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Kinetics of OH radical reaction with allyl alcohol (H2CCHCH2OH) and propargyl alcohol (HCCCH2OH) studied by LIF
30 November 2001
Chemical Physics Letters 349 (2001) 279±285 www.elsevier.com/locate/cplett
Kinetics of OH radical reaction with allyl alcohol H2C@CHCH2OH and propargyl alcohol HCBCCH2OH studied by LIF Hari P. Upadhyaya *, Awadhesh Kumar, P.D. Naik, A.V. Sapre, J.P. Mittal Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received 13 August 2001; in ®nal form 8 October 2001
Abstract The rate constants for the reactions of hydroxyl radical (OH) with two unsaturated alcohols (ROH) namely, allyl alcohol H2 C@CHCH2 OH and propargyl alcohol HCBCCH2 OH in the gas phase have been measured. The kinetic measurements were carried out using laser photolysis (LP) combined with laser induced ¯uorescence (LIF) technique at room temperature over a pressure range of 10±20 Torr. The bimolecular rate constants for the reactions OH ROH ! products, are determined at room temperature to be 3:7 0:5 10 11 , 9:2 1:4 10 12 cm3 molecule 1 s 1 , respectively, for allyl alcohol and propargyl alcohol. The measured rate constants in combination with ab initio molecular orbital calculation provide a better understanding of the structure-reactivity rules. Ó 2001 Published by Elsevier Science B.V.
1. Introduction The fate of oxygenated organic hydrocarbons that are released into the atmosphere in the course of various human activities is of great environmental concern. It is well known that reactions of the OH radical with organic compounds are the most important processes both for atmospheric and combustion chemistry. The reaction with OH radicals is the main loss process in the troposphere both for hydrogen containing organic and unsat-
*
Corresponding author. Fax: +91-22-5560750. E-mail address: [email protected] (H.P. Upadhyaya).
urated compounds. The atmospheric lifetimes of various substrates injected into the troposphere are mainly determined by the rates at which they react with most abundant OH radical in troposphere. Precise and accurate measurements of the rate constants of the OH radical with atmospherically important substrates, an area of current research in chemical kinetics, are thus vital for modeling the atmospheric reaction schemes. In addition to compilation of measured kinetic data, development of the methods for predicting the reaction rate constants by theoretical and empirical approaches is required for predicting the kinetics of numerous chemical reactions, which are otherwise not studied experimentally, for environmental assessment [1].
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 2 1 8 - 0
280
H.P. Upadhyaya et al. / Chemical Physics Letters 349 (2001) 279±285
2. Experimental
In this context, we have studied the kinetics of reaction between OH radical with two very important unsaturated alcohols namely allyl H2 C@CHCH2 OH and propargyl HCBCCH2 OH alcohols. Allyl alcohol is used as an intermediate in the manufacture of number of chemicals [2], while propargyl alcohol is used as corrosion inhibitors in industries [3±5]. At present, insucient information is available to assess the atmospheric fate of these species. Like other unsaturated alcohols, these compounds are expected to be rather short-lived in the atmosphere en-route reaction with OH, O3 , and NO3 . The rate coecient for reaction of OH with allyl alcohol was reported recently [6,7], but that with propargyl alcohol is not available in the literature. The kinetic data are used to assess the atmospheric lifetime and fate of these alcohols. In addition to the atmospheric importance the above set of reactions studied can provide an example of the radical± molecule reactivity trend correlating the ionization potential (IP) of the electron donor and electron anity (EA) of the electro®le. The rate constants for OH + allyl alcohol and OH + propargyl alcohols were evaluated in ¯ow condition in the pressure range of 10±20 Torr to be 3:7 0:5 10 11 and 9:2 1:4 10 12 cm3 molecule 1 s 1 , respectively (cf. Table 1). The rate constants so evaluated are discussed in light of the reactivity trend in radical/molecule reaction. It was observed that within the framework of empirical IP±EA correlation, the above set of studied reactions seems to be following the reactivity trend.
