A non-empirical LCAO MO SCF investigation of electronic relaxation accompanying core ionization in a series of saturated alkanes

Ab initio LCAO MO ΔSCF binding energies and relaxation energies have been computed at the STO-4.31G level for linear and branched alkanes (up to C₆) and are compared with experimental data. The structural dependence of binding energies and relaxation energies is discussed, and it is concluded that binding enegy shifts in these alkanes (compared with methane) arise mainly from relaxation energy changes.     

Non empirical LCAO SCF MO investigations of electronic reorganizations accompanying core ionizations

Calculations have been carried out on an extensive series of molecules for both the neutral species and core ionized states. Substituent effects on C_1s , N_1s , O_1s , and F_1s levels have been investigated and where available comparison has been drawn with experiment. Comparison with Koopmans' theorem has allowed a relatively detailed study of change in relaxation energies as a function of substituent effect on a given core level. Whilst for C_1s levels the computed shifts in core binding energies are approximately linearly related to differences in relaxation energies for the N_1s , O_1s , and F_1s levels, the relative electronegativity of the substituent can invert this correlation. The empirical correction of Koopmans' theorem for differences in relaxation energies at different sites has been investigated for large molecules. The results compare well with direct hole state calculations.     

A semi-empirical SCF LCAO MO study of the hindered internal rotation in formamide

A molecular orbital calculation in the CNDO/2 approximation has been used to determine the possible forms of the transition state for the hindered rotation in formamide, and geometrical optimisation is shown to be important in the determination of the theoretical energy barrier.     

All-electron SCF LCAO MO calculations on various conformations of cis- and trans-stilbene

Ab initio SCF calculations of cis- and trans-stilbene at different conformations were performed using two program systems. Minimal energy is obtained for cis-stilbene when the phenyl rings are rotated by 52 ° out of the molecular plane. The deviation from planarity due to steric hindrance is smaller for the trans isomer yielding a rotational angle of 19 °. The trans isomer is calculated to be more stable by 5.7 kcal/mole than the cis isomer, confirming the experimental estimate according to which the energy of isomerization is about 3 kcal/mole. This is an improvement over semiempirical calculations which predict a lower energy for the trans configuration.     

Photoelectron spectroscopy and ab initio LCAO MO SCF calculations on thiazyl fluoride

The He l, He II and Ne I photoelectron (p.e.) spectra of thiazyl fluoride. NSF, are compared with ab initio LCAO MO SCF calculations via Koopmans' theorem. The calculations predict the valence formulation NSF, and the p.e. spectrum is consistent with this. Comparisons are drawn with the isoelectronic molecule SO₂     

Cyclooctatetraene and related systems. An ab initio SCF LCAO MO study

Minimal basis set non-empirical SCF LCAO MO calculations on cyclooctatetraene I, its dianion II and its bicyclic valence tautometer III lead to a ring inversion barrier of 17.8 kcal/mole for the parent molecule, a planar D_8h structure for the dianion, and an energy difference of 3.9 kcal/mole between I and III.     

Repulsion integral parametrization of the SCF LCAO MO CI method for nitrogen heteroaromates

Parametrization of Coulomb repulsion integrals for a number of nitrogen heteroaromates has been studied within the P-P-P method. The calculation was performed for the Mataga-Nishimoto and Ohno approximation as well as for the weighted mean from the two approximations. The latter approach gives good agreement with experimental data for both the first singlet and triplet excited state, particularly in the case of one ring molecules.     

A discussion of the bonding in ferrocene from an ab initio LCAO MO SCF calculation

The nature of the bonding in ferrocene is discussed on the basis of an ab initio LCAO MO SCF calculation, with the emphasis put on a number of controversial points such as: i) the relative ordering of the orbitals e_2g(3d) and a_1g(3d); ii) the relative ordering of the 3_1u and e_1g ligand π orbitals; and iii) the degree of participation of the metal 4s and 4p orbitals to the bonding.     

Non-additivity of intermolecular forces in He3 within the SCF LCAO MO approximation

The deviation of the interaction energy of the He₃ system from pairwise additivity is calculated by the SCF LCAO MO method. Our results are in qualitative agreement with existing perturbation theory calculations, but differ considerably in magnitude.     

Avoiding the use of integral lists in the ab initio LCAO SCF MO method

The suggestion is made that the storage of long lists of two-electron repulsion integrals in the LCAO SCF MO method could be avoided if such integrals were evaluated at each SCF cycle and an initial set of eigenvectors were available to lead to rapid convergence. Results are presented showing that the simulated ab initio molecular orbital (SAMO) technique provides an excellent set of initial eigenvectors for the SCF procedure.     

A non-empirical LCAO MO SCF investigation of electronic relaxations accompanying core ionizations in the series X2 and HX (X = F, Cl and Br)

A theoretical analysis has been made of differences in relaxation energies for photoionization from the core levels of the series X₂, HX for X = F, Cl, Br. It is demonstrated that whilst the charge in relaxation energies is largest for F₂ with respect to HF, the contribution to the shifts in core levels is relatively larger for the series X₂ and HX for X = Cl, Br. It is further shown that shifts in binding and relaxation energies show very little dependence on the core levels studied.     

Study of the possible isomerization of the phenyl radical employing the nonempirical MO LCAO SCF method

Conclusions Prototropic rearrangement in the phenyl radical cannot proceed under ordinary conditions in view of the high energy barrier.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 460–462, February, 1979.     

π-Electron ring currents and magnetic properties of porphyrin molecules in the MO LCAO SCF method

The coupled variant of double-parameter perturbation theory in the MO LCAO SCF method in the London approximation has been used for the calculation of π-electron current distributions in the molecules of porphin and its derivatives. The chemical shifts of¹H-NMR have been computed on the basis of calculations of ring currents and charge distributions. It is shown that π-electron ring currents are responsible for the dominant contribution to the shielding of protons. The theoretical and experimental values of proton chemical shifts are in a good agreement. Chemical shifts of the¹³C and¹⁵N nuclei have also been estimated. Two aromaticity scales are proposed for the compounds under study based on the calculations of the π-electron contribution to the diamagnetic susceptibility and of π-electron currents, respectively.     

An approximation for gaussian d-orbital exponents of second-row atoms for LCAO MO SCF studies

It is demonstrated that the radius corresponding to the shell of maximum electron density for molecularly optimized 3d orbitals of a second-row atom is approximately equal to the same radius for the atom-optimized 3p orbitals. An approximation for expanding a single gaussian d-orbital exponent into a larger set of exponents is also offered.