A Quantitative Study of Alkyl Radical Reactions by Kinetic Spectroscopy.

IV. The Flash Photolysis of Azopropanes

HIROYUKI ADACHI and NORMAN BASCO Lkpartment of Chemistry, l?niuersity of British Columbia, Vancoucrr, 8.C' , V67' I Y6'.

Canada

Abstract

T h e flash photolysis of azo-n-propane and of azoisopropane has been studied by kinetic spectroscopy. Transient absorption spectra in the region of 220-260 nm have been assigned to the n-propyl and isopropyl radicals. For the n-propyl radical, cmax = 744 f 39 limo1 cm a t 245 nm and the rate constants for the mutual reactions were measured to he k , = (1.0 f 0.1) X 1Oln I/mol sec (Combination) and k d = (1.9 f 0.2) X loy I/mol sec (disproportionation). For the isopropyl radical, cmax = 1280 f 110 I/mol cm a t 238 nm, with k , = ( i . 7 f 1.6) X loy I/mol sec and k d = (5.0 f 1.2) X loy h o l sec.

T h e rate constant for the dissociation of the vibrationally excited triplet s ta te of the azo- propanes into radicals was measured from the variation in the quantum yield of radicals with pressure. For azo-n-propane k i = (6.6 f 1.3) X 10' sec-I, and for azoisopropane k z = (1.6 f 0.4) X lo8 sec-I. Collisional deactivation o f the vibrationally excited singlet and triplet states was found to occur on every collision for n-pentane; hut nitrogen and argon were inef- ficient with a rate constant of 1.1 X 10") I/mol sec.

Spectra observed in the region of 220-260 and 370-400 nm are attributed to the cis isomers of the parent trans-azopropanes. These are formed, as permanent products, in increasing amounts a s the pressure is increased.

Introduction

The first, and still the most recent determination of the rate constant for the mutual reaction of n-propyl radicals was published in 1956 [ l ] . From the photolysis of di-n-propyl ketone a t 100°C, using the rotating sector technique, the value k 1 = k la + h I b = 6 X 10" l/mol sec was obtained (with h l , / h l b = 8) for the reactions

( l a ) 2n-C3H7

(1b) 2n-C3H7 - C:<HS + C:~HG

After considering possible experimental errors, this value was thought to be too high by a factor of about 20, so that reaction (1) occurred a t every collision.

International .Journal of Chemical Kinetics, Vol. 13,367-384 (1981) (c 1981 John Wiley & Sons, Inc. CCC 0~~8-8066/81/040X67-18$01.80

368 AI)ACHI A N D HASCO

Since then literature rate constants for other reactions of the n-propyl radical have been variously based on assumed values of k la between about 5 X lo9 and 1 X 10" l/mol sec.

The same technique was used for the isopropyl radical with isobutyral- dehyde as the radical source [2]. For the reactions

(2a) 2 iso-C:jH7 - CGH14 (DMB)

(2b) 2 iso-C:jH.; - C:~HH + C:jH6

the value k2 = k2a + h2b = 1.3 X 10" l/mol sec (with k:,b/kZa = 0.65) ob- tained was again too close to the collision number.

The radical buffer method [3] gave a much lower value, log k2a = 8.6 f 1.1 l/mol sec a t 415 K. The error limits here arise mostly from the uncer- tainty in the value for the A H , of the isopropyl radical, taken to be 76.1 f 6.3 kJ/mol a t 298 K.

An absolute measurement by the very-low-pressure pyrolysis (VLPP) technique [4] yielded log k2, = 9.5 f 0.2 over the temperature range of 683-808 K and this value can therefore be reconciled to the radical buffer method data. An even higher value of 86 kJ/mol for A H , (298) has since been suggested [5] for the radical.

Using the molecular modulation spectrometry technique, Parkes and Quinn [6] found a transient ultraviolet absorption on the photolysis of azoisopropane (AIP) or di-isopropyl ketone, which could be assigned to the isopropyl radical. They measured a maximum extinction coefficient of (1.0 f 0.15) X lo3 l/mol cm a t 233 nm and found k2a = (5.0 f 1.2) X lo9 l/mol sec with h2a/k2h = 0.65 f 0.05 a t room temperature. These results were confirmed by Arrowsmith and Kirsch [7] using the same technique, and the study was extended to cover the temperature range of 301-424 K. Extrapolation of the Arrhenius parameters obtained to 750 K gave a value of k2a about twice that obtained by VLPP [4].

