Nonadditive effects of polysubstitution in CH4 and CH3: a test of abinitio and semiempirical MO methods

1983 ◽  
Vol 61 (7) ◽  
pp. 1567-1572 ◽  
Author(s):  
N. Colin Baird
Keyword(s):  
Basis Set ◽  
Semiempirical Methods ◽  
Valence Shell ◽  
Bond Lengths ◽  
Hartree Fock ◽  
Nonadditive Effects ◽  
D Orbitals ◽  
Split Valence ◽  
Abinitio Calculations

The effects to the bond lengths and molecular energy of making multiple substitutions at the same carbon atom in methane and the methyl free radical are studied using various molecular orbital methods. All abinitio calculations were based upon the Hartree–Fock formalism (unrestricted in the case of free radicals) and employed the STO-3G (with d orbitals on chlorine), 3-21G, and 4-31G bases, the last both with and without a set of Gaussian d orbitals on the carbon. The semiempirical methods used were the MINDO/3 and MNDO methods of Dewar and co-workers; computations for polysubstituted ethanes by these two methods also arc reported. The abinito methods which use a split valence shell basis set account very well for the trends in bond lengths and heats of formation, at least when the polysubstituent is fluorine or hydroxyl. In contrast, the semiempirical calculations and the abinitio STO-3G results gave very poor results. Finally, the role of interactions between the AH bonds in a variety of hydrides AHn is illustrated using experimental energetics.

From Atomic Orbitals to Molecular Orbitals and Chemical Bonds

2020 ◽  
pp. 150-187
Author(s):  
Jochen Autschbach
Keyword(s):  
Hydrogen Molecule ◽  
Atomic Orbital ◽  
Plane Waves ◽  
Basis Set ◽  
Orbital Method ◽  
Chemical Bonds ◽  
Atomic Orbitals ◽  
Hartree Fock ◽  
Generalized Matrix

It is shown how an aufbau principle for atoms arises from the Hartree-Fock (HF) treatment with increasing numbers of electrons. The Slater screening rules are introduced. The HF equations for general molecules are not separable in the spatial variables. This requires another approximation, such as the linear combination of atomic orbitals (LCAO) molecular orbital method. The orbitals of molecules are represented in a basis set of known functions, for example atomic orbital (AO)-like functions or plane waves. The HF equation then becomes a generalized matrix pseudo-eigenvalue problem. Solutions are obtained for the hydrogen molecule ion and H2 with a minimal AO basis. The Slater rule for 1s shells is rationalized via the optimal exponent in a minimal 1s basis. The nature of the chemical bond, and specifically the role of the kinetic energy in covalent bonding, are discussed in details with the example of the hydrogen molecule ion.

Structure and bonding in square-planar chalcogen rings

1992 ◽  
Vol 70 (2) ◽  
pp. 348-352 ◽  
Author(s):  
Leif J. Saethre ◽  
Odd Gropen
Keyword(s):  
Potential Model ◽  
Population Analysis ◽  
Atomic Charge ◽  
Molecular Structures ◽  
Basis Set ◽  
Single Bond ◽  
Bond Lengths ◽  
Square Planar ◽  
Structure And Bonding ◽  
D Orbitals

The molecular structures of square-planar X42+, X4+, and X4 (X = S, Se, Te) have been calculated using the effective core potential model. For X42+ the agreement between experimental and calculated values is excellent provided that d orbitals are included in the basis set. For the hypothetical molecules X4+ and X4 the bond lengths are found to increase dramatically as one and, subsequently, two electrons are added to the systems. Extensive population analysis shows that this increase is almost exclusively due to loss of bonding in the π system, whereas the bonding in the σ system remains relatively unaltered. These results make it possible to predict covalent single bond radii for S, Se, and Te for which the influence of π repulsion is removed. From the calculated variation of bond lengths with atomic charge, bond lengths are predicted for a series of planar disulphide rings. Keywords: structure, bonding, chalcogen, theoretical, ECP.

Ab initio studies of isocyanatoborane and difluoroisocyanatoborane; bent or linear BNCO chains?

1991 ◽  
Vol 69 (6) ◽  
pp. 1000-1005 ◽  
Author(s):  
Susan Ellis ◽  
Edward G. Livingstone ◽  
Nicholas P. C. Westwood
Keyword(s):  
Carbon Atom ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Potential Surface ◽  
Energy Difference ◽  
Basis Set ◽  
Basis Sets ◽  
Hartree Fock ◽  
Split Valence ◽  
Valence Basis Set

