ARTICLE pubs.acs.org/JPCA
Photochemistry and Photophysics of n-Butanal, 3-Methylbutanal, and 3,3-Dimethylbutanal: Experimental and Theoretical Study Jovan M. Tadic,*,† Geert K. Moortgat,‡ Partha P. Bera,† Max Loewenstein,† Emma L. Yates,† and Timothy J. Lee† † ‡
NASA Ames Research Center, Moffett Field, Mountain View, California 94035, United States Atmospheric Chemistry Department, Max-Planck-Institut f€ur Chemie, P.O. Box 3060, 55020 Mainz, Germany
bS Supporting Information ABSTRACT: Dilute mixtures of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal in synthetic air, different N2/O2 mixtures, and pure nitrogen (up to 100 ppm) were photolyzed with fluorescent UV lamps (275380 nm) at 298 K. The main photooxidation products were ethene (n-butanal), propene (3-methylbutanal) or i-butene (3,3-dimethylbutanal), CO, vinylalcohol, and ethanal. The photolysis rates and the absolute quantum yields were found to be dependent on the total pressure of synthetic air but not of nitrogen. At 100 Torr, the total quantum yield Φ100 = 0.45 ( 0.01 and 0.49 ( 0.07, whereas at 700 Torr, Φ700 = 0.31 ( 0.01 and 0.36 ( 0.03 for 3-methylbutanal and 3,3-dimethybutanal, respectively. Quantum yield values for nbutanal were reported earlier by Tadic et al. (J. Photochem. Photobiol. A 2001 143, 169179) to be Φ100 = 0.48 ( 0.02 and Φ700 = 0.32 ( 0.01. Two decomposition channels were identified: the radical channel RCHO f R + HCO (Norrish type I) and the molecular channel CH3CH(CH3)CH2CHO f CH2CHCH3 + CH2dCHOH or CH3C(CH3)2CH2CHO f CHC(CH3)CH3 + CH2dCHOH, (Norrish type II) having the absolute quantum yields of 0.123 and 0.119 for 3-methybutanal and 0.071 and 0.199 for 3,3-dimethylbutanal at 700 Torr of synthetic air. The product ethenol CH2dCHOH tautomerizes to ethanal. We have performed ab initio and density functional quantum (DFT) chemical computations of both type I and type II processes starting from the singlet and triplet excited states. We conclude that the Norrish type I dissociation produces radicals from both singlet and triplet excited states, while Norrish type II dissociation is a two-step process starting from the triplet excited state, but is a concerted process from the singlet state.
1. INTRODUCTION Photodissociation of aldehydes represents an important source of free radicals in the lower atmosphere and thus may significantly influence the atmospheric oxidation capacity.13 n-Hexanal, n-heptanal, and higher aldehydes have been observed in ambient air and in emissions of various plants, especially grasses, with comparable emissions rates to the monoterpenes.46 The examination of longer chain aldehydes has recently come into the research focus, providing quantum yield and decomposition pattern data necessary for atmospheric modeling.713 Aldehydes are an important source of chain-initiating radicals throughout the Earth’s troposphere via their Norrish type I photodecomposition channel.14 Aldehydes also appear to be key components of secondary organic aerosol formation, contributing to polymerization reactions.15,16 The smallest members of the aldehyde family, formaldehyde, acetaldehyde, and propanal, have been studied extensively.1725 Aldehydes with four or more carbons have additional reaction pathways, like photocyclization and intramolecular rearrangement, and have been studied from the point of view of their atmospheric importance, providing data necessary for modeling and the understanding of complex atmospheric reaction patterns they are involved in. An extensive r 2011 American Chemical Society
literature review reveals that few important issues have remained unsolved, specifically concerning some photophysical aspects of aldehyde photolysis. Paulson et al.12 pointed out after theoretical examinations, that the very existence of Norrish type I process (∼10%) cannot be understood from an energetically point of view if initial photoexcited state of n-heptanal is solely the triplet state. It has been earlier proposed that aldehyde decomposition generally proceeds from the excited triplet state produced after fast intersystem crossing from the initially generated excited singlet state.26,27 Tadic et al.13 proposed further examination of the aldehyde photolysis mechanism on the pressure of pure oxygen to try to distinguish products based on initial aldehyde triplet and singlet states, as triplet state should be more efficiently quenched by the triplet ground electronic state of oxygen. In another study, Tadic et al.28 concluded that, in the case of noctanal, the very existence of the Norrish type I products is not clear as there are energetically more favorable decomposition Special Issue: A. R. Ravishankara Festschrift Received: September 8, 2011 Revised: November 2, 2011 Published: November 03, 2011 5830
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routes. In addition, the Norrish type I/II ratio suddenly changes from ∼2.12 to ∼0.25 between n-butanal and n-pentanal and remains similar (0.210.38) for all higher straight chain aldehydes.8 Theoretical explanation for this unusual behavior is unclear, as n-butanal has available γ-H atoms and, consequently, the Norrish type II channel should be energetically more favorable. In this paper, the products and absolute quantum yields obtained in the pressure range 100700 Torr from the photolysis of small quantities of n-butanal, 3-methylbutanal, and 3,3dimethylbutanal in air and nitrogen, using broad band emission lamps (275380 nm), are reported together with a theoretical investigation of the reactions of triplet and singlet electronic excited states of aldehydes that result from photoexcitation, using density functional theory (DFT) and ab initio quantum chemistry methods.
