4728
J . Phys. Chem. 1991, 95, 4128-4135
the Franck-Condon integral, depending on whether the electron transfer is charge separation or charge shift; the effect should be large for charge shift reactions. Now, back electron transfer within the geminate pair would appear to be charge shift from Ru3+to MV'+ and, thereby, would be expected to have a moderate dependence on exergonicity. However, since the charge change (3+ 2+) of the Ru center coordinated to the large ligands would not be felt by the dipolar solvent molecules surrounding the complex ion, the back-electron-transfer reaction, in which the charge on MV'+ is increased to 2+, is regarded as being of the charge separation type. In this case, the dependence of the electron transfer rate on AGO is predicted to be very weak.
-
Summary The quenching of the lowest excited states of Ru(II)-diimine complexes by MV2+in H20/CH3CN solution yields the oxidized photosensitizer and reduced quencher with an efficiency (0.1-0.2) that is only weakly dependent on the AGO of the backelectron-transfer reaction in the range -1.4 to -2.0 eV. By assuming that the rate constant for the escape of the solvent-caged geminate redox pair into bulk solution is approximately constant for most of the complexes studied, and may be lower for those
complexes that possess larger ligands, we find the dependence of the rate constant of back electron transfer in the highly exergonic region on AGO to be very weak, decreasing somewhat as the exergonicity of the reaction increases in the "inverted Marcusn region, unlike the strong dependence and 'bell-shaped" behavior shown for reductive quenching. The reason for this behavior is not completely clear, but it is suggested that specific interactions between the geminate pair species and the molecules that make up the cage may be very important, especially in mixed solvents. Acknowledgment. This research was supported in part by the Ministry of Education, Science, and Culture of Japan (Grant 62213022) and in part by the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy (Grant DE-FG02-86ER13603). The collaboration between T.O. and M.Z.H. is part of the US.-Japan Program of Cooperation in Photoconversion and Photosynthesis (NSF Grant INT-8708425). The authors thank Dr.Koichi Nozaki for his determination of the redox potentials in 4:l H20/CH3CN. Supplementary Material Available: Absorption spectra of nine Ru'Ldiimine complexes in H20/CH3CN (9 pages). Ordering information is given on any current masthead page.
Reactions of NF(alh) with Nitrogen, Oxygen, and Carbon Atoms K. Y. Du and D. W . Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: August 9, 1990; In Final Form: December 28, 1990)
The total quenching rate constants and the products from the reactions of NF(alA) with N, 0,and C atoms have been studied in a flow reactor at room temperature. The rate constants increase by nearly 2 orders of magnitude in the series; kN = ( 5 f 2) X lO-I3, ko = (6 f 2) X 10-12, and kc = (2.7 f 1.2) X lo-" cm3 molecule-I s-I. The primary product from N atom quenching probably is NF(X3Z-), reactive uenching by 0 atoms gives NO(X) + F, and the carbon atom reaction gives CN(A211),CN(B22+),and probably CN(X12+). Energy transfer to give C(ID) + NF(X3Z-) may be a minor channel in competition with direct CN(A and B) formation. The reaction of C atoms with NF(b'Z+) also gives CN(A and B) chemiluminescence. There is a propensity for NF(a) to give more CN(A) and for NF(b) to give more CN(B). The mechanisms for these reactions are discussed.
Introduction Utilization of the chemical energy stored in the NF(a'A,1.42 eV) system is likely to involve a highly reactive environment. Therefore, an understanding of the chemistry of NF(a) with reactive atoms will be essential. The rate constant with F atoms, (4 f 2) X IO-" cm3 atom-l s-l at 300 K, was reported earlier by the authors.' The reaction with H atoms2 has a similar rate cm3 atom-' s-I at 300 K. In constant, k H = (3.1 f 0.6) X this work, we have studied the reactions of ground-state N, 0, and C atoms using a flow reactor; total quenching rate constants and product states were investigated. Only the C atom reaction gives chemiluminescence. In previous work1v3from this laboratory, quenching rate constants for NF(a) from a variety of stable molecules were reported, including a separate study with halogen molecules, which tend to have large rate constants.Ib The NF(a) state has a wide range of reactivity with small rate constants for molecules that quench by E-V energy transfer, but larger rate (1) (a) Du, K. Y.; Setser, D. W. J . Phys. Chem. 1990,942425. (b) Du, K. Y.; Setser, D. W. SPIE Con/. High-Power Gas h e r s 1990,1225, 523. ( 2 ) (a) Dayis, S. J.; Rawlins, W. T.; Piper, L. 0.J. Phys. Chem. 1989,93, 1078. (b) Davis, S. J.; Piper, L. G. J. Phys. Chem. 1990,944525, (c) Hovis, F. E.; Whitefield, P. D.; Lilenfeld, H. V.; Bradburn, G. R. J . Phys. Chem. 1988, 92. 5133. (3) Quinones, E.; Habdas, J.;Sctser, D. W.J. Phys. Chem. 1987, 91, 5155.
