Ab Initio UHF Research Articles

Ab initio studies on the electronic structure of the FSO radical

The geometries of the FSO radical in the ground ( 2A″) and first excited ( 2A′, n → π*) state have been calculated with the ab initio UHF SCF gradient method. The geometry in the first excited state is predicted to be r so = 1.69 3 Å, r SF = 1.60 g Å, and <FSO = 95.5°. The force constants, vibrational frequencies and dipole moments have also been calculated. The predicted 2A′ ← 2A″ transition energy, calculated at the optimized geometries with an UHF-NO-CI method, is 3.9 eV for the vertical excitation, 0.9 eV for the vertical emission and 2.6 eV for the electronic term value. The oscillator strength is also calculated. A brief discussion is given on the role of bond functions on the calculated geometry.

Electronic structures of the free radicals derived from ascorbic acid and α-hydroxytetronic acid

Calculations on several free radicals derived from ascorbic acid, and α-hydroxytetronic acid are reported. The calculations have been carried out both with the INDO method and the ab initio UHF method. The calculated spin densities are only consistent with the assignment of the structure of the predominant radical derived from these molecules to the anion radical.

Ab initio UHF and UHF NO CI approaches for quasi-degenerate systems: methylene peroxide (CH 2OO)

The ab initio UHF and multi-reference (MR) UHF natural-orbital (NO) CI methods have been applied to elucidate geometrical structures and electronic energy levels of methylene peroxide which is regarded as a typical example of a quasidegenerate system. The results by MR UHF NO CI are in good agreement with those of the GVB CI method. It is shown that the present CI is closely related to the extended Hartree-Fock CI approach, which is equivalent to the GVB CI or MC SCF CI for diradical species.

Study of the predissociation of NH3in the 3sA"2state from ab initio UHF calculations

Predissociation of the A2 state in NH3 is investigated from ab initio UHF calculations of the relevant cross section of the potential energy surface. Emphasis is placed on dissociation products and the role of the Rydberg 3s orbital. The force constants, Rydberg ionisation limit, dissociation barrier and dissociation products are systematically calculated and comparison with experiment is made. A critical examination of a previous hypothesis concerning the predissociation is made and some new aspects are obtained.

Problem of spin contamination and basis set in ab initio UHF spin density calulations

Problem of spin contamination and basis set in ab initio UHF spin density calulations

ChemInform Abstract: ON THE DETERMINATION OF BOND ANGLES OF TRIATOMIC RADICALS FROM ELECTRON SPIN RESONANCE DATA

AbstractFür das neutrale Radikal BH; und die isoelektronische Reihe Nä′, N02, CO; , B03′ und BF2 werden ab initio‐UHF(unrestricted Hartree‐Fock)‐ und vollständige CI(configuration interaction)‐Berechnungen durchgeführt, um die Zuverlässigkeit einer Gleichung von Coulson, in der der Bindungswinkel mit der Orbitalhybridisierung am Zentralatom verknüpft ist, theoretisch zu testen.

Analysis of EPR with large quadrupole interaction

A method has been developed to analyze EPR spectra for the case when the quadrupole coupling and the hyperfine coupling are of comparable magnitudes but much smaller than the electronic Zeeman term. A computer program which calculates the EPR spectrum for a finite Gaussian or Lorentzian linewidth has been written. The program is particularly suitable for cases with several interacting nuclei. The method has been applied to confirm the experimentally observed planar structure of ·CHCl 2. Ab initio UHF calculations have further confirmed this structure. Geometry optimization of · CH 2Cl has shown that the radical can be assumed planar at least on the ESR time scale.

The structure and hyperfine splittings of simple phosphoranyl radicals

The ab initio UHF method has been employed to calculate equilibrium geometries and isotopic hyperfine coupling constants for the radicals PH 2, PF 2, PH 4 and PF 4.

Influence of spin contamination and basis set on electrostatic potential and Hfs coupling constants of organic radicals

Ab initio UHF wavefunctions for organic radicals have been obtained with different basis sets, before and after spin annihilation. It has been verified that the shapes of the electrostatic potential surfaces of these radicals are basis and spin contamination independent, at difference from hfs coupling constants.

On the determination of bond angles of triatomic radicals from electron spin resonance data

Ab-initio UHF and complete Cl calculations using minimal basis sets of atomic orbitals have been made for the neutral radical BH2 and the isoelectronic series of radicals N32–, NO2, CO2–, BO22–, and BF2. The purpose is to test theoretically the reliability of an equation derived by Coulson linking the bond angle with the orbital hybridisation on the central atom. It is concluded that this equation is qualitatively reliable.

