An NMR Study of the Occupation of C 60 Interstitial Sites by Oxygen Molecules
- PDF / 322,697 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 78 Downloads / 177 Views
AN NMR STUDY OF THE OCCUPATION OF C60 INTERSTITIAL SITES BY OXYGEN MOLECULES
Roger A. Assink, Douglas A. Loy, James E. Schirber and Bruno Morosin Sandia National Laboratories, Albuquerque, NM 87185
ABSTRACT The 1 3 C NMR of FCC C 6 0 under magic angle spinning (MAS) conditions yields linewidths on the order of 1 Hz at fields of 4.7 T. The spectrum consists of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The intensity of the minor resonance relative to the primary resonance was found to vary from 0.6 to 5.5 % depending on the sample history. The downfield shift obeys Curie's law and is attributed to the Fermi contact coupling interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C6 0 molecules. Exposure of the sample to 1 kbar oxygen for 1 3/4 hours resulted in a spectrum of 7 evenly spaced resonances corresponding to 0 to 6 of the adjacent octahedral interstitial sites being filled with oxygen molecules. At ambient pressure, the oxygen diffused out of the lattice on time scales ranging from hours to days.
INTRODUCTION The identification of the fullerene C60 by Kroto et. al. (11 and its production in large quantities by Kratschmer et. al. [2] have generated considerable interest in fullerene structure and behavior. In this paper we present evidence that the chemistry of solid C60 can be significantly affected by surrounding gases. High 13 resolution C NMR spectroscopy is used to show that under ambient conditions, oxygen molecules occupy the octahedral interstitial sites of the FCC C60 lattice. A substantial fraction of these sites are filled by gas molecules when the fullerene is exposed to high pressure oxygen. OXYGEN/C6
0
ASSOCIATION
High resolution MAS 1 3 C NMR spectra of solid C 6 0 are shown in Figs. 1A, B and C. Fig. 1A shows the fully relaxed spectrum of C60 purified in our laboratory [3A]. In addition to the primary resonance at 143.7 ppm, there is a minor resonance at 144.4 ppm with 4.5 % of the total intensity. The spin-lattice relaxation time of the primary peak is 71±7 s while the spin-lattice relaxation time of the minor peak is 0.26±0.03 s. To search for less intense peaks we accumulated a large number of free induction decays at short repetition times. Under such conditions, only the minor peak would exhibit quantitative intensity. The spectra shown in Figs lB and 1C were recorded using a 1 s repetition rate and establish the existence of an additional resonance at 145.1 ppm whose intensity is only 0.14 % of the total intensity. Mat. Res. Soc. Symp. Proc. Vol. 270. 01992 Materials Research Society
256
A
?
D
0.8 0.6
.•*1
04 u
0.2 0 0
. . . . .. . . . . . . . . . . . . 0.001 0.002 0.003 0.004 1000/T (K")
0.005
C
F*-' 148
,
,
I 146
.
I 144
.
I 142
.
1 140
PPM
Fig. 1. High resolution 1 3C NMR spectra were recorded at 50.2 MHz using MAS. (A) Fully relaxed spectrum using a 240 s wait time and 256 accumulations. (B) Partially relaxed spectrum with a 1 s wait time and 9742 accumulations. (C) The pr
Data Loading...
