Torsional processes

Dielectric absorption on furan-2-carboxaldehyde has been measured (78JCS(F2)727 81ZPC147). Even in a polymer matrix, the energy barrier obtained for internal rotation is close to that determined with other experimental techniques, suggesting a low influence of the surrounding medium on the torsional process. 

Isomerization around a double bond may be regarded as a torsional process involving rotation around the bond axis in this system. Activation energies for thermal isomerization of alkenes are 104-270 kJ mol-1 55,56. The delocalization of the 71-electrons of the double bond in the ground state of the molecule is expected to lower the activation energy for this process. In enamines, a simple resonance structure provides one way for a qualitative description of such a situation. 

The activation energy for a torsional process around a covalent bond depends, among other factors, on the 71-electron density associated with this bond. It is of interest to investigate whether the energy barrier for rotation around a carbon-carbon double bond can be sufficiently reduced to allow the establishment of the dynamic equilibrium between the isomers 26 and 28 in the ground state of the system. The complete equilibrium must also include the transformations 26 29 and 28 30, which are associated with restricted rotation around the C—N bond (Scheme 1). 

Discussion of the physicochemical sizes of substituents on the basis of conformational analysis of the torsional process of substituted cyclohexanes, cyclophanes, and diaryl derivatives have been summarized by Schlosser [5]. The energies required for the conformational exchange of an equatorial position to the axial position for each substituent in cyclohexane are listed in Table 1.47 [6]. For fluorine, the energy difference between the axial and equatorial environment is negligibly small as compared with methyl, while that of CF3 seems larger than that of z-Pr so far observed in the cyclohexane torsional system. From the... 

For conformational changes involving torsions, the rotational barriers can be determined in the following way. A potential-energy function, for example in the form of a truncated Fourier series (18), where the are unknown parameters, is used in the Schrodinger equation for the torsional process ... 

Szymczak JJ, Barbatti M, Lischka H (2009) Is the photoinduced isomerization in retinal 4 protonated Schiffbases asingle- or double-torsional process J Phys Chem A 113 11907... 

Szymczak, J. J., Barbatti, M., Lischka, H. (2009). Is the photoinduced isomerization in retinal protonated schiff bases a single- or double-torsional process The Journal of Physical Chemistry A, 113(43), 11907-11918. doi 10.1021/jp903329j. 

Ab initio SCF MO calculations and frontier molecular orbital analysis (FMO) reveal a preference for an inversion mechanism for the topomerization of the cation over a torsional process. The calculations gave for planar inversion (i.e. changes from trans position to cis position towards without rotation around the N-N bond) a considerably lower barrier (123 kJ/mol) than for rotation around the N-N bond (309.8 kJ/mol) [16, 23] for earlier studies, see [24, 25]. 

The first study in which a full CASSCE treatment was used for the non-adiabatic dynamics of a polyatomic system was a study on a model of the retinal chromophore [86]. The cis-trans photoisomerization of retinal is the primary event in vision, but despite much study the mechanism for this process is still unclear. The minimal model for retinal is l-cis-CjH NHj, which had been studied in an earlier quantum chemisti7 study [230]. There, it had been established that a conical intersection exists between the Si and So states with the cis-trans defining torsion angle at approximately a = 80° (cis is at 0°). Two... 

In an ambitious study, the AIMS method was used to calculate the absorption and resonance Raman spectra of ethylene [221]. In this, sets starting with 10 functions were calculated. To cope with the huge resources required for these calculations the code was parallelized. The spectra, obtained from the autocorrelation function, compare well with the experimental ones. It was also found that the non-adiabatic processes described above do not influence the spectra, as their profiles are formed in the time before the packet reaches the intersection, that is, the observed dynamic is dominated by the torsional motion. Calculations using the Condon approximation were also compared to calculations implicitly including the transition dipole, and little difference was seen. 

Dmparison of various methods for searching conformational space has been performed cycloheptadecane (C17H34) [Saunders et al. 1990]. The methods compared were the ematic search, random search (both Cartesian and torsional), distance geometry and ecular dynamics. The number of unique minimum energy conformations found with 1 method within 3 kcal/mol of the global minimum after 30 days of computer processing e determined (the study was performed in 1990 on what would now be considered a / slow computer). The results are shown in Table 9.1. 

Stretching, bond bending, torsions, electrostatic interactions, van der Waals forces, and hydrogen bonding. Force fields differ in the number of terms in the energy expression, the complexity of those terms, and the way in which the constants were obtained. Since electrons are not explicitly included, electronic processes cannot be modeled. 

Other Fiber Deformations. Deformations such as bending, torsion, shear, and compression are of practical importance in textile apphcations. Bending and twisting of yams, both influential in the development of bulk and stretch in filament yams, are also important in the production of staple yams. Bending characteristics are important in cmsh resistance in carpets. Bending and shear are factors that influence the hand and drape of apparel fabrics, whereas compression influences the recovery of fabrics after such processes as winding. 

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