Ab Initio SCF MO Research Articles

Photoelectron angular distribution and assignments of photoelectron spectra of nitrogen dioxide, nitromethane and nitrobenzene

Photoelectron bands closely associated with the NO 2 group have been identified for CH 3NO 2 and C 6H 5NO 2 in the gas phase by determining the asymmetry parameter (β) of the photoelectron angular distribution with HeI and NeI radiation. For NO 2, β values for the 3,1 A 2 and 3,1 B 2 ionic states have been obtained which are associated with the out-of-plane NO 2 orbitals. Ab initio SCF MO CI calculations of ionic states have been carried out for NO 2 and CH 3NO 2. For CH 3NO 2, three ionization bands observed with different β values (0.08, 0.20 and 0.14 for NeI radiation), appearing in the region below ≈ 12 eV have been assigned to the (n + O) −1, (π 2) −1 and (n − O) −1 ionic states. For C 6H 5NO 2, on the basis of ab initio SCF MO calculations, it has been concluded that, besides the first two bands with β(HeI) = 0.7–0.8 previously assigned to the π orbitals of the benzene ring, the next three bands with a β(HeI) value of 0.11 appearing in the 10.7–12.0 eV region are assigned to the NO 2 orbitals.

Optimization of equilibrium geometries and transition structures

AbstractA modified conjugate gradient algorithm for geometry optimization is outlined for use with ab initio MO methods. Since the computation time for analytical energy gradients is approximately the same as for the energy, the optimization algorithm evaluates and utilizes the gradients each time the energy is computed. The second derivative matrix, rather than its inverse, is updated employing the gradients. At each step, a one‐dimensional minimization using a quartic polynomial is carried out, followed by an n‐dimensional search using the second derivative matrix. By suitably controlling the number of negative eigenvalues of the second derivative matrix, the algorithm can also be used to locate transition structures. Representative timing data for optimizations of equilibrium geometries and transition structures are reported for ab initio SCF–MO calculations.

A study of bullvalene by ab initio SCF MO calculations

A study of bullvalene by ab initio SCF MO calculations

Ab initio SCF MO study of the hydrogen-bonded dimers of cyanogen with HF and HCl

Large-scale gaussian orbital SCF MO calculations are presented for the hydrogen-bonded complexes NCCN…HF and NCCN…HCl. Calculated equilibrium geometries, hydrogen-bond dissociation energies and selected one-electron properties are given to supplement available experimental data. Changes of electron distribution on complex formation are discussed in terms of Mulliken population indices.

Stability and bonding of disilyne and its isomers: A generalized valence bond‐effective potential study

AbstractWe have employed an effective potential and a single‐zeta basis in SCF–MO computations to estimate the relative stability of linear disilyne HSiSiH and five isomeric structures defined in earlier all‐electron ab initio SCF–MO computations. The effect of electron correlation has been estimated by generalized valence‐bond (GVB) computations for the five valence electron pairs of these structures. All our computations indicate that linear disilyne is the least stable structure and that H2SiSi, the silicon analog of vinylidene carbene, is the most stable structure. In these structures silicon occurs in divalent and tetravalent states. The nature of silicon bonding in these valence states is illustrated by contour diagrams of the GVB orbital pairs.

An ab initio SCF MO study of the molecular structure and properties of the hydrogen-bonded dimers of ethyne and ethene with HF and HCl

Large-scale gaussian orbital SCR MO calculalions are presented for the title molecules. Calculated equilibrium geometrics, hydrogen-bond dissociation energies, and one-electron properties are given to supplement experimental data for C 2H 2HCl. Changes of electron distribution on complex formation are discussed in terms of Mulliken population indices.

An ab initio approach to the mechanism of the Grignard reaction

The mechanism of the Grignard reaction is investigated for the first time in the light of ab initio SCF MO theory. The possible advantage of a single-electron transfer over a polar mechanism is discussed.

