The molecular structures of cis-3-hexene and trans-3-hexene in the gas phase by electron diffraction and molecular mechanical calculations

The molecular structures of cis-3-hexene and of trans-3-hexene in the gas phase have been determined by electron diffraction combined with molecular mechanical calculations. For cis-3-hexene the data indicate the presence of the (+ac, +ac) and the (?ac, +ac) forms. In trans-3 -hexene three rotamers were observed, with an energy sequence E(+ac, +ac) ≈ E(?ac, +ac) < E(ac, sp). The refined r_α⁰-structural parameters are: cis-3-hexene: C-H = 1.073 Å, CC = 1.330 Å, C(sp²)-C(sp³) = 1.505 Å, ∠CCH(in CH₂) = 111.1°, ∠CCC = 111.4°, ∠(CC-C) = 129.1° trans-3-hexene: C-H = 1.078 Å, CC = 1.342 Å, C(sp²)-C(sp³) = 1.506 Å, ∠CCH(in CH₂) = 109.3°, ∠CCC = 112.8, ∠CC—C = 124.1°The agreement between calculated and experimental geometries and vibrational amplitudes is good.     

Attractive-dominant and repulsive-dominant hydrocarbon reactions

The difference between the expectation values of the total electronic kinetic energy operator (ΔE_K), and the operators accounting for the Coulombic interactions between the electrons and nuclei (ΔV_en), between all pairs of electrons (ΔV_ee), and between all pairs of nuclei (ΔV_nn) for the product and reactant species in a wide variety of hydrocarbon reactions are calculated using single determinant basis set data reported in the literature. Following Allen, their contributions to ΔE_T, the difference between the corresponding total molecular energies and thus the reaction heat, are grouped together as a repulsion energy term, ΔE_rep = ΔE_K + ΔV_ee + ΔV_nn, and an attraction energy term ΔE_attr = ΔV_en. For all but 2 of the 71 individual reactions considered in this paper, the experimental reaction heat at 0°K corrected for zero-point energy contributions, (ΔH)_zpe, is the result of near compensation between far larger ΔE_rep and ΔE_attr terms, in sharp contrast to the much smaller ΔE_rep and ΔE_attr terms which are characteristic of many molecular rotation processes. By matching the sign of (ΔH)_zpe with that of ΔE_rep or ΔE_attr, as the case may be, the reactions are classified as attractive-dominant or repulsive-dominant (46 in the former class and 23 in the latter), a property which is independent of the direction in which the reaction is written. The sign and magnitude of ΔV_ee, ΔV_nn, and ΔV_en and reaction category are discussed in relation to the various kinds of structural change involved in going from reactants to products. For the vast majority of reactions, the numerical relationship ΔV_ee ≈ ΔV_nn has been found to hold to within a few percent.     

A molecular orbital study of the cis-trans energy difference in glyoxal

The barrier to internal rotation around the C-C bond in glyoxal, and the energy difference between the cis and the trans form of the molecule have been estimated using (7,$\text{3}\text{4}$) and (9,$\text{5}\text{4}$) Gaussian basis sets including polarization functions. The predicted energy of the cis form relative the ground state trans form was found to be substantially higher (4.9–6.3 kcal mol^?1) than the experimental value of 3.2 kcal mol^?1.     

Synthesis and structure of some cis-and trans-myrtanylstannanes

Enantiomerically pure cis- and trans-myrtanylstannanes cis-MyrSnPh₃ (1), trans-MyrSnPh₃ (2), cis-MyrSnPh₂Cl (3), trans-MyrSnPh₂Cl (4), cis-MyrSnPhCl₂ (5), trans-MyrSnPhCl₂ (6), cis-MyrSnCl₃ (7), trans-MyrSnCl₃ (8) were synthesized and fully characterized by ¹H, ¹³C and ¹¹⁹Sn NMR spectroscopy. The molecular structures of 1, 3, 6, 7, and [trans-MyrSn(OH)Cl₂ · H₂O]₂ (8a) a hydrolysis product of 8, were determined by X-ray crystallography.     

