2553
J. Phys. Chem. 1992, 96,2553-2561 excimer formation (ca. 4-5 kcal/mol) marginally larger than the entropy of a~sociation.'-~Since the singlet lifetimes of P N and PA are similar to that of PE, the failure to observe self-quenching for PN and PA must result from smaller enthalpies of formation (50% quenching of NF(a). formed by quenching of NF(a), the Br2* afterglow would be hard There was no significant NF(b-X) emission upon introducing the to observe for this relatively low [ Br] .
+
-
The Journal of Physical Chemistry, Vol. 96, No. 6,1992 2557
Quenching Reactions of NF(a’) by Halogens
2o
3
A
0
2
E
1
0 3
2
1
0
4
cm-l
[IF]/lO”molecule
Figure 5. First-order quenching plots for NF(a) by IF for various concentrationsof [HN,], and [F],: (a) [HN310and [F], are 1.5 X 10l2and 10 X 10l2molecule cm-,, At = 39 ms; (b) [HN,], and [F], are 1.5 X 10” and 10 X 10l2molecule cm-,, At = 65 ms; (c) [HN,], and [F], are, 2.8 X lo’*and 13 X 10l2molecule cm-,, At = 96 ms; (d) [HN3I0and [F], are 1.5 X 10l2and 4.0 X 10l2molecule cm-,, At = 88 ms; (e) [HN,], and [F], are 1.5 X 10l2and 4.0 X 10l2 molecule c d , At = 163 ms.
N,(A) flow. The NF(b-X) signal was calibrated for the initial [NF(a)] and the [Nz(A)] by adding enough I F to convert all the NF(a) to NF(X). These data showed that less than 1% of the quenching of NF(a) by Clz could give NF(X). By inference, the quenching of NF(a) by ClZis chemical in nature. C. Quenching by IF. The F + CF31reaction has a rate constant of (1.0 f 0.3) X lo-’, cm3 s-I and converts CF31to I F within -5 ms; thus the remainder of the reactor could be used to study quenching of NF(a) by IF. Under conditions of excess [F],, the metered CF31flow gives the [IF]. A 2% mixture of CF31 in Ar was introduced at the reagent inlet for [NF(a)] of (2-8) X 10” molecule ~ m - ~Several . pseudo-first-order plots are shown in Figure 5. The rate constants were (7.2-13.0) X lo-” cm3 s-I; the uncertainty mainly arises from the time allotted for converting CF31to IF. By selection of the more reliable data, a rate constant of (10 f 3) X lo-” cm3 s-l was assigned. In contrast to the quenching of NF(a) by IC1 and Iz, vide infra, there was no enhancement in the NF(b-X) emission intensity for any range of [IF]. Also, the IF(B-X) emission was never observed. Inspection of Figure 5 shows the curious result that [IF] is only -2 X 10” molecule ~ m - that ~ ; is, pseudo-first-order quenching conditions were not established. This strongly implies that I F is being recycled in a physical quenching mechanism. I F + NF(a)
-
IF(X,u’?
+ NF(X)
+ IF(X)
-
N,(x)
+ IF(B~II~+)
(7)
The IF(B-X) intensity vs [CF31] with and without [HN,] added is shown in Figure 6A for conditions of excess [F],. The IF(B-X) emission intensity was constant for a [NF(a)],/[IF], ratio of 1-10, implying that [IF] is constant. These I F quenching data show that the reaction rate with NF(a) is fast, NF(X) is formed, and IF(X) is not remoued. The apparent slight increase in IF(B-X) intensity with added HN, is due to the overlap between IF(B+X) and NF(b,u’-+X,u’+l) bands. Finally, an experiment was done to show that NF(X) does not react with I F by observing the NF(b-X) intensity vs [HN,], for three different [IF] from 1.8 X 10’’ to 3.6 X 10” molecule ~ m - ~The . results are shown in Figure 6B. The linearity of the plot up to 1.4 X 10l2molecule cm-3 shows that NF(X) is not removed by IF. The curvature in the plot for high [NF(a)] is a consequence of the removal of NF(a) by bimolecular self-destruction. Thus, we conclude that I F is not
-
2
-
15-
X
4
$ t-
10-
10 ( H N ~ ) I I O ” molec
0
20
30
cm-3
Figure 6. (A) IF(B-X) emission intensity from N2(A)+ IF(X) with and without added [HN,] but always with excess [F],. The IF(B-X) emission was observed slightly beyond the N,(A) inlet: ( 0 )[HN,], = 1.4 X 10l2molecule (*) without HN,. In both cases [F], = 4.8 X lo1*atom cm-, and At = 163 ms. (B) NF(b-X) emission intensity vs [HN,] for different [IF] and fixed [F], = 1.9 X lo1, atom cm-’: (0) [IF] = 1.82 X 10”; (*) [IF] = 2.5 X 10”; (0)[IF] = 3.6 X 10” molecule ~ m - ~ .