The experimental determination of the rate constant was performed using a setup used for photodissociation dynamics studies and it is described elsewhere [8]. The kinetic measurements were carried out under the pseudo-®rst-order conditions by using laser photolysis (LP) and laser induced ¯uorescence (LIF) technique under ¯ow condition to minimize the buildup of the reaction products. Premixed gas mixture (0.3±0.6%) with Ar as a buer gas was allowed to enter the reaction cell through a needle valve. The pressure in the cell was measured with a capacitance manometer and was varied from 10±20 Torr. OH radicals were produced by photodissociation of parent compounds, namely allyl and propargyl alcohol, with an ArF excimer laser at 193 nm (Lambda Physik, Compex 102). The maximum initial concentration of the OH radical was estimated about 4 1012 molecules cm 3 using the measured excimer laser ¯uence, absorption cross-section of the precursor at 193 nm and taking quantum yield as unity for the OH production. Absorption cross-section for allyl alcohol is taken as 6:23 10 18 cm 2 [9]. Experiments were typically conducted for photolysis ¯uence of 0:2±0:8 mJ cm 2 . Following reaction initiation, time resolved OH pro®les were measured as a function of alcohol concentration. Decay of OH concentration was observed by monitoring the LIF signal from OH. For this purpose dierent J lines such as P1 (2), Q1 (3), etc., of A X(0±0) vibrational band near 308 nm were chosen and
Table 1 Rate constant of OH with unsaturated alcohols Rate coecient cm3 molecule OH H2 C@CHCH2 OH (allyl alcohol)
OH HCBCCH2 OH (propargyl alcohol)
1
Technique
Rate coecient with parent alkene [21,22] cm3 molecule 1 s 1
Relative rate method Atmospheric pressure of air Relative rate method Atmospheric pressure of air Laser induced ¯uorescence 10±20 Torr of Ar
H2 C@CHCH3 (propene) 2:6 10 11
Laser induced ¯uorescence 10±20 Torr of Ar
HCBCCH3 propyne 2:95 10 12
s 1
Papagani et al. [6]
5:46 0:35 10
Orlando et al. [7]
4:5 0:6 10
11
Present study
3:7 0:5 10
11
Present study
9:2 1:4 10
12
11
H.P. Upadhyaya et al. / Chemical Physics Letters 349 (2001) 279±285
experiments were done in these transition lines. The probe light was generated using the BBO doubled output of a pulsed dye laser running on a Dye (DCM special kmax 625 nm). The dye laser (Quantel TDL 90) was pumped by the second harmonic of a Nd:YAG laser (Quantel) running at 532 nm. To minimize the diusion of probe species the photolysis volume was always kept more than the probe volume. The resulting broadband ¯uorescence was collected through collecting optics and band pass ®lter combination with a photomultiplier. The band pass ®lter was centered at 310 nm with a FWHM of 10 nm. To minimize the signal due to the scattered light of laser and background ¯uorescence the detection of OH ¯uorescence was electronically gated. The gate opens after 20 ns of the probe laser light and remains open for 300 ns for collection of ¯uorescence. To prevent the saturation of the ¯uorescence signal, the probe energy was kept very low (2 lJ/pulse). The decay was measured by a fast oscilloscope (Lecroy, 9350 A) and integrated and saved. The timing between photolysis and the probe lasers was controlled by a digital delay generator. The whole system was controlled by a computer through a GPIB card. The reaction kinetics was measured by storing the integrated ¯uorescence as a function of time delay between the photolysis and the probe lasers. The OH produced from the parent molecules is rotationally hot and hence the decay kinetics was measured after the OH radical get thermalized. In our experimental conditions it has been observed that OH radical gets thermalized after 15 ls and the data taken after 15 ls are analyzed for rate constant evaluation. The ratios of substrate to OH was kept about 600±1000 satisfying the condition of pseudo-®rst-order kinetics. Allyl alcohol (Merck 99+%) and propargyl alcohol (Merck 99+%) were used as supplied after several freeze-pump-thaw cycles.
dOH kOHROH; dt
281
1
where k is the bimolecular rate constant for the reaction of OH with ROH. Under the present experimental conditions, in which the reactant ROH is in large excess (more than three order of magnitude) over OH, pseudo®rst-order conditions are maintained and the integrated form for Eq. (1) in which ROH 6 f t becomes lnOH=OH0
k 0 t;
2
0
where k is the experimentally measured pseudo®rst-order decay constant and t is the interaction time given by delay between photolysis and probe laser pulse. OH0 is the hydroxyl radical concentration at t 0. Because ROH > 1000OH0 in all reactive experiments, exponential decay of OH radical was observed with a decay rate constant k 0 . A typical experimental curve is shown in the inset of Figs. 1 and 2 for ally alcohol and propargyl alcohol, respectively. The k 0 is given by kROH kd , where kd is the loss of OH radical from the probe volume due to diusion. The pseudo-®rst-order rate constant decay rates were obtained from a weighted linear least-square-®t of the logarithm of the OH ¯uorescence signals vs delay time. The bimolecular rate constant k was then obtained from the slope
3. Results and discussion 3.1. Presentation of kinetic data The decrease in the OH radical concentration with time follows the rate law
Fig. 1. Plot of the pseudo-®rst-order rate constant k 0 vs concentration for allyl alcohol. Inset: temporal pro®le OH LIF intensity vs pump-probe delay for allyl alcohol concentration of 2 1015 molecule/cc at a total pressure of 10 Torr.
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H.P. Upadhyaya et al. / Chemical Physics Letters 349 (2001) 279±285
k II
k0 T M 1 k0 T M=k1 T
Fc 1log k0 T M=k1 T
Fig. 2. Plot of the pseudo-®rst-order rate constant k 0 vs concentration for propargyl alcohol. Inset: temporal pro®le OH LIF intensity vs pump-probe delay for propargyl alcohol concentration of 5 1015 molecule/cc at a total pressure of 15 Torr.