An ab initio study of the geometry and energy of the four planar conformers of glyoxylic acid and the glyoxylate ion

The structures and energies of the four planar conformers of glyoxylic acid and the glyoxylate ion have been studied ab initio using the unscaled 4—31G basis set with full geometry optimization. Changes in the CO, OH and CO bond lengths in the conversion of the cc conformer into the ct and tt conformers, and into the tc conformer, are consistent with the formation of four-membered and five-membered hydrogen-bonded ring structures, respectively. Changes in the distances between the nearest non-bonded atoms around each C atom reveal that the internal geometry of the CHO and COOH groups is significantly affected by cis—trans isomerization with respect to the OCCO backbone, and that the geometry of the CHO group is affected by proton dissociation from the COOH group. Furthermore, the movement of the component atoms in each functional group, characterized as clockwise or anticlockwise about the C atom, results in some cases in a rotation of the functional group as a whole. Whereas experiment shows the tc conformer to be more stable than the tt by 1.2 ± 0.5 kcal mol^?1, the calculations find the tt conformer to be the most stable, separated in energy from the ct, tc and cc conformers by 0.4, 1.4 and 10.7 kcal mol^?1, respectively. Augmentation of the 4—31G basis set in several forms, and use of (9,5/4) and (9,5/4,1) basis sets, only served to decrease slightly the tt/tc energy difference, not change the sign. The calculated proton affinity of the glyoxylate ion with respect to the tt conformer is 342.7 kcal mol^?1, compared to 357.7 kcal mol^?1 for the formate ion. The expectation energy differences Δ V_nn, Δ V_ee and Δ V_en for the cis—trans isomerization of the ct and cc conformers are opposite in sign to those for the glyoxal reaction, and in magnitude they all far exceed the ΔE_T values, which shows that hydroxyl group substitution has a much greater influence than a comparison of only the ΔE_T values would suggest.     

Palladium-catalyzed reduction of alkynes employing HSiEt3: stereoselective synthesis of trans- and cis-alkenes

The palladium-catalyzed semi-hydrogenation of alkynes to trans- or cis-alkenes employing HSiEt₃ as the reductant is developed. The CuSO₄ played a significant role for the trans/cis stereoselectivity. The labeling study revealed that the two olefinic hydrogen atoms came from HSiEt₃ and H₂O, respectively.     

Hydrogen bonding in cis and trans 1-t butylcyclohexan-4-ol

The homogeneous autoassociation of cis and trans 1-t-butylcyclohexan-4-ol has been investigated by means of IR spectroscopy. The temperature and concentration dependences of this interaction have been examined in CCl₄ as solvent. The results demonstrate a lower degree of association in the case of the cis compound, especially because of the steric hindrance of the acceptor function of this substance. The enthalpy of associatio-n-ΔH is found at about 13 [kJ/mole] for the trans compound and may be somewhat lower for the cis alcohol. The absorbance of the associated OH band of the trans compound is constant with varying concentration and/or temperature, but for the cis alcohol it increases with increasing concentration and decreasing temperature.     

Practical synthesis of four different pseudoenantiomeric organocatalysts with both cis- and trans-substituted 1,2-cis-cyclohexanediamine structures from a common intermediate

A new and convenient approach has been deviced for the practical synthesis of structurally robust, four different pseudoenantiomeric amino Tf-amido organocatalysts with the unique cis- and trans-substituted 1,2-cis-cyclohexanediamine structures. These pseudoenantiomeric organocatalysts are easily prepared by the Diels-Alder strategy of 2-phenyl-1,3-butadiene and maleic anhydride, and their chemical behavior was investigated by their application to asymmetric aldol synthesis for the practical synthesis of both enantiomeric aldols.     

Facile stereoselective synthesis of cis- and trans-3-alkoxyazetidin-2-ones

A highly stereoselective synthesis of cis- and trans-3-alkoxy-3-phenyl/benzylthioazetidin-2-ones is described. The reaction of α-chlorosulfide-β-lactams with various alcohols catalyzed by a Lewis acid such as ZnCl₂ in the presence of molecular sieves (3-4 Å) leads to cis-3-alkoxy-3-phenyl/benzylthio-β-lactams whereas treatment of potassium 2-alkoxy-2-phenylthioethanoate with appropriate Schiff's base using POCl₃ in the presence of triethylamine leads to the formation of trans-3-alkoxy-3-phenylthioazetidin-2-ones as major products.     