2. EXPERIMENTAL DETAILS The apparatus employed in this work has been described elsewhere29,30 and will be briefly discussed here. The central part of the apparatus is a 44.2 L (1.40 m length and 20 cm diameter) quartz cell equipped with two independent sets of White-optic mirror arrangements. Sapphire-coated aluminum mirrors were used in the infrared region (l = 33.6 m) for the measurements of the educts and products. Infrared spectra at 0.5 cm1 resolution (4504000 cm1) were measured with a Bomem DA8-FTIR spectrometer. This method provides the possibility of simultaneous detection and monitoring of all the IR-active products and the starting material. Photolysis was achieved with six radially mounted lamps, TL/12-sunlamps (275380 nm, Philips 40W). Spectra were taken every 510 min with a total irradiation time of 3050 min. The extent of the conversion of initial compound was approximately 30%. Experiments were carried out at room temperature (298 K), at pressures between 100 and 700 Torr (1 Torr = {101325/ 760}Pa), with an initial concentrations of approximately 100 ppm. Carbonyl compounds were obtained from Sigma-Aldrich Co. with purity higher than 98%. The calibration of vinylalcohol was done indirectly, from the amount of formed ethanal and the converted vinylalcohol (peak decrease), because the tautomerization of these compounds is 1:1 (ketoenol tautomerism) according to CH2 dCH OH a CH3 CHdO
ð1Þ
3. QUANTUM MECHANICAL METHODS Quantum chemical computations were carried out using ab initio and density functional theory (DFT) methods. Becke’s three-parameter exchange functional31 was used along with the correlation functional of Lee, Yang, and Parr (LYP).32 Pople’s 6-31+G(d)33,34 split valence basis set was used along with the above-mentioned methods. The geometries of all molecular species were fully optimized. Vibrational frequency calculations were performed at the above-mentioned levels of theory to confirm that minimum energy structures have no imaginary frequencies and transition structures (TS) have the appropriate imaginary frequency. For further refinement of energies of the triplet states, ab initio quantum chemical computations were performed using unrestricted MøllerPlesset perturbation theory (UMP2) in conjunction with Dunning’s correlation consistent valence triple-ζ basis set (cc-PVTZ).35 The spin
contamination in unrestricted calculations was found to be low. The ÆS2æ value for a pure triplet state should be 2.00, and the largest ÆS2æ value among all the UMP2 calculations was 2.05 for the type II dissociation transition state for the 3-methylbutanal. ÆS2æ values for all other triplet calculations were less than 2.05. For the singlet excited states, the configuration interaction singles with perturbative doubles [CIS(D)]36 method was used in conjunction with the cc-pVTZ basis set. The Q-Chem3.2 quantum chemical package was used for all the calculations.
4. MECHANISMS All aliphatic aldehydes exhibit a weak absorption band in the wavelength range 240360 nm as a result of a symmetry forbidden nπ* transition.26,37 There have been a number of studies devoted to the photodissociation of the simplest alkyl aldehydes, such as HCHO, CH3CHO, C2H5CHO,1725 and a recently growing number devoted to longer chain aldehydes, such as C3H7CHO,8 C4H9CHO,7,8 i-pentanal and t-pentanal,9 nhexanal,10,11 n-heptanal,1113 and n-octanal.28 Photodissociation patterns of other aldehydes suggest that processes 2 and 4 and 3 and 5 play an important role in the photolysis of longer chain aldehydes: 3-methylbutanal þ hv f CH3 CHðCH3 ÞCH2 þ HCO ðNorrish type IÞ
ð2Þ
f CH2 CHCH3 þ ½CH2 dCHOH ðNorrish type IIÞ
ð3Þ
3; 3-dimethylbutanal þ hv f CH3 CðCH3 Þ2 CH2 þ HCO ðNorrish type IÞ
ð4Þ
f CHCðCH3 ÞCH3 þ ½CH2 dCHOH ðNorrish type IIÞ
ð5Þ
Processes 2 and 4 represent the fragmentation into free radicals, with an enthalpy of around 84 kcal mol1, corresponding to a photochemical threshold of around 340 nm.7 Processes 3 and 5, which are common to molecules with a γ-hydrogen atom, are an intramolecular rearrangement with enthalpy around 18 kcal mol1 (λ e 1454 nm).7 The enthalpy change for processes 3 and 5 was calculated assuming that the keto-form of the acetaldehyde is formed in the primary step. This assumption is not correct, and, therefore, the enthalpy change for this reaction should be adjusted for the difference between the heats of formation of enol and keto forms of acetaldehyde (the equilibrium constant for ketoenol equilibrium is 5 106, and the corresponding ΔG value is ∼17 kcal mol1).38 In our previous studies, the total contribution of the Norrish type I and II processes in the case of n-butanal, 8 n-pentanal,8 n-hexanal, 10 n-heptanal,13 and n-octanal28 photolysis is, however, less than 100%, indicating further reaction pathways (the calculation is made on carbon balance data). Paulson and Orlando14 and Tang and Zhu11 estimated the yield of cyclic alcohols 1530% and 40% in the case of n-heptanal, respectively, which correlates well with the carbon balance of Norrish type I and II products found in our studies.
