Showing papers on "Pi interaction published in 1974"

A “double-zeta” type wavefunction for an organometallic: Bis-(π-allyl)nickel

Abstract: A wavefunction which is of double-zeta quality at the level of the valence orbitals [based on a (11, 7, 5/8, 4/4) gaussian basis set contracted to (4, 3, 2/3, 2/2)] is reported for thebis-(π-allyl)nickel molecule. Independant SCF calculations for two ionized states substantiate the conclusion reached previously for a number of organometallics with a minimal basis set that Koopmans' theorem is not valid for these molecules, namely that the highest occupied orbital from the ground state calculation for the neutral molecule is mostly a ligand π orbital whereas the lowest ionization potential corresponds to the removal of an electron from a molecular orbital which is mostly a metal 3d orbital. The nature of the bonding inbis-(π-allyl)nickel is discussed on the basis of the possible interactions between the metal orbitals and the π orbitals of the allyl group. The interaction between the filled nonbonding π orbital of the allyl group and the empty 3dxz orbital of the Ni atom appears responsible for most of the bonding, together with some backbonding through an interaction between the 3dx2−y2and 3dxyorbitals and the σ and π orbitals of the ligands. The computed value for the rotation barrier about the C-C allyl bond, 90 kcal/mole, rules out this rotation as one of the possible mechanisms which account for the equivalence of the terminal hydrogens in the proton magnetic resonance spectra of π-allyl complexes.

Spezifisch π→π*‐induzierte Reaktionen von γ‐Dimethoxymethylcyclohexen‐2‐onen: 1,3‐Umlagerung und Wasserstoffabstraktion durch das α‐Kohlenstoffatom

Abstract: When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S2(π,π*) state, certain unimolecular reactions can be observed to compete with S2 S1 internal conversion. These reactions do not occur from the S1(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8, 9 32 + 33), γ α formylacetal migration (cf. 6 27, 8 30, 9 34, 12 37), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 28, 8 31, 12 38, 20 40 + 41). The quantum yield of the ring closure 20a 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the ππ* absorption band (238–254 nm), but Φ (40a + 41a) decreases at higher concentrations. According to the experimental data the reactive species of these specifically ππ*-induced transformations is placed energetically higher than the S1(n,π*) state, and it is either identical with the thermally equilibrated S2(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar ππ*-induced cyclization ( 42) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n π and ππ* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones.

Synthetic Photochemistry. III. The Addition Reaction of Methyl Acetopyruvate to Cycloheptatriene: An Ene Reaction and (4+2)π and (6+2)π Cycloadditions

Abstract: The irradiation of methyl acetopyruvate with cycloheptatriene by means of a high-pressure mercury lamp has yielded (6+2)π and (4+2)π cycloadducts, together with tropylcarbinol derivatives formed by a formal abstraction of allylic hydrogen by the excited carbonyl group, this being the major process in this reaction

Interactions between purine derivatives: Electronic spectral studies. II. Excition interactions in the dimer of 6‐methylpurine in aqueous solution

Abstract: From a study of the concentration dependence of the ultraviolet absorption spectra of aqueous solutions of 6-methylpurine, the spectra of monomeric and dimeric species have been derived. The dichroic spectrum of 6-methylpurine in stretched poly(vinyl alcohol) films has been determined. The absorption envelope of 6-methylpurine up to 44,000 cm−1 appears to include a low-intensity nitrogen (π*,n) transition at the low wavenumber tail and two additional electronic transitions of dominant intensities. Both the experimental dichroic ratios and theoretical calculations suggest that the letter two transitions are in-plane (π*,π) types. Analysis of the dimer species spectrum in terms of exciton interactions between molecules in “sandwich” conformation has been carried out. The interaction between the moments of the lowest energy (π*,π) transitions of the molecule in the dimer seems to result in transfer rates of energy of the order of 1013 sec−1 between the two molecules.

π electron structure and reactivities of isomeric pyrrolo-sym-triazoles

Abstract: The π electron structures and energies of the singlet π → π * transitions of a number of isoelectronic analogs of indolizine were calculated by the MO LCAO method within the semiempirical self-consistent field (SCF) approximation. In contrast to the calculations made by the simple MO LCAO method, the π electron density distribution obtained is in better agreement with the direction of electrophilic substitution. On the basis of an analysis of the electronic spectra, it was concluded that the 6-phenyl group is not coplanar.

An INDO molecular orbital study of [1,3]π interactions in 2-substituted allyl cations

Abstract: The INDO molecular orbital technique, including a partial geometry search, has been applied to three 2-substituted allyl cations and the parent allyl cation. It is deduced that electron-releasing substituents favour π bonding between the terminal carbons. The 2-oxidoallyl species is found to represent an extreme example of this effect, the [1,3]π interaction being so pronounced that 2-oxidoallyl appears to be best described as a monohomocyclopropenone.

Photoelectron spectra and molecular properties. 30, Pi-interactions in H3Si- and H3C-substituted acetylenes

Abstract: The photoelectron (pe) spectra of mono- and disubstituted silyl- and methylacetylenes are assigned by comparison with the corresponding ionization potentials of acetylene, disilane, and ethane. The observed π splittings can be rationalized within a parameterized hyperconjugation model. Calculations using a modified CND0/2 procedure with and without inclusion of 3d orbitals in the basis set demonstrate that the usual parameters tend to overemphasize p,d, back-bonding. To rationalize available experimental data, 3d si orbitals need not to be considered explicitly-neither for the different dipole moments of methyl- and silylacetylenes nor for the general pe spectroscopic pattern of silicon compounds, i.e., high first ionization potential, reduced spin-orbit coupling, or band broadening.