INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, VOL. V I I I , 959-969 (1976)

Excited-State Kinetics in the Photolysis of Azoethane and Hexafluoroazomethane at 366 nm, up to One Atmosphere Pressure,

and from Room Temperature to 150°C

G. 0. PRITCHARD, IT. 11. SERVEDIO, and P. E. JIARCHANT Department oj Chemistry, University oj California, Santa Barbara, Calijornia 95106

Abstract

The photochemistry of azoethane and hexafluoroazornethane at 366 nm has been reinvesti- gated up to 1 atni pressure, and over a range of temperature from 27 to 150°C. The Stern- Volnier type quenching plots primarily demonstrate the decomposition of a single electronic and vibrationally excited state for azoethane, but competitive photodissociation from two different electronic and vibrationally excited states, which was previously postulated for hexafluoroazomethane and azoisopropane, is confirmed for hexafluoroazomethane. I t is concluded, however, that two different electronic and vibrationally excited photodissociating states are present in azoethane photolysis, but that one of them is difficult to detect, a t least by the present approach.

Photosensitization with biacet,yl a t 436 nni also causes the dissociation of azoethane, and this is probably from the vibrationally equilibrated first triplet state. The energy barrier for this process was found to be 5.0 kcal/mol.

Introduction

The gas-phase photolysis of azoethane in the 366-nm region has been the subject of three prior investigations [l-31. The nitrogen quantum yield data can be displayed as linear Stern-Volmer type' quenching plots, in accordance with a simple photochemical activation mechanism, in which subsequent photolytic decomposition of the azoethane competes with collisional deactiva- tion. More recently, in investigations of the photolysis of hexafluoroazomethane (41 and azoisopropane [5], distinct curvature has been observed in the quenching plot.s, if the nitrogen quantum yields are determined over a sufficient range of pressure. This curvature is ascribed [4, 51 to decomposition from both the vibrationally excited first singlet (&) and triplet ( T I ) states.

Although the Stern-Volrner equation originally relates to the electronic quenching of an excited electronic state, it is convenient to use the terminology for vibrational relaxation; more than one (distinguishing equilibrated states) electronic state is involved in any case.

959

@ 1976 by John Wiley & Sons, Inc.

960 PRITCHARD, SERVEDIO, AND MARCHANT

In this work we have reinvestigated the excited-state kinetics and photo- lytic decomposition of azoethane and hexafluoroazomethane, up to a pressure of 1 atm, and from room temperature to 150"C, at 366 nm. The original experiments of Weininger and Rice [ l ] on azoethane went to almost 2 atm, but the data are too scattered for the detection of any curvature. Cerfontain and Iiutschke [2] examined the photolysis of azoethane at several temperatures from 28 to 152"C, but a t less than 200 torr pressure. In subsequent experiments with azoethane Worsham and Rice [3] also worked below 200 torr, and only a t room temperature, but they used excitation a t three different wavelengths of 352, 366, and 378 nm, respectively. In their investigation of the photolysis of hexafluoroazomethane Wu and Rice [4] extended the pressure range up to 1 atm, but again worked only a t room temperature.

The purpose of the work reported here is to ascertain if there is curvature in the azoethane Stern-Volmer plot a t higher pressures; this is not certain, and it has been speculated to occur [3, 41. In the event that curvature was not observed, it was thought to be worthwhile to corroborate the curvature observed by Wu and Rice [4] in the hexafluoroazomethane experiments, and to extend the range of temperature.

The simplest activated molecule reaction scheme is [ 1-31

(a) A + h v - + A *

(1) A* -+ N2 + 2R

(11) A* + A(1I) -+ A + A(1I)

where A is an azoalkane molecule, R is an ethyl or a trifluoromethyl radical, and the asterisk represents electronic and/or vibrational photoexcitation. Assuming a steady state for A* results in the familiar Stern-Volmer type expression

= 1 + ( h r / h ) [ A ]

All the more recent data on azoethane obey this expression quite well over the range of pressures studied, except that there is deviation with some of the intercepts from unity [2, 31. With the possibility of curvature in the plots (4, 51 these deviations need no longer be considered to be significant [5].2 Worsham and Rice [3] do, in fact, introduce a slight curvature from their theoretical treatment, which embodies the thermal energy distribution of the ground electronic state. However, any detectable curvature falls well within their experimental uncertainty, and the curves can be assumed to be linear in accord- ance with the Stern-Volmer equation [3]. Bowers [6] has carried out similar calculations on both the azoethane and hexafluoroazomethane data [3, 41. He

2 Not significant in the sense that there is no need to postulate that an A* molecule under- goes a unimolecular process, e.g., internal conversion [2], to another electronic state which is incapable of dissociation [2, 31.

