FTIR spectroscopy of tetracarbon radicals in solid argon

Abstract

A Fourier transform infrared isotopic study of the products from the VUV photolysis of diacetylene and 1,3-butadiene trapped in solid argon has enabled the identification of the 1543.4 cm$\sp{-1}$ frequency as the $\nu\sb2$ vibration of a nearly linear C$\sb4$ molecule. The assignment is confirmed by the observation of the frequencies for seven carbon-13 substituted isotopomers of the molecule. These frequencies are: 1528.8 cm$\sp{-1}$ for:C=C=C=$\sp{13}$C:; 1527.5 cm$\sp{-1}$ for:C=C=$\sp{13}$C=C:; 1513.8 cm$\sp{-1}$ for:C=$\sp{13}$C=C=$\sp{13}$C:; 1513.8 cm$\sp{-1}$ for:C=$\sp{13}$C=$\sp{13}$C=C:; 1511.6 cm$\sp{-1}$ for:$\sp{13}$C=C=C=$\sp{13}$C:; 1498.8 cm$\sp{-1}$ for:C=$\sp{13}$C=$\sp{13}$C=$\sp{13}$C:; and 1497.7 cm$\sp{-1}$ for:$\sp{13}$C=C=$\sp{13}$C=$\sp{13}$C:. The fact that the C$\sb4$ molecule trapped in argon matrix is slightly cis-bent has been determined by the detection of an infrared absorption band at 1699.8 cm$\sp{-1}$, which has been assigned to the $\nu\sb2$ + $\nu\sb6$ combination band for C$\sb4$ with such a geometry. For five $\sp{13}$C-substituted isotopomers of the slightly cis-bent C$\sb4$ molecule, the $\nu\sb2$ + $\nu\sb6$ combination band are found at: 1683.4 cm$\sp{-1}$ for:C=C=C=$\sp{13}$C:; 1681.8 cm$\sp{-1}$ for:C=C=$\sp{13}$C=C:; 1667.2 cm$\sp{-1}$ for:C=$\sp{13}$C=C=$\sp{13}$C:; 1667.2 cm$\sp{-1}$ for:C=$\sp{12}$C=$\sp{12}$C=C:; and 1663.2 cm$\sp{-1}$ for:$\sp{13}$C=C=C$\sp{13}$C:. The results has, for the first time, unambiguously determined the molecular structure of C$\sb4$, which would otherwise have still remained a subject of controversy. An FTIR isotopic study of the C$\sb4$H (butadiynyl) radical produced by trapping the products from the vacuum ultraviolet photolysis of diacetylene (C$\sb4$H$\sb2$) and 1,3-butadiene (C$\sb4$H$\sb6$) in solid argon at 10 K has been carried out in the midinfrared from 400-3900 cm$\sp{-1}$. A variety of carbon-13 and deuterium substituted C$\sb4$H$\sb2$ and C$\sb4$H$\sb6$ parent molecules were used to produce various isotopomers of C$\sb4$H. On the basis of their isotopic behavior two absorptions at 3307.4 and 2083.9 cm$\sp{-1}$ have been assigned to the $\nu\sb1$, C-H stretching and $\nu\sb2$, C$\equiv$C stretching modes. The previous identification of the other C$\equiv$C stretching mode, $\nu\sb3$ = 2060.6 cm$\sp{-1}$, which was based on the effects of deuteration, has been confirmed and the corresponding frequencies for various $\sp{13}$C-substituted isotopomers have been measured. The following frequencies have been obtained for $\nu\sb1$, $\nu\sb2$ and $\nu\sb3$ for C$\sb4$H isotopomers: 3307.2, 2078.9, and 2023.8 cm$\sp{-1}$ for H-C$\equiv$C-C$\equiv\sp{13}$C; 3307.2, 2069.9, and 2032.5 cm$\sp{-1}$ for H-C$\equiv$C-$\sp{13}$C$\equiv$C; 3307.2, 2066.8, and 2054.0 cm$\sp{-1}$ for H-C$\equiv\sp{13}$C-C$\equiv$C;; 3293.3, 2048.9, and 2057.4 cm$\sp{-1}$ for H-$\sp{13}$C$\equiv$C-C$\equiv$C; 3292.5, 2050.6, and 2024.6 cm$\sp{-1}$ for H-$\sp{13}$C$\equiv$C-C$\equiv\sp{13}$C; 3292.5, 2050.6, and 2030.0 cm$\sp{-1}$ for H-$\sp{13}$C$\equiv$C-$\sp{13}$C$\equiv$C; 3306.3, 2063.3, and 2019.5 cm$\sp{-1}$ for H-C$\equiv\sp{13}$C-C$\equiv\sp{13}$C; 3306.3, 2050.6, and 2018.0 cm$\sp{-1}$ for H-C$\equiv\sp{13}$C-$\sp{13}$C$\equiv$C; 3290.1, 2007.9, and 1981.6 cm$\sp{-1}$ for H-$\sp{13}$C$\equiv\sp{13}$C-$\sp{13}$C$\equiv\sp{13}$C; and 2579.3, 2056.5, and 2049.7 cm$\sp{-1}$ for D-C$\equiv$C-C$\equiv$C.