Effects Of Steric Strains Research Articles

X-ray spectroscopic identification of strain and structure-based resonances in a series of saturated carbon-cage molecules: Adamantane, twistane, octahedrane, and cubane

Novel nanocarbons such as fullerenes, nanotubes, graphene, and nanodiamond reside at the cutting edge of nanoscience and technology. Along with chemical functionalization, geometric constraints (such as extreme curvature in nanotubes or defects within or at the surfaces of diamond nanoparticles) significantly alter the electronic states of the nanocarbon material. Understanding the effects of steric strain on the electronic structure is critical to developing nanoelectronic applications based on these materials. This paper presents a fundamental study of how strain affects the electronic structure in a benchmark series of some fundamental saturated carbon cage compounds. Adamantane, C10H16, the smallest diamondoid and arguably the smallest nanodiamond crystallite, has carbon atoms essentially commensurate with diamond lattice positions and possesses by far the least molecular strain of this series. Twistane also is a C10H16 isomer but the fixed cyclohexane twist conformation of the central ring introduces additional strain into the cage. Octahedrane [(CH)12] and cubane [(CH)8] are considerably more strained, culminating in cubane where carbon–carbon bonds lie either parallel or orthogonal to one another. Using gas-phase near-edge x-ray absorption fine structure spectroscopy to probe the unoccupied electronic states, we observe two major progressions across this series. First, a broad C–C σ* resonance in the absorption splits into two more narrow and intense resonances with increasing strain. Second, the first manifold of states previously associated with tertiary C–H σ* in the diamondoid series appears to broaden and shift to lower energy. This feature is more than twice as intense in cubane than in octahedrane, even though these two molecules have only tertiary carbons, with the chemical formula (CH)x. The spectral differences are entirely due to the shape of the molecules; in particular, in cubane, the features arise from a high degree of p-p interaction between parallel C–C bonds. In contrast to the conventional wisdom that near-edge x-ray absorption is primarily an atomically localized spectroscopy, molecular shape and associated strain lead to the dominant features in spectra acquired from this fundamental series of carbon cage structures.

Christoph Rüchardt (1929–2018)

Rüchardt addressed a multitude of questions in physical-organic chemistry, putting strong emphasis on a deep and accumulated understanding of reactivity, stability, mechanisms, and intermediates. His first independent projects were dedicated to radical and sextet rearrangements, before he started extensive research on the formation of radicals, in particular emerging from azo compounds, peroxides, and highly substituted alkanes. The last compound class determined his work over nearly thirty years, leading to the systematic evaluation of the stability of the carbon–carbon bond from different angles, with a particular emphasis on steric effects. The creation of a complete reaction coordinate required not only kinetic data, but also the careful execution of thermochemical measurements in order to quantify polar, steric, anomeric, and captodative effects on radical stability. Accordingly, the arsenal of methods was extended by modern combustion calorimetry and the respective contemporary computational methods, the accuracy of the latter being challenged by experimental heats of combustion and transition-state energies. With the determination of the heat of combustion of C60 , Rüchardt created a quantitative basis for the understanding of σ-strain and stability–reactivity relationships regarding the most prominent fullerene. In addition, his great knowledge of radical mechanisms allowed extension of the synthetic use of noncatalyzed transfer hydrogenation and transfer hydrogenolysis. With his many studies, in particular those on the effect of steric strain on the carbon–carbon bond, conformational analysis of complex hydrocarbons and stability, structure and reactivity of carbon radicals, did he not only enrich organic chemistry textbooks, but also gave important insights into the refinement of technically useful radical initiators, for cracking procedures based on thermal decomposition, and for coal hydrogenation. The scientific merits of Rüchardt were widely recognized with many awards, an honorary doctorate, and memberships of scientific academies. In addition to his scientific activities, Rüchardt was highly committed to social and educational issues. As Rector of the University of Freiburg (1987–1991), he initiated the foundation of, among others, the Faculty of Engineering. In 1996, he was awarded the Officer's Cross of the Order of Merit of the Federal Republic of Germany. Rüchardt was a great role model for his students, acting with integrity and correctness, and was well-organized in thought and action. He was also archetype, motivator, and guiding spirit for his co-workers through organization, discipline, and unselfish commitment. With his generosity and systematic support for young academics, he set up excellent working conditions for his approximately 90 PhD students and 4 habilitation candidates (Giese, Langhals, Lüning, Schmittel). Almost 300 publications, 94 of them together with his assiduous and congenial companion Dr. Beckhaus, illustrate his work and contribution to science. Rüchardt was married to his wife Käte for over 60 years and they had three children. His family gave him strength and unconditional support for his far-reaching scientific, university, political, and social activities. As a result, his life and work will not only remain unforgotten in the scientific community but also at the University of Freiburg.

