Molecular mechanics studies on isogermacrone

Molecular mechanics studies on the distribution of conformers with parallel and crossed double bonds at C-2 and C-7 of the ten-membered ring ketone isogermacrone led to a 6.7 kJ mol^?1 difference between the two most stable conformers the barrier between them being approximately 66.9 kJ mol^?1.     

Molecular structures of trinitromethane derivatives

The molecular structures of trinitromethane derivatives XC(NO₂)₃ (X = F, Cl, Br, NC, NF₂, N₃) were studied using the density functional approach. The rules for changing the configurations of substituents in these compounds were revealed. Acceptability of the method employed for the calculations of trinitromethane derivatives is discussed.     

Molecular structures and reactivities of 1,3-diphthalimidobenzene derivatives

X-ray diffraction analysis of 4,6-di(morpholin-4-yl) and 4,6-di(2,2,2-trifluoroethoxy) derivatives of 1,3-diphthalimidobenzene, which are potent reagents in the synthesis of heterocyclic polymers, was carried out. The conjugation between the π-systems of the benzene ring and of the phthalimide and morpholine substituents is distorted due to the rotation of the substituents about the C(Ar)−N bonds. The AMI calculations demonstrated that the hydrogen atoms of the methylene groups are “acidic”, which is favorable for condensation reactions. Steric hindrances to intramolecular condensation were estimated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1155–1161, June, 1998.     

Thio-claisen rearrangements of some ketene derivatives

A number of 1-allylthio and 1-crotylthio 1-aminoalkenes derived from ketones and active methylene compounds has been prepared and rearranged to the corresponding thioamides (thio-Claisen rearrangement). Besides rearrangement products, small amounts of 1-alkyl-thio-N-alkyl-N-phenyl-1-aminoalkenes have also been isolated. When the ketene derivatives did possess a H-atom on nitrogen, cleavage of the formed thioamide produced phenyl isothiocyanate and substituted active methylene compounds.     

Molecular structures of acetylene derivatives of tin

The electron diffraction data on bis(trimethylstannyl)acetylene, Me₃SnC≡CSnMe₃, were analyzed in the framework of the one-dimensional dynamic model of free internal rotation of the SnMe₃ group about the axis of the Sn−C≡C−Sn linear fragment. The root-mean-square amplitudes and harmonic shrinkage corrections used in the analysis were calculated from the scaled quantum-chemical force field (i) taking into account nonlinear relations between Cartesian and internal vibrational coordinates at the first-order level of perturbation theory (h1) and (ii) using a conventional approach (h0). Ther _h1 parameters of internuclear distances describe the equilibrium geometry of the Me₃SnC≡CSnMe₃ molecule much better than the commonly accepted parametersr _α ≡r _h0 . Substituent effects on the geometry of the acetylene fragment are discussed.     

Molecular structures of acetylene derivatives of tin

The geometry and force fields of the bis(trimethylstannyl)acetylene molecule (a conformer withD _3d symmetry corresponding to a minimum of the total energy of the molecule) were calculated by the RHF and MP2(fc) methods. The effective core potential in SBK form with the optimized 31G^* valence basis set was employed in the case of Sn atoms. The 6–31G^** and 6–311G^** basis sets were used for carbon and hydrogen atoms. Vibrational spectra of the light and perdeuterated isotopomers of bis(trimethylstannyl)acetylene were interpreted using the procedure of scaling the quantum-chemical force fields. For Part 5, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 616–626, April, 2000.     

Molecular mechanics calculations on the structures and conformational properties of [8]circulene and some related phane compounds

Molecular mechanics calculations on [8]circulene and some related compounds have been performed in order to gain insight into the geometrical and conformational properties of these molecules [8]Circulene is predicted to undergo a facile “bond-shift” process between two saddle shaped conformations. The relation between the geometrical and conformational properties of 1 and 2 and the so far unsuccessful attempts to synthesize [8]circulene is discussed.     

