The fluorine-NHC gauche effect: a structural and computational study

Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of these species is a flanking fluoroethyl group, which is either freely rotating or embedded within a bicyclic framework. Solid state analysis confirmed a gauche conformational preference in all cases with the fluorine adopting a syn clinal arrangement (?_[NCCF]～60°) with respect to the triazolium nitrogen at the vicinal position of the NHC. A density functional theory analysis was employed to quantify these effects and evaluate the influence of electronic modulation of the carbenic carbon [(CN⁺); neutral carbene (C:); metal-bound carbene (CM)], on the relative gauche/anti preference, thus highlighting the potential of this conformational phenomenon as a useful molecular design strategy for controlling the topology of organometallic complexes.     

Fluorinated organocatalysts for the enantioselective epoxidation of enals: molecular preorganisation by the fluorine-iminium ion gauche effect

The fluorine-iminium ion gauche effect is triggered upon union of a secondary β-fluoroamine and an α,β-unsaturated aldehyde, providing a useful strategy for controlling the molecular topology of intermediates that are central to organocatalytic processes. The β-fluoroamine (S)-2-(fluorodiphenylmethyl)pyrrolidine (1) is an effective catalyst for the enantioselective epoxidation of α,β-unsaturated aldehydes. A process of structural editing has revealed that the efficiency of this catalyst is due to the (fluorodiphenyl)methyl group when it is embedded in a β-fluoroiminium motif. Epoxidations of challenging cyclic α,β-disubstituted, β,β-disubstituted and α,β,β-trisubstituted enals catalysed by 1 proceed with excellent levels of enantiocontrol (up to 98?%?ee).     

The gauche effect : On the nature of the interaction between electronegative substituents in trans-1,2-disubstituted cyclohexanes

The positions of the aa/ee conformational equilibria of a series of trans-1,2-disubstituted cyclohexanes containing etectronegative substituents have been determined experimentally by NMR, and calculated theoretically by classical means. It is found that the classical analysis of the results, which takes into account explicitly steric and electrostatic interactions between gauche substituents, is not adequate, because residual attraction remains between the substituents.     

The effect of shear flow on the conformation of polyelectrolyte brushes

The behavior of flat polyelectrolyte brushes under the action of a lateral force or flow was studied. Special attention was focused on the case when a lateral force acts on a brush that occurs near the point of phase transition from the swollen state to the collapsed state. The difference between phase transitions in a brush induced by isotropic and anisotropic interactions is analyzed. As examples of such transitions, the collapse of a polyelectrolyte brush upon cooling and the nematic collapse of an anisotropic brush are considered. It was shown that lateral force (flow), exerting a marked effect on the nematic collapse of an anisotropic brush, has no practical effect on the collapse of a brush with isotropic interactions.     

Hydrogen bond effect of azido polyurethane elastomer by dynamic mechanical analysis

Hydrogen bond effects in azido polyurethane elastomers (APUE) have been studied by dynamic mechanical analysis (DMA) and the results show that the hydrogen bond effect has stronger temperature dependence. The activation energy of hydrogen bond dissociation (E_a) and the hydrogen bond density (vs/V) have been evaluated from the elastic modulus–temperature relationship. The calculated E_a in this work is much higher than the reported values of normal polyurethane elastomer (PUE). The values of E_a are 81.3, 68.1, 53.3, and 42.3 kJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐1 (CPPB/HDI trimer elastomer); 94.6, 75.8, 48.4, and 36.9 kJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐2 (APPB/HDI trimer elastomer); 82.1, 74.4, 59.8, and 46.5 kJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐3 (APPB/HDI trimer/EG elastomer); 145, 124, 88.0, and 75.5 kJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐4 (APPB/HDI trimer/BD elastomer); and 72.2, 64.3, 49.8, and 39.9 KJ/mol at 150, 110, 50, and 20 Hz, respectively, for PUE‐5 (APPB/HDI trimer/HD elastomer). The DMA estimations are semiquantitative for it ignores other physical crosslinking effects and the results give relative order of vs/V and E_a. The values of vs/V of crosslinked APUE (PUE‐3, PUE‐4, and PUE‐5) are much higher than PUE‐2. The test frequency could affect the values of vs/V and higher frequency would minify the difference of the values of vs/V for two given temperatures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2841–2851, 2006     

Theoretical study and X-ray determination of bianthronyl [corrected] : long C-C bond length and preferred gauche conformation

