Showing papers on "Steric effects published in 1988"

A revisit to the three-dimensional structure of B-type starch

Abstract: A new three-dimensional structure of B-starch is proposed in which the unit cell contains 12 glucose residues located in two left-handed, parallel-stranded double helices packed in a parallel register; 36 water molecules are located between these helices. Chains are crystallized in the hexagonal space group P61, with lattice parameters a = b = 1.85 nm, c = 1.04 nm. The space group symmetry was derived from an exhaustive analysis of the large body of structural studies published so far. Diffraction data used in this work were taken from the previously reported x-ray fiber diffractogram [H.C. Wu and A. Sarko (1978), Carbohydrate Research, 61, 7–25] after adequate reindexing. The final R factor is 0.145 for the three-dimensional data. The repeating unit consists of a maltose molecule where the glucose residues have the 4C1 pyranose conformation and are α(1 → 4) linked. The conformation of the glycosidic linkage is characterized by torsion angles (Φ, Ψ) that take the values (83.8°, −144.6°) and (84.3°, −144.1°), whereas the valence angles at the glycosidic bridge have a magnitude of 115.8° and 116.5°, respectively. The primary hydroxyl groups exist in a gauche–gauche conformation. There is no intramolecular hydrogen bond. Within the double helix, interstrand stabilization is achieved without any steric conflict and through the occurrence of O(2)…O(6) type of hydrogen bonds. The model presented here, with an hydration around 27% w/w, corresponds to a well-ordered crystalline sample, since all the water molecules could be located with no apparent sign of a disorder. Half of the water molecules are tightly bound to the double helices; the remainder forms a complex network centered around the sixfold screw axis of the unit cell. The consistency of the present structural model, with both physicochemical and biochemical aspects of the crystalline component of tuber starch granules, is analyzed.

“Proton Sponges” and the Geometry of Hydrogen Bonds: Aromatic Nitrogen Bases with Exceptional Basicities

Abstract: Certain aromatic diamines (the “proton sponges”) are found to have exceptionally high basicity constants: this is due to spatial interaction of the basic centers, which are in close proximity. The two factors which are most important in causing this effect are, on the one hand, the extreme steric strain in these systems and the destabilizing effect of the overlap of the nitrogen lone pairs of the neutral diamines and, on the other, the strong NċHċN hydrogen bonds which are formed on monoprotonation and which lead to a considerable relaxation of the steric strain. By the systematic variation of the structures of such aromatic diamines we have been able to study these effects as a function of steric factors, in particular of the geometry and the bond length of the NċHċN hydrogen bonds, by means of X-ray structural analysis. The hydrophobic shielding of the basic centers and the NċHċN hydrogen bonds, which was characteristic of the “proton sponge” compounds studied previously, is indeed responsible for the extremely low rate of protonation and deprotonation of these compounds; however, it apparently has no influence on their high thermodynamic basicity. The recent synthesis and basicity determination of a new type of “proton sponge” with no hydrophobic shielding whatever show that not only very strong but also kinetically active bases are accessible using the “proton sponge” concept. Their unusual properties, which are discussed here as the result of steric interactions between two basic centers, provide examples of the fact that cooperative steric interactions of reactive structural elements can lead to properties which cannot be derived from an isolated consideration of the various functional groups. Such “proximity effects” are certainly of general importance in chemistry and biochemistry; the study of their structure-function relationships is worthy of closer consideration.

Observation of steric effects in gas–surface scattering

Abstract: Molecules adsorbed on a surface are known to show preferential orientations, and in particular, the interaction potential between a linear molecule and a surface depends on the orientation of the molecular axis. But the fact that the molecules eventually adsorbed are orientated with respect to the surface is not evidence that the dynamics of gas–surface interactions is governed by the initial molecular orientation in the gas phase. For example at very low velocities the molecule might achieve its optimum orientation adiabatically during its approach to the surface. Dependence of scattering dynamics on molecular orientation can also be degraded as a result of surface structure and surface vibrations. There have been, experimental studies, however, which suggest that orientational or steric effects could influence gas–surface scattering1–4. Thus, the much larger rotational excitation for NO- than for N2-scattering from Ag(lll) indicates that large anisotropies can occur in a potential for gas–surface dynamics (G. O. Sitz et al. manuscript in preparation). The double rotational rainbow observed in the rotational state distribution for NO/Ag(111) (refs 3 and 4) can also be interpreted as a manifestation of this anisotropy5–9. These studies all indicate that steric effects could be important and here we report on scattering of orientated NO from Ag(111) to provide direct experimental evidence for the importance of such effects in gas–surface interactions.

