Showing papers on "Double bond published in 1984"

Chemical Bonding in Higher Main Group Elements

Abstract: Many concepts used for a qualitative description of chemical bonding that originated in the early days of theoretical chemistry have been vindicated recently by quantum chemical calculations, at least as far as first row elements are concerned. However, many concepts that have been justified for first row elements (Li to Ne) cannot—contrary to widespread belief—be generalized to the higher main group elements. This applies particularly to the concept of hybridization, which should be viewed with considerable caution. The essential difference between the atoms of the first and higher rows is that the cores of the former contain only s-AOs, whereas the cores of the latter include at least s- and p-AOs. As a consequence, the s and p valence AOs of first row atoms are localized in roughly the same region of space, while the p valence AOs of higher row atoms are much more extended in space. This has the consequence that for the light main group elements both lone-pair repulsion and isovalent hybridization play a greater role than for the heavy main group elements. Furthermore, this implies that single bonds between first row elements are weak and multiple bonds are strong, whereas for the second or higher row elements single bonds are strong and multiple bonds weak. The “extended valence” (violation of the octet rule) observed in compounds of higher main group elements has very little to do with the availability of d-AOs but is due rather to the size of these atoms and thus to the reduced steric hindrance between ligands and, to a lesser extent, also to the lower electronegativity of the heavy atoms. A model based on the concept of electron-rich multicenter bonds is certainly closer to reality than one involving hybrids with the participation of d-AOs. The XO bonds in phosphane oxides, sulfoxides, oxo acids and related compounds are better formulated as semipolar rather than as true double bonds, even if they behave in some respects like double bonds.—The growing interest of theorists in compounds of higher main group elements parallels new and, in some instances, spectacular results of experimental research on the chemistry of these elements.

Excited‐state torsional dynamics of cis‐stilbene from resonance Raman intensities

Abstract: Resonance Raman spectra of cis‐stilbene have been obtained using excitation at 356, 299, 282, and 266 nm. Intense Raman lines at 403 and 560 cm−1 are observed in the frequency region characteristic of double bond torsions. Resonance spectra of trans‐stilbene show no Raman lines of comparable intensity which can be assigned to double bond torsions. These results suggest that electronically excited cis‐stilbene twists about its central double bond much more rapidly than does the trans isomer. To quantitate these observations, the Raman lines of cis‐stilbene at 165, 403, 560, and 963 cm−1 are assigned to specific out‐of‐plane normal modes by comparison with QCFF/PI calculations. We then model the excited state surface as a linear dissociative potential along the 560 cm−1 double bond torsion and a harmonic potential along all other modes. The experimental Raman intensities are used to determine the slope of the excited state torsional potential and the excited state origin displacements in the harmonic modes....

Substituent Effects on the Carbon-Silicon Double Bond. Monosubstituted Silenes

Abstract: A series of isomeric silenes H2C=SiHR (1) and H2Si=CHR (2) where R = CH3, SiH3, F, OH, OSiH3, CN, and NO2 were studied at the RHF/3-21G and 6-31G* levels. The two basis sets give in general similar results. The calculated r(C=Si) are (3-21G, values (in A) for 1 followed by 2) 1.717 (R = H), 1.716, 1.725 (CH,), 1.724, 1.721 (SiH,), 1.698, 1.730 (F), 1.705, 1.746 (OH), 1.705, 1.749 (OSiH3), 1.711, 1.727 (CN), 1.707, and 1.726 (NO,). The experimental r(C=Si) of 1.764 A in (Me3Si),Si=C(adamantyl)(OSiMe3) is electronically elongated and consistent with our calculations and with a r(C=Si) of 1.70 8, in unperturbed silenes but contrasts with the electron diffraction measurement of 1.83 A. The 2 - 1 energy differences (AE) are determined by the bond energies of Si-R vs. C-R and of Si-H vs. C-H: AE (6-31G*, kcal mol-') 8.9 (R = CH,), -3.6 (SiH3), 53.9 (F), 41.2 (OH), 51.1 (OSiH3), 3.0 (CN), 2.7 (NO,). Substituent effects (SE) on the thermodynamic stability of the C=Si bond are generally small. The calculated energies of the equations 1 + SiH4 - 1, R = H and H3SiR, 2 + CH, -. 2, R = H and H3CR, that also model the SE on the dimerization energies of 1 and 2 are, respectively, (6-31G*, kcal mol-'), R = CH3, 0.1, 0.3, H3Si, 2.9, 6.0, F, -2.0, -7.8, OH, 0.3, 0.4, OSiH3, -0.2, -2.8, CN, -1.4, 3.8, and NO,, -3.4, 2.4. SE on the charge distributions and on the energies and coefficients of the frontier orbitals of 1 and 2 are evaluated and used for analyzing the factors that control their kinetic stability. A "reversed polarity" of the n-bond, i.e., C6+=Si6-, is the most important single electronic factor that reduces the reactivity of silenes; the energies of the a- and a*-orbitals are less significant.