Five X-ray crystallographic structures of bianthronyl (1, 2b-5b) revealed the presence of long central C(sp3)-C(sp3) bonds (> or =1.60 A) and mostly gauche conformation. DFT calculations were carried out to study the structural features of bianthronyl and related model molecules [corrected]. Our B3LYP/6-31G gas-phase calculation results suggest that steric effect plays a role in causing the long C-C sigma bond length (>1.60 A), nonplanarity of the anthracenone moiety (>20 degrees ), and preferred gauche conformation. However, when the gauche preference is weak, environmental effects such as solvation and packing forces can reverse it. As the degree of nonplanarity of the anthracenone moiety is consistently larger in the anti conformation than in the gauche and the ease of achieving nonplanarity is determined by flexibility of the central six-membered rings directly connected to the C(sp3)-C(sp3) bond, one way to control the structure and anti-gauche energy difference of closely related compounds is by changing the clamping group (Z) in the central six-membered ring.     

Discovery of the gauche form in dichloroethane

Our study of internal rotation was carried out under the guidance of Professor San—ichiro Mizushima. Dipole moment measurements of 1,2-dichloroethane in non-polar solvents revealed a temperature dependence of the moment, which was interpreted as an indication of the existence of hindered rotation about the central CC bond, contrary to the original assumption of free rotation by van't Hoff and Le Bel. Raman Spectroscopy confirmed that the solid phase consists of the trans form only, whereas in the liquid and in the gas phases a second stable isomer coexists with the trans form. This second form was called > gauche< by Mizushima and its geometrical structure was established by electron diffraction. The potential function for internal rotation is considered with regard to its experimental determination and its interpretation by ab initio calculations.     

The effect of tethered polymers on the conformation of a lipid membrane

We consider the effect of tethered macromolecules on the stability of the flat conformation of a lipid bilayer in a dense brush regime when anchored chains are significantly overlapped. On the basis of a scaling approach we have shown that redistribution of polymer chains from inner to outer layers reduces the free energy of the system and stabilizes spherically bent configurations of the membrane.     

The effect of the physical state on the equilibrium conformation of 2-arylpyrroles

The equilibrium torsion angles of 2-arylpyrroles in the liquid and solid phases were estimated by UV spectroscopy. In solution, compounds containing no substituents in positions 1, 3, and 2′ possess an average torsion angle of 24°, those containing one substituent have an angle of 29°, and in the case of two and three substituents, the angles are 53° and 65°, respectively. Phase transitions lead to flattening of the molecules in almost all cases. The average torsion angles in the compounds with no substituents in positions 1, 3, and 2′ decrease by ≈5° on passing from the gas to the liquid state and by ≈25° on going from the liquid to the solid state. The geometric parameters of 2-arylpyrroles with one or two substituents in positions 1, 3, and 2′ are less sensitive to phase transitions, while trisubstituted derivatives even retain their equilibrium conformations upon phase transitions. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1472–1479, August, 1997.     

Evidence for a gauche minor conformer of 1,3-butadiene

We present experimental Raman data on torsional transitions that prove that the less stable conformer of 1,3-butadiene has a gauche structure. It is shown that in order to predict the relative intensities of the “gauche” transitions with respect to the “trans” transitions, one has to include a second-order term in the Fourier expansion of the molecular polarizability.     

The effect of the hydrogen bonds on the molecule conformation in 2-biphenylmethanol

Infrared spectra of hydrogen-bonded organic compound 2-biphenylmethanol has been studied in the wide temperature range from 320 to 12 K. It has been found that IR spectra drastically changed with temperature in the 700–850, 1290–1500, and 3100–3600 cm⁻¹ spectral regions, where deformational vibrations of phenyl rings and methanol group, as well as ν(OH) stretching vibration manifest themselves. The analysis of the computer simulation of the IR spectra for various 2-biphenylmethanol conformers, which are characterized by different combinations of angles of the mutual orientation of the phenyl rings relative to each other and a methanol group relative to the phenyl ring allowed us to conclude that the hydrogen bonding results not only in the shortened of the intermolecular bonds, but also in the increasing of the angle between phenyl ring and methanol group. In other words, hydrogen bonding leads to the changes in the molecule conformation.     

Rearrangement energies for radicals of azido nitroaromatic compounds

Dissociation energies of C–N₃ bonds have been determined on the basis of data on the enthalpies of formation for a series of azido nitroaromatic compounds and the enthalpies of formation of radicals. Using fundamental relationships of chemical physics, a procedure has been suggested to calculate the energy of rearrangement of molecule fragments into radicals on the basis of special properties of rearrangement energy and the sums of average thermochemical energies for bonds comprising radical fragment in molecule. This calculation procedure provided a possibility to determine the energy of the N₃ moiety transformation into N₃ radical and the rearrangement energies of nitroaromatic radicals.     