of the least-squares straight line through the plot of k 0 vs [ROH] as illustrated in Figs. 1 and 2. The bimolecular rate constants for the reaction OH ROH ! products, were determined at room temperature to be 3:7 0:5 10 11 , and 9:2 1:4 10 12 cm3 molecule 1 s 1 , respectively, for allyl alcohol and propargyl alcohol. It should be noted here that the error represents one standard deviation in the experimental data. The very recent measurements on the reaction of OH with allyl alcohol by Orlando et al. and Atkinson and co-workers give a value of 4:5 0:6 10 11 and 5:46 0:35 10 11 cm3 molecule 1 s 1 . However, a value of 1:0 10 11 cm3 molecule 1 s 1 was estimated in the aqueous solution at pH 7.0, using pulse-radiolysis technique [24]. Both earlier measurements [6,7] were carried out at atmospheric pressure in environment chamber ®lled with puri®ed air. The observed rate constant in the present study for the allyl alcohol is a bit lower compared to earlier studies. It is well known that the reaction of OH to unsaturated hydrocarbons proceeds through addition mechanism [10] and shows negative temperature dependence, a characteristic of barrier less reaction. This type of reactions also shows pressure dependence behavior. Therefore our measurement at 10±20 Torr that is lower than the rate constant at 760 Torr is expected. The rate constant in fall-o region can be calculated according to the formula given by Troe [11,12]
2
1
;
3
where k0 is the termolecular rate constant at very low pressure, M is the concentration of third body, and k1 T is the high pressure limiting rate constant. Taking k0 , Fc as similar to OH + propene reaction [13] and the k1 value from [6,7] the rate calculated at 10±20 Torr of buer gas is 3:79±3:89 10 11 cm3 molecule 1 s 1 . This is in excellent agreement with our measured value. However, we cannot attach much importance to the quantitative comparison because of experimental data taken in a limited pressure range. Another reason for this dierence may arise due to dierent reaction environments used. Both earlier groups have used a puri®ed air as a buer whereas in the present study we used Ar as a buer gas. Also the consumption of OH radical in secondary reactions cannot be ruled out in previous studies. However, because of limited pressure range under study, further studies covering a broader pressure range will be undertaken to fully characterize the fall-o region of both the reactions. As mentioned earlier, this type of reactions proceeds through addition mechanism to the double or triple bond. Many experimental studies indicate a general trend in the reactivity of radical± molecule addition reactions which correlate very well with IP of the electron donor molecule and the EA of the radical having electron withdrawing properties. Thus the magnitude of reaction rate can be empirically determined by the relative magnitude of dierence of these quantities (IP EA), which determine the extent of partial electron transfer [14,15]. Thus the reactivity trend of these reactions can be correlated with the IP of hydrocarbon and EA of electro®le. The observed rate constants for studied reactions are compared with the parent-unsubstituted hydrocarbons. It is clearly seen that the rate has been enhanced for both the reactions. This qualitative observation is examined with the IP EA trend for radical± molecule reaction. The electro®le (OH) remaining the same, the IP for both the molecules were examined. The IP of propene is 9.73 eV and that of substituted alcohol (allyl alcohol) is lower (9.67 eV)
H.P. Upadhyaya et al. / Chemical Physics Letters 349 (2001) 279±285
and this is re¯ected in its increased rate constant compared to propene. But similar dependence of the rate constant does not hold for the propyne and propargyl alcohol. The IP of propyne (10.37 eV) is less than the propargyl alcohol (10.49 eV) [16] so the rate is expected to be greater for propyne. Thus the rate constant of propyne and propargyl alcohol with OH shows a reverse correlation in terms of IP. To have a better understanding of this observed reactivity trend we performed the molecular orbital analysis at PM3 level. As IP can be directly correlated with the frontier orbitals, i.e., HOMO/LUMO, the nature of these frontier orbital have been examined. It has been seen that while the HOMO/LUMO of propene and allyl alcohol is very much similar in nature, there is dissimilarity in the LUMO of propyne and propargyl alcohol. While the nature of LUMO is purely a 2py character for propyne, it is purely 2pz character for propargyl alcohol. The HOMO is purely a 2pz for these molecules. This dierence in the nature of LUMO is responsible for the high IP of propargyl alcohol. At this point the rate enhancement nature of OH substituent can be discussed. It seems that the OH substitution facile the addition reaction. An enhancement in the rate constant has been observed for various
283
alkenes. The enhancement factor comes around 2 for alkenes and 4 for alkynes. The calculated rate constant can be used to estimate the tropospheric lifetime of allyl alcohol and propargyl alcohol. For average [OH] concentration of 2 106 molecules cm 3 [17,18] the tropospheric lifetime have been calculated to be 2 and 5 h, respectively, for allyl alcohol and propargyl alcohol. Using the rate constant [19] value of 1:44 10 17 cm3 molecule 1 s 1 for the reaction of allyl alcohol with O3 , the tropospheric lifetime has been calculated with reference to O3 as 13.8 h taking an average O3 concentration as 1:4 1012 molecules cm 3 . In the previous studies by Orlando et al. and Atkinson and co-worker it is dicult to know the OH attacking carbon center as end product is same for both the radicals formed after addition of OH to two dierent carbon atom of the double bond. Hence it is desirable to know the mechanism of the OH attack to these double and triple bonded unsaturated alcohols. As mentioned earlier, this type of reactions proceeds via barrier less transition. Hence the relative energy or the stability of the radical formed after the OH addition to these alcohols can throw light on the nature of attack for OH radical. For this an ab initio calculation
Fig. 3. Diagram for relative energy calculated at QCISD(T)/6-311G level of theory for the dierent adduct formed in the reaction between OH radical with allyl alcohol and propargyl alcohol. (Values in the parenthesis are at MP4 and MP2 level.)
