Molecular conformation of cyclenes: V. Cyclodecadienes

The strain energy of 1,3-, 1,4-, 1,5- and 1,6-cyclodecadienes has been calculated as a function of various geometric parameters. The relative stability of the minimum-energy forms is discussed and, where possible, compared with thermodynamic and electron diffraction data. In the 1,5- and 1,6-isomers agreement with experimental data is obtained if the transannular interaction between the double bonds is taken into account.     

Molecular conformation of cyclenes: Part IX. Asymmetrical 1,3,6-cyclononatriene and 1,3,6-cyclodecatriene

The strain energy of the most probable conformers of 1,3,6-cyclononatriene and 1,3,6-cyclodecatriene, which are asymmetrical compounds, has been calculated as a function of various geometrical parameters by means of the semiempirical Hendrickson's treatment, partially modified. The most stable conformation of 1,3,6-cyclononatriene is a member of the all-cis family, while a transcis-trans structure is the most stable among the 1,3,6-cyclodecatriene conformations. In this latter compound however, the energy minima of the all-cis, trans-cis-cis, and cis-cis-trans conformers also appear to be close to each other. Some possible interconversions are also discussed.     

Molecular conformation of cyclenes: Part VI. MINDO/2' study of 1,3-cycloheptadiene,cis- and trans-cyclooctene

Quantum mechanical calculations indicate that the most stable conformation for 1,3-cycloheptadiene is a C_s semi-planar form, for trans-cyclooctene a C₂ twist form and for cis-cyclooctene a form without symmetry. An equilibrium between two C₁ forms and the C_s form can be suggested as a consequence of the negligible rotational barrier in 1,3-cycloheptadiene; no obviously preferred conformations exist in cis-cyclooctene, where the molecule is quite flexible. Theoretical results are consistent with the experimental data available.     

The vibrational spectra, molecular structure and conformation of organic azides : Part VI. Azidoacetonitrile

Azidoacetonitrile (azidoethanenitrile) has been synthesized and the structure determined by electron diffraction from the vapour. IR spectra have been recorded of the vapour, liquid, solutions, solid at ca. 90 K and of the molecule trapped in argon and nitrogen matrices at 15 K. Raman spectra of the cooled liquid and of the amorphous and crystalline solids have also been obtained.

The electron diffraction results show the conformation to be gauche around the C---N bond with a dihedral angle of 52°(5) from syn and the NNN angle 173°(3) oriented anti to the C---N bond.

The following bond distances (r_a) between the heavy atoms were obtained: r_CN = 115.4(5) pm, r_C---C = 146.5(15) pm, r_C---N = 147.5(6) pm, r_N---N = 124.5(5) pm and r_NN = 113.5(4) pm.

Very large changes in both frequency and intensity have been observed in the vibrational spectra upon crystallization which may be due to intermolecular associations formed by the azido group but which can also be explained by a change of the molecular conformation from only gauche in the vapour and liquid phases to anti in the crystal phase.     

Molecular mechanics and molecular shape: Part VI. The response of simple molecules to bimolecular association

A molecular-mechanical force field, originally developed to calculate the structure and conformation of single molecules, has been applied to pairs of identical molecules lying at close proximity. The prototypes examined are very simple - short alkanes and short haloalkanes - but do include cases with conformational diversity and possible electrostatic effects. The optimized mutual orientations include the parallel alignment of linear alkanes, head-to-tail orientations of haloalkanes, as well as the head-to-head association, known from the crystallographic literature. The effect of aggregation on the conformation of the separate partners has been studied, and the intramolecular response characterized: in all cases examined, the partners contract. The computed energy gains are of the expected order of magnitude, the main contribution coming always from what molecular-mechanists label “non-bonded attraction”. Graphical representations of optimized orientations show the “protrusion-in-cleft” matching of molecular surfaces.     

Effect of substituents at atoms of double bonds on the conformation of the 1,3-cyclohexadiene ring

The effect of alkyl substituents at atoms of double bonds on the conformation of the 1,3-cyclohexadiene ring was studied using a molecular-mechanics method. It was shown that the introduction of substituents results only in a change of the degree of ring puckering. It is concluded that there is equilibrium between flattening and unflattening factors, which then governs the constancy of the ring conformation.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 4, pp. 334–337, July–August, 1992.     

