Rotational isomeric state analysis

Polymers of + ) catechin and (-)-epicatechin have an intrinsic fluorescence because chromophores are an integral part of each monomer unit. The time-resolved emission from well-characterized dimers can be used to determine the relative populations of two rotational isomers at the interflavan bond between monomer units. When combined with the solid-state conformations, molecular mechanics calculations, and rotational isomeric state analysis, the interpretation of the time-resolved fluorescence leads to the unperturbed dimensions of the polymers. Significant population of both rotational isomers causes the chains to have unperturbed dimensions comparable with those in atactic polystyrene molecules of the same molecular weight. 

Determination of the unperturbed dimensions of the polymeric pro-cyanidins presents special challenges. Well-characterized samples of high molecular weight are not readily available, and the solubility characteristics of the high polymers are incompatible with the application of the classical techniques for the measurement of the dimensions of a macromolecule. An alternative route to the unperturbed dimensions exploits structural determinations in the solid state (8, 9), spectroscopic studies of well-characterized oligomers in dilute solution (iO, ii), molecular mechanics (MM2 software) (i2) calculations (i3, 14), and rotational isomeric state analysis (15-17) to provide a realistic description of the dimensions of the high polymers. A vital piece of information comes from the time-resolved fluorescence of the monomers and oligomers of well-defined covalent structure. The fluorescence measurements also show promise for the characterization of the com-... 

The description of the emission in the constrained dimer by equation 1, and the successful correlation of the populations deduced for the peracetylated dimer from 400-MHz proton NMR, with the preexponential factors in equation 2 show that the heterogeneity of the emission in the unconstrained dimers arises from the population of two rotational isomers at the interflavan bond. The populations of these rotational isomers in the free phenol forms can be deduced from the preexponential factors on the third and fourth lines of Table 1. This assignment of the populations provides the necessary ingredient for a rotational isomeric state analysis of the unperturbed dimensions of the polymers. 

Rotational Isomeric State Analysis of the Unperturbed Dimensions... 

Abe, A., and Furuya, H., Molecular ordering in liquid crystals carrying flexible hydrocarbon tails a rotational isomeric state analysis of D-NMR data of Emsley et al. on 4-n-aIkyW -cyanobiphenyls (nCB), Mol. Cryst. Liq. Cryst., 159, 99-114 (1988a). 

Sasanuma, Y, and Abe, A., Conformational anisotropy of n-alkanes dissolved in a nematic solvent a rotational isomeric state analysis of proton-proton dipolar coupling and deuterium quadrupolar splitting data, Polym. J., 23, 117-125 (1991). 

Abe A, Furuya H. Orientational characteristics of main-chain polymer liquid crystals as revealed by the rotational isomeric state analysis of deuterium NMR spectra. Macromolecules 1989 22 2982-7. 

In a typical analysis of a polymer chain, the experimental values of configuration-dependent properties and their temperature coefficients are compared with the results of rotational isomeric state calculations. These comparisons yield values of the energies for the various rotational states about the backbone bonds, and these conformational preferences can then be used to predict other configuration-dependent properties of the chains. It is also possible to obtain such conformational information from potential energy calculations, using the methods of molecular mechanics.39,46 52... 

R. A. Scott and H. A. Scheraga,/. Chem. Phys., 44, 3054 (1966). Conformational Analysis of Macromolecules. II. The Rotational Isomeric States of the Normal Hydrocarbons. 

Relationship to Electronic Properties. As a result of the close connection between bond conformation and electronic properties (4), the analysis of chain conformation in the polysilylenes has been of interest to researchers in this field, both from the experimental and theoretical viewpoints. As reported by Trefonas et al. (5), most asymmetrically substituted alkyl polysilylenes in solution at room temperature display an electronic absorption with ranging from 303 to 309 nm. The variable-temperature absorption spectrum of PMHS is shown in Figure 4 (4). At room temperature, max is 308 nm, and as the solution is cooled, there is a continuous red shift with the X x reaching 328 nm at -95 °C. Some workers 4, 6) suggest that this observation is a reflection of an increasing population of trans rotational states in the silicon backbone as the temperature is lowered. This suggestion is supported by the finding that these spectra can be adequately modeled by a rotational isomeric-state treatment (4). 

T. P. Straatsma and J. A. McCammon, J. Chem. Phys., 90, 3300 (1989). Treatment of Rotational Isomers in Free Energy Evaluations. Analysis of the Evaluation of Free Energy Differences by Molecular Dynamics Simulations of Systems with Rotational Isomeric States. 

R 9 Y. Sasanuma, Intramolecular Interactions of Polyethers and Polysulfides, Investigated by NMR, Ab Initio Molecular Orbital Calculations, and Rotational Isomeric State Scheme An Advanced Analysis of NMR Data , p. 213 Vol. 50, 2003... 

Sasanuma, Y., Conformational analysis of chain molecules in liquid crystalline phases by a rotational isomeric state scheme with maximum entropy method I. H— H dipolar couplings from n-alkanes dissolved in a nematic solvent, Polym. J., 32, 883-889 (2000a). 

Sasanuma, Y., H. Kato, and A. Kaito. 2003. Conformational analysis of poly(di- -butylsilane), poly(di-M-hexylsilane), and poly(methyl- -propylsilane) by a rotational isomeric state scheme with molecular dynamics simulations. J Phys Chem B 107 11852. 

Atactic poly(methyl methacrylate)-Q FTIR spectroscopic analysis of the conformational energy differences between rotational isomeric states is presented. [32]... 

The potential use of solid-state CPMAS NMR to detail the molecular structure of PP is demonstrated by many authors and is nicely reviewed in a recent paper by ToneUi [3]. The key parameter in such an analysis is the -y-gauche effect which enables the NMR chemical shifts of vinyl polymers to be assigned and provides an opportunity to test or derive rotational isomeric state (RIS) model descriptions of their conformational characteristics. 

Based on relative intensity and, particularly, bandshape of skeletal bands, it has become possible to obtain the relative energy difference between various rotational isomeric states. In the first study of this type, Snyder and co-workers analyzed low frequency (0-600 cm ) Raman spectra of re-alkanes in the liquid state (35). The method was subsequently applied to analysis of the low frequency Raman spectrum of molten state isotactic polypropylene. The vibrational spectroscopic analysis was successfully used to differentiate the correct model governing the chain (80) and has been extended to analysis of higher frequency vibrations (0-1500 cm" ) of liquid re-alkanes (36). 

Conformational Analysis.— The major effort in this area has been directed towards empirical correlations of chemical shift and conformation. Tonelli and co-workers have examined the stereosequence dependence of C chemical shifts in polypropylene (PP) poly(vinyl chloride) (PVC), and polystyrene (PS) using the rotational isomeric-state scheme of polymer statistical mechanics. The structural variation of C shifts was shown to correlate well with the occurrence of three-bond gauche (y) interactions between carbon atoms together with y interactions between carbon and chlorine atoms in PVC and ring-current shifts in PS. It has been... 

In the following an outHne of the model and some of the assumptions used in the analysis are described [129]. The schematic sketch of the model is given in Fig. 25. In the model, conformational transitions are represented by a jump motion from a conformation (a rotational isomeric state) to another. In each rota-... 

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