PHOTOLYSIS O F AZOETHANE AND HEXAFLUOROAZOMETHANE 961

I 0 o/

3.0

2*oby I .o

9 4 3 O I

I I I I I 1 0 (0 20 30 40

Figure 1 . in niol/I., a t room temperature, and 366 nm. above 1.6 X Worshani and Rice 131.

Plot for azoethane of reciprocal N2 quantum yield vs. concentration Open symbols-present work;

Closed symbols-from mol/l. n-C4HlQ in the added gas. The curve is reproduced in Figure 2.

concludes that the very significant curvature seen in the hexafluoroazomethane data [4,6] is best rationalized on the basis that more than one electronic state is involved in the photodissociation.

Recently a report on the photolytic decomposition of azoethane at very high helium pressures (6-140 atm) has appeared (71. The search for Stern-Volmer curvature due to the possible nonrandomness of the energy distribution in the excited molecule is not to be confused with curvature a t low (i.e., 0-1 atm) pressures which might be due to the participation of more than one electronic energy level discussed above [7].

Experimental and Results

The experimental procedures and details are described elsewhere [ 5 ] . Azo- ethane and hexafluoroazomethane were obtained chromatographically pure from Merck, Sharp, and Dohm of Canada, Ltd.3 Azomethane was used as an actinometer. In the azoethane experiments n-butane was used as a vibrational relaxer above 30 torr, and biacetyl was used as the triplet sensitizer in the photo- sensitization experiments [ 5 ] . In the hexafluoroazomethane experiments hexafluoroethane was used as a vibrational relaxer above 100 tom.

The data on the direct photolysis of azoethane at room temperature are given in Figure 1, together with the data of Worsham and Rice [3] a t 366 nm.

The authors thank Professor Sidney Toby of Rutgers University for a gift of some hexa- fluoroazomethane.

962 PRITCHARD, SERVEDIO, AND MARCHANT

Figure 2. Plot for azoethane of reciprocal of NP quantum yield vs. concentra- tion in mol/l., a t various temperatures, and 366 nm. Open symbols-present work; closed symbols-from Cerfontain and Kutschke [a]. The lines in the inset are taken from our present data (large figure) through unity intercept,. The closed symbols are for 78, 116, and 152"C, respectively.

Five other data points (not shown in Figure 1) obtained by Terry and Iiutrell [S] between 20 and 100 torr in an ethyl radical study also agree with these results. The scatter in the intermediate pressure range was found to be due, a t least in part, to the mixing procedure employed for the two gases, and this was changed as we progressed to higher pressures. Our data a t higher temperatures are given in Figure 2. The thermal dark reaction correction at 150°C amounts to -27% of the Nz yield. Cerfontain and Kutschke's data [ 2 ] , including their room temperature results, a t 366 nm are also included in Figure 2 for com- parative purposes.

The data on the biacetyl triplet photosensitized decomposition at 436 nm are given in Table I. It can be seen that the photodissociation of the biacetyl is virtually completely quenched by the azoethane. The increase in the CO yield with temperature, in the absence of azoethane, is expected [9]. (We also found that high concentrations of butane appeared to quench the photodissociation of the biacetyl.) An Arrhenius plot of the sensitized Nz yield (corrected for the nonsensitized Nz formation) is shown in Figure 3.

The data on hexafluoroazomcthane are given in Figure 4 where the room temperature results of Wu and Rice [4] are represented by the dashed line. Correction for the dark reaction a t 150°C amounted to -5% of the NZ yield. There is apparently some small systematic difference in the two sets of room

The variations are discussed later in the paper.