Azoalkanes are well known to be clean photolytic sources of thermalized alkyl radicals. The photolysis of AIP was first studied by Durham and Steacie [8] and they established the basic mechanism. Absorption of ra- diation a t 366 nm produces an excited molecule which can either return to the ground state by collision or dissociate into isopropyl radicals and N2:

(3) hi,

M AIP + AIP* - 2 isopropyl + N:,

The radicals either combine or disproportionate [reactions (2a), (2b)I since the only alternative, hydrogen abstraction,

(4) C3H7 + C?H7N2C:{H7 - C:jH8 + C:<H6N2 C:jH.; has an activation energy of 27 kJ/mol. In a similar study Riem and Kutschke [9] measured k4/(h2a)1/2 in the range of 334-400 K and also the

FI.ASH PHOTOLYSIS OF AZOPHOPANES 369

relative rates for dissociation, internal conversion, and collisional deacti- vation for the excited AIP molecule. They also suggested that internal conversion occurs through isomerization to the cis ground state. The relative unimportance of the abstraction reaction was confirmed by Berkley et al. [ 101 who found

The situation with regard to the photolysis of azo-n-propane (ANP) is entirely similar. Kerr and Calvert [ l l ] found that, below 215OC, the products were almost entirely nitrogen, propane, propene, and n -hexane, since the abstraction reaction (4) had an activation energy of 33 kJ/mol. A similar value was found by Berkley et al. [lo] who measured

k4 1750 log (s) = 4.8 - 1 (cm:'/mol set)'"

7

A study of the variation of the quantum yield of Ng by Terry and Futrell [ 121 showed that the mechanism for photolysis a t 366 nm was analogous to that of AIP [reaction ( 3 ) ] , with ( l a ) and ( Ib) the only significant radical reactions.

More recently, studies of the photochemistry of azopropanes have been mainly concerned with the behavior of the excited state. The inadequacy of the simplest model [reaction ( 3 ) ] was shown by the observation that cis-trans isomerization occurs, as well as decomposition, on exposing either trans- or cis-AIP to 366 nm radiation in the gas phase [13,14]. The cis isomer is relatively unstable and its thermal (or photochemical) decom- position may contribute to the observed yield of nitrogen. Furthermore, curvature has been detected in the Stern-Volmer type plot of l/4( N?) against [MI, showing that more than one excited state is involved [14-

Various detailed mechanisms have been proposed to explain these ob- servations [13-171, and these have been critically discussed by Pritchard, Marchant, and Steel [17].

We report here a study of the flash photolysis of ANP and AIP a t room temperature in which the reactions of n -propyl and of isopropyl radicals, the variation of quantum yield of radicals with pressure, and the formation o f the cis isomers are followed by kinetic spectroscopy.

I 71.

Experimental

ANP or AIP (2.2-5.5 X mol/l), either alone or in the presence of n-pentane (1.6 X 10-4-4.7 X lo-:< mol/l), nitrogen (1.9-4.0 X mol/l), or argon (4 X lo-' mol/l) were photolyzed using flash energies of 1080 or 1270 J .

370

t

ADACHI AND BASCO

0 05 10

Time ( m s e c ) Figure 1. oscilloscope output. Am-isopropane pressure 5.5 torr.

Variation of absorbance a t 240 nm with time; pen recording o f digital

The temporal variation of absorbance a t various wavelengths in the range of 220-400 nm was recorded using the photoelectric technique described in detail in previous publications [18,19]. The analytical light source, a 150-W xenon lamp, was intensified for a period of up to 10 msec by applying a square-wave voltage pulse. The collimated output of this lamp was split and the resulting parallel beams were passed through two identical reaction vessels and were focused on the slit of a 1-m-focal-length spectrometer (Jarrell Ash 78-420). The emerging beams were directed to two balanced photomultipliers and the output from each of these, I0 and I , was measured on a digital storage oscilloscope (Nicolet 1090A) interfaced to a calculator (HP 9825A). Up to 2048 readings of log ( I ( ) /Z ) , taken at intervals of 0.5 psec or longer, were stored on magnetic tape for subsequent analysis. The re- sults of typical experiments are shown in Figures 1 , 2 and 3. These are pen recordings of the output of the digital oscilloscope after the original data had been transferred back to it from the calculator as log (Zo/Z) (Figures 1 and 3) or its reciprocal (Figure 2).

0 0 0.1 0.2 03 0.4

Time ( msec Figure 2. 240 nm. Am-isopropane pressure 5.0 torr.

Second-order plot for the decay of the isopropyl radical spectrum a t

FLASH PHOTOI.YSIS OF AZOPROPANES 371

I

I . . . . . . . ' 8

0 05 10

T I me ( msec )

Figure 3. tal oscilloscope output. Am-isopropane pressure 5.5 torr.

Variation of absorbance a t 227.5 nm with time; pen recording of digi

The cylindrical reaction vessels, 914 mm long and of 20 mm internal di- ameter, were of Pyrex with Suprasil end windows. With the photolytic radiation thus restricted to wavelengths longer than about 280 nm, there was virtually no interference from scattered light a t the shorter wavelengths used for all kinetic measurements. Furthermore a double gas filter con- taining chlorine and bromine, placed before the spectrometer slit, removed radiation between about 280 nm and 520 nm, which could have been scat- tered within the spectrometer. With these precautions it was possible to make absorbance measurements during the period of the photoflash and thus apply an accurate extrapolation procedure for the evaluation of the molar extinction coefficients of the n-propyl and isopropyl radicals.