NMR study of the structure of simple molecules adsorbed on metal surfaces: C/sub 2/H/sub 2/ on Pt

Abstract: The authors have used NMR to determine the structure of C/sub 2/H/sub 2/ adsorbed at room temperature on small platinum particles by probing the /sup 13/C-/sup 13/C, /sup 13/C-/sup 1/H, and /sup 1/H-/sup 1/H dipolar interactions among the nuclei in the adsorbed molecules. They find a model of (77 +- 7)% CCH/sub 2/ and (23 +- 7)% HCCH. The C-C bond length of the CCH/sub 2/ is 1.44 +- 0.02 A, midway between a single and double bond, suggesting that both C atoms bond to the surface.

Synthesis and structures of compounds containing double bonds between the heavier Group VA elements: diphosphenes, diarsenes, phosphaarsenes, and phosphastibenes

Abstract: Preparation des composes (Me 3 Si) 3 CP=PC(SiMe 3 ) 3 , [2,4,6-(t-Bu) 3 C 6 H 2 ]P=PC(SiMe 3 ) 3 , [2,4,6-(t-Bu) 3 C 6 H 2 ]P=M=H(SiMe 3 ) 2 avec M=P. As et Sb et [2,4,6-(t-Bu) 3 C 6 H 2 ]As=AsCH(SiMe 3 ) 2 . Etude des structures cristallines de trois d'entre eux. Discussion sur la nature de la double liaison a partir des donnees RX et RMN

Picolinyl derivatives for the structural determination of fatty acids by mass spectrometry: Applications to polyenoic acids, hydroxy acids, di‐acids and related compounds

Abstract: A rapid, mild procedure is described for the synthesis of picolinyl esters of branched-chain, cyclic and unsaturated fatty acids, dicarboxylic acids and hydroxy acids by the reaction of the derived acid chloride with 3-pyridylcarbinol. The mass spectra of these derivatives contained abundant diagnostic ions resulting from radical-induced cleavage of the hydrocarbon chain following random hydrogen abstraction by the nitrogen atom of the pyridine ring. Ions thus represented cleavage, without further rearrangement, at each carbon-carbon bond and enabled methyl groups and double bonds to be located. The spectra of the polyenoic acids were more complex than those of their monoenoic analogues, but major ions whose mass differences could be represented by the series —14, 26, 14, 26, 14— could be used to localize the double bonds. The position of the double bond most remote from the carboxylic ester group could be established by the position of an abundant ion produced by loss of an allylic radical containing the terminal carbon atoms of the chain. Derivatives of the hydroxy acids, although not prepared in quantitative yield, also gave diagnostic spectra. The spectra of the derivatives of the dicarboxylic acids contained abundant molecular ions and again showed diagnostic cleavages of the hydrocarbon chain.

