Theoretical Investigation on Stability of the C·H 2 OCl Radical

The reaction of ClO with CH 3 OO forms CH 3 OCl, which is thought to be an important species in ozone destruction over the Artic and Antarctic polar regions. Principal processes for CH 3 OCl loss include abstraction of an H atom by OH or Cl, where the C·H 2 OCl radical is formed. This radical can dissociate to Cl + CH 2 O products via β scission (elimination) of a Cl atom with formation of a strong carbonyl π bond; or if stable, C·H 2 OCl will react with O 2 to form a peroxy radical. Structures and the thermochemical properties (enthalpy,, entropy S (298), and heat capacity) of the C·H 2 OCl radical, a transition state for its dissociation to CH 2 O + Cl (TSCH 2 O−Cl) and a formaldehyde−Cl atom adduct (ACH 2 O∼Cl), are estimated by ab initio and density functional calculations. Geometries are optimized 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 estimation of energy are performed with B3LYP/6-311+G(3df,2p) and QCISDT/6-31G(d,p) and with composite methods of CBS-Q and G3/MP2. Density functional calculations do not predict the existence of a stable C·H 2 OCl radical; only a lower energy, loosely bound adduct and final products CH 2 O + Cl. HF and MP2 calculations optimize to a C·H 2 OCl radical structure with an O−Cl bond (MP2) of 1.72 Å. values on this MP2 structure are calculated using four different working (isodesmic) reactions. Standard enthalpy based on CBS-Q//MP2/6-31G(d,p) energies with isodesmic reaction analysis on this MP2 structure results in of 32.39 ± 2.21 kcal/mol; this is in excellent agreement with the high level calculations of Espinosa-Garcia (32.0 ± 3.5). MP2 calculations also predict an early transition state for C·H 2 OCl dissociation with an O−Cl bond of 1.78 Å. 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 radical; i.e., no barrier to C·H 2 OCl dissociation is found. Products CH 2 O + Cl and a loosely bound adduct of ACH 2 O∼Cl have much lower energies at −1.86 ± 2.21 and 0.26 ± 2.21 kcal/mol, respectively. We conclude from analysis of the enthalpy values of the three structures (adduct, TST, and products) that the HF- or MP2-optimized C·H 2 OCl structure does not exist as a stable radical and that density functional calculations provide more realistic insight into the chemistry of this species. Abstraction of H atoms from CH 3 OCl by active radicals such as OH, Cl, O, H, ..., etc., result in formation of CH 2 O plus a Cl atom.