The only significant products of the photolysis were nitrogen, propane, propene, and either n-hexane or 2J-dimethyl butane. These were sepa- rated into two fractions and a residue by distillation under vacuum. The number of moles of each fraction was calculated by measuring its pressure in a known volume with a McLeod gauge. The first fraction, nitrogen, was isolated a t 73 K (He through liquid nitrogen) and the second, which com- prises propane and propene, a t 156 K (iso-amyl alcohol slush). The molar yield of the CfiH14 products was estimated as the difference between the molar yield of nitrogen and one half of the molar yield of the C:% fraction.

1 ,l'-Azo-n-propane and 2,2'-azoisopropane were supplied by Merck, Sharp and Dohme (Canada) Ltd. Analysis of these compounds by vapor-phase chromatography showed that the sole impurities were traces of the normal decomposition products nitrogen, propane, propene, n - hexane, and 2J-dimethyl butane. These were removed by exhaustive degassing in freeze-thaw cycles under vacuum, followed by repeated bulb-to-bulb distillations a t the appropriate temperatures. The n-pentane (98%) was supplied by Matheson, Coleman and Bell and was purified by two distillations, the first with a 50-cm-long distillation tower and the second with a 100-cm tower in a stream of nitrogen. No impurities were then found by vapor-phase chromatography, and the product was stored over Linde molecular sieve 4A. Nitrogen (99.9995%) and argon (99.9995%) were obtained from Matheson Company and passed through cold traps.

All experiments were done under isothermal conditions a t room tem- perature.

Results and Discussion

Spectra

Two types of absorption were observed after flash photolyzing ANP or AIP. The first (and under most conditions predominant) absorption was the expected transient in the regions of 230-260 nm (ANP) or 225-260 nm (AIP) which were attributed to the n-propyl and isopropyl radicals. A representative trace of the temporal behavior of this type is shown in Figure 1. The decay of this absorption could be followed for about three half-lives, as shown in Figures 1 and 2, and was found to be strictly second order over the entire period (Figure 2).

The second type was a permanent (on our time scale) absorption in the regions of220-260 nm and 370-400 nm (ANP) and 220-240 nm (AIP). The strength of this absorption increased as the pressure of added inert gas was increased while the transient spectrum became weaker. With ANP or AIP alone, the relative strength of the permanent absorption increased with increasing pressure and, under favorable conditions, the two absorptions were of comparable strength below about 230 nm. Figure 3 shows the temporal behavior of the total absorbance for pure AIP a t 227.5 nm.

The spectra of these permanent products were obtained by measuring the residual absorbance A a a t various wavelengths under suitable condi- tions and are shown in Figure 4. The spectrum obtained for ANP in the

t

h ( n m ) Figure 4. n -propane; (0) am-isopropane.

Spectrum attributed to the C L S isomers of the azopropanes. ( 0 ) Azo-

FI.ASH PHO‘I’OLYSIS OF AZOI’HOI’ANES 37:3

370-400-nm region was much weaker with a maximum observed absorbance a t 375-380 nm of only 0.017, decreasing to -0.005 a t 365 nm and 400 nm. (The chlorine and bromine filters were, of course, removed for these ex- periments and replaced by a Corning 7-54 o r 5-57 glass filter.)

The spectra of the transient species, n -propyl and isopropyl radicals, were obtained from experiments where the transient absorption was predomi- nant. Allowance for any small contribution to the total absorbance from the permanent product could be made, where necessary, by using the re- sidual absorbance A , as the baseline. To compare absorbances on an equal basis and to enable the absolute values of the molar extinction coefficients to be determined, the net absorbance A,, was obtained by an extrapolation procedure. This procedure, which uses the ability to make absorbance measurements during the period of the photoflash, was similar to that described earlier [ 19,201, though the use of the digital oscilloscope/calculator combination allowed the calculations to be done automatically. The loss of absorbance, AA, during the period from the beginning of the photoflash until the time t o when decay measurements begin (25-35 psec) was calcu- lated as a summation AA = 2 dA,. The loss dA, over the very small in- terval At (20.5 psec) between measurements is given to a more than ade- quate approximation by dA, = At (- dA/dt ) = At-S - ( A , ) where S is the slope of the plot of 1/A against time after the flash and ( A , ) is the average absorbance during the interval. The net absorbance is that which would have been observed if all the transient species produced were present; it is given by A, = AA + Ao, where A0 is the measured absorbance a t to. The values of S and of A() used in practice were those obtained from a least- squares f i t of the absorbance data to the equation A = A,)/( 1 + S-A(rt) rather than to the nominally equivalent linear form using l / A as the vari- able. (No further advantage was achieved by using I ( ) / Z , rather than its logarithm, as the variable.) The value of A0 obtained in this way was very close to the directly measured value of the absorbance a t time to. This extrapolation procedure introduces a negligible error into the net absorb- ance, particularly since the loss of absorbance AA is only about 20% of A,. These experiments thus yielded a plot of absorbance against wavelength for both radicals. The absolute values of the extinction coefficients were obtained in separate series of experiments a t a single wavelength performed under the most favorable conditions both for this purpose and for kinetic measurements. Since the total concentration of radicals produced is twice the yield of nitrogen “21, the molar extinction coefficient a t any wavelength c ( X ) is related to the net absorbance a t that wavelength A,,(X) by the equation

