SCF CI Method Research Articles

INDO/SCF-CI calculations and structural spectroscopic studies of some complexes of 4-hydroxyacetanilide

The electronic energies among different possible structures of 4-hydroxyacetanilide (paracetamol) (PA) molecule, were calculated using INDO method and it has been concluded that its structure has C s point group symmetry of the cis-form. The ionization potential, electron affinity, dipole moment and binding energy have been calculated. The calculated electronic transitions of the cis-form of PA using SCF-CI method have good coincidence with the electronic absorption spectrum. The temperature effect on the electronic spectrum of PA confirms the presence of one conformer only. The electronic spectra of PA compound were studied in different polar- and non-polar solvents and the hydrogen bonding as well as the orientation energies of the polar solvents were determined from the mixed solvents studies. Complexes of PA with various metal ions such as, Cu(II), Zn(II) or Fe(II) ions of ratio 2:1, respectively, have been prepared and their structure has been confirmed by elemental analysis, atomic absorption spectra, IR spectra and 1H NMR spectra and finally it can be concluded that the structure of the complexes has C 2h point group symmetry in which two PA molecules are chelated to any one of the metal ions, Cu(II), Zn(II) and Fe(II) ions.

An ab initio potential energy surface of Ne–LiH

The intermolecular potential energy surface for Ne–LiH is calculated using the fourth-order Møller–Plesset perturbation approach with a large basis set consisting of the midpoint bond function set 3s3p2d. Two local potential minima are found to be located at the linear Ne–LiH ( θ=180°) and Ne–HLi ( θ=0°) geometry. The MP2 potential is also calculated to check the convergence of the potential. In addition, the vibrational energy levels are calculated using the vibrational SCF-CI method. Both MP2 and MP4 potentials support three bound vibrational states for the Ne–LiH system.

An SCF-CI method for determining the potential energy surface of a triatomic molecule

A self-consistent-field (SCF)-configuration interaction (CI) (SCF-CI) method for determining the potential energy surface of a triatomic molecule from the observed vibrational band origins has been suggested. By this method, the SCF-CI procedure in the internal coordinates is used to calculate the vibrational bond origins and their first derivatives with respect to parameters in the potential energy function using the exact vibrational Hamiltonian, and the optimizer LMF in the nonlinear-squares problem is employed to optimize parameters in the potential energy function. This approach is used to optimize the potential energy function of the water molecule. The standard deviation of this fitting to the 70 observed band origins is 1.154cm-1.

Structure and spectroluminescence properties of derivatives of 1,8-naphthoylene-1',2-benzimidazole with substituents of various electronic types

A series of quantum-chemical calculations using the CNO SCF CI method were carried out to account for the nontraditional influence of substituents of various electronic types on the spectroluminescence characteristics of naphthoylenebenzimidazole. It is shown that the principal direction of electron-density redistribution during excitation of this molecule is the charge transfer from its benzene fragment to its naphthalene fragment, and the most appreciable change in spectrofluorescence characteristics compared to the unsubstituted compound is achieved by introducing electron-donor substituents into the benzimidazole fragment, and in electronacceptor characteristics, into the naphthalene fragment of the naphthoylenebenzimidazole molecule.

Charge transfer in slow N2++H collisions

We have performed quantal calculations of charge transfer in N2++H collisions in the range of centre of mass energies 0.1<E<100 eV amu-1. Molecular states were calculated using ab initio SCF-CI methods. Radial and angular couplings between the adiabatic molecular states were evaluated with allowance for a common translation factor. In agreement with earlier calculations, we find that charge transfer into the N+(2s2p33D0) share is dominant in the energy range considered and relevant to the study of photoionized nebulae. However, the cross sections and the rate coefficients that we have computed are smaller than the results of the earlier work by factors of about 1.5.

Polarized one-electron potentials fitted by multicenter polarizabilities and hyperpolarizabilities. Ab initio SCF-CI calculation of water

The electrostatic potential is usually evaluated by putting a test charge at several points on an appropriate molecular surface. The polarization effect is an indispensable long-range interaction as well as the electrostatic interaction. To evaluate the polarization effect, it is necessary to relax the wavefunction under the perturbed Hamiltonian. In the present work the polarized molecular potentials based on the ab initio SCF-CI method have been fitted by multicenter polarizabilities and hyperpolarizabilities. The polarization energies and induced dipole moments calculated by using the fitted parameters have been significantly improved by including the first hyperpolarizabilities in the case of a water molecule.