Binuclear cyclopentadienylmetal cyclooctatetraene derivatives of the first row transition metals: effects of ring conformation on the bonding of an eight-membered carbocyclic ring to a pair of metal atoms

Binuclear Cp(2)M(2)(μ-C(8)H(8)) derivatives have been synthesized for M = V, Cr, Co, and Ni and have now been studied theoretically for the entire first row of transition metals from Ti to Ni. The early transition metal derivatives Cp(2)M(2)(μ-C(8)H(8)) (M = Ti, V, Cr. Mn) are predicted to form low-energy cis-Cp(2)M(2)(μ-C(8)H(8)) structures with a folded C(8)H(8) ring (dihedral angle ～130°) and short metal-metal distances suggesting multiple bonding. These predicted structures are close to the experimental structures for M = V, Cr with V≡V and Cr≡Cr bond lengths of ～2.48 and ～2.36 ?, respectively. The middle to late transition metals form trans-Cp(2)M(2)(μ-C(8)H(8)) structures (M = Mn, Fe, Co, Ni) with a twisted μ-C(8)H(8) ring and no metal-metal bonding. The hapticity of the central μ-C(8)H(8) ring in such structures ranges from five for Mn and Fe to four for Co and three for Ni and thus depend on the electronic requirements of the central metal atom. This leads to the favored 18-electron configuration for both metal atoms in the singlet Fe, Co, and Ni structures but only 17-electron metal configurations in the triplet Mn structure. In addition, the late transition metals form trans-Cp(2)M(2)(μ-C(8)H(8)) structures (M = Fe, Co, Ni), with the tub conformation of the μ-C(8)H(8) ring functioning as a tetrahapto (M = Fe, Co) or trihapto (M = Ni) ligand to each CpM group. A μ-C(8)H(8) ring in the tub conformation also bonds to two CpFe units as a bis(tetrahapto) ligand in both singlet and triplet cis-Cp(2)Fe(2)(μ-C(8)H(8)) structures.     

Molecular fragmentations: Part IV. The mass spectral fragmentation of carbon tetrachloride

MINDO/3 calculations have been made of the molecular structures and energies of seven isomeric forms of the molecular cation (CCl₄)⁺, of the mass spectral fragment pairs:

and also of a number of neutral fragment pairs. Reaction energy profiles have been calculated for two fragmentations of (CCl₄)⁺, into [(CCl₂)⁺ + Cl₂], and into [(CCl₃)⁺ + Cl^?], in the latter of which the reaction proceeds via a rather stable intermediate; for the fragmentation of three electronic states of (CCl₃)⁺ into [(CCl₂)⁺ + Cl^?], where the ground singlet state and first triplet state of (CCl₃⁺ yield the ground doublet state of (CCl₂)⁺, but the first excited singlet of (CCl₃)⁺ yields the first excited doublet of (CCl₂)⁺ ; and for the fragmentation of the ground state of (CCl₃)⁺ into [(CCl)⁺ + Cl₂].     

Ab initio studies of structural features not easily amenable to experiment : Part 8. The structural consequences of the anomeric effect in compounds with disubstituted tetrahedral carbon atoms

The geometries of three conformations of FCH₂OH and four conformations each of NH₂CH₂NH₂ and NH₂CH₂OH are completely refined by ab initio calculations on the 4–21G level. It is found that most characteristic structural and conformational properties of such systems can be reliably predicted on the basis of a simple anomeric orbital interaction model. The extension of this model to all compounds in which two electronegative substituents with non-bonding lone pairs or bonding π-electrons are attached to the same tetrahedral carbon atom, including polymer systems such as proteins, seems to be useful.     

Substitution in the hydantoin ring. Part IX. N-cyanohydantoins

The N₁-and N₃-cyanohydantoins, a new series of derivatives, were prepared by reaction of the parent hydantoin with a base and a cyanogen halide. Analysis of ir, pmr, and mass spectral data allowed the assignment of ring position-3 to the cyano group in derivatives IIa,b of 1,3-unsubstituted hydantoins. 3-Cyanohydantoins can transfer the cyano substituent to strong nucleophiles via an addition-elimination process.     

Acetylenic ketones. Part VI. Reaction of aroylphenylacetylenes with hydrazine derivatives

The reaction of aroylphenylacetylenes (I) with acyl- or aroylhydrazines (II) gave ω-aroyl-acetophenone-N-acyl or N-aroylhydrazones (IV). The latter gave upon treatment with methanolic potassium hydroxide and with acetic anhydride in the presence of sodium acetate, the corresponding pyrazoles (V) and the N-acetylpyrazoles (VII and VIII), respectively. The acetylenic ketones ( 1 ) also reacted with methylhydrazine and 1,1-dimethylhydrazine to give 5-aryl-1-methyl-3-phenylpyrazoles (XII), and 1,1-dimethylhydrazine derivatives (XIII), respectively. When the latter compounds were heated with acetic anhydride, they gave the N-methylpyrazoles (XII).     