(6)
The physical quenching mechanism was confirmed in three ways. First, a flow of N,(A) was added at the end of the reactor under conditions such that the NF(a) had been quenched by IF, and strong NF(b-X) emission was observed. As demonstrated in Figure 1, the yield for NF(X) from (6) seems to be unity. As a second test, we ran an experiment with [NF(a)], > [IF], and monitored [IF(X)] a t the end of the reactor by using the N2(A) excitation-transfer reaction.’* N,(A)
0
l t
0
0
2
4
6
8
10
(ICI]/IO” m o l e c cm-’
Figure 7. First-order quenching plots for NF(a) by IC1 for various [HN,],, [F],, and At: (a) [HN,], and [F], = 1.3 X 10l2and 2.4 X 10l2 molecule cm3, At = 42 ms; (b) [HN,], and [F], = 1.9 X 10l2and 2.4 X l o t 2molecule cm3, At = 29 ms; (c) [HN,], and [F], = 2.0 X 10l2 and 4.5 X 10l2molecule cm-,, At = 59 ms.
consumed in the quenching of NF(a) and that NF(X) does not react with IF. D. Quenching by ICl. A 5% mixture of IC1 in Ar was prepared on the same day that the experiment was performed. Since F atoms would reactga with IC1 to generate IF, experiments were done with [HN,], 5 [F],. The NF(a) quenching rate by IC1 is fast and the data appear to follow pseudo-first-order kinetics, as demonstrated by the results in Figure 7 . The rate constant is large, and the average of three independent experiments was k = (5.0 1.0) X lo-” cm3 s-]. Although the data appear to follow
Du and Setser
2558 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 107
-
1
"1
V
O J 10
I
1
,
I
,
I
-
,
I
-1
1
b O0
Figure 9. Plot of In [NF(a)] as a function of added [I2]for six sets of results with various reaction times and [F],/[HN,]o: (A) [F]o/[HN~]~ = 2.0, [NF(a)] = 9 X 10" molecule cm-,, At = 100 ms, P = 3.5 Torr; (A)IF],/[HN,J, = 1.6, [NF(a)] = 6 X 10" moleculecm-', At = 78 ms, P = 2.7 Torr; (W) [F]o/[HN3],= 1.4, [NF(a)] = 6 X 10" molecule cm-,, At = 95 ms, P = 2.7 Torr; (0)[Fl0/HN3], = 0.85, [NF(a)] = 2 X 10" At = 95 ms, P = 2.7 Torr; ( 0 ) [FIo/[HNs]o = 2.0, molecule [NF(a)] = 7 X 10" molecule cm-,, At = 98 ms, P = 2.0 Torr; (0) [F],/[HN,], = 2.0, [NF(a)] = 6 X 10" molecule At = 85 ms,P = 0.9 Torr.
