Wavelength-Dependent Photolysis of i-Pentanal and t-Pentanal from 280 to 330 nm

Abstract

Photodissociation of i-pentanal ((CH3)2CHCH2CHO; 3-methylbutanal) and t-pentanal ((CH3)3CCHO; 2,2-dimethylpropanal) in the 280−330 nm region was investigated at 5 nm intervals by using dye laser photolysis in combination with cavity ring-down spectroscopy. Absorption cross-sections of i-pentanal and t-pentanal were obtained. Both aldehydes exhibited a broad absorption band peaking at 295 nm, with maximum absorption cross-sections of (5.90 ± 0.22) × 10-20 and (4.26 ± 0.07) × 10-20 cm2, respectively, for i-pentanal and t-pentanal. The formation yield of the HCO radical from photolysis of i-pentanal and t-pentanal was determined by monitoring HCO absorption at 613.8 nm. Except for 325 and 330 nm photolysis of i-pentanal, the HCO yield was found to be independent of aldehyde pressure (2−12 Torr) and total pressure (20−400 Torr). For the photodissociation of t-pentanal, the HCO yield was also determined in the 0.5−2 Torr range and was found to increase with t-pentanal pressure. The variation of HCO radical yield with photolysis wavelength was determined. The peak HCO yields from t-pentanal and i-pentanal photolysis were 0.92 ± 0.11 and 0.40 ± 0.08 at 315 nm, respectively, where uncertainty reflects experimental scatter only. The difference in the peak HCO yield from t-pentanal and i-pentanal photolysis was attributed to the opening up of the Norrish II channel for i-pentanal. End products from closed-cell photolysis of i-pentanal and t-pentanal were investigated at 290, 310, and 330 nm by using HPLC. Acetaldehyde was found to be a product from the photodissociation of i-pentanal/N2 mixtures. Photolysis rates of i-pentanal and t-pentanal to form HCO were calculated for two representative atmospheric conditions (noontime at sea level and 40° N latitude on January 1 and on July 1) and were compared to those obtained from acetaldehyde photolysis.

Modelling the evolution of organic carbon during its gas-phase tropospheric oxidation: development of an explicit model based on a self generating approach

“…peroxy acetyl nitrate Atkinson et al (1999) a quantum yields are from Desai et al (1986); b this structure was also used for di substituted aldehydes on the α carbon; c quantum yields of the radical channel are from Zhu et al (1999), quantum yield for the Norrish type II reaction are from Moortgat et al (1999) for n-pentanal; d enols (>C=C(OH)−) produced by the Norrish type II reaction pathways are assumed to rearrange to the carbonyl form (>CH−CO−); e the more labile H is used to perform the Norrish II reaction if distinct H are available on the γ carbon; f dissociation occurs at the weakest CO−C bond. Bond energies are estimated using the Benson method (Benson, 1976); g total quantum yield based on 2-butanone values from Raber and Moortgat (1995); h this reaction is also used for −CH 2 −CO−C≡ structures; i average value of 3-pentanone and 2,4-dimethyl-3-pentanone; j this reaction is also used for >CH−CO−C≡ and ≡C−CO−C≡ structures; k branching ratios for the molecular and radical channel set to 0.7 and 0.3, respectively.…”

NO spin trapping and EPR studies on the photochemistry of aliphatic aldehydes

“…As mentioned above, except for the primary -bond cleavage, isopentanal also undergoes a Norrish II process to form acetaldehyde [Eqn (6)] under UV light owing to the lower enthalpy of reaction,  f H 298°D 21.6 kcal mol 1 . 9 The relative quantum yield of Norrish II process is 0.7 for isopentanal. 9 Hence acetaldehyde is produced.…”

“…9 The relative quantum yield of Norrish II process is 0.7 for isopentanal. 9 Hence acetaldehyde is produced. The observation of 1 is reasonable.…”

UV absorption spectra and self‐reaction rate constants for primary peroxy radicals arising from the chlorine‐initiated oxidation of carbonyl compounds

“…In laboratory studies, Cl atoms are often used to generate peroxy radicals when the way of formation of OH radicals, like UV flash-lamp photolysis or Kr-F laser photolysis of H 2 O 2 may also photolyze the peroxy radical precursor. This is the case for precursors like pivalaldehyde (CH 3 ) 3 CCHO which may be photolyzed because of its large absorption band around 280-290 nm [4]. Branching ratio of the (pivalaldehyde + Cl) reaction has been reported recently [5], using both FTIR and UV measurements:…”