Carbon Clusters: Thermochemistry and Electronic Structure at High Temperatures

Abstract

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.

Comprehensive Study of HCN: Potential Energy Surfaces, State-to-State Kinetics, and Master Equation Analysis

“…27,28 In the future, StS kinetics made available in this work can be used to study coarse grain models 29−36 and develop accurate models for ablation and pyrolysis chemistry. 37 The associated diatom + atom systems with the three PESs are shown in Figure 1.…”

“…Our new PESs are used for quasi-classical trajectory (QCT) calculations to generate the rovibrational StS kinetic database for the molecule-atom collisions. A detailed analysis of the StS kinetics is presented by studying isothermal heat bath simulations using the PLATO library. , In the future, StS kinetics made available in this work can be used to study coarse grain models − and develop accurate models for ablation and pyrolysis chemistry . The associated diatom + atom systems with the three PESs are shown in Figure .…”