Structural analysis of regular copolymers of Butadiene and methylene with repeat unit [But-(CH2)n-But] by the use of model compounds—III. An analysis of the factors which influence the ratio of cis and trans 1,4 addition

In previous papers the 1,2 to total 1,4 ratios in copolymers of butadiene of repeat unit-[But-But-R]¹, where R is an alkyl ligand, have been determined by preparing model compounds HR-But-But-RH under identical conditions. Use of capillary column GLC enables the cis and trans isomers to be separated and the effect of varying experimental parameters on the cis/trans ratio is examined. This ratio is shown to be more sensitive to reaction temperature than the 1,2 to total 1,4 ratios previously studied. Results indicate that when R = (CH₂)_n the cis/trans ratio is independent of n for values of n > 1.     

Intramolecular noncovalent interactions: Bis(toluene)chromium(0) conformers

The structure of trans- and cis-conformers of bis(toluene)chromium(0) and the intramolecular interactions in them are studied by means of MP2 and density functional theory along with topological and NBO analyses. It was concluded for the first time that the locations of two hydrogen atoms of each methyl group between the phenyl ring planes in the most stable cis-conformer of bis(toluene)chromium(0) are largely determined by the stabilizing interactions of methyl C-H bonds and their hydrogen atoms with the metal atom and chromium-carbon bonds. It was concluded from the obtained data that the C-H...Cr contacts in the studied conformers should be considered as hydrogen bonds rather than agostic interactions. Contrary to the existing conception of the quantum theory of atoms in molecules, repulsive interaction is shown to occur between the pairs of hydrogen atoms of the two methyl groups in bis(toluene)chromium(0) cis-conformers rather than the stabilizing hydrogen-hydrogen interactions.     

Electrochemical studies and photoconversion of cis-and trans-6,6′-diethoxythioindigo

The reduction of cis- and trans-6,6′-diethoxythioindigo (DETI) at a platinum electrode in N,N-dimethylformamide (DMF) solution was studied by cyclic voltammetry and controlled potential coulometry. The cis-form, produced by photoisomerization of the trans-form, is reduced at less negative potentials than the trans-. The trans-isomer produces a fairly stable radical anion upon one-electron reduction; the cis-radical anion isomerizes rapidly to the trans-form and also dimerizes much more rapidly than the trans-isomer. A photogalvanic cell based on this system was devised but showed low efficiency for conversion of radiant energy to electricity.     

Decomposition of cis-and trans- potassium diaquobis(oxalato)chromate(III)

The decomposition of cis- and trans-K[Cr(C₂O₄)₂](H₂O)₂] has been studied using differential scanning calorimetry. Dehydration occurs as the first step with activation energies being 27.5 and 13.9 mol^?1, respectively, for the cis and trans complexes. After dehydration, continued heating results in loss of CO amd CO₂. For the trans complex, an additional endothermic peak is seen and the mass loss indicated that CO has been lost in a single step. In both cases, the final product indicated by mass loss data is KCrO₂.     

Ab initio studies of the 3nπ* states of glyoxal and methyl glyoxal

Results of ab initio calculations on the ³nπ* states of glyoxal and methyl glyoxal are reported. The symmetry unconstrained ΔE_SCF transition energies, unlike the symmetry constrained results, are found to be very similar. These calculations explain some anomalies which have appeared in the literature recently.     

Xenicane Diterpenes Revisited: Thermal (E)→(Z) isomerization and conformational motions. A unifying picture

Kinetic and equilibrium studies show that typical xenicanes such as dictyolactone ( 1 ) and 4-hydroxydictyolactone ( 3 ) undergo slow conformation medium-ring flipping between the predominant trans-( 1a or 3a ; Me(20) trans to H–C(3)) and the minor cis-conformers ( 1b or 3b ; Me(20) cis to H–C(3); see Scheme 1). The formation of the latter is inhibited in heterocyclic-ring-opened congeners such as 18-acetoxy-4-hydroxydictyo-19-al ( 7 ). Molecular-mechanics calculations suggest that typical-xenicane cis-conformers are disfavoured by mainly C(4)–C(5) torsional strain. This is confirmed by the observation of two sizably populated cis-and trans-conformers for the unnatural 4-oxoxenicanes 10–12 . Unusually facile thermal (E)→(Z) isomerization of xenicanes 1,3,10–12 , and 7 is also observed (→ 13–17 and 9 , resp.; Scheme 3), reflecting great strain relief in the transition state. Conflicting results in the literature now fit into this scheme which provides a basis for unravelling recognition phenomena with these biologically active systems.     

