Electronic conformational dependence

JB Koerner, T Ichiye. Conformational dependence of the electronic properties of [Ee(SCH3)4] - 2-. J Phys Chem B 101 3633-3643, 1997. 

The effectiveness of equatorial vs. axial bystanders at promoting the 1,2-H shift (MeO > Me > Ph) may be related to their ability to stabilize the partial positive charge that arises at the migration origin during the 1,2-H shift. In this scenario, the lone pairs of the MeO group are superior to the hyperconjugative and inductive properties of Me, whereas the conformationally dependent h-electron release of Ph is the least effective. 

The Fe-protein has the protein fold and nucleotide-binding domain of the G-protein family of nucleotide-dependent switch proteins, which are able to change their conformation dependent on whether a nucleoside diphosphate (such as GDP or ADP) is bound instead of the corresponding triphosphate (GTP or ATP). However, nucleotide analogues, which induce the conformational switch of the Fe-protein, do not allow substrate reduction by the MoFe-protein, nor does reduction of the MoFe-protein by other electron-transfer reagents (whether small proteins or redox dyes) drive substrate reduction. Only the Fe-protein can reduce the MoFe-protein to a level that allows it to reduce substrates such as... 

The foregoing examples of differential reactivities of rotamers may be summarized by saying that the reactivity is controlled by the steric factor. The difference in the reactivities of rotamers of 9-(2-bromomethyl-6-methyl-phenyl)fluorene (56) in SN2 type reactions falls in the same category (176). However, the substituent effect is not limited to a steric one there can be conformation-dependent electronic effects of substituents as well. A pertinent example is found in the reactivity of the bromomethyl compound (56) when the rotamers are heated in a trifluoroacetic acid solution (Scheme 10). The ap form gives rise to a cyclized product, whereas the sp form remains intact (176). The former must be reacting by participation of the it system of the fluorene ring. 

The two representations shown here are actually two different conformers of ethane there will be an infinite number of such conformers, depending upon the amount of rotation about the C-C bond. Although there is fairly free rotation about this bond, there does exist a small energy barrier to rotation of about 12kJmol due to repulsion of the electrons in the C-H bonds. By inspecting the Newman projections, it can be predicted that this repulsion will be a minimum when the C-H bonds are positioned as far away from each other... 

An empirical increment system permits prediction of charge distribution in a,/ -unsaturated carbonyl compounds, assuming additivity of electronic effects and neglecting the conformational dependence of carbon-13 chemical shifts [290]. Moreover, carbonyl and alkenyl carbon shifts of a, /3-unsaturatcd ketones may be used to differentiate between planar and twisted conjugated systems, as shown in Table 4.29 [291] and outlined for phenones in Section 3.1.3.8. 

In the above example, we have not given attention to the shieldings of the carbon atoms directly participating in the twisting bond C6-C7. To understand the conformation dependence of the C6 and C7 shieldings, a special treatment is required. For simplicity, we consider a four 71-electron system such as HEX. It is now assumed that the x, y and z axes coincide with the direction of the principal axes for o33, a22 and on, respectively. Then, the rotation of the C2-C3 bond modifies the pz orbitals of these carbons, resulting in a distortion of the 7C-orbitals, which is represented by the mixing of the pz orbital with px y orbital defined as... 

Contrary to the experimental electron density, the electrostatic potential is conformation dependent. Figure 18 shows the ORTEP view of /-butylCOproline-histidine-methylamide (tbuCOprohisNHMe) [55] which exhibit a folded conformation due to an intramolecular hydrogen bond (P-tum) between 0 and N3H (N3—O, = 2.935 A) as a consequence, hydrogen bond occurs between the histidine N4 and the N2 hydrogen (N4—N2 = 3.205 A). The effect of the pturn on the electron density has been discussed in refs. 28 and 64. 

Rigid Molecule Group theory will be given in the main part of this paper. For example, synunetry adapted potential energy function for internal molecular large amplitude motions will be deduced. Symmetry eigenvectors which factorize the Hamiltonian matrix in boxes will be derived. In the last section, applications to problems of physical interest will be forwarded. For example, conformational dependencies of molecular parameters as a function of temperature will be determined. Selection rules, as wdl as, torsional far infrared spectrum band structure calculations will be predicted. Finally, the torsional band structures of electronic spectra of flexible molecules will be presented. 

Exciton theory deals with the theory of electronic excitation processes in ordered arrays of absorbers (Kasha, 1959). Applications of exciton theory to protein problems have not been numerous, but some important results, especially on the conformation-dependence of the peptide absorption around 2000 A, will be discussed in the section on peptide-bond absorption. 

The fundamental questions are What is the microscopic mechanism or driving force for the transition, and what physical factors are important Two distinct possibilities have been advanced side-chain crystallization (5, 6, 17-19), which is postulated to induce polymer backbone ordering, and conformation-dependent polymer-solvent interactions that arise explicitly from electron delocalization and that stabilize an ordered rodlike conformation (20-24). Side-chain crystallization remains a qualitative suggestion that has not been developed to the point where it has predictive power and can be critically tested. However, in the solid state, the enhanced importance of packing effects makes such a mechanism more plausible (18, 19). 

In this chapter, the theory of conformation-dependent polymer-solvent interactions, which was developed in detail by Schweizer (20-22) for soluble TT-conjugated polymers, will be used to explain both qualitatively and quantitatively a large body of observations on the polysilylenes (23, 24). The same theory has been used recently to interpret qualitatively order-disorder phenomena and the electronic thermochromism of TT-conjugated-polymer solutions and films (25, 26). The study presented in this chapter represents part of an ongoing effort to understand in a unified fashion both the optical properties (27-30) and order-disorder transitions (20-24) of flexible, conjugated-polymer solutions. 

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