5. RESULTS AND DISCUSSION 5.1. Products. The photooxidation experiments were carried out in a long-path quartz cell with the detection of precursors and 5831
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The Journal of Physical Chemistry A products by FTIR spectroscopy. Although the UV spectra for aliphatic straight chain aldehydes above C4 are practically identical,39 it is not the case for branched chain aldehydes (see Figure 1). Detailed information on the absorption spectra is found in the Supporting Information. Major products observed in the photolysis of 3-methylbutanal (a) and 3,3-dimethylbutanal (b) were CO (νmax = 2037 2235 cm1), propene (a; 880940, 980995, 14301485, 16201680, 28303150 cm1), i-butene (b; 882892, 13601400, 14301485, 16301680, 28303020, 3060 3110 cm1), ethenol (947.6, 1078, 1118.3, 1259.8 cm1), and ethanal (1348.51355.5 cm1). Figure 2 shows the concentrationtime profiles for the two products formed in the photolysis of 3-methylbutanal and 3,3-dimethylbutanal used for the estimation of Norrish type I/II absolute yields and their ratios. Propene and i-butene are formed exclusively as a primary product in the reactions. Ethanal is secondary product arising from ethenol tautomerization (reaction 1) and undergoes further photolysis, which was described in our previous studies on n-butanal to noctanal.8,10,13,28 In this study, only the identification of ethanal was done.
Figure 1. Absorption cross sections of n-pentanal,8 n-butanal,8 and 3-methylbutanal.9,39.
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Norrish type I reaction gives two radicals, which immediately react with oxygen. One of the products, formyl radical HCO is quantitatively converted to CO and HO218,39,40 according to HCO þ O2 f HO2 þ CO
ð6Þ
This process is followed through the rate of production of CO (20942096 cm1). The remaining alkyl radicals are oxidized forming alkylperoxy radicals: R þ O2 þ M f RO2 þ M
ð7Þ
The RO2 radicals subsequently react with HO2, recombine or disproportionate, to produce a variety of products (reactions 811):41,42 RO2 þ HO2 f ROOH þ O2
ð8Þ
2RO2 f 2RO þ O2
ð9Þ
f ROH þ R 0 H0 CHO þ O2
ð10Þ
RO þ O2 f R 0 H0 CHO þ HO2
ð11Þ
However, due to the estimated low reaction rate constant for the recombination and disproportionation, reactions 9 and 10,42 the major expected products are alkyl-hydroperoxides. These peroxides were not observed in the FTIR spectrum. Their concentrations were either below the detection limit (∼1 mTorr) or the signals interfered with those of other compounds. No evidence of other aldehydes produced via channels 10 and 11 were observed. Both propene and i-butene are stable molecular species and were used to calculate the yields of the Norrish type II processes. The coproduct of the reactions is ethenol CH2dCHOH, which tautomerizes slowly to ethanol via reaction 1. Ethenol concentration reaches a maximum after ∼40 min photolysis, and its subsequent decay is caused by its conversion to ethanal. Figure 3 shows profiles of product concentrations (partial pressures) of CO and i-butene versus loss of initial 3,3-dimethylbutanal. The analogous plot was made for 3-methylbutanal. According to the suggested mechanism, the yields of propene or i-butene should be identical to the sum of the yields of ethenol and ethanal for both examined aldehydes, because all compounds are products of the same decomposition channel. CO originating
Figure 2. Photolysis of 3-methylbutanal and 3,3-dimethylbutanal: time profile, variation of the partial pressures of products. Curves of CO (A) show that CO is not only the primary product, but also a product of the reactions following a primary step. 5832
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Scheme 1. Cyclization Pathways for Two Aldehydes
Figure 3. Photolysis of 3,3-dimethylbutanal: products formed vs loss of 3,3-dimethylbutanal. See comments in Figure 2.