The disilenes: chemistry of silicon-silicon double bonds

Abstract: Three stable disilenes, Mes(R)Si=Si(R)Mes where R = mesityl, tert—butyl and (Me3Si)2N, have been prepared by photolysis of trisilanes and dimerization of the resulting ilylenes. X-ray crystallographic studies show Si=Si distances of "215 pm, about 20 pm shorter than for a Si—Si single bond. The silicon and attached carbon atoms are planar in the tert-butylmesityl compound but anti-pyramidal in tetramesityldisilene. Stereoisomers (cis—trans) have been identified for two of the disilenes and tI1Tr interconversions have been investigated. Disilenes are reactive toward both polar and radical reactions.. With oxygen, disilenes form adducts with unique four—membered ring structures

Polymerization of acetylenes and cyclic olefins induced by metal carbynes

Abstract: : Trans-(bromotetracarbonyl(phenylmethylidyne)tungsten) and related metal-carbynes induce acetylenes and cycloalkenes to polymerize. The acetylenes include examples that are monosubstituted, disubstituted, and unsubstituted, as well as the first reported functionalized acetylenes in which the functional groups (the nitrile, ester, and halogen functions were studied) are not attached to the triple bond. The cycloalkenes yield polyalkenamers whose double bonds are largely cis. This last polymerization is speeded by the presence of oxygen. The stereochemistries of the polyacetylenes and the polynorborneamer produced when the initiator is the metal-carbyne are similar to those of the polymers produced when the initiator is pentacarbonyl(methoxyphenylmethylene) tungsten. A possible mechanism is presented to account for why the metal-carbynes behave as though they were sources of reactive metal-carbenes. (Author)

Method for hydrogenation of polymer

Abstract: Selective hydrogenation of unsaturated double bonds in conjugated diene units of a conjugated diene polymer resulting from polymerization of copolymerization of a conjugated diene is effected by a method which comprises hydrogenating the conjugated diene polymer in an inert solvent in the presence of: (A) at least one bis-(cyclopentadienyl) titanium compound or (B) at least one hydrocarbon lithium compound containing at least one lithium atom in addition to (A) the aforementioned at least one bis-(cyclopentadienyl) titanium compound as a catalyst (providing that the presence of both the two components, (A) and (B), is essential where the conjugated diene polymer under treatment is other than a living polymer containing a lithium atom in the polymer chain thereof).

Oxo-analogs of mevinolin-like antihypercholesterolemic agents

Abstract: Mevinolin-like compounds of the structural formula: are disclosed wherein: R' is, e.g., hydrogen or C1-4alkyl; E is-CH2CH2,-CH=CH-, or-(CH2)3-; and Z is wherein X is-O-or-NR9wherein R9 is hydrogen or C1-3alkyl; R7 is C2-8alkyl; and R6 is hydrogen or-CH3; I wherein R10, R" and R12 are independently, e.g., hydrogen, halogen or C1-4alkyl; wherein n is 0-2 and R14 is halo or Ci-4alkyl; or wherein the dotted lines represent possible double bonds there being 0,1 or 2 double bonds; m represents 1, 2 or 3; and R16 is methyl, hydroxy, C1-4alkoxy, oxo, or halo. Those compounds are potent HMG-CoA reductase inhibitors possessing one less asymmetric center.

Chemistry, including the X-ray structure, of bis[bis(trimethylsilyl)methylgermanium(II)], R2GeGeR2[R = CH(SiMe3)2], a stable compound having a metal–metal ‘double bond,’ a dimetallene

Abstract: Reaction of MgClR(OEt2)[R = CH(SiMe3)2] with GeCl2· dioxane in OEt2 yields the bright yellow crystalline Ge2R4[having a trans-folded C2h Ge2C4 framework with a fold angle of 32°, a germanium environment intermediate between pyramidal and planar (sum of angles at Ge = 348°) and a GeGe bond length of 2.347(2)A(ca. 4% shorter than in elemental germanium)], which behaves chemically as a singlet germylene eR2.

Reactions of alkenes with [hydroxy(tosyloxy)iodo]benzene: stereospecific syn-1,2-ditosyloxylation of the carbon-carbon double bond and other processes

Abstract: Etude de la reaction de ditosyloxylation de butene-2, pentene-2, hexene-3, octene-4, cyclohexene, norbornene, styrene, diphenyl-1,1 ethylene et hexadiene-2,4

Carbene complexes of zirconium. Synthesis, structure, and reactivity with carbon monoxide to afford coordinated ketene