N-アリルフロイルアミドの分子内Diels-Alder反応：アミド配座異性ならびに立体ひずみの効果

N-アリルフロイルアミドの分子内Diels-Alder反応：アミド配座異性ならびに立体ひずみの効果

Intramolecular Diels–Alder reaction of N-allyl 2-furoyl amides: effect of steric strain and amide rotational isomerism

Intramolecular Diels–Alder reactions of various N-allyl 2-furoyl amides with different substituents on the nitrogen atom were investigated. The reaction of amides having bulky substituents proceeded at a faster rate than the analogs whose substituents were of less bulkiness. From the systematic experimental survey of the substituent effects and the energetic evaluation based on the DFT calculations at the B3LYP/6-31G(d) level, the enhanced reactivity was ascribed to the relief of steric strain upon cyclization rather than the amide rotational isomerism governed by the bulky substituents.

Structure and coordination properties of sterically strained meso-alkyl-substituted Zn porphyrin

The formation of molecular complexes of zinc 5,15-dipentyl-2,3,7,8,12,13,17,18-octamethylporphin (ZnP) with nitrogen bases (L = 2-methylimidazole (2-Me-Im), imidazole (Im), pyridine (Py), 3,5-dimethylpyrazole (DMP), and dimethylformanmide (DMF)) was studied by spectrophotometric titration and quantum-chemical calculations. The stability constant was found to increase linearly with the basicity of the extra ligand and to be proportional to the shift of the fundamental absorption bands in the UV/Vis spectra. The structure of the compound formed as a result of the intermolecular interaction of the macrocyclic complex with a base was determined. The structures of zinc porphyrin and its extra complexes were optimized by the PM3 quantum chemical technique. The geometric and energetic characteristics of the compounds were obtained. A correlation between the calculated interaction energy of the central metal atom and the nitrogen atom of the extra ligand, on the one hand, and the stability of Zn porphyrin extra complexes, on the other hand, was established. The dependence of the zinc-extra ligand bond strength on the basicity of the additional molecular ligand was established. An effect of steric strain on the coordination properties of the metal porphyrin was noted.

Molecular structure of mono- and 1,2-aminoderivatives of cyclohexane: Steric strain effects as determining factors

The conformational study of cyclohexylamine and 1,2-cyclohexylamine derivatives was performed. The potential energy surface of every compound was calculated at B3LYP/6-31G(d) and the conformational energy minima were further optimized at B3LYP/aug-cc-pVDZ level of theory. The geometrical parameters and the electronic energy of each conformer were determined. The internal energy, enthalpy, entropy, Gibbs energy and the relative weight of each conformer in the structure of the respective isomer were calculated at 298.15 K. The steric hindrances in the crowded molecules resulting from the replacement of hydrogens of cyclohexane by amine groups are pointed out by short non-bonded H ↔ H distances and by the consequent bond and dihedral angles distortions relatively to the cyclohexane structure.

Production of Highly Homogeneous Si(100) Surfaces by H2O Etching: Surface Morphology and the Role of Strain

The etching of Si(100) surfaces in ultrapure water was studied with a combination of infrared spectroscopy (FTIR) and scanning tunneling microscopy (STM). While the FTIR results show that the initially rough H/Si(100) surface becomes highly homogeneous during etching, a phenomenon generally associated with surface smoothing, STM images reveal that the homogeneity is associated with the formation of well-defined etch hillocks. After many hours of etching, the resulting H-terminated surface is composed of stripes of atomically flat Si(100) terminated by etch hillocks bounded by {111}- and {110}-oriented microfacets. Polarization analysis of the Si-H stretching modes provides strong evidence for uniform dihydride-termination of the flat regions, with the narrow (approximately 25 A) width of these stripes allowing for relaxation of steric strain between neighboring dihydrides. The unusual hill-and-valley etch morphology is attributed to the effects of steric strain on the reactivity of sites on the etched surface.