Table 2. Absolute Yields of Norrish Type I and II Decomposition Products in 3-Methylbutanal and 3,3-Dimethylbutanal Photolysis in Synthetic Air (Errors Represent the Experimental Scatter) 3-methylbutanal
Table 1. Relative Yields of Norrish Type I and II Products in 3-Methylbutanal and 3,3-Dimethylbutanal Photolysis (Errors are Represented by Experimental Scatter) 3-methylbutanal
3,3-dimethylbutanal
3,3-dimethylbutanal
total
Norrish
Norrish
Norrish
Norrish
pressure
type I
type II
type I
type II
100
39.84
31.43
16.56
59.24
100
37.61
35.06
21.13
63.41
13.14
46.82
24.93
55.39
total
Norrish
Norrish
Norrish
Norrish
300
37.50
38.78
pressure
type I
type II
type I
type II
300
42.67
40.45
500
40.54
41.05
500
34.55
38.16
700
47.87
40.21
23.15
51.41
700 average
37.26 39.73 ( 8.14
41.01 38.27 ( 6.84
19.78 ( 6.64
55.25 ( 8.43
100
55.90
44.10
21.84
78.16
100
51.76
48.24
33.21
66.79
300
49.16
50.84
27.98
72.02
300
51.33
48.67
500
49.69
50.31
31.04
68.96
500
47.52
52.48
700 700
54.35 47.60
45.65 52.40
31.05
69.95
total
50.91 ( 10.39
49.09 ( 10.39
29.02 ( 7.18
70.98 ( 7.18
from the reaction of HCO radical with oxygen can be treated as a primary product, but increase of the yield at long conversion times is observed, which can be attributed to the photolysis of secondary products such as ethanal, lower carbonyls, and so on. Theoretically, another possible source of CO could be the reaction RCHO þ hv f RH þ CO
ð12Þ
The stable products of those reactions, i-butane and neopentane, were not identified in our experiments. Decarbonylation was not observed in the recent similar investigation of n-heptanal photolysis.12 The relative yields of Norrish type I and II products for 3-methylbutanal and 3,3-dimethylbutanal molecules are shown in Table 1. The yield is deduced from the quantities of primary formed CO (Norrish Type I) and propene or i-butene (Norrish type II) for different total pressures. Both molecules 3-methylbutanal and 3,3-dimethylbutanal could undergo photocyclization. 3-Methylbutanal gives both cis/trans isomers of cyclobutanol derivatives, while 3,3-dimethylbutanal gives only one product (Scheme 1). The single product in the case of 3,3-dimethylbutanal is due to the lack of available
δ-H atoms resulting in no other possible 3,3-dimethylbutanal cyclization products. This mechanism is observed in the photolysis of n-pentanal,43 and suggested as the important mechanism in the photolysis of n-heptanal accounting for ∼30% of the total product yields.12 The cleavage of the intermediate biradical is probably the dominant pathway compared to cyclization as a consequence of the entropic effect and both cyclization and fragmentation are essentially barrierless, as was suggested for the photolysis of similar 2-pentanone.44 These compounds were not detected in our experiments, but upper limits of the yields could be estimated based on carbon balance data. For n-butanal it can hardly exceed 10%, while is case of the other two aldehydes, it can be 22 ( 10% for 3-methylbutanal and 25 ( 15% for 3,3-dimethylbutanal. It should be emphasized that, in the case of similar ketones, photocyclization was suggested to proceed mainly for triplet molecules through a biradical intermediate, while a different mechanism was proposed for singlet molecules. This conclusion was derived from the fact that the increase in concentration of the triplet quencher decreased the ratio of cyclization to elimination.4547 In earlier studies on similar ketones it has been shown that the strength of γ CH bond plays an important role in the spin state distribution upon photoexcitation, as well as in lifetimes of both states.45 Steric and inductive effects were analyzed as well, but not the influence of the number of available γ-H atoms on the Norrish type I/II ratio.45 5833
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Table 3. Absolute Quantum Yield Values in n-Butanal,8 3-Methylbutanal, and 3,3-Dimethylbutanal Photolysis at Different Pressures of Synthetic Air (Errors are Represented by Experimental Scatter) absolute quantum yields in synthetic air n-butanal8
total pressure (Torr)
3-methylbutanal 3,3-dimethylbutanal
100
0.47
0.47
0.56
100
0.50
0.45
0.43
100
0.47
0.46
0.50
100
0.47
-
0.49
100
-
-
0.55
average
0.48 ( 0.02
0.45 ( 0.01
0.49 ( 0.07
300 300
0.43 0.44
0.42 0.40
0.42 0.46
300
0.44
0.42
average
0.44 ( 0.01
0.41 ( 0.01
0.44 ( 0.02
500
0.30
0.35
0.36
500
0.37
0.35
0.42
500
0.37
0.34
0.42
average
0.35 ( 0.05
0.35 ( 0.01
0.39 ( 0.03
700 700
0.32 0.33
0.31 0.30
0.34 0.39
700
0.32
-
0.34
average
0.32 ( 0.01
0.31 ( 0.01
0.36 ( 0.03
Assuming that the formation of CO2 is an artifact, the sum of both processes Norrish type I and II is 78 ( 10% and 75 ( 15% in case of 3-methylbutanal and 3,3-dimethylbutanal, respectively, indicating the presences of other unidentified product channels (Table 2). The results were obtained using CO and propene or i-butene obtained as primary products, calculating the carbon content of all products obtained in those channels and comparing it to the amount of carbon consumed in the photolysis. The number of available γ-CH atoms obviously plays an important role in the Norrish type I/II ratio, in addition to the strength of γ-CH bond and inductive effects. 5.2. Absolute Quantum Yields. One of the objectives of the study was to determine the dependence of the absolute quantum yield on the total pressure. n-Butanal was used as the actinometer, with absolute quantum yields recently reported in our previous study.8 From the decay of 3-methylbutanal and 3,3-dimethylbutanal concentrations (partial pressures), the photolytic rates were deduced for the different pressures by plotting the natural logarithms of concentrations versus time (first order decay), and performing a least-squares fit. From these results, overall quantum yields were calculated according to the eq 1. The photolysis rates for both compound Kphot(C) and actinometer Kphot(Act) could be directly measured, and the terms ΣOV(C) and ΣOV(Act) represent the calculated overlap of lamp emission and absorption spectrum of the substrate. Φint ðCÞ ¼