O
0
o
l
C
1 * *
#r 1
0 0
15
10
[ I ~ ] /IO'' motecule cm-'
i
'
5
0 0
20
I
*I 40
H t
I
*
60
I
I * . 80
I*
100
Timerms Figure 8. Enhancement of the NF(b-X) emission intensity along the reactor for low [ICI]: [ICI] = (a) 2.75 X lo", (b) 1.7 X lo", (c) 0
molecule cm-). first-order kinetics, the [IC11 5 [NF(a)], which suggests that the mechanism is physical quenching. Upon the addition of a small [IC11 to the reactor, such that 10.85 quenching of NF(a) occurred, the NF(b-X) green emission was actually enhanced along the reactor by a factor of 3-4; see Figure 8. The enhancement required the presence of NF(a). If [ICI] was increased to obtain greater quenching of NF(a), the NF(b) emission was removed. The enhancement of NF(b) also was observed upon addition of I,; the mechanism for the enhancement of [NF(a)] will be more fully discussed in the next section. However, the likely mechanism is the generation of I atoms, which could be by chemical reaction (NFC1 I) or a second step involving ICl(u) and NF(a). Thus, distinguishing between physical and chemical quenching in the first step becomes of interest. Checking for NF(X) is more difficult in the IC1 system than for IF, because excess [F] cannot be used to entirely convert all the [HN,] to [NF(a)]. In order to obtain higher [NF(a)] without [F], high [HN,], and [F], were used with [F], = 1.5[HN3],. With [NF(a)] of 2 X 10" cm-, (which was determined by comparing the NF(a-X) intensity to that from a calibration plot for known [NF(a)], the formation NF(X) from the quenching of NF(a) by IC1 was observed by the N2(A) method. Next, the [NF(X)] measurement was done by comparing the NF(b-X) emission from quenching by IC1 with the [NF(X)] obtained by quenching with I F for the same [N2(A)] and [NF(a)]. The comparison suggested that the branching fraction for NF(X) formation is 0.85 f 0.1 from quenching by IC1. Two quenching experiments were done by adding excess [F] to a flow of ICl. Although there may be a minor component giving C1F + I,9a the IC1 is largely converted to IF. The quenching rate cm3 SS;, which constant for these conditions was 8.1 i 0.4 X
+
is close to the value measured using CFJ as the I F source. This value is larger than measured for ICl, and a somewhat smaller quenching rate constant for IC1 than for IF is confirmed. Finally, this experiment suggests that the reaction of NF(a) with C1 atoms must have a rate constant of less than 1 X 10-I'cm3 s-I. E. Quenching by 12. The quenching rate by I2 is extremely fast, and the addition of small amounts of I2 greatly reduced the [NF(a)]. Some typical quenching plots are shown in Figure 9 for various ratios of [F], and [HN310and reaction times; the 12-Ar mixture was freshly prepared for each experiment. Even though the [I2lO< [NF(a)], for nearly all experiments, the plots show first-order kinetics for a factor of 5 change in [NF(a)]. The apparent quenching rate constant is (1.5 f 1.0) X 10-Io cm3 s-I. Changing the pressure from 0.9 to 3.5 Torr and the [F],/[HN,], ratio from 0.75 to 2 gave no apparent change in kI,. The quenching plots seemed to be reliable to within our ability to control the I2 concentration. In addition to the extensive quenching of NF(a), an increase in NF(b-X) intensity was observed upon addition of I2 for low fractional quenching of NF(a). Experiments were done with N2(A) to monitor the formation of NF(X). The results are very similar to IF, and the addition of small amounts of I2 give strong NF(b-X) emission from the N2(A) reaction. Calibration of the NF(b-X) emission by addition of I F indicated that 90% of the NF(a) is converted to NF(X) by I2 quenching. The N,(A) I2 reaction does not give 12(B-X) emission, and there was no easy way to monitor the degree of I2 dissociation by the NF(a) reaction in our apparatus. The NF(a-X) and NF(b+X) intensities were monitored along the flow reactor in the absence and presence of a low [I2]; see Figure 10. (In the absence of 12, the [NF(b)] is at steady state from formation via energy pooling from 2NF(a) and removal by radiation (7 = 20 ms)). For [I2] = 1.5 X 1O'O molecule cm-, and [NF(a)], = 0.5 X 10I2molecule the [NF(a)] is reduced, but the [NF(b)] is enhanced by a factor of -2. In a similar experiment with ICl, the NF(b) was actually enhanced by a factor of nearly 3-4 and the NF(b) profiles had a more rapid rise time; see Figure 8. The variation of [NF(a)] and [NF(b)] with [I2] for a reaction time of 79 ms is shown in Figure 1 1. The maximum in the [NF(b)] vs [I2] plot is indicative of the need to have both I* and NF(a) present simultaneously. The NF(b) excitation mechanism probably requires two additional NF(a) molecules after I(2P3,2)is generated.