Structure and stability of hydrogen rings

The thermochemical stability of hydrogen rings is studied using ab initio SCF-CI methods. The rings include H 3 + , H 4 , H 5 + , H 5 - , H 6 , H 7 + , and H 8 . Optimized geometries for the D nh rings are reported at theoretical levels from SCF to multireference CI. Vibrational frequencies are reported at the optimized geometries, in order to establish whether the ring geometries are absolute minima, relative minima, or saddle points on the potential surface

SPECTROSCOPIC DETERMINATION OF Lin ([MATH]) CLUSTERS STRUCTURE

Lithium clusters up to Li 100 are produced in a seeded molecular beam using a special oven able tu sustain temperature up to 1800 K. Optical spectra were recorded by two photon ionization spectroscopy for Li 3 and dy depletion for larger clusters because of fast dissociation in excited states. For Li 3 , clear vibronic and rotational structures have been measured leading to a complete determination of the geometry and potential surfaces of the ground state of the two observed excited states. For larger clusters, no vibronic structure was observed except in Li 4 for which a residual vibronic pattern was obtained. Optical spectra have been compared to SCF-CI calculations [1]. Since the calculated spectra strongly depend on the ground state geometry, it is possible to determine the cluster geometry. In Li 4 , the experimental absorption spectrum is in complete agreement with the theoretical spectrum obtained for the planar rhombus geometry while the theoretical spectrum calculated for the other isomer slightly higher in energy (deformed tetrahedron) is completely different. This is the first confirmation of the planar rhombus geometry of Li 4 . For Li 7 and Li 8 , the experimental results are also in agreement with the geometries calculated by the SCF-CI method. In Li 6 , the calculations lead to three isomers having very similar energy [2] : the planar D 3h structure, the quasi planar C 5v pentagonal pyramid and the 3D trypyramidal C 2v geometry. Only this last geometry gives a theoretical absorption spectrum in agreement with the experimental one. The two other structures lead mainly to a hyge peak at 2.1 eV in complete contradiction with the experimental results. Experiments are in progress to extend our measurements up to Li 40

Quenching of the 3P and 1P states of Mg by acetylene and possible formation of vinylidene. A theoretical ab initio SCF CI study

Physical and chemical quenching of 3P and 1P states have been studied by theoretical ab initio SCF CI methods. The singlet and the triplet are attractive, yielding C 2v triangular π exiplexes. No secondary minimum corresponding to an open form has been found on the singlet surface, whereas the triplet surface exhibits a very shallow secondary minimum corresponding to a a complex. Physical quenching is expected to be efficient for both states. The initial energy of the singlet system is large enough to induce a hydrogen transposition yielding vinylidene magnesium, which, in turn, can easily undergo a CMg rupture and produce vinylidene. This channel might compete with an insertion reaction of Mg into a CH bond.

X-ray photoelectron, Auger electron and ion fragment spectra of O2 and potential curves of O22+

The O22+ ion has been studied by means of electron-impact-induced and photon-induced Auger electron spectroscopy and oxygen ion fragment spectroscopy of O2. The oxygen ion kinetic energy spectrum was recorded by inverting the relevant potentials of an electron spectrometer for the detection of positive particles. The 4 Sigma - and 2 Sigma - O 1s initial core hole states have been studied using monochromatised X-ray photoelectron spectroscopy. Potential energy curves for a number of electronic states of the O22+ dication have been calculated with the complete active space SCF (CASSCF) and multireference contracted CI (MRCCI) methods with a one-particle basis set of medium size ((8s, 6p, 2d)). An analysis of the O2 Auger electron spectrum based on the computed potential curves of O22+ is presented. The autoionisation satellites are analysed and these lines correspond to molecular singly ionised final states. One line at 510.7 eV, however, is associated with an atomic-like transition. Two shake-up Auger satellites are identified by a comparison with a recent O 1s shake-up spectrum from O2.

Theoretical study of the CO 2+ dication

Potential energy curves for a number of electronic states of the doubly charged CO 2+ dications have been calculated with the complete active space SCF (CASSCF) and multireference contracted CI( (MRCCI) methods with a one-particle basis set of medium size ([8s, 6p, 2d]). The computed results are compared with results obtained from experimental investigations such as Auger spectroscopy, time-of-flight mass spectrometry, energy gain spectroscopy, double electron capture and ion translational energy spectrometery.

Low-lying singlet states of MgCl +

The potential energy curves of the X 1Σ + and A 1Π states of the MgCl + cation have been computed using CAS SCF and MR CI methods, including core-valence correlation, combined with large basis sets. The results for the X 1Σ + state give D e=3.20 eV and r e=3.97 a 0 while the A 1Π state has a D e=0.41 eV and a substantially longer r e of 5.19 a 0. The present work supports the interpretation of the recent experimental observation of MgCl + photodissociation as A 1Π-X 1Σ + occurring from high- v″ levels.

