Experimental and theoretical studies of isoprene reaction with NO3

The reaction of isoprene with nitrate radicals (NO3) has been investigated using combined experimental and theoretical approaches. A fast-flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection was used to measure the rate constant of the NO3-isoprene reaction, yielding a valu e of (7.3 +/- 0.2) x 10(-13) cm(3) molecule(-1) s(-1) in the pressure range of 5-7 Torr and at 298 +/- 2 K. The reaction product, the NO3-isoprene add uct radical, was directly detected using the CIMS method. In addition, dens ity functional theory and ab initio molecular orbital calculations have bee n employed to determine the structures and energies of the NO3-isoprene add uct isomers. Geometry optimizations were performed using density functional theory at the B3LYP/6-31G(d,p) level, and the single-point energies were c omputed using second-order Moller-Plesset perturbation theory and the coupl ed-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). At the CCSD(T)/6-31G(d) level of theory, the zero-point-corrected energies of the NO3-isoprene addu ct radicals are 15 to 31 kcal mol(-1) more stable than the separated NO3 an d isoprene, and the isomers of terminal NO; additions are more energeticall y favorable than those of internal NO3 additions. The rate constants of the formation of the NO3-isoprene adduct radicals and their isomeric branching have been calculated using the canonical variational transition state theo ry.