J . Phys. Chem. 1991, 95,1158-7162
7758
Kinetlcs and Mechanism of the Reaction of NFCI, with Hydrogen Atoms D.B. Exton, J. V. Gilbert, and R. D. Coombe* Department of Chemistry, University of Denver, Denver, Colorado 80208 (Received: April 15, 1991)
The reaction of gaseous NFCI2 with excess H atoms has been observed in a continuous flow reactor. The reaction produces excited singlet states (alA, blZ+) of N F and NCI and is thought to proceed by a two-step mechanism in which H atoms react with NFCI2 to produce primarily HCI and NFCI, followed by the H + NFCl reaction which proceeds to both HCI + NF and HF + NCI. The rate constant of the first step, H + NFCI2, is (2.6 f 0.2) X cm3 s-I at 298 K. The rate constant of the second step, H + NFCI, was too great to be accurately measured with our apparatus. Overall, the two-step process shows a roughly 10-fold preference for the production of NF(alA) over NCl(a'A). The generation of both NF(alA) and NCl(alA) in this system results in energy pooling among these species which produces NF(blZ+). This process is significantly enhanced by the admission of HI to the reaction medium.
Introduction Reactions which lead to excited singlet nitrenes are among the few in which dynamical constraints dictate the formation of electronically excited species with great specificity. Systems leading to the production of the aIA and blZ+ states of N F and NCI are of particular interest due to the possible utility of these species as energy carriers in short-wavelength chemical laser systems.'" Experiments have shown7-I6that these excited species can be formed efficiently from HID atom reactions with NF2 and NCI2, as well as from processes involving azides, Le., atom + N3 reaction^^-^^-^^ or thermal dissociation of XN3.4*6These methods have been used to generate4s6J5local densities of NF(a) or NCl(a) in excess of cm-3. As a consequence of the production of high local densities of NX(a'A) species, observation of energy pooling processes among these species has been possible.6 Such processes offer the possibility of efficient production of large densities of higher energy states. Recent work in our laboratory has investigated the reaction of NCI, with excess H atoms.7 The data are consistent with a mechanism in which the H NCI, reaction produces NC12, which then reacts with H atoms to produce excited NCI:
+
H
+ NC12
The rate constant for H (I) 171. (2) (3) (4)
-
HCI
+ NCl(alA,blZ+)
(1)
+ NCI, was found to be (4.0 f 0.4) X
Tennyson, P. H.; Fontijn, A.; Clyne, M. A. A. Chem. Phys. 1981,62,
Herbelin, J. M.; Klingberg. R. A. Inr. J . Chem. Kiner. 1984, 16, 849. Coombe, R. D.;Pritt, A. T., Jr. Chem. Phys. Lett. 1978, 58, 606. Benard, D. J.; Winker, B. K.; Seder, T. A.; Cohn, R. H. J . Phys. Chem. 1989, 93, 4790. ( 5 ) Herbelin, J. M.; Kwok, M. A.; Spencer, D. J. J . Appl. Phys. 1978,49, 3750. (6) Benard, D. J.; Chowdhury, M. A.; Winker, B. K.; Seder, T. A,; Michels, H. H. J . Phys. Chem. 1990, 94, 7507. (7) Exton, D. B.; Gilbert, J. V.; Coombe, R. D. J . Phys. Chem. 1991,95, 2692. ( 8 ) Clyne, M. A . A.; White, 1. F. Chem. f h y s . Lett. 1970, 6, 465. (9) Malins, R. J.; Setser, D. W. J . fhys. Chem. 1981, 85, 1342. (IO) Herbelin, J . M. Chem. Phys. Leu. 1976, 42, 367. (11) Herbelin, J. M.; Cohen, N. Chem. Phys. Leu. 1973, 20, 605. (12) Cheah, C. T.; Clyne, M. A. A.; Whitefield, P. D. J . Chem. Soc., Faraday Trans. 2 1980, 76, 7 1 I . ( I 3) Cheah, C. T.; Clyne, M. A. A. J . Chem. Soc.,Faraday Trans. 2 1980, 74, 1543. (14) Koffend. J. B.; Gardner, C. E.; Heidner, R. F. J . Chem. Phys. 1985, 83, 2904. (15) Herbelin, J. M.; Spencer, D. J.; Kwok, J. A. J . Appl. Phys. 1977.48, 3050. (16) Heidner, R. F.; Helvajian, H.; Holloway, J. S.;Koffend, J. B. J . Phys. Chem. 1989, 93, 7818. (17) Clark, T. C.; Clyne, M. A. A. Trans. Faraday SOC.1970.66. 877. (18) Pritt, A. T., Jr.; Coombe, R. D. Inr. J . Chem. Kinet. 1980, 12, 741. (19) Pritt, A . T., Jr.; Patel, D.; Coombe, R. D. J . Mol. Spectrosc. 1981, 87, 401. (20) Habdas, J.; Setser, D. W. J . Phys. Chem. 1989, 93, 229.