+
NF(a)
-
+ I(2P3,2)
I(2Pl,2)+ NF(a)
I(2Pl,2)+ NF(X)
AH,' = -3855 cm-' (8a)
NF(b)
+ I(2P3,2)
AH,' = -110 cm-! (8b)
The Journal of Physical Chemistry, Vol. 96, No. 6 , 1992 2559
Quenching Reactions of NF(a') by Halogens
TABLE I: Quenching Rate Constants for Halogen Molecules reagent F2 C12 CIF Br2
I /
U
t -
12
"il
I 50
0
100
20
15 h
X h
I
10
LL
Z
v
-
5
0
50
150
100
Reaction Time (ms)
Figure 10. Relative concentrations of NF(a) (A) and NF(b) (B)as a function of the reaction time with (A, 0 ) and without (A, 0)[I2] = 1.5 X l o i omolecule The [F], and [HN310were 2.4 X 10l2 and 1.4 X l o i 2 molecule ~ m - respectively. ~, 20,
I
0 0
I
cm3 s-I
40 i 5 150 f 30 50 f 10 1350 f 200 1550 f 300 800 i 300 1100 f 200 300 i 100
150
Reaction Time (ms)
n
IC1 IF
NF(b),'
"Taken from ref 9a. bTaken from ref 1 .
0
v
NF(a), cm3 s-' 0.32 f 0.03b 5.8 f 0.6 76 f 10 380 f 60 1500 f 1000 500 f 100 1000 f 300 180 f 40
3
9
6
[ I 21/1 0'' molecute
12 -3
cm
Figure 11. Plots of the NF(a+X) and NF(b+X) relative intensities observed at 79 ms as a function of added [I2]. The [F], and [HN,], were 2.4 X l o i 2 and 1.4 X 10l2 molecule ~ m - respectively. ~,
Based upon the rate constant for the reverse of (8b) and the 300 K equilibrium constant, the rate constant for (8b) has been assignedgaqMas 5.7 X lo-" cm3s-]. If reactions 8 are the mechanism for NF(b) excitation, then an explanation of the source of I in the I2 and ICl, but not I F systems, is required. Searches for other weak emission in the 400-850-nm range from the NF(a) I2 system were made, but no emission was observed for the range of concentration used in this work. F. Quenching of NF(a) by I Atoms. Some qualitative experiments were done to study reaction 8a. The I atoms were generated by the reaction of 0 atoms with 12,23the rate constants for (9a) and (9b) are 1.4 X and 5 X lo-" cm3 s-!, respec0 + I, IO + I (9a)
+
IO + 0
- o2+
I
(9b)
tively. A small prereactor was attached to the reagent inlet and (23) Atkinson, R.;Baulch, D. L.; Cox, R. A,; Hampson, F. R., Jr.; Kerr, .I. A.; Troe, J . J . Phys. Chem. Ref.Data 1989, 18, 915.
a flow of 0 atoms was generated from a microwave discharge in N2024to avoid the production of 02(a) from a discharge in 02. Neither the N 2 0 nor 0 causes quenching of NF(a) for the concentration ranges used.',2 Upon striking the microwave discharge in the N20-Ar flow for a given flow of I2 in the reactor, the [NF(a)] actually increased, indicating that kl < k12for quenching of NF(a). The introduction of the 0 atom flow to the reactor containing NF(a) and I2greatly enhanced the NF(b-X) emission intensity, but a slight induction period along the reactor for the increase in the N F ( b X ) emission existed, consistent with the time required for generating [I*]. By changing the I2 flow for constant excess [O], an estimate for kI was made based on the quenching of [NF(a)] vs [I]. Because of the uncertainty in the conversion of [I2] to [21] for the relatively short reaction time, the value assigned to kl is uncertain, although a computer simulation indicates that 90% of I2 should have been converted to I. A value of 1.8 X lo-" cm3 s-l was consistent with the available limited data. Certainly kl must be larger than 1 X lo-" cm3 s-l and smaller than k12.
