Bottom-Up Formation of Antiaromatic Cyclobutadiene (c-C₄H₄) in Interstellar Ice Analogs

Abstract

Antiaromatic cyclobutadiene (c-C 4 H 4 ) is the simplest prototype of [n]annulenes and a key reactive intermediate with significant ring strain, serving as the model compound for antiaromatic systems in organic chemistry. Here, we report the first bottom-up formation of cyclobutadiene in low-temperature acetylene (C 2 H 2 ) ices exposed to energetic electrons. Cyclobutadiene was isolated and detected in the gas phase upon sublimation utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry along with ultraviolet photolysis studies. These findings advance our fundamental understanding of the exotic chemistry and preparation of highly strained antiaromatic cycles through nonequilibrium chemistry in interstellar environments, thus affording a possible route for the formation of highly strained molecules such as the hitherto elusive tetrahedrane (C 4 H 4 ). Because acetylene is a major product of the photolysis and radiolysis of methane (CH 4 ) ice, an abundant component of interstellar ices, our results suggest that cyclobutadiene can likely be formed in methane-rich ices of cold molecular clouds.