Kphot ðCÞ ΣOVðCÞ Kphot ðActÞ ΣOVðActÞ Φint ðActÞ
ðeq1Þ
These integrals are not equal to unity and their values are estimated using relative ratios obtained from the analysis of the cross sections of similar molecular species with different groups
presented (iso- and tert-) and the end of the chain. For example, the cross section of 3-methylbutanal measured by Lanza et al.48 is ∼4% lower than our value for n-butanal measured earlier,8 so that factor (1.04) influenced all the quantum yield calculations. For 3,3-dimethylbutanal there are no available data for the cross sections so we used the relative ratio between the cross sections of propanal and 2,2-dimethylpropanal (t-pentanal)9 to calculate quantum yields. This conclusion cannot be completely justified, as in the case of 3,3-dimethylbutanal the chain is longer, tert-group more isolated from carbonyl group and the actual cross section could be somewhat higher. The multiplication factor obtained in this way was 1.4. In all cases the absolute quantum yield dependency on the total pressure was observed. The absolute quantum yields data are summarized in Table 3 for all experiments performed at total pressures synthetic air of 100, 300, 500, and 700 Torr, and are also shown in the form of a SternVolmer plot in Figure 4a. Slightly higher values for 3,3-dimethylbutanal also suggest that the correction factor obtained from the cross section analysis of propanal and 3,3-dimethylpropanal could be somewhat lower (probably not more than 1015%) than we assumed earlier. Because the intercept at zero pressure is not equal to 1 (the intercepts are 1.96 and 1.85 for 3-methylbutanal and 3,3dimethylbutanal, respectively), which should be the case if collisional deactivations were the only relaxation processes besides the photodecompositions, it seems very probable that there are other energy-dissipating processes (the triplet states of 3-methylbutanal and 3,3-dimethylbutanal could deactivate by phosphorescence, etc.).10 The analysis of the femtosecond dynamics of similar processes in ketones revealed that the dominant deactivation path is internal conversion.44 The interaction of the photoexcited molecules with the walls, resulting in the relaxation to the ground state, is of minor importance because of the large volume-to-surface ratio of the reaction cell. The slopes of the fitting lines, described by 1.798 103 P and 1.381 103 P for 3-methylbutanal and 3,3-dimethylbutanal, respectively, correspond to the sensitivities of absolute quantum yields on the total pressure of synthetic air P (in Torr). The total quantum yield Φtot can be calculated from the following eqs 1/Φtot = 1.96 + (1.798 103 P) and 1/Φtot = 1.85 + (1.381 103 P) for 3-methylbutanal and 3,3dimethylbutanal, respectively. Using the estimated absolute yields for Norrish type I and II decomposition products from Table 2 and the absolute quantum yield values from Table 3, it is possible to calculate the absolute contributions at atmospheric conditions (700 Torr): for Norrish type I (radical) process, j(I) = 0.123 ( 0.002 and j(I) = 0.071 ( 0.002 and Norrish Type II (molecular) process j(II) = 0.119 ( 0.002 and j(II) = 0.199 ( 0.002 for 3-methylbutanal and 3,3-dimethylbutanal photolysis, respectively. The total quantum yield at 700 Torr is 0.31 for 3-methylbutanal and 0.36 for 3,3-dimethylbutanal so that the contribution of both decomposition channels amounts to ∼78 and ∼75%, respectively. The slope of the curves in SternVolmer plot (Figure 4a) for 3-methylbutanal and 3,3-dimethylbutanal are similar to previously reported values for n-butanal and other C5C8 straight chain aldehydes.8,10,13,28 Table 4 summarizes the results of our previous investigations, both in terms of SternVolmer slopes and absolute quantum yields of Norrish type I/II processes in higher aldehyde photolysis. It shows that absolute quantum yields of the Norrish type I process (free radical channel) in the case of 3-methylbutanal is more similar to n-butanal and, 5834
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Figure 4. (A) Pressure dependency of 1/(absolute quantum yield) in 3-methylbutanal and 3,3-dimethylbutanal, at different total pressures of synthetic air (SternVolmer plot); (B) Pressure dependency of 1/(absolute quantum yield) in n-butanal photolysis, at different total pressures of synthetic air and N2 (SternVolmer plot); (C) Pressure dependency of 1/(absolute quantum yield) in n-butanal photolysis, at different total pressures of pure O2 (SternVolmer plot); (D) Pressure independency of the carbon balance and Norrish type I absolute and relative yields in n-butanal photolysis, at different partial pressures of O2 in 700 Torr O2/N2.
