Infrared studies on the spectra and structures of novel carbon molecules

Abstract

Carbon clusters are formed in the laboratory by trapping the products from the Nd-YAG laser evaporation of graphite in argon matrices held at ~10 K. Linear carbon chains have been the subject of extensive theoretical and experimental studies over many years and are important in diverse areas as astrophysics, studies of the fullerenes, and the chemistry of fuel combustion. FTIR measurements of vibrational fundamentals and carbon-13 isotopic shifts, coupled with the predictions of theoretical density functional theory calculations and a recently developed theoretical tool, the deperturbation method have been successfully employed to identify long linear C[subscript]n (n [greater than or equal to] 7) carbon chains. The development of a process to produce carbon rods highly enriched with 13C enabled the observation of well-resolved isotopic spectra of linear 13C[subscript]n carbon clusters (n =3-18).^The identifications are facilitated by the measurement of both the isotopic pattern for single 12C-substituted (12C13C[subscript]n-1) isotopomers and the "mirror" isotopic pattern for single 13C-substituted (13C12C[subscript]n-1) isotopomers. As a result of this work it is now routinely possible to achieve experimental reproducibility of isotopic shift patterns, which enables comparison with theoretical predictions.The combination of experimental improvements and theoretical results, has led to the identification of 13C isotopic shifts for sixteen vibrational fundamentals belonging to eight different long chains, C[subscript]n (n [greater than or equal to] 7) species.^The fundamentals identified include the v4(su) = 2128.1 cm-1 mode of C7; the v5(su) = 2078.2, v6(su) = 1998.2, and v7(su) = 1601.0 cm-1 modes of C9; the v6(su) 2074.2 and v7(su) = 1915.7cm-1 modes of C10; the v7(su) = 1946.1, v8(su) = 1856.7, and v9(su) = 1360.0 cm-1 modes of C11; the v5(su) = 2071.4 and v6(su) = 1710.5 cm-1 modes of linear C8; the v7(su) = 2140.6, v8(su) = 997.3 and v9(su) = 1817.9 cm-1 of linear C12; the v10(su) = 1999.3 cm-1; mode of linear C15; and the v12(su) = 2001.0 cm-1 mode of linear C18. In addition, isotopomer absorptions have been identified for the following modes that are normally IR inactive until isotopic substitution when they become observable: v1(sg) mode of C7, the v1(sg) mode of C9,v1(sg) and v2(sg) mode of C10, v2(sg) and v1(sg) mode of C11, v3(sg) v3(sg) modes C15 and the v3(sg) mode of C18.