The compound C60 (MW = 720), also known as Buckminsterfullerene or \"Bucky Balls
ID: 530718 • Letter: T
Question
The compound C60 (MW = 720), also known as Buckminsterfullerene or "Bucky Balls" can be isolated from the soot that forms when creating an electric are between two graphite electrodes in an atmosphere of helium gas. The molecule is spherical with 32 faces, 12 of which are pentagons and 20 of which are hexagons. The molecule displays a single resonance in^13 C NMR. In addition to C60 another compound can be isolated from the soot having the MW of 724. During the formation of the spherical structure, an approximate sphere having a "hole" first forms, and subsequently closes to the final structure. Given the information, answer the following: a) Calculate the intensity of the M+1 peak of C60 as a percent of the molecular ion peak (m/z 720 = 100%). b) Would you expect to see a significant M+2 peak? Explain. c) Explain why the^13 C NMR only shows one resonance. d) Propose a structure for the 724 molecular weight product. Note that you needn't draw out the full structure; simply explain it in a sentence.Explanation / Answer
Answer to Qa)
The ratio of PM+1 / PM is given as:
PM+1 / PM = w[c/ (100-c)]
Where,
the probability of monoisotropic C60 is PM,
the probability of its M+1 ion is PM+1
c is the natural abundance of 13C in the mixture of 12C and 13C = 1.1 %
w is the number of carbon atoms in the compound = 60
PM+1 / PM = 60[1.1/ (100-1.1)]
= 66 / 98.9
= 0.6673
If the intensity of monoisotropic C60 peak is 100%, then the intensity of M+1 peak will be 66.73%
Answer to Qb)
The ratio of PM+1 / PM is given as:
PM+2 / PM = w(w-1)c2/ 2(100-c)2
= 60 (60-1) (1.1)2 / 2(100-1.1)2
= 60 • 59 • 1.21 / 2 • 9781.21
= 0.2189
If the intensity of monoisotropic C60 peak is 100%, then the intensity of M+2 peak will be 21.89 %. This intensity is substantial and hence the M+2 peak will be observed.
Answer to Qc)
The C60 fullerene is a highly symmetrical truncated icosahedron structure. Thus all carbon atoms are identical and hence only one peak in 13C NMR is observed.
Answer to Qd)
The difference in m/z of MC60 and C60 suggests the presence of a molecule with molar mass of 4.0 amu trapped during the synthesis of the C60 bucky balls. Since the C60 bucky balls were synthesized in He2 atmosphere, the gas must be He2. This endohedral fullerene can be shown by drawing a He2 molecule inside the icosahedron.
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