Epoxidation by dimethyldioxirane. Electronic and steric effects

Abstract: Etude de l'epoxydation d'une serie de styrenes substitues et d'alcenes di- et monosubstitues par le dimethyldioxiranne dans l'acetone. La presence d'eau accroit la vitesse de l'epoxydation

The Chelate, Cryptate and Macrocyclic Effects

Abstract: The chelate, macrocyclic, and cryptate effects are analyzed. It is concluded that for all three effects considerable stabilization is derived from the greater basicity induced in donor atoms as ethylene bridges are added. Further considerations of importance in these effects are (1) desolvation effects, where steric constraints to solvation of the donor atoms in the free ligand lead to increased complex stability, (2) enforced dipole-dipole repulsion in the ligand, which is relieved on complex formation, and (3) structural preorganization of the ligand such that the donor atoms in the free ligand are already correctly oriented for complex formation. Only for the chelate effect is entropy of paramount importance, where it is derived from a cratic effect. It is emphasized that the level of preorganization of macrocycles, and to a lesser extent cryptands, is much lower than commonly realized. Newly emerging types of more highly preorganized ligands are discussed.

Kinetics and mechanism of the insertion of olefins into niobium- and tantalum-hydride bonds: a study of the competition between steric and electronic effects

Abstract: The mechanism of insertion of olefins into transition metal-hydride bonds has been examined for a series of olefin hydride complexes of niobocene, permethylniobocene, tantalocene, and permethyltantalocene. The insertion rates were measured for several meta-substituted styrene complexes of permethylniobocene and found to be comparable to those found for the para-substituted analogues, indicating that the electronic effects are largely inductive in origin. An X-ray crystal structure determination for the parent styrene complex (P/sub bca/; a = 14.626 (3) A, b = 18.134 (4) A, c = 18.147 (4) A; ..cap alpha.., ..beta.., ..gamma.. = 90/sup 0/) offers an explanation for this observation: the phenyl ring is bent out of resonance with the olefin ..pi.. system by 32/sup 0/. A comparison of the rates for the entire series of ethylene compounds (where steric effects are minimal) indicates that there are significant electronic effects acting within this family (k/sub Nb/ > k/sub Cp/; Cp = (eta/sup 5/-C/sub 5/H/sub 5/), Cp* = (eta/sup 5/-C/sub 5/Me/sub 5/)). In contrast to the pentamethylcyclopentadienyl systems for which steric interactions appear to dominate, the normal cyclopentadienyl compounds are virtually insensitive to sterics, allowing for the existence of two isomeric forms of the ..cap alpha..-olefin hydride complexes (endo, olefin substituentmore » central, and exo, olefin substituent lateral in the equatorial plane) in roughly 1:1 proportion.« less

Biomimetic oxidation with molecular oxygen. Selective carbon-carbon bond cleavage of 1,2-diols by molecular oxygen and dihydropyridine in the presence of iron-porphyrin catalysts

Abstract: The selective carbon-carbon bond cleavage of 1,2-diols in the presence of an iron-porphyrin complex, molecular oxygen, and 1-benzyl-3-carbamoyl-1,4-dihydropyridine is reported. The C-C bonds of aryl-substituted ethane-1,2-diols were cleaved exclusively to aldehydes or ketones as the oxidation products at room temperature. The reaction rates were influenced by the steric hindrance of the substituents both in the catalysts and diols, but no differences in the reactivities were observed between the two stereo isomers (meso and dl) of diols. A kinetic analysis of this bond cleavage reaction is consistent with the reaction mechanism consisting of the initial binding of diol on the active catalyst forming an intermediate complex and its subsequent breakdown in the rate-determining step of the catalytic cycle. The initial binding step is favorable for electron-deficient diols and is influenced by steric hindrance, whereas the rate-determining bond cleavage step is accelerated by electron-rich diols and unaffected by the steric effect. The mechanism of this diol cleavage reaction is discussed on the basis of these observations.