Discussion The kQ values summarized in Table I are the recommended values. The larger uncertainties relative to rate constants reported by stable molecules2 are due to the chemical complexity in the reactor and/or to the difficulty of handling and metering halogens. The rate constants' for CF31 and CH31 are 2.4 X and 23 X cm3 s-l, so the mere presence of I atoms in a molecule does not guarantee rapid quenching of NF(a). The results in Table I show that the quenching constants of 12,ICl, and I F are comparable to those for NF(b). However, the lighter halogens quench NF(b) more rapidly than NF(a). The primary products of the reaction of NF(a) with X2 and XY have not been fully identified, but, 12, ICl, and I F seem to give mainly NF(X), whereas C12, Br2, and probably ClF do not give NF(X) and by implication their quenching is by chemical reaction. These chemical pathways will be discussed for C12and Br2 before the quenching by IF, IC1, and I2 is considered. Both abstraction (the 'A" component) and insertion (the iAfcomponent) pathways can be envisioned from the correlation diagram of Figure 12. NF(a)
+ C12
-
NFCl
+ C1
NFCl2(%,IAf)
( 1Oa)
(lob)
The NFC12(%) molecules will be vibrationally excited and can decompose by various competing pathways unless stabilized by collisions with the Ar carrier gas. NFCl,(%)
-
-
-+
-
NFCl
+ C1
NF(X) NCl(X)
+ C12
+ C1F
(1 la) (1lb) (1 IC)
The enthalpy changes for reactions 10a and 10b were estimated as --31 and --84 kcal mol-', respectively, from D(NF-C1) = D(NFClC1) 53 kcal mol-'.25-27 Although the thermochemistry (24) Piper, L. G.; Rawlins, W. T. J . Phys. Chem. 1986, 90, 320. (25) Zhang, F. M.; Oba, D.; Setser, D. W. J . Phys. Chem. 19f37, 91, 1099. (26) Bettendorff, M.; Peyerimhoff, S. D. Chem. Phys. 1986, 104, 29.
2560 The Journal of Physical Chemistry, Vol. 96, No. 6,I992
Du and Setser rate constant for quenching by 12, IF, and IC1 is implied. This could be the interaction at long range between the 3A" entrance channel potential, correlating to NF(X) IX, and the IA" potential, which may become attractive for I, and IX compounds. The strong spin-orbit coupling provided by the I atom apparently facilitates spin inversion; the mechanism probably is similar to the E-V quenching"-l5 for 02(a) + 12. Although the fundamental nature of these E-V conversion processes may not be well understood, utilization of the rapid quenching of O,(a) and NF(a) to generate high concentrations of vibrationally excited IF, IC1, and I2 molecules should be ~ s e f u 1 . l ~The failure to generate I(2Pli2)atoms from IF for our relative low [NF(a)] is consistent with the larger bond energy (2.88 eV) relative to I, (1.54 eV) or IC1 (2.15 eV). For the latter two cases, the energy of two NF(a.) molecules can cause dissociation, but 3NF(a) are required to dissociate IF. An alternative explanation, excitation transfer with I, and IC1 to give the A'3112 electronic states followed by interaction witb another NF(a) to give dissociation, was rejected because the yield of NF(b) should have been much larger according to calculations for such a model. Nevertheless, this explanation would differentiate IC1 and I, from IF, since the IF(At3112)state is too high in energy to be formed by collision with NF(a). Excitation of Br2(A'311,) would require 1700 cm-I and can be discounted.29a Excitation transfer from 12(Aor A') to O2to give 02(a) has been observed in the solid state.29b A simulation of NF(b) formation in the 1, system was done by integrating the rate equations using the mechanism
Kcallmole
+
\ '\
look
\ ,
0
A"
l
/'/
/ / ' / /
, , / /
/
lA"
40
t
0
ii
\
ii
t
NFC t2hS I
Figure 12. Correlation diagram of the NF(X) and NF(a) states with CI2 showing the possible exit channels. The AHH,"of NFCI,, NFC1, and NCI were estimated to be -4 f 4, 22 f 4, and 8 f 5 kcal mol-', respectively. The AHY(NF) was taken as 50 kcal mol-'.
