This paper studies the thermochemistry
and electronic structure
of small carbon clusters and hydrocarbons, which are major constituents
of pyrolysis gases released into the boundary layer of ablating heat
shields. Our focus lies on clusters of up to four carbon atoms. Among
other molecules, thermochemistry data for molecules such as C3H and C4H have been determined using the Weizmann-1
(W1) method. These molecules have very limited thermochemistry data
recorded in the literature, thereby necessitating new and accurate
computations of required properties such as electronic energies of
low-lying states, heats of formation, harmonic frequencies, and rotational
constants. A study of electronically excited states of these molecules
computed using the equations of motion coupled cluster singles doubles
method revealed C4 and C4H to be potential sources
of radiation absorption in the boundary layer. The excited electronic
states of interest are studied further to obtain their optimum geometries,
rotational constants, and vibrational frequencies. Moreover, we also
study the effect of low-lying excited electronic states on the partition
function to assess their effect on the thermodynamics of these pyrolysis
gases in the high-temperature regime. Neglecting the excited electronic
states records a maximum difference of 12% in the computed specific
heat capacity values, C
p values. Finally,
comparisons of the equilibrium mole fractions obtained using the thermodynamics
computed in this paper with the existing state-of-the-art tables used
for hypersonic applications (e.g., JANAF and Gurvich
Tables) show an order of magnitude difference in the mixture compositions.
It is shown that the rhombic isomer of C4 (1Ag), which is energetically close to the ground state
(3Σg
–) and usually neglected in composition calculations,
contributes to a 28% increase in the equilibrium mole fraction of
the C4 molecule.