Origin and Consequences of Steric Strain in the Rhodopsin Binding Pocket

To study the origin and the effects of steric strain on the chromophore conformation in rhodopsin, we have performed quantum-mechanical calculations on the wild-type retinal chromophore and four retinal derivatives, 13-demethyl-, 10-methyl-13-demethyl-, 10-methyl-, and 9-demethylretinal. For the dynamics of the whole protein, a combined quantum mechanics/molecular mechanics method (DFTB/CHARMM) was used and for the calculation of excited-state properties the nonempirical CASSCF/CASPT2 method. After relaxation inside the protein, all chromophores show significant nonplanar distortions from C10 to C13, most strongly for 10-methylretinal and least pronounced for 9-demethylretinal. In all five cases, the dihedral angle of the C10-C11=C12-C13 bond is negative which attests to the strong chiral discrimination exerted by the protein pocket. The calculations show that the nonplanar distortion of the chromophore, including the sense of rotation, is caused by a combination of two effects: the fitting of both ends to the protein matrix which imposes a distance constraint and the bonding arrangement at the Schiff base terminus. With both the counterion Glu113 and Lys296 displaced off the plane of the chromophore, their binding to N16 exerts a torque on the chromophore. As a result, the polyene chain, from N16 to C13, is twisted in a clockwise manner against the remaining part of the chromophore, leading to a C11=C12 bond with the observed negative dihedral angle. Shifts of the absorption maxima are reproduced correctly, in particular, the red shift of the 10-methyl and the strong blue shift of the 9-demethyl analogue relative to the wild type. Calculated positive rotatory strengths of the alpha-CD bands are in agreement with the calculated absolute conformation of the mutant chromophores.

Effects of steric strain on the bonding in zinc metallocenes: the structure of [(C 3H 7) 4C 5H] 2Zn

Zinc diiodide reacts with the potassium salts of 1,2,4-tri and 1,2,3,4-tetra(isopropyl)cyclopentadiene to form the corresponding zinc metallocenes in high yield. Both compounds are highly air-sensitive, but [(C 3H 7) 4C 5H] 2Zn is isolated as a high-melting crystalline solid, whereas [(C 3H 7) 3C 5H 2] 2Zn is a waxy solid that melts just above room temperature. Like other structurally characterized zincocenes, [(C 3H 7) 4C 5H] 2Zn has an asymmetric “slipped-sandwich” structure with one η 1- and one approximately η 5-ring. The ZnC distance for the η 1-ring of 2.223(4) Å is the longest yet observed in a zincocene. The ZnC distances for the η 5-ring cover a larger than usual range (0.52 Å), and include the shortest ZnC ring bond reported in a cyclopentadienyl zinc complex (1.991(4) Å). These extremes in structural values reflect the increased slippage of the rings in [(C 3H 7) 4C 5H] 2Zn, which is a consequence of the exceptional steric bulk of the [(C 3H 7) 4C 5H] − anion. Variable temperature NMR data for [(C 3H 7) 4C 5H] 2Zn are consistent with the existence of an asymmetric structure in solution.

Nitrogen NMR assessment of interactions between nitroso, cyano and acetyl groups across aliphatic systems

AbstractThe nitrogen NMR shieldings (Δσ, equivalent to —Δδ on the frequency scale of chemical shifts) of the NO group in nitrosoalkanes are shown to be a sensitive probe to electron‐withdrawing effects of substituents and to steric strain effects. The former produce high‐field (low‐frequency) shifts, which can amount to about 90 ppm, while the latter result in considerable low‐field (high‐frequency) shifts of the NO nitrogen resonance. The CN nitrogen shieldings show less pronounced low‐field (high‐frequency) shifts under the influence of electron acceptors. The signs and magnitudes of the observed effects are explained in terms of semi‐empirical molecular orbital calculations of the shieldings and molecular mechanics evaluations of strain energies. The overall picture of relationships between the NO and CN nitrogen shieldings and electron charge migration within the molecules concerned is in accord with that obtained in earlier studies of solvent‐induced effects on the nitrogen shieldings in alkyl cyanides and nitro compounds.