where 1 is the length of the reaction vessel. The use of the Beer-Lambert

374 ADACHI ANI) HASCO

law is justified by the fact that the spectra are continuous with no resolvable fine structure. The yield of N2 was generally in the range of 3-6 X mol/l.

The extinction coefficient of the n-propyl radical was measured at 247.5 nm. From 23 experiments using 10 torr of ANP, with no added inert gas, we calculated 6 (247.5) = 722 f 38 I/mol cm. With this absolute value the extinction coefficients over the wavelength range investigated were cal- culated from the known relative values of the net absorbance A , . The resulting spectrum is shown in Figure 5. At the maximum, the extinction is estimated to be 744 f 39 l/mol cm a t 245 nm.

The extinction coefficient of the isopropyl radical was measured a t 240 nm, again slightly on the long-wavelength side of the maximum near 238 nm. From five series of 28 experiments, using pure AIP (4.5-10 torr), we found t(240) = (1.29 f 0.09) X lo3 l/mol cm. A further three series of 27 experiments using AIP (-10 torr) + n-pentane (:34,58, or 86 torr) yielded the essentially identical value ~ ( 2 4 0 ) = (1.23 f 0.12) X 10:' l/mol cm. Combining all results gives t(240) = (1.26 f 0.11) X loi l/mol cm. I t would appear, from the agreement between the two sets of results, that there was no appreciable decomposition of the cis isomer produced in the second set. Using the value for ~ (240) and the known relative values of A , , the absolute values for the extinction coefficient over the range of 220-260 nm were calculated. The resulting spectrum is shown in Figure 6. Our value = (1.28 f 0.11) X lo3 l/mol cm at 238 nm is rather higher than the maximum value t(233) = (1.0 f 0.15) X lo3 l/mol cm, found by Parkes and Quinn [GI.

0 1 1 . 1 1 1 1 1 230 240 zx) 260

A ( n m ) Figure 5. Spectrum attributed to the n-propyl radical. Two separate series of experiments, 0 and A, were done under nearly identical conditions, with 10 torr am-n -propane.

h - 'E -" 1000- 'S

u) E" v

- 4 500-

W 0

v

220 230 240 250 260 A ( n m )

01' ' ' ' ' . ' . '

Figure 6. Spectrum attributed to the isopropyl radical. Azo-isopropane pres sure 4.5 ( A ) , 5.0 ( n ) , o r 5.5 (0) torr.

Rate Constants

The decay of the n-propyl and of the isopropyl radical spectra followed second-order kinetics and the absorbance measurements were fitted to the equation A = A o / ( l + S-A(,-t) , as described earlier. The rate constant h s for the mutual reaction

(5) 2 C:{H7 -+ products

is related to the slope S(X) a t any wavelength by the equation

h5 = S(X) * C(X) - f/2

For the n-propyl radical we calculate the mean value S(247.5) = (3.60 f 0.31) x 105sec-l from the 23 experiments used to measure the absolute extinction coefficient of the radical. An additional 41 experiments were performed a t 242.5 nm, a wavelength a t which the extinction coefficient is the same as a t 247.5 nm. These included 18 experiments with pure ANP (4 or 7 torr) and 23 with ANP + n-pentane mixtures (7 torr + 3,7 or 10 torr). The mean value, S(242.5) = (3.61 f 0.49) X 1Oj sec-', obtained from these is identical with the previous result. Combining all results with the mean value of the extinction coefficient, we find h 1 = (1.19 f 0.20) X 1 O l o I/mol sec. The same result, hl = (1.19 f 0.13) X 1Olo I/mol sec, is obtained as the mean of 23 values of hl calculated from those experiments in which both S(X) and ( ( A ) were measured.

