Coupled oscillator models

Stereochemical information can clearly be derived from the analysis of VCD couplets using the coupled oscillator model. While the magnitude of m and P),... 

Studies of molecules containing pairs of OH or C=0 oscillators have provided the most direct application of the degenerate coupled oscillator model of VCD to determine molecular conformation in solution. 

The COSMO solvent model has been used to simulate the influence of water on the electronic spectrum of A -methylacetamide [81], and the results was compared with the results of molecular dynamics simulations (where the electronic spectrum were calculated as an average over 90 snapshots from MD simulations). Most of the hydration effects were found to come from the first solvation shell hydrogen-bonded water molecules, and the continuum model does not properly account for these effects. The rotatory strengths were not calculated directly in ref. [81]. However, the results were used to model ECD spectra of peptides via the coupled oscillator model, with satisfactory result. 

These spectral data were interpreted via a straightforward coupled oscillator model, and calculations were carried out to find a molecular conformation in < > and space around the center alanine residue which best reproduces the observed spectral data. The computational methods employed were discussed above (cf. Section 3) and detailed arguments are given in the original report [23]. A similar, yet not as detailed, effort had been published before for peptide models in non-aqueous solution [46]. 

The tripeptide L-Pro-L-Pro-L-Pro in aqueous solution was studied by Keiderling and coworkers [47], and exhibits VCD spectra similar to those observed for L-Ala-L-Ala-L-Ala, although the couplet is more negatively biased. Similarly, the protected (L-ProH shows a very similar VCD spectrum [39], No analysis of these results in terms of a solution conformation based on the coupled oscillator model was carried out. However, there is clear evidence that in aqueous and non-aqueous solutions, distinct solution conformation is reached at the level of the tripeptide, and persists similarly up... 

A theoretical explanation for the chirality induced upon formation of inclusion or intercalation complexes was given by Schipper and Rodger, who developed symmetry rules based on a coupled-oscillator model [26]. These workers were able to obtain a correlation of the sign of the induced CD with... 

Wilson CJ, Callaway JC (2000) Coupled oscillator model of the dopaminergic neuron of the substantia nigra. 

Still, in many cases photoderacemization of racemic mixtures has been successful. Irie et al. announced [54] the detection of the effect, but a detailed study of this system was published only as a patent [55] in CH2CI2 solutions of 1,l -binaphthyl they found values of [a]345 =-JX75-and 4- 0.7 for 1- and r-cpl with a 270-300 nm band of a 500 W Xe source at room temperature in ca. 5 min. Here two aromatic units are connected by a single bond. These systems are realizations of Kuhn s coupled oscillator model of optical activity [56]. The steric overlap provides chirality, ground state enantiomerization has AIv 92 kJ mol 1 [57], and the triplet state enantiomerizes fast, with 8 kJ mol 1. 

Coupled oscillator models are extensions to the simple models developed for electronic circular dichroism. They are well known under the name exciton theory (see e.g. Harada and Nakanishi, 1972). These models, extended to vibrational transitions, describe the coupling of pairs of electric dipole transition moments. They predict equal amounts of positive and negative VCD intensity ... 

G. Holzwarth and I. Chabay, Optical activity of vibrational transitions a coupled oscillator model, J. Chem. Phys., 57(4), 1632-1635 (1972). 

By resolution of both light-absorption and CD bands of ARN and FAD and by the use of both experimental and theoretical values for transition moments, conformations of the relevant molecules were deduced. However, because of difficulties in accurate assignment and determination of the various transition moments and because of the limitations inherent in calculations using the coupled oscillator model and dipole-dipole interaction potentials, only a qualitative picture of the conformations involved could be expected (12). Generally, reciprocal relations between known transitions were taken as evidence for juxtaposition of the corresponding chromophores (12). The optical data obtained for coenzymes or their model compounds may be utilized in favorable cases for estimating structural features in the bound state, in which the interactions with the protein environment will also have to be evaluated. 

Fig. 4.3. In the coupled-oscillator model of two atoms interacting with the dispersion force, a portion of the negative charge cloud in each atom is instantaneously displaced from the nucleus, to give the atoms instantaneous dipole moments of magnitudes

Holzwarth G, Chabay I (1972) Optical activity in molecular vibrations a coupled oscillator model. J Chem Phys 57 1632-1635... 

Raman spectra of oligomers, have been explained in terms of a coupled oscillator model (42,48). Detailed understanding of the vibrations found for oligomers yields considerable insight into vibrational spectra obtained for polymers. 

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