0022-365419 112095-4728302.50/0
constants for molecules that can act as strong Lewis bases and for unsaturated molecules. All of the previous work, as well as the present study, utilized the 2F + HN, reaction system as the source of NF(a) in a flow reactor.'^^ The thermochemically allowed reaction channels with N, 0, and C atoms are listed in eq 1; we used AHfo(NF) = 50 kcal mol-l N(4S) + NF(alA)
-
-
N(4S) + NF(X3Z-); AHoo= -32.8 kcal mol-'
N2(XIZi) + F(2P); AHo,, = -177 kcal mol-'
-
+ F(zP); N2(B3111,) + F(zP);
N2(A3Z:)
AHo,, = -33.4 kcal mol-I
AHoo= -6.5 kcal mol-!
(1)
The most recent ab for evaluation of the thermochemi~try.~~~ initio calculationss favor a AHfo(NF) value of 55 kcal mol-I, but Within the uncertainty for the the issue is not fully resol~ed.~ AHfo(NF), the formation of N2(B) is thermoneutral. The formation of NO(A and B) by (2) is endoergic, and only physical (4) Du, K. Y.; Sctser, D. W. Chem. Phys. Lett. 1988, 153, 393. ( 5 ) (a) Montgomery Jr., J. A.: Peterson, G. A.; AI-Laham, M. A,; Mantzaris, J . Chem. Phys. Len. 1990, 169, 497. (b) Michels, H. H.; Montgomery Jr., J. A. To be. published.
0 1991 American Chemical Society
Reactions of NF(a'A) with N, 0, and C Atoms
-
The Journal of Physical Chemistry, Vol. 95. No. 12, 1991 4729
quenching and NO(X) need to be considered. O(3P,)
+ NF(a'A) -c
NO(X211)
O(3P,)
+ NF(X3Z-);
+ F(2P);
-
Mi crowavo
fTT
Moo = -32.8 kcal mol-'
Moo = -102 kcal mol-'
+ NF(a'A)
-
+
C(3P,)
+ NF(X%);
a
Moo =
-32.8 kcal mol-'
+ NF(X3Z-); Moo = -3.7 CN(X2Z+;A211;B2Z+)+ F(2P); M o o C('D2)
kcal mol-'
+
+ HF;
AHoo= -33 kcal mol-' (4) a spin-allowed pathway of similar exoergicity. The rather small rate constant implies a significant activation energy barrier, which is surprising for a spin-allowed, exoergic reaction involving an atom. The N(2D) product subsequently reacts2bwith NF(a) to give N2(B311) and possibly other N2* states with a rate constant cm3 molecule-' s-'. Clyne and co-workers of (2.5 f I.lf X encountered reactions 1 and 2 as possible secondary reactions when they investigated 0 and N atom reactions with NF2? However, they could not assign rate constants or products, since the concentration of NF(a) was not known. The reaction of NF(a) with carbon atoms has not been previously explored. N(2D)
Experimental Methods
The experimental apparatus and the calibration methods for the flow reactor have been described in detail elsewhere,' and only the modifications of the reactor necessary to obtain the N, 0,and C atoms are given here; see Figure 1. The Pyrex flow reactor, 80 cm in length and 6.4 cm in diameter, was completely coated by halocarbon wax. A 1500 L min-' mechanical pump provided a linear flow velocity of 700 cm s-' at 2-4 Torr. The flow was steady for pressures less than 5 Torr; however, fluctuations in pressure and flow developed above about 7 Torr. The NF(a) concentration was monitored by the intensity of the NF(a-X) transition at 874 nm. For observation of ultraviolet emission, three quartz windows were attached to one side of the reactor by epoxy glue; they were located 4, 27, and 58 cm downstream from the reactive atom inlet, which corresponded to reaction times of 5, 31, and 85 ms, respectively. The Ar carrier gas was taken from tanks and purified by passing it through a commercial sieve trap (Matheson 1306), which mainly removes the O2 impurity, and two homemade traps filled with molecular sieves cooled to 196 K to remove the water. After each experiment, the molecular sieves filled traps were baked at 500 K under vacuum. The NF(a) molecules were produced in the prereactor section by the following reactions with a 3-fold excess of [FIo over [HN,],. F
F + N3
- + + HN3
NF(a)
HF
+ N3;
kSs = 1
N2; kSb= (4 f 2)
X X
cm3 s-I
(sa)
lo-" cm3 s-I (Sb)
The 24-cm-long prereactor, corresponding to a reaction time of ~~~~
C
To Pump
Coating
