Ab Initio SCF MO Method Research Articles

Theoretical studies on the cycloaddition reaction of ethylene and formaldehyde

The potential energy surface of cycloaddition (H2CCH2 + H2CO) has been investigated by using the ab initio SCF MO method at extended (3–21G) level. All the critical points were optimized by energy gradient technique and characterized by computing the Hessian matrices with analytic method. The results of computations indicate that: (i) Stepwise reactions involving diradicaloid intermediates and transition states are more favorable than the concerted ones. (ii) There are two compatible stepwise reaction mechanism, one is to form C—C bond firstly, while the other is to form C—O bond at first. (iii) For the reverse reaction-thermolysis of oxetane, the cleavage of C—O bond is preferred over that of C—C bond.

Difference in the Potential Energy Surfaces from the Fragment MO Method and Conventional Ab Initio SCF-MO Method. A Case of a Surface for Ring Rotation of (−)-Epicatechin Gallate Using the STO-3G Basis Set

Abstract Difference in the potential energy surfaces (ΔE) obtained from the fragment MO (FMO) method and conventional ab initio Hartree–Fock SCF-MO (HFMO) method was investigated using the best fragmentation pattern of (−)-epicatechin gallate. In the lower energy region (<23.06 kcal mol−1 from the minimum), the average ΔE at each grid point was 1.1 kcal mol−1. The FMO method is applicable to quantum MD calculation of large molecular system at around the standard temperature.

Fragmentation Position Dependency of the Total Energy and Atomic Charge Difference between the Fragment MO Method and Conventional Ab Initio SCF-MO Method. A Case of (−)-Epicatechin Gallate with STO-3G Basis Set

Abstract The fragmentation pattern dependence of the total energy and atomic charge difference between the fragment MO method and conventional ab initio Hartree–Fock SCF-MO method (HFMO) was investigated using 44 patterns of (−)-Epicatechin gallate. Both differences were sensitive to the fragmentation pattern. A “best” pattern for which results comparable to HFMO can be achieved, exists.

Conformational study of 1,2-diaminoethane by combined ab initio MO calculations and Raman spectroscopy

A conformational analysis of the linear polyamine 1,2-diaminoethane was performed by both Raman spectroscopy and ab initio SCF-MO methods (with and without inclusion of water solvent effects). Ten different conformers were detected by ab initio calculations, the most stable ones being TGG′ and GGG′. When considering the solvent, the conformations having a higher dipole moment were found to be favoured over the less polar ones. The Raman spectra of this amine obtained for the pure liquid, the aqueous solution and the solid sample, allowed to conclude that its conformational preferences are determined by the relative importance of intra- versus intermolecular hydrogen bonds.

γ-Dissociations of 3-alkylbicyclo[1.1.1]pent-1-yl radicals into [1.1.1]propellane and alkyl radicals: verification of a theoretical prediction

The γ-dissociations of 3-substituted bicyclo[1.1.1]pent-1-yl radicals and cyclobutyl radicals were investigated by abinitio SCF MO (HF, MP2, MP3 and MP4/6–31G*) and density functional methods (B3LYP/6–31G*). The transition states were found to resemble the product alkyl radical and [1.1.1]propellane or bicyclo[1.1.0]butane. Calculated endothermicities and energy barriers were comparatively low for loss of the t-butyl radical from the 3-t-butylbicyclo[1.1.1]pent-1-yl radical, which suggested that this dissociation would be significant under laboratory conditions. The dissociation was verified experimentally by means of the reaction of 1-iodo-3-t-butylbicyclo[1.1.1]pentane with tributyltin hydride. Arrhenius parameters for this dissociation were determined by end product analysis. The SCF MO and density functional calculations resulted in much higher endothermicities and energy barriers for γ-dissociations of cyclobutyl radicals, hence neither the formation of bicyclo[1.1.0]butane nor alkyl radical addition to this bicyclic compound was predicted to be important.

Importance of steric hindrance in linear coordination of Lewis acids with α-enones

The complexation of α-enones [(E)-MeC(H)C(H)COR] with SnCl4 and TiCl4 has been studied with NMR spectroscopy as well as semiempirical and ab initio SCF-MO methods, which indicate a preference for linear coordination of Lewis acids with α-enones due to the steric effect of R group (R = But, Ph, 2,4,6-Me3C6H2, 4-MeOC6H4) in the s-cis conformation.

(SiC)60' an Indealized Inverse Superatom?

ABSTRACT Geometry-optimization of a hypothetical double fullerene C60 @ Si60 with semi-empirical PM3 and ab initio DZ ECP SCF-MO methods produces sixty new Si-C bonds between the outer silicon and inner carbon shells. The formal polymorph of silicon carbide thus obtained is designated as Iit-(SiC)60 (1). Analysis of the energy-minimum structure reveals reasonable bond lengths and stable electronic configuration. One of the interesting aspects of 1 is that it can be regarded as an inverse superatom having electron-deficient curved surface.