cm3 s-l and the rate constant for reaction 1 is near cm3 Both of these rate constants are greater than those for the analogous H + NF, systems.I2Tz1 This reaction offers an interesting alternative to thermolysis of CIN, as a source of NCl(alA), a potentially important energy storage agent for I* (2Pl 2) lasersa6 The work presented in this paper represents an initial study of the analogous reaction of H atoms with NFCI2. NFCI2 demonstrates far greater stability and storage potential than does NCI3, and also offers the possibility of simultaneous generation of NF(alA) and NCl(alA) by branched processes. The initial reactions with H atoms can proceed to either NCI2 or NFCI: S-I.
,,
+ NFCl
HCI
H
+
NFClp
HF
+
NClp
(24 (2b)
Subsequent reaction of the NCI2 produced in (2b) with H atoms will produce excited NCI as in (1). The reaction of NFCl with H atoms can produce either excited N F or NCI: H
+
NFCl
/I
HCI
HF
+
+
NF(a,b)
(3a)
NCl(a.b)
(3b)
An understanding of the branching fractions in these reactions should reveal information about the detailed dynamics of these processes. Additionally, cogeneration of NCl(a) and NF(a) presents the possibility of near-resonant energy pooling among these species, as was recently demonstrated by Benard and coworkers6 in an NFINCI system based upon the thermolysis of FN,/CIN3 mixtures. Specifically, NF(a) can be pumped to the b'Z+ state by collisions with NCl(a): NF(a)
+ NCl(a)
-
NF(b)
+ NCI(X)
(4)
While this process does occur,it is apparently slow due to violation of spin conservation. Benard has shown, however, that it can be efficiently catalyzed by I atoms: NCl(a)
+ I(2P3,2)
I*(2Pl,,)
-
+ NF(a)
+ I*(2Pl/2) I(2P3/2)+ NF(b)
NCI(X)
(5) (6)
Herbelin and co-workers5 have found that reaction 6 is both very cm3 s-I) and efficient. NF(b) densities greater rapid (k 2 than 1014cm-3 have been generated in this manner.5 In theory, these processes may occur in the H + NFC12 system with the admission of I atoms. The objectives of the present work were to investigate processes 2 and 3 and to determine their rate constants and the relative (21) Rabideau, S. W. J . Magn. Reson. 1973, 1 1 , 163.
0022-365419 112095-1158%02.50/0 0 1991 American Chemical Society
Reaction of NFCI, with Hydrogen Atoms
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7759
proportions of NF(a) and NF(b) produced by the system. The NF(a)/NCl(a) energy pooling process was observed, along with the catalytic role of I atoms in the system. These studies serve to show that the reaction of H atoms with NFCl, is indeed a viable method for the chemical generation of these important metastable species.