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was performed using G92 program [20]. The relative energies for all the radicals were computed at dierent levels of theory, for example, QCISD(T), fourth- and second-order Mùller±Plesset theory using 6-311G(d,p) basis set. The energies were corrected for zero point energy using vibrational frequencies which were calculated after geometry optimization at HF/6-311G level of theory. The outcome of this computation is shown in Fig. 3. From ®gure it is evident that the radical produced after the attack of OH to the carbon atom attached to the OH group is more stable in both types of alcohols. So the OH addition prefers the carbon atom of double/triple bond attached to ACH2 OH group. This preferential attack of the OH at the C atom of the double/triple bond leads to the formation of primary radical. In contrast to the general rule of higher stability of a secondary radical, the primary radical is more stable in these reactions. Apart from the addition to the double/triple bond another minor channel namely H-atom abstraction may occur in these types of reactions. A value of 5% was evaluated for the H-atom abstraction channel in the reaction of OH with allyl alcohol by end product analysis [6,7]. However, a value of 12% was estimated for the H-atom abstraction for the OH with allyl alcohol in aqueous solution studied by pulse-radiolysis technique [24]. Although not conclusive but it is interesting to observe that in electrophile + substituted alkenes reaction the electrophile prefers to attack the carbon atom in which the positive inductive eect (electron donating) group is attached or it will prefer not to attack the carbon atom to which a negative inductive eect (electron withdrawing) group is added [15,23]. 4. Conclusion The reaction of the OH radical with unsaturated alcohols, allyl alcohol and propargyl alcohol, was studied in ¯ow condition at a pressure range of 10±20 Torr using LP±LIF detection of OH radical. The bimolecular rate constants for the reactions OH ROH ! products, are determined
at room temperature to be 3:7 0:5 10 11 , and 9:2 1:4 10 12 cm3 molecule 1 s 1 , respectively, for allyl alcohol and propargyl alcohol. There is a clear enhancement of the rate constant for the OH-substituted unsaturated hydrocarbons. Ab initio calculations show the preferential attack of OH at the carbon atom attached to the ACH2 OH group. Acknowledgements Authors thank Dr. Y.-C. Lee, National Tsing Hua University, Hsinchu, Taiwan, and Dr. D.K. Maity for their generous assistance in ab initio calculations. We acknowledge Dr. T. Mukherjee for his keen interest throughout this work. We thank the referee for his valuable suggestions. References [1] R. Atkinson, Chem. Rev. 86 (1986) 69, and references therein. [2] P.H. Howard (Ed.), Handbook of Environmental Fate and Exposure Data for Organic Chemicals: Large Production and Priority Pollutants, vol. 1, Lewis Publishers, MI, 1989. [3] E. Duwell, J. Electrochem. Soc. 109 (1962) 1013. [4] Y. Feng, K.S. Siow, W.K. Teo, A.K. Hsieh, Corrosion Sci. 41 (1999) 829, and references therein. [5] B.B. Pati, P. Chatterjee, T.B. Singh, D.D.N. Singh, Corrosion 46 (1990) 354. [6] C. Papagani, J. Arey, R. Atkinson, Int. J. Chem. Kinetics 33 (2001) 142. [7] J.J. Orlando, G.S. Tyndall, N. Ceazan, J. Phys. Chem. A 105 (2001) 3364. [8] P.D. Naik, H.P. Upadhyaya, A. Kumar, A.V. Sapre, J.P. Mittal, Chem. Phys. Lett. 340 (2001) 116. [9] UV Atlas of Organic Compounds, vol. III, Butterworths, London, 1967 (pg. A1/1). [10] R. Cvetanovic, D.L. Singleton, Rev. Chem. Intermed. 5 (1984) 183. [11] J. Troe, J. Chem. Phys. 66 (1977) 4745, see also p. 4758. [12] J. Troe, J. Phys. Chem. 83 (1979) 114. [13] R. Atkinson, D.L. Baulch, R.A. Cox, R.F. Hampson, J.A. Kerr, M.J. Ross, J. Troe, J. Phys. Chem. Ref. Data 26 (1997) 521. [14] J.P.D. Abbatt, J.G. Anderson, J. Phys. Chem. 95 (1991) 2382, and references therein. [15] H.P. Upadhyaya, A. Kumar, P.D. Naik, A.V. Sapre, Chem. Phys. Lett. 321 (2000) 411. [16] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 74th ed., CRC Press, Boca Raton, 1993±1994, p. 6.
H.P. Upadhyaya et al. / Chemical Physics Letters 349 (2001) 279±285 [17] R.G. Prinn, R.F. Weiss, B.R. Miller, J. Huang, F.N. Aleya, D.M. Cunnold, P.J. Fraser, D.E. Hartley, P.G. Simmonds, Science 269 (1995) 187. [18] R. Heinn, P.J. Crutzen, M. Heimann, Global Biogeochem. Cycles 11 (1997) 43. [19] D. Grosjean, E. Grosjean, E.L. Williams II, Int. J. Chem. Kinet. 25 (1993) 783. [20] M.J. Frisch et al., GA U S S I A N 92, Revision E.1, Gaussian, Inc., Pittsburgh, PA, 1992.