NF(a)
+ I2
-
-
12(X,u) + NF(X)
12(A')
+ NF(X)
The 12(X,u)can either be vibrationally relaxed by collisions with Ar or be dissociated by interaction with another NF(a). 12(X,u) + Ar 12(X,u)
-
+ NF(a)
12(A') + NF(a)
.. . 9
11
13
15
Ion i I atio n E n e r g y(eV1
Figure 13. Correlation between the ionization energy of X, and kx2: I, ( I ) , IC1 (21, IF (3), Br2 (4), C12 (5), CIF ( 6 ) , and F2 ( 7 ) .
of reactions 10 and 11 is not well documented, the qualitative implications are not dependent upon small changes in the thermochemistry. In order to place the NCI exit channels on the plot, we used AHfO(NC1) = 74 kcal mol-'.26 An insertion mechanism is supported by the correlation of the magnitude of the rate constants with the ionization energy of X2 or XY; see Figure 13. Similar correlations have been found for the reaction of NF(a)' and Si(CH,)2 with olefins,28and they generally are indicative of an addition reaction. The dissociation channel giving NFCl + C1 (or NFBr + Br) must be dominant relative to C12 or ClF elimination from NFC12,since NF(X) was not found as a product. If the halogen contains iodine, NF(X) can become the major primary product. This could be from strong chemical interaction with (1 1 b) being the dominant decay channel, rather than dissociation to NFX + I. But D(1-NFI) < D(C1-NFC1) and dissociation is spin allowed, and a different mechanism with a large (27) (a) Exton, D. B.; Gilbert, J. V.; Coombe, R. D. J . Phys. Chem. 1991, 95, 2692. (b) Arunan, E.; Gilbert, J . V. Setser, D. W., to be submitted to J . Phys. Chem. (c) Conklin, R. A,; Gilbert, J. V. J . Phys. Chem. 1990,94,3027. (28) Baggott, J. E.; Blitz, M . A.; Frey, H. M.; Lightfoot, P. D.; Walsh, R. J . Chem. SOC.,Faraday Trans. 2 1988, 84, 5 15.
12(X,lowu) + Ar
-
-
21 + NF(X)
21
+ NF(X)
The quenching of 12(A') was not allowed in the model; thus it represents a permanent reservoir for formation of I atoms. These steps were followed by (8a) and (8b) plus the quenching of I(2P1i2) by I,. Since the absolute yield of NF(b) could be approximately determined from calibration vs the 'background" NF(b) provided by the 2NF(a) energy-pooling reaction,] the [NF(b)] and the time and [I,] dependence of NF(b) formation are constraints. The results from numerical integration of the rate equations indicated that formation of 12(X,u)in the reaction with I2 must be the main channel, and that most of the 12(X,u) molecules are vibrationally relaxed for our conditions, otherwise the NF(a) yield would have been much higher than we observed. The thermal d i s s o ~ i a t i o n ~ ~ of I2(AI3II2)also could contribute to I formation, which would lead to too high an NF(b) concentration. Apparently 12(A'3n:2) is not an important product, unless it is quenched readily, which seems unlikely. The higher [NF(b)] from the IC1 system can be explained by slower vibrational relaxation of IC1 or by slowier quenching of I(2Pl,2) by IC1. Although many details of the mechanism remain to be refined, the qualitative features are in reasonable accord with the data. Conclusions The quenching rate constants for NF(a) by molecular halogens systematically increase by 3 orders of magnitude in the F2 to I2 series. There also seems to be a change in mechanism with chemical reaction over an entrance channel barrier being important for the lighter halogens, whereas E-V transfer giving NF(X) is dominant for 12, ICl, and IF. These NF(a) reactions provide: a
(29) (a) Langen, J.; Lodemann, K. P.; Schurath, U. Chem. Phys. 1987, 112, 393. (b) Bohling, R.; Becker, A. C.; Minaev, B. F.; Seranski, K.; Schurath, U.Chem. Phys. 1990, 142, 445. (30) Tellinghuisen, J. J . Phys. Chem. 1984, 88, 6084; 1986, 90, 5108.
