Showing papers in "Israel Journal of Chemistry in 1977"

Studies on Warburganal, Muzigadial and Related Compounds

Abstract: Warburganal and muzigadial are two new sesquiterpenes isolated from a medicinal plant. They are two of the strongest antifeedants against the monophagous African army worm; in addition they possess potent molluscicidal activity and other antibiotic properties. A general account is given on these aspects as well as the spectral data leading to determination of their structures.

V. One‐Electron Density Functions and Many‐Centered Finite Multipole Expansions

Abstract: The one-electron density function, ρ(r), (in principle deduced from elastically scattered X-ray intensities) is the probability distribution function of an electron, averaged over the positions of all other electrons. A partitioning of ρ(r) into constituent parts is an intellectual exercise that does not lend itself to unique measurement from elastic X-ray scattering experiments. It is shown that in the limit of perfect data and an infinite Ewald sphere, a least-squares fit with a many-centered finite multipole expansion of the charge density about the N nuclei will necessarily satisfy the q-centered multipoles of the molecule for q = 1, 2,…, N. This means that a large number of static-charge physical properties (averages over ρ(r)) are correctly given. Several expressions for averages of ρ(r) or over FH are given. It is shown that outer moments, such as atomic charges, dipole moments and quadrupole moments, always depend on a shape function. On the other hand, inner moments such as potentials, electric forces, and electric field gradients, may be represented by direct Fourier analysis of FH (obs) (suitably phased, of course). Nuclear vibrations have been neglected throughout the discussion.

XVII. Spatial Partitioning of Charge Density

Abstract: Quantitative indices of a molecular charge distribution are provided by the values of volume integrals of the deformation density times a set of appropriate sampling functions. These integrals may, for example, represent the net charges in particular atomic or bond regions. A sharing function derived from the promolecule density defines a general partitioning of any molecule into bonded-atom fragments, whose net charges and multipole moments determine the electrostatic properties of the molecule. Expressions are available for estimating the experimental accuracy of such indices but they are not easy to evaluate nor very reliable.

Diterpenoid Irritants and Cocarcinogens in Euphorbiaceae and Thymelaeaceae: Structural Relationships in View of their Biogenesis

Abstract: Diterpene esters exhibiting toxic, irritant, cocarcinogenic as well as antileukemic activities have been isolated from many Euphorbiaceae and Thymelaeaceae species. They contain the structurally closely related polycyclic skeletons of tigliane, daphnane and ingenane. They are being used in all facets of cancer research as both tumor promoters and inhibitors. They are also considered as second-order carcinogenic risk factors in the etiology of human cancer. The authors review possible biosynthetic pathways for the plant diterpenes and discuss structural relationships in view of their biogenesis.

Appendix 2. A Deformation Density Refinement Program

Abstract: A computer program is described that permits the refinement of the static charge deformation in a flexible atom-centered multipole expansion, together with positional and vibrational parameters, against X-ray and neutron data. Other options allow for rigid-body constraints on some or all of the atomic vibration tensors and the imposition of local symmetry on a set of atoms such as a methyl group.

Biogenetic Aspects of Sesquiterpene Lactone Chemistry

Abstract: Current notions concerning the biogenesis of this large group of naturally occurring substances are discussed with emphasis on stereochemical aspects.

IV. Symmetry and its Implications

Abstract: Significance of symmetry in representing real- and reciprocal-space properties of a crystal and of its constituents is discussed in terms of symmetry eigenfunctions. The formation of the site-symmetrized multipole expansion for the 32 crystal point symmetries is shown, and the mutual relations between the real- and reciprocal-space expansions are discussed. The origin of the symmetry-based phase relations, equivalences and extinction rules of structure amplitudes is pointed out. Distinction is made between the accurate and approximate ones arising from the true space-group symmetry and from the approximate free-particle symmetry of the constituents, respectively. Finally, the symmetry imposed by reality of charge density and its violation through anomalous dispersion are discussed in terms of symmetric and antisymmetric contributions of atomic charge densities.

XVI. Representations of the Electron Density and its Topographical Features

Abstract: The various ways of presenting the topographical features of electron densities and difference densities are discussed and illustrated with graphical examples A systematic terminology is introduced

21,20‐Anhydromelianone and Melianone from Simarouba amara (Simaroubaceae); Carbon‐13 NMR Spectral Analysis of Δ7‐Tirucallol‐Type Triterpenes

Abstract: Further examination of the hexane extract of the root bark of Simarouba amara (Simaroubaceae) has led to the isolation of two side chain oxygenated Δ7-tirucallol-type triterpenes in addition to the already reported compounds 4 and 5. One has been found to be identical with the known melianone 8 and the second is a new compound and has been shown to be 21,20-anhydromelianone 11. The 13C-NMR spectra of nine Δ7-tirucallol derivatives have been analysed and the data obtained have been applied to the confirmation of the structure of the new substance 11.