Table 4. Slopes in SternVolmer Plots and Absolute Quantum Yield Values for Norrish Type I/II Processes at 700 Torr of Synthetic Air for n-Butanal,8 n-Pentanal,8 n-Hexanal,10 nHeptanal,13 n-Octanal,28 3-Methylbutanal, and 3,3Dimethylbutanal SternVolmer quantum yield quantum yield ratio type compound
slope
Norrish type I Norrish type II I/type II 3
n-butanal
1.931 10
0.199
0.094
2.12
n-pentanal
7.771 104
0.043
0.176
0.24
n-hexanal
4.758 104
0.087
0.232
0.38
n-heptanal
1.169 103
0.031
0.118
0.26
n-octanal
1.061 103
0.022
0.109
0.21
3-methylbutanal
1.798 103
0.123
0.119
1.03
3,3-dimethylbutanal 1.381 103
0.071
0.199
0.36
as such, represents a significant source of free radicals in the atmosphere. The photolysis of 3,3-dimethylbutanal is more similar to the photolysis of C5C8 straight chain aldehydes in terms of Norrish type I contribution and represents a minor
source of free radicals in the atmosphere (see section 6, Atmospheric Implications). It is apparent from Table 4 that C5 and higher aldehydes exhibit similar Norrish type I/II ratio, with an exception of 3-methylbutanal. In the case of 3-methylbutanal the higher Norrish type I/II ratio also corresponds to the higher sensitivity of photolysis on total pressure (slope 1.798 103 compared to ∼1 103 or lower for the rest of C5C8 series of aldehydes). This is probably an indication that the distribution of spin states is more similar to the case of n-butanal, while the increased importance of Norrish type II in case of 3-methylbutanal compared to n-butanal may be due to the fact that there are twice as many γ-H atoms available compared to n-butanal. So it is not only the thermodynamics or photophysics, but pure probability that is responsible for increased Norrish type II importance. In section 5.4 it is shown that the barrier height for the Norrish type II process starting from first excited singlet of 3-methylbutanal is slightly lower compared to the other two examined aldehydes. Norrish type II dissociation competes with ISC and may point to the fact that more of Norrish type II 5835
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The Journal of Physical Chemistry A dissociation in the case of 3-methylbutanal occurs from excited singlet state relative to the other two aldehydes. 5.3. Nature of Spin States. Although it has been proposed that the photodecomposition of aldehydes proceeds from excited triplet state (T1) obtained after fast intersystem crossing from the first excited singlet,26,27 later experimental findings in higher aldehydes photolysis inspired authors to doubt that conclusion.8,10,12,13,22,28 The main reasons for this conclusion are based on the substantially different Norrish type I/II ratios in the case of n-butanal photolysis compared to other higher straight chain aldehydes,8 relatively higher sensitivity to total pressure in the n-butanal photolysis,8 and the fact that the quantum chemical calculations showed that there are much more favorable decomposition routes from the analysis of the reaction enthalpies and energy barriers.12,28 Because theoretical investigations starting from triplet states were not able to explain the existence of Norrish type I products channel, authors presumed that the spin state responsible for this channel might be the first excited singlet state (S1) or vibrationally excited ground singlet state (S0*).13 Our working hypothesis was that, if products are partially formed from two different spin states, the more efficient quenching of one of them would result in different product ratios and absolute yields. It is well-known that oxygen molecules in the ground state act as efficient triplet quenchers. To check the hypothesis a series of experiments in pure N2 was performed. Figure 4b shows n-butanal photolysis quantum yield dependency on the total pressure of pure N2 and synthetic air in the form of a SternVolmer plot. It is obvious that excellent agreement was obtained in terms of the zero pressure quantum yield values (intercept of the Y axis from two sets of experiments, 1.81 and 1.85). The conclusion is that the oxygen present in synthetic air is efficiently quenching the photoexcited aldehyde molecules. In Figure 4c, a SternVolmer plot of the n-butanal quantum yield dependency on total pressure of pure O2 is shown. To further confirm that the quenching process itself does not change the Norrish type I/II yields ratio, a series of experiments was performed under constant total pressure of 700 Torr but different partial pressures of oxygen. Figure 4d shows that the Norrish type I absolute and relative quantum yields (and, consequently, Norrish type II absolute and relative quantum yields and their ratio) and the carbon balance in the case of nbutanal photolysis remains insensitive to the change in oxygen partial pressure. This is consistent with the earlier observations that the HCO radical yield is not sensitive to changes in total pressure in aldehyde photolysis.7,9 It could be assumed that, because there is no observed dependency of the quantum yield on total pressure of N2 and, thus, O2 is solely responsible for the quenching effects, then the dependencies observed by changing the partial pressure of oxygen in O2/N2 mixtures and on pure oxygen should correlate with the partial pressure of O2. Figure 5 shows that, for higher O2/N2 ratios (above 400 Torr) and for very low O2/N2 mixing ratios approaching zero partial O2 pressure, it is really the case. However, there is a range of the O2 partial pressure (∼0300 Torr) where it seems that the N2 exhibits a synergistic effect in quenching photoexcited molecules. One possible explanation could be given by the pressure induced intersystem crossing (ISC). The singlet state is converted to triplet more efficiently at higher total pressures, and triplet is quenched more efficiently than singlet. If the decomposition proceeds only from the singlet state then there is no triplet state involved at all, or the relaxation from the
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Figure 5. Absolute quantum yield dependency on (a) O2 partial pressure in 700 Torr O2/N2 mixture and on (b) pure O2 (Torr).