J . Phys. Chem. 1992, 96, 2561-2567 way to generate high concentrations of vibrationally excited IF, IC1, and I2 molecules in a flow reactor. The secondary interaction of NF(a) with the vibrationally excited I2 and IC1 molecules probably gives I atoms. The formal NF(a) quenching kinetics of these iodine-containing molecules are intriguing because pseudo-first-order kinetics are observed even though [NF(a)] > [Q],because the 12, IF, and IC1 molecules are recycled. Qual:itative measurements gave the quenching rate constant for NF(a)
2561
by I atoms as -1.8 X lo-" cm3 s-I; this excitation-transfer constant seems to be 2-3 times smaller than for 02(a) 1.31 At least, CI2 and I F do not react rapidly with NF(X).
+
Acknowledgment. This work was supported by the Air Force Office Of Scientific Research (Grant 88-0279)* (31)
Young, A. T.; Houston, P. L. J . Chem. Phys. 1983, 78, 2319.
Diode Laser Probes of the Product Distribution of Coordinatively Unsaturated Iron Carbonyls Produced following Excimer Laser Photolysis of Fe(CO), in the Gas Phase Robert J. Ryther and Eric Weitz* Department of Chemistry, Northwestern University, Evanston, Illinois 60208-31 13 (Received: September 9, 1991; In Final Form: December 2, 1991)
The distributions of photoproductscreated by single photon photolysis of Fe(CO)5at 193,248, and 35 1 nm have been determined by using a tunable infrared diode laser to monitor the concentration of Fe(CO)5 consumed in reactions with each photoproduct. The relative concentrations of the iron carbonyl photofragments that form are as follows: on 193-nm photolysis, 90 i 5% Fe(C0)2and 10 f 5% Fe(C0); on 248-nm photolysis, 64 f 7% Fe(CO)3 and 36 f 7% Fe(C0)2; and on 351-nm photolysis, 39 5% Fe(C0)4 and 61 i 5% Fe(C0)3. A pressure dependence of the 248-nm product yield is observed, and the reported branching ratio is for 3 Torr of total pressure. Peak absorption coefficients of these species are reported. In addition to these products, which are produced in their ground electronic states, species best assigned as excited electronic states of Fe(C0)4 and Fe(CO), are produced following 35 1- and 248-nm photolysis, respectively. These product distributions are compared to those reported in other studies, and evidence is presented that for at least some of the wavelengths employed in this study the dissociation of Fe(CO)5 involves multiple potential energy surfaces.
