The benzene+OH potential energy surface: intermediates and transition states

Abstract

The potential energy surface for the interaction between benzene and hydroxyl radical is studied in detail using quantum mechanical methods, with a particular focus on the hydrogen abstraction pathway. Geometric parameters are optimized using a variety of density functional methods as well as perturbation theory. Energies are refined using coupled cluster singles and doubles with perturbative triples [CCSD(T)] extrapolated to the complete basis set limit. At our most reliable level of theory, complexation energies are found to be (with zero-point corrected energies in parentheses) 3.7 (2.8) kcal/mol for the benzene-hydroxyl radical complex and 2.9 (-1.7) kcal/mol for the phenyl radical-water complex. The barrier to H abstraction lies 6.5 (4.2) kcal/mol above the infinitely separated benzene and hydroxyl radical monomers.

Evaluation of DFT methods to study reactions of benzene with OH radical

“…Unfortunately, there has been little study of these individual energetic quantities at high levels of theory. One very recent work, however, has reported values [29] for these quantities, which are listed in the last row of Table 5.…”

“…The exception is PBEPBE which goes directly to products in either case, with no barrier, and seems ill-suited to study of these processes. [29] …”

“…As indicated above, there have been a number of studies of both the H· abstraction and the OH· addition process of benzene, using fairly high levels of theory [24,25,27,29,46]. In addition to various specific levels of theory, the results differ also in whether or not zero-point vibrational terms have been added to the electronic energies.…”

“…The later dissociation to products C 6 H 5 and HOH, E 4 , requires a bit more energy, between 2 and 3.5 kcal/mol. The CCSD(T) computations [29] found values of about 3 kcal/mol for both of these complexation energies. The underestimates of the DFT binding energies, particularly that between C 6 H 6 and OH·, are likely due at least in part to their failure to properly include dispersion forces.…”

“…Reactions of OH· and benzene were examined in 2009 [28] from both experimental and computational perspectives, and more recently with the most accurate calculations to date [29]. The reactions of naphthyl radical with acetylene were studied and it was shown that calculations could reach agreement with experiment [30].…”