PHOTOLYSIS O F AZOETHANE AND HEXAFLUOROAZOMETHANE 963

TABLE I. Biacetyl photosensitized dissociation of azoet,hane a t 436 nm. a

Temp., OC [Azoethane] x 103 [Biacetyl] x 1 6 N2 CO

-

_____ ~~~

27 1.6 11.5 -

1.6 - 123

1.6 1.6 26.0 -

70 1.6 12.0 - 1.6 - 229

1.6 1.6 50.0 4.0

120 1.6 26.0 - 1.6 - 2192

1.6 1.6 125 71.0

Product yields are in units of relative gas chroniatographic “Concentrations are in mol/l. peak areas.

2.c

1 .o 2.4 2.6 2.8 3.0 3.2 3.4

103/ TO K

Figure 3. temperature for azoethane and biacetyl.

Arrhenius plot of logarithm of the sensitized N, yield vs. reciprocal

964 PRITCHARD, SERVEDIO, AND MARCHANT

4.0

, 3.0 +ii2

2.0

1.0 I I I I I I 0 10 20 30 40

[MI x lo3

Figure 4. Plot for hexafluoroazomethane of reciprocal NZ quantum yield vs. concentration in mol/l., a t various temperatures, and 366 nm; above j .4 X 10-3 mol/l. CzF6 is the added gas. The room temperature data of Wu and Rice [4] are shown by the dashed line.

temperature data.* An expanded plot of the lower pressure experiments is given in Figure 5 , which is the region of interest and where curvature clearly occurs. There is excellent agreement (allowing for the small systematic difference) between Wu and Rice's work [4] and the present work. Some additional very-low-pressure data points, obtained by Dacey, Mann, and Pritchard [lo] in a circulating system, are also given in Figure 5 , and they show good agreement with both sets of values.

Discussion

We retain the mechanism given for the photolysis of azoisopropane [5] for our present discussion on azoethane and hexafluoroazomethane where the superscripts have their usual significance :

(a> A + hv -+ IA*

(1)

(2) (3) 'A* -i 3A*

'A* -+ Nz + 2R

'A* + JI -i 'Ao + M

We do not consider this discrepancy to be serious. At 722 torr Wu and Rice (41 report a value of p ~ * = 0.26 and we find p~~ = 0.21 a t the same pressure. Corrections for reflected and scattered light are never the same in different optical setups [ 2 4 , 111. Cerfontain and Kutschke (21 estimate an uncertainty in the actinometer measurements of &lo%, which be- comes the uncertainty in the (D--1 scale. The two p values given in this footnote span the range of q~~ = 0.235 f 10% of that value.

PHOTOLYSIS OF AZOETHANE AND HEXAFLUOROAZOMETHANE 965

2.5

2.0

1.5

2.5 5.0 1.5 10.0

[MI x lo3

Figure 5 . Plot for hexafluoroazomethane of reciprocal N, quantum yield vs. concentration in mol/l., a t room temperature, and 366 nm, in the low-pressure region. C-present work; .-from Wu and Itice [4] (the half- filled circles have added C2Fs); 0-from Dacey, bfann, and Pritchard [lo] a t 25 and 37°C. The quenching efficiencies of (CF3)2N, and C,F, are identical [4].

3A* -+ hT2 + 2R 3A* + M -+ 3A0 + M

'A0 -+ 3A0

3A0 4 Nf + 2R 3A0 --f A

The superscripts 1 and 3 represent the multiplicity of the excited electronic states, while an asterisk denotes a molecule with more vibrational energy than a vibrationally equilibrated m'olecule with superscript zero [ 121. The intersystem crossings, reactions (6) and (S), are more complicated reactions involving vibrationally excited intermediates, such as 'A0 ---f 3A*, and 3A0 -+ A**, but in the absence of evidence to the contrary, it is assumed that they may be repre- sented over the whole range of pressure studied by the simple first-order processes shown [13].5 Other nondissociative reaction channels for return to the ground state are possible, such as the internal conversion IAo + A**, but we confine ourselves to reaction (S), which is sufficient to be consistent with values of (py, < unity.

The double asterisk represents a highly vibrat,ionally excited ground-state molecule, with -53 kcal/mol [ 141 of excess vibrational energy, which is subsequently collisionally deactivated. Similarly 3A* + M + 3A0 + M occurs, and it is kinetically sufficient. to write reaction (6). Our data preclude the measurable decomposition of vibrationally excited mole- cules which have already undergone a deactivating collision. See the following discussion.