Methyl 3-[2-(tert-Butylthio)phenyl]propenoate

The effects of steric strain in methyl 3-[2-(tert-butylthio)phenyl]propenoate (C 14 H 18 O 2 S) are seen in the distribution of C-C bond lengths within the phenyl ring and in the placement of the tert-butyl substituent

ChemInform Abstract: Thermodynamics of Vinyl Ethers. Part 28. Effects of Steric Strain on the Stability of a C = C ‐ Ph System.

AbstractThermodynamics of Vinyl Ethers.

Thermodynamics of Vinyl Ethers. XXVIII. Effects of Steric Strain on the Stability of a C=C--Ph System.

Les reactions concernees sont l'isomerisation de methoxy-2 phenyl-3 propenes diversement substitues en methoxy-2 phenyl-1 propene correspondants et l'isomerisation de methoxy-2 dihydro-1,4 naphtalenes en derives dihydro-«3,4» correspondants

The effect of steric strain in the bay-region of polycyclic aromatic hydrocarbons: Tumorigenicity of alkyl-substituted benz[ a]anthracenes

3,6-Dimethylcholanthrene (3,6-DMC) and 7,11,12-trimethylbenz[ a]anthrene (7,11,12-TMBA) were tested for tumor-initiating activity on mouse skin as an approach to evaluate the potential role of steric strain in the bay-region on tumorigenic potency. Methyl-substitution at the 6-position of 3-methylcholanthrene (3-MC) increases steric strain in the bay-region of the hydrocarbon as it does at the 12-position of benz[ a]anthracene (BA) causing both hydrocarbons to become non-planar. 3,6-DMC had at least 2- to 3-fold higher tumor-initiating activity than did 3-MC. Introduction of an 11-methyl group in 7,12-dimethylbenz[ a]anthracene (7,12-DMBA) results in the formation of a more highly hindered (buttressing effect) hydrocarbon. 7,11,12-TMBA had 5% or less of the tumor-initiating activity of 7,12-DMBA, although the hydrocarbon still had relatively high tumorigenic activity on mouse skin. The results obtained with 3,6-DMC and studies reported previously with other methyl-substituted hydrocarbons, show that hydrocarbons possessing steric strain in the bay-region of the molecule can have enhanced tumorigenic activity. The basis of this steric effect remains unclear, however, as a result of the decreased tumorigenic activity of the 11-methyl-substituted derivative of 7,12-DMBA. The weak tumor-initiating activity of BA was enhanced at least 4- to 8-fold as a result of methyl-substitution at the 6- and 8-positions (6,8-dimethylbenz[ a]anthracene). The higher tumorigenic activity of 6,8-dimethylbenz[ a]anthracene compared to BA is consistent with a presumed decrease in metabolic detoxification of the dimethyl-substituted derivative at the 5,6- and 8,9-double bonds.

Chemical effects of steric strains. 25. Steric effects as a factor in the stability of addition compounds of borinane with amines

Chemical effects of steric strains. 25. Steric effects as a factor in the stability of addition compounds of borinane with amines

Chemical effects of steric strains. 24. Carbon-13 NMR study of the interaction of 9-borabicyclo[3.3.1]nonane with amines of increasing steric requirements

Chemical effects of steric strains. 24. Carbon-13 NMR study of the interaction of 9-borabicyclo[3.3.1]nonane with amines of increasing steric requirements

Chemical effects of steric strains. Part XXIII. Steric effects as a factor in the stability of addition compounds of amines with 9‐borabicyclo[3.3.1]nonane

