Conformational polymorphs of 1,1,2,2-tetrachloroethane: pressure vs. temperature

Directional Cl···Cl type I and II interactions govern the low-density aggregation of 1,1,2,2-tetrachloroethane molecules in synclinal conformation in the crystalline state at low temperature, whereas the dense molecular packing in high-pressure is achieved for the antiperiplanar conformers and electrostatically less favored Cl···Cl contacts. The mechanism of transformation between loose and dense associations involves the collapse of Cl···Cl contacts and conformational conversion.     

A normal coordinate analysis of 1,1,2,2-tetracyanoethane

The i.r. and Raman spectra of 1,1,2,2-tetracyanoethane indicate the presence of only the trans rotamer in the solid state. The fundamentals are identified and compared with frequencies calculated with the aid of a normal coordinate analysis. Assignments made for the fundamental frequencies are accurate to an average deviation of 2.6 %.     

Conformational analysis of tetraarylethanes

NMR evidence establishes that both diastereomers of 1,2-diphenyl-1,2-bis(4-pyridyl)ethane (2), identified by optical resolution of the racemic form, exist predominantly in the anti conformation. Furthermore, empirical force field calculations show that the gauche conformer of 1,1,2,2-tetrakis(2,6-dimethylphenyl)ethane (3) is less stable by ca. 10 $\text{kcal}\text{mol}$ than the anti structure. It thus appears that neither polar effects nor steric congestion are effective in reversing the marked preference of 1,1,2,2-tetraphenylethane (1) and other unclamped tetraarylethanes for an anti ground state. In contrast, as predicted by empirical force field calculations and confirmed by X-ray and NMR evidence, the ground state structure of 9,9'-bifluorenyl (4) is gauche. The conformational behavior of 1–4 is discussed in terms of the intramolecular aryl ring stacking in clamped and unclamped tetraarylethanes.     

Conformational analysis of cinnamanilides

The conformational analysis of cinnamanilides has been carried out using IR spectroscopy. All the anilides studied were found to exist as equilibrium mixtures ofs-cis ands-trans forms in benzene. Thes-cis form was predominant over thes-trans in all the anilides except in thep-nitro anilide in which thes-trans form was predominant. The relative stabilities of the conformers were found to depend upon the electrostatic repulsions between the anilide nitrogen and the β-carbon atom in thes-trans form and those between the π-electrons of the C=O and C=C bonds in thes-cis form.     

Conformational analysis of dimethylmethylphosphonate

The results of a CNDO/2 conformation analysis on dimethylmethylphosphonate are reported. Six stable conformers were found; their relative stabilities can be understood in terms of steric hindrance and gauche effect. Calculated barriers to internal rotation around P—O and O—C bonds are tabulated.     

Conformational analysis of double-helical polynucleotides

The intramolecular interaction energy of the regular double-helical polynucleotide as a function of variables that determine the mutual position of base pairs and sugar pucker was calculated using atom–atom potentials. The calculations showed the existence of two valley-like regions with minimal values on the energetic surface. One of them corresponds to the A family of nucleic acids, the other to the B family. The points that correspond to the models constructed by means of x-ray data are placed in a conformational space near the lines that describe the position of the bottom of the valleys.     

Conformational analysis of 1,3-oxathiane

Computer simulation of pathways of conformational isomerization of 1,3-oxathiane molecule carried out with the help of HF/6-31G(d), MP2.6-31G(d)/HF/6-31G(d), and PNE/3z quantum-chemical approximations showed that interconversion between the degenerate in energy chair conformers proceeds through seven independent pathways: directly and via six flexible forms. Potential energy surface contains eight minimum points including chair conformers and enantiomeric pairs of twist forms, and also five transition states, among them different modification of semi-chair, symmetric and unmmetrical boat. Molecular dynamics methods show that flexible forms at room temperature convert into one another and into the chair conformers.     

Conformational analysis of medium-ring ketones

The conformations of cyclooctanone, cyclooctane-1,5-dione, cyclononanone, ayclodecanone, cyclodecane-1,6-dione, and cycloundecanone are discussed.     

Conformational analysis of 1,4-heterophosphinanes

Conformational analysis of 1,4-heterophosphinanes in solution, based on a combination of the dipole moment and Kerr effect methods, molecular mechanics, and quantum-chemical calculations (B3LYP/6-31G^*) was performed. It was established that, regardless the second heteroatom in the six-membered phosphorus-containing heterocycle (oxygen, sulfur, or silicon) and phosphorus coordination, these compounds prefer a chair conformation with an equatorial exocyclic phenyl substituent.     

Conformational analysis of aromatic molecules

The mechanical model used in conformation calculations is discussed. A method is developed for strained aromatic systems with possible out-of-plane deformations. An approximate analysis is presented to reveal criteria for planarity.     

Conformational analysis of hydrogen polyoxides

The semiempirical AM I method was used for the calculation of conformational potential energy surfaces of hydrogen trioxide and tetraoxide. The most stable conformation of trioxide is itsanti-form, which is characterized by the torsion angle = 82.3°, and that of tetraoxide is theanti, anti-form with = 80.7° (H-O-O-O) and 74.1° (O-O-O-O). The structures of the stable conformers are determined by the interaction between lone clectron pairs of oxygen atoms. The dipole-dipole interaction of OH groups is weaker than that in hydrogen peroxide. The results were used for the estimation of the polyoxide increment of the Benson's additivity scheme, _f H°[O-(O)₂] = 48±6 k1 mol^–1.Translated from lzveshyn Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1356-1361, June, 1996.     

