Photodissociation of alkyl bromides

Volume

53, number

1

CHEMICAL

PHOTODISSOCIATION

OF ALKYL

PHYSICS

LETTERS

1 January

1978

BROMIDES

W-L. EBENSTEIN, J.R. WIESENFELD

’ and G.L. WOLK

Deparrment of B~emistiy. Cornell University. Ithaca, New York 14853. USA Received

i September

1977

The branching ratios for the production of Br(4 *Pr /a) and Br(4 * P a,~) following the broadband flash photolysk of the *Yl bromides, CH& and CaHsBr, and the per-fluorinated molecules, CFaBr, CaFsBr and n-CaF,Br, have been determined using time-resolved atomic absorption spectroscopy. The production of electronically excited bromine atoms is shown to be inefftcient in the case of the aIky1 bromides while the pertluorinated molecules yield decreasing amounts of Br(4 *P112) as the molecular complexity increases, i.e., CFsBr > C2F5 Br > C3F7Br_ It is abo shown that the hydrogenated bromides deactivate electronically excited atoms almost two orders of magnitude faster than do the perfluorinated bromides.

1. Introduction

Br(4 2P,,2) + Br(4 2P3/2) + hu 0 = 2713

The production of electronically excited atoms during the photodissociation of small molecules has received much attention in recent years, with special emphasis on the photolysis of the aiky1 iodides and their perfluorinated analogs [l--4]. The alkyd iodidk experimentswere stimulated in part by the observation [S] of laser action on the rnagneticdipole allowed transition,

As the kinetics of Br(4 *Pla) (hereafter Br*) following flash photolysis of the parent molecule are of importance in the Br* laser and in systems utilizing alkyl bromides as sources of ground or excited state bromine atoms, we have also measured rates of deactivation of Br* by the Parent molecules and propose possible mechanisms for these processes.

I(5 *Pl&

* I(5 *P3j2) f hv 0 = 13 15 nm)

2. Experiment21

and the need to elucidate the mechanism of excited atom production during photolysis. The results of these investigations have led to a qualitative theory [4] regarding the effect of the parent molecule structure on the yields of the electronically excited species. The theory relates the observed correlation between alkyl group structure and the relative production of excited state atoms by using simple sym-

metry arguments which take into account the role of spin-orbit

coupling in the photodissociative

state

WIThe purpose of this investigation then is to test the theory and to determine the utility of various aikyl bromides as sources for the analogous bromine atom laser [7], +

Camille and Henry Dreyfus Teacher-Scholar.

nm).

The apparatus utilized in these experiments has been described previously [3,4,8] - A dilute solution of the alkyl bromide (typically l-50 X 10m3 torr) in an argon buffer (Ptotal = 20 or 30 torr for perfluorinated species, 60 torr for alkyl bromides) was exposed to a light pulse from a Kr-fdled flashlamp (Q/a x 10 ps, E = 100 J). Thermahzation of species in the quartz reaction vessel was insured by the great excess of argon buffer gas. The temporal profiles of [Br*] f or [Br] t (Br = Br(4 2P3,z)) after the flash were monitored by observing the attenuation of atomic resonance radiation at 154.1 nm (5 4P3,2 The + 4 2p3,2) or 153.2 nm (5 2Pl,2 f 4 2Pl& resonance transitions -were excited in a continuously operating, microwave-powered electrodeless discharge through a 10% Br2-Ar mixture (PtoM = 0.2 tot-r). 185

