A FOURIER TRANSFORM INFRARED ISOTOPIC STUDY OF THE $\nu_{4}$ STRETCHING MODES OF THE $C_{5}$ AND $C_{6}$ CLUSTERS IN Ar AT 10 K.$^{1}$

Abstract

The $C_{5}$ carbon cluster has been produced by trapping in Ar the products of the VUV-photolysis of 2-methyl-l,3-butadiene or by the high temperature evaporation of graphite. The $\nu_{3}(\sigma_{u}$) stretching mode was earlier observed at $2164.3 cm^{-1}$ and a band at $1544 cm^{-1}$ was attributed to the $\nu_{4}(\sigma_{u}$) vibration of $C_{5}$, but was later assigned to the $\nu_{3}(\sigma_{u}$ vibration of $C_{4}^{3}$. In the present work, the second $C=C$ stretching mode, $\nu_{4}(\sigma_{u}$ of $C_{5}$, has been found at $1446.6 cm^{-1}$. Measurements have been made for all twenty $^{13}C$-substituted isotopomers of linear $C_{5}$. Both the $\nu_{4}$ fundamental frequency and the isotopic shifts are in agreement with recent ab initio calculations done al the coupled electron pair approximation and MP2/DZP $levels.^{4,5}$ Also produced in the high temperature evaporation of graphite was the $C_{6}$ carbon cluster. The present work reports the continuation of the assignment of the $1197.3 cm^{-1}$ band, through extensive $^{13}C$ substitution, to the $\nu_{4}$ stretching mode of $C_{6}$, The results of both the original assignment and the isotopic measurements agree well with recent ab initio calculations done at the $MP2/6-31G^{*}$ $level.^{6}$

$^{1}$Supported by the W.M. Keck Foundation and the Welch Foundation (Grant P-0786). $^{2}$M. Vala. T.M. Chandrasekhar. J. Szezepanski, R. Van Zee, and W. Weltner, JR. J. Chem. Phys. 90, 595 (1989). $^{3}$L.N. Shen and W.R.M. Graham. J. Chem. Phys. 91, 5115 (1989). $^{4}$P. Botschwina and P. Sebald, Chem. Phys. Lett 160, 485 (1989). $^{5}$D.W. Ewing and I. Shavitt, J. Chem. Phys., in press. $^{6}$J,M.L. Martin, J.P. Francois, and R, Gijbels. J. Chem. Phys. 93, 8650 (1990).

Author Institution: Department of Physics, Texas Christian University