On the stabilities of the singly and doubly negative atomic ions of Si, P, S, and Cl

The first and second electron affinities of four second-period elements, Si, P, S, and Cl were investigated with the CI(SD), CEPA, MC SCF and MC CI(SD) methods. The most accurate MC CI(SDQ) calculations yield errors of 0.08, 0.23, and 0.18 eV in the first electron affinity values of Si, S, and Cl, respectively. The error for the P atom is 0.4 eV even at the MC CI(SDQ) level of approximation. All calculated second electron affinities were obtained negative, irrespective of the inclusion of electron correlation, i.e., the doubly negative atomic ions are not stable in the gas phase. The investigation of a few high spin states of Si 2− and P 2− gives no hint for metastable states.

Gauge-dependent ket calculation of the magnetic properties of molecules: Part I. Method and application to H 2, H 2O and CO

A gauge-invariant ket function theory of magnetic properties is presented. The perturbed state, which depends explicitly on the strength of the external fields, is determined variationally as a linear combination of Slater's determinants associated with a gauge transformation factor. This “mixed” procedure combines advantages of both the polynomial and the familiar SCF-CI methods; it is applied, with inclusion of electronic correlation, to H 2, H 2O and CO.

Accurate MR CI studies of the N 2 ground state

Extensive calculations of the ground-state energy of the N 2 molecule have been performed by means of the MC SCF and MR CI(SD) methods which are defined in terms of a 20-configuration reference space including all the configurations required for a correct description of the dissociation of the triple bond. The AO basis contained the 3d2flg polarization set. The whole potential curves have been obtained using the same approach. The calculations yields the lowest total energies and the most accurate results for D e ( = 9.629 eV) and R e ( = 2.078 au) obtained so far in extensive variational calculations. Further evaluation of the quality of our potential curves is made by comparing vibrational intervals, rotational constants and several spectroscopic constants with experimental values. A very satisfactory agreement has been found.

The electronic structure of the disilavinylidene anion

The electronic structure of the disilavinylidene (H 2Si = Si) − anion is investigated with the MC SCF CI and CEPA methods. In addition to the doublet ground state ( 2B 2) with an electron affinity of 1.65 eV, an excited state ( 4A'') is found which is still stable by about 0.6 eV.

The harmonic and anharmonic force fields and vibration–rotation constants of ClNO, FNO, FSN, and ClSN from SCF and CI a b i n i t i o computations

The harmonic and cubic force field of the triatomic molecules ClNO, FNO, ClSN, and FSN are evaluated by using SCF and various CI methods. The vibration–rotation constants derived from the force field are compared to the experimental values. The computed cubic force field is found to be accurate at the SCF level for ClNO and FNO, despite severe limitations of the HF description for the latter molecule. CI effects are important for some of the harmonic components of the force field of FNO and for its α vibration–rotation constants. The values of the vibration–rotation interaction constants have been predicted for ClSN and FSN.

Experimental and theoretical study of configuration interaction states of CO+2

Configuration interaction states of CO+2 have been observed as satellites in a high resolution photoelectron spectrum of CO2 using an angle resolved electron spectrometer and far UV synchrotron radiation in the 30–55 eV photon energy range. Ten satellites have been identified in the 22–40 eV binding energy region and classified as the result of outer and inner valence ionization processes on the basis of their asymmetry parameter ( β) values. Theoretical calculations of configuration interaction states of CO+2 using an ab-initio SCF-CI method show that three-hole-two-particle excited configurations are necessary to explain the line positions and intensities. The calculated satellite spectrum, corrected for transition moments at 45 and 1254 eV, shows an excellent agreement with experiment and allows assignments of both outer and inner valence satellite lines, with Πg, Πu, Σ+g, and Σ+u symmetries.

Ab initio calculations on the excited states of π-systems. II. Valence excitations in diacetylene

The electronic ground state and the first valence excited states of diacetylene ( 3Σ + u, 1,3Δ u, 1,3Σ − u) are investigated with the aid of ab initio methods applying basis sets of double-zeta and triple-zeta + d quality. SCF, SCF CI, and MC SCF CI methods have been applied. We report equilibrium geometries, vertical and adiabatic excitation energies, and discuss two-dimensional bending potentials in detail. Both C 2h- and C 2v-type structures are considered. For the lowest singlet states harmonic vibrational analysis of totally symmetric modes has been carried out. Previous experimental assignments of singlet excited states of diacetylene are essentially confirmed in this study and new results on the energetics of triplet excited states are reported.

Ab initio calculations on the excited states of π-systems. I. Valence excitations in acetylene

The electronic ground state and the first valence excited states of acetylene ( 3Σ + u, 1,3Σ − u and 1,3Δ u) are investigated with the aid of ab initio methods applying large and flexible basis sets. Electron correlation has been taken into account on various levels of sophistication, including SCF CI, MC SCF and MC SCF CI methods. Equilibrium geometries are determined for all states mentioned, excitation energies, vertical and 0–0 transitions are reported and the dependence of the computed results on the type of wavefunction is discussed. Harmonic force constants and vibrational frequencies are calculated for all states treated in this paper and compared to experimental values.