Experimental Section Chlorofluoramines were readily synthesized, utilizing the method reported by Gilbert et al.,, Although this method can successfully generate large quantities of NFCIZ,the product also contains a substantial C12 impurity. In the present experiments, the CI2was successfully removed from the NFCI,. The reaction products were pumped through a series of stainless steel U-traps held at 193 K (CO,/methanol), 157 K (ethanol/liquid N2), and 77 K (liquid N,). Spectroscopic analysis indicated that the 193 K trap contained water and other impurities, the 157 K trap contained NFClz and CI2, and the 77 K trap contained mostly NF2Cl plus CI2 and a trace of NFCI,. All except the contents of the 157 K trap were pumped away. In order to further separate CI, from NFCl,, repeated separations were performed utilizing 157 and 77 K baths, condensing NFClz and some C12 at 157 K, and CI2only at 77 K. This was continued until the 157 K trap contained pure NFCI,, as verified by IR (Nicolet Model 5DXC FTIR spectrometer) and UV (Beckman DU-50 spectrophotometer) analysis. A 7.5-cm cell equipped with quartz windows was used for UV absorption measurements in order to determine the NFCI, extinction coefficient at 270 nm. Pressure was measured with an inductance manometer (Validyne). For experimental purposes, 5-6 Torr of the purified NFClz was transferred to a glass bulb which was then filled to 600 Torr with argon (99.999%). The remaining NFCl, was stored indefinitely at 77 K under flowing nitrogen. UV analysis of the contents of the bulbs indicated that NFCI, was very stable in the bulb, provided that the inner surface of the bulb was clean. The concentration of NFCI, in the bulb was determined by UV absorption measurements. The flow apparatus utilized in the experiments was the same as used previously in our laboratory for studies of the reaction of NCI3 with hydrogen atoms, with the exception that the flow reactor was coated with Halocarbon wax. H or D atoms were produced by admitting H2 or D2 to a stream of F atoms generated by a microwave discharge through F,/Ar. The amine was admitted through a second injector downstream. Before the deuterium experiments were performed, the flow system and gas handling lines were thoroughly passivated by filling the apparatus with D2 at atmospheric pressure for 48 h. Following passage through a metal metering valve, the NFCI2/Ar mixture from the bulb was admitted to the H atom flow IO cm downstream from the F + H2 mixing zone, via the inner tube of the sliding injector. As this compound is more stable than NCI, and does not readily decompose on metal surfaces, flow rates of the Ar/NFCI2 mixture were measured directly with a calibrated mass flow meter. In order to generate I atoms, HI was admitted to the flow system from a bulb which was 10% HI in U H P argon. The HI/Ar mixture entered the gas stream with the H,, and both subsequently reacted with fluorine atoms. The light detection system employed in these experiments was identical with that used in the previous NCI, studies. A 0.275-m monochromator was used to disperse the emission. Near-IR emission was measured in second order with a grating blazed at 2 gm. The response of the detection system in the near-IR region was calibrated by measuring chemiluminescence from the 0 + N O reaction and comparing it with the known emission spectrumaZ3 For this purpose, air was bled into the system through the microwave discharge to generate oxygen atoms, which then reacted with N O which was added upstream of the reaction zone. (22) Gilbert, J. V.; Conklin, R. A.; Wilson, R. D.; Christe, Fluorine Chem. 1990.48. 36 I .
K. 0. J.
(23) Vanpee, M.; Hill; K. D.; Kineyko, W. R. Am. Inst. Aeronouf. As-
tronaut. 1971, 9, 135.
Figure 1. UV absorption spectrum of NFCll recorded with a Beckman DU-50 spectrophotometer.
First-order emission near 2 gm from the 0 + N O reaction was blocked with a water-filled 1.0-cm cell equipped with quartz windows.
