Density functional, single and multireference perturbation theory study ofthe reaction (3)Sigma O-g(2)+HOCH2CH2 center dot -> HOO center dot+HOCH=CH2, modeling an important step in tropospheric benzene oxidation

The one-step transformation of hydroxycyclohexadienyl radical into phenol b y O-2 is modeled by the title hydrogen-abstraction reaction, which converts the simplest beta-hydroxy radical to an enol. The reaction is studied by d ifferent quantum-mechanical methods, to assess which level of theory is sim ultaneously reliable and affordable enough to investigate relatively large aromatic systems. Density functional theory (DFT(B3LYP)), unrestricted Moll er-Plesset perturbation theory to the 2nd order (UMP2), and complete active space multiconfiguration self-consistent field (CAS-MCSCF) optimizations a re first carried out to determine stable and transition structures. Then, m ore accurate energetics are determined by spin-projected single-reference P MP4//UMP2 calculations (which are compared with coupled cluster CCSD(T)//UM P2 results), and by two multireference second-order perturbation methods (M R-PT2), based on CAS-MCSCF wave functions and structures. With an (11,9) ac tive space and the 6-311G(d,p) basis set, the MR-PT2 estimates for the ener gy barrier and reaction energy are: 14.5 and -12.1 kcal mol(-1) (CAS-PT2), and 8.3 and -13.4 kcal mol(-1) (MC-QDPT2). These estimates fall between the DFT(B3LYP)/6-311G(d,p) (3.3 and -19.1 kcal mol(-1)) and PMP4/6-311G(d,p) v alues (17.2 and -10.7 kcal mol(-1)). Single-point energy computations using larger basis sets are also discussed. The DFT(B3LYP) method tends to under estimate the barrier for H abstraction; the PMP4 barrier is likely to repre sent an upper bound, given that the single-reference perturbation expansion does not converge very efficiently. For extensions of the study to aromati cs, DFT could be deemed to be an acceptable compromise between reliability and feasibility. (C) 1999 American Institute of Physics. [S0021-9606(99)307 13-3].