Showing papers on "Steric effects published in 1979"

Ethylene complexes. Bonding, rotational barriers, and conformational preferences

Abstract: Rotational barriers and conformational preferences are a primary probe of bonding in olefin complexes. Such bar- riers in ethylene-ML2-s are analyzed in terms of differential interactions between the frontier orbitals of the ML, fragment and the ethylene 7r and x*. It is found that the large barrier to internal rotation about the M-ethylene axis in ethylene-ML2 complexes, favoring the in-plane orientation, is due to loss of overlap between ethylene 7r* and an ML2 b2 orbital-the domi- nant interaction in these compounds. An analogous situation exists for rigid rotation in ethylene-ML4 within the trigonal-bipy- ramidal geometry. A much lower energy pathway for this complex is found if rotation is accompanied by pseudorotation. The barrier in square-planar ethylene-ML3 compounds of the Zeise's salt type, on the other hand, is largely set by steric factors which favor the upright geometry. Various strategies are devised to lower the barrier or reverse the conformational preference in these complexes. This may be accomplished by changing the electronic or steric properties of the ligands on the metal or the ethylene. Finally unsymmetrically substituted olefin complexes are examined. In the ML3 case the metal-carbon bond to the carbon bearing the weaker donor or weaker acceptor should be the stronger or shorter one. In the ML2 and ML4 complexes of ethylene the acceptor effect is accentuated, that of the donor less important.

13C NMR of polycyclic aromatic compounds. A review

Abstract: The analysis of the 13C NMR spectra of polycyclic aromatics is discussed briefly. Basic trends of chemical shifts are mentioned, but the emphasis is placed on substituent-induced chemical shifts (SCS). Semi-empirical approaches and regressional analysis are treated. The factors controlling SCS are discussed and steric, mesomeric and π-inductive effects are analysed. CH, CF, CC, CP and C, Metal coupling constants are investigated and the influence of steric effects, bond order, mesomerism and angle distortions in relation to some of these coupling constants is discussed. Relaxation times are described in a series of compounds. The effects of dissolved oxygen or radicals are shown and the use of T1 as a monitor of molecular tumbling is depicted. The impact of 13C NMR on the understanding of charged aromatic species, both positive and negative ions, is mentioned and new information about reaction intermediates in electrophilic aromatic substitution is outlined. The possibility of using 13C NMR to investigate charge transfer complexes is also discussed. Among other subjects treated are automerization, deuterium exchange and biosynthetic incorporation of labelled materials and, finally, quantitative analysis is briefly touched upon.

The Effects of Terminal Groups in 4–Aminoazobenzene and Disperse Dyes Related Thereto

Abstract: The changes brought about by structural variations on the properties of disperse dyes derived from 4–aminoazobenzene and related compounds are described and discussed, with particular reference to modifications in terminal substituents. Relationships between electronic effects, steric effects, tautomeric equilibria and colour, and between fastness properties and dye structure are examined.

Conformation and Optical Activity of all‐trans, mono‐cis, and di‐cis Carotenoids: Temperature Dependent Circular Dichroism

Abstract: The CD. spectra of carotenoids with an asymmetric centre at C(3) have the following unusual features: (1) All-trans and di-cis compounds with two end-rings, at least one of which possesses an asymmetric C-atom, have very similar CD. spectra, whereas the corresponding mono-cis compounds give mirror-image CD. spectra; (2) In carotenoids or apocarotenoids with only one end-ring all-trans and mono-cis compounds have the same CD. spectra; (3) The CD. spectra of such carotenoids are strongly temperature dependent either increasing in magnitude or completely changing in sign upon cooling. These properties have been rationalized with the aid of a model with takes the total chromophore of the carotenoid as being intrinsically chiral with symmetry C2. It seems that the chirality arises not only from the presence of the hydroxyl group of an asymmetric carbon atom, C(3), which occupies an equatorial position thereby locking the conformation of the end-ring, but also from the steric hindrance across the formal single bond C(6), C(7), linking the end-ring to the chain and thus creating a chiral π-system. (The twist about the C(6), C(7)-bond acquires a handedness because of the predominance of one conformational form of the end-ring. In this way, the double bonds of the end-ring become twisted out of the plane of the chain with one hand predominating. Thus the whole conjugated system becomes chiral). The reversal of sign between the trans (and di-cis) and mono-cis compounds is due to a tilt of the 2-fold symmetry axis and thereby a change of chirality. The temperature dependence stems from the varying population of forms of different twist of the end-group relative to the chain. Compounds with 7, 8-triple bonds also show distinct CD. spectra and a sign change between all-trans and mono-cis isomers in addition to temperature dependence. The latter property demonstrates that some steric hindrance between the end-ring and the main chain is present in these compounds. Some suggestions for the origins of the sign patterns and band intensities of the CD. and absorption spectra are included.

