Theoretical investigation on stability of the C center dot H2OCl radical

The reaction of ClO with CH3OO forms CH3OCl3, which is thought to be an imp ortant species in ozone destruction over the Artic and Antarctic polar regi ons. Principal processes for CH3OCl loss include abstraction of an H atom b y OH or Cl, where the C .H2OCl radical is formed. This radical can dissocia te to Cl + CH2O products via beta scission (elimination) of a Cl atom with formation of a strong carbonyl pi bond; or if stable, C .H2OCl will react w ith O-2 to form a peroxy radical. Structures and the thermochemical propert ies (enthalpy, DeltaH(f298)degrees entropy S(298), and heat capacity) of th e C .H2OCl radical, a transition state for its dissociation to CH2O + Cl (T SCH2O-Cl) and a formaldehyde-Cl atom adduct (ACH(2)O similar to Cl), are es timated by ah initio and density functional calculations. Geometries are op timized and frequencies are estimated using MP2/6-31G(d,p), B3LYP/6-31G(d,p ), or MP2/6-31G(d) level calculations. Single point calculations for estima tion of energy are performed with B3LYP/6-311 +G(3df,2p) and QC1SDT/6-31G(d ,p) and with composite methods of CBS-Q and G3/MP2. Density functional calc ulations do not predict the existence of a stable C .H2OCl radical; only a lower energy, loosely bound adduct and final products CH2O + Cl. HF and MP2 calculations optimize to a C .H2OCl radical structure with an O-Cl bond (M P2) of 1.72 Angstrom. DeltaH(f298)degrees values on this MP2 structure are calculated using four different working (isodesmic) reactions. Standard ent halpy based on CBS-Q//MP2/6-31G(d,p) energies with isodesmic reaction analy sis on this MP2 structure results in DeltaH(f298)degrees of 32.39 +/- 2.21 kcal/mol; this is in excellent agreement with the high level calculations o f Espinosa-Garcia (32.0 +/- 3.5). MP2 calculations also predict an early tr ansition state for C .H2OCl dissociation with an O-Cl bond of 1.78 Angstrom . Analysis of the MP2/6-31G transition state structure, using either CBS-Q or G3/MP2, yields an enthalpy some 4.4 kcal/mol lower than the stable radic al; i.e., no barrier to C .H2OCl dissociation is found. Products CH2O + Cl and a loosely bound adduct of ACH(2)O similar to Cl have much lower energie s at -1.86 +/- 2.21 and 0.26 +/- 2.21 kcal/mol, respectively. We conclude f rom analysis of the enthalpy values of the three structures (adduct, TST, a nd products) that the HF- or MP2-optimized C .H2OCl structure does not exis t as a stable radical and that density functional calculations provide more realistic insight into the chemistry of this species. Abstraction of H ato ms from CH3OCl by active radicals such as OH, Cl, O, H,..., etc., result in formation of CH2O plus a Cl atom.