Calculation of electrostatic potentials and fields inside zeolite cavities

1988 ◽  
Vol 53 (10) ◽  
pp. 2308-2319 ◽  
Author(s):  
János G. Ángyán ◽  
György Ferenczy ◽  
Péter Nagy ◽  
Gábor Náray-Szabó
Keyword(s):  
Ab Initio ◽  
Lone Pair ◽  
Electrostatic Potentials ◽  
Basis Set ◽  
Mean Deviation ◽  
Minimal Basis ◽  
Approximate Methods ◽  
Monopole Approximation ◽  
Van Der Waals Surface ◽  
Modified Formula

We present a modification of our bond increment method for the calculation of molecular electrostatic potentials and fields inside zeolite cavities. Introducing a variant of the Mulliken approximation for the off-diagonal matrix elements of the potential and optimizing the parameters of the modified formula, we achieved much better agreement with ab initio STO-3G minimal basis set results than with the original version. For a representative set of 10 small molecules the standard mean deviation between potentials calculated on the van der Waals surface with the ab initio and approximate methods is 9·1 kJ/mol. The relative error decreases from 21 to 9 per cent for the lone-pair regions of molecules modelling zeolite cavities. Applying the modified bond increment method for a realistic faujausite model we have found that the potential and field are almost exclusively of long-range origin. This means that, if using appropriate atomic charges, the monopole approximation gives correct results for electrostatic potentials and fields inside zeolite cavities.

Anomalous stabilizing and destabilizing effects in some cyclic π-electron systems

1993 ◽  
Vol 71 (8) ◽  
pp. 1123-1127 ◽  
Author(s):  
Peter Politzer ◽  
M. Edward Grice ◽  
Jane S. Murray ◽  
Jorge M. Seminario
Keyword(s):  
Ab Initio ◽  
Lone Pair ◽  
Electrostatic Potentials ◽  
Electron Delocalization ◽  
Isodesmic Reaction ◽  
Computational Studies ◽  
Electron Systems ◽  
Molecular Electrostatic Potentials ◽  
Lone Pairs

Ab initio computational studies have been carried out for three molecules that are commonly classed as antiaromatic: cyclobutadiene (1), 1,3-diazacyclobutadiene (7), and 1,4-dihydropyrazine (6). Their dinitro and diamino derivatives were also investigated. Stabilizing or destabilizing energetic effects were quantified by means of the isodesmic reaction procedure at the MP2/6-31G*//HF/3-21G level, and calculated molecular electrostatic potentials (HF/STO-5G//HF/3-21G) were used as a probe of electron delocalization. Our results do not show extensive delocalization in the π systems of any one of the three parent molecules. The destabilization found for 1 and 7 is attributed primarily to strain and to repulsion between the localized π electrons in the C=C and C=N bonds, respectively. However, 6 is significantly stabilized, presumably due to limited delocalization of the nitrogen lone pairs. NH2 groups are highly stabilizing, apparently because of lone pair delocalization. NO2 is neither uniformly stabilizing nor destabilizing.

Ab initio calculations on tetramethoxymethane

1988 ◽  
Vol 66 (8) ◽  
pp. 2041-2044 ◽  
Author(s):  
R. J. McEachern ◽  
J. A. Weil ◽  
P. G. Mezey
Keyword(s):  
Dipole Moment ◽  
Ab Initio ◽  
Electron Diffraction Study ◽  
Energy Difference ◽  
The Other ◽  
Geometric Configuration ◽  
Basis Set ◽  
Mo Calculations ◽  
Minimal Basis ◽  
Atom System

Minimal basis set ab initio SCF-MO calculations were performed on the 21-atom system of tetramethoxymethane (tetramethyl orthocarbonate). The geometric configuration of this model was optimized in two conformations, one having quasi-S4 symmetry and the other D2d symmetry. The S4 conformation was found to be 8 kJ mol−1 lower in energy than the D2d conformation, at the STO-3G level. The calculated energy difference is consistent with the recently measured geometric configuration of crystalline tetrabenzyl orthocarbonate. The calculated values of the bond lengths and angles were compared to the results of an electron diffraction study of the methyl species, and agree well with experiment. The theoretical electric dipole moment was calculated to be 0.01 D.

scholarly journals Ab initio study of the mechanism of the reaction ClO + O --> Cl + O2

2021 ◽  
Vol 66 (1) ◽  
Author(s):  
S. Naskar ◽  
G. Nandi ◽  
T. K. Ghosh
Keyword(s):  
Reaction Mechanism ◽  
Ab Initio ◽  
Minimum Energy ◽  
Transition States ◽  
Basis Set ◽  
Heat Of Reaction ◽  
Minimal Basis ◽  
Dissociation Energies ◽  
Correlation Consistent ◽  
Equilibrium Geometries

Abstract. Ab initio investigation on the reaction mechanism of ClO + O --> Cl + O2 reaction has been performed using correlation consistent triple zeta basis set. The geometry and frequency of the reactants, products, minimum energy geometries and transition states are obtained using MP2 method and energetics are obtained at the QCISD(T)//MP2 level of theory. Primarily, a possible reaction mechanism is obtained on the basis on IRC calculations using MP2 level of theory. To obtain true picture of the reaction path, we performed IRC calculations using CASSCF method with a minimal basis set 6-31G**. Some new equilibrium geometries and transition states have been identified at the CASSCF level. Energetics are also obtained at the QCISD(T)//CASSCF method. Possible reaction paths have been discussed, which are new in literature. Heat of reaction is found to be consistent with the experimental data. Bond dissociation energies to various dissociation paths are also reported.