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[21] NIST Chemical Kinetics Data base, Version 5.0, Gaithersburg, MD 20899, USA. [22] R.B. Boodaghians, I.W. Hall, F.S. Toby, R.P. Wayne, J. Chem. Soc. Faraday Trans. 2 83 (1987) 2073. [23] H.P. Upadhyaya, P.D. Naik, U.B. Pavanaja, A. Kumar, R.K. Vatsa, A.V. Sapre, J.P. Mittal, Chem. Phys. Lett. 274 (1997) 383. [24] M. Simic, P. Neta, E. Hayon, J. Phys. Chem. 77 (1973) 2662.
Chemical Physics Letters 349 (2001) 279±285 www.elsevier.com/locate/cplett
Kinetics of OH radical reaction with allyl alcohol H2C@CHCH2OH and propargyl alcohol HCBCCH2OH studied by LIF Hari P. Upadhyaya *, Awadhesh Kumar, P.D. Naik, A.V. Sapre, J.P. Mittal Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received 13 August 2001; in ®nal form 8 October 2001
Abstract The rate constants for the reactions of hydroxyl radical (OH) with two unsaturated alcohols (ROH) namely, allyl alcohol H2 C@CHCH2 OH and propargyl alcohol HCBCCH2 OH in the gas phase have been measured. The kinetic measurements were carried out using laser photolysis (LP) combined with laser induced ¯uorescence (LIF) technique at room temperature over a pressure range of 10±20 Torr. The bimolecular rate constants for the reactions OH ROH ! products, are determined at room temperature to be 3:7 0:5 10 11 , 9:2 1:4 10 12 cm3 molecule 1 s 1 , respectively, for allyl alcohol and propargyl alcohol. The measured rate constants in combination with ab initio molecular orbital calculation provide a better understanding of the structure-reactivity rules. Ó 2001 Published by Elsevier Science B.V.
1. Introduction The fate of oxygenated organic hydrocarbons that are released into the atmosphere in the course of various human activities is of great environmental concern. It is well known that reactions of the OH radical with organic compounds are the most important processes both for atmospheric and combustion chemistry. The reaction with OH radicals is the main loss process in the troposphere both for hydrogen containing organic and unsat-
*
Corresponding author. Fax: +91-22-5560750. E-mail address: [email protected] (H.P. Upadhyaya).
urated compounds. The atmospheric lifetimes of various substrates injected into the troposphere are mainly determined by the rates at which they react with most abundant OH radical in troposphere. Precise and accurate measurements of the rate constants of the OH radical with atmospherically important substrates, an area of current research in chemical kinetics, are thus vital for modeling the atmospheric reaction schemes. In addition to compilation of measured kinetic data, development of the methods for predicting the reaction rate constants by theoretical and empirical approaches is required for predicting the kinetics of numerous chemical reactions, which are otherwise not studied experimentally, for environmental assessment [1].
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 2 1 8 - 0
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H.P. Upadhyaya et al. / Chemical Physics Letters 349 (2001) 279±285
2. Experimental
In this context, we have studied the kinetics of reaction between OH radical with two very important unsaturated alcohols namely allyl H2 C@CHCH2 OH and propargyl HCBCCH2 OH alcohols. Allyl alcohol is used as an intermediate in the manufacture of number of chemicals [2], while propargyl alcohol is used as corrosion inhibitors in industries [3±5]. At present, insucient information is available to assess the atmospheric fate of these species. Like other unsaturated alcohols, these compounds are expected to be rather short-lived in the atmosphere en-route reaction with OH, O3 , and NO3 . The rate coecient for reaction of OH with allyl alcohol was reported recently [6,7], but that with propargyl alcohol is not available in the literature. The kinetic data are used to assess the atmospheric lifetime and fate of these alcohols. In addition to the atmospheric importance the above set of reactions studied can provide an example of the radical± molecule reactivity trend correlating the ionization potential (IP) of the electron donor and electron anity (EA) of the electro®le. The rate constants for OH + allyl alcohol and OH + propargyl alcohols were evaluated in ¯ow condition in the pressure range of 10±20 Torr to be 3:7 0:5 10 11 and 9:2 1:4 10 12 cm3 molecule 1 s 1 , respectively (cf. Table 1). The rate constants so evaluated are discussed in light of the reactivity trend in radical/molecule reaction. It was observed that within the framework of empirical IP±EA correlation, the above set of studied reactions seems to be following the reactivity trend.