For the isopropyl radical the slope S(240) was measured in four series of experiments. Series A comprised 41 experiments with pure AIP (5 or 10 torr). In 28 of these, series A], the extinction coefficient ~ (240) was also measured and values of h2 were obtained directly. Series I3 comprised 76

376 A D A C H I AND HASCO

experiments with mixtures of AIP (10 torr) and n-pentane (3 -86 torr). In 27 of these, series B1, ~ ( 2 4 0 ) was also measured. Series C comprised 37 experiments with mixtures of AIP (9 torr) and nitrogen (348 or 740 torr), and in series D, argon (731 torr) was used as the inert gas in 23 experi- ments.

Let us consider first those experiments in which individual values for h2 were obtained. For series Al, k2 = (1.39 f 0.20) X 1010 l/mol sec, and for series B1, h2 = (1.20 f 0.25) X l/mol sec. These do not differ sig- nificantly and, accordingly, we adopt the average from all 55 experiments, h2 = (1.29 f 0.24) X 1 O ' O l/mol sec. In turn, this is very close to the value h z = (1.27 f 0.20) X 10'O l/mol sec calculated from all 117 experiments of series A + B and using the average value of (1.26 f 0.11) X 10:j l/mol cm for ~ ( 2 4 0 ) and S(240) = (2.20 f 0.27) X lo5 sec-'.

From series C, S(240) = (1.85 f 0.20) X lo5 sec-' and from series D, S(240) = (1.96 f 0.13) X lo5 sec-'. Combining these and using 6(240), we obtain h2 = (1.09 f 0.20) x 1O1O l/mol sec.

It therefore seems reasonable to combine all four series to obtain the value hp = (1.21 f 0.27) X 1O1O l/mol sec. However, there is a reason for believing that the slope obtained from the nitrogen series (and possibly, to a lesser extent, the argon series) may be a little too low. On reflashing a mixture of AIP and nitrogen, the value of S(240) obtained was generally slightly higher than that obtained from the fresh mixture. I t seems possible that this effect was due to a trace (say 1-5 ppm) of oxygen in the nitrogen which reacts with a small fraction of the isopropyl radicals to give isopropyl peroxy radicals. These have a maximum absorbance a t 240 nm with an extinction coefficient almost identical to that of the isopropyl radical [21,22]. Since this absorption remains essentially constant over the period of measure- ment of the isopropyl radical decay, the value of S(240) is reduced. Al- though this effect rapidly disappears due to the permanent removal of the oxygen, the possibility remains that it leads to a systematic error ap- proaching the random error in magnitude. Accordingly, we prefer to adopt the value h2 = (1.27 f 0.26) X 1 O 1 O l/mol sec obtained from series A + B only.

From the work of Berkley e t al. [ lo], one would expect the reactions

(4)

(6)

C3H7 + C3H7 N2C3H7 + C:jHa + C ~ H G N ~ C : { H ~

C3H; + C5H12 + C:iHx + C ~ H I I to be quite negligible for thermalized radicals a t room temperature. The fact that the value of h2 obtained is independent of AIP concentration (series A) or of n-pentane concentration (series B) confirms this and shows that no excited radical reactions need be considered.

From measurements of the yield of nitrogen and of propane + propene from ANP, we calculate for the disproportionation/combination reactions (lb) and ( la ) the ratio hlb/hla = 0.19. This is in reasonable agreement with

the value of 0.16 measured by Kerr [ 11,231 or 0.154 obtained by Terry and Futrell [ la] . Adopting our value gives kl, = (1.0 f 0.1) X 1 O l o l/mol sec and k I b = (1.9 f 0.2) X lo9 l/mol sec as the rate constants for the n-propyl radical reactions. Using the lowest value for the ratio I121 would give klk)

= (1.6 f 0.2) X lo9 l/mol sec and leave k l a unchanged. For the isopropyl radical, from similar measurements we find k2tJhYc,

= 0.655, in good agreement with the literature values of 0.60 [7],0.65 [6], and 0.69 [ 12,241. Thus the rate constants for the isopropyl radical reactions are k2, = (7.7 f 1.6) X lo9 l/mol sec and k21, = (5.0 f 1.0) X lo9 l/mol sec. These values are about 50% higher than those o f k2, = (5.0 f 1.2) X 10" l/mol sec and k21, = (3.2 f 0.8) X 10" l/mol sec given by Parkes and Quinn [6] or of h2, = (5.7 f 4.1) X loy l/mol sec and h2t) = (3.4 f 2.5) X lo9 l/mol sec measured a t 301 K by Arrowsmith and Kirsch [7]. The difference with the former arises because our values for c(X) and k / c ( X ) are each about 25% higher. When the error limits are taken into account, the results may not be significantly different.