Volume 53, number 1

The transient absorption signal was monitored by employing an EMI solar blind photomultiplier tube and digitized with a Biomation 802 transient recorder. Typically 16-64 runs were averaged in order to enhance the signal. Gases in the reaction vessel were swept out between individual kinetic NIS. CF3Br, C,F,Br, and n-C3F7Br {PCR, Inc.) were first degassed and then purified by repeated distillations. CF3Br was distilled from a 40% pentane/60% ethanol slush (138 K) to liquid N2, C2F5Br was distilled from a toluene slush (178 K) ro liquid N2, and C2F7Br was distilled from an acetone slush (187 K) to liquid N2. CH3Br (99.5%, Matheson) and C2HSBr (Fisher Certified) were similarly degssed and distilled, CH3Br from an acetone slush to liquid N2, and C2HSBr from a chlorobenzene slush (228 K) to liquid N2. Research grade H2 (Matheson) was used without further purification. Ultra high purity argon (Matheson) was passed over a molecular sieve at 193 K immediately before use. All mixtures were prepared on a Hg-free glass vacuum line which was evacuable to =10e6 torr. The composition of the gas mixture in the reaction vessel was determined using calibrated floating ball rotameters Pressures were measured with a glass Bourdon gauge and standard test gauges.

3. Results The removal of Br* was monitored using the resonance transition at 153.2 nm, the absorption signal being related to [Br*] r by W&)

= (eGBr*] r)7,

(1)

where the constants have been previously described [9] _ The intensity of the radiation before the flash, IO, was measured using the pretrigger record feature of the transient recorder_ An experimental value of y (typically 0.60-0.90) was determdned over the range of observed absorption by measuring the absorbance of Br* at time t = 0 (~20 w after the flash) while varying the pressure of the source gas. The pseudofust order rate constant, k, derived from the transient absorption measurements ([Q] > [Br*] t) may be related to the bimolecular rate constant for deactivation, kQ. by k=kQ[QI 186

+K,

1 January

CHEMICALPHYSICS LETTERS

1978

Table 1

sctirce‘

Q,

P*

CF3Br

2.0

0.66 + 0.07

Cz Fr. Br n-CsF7Br CHs3r C2H5Br

o-9
0.2
O-48 f 0.02 co.10

0.15 c 0.12
10” k (cm3 molec-’ s-l) _

1.20 f 0.2 0.5 a) 2-7 20.4 3.8 t 0.3 77 +7 121 r8

a) Ref. [ll]. where K represents removal of Br* by spontaneous emission, diffusion to the walls of the reaction vessel and quenching by impurities in the samples of alkyl bromides or argon buffer- A plot of k versus [Q] ([Q] = allcyl bromide pressure) gives a straight line Of Slope kQ . The results of these kinetic experiments are presented in table 1 and fig. 1 - They clearly indicate that the hydrogenated bromides are -100 times more efficient at deactivating Br* than are the perfluorinated bromides. The branching ratios (+ = [Br*]o/[Br]O) and fractional yields of Br* @* = [Br*lO/( [Br*] o + [Br]o)) were determined using the following technique. A given pressure of alkyl bromide was photolyzed in the absence of all gases other than the argon buffer (fig. 2a). The absorption measured immediately following the flash then represents [Br], , the concentration of ground state Br atoms produced photolytically. The addition of an excess (~0.1 torr) of H2 to the photolysis mixture described abcve results in the rapid (fl12 = (kH2 [Hd)-l = lo-’ s) relaxation of Br* produced in the flash. All of the Br* deactivated in this fashion relaxes to form ground state atoms; reactive processes play no role here, as formation of HBr + H from Br* + H2 is 7 k&/mole endothermic. Therefore the absorption immediately following the flash (fig. 2b) may he related to the total production of bromine atoms, [Bi”lo + [Brlo. This technique has been previously applied to the reaction of excited iodine atoms with HBr and DBr [S] . The kinetic equations governing the disappea.ance of Br* and the appearance of Br have been thoroughly described in ref. [S] and will not be repeated here. In the’ absence of a reactive species, the equations for @ and p* are simply:

Volume 53, number 1

CHEMICAL

PHYSICS

1 January 1978

LETTERS

66

56

-

_

50

i P 2

3.9 -

7” t% -r x. n)

42

3.4-

‘0

I8

3

0

0.4

0.2

16’“~

Density

I

1

I

L

0.6

0.8

1.0

1.2

(molecule

1.41 0.1

I

1

0.5

cni3)

