Aromaticity of Even-Number Cyclo[n]carbons (n = 6-100).

Glib V Baryshnikov,Dage Sundholm,Rinat T Nasibullin,Rashid R Valiev,Theo Kurten,Hans Ågren

doi:10.1021/acs.jpca.0c09692

Abstract

The recently synthesized cyclo[18]carbon molecule has been characterized in a number of studies by calculating electronic, spectroscopic, and mechanical properties. However, cyclo[18]carbon is only one member of the class of cyclo[n]carbons—standalone carbon allotrope representatives. Many of the larger members of this class of molecules have not been thoroughly investigated. In this work, we calculate the magnetically induced current density of cyclo[n]carbons in order to elucidate how electron delocalization and aromatic properties change with the size of the molecular ring (n), where n is an even number between 6 and 100. We find that the Hückel rules for aromaticity (4k + 2) and antiaromaticity (4k) become degenerate for large Cn rings (n > 50), which can be understood as a transition from a delocalized electronic structure to a nonaromatic structure with localized current density fluxes in the triple bonds. Actually, the calculations suggest that cyclo[n]carbons with n > 50 are nonaromatic cyclic polyalkynes. The influence of the amount of nonlocal exchange and the asymptotic behavior of the exchange–correlation potential of the employed density functionals on the strength of the magnetically induced ring current and the aromatic character of the large cyclo[n]carbons is also discussed.

Highlights

Since the recent synthesis of cyclo[18]carbon,[1] a large number of studies of spectroscopic,[2−4] aromatic,[5−10] structural,[5,10−14] mechanical,[15] and electronic properties[7,16−21] of cyclo[n]carbons have been reported
In addition to the strong bond length alternation (BLA) that appears in C18 and the larger cyclo[n]carbons (n = 22−100), the bond angle alternation (BAA) disappears for C18 and larger cyclo[n]carbons (Figure 1, Table S2)
A series of cyclo[n]carbons with an even number of carbon atoms in the range of n = 6−100 were studied computationally with the focus on an analysis of the magnetically induced current density, which was used for assessing their aromatic character

Summary

■ INTRODUCTION

Since the recent synthesis of cyclo[18]carbon,[1] a large number of studies of spectroscopic,[2−4] aromatic,[5−10] structural,[5,10−14] mechanical,[15] and electronic properties[7,16−21] of cyclo[n]carbons have been reported. They reported ionization potentials, electron affinities, fundamental gaps, occupation numbers of the frontier orbitals, and the relative energy with respect to the linear isomers Their calculations yielded similar trends as obtained in earlier studies.[8,25,28] For small cyclo[n]carbons with n < 32, there is a clear difference between the properties of the rings with (4k + 2) and (4k) carbon atoms, whereas for larger cyclo[n]carbons, the molecules with (4k + 2) and (4k) carbon atoms have similar properties. In the singlepoint CASSCF calculations, we used the 6-31G(d,p),39−41 6311G(d,p),[40−42] and cc-pVTZ43 basis sets and the BHLYP/ def2-TZVP-optimized molecular structures because this level of theory gives the expected and most likely correct dependence of the strength of the magnetically induced ring current on the size of the ring. Orbital with an Occupation Number Near 2 Obtained in the CASSCF Calculation and the Weight (w) of Closed-Shell Determinant for the S0 State Using Different Basis Sets molecule

C30 C40 C50 C60 C70 C80 C90 C100

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES

25. Supplementary