Results and Discussion UV Spectra. Figure 1 shows the UV spectrum of purified NFCl,, measured as described above. The previously reported spectrumz4showed significant contamination by C12, which has been successfully removed for these experiments. The broad structureless peak at 270 nm indicates a dissociative excited state which should undergo ready photodissociation with pulsed laser sources. Based upon a Beer's Law determination, the extinction coefficient (decadic) was found to be 230 f 20 L mol-I cm-I a t 298 K for A, = 270 nm. Chemiluminescence. Following admission of the NFC12/Ar mixture to the gas stream containing H atoms, a flame with both green and red elements was produced, with the green being longer lived in the flow reactor. The visible emission spectrum shown in Figure 2A, recorded with an NFClZdensity of 5.2 X 1O1Icm-3 and H atom density of 2.6 X IOt3 ~ m - displays ~, features at 528.8 and 664.8 nm which are identified respectively as the b'Z+ X31T transitions2s*26in N F and NCl. In the near-IR region, bands at 874 and 1080 nm are identified as the alA X3Z- transition~I~-~' in N F and NCI, respectively. A representative IR spectrum (NFC1, = 7.8 X 10" cm-,, H = 2.8 X lot3~ m - is ~ shown ) in Figure 3A. The production of NF(a,b) indicates that the reaction must proceed through generation of NFCl(2a) followed by (3a), at least in some measure. The observed chemiluminescence in both the visible and near-IR regions was dependent upon the presence of both NFCI, and H atoms. In order to gain an understanding of the dominant pathways in the system, the relative amounts of NCI and N F present in the steady state were determined, based upon the NCl(a) and NF(a) emission intensities. Following calibration of the light detection system, populations of the a'A states were calculated from the measured intensities and known radiative rates. Lifetimes of 1.4 s for NCl(a) and 5.6 s for NF(a) were used to determine the radiative rates. While the lifetime of NF(a) is well known: values for the radiative lifetime of NCl(a) reported in the literature show significant v a r i a n ~ e . ~ Recent ~ * ~ ~ ~ o r k , however, ~ , ~ ~ tends to
-
-
Conklin, R. A.; Gilbert, J. V. J . Phys. Chem. 1990, 94, 3027. Douglas, A. E.; Jones, W. E. Can. J . Phys. 1966, 44, 2251. Colin, R.; Jones, W. E. Can. J. Phys. 1967, 45, 301. (27) Jones, W. E. Can. J. Phys. 1967.45, 21. ( 2 8 ) Coombe, R. D.; VanBenthem, M. H. J . Chem. Phys. 1984,8I,2984. (29) Becker, A. C.; Schurath, U. Chem. Phys. Lerr. 1989, 160, 586. (30) Yarkony, D. R. J . Chem. Phys. 1987,86, 1642.
7760 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Exton et al.
1
-
IF b - X
A
NFa-X
H+WFClr
H + NFCfZ
NCI b-X
1
1
I
825
875
915 r w k y l b , I.
925
loa
lo75
I
1
1
1 I125
I
B
D+WFC~
I
NFo-x
-v-,
Figure 3. Near-IR emission spectra produced by the reaction of (A) NFCl2 (6.5 X 10" cm-') with excess H atoms (2.8 X IO" cm-') and (B) NFCI, (8.7 X 10" cm-') with excess D atoms (3.8 X loi3cm-9. Electrometer sensitivity was doubled in (B).
2 200-
I' I
500
150-
NF b-X
100
D + NFCfZ
-
50 -
0.
550
600 Wovtltnqth, nm
650
Figure 2. (A, B) Visible emission spectra produced by the reaction of excess H atoms ( > I 0'' cm-') with NFC12. The density of NFCl2 was 5.2 X 10" in (A) and increased to 2.1 X I O i 2 in (B). (C) Spectrum of visible emissions produced by the reaction of a comparable density of D atoms with NFC12 (2.2 X loi2cm-7.
support a lifetime of 1.4 s. The N F and NCI emission intensities were measured at their respective maxima, which were found to overlap one another at short distances down the flow reactor. The [NCI]/[NF] ratio was determined to be 0.05 f 0.01, suggesting that reactions 2a and 3a are surely d ~ m i n a n t . ~ ' When the flow of NFCl, into the system was increased, emission was observed which was identified as N2 first positive bands (B3& as is seen in Figure 2B. The NFCl2 density for this
-.
