scholarly journals Exhaustive exploration of the conformational landscape of mono- and disubstituted five-membered rings by DFT and MP2 calculations

RSC Advances ◽  
2019 ◽  
Vol 9 (42) ◽  
pp. 24134-24145 ◽  
Author(s):  
Carlos A. Stortz ◽  
Ariel M. Sarotti
Keyword(s):  
Ab Initio ◽  
Membered Ring ◽  
Basis Set ◽  
Mp2 Calculations ◽  
Conformational Landscape ◽  
Dft And Mp2 Calculations

The conformational landscape of 22 different non, mono-, and disubstituted compounds with a five-membered ring was thoroughly explored by ab initio (MP2) and DFT (B3LYP and M06-2X) methods with the 6-311+G** basis set.

On the possible formation of Si=O, Si=S, and Si=Se double bonds via the reaction of silylenes with oxirane, thiirane, and selenirane, respectively. An ab initio theoretical study

2000 ◽  
Vol 78 (11) ◽  
pp. 1496-1510 ◽  
Author(s):  
Yitzhak Apeloig ◽  
Stepan Sklenak
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Gas Phase ◽  
Theoretical Study ◽  
Single Step ◽  
Membered Ring ◽  
Basis Set ◽  
Double Bonds ◽  
Ring Compounds ◽  
Encounter Complex

The reactions between silylene, H2Si, and the three-membered ring compounds, oxirane (9), thiirane (10), and selenirane (11), which provide possible routes to Si=X (X = O, S, Se) double bonds were studied by ab initio calculations at the MP2 and QCISD correlated levels of theory employing the polarized 6-31G** basis set. The calculations show that the three reactions are all highly exothermic, (–61.8, –45.2, –52.9 kcal/mol at MP2/6-31G**//MP2/6-31G** for X = O, S, Se, respectively). In the gas phase, at 0 K, these reactions are predicted to be also spontaneous (i.e., the calculated transition states are lower in energy than the reactants). At 298 K, entropy contributions result in small barriers on the ΔG surface for X = O, S (7.4 kcal/mol for both reactions at QCISD (full)/6-31G**), but for X = Se the reaction remains spontaneous. Thus, the calculations suggest that these reactions are viable routes for the preparation of compounds with Si=X (X = O, S, Se) double bonds. The first step in all the reactions is the barrier-less formation of an encounter-complex between the silylene and the X atom of the precursor. For X = O, S, these complexes are predicted to be sufficiently stable to be observed in a matrix. The reaction steps which follow depend on X; for X = O fragmentation of the silylene-XC2H4 complex proceeds in two steps via a biradical intermediate, while for X = S and X = Se the fragmentation occurs in a single step. The first ab initio calculations for H2Si=Se are reported.Key words: silylene, silanone, silanethione, silaneselone.

Theoretical studies of azulene and its derivatives

2000 ◽  
Vol 78 (2) ◽  
pp. 224-232 ◽  
Author(s):  
Bo-Cheng Wang ◽  
Yun-Shan Lin ◽  
Jian-Chuang Chang ◽  
Pei-Yu Wang
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Relative Stability ◽  
Basis Set ◽  
Basis Sets ◽  
Ground State Structure ◽  
Geometrical Isomers ◽  
Bromination Product ◽  
Geometry Analysis ◽  
Dft And Mp2 Calculations

