J. Phys. Chem. A 2010, 114, 3523–3532
3523
Atmospheric Chemistry of Isopropyl Methyl Methylphosphonate and Dimethyl N,N-Dimethylphosphoroamidate Sara M. Aschmann,† Ernesto C. Tuazon,† William D. Long,† and Roger Atkinson*,†,‡ Air Pollution Research Center, Department of EnVironmental Sciences, and Department of Chemistry, UniVersity of California, RiVerside, California 92521 ReceiVed: December 9, 2009; ReVised Manuscript ReceiVed: January 26, 2010
Rate constants for the reactions of OH radicals with isopropyl methyl methylphosphonate [(CH3)2CHOP(O)(CH3)OCH3; IMMP] and dimethyl N,N-dimethylphosphoroamidate [(CH3O)2P(O)N(CH3)2; DMDMPA] have been measured over the temperature range 283-350 K and atmospheric pressure of air using a relative rate method. The rate expressions obtained were 2.72 × 10-13 e(1642(144)/T cm3 molecule-1 s-1 (6.72 × 10-11 cm3 molecule-1 s-1 at 298 K) for OH + IMMP and 6.05 × 10-13 e(1185(144)/T cm3 molecule-1 s-1 (3.23 × 10-11 cm3 molecule-1 s-1 at 298 K) for OH + DMDMPA. In addition, rate constants for the reactions of NO3 radicals and O3 with IMMP were measured at 296 ( 2 K, of (4.8 ( 2.1) × 10-16 and 290 nm) and reactions with OH radicals, NO3 radicals, and O3.1 Organophosphorus compounds are widely used as pesticides2 and may be present in the atmosphere, where they can undergo transport and chemical reactions. To date, kinetics of the gasphase reactions of a number of organophosphorus compounds of structures (RO)nP(X)(SR)3-n and (RO)2P(X)Y, where R ) CH3 or C2H5, X ) O or S, and Y ) H, CH3, C2H5, NH2, NHCH3, N(CH3)2, OCHdCCl2, and Cl, with OH radicals, NO3 radicals, and O3, have been studied,3–13 with a number of the OH radical reaction rate constants being measured as a function of temperature.12,13 For those organophosphorus compounds for which data are available, reaction with OH radicals is calculated to dominate in the atmosphere.3–6,8,10,12 The products of the reactions of OH radicals with a number of these organophosphorus compounds have been investigated,8,10–12,14 most extensively for dimethyl methylphosphonate [(CH3O)2P(O)CH3; DMMP],10 triethyl phosphate [(C2H5O)3PO; TEP],8 and O,O,O-triethyl phosphorothioate [(C2H5O)3PS; TEPT].12,14 Based on the products observed from the OH radical initiated reactions of DMMP and TEP and the significant deuterium isotope effect observed for the DMMP-d9 reaction,8,10 the reactions of alkyl phosphates and alkyl phosphonates [(RO)3PO and (RO)3-xP(O)Rx] are postulated to proceed as shown in Scheme 1. For DMMP and TEP (and presumably also for the other alkyl phosphonates studied to date), the OH radical initiated reactions appear to proceed mainly by H-atom abstraction from the -OCH3 or -OC2H5 groups, with the resulting >P(O)OCH(O•)R (R ) H or CH3) alkoxy radical * To whom correspondence should be addressed. Telephone: (951) 8274191. E-mail:
[email protected]. † Air Pollution Research Center. ‡ Department of Environmental Sciences and Department of Chemistry.
rearranging through a five-member transition state to >P(O)OH + RC•O (Scheme 1).8,10 However, for organophosphorus compounds containing -OCH(CH3)2 group(s), the alkoxy radical formed after H-atom abstraction from the tertiary C-H bond in the -OCH(CH3)2 group cannot undergo this rearrangement. Therefore, to further investigate the products and mechanisms of these OH + organophosphorus compound reactions, in this work we have carried out kinetic and product studies of the gas-phase reaction of OH radicals with the relatively volatile model compound isopropyl methyl methylphosphonate [(CH3)2CHOP(O)(CH3)OCH3; IMMP]. In addition, we have studied the kinetics and products of the reaction of OH radicals with dimethyl N,Ndimethylphosphoroamidate [(CH3O)2P(O)N(CH3)2; DMDMPA], a simple model alkyl phosphoroamidate, over the temperature range 284-350 K, and measured rate constants for the reactions of NO3 radicals and O3 with IMMP at room temperature (the corresponding rate constants for DMDMPA have been reported previously,5 with upper limits to the rate constants for the reactions of NO3 radicals and O3 with DMDMPA of
