Browsing by Author "Stevenson, Steven A."

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    Chromatography and purification of endohedral metallofullerenes
    Stevenson, Steven A. (Virginia Tech, 1995)
    At the conception of this research, a separation methodology for obtaining purified mctallofullerene [Am@C2n, m = # of metal atoms, A, and C2n = # of carbons in the surrounding cage] samples was not yet developed. Isolation of these metal-encapsulated fullerenes was strongly desired for characterization of their physical and chemical properties. Predicted applications for these novel species include their use as possible superconductors, catalysts, and non-linear optical devices. However, initial purification efforts have been hindered by several difficulties. These factors include a low abundance (< 1%) in the raw extract, uncertain stability in aerobic environments, co-elution of Am@C2n with empty-cage fullerenes, and the need for selective chromatographic detection. In this research, these difficulties have been overcome with the development of a continuous-flow, on-line HPLC-EPR apparatus. Advantages include a selective, non-invasive detector with chromatographic separations being performed in a controlled anaerobic environment. This on-line approach permits the selective detection of only those metallofullerenes with an odd-number of encapsulated atoms. The ability to continually monitor separations of these paramagnetic species ultimately permits the optimization of chromatographic parameters. The methodology developed from this on-line HPLC-EPR approach has ultimately resulted in purified empty-cage (C₆₀, C₇₀...C₉₆) and metallofullerene samples (Sc₂@C₇₄, Sc₂@C₇₆, Sc₂@C₇₈, Sc₂@C₈₀, Sc₂@C₈₂, Sc₂@C₈₄ - two isomers, Sc₂@C₈₆, Sc₂@C₈₈, Sc₂@C₉₀, Sc₃@C₈₂, Sc₄@C₈₂, La₂@C₇₂, Er@C₈₂, Er₂@C₈₂ - two isomers, and Er₂@C₉₂).
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    Endohedral metallofullerenes and method for making the same
    (United States Patent and Trademark Office, 2001-10-16)
    A family of trimetallic nitride endohedral metallofullerenes and their preparation are described. The trimetallic nitride endohedral metallofullerenes have the general formula A.sub.3-n X.sub.n @C.sub.m where n ranges from 0 to 3, A and X may be trivalent metals and may be either rare earth metal or group IIIB metals, and m is between about 60 and about 200. Further, the A.sub.3-n X.sub.n @C.sub.68, A.sub.3-n X.sub.n @C.sub.78, A.sub.3-n X.sub.n @C.sub.80 families of endohedral fullerenes are described. The trimetallic nitride endohedral metallofullerenes are produced by charging a reactor with a cored graphite rod that has been filled with a metal oxide graphite mixture. The metal oxides correspond to the metals for A and X. The graphite rod is arc discharged in a helium and nitrogen atmosphere to produce the desired trimetallic nitride endohedral metallofullerenes.
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    Enhanced nonlinear optical response of an endohedral metallofullerene through metal-to-cage charge transfer
    Heflin, James R.; Marciu, D.; Figura, C.; Wang, S.; Burbank, P.; Stevenson, Steven A.; Dom, H. C. (AIP Publishing, 1998-06)
    A new mechanism for increasing the third-order nonlinear optical susceptibility, X-(3), is described for endohedral metallofullerenes. A two to three orders of magnitude increase in the nonlinear response is reported for degenerate four-wave mixing experiments conducted with solutions of Er-2@C-82 (isomer III) relative to empty-cage fullerenes. A value of - 8.7x 10(-32) esu is found for the molecular susceptibility, gamma(xyyx), of Er-2@C-82 compared to previously reported values of gamma(xxxx) = 3 x 10(-34) esu and gamma(xyyx) = 4 x 10(-35) esu for C-60. The results confirm the importance of the metal-to-cage charge-transfer mechanism for enhancing the nonlinear optical response in endohedral metallofullerenes. (C) 1998 American Institute of Physics.
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    LC - ¹³C NMR utilizing dynamic nuclear polarization (DNP) for signal enhancement
    Stevenson, Steven A. (Virginia Tech, 1992-06-05)
    The primary difficulty for successful LC - ¹³C NMR (whether ¹H or ¹³C) is overcoming the relatively low sensitivity of NHR as a chromatographic detector. For the ¹H nuclide this is much less of a problem; the sensitivity ;s approximately 6000 times more sensitive than that of ¹³C nuclei. For this reason, much of the literature focuses on LC - ¹H NMR. To ever successfully realize LC - ¹³C NMR, it is mandatory that an augmentation of ¹³C signal intensity must be effectuated to overcome this sensitivity deficit (~ three orders of magnitude). To satisfy this requirement, our laboratory has utilized dynamic nuclear polarization (DNP) to ameliorate these otherwise weak or non-existent signals. For favorable molecules, sensitivity recoveries of up to two orders of magnitude have been developed. This improvement (relative to 'H) narrows the sensitivity gap between 'H and ¹³C NMR detection of chromatographically separated analytes. Despite the fact that relatively large injection volumes were required in most LC experiments, the wealth of structural information inherent to ¹³C NMR justifies any attempt to successfully couple nuclear magnetic resonance to liquid chromatography. In addition, DNP was utilized in a series of SLIT and LLIT experiments where a test mixture was recycled through a NMR spectrometer. Results indicate that ¹³C spectra were obtained with a significantly higher signal-to-noise ratio in a shorter amount of analysis time relative to experiments where DNP was not employed for signal enhancement.