The rare gas xenon contains two NMR-sensitive isotopes in high natural abundance. The nuclide '"Xe has a spin of 'h; '31Xe is quadrupolar with a spin of 3/2. The complementary NMR characteristics ofthese nuclei provide a unique opportunity for probing their environment. The method is widely applicable because xenon interacts with a useful range of condensed phases including pure liquids, protein solutions, and suspensions of lipid and biological membranes. Although xenon is chemically inert, it does interact with living systems; it is an effective general anesthetic. We have found that the range of chemical shifts of '"Xe dissolved in common solvents is ca. 200 ppm, which is 30 times larger than that found for 13C in methane dissolved in various solvents. Resonances were also observed for 131Xe in some systems; they were broader and exhibited much greater relaxation rates than did '29Xe. The use of '29Xe NMR as a probe of biological systems was investigated. Spectra were obtained from solutions of myoglobin, from suspensions of various lipid bilayers, and from suspensions of the membranes of erythrocytes and of the acetylcholine receptor-rich membranes of Torpedo californica These systems exhibited a smaller range ofchemical shifts. In most cases there was evidence of a fast exchange of xenon between the aqueous and organic environments, but the exchange was slow in suspensions of dimyristoyl lecithin vesicles.Xenon NMR spectroscopy is a potential probe of the structural and dynamic aspects ofthe molecular environment ofthe xenon atom in physical and biological media. Natural xenon contains 26% '29Xe which has spin I = 1/2 and 21% 131Xe which is a quadrupolar nucleus and has spin I = 3/2. The NMR sensitivity of '29Xe is relatively large and the solubility of xenon in most liquids is high for an apolar gas-e.g., from 4.3 mM in water to 166 mM in isooctane at 1 atm and 273 K-so the NMR spectra ofboth isotopes can be observed with commercial multinuclear spectrometers. The chemical shift of the xenon atom is especially reflective of its environment due to its large, polarizable, electron cloud. In xenon compounds, shifts up to 4000 ppm have been observed (1). In the free xenon atom, the effects of the medium can produce sizable shifts. Such shifts have been observed in pure liquid and gaseous xenon as well as in gas mixtures of xenon with a second component (1). However this effect had not been studied in condensed phases. We have observed solvent-dependent shifts over a range of ca. 200 ppm, a range that is much larger than the solvent shifts of 13C and 19F (2, 3).Xenon interacts with many biological systems including myoglobin (4) and hemoglobin (5). It is also soluble in lipid bilayers: membrane/gas partition coefficients vary from 0.4 (at 20°C) in erythrocytes (6) to 1.3 (at 25°C) in egg lecithin (unpublished data). Its most striking pharmacological property is its ability to induce general anesthesia; its efficacy is comparable to that of nitrous oxide (7). The physicochemical mechanism of anestheti...

A new spectrophotometric assay has been used to determine the gross concentration of cardiac glycoside in individual monarch butterflies. Adults sampled during the fall migration in four areas of eastern North America exhibited a wide variation in cardiac glycoside concentration. The correlation between spectrophotometrically measured concentrations and emetic dose determinations supports the existence of a broad palatability spectrum in wild monarch butterflies. The cardiac gylcoside concentration is greater in females than in males and is independent of the dry weight of the butterflies; contrary to prediction, both the concentration mean and variance decrease southward. The defensive advantage of incorporating cardiac glycosides may be balanced by detrimental effects on individual viability.