Electron-spin resonance and theoretical studies of radicals formed from trimethylamine borane : the radicals Me2[Ndot]-BH3and (Me3N-BH3)-

Exposure of trimethylamine borane to 60Co γ-rays at 77 K gave, initially, a four-line E.S.R. spectrum assigned to the radical anion, Me3[Ndot]-BH3 -, having a large (58 G) hyperfine coupling to boron. Theoretical calculations suggest that the radical resembles a perturbed .BH3 - in which the normally planar unit still has considerable pyramidal character. After annealing above 77 K this species was lost and a multiline, isotropic spectrum grew in which, by comparison with spectra obtained from Me3N-BD3, is shown to belong to Me2[Ndot]-BH3 radicals. This species is of interest because of the large 1H coupling to the -BH3 protons (ca. 45 G). This coupling, qualitatively ascribed to hyperconjugation, is correctly reproduced by theoretical (ab initio UHF) calculations.

Ab initio unrestricted Hartree–Fock calculations. Part 9.–Hyperfine coupling constants in the ethyl radical

Calculated values for the anisotropic hyperfine coupling tensors for the ethyl radical by the ab initio UHF method with spin annihilation are in good agreement with experiment. The results are critically compared with those from semi-empirical calculations.

Satellite peaks in the high energy photoelectron spectra of some small first row molecules

The gas phase high energy photoelectron spectra of CH4, NH3, H2O, N2, O2, CO and CO2 have been recorded, and in all cases weak satellite peaks to high binding energy of the main ionization peak are observed. These peaks are assigned to transitions to ionic states in which valence electron excitation as well as core ionization has occurred. The intensity and position of these peaks, relative to the main ionization peak have been estimated from ab initio UHF calculations on the core hole states, which in general allow assignment of the satellite peaks in terms of orbital transitions of the core hole ion.

An MO theoretical treatment for the solvated electron in polar solvents using simplified models

The solvated electrons in polar solvents such as H 2O, NH 3, HF, MeOH and EtOH are treated using the dimer model and the monomer model. For the dimer model. The unrestricted Hartree-Fock method with the INDO approximation is applied and the calculation of the monomer model is performed by the INDO and ab initio UHF methods; in either case, the additional atomic orbitals for the excess electron are used. It is found conclusively that the properties of the solvated electron in H 2O, NH 3 and HF can be interpreted successfully even when the dimer model is used. Particularly with regad to the inner proton spin density, the result is scarcely affected by the choice of the model or the method of calculation. Concerning the properties of MeOH and EtOH, several reasonable results can also be obtained by using the dimer model.

Ab initio UHF calculations. Part 8.—Spin contamination in the cyanide radical

Ab initio UHF calculations on the cyanide radical have been extended to reduce the excessive amount of spin contamination which is sometimes characteristic in the UHF method. The “spin constrained UHF method” minimizes the function αEUHF+(1–α)〈S2〉UHF for values of α between 0 and 1, to produce UHF-type wave functions. The coefficients of the contaminating spin states are determined for selected values of α. By an extrapolation procedure the hyperfine coupling constants arising from the doublet-state component of the UHF wave function (α= 1) are estimated to be a(13C)= 238 G and a(14N)=–4.2 G.

Ab initio UHF calculations. Part 7.—Possible evidence for environmental effects on the radicals BH–3, CH3and NH+3

The radicals BH–3, CH3 and NH+3 are compared using ab initio UHF calculations after annihilation of the contaminating quartet spin state. Although there is a consistency in the trends predicted for the properties, these are not in accord with those determined experimentally. Although the out-of-plane zero-point energy vibration has a dominating influence, it is concluded that the environment must determine the physical properties of BH–3.

Ab initio UHF calculations.—Part 5.–HBO–, HCO, HCN–and hyperconjugation in H2CN

The hyperfine coupling constants for H2CN, HBO–, HCO, and HCN– have been calculated using a minimal basis set of SCF atomic orbitals by the ab initio UHF method after annihilation of the contaminating quartet spin state. The wave functions are discussed with reference to the concept of hyper-conjugation. It is concluded that the delocalization of the unpaired electron in all the radicals studied has an identical origin.

Comparison of minimization procedures for UHF wave functions

The Roothaan, McWeeny and Fletcher methods of minimizing the electronic energy in the SCF method are compared. The CN radical is used as an example using the abinitio UHF method. It is concluded that a combination of the Fletcher and Roothaan methods should be generally applicable and also highly efficient.

Ab initio UHF calculations. Part 4.—The radical ion NH+3

The hyperfine coupling constants for NH+3 are calculated to be aN= 10.57, aH=–22.65 G using the ab initio UHF method after spin annihilation of the quartet spin state and optimization of the NH bond length and hydrogen atom orbital exponent. It is calculated that the effect of the out-of-plane zero-point energy vibration modifies the hyperfine coupling constants to aN= 16.92, aH=–20.02 G.

Ab initio UHF calculations of hyperfine coupling constants

Ab initio UHF calculations using a minimal basis set of orbitals for the radicals H 2S −, H 2S +, NH 3 +, CH 3, CH 3 − and NH 2 predict the hyperfine coupling constants to all nuclei with a consistent accuracy if SCF atomic orbitals are used.