Zurek Eva, Autschbach Jochen
Contribution from the Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany.
J Am Chem Soc. 2004 Oct 13;126(40):13079-88. doi: 10.1021/ja047941m.
The electronic structure and (13)C NMR chemical shift of (9,0) single-walled carbon nanotubes (SWNTs) are investigated theoretically. Shielding tensor components are also reported. Density functional calculations were carried out for C(30)-capped and H-capped fragments which serve as model systems for the infinite (9,0) SWNT. Based on the vanishing HOMO-LUMO gap, H-capped nanotube fragments are predicted to exhibit "metallic" behavior. The (13)C chemical shift approaches a value of approximately 133 ppm for the longest fragment studied here. The C(30)-capped SWNT fragments of D(3d)/D(3h) symmetry, on the other hand, are predicted to be small-gap semiconductors just like the infinite (9,0) SWNT. The differences in successive HOMO-LUMO gaps and HOMO and LUMO energies, as well as the (13)C NMR chemical shifts, converge slightly faster with the fragment's length than for the H-capped tubes. The difference between the H-capped and C(30)-capped fragments is analyzed in some detail. The results indicate that (at least at lengths currently accessible to quantum chemical computations) the H-capped systems represent less suitable models for the (9,0) SWNT because of pronounced artifacts due to their finite length. From our calculations for the C(30)-capped fragments, the chemical shift of a carbon atom in the (9,0) SWNT is predicted to be about 130 ppm. This value is in reasonably good agreement with experimental estimates for the (13)C chemical shift in SWNTs.
对（9,0）单壁碳纳米管（SWNTs）的电子结构和（13）C NMR化学位移进行了理论研究。还报告了屏蔽张量分量。对作为无限（9,0）SWNT模型系统的C（30）封端和H封端片段进行了密度泛函计算。基于HOMO-LUMO能隙的消失，预测H封端的纳米管片段表现出“金属”行为。对于此处研究的最长片段，（13）C化学位移接近约133 ppm的值。另一方面，预测具有D（3d）/D（3h）对称性的C（30）封端的SWNT片段与无限（9,0）SWNT一样是小能隙半导体。连续的HOMO-LUMO能隙以及HOMO和LUMO能量的差异，以及（13）C NMR化学位移，与片段长度的收敛速度比H封端的管稍快。对H封端和C（30）封端片段之间的差异进行了详细分析。结果表明（至少在目前量子化学计算可及的长度下），由于H封端系统因其有限长度而产生的明显伪影，它们对于（9,0）SWNT来说是不太合适的模型。根据我们对C（30）封端片段的计算，预测（9,0）SWNT中碳原子的化学位移约为130 ppm。该值与SWNTs中（13）C化学位移的实验估计值相当吻合。