Certainly, the similarity in the results obtained by Parkes and Quinn [C;] and by ourselves provide mutual support that the spectrum observed and the rate constants measured are those of the isopropyl radical. This, in turn, supports our assignment of the n-propyl radical spectrum. Although, in view of what is known about the photochemistry of the azoalkanes, there cannot be any serious doubt as to the spectral assignments, the additional evidence provided by these studies may be summarized as follows. First, the same spectrum is observed following the photolysis of AIP and di-iso- propyl ketone [6]. Second, the measured extinction coefficient, based on the yield of nitrogen, is of the order of 1 x 10:j l/mol cm for both transients, and the species responsible must therefore almost certainly be a major product of the photolysis in each case. Third, the decay of both spectra obeys second-order kinetics, and if the observed rate constants, of about 1.3 X 10"' l/mol sec, applied to a hypothetical minor product, the value would exceed the collision number. Of course the immediate major product of absorption of radiation about 300 nm is an excited state of AIP or ANP, and this (or another excited state) either dissociates or is collisionally deactivated. Since such species would decay by first-order kinetics and have much too short a lifetime (and one that depends on the total pressure), they obviously could not account for the observed spectra. An excited state from which dissociation did not occur (such as a vibrationally relaxed triplet state) would either have a decay rate which was pressure dependent o r a concentration which increased with pressure. This is contrary to the be- havior observed for the transient spectra, discussed in detail in the next section, which provides good evidence for the assignment to radicals. Fi- nally, it has been found that, for oxygen + AIP [6,22] or oxygen + ANP [22] mixtures, the transient spectra observed in the absence of oxygen are re- placed by relatively long-lived spectra of the alkylperoxy radicals. These

378 ADACHI A N D HASCO

would be expected to arise from the direct reaction of the radicals with oxygen, and this view is supported by the observation [22] that the yield of peroxy radicals is decreased as the pressure of oxygen is increased much above that which is required for the reaction to compete with the mutual radical reactions. This tends to rule out the possibility that they are pro- duced directly from an excited state of the parent molecule.

Table I summarizes our results from the flash photolysis of azoal- kanes.

Yield of Radicals and cis-trans Isomeritation

We observe a continuous reduction in the yield of radicals when in- creasing pressures of inert gas are added to ANP or AIP. This reduction is the same whether it is measured from the net absorbance A,, of the transient or from the yield of the permanent products nitrogen and propane + propene.

The same effect is observed, but as a relative decrease when the pressure of ANP or AIP is increased in the absence of added inert gas. For example, in one series the flash photolysis of 5 torr of AIP yielded 3.5 X mol/l of nitrogen, while 10 torr gave 5.7 X mol/l. In another series the yield of nitrogen was reduced by about 30% as the pressure of added n-pentane was increased from 34 to 86 torr. In both series the net absorbance due to the isopropyl radical changed in exactly the same ratio. The reduction in the radical concentration produced by added inert gas was accompanied by an increase in the residual absorbance A, . Likewise, for pure ANP or AIP A , increased more than proportionally as the pressure was in- creased.

These observations are consistent with what is known about azoalkane photochemistry. The relevant reactions from the mechanism proposed by Pritchard, Marchant, and Steel [ 171 are, with their nomenclature,

I So+hv+SY

TABLE I . photolysis.

Spectra and rate constants of mutual interaction of alkyl radicals by flash

A,,, lo-:< cmax 10-l" k , (I/mol sec) kh(l/mol sec) Radical (nm) (I/mol cm) (combination) (dis~roport ionat ion)

- Methyl [20,25] 216.4 9.5 3.2 Ethyl [26] 247 0.48 1.24 0.16 n-Propyl 245 0.74 1 .o 0.19 Isopropyl 238 1.3 0.77 0.50

FLASH PHOI'OI.YSIS OF AZOPHOPANES

s y + M - S y + M

379

Sy - ( Y t rans + (1 - t ~ ) cis-SO

T: - /3 t rans + (1 - d) cis-SO

Here, Sy and T'; are vibrationally excited singlet and triplet molecules, S:' and Ty the corresponding collisionally deactivated singlet and triplet molecules, and SO is the electronic ground state.

The total radical concentration produced, [R], is given by

[R] = 21 - a - b/(k,T + kcoll [MI)

where 1 is the absorbed light (Einsteidl) ,

l / a = h& + kcoll - [M] + kIsc

and

b = k z (k,T + kcoll - [MI) + k l - krsc

so that

For ANP or AIP in the absence of inert gas, [MI = [AP], the concentration of the azopropane. A t the concentrations used, absorption is very weak so that I = c.[AP]. Furthermore,

so that expressions a and b (and particularly their product) are essentially constant. Then

1 _ - ( 7 )

and a plot of [R]-' or (absorbance)-' against [API-' should be linear with

slope intercept kd' = kcoil.

380 AI)A('HI ANI) HAS( '0

60

40

Such plots for ANP and AIP are shown in Figures 7 and 8, respectively. From these data, using hc(,ll = 3 X l o l l l/mol sec, we obtain h r (ANP) = (6.6 f 1.3) X lo7 sec-' and h&' (AIP) = (1.6 f 0.4) X lo8 sec-I. These values are in remarkably good agreement with the values hl' (ANP) = 5 X lo7 sec-' and h z (AIP) = 1.7 X lo8 sec-' given by I'ritchard et al. [ 171.