Observed wurtzite derivatives and related dipolar tetrahedral structures

Eight lower-symmetry ordered derivatives of the wurtzite type and an additional six related dipolar tetrahedral structure types were observed. All these structures are based on hexagonal closest packing of anions. Their mutual symmetry relationships can be summarized in a Bärnighausen diagram. Dipolar tetrahedral structures are characterized by centrosymmetric space groups, thus resulting in a dipolar distribution of the coordination tetrahedra. Dipolar tetrahedral structures often exhibit disorder and random statistical occupations and several good ionic conductors are found among them. The wurtzite-type derivatives are characterized by polar space groups and all coordination tetrahedra in them are pointing in the same direction. Some simple, geometrically possible wurtzite derivatives have never been observed. Their absence can be rationalized on the basis of a poor geometrical fit between tetrahedral chains as demonstrated by computer simulations of their hypothetical structures. The ratio ? (quotient of the thickness of one hexagonal layer in [001] over one measure of the mean diameter of one atom within this layer) is useful for ordering the wurtzite derivatives on the basis of their chemical composition, since ? increases from the oxides, over the nitrides to the sulfides. The ? ratio also distinguishes clearly between the oxidic wurtzite types (0.79) and the tetrahedral rutile types of β-BeO derivatives (0.86), while the dipolar tetrahedral types have ? values around 0.82, that is, close to the value expected for hexagonal closest packing.     

Enumeration of wurtzite derivatives and related dipolar tetrahedral structures

Pólya's theorem and related results are used to count and classify by space group all possible wurtzite derivatives with small unit cells having composition ABX₂, AB₂X₃, AB₃X₄, or ABC₂X₄. The same arguments are applied to the dipolar tetrahedral structures, which resemble wurtzite but have a different pattern of occupancy of the tetrahedral voids in the hcp anion framework. Covalent molecular orbital and electrostatic calculations are used to study the two real and eight hypothetical structures for Li₃PO₄. Both predict shared edges to be destabilizing, in keeping with Pauling's rules.     

Molecular structures of some hydroxyazo compounds and their derivatives

The i.r. absorption spectra of 14 o-hydroxyazophenyl derivatives with increasing number of fused benzene rings of the aryl moiety are studied in solid state, in CCl₄ solution and in nujol mull. The i.r. spectral bands are assigned and indicate that these compounds exist in two tautomer forms. The OH group of the aryl moiety ortho to the azo group is responsible for the hydrazone structure. If solutions in CCl₄ are exposed to sunlight the strongly coloured compounds undergo a rapid bleaching, leading to a destruction of the azo group accompanied by an increase of hydrazone structures.     

Synthesis of quinoline derivatives using ketene dithioacetals

Methyl 3-(N-arylamino)-2-cyano-3-methylthioacrylates 3a-h , which are readily prepared by the reaction of the ketene dithioacetal, methyl 2-cyano-3,3-bis(methylthio)acrylate ( 1 ) with arylamines, was heated at reflux in diphenyl ether to give the corresponding 2-methylthio-4-hydroxyquinoline-3-carbonitriles 4a-h in 14–77% yields. The reaction of 1 with excess aryl amines in diphenyl ether at reflux gave also the corresponding 2-arylamino-4-hydroxyquinoline-3-carbonitriles 6a-d . The 2-methylthioquinoline-3-carbonitriles 4a-h , thus obtained, are useful intermediates for the synthesis of various quinoline derivatives.     

Molecular orbital studies on ice-like structures

Molecular orbital (MO) calculations were carried out on a series of ice-I _h and ice-I _c type lattices. These lattices were assigned dimensions which approximate conditions in liquid water (O-O = 2.86 Å) and, in addition, the dimensions of the ice-I _h lattices were changed to ascertain the influence of lattice expansion and contraction. Component parts of several lattices were investigated as were lattices lacking one or two individual monomeric units. Results are in accord with current experimental approximations. As the ice-I _h lattice is expanded, the stabilization energy diminishes. In the liquid model, ice-I _h structures are generally more stable than those of ice-I _c ; component rings of the lattice models of ice-I _c determine the stabilities of these models, and a stable ring system can stabilize an unstable system. An ice-I _h lattice model lacking a single monomer is stable. The trend of charges at the oxygen centres closely correlates with the charges on the central oxygen atom of the appropriate trimer.     

Molecular mechanics (MM3) parameterization of hydroxylamine and methyl derivatives

Based on results of electron diffraction, gas phase infrared spectroscopy (IR), and MP2/6-31 + G* ab initio calculations, a set of molecular mechanics (MM3) parameters was developed for molecules containing the N(sp³)—O(sp³) moiety. Using this set of parameters, MM3 is able to reproduce structures (bond lengths and bond angles) and vibrational spectra satisfactorily. © 1994 by John Wiley & Sons, Inc.     