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The solution structure and dynamics of {(17j-CsMej)zZrNz}zN2, ((11j-CsMej)zZr(CO)I2N2, {(q5-CsMe5)2Zr-(PF3)}2x>, and ((q5-CjMej)2TiN2)2N2 have been studied by IH N M R spectrometry. {(q5-CsMej)2ZrN2J2N2 is observed to undergo mutual exchange of pentamethylcyclopentadienyl ligands between the two sites of the molecule on the time scale of the order of the ' H N M R experiments at 10 OC. Variable-temperature I5N N M R experiments for {(q5-CjMej)2Zr-('5N2)12('5N2) were also carried out, and the results are interpreted on the basis of dissociative exchange of the two terminal dinitrogen ligands with free dissolved N2. The observation that nitrogen dissociation is 5-10 times faster than [q5-CsMes] site exchange suggests a mechanism for ring interchange involving stepwise dissociation-association of terminal IV2 ligands with "inversion" at the zirconium centers. Activation parameters calculated from I5N N M R data are E , = 11 kca1,mol-' and AS* = t I O eu. The observed temperature dependence of the ' H N M R spectra for the isostructural complex ((qs-CsMej)2TiN2}2Nzsuggests that the same mechanism is operative. (($-CsMej)2Zr(C0)}2N2, on the other hand, does not undergo C O dissociation at a rate sufficient to observe [$-CjMes] ring site exchange by ' H N M R spectrometrq even a t 64 "C.Bis(pentamethylcyclopentadienyl) derivatives of titanium and zirconium have proved to be useful congeners to their bis(cyclopentadieny1) analogues by virtue of enhanced stability, solubility, and crystallizability. Dinitrogen complexes of (q5-C5Me5)2Ti and (v5-C5Me5)2Zr are of particular interest in view of the ready protonation and reduction to hydrazine of their ligated N2.3-8 We have recently reported the solid-state structures of {(v5-C5Me5)2Ti)2Nz8 and ((v5-C5Me5)2-ZrN2J2N2' as determined by single-crystal x-ray diffraction methods. I n this paper we report the results of an NMR and IR study of the solution structure and dynamics of {($-CsMe5)2ZrNz}2Nl, its carbonyl and PF3 derivatives, and the titanium analogue. Experimental SectionPhysical Measurements. ' H N M R spectra were recorded on a Varian H R 220 (CW) spectrometer. IsN N M R spectra were obtained at 18.25 M H z on a Bruker WH180 (FT) spectrometer. Computersynthesized spectra were obtained using DNMR3, a general N M R line-shape program with symmetry and magnetic equivalence factoring, written by Binsch and Kleier.9 Infrared spectra were obtained on Perkin-Elmer 180. 225, and 457 and Beckman IR-12 spectrophotometers.Materials. All manipulations were performed on a vacuum line, in a glovebox which was evacuated to <0.05 Torr and filled just prior to use with either prepurified argon or nitrogen, or in a Vacuum Atmospheres glovebox under nitrogen. Nitrogen used in the experiments was prepurified grade rendered rigorously oxygen-and water-free by passage over MnO on vermiculitelo and activated 4A molecules sieves. Toluene, benzene, and 30-60 OC petroleum ether were purified by vacuum transfer first from LiAIH4 and then from "titanocene".Il I ,2,3,4,5-Pentamethylcyclopentadien...

The medium shift of 129Xe was measured in a number of liquid alkanes, monosubstituted benzenes, and a set of ten test solvents where the magnetic anisotropy is negligible. The shifts arise from van der Waals shielding, but in the test solvents they do not correlate well with the predicted reaction field. A good correlation is found for the n-alkanes where the repulsive part of the shift is assumed constant. Branched alkanes show positive deviations which are ascribed to enhanced repulsive interactions. Shifts in monosubstituted benzenes correlate well with prediction except for solvents with low-lying excited states which show negative deviations. It is concluded that the reaction field model is valid within a group of closely related solvents, but it fails when extended to collections of randomly selected solvents. IntroductionRecently we undertook an NMR study of the interaction of xenon with biological systems. In the course of this work we expect to utilize the chemical shift of lBXe to determine the locus of xenon when it is present in complex systems such as cell membrane suspensions. For this work, it is important to have an understanding of the factors responsible for xenon shifts in the absence of specific interactions. In order to elucidate the mechanism of the nonspecific medium shift, we measured the 129Xe shift in a number of simple solvents, and we have attempted to explain the results in terms of a reaction field model.The magnetic shielding of a molecule dissolved in a solvent differs significantly from its value in the isolated state. Part of this difference is connected with the bulk magnetic susceptibility of the sample. The remaining

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