Isotope scrambling in the formation of cyanopolyynes by laser ablation of carbon particles in liquid acetonitrile

Abstract

Cyanopolyynes, H(C C) n C N ( n = 3–6), were formed by laser ablation of carbon particles in liquid acetonitrile. The molecules were separated according to the size n and characterized by UV absorption spectroscopy, mass spectroscopy in combination with gas-chromatographic separation, and nuclear magnetic resonance (NMR) spectroscopy. For the study of nascent carbon cluster distribution during the growth of the long carbon chain molecules, isotopomer distribution was analyzed by NMR spectroscopy for the product molecules. Two cyanopolyynes of HC 7N and HC 9N were isolated from solutions after laser ablation of isotope-enriched carbon powder (99% 13C) in liquid acetonitrile, CH 3CN, of natural isotopic abundance (1.1% 13C). With the observed chemical shift, δ, and spin–spin coupling constants, J CH and J CC, spectral simulation was made to determine relative abundance of possible isotopomeric forms for HC 9N. We found that the isotope of 12C, mostly from solvent molecules, contributes substantially for the part of carbon in the cyano group, –C N, in HC 9N. The isotopomer distribution observed for the sequence of H–C C–C C– was fairly explainable by a binomial, random distribution of the two carbon isotopes of 12C and 13C, reducing the concentration of 13C to 76–55%.