The experimental determination of the rate constant was performed using a setup used for photodissociation dynamics studies and it is described elsewhere [8]. The kinetic measurements were carried out under the pseudo-®rst-order conditions by using laser photolysis (LP) and laser induced ¯uorescence (LIF) technique under ¯ow condition to minimize the buildup of the reaction products. Premixed gas mixture (0.3±0.6%) with Ar as a buer gas was allowed to enter the reaction cell through a needle valve. The pressure in the cell was measured with a capacitance manometer and was varied from 10±20 Torr. OH radicals were produced by photodissociation of parent compounds, namely allyl and propargyl alcohol, with an ArF excimer laser at 193 nm (Lambda Physik, Compex 102). The maximum initial concentration of the OH radical was estimated about 4 1012 molecules cm 3 using the measured excimer laser ¯uence, absorption cross-section of the precursor at 193 nm and taking quantum yield as unity for the OH production. Absorption cross-section for allyl alcohol is taken as 6:23 10 18 cm 2 [9]. Experiments were typically conducted for photolysis ¯uence of 0:2±0:8 mJ cm 2 . Following reaction initiation, time resolved OH pro®les were measured as a function of alcohol concentration. Decay of OH concentration was observed by monitoring the LIF signal from OH. For this purpose dierent J lines such as P1 (2), Q1 (3), etc., of A X(0±0) vibrational band near 308 nm were chosen and
Table 1 Rate constant of OH with unsaturated alcohols Rate coecient cm3 molecule OH H2 C@CHCH2 OH (allyl alcohol)
OH HCBCCH2 OH (propargyl alcohol)
1
Technique
Rate coecient with parent alkene [21,22] cm3 molecule 1 s 1
Relative rate method Atmospheric pressure of air Relative rate method Atmospheric pressure of air Laser induced ¯uorescence 10±20 Torr of Ar
H2 C@CHCH3 (propene) 2:6 10 11
Laser induced ¯uorescence 10±20 Torr of Ar
HCBCCH3 propyne 2:95 10 12
s 1
Papagani et al. [6]
5:46 0:35 10
Orlando et al. [7]
4:5 0:6 10
11
Present study
3:7 0:5 10
11
Present study
9:2 1:4 10
12
11
H.P. Upadhyaya et al. / Chemical Physics Letters 349 (2001) 279±285
experiments were done in these transition lines. The probe light was generated using the BBO doubled output of a pulsed dye laser running on a Dye (DCM special kmax 625 nm). The dye laser (Quantel TDL 90) was pumped by the second harmonic of a Nd:YAG laser (Quantel) running at 532 nm. To minimize the diusion of probe species the photolysis volume was always kept more than the probe volume. The resulting broadband ¯uorescence was collected through collecting optics and band pass ®lter combination with a photomultiplier. The band pass ®lter was centered at 310 nm with a FWHM of 10 nm. To minimize the signal due to the scattered light of laser and background ¯uorescence the detection of OH ¯uorescence was electronically gated. The gate opens after 20 ns of the probe laser light and remains open for 300 ns for collection of ¯uorescence. To prevent the saturation of the ¯uorescence signal, the probe energy was kept very low (2 lJ/pulse). The decay was measured by a fast oscilloscope (Lecroy, 9350 A) and integrated and saved. The timing between photolysis and the probe lasers was controlled by a digital delay generator. The whole system was controlled by a computer through a GPIB card. The reaction kinetics was measured by storing the integrated ¯uorescence as a function of time delay between the photolysis and the probe lasers. The OH produced from the parent molecules is rotationally hot and hence the decay kinetics was measured after the OH radical get thermalized. In our experimental conditions it has been observed that OH radical gets thermalized after 15 ls and the data taken after 15 ls are analyzed for rate constant evaluation. The ratios of substrate to OH was kept about 600±1000 satisfying the condition of pseudo-®rst-order kinetics. Allyl alcohol (Merck 99+%) and propargyl alcohol (Merck 99+%) were used as supplied after several freeze-pump-thaw cycles.
dOH kOHROH; dt
281
1
where k is the bimolecular rate constant for the reaction of OH with ROH. Under the present experimental conditions, in which the reactant ROH is in large excess (more than three order of magnitude) over OH, pseudo®rst-order conditions are maintained and the integrated form for Eq. (1) in which ROH 6 f t becomes lnOH=OH0
k 0 t;
2
0
where k is the experimentally measured pseudo®rst-order decay constant and t is the interaction time given by delay between photolysis and probe laser pulse. OH0 is the hydroxyl radical concentration at t 0. Because ROH > 1000OH0 in all reactive experiments, exponential decay of OH radical was observed with a decay rate constant k 0 . A typical experimental curve is shown in the inset of Figs. 1 and 2 for ally alcohol and propargyl alcohol, respectively. The k 0 is given by kROH kd , where kd is the loss of OH radical from the probe volume due to diusion. The pseudo-®rst-order rate constant decay rates were obtained from a weighted linear least-square-®t of the logarithm of the OH ¯uorescence signals vs delay time. The bimolecular rate constant k was then obtained from the slope
3. Results and discussion 3.1. Presentation of kinetic data The decrease in the OH radical concentration with time follows the rate law
Fig. 1. Plot of the pseudo-®rst-order rate constant k 0 vs concentration for allyl alcohol. Inset: temporal pro®le OH LIF intensity vs pump-probe delay for allyl alcohol concentration of 2 1015 molecule/cc at a total pressure of 10 Torr.
