1 Supporting Information for An approach to the Atmospheric Chemistry of Methyl Nitrate and Methylperoxy Nitrite. Chemical Mechanisms of Their Formation and Decomposition Reactions in the Gas Phase Juan F. Arenas, Francisco J. Avila, Juan C. Otero, Daniel Peláez, and Juan Soto* Index of Tables and Figures: Table S1a. Geometrical parameters for CH 3 ONO 2 . ............................................................... 2 Table S1b. CASPT2(14,11) and CAS-SCF(14,11)/ANO-L geometrical parameters of Sd1 and Sd2........................................................................................................................................ 3 Table S1c. CAS-SCF(14,11)/ANO-L geometrical parameters of cis- and trans- CH 3 OONO. ................................................................................................................................. 4 Table S1d. Geometrical parameters for Ci1 (S 1 /S 0 ) and Ci2(S1/S0) conical intersections(CAS-SCF(14,11)/cc-pVDZ................................................................................. 5 Table S1e. Geometrical parameters of the transition state (Ts1) for CH 2 O elimination from CH 3 ONO 2 , CH 3 ONO 2 → CH 2 O + HONO. ................................................. 6 Table S2. B3-LYP/aug-cc-pVDZ geometrical parameters of cis- and trans- CH 3 OONO and fictitious transition states for dissociation (Ts2 and Ts3)............................. 7 Table S3. Energetic and kinetic parameters for the isomerization and elimination reactions of CH 3 ONO 2 and CH 3 OONO at the B3-LYP/aug-cc- pVDZ level. ................................................................................................................................ 8 Table S4. B3-LYP/aug-cc-pVDZ geometrical parameters of the transition state (Ts4) for CH 3 ONO 2 → cis-CH 3 OONO rearrangement. .......................................................... 9 Table S5. B3-LYP/aug-cc-pVDZ geometrical parameters of the transition states Ts5 and Ts6 . .............................................................................................................................. 10 Table S6. Cartesian Coordinates in Å for Ts7 (B3-LYP/aug-cc-pVDZ)............................... 11 Figure S1. (a) B3-LYP/aug-cc-pVDZ IRC starting at the transition state Ts2. (b) B3-LYP/aug-cc-pVDZ IRC starting at Ts3. ....................................................................... 12 Figure S2. B3-LYP/aug-cc-pVDZ IRC starting at Ts4. ......................................................... 13 Figure S3. Potential energy surfaces for CH 3 ONO 2 → trans-CH 3 OONO reaction (a) S 0 and S 1 surfaces at the CAS(14, 11) level, (b) S 0 surface showing molecular arrangements of the four corners of the surfaces, labels denote coordinates. .......................................................... 14 Figure S4. B3-LYP/aug-cc-pVDZ IRC starting at Ts1. ......................................................... 15 Figure S5. B3-LYP/aug-cc-pVDZ IRC starting at (a) Ts5 and (b) Ts6................................. 16