Photochemistry of (CH3)3CC(O)H at 254 nm

DIANA P. HENAO A.; GUSTAVO A. ARGÜELLO; FABIO E. MALANCA

Carlos Paz

Congreso; 13th Latin American Conference on Physical Organic Chemistry (CLAFQO-13); 2015

Sociedad Argentina de Investigación en Química Orgánica

Aldehydes have natural and anthropogenic sources, primarily associated with vehicle exhaust, industrial activity, and secondarily with the oxidation of volatile organic precursors. Chemical removal of aldehydes from the atmosphere can occur by gas phase reaction with OH and NO3 radicals or by photolysis at wavelength that decreases with altitude. Photolysis of trimethyl acetaldehyde (2,2 dimethyl propionyl aldehyde, (CH3)3CCHO, TMA) is an important loss process that leads to its degradation in the atmosphere. In this work, we present the determination of the quantum yield at 254 nm. UV absorption cross sections (s) were measured from 200 to 400 nm using a standard UV gas cell and a UV?vis spectrophotometer with a diode array detector. Photolysis experiments were performed using a quartz cell and a low-pressure mercury lamp (15 W). TMA was irradiated alone and in the presence of cyclohexane (90 mbar) to trap the radicals formed so as to ensure that they did not contribute to secondary reactions. The analysis of the infrared spectra reveals the formation of carbon monoxide, isobutane and formaldehyde as a consequence of three different processes.The appearance of the products could be rationalized assuming several possible pathways for the photolytic rupture: (CH3)3CCHO ® (CH3)3C· + HC(O)·                                                                       (1a)                      ® (CH3)3CC(O)· + H·                                                                    (1b)                          ® (CH3)3CH + CO                                                                        (1c) The quantum yield was measured employed CF3C(O)Cl (s254nm = 6.86 x 10-20 cm2 molecule-1, f @ 1.0) as actinometer. Quantification of the disappearance of TMA leads to a global value of (0.60 ± 0.05) for the total quantum yield, in good agreement with the values derived for similar aldehydes by other workers, for instance 0.65 for propanal  and 0.63 for isobutanal. The quantum yield determined for the C-C scission has a value of fa = 0.32, while the sum (fb + fc) has a value of 0.30. These results also agree with those reported for similar: 0.28 and 0.32 for propanal; 0.20 and 0.40 for isobutanal.