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k II
k0 T M 1 k0 T M=k1 T
Fc 1log k0 T M=k1 T
Fig. 2. Plot of the pseudo-®rst-order rate constant k 0 vs concentration for propargyl alcohol. Inset: temporal pro®le OH LIF intensity vs pump-probe delay for propargyl alcohol concentration of 5 1015 molecule/cc at a total pressure of 15 Torr.
of the least-squares straight line through the plot of k 0 vs [ROH] as illustrated in Figs. 1 and 2. The bimolecular rate constants for the reaction OH ROH ! products, were determined at room temperature to be 3:7 0:5 10 11 , and 9:2 1:4 10 12 cm3 molecule 1 s 1 , respectively, for allyl alcohol and propargyl alcohol. It should be noted here that the error represents one standard deviation in the experimental data. The very recent measurements on the reaction of OH with allyl alcohol by Orlando et al. and Atkinson and co-workers give a value of 4:5 0:6 10 11 and 5:46 0:35 10 11 cm3 molecule 1 s 1 . However, a value of 1:0 10 11 cm3 molecule 1 s 1 was estimated in the aqueous solution at pH 7.0, using pulse-radiolysis technique [24]. Both earlier measurements [6,7] were carried out at atmospheric pressure in environment chamber ®lled with puri®ed air. The observed rate constant in the present study for the allyl alcohol is a bit lower compared to earlier studies. It is well known that the reaction of OH to unsaturated hydrocarbons proceeds through addition mechanism [10] and shows negative temperature dependence, a characteristic of barrier less reaction. This type of reactions also shows pressure dependence behavior. Therefore our measurement at 10±20 Torr that is lower than the rate constant at 760 Torr is expected. The rate constant in fall-o region can be calculated according to the formula given by Troe [11,12]
2
1
;
3
where k0 is the termolecular rate constant at very low pressure, M is the concentration of third body, and k1 T is the high pressure limiting rate constant. Taking k0 , Fc as similar to OH + propene reaction [13] and the k1 value from [6,7] the rate calculated at 10±20 Torr of buer gas is 3:79±3:89 10 11 cm3 molecule 1 s 1 . This is in excellent agreement with our measured value. However, we cannot attach much importance to the quantitative comparison because of experimental data taken in a limited pressure range. Another reason for this dierence may arise due to dierent reaction environments used. Both earlier groups have used a puri®ed air as a buer whereas in the present study we used Ar as a buer gas. Also the consumption of OH radical in secondary reactions cannot be ruled out in previous studies. However, because of limited pressure range under study, further studies covering a broader pressure range will be undertaken to fully characterize the fall-o region of both the reactions. As mentioned earlier, this type of reactions proceeds through addition mechanism to the double or triple bond. Many experimental studies indicate a general trend in the reactivity of radical± molecule addition reactions which correlate very well with IP of the electron donor molecule and the EA of the radical having electron withdrawing properties. Thus the magnitude of reaction rate can be empirically determined by the relative magnitude of dierence of these quantities (IP EA), which determine the extent of partial electron transfer [14,15]. Thus the reactivity trend of these reactions can be correlated with the IP of hydrocarbon and EA of electro®le. The observed rate constants for studied reactions are compared with the parent-unsubstituted hydrocarbons. It is clearly seen that the rate has been enhanced for both the reactions. This qualitative observation is examined with the IP EA trend for radical± molecule reaction. The electro®le (OH) remaining the same, the IP for both the molecules were examined. The IP of propene is 9.73 eV and that of substituted alcohol (allyl alcohol) is lower (9.67 eV)
H.P. Upadhyaya et al. / Chemical Physics Letters 349 (2001) 279±285
and this is re¯ected in its increased rate constant compared to propene. But similar dependence of the rate constant does not hold for the propyne and propargyl alcohol. The IP of propyne (10.37 eV) is less than the propargyl alcohol (10.49 eV) [16] so the rate is expected to be greater for propyne. Thus the rate constant of propyne and propargyl alcohol with OH shows a reverse correlation in terms of IP. To have a better understanding of this observed reactivity trend we performed the molecular orbital analysis at PM3 level. As IP can be directly correlated with the frontier orbitals, i.e., HOMO/LUMO, the nature of these frontier orbital have been examined. It has been seen that while the HOMO/LUMO of propene and allyl alcohol is very much similar in nature, there is dissimilarity in the LUMO of propyne and propargyl alcohol. While the nature of LUMO is purely a 2py character for propyne, it is purely 2pz character for propargyl alcohol. The HOMO is purely a 2pz for these molecules. This dierence in the nature of LUMO is responsible for the high IP of propargyl alcohol. At this point the rate enhancement nature of OH substituent can be discussed. It seems that the OH substitution facile the addition reaction. An enhancement in the rate constant has been observed for various
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alkenes. The enhancement factor comes around 2 for alkenes and 4 for alkynes. The calculated rate constant can be used to estimate the tropospheric lifetime of allyl alcohol and propargyl alcohol. For average [OH] concentration of 2 106 molecules cm 3 [17,18] the tropospheric lifetime have been calculated to be 2 and 5 h, respectively, for allyl alcohol and propargyl alcohol. Using the rate constant [19] value of 1:44 10 17 cm3 molecule 1 s 1 for the reaction of allyl alcohol with O3 , the tropospheric lifetime has been calculated with reference to O3 as 13.8 h taking an average O3 concentration as 1:4 1012 molecules cm 3 . In the previous studies by Orlando et al. and Atkinson and co-worker it is dicult to know the OH attacking carbon center as end product is same for both the radicals formed after addition of OH to two dierent carbon atom of the double bond. Hence it is desirable to know the mechanism of the OH attack to these double and triple bonded unsaturated alcohols. As mentioned earlier, this type of reactions proceeds via barrier less transition. Hence the relative energy or the stability of the radical formed after the OH addition to these alcohols can throw light on the nature of attack for OH radical. For this an ab initio calculation
Fig. 3. Diagram for relative energy calculated at QCISD(T)/6-311G level of theory for the dierent adduct formed in the reaction between OH radical with allyl alcohol and propargyl alcohol. (Values in the parenthesis are at MP4 and MP2 level.)