Synthesis of (Z)-alkene-containing cis-proline dipeptide mimetics using samarium(II) diiodide (SmI2)-mediated reductive alkylation reaction

The amide bond between an amino acid and proline can take the trans- or cis-conformation. The conformation influences both the structure and function of peptides and proteins. Therefore, constrained mimetic, which corresponds to Pro-dipeptides whose amide bond is replaced with an (E)- or (Z)-alkene, is a useful bioprobe for elucidating the structure-function relationship of peptides and proteins. Herein, we report the synthesis of cis-(Z)-alkene-containing Pro-dipeptide mimetics via a samarium(II) diiodide (SmI₂)-mediated reductive alkylation reaction.     

cis–trans Geometrical isomerization of bis(2-pyridinecarboxylato)nitrosylruthenium complex accompanying electrochemical one-electron reduction

cis-[Ru(NO)(CH₃CN)(pyca)₂] and trans-[Ru(NO)(OH)(pyca)₂] (pyca = 2-pyridinecarboxylato) were synthesized and characterized by X-ray crystallography. Electrochemical behaviors of cis-[Ru(NO)(CH₃CN)(pyca)₂] and cis-[Ru(NO)(CH₃O)(pyca)₂] in acetonitrile were studied. These complexes showed two reduction processes in CH₃CN. The controlled potential electrolyses of cis-[Ru(NO)(CH₃O)(pyca)₂] in a methanol–acetonitrile mixed solution were performed at the potential of the first reduction process. trans-[Ru(NO)(CH₃O)(pyca)₂] was isolated from the electrolyzed solution and characterized by IR and CV. The cis–trans geometrical change reaction occurred in the electrochemical one-electron reduction of cis-[Ru(NO)(CH₃O)(pyca)₂].     

PhP-PPh group bound to 1,8-positions of naphthalene: Preparation of cis isomer and synthesis of binuclear complex

A thermodynamically less stable cis isomer of 1,2-diphosphacycle was prepared from the corresponding trans isomer. Diphosphine, in which a PhP-PPh bond bridges the 1,8-positions of naphthalene, 1,2-diphenyl-1,2-dihydronaphtho[1,8-cd][1,2]diphosphole (1), was first prepared according to a previously reported method, and the trans isomer of 1 was irradiated in tetrahydrofuran with UV-vis light to reach equilibrium with cis-1 in a trans:cis ratio of 1:2. When a similar photochemical conversion was carried out using a saturated hexane solution of trans-1, cis-1 was precipitated in a good yield of 94%. The configuration of cis-1 was confirmed by X-ray analysis. Both cis- and trans-1 diphosphine ligands were used for the preparation of binuclear gold complexes. The crystal structure of (μ-cis-1)-[AuCl]₂ demonstrated that the two lone pairs of cis-1 are suitably directed for arrangement of the two gold centers in close proximity to each other. The two independent (μ-cis-1)-[AuCl]₂ molecules in the crystal were found to form a dimer through the multiple intermolecular interaction among the gold centers.     

Etude du mecanisme de photolyse des complexes cis- et trans-[PtCl2(C2H4)(4-CH3C5H4N)]. Influence de la concentration et de la longueur d'onde

Photodimerization, photoisomerization and photosubstitution quantum yields are measured for cis- and trans-[PtCl₂(C₂H₄)(4-CH₃C₅H₄N)], at various concentrations and wavelengths. Dissociation of the platinumethylene bond o?curs with a quantum yield nearly unity when the cis-compléx is irradiated in the charge transfer bands 5d → π^*(C₂H₄). Dissociation is also observed, but with a lower efficiency, at longer wavelengths. A cis → trans-photoisomerization reaction, probably via a low energy dd excited state is observed at 313,366 and 405 nm, with a constant quantum yield.     

Triazolopeptides: chirospecific synthesis and cis/trans prolyl ratios of structural isomers

As cis/trans prolyl isomerization plays a crucial role in various biological processes, peptide mimics capable of modifying the cis/trans Xaa-Pro ratio are of particular interest. A practical approach toward proline derived triazolopeptides employing [3+2] azide-alkyne cycloadditions as the key reaction step and the analysis of their cis/trans prolyl ratios are reported. Structural investigations indicated the adjustability of both the cis-percentage and the conformational stability toward intramolecular H-bonding effects.