NO-induced adaptive antiaromaticity in cyclobutadiene

Cyclobutadiene (CBD) displays aromaticity in the lowest-lying triplet excited state (T1) according to Baird's 4n electron rule. Hence, antiaromatic CBD in the T1 state has never been reported so far. Here we demonstrate via density functional theory (DFT) calculations that the CBD ring could possess dual antiaromaticity in the lowest singlet state (S0) and T1 states (termed as adaptive antiaromaticity), which is supported by various aromaticity indices including NICS, ACID, Delta BL, ELF and ISE. Furthermore, the spin density localization on the substituent is the key factor for NO-substituted CBD to maintain antiaromaticity in the T1 state. The principal interacting spin orbital (PISO) analysis suggests that the nitrogen atom in the NO substituent tends to form a double bond with the carbon atom in the CBD ring, blocking the delocalization into the CBD ring and resulting in the antiaromaticity in the T1 state. Our findings represent a step forward in aromatic chemistry. Adaptive antiaromaticity in NO-substituted cyclobutadiene was first demonstrated via density functional theory calculations due to the significant contribution of the substituent to the spin density in the lowest triplet state.

Automated summary

This study demonstrates through density functional theory calculations that the cyclobutadiene (CBD) ring could exhibit dual antiaromaticity in the lowest singlet and triplet states. It also identifies the localization of spin density on the substituent as the key factor in maintaining antiaromaticity in the triplet state.