*
I. Introduction A number of recent studies have focused on the photodissociation dynamics of metal carbonyls in the gas phase. Of particular interest is the observation that species with multiple site of coordinative unsaturation can be produced by absorption of a single photon.l-I8 To date, iron pentacarbonyl is the best studied of such systems. Yardley et al. photolyzed Fe(CO)5 in the ultraviolet and used PF3to chemically trap the unsaturated photoproducts with product yields determined by G C analysis.] Vernon and co-workers used a crossed laser-molecular beam system to study (1) Yardley, J. T.; Gitlin B.; Nathanson, G.; Rosan, A. M. J . Chem. Phys. 1981, 74, 370. (2) Ray, U.; Brandow, S. L.;Bandukwalla, G.;Venkataraman, B. K.; Zhang, Z.; Vernon, M. J . Chem. Phys. 1988,89, 4092. (3) Venkataraman, B. K.; Bandukwalla, G.;Zhang, Z.; Vernon, M. J . Chem. Phys. 1989, 90, 5510. (4) Waller, I. M.; Hepburn, J. W. J . Chem. Phys. 1988, 88, 6658. ( 5 ) Waller, 1. M.; Davis, H.F.; Hepburn, J. W. J. Phys. Chem. 1987,91, 506. (6) Seder, T. S.; Ouderkirk, A. J.; Weitz, E. J . Chem. Phys. 1986, 85, 1977. (7) Ryther, R. J.; Weitz, E. J . Phys. Chem. 1991, 95, 9841. (8) Ryther, R. J. Ph.D. Thesis, Northwestern University, 1991. (9) Bogdan, P. L.; Weitz, E. J. Am. Chem. SOC.1989, 111, 3163. (IO) Bogdan, P. L.; Weitz, E. J . Am. Chem. SOC.1990, 112, 639. (1 1) Seder, T. A.; Church, S. P.;Weitz, E. J . Am. Chem. SOC.1986,108, 4721
(12) Seder, T.; Ouderkirk, A.; Church, S.; Weitz, E. ACS Symp. Ser. 1987, No. 333, 8 1. (13) Seder, T. A.; Church, S. P.: Weitz, E. J . Am. Chem. SOC.1986, 108, 1084. (14) Ishikawa, Y.; Hackett, P. H.; Rayner, D. M. J. Phys. Chem. 1988, 92, 3863. ( 1 5) Fletcher, T. R.; Rosenfeld, R. N. J . Am. Chem. Soc. 1988,110, 2097. (16) Rosenfeld, R. N.; Ganske, J. A. J . Phys. Chem. 1989, 93, 1959. (17) Rayner, D. M.; Ishikawa, Y.; Brown, C. E.; Hackett, P. A. J . Chem. Phys. 1991, 94, 5471. (18) Ishikawa, Y.; Brown, C. E.; Hackett, P. A,; Rayner, D. M. J. Phys. Chem. 1990, 94, 2404.
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the photodissociation of Fe(C0)5,2,3detecting the iron carbonyl photofragments using time-of-flight mass spectrometry. Hepbum and co-workers also studied the photolysis of Fe(CO)S in a molecular beam, employing vacuum ultraviolet laser-induced fluorescence to detect vibrational, rotational, and translational distributions of the photoejected CO. They then used the energy distribution of the ejected CO, along with literature values for the iron420 bond energies for the different iron carbonyl species to calculate a product distribution for each UV photolysis ~avelength.~,~ Although there are some significant differences in product distributions reported in these studies, they all concur that the dissociation mechanism involves stepwise loss of CO ligands which results in an increase in the degree of unsaturation of the photoproducts with increasing photon energy. This picture is supported by transient infrared spectroscopic studies of product distributions as a function of photolysis wavelengthe6 Further, both molecular beam studies conclude that sequential loss of CO can be rationalized by a statistical model. The current study makes use of fast time-resolved IR spectroscopy (TRIS) to quantitatively determine the UV wavelength dependent product distribution of the photofragments produced by the gas-phase photolysis of Fe(CO)S. A diode laser has been employed in this study instead of the more commonly used C O laser, which has typically been the probe source of choice for gas-phase studies of coordinatively unsaturated metal carbonyls. The CO laser by necessity lases at frequencies corresponding to transitions of excited-state CO. Thus transient IR spectra of metal carbonyls obtained with a CO laser may contain contributions, at a given wavelength, from both metal carbonyls and internally excited CO. As pointed out previously, this makes quantitative determinations of product distributions and absorption coefficients of coordinatively unsaturated species difficuk6 However, with a diode laser probe it is possible to easily distinguish metal carbonyl absorptions from absorptions of internally excited CO. This has 0 1992 American Chemical Society