Theoretical study of the reaction mechanism and migratory aptitude of the pinacol rearrangement

The reaction pathways of the pinacol rearrangement are examined for some monosubstituted protonated 1,2-ethanediols by means of ab initio SCF MO methods. The relative activation energies are evaluated for both stepwise and concerted pathways. Gas-phase calculations show the concerted mechanism dominates over the traditionally supported stepwise mechanism via a carbonium ion intermediate. These results agree well with experiments in the gas phase and in aprotic solvents. The migratory aptitude is also calculated for some substituents. The high migratory aptitude of vinyl and cyclopropyl groups is compatible with the participation of π- or pseudo-π-orbitals to stabilize the transition states. The relatively low migratory aptitudes of alkyl and alkynyl groups are also reproduced

Molecular and Electronic Structures of Model Compounds of Doped Polyacetylene

Abstract Using the ab initio HF SCF MO method optimized molecular structures were calculated on twenty five model compounds of doped polyacetylene, which were charge-transfer complexes between dopants (Na, Li, and Cl atoms) and trans-polyenes (C6H8, C8H10, C12H14, C14H16, and C16H18) or trans-odd polyene radicals (C3H5·, C5H7·, C7H9·, C9H11·, C11H13·, and C13H15·). In alkali metal-doped complexes, each alkali atom is located at the vertex of the trigonal pyramid, whose basal triangle consists of an allylic group, (CH)3. On the other hand, each chlorine atom is coplanar with the (CH)n chain and is connected with an allylic group by hydrogen bonding. The allylic group acts as a bidentate or tridentate ligand, forming an ionic closed-shell structure with the dopants. The molecular orbitals of the singly doped complexes are correlated with those of parent trans-polyene or odd polyene radical chains. The molecular orbitals of doubly doped complexes are analyzed on the basis of those of singly doped complexes. The molecular structure of the polson unit is presented, which is important for heavily doped polyacetylene to be metallic.

Conformational analysis of carbonyl and thiocarbonyl esters: the HC(=X)YXXXCH(CH 3) 2 (X,Y = O or S) internal rotation

The YXXXCH(CH 3) 2 internal rotation in the HC(=X)YCH(CH 3) 2, (X,Y = O or S) series of molecules was studied by the ab initio SCF-MO method using 3–21G and 3–21G + d(0.65S) basis sets. Energies and structures of several conformations of these molecules, determined by gradient geometry refinement, are reported and used to assess the effects of oxygen-bysulphur substitution on molecular properties. The nature and relative importance of intramolecular interactions involving both the −CH(CH 3) 2 and the HC(=X)Y (X,Y = O or S) fragments are also discussed.

Ab initio studies on the thermolysis of azetidine

AbstractThe reaction mechanism of the thermolysis of azetidine to form ethylene and methylenimine has been studied by ab initio SCF MO method at STO–3G and 3–21G levels. Two possible stepwise pathways are explored. One is the breaking of C—C bond as the first step, while the other is the breaking of C—N bond. All the stationary points on the potential energy surface (PES) are fully optimized. MP2/3–21G single point calculations on all stationary points and MCSCF/STO‐3G computations for some stationary points are also carried out. The calculations indicate that azetidine decomposes via biradicaloid intermediates and the cleavage of C—N bond is preferable to that of C—C bond.

Complexes between formaldehyde and titanium tetrachloride. An ab initio study

The complexes of titanium tetrachloride with one or two molecules of formaldehyde have been studied with ab initio SCF-MO methods. Several structures of the H 2 CO-TiCl 4 , (H 2 CO) 2 -TiCl 4 , and (H 2 CO-TiCl 4 ) 2 complexes have been considered and the rearrangements between them have been discussed. The interaction between formaldehyde and TiCl 4 has been analyzed in terms of molecular orbital interactions, and the effects produced by complex formation have been rationalized

Energy Band Structure for Metallic Polyacetylene

ABSTRACTThe optimized molecular structures of doped polyacetylene model compounds are studied by the ab initio SCF MO method. The calculated structures are assigned to polaron, charged soliton and poison unit. The one dimensional energy band structures of the charged soliton-antisoliton and the polson-antipolson lattices are investigated. The latter one gives a quasi-metallic energy band. The results of the energy band calculation are consistent with the electrical properties and ultraviolet photoemission spectra of the doped polyacetylene.

Ab initio and semiempirical molecular orbital studies of alpha and beta lithiated vinyl ethers

Ab initio and semiempirical SCF MO methods are used to study models for vinyl ether carbanions and lithiocarbanions.

