Bottom-Up Formation of Antiaromatic Cyclobutadiene ( c -C 4 H 4 ) in Interstellar Ice Analogs.

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 non-equilibrium 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.