For mixtures of azopropanes with inert gas, eq. ( i ) hecomes

- -

-

1 - ( h z + hcoll(AP) - [AP] + h, . (M) - [MI) 1 _ -

[R] B a - b - c - [AP]

-

I I I I I I

where h,(M) is the rate constant for the deactivation of S'y and TI by M,

1 5 t 15 -

I 1 I I I

OO 2 4 6 I

103/[AlP] ( P mole' 1 Figure 8. Variation of the absorbance o f the isopropyl radical am-isopropane (AIP) pressure. Each point is the average of4- 1

I 1 I I I

2 4 6

103/[AlP] ( P mole' 1 Figure 8. Variation of the absorbance o f the isopropyl radical am-isopropane (AIP) pressure. Each point is the average of4- 1

a t 240 nm with 0 experiments.

F I A S H I'HO'I'O I I YS IS 0 F AZO I ' l iO PA N ES 38 1

and h,,ll(AP) is the corresponding rate constant for azopropanes-assumed to be the collision number, 3 X 10" l/mol sec. In expressions a and b given earlier, h,,,ll(AP)[AP] + h,(M)-[M] replaced ~, . , , II[AP].

Thus for sufficiently low values of [MI, such that the product a X b re- mains effectively constant, a plot of [R]-' or (absorbance)-' against [MI, for a given [AP], should be a straight line. This is indeed found to be the case for mixtures of ANP + n-pentane over the range studied, and the re- sults are shown in Figure 9. The rate constant h,(n-pentane) is related to the slope and intercept of this graph by the equation

slope intercept

h, ( n -pentane) = - (ha'+ h,(,ii(AP) - [ A P J )

and we calculate h,(n-pentane) = 1.9 X 10" I/mol sec directly from the graph.

However, although the value of the product a-b increases quite slowly with increasing concentrations o f n-pentane (so that no curvature of the plot should be detectable), the slope of the line is decreased to a significant extent. Accordingly, the results were recalculated after applying correc- tions for the variation in a h . For each concentration of n-pentane, the value of a-6 was calculated using k b (ANP) = 4 x lox sec-' and h,sc(ANP) = 8 X lo9 sec-' [ l T ] with h z (ANI') = 6.6 X 10; sec-] and h, (n-pentane) = 1.9 X 10" l/mol sec. Each experimental value of (absorbance)-' was multiplied by the correction factor (a -h ) / (a .h )o , where (a&)( ) is the value of a-b in the absence of n -pentane. This correction factor is important (though it lies in the narrow range of 1-1.13) since a plot of [ (a .b) / (a-h)o]- l /A against [n-pentane] then gives h, (n-pentane) = 2.4 X 10" l/mol sec. Using this value to recalculate the correct ion factor then gives the self-consistent value k , (n-pentane) = 2.6 X 1 0 ' I/mol sec so that rz-pentane deactivates the 5'; and 7'; states on every collision.

The concentration of n -pentane required to reduce the amount of AIP photolyzed was much higher than is the case for ANP. This is to be ex-

' 0 2 4 6 8

1 o4 [n-pentane](mo~eP) Figure 9. n-pentane pressure. Each point is the average of A 14 experiments.

\'ariation o f theat)sort)ance (11 the n - p r ~ ~ p y l radical at 2 4 2 5 nm with

382 ADACHI AND HASCO

pected from the difference in the values of h z for the two compounds. The results obtained are not of sufficient accuracy to show the expected cur- vature in the plot of (absorbance)-l against [n-pentane], and the value h, (n- pentane) = 1.0 x 10' I/mol sec can be calculated from the slope and intercept of the straight line. However, the product a 4 is far from constant over the range of n-pentane concentration studies, and this value of h,(n-pentane) must be much too low. However, it can be used with our value of h r (AIP) = 1.6 X lo8 sec-I and the values h.2 (AIP) = 1 X lo9 sec-' and hrsc (AIP) = 1.8 X 109 sec-' given by Pritchard et al. [ lr] to calculate the correction factor (a-b)/(a-b)o for each value of [n-pentane] used. Using the resulting new value for h,(n-pentane) to recalculate the correction factor finally gives the self-consistent value h,(n-pentane) = 3.0 X 10" l/mol sec. The agreement with the value from the ANP + n-pentane ex- periments is closer than might have been expected. The final plot of [(a-b)/(a.b)o]-l/A against [n-pentane] is shown in Figure 10.