CuCl2对聚乙二醇（PEG）的构象和结晶的影响

用傅里叶变换红外光谱法(FT-IR)、差示扫描量热法(DSC)、广角 X射线衍射法(WAXD)研究了少量过渡金属盐氯化铜(CuCl2)对聚乙二醇(PEG)构象和结晶行为的影响. Cu2＋与PEG分子链中的氧原子有配位键生成, 通过溶液电导率的测定, 发现一个Cu2＋大约与3个PEG链节单元形成配位结构, 从而使PEG的构象和结晶行为发生变化, 随着体系中CuCl2含量的增加, PEG分子链中OC—CO旁式构象相对反式构象含量逐渐增加, 在CuCl2含量(w)为7%左右时达到极大值, 在CuCl2含量(w)为9%时, 反而是反式构象相对旁式构象含量多些. 混合物中PEG的结晶度随着体系中CuCl2含量的增加逐渐降低, 含量(w)为9%时, 又有所提高.     

Searching the "biologically relevant"conformation of dopamine: a computational approach

We report here an exhaustive and complete conformational study on the conformational potential energy hypersurface (PEHS) of dopamine (DA) interacting with the dopamine D2 receptor (D2-DR). A reduced 3D model for the binding pocket of the human D2-DR was constructed on the basis of the theoretical model structure of bacteriorhodopsin. In our reduced model system, only 13 amino acids were included to perform the quantum mechanics calculations. To obtain the different complexes of DA/D2-DR, we combined semiempirical (PM6), DFT (B3LYP/6-31G(d)), and QTAIM calculations. The molecular flexibility of DA interacting with the D2-DR was evaluated from potential energy surfaces and potential energy curves. A comparative study between the molecular flexibility of DA in the gas phase and at D2-DR was carried out. In addition, several molecular dynamics simulations were carried out to evaluate the molecular flexibility of the different complexes obtained. Our results allow us to postulate the complexes of type A as the "biologically relevant conformations" of DA. In addition, the theoretical calculations reported here suggested that a mechanistic stepwise process takes place for DA in which the protonated nitrogen group (in any conformation) acts as the anchoring portion, and this process is followed by a rapid rearrangement of the conformation allowing the interaction of the catecholic OH groups.     

Salt effect on cationic polyacrylamide conformation on mica studied by single molecule "pulling" with scanning probe microscopy

The effect of salts on adsorbed polyelectrolyte conformations has been studied extensively over the past three decades. Previous researchers have proposed that increasing salt concentration results in larger loops and tails for weak polyelectrolytes adsorbed on a surface. However, no experimental verification of this theory has been published. In this work, we present experimental verification acquired by "pulling" single molecules of a polyelectrolyte from a mica surface using a scanning probe technique. We also present a new method for analyzing changes in adsorbed polymer tail lengths. We demonstrate that increasing solution salt concentration correlates with both loop and tail lengths of an adsorbed low charge density cationic polyacrylamide on a mica substrate.     

The use of aryl azido ATP analogs as photoaffinity labels for myosin ATPase.

The synthesis of ATP analogs containing a photoactive aryl azido grouping coupled to the 3' hydroxyl of ATP is described. The potential effectiveness of these analogs in the investigation of nucleotide-binding regions is outlined and this effectiveness demonstrated by their photodependent inhibition of subfragment 1 ATPase. The use of 14C-labeled azido ATP demonstrates an almost stoichiometric covalent binding of the analog. Because of their potential application to other systems, a number of reactions describing the reactivity of the 3' hydroxyl of the nucleotide ribose are outlined in an Appendix.     

The effect of 2'-fluorine substitutions on DNA i-motif conformation and stability

(1)H and (19)F NMR, and UV thermal melting studies have established that the stability of d(TCCCCC) is enhanced by the inclusion of a single 2'-fluorine-modified deoxycytidine residue; the results support the notion of the importance of sugar-sugar contacts in stabilising i-motifs in general and reveal that solvation is the cause of the instability of RNA equivalents.     

A theoretical study of the effect of copper(I) chloride on the azido/tetrazole isomerism

B3LYP/6-31G(d), B3LYP/6-311++G(d,p), and CBS-QB3 calculations on the effect of CuCl on the azido/tetrazole isomerism have been performed. The cases of 2-azidopyridine and 2-azido-1H-imidazole have been selected as examples of heteroaromatic six- and five-membered rings. All minima and transition states have been characterized.