The possible direct formation of electronically excited product molecules is an intrinsic feature of many NF(a) reactions, and identification of the product states, as well as assigning total rate constants for the reactions, is important. Since NF(a) is a singlet state, the spin-allowed product states are defined by the atomic reactant, and it should be possible to devise schemes for selective generation of product states. The H atom reaction seems to yield mainly N(zD), even though formation of NH(X3Z-) F also is
-
b
(3)
-133; -107; -59.2 kcal mol-'
H(2S) + NF(a'A)
~~ii_ll To pressure gauge
(2)
The C(3P) reaction has several possible exit channels. C(3P,)
ColdGothode
~
(6) (a) Chcah, C. T.; Clync. M. A. A. J . Phorochem. 1981, IS, 21. (b) Cheah. C. T.; Clync, M. A. A.; Whitefield, P. D. J . Chem. Soc., Faraday Trom. 2 1980, 76, 71 1. (c) Cheah, C. T.; Clyne, M. A. A. J . Chem. Soc., Faraday Trans. 2 1980, 76, 1543.
F i 1. Schematic diagram of the flow reactor showing the microwave discharge used for generating the reactive atoms and the hollow cathode discharge used for generating N2(A) from the Ar('Po3) atoms. The F/HN3 prereactor section is to the left of the reactive atom inlet. a, HN3 inlet; b, F atom inlet; c, main Ar flow inlet; d, atom precursor flow; e, NO or CHI inlet; f, Ar flow to cold cathode discharge; g, N2 inlet to generate N2(A).
32 ms, allowed complete conversion of N3 to NF(a) before the quenching atom flow was introduced into the reactor. The HN,, F, and Ar buffer gas flows were introduced at the entrance to the prereactor section and formed a homogeneous NF(a) mixture in the Ar carrier. The F atoms were generated by passing a 15% CF4/Ar mixture through a microwave discharge. The F concentration was determined by the CF31 titration reaction, using the H F emission from C2H6to monitor [F]. Two F atoms were obtained per CF4 molecule for the [CF,] range (0.2-1.2) X 10l2 molecules ~ m - the ~ ; efficiency of the CF, dissociation decreased for higher [CF,]. The reactive atom inlet was attached to the main reactor by a vertical O-ring joint placed 24 cm downstream from the HN3 inlet. The discharge tube was 1.2 cm in diameter. Nitrogen a t o m were generated by passing a N2/Ar mixture in a range of 10-100 pmol s-' through the microwave discharge. The N2, taken from commercial tanks quoted as 99.99% pure, was placed in two 12-L glass reservoirs and used without further purification. Since the fractional dissociation of N2 was low, 10[NF(a)], and short reaction times, 40 ms, were used. Since [CF,] < [NF(a)], the effect of CF2 on the concentration of 0 atoms was minor. Figure 6 shows the semilog plots of the NF(a--X) intensity vs the 0 atom concentration. Five independent experiments were done. The average value of the rate constant, ko. obtained by dividing the slopes by the reaction times was (6.0 1 .O) X cm3 s-l. The uncertainty in the calibrations for At and [O] suggest a larger uncertainty, and (6.0 2.0) X cm3 s-I is more realistic. Examination of the product from reaction 2 was attempted by using N2(A) as an excitation source and searching for emission from NO(A) and NF(b), which would identify the presence of NO(X) and NF(X), respectively.' The authenticity of the N,(A) generator was confirmed by directly adding N O to the reactor and observing the NO(A-X) emission through a quartz window, 4 cm downstream from the N2(A) source inlet. In order to calibrate for [NO], a plot of the NO(A--X) intensity vs [NO] for the specific [N2(A)] of the given experiment was made before 0 atoms were added to the reactor. Then, the microwave discharge was activated and nitrogen atoms were generated to remove some of the N O to obtain 0 atoms. The N O formation experiments were conducted with excess [NO] and [F] to avoid effects from the reactions of N atoms with the newly formed N O or of
+
Du and Setser TABLE XI: NO Fonnntion IhW
6.7 12.0 11.0 11.0 11.0
19.0 18.0 52.0 52.0 52.0
8.7 3.6 5.8 4.0 0
2.5 5.9 6.0 & 5.6 6.0
*
0.5 1 0.5 0.8 0.5
0.37 0.50 0.54 0.50 0.54
OAll concentrations are in 10" molecules cm-'.