Experimental observation of steric transition phenomena in sedimentation field-flow fractionation

Abstract: The steric transition region of field-flow (FFF) is described as that part of a fractogram, found at very high retention volumes, in which normal FFF undergoes a transition to steric FFF by virtue of increasing particle diameter. The steric transition region is treated theoretically by assuming first that the steric factor ..gamma.. is constant and second that it is related by a simple power law to particle diameter. The authors then report the first experiments in which fractograms display the characteristic signature predicted by theory for the steric transition: a narrow terminal peak followed by a rapid dropoff to base line. It is shown that the steric transition point, which coincides with the dropoff, is displaced to higher retention volumes with increasing field strength, approximately as expected. The expected steric transition phenomena are further confirmed by collecting narrow fractions of a polydisperse poly(vinyl chloride) sample near the steric transition point and subjecting them to electron microscopy. The particle size distribution of the fractions is found to be sharply bimodal, in accordance with steric transition theory. However, satisfactory agreement between the measured particle diameters and the theoretical expressions is found only by application of the more complicated equations in which ..gamma..more » is assumed to be size dependent.« less

Steric contributions to the thermodynamics of electron transfer in cobalt(III) hexaamine complexes

Abstract: Molecular mechanics modeling of the Co(III) and Co(II) oxidation states for a number of complexes with six saturated amine donor atoms shows that differences in the change in strain energy on reduction of Co(III) result from the different abilities of ligands to accommodate the change in bond lengths that occurs concomitant with reduction

Organotin biocides. Part 11. Triphenyltin benzoates: electronic versus steric control of structure

Abstract: Twenty four triphenyltin derivatives of substituted benzoic acids have been synthesised. On the basis of i.r., 119Sn n.m.r., and Mossbauer spectroscopic data, structures have been assigned to these compounds as either tetrahedral monomers or trans-SnR3O2 polymers. The choice of structure in these systems is dictated by both steric and electronic effects, and for two compounds (2,6-dichloro- and 2-nitro-benzoates) evidence suggests that both structures occur. The factors which influence the preferred structure are discussed.

Molecular mechanics parameters for organosilicon compounds calculated from ab initio computations

Abstract: Molecular structures of 26 organosilicon compounds have been optimized using ab initio calculations at the 3-21G* (modified) level. From these optimized structures, the internal coordinates have been deformed and the variation of the total molecular energy has been studied. Parameters for stretching and bending deformations are reported herein. The bending potential for the SiOSi bond which has an unusual flexibility is also included. Nonbonding interactions are described in terms of steric and electrostatic potentials. For systems which do not include bond resonance effects, torsional behavior is well described by steric potentials with van der Waals radii 20% larger than the previous values and simple electrostatic potential (monopole-monopole) with net atomic charges obtained from ab initio or Extended Huckel Theory calculations. The nonbonding potentials, as defined in this study, have an advantage in that they allow for the computation of torsional barriers without torsional potentials, in the case of single bonds where no additional electronic effects interfere. As an example, it is shown that no torsional potentials are necessary to estimate the torsional barriers in the case of ethane. The newly defined potentials are used to study the torsional barrier in hexamethyldisiloxane and the conformation of octamethylcyclotetrasiloxane (D4). The most stable calculated conformation of D4, coincides with the experimentally determined structure. This study shows that the most stable conformation is determined by the steric repulsion of methyl groups.

Steric effects in molecular adsorption

Abstract: The results of a molecular beam surface scattering experiment with preferentially orientated NO on Ag[111] are given. In adsorption molecules oriented with the O end towards the surface have a larger trapping probability.

The Quantitative Analysis of Steric Effects in Heteroaromatics

Abstract: Publisher Summary This chapter describes the quantitative analysis of steric effects in heteroaromatics. This chapter illustrates that, important progress in the quantitative analysis of steric effects in heteroaromatics has been made recently by linear free energy relationships (LFER), geometrical, and theoretical methods. Heterocycles have been used to propose and exemplify new concepts and new parameters in physical organic chemistry. This chapter is concerned with the importance of steric effects in rationalizing the reactivity and dynamic stereochemistry of heteroaromatics and with recent attempts to bridge the gap between them. There has been a decisive evolution in the treatment of steric effects in heteroaromatic chemistry. A better knowledge of static and dynamic stereochemistry has helped greatly in understanding not only intramolecular but also intermolecular steric effects associated with rates and equilibria. Quantum and molecular mechanics calculations will certainly be used in the future to a greater extent. Heteroaromatics have features in common with aromatic hydrocarbons. However, the variety of geometrical situations, the number of heteroatoms and their associated reactivity, and the ease of monitoring stereochemistry often associated with the presence of heteroatoms all have made, and continues to make, heterocycles exceptional probes for better understanding of reactivity, structures, and the relations between them.

