Thermodynamic properties of linear carbon chain molecules with conjugated triple bonds: Part I. Polyacetylenes, H(CC) nH ( n = 2–6), cyanopolyacetylenes, H(CC) nCN ( n = 1–5), and dicyanopolyacetylenes, NC(CC) nCN ( n = 1–4)

Abstract

The available structural parameters, fundamental frequencies and enthalpies of formation, if any, for polyacetylenes, C 2 n H 2 ( n = 2–6), cyanopolyacetylenes, HC 2 n+1 N ( n = 1–5), and dicyanopolyacetylenes, C 2 n N 2 ( n = 2–5), which are known to be interstellar molecules, were critically evaluated and the recommended values selected. Molecular constants and enthalpies of formation for most of the molecules were estimated as the experimental values for these compounds are not available. The similarity transference procedure has been developed for the calculation of bond distances and force constants. Using a rigid-rotor and harmonic-oscillator approximation, this information was utilized to calculate the ideal gas thermodynamic functions for 14 molecules. The calculated values of heat capacities, C p°, Gibbs energies, -( G°- H 0°)/ T, and enthalpies, H°- H 0°), were fitted to the suitable polynomial functions. The constants of these polynomials allow the values of C p°, -( G°- H 0°)/ T) and H°- H 0°)/ T for considered compounds between 298.15 and 2000 K to be calculated. These constants together with the values of enthalpy of formation, Δ f H° (298.15 K), estimated using the group additivity method are presented in the paper.