I. Basic Concepts of Quantum Chemistry for Electron Density Studies

Abstract: A systematic treatment of those concepts of quantum chemistry relevant to electron-density studies is given. The discussion is at an elementary level and encompasses not only the familiar concepts but more modern developments as well.

The Influence of Weak Magnetic Fields on Radiationless Processes

Abstract: Magnetic effects on the fluorescence from singlet excited states represent an interesting new method of studying the behaviour of excited states. Here the intersystem crossing in a weak magnetic field in both protonated and deuterated glyoxal was studied. Fluorescence quenching was observed at various pressures with different collision partners as a function of the magnetic field. Time resolved emission and excitation spectra were observed and quenching rate constants were determined with and without magnetic field. The collision free fluorescence lifetime of glyoxal was found to be unaffected by the magnetic field. The magnetic fluorescence quenching effect is only observed in the presence of colliders and is seen as an enhancement of intersystem crossing. In protonated glyoxal all phosphorescence is collision dependent, whereas in the deuterated species there exists zero pressure phosphorescence. The magnetic effect is here interpreted as being due to the interaction of the singlet with the triplet state, which is magnetically sensitive. In particular additional intersystem crossing mechanisms are postulated to be operative in the presence of a magnetic field. A grouptheoretical method predicts which new frequencies must be involved in this new process, and this activity is experimentally confirmed in the emission spectrum.

Techniques for Signal Assignment in 13C-NMR Spectroscopy of Naturally Occurring Aromatic Compounds

Abstract: Techniques of signal assignment of 13C-NMR spectra of organic compounds, and in particular those of natural origin, are reviewed. They include symmetry, chemical shift considerations, spin-spin coupling (both one-bond and long-range), spin relaxation (T1, NOE), isotopic enrichment and complexing agents. Special emphasis is given to the effects of oxygenated substituents on aromatic rings.

XV. Assessment of Accuracy

Abstract: The uncertainty of X-ray structure factors is discussed. Formulae are given for the estimation of variances and covariances in experimental electronic charge and deformation densities, and in related quantities, e.g. atomic charges.

Cembranolids from Marine Origin (Red Sea), Survey, and Isolation of a New Sinulariolide Derivative

Abstract: The hitherto isolated and identified cembrane derivatives from alcyonarians of the Gulf of Elat (Red Sea) are briefly reviewed, including the thus far unreported isolation of nephtenol and 16-deoxysarcophine from several soft-corals. The structure of a new sinulariolide derivative, 11-episinulariolide acetate, isolated together with 11-dehydrosinulariolide, from two (out of seven tested) sinularia species is illustrated.

Exciplex Formation between Indole Derivatives and Polar Solutes

Abstract: The interaction between 3-methylindole (3-MeI) in cyclohexane with the polar molecules ethyl acetate (EA) and dimethylacetamide (DMA) was studied. The kinetic analysis of the fluorescence data shows that 3-MeI forms an excited state complex with EA in a diffusion controlled process. The temperature dependence of the exciplex formation indicates that its dissociation rate constant too is of appreciable magnitude. UV, IR and fluorescence measurements all indicate that DMA forms complexes with 3-MeI both in the ground and in the excited states. The latter complex is rather strong i.e. its dissociation rate is negligibly small. The findings imply that the fluorescence properties of tryptophan residues within a protein often must be determined by their interaction with neighboring polar groups rather than by the degree to which they are exposed to the ambient solution.

Chemical Analysis of Hybrids of Withania somnifera L. (Dun.). 1. Chemotypes III (Israel) by Indian I (Delhi)

Abstract: Reciprocal crossbreeding between Withania somnifera chemotype III (Israel) and chemotype Indian I (Delhi) were performed. The F1 offspring of this cross was characterized by significant changes in the main steroidal lactone constituents of the parent plants. From the withaferin A (9) and the withanone (2) both present as major constituents in the parent Indian I, the former had disappeared in the hybrid and was replaced by withanolide D (10), whereas the latter was still present, though in about one third of the original quantity. It was accompanied by 17-deoxy-20-hydroxy withanone (2) and by withanolide T (1); the enzymatic system leading to hydroxylation at C20 (20αF-OH) has a dominant character. Two new withanolides have been isolated from this F1 offspring, namely withanolide T (1) (20αF-hydroxy withanone) and withanolide U (5) (4β-hydroxy-withanolide G). The structures assigned to these two new compounds are based on spectral evidence (NMR, IR and UV) and analysis of their fragmentation under electron impact.