Ab initio calculating at the 3-21G, 6-31G*, 6-31G**, 6-311G*, and 6-311G** Hartree–Fock levels, have been performed on the unknown H2BNCO and F2BNCO molecules in order to establish the geometries of these isoelectronic propadienone analogues. For H2BNCO the double split valence basis sets lead to linear BNCO chains, whereas either a triple split valence basis set, or the inclusion of correlation to second order (MP2/6-31G*) gives trans-bent structures. These have angles at nitrogen of 153.6° (6-311G*) or 149.9° (MP2/6-31G*), with the potential surface for angle bending extremely flat, and 0.5 kJ mol−1 (6-311 G*) or 1.12 kJ mol−1 (MP2/6-31 G*) separating the C2v and Cs structures. For the bent structures there is a small trans-bend (4–6°) at the carbon atom. The F2BNCO molecule is also linear at the 3-21G level, but is, however, already trans-bent (145.6° at nitrogen, 175.9° at carbon) at the 6-31G* Hartree–Fock level; 1.47 kJ mol−1 separates the bent and linear structures. The triple split valence basis set 6-311G* leads to a further decrease in the angle at nitrogen (141.0°), and a similar NCO angle (175.9°), with the bent structure favoured by 3.85 kJ mol−1. MP2/6-31G* calculations give a minimum with an angle at nitrogen of 140.2°, and a bent-linear energy difference of 3.58 kJ mol−1. Key words: ab initio calculations, isocyanatoboranes, structures, quasilinearity, propadienone analogues.

Abinitio Calculations of the Barriers to Rotation in Some Monosubstituted Benzenes

1986 ◽  
Vol 39 (7) ◽  
pp. 1157 ◽  
Author(s):  
S Marriott ◽  
RD Topsom
Keyword(s):  
Electron Transfer ◽  
Ab Initio ◽  
Benzene Ring ◽  
Theoretical Calculations ◽  
Geometry Optimization ◽  
Basis Set ◽  
Monosubstituted Benzenes ◽  
Split Valence ◽  
Good Agreement ◽  
Abinitio Calculations

Theoretical calculations at the ab initio level have been made of the twofold rotational barriers of some monosubstituted benzenes, and the effects both of basis set and of geometry optimization have been examined. Values at the STO-3G//STO-3G level are in good agreement with experiment. The use of split-valence sets, even with geometry optimization, is much less satisfactory, and this may be related to an overestimation of π-electron transfer between the substituent and the benzene ring.

Theoretical Coupling and Stability of Boronic Acid Adducts with Catecholamines

2019 ◽  
Vol 16 (4) ◽  
pp. 467-475
Author(s):  
Eugeniy Demianenko ◽  
Alexey Rayevsky ◽  
Marvin A. Soriano-Ursúa ◽  
José G. Trujillo-Ferrara
Keyword(s):  
Drug Design ◽  
Boronic Acids ◽  
Relative Strength ◽  
Basis Set ◽  
Para Position ◽  
Hydroxyl Group ◽  
Meta Position ◽  
The Us ◽  
Split Valence

Background: Catecholamines combined with boric/boronic acids are attractive chemical agents in drug design because some of their adducts have shown interesting biological activity. Scant information exists about their stability. Objective: The aim of the present theoretical study was to explore the role of boron in molecules that combine catecholamines and boric/boronic acids, with a particular interest in examining stability. Method: The methodology was based on the US GAMESS program using DFT with the B3LYP exchange-correlation functional and the 6-31G (d,p) split-valence basis set. Results: According to the current findings, the boron-containing compounds (BCCs) exhibit weaker bonding to the hydroxyls on the ethylamine moiety than to those in the aromatic ring. The strongest binding site of a hydroxyl group was often found to be in meta-position (relative to ethylamine moiety) for boron-free compounds and in para-position for BCCs. Nonetheless, the methyl substituent in the amino group was able to induce changes in this pattern. We analyzed feasible boronsubstituted structures and assessed the relative strength of the respective C-B bonds, which allowed for the identification of the favorable points for reaction and stability. Conclusion: It is feasible to form adducts by bonding on the amine and catechol sides of catecholamines. The presence of boron stabilizes the adducts in para-position. Since some of these BCCs are promising therapeutic agents, understanding the mechanisms of reaction is relevant for drug design.

Hartree-Fock Stability and Broken Symmetry Solutions of O2- and S2- Anions in External Confinement

2007 ◽  
Vol 72 (2) ◽  
pp. 197-222 ◽  
Author(s):  
Filip Holka ◽  
Pavel Neogrády ◽  
Miroslav Urban ◽  
Josef Paldus
Keyword(s):  
Model Systems ◽  
Basis Set ◽  
Broken Symmetry ◽  
Basis Sets ◽  
Gaussian Basis Sets ◽  
Hartree Fock ◽  
Harmonic Confinement ◽  
Electron Correlation Effects ◽  
Doubly Charged

We study the effect of a confining potential on systems that exhibit Hartree-Fock (HF) instabilities, and thus admit broken symmetry (BS) HF solutions, by relying on the O2- and S2- doubly-charged anions as model systems. We find that with the increasing strength of the external harmonic confinement potential, W(r) = 1/2(ωr)2 (with 0.0 ≤ ω < 0.2), the BS solutions are systematically eliminated. We use extended, diffuse, doubly-augmented Gaussian basis sets up to and including d-aug-cc-pV6Z, and find that the number and the character of BS solutions exhibit significant basis set effects. These basis sets were further extended by additional ghost basis functions, located away from the atomic center. The role of the electron correlation effects for the BS HF solutions was examined by the CCSD(T) method. In addition to modelling the confinement by the harmonic-like potential W(r), we also examined a more realistic "confinement", realized by a grid of point charges modelling the crystal structure of MgO. Again, we find that the HF instabilities and the implied BS solutions disappear with the increasing magnitude of the model charges simulating the crystal environment. At the same time, the O2- anion is energetically stabilized with respect to both the O- anion and the neutral oxygen atom.

