Structural details of small non-IPR fullerenes: Experimental and theoretical insights

Abstract

The investigation of non-IPR (isolated pentagon rule) fullerenes is limited to theoretical studies since there is simply no or very little sample available that allows investigating these compounds. The discovery of fullerenes in crude oil provided a suitable sample system. The information that might be gathered about these non-IPR fullerenes can be useful for developing synthetic routes and study their properties for potential future applications. Investigations using high-resolution mass spectrometry (HRMS) were conducted to understand the stability of these fullerenes. The experimental results are supported by density-functional-theory (DFT) calculations. The outcome of HRMS and DFT provided results on relative quantity, stability and basic mechanistic fragmentation information. The results showed that non-IPR fullerenes were present in higher abundances than the most stable C₆₀-buckminsterfullerene. The investigation of C₃₀ to C₄₄ indicates C₃₂ and C₃₆ to be the most stable congeners, which is in accordance with the existing theory. The mechanism of fragmentation is, in general, marked by the loss of C₂ and a subsequent Stone-Wales-transformation into an energetically more stable isomer. The fragmentation behavior differs with size as the smaller fullerenes disintegrate easier instead of losing C₂-units. This study shows that crude oil makes up a very useful sample system for the investigation of non-IPR fullerenes.