Charge calculations in molecular mechanics. V. Silicon compounds and π bonding

A previously published scheme for the calculation of partial atomic charges has been extended to include silicon, and has been parameterized for a range of Si? X bonds (X?C,H,O,F,Cl,Br). For the silicon–halogen and silicon–oxygen bonds, a comparison is made between charges calculated with and without the inclusion of π-bonding. An extensive data set consisting of experimental geometries and dipole moments for the silicon compounds considered is presented and this leads to the selection of standard Si? X bond lengths. The calculated dipole moments for the above compounds are in good agreement with those obtained experimentally only when the π charges are included. A comparison has also been made between the partial charges from this scheme and those obtained from computational methods using the Mulliken population analysis. There is considerable disagreement between the methods. Finally, the implications of the charges and structural data are considered in terms of application to zeolite systems.     

Surfaces and roughening

Some recent results in the application of statistical mechanics to surfaces are discussed. Only exactly soluable models are described. First, we consider phase separation below the critical temperature in uniaxial ferromagnets and their analogs. We then consider the determination of the equilibrium shape of a crystal having fixed volume, given the orientation-dependent surface tension, using the Wulff construction.     

The NMR spectra of the porphyrins. 17—Metalloporphyrins as diamagnetic shift reagents,structural and specificity studies

The nature of the metalloporphyrin-ligand complexes produced by zinc, magnesium and cobalt porphyrins with basic ligands has been investigated using the diamagnetic ring current shifts of the porphyrin on the ligand protons. The metal to nitrogen bond lengths in some metallo-meso-tetraphenylporphyrin (pyridine) complexes have been determined and compared with the data of the crystalline complexes. The geometry of the Zn meso-tetraphenylporphyrin complexes with 2-picoline, quinoline and isoquinoline has been investigated. Steric interactions between the ligand and the porphyrin in 2-picoline and quinoline produce a dramatic increase in the Zn? N bond length when compared to the unstrained analogues pyridine and isoquinoline. This large increase is associated with comparatively minor angle distortions in the complex. The specificity of the Zn meso-tetraphenylporphyrin complexation shifts has been determined for a range of benzyl and butyl compounds. The complexation shift is linearly related to the basicity of the ligand for a wide range of basicities.     

Matrices and eigenfunctions induced by Markov maps

The matrix M induced by a Markov map has 1 as the eigenvalue of maximum modulus. If M is also irreducible, then the algebraic and geometric multiplicities of the eigenvalue 1 are also 1. Let g=(g₁,g₂…g_n) be an n-row-vector such that g₁=c> and g_ic, i?2. Then every such g is the unique (upto constant multiples) left eigenvector associated with the eigenvalue 1 of a matrix M induced by a Markov map.     

Difficulties in the Further Development of Venous Hemodynamics

The validity of hemodynamic theories developed for the arterial tree is examined with respect to their applicability to the venous system. The Navier-Stokes and continuity equations are given and the assumptions (in terms of these equations) found useful and justifiable for arterial theories are briefly described. The validity of these assumptions with respect to the venous system is considered. The significance of the dissimilarities in terms of computer simulation (analog and digital) is discussed. It is concluded that in regions which do not collapse much of the theory developed for the arterial system is applicable; in those regions which do collapse, theories based on the nonlinear Navier-Stokes equation should be considered.     

The quantum efficiency of the primary processes in formaldehyde photolysis at 3130 Å and 25°C

The mechanism of the photolysis of formaldehyde was studied in experiments at 3130 Å and in the pressure range of 1–12 torr at 25°C. The experiments were designed to establish the quantum yields of the primary decomposition steps (1) and (2), CH₂O + hν → H + HCO (1): CH₂O + hν → H₂ + CO (2), through the effects of added isobutene, trimethylsilane, and nitric oxide on Φ_CO and Φ. The ratio Φ_CO/Φ was found to be 1.01 ± 0.09(2σ) and (Φ + Φ_CO)/2 = 1.10 ± 0.08 over the range of pressures and a 12-fold change in incident light intensity. Isobutene and nitric oxide additions reduced Φ to about the same limiting value, 0.32 ± 0.03 and 0.34 ± 0.04, respectively, but these added gases differed in their effects on Φ_CO. With isobutene addition Φ_CO/Φ reached a limiting value of 2.3; with NO addition Φ_CO exceeded unity. The addition of small amounts of Me₃SiH reduced Φ to 1.02 ± 0.08 and lowered Φ_CO to 0.7. These findings were rationalized in terms of a mechanism in which the “nonscavengeable,” molecular hydrogen is formed in reaction (2) with ?₂ = 0.32 ± 0.03, while the “free radical” hydrogen is formed in reaction (1) with ?₁ = 0.68 ± 0.03. In the pure formaldehyde system these reactions are followed by (3)–(5): H + CH₂O → H₂ + HCO (3); 2HCO → CH₂O + CO (4); 2HCO → H₂ + 2CO (5). The data suggest k₄/k₅ ? 5.8. Isobutene reduced Φ by the reaction H + iso-C₄H₈ → C₄H₉ (20), and the results give k₂₀/k₃ ? 43 ± 4, in good agreement with the ratio of the reported values of the individual constants k₃ and k₂₀.     

The use of chiral solvents to distinguish meso from d or 1 diastereoisomers

The 56·4 MHz ¹⁹F spectrum of a mixture of meso and dl 1,2-difluoro-1,2 dichloroethane in L -bornyl acetate resolved the ¹⁹F resonances of the separate d and l stereoisomers, thus identifying the dl spectrum, but not those of the meso isomer. These observations are considered with other examples of the use of chiral solvents to distinguish meso and dl isomers.     

Photochemie substituierter cycloheptatriene—VI : Diarylsubstituierte cycloheptatriene

The substituent effect of the dimethylamino group in diarylcycloheptatrienes, on the one hand, leads to a strict selection of the reaction way of the sigmatropic hydrogen shift. On the other hand, these substituents in 1,4-positions give rise to the reversible photo valence isomerization cycloheptatriene-bicycloheptadiene. The quantum yield of the sigmatropic H-shift depends strongly on the wavelength of the exciting light. It has been shown by sensitizing and quenching experiments that the intramolecular cyclisation does not proceed through triplet manifold or fluorescent minimum, but through an independent “funnel” in the singlet state. The ring opening is possible not only photoinduced, but also thermically with small activation energy.     

The fixed conformation of the leucyl side-chain in a tripeptide

The complete analysis of the ¹H NMR spectrum of the leucyl side-chain of a tripeptide allows the deduction of the conformation of this side-chain. In the peptide, in contrast to the free amino acid, the side-chain is in a single fixed conformation.