Ir Y
I
I
I
Figure 7. The CN(A-.X) spectrum in the 500-700-nm range. The 9.6 X 10l2,and 36 X 10I2 [NF(a)], [F], and [CH,] were 3.5 X molecules ~ m -respectively; ~, Af = 17.4 ms.
N3 with the 0 atoms. To increase the sensitivity of this experiment, the excess [NO] was controlled to be 1-3 times the [NF(a)] and the [O] range was 2-10 times the [NF(a)]; so, the NF(a) was completely removed. The [NO] generated from the reaction of the 0 atoms with NF(a) was calculated based upon the observed NO(A--X) intensity and the intensity vs concentration plot prepared for each experiment. We list five sets of experiments [NOIW, in Table 11, including the [NF(a)lo, [O],, [NO],,-, and the ratio between [NO]d and [NF(a)lo. The data clearly show that NO(X) is a major product. However, there was no increase in the NF(b) intensity from reaction with N2(A) and there was no evidence to indicate that NF(X) was a product. The formation of NO(X) was confirmed by observing an enhancement of the NO(A+X) emission when a microwave discharge in O2 ( 5 % ) was used as an alternative 0 atom source. However, quantitative conclusions could not be drawn because the 0 atom concentration was not measured. These experiments show that N O is formed as a product; however, since 0 atoms were always in excess, the mechanism for N O formation could be quenching of NF(a) to NF(X) followed by fast reaction of ground-state NF(X) with the excess 0 atoms, as well as direct reaction with NF(a). The C(3P)Reacfion System. Two sources of C atoms were used to address the question of whether the observed CN(A and B) chemiluminescence was from ground-state carbon atoms. The NF(a) quenching data were obtained from carbon atoms generated by reacting excess F atoms with CH4. These results then were confirmed by using the Ar(3Po,2)+ CO reaction, which is known to give only C(3P) atoms," by comparing the CN(A and B)
The Journal of Physical Chemistry, Vol. 95, NO. 12, 1991 4733
Reactions of NF(alA) with N, 0, and C Atoms
with NF(alA). We found the same CN(A-X) spectrum as when using F CH, as the C atom source. The vibrational distribution for CN(A) was assigned from the emission spectrum at 2 Torr. The band areas of the AD 3,4, and 5 sequences were used to estimate the vibrational distribution using the Franck-Condon factors of S~ind1er.I~The distribution favors the mid-u levels with P?-Pl? = 1.1:1.5:1.4:0.9:0.5:0.35:0.14. There undoubtedly is population in the lower levels, but scattered light from the discharges made these assignments difficult. Vibrational relaxation of CN(A) in Ar is rapid, and there will be some difference between the observed and the nascent CN(A) vibrational distributions. The rate constant for CN(A) formation was estimated by comparing the relative intensities from CN(A-X) and NF(a-X). Since the CN(A) concentration is in steady state, the rate constant for CN(A) formation is related to the CN(A) and NF(a) intensities in a simple way:
+
2-
0 30 45 R i i c t i on T i m i ( m s ]
15
0
60
d[CN(A)l /dt = k c ~ ( ~ ) [ N F ( a[Cl )l
- TCN(A)-~[CN(A)I= 0 (1 la)
ICN(A)
r t
\
-50
-"I
~ C N ( A )=
\
-70 5
10
15
= 7CN(A)-'[CN(A)I
20
25
( ~ l ~ ~ r ~ ] i i ~ " n coml' i c u ~ ~ i
Fipn 8. (A) Decay of the CN(A-+X) emission intensity vs reaction time for various [NF(a)loconcentrations. The NF(a) concentrations (in IOi2 molecules cm-') are (0)2.0, (A) 1.9, (A) 1.5, (B) 1.4, and ( 0 )0.82. (e) Plot of the pseudo-first-order CN(A-.X) decay constants obtained from (A) vs the NF(a) concentration.
