Some early shock tube experiments done by Prakash and Park estimated that the reduction in radiative flux could be as large as 12.5% with strong absorption of VUV radiation at temperatures between 3500-4000K. These experiments were conducted with shock heated acetylene and methane and the absorption was attributed to C 3 and C 2 H. Studying the composition of the pyrolysis gases injected into the boundary layer by carbonaceous ablating heat shields revealed that C 3 is the predominant pyrolysis species. Having a prominent spectral feature in the VUV (centered at 160 nm ), it is proposed that C 3 can absorb the radiative heating due to emission by atomic nitrogen at 174.29 nm. The relevance and importance of studying small carbon clusters and hydrocarbons, C n and C n H (n = 1,5), has urged to authors to study their properties in this paper. Our focus primarily lies on the thermodynamics and spectroscopic properties of these molecules at high temperatures relevant to ablating boundary layer conditions since these are major constituents of the pyrolysis gases.