I

0.9

ICY

x Density

F& 1. Plots of pseudo-tint-order rate coefticients for removal of Br(4 2Pr,z) versus density a Cz H5 Br; 0~) o CF33r, p C2 Fs Br, 0 C3F7 Br. For the sake of clarity, onIy a ‘._ wesentative

,

1.3 (molecule

t

I

1.7

2.1

2.5

cms3)

of deactivating gas. (a) o CH3&, sample

ofdata

@MS

is S~OWR.

is

zj

IOO-

g

80

z f

60-

$j

Ml-

2s

20-

3

4.3

0

I

2

3 IO3

d t-i x Time bed

6

Fig. 2. Temporal PrOfikS Of [%
Cp= (AC;

p”=(Ag

- A$

)/A$’

- A8-l )/AZ;

8 s

7

b

-

0

IO

20

1,)

30

40

I,,

50

60

)

70

IO3 x Ttmefseci PhOtOiySiS

,

(2) ,

where AH, = (d [Br] ,, + d [Br*] o)r, the absorbance

Of 0.017 torrCF3Br

1

in 30

1Oz

AC

(a) pHz = 0, (b) pHt = 0.22

FOIL

measured in the presence of H2 and A0 = (Er[Br]#, the initial absorbance measured in the absence of H,. Alternatively Q1may be written as

(3)

exp(GH2 /r) - axP(%/?) cp =

exp(Q&)

’

187

Volume

53. number

C!ii&iCRL

1

PHYSICS

since for an optically thin absorber,

1 January

LETTERS

1978

1.6 t

(9

B*l t=o= K*pR& (where P reflects the photolytic yield of bromine atoms and PRB~ = partial pressure of the bromide). Substitution of (5) into (1) gives Inln(lo/I),,o

= const. + y In PRBr,

(6)

so that fix = constant of eq. (6). Plots of In A, where AX =A0 or A,, versus In pRBr derived from the data (figs. 3 and 4) verify this relationship over the range of absorbances measured as well as yield values for -r-.Typical values for 7 for the ground state transition (154.1 nnr) range from 0.3-0.5. Values of + and p* were calculated using both eqs. (2) and (4) and the agreement is reasonable. However errors introduced into eq. (4) due to the long-term drifts in flashlamp output and resonance lamp profile lead to larger scatter in tl’le results.

r

2-6 r 2.d 2.3

I

I

2.7

3.1 ‘”

3.5

PC3F7i3r

3.9

(P)

Fig. 4. As fig. 3, but CsF,Br is the source gas. The absence of a significant increase in absorption by the ground state atoms upon addition of H2 shows that relatively few excited atoms are produced in the initial photoiysis.

Calculated values for Cpand p* are shown in table 1. The results clearly show decreased production of BP versus Br, as one proceeds through the homologous series,

2.0

CF3Br > C2F5Br > C3F7Br. In the case of the hydrogenated bromides, as in the case of the hydrogenated iodides [3], the branching ratios are smaller and decrease with increasing molecular complexity. It should be noted that this technique is probably superior to that used for the alkyl iodides [3,4] since here we essentially measure the [Br*10 directly, whereas in the previous experiments, the [I*]o was extrapolated from measurements of [I] r_ where t nrsx = time of maximum I concentration.

o =A,

36

4. Discussion

t

3.&-----1.6

2.2

2.6

‘” ‘CF,

3-o

3.4

3.8

Br (p)

Fig. 3. Plot of -inIn(Zo/Z)o versus in pCF3Br demOnStrati?g the validity of eqs. (4) and (6). The increased absorption observed when H2 is added to the reaction mixture represents the deactivation of Br(4 2Pt,2) :o Br(4 *P3&. Only a sample of data points is shown here.