chemiluminescence spectra from the two sources. In both cases the CN(A-X) emission seemed to be first order in both [C] and [NF(a)]; a spectrum is shown in Figure 7 for the 500-800-nm region. The emission at longer wavelength is obscured by the intense Ar(4p-4~)emission lines from the microwave discharges. Since the radiative lifetime of CN(A) is 7 ps, the CN(A) concentration is in steady state and it can be used to monitor the [C] along the reactor, if [NF(a)] is constant. Therefore, experiments were designed using excess [NF(a)] relative to [C] so that pseudo-first-order decay of [C] would exist. Fortunately, the reaction rate for (3) is sufficiently rapid that a significant change in [C] was observed for the accessible range of [NF(a)]. The decay rate of the [C], monitored by the CN(A-X) emission intensity along the reactor, is shown in Figure 8A for [NF(a)] = 0.8 X 10I2,1.4 X 1012,1.5 X 1012,1.9 X 10l2, and 2.0 X 10l2 molecules ~ m - ~These . pseuddirst-order rate constants are plotted vs the [NF(a)] values in Figure 8B to obtain the second-order rate constant. The average value is (2.5 f 1.0) X lo-" cm3 molecule-' s-l. There may be an appreciable loss rate of the [C] to the walls. Since we were able to correlate the total decay rate of the [C] with [NF(a)] in Figure 8B, the quoted rate constant should give a measure of the rate constant for reaction 3. The CN(A-X) spectrum was recorded for various reaction times and [NF(a)]. The spectrum seemed to be invariant for constant Ar pressure. In order to confirm that the formations of CN(A) was from ground-state C(3P) reactions, the Ar*/CO reaction was used as an alternative C atom source. The presence of C(3P) was confirmed by introducing OCS (or SO2) to the C atom flow." The OCS reaction gives strong CS(a-X) emission and served as an excellent test for C(3P) in our range of concentration. The SOz reaction gave much weaker SO(A-X) emission and was a much less satisfactory test for C(3P). After demonstrating the presence of the C(3P) atoms by the COS and SO2test reactions, the C(3P) from the Ar('PO2) source was reacted
ICN(A) 7;' [CI INFW
-
(1 1b) (1 IC)
In eq 11 7[l and rCN(A)-I are the reciprocals of the radiative lifetimes of NF(a) and CN(A) (5.6 s and 7 ps), respectively. The intensities are the relative photon emission rates, which are proportional to the recorded band areas. The observed emission from CN(A,u' = 6 and 7) was scaled in the following way to obtain ZcN(Ap First the Franck-Condon factors for u' = 6 and u' = 7 were used to allow for the bands from u' = 6 and 7 that were not observed. Then the emission from u'> 7 and u'< 6 was included, using the vibrational distribution mentioned above. The levels below u' = 4 were all assigned a relative population of 1.1. The and ZNF(,) mast uncertain quantity in (1 IC) is the [C]. The Zm(A) were recorded for [CH,] = 2 X 10l2and [F] = 1.6 X lOI3atoms ~ m - ~Based , upon the known rate constants of F with CH, and CH3 and the estimated values for CH2 and CH, the muximum [C] would be 1 X 1OI2 atoms cm3. This concentration gives a lower limit to kCN(A) of 21.3 X 10-l2 cm3 s-', which would correspond to a branching ratio of 20.05 for CN(A) formation. If the [C] is less, then the branching ratio is larger. We conclude that CN(A) formation is a significant component of reaction 3, but it is not the only product. In fact, CN(B,u12-X) emission also was always present in the carbon atom reaction system. The CN(B-X) and CN(A-X) emission intensities were both monitored for some experiments using the F/CH4 source of C atoms. For a fixed At, the ratio of CN(B)/CN(A) emission intensity was constant (approximately 0.2) as the NF(a) concentration was changed, providing that the [NF(b)J/[NF(a)] ratio was constant. However, the CN(B)/ CN(A) ratio did decline with distance along the flow reactor, which is evidence that the formation mechanism for CN(B) must differ somewhat from that of CN(A). The [NF(b)] is higher at the front of the reactor because NF(b) is formed by the V-E transfer reactionI2 between NF(a) and the HF(u) formed by F + CHI. The suspicion that CN(B) formation correlated with the higher concentration of NF(b) was directly tested by adding C atoms to a reactor containing NF(b) generated by the interaction of Ar(3Po,2)and NFz.I6 When C atoms were added to the NF(b) flow reactor, CN(A) and CN(B) emissions were both observed; the CN(B)/CN(A) ratio was -0.4. Thus, NF(b) certainly reacts with C(3P) atoms to give both CN(A) and CN(B). However, the [NF(b)]/[NF(a)] ratio in the NF(a) reactor was only -0.02, and NF(b) cannot be the only source of CN(B). A
-
~
(15) Spindler, R. J. J . Quanr. Specrrosc. Rudiat. Transfer 1965, 5, 165. (16) (a) Cha, H.; Setser, D. W. J . Phys. Chem. 1989. 93, 235. (b) Du, K. Y.; Setser, D. W. J . Phys. Chem., submitted for publication. Enhanced NF(b) concentrations were obtained by passing the NF2 flow through the
hollow cathode discharge with At.