Cis‐trans double bond distribution in poly(1,3‐cyclopentylenevinylene); dependence on metathesis catalyst, cocatalyst, and additives

Abstract: About one hundred samples of poly(1,3-cyclopentylenevinylene) (2) were prepared by ring-opening polymerization of norbornene using a variety of metathesis catalysts based mainly on WCl6 or MoCl5, with EtAlCl2, BuLi, Ph4Sn, and (CH2CHCH2)4Sn as cocatalysts, and with methyl acrylate, diethyl maleate or diethyl fumarate as additives. Ir, Re, V, and Os compounds were also used as catalysts. The double bond pair sequences, trans-trans (tt), trans-cis (tc) etc. were determined from the 13C NMR spectra of these samples and the ratios rt(=tt/tc) and rc(=cc/ct) examined as a function of the fraction of double bonds σc having cis structure. For σc up to 0,35 all polymers showed random distribution of cis and trans structures (rtrc = 1), corresponding to a single type of propagating species. Polymers having σc = 0,35–0,85 invariably gave rtrc ≥ 1, with values greater than 5 in some cases. It is proposed that steric crowding of the active site leads both to high values of σc and to restriction of rotation about the metal-carbene bond of the propagating metalcarbene species, such as to give rise to kinetically distinct conformations which tend to regenerate their own kind on addition of monomer.

Chemistry of singlet oxygen—xxviii*. steric and electronic effects on the reactivity of sulfides with singlet oxygen

Abstract: —The rates of reaction of singlet oxygen with a number of alkyl and aryl sulfides have been determined in CH3OH. The rate for diethylsulfide is 1.71 ± 0.06 × 107M-1 s-1. The addition of methyl groups α to the sulfur causes an approximate tenfold decrease in rate for each symmetrical pair. A similar effect is caused by replacement of alkyl by phenyl groups. The rates of substituted thioanisoles correlate well with σ(ρ= -1.6). but poorly with EP/2 (half-peak oxidation potentials). A mechanism involving nucleophilic reaction of the sulfide with oxygen, rather than charge transfer is suggested.

Propellanes. xlvi. steric and electronic effects as observed in reactions of propellanes

Abstract: The use of different propellane substrates indicates for a number of chemical reactions that either repulsive steric interactions of the reactants or attractive interactions of the secondary orbital type control the configuration of the product.

270‐MHz 1H Nuclear‐Magnetic‐Resonance Study of Met‐enkephalin in Water

Abstract: 1H spectra at 270 MHz of the Met-enkephalin pentapeptide (Tyr-Gly-Gly-Phe-Met) in aqueous solution, as a function of pH and of temperature, are reported. The analysis of the chemical shifts, line widths, coupling constants and rotamer populations around χ1 and χ2, suggests the existence of three conformational regions in the pH range 1–10, due to some rearrangements of the first two amino acids, Tyr-1 and Gly-2. A first conformational transition occurs corresponding to the N-terminus titration, with an exchange rate between conformational states which is not too fast. The second transition occurs concomitantly with the phenolic group titration. The conformational states of the cationic and zwitterionic forms are probably very similar and are characterized by the absence of intramolecular hydrogen bonds and of head-to-tail interactions. A bent structure, with the Tyr-1 ring Facing the Gly-2 could contribute to the equilibrium of the conformational states of Met-enkephalin in the pH range 8–10. This structure could be destabilized as a consequence of the phenolic group deprotonation. The Met-5 and Phe-4 residues appear to have little sensitivity to the conformational transitions of the backbone and both show a settling of the side chains corresponding to the carboxyl group titration, thus showing a reciprocal steric hindrance. The temperature coefficients of the chemical shifts of amide protons at acidic pH are in the range of solvated peptides. In the temperature range 0–75°C, the 3JαNH coupling constants were independent of temperature.

13 C nuclear magnetic resonance spectra. Part 10. Substituent effects on the 13C chemical shifts of adamantanes, diamantanes, and triamantanes

Abstract: The 13C n.m.r. data of a variety of substituted adamantanes, diamantanes, and triamantanes are presented. The α-substituted chemical shifts (SCS) can be described by gauche XC interactions, the magnitudes of which decrease in the sequence BrC > HC > OC ≳ CC. Similarly, gauche interactions of the types HX and CX contribute to the β-SCS, but additionally the β-SCS are increased, if the β-carbon atoms possess antiperplanar γ′-carbon atoms. Steric interaction of the substituent with gauche γ-carbon atoms seem to cause rather downfield rather than upfield shifts of the γ-signals. The hyperconjugative interaction of parallel Cα–Cβ- and Cβ′–Cα′-bond orbitals increase the γanii-SCS of the hydroxy and, to a smaller extent, the bromine substituent.