The results of ab initio calculations with the 6-31G basis sets on azulene and its derivatives (including azulenequinones and diazoazulenequinones) are presented in accordance with considerations of their structures and bonding. Azulene is a non-alternant compound with ten π electrons and has either a Cs or C2v symmetry depending on the different carbon bonding. The semiempirical and HF ab initio calculations converge to a Cs symmetry and the DFT and MP2 calculations converge to a C2v symmetry as a ground state structure of azulene. The CIS calculations describe the excited state of azulene and the first excitation energy (S0 - S1) is 533 nm (CIS/6-31+G*), which could illustrate the azure color of azulene. According to the geometry analysis, there are 16 geometrical isomers in azulenequinone conjugated diketones of azulene. Ab initio calculation with the 6-31G basis set generates 1,5- and 1,7-azulenequinone being the most stable isomers of azulenequinone. Theoretically, the relative stability of the bromination product of azulenequinones indicates that 7-bromo-1,5-azulenequinone and 3-bromo-1,7- azulenequinone (for monobromoazulenequinones) and 3,7-dibromo-1,5- azulenequinone and 3,5-dibromo-1,7-azulenequinone (for dibromoazulenequinones) are more stable isomers. The product of diazotization of amino- bromoazulenes is diazoazulenequinone in which a diazo group replaces a ketone group. Isomeric 1,8- and 1,2-diazoazulenequinones are the most stable isomers of diazoazulenequinone according to the theoretical consideration. Due to the resonance and relative stability, diazoazulenequinone may easily extrude nitrogen and form the corresponding triplet ketocarbene intermediate and electronic isomers that undergo photoreaction with THF leading to a polyether bridged azulene (crown type ether). The cyclic reactions in diazoazulenequinone are also studied.Key words: azulene, azulenequinone, diazoazulenequinone, ab initio.

Model Narrow Nanotubes Related to C36, C32 and C20: Computationa Insight

2001 ◽  
Vol 706 ◽  
Author(s):  
Zdeněk Slanina ◽  
Filip Uhlík
Keyword(s):  
Ab Initio ◽  
Significant Role ◽  
Lower Energy ◽  
Parametric Method ◽  
Membered Ring ◽  
The Other ◽  
Basis Set ◽  
Related Model ◽  
Hartree Fock ◽  
Non Local

AbstractVery recently, narrow nanotubes have been observed with a diameter of 5 Å and even with a diameter of 4 Å. It has been supposed that the narrow nanotubes are closed by fragments of C36 and C20 fullerenes. The contribution reports computations on related model nanotubes with stoichiometries like C84, C96 or C80. Computations are carried out at the PM3 (Parametric Method 3), SaM1 (Semi-Ab-Initio Model 1), HF/4-31G (Hartree-Fock SCF approach with the standard 4-31G basis set), and B3LYP/6-31G* (Becke's three parameter functional with the non-local Lee-Yang-Parr correlation functional using the standard 6-31G* basis set) levels, though the geometry optimizations are performed only at the semiempirical levels. Two C36 fullerenes are considered, D6h and D2d, and, for example, at the PM3 level and with the C84 nanotube stoichiometry the D2d cage closure gives a lower energy (by 185 kcal/mol and diameter of 5.42 Å). There is another possible candidate, C32 cage with a D4d symmetry. At the PM3 level and with the C96 nanotube stoichiometry the D4d closure has the nanotube enrgy lower by 210 kca/mol (with the nanotube diameter of 5.43 Å) compared to the D6h nanotube closure. On the other hand, four-membered rings should not play a significant role in the narrow nanotubes with the diameter of 4 Å, where the dodecahedron-related closure should be exclusive as a four-membered ring containing structure is located already much higher in energy.

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 The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings

2021 ◽  
Author(s):  
Christopher Antony Ramsden ◽  
Wojciech Piotr Oziminski
Keyword(s):  
Ab Initio ◽  
Type A ◽  
Nbo Analysis ◽  
Ring Systems ◽  
X Ray ◽  
X Ray Crystallography ◽  
Bond Model ◽  
Lone Pairs ◽  
Mp2 Calculations ◽  
Good Agreement

AbstractBased on structures determined by X-ray crystallography, ab initio MP2 calculations on type A mesoionic rings give geometries in good agreement with observed values. A study of four mesoionic ring systems, each with exocyclic oxygen, nitrogen or carbon groups, shows that the presence and configuration of exocyclic lone pairs significantly influences the geometry and configurational preference. Using a localised bond model and NBO analysis, these effects are rationalised in terms of an anomeric interaction of lone pairs with the antibonding orbitals of adjacent σ bonds. In agreement with experiment, similar effects are calculated for pyran-2-imines.

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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