4134 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 special experiment was done using the Ar(3Po,2)+ CO system to observe the CN(B)/CN(A) ratio from C(3P) + NF(a). The F/HN3 reaction system was run with [FIo -1.5 [HN,] in order to minimize the [NF(b)]. Upon addition of C atoms, CN(A) and CN(B) were both observed with a ratio of 0.16. Thus, NF(a) has a higher propensity to give CN(A) and NF(b) has a higher propensity to give CN( B), but neither reaction exclusively gives CN(A) or CN(B). Although there is some collisional mixing1’ between the CN(A,o’=lO) and CN(B,o’=O) states, this cannot explain the CN(B) concentration that was observed in the NF(a) and NF(b) reactions; we conclude that the CN(B) must be formed mainly by reaction with NF(a). Some reservation must be maintained about the possible role of energy transfer to C(lD) as a primary step, followed by chemical reaction with NF(a). Although the CN(A) intensity seemed to be first order in [NF(a)], the data do not exclude some branching to C(lD) followed by a fast secondary reaction with NF(a) to give CN(A,B). During the course of investigation of the C(3P) reaction with NF(a), we also studied the C(3P) + N 3 reaction. In these experiments N3 was generated by setting [F], = [HN,], and the carbon atoms were generated by the Ar(3Po,2) CO source. To our surprise neither CN(A) nor CN(B) was observed. If [F], was increased to generate NF(a), in the prereactor, then the CN(B,A) chemiluminescence would be observed. A factor of 10 reduction in the chemiluminescence intensity from the N3 experiment seems conclusive. The previously reported’” CN(A,B) chemiluminescence from C(3P) + N3 probably had a large contribution from the C(3P) NF(a) reaction. In view of the chemiluminescence from N, P,As, 0, S,and Se reactions with N3,I4J*the failure to observe CN(A,B) from C(3P) was startling. One explanation is that the C atoms add to N3 to generate NCN or N2C rather than abstracting a N atom to form CN.