The crystal structure of the inclusion compound between cycloveratril, benzene and water

Abstract: A monoclinic phase of the 1: 0.5 : 1 inclusion compound between cycloveratril, benzene and water crystallizes in space group C2/c, with a = 33.908 (9), b = 9.629 (3), c = 22.748 (5)/k, fl = 134.02 (1) °, Z = 8. The cycloveratril:benzene ratio was confirmed by gas chromatography. The structure was refined to a final R of 0.057. The host molecules have an umbrella shape with their methyl groups approximately coplanar with the benzene rings and are stacked along b. The guest molecules occupy the cavities of cages formed by methyl groups. The water molecules probably form bifurcated hydrogen bonds with the O atoms of cycloveratril. Goldup, Morrison & Smith, 1965). Thus this compound was recognized as 2,3,7,8,12,13-hexamethoxy5,10-dihydro- 15 H-tribenzo [a,d,g] cyclononene (cycloveratril, CVT), for which steric requirements favour a crown conformation for the cyclononatriene ring (Fig. 1).

Crystal structure of dichlorobis(1-methylcytosine)cadmium(II)

Abstract: Crystals of CdCl2(1-methylcytosine)2 belong to space group Cc, with a = 10.571(7), b = 24.35(2), c = 7.097(3) A, β = 57.33(4)°, and Z = 4. The structure was refined over 1988 independent reflections to an R factor of 0.026. The structure consists of monomeric molecules in which cadmium has a (4 + 2)-coordination. Four strong bonds, two Cd—Cl (2.497, 2.485 A) and two Cd—N(3) (2.281, 2.296 A), define an approximate tetrahedron around the metal. Two carbonyl oxygens take part in weak Cd—O bonding interactions (2.677, 2.780 A). The amino groups form only very weak intramolecular hydrogen bonds with chlorine and the difference >30° between angles Cd—N(3)—C(4) and Cd—N(3)—C(2) is ascribed to Cd—O bonding. Similar effects for other metal complexes are discussed in terms of steric hindrance of the amino group and bonding of the carbonyl group.

Structural studies of steric effects in phosphine complexes. Part 4. Synthesis and crystal and molecular structure of trans-di-iodobis(tri-o-tolylphosphine)platinum(II)–dichloromethane (1/1)

Abstract: The title complex has been synthesized and crystallizes in a 1 : 1 ratio with dichloromethane in the monoclinic space group C2/c with a= 22.100(3), b= 9.961 (3), c= 21.516(3)A, β= 113.19(1)°, and Z= 4. The structure has been solved by Patterson and Fourier methods and refined by full-matrix least-squares techniques to a final R of 0.036 for 3 073 observed reflections measured by diffractometer. The dichloromethane is disordered over three equally populated sites around the crystallographic two-fold axis. The trans-[Ptl2{P(C6H4Me-o)3}2] molecule resides on a crystallographic centre of symmetry with Pt–I and Pt–P distances of 2.622(1) and 2.348(2)A respectively. The failure of this complex to undergo intramolecular metallation can be explained by the steric effects of the P(C6H4Me-o)3 ligand, which are described by cone-angle calculations (θ= 183°) and a ‘ligand profile’.

Steric effects on excimer formation in isotactic and atactic polystyrene and poly(ar‐methylstyrene)s in solution

Abstract: Effects of tacticity and steric hindrance on excimer formation were investigated in isotactic and atactic polystyrene, poly(o-methylstyrene), poly(m-methylstyrene), and poly(p-methylstyrene) in the presence and absence of a quencher (CCl4). The calculated rate constants for excimer formation in the isotactic polymers except for poly(o-methylstyrene) were almost the same and larger than those in the corresponding atactic polymers. These results indicate that excimer formation was due to not only rotational sampling but also energy migration to trapping sites. It was found that steric hindrance on excimer formation was intimately related to the excition diffusion length in the polymer chain.

Dechlorination of Polychlorinated Biphenyls by UV-irradiation. V. Reaction of 2,4,6-Trichlorobiphenyl in Neutral and Alkaline Alcoholic Solution

Abstract: 2,4,6-Trichlorobiphenyl and seven other chlorobiphenyls have been photolyzed in both alkaline and neutral 2-propanol in order to investigate the reactivity and selectivity towards dechlorination. Predominant dechlorination at the 2-position has been confirmed for all PCBs in neutral solution and interpreted in terms of the steric and electronic effects of the phenyl group. In the presence of alkali, however, competitive elimination between the ortho and para chlorine atoms of 2,4-dichlorobiphenyl and 2,4,6-trichlorobiphenyl occurred. The photo-dechlorination of 2,3- and 2,5-dichlorobiphenyls took place however only at the ortho position. The dechlorination ratios (ortho/para) were 7.5 and 0.8 for 2,4-dichlorobiphenyl and 2,4,6-trichlorobiphenyl, respectively. This characteristic of the ortho chlorine atom has been elucidated assuming the steric and electronic effects of the phenyl group in the radical anion. A correlation has been found between the reduction potential and the reactivity of PCB in alkaline...