Electronic aspects of LADH catalytic mechanism

AbstractElectronic aspects of the catalytic mechanism of liver alcohol dehydrogenase (LADH) are studied with the help of ab initio analytical gradient SCF MO methods. Three points are considered: (i) role of the catalytic zinc; (ii) geometry and electronic structure of the transition state for the hydride transfer reactions; and (iii) factors affecting the energy gap for the hydride transfer step, namely, substrate binding to zinc, reaction field, and serine 48 effects on the potential energy profile. The coordination sphere of the catalytic zinc has been modeled with an ammonia molecule and two SH− groups; complexes with CH3O−, CH3OH, and CH2O have been studied; a (6, 2, 2, 2, 1/6, 2, 1/3, 2) basis set has been used for Zn++; a (5, 2, 1, 1/3, 2) was used for oxygen, carbon, and sulfur; and a (3, 1) was used for hydrogen atoms. The hydride step was studied with two model systems: pyridinium cation/1,4‐dihydropyridine coupled to the CH3O−/CH2O reaction, and cyclopropenyl cation/cyclopropene coupled to the CH3O−/CH2O system. For the latter, the role of Ser48 has been studied at the supermolecule level. The calculation on the hydride transfer step has been done at a 4–31G basis set level. The results obtained shed new light on the sources of catalytic activity of liver alcohol dehydrogenases.

MO study of the possibility of a concerted mechanism in the pinacol rearrangement

AbstractThe ab initio SCF‐MO method was employed to examine the possibility of a reaction pathway without a carbocation intermediate in the pinacol rearrangement. The molecular geometries of the transition states were obtained for the 1,2‐hydride shift together with H2O elimination, starting from the various methyl‐substituted protonated 1,2‐diols. It was found that the activation energies depend strongly on the substitutents. A comparison of the relative energies between β‐hydroxycarbonium ions and the transition states of the concerted mechanism suggests that the stepwise mechanism is less favourable than the concerted path in each case.

The CH3CH2⊖ internal rotation in thiopropionic acid as studied by ab initio SCF-MO method

The CH3CH2⊖internal rotation in thiopropionic acid was studied by ab initio SCF-MO calculations using 3–21G and 3–21G+d(0.65S) basis sets. Energies and structures of several conformations of the two tautomeric forms of this molecule (the thiol and thion forms) determined by gradient geometry refinement are reported, and some conformationally dependent local geometry trends are discussed. The results are compared with available data on propionic and dithiopropionic acids to assess the effects of substitution of oxygen by sulphur on molecular properties. While the CH3-CH2-CO axis adopts preferentially the syn conformation (C-C-CO dihedral angle equal to 0°), the lowest-energy conformation of the CH3-CH2-CS axis occurs for a C-C-CS dihedral angle in the 100–120° region. However, both axes possess energy maxima near 60° and at 180°. The calculated molecular atomic charges, dipole moments and ionization potentials are reported and compared with available experimental data.

Ab initio and INDO SCF MO study of monosubstituted hydrazyl radicals

The unrestricted Hartree-Fock ab initio SCF MO method was applied to the monosubstituted hydrazyl radical XN-NH 2 (X = H,CH 3,HC(O),CH 3C(O) or C 6H 5) and [XNH-NH 2] .+ (X = H or CH 3). The optimized geometrical parameters for these radicals obtained via analytical gradient procedures are reported, and the isotropic hyperfine splitting constants calculated both directly from the ab initio wavefunctions and also corrected by employing a recently proposed spin annihilation method. In addition, the INDO method was used in conjunction with these same optimized geometrical parameters to calculate a further set of values for these isotropic hyperfine splitting constants. In general, satisfactory agreement was obtained between the corrected ab initio derived splittings, those from INDO, and experimentally measured values.

Electronic structure of the CuO 2-plane of cuprate superconductors

Electronic band structure of the CuO 2-plane common to cuprate superconductors is calculated with a novel approach, assuming that the plane consists of CuO 6- 4 and Cu 2+ ions. Cluster orbitals of a CuO 6- 4 ion are calculated with ab initio UHF SCF MO method. In the CuO 2-plane, the highest occupied MO (HOMO) is antibonding combination of π type O(2p x,y ) orbitals. The energy band structure is calculated by tight binding approximation with each MOs. Mid-infrared exciton bands of the doped CuO 2 plane are explained as interband transitions to the π holes of O(2p x,y ) antibonding type HOMO band from the degenerate O(2p x ) and O(2p y ) type next HOMO bands.

Ab initio study of vibrational frequencies, force field and normal coordinate analysis of the cis- and trans-isomets of nitrosomethanol

In this paper, the equilibrium geometries of two isomers of the newly found compound — nitrosomethanol—have been optimized by ab initio SCF MO method with 3–21G basic set by gradient technique. And the second derivatives of potential energy (i. e. the force constant matrix elements) have been calculated analytically. Hence the entire force fields of the two isomers of nitrosomethanol have been obtained theoretically. The theoretical vibrational frequencies and the corresponding normal modes were obtained and compared with the experimental values, and the structures of two isomeric forms of nitrosomethanol are established.