Carbon Clusters: Thermochemistry and Electronic Structure at High Temperatures.

Area of Science:

• Aerospace Engineering
• Computational Chemistry
• Chemical Physics

Background:

• Pyrolysis gases, primarily small carbon clusters and hydrocarbons, are key components in the boundary layer of ablating heat shields.
• Limited thermochemical data exists for crucial molecules like C3H and C4H, hindering accurate modeling.

Purpose of the Study:

• To compute accurate thermochemistry and electronic structure data for small carbon clusters (up to four carbons) and hydrocarbons.
• To investigate the role of electronically excited states in the thermochemistry and radiative properties of these species.
• To assess the impact of these findings on hypersonic applications and compare with existing data.

Main Methods:

• Employed the Weizmann-1 (W1) composite method for high-accuracy thermochemistry calculations.
• Utilized the equations of motion coupled cluster singles doubles (EOM-CCSD) method to study electronically excited states.
• Calculated electronic energies, heats of formation, harmonic frequencies, and rotational constants.

Main Results:

• Determined thermochemistry data for C3H and C4H, including low-lying electronic states.
• Identified C4 and C4H excited states as potential sources of radiation absorption in the boundary layer.
• Found that neglecting excited states can lead to up to 12% error in specific heat capacity (Cp) values.
• Observed an order of magnitude difference in mixture compositions compared to JANAF and Gurvich Tables.
• The rhombic isomer of C4 significantly increases its equilibrium mole fraction by 28%.

Conclusions:

• Accurate thermochemistry, including excited states, is essential for modeling pyrolysis gases in high-temperature regimes.
• The computed data provides a more accurate basis for hypersonic applications, highlighting discrepancies with current standard tables.
• The inclusion of typically neglected isomers like rhombic C4 is critical for accurate composition predictions.