Non-empirical LCAO MO SCF Calculations Research Articles

A non-emperical LCAO MO SCF investigation of the ground and core ionized states of tetrahedrane and cyclobutadiene

Non-empirical LCAO MO SCF calculations have been carried out on the ground and localised core hole states of tetrahedrane and cyclobutadiene to investigate the suitability of ESCA as a technique for distinguishing between structurally related, isomeric hydrocarbons. Differences in energy are found to be considerably magnified in the core hole state manifold and shifts in core binding energies and shake-up structures are shown to be distinctive.

A non-empirical LCAO MO SCF and experimental investigation on the core-ionized states of acetylacetone and some of its enol tautomers

Non-empirical LCAO MO SCF calculations have been performed on the ground and core-hole states of acetylacetone for a range of geometries. The theoretical studies have been complemented by gas-phase ESCA studies of both the O 1 s and C 1 s core levels. A comparison of the theoretical and experimental data shows excellent agreement for an unsymmetrical C s enol structure and a discussion is given of the low-energy shake-up satellites accompanying both C 1 s and O 1 s core ionization. Consideration is also given to the relative energies of the various tautomeric model systems as a function of the hole states.

A theoretical investigation of the ground and core hole states of linear and bent NiCO; a prototype for CO adsorbed on nickel

Non-empirical LCAO MO SCF calculations within the Hartree-Fock formalism have been performed on linear and bent conformations of NiCO, for both the states arising from the 3d 94s 1 and 3d 10 electronic configurations of Ni, for ground and core ionized species. Binding and relaxation energies are computed for the nickel atom, the free ligand (CO) and the absorbed species (NiCO). Comparison with available experimental data reveals good agreement for the shifts in core binding energies for the bent, 3d 94s 1 NiCO system. The main contribution to these binding energy shifts is found to be due to relaxation effects, which may be rationalized in terms of atomic population and bond overlap changes accompanying core ionization.

A theoretical investigation of single orbital relaxation energy contributions accompanying core ionization of N 2

Non-empirical LCAO MO SCF calculations within the Hartree—Fock formalism have been used to investigate the variation of binding energies and relaxation energies consequent upon core ionization as a function of bond-length variation and applied electric field for the nitrogen molecule. Single orbital contributions to the total relaxation energies are presented, and are shown to give considerable insight into the electronic reorganization processes accompanying core ionization. Comparison is made throughout with results obtained in a previous study of the isoelectronic species CO. An investigation of the effect of an electric field on the potential energy surfaces of ground and core hole states, as evidenced by band profile analyses, is also considered.

Single orbital relaxations accompanying core ionization in the isoelectronic series CH4, NH3, H2O, HF and Ne

Abstract Single orbital contributions to the total relaxation energy accompanying core ionization are computed by means of non-empirical LCAO MO SCF calculations within the ΔSCF formalism for the isoelectronic series CH4, NH3, H2O, HF and Ne. Individual contributions to the total relaxation energy associated with the valence levels of essentially core-like 2s character are also presented.

A theoretical investigation of the electric field dependence of core ionization phenomena in carbon monoxide

The influence of an applied electric field on core binding and relaxation energies accompanying core ionization has been theoretically investigated by means of non-empirical LCAO MO SCF calculations within the ASCF formalism, on a simple prototype molecule (carbon monoxide). Single orbital contributions to the total relaxation energies, together with Mulliken population analyses and density difference contours, are presented. Investigations of the effect of an electric field on shake-up transitions, and potential energy surfaces, as evidenced by band profile analysis, are also considered.

ChemInform Abstract: MOLECULAR CALCULATIONS WITH THE MODPOT, VRDDO, AND MODPOT VRDDO PROCEDURES. II. CYCLOPENTADIENE, BENZENE, DIAZOLES, DIAZINES, AND BENZONITRILE

A procedure for carrying out nonempirical LCAO–MO–SCF calculations on large molecules is outlined. Use is made of an approximation for neglecting some one‐ and two‐electron integrals proposed by Wilhite and Euwema. The differential overlap between a pair of basis functions is set to zero if the pseudo‐overlap is less than the adjustable threshold. We refer to this method as variable retention of diatomic differential overlap (VRDDO). In addition, only the valence electrons may be explicitly considered. The influence of the core electrons is taken into account by using the model potential (MODPOT) Hamiltonian suggested by Bonifacic and Huzinaga. The two methods can be used together resulting in the MODPOT/VRDDO procedure. VRDDO, MODPOT, and MODPOT/VRDDO calculations are reported for cyclopentadiene, benzene, pyrazole, imidazole, pyridazine, pyrimidine, pyrazine, and benzonitrile. The accuracy of the VRDDO calculations is very good. Orbital energies and gross atomic populations differ from the reference val...

