related aspects of the ONCN potential surface

1978 ◽  
Vol 31 (11) ◽  
pp. 2349 ◽  
Author(s):  
BG Gowenlock ◽  
L Radom
Keyword(s):  
Experimental Data ◽  
Ab Initio ◽  
Molecular Orbital ◽  
Related Species ◽  
Potential Surface ◽  
Basis Sets ◽  
Molecular Orbital Calculations ◽  
Hartree Fock ◽  
Split Valence ◽  
Good Agreement

Ab initio molecular orbital calculations using the restricted Hartree-Fock approach have been carried out for nitrosyl cyanide and related species on the ONCN potential surface. Full geometry optimizations have been performed with the minimal STO-3G and split-valence 4-31G basis sets. Calculated (4-31G) geometries are in good agreement with available experimental data as are the energy changes in the reactions ONCN → NO + CN and NO + CN → N2 + CO. Possible mechanisms are discussed.

AB INITIO CALCULATIONS AND KINETICS STUDY FOR THE DIRECT HYDROGEN ABSTRACTION REACTION OF HYDROGEN SELENIDE WITH HYDROGEN ATOM

2011 ◽  
Vol 10 (05) ◽  
pp. 629-639
Author(s):  
YUE ZHANG
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Transition State Theory ◽  
Hydrogen Abstraction ◽  
State Theory ◽  
Forward Rate ◽  
Variational Transition State Theory ◽  
Abstraction Reaction ◽  
Hydrogen Abstraction Reaction ◽  
Good Agreement

We present a direct ab initio dynamics study of thermal rate constants of the hydrogen abstraction reaction of H 2 Se + H → SeH + H 2. The QCISD and CCSD(T) methods were employed to optimize the geometries of stationary points and to calculate the harmonic vibrational frequencies. The split-valence 6-311 + G(d,p) and correlation-consistent cc-pVTZ basis sets big enough to describe the geometries and vibrational frequencies of the species involving in the title reaction. The energies obtained at the QCISD(T)/6-311 ++ G(3df,3pd)//QCISD/6-311 + G(d,p) level of theory is able to compare to those calculated at the CCSD(T)/6-311 ++ G(3df,3pd)//CCSD(T)/6-311 + G(d,p) level of theory. The energies of all the selected points along the minimum energy path (MEP) were refined at the QCISD(T)/6-311 ++ G(3df,3pd) level of theory. The barriers were obtained at the both QCISD(T)/6-311 ++ G(3df,3pd)//QCISD/6-311 + G(d,p) and CCSD(T)/6-311 ++ G(3df,3pd)//CCSD(T)/6-311 + G(d,p) levels of theory are in good agreement with experimental one. The forward rate constants were evaluated with both canonical variational transition state theory with small curvature tunneling correction (CVT/SCT) and improved canonical variational transition state theory with small curvature tunneling correction (ICVT/SCT) in the temperature range of 200–2500 K. The calculated forward rate constants of the reaction at the QCISD(T)/6-311 ++ G(3df,3pd)//QCISD/6-311 + G(d,p) level of theory are in good agreement with available experimental data.

scholarly journals DFT calculation and assignment of vibrational spectra of aryl and alkyl chlorophosphates

2014 ◽  
Vol 12 (2) ◽  
pp. 153-163
Author(s):  
Viktor Anishchenko ◽  
Vladimir Rybachenko ◽  
Konstantin Chotiy ◽  
Andrey Redko
Keyword(s):  
Vibrational Spectra ◽  
Basis Sets ◽  
Heavy Atoms ◽  
Wide Range ◽  
Complete Assignment ◽  
Key Factor ◽  
Polarization Functions ◽  
Experimental Values ◽  
Semi Empirical ◽  
Good Agreement