188

As may be seen from table 1, the akyl bromides are relatively irreffrcient photolytic sources of Br*, the only molecules capable of producing laser action (threshold requires @ > OS) being CFsBr and _ C2FSEr. In addition the branching ratios are substantially smailer than those observed for RI [3,4]. This observation may be viewed iir the context of

Vo!ume 53, number 1

CHEMICAL

PHYSICS

the earlier theory devised to explain the dependence of branching ratio on source molecule structure of the alkyl iodides [4]. The production of excited bromine atoms will result following direct population of the lowest excited Af state of the alkyl bromide RBr(AI)

%Br(Ai),

RBr(AT)+R-

f Br(4 2P1,&

‘Ihe lower yields of excited bromine atoms may then simply be explained in terms of two reinforcing phenomena. Firstly, the spin-orbit splitting in the alkyl bromides will be inherently smaller than in the iodides due to the lower atomic number of bromine. Secondly, the electronegativity of bromine is higher than that of iodine, thereby resulting in the energetic stabilization of zwitterionic valence struccontures [IO] of the form R%r-, where Br-(‘So) tributes nothing to the magnitude of the spin-orbit coupling in the molecular excited state. Thus, the AZ state cannot accurately be discussed in terms of strong spin-orbit coupling and the correlation between this molecular state and R + Br*, which is valid only in the case of strong coupling, is relatively poor. The yield of electronically excited bromine atoms in the photolysis of alkyl bromides is relatively low when compared to the corresponding observations in iodine. It should be further noted that the perfluorinated alkyl bromides display lower branching ratios for the formation of Br* as the molecular complexity increases in apparent contradiction to the trend observed in the analogous iodides where the values of Cpare CF$ < C2F51 < &Z,F,I [3]. No simple explanation of this observation may be extracted fi-om the qualitative theory. The data do suggest, however, that for alkyl bromides for which the allcyl radicals display very similar ionization potentials (and hence would show comparable contributions of zwitterionic struc-

LETTERS

ture), secondary structural

1 January 1978

effects will play a signif-

icant role in determining the branching ratio. The alkyl bromides are clearly seen to be far more efficient in deactivating Br* than are the perfluorinated molecules. This is presumably due to the relatively close resonance between the magnitude of the electronic quantum in bromine, 3686 cm-‘, and the C-H stretch, ca. 3000 cm-‘, which can significantly enhance the efficiency of electronic-vibrational energy transfer [8] _ No such close resonance occurs in the case of the perfluorinated molecules. Alternatively, the deactivation of Br* may take place via an atom transfer reaction, Brz + RBrb + RBr, + Brb, although it is difficult to understand why such a process would be over one order of magnitude more efficient for the hydrogenous bromides.

References 111 R.J. D~*novan, F.G.M. Hathom and D. Husain, Trans. Faradzy Sot. 64 (1968) 3192. S.J. Ri!ey and K.R Wilson, Discussions Faraday Sot. 53 (1972) 132. [31 T. Donahue and J.R Wiesenfeld, Chem. Phys. Letters 33 (1975) 176. I41 T. Donahue and J-R. Wiesenfeid, J. Chem. Phys. 63 (1975) 3130. [51 K. Hohla and K-L. Kompa, in: Handbook of chemical lasers, *ds. R.W.F. Gross and J.F. Bott (\Wey, New York, 1976) p_ 667. [61 A.& Nlkolskii, Opt Spectry. 29 (1970) 560. [71 J-D. Campbell and J-V-V. Rasper, Chem. Phys. Letters LO (1971) 436. [81 J.R. Wiesenfeld and G.L. \VoIk, I. Chem. Phys 65 (1976) 1506. PI P.D. Foo, T. Lohman, J. Podobke and J-R. Wesenfeld, J. Phys. Chem. 79 (1975) 414. [LOI 1V.G. Dauben, L. Salem and N-J. Turro, Accounts Chem. Res. 8 (197.5) 41. [ii1 R J. Donovan and D. Husain, Trans. Faraday Sot. 62

PI

<1966) 2643.

189

_