triplet state is nondecaying in nature. Obvious the influence of the partial pressure of oxygen on the decomposition yield and the absence of that influence in case of nitrogen suggest involvement of the triplet state. In addition, if only the singlet state is involved in the decomposition, then an increase of the rate of ISC should lead to the decrease of the quantum yield, which is not the case as demonstrated in experiments in pure nitrogen, while the influence of the nitrogen itself is demonstrated in experiments done in mixtures O2/N2. If the decomposition proceeds only from the triplet state, then the increase of the total pressure in mixture O2/ N2, and consequently, more efficient ISC should lead to higher quantum yield in mixtures compared with the pure oxygen with the same pressures of oxygen. In contrast, the quantum yield is lower in mixtures. If lower singlet concentrations in equilibrium cause lower quantum yields, then the singlet state is also involved in the decomposition. The only plausible assumption is that both spin states are involved in the decomposition. In this case intensifying ISC could increase or decrease the quantum yield, depending on the ratio of the rates of nondecaying relaxation processes from both states compared to the decomposition. Because pure N2 does not influence the decomposition, it implies that the ratio of the rates of nondecaying processes and decomposition from both states are very similar. In the case of similar aliphatic ketones, it has been proposed that both states are involved in the decomposition.4649 The conclusion was derived from the fact that the increase in the concentration of efficient triplet quencher leads to the decrease of the quantum yield down to the certain concentration and then remains constant. It has earlier been reported that singlet/triplet ratio in case of aliphatic ketones strongly depends on the strength of γ-CH bond.45 The quantum yield of ISC decreased markedly as the γ-CH bond strength decreased. It perfectly matches the observed difference in the Norrish type I/II ratio in the photolysis of n-butanal and n-pentanal, reported in our previous work.8 A primary γ-CH bond in n-butanal, 3-methylbutanal and 3,3dimethylbutanal are stronger than the secondary γ-CH in npentanal and higher aldehydes. The similar phenomenon is observed in the case of 2-pentanone and 2-hexanone, where the ratio of the number of molecules decomposing from singlet and triplet states changes from ∼1:8 in the case of 2-pentanone 5836
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Table 5. Absolute Energies (Atomic Units, Hartrees) and Barrier Heights for the Norrish Type II and I Reactions through the Triplet Excited States of n-Butanal, 3-Methylbutanal, and 3,3-Dimethylbutanal Starting from the Triplet State intermediate (Hartree)
n-butanal
232.353546
232.347190
232.332902
13.0
3-methylbutanal
271.668862
271.662161
271.647772
13.2
3,3-dimethylbutanal
310.983362
310.976513
310.962935
12.8
n-butanal
231.919852
231.931110
231.905092
9.3
3-methylbutanal
271.160700
271.171191
271.146775
8.7
3,3-dimethylbutanal
310.403678
310.403486
310.389320
9.0
Type II
transition state (Hartree)
barrier height (kcal mol1)
reactant (Hartree)
B3LYP/6-31+G(d)
Type II
UMP2/cc-pVTZ
Type I
B3LYP/6-31G+(d)
n-butanal 3-methylbutanal
232.353546 271.668862
232.336753 271.648741
10.5 12.6
3,3-dimethylbutanal
310.983362
310.966283
10.7
Type I
UMP2/cc-pVTZ
n-butanal
231.921656
231.911630
9.4
3-methylbutanal
271.163013
271.145002
11.3
3,3-dimethylbutanal
310.406345
310.391046
9.6
Figure 6. Norrish type I (left) and II (right) decomposition processes of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal are presented here at the B3LYP/6-31+G(d) level of theory. Numbers on the transition states represent barrier heights (in kcal mol1) for butanal, 3-methylbutanal, and 3,3-dimethylbutanal.
to ∼1:1.3 in the case of 2-hexanone.47 The reported sensitivity of n-butanal photolysis quantum yield on the total pressure of synthetic air (slope of the curve in Stern Volmer plot 1.931 103) is more than two times higher than that of n-pentanal
(7.771 104),8 as seen in Table 4. The decomposition from the triplet state is relatively more important in case of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal as their γ-C atoms are terminal ones. Similarities between sensitivities of quantum yields on total pressure in case of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal support the conclusion that similar γ-CH bond strengths lead to similar quantum yield S1/T1 ratio. Further work is required to understand the nature of other relaxation processes, examination of possible phosphorescence and fluorescence phenomena, and complete theoretical examinations of transition states for Norrish type I and II processes. 5.4. Quantum Chemical Calculations. Structures, energies, barrier heights, and reaction enthalpies were calculated using B3LYP/6-31+Gd) and MP2/cc-pVTZ levels of theories as described in the Experimental Details. Reactions occurring through both the triplet and singlet excited electronic states were investigated for n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal. After initial excitation to the excited singlet state, the reaction can proceed in the singlet state, or after an intersystem crossing, the reaction can proceed through the triplet state. Both Norrish type I and II processes starting from the triplet photoexcited state were investigated using B3LYP density functional with 6-31 +G(d) basis set. In the triplet state the Norrish type I and II processes are found to compete with each other with similar barriers for decomposition, type I being slightly favorable, as seen in Table 5. Type II process is assumed to proceed through a twostep mechanism. The first step is a 1,5-hydrogen atom transfer leading to a stable intermediate. The second step is the dissociation of the intermediate. The barrier to the 1,5-hydrogen atom transfer process was found to be 13.0, 13.2, and 12.8 kcal mol1 for n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal, respectively, at the B3LYP/6-31+G(d) level (Figure 6). The barrier heights calculated using UMP2/cc-pVTZ are 9.3, 8.7, and 9.0 kcal mol1. This indicates that the observed differences in the product ratios in these three butanals photodecompositions are due mainly to the availability of the hydrogen atom in the delta 5837
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through the triplet state. At the CIS(D) level, the transition state for dissociation starting from the singlet excited state indicates that the photoexcited butanal dissociates into Norrish type II fragments via a cyclic transition state in which delta hydrogen atom is transferred to the oxygen. Interestingly, transition barrier for Norrish type II dissociation of butanal is only 3.6 kcal mol1 at the CIS(D)/6-31+G(d) level (Figure 7). Dissociation barriers, presented in Table 6, for 3-methylbutanal and 3,3-dimethylbutanal are 3.1 and 3.3 kcal mol1 (Figure 7). Obviously the barrier height for the singlet photodissociation is smaller compared to that through the triplet state for type II dissociation of butanal. This theoretical prediction is also supported by the observation from the experiment that the reaction partially proceeds through the singlet state. The decomposition from the singlet state apparently competes with intersystem crossing and internal conversion.
