A linear correlation with Q is found for the shifts of protons or fluorines placed ortho or cis to the substituent in monosubstituted benzenes, ethylenes, propenes, monofluorobenzenes, and perfluorobenzenes. The substituent X corresponds to H, F, CI, Br, I, and Q equals P/Ir3 where P is the polarizability of the C-X bond, r is the C-X bond length, and I is the first ionization potential of atom X. T h e correlation is useful for predicting some as yet unlanown shifts in the above compounds. The significance of this correlation is discussed in an inconclusive manner.

The optimized g-tensor parameters for the oxidized form of Rhodobacter capsulatus cytochrome c2 in solution were obtained using a set (50) of backbone amide protons. Dipolar shifts for more than 500 individual protons of R.capsularus cytochrome c2 have been calculated by using the optimized g-tensor and the X-ray crystallographic coordinates of the reduced form of R. capsulatus cytochrome c2. The calculated results for dipolar shifts are compared with the observed paramagnetic shifts. The calculated and the observed data are in good agreement throughout the entire protein, but there are significant differences between calculated and experimental results localized to the regions in the immediate vicinity of the heme ligand and the region of the front crevice of the protein (residues 44-50, 53-57, and 61-68). The results not only indicate that the overall solution structures are very similar in both the reduced and oxidized states, but that these structures in solution are similar to the crystal structure. However, there are small structural changes near the heme and the rearrangement of certain residues that result in changes in their hydrogen bonding concomitant with the change in the oxidation states; this was also evident in the data for the NH exchange rate measurements for R. capsulatus cytochrome c2.

At temperatures below -40 "C the proton magnetic resonance of a,a,2,4,6-pentachlorotoluene in toluene-d8 or methylcyclohexane is an ABX spectrum where X is the methine proton. The C-H bond of the dichloromethyl group lies in the plane of the ring and its proton couples to one ring proton only. As the temperature increases the mean lifetime before interconversion of the "mirror image" isomers decreases and at 70 "C the proton spectrum is A2X. A density matrix formalism is used to calculate the line shape as a function of the lifetime before exchange. The free energy barrier to rotation of the dichloromethyl group is 15.0 + 0.1 kcal/mole in both solvents. In toluene-d8 the entropy of activation is near zero. The high energy transition state is very likely one in which the C-CI bonds of the ring and the dichloromethyl groups are eclipsed. Canadian Journal of Chemistry, 48, 1558 (1970) IntroductionExtensive nuclear magnetic resonance (n.m.r.) studies of the rotational isomers of halogenated ethanes are described in the recent literature (1-4). Reliable thermodynamic parameters for the isomers in their ground state as well as rotational barriers are only obtainable if the n.m.r. spectra of the individual isomers can be observed (1, 2). In common with the halogenated ethanes, toluene derivatives present the problem of rotation about an essentially single carbonxarbon bond. In toluene itself the sixfold barrier to rotation of the methyl group lies well below 200 cal/mole in the gaseous and solid states (5, 7) but it has become clear that in toluene derivatives containing halogen atoms both in the methyl groups and in the ortho ring positions (7),3 the barrier to rotation can be large enough for convenient study by dynamic n.m.r. (d.n.m.r.) methods (4).In this laboratory a series of such compounds are under investigation. Previous studies were confined to fluoromethyl toluene derivatives in which the n.m.r. spectra of individual rotamers were not observed (8). In this paper we describe the analytical techniques involved in our studies and report the barriers to rotation about the C-C bond of a,a,2,4,6-pentachlorotoluene (PCT) in toluene-d8 (C,D8) and in methyl cyclohexane (C6HllCH,).

WEFT-NOESY and transfer WEFT-NOESY NMR spectra were used to determine the heme proton assignments for Rhodobacter capsulatus ferricytochrome c 2 . The Fermi contact and pseudo-contact contributions to the paramagnetic effect of the unpaired electron in the oxidized state were evaluated for the heme and ligand protons. The chemical shift assignments for the 1 H and 15 N NMR spectra were obtained by a combination of 1 H-1 H and 1 H-15 N two-dimensional NMR spectroscopy. The short-range nuclear Overhauser effect~NOE! data are consistent with the view that the secondary structure for the oxidized state of this protein closely approximates that of the reduced form, but with redox-related conformational changes between the two redox states. To understand the decrease in stability of the oxidized state of this cytochrome c 2 compared to the reduced form, the structural difference between the two redox states were analyzed by the differences in the NOE intensities, pseudo-contact shifts and the hydrogen-deuterium exchange rates of the amide protons. We find that the major difference between redox states, although subtle, involve heme protein interactions, orientation of the heme ligands, differences in hydrogen bond networks and, possible alterations in the position of some internal water molecules. Thus, it appears that the general destabilization of cytochrome c 2 , which occurs on oxidation, is consistent with the alteration of hydrogen bonds that result in changes in the internal dynamics of the protein.

A series of functionalized polynorbornenes containing pendent ether- or ester-bridged poly(aromatic ether) chains were prepared. The ether-bridged norbornene complex was synthesized via cyclopentadienyliron-mediated nucleophilic aromatic substitution reactions. This methodology, combined with that of dicyclohexylcarbodiimide-mediated coupling, allowed for the formation of novel oligomeric aryl ether and ester substituted norbornene complexes. Photolytic demetallation gave the monomers in good yields. Structural identification of the exo and endo isomers of both the metallated and demetallated norbornene derivatives was accomplished using HH and CH COSY NMR techniques. Ring-opening metathesis polymerization (ROMP) of these monomers using RuCl3(hydrate) and (Cy3P)2Cl2Ru=CHPh allowed for the preparation of the functionalized polynorbornenes. Thermal analysis of the resulting polymeric materials demonstrated greater thermal stability as the number of aryl ether groups increased.Key words: aromatic ethers, cyclopentadienyliron, polynorbornene, ROMP, ruthenium catalysts.