The same situation is obtained with experiments with mixtures of AIP with nitrogen or argon up to 1 atmosphere pressure. The plot of 1/A against [MI again appears to be a straight line which yields the value h,(Ar, N2) = 5.2 x lo9 l/mol sec. Using this, as before, to calculate the correction factor for each point finally gives the self-consistent value h,.(Ar, N2) = 1.1 x lo1() l/mol sec, and the plot is shown in Figure 10.

The permanent products whose spectra are observed in the regions of 220-260 nm (Fig. 4) and 370-400 nm are, almost certainly, the cis isomers of ANP and AIP. Other products could be formed following the hydrogen abstraction reaction (4), and some of these, including the isomer propanol propylhydrazone, have been characterized [lo]. Much of this last product

40 -

o o " " " 2 4 6

1 o3 [n-pentone] 1 0' [ NZ, At-] (mole 1-l)

Figure 10. Variation of the absorbance o f the isopropyl radical a t 240 nm with the pressure of n-pentane ( 0 ) . nitrogen (0). and argon (v). Each point is the av- erage of about 10 runs, except 21 for argon.

was thought to have formed in a heterogeneous reaction and the rest by disproportionation o r abstraction reactions radicals produced in reaction (4). Under the present conditions of room temperature and very high light intensity, this reaction (4 ) is quite negligible compared to the mutual re- action of the alkyl radicals.

Any product of reaction (4) would be present in such minute concen- trations as to require an impossibly large extinction coefficient to be de- tectable spectroscopically. On the other hand, cis-trans isomerization is well known to occur to a major extent with moderate additions of inert gas [13,14].

I t is possible to combine the absorbance measurements made on the radical and the isomer spectra to calculate approximate values for the ex- tinction coefficients of the cis isomers in the ultraviolet.

The relative concentrations of the cis isomer, cis-AP, and the radical are given by the expression

with h,, ,11[M] = h,,,ll(AP)[AP] + hJn-pentane) [n-pentane]. Using the values (Y = 0.5, /3 = 0.94, and h ~ s c (AIP) = 1.8 X lo9 sec-' [ 171,

we find c(ci.s-AIP) = 890 I/mol cm a t 23.5 nm, 1370 l/mol cm a t 232.5 nm, and 2520 l/mol cm a t 227.5 nm in one series of experiments with 34 torr of added n-pentane. In another series of five experiments with 58 torr of n-pentane we calculate f(cis-AIP) = 610 l/mol cm a t 237.5 nm. Thus cmax (c i s -AIP) 2 3.9 x 10:j l/mol cm near 223 nm.

A similar calculation, using the same values for (Y and 8, with hrs(. ( A N P ) = 8 X 109 sec-I, gave ( (c i s -ANP) = 910 l/mol cm a t 242.5 nm. This result was obtained as an average of five experiments using mixtures of A N P (7 torr) and n-pentane (10 torr). I t is in very satisfactory agreement with the value c(cis-ANP) = I160 l/mol cm obtained from a mixture of A N P (10 torr) and argon (504 torr) which was calculated using the measured value h,.(Ar) = 1.1 X 10'Ol/mol sec. Thus c,,;,, (c i s -ANP) = 5.5 X 10:'l/mol cm near 2 3 1 nm.

Clearly, these calculations are subject to considerable uncertainties-not the least of which arise from the values of tv and /j used. The situation is worse in the case of A N P because about three quarters of the cis isomer is calculated to be produced from the T',' state. If 4'3 were as low as 0.5, the extinction coefficient calculated above would be too high by a factor of about 6. For AIP we calculate that most of the cis isomer is produced from the $'state; but even so. using a value o f 0 5 for 13 would lower the extinction coefficient by a factor of about 2.5. Fogel and Steel [ 141 have reported that gaseous cis-AIP has an absorption maximum at 380 nm with tmax = 70 l/mol cm compared to an c,;,, of 9.4 l/mol cm a t 355 nm for t r ans -AIP . I t has not been possible to detect this absorption f'ollowing the flash photolysis o f ~ r o n s - A I P in the present study. However, i t seems highly probable that

384 AI)A('HI AND HASCO

the very weak absorption we observed around 380 nm from ANP is due to the corresponding symmetry allowed n--H* transition of cis -ANP. A preliminary estimate, based on the value for c(ANP) a t 242.5 nm calculated above, suggests that the extinction coefficient a t 380 nm could be as high as 380 I/mol cm. In view of the uncertainties in this calculation and the weakness of the spectrum, this may not be irreconcilable with the corre- sponding value for cis- AIP quoted. I t would appear that an independent, quantitative determination of the absorption spectra of the cis-azopropanes would allow the technique of kinetic spectroscopy to make a significant contribution to the photochemistry of the azopropanes.

Acknowledgment

This work was supported by the Natural Sciences and Engineering Re- search Council and by the Atmospheric Environment Service of Canada.

Ri bliography

Received ?July 16, 1980 Accepted October 3, 1980