Conformational analysis of substituted polyacetylenes

A conformational analysis of isolated chains of polymethylacetylene (PMA), polypentylacetylene (PPA), and poly(t-butylacetylene) (PTA) was carried out taking into account interactions between nonbonded atoms and torsional potentials. It was found that the trans configuration of all three polymers is more stable than the cis configuration, the difference in potential energy between the trans and the cis isomers however being very small for PTA, leading to the possibility of observing a trans^→ cis isomerization in some solvents. The calculations show that the substituted polyacetylenes are not found in a planar conformation, the larger deviations from planarity being found with the bulkier substituents: PTA > PPA > PMA. A correlation could be established between the UV absorption limit of the samples and the minimum torsional angle of the potential-energy functions. This relation predicts that the absorption limit is shifted to long wavelengths on increasing the planarity of the molecule. Moreover, UV spectra could be calculated from the potential-energy functions, and it is shown that the potential-energy functions of other substituted polyacetylenes can be calculated from their experimental UV spectra.     

Conformational analysis of chlorocholine analogues

The high-resolution proton magnetic resonance spectra of the methylene groups of 2-chloroethyl trimethyl ammonium chloride (I) and 2-chloroethyl dimethyl ammonium chloride (II) were studied by iterative computer calculation. From the values of the spectral parameter L it is concluded that in the case of compound (I) the enthalpy difference between the antiperiplanar and the synclinal conformers is very low, probably, emphasis lies on the antiperiplanar conformation. Compound (II) exists in a dominant synclinal conformation. PCILO calculations of 2-chloroethyl trimethyl ammonium yield the antiperiplanar conformation as the most stable one in agreement with experiment. The energy difference between the antiperiplanar and the syn/anticlinal conformation is 0.7 kcal mole^?1. 2-Chloroethyl dimethyl ammonium, 2-chloroethyl methyl ammonium and 2-chloroethyl ammonium in our PCILO calculations exist in a predominant synclinal conformation. The calculated charge distributions demonstrate that the quaternary nitrogen atom is approximately neutral, the “positive” charge is distributed among the methyl groups and hydrogen atoms attached to the nitrogen atom.     

Conformational analysis of 1,2,3,4-tetrahydroisoquinolines

Ring and nitrogen inversion account for the conformational equilibria of 3-phenyl-1, 2,3, 4-tetrahydroiso-quinolines. In order to quantitate the relative contribution of each conformer to the equilibrium, we undertook a molecular mechanics study on several substituted 3-phenyl-1, 2, 3, 4-tetrahydroisoquinolines. Predictions from calculations were checked against cmr chemical shift data. No boat conformation contributed significantly to the equilibrium. A general result of our calculations is that in all cases the 3-phenyl group in the equatorial position is strongly favored (by at least 2.50 kcal/mole). For 3-phenyl-1, 2, 3, 4-tetrahydroisoquinolines without substitution at nitrogen, N-H in equatorial position is preferred over the axial conformer, although the energy difference between both is always small (0.30–1.10 kcal/mole). For the cis-1,3-disubstituted compounds the le'3e conformers are the only species present (at least 99.8%). The calculated energy differences between the la′3a conformer and the le′3e conformer are always large (3.80–6.10 kcal/mole for the NHe conformers and 3.60–3.80 kcal/mole for the NHa conformers). The lack of a γ_1a upfield shift at C3 also points to the preference for the pseudoequatorial-equatorial conformer. For N-methyl-3-phenyl-1,2,3,4-tetrahydroisoquinoline a preference for the NMe group in the equatorial position is predicted (0.60–2.00 kcal/mole). The small downfield shift at C4 (γ_Na = 0.5 ppm) is consistent with the equatorial NMe preference. For the cis-1,2,3-trisubstituted compounds no significant γ_1a effect at C3 (γ_1a = -0.2 and 1.0 ppm) or γ_Na effect at C4 (γ_Na = 0.1 and 0.4 ppm) is observed. For these compounds, deformations due to steric congestion are evidenced by the deviation from the values of the C4a-C8a-Cl-N and C4a-C4-C3-N torsional angles, as compared to less crowded 3-phenyl-1,2,3,4-tetrahydroisoquinolines. Here the heterocyclic ring adopts a distorted half-chair conformation.     

Chemistry of 1,1,2,2-tetracyanoethane

Conclusions 1-(N,N-Dimethylhydrazino)-4-methyl-2,2,3,3-tetracyanocyclopentane is isomerized reverslbly in solution, with inversion of the configuration of the tetrahedral carbon atom in position 1.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1075–1079, May, 1989.     

Chemistry of 1,1,2,2-tetracyanoethane. Reaction of 1,1,2,2-tetracyanoethane with α-dicarbonyl compounds

1,1,2,2-Tetracyanoethane reacts with -dicarbonyl compounds in water to form 7-amino-4-hydroxy-1,8-dicyano-3-aza-6-oxabicyclo-[3.3.0]oct-7-en-2-ones. Their structures were confirmed by IR,¹³C NMR and mass spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 821–824, June, 1990.     

Conformational analysis by energy embedding

The distance geometry approach to conformational calculation has been shown to be very effective at producing large molecular structures satisfying many given, long-range constraints on the interatomic distances. I now present a significant extension of the method that handles strictly geometric constraints as well as before while also locating conformers of very low energy. The main feature of the algorithm is a projection of the molecule from a low energy conformation in a high dimensional space to three dimensions in such a way as to perturb the energy as little as possible. Tests of the method on very small systems with simple energy functions completely explored by independent means show that the global minimum of energy is sometimes attained. In every case the final energy is very low, and geometric constraints are completely satisfied.