Trans states

The columns are indexed by the three states of bond i, the rows are indexed by the three states of bond i—1, and the order of indexing is t, g+, g. The statistical weight denoted by cr, which is approximately exp (-250/T), is the first-order weight of a gauche state relative to a trans state. The second-order interaction, which arises from the pentane effect where a bond pair is g gT, is weighted by to, which is approximately exp (-1000/T). 

Figure 2. To the left, quasi-diabatic potential energy surfaces in the B3LYP/cc-pvtz Dunning s basis set. AA represents a cis state (solid line) BB a trans state (solid line) AB is the excited diradical state spin singlet (dashed line) Triplet is the diradical state S=1 (dotted line). To the right, extrapolated diabatic potential energy surfaces for the same states. The angle used to plot energy entries is a = 2 0. All calculations were done with Gaussian 98 [23].

The electron-phonon operator is a tensor product between the electronic dipole and the nuclear dipole operators. A mixing between the AA and BB via the singlet-spin diradical AB state is possible now. A linear superposition of identical vibration states in AA and BB is performed by the excited state diradical AB. If the system started at cis state, after coupling may decohere by emission of a vibration photon in the trans state furthermore, relaxation to the trans... 

Stress-temperature coefficients are determined for cross-linked networks of PE and polyisobutylene elongated in the amorphous state. Interpretation of the indicated temperature coefficient of 0 for PE according to the three-fold potential model for rotation around the C—C bonds is consistent with an energy difference of 2.1 kJ mol-1 between gauche and trans states. The small temperature coefficient for isobutylene is due to steric interactions affecting bond rotations. 

Owing to the inequality of the bond angles, a decrease in the trans population increases 0lnl2. Hence, the positive temperature coefficient for this ratio denotes a lower energy for the trans state. This energy is attributed to favorable interaction between CH3 pairs separated by 380 pm in the planar (trans) conformation. The larger value observed for the ratio in a less polar medium is in the direction predicted for enhanced electrostatic interaction within a chain of partially ionic Si—0 bonds. 

The conformational energies derived from gas phase NMR of 1,2-dimethoxyethane can be most reliably compared with the results of theoretical calculations where a single molecule is treated. Adoption of the Jt and Jg values obtained leads to the energy difference Eio) = - 1.26 kJ mol-1 between the gauche and the trans state. This result is consistent with E(c) < 2.1 kJ mol-1 obtained from measurements in cyclohexane. 

In this study, the dipole moments at 298 K and the corresponding temperature coefficient of PDTC are calculated in the RIS approximation. The results are compared to the values determined by experiment. The results indicate that an extra stabilization energy of about 3.8 kj mol-1 must be added to these gauche states relative to trans state. This provides evidence of a large attractive sulfur gauche effect in the polysulfides, about equal in magnitude to that found for the analogous POM. 

Values of the mean-square dipole moment of PNA are determined from measurements of dielectric constants and refractive indices of the polymer in benzene. The dipole moment ratio and the temperature coefficient of both the dipole moment and the unperturbed dimensions are critically interpreted using the RIS model. Good agreement between theory and experiment is obtained by assuming that the gauche states about C(CH3)2— CH2 bonds have an energy 2.5 kJ mol-1 lower than the alternative trans states. 

All ester groups in the PET chain are assigned confidently to be planar trans. The restriction of bonds 1 and 3 to the trans states and bond 2 to a choice between cis and trans leads to the simple statistical weight matrices. The length of the span of the terephthaloyl residue in PET guarantees independence of the conformations of successive repeating units of the chain. 

E(y), governing the occurrence of cis versus trans states for the virtual C -C bond, was taken to be 0.210 kJ mol-1 i.e., the trans orientation of the two ester groups is slightly preferred owing to the dipole-dipole interactions. 

A realistic RI5 model is used to estimate the relative probabilities of the formation of various types of short branches in ethylene-vinyl acetate copolymers that are rich in ethylene. Butyl is predicted to be the most common short branch in all of the copolymers examined, although it Is less common in the copolymers than in low-density PE. The major factor responsible for the suppression of the Rq4 backbiting intrachain radical transfer is the increased preference for trans states at the main chain bonds flanking the attachment site for an Isolated acetoxy side chain. 

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