Abstract: Treatment of Cp/sub 2/Zr(L)(CO) (Cp = C/sub 5/H/sub 5/; L = PMe/sub 3/, CO) with Cp*/sub 2/ZrH/sub 2/ (Cp* = C/sub 5/Me/sub 5/) affords zirconium oxycarbene complexes, Cp/sub 2/(L)Zr double bond CHO single bond Zr(X)Cp*/sub 2/ (L = PMe/sub 3/, X = H, I; L = CO, X = H), that represent some of the first examples of group 4 metal-to-carbon multiple bonding. The first X-ray diffraction structure determination of a zirconium carbene complex, that of Cp(PMe/sub 3/)Zr double bond CHO single bond Zr(H)Cp*/sub 2/.C/sub 6/H/sub 6/, is reported (C2/c, a = 27.318 (4) A, b = 19.895 (3) A, c = 19.932 (5) A ..beta.. = 132.188 (10)/sup 0/, Z = 8) and shows a very short Zr-C bond length of 2.117 (7) A. Treatment of Cp/sub 2/(CO)Zr double bond CHO single bond Zr(H)Cp*/sub 2/ with CH/sub 3/ I or Cp/sub 2/(PMe/sub 3/)Zr double bond CHO single bond Zr(I)Cp*/sub 2/ with CO affords the zirconium substituted enediolate zirconacycle, Cp*/sub 2/ZrOCH double bond C(Zr(I)Cp/sub 2/)O, which has been characterized by an X-ray diffraction study (P2/sub 1//c, a = 15.866 (4) A, b = 10.673 (3) A, c = 20.561 (4) A, ..beta.. = 105.5 (2)/sup 0/, Z = 4). Thismore » complex most likely forms by coupling of the zirconoxycarbene and a carbonyl to give a metal-coordinated ketene intermediate that subsequently rearranges to the isolated product. An isotopic crossover experiment has demonstrated that the new carbon-carbon bond is formed in an intramolecular coupling step. The ketene intermediate can be trapped by dissolving Cp/sub 2/(CO)Zr double bond CHO single bond Zr(H)Cp*/sub 2/ in pyridine, giving Cp/sub 2/(py)Zr(O double bond C double bond CHOZr(I)Cp*/sub 2/). Treatment of the isolated ketene complex with CH/sub 3/I in benzene gives the enediolate zirconocycle; in pyridine Cp/sub 2/(py)Zr(O double bond C double bond CHOZr(I)Cp*/sub 2/) can be observed spectroscopically. 38 references, 4 figures, 5 tables.« less

Fourier transform infrared spectroscopic characterization of aromatic bismaleimide resin cure states

Abstract: Interesting polyimide materials, possessing good mechanical and thermal properties, are obtained by homopolymerization or reaction of 4,′4-bis(maleimidodiphenylmethane) with a diamine. Fourier transform infrared spectroscopy has been used to characterize the crosslinking of such materials using maleimide and amine absorption bands. Amine group reaction on double bonds is readily achieved and appears to be insensitive to the temperature of curing. On the other hand, the decrease of maleimide double bonds is strongly dependent on the reaction temperature. The residual amount of double bonds present in the cured material is a function of the temperature: a linear relationship holds between residual double bond concentration vs. curing temperature. The general behavior during crosslinking of this kind of polyimide was related to glass transition temperature changes.

A new double bond metathesis reaction: conversion of an niobium:niobium and an nitrogen:nitrogen bond into two niobium:nitrogen bonds

Abstract: Nb 2 Cl 6 (Me 2 S) 3 reagit avec C 6 H 5 N=NC 6 H 5 pour donner le compose [NbCl 2 (Me 2 S)(NC 6 H 5 )] 2 (μ-Cl) 2 , caracterise par spectrometrie de masse, IR, RMN et diffraction RX. Cristallisation dans le systeme monoclinique, groupe C2/c et affinement de la structure jusqu'a R=4,89%. Coordination octaedrique deformee autour des atomes Nb

Multiply bonded dimetal complexes containing bis(diphenylphosphino)methane bridges: complexes possessing rhenium-rhenium double bonds and a tungsten-tungsten single bond

