• PDF / 292,176 Bytes
• 6 Pages / 414.72 x 648 pts Page_size
• 18 Downloads / 320 Views

DOWNLOAD

REPORT

---

I&

0 (eq

1)

The synthesis of derivatives is readily followed by electrospray MS. Electrospray is a solution MS technique whose major requirement is that ions already exist in solution. Thus the application of electrospray MS to fullererenes requires a method for making the uncharged fullerene derivatives visible (chargeable). We have found that tagging of fullerenes with the diazo-crown reagent 1 allows ready analysis of fullerene products [4].

357

Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society

The analysis of fullerene derivatives therefore involves the reaction of 1 with the compound of interest in benzene, followed by addition of KOAc or NaOAc in methanol. Electrospray of this solution produces a very clean spectrum containing ions due to complexation of the crown ether group with Na÷ or K'. MXK÷

MK÷

MX 2 K+

MX3 K÷ M 4K

J

f

M4XsK÷

MX6K MX7K*

19I 11II1290

ON 1 1399 149 19N I 19 179I 199

IO999

m/z Figure 1. Electrospray MS spectrum of enone photoproducts in equation I (cf. reference 3). Electrospray MS of the tagged reaction mixture from equation 1 in benzene/methanol containing KOAc shows the [2+2]-photocycloaddition of multiple units of 3methylcyclohexenone to C60 (Figure 1). Of course, mass spectrometry only detects compounds differing in mass. While the electrospray MS method is excellent for identification of adducts of C60 , it cannot distinguish isomers. The multiple enone additions to C60 , while giving many different isomers, give only one peak in the electrospray MS spectrum. HELIUM NMR Thus we have begun exploring the new technique of helium-3 NMR for identifying C60 isomeric derivatives. One unique aspect of the soccer-ball shaped C60 molecule that had been recognized since its discovery in 1985 was the potential for trapping an atom inside the carbon cage [5]. Work on these so-called endocyclic compounds has been a major focus of research. Preparation and isolation of compounds, termed metallofullerenes and having the nomenclature M@C60, is difficult and only very small quantities have thus far been characterized. A potentially more interesting development in this area involves filling C60 with inert gases and particularly with an isotope of helium-- 3He [6]. This isotope, available in quantity relatively cheaply via decay of tritium, is NMR active, i.e. has a spin of 1/2, resonates at 381 MHz, and has NMR sensitivity similar to 'H. Thus, 3He NMR spectrometry can be used to examine fullerene structure [7]. Earlier this year, only one 3He NMR line was known, the signal for gaseous 3He -- there were no helium compounds [6]. The discovery of 3He@C 6o and its relative 3He@C 70 suddenly opens a whole new field of helium molecule chemistry and 3He NMR spectroscopy. We were able to obtain a sample of 3He@C 60 and carry out several chemical reactions. We have produced two of the first isomeric helium compounds, cis and trans 3He@C 60 -enones (Figure 2). One can observed distinct chemical shifts for the new helium compound isomers (relative to gaseous helium and 3He@C 6o