ECE and disproportionation: Part VI. General resolution. Application to potential step chronoamperometry

The competition between homogeneous and heterogeneous electron transfers in ECE-DISP mechanisms: A+eB BC C+eD B+C→A+D is analyzed in the context of potential step chronoamperometry. Starting from the expressions of the apparent number of electrons for the limiting situation, ECE, DISP1, DISP2, the transition between two of these “pure” mechanisms is systematically investigated. The general system is then analyzed for large values of the kinetic parameters, i.e., in the particular case of a stationary state arising from mutual compensation of diffusion and chemical reaction (“pure kinetic” conditions). This provides an estimation of the range of parameters where numerical analysis of the general case is actually necessary together with tests of accuracy of the numerical computation procedures. In this framework the analysis of the most general case involving no particular assumption about the magnitude of the thermodynamic and kinetic parameters leads to a three-dimensional kinetic zone diagram which allows to predict the effects of the intrinsic (equilibrium and rate constants) and operational (time, concentration) parameters on the displacement of the system from one limiting situation to the other. On these bases, the practical effectiveness of single-step techniques in discriminating between the limiting mechanisms is discussed. It is shown that the discrimination between DISP2 and either ECE and DISP1 can be easily carried out in terms of either concentration or time dependence of the current response. The discrimination between ECE and DISP1 is much more difficult and requires a high experimental accuracy to be effective.     

Pyrazoles VI: The electron-releasing capacity of the pyrazole ring

UV and PMR spectral data and C?O frequencies of some methylpyrazoles containing in the 3-, 4- or 5-position, a formyl-, acetyl- or ethoxycarbonyl group are reported. These data confirm earlier conclusions that, in particular, the 4-pyrazolyl group acts as an electron releasing group. The syntheses of a number of formyl-, acetyl- and ethoxycarbonyl pyrazoles are described. In addition, some 4-dicyanovinyl- and 4-tricyanovinylpyrazoles were investigated.     

Motion of gold atoms on carbon in the aberration-corrected STEM.

The movement of heavy atoms on a thin carbon substrate is readily observed using a sub-Angstrom electron probe. The observed movement is consistent with an electron beam activation mechanism whereby atoms are occasionally detached from bonding sites, allowing rapid diffusion to new sites that may be quite far from the original. The bonding sites are most often observed to lie at defects, steps, and other asperities in the substrate. Formation of three-dimensional clusters can occur during diffusion of several isolated atoms. Coalescence and dissolution of larger clusters and islands both occur under varying observation conditions, but island coalescence appears most probable for islands that are greater than 2 nm in size.     

Tetraphenyl-p-benziporphyrin: a carbaporphyrinoid with two linked carbon atoms in the coordination core

Replacement of one of the pyrrole rings with a p-phenylene unit transforms porphyrin into p-benziporphyrin (1), an aromatic carbaporphyrinoid that locates two connected carbon atoms in the coordination core. p-Benziporphyrin forms a complex with cadmium(II) (2) with an unprecedented eta(2) Cd(II)-arene interaction.     

Synthesis of six-membered cyclic compounds,containing the carbon atoms of the o-carborane nucleus and one heteroatom in the ring

Russian Chemical Bulletin - Bis-(o-carboranyl)methoxymethane was synthesized from monolithium-o-carborane and 1,1-dichloro-methyl methyl ether, from which were obtained six-membered carborane...     

Conservation of the skeletal conformation in the ring decomposition of dimethylcyclohexanes

By comparing the composition of products formed during the -radiolysis of structural and geometrical /cis and trans/ isomers of dimethylcyclohexanes, the relationship between molecular structure and ring decomposition reactions was investigated. The yields of products that exist in geometrical isomeric forms /e.g. 2-butenes, 2-octenes/ were considerably different in the irradiation of cis- and transdimethylcyclohexanes: the products reflected the conformation of the decomposed molecule. It was assumed that the ring decomposition is a very fast process taking place within about 1 ps. If biradical intermediates are involved in the reactions, their further transformations occur before attaining conformational equilibrium.     

Probing furanose ring conformation by gas-phase computational methods: energy profile and structural parameters in methyl beta-D-arabinofuranoside as a function of ring conformation

The potential energy surface of methyl beta-D-arabinofuranoside (3) has been studied by ab initio molecular orbital (HF/6-31G) and density functional theory (B3LYP/6-31G) calculations via minimization of the 10 possible envelope conformers. The partial potential energy surface identified that the global minimum and lowest energy northern conformer was E(2). In the HF calculations, (2)E was the most stable southern conformer, while the density functional theory methods identified (4)E as the local minimum in this hemisphere. Additional calculations at higher levels of theory showed that the B3LYP-derived energies of many of the envelope conformers of 3 are dependent upon the basis set used. It has also been demonstrated that B3LYP/6-31+G//B3LYP/6-31G single point energies are essentially the same as those obtained from full geometry optimizations at the B3LYP/6-31+G level. The northern and southern minima of the B3LYP/6-31+G surface are, respectively, the E(2) and (2)E conformers. The B3LYP/6-31G geometries were used to study the relationship between ring conformation and various structural parameters including bond angles, dihedral angles, bond lengths, and interatomic distances.