A molecular orbital study of the conformation (inversion and rotational barriers) and electronic properties of sulfamide

1989 ◽  
Vol 67 (12) ◽  
pp. 2227-2236 ◽  
Author(s):  
Otilia Mó ◽  
José L. G. De Paz ◽  
Manuel Yáñez ◽  
Ibon Alkorta ◽  
José Elguero ◽  
...  
Keyword(s):  
Multiple Bond ◽  
Amino Groups ◽  
Orbital Exponent ◽  
Nitrogen Inversion ◽  
Rotational Barriers ◽  
Polarization Functions ◽  
D Orbitals ◽  
Bond Character ◽  
Abinitio Calculations

Abinitio calculations have been used to study the conformational potential surface of sulfamide, by considering the S—N bond rotations and the nitrogen inversion processes. The lowest energy conformation (b) is found for a cis–trans arrangement of the amino groups, although conformations with cis–cis (a), trans–trans (c), and near staggered (c′) arrangements lie close in energy. Nitrogen inversion barriers are very low, and consequently one may expect forms b and c′ to be the only ones present in the gas phase. Conformer a is very polar, its dipole moment being twice that of b, so it may be favored in condensed media or in polar-solvent solutions. The relative stability of the different isomers is governed by interactions between the amino protons and between the nitrogen lone pairs. Our results show that d–π backbonding, involving the d orbitals on sulfur, is responsible for the multiple bond character of the S—O linkage, but is very small in the S—N interactions. The role of the sulfur d-orbital exponent, when a 6-31G* basis is used, is analyzed on a series of model compounds containing SII, SIV, and SVI. Although the inclusion of d functions on sulfur is crucial to describing correctly the bonding in sulfamide, the results obtained do not change appreciably if a second set of d functions is centered on sulfur. Nevertheless, only when polarization functions are also included for first-row atoms is the description of the system reliable. Keywords: sulfamide, inversion and rotational barriers, sulfur d-functions.

Ab initio and MNDO Calculations of the 35Cl NQR Parameters of Some Organic and Inorganic Molecules

1996 ◽  
Vol 51 (5-6) ◽  
pp. 549-553 ◽  
Author(s):  
Valentin P. Feshin ◽  
Mikhail Yu. Konshin
Keyword(s):  
Ab Initio ◽  
Basis Set ◽  
Hartree Fock ◽  
Mndo Calculations ◽  
Inorganic Molecules ◽  
Asymmetry Parameters ◽  
Linear Correlations ◽  
Split Valence ◽  
Valence Basis Set ◽  
Good Agreement

Abstract The results of ab initio and MNDO calculations of the Cl2C = CHOCH3 , XCOCl (X = CH3 , OCH3 and COCl), 4-ClC6H4CH2Cl, (CNCl)3 and PCl5 with total optimization of their geometry are presented. The ab initio calculations were executed using Hartree-Fock theory and the split valence basis set 6-31G* (RHF/6-31 G*//RHF/6-31G*). Using the calculated p-orbital populations of the CI atoms in these molecules the 35Cl NQR frequencies and asymmetry parameters of the EFG at the 35Cl nuclei have been determined. When the populations of the less diffuse components of orbitals in the split valence basis set are used the calculated and experimenal ν and η values are in good agreement. Linear correlations between these calculated and corresponding experimental ν and η values are obtained. The causes of the nonconformity of the earlier calculated ν and η values and their experimental ones are analysed.

MP2 or B3LYP: computed bond distances compared with CCSD(T)/cc-pVQZ

2018 ◽  
Vol 96 (3) ◽  
pp. 336-339 ◽  
Author(s):  
Delano P. Chong
Keyword(s):  
Small Molecules ◽  
Density Functional ◽  
Basis Set ◽  
Basis Sets ◽  
Mean Absolute Deviation ◽  
Slater Type Orbital ◽  
Absolute Deviation ◽  
Bond Lengths ◽  
Hartree Fock ◽  
The Mean

The equilibrium bond lengths of 41 small molecules are calculated by Gaussian09 and ADF2013 programs. We use five different basis sets: 6-31G*, cc-pVDZ, 6-311G+(2d,p), cc-pVTZ, and cc-pVQZ, for six different methods: Hartree-Fock, MP2, MP3, CCSD, CCSD(T), and B3LYP. The reliability of each level of theory on 89 bond lengths compared with CCSD(T)/cc-pVQZ is examined in terms of the mean absolute deviation. In particular, basis set dependence of the relative reliability of the two popular methods MP2 versus B3LYP is important to computational chemists. In addition, the efficient even-tempered basis set of Slater-type orbital called et-pVQZ, available in the ADF2013 program, is tested with the popular density functional B3LYP.

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