Photoformation of “Z” (cis) Isomers in Diaryl‐ and Triaryl‐ Azomethines, Part II.1 Electronic and NMR Spectra of Methyl Derivatives of Benzylideneaniline (C6H5—CH=N—C6H5)

Abstract: UV irradiation of benzylideneaniline, A, and its methyl derivatives B–E in aliphatic hydrocarbon solvents or CH2Cl2 results in extensive conversion into the corresponding “Z” (cis) isomers. The process is reversible both photochemically and thermally, and no pronounced side reactions were observed. At temperatures below about — 70° the Z modifications are thermally stable and their electronic and NMR spectra were measured by static methods. The extent of photoisomerization was estimated by means of the NMR spectra. The changes in electronic and NMR spectra resulting from the E Z conversion were compared with those observed in the corresponding derivatives of stilbene and azobenzene. Photosensitized Z E at below — 70° was observed with biacetyl, but not with dye sensitizers having triplet energies below about 50 kcal · mol−1. Quantum yields for the direct photoisomerization in both directions were calculated, and are in the range of 0.1—0.6. As distinct from stilbenes and azobenzene derivatives, no appreciable temperature dependence of these yields was found, in the range — 70° to – 180°.

A New Flavolignan of Biogenetic Interest from Aegilops ovata L.—Part I†

Abstract: Two compounds, tricin and a related new flavolignan have been isolated from Aegilops ovata L. dispersal units. Evidence leading to assignment of structure of this new compound is presented together with 1H and 13C-NMR chemical shifts of related model compounds. These phenolics found together in the same plant species with a previously discovered lignan possessing activity as a germination inhibitor, raise the possibility of a new biogenetic link.

VIII. What Systems Can Be Studied, Have Been Studied, and Should Be Studied?

Abstract: The status of charge density studies is reviewed with special emphasis on factors limiting its potential, on a priori estimates of position-dependent standard deviations and on present and possible future applications of the method.

The Flavonoids: Indispensable Additions to a Recent Coverage

Abstract: While the flavonoids are biosynthetically and constitutionally a homogeneous group of compounds, the neoflavonoids, as presently defined, may be biosynthetically and are constitutionally a heterogeneous group. Biogenetic considerations which lead to these conclusions and reviews on occurrence, structural elucidation and synthesis are presented for 1,3-diarylpropanes and flavans (flavonoids), as well as for cinnamylphenols and 2-aryl-3-methylbenzofurans (neoflavonoids), with the purpose of providing addenda to the 1975 monograph, The Flavonoids, edited by Harborne, Mabry and Mabry for Chapman and Hall, London.

VI. Charge Deformation Models

Abstract: Basic construction principles of charge-density models based on independent deformed atoms are presented. The asymptotic validity of the rigid free-atom model in reciprocal space; mathematical properties of possible radial bases of multipole expansions; requirements put by symmetry, by regularity, by approximate free-atom nature, by approximate harmonicity of motion and by N-representability on the deformation models; and the connection between orthogonality and independence of model parameters are discussed together with consequent conclusions about the analysis of diffraction data in terms of deformation models.

II. Diffraction Physics

Abstract: Starting from the Schrodinger equation, the basic theory of scattering of X-rays by electrons is given. The kinematic scattering, based on the first Born approximation, is treated in detail. The theory of phenomena, corresponding to a breakdown of this approximation, such as multiple and anomalous scattering, is given in outline. Kinematic scattering by a finite crystal is treated by using the convolution method. The consequences of finiteness of the crystal and of a series in a Fourier synthesis of the electron density are indicated.

The Varied and Fascinating Chemistry of Marine Mollusks

Abstract: With illustrations from a single phylum of marine invertebrates, the mollusks, this review presents a cross section of recent marine natural product research. The origins of this research, biological, biomedical, chemical, or serendipitous, are presented as well as the results.