The simplified ab initio method calculation of linear polarizability

1973 ◽  
Vol 26 (5) ◽  
pp. 921 ◽  
Author(s):  
RD Brown ◽  
GR Williams
Keyword(s):  
Ab Initio ◽  
Small Molecules ◽  
Basis Set ◽  
Basis Sets ◽  
Orbital Method ◽  
Polarizability Anisotropy ◽  
Minimal Basis ◽  
Hartree Fock ◽  
Numerical Performance ◽  
Experimental Values

The simplified ab-initio molecular-orbital method described previously is particularly suited to the calculation of polarizabilities by the non-perturbative coupled Hartree-Fock technique. Trial calculations on CO and HF, for which comparison with corresponding ab-initio calculations is possible, show that the method gives an adequate numerical performance. Minimal basis set calculations in general tend to give values that are considerably too low because of inadequate flexibility of the basis and this is the origin of the large discrepancy between theory and experiment, especially for small molecules. ��� Results are also reported for N2O and O3. For these larger systems the SAI results with minimal basis sets are noticeably nearer experimental values. The polarizability anisotropy for N2O is particularly well reproduced by the SAI method. �

The Rotational Barrier About the Disulphide Bridge in Dimethyl Disulphide: An Ab Initio Study

1984 ◽  
Vol 39 (5) ◽  
pp. 495-498
Author(s):  
V. Renugopalakrishnan ◽  
R. Walter
Keyword(s):  
Ab Initio ◽  
Rotational Barrier ◽  
Basis Set ◽  
Basis Sets ◽  
Disulphide Bridge ◽  
Minimal Basis ◽  
Gaussian Basis Sets ◽  
Type Function ◽  
Dimethyl Disulphide ◽  
Extended Basis

An ab initio molecular orbital technique was used to investigate the rotational barrier about the disulphide bridge in dimethyl disulphide. Various minimal and extended basis sets were used in the calculations. The chosen minimal basis set was the STO-3G set, and the extended basis sets were the STO 4-31G set, the Dunning and Hay set consisting of contracted Gaussian basis sets: [2s], [3s, 2p] and [6s, 4p] for H, C, and S atoms, and the Dunning and Hay basis set augmented with a d-type function on S atoms. The total energy was calculated as a function of the torsion angle about the disulphide bond. The barrier to rotation about this bond was found to be two-fold in nature, in accordance with previous findings. The heights of the barriers were observed to depend upon the basis set and input geometry. For our particular choice of basis sets and input geometry, the calculated value of the eis and trans barriers ranged from 12.68 to 16.49 kcal/mol and from 6.23 to 8 kcal/mol, respectively. Inclusion of a d-type function in the basis sets was found to result in better agreement between the calculated and experimental values, thereby emphasizing the need for considering 3d orbitals of sulphur in MO calculations

Abinitio studies of complexes between SiF4 and ammonia

1984 ◽  
Vol 62 (1) ◽  
pp. 27-31 ◽  
Author(s):  
J. M. Chehayber ◽  
S. T. Nagy ◽  
C. S. Lin
Keyword(s):  
Dipole Moment ◽  
Complex Formation ◽  
Solid Phase ◽  
Lone Pair ◽  
Basis Set ◽  
Minimal Basis ◽  
Geometrical Isomers ◽  
Calculation Results ◽  
Positively Charged ◽  
Rule Out

Abinitio minimal basis set STO-6G calculations show axial-SiF4•NH3 and trans-SiF4•2NH3 to be energetically more stable with respect to SiF4 and ammonia. All other geometrical isomers are 1.0–2.0 eV higher.Our calculation results rule out the steric hindrance between ligands as the reason SiF4•2NH3 is, according to experimental evidences, cis while complexes of bulkier ligands are trans.Since the cis-SiF4•2NH3 has a large electric dipole moment and a suitable charge arrangement it is conceivably the favoured form in the solid phase. The cis → trans rearrangement may take place under conditions where the advantages of the cis are not operative.Although there is some degree of covalency in the Si—N bond it remains largely electrostatic, with the lone pair of ammonia attracted to the positively charged Si atom. The lone pair does not appear to be extensively altered by the complex formation.

An ab initio Molecular Orbital Study of Bonding in Fluorocarbonium Ions

1971 ◽  
Vol 49 (22) ◽  
pp. 3708-3713 ◽  
Author(s):  
N. C. Baird ◽  
R. K. Datta
Keyword(s):  
Ab Initio ◽  
Molecular Orbital ◽  
Basis Set ◽  
Molecular Orbital Calculations ◽  
Minimal Basis ◽  
Fluorine Substitution ◽  
Abstraction Reaction ◽  
Carbonium Ions ◽  
Hydride Abstraction ◽  
Resonance Stabilization

Ab initio molecular orbital calculations are reported for the series of carbonium ions (CH3)+, (FCH2)+, and (F2CH)+ and for their neutral molecule counterparts CH4, CH3F, and CH2F2. The energies and wavefunctions for the carbonium ions have been calculated both with and without including the carbon 2pπ orbital in the minimal basis set in order to unravel the inductive destabilization and resonance stabilization due to fluorine substitution. The increase in bonding energy with multiple fluorine substitution is less than linear, due primarily to nonadditivity in the dative carbon–fluorine π bonding. The "saturation" effect noted previously for the hydride abstraction reaction enthalpies is shown to be due primarily to stability effects in the neutral molecules themselves rather than to energetic effects of the carbonium ions.

Ab initio minimal basis set calculations on C60Mu

1993 ◽  
Vol 207 (1) ◽  
pp. 31-40 ◽  
Author(s):  
T.A. Claxton ◽  
S.F.J. Cox
Keyword(s):  
Ab Initio ◽  
Basis Set ◽  
Minimal Basis
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