Molecular calculations with the MODPOT, VRDDO, and MODPOT/VRDDO procedures. II. Cyclopentadiene, benzene, diazoles, diazines, and benzonitrile

A procedure for carrying out nonempirical LCAO–MO–SCF calculations on large molecules is outlined. Use is made of an approximation for neglecting some one- and two-electron integrals proposed by Wilhite and Euwema. The differential overlap between a pair of basis functions is set to zero if the pseudo-overlap is less than the adjustable threshold. We refer to this method as variable retention of diatomic differential overlap (VRDDO). In addition, only the valence electrons may be explicitly considered. The influence of the core electrons is taken into account by using the model potential (MODPOT) Hamiltonian suggested by Bonifacic and Huzinaga. The two methods can be used together resulting in the MODPOT/VRDDO procedure. VRDDO, MODPOT, and MODPOT/VRDDO calculations are reported for cyclopentadiene, benzene, pyrazole, imidazole, pyridazine, pyrimidine, pyrazine, and benzonitrile. The accuracy of the VRDDO calculations is very good. Orbital energies and gross atomic populations differ from the reference values by only 0.001–0.002 a.u. The maximum error in the total energy is 0.0024 a.u. and in the dipole moment is 0.009 a.u. Valence electron-only calculations with the MODPOT and MODPOT/VRDDO procedures yield orbital energies and charge densities that the quite good in comparison with the ab initio calculations taking all electrons into consideration. Orbital energies and gross atomic populations differ from the reference values b by 0.001–0.011 a.u. The maximum error in the dipole moment is 0.018 a.u. For benzonitrile, the largest molecule studied, introduction of the VRDDO procedure results in a 25% saving in computer time. The MODPOT calculation is three times faster than the ab initio reference calculation, while the MODPOT/VRDDO calculation is four times faster.

A theoretical investigation of electronic reorganizations accompanying core and valence ionization in simple hydrogen bonded systems

Non-empirical LCAO MO SCF calculations have been carried out on the ground state and core ionized states of some hydrogen bonded dimers, and in the particular case of H2O the trimer has also been investigated. Comparison of absolute and relative binding energies and relaxation energies with respect to the corresponding monomers reveals that substantial changes occur in going to the associated species. The relaxation energies for a given core hole are shown to increase on going from monomer to dimer indicating that intermolecular contributions to relaxation energies are of the same sign irrespective of the sign for the shift in core binding energy. Creation of a core hole in the dimer species is shown to give rise to substantial changes in hydrogen bond energies compared with the neutral species. In the particular case of valence holes dominantly of 2s and 2p character it is shown that trends in shifts and relaxation energies parallel those for the core hole states.

Relative energies of the ground and core hole states of the 1-propyl and 2-norbornyl carbocations

Non-empirical LCAO MO SCF calculations on the ground state and localized C1s hole states of 1-propyl and 2-norbornyl carbocations have been carried out at the STO·4·31G level employing optimised geometries; the energetic preferences of the core ionized species are found to magnify energy differences which are small for the ground-state system.

A NON-EMPIRICAL LCAO MO SCF INVESTIGATION OF SOME ASPECTS OF STRUCTURE AND BONDING IN THE THIATHIOPHTHEN RING SYSTEM

Abstract Non-empirical LCAO MO SCF calculations have been performed on prototype symmetrical and unsymmetrical thiathiophthen ring systems to investigate structure and bonding as a function of change in geometry. Substituent effects have been investigated and it is shown that within the theoretical limitations the energy differences between symmetrical and unsymmetrical structures are quite small. Comparison has been made with UPS and ESCA data pertaining to valence and core levels respectively and some consideration given to the thiathiophthen radical anion and dianion ring systems.