AbstractDFT calculations of vibrational spectra of chlorophosphates using wide range of basis sets and hybrid functionals were performed. Good agreement between calculated and experimental vibrational spectra was reached by the combination of non-empirical functional PBE0 with both middle and large basis sets. The frequencies of the stretching vibrations of the phosphate group calculated using semi-empirical functional B3LYP for all basis sets deviate significantly from the experimental values. The number of polarization functions on heavy atoms was shown to be a key factor for the calculation of vibrational frequencies of organophosphates. The importance of consideration of all the stable rotamers for a complete assignment of fundamental modes was shown.

scholarly journals A QM/MM Study of Acylphosphatase Reveals the Nucleophilic-Attack and Ensuing Carbonyl-Assisted Catalytic Mechanisms

2020 ◽  
Author(s):  
zheng zhao ◽  
Phil bourne ◽  
Hao Hu ◽  
Huanyu Chu
Keyword(s):  
Energy Barrier ◽  
Potential Energy Surfaces ◽  
Catalytic Mechanism ◽  
Interaction Networks ◽  
Nucleophilic Attack ◽  
Reaction Coordinates ◽  
Transition State Stabilization ◽  
Catalytic Mechanisms ◽  
Energy Surfaces

Acylphosphatase is one of the vital enzymes in many organs/tissues to catalyze an acylphosphate molecule into carboxylate and phosphate. Here we use a combined <i>ab initio</i> QM/MM approach to reveal the catalytic mechanism of the benzoylphosphate-bound acylphosphatase system. Using a multi-dimensional reaction-coordinates-driving scheme, we obtained a detailed catalytic process including one nucleophilic-attack and then an ensuing carbonyl-shuttle catalytic mechanism by calculating two-dimensional potential energy surfaces. We also obtained an experiment-agreeable energy barrier and validated the role of the key amino acid Asn38. Additionally, we qualified the transition state stabilization strategy based on the amino acids-contributed interaction networks revealed in the enzymatic environment. This study provided usefule insights into the underlying catalytic mechanism to contribute to disease-involved research.

scholarly journals Popular Integration Grids Can Result in Large Errors in DFT-Computed Free Energies

2019 ◽  
Author(s):  
Andrea N. Bootsma ◽  
Steven Wheeler
Keyword(s):  
Transition State ◽  
Molecular Orientation ◽  
Density Functional ◽  
Basis Sets ◽  
Free Energies ◽  
Functional Theory ◽  
Metal Free ◽  
Stereoselective Reactions ◽  
Functionalization Reaction ◽  
Transition State Structures

<div>Density functional theory (DFT) has emerged as a powerful tool for analyzing organic and organometallic systems and proved remarkably accurate in computing the small free energy differences that underpin many chemical phenomena (e.g. regio- and stereoselective reactions). We show that the lack of rotational invariance of popular DFT integration grids reveals large uncertainties in computed free energies for isomerizations, torsional barriers, and regio- and stereoselective reactions. The result is that predictions based on DFT-computed free energies for many systems can change qualitatively depending on molecular orientation. For example, for a metal-free propargylation of benzaldehyde, predicted enantioselectivities based on B97-D/def2-TZVP free energies using the popular (75,302) integration grid can vary from 62:38 to 99:1 by simply rotating the transition state structures. Relative free energies for the regiocontrolling transition state structures for an Ir-catalyzed C–H functionalization reaction computed using M06/6-31G(d,p)/LANL2DZ and the same grid can vary by more than 5 kcal mol–1, resulting in predicted regioselectivities that range anywhere from 14:86 to >99:1. Errors of these magnitudes occur for different functionals and basis sets, are widespread among modern applications of DFT, and can be reduced by using much denser integration grids than commonly employed.</div>

Molecular orbital and 1H nuclear magnetic resonance studies of the inversion potentials of thianthrene and thioxanthene

1991 ◽  
Vol 69 (6) ◽  
pp. 927-933 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Christian Beaulieu
Keyword(s):  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Basis Set ◽  
Basis Sets ◽  
Inversion Barrier ◽  
H Nmr ◽  
Methylene Groups ◽  
Spin Spin Coupling ◽  
Split Valence