Adducts of trimethylaluminium with phosphine ligands; electronic and steric effects

Abstract: Complexes of the type Me3AlPR2R′ have been synthesised and studied by 1H, 13C, and 31P n.m.r. spectroscopy. The change in 31P chemical shift on co-ordination, Δ(31P), decreases, becoming more negative, with sterically bulkier phosphines and with increasing donor ability. The aluminiummethyl 13C resonance shifts downfield with increasing phosphine steric bulk, and with increased donor ability. Steric effects, however, are found to be the major factor contributing to the changes in both Δ(31P) and methyl 13C chemical shifts.

Stereochemical Behavior of Intermediary Compounds in the Amine-Catalyzed Knoevenagel Reaction

Abstract: Treatment of the active methylene compounds and aldehydes with a catalytic amount of a secondary amine produces thermodynamically stable alkenes, and the intermediary amino compounds are isolated. The reaction involves many reversible steps, and the stereochemistry of products is determined in the elimination step from the intermediary compounds via stable planar carbanions, in which the small difference in the steric requirements of two electron-withdrawing groups is effective. Steric and electronic effects in the intermediates sometimes bring about the carbon–carbon bond fission with recovery of active methylene compounds, and these effects prevent the Knoevenagel reaction from occurring.

Stochastic method for finding molecular conformations

Abstract: This is a stochastic method for determining the likely conformations of a molecule by starting with an initial molecular structure along with given atomic coordinate positions and defined bonds between atoms. Each of the atom's initial X, Y and Z coordinates are modified by the combination therewith of random numbers to create a new random coordinate position for each atom. A predetermined constraint is placed on the distance of each new coordinate from each initial coordinate. The steric energy of the reconfigured molecule is then calculated, stored and a new set of randon numbers combined with the atomic coordinates and the steric energy of the new structure calculated. The process is repeated until most, if not all, steric minima are detected and the structures associated therewith are determined.

Conformational and steric aspects of the inhibition of phenylethanolamine N-methyltransferase by benzylamines.

Abstract: Compounds of the benzylamine (BA) class are potent inhibitors of phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28). Restriction of the aminomethyl side chain through its incorporation into a cyclic framework as in 1,2,3,4-tetrahydroisoquinoline (THIQ) or 2,3,4,5-tetrahydro-1H-2-benzazepine (THBA) results in enhanced potency as an inhibitor, suggesting a conformational effect in the binding of BAs to the active site; however, these ring systems still retain a high degree of flexibility. We have synthesized a series of conformationally defined analogues of benzylamine in order to probe the effect of conformation, as well as the influence of steric bulk, on PNMT inhibition by this class of ligands. In addition, 1-, 3-, and 4-methyl-substituted THIQs were synthesized and evaluated as flexible models for steric bulk tolerance about this ring system. Substitution by a methyl group on either benzylic position of THIQ results in diminished activity as a PNMT inhibitor; however, 3-methyl-THIQ shows enhanced activity as an inhibitor vs THIQ itself. Full conformational restriction of the BA side chain in analogues 4-8 results in a dramatic loss in inhibitor potency. We attribute this effect to a negative steric interaction between the alkyl bridging units above (or below) the heterocyclic ring systems and an active-site amino acid residue. Conformational restriction of THIQ employing a bridging unit that is not located above (or below) the ring system results in only slightly diminished activity compared to THIQ itself. The relative activities of 4-8 were examined in terms of the conformational descriptors tau 1 and tau 2. Although there is no correlation between tau 1 and activity as a PNMT inhibitor, a qualitative relationship between tau 2 (endo or exo) and activity with PNMT is apparent. We believe that the binding of the N-H and/or N-lone pair of electrons may influence the spatial orientation of these molecules at the active site, resulting in positive binding interactions for compounds 4 and 8 and negative interactions for analogues 5-7. The results from the current investigation are compared to those obtained from a similar study involving conformationally defined amphetamines.