Structure and bonding in diazocyclopentadiene and some of its core and valence ionized states

Structure and bonding in diazocyclopentadiene and its core ionized states has been investigated by non-empirical LCAO MO SCF calculations and by ESCA. Shake-up satellites accompanying core ionizations have been interpreted in terms of the sudden approximation and CNDO formalism. Theoretical studies of the lower valence ionized states confirm the assignment of the recently published UPS(Hel) experimental data.

Nonempirical LCAO MO SCF studies of the group IIa dihalides BeF2, MgF2, and CaF2

Nonempirical LCAO MO SCF calculations are reported for the ground, C2ν states of the Group IIa dihalides, BeF2, MgF2, and CaF2. These calculations demonstrate the importance of 3d orbitals in the bonding trends of the dihalides and, hence, in the determination of the equilibrium bond angles and the bending force constants. The calculations on BeF2 indicate that d orbitals play an important role in the bonding but do not preferentially alter the general features of the total energy curve at any bond angle. In the case of MgF2, d orbitals are found to preferentially lower the energy of the nonlinear configurations. However, for both species, the ground-state equilibrium bond angle is predicted to be 180°. Configuration interaction studies also support the fact that, in the gas phase, the linear configuration is most stable. On the basis of s(1s, 2s, 3s, 4s, 2p, and 3p orbitals on Ca) basis set calculations, it is predicted that CaF2 is linear. However, when 3d orbitals are added to the s basis set, the predicted equilibrium bond angle is 145°. This is in good agreement with the value of 141° deduced from matrix-isolation spectroscopy. The bonding in these molecules is discussed with reference to the nonempirical molecular correlation diagram.

Model potential energy surfaces for approach of an electrophile to acetylene and fluoroacetylene

Non-empirical LCAO MO SCF calculations have been made on cross sections through prototype potential energy surfaces for the initial approach of an electrophile to acetylene and fluoroacetylene. The electrophile has been simulated by a unit positive charge and this is shown to give a computationally inexpensive means of providing information on such processes.

A non-empirical LCAO-SCF-MO investigation of cross sections through the potential energy surface for the [C 2H 2Cl] + systems; comparison with the [C 2H 5+] and [C 2H 4F] + systems

Non-empirical LCAO MO SCF calculations are reported on cross sections through the C 2H 4Cl + system and comparisons are drawn with the C 2H 5 + and C 2H 4F + systems. Barriers to rotation in the classical 1- and 2-substituted ethyl cations have been computed and an investigation made of the bridged chloronium and fluoronium ions. The results suggest that the relative stabilities of bridged ions with respect to the corresponding classical 2-substituted ethyl cations increase in the order H < F < Cl. The results are discussed in terms of available experimental data and consideration given to correlation and solvation energy effects.

Nonempirical LCAO MO SCF and CI Studies of the Low Lying Electronic State of the HOO Radical

Nonempirical LCAO MO SCF and configuration interaction calculations are reported for the ground state and the low lying 2A′ state of HO2. The first excited 2A′ state of HO2 is found to be approximately 17 kcal above the ground state. The correlation diagram for the reaction H+O2⇌OH+O and the importance of the low lying 2A′ state of HO2 are discussed.

Nonempirical LCAO MO SCF Calculation on Acetylene, Vinylidene Carbene, and the Vinyl Cation

Nonempirical LCAO MO SCF calculations using Gaussian-type functions have been used to predict the geometries of acetylene, vinylidene carbene, π-protonated acetylene, and the vinyl cation. The computed geometry and molecular ionization potential of acetylene give good agreement with experimental data. The vinyl cation is predicted to be more stable than π-protonated acetylene by about 18 kcal/mole.

The proton affinities of the acetylene molecule, and of the acetylide and diacetylide ions

Non-empirical LCAO–MO–SCF calculations using Gaussian basis sets were used to predict the geometry of the acetylide ion, proton affinities of 547·5, 391·6, and 166·1 kcal/mole being computed for C22–, C2H–, and C2H2, respectively.

The electronic structure of borazine

Non-empirical LCAO–MO–SCF calculations have been performed on planar (D3h) and non-planar (C2) configurations of borazine; it is shown that the planar structure is energetically preferred by a substantial margin.

The electronic structure of bridge-protonated fluoroethylene and the interconversion of 1- and 2-fluoroethyl cations

Nonempirical LCAO–MO–SCF calculations have been performed on an idealized reaction co-ordinate relating 1- and 2-fluoroethyl cations through bridgeprotonated fluoroethylene.