Molecular mechanics,quantum mechanics,potentiometric, and conductometric studies on the complexes of some rare earth metals with 5‐azorhodanine derivatives

The geometry of metal ions (La³⁺, Ce³⁺, UO, and Th⁴⁺) complexes with 5‐azorhodanine derivatives was optimized at the level of molecular mechanics. Two stoichiometric ratios of metal to ligand (i.e., 1:1 and 1:2) were investigated. Tetracoordinate and hexacoordinate of each stoichiometric ratio have been studied. Effect of substitution in the ligand on the geometry of the complexes was discussed in the light of electron donating–accepting properties of these substituents. The influence of the nuclear effective charge of the central metal ions on the metal–ligand (M–L) bonding was discussed and the effect of the number of ligands on the M–L bond length was also discussed and correlated to the experimental results. The total energies of the different metal complexes were computed using the extended Huckel method. The effect of substituents in ligand, metal type, and stoichiometry of the complexes on the complex total energies were discussed. Stability constant of (La³⁺, Ce³⁺, UO, and Th⁴⁺) metal ions with 5‐azorhodanine derivaties have been determined potentiometrically in 0.1 M KCl and 50% (v/v) ethanol–water mixture. The order of the stability constants of the formed complexes was found to be La³⁺ < Ce³⁺ < UO < Th⁴⁺. The influence of substituents on the stability of the complexes was examined on the basis of electron‐repelling property of the substituent. The effect of temperature on the stability of the complexes formed was studied and the corresponding thermodynamic parameters (ΔG, ΔH, and ΔS) were derived and discussed. The stoichiometries of these complexes were determined conductometrically and indicated the formation of 1:1 and 1:2 (metal:ligand) complexes. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Molecular mechanics studies (MM4) of sulfides and mercaptans

The MM4 force field has been extended to the title class of compounds. The vibrational spectra, structures, conformational equilibria, and heats of formation have been studied for 47 conformers of 29 compounds. In general, the properties may be calculated with accuracy that is competitive with that for hydrocarbons. The structures are better fit than previously because of the inclusion of a torsion–bend interaction term, which has its origin in the lone pair (Bohlmann) effect. Available experimental data do not suffice to yield detailed torsional potentials, or geometries as a function of torsion angle, and these quantities were determined by ab initio calculations at the MP2/6-31G* level. The rms error in the calculated frequencies of seven representative structures (with a total of 64 experimental and 96 ab initio frequencies) is 25 cm⁻¹. The heats of formation for 23 compounds have a weighted rms error of 0.36 kcal/mol. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1827–1847, 1997     

Conformational analysis—CXIV : Molecular mechanics studies of sulfoxides

The conformational characteristics of dimethyl sulfoxide and a number of 6-membered ring sulfoxides have been studied. The thiane, 1,3- and 1,4-dithiane and 1,3,5-trithiane ring systems with various oxide substitutions have been examined. It is found that the chair forms are more stable than the twist or boat in all cases. The energy profiles of the twist-boat manifold are in many cases highly unusual, and quite different from anything so far known experimentally. The axial-equatorial preference of the oxygen is highly variable, depending on the steric and electrostatic interactions found in the particular case.     

Theoretical studies of five-membered ring ketene acetals

Cyclic ketene acetals are a class of organic molecules characterized by a nucleophilic exo-methylene carbon attached to a carbon with two adjacent O, N, or S atoms. We have carried out a systematic computational study of a series of five-membered cyclic acetals like 2-methylene-1,3-dioxolane and its OS, SS, NO, NS, and NN analogs as well as all the protonated species. The calculations were performed at the MP2 level using a triple zeta plus polarization basis set. The nucleophilicity was discussed in terms of geometrical factors, calculated atomic charges, calculated chemical shifts, and proton affinities. All the six neutral species were strong nucleophiles. The NN analog was predicted to be the strongest and the SS analog the weakest nucleophile.     

Molecular mechanics studies of enthalpies of formation and strain energies of azoxyalkanes

A force field has been developed, on the basis of available experimental and partial retention of diatomic differential overlap (PRDDO) data, to permit molecular mechanics calculations on azoxyalkanes. The calculated structures and enthalpies of formation are in fair agreement with experimental results. The calculated enthalpies of formation and strain energies are compared with the corresponding quantities for analogous azoalkanes.