Influence of alkali and alkaline earth salts on the internal rotational barriers in N,N-dimethylbiuret. Experimental and theoretical studies

The effect of alkali and alkaline earth salts on the barriers to internal rotation about the CNR 2 (R = H, CH 3) bonds in N,N-dimethylbiuret has been studied by means of 1H n.m.r. line shape and analysis. The 1:1 complexes of the ligand with the ions of Li, Na, K, Be, Mg and Ca have been investigated by ab initio MOSCF calculations with minimal GLO basis sets. The changes of the activation energies for the rotational barriers, the influence of methyl N-substitution on complex stability, and the geometry of the complexes are discussed on the basis of the theoretical results.

Ab initio MO-SCF study of prototropic transformations of non-substituted dihydropyridines

The equilibrium systems formed by ionization of all the isomeric dihydropyridines I, II, III, IV and V with the particles H3O+, H2O and OH- have been characterized on the basis of the total energy changes calculated with the use of the STO-3G and 4-31G basis sets. Relative thermodynamic stabilities of the corresponding dihydropyridine forms are discussed.

Quantitative orbital analysis of ab initio SCFMO computations: Part II. Conformational preferences in H 2NOH and H 2NSH

The results obtained in a quantitative orbital analysis of the ab initio SCFMO computations performed on hydroxylamine and thiohydroxylamine are described. The analysis is based on the use of fragment localized MOs and the energy effects associated with their interactions are estimated, in the framework of the ab initio SCF—MO computations, using PMO expressions. In particular the factors are analyzed which control the conformational preference in these molecules, and the effect of the sulphur 3 d orbitals upon conformational stability is discussed.

A comparative analysis of various gradient methods for geometry optimization at the ab initio SCF-MO level

Various techniques, already suggested for improving the computational efficiency of the force relaxation method, have been applied here in conjunction with the variable metric algorithms suggested by Murtagh and Sargent and by Fletcher. Various tests have been performed to assess the computational efficiency of the resulting minimization procedures. In addition, procedures where the steps are defined in terms of a parabolic fitting of the forces in approximated normal coordinates have been tested. In all cases the forces are computed analytically.

Force constants of trans and cis N-methylformamide from Ab initio SCF MO calculations

Ab initio SCF MO studies on the equilibrium geometries, force constants, vibrational frequencies, and dipole moment derivatives of trans and cis N-methylformamide have been carried out using the 4-31G and 4-31G* basis sets and an energy gradient method. The trans form was calculated to be 1.4 kcal/mol more stable than cis, the one in agreement with the experimental result. Discussions were made on the trans-cis differences in the geometry, force constants, and normal vibrations.

Ab initio molecular orbital calculation of fe‐porphine with a double zeta basis set

AbstractAn ab initio LCAO SCF MO calculation was performed on planar Fe‐porphine with a double zeta basis set consisting of 300 CGTO's. SCF wave functions of several states of Fe‐porphine and its cation were obtained. The net charge of Fe is in the range of 1.39 to 1.53. The highest occupied orbital is ascertained to be a pure porphine π‐MO, 1a1u. The calculated ionization potentials of the two highest occupied orbitals, 1a1u and 5a2u are 5.98 and 6.43 eV, respectively. They are in good agreement with experiments. The role of the porphine macrocycle on the oxidation of Fe is discussed in terms of gross atomic populations and with contour maps of the density difference.

The α-helix as an ion channel. An ab initio molecular orbital study

Minimal basis set ab initio SCF MO expectation values of the electrostatic potential of an octa-alanine α-helix are reported. The energy profile for a proton moving through the interior of the helix is given. It is suggested that the internal electric field of a helix may be essential in biological ion pumps.

Ab initio MO—SCF calculations of equilibrium geometry and vibrational structure for the bichloride ion, HCl 2−

The potential energy surface for the bichloride ion has been computed with ab initio methods. The potential was found to be of double-minimum type with a very small barrier. The equilibrium conformation has HCl distances of 1.39 and 1.85 Å and a hydrogen-bond energy of 20 kcal mol −1 relative to isolated HCl molecules and Cl − ions. The vibrational frequencies have also been evaluated and are found to agree well with the experimental values.