+
+
Discussion Systematic study of the reactions of the three electronic states of N F with various reactive atoms offers the possibility to probe the multitude of potentials in these triatomic systems. Related work with N H is in progress in other laboratories.’”2 The X32state is expected to have the reactivity of a free radical, and usually one of the potentials from NF(X3Z-) plus a reactive atom will correlate to ground electronic state products. On the other hand, the aIA state could have either free-radical character ( r F r y configuration) or a reactivity associated with an empty orbital ( ? r X l . y ’ configuration). The H atom reaction with NF(a) should be the simplest example. A correlation diagram, not shown here, suggests that H + NF(X) should give N(4S) + HF by either an abstraction reaction over a quartet potential surface or by a curve crossing in the exit channel between the bound X(2Aff)potential associated with ground-state H N F and the H F + N(4S) quartet potential. The latter pathway could resemble the H + SF(X211) reaction, which gives H F S(3P) via a curve crossing between the HSF(X) singlet potential and the repulsive potential leading to S(3P) HFSZ3The H NF(a’A) reaction is thought to give mainly H F + N(2D).2 However, the N(2D) + HF and NH(X3Z-) + F(2P) product channels have almost the same energy, -55 kcal mol-’ above N(4S) HF. Both of these product channels have several 2A’ and 2Af’potentials that could correlate to NF(a’A)
+ +
+
+
(17) (a) Duewer, W. H.; Setser, D. W.; Coxon, J. A. J. Chem. Phys. 1972, 56, 4355. (b) Furio, N.; Ali, A.; Dagdigian, P. J. J . Chem. Phys. 1986, 85, 3860.7098. This modem work reports collisional transfer between the CN(A) and CN(X) states but includes reference to the analogous 8-A transfer studies. ..-~ .. (18) (a) Henshaw, T. L.;McElwee. D.; Stedman, D. H.; Coombe, R. D. J. fhys. Chem. I=, 92,4606; 1987,91,2538. (b) Ongstad, A. P.; Henshaw, T. L.; Lawconnell, R. 1.; Thorpe, W.G. J . Phys. Chem. 1990,943602,6724. (c) May, D.J.; Coombe, R. D. J . Phys. Chem. 1989, 93, 520. (19) Hack, W.; Rathman, K. J . Phys. Chem. 1990, 91, 4157. (20) Hack, W.; Rathman, K. J . fhys. Chem. 1990. 91, 3636. (21) Hack, W.; Wagner, H. Gg.; Wilma, A. Be?. Bumen-Ges. fhys. Chem. 1988, 92,620. (22) Freitag, F.; Rohrer, F.; Stuhl, F. J . fhys. Chem. 1989, 93, 3170. (23) Wategaonkar, S.;Setser, D. W. J. Chem. Phys. 1989, 90,6223.
Du and Setser NF
+
N
T
200
4 2 * , 3 y
/
500
21’
I
+
Figure 9. State correlation diagram for the N(‘S) NF(alA) reaction system. The entrance and exit channels have been placed on the correct
thermochemical scale. TABLE HI: Ouenchiiu Rate Constants for Reactive Atoms ~
atom N(S) o(3p) C(W F(~P)C
H(2S)d N(2D)d
rate constantu ( 5 f 2) X IO-”
*
(6 2) X (2.7 i 1.0) x 10-11 (4 f 2) x 1 0 4 3 (3.1 f 0.6) X 1O-I) (2.5 f 1.1) x 1O-Io
~~
probable product NF(X3Z-) + N(%) NO(XZll)+ F(2P)
+
C N ( A * ~ ) F(ZP)~
NF(X’Z-) + F(2P) HF(’Z+) + N(2D) N2(B311,) + F(2P)
-
“Measured at 300 K and in units of cm3 molecule-l s-I. bCN(B) also is formed with CN(B)/CN(A) 0.2. cSee ref 1. dSee ref 2.
+ H(*S). It is not obvious why N(2D) should be the only product from NF(aIA) + H(2S), and very careful measurements for the N(2D) vs NH(X) yields are needed to address this point. The ordering of excited states for the H N F radical is expected to be A(2Af) and fi(2A’’) by analogy to NH2. Thus, the rx4r; component, which correlates to an A’ excited H N F state, must be the reactive pathway for H + NF(a). The small rate constant for H + NF(a) implies a barrier in the entrance channel for the addition of H atoms to the r2-r; component. The r X r component will correlate to a higher energy potential of HNF(b2AN). The N(4S) and F(2P) reactions with NF(a) have similar rate constants. We suspect that both proceed by electronic-state relaxation with formation of NF(X) in the primary step. For the F atom case there seems to be only one pathway, and the mechanism presumably involves internal conversion between a NF2* excited doublet state arising from NF(a) F(2P) and another doublet state correlating to NF(X3Z) + F(2p). The small rate constant could be a consequence of an unfavorable interaction, such as between A’ and A” states, or from a bamer in the entrance channel. The situation for N(4S) quenching is less clear, even though there are fewer states because of the 4S symmetry. A correlation diagram is given in Figure 9 for aid in the discussion. The entrance channel has only 4Af and 4Af’potentials, and these correlate to N2(B) and N2(A). We certainly expected to generate N2(A) via the 4A’f channel, but the experiments conclusively show that this is not the reaction pathway. Evidently, the 4A’f potential has a bamer and does not lead to N2(A) + F at room temperature. Interaction between the repulsive 4Afpotentials from NF(X) and either the 4A’ or 4Affpotentials from N(4S) + NF(a) must lead to quenching. Although not yet studied, the NF(X3Z-) molecules probably react rapidly with N(4S) to give N2(X) + F. So, the net result from adding excess N atoms to NF(a) would be formation of vibrationally excited N2(X), but the mechanism would involve two consecutive steps. For the currently accepted thermochemistry, formation of N2(A3Z:) + F is slightly endoergic from NF(X) + N. In any case, N2(A) was not an important product from either a primary or secondary step. Although the rate constants for H, F, and N are all similar (see Table 111). the microscopic pathways for the reaction mechanisms appear to be quite different. The reaction of N(2D) with NF(a), and possibly
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The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4135
Reactions of NF(aiA) with N, 0, and C Atoms
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to give CN(A) chemiluminescence could be fast. Direct probing will be required to prove/disprove the possible formation of C(lD). The reaction of NF(b) with C(3P)was shown to give both CN(A) and CN(B), even though the correlation diagram suggests that NF(b) would not react to give CN(A and B). Obviously, more detail than just a state-testate correlation diagram will be needed to understand the interactions between the large number of potentials in the C + N F system.
