Size effects in large cyclo[26]carbon: a theoretical study

Cyclo[n]carbons are an emerging class of size-dependent carbon allotropes with promising application potential, while cyclo[26]carbon, as an even-numbered carbon ring with n > 20, remains insufficiently explored. This study systematically investigates the bonding properties, electron delocalization, vibrational modes, and aromaticity of cyclo[26]carbon. Computational results reveal a pronounced bond length alternation (BLA) pattern, with the shorter CC bonds exhibiting stronger π interactions and enhanced electron delocalization. The molecule displays characteristics of dual aromaticity, albeit with overall weak aromatic strength. Furthermore, electrostatic potential (ESP) and average local ionization energy (ALIE) analyses demonstrate the electrophilic properties of the short CC bonds. Ab initio molecular dynamics simulations indicatethat this molecule has certain stability under vacuum at low to moderate temperatures. Electron spectra analysis clarifies the respective contributions of πⁱⁿ and π^out orbitals to electronic excitations. Nonlinear optical property calculations show that the mean second-order hyperpolarizability of cyclo[26]carbon is 4.5-5.0 times higher than that of cyclo[18]carbon.This study deepens the theoretical understanding of this system and provides a foundation for its potential applications and the design of related derivatives.