Figure 7. Norrish type I concerted decomposition process of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal from first excited singlet state are presented here at the CIS(D)/6-31+G(d) level of theory. Numbers on the transition states represent barrier heights (in kcal mol1) for nbutanal, 3-methylbutanal, and 3,3-dimethylbutanal.
Table 6. Norrish Type II Reaction Barrier Heights for Concerted Dissociation through Singlet Excited States of n-Butanal, 3-Methylbutanal, and 3,3-Dimethylbutanal reactant
transition state
barrier height
(Hartree)
(Hartree)
(kcal mol1)
n-butanal
231.571615
231.565929
3.6
3-methylbutanal
270.747246
270.742265
3.1
3,3-dimethylbutanal 309.933033
309.927710
3.3
position. In other words, larger fractions of the volumes in phase space of the photoexcited molecule lead to the Norrish type II decomposition. The dissociation barriers of the alpha cleavage, that is, for the Norrish type I process, are comparable to the type II but slightly lower. There is not much variation in the barriers to dissociation for the three butanals investigated here, indicating that the methyl substitution at the tertiary position does not influence the alpha cleavage. The relative energies and dissociation barriers for alpha cleavage are also presented in Table 5. The dissociation barriers for butanal, 3-methylbutanal, and 3,3-dimethylbutanal are 9.3, 8.7, and 9.0 kcal mol1, respectively. The barrier heights calculated using UMP2/cc-pVTZ are 9.4, 11.3, and 9.6 kcal mol1. The other possibility is that the photodissociation occurs through the singlet excited electronic state before the intersystem crossing can occur. This possibility has also been explored theoretically by ab initio configuration interaction singles (CIS) and configuration interaction singles with perturbative doubles (CIS(D))36 methods, which includes dynamic correlation, along with 6-31+G(d) basis set. It is important to properly include dynamic correlation effects while computing energies and geometries of molecules in their excited states.50,51 Our quantum chemical results are based on the assumption that the Norrish type II photodissociation from the singlet excited state is a one-step concerted process unlike the type II dissociation
6. ATMOSPHERIC IMPLICATIONS The main degradation processes of carbonyl compounds are controlled by photolysis and by the reaction with OH radicals. The atmospheric lifetime of 3-methylbutanal and 3,3-dimethylbutanal can be estimated from the knowledge of the OH reaction rate constant and the photodissociation rate. The rate constants for OH reactions were recently measured for 3-methylbutanal52 and 3,3-dimethylbutanal53,54 and are 2.97 1011 cm3 molecule1 s1 and 2.73 1011 cm3 molecule1 s1, respectively, resulting in the reactive lifetime of 9.4 h of 3-methylbutanal under atmospheric conditions (assuming OH concentration 1 106 molecules cm3).43 Similar value of 10.2 h could be calculated for 3,3-dimethylbutanal. Jimenez et al.52 calculated photolytic lifetime of 3-methylbutanal to be less than 10 h and, from our measurements of the quantum yield of 3,3-dimethylbutanal, it can be concluded that it has the similar lifetime. Photolytic lifetime for n-butanal is earlier reported to be ∼28 h.8 It is obvious that in case of 3-methylbutanal and 3,3-dimethylbutanal photolytic and reactive lifetimes are very similar, and that those two removal processes compete among each other, while the reaction with OH radical is the dominant process in the case of n-butanal. In view of the results obtained in this study, combined with the absolute quantum yields and branching ratios for radical formation (Tables 2 and 3), the absolute radical yield at atmospheric conditions can be estimated for n-butanal as 62.3 0.32 = 19.9%, for 3-methylbutanal 39.7 0.31 = 12.3%, and for 3,3-dimethylbutanal 19.8 0.36 = 7.1%. 7. CONCLUSIONS In this work we experimentally and theoretically examined three structurally similar aldehydes to assess the importance of factors governing the decomposition of aldehydes toward Norrish type I or II products. Besides that, we reported photochemical parameters necessary for atmospheric modeling and evaluated the photooxidation of examined aldehydes as a potential free radical source. For the first time, theoretical examination of the first excited state was reported, and obtained results provided new insights into the photochemistry of higher aldehydes. Reported results are similar to the results obtained in the examination of the photochemistry of ketones. We analyzed the effect of the presence of oxygen in the bath gas, Norrish type I/II products distribution, implications of different quenching extents 5838
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The Journal of Physical Chemistry A to the decomposition patterns, pressure sensitivities, and their relation with spin states distribution, and assessed free radical production capacities. Besides that, a method was suggested for the implicit identification of unknown compounds found in other similar studies that could help in understanding the carbon balance lower than unity in this and other similar studies.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed information on the absorption spectra. Here we evaluate the absolute absorption cross sections of 3-methylbutanal, as measured by various groups,7948 and make an estimation of the absorption cross sections of the 3,3-dimethylbutanal. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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