Thermal isomerization and cyclization of 1-naphthylacetylene: Experimental results, quantum chemical and transition state theory calculations

The isomerization of 1-naphthylacetylene diluted in argon was studied behind reflected shock waves in a 2 in i.d. single pulse shock tube over the temperature range 1000-1250 K and overall densities of ∼3 × 10 ^-5 mol/cm³. The only reaction product found in the post shock mixtures was acenaphthylene. The first order rate constant of the isomerization was found to be k = 2.08 × 10¹² exp(-54.2 × 10³/RT) s^-1 where R is expressed in units of cal/K mol. Potential energy surfaces of the cyclization reaction 1-naphthyl-acetylene -→ acenaphthylene and the isomerization 1-naphthylacetylene -→ 2-naphthylacetylene were calculated using the Becke three-parameter hybrid method with Lee-Yang-Parr correlation functional approximation (B3LYP). Structure, energy and frequency calculations were carried out with the Dunning correlation consistent polarized double ξ (cc-pVDZ) basis set. The rate constant (k_&infin) for the 1-naphthylacetylene -→ acenaphthylene cyclization was calculated using transition state theory, the value obtained is k_∞ = 3.52 × 1012 exp(-55.9 × 10³/RT) s^-1, where R is expressed in units of cal/K mol. The agreement between the experiment and the calculations is very good. RRKM calculations were done to transfer k_∞ to the pressure of the single pulse shock tube experiments. In view of high temperature and the large molecule involved the deviation from k_∞ is very small. The isomerization 1-naphthylacetylene → 2-naphthylacetylene proceeds via the formation of an unstable intermediate 1,2-naphthalenocyclobutene and has a high barrier of <83.5 kcal/mol. In view of this high barrier, the isomerization cannot compete with the cyclization that proceeds with a barrier of ∼56 kcal/mol.