Fourier transform infrared spectroscopic studies of small pure carbon clusters

Abstract

Fourier transform infrared studies of small pure carbon linear chains have been carried out by trapping the products of the vacuum ultraviolet photolysis of 2-methyl-1,3,-butadiene and by the high temperature evaporation of graphite, in solid Ar at 10 K. Isotopic data obtained for each of the species studied enabled the unambiguous assignments. Comparison of these assignments with theoretical calculations yielded very good agreement. The results of the presented research have resulted in new information on two pure carbon linear chains, C$\sb5$ and C$\sb9$, and have cleared up considerable confusion surrounding previous assignments to the linear C$\sb6$ carbon cluster. In the course of the C$\sb9$ study, however, it was also determined that in certain circumstances extreme care must be taken when making assignments based on isotopic substitutions. The $v\sb4(\sigma\sb{u}$) antisymmetric stretch at 1446.6 cm$\sp{-1}$ of the linear C$\sb5$ carbon cluster has been identified for the first time by the analysis of isotopic data. This assignment agrees well with the predicted frequencies of both Botschwina and Sebald (1989) and Ewing and Shavitt (1992). Several isotopic bands belonging to the $v\sb3(\sigma\sb{u}$) antisymmetric stretch were also reassigned as a result of the better resolution available in the present study. Both the $v\sb4(\sigma\sb{u}$) and $v\sb5(\sigma\sb{u}$) assignments for the linear C$\sb6$ carbon cluster were confirmed through the analysis of isotopic data. Prior to publication of this work, it had been suggested by Neumark et al. (1993) that the assignments particularly for $v\sb5(\sigma\sb{u}$) were incorrect on the basis of force constant adjustment calculations. However, publication of the present work with the isotopic evidence, which definitively proved that the proposed assignments were correct, Neumark et al. agreed and concluded their difficulty resulted from the large anharmonicity involved with the $v\sb5(\sigma\sb{u}$) mode. A second antisymmetric stretch, $v\sb7(\sigma\sb{u}$), for the linear C$\sb9$ cluster has been identified for the first time at 1601.0 cm$\sp{-1}$. This assignment is supported by isotopic data and an extensive number of theoretical calculations. The agreement between the observed isotopic bands and those predicted by theory is very good. The previous assignment of the $v\sb6(\sigma\sb{u}$) mode of C$\sb9$ to 1998.0 cm$\sp{-1}$ could not be confirmed because of the mode's extreme sensitivity on $\sp{13}$C isotopic substitution. This theory of "isotopic sensitivity" was tested in an isotopic study of previously identified modes of linear C$\sb7$ and found it accurately predicted the behavior of vibrational modes under $\sp{13}$C substitution. The C$\sb7$ and C$\sb9$ studies therefore indicate that consideration of the isotopic sensitivity is very important in making vibrational assignments for larger carbon species on the basis of isotopic substitutions.