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was performed using G92 program [20]. The relative energies for all the radicals were computed at dierent levels of theory, for example, QCISD(T), fourth- and second-order Mùller±Plesset theory using 6-311G(d,p) basis set. The energies were corrected for zero point energy using vibrational frequencies which were calculated after geometry optimization at HF/6-311G level of theory. The outcome of this computation is shown in Fig. 3. From ®gure it is evident that the radical produced after the attack of OH to the carbon atom attached to the OH group is more stable in both types of alcohols. So the OH addition prefers the carbon atom of double/triple bond attached to ACH2 OH group. This preferential attack of the OH at the C atom of the double/triple bond leads to the formation of primary radical. In contrast to the general rule of higher stability of a secondary radical, the primary radical is more stable in these reactions. Apart from the addition to the double/triple bond another minor channel namely H-atom abstraction may occur in these types of reactions. A value of 5% was evaluated for the H-atom abstraction channel in the reaction of OH with allyl alcohol by end product analysis [6,7]. However, a value of 12% was estimated for the H-atom abstraction for the OH with allyl alcohol in aqueous solution studied by pulse-radiolysis technique [24]. Although not conclusive but it is interesting to observe that in electrophile + substituted alkenes reaction the electrophile prefers to attack the carbon atom in which the positive inductive eect (electron donating) group is attached or it will prefer not to attack the carbon atom to which a negative inductive eect (electron withdrawing) group is added [15,23]. 4. Conclusion The reaction of the OH radical with unsaturated alcohols, allyl alcohol and propargyl alcohol, was studied in ¯ow condition at a pressure range of 10±20 Torr using LP±LIF detection of OH radical. The bimolecular rate constants for the reactions OH ROH ! products, are determined
at room temperature to be 3:7 0:5 10 11 , and 9:2 1:4 10 12 cm3 molecule 1 s 1 , respectively, for allyl alcohol and propargyl alcohol. There is a clear enhancement of the rate constant for the OH-substituted unsaturated hydrocarbons. Ab initio calculations show the preferential attack of OH at the carbon atom attached to the ACH2 OH group. Acknowledgements Authors thank Dr. Y.-C. Lee, National Tsing Hua University, Hsinchu, Taiwan, and Dr. D.K. Maity for their generous assistance in ab initio calculations. We acknowledge Dr. T. Mukherjee for his keen interest throughout this work. We thank the referee for his valuable suggestions. References [1] R. Atkinson, Chem. Rev. 86 (1986) 69, and references therein. [2] P.H. Howard (Ed.), Handbook of Environmental Fate and Exposure Data for Organic Chemicals: Large Production and Priority Pollutants, vol. 1, Lewis Publishers, MI, 1989. [3] E. Duwell, J. Electrochem. Soc. 109 (1962) 1013. [4] Y. Feng, K.S. Siow, W.K. Teo, A.K. Hsieh, Corrosion Sci. 41 (1999) 829, and references therein. [5] B.B. Pati, P. Chatterjee, T.B. Singh, D.D.N. Singh, Corrosion 46 (1990) 354. [6] C. Papagani, J. Arey, R. Atkinson, Int. J. Chem. Kinetics 33 (2001) 142. [7] J.J. Orlando, G.S. Tyndall, N. Ceazan, J. Phys. Chem. A 105 (2001) 3364. [8] P.D. Naik, H.P. Upadhyaya, A. Kumar, A.V. Sapre, J.P. Mittal, Chem. Phys. Lett. 340 (2001) 116. [9] UV Atlas of Organic Compounds, vol. III, Butterworths, London, 1967 (pg. A1/1). [10] R. Cvetanovic, D.L. Singleton, Rev. Chem. Intermed. 5 (1984) 183. [11] J. Troe, J. Chem. Phys. 66 (1977) 4745, see also p. 4758. [12] J. Troe, J. Phys. Chem. 83 (1979) 114. [13] R. Atkinson, D.L. Baulch, R.A. Cox, R.F. Hampson, J.A. Kerr, M.J. Ross, J. Troe, J. Phys. Chem. Ref. Data 26 (1997) 521. [14] J.P.D. Abbatt, J.G. Anderson, J. Phys. Chem. 95 (1991) 2382, and references therein. [15] H.P. Upadhyaya, A. Kumar, P.D. Naik, A.V. Sapre, Chem. Phys. Lett. 321 (2000) 411. [16] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 74th ed., CRC Press, Boca Raton, 1993±1994, p. 6.
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[21] NIST Chemical Kinetics Data base, Version 5.0, Gaithersburg, MD 20899, USA. [22] R.B. Boodaghians, I.W. Hall, F.S. Toby, R.P. Wayne, J. Chem. Soc. Faraday Trans. 2 83 (1987) 2073. [23] H.P. Upadhyaya, P.D. Naik, U.B. Pavanaja, A. Kumar, R.K. Vatsa, A.V. Sapre, J.P. Mittal, Chem. Phys. Lett. 274 (1997) 383. [24] M. Simic, P. Neta, E. Hayon, J. Phys. Chem. 77 (1973) 2662.