966 PKITCHARD, YERVEDIO, AND MAHCHANT

I n all the acyclic azoalkane data [2-5, 8, 111 there is no detectable concavity in the initial portion of the Stern-Volmer plots, which occurs with hexafluoro- acetone [15, 161, so that unit collisional deactivation efficiency can be assumed [ 171. This also provides inferential evidence that decomposition does not occur via an internal conversion of the initially photoexcited molecules to a very highly vibrationally excited ground-state molecule,

(10) A** -+ N, + 2R

If all the light energy eventually appears as vibrational energy the molecules in the A** state in reaction (9) will contain about 78 kcal/mol more vibrational energy than vibrationally equilibrated molecules. If sufficient vibrational energy is not removed on a single collision to prevent decomposition, product formation can occur from more than one level on the vibrational cascade, which should produce a concave (due to an initial zero slope) Stern-Volmer plot at low pressures [17]. If, on average, 10-12 kcal/mol is lost per collision with a poly- atomic quenching molecule [IS], some concavity could be discernible. Aetiva- tion energy values for the pyrolyses of acyclic azoalkane molecules are generally close to 50 kcal/mol [19]. Consideration of the energetics involved has also led other n-orkers to conclude [3 , 14,201 that reaction (10) is not the dissociative pathway.

Alternatively if the A** molecule in reaction (9) is effectively quenched from further decomposition by a single collision, the quenching plot should be linear. This is not the case with hexafluoroazomethane as Wu and Rice [4] have shoun, and which is completely corroborated by the present work, nor is it the case with azoisopropane [5]. Further, reactions [9] and (10) do not provide an explana- tion for the limiting high-pressure quantum yield -px2 n-hich is observed in azoisopropane photolysis a t 160°C [5]. Reaction pathways which populate the vibrationally equilibrated triplet state and lekd to the possible occurrence of reaction (7) do provide an explanation for the -pN2 values. This is entirely analogous to the photochemistry of hexafluoroacetone [la, L5].

The curvature seen in the hexafluoroazomethane and azoisopropane [5] quenching plots must indicate the existence of two, distinct vibrationally excited states with differing lifetimes. The postulate of a “hot” intersystem crossover, reaction (3), which is competitive with reaction ( l ) , is consistent with the experimental data. The molecules in the 3A* state are longer lived than those in the ‘A* state, so that they are effrctively quenched by collisions a t a relatively lower pressure [4,5]. In Figure 4 the curvature in the plots is still discernible a t 70 and 120”C, in that the best straight lines drawn through the data produce high (> unity) intercepts. The significancc of intercepts is discussed more fully below with respect to azoethane.

PHOTOLYSIS OF AZOETHANE AND HEXAFLUOKOAZOMETHANE 967

The present data on azoethane show linear Stern-Volmer quenching plots \\ ith 110 evidence of curvature, up to a pressure of 1 atm. Similarly Chervinsky and Oref [7] obtained a linear plot, a t room temperature, with up to 140 atm of added hclium. Arbitrarily assuming intercepts of unity, as we have done for our data in Figures 1 and 2, the linearity of the curves clearly shows that there is only one photodissociating state for azoethane over the range of pressure studied by us. The data do not distinguish betneen reactions ( l ) , (4), or (10) for the dissociative mode, provided deactivation occurs on a single collision, and the simplified mechanism, reactions (I) and (11) which are given in the Introduction, is sufficient.

Hen-ever, Worsham and Rice [3] have pointed out that extrapolations of the azocthane data consistently show intercepts which are slightly greater than unity, although it is difficult to substantiate these deviations from unity outside the experimental error [3]. The very careful and reliable work of Cerfontain and Iiutschke I21 below 200 torr shorn the occurrence of high intercepts, which decrease in value toward unity as the temperature is raised. Cerfontain and Iiutschke's data are given in Figure 2 , ~ i t h the expcriments done above room temperature magnified in the inset.6 We can conclude, as Rice and coworkers [ 3 , 4 ] have previously suggested, that curvature occurs a t very low pressures, and that tno decomposition processes with different rates do in fact occur. There is no a pr ior i reason to assume that curvature should be manifest in the same pressurc region for all acyclic azoalkanes. The original data on both azoiso- propane [21] and perfluoroazoethane [ 111 were interpreted to give high intercepts, but lines which exhibit curvature can also be draxn through the data points.