Abstract: Preparation et caracterisation structurale des complexes Re 2 Cl 6 (dppm) 2 et Re 2 Cl 5 (OR)(dppm) 2 avec R=CH 2 CH 3 . Etude de l'influence sur la liaison M−M du remplacement d'un coordinat Cl par OR en determinant la structure cristalline de W 2 (μ-O-i-Pr) 2 (O-i-Pr) 6 Cl 2 . Les proprietes spectroscopiques et electrochimiques des especes dirhenium tel que Re 2 Cl 6 (dppa) 2 , Re 2 Cl 4 (dppa) 2 et Re 2 Cl 4 (PMePh 2 ) 2 (dppa) montrent une proche similitude avec les analogues dppm

Catalysts and process for olefin conversion

Abstract: A process for disproportionation of olefins is disclosed employing a novel catalyst comprising a silica support treated with an oxide or oxide precursor of tungsten and a Group I oxide or compound convertible to the oxide in admixture with a double bond isomerization catalyst.

Conformationally restricted retinoids.

Abstract: A series of conformationally restricted retinoids was synthesized and screened in two assays used to measure the ability of retinoids to control cell differentiation, namely, the reversal of keratinization in tracheal organ culture from vitamin A deficient hamsters and the inhibition of the induction of mouse epidermal ornithine decarboxylase by a tumor promoter. These compounds had bonds corresponding to selected bonds of the E-tetraene chain of retinoic acid (1) held in a planar cisoid conformation by inclusion in an aromatic ring. The meta-substituted analogue 3 of 4-[(E)-2-methyl-4-(2,6,6-trimethylcyclohexenyl)-1,3-butadienyl+ ++]benzoic acid (2) was far less active than 2 in both assays. In contrast, the vinyl homologue of 2 (4) and the 7,8-dihydro and 7,8-methano analogues (5 and 6) had activity comparable to that of 2. Analogues of 4-[(E)-2-(1,1,4,4-tetramethyl-1,2,3,4-tetrahydro-6-naphthyl)propenyl] benzoic acid (7) were also screened. Replacement of the tetrahydronaphthalene ring of 7 by a benzonorbornenyl group (9) significantly reduced activity, as did removal of the vinylic methyl group from 9 (10). Replacement of the propenyl group of 9 by a cyclopropane ring (12) also reduced activity. Replacement of the tetrahydronaphthalene ring of 7 by 4,4-dimethyl-3,4-dihydro-2H-1-benzopyran and -benzothiopyran rings (13 and 14) also decreased activity. Inclusion of the 7,9 double bond system of 1 in an aromatic ring (15 and 16) reduced activity, whereas inclusion of the 5,7 double bond system in an aromatic ring enhanced activity (7 and 19). Inclusion of the 11,13 and 9,11,13 double bond systems in aromatic rings (2 and 18) also reduced activity below that of 1. Retinoic acid, 7, 13, 14, and 19 inhibited papilloma tumor formation in mice. Toxicity testing indicated that 7 was more toxic than 1, 13, 14, and 19, 19 was more toxic than 1, and 13 and 14 were less toxic than 1.

Fluorine-containing Imines

Abstract: Data on the methods of synthesis, certain physicochemical properties, and reactivities of fluorine-containing imines of known types are surveyed and the characteristic features of the variation of their properties due to the effects of the fluorine atoms and polyfluoro-substituents at the C=N bond are examined. The theoretical and applied aspects of the development of the chemistry of fluorine-containing compounds with a nitrogen–carbon double bond are discussed. The bibliography includes 240 references.

Structure of ascorbic acid and its biological function

Abstract: The four O-H bands of ascorbic acid could be assigned by means of infrared investigations It could be shown by electron spin resonance and nuclear magnetic resonance measurements that the radical sodium ascorbate is formed by a cyclic side-chain structure resulting in a loss of C(6)-OH and C(3)-OH The C(2) = C(3) double bond is still maintained as could be shown by infrared and ultraviolet absorption spectroscopy In the case of complete oxidation of ascorbic acid to dehydroascorbic acid, C(6)-OH is reestablished (indicating the reopening of the furanoid ring), while C(2)-OH as well as the C(2) = C(3) double bond have disappeared due to the deprotonation of C(2)-OH and C(3)-OH In the case of isoascorbic acid and its radical potassium isoascorbate similar results are obtained with one distinct difference: in the case of isoascorbic acid, C(2)-OH does not appear while C(3)-OH exhibits a shoulder