XII. The Combination of Aspherical-Atom Least-Squares Refinements with Fourier Methods in the Study of Electron Distributions

Abstract: Since electron density fitting in real and reciprocal space are equivalent, the convergence of aspherical-atom least-squares refinements may be judged from Fourier density maps. Residual densities, least-squares model densities and X–N or X–X difference densities are defined and used in the analysis. A further criterion is provided by comparison of atomic parameters from multipolar refinements with neutron or X-ray high-order results. Derived molecular properties such as dipole moments can be obtained from the least-squares results and compared with quantities from direct-space integration. Expressions for both methods of integration are presented.

Electronic Overlap Population as Reactivity Measure. VII. Substituent Effects on the Photocyclization of Stilbenes

Abstract: Excited state electronic overlap populations for atom pairs forming the new bond were found to correlated satisfactorily with the photocyclization reactivity of a series of para and meta substituted stilbenes.

III. Total X-ray Scattering and Two-Electron Density Functions

Abstract: Within the Waller–Hartree theory it is possible to map out the interelectron density function (the diagonal elements of the intracule matrix) from an analysis of total X-ray scattering intensities. Theoretical studies of He and H2 show that electron correlation effects substantially influence total X-ray scattering intensities, but do not appreciably alter the elastic X-ray scattering intensity. On the other hand, theoretical studies of Be(1S), Ne(1S) and CO(1Σ*) indicate that electron correlation influences both total and elastic X-ray scattering intensities by comparable amounts (∼ 1–2%), but at differing regions of the scattering vector variable.

Application of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) to Spin-Forbidden Processes

Abstract: Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) is extended to deal with thermal reactions in which spin multiplicity changes. Only minor revisions are required in the theoretical foundation previously established for OCAMS. For reactions in which triplets may be interconverted with singlets, the use of geminals rather than one-electron wave functions allows us to provide a symmetry criterion for determining whether spin-orbit coupling can induce the interconversion and—if so—which component of the triplet state would participate in it. A number of examples illustrate the analysis, including: intersystem crossing between 1A1 and 3B1 CH2; the direct production of 3CH2 by thermolysis of singlet diazomethane; and the stereospecific trapping of 3CH2 by ethylene. We also analyze the intriguing reactions of Dewar-benzene and 1,2-dioxetanes, described as “photochemistry without light”, and clarify the complex manifold of thermolysis reactions of methylenepyrazoline.

VII. Vibrational Averaging of X‐Ray Scattering Intensities

Abstract: The adiabatic potential for vibrational wavefunctions is given in a Preface. The vibrational average of ℐXRel(K;Q) is studied by taking a canonical-ensemble average of the states of motion for the nuclei. A simple example for an oriented diatomic molecule (one vibration) is used to illustrate the sin θ/Λ dependence of ℐBragg and ℐTDS. The averaged X-ray intensity, ℐav, is then fully developed (as done by Born) within the harmonic-oscillator and rigid-pseudoatom approximations for molecules and a crystal lattice. It is shown that generalized X-ray scattering factors, characteristic of static charge properties, cannot in general be extracted from FOBSBragg. With the rigid-pseudoatom model, relationships betweeen the dynamical matrix and electric-field gradients and charge densities are given. In this way self-consistency between Born's scattering matrix and pseudoatoms can be imposed on charge-density models.

Azulene Revisited: Picosecond Decay of the S1 State in the Gas Phase and in Solution

Abstract: In this paper we report the results of an experimental study of picosecond time-resolved relaxation dynamics of the S1 state of the azulene molecule in solution and in the low pressure gas phase, utilizing consecutive two-photon excitation S0 S1(v) S2 (or S3), with the exciting and the monitoring picosecond pulses chosen at the same wavelength. The nonradiative electronic relaxation times for azulene in solution were τnr = 4 ± 3 ps for S(v = 5), excited at 530 nm, and τnr = 3 ± 2 ps for S(v = 2), excited at 625 nm, while for the “isolated” molecule case τnr = 4 ± 3 ps for S(v = 2), excited at 625 nm. The electronic relaxation rate of the S1 manifold exhibits only a weak dependence on the excess vibrational energy Ev up to Ev = 4800 cm−1. The electronic relaxation rate is practically unaffected by solvent perturbations, as appropriate for the statistical limit of intramolecular radiationless transitions.

Polarized Absorption and Emission Spectra of 4,4′-Dimethylstilbene

Abstract: The polarized absorption and emission spectra of 4,4′-dimethylstilbene in frozen isotropic solutions and in an oriented matrix are described. A mathematical treatment for interpretation of the experimental data is given. The polarizations of the different electronic transitions were compared with theoretical predictions.