AbstractAn infrared technique was developed to establish the equilibrium involved in the reaction of 9‐borabicyclo[3.3.1]nonane with representative amines under standard conditions (25.0°C, cyclohexane or THF solution): (9‐BBN)2 + 2 Amine ⇌ 2 9‐BBN: Amine. Coordination is complete with primary amines, even as hindered as tert‐butylamine. It is complete with diethylamine, but incomplete with diisopropylamine. It is incomplete even with the simple triethylamine. However, the cyclic structure corresponding to triethylamine, namely, quinuclidine, forms a stable adduct. Pyridine is fully coordinated, but the extent of coordination decreases with increasing steric requirements of the 2‐alkyl substituent, becoming essentially zero with 2‐tert‐butylpyridine and 2,6‐dimethylpyridine. Thus the results with the unusual Lewis acid, 9‐borabicyclo[3.3.1]nonane parallel closely the earlier results for the stability of addition compounds produced by the reaction of such amines with trimethyl‐borane, boron trifluoride, and borane.

Chemical effects of steric strains. 22. Rates of solvolysis of dimethyl(2-norbornyl)carbinyl p-nitrobenzoates and of the quaternization of 2-dimethylaminonorbornanes. A critical examination of the significance of steric factors in the chemistry of exo- and endo-norbornyl derivatives

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChemical effects of steric strains. 22. Rates of solvolysis of dimethyl(2-norbornyl)carbinyl p-nitrobenzoates and of the quaternization of 2-dimethylaminonorbornanes. A critical examination of the significance of steric factors in the chemistry of exo- and endo-norbornyl derivativesHerbert C. Brown and M. RavindranathanCite this: J. Am. Chem. Soc. 1978, 100, 6, 1865–1869Publication Date (Print):March 1, 1978Publication History Published online1 May 2002Published inissue 1 March 1978https://doi.org/10.1021/ja00474a034RIGHTS & PERMISSIONSArticle Views115Altmetric-Citations14LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (594 KB) Get e-Alerts Get e-Alerts

Cationic ruthenium systems. Part 2. Synthesis, characterization, and X-ray structure of hydridopentakis(dimethylphenylphosphine)-ruthenium(II) hexafluorophosphate: a discussion of the effects of steric strains on the reactivity of the complex

The complex [RuH(PMe2Ph)5][PF6] has been prepared by treatment of a methanolic solution of [RuH(cod)(NH2-NMe2)3][PF6](cod = cyclo-ocata-1,5-diene) with PMe2Ph under argon and has been characterized by microanalytical, i.r., and 1H n.m.r. data and by a three-dimensional X-ray structure determination. The colourless crystals of [RuH(PMe2Ph)5][PF6], M= 938, are orthorhombic, space group P212121, a= 21.55(3), b= 19.08(3), c= 10.38(2)A, U= 4 269 A3, Dm= 1.48, Dc= 1.46 g cm–3, Z= 4. The structure has been solved by the heavyatom method and refined by least squares to R 0.061 for 1 954 observed reflections collected on a diffractometer using graphite-monochromatized Mo-Kα radiation. The co-ordination about the ruthenium is distorted octahedral. The length of the bond to the phosphine trans to the hydride ligand (Ru–P 2.48 A) is significantly longer than those to the other four phosphine ligands (mean Ru–P 2.40 ± 0.01 A). There is considerable strain within the cation caused by non-bonded repulsions between the methyl and phenyl groups on different phosphine ligands. A discussion of the build-up of steric strain in co-ordination complexes containing PMe2Ph ligands is presented.

Chemical effects of steric strains. XXI. Solvolysis of tertiary methyl- and tert-butylcarbinyl p-nitrobenzoates. Evidence that steric effects are more important in rigid bicyclic systems than in acyclic and alicyclic systems

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChemical effects of steric strains. XXI. Solvolysis of tertiary methyl- and tert-butylcarbinyl p-nitrobenzoates. Evidence that steric effects are more important in rigid bicyclic systems than in acyclic and alicyclic systemsEdward N. Peters and Herbert C. BrownCite this: J. Am. Chem. Soc. 1975, 97, 10, 2892–2895Publication Date (Print):May 1, 1975Publication History Published online1 May 2002Published inissue 1 May 1975https://doi.org/10.1021/ja00843a046RIGHTS & PERMISSIONSArticle Views696Altmetric-Citations16LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (436 KB) Get e-Alerts Get e-Alerts