Electronic quenching of OH(A, v?=1) by ROH (R= CH3, C2H5 , C3H7 and C4H9): the role of Hydrogen-Bonded complexes

FEDERICO HERNÁNDEZ; GUSTAVO PINO; JULIANA PALMA ; MARCELA C. CAPELLO

Mendoza

Congreso; )th International Meeting on Photodynamics and Related Aspects; 2016

Electronic quenching of OH(A, ?=1) by ROH (R= CH3, C2H5, C3H7 and C4H9): the role of Hydrogen-Bonded complexes.Federico J. Hernándeza, Marcela C. Capelloa, Juliana I. Palmab and Gustavo A. Pinoaa INFIQC (UNC, CONICET) Dpto. de Fisicoquímica, Fac. de Ciencias Químicas, Centro Láser de Ciencias Moleculares. Universidad Nacional de Córdoba, Ciudad Universitaria, Pabellón Argentina, 5000. Córdoba, Argentina.b Universidad Nacional de Quilmes. Roque Sáenz Peña 352 Bernal, B1876BXD, Buenos Aires, ArgentinaThe hydroxyl radical (OH) is an atmospherically important radical, critical in the oxidation chemistry of the atmosphere. OH is often detected in these environments, using laser induced fluorescence (LIF) on its A2Σ+  X2П transition. LIF signals are frequently generated under saturation conditions at atmospheric pressure, consequently, collisional quenching is an important process that needs to be considered for correct interpretation of the spectra. From a dynamical point of view, it has been experimentally observed the formation of intermediate van der Waals (vdW) as well as Hydrogen Bonded (HB) complexes in reactions involving OH, in both X and A states, with several compounds of atmospheric interest, among them, CH3OH and C2H5OH3,4. In this work we study the electronic quenching of OH (A2Σ+) with several alcohols (CH3OH, C2H5OH, 1-C3H7OH, 2-C3H7OH, 1-C4H9OH and 2-C4H9OH). For these series of alcohols (ROH) we found that kq for OH(A) decreases as the length of the side chain of the alcohol increases (Figure 1-right) at variance with the reaction rate constant of OH(X) with the same compounds that increases with the size of the side chain (Figure 1-left) as expected for H abstraction reactions. Theoretical calculations suggest that HB complexes play a crucial role in the dynamics of the process. On one side it determined the large value for all the kq determined and on the other side it can explain the trend of kq with the side chain length. We have associated the fact that kq decrease as the carbon chain increases to a mechanism involving long-range attractive forces, which are important for the head-on collision between OH(A) and the OH group of the ROH. The steric factor associated with larger side chain of the ROHs, will render a lower effective quenching coefficient, as previously observed in related systems5.