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C N ( X *I1:‘1 + F ‘pU
NFtC
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Figure 10. State correlation diagram for the C(lP) NF(alA) reaction system. The inclusion of the C(lD) NF(X3Z-) states greatly affects the diagram. Since many of the asymptotic states are expected to be repulsive, this correlation diagram based only on energy ordering of states could bc modified by more detailed considerations. There are multiple states associated with the following correlation lines of the diagram: 3A’(Z), (NF(X) C3P - CN(X) F2P): 3A‘(Z) IA”(3), (NF(X) C I D CN(A) F2P): ’A’(Z), (NF(a) C3P - CN(B) F2P): there are multiple states arising from the following asymptotic limits: 5A’(2), (NF(X) C3P);3A”(Z), (NF(a) C3P);IA”(2), (NF(b) C3P):lA’(3) lA”(3), (CN(A) F2P): ’A’(Z), (CN(B) F2P). The entrance and exit channels have been placed on the correct thermochemical scale.
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even with NF(X), has a large rate constant and gives high yields of N2(B).2b The correlation diagram (not shown) for 0 NF(a) has three A’ and three A” triplet potentials arising from the entrance channel. One of the 3A’ and two of the 3A’’ potentials correlate to NO(X) + O(3P). Thus, reactive quenching with a large rate constant would be expected and is in accord with the experimental finding. Our measurement for the NO(X) yield is not particularly accurate, but we interpret the conversion factor (-0.5) of NF(a) to NO(X) as an indication of high and probably unit conversion efficiency. The formation of NO probably is direct, but the data would be compatible with NF(X) formation followed by a fast secondary step with 0 atoms. The entrance channel part of the C(3P) + NF(a) correlation diagram (see Figure 10) resembles that for O(3P), but the product side is quite different because of the C(lD) NF(X) channel and the larger bond energy of CN relative to NO. The diagram implies a high conversion efficiency of NF(a) to CN(A) and CN(B). However, some of these 3A’ or 3A’’ potentials could interact with the potentials of similar symmetry correlating to CN(X) or to NF(X) C(lD). Our data do not preclude the possibility that some C(lD) is formed, since the subsequent reaction with NF(a)
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Conclusions The rate constants for the reactions of N, 0,and C atoms with to 2.7 X NF(aiA) at room temperature increase from 5 X lo-” cm3 s-l in this series. The 0 and C atom reactions appear to proceed by direct reactive quenching, but the N atom reaction probably gives electronic quenching to NF(X3Z-) as the primary step. In general terms, the NF(aiA) state is not so reactive with atoms as might have been expected for an electronically excited diatomic radical. These results were discussed in terms of state correlation diagrams. The reactivity of the xxx and uxLuyZ components of the NF(aiA) state toward H, F, N, d,and C atoms was considered. The rx%ryZ configuration seems to be the more reactive component, whereas the r x x ycomponent correlates to higher energy molecular states. Even the U , ~ - U ; component of NF(a’A) seems to have a potential energy barrier to reaction with many open-shell atoms. For the H, N, 0,and C atom reactions, the main interaction should be with the N atom end of N F in contrast to metal atom reactions where the large electron affinity of F is expected to dominate the r e a ~ t i v i t y . ~ ~ The quenching rate constants for H, N, and 0 atoms can be compared with those for 02(a1$),25-27 which are 1.8 X lO-I4, 2.7 X lO-I5, and