Acyclic azoalltanes photodecompose in solution [ 141, and thc photodecomposi- tiori of azomethane, azoethane, azoisopropane, and hexafluoroazomethane in the liquid phase have all been observed [22, 231. The quantum yield reported for szoisopropane has a value of 0 3 q N 2 = 0.025 in isooctane solution a t room tempera- ture [22], which is equivalent to a pressure of 1000 atm. We can assume that this is the pressure-independent limiting quantum yield resulting from the decomposition of a vibrationally equilibrated state. As the temperature is raised, values of mq~2increase due to the existence of a triplet energy barrier to decomposition [ 171. Similar energy barriers are reported in ketone photo- chemistry, since the 3A0 state has a lifetime long enough to achieve vibrational cyuilibrium and hence to dissociate thermally [12, 13, 15, 171.

In order to establish the possible dissociation of triplet-state molecules, and to givcb some further substantiation to the proposed mechanism, we attempted to

"There 1s some discrepancy between the 116°C data of Cerfontain and Kutschke (black squares) and the extrapolated line from our high-pressure data a t 100°C. The puipose of the present work was to look for curvature a t higher pressures and not to obtain exact agreement with Cerfontain and Kutschke's very reliable data at lower pressures. The agreement IS

excellent a t 27-28°C and 150-132°C and satisfactory at 70-78°C.

-____

968 PRITCHARD, SERVEDIO, AND MARCHANT

photosensitize the decomposition of azoethane with biacetyl. Table I demonstrate that such a process does occur,

The data in

3 6 + A -+ B + 3A

(12) ‘A + Nz + 2R

The energy of the triplet in biacetyl is ET = 55 kcalimol [14], and the triplet energy levels of acyclic azoalkanes are estimated to be similar; Collier, Slater, and Calvert calculate ET ‘v 53 f 3 kcal/mol for azoisobutane [14]. The triplet state azoethane molecules formed in reaction (11) must therefore be close to vibrational equilibrium. An Arrhenius plot is made for the dissociation in Figure 3, and an activation energy barrier of 5 kcal/mol is obtained from the graph. If we infer that reactions (7) and (12) represent the same process, we can conclude that in the direct photolysis, and at high enough pressures, limiting values of m q N 2 should be obtained, which correspond to the liquid-phase quantum yield a t the same temperature. 1 atm of polyatomic qucncher used in the present work, or 140 atm of Hc [7], are insufficient to cause clearly detectable leveling off in the Stern-Volmer plots for azocthane, and the photodecomposition of the vibrationally excited state is still dominant under these ~ondi t ions .~

Summary

(1) We present a consistent mechanistic picture for the photodissociation a t 366 nm for azoethane, hexafluoroazomethane, and azoisopropane, which involves two decomposition processes, with different rates, from tn o distinct vibrationally excited states. The evidence is inferential for azoethane, since it is based upon the high (> unity) intercepts which were obtained by Cerfontain and Icutschke [2] ; however their work can be accepted as being extremely reliablr..

(2) The mechanism also a l h s for the photodecomposition of the vibra- tionally equilibrated lowest excited triplet state, \\ hich provides for the eventual onset of a pressure-independent limiting quantum yield, and accounts for liquid-phase photolysis.

(3) Due to the inherent scatter \\ hich often is obtained in the quantum yield determinations, for cxamplc, Figure 1 of Chervinsky and Oref [7], which seems to arise despite very careful experimental procedures, it is difficult to detect curvature or high intercepts, and draw appropriate mechanistic conclusions, unless the curvature or intercepts arc very distinct.

These are taken to be the S1 and T I electronic states.

Chervinsky and Oref [7] give a lot of weight to the single datum point a t 140 atm in establishing their straight-line plot. An alternat,ive line could be drawn showing curvature with the leveling off beginning at around 80 atm, suggestive of a value m ‘ p ~ ? * 0.1. A de- finitive conclusion must await further work, including a liquid-phase determination of ‘ p ~ ~ .

We intend to pursue some of these measurements.

PHOTOLYSIS OF AZOETHANE AND HEXAFLUOROAZOMETHANE 969

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Received September 12, 1975 Revised ,July 8, 1976