EPR and a b i n i t i o SCF–MO studies of the Si⋅H–Si system in the E′4 center of α-quartz

The E′4 center in irradiated α-quartz has been studied by single-crystal EPR. The unpaired electron is shared mainly by two silicon ions Si(1, 2) with the larger fraction on Si(2). The spin-Hamiltonian parameter matrices ḡ, Ā1H, Ā29Si(1, 2) at 40 K, as well as Ā29Si(1, 2) at 300 K, are reported. A model of the E4′ center, consisting of an O− vacancy between Si(1, 2), with a hydride ion bonded to Si(1), is presented. Ab initio SCF–MO calculations for a 15-atom cluster, yielding the minimum energy atom configurations, have supported this model by giving good qualitative agreement with the EPR data. The temperature dependence of the hyperfine matrices Ā29Si(1, 2) is explained by Boltzmann population of the vibrational levels in the asymmetric double-minimum potential of the hydrogen bond system Si(2) ⋅ H–Si(1).

Electronic structure of dirhodium tetracarboxylate complexes by the AB initio SCF MO method

Electronic structures of dirhodium tetracarboxylate (DRTC) complexes with ligand (L) free, L = OH 2, NH 3, and PH 3 have been calculated by the ab initio SCF MO method with a STO-3G basis set. The Rh-Rh bond is a weak single bond. The electronic configuration is η 4δ 2η *4δ *2σ 2 for L free, L = OH 2,and NH 3, and δ 2η 4η *4δ *2σ 2 for L = PH 3. The HOMO is σ for all the complexes, and for L = PH 3, this is in accord with an ESR experiment

A Comparison of Mesquac- and “Full” AB Initio MO-Wavefunctions

The quality of wavefunctions for complex systems derived by either “full” ab initio MO-SCF calculations or within the MESQUAC-MO framework [1] is investigated comparing chemically interesting quantities as atomic and overlap populations and the character of the chemical bond. A direct relation between the results of both methods is shown to exist, allowing an extrapolation of the much less time consuming MESQUAC computations to the “full” MO SCF level. Hydrate complexes of main group and transition metal ions have been chosen for some practical applications

Effects ofN-protonation on the electronic structure of morphine and naloxone

The FSGO molecular fragment technique has been employed to perform ab-initio SCF-MO calculations on morphine and naloxone in the free-base and N-protonated forms. Particular features of electronic structure which have been investigated are molecular-orbital (MO) density distributions, energies and ordering, as well as the correlation of specific orbitals between different molecular species. It has been found that high-energy occupied and low-energy unoccupied MOS are centered on the aromatic moiety of both drugs, enabling the ring to act either as an electron donor or as an acceptor in a charge-transfer interaction. Comparison of corresponding MOs in the free base and the acid cation of both morphine and naloxone demonstrates that the field of the cationic head causes a dramatic downward shift in orbital energies. Thus in the N-protonated drugs the acceptor capability of the benzene ring is greatly enhanced at the expense of its suitability as a donor. Furthermore, the allyl group of naloxone becomes highly activated as an electron acceptor through N-protonation. Investigation of the molecular electrostatic potential of the free-base species has revealed preferred avenues of approach for electrophilic and nucleophilic entities. However, the N-protonated species shows no regions of attraction for electrophiles. In both the free base and the acid cation, the vicinities of the nitrogen atom and the 3-hydroxy group are found to be favorably disposed for electrostatic interactions with receptor entities possessing the proper charge or polarity. Based on the findings with regard to electronic structure, a simple model has been suggested to explain the binding of morphine and naloxone at the opioid receptor.

Ab initio calculations on the clustering energies and stable structures of H(M) + + M → H(M) 2+ (M = CO and N 2)

The clustering energies of the reactions, H(M) ++ M → H(M) + 2 (M = CO and N 2), were calculated by ab initio single configuration LCAO SCF MO and SCEP methods. The calculated energies are in good agreement with the experimental enthalpies.