The inversion potentials, obtained from STO-3G, STO-3G(*), 3-21G, 3-21G(*), and 4-31G basis sets, are reported for thianthrene and thioxanthene, molecules in which both or only one of the methylene groups have been replaced by sulfur in 9,10-dihydroanthracene. Comparison with the available experimental data suggests that the split-valence bases lead to an overestimate, possibly by about 10 kJ/mol, of the inversion barrier in the crystal, whereas the STO-3G and STO-3G* basis sets underestimate this barrier. It appears that the inversion barrier for thianthrene is much lower in solution than in the crystal. The long-range coupling constants between the methylene and ring protons for thioxanthene in solution are consistent with an inversion barrier somewhat smaller than those obtained with the split-valence bases but rather larger than those predicted with the STO-3G basis set. The bond lengths and angles in the equilibrium structures of the two molecules, as computed with the 3-21G(*) basis, agree reasonably well with those in their crystals, except that the theoretical folding angles are smaller than measured. These discrepancies become less marked when expectation values are calculated from the theoretical inversion potentials at finite temperatures. Key words: MO calculations, inversion potentials of thianthrene and thioxanthene; 1H NMR, thioxanthene; spin–spin coupling constants, long range, in thioxanthene.

Reaction kinetics of hydrogen atom abstraction from isopentanol by the H atom and HO2˙ radical

2018 ◽  
Vol 20 (16) ◽  
pp. 10895-10905
Author(s):  
Prajakta Rajaram Parab ◽  
K. Alexander Heufer ◽  
Ravi Xavier Fernandes
Keyword(s):  
Hydrogen Atom ◽  
Transition State ◽  
Reaction Kinetics ◽  
Hydrogen Atom Abstraction ◽  
Abstraction Reaction ◽  
Kinetics Of ◽  
Ho2 Radical

The formation of the transition state for the H-abstraction reaction from isopentanol by the HO2˙ radical.

Thermal and photochemical study of CH3OH and CH3OH–O2 astrophysical ices

2020 ◽  
Vol 500 (1) ◽  
pp. 1188-1200
Author(s):  
Killian Leroux ◽  
Lahouari Krim
Keyword(s):  
Interstellar Medium ◽  
Organic Compounds ◽  
Organic Molecules ◽  
Uv Light ◽  
Hydrogen Abstraction ◽  
Insertion Reaction ◽  
Abstraction Reaction ◽  
Uv Photons ◽  
Astrophysical Ices ◽  
Complex Organic

ABSTRACT Methanol, which is one of the most abundant organic molecules in the interstellar medium, plays an important role in the complex grain surface chemistry that is believed to be a source of many organic compounds. Under energetic processing such as ultraviolet (UV) photons or cosmic rays, methanol may decompose into CH4, CO2, CO, HCO, H2CO, CH3O and CH2OH, which in turn lead to complex organic molecules such as CH3OCHO, CHOCH2OH and HOCH2CH2OH through radical recombination reactions. However, although molecular oxygen and its detection, abundance and role in the interstellar medium have been the subject of many debates, few experiments on the oxidation of organic compounds have been carried out under interstellar conditions. The present study shows the behaviour of solid methanol when treated by UV light and thermal processing in oxygen-rich environments. Methanol has been irradiated in the absence and presence of O2 at different concentrations in order to study how oxidized complex organic molecules may form and also to investigate the O-insertion reaction in the C–H bound to form methanediol HOCH2OH through a CH3OH + O(1D) solid-state reaction. The adding of O2 in the thermal and photochemical reaction of solid methanol leads to the formation of O3, H2O and HO2, in addition to three main organics, HCOOH, CHOCHO and HOCH2OH. We show that in an O2-rich environment, species such as CO, CH4, HCO, CH3OH and CHOCH2OH are oxidized into CO2, CH3OH, HC(O)OO, HOCH2OH and CHOCHO, respectively, while HCOOH might be formed through the H2CO + O(3P) → (OH + HCO)cage → HCOOH hydrogen-abstraction reaction.

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