Comment on "Stability of Small Carbon-Nitride Heterofullerenes"

3 Pages·0.1 MB·English

by  Guangyu Sun

Comment on “Stability of Small Carbon-Nitride Heterofullerenes”
Guangyu Sun1, Rui-Hua Xie2, and Marc C. Nicklaus1
1Laboratory of Medicinal Chemistry, NCI-Frederick, Frederick, MD 21702
2National Institute of Standards and Technology, Gaithersburg, MD 20899-8423
(February 2, 2008)
3
0 In a recent Letter [1], Schultz et al. proposed the Natomsintotwogroupsofthreeatomsandsubstituting
0 existence of heterofullerenes Cn−xNx (40 ≤ n ≤ 50) differentgroupsofcarbonatomsinC50:270gives64pos-
2
based on the pattern of the peaks in the mass spectrum sible structures. We first performed calculations at PM3

n of carbon-nitride nanostructures produced by the arc- and B3LYP/STO-3G levels, which gave similar relative
a
discharge technique. Using PM3 semiempirical calcula- energies. Three structures have relative energies lower
J
tions, they predicted the most stable structures of the than 16 kcal/mol, whereas all others have relative ener-
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2 N-substituted fullerenes, C33N9 and C44N6, to have C3 gieshigherthan40kcal/mol. Furthercalculationsatthe
symmetry. While some levelof symmetry seems to max- B3LYP/6-31G*level gave relative energies as 0.00 (D3),
] imize stability, the highest symmetry often sacrifices it, 6.78(C3)and9.63(D3)kcal/mol,confirmingresultsfrom
i
c asinthecaseoffullereneC84[2]. UsingPM3anddensity lower levels. The HOMO-LUMO gaps, 2.70 eV, 2.85 eV
s functionaltheory(DFT), wearguethatahighlysystem- and 2.58 eV, respectively, are large by DFT standards,
-
l atic investigationis neededto determine the most stable indicating their kinetic stability. Fig.2 shows the most
r
t structures of both pristine and N-substituted fullerenes. stable structure of C50:270 with N6-substitution having
m
We demonstrate this explicitly for two particular cases, D3 symmetry.
t. C40 and C44N6, for which the authors of Ref. [1] only In summary, quantum chemical calculations at both
a
considered four and two isomers, respectively. DFTandPM3levelsindicatethateitheracompletesur-
m
Mathematically, there are 40 possible isomers for C40 vey or a more rational selection scheme is needed to de-
-
d [3]. We optimized the geometries of the 37 isomers termine the ground state structures of fullerenes.
n with non-vanishing HOMO-LUMO gaps using the PM3
o Hamiltonian and the B3LYP functional with STO-3G,
c
3- 21G and 6-31G* basis sets as implemented in Gaus-
[
sian 98 [4]. The closed-shell formalism was used and the

1 topological symmetry was maintained throughout. Iso-
v
mers 1, 30 and 40 (note: the isomer index follows Ref.
4 [1] D. Schultz, R. Droppa, F. Alvarez, and M.C. dos Santos,
9 [3]) have zero HOMO-LUMO gaps at the Hu¨ckel levelof Phys. Rev.Lett. 90, 015501 (2003).
4 theory. Geometricconvergencecouldnotbe achievedfor [2] G. Sun and M. Kertesz, J. Phys. Chem. A 105, 5212
1 these cases at the PM3 level and no further calculation (2001).
0 was performed. [3] P.W. Fowler and D.E. Manolopoulos, An Atlas of
3
0 The most stable isomer of C40 is predicted to be iso- Fullerenes (Clarendon Press, Oxford, 1995).
/ mer38(D2)byalllevelsoftheory. Analysisoftheheats [4] M.J. Frisch et al., Gaussian 98, Revision A11.3, Gaussian
at offormation(notshown)indicatedthatisomer38isnot Inc., Pittsburgh, PA,2002.
m among the four isomers studied in Ref. [1], although the [5] B.L. Zhanget al., J. Chem. Phys.97, 5007 (1992).
[6] E. Albertazzi et al., Phys. Chem. Chem. Phys. 1, 2913
- authors did not identify the isomers. Isomers with low
d relative energies include isomer 39 (D5d, 10.9 kcal/mol) (1999).
n
o and isomer 31 (Cs, 16.1 kcal/mol), see Fig. 1. All other
c isomershaveenergieshigherthan20kcal/mol. Thesere-
: sults are in agreementwith earlier predictions [5,6]. The Caption of Figures
v
i stabilities from different levels of theory agree with each
X
other.
ar In Ref. [1], Schultz et al. showed an isomer with C3 Fig.1: RelativeenergyofisomersoffullereneC40. The
symmetry as the most stable structure for C50 substi- isomer index follows Ref. [3].
tuted with six nitrogen atoms (Fig.4b in Ref. [1]). Here
we present a complete search of the N6-substituted iso- Fig.2: The moststable structureofC50:270withN6-
mers withinthe frameofC50:270(D3). Dividing the six substitution has D3 symmetry.
1
Fig 1/PRL/Sun et al.
200
PM3
180
b3lyp/sto-3g
mol)116400 bb33llyypp//36--2311gg*
al/120
kc100
y ( 80
g
r 60
e
n 40
e
20
0
1 4 7 10 13 16 19 22 25 28 31 34 37 40
isomer index
Fig 2/PRL/Sun et al.