(31) This conclusion is supported by recent work by Gilbert, Setser, and co-workers (unpublished). (32) Parse, R. W.B.; Gaydon, A. G . The Idenrification of Molecular Spectra, 4th cd.: Wiley: New York, 1976.
I
spectrum was about 3 times greater than for that shown in Figure 2A, with a comparable H atom density. In the H + NF2system, to the a similar effect has been r e p ~ r t e d , ' I - ~and ~ * ~ascribed ' following sequence of reactions:
- +
+ H HF + N(2D) N,(B) + F N(zD) + NF(a) NF(a)
N,(B)
Nz(A)
h~
(7) (8) (9)
in the H + NC13 experiments reported previously, N2(B) was not observed, suggesting that its production in the present system is tied to the generation of NF(a) in reaction 3a. Kinetics. Of the NX(a,b) species observed in these experiments, only the NCl(b) has a lifetime35short enough such that the time dependence of its emission tracks the actual reaction rates. For the others (NCl(a), NF(a), and NF(b)), the long lifetimes dominate the time profiles. As was observed in the H + NC13 system,' (33) Davis, S. J.; Rawlins, W. T.; Piper, L. G . J. Phys. Chem. 1989,93, 1078. (34) Davis, S.J.; Piper, L. G. J . Phys. Chem. 1990, 94, 4515. (35) Barnes, I.; Becker, K. H.; Fink, E. H. Chem. Phys. Lett. 1979, 67, 314.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7761
Reaction of NFCI, with Hydrogen Atoms
160
(:I,------------1 I‘
a
,‘
i 100
JY‘
60
NCI I b l
40
,/v
20
**-d
0 0
10
20
30
40
50 60 70 BO NFCI~ Oenuty I 1o”c6’1
90
100
110
120
Fwre 5. Emission intensity of NCI(b) (solid circles) and NF(b) (open circles) vs initial NFCI2density. The solid line represents a linear fit to the data, and the dashed line corresponds to a 1.6-orderdependence on NFCI2density, based upon a power function fit. the time behavior of the NCl(b) emission intensity should display a rise and fall, based upon the simplified rate expression
For this expression to be valid, kdrthe rate constant for pseudofirst-order decay of NCl(b), must be much larger than k2[H]and k3[H], where k, and k , are rate constants for reactions 2a and 2b and 3a and 3b above. Therefore, H atom densities were kept ) , still in large excess over NFC12 (-5 low (-5 X IO” ~ m - ~while x 10” cm-,). The NCl(b-X) time profiles exhibited a very rapid rise and much slower fall in emission intensity. The rapid rise made it impossible to extract rate information from this portion of the time profiles. However, the well-resolved decays were easily fit to a single-exponential term, using RS-1 data analysis software operated on an IBM RT-6 152 microcomputer. Figure 4 shows a plot of the exponential decay rate vs the initial H atom density, for H atom densities ranging from 2 X lo1, to 1 X IOl4 ~ m - The ~. slope of this plot gives a rate constant k = (2.6 f 0.2) X IO-’, cm3 s-l. While it is not possible from these experiments to specify whether this rate constant corresponds to reaction 2 or 3, it is possible to make a prediction, based upon the known rate constants cm3 s-l) for the analogous H + NF3 reaction2I ( k = 4.3 X and the subsequent H NF2 reactiong ( k = 1.3 X 1O-Il cm3 s-l). In this case, the initial halogen atom abstraction is much slower than the subsequent HX elimination. If the H + NFCI, system behaves similarly, reaction 2 is the slowest process and should be assigned the measured rate constant. A similar conclusion was drawn for the H + NCI, system. It can be seen from these reaction rates that the presence of CI greatly increases the reactivity of the halogenated amine compounds. Given the assignment of the measured rate constant to reaction 2, the intercept at [HI = 0 represents the rate of loss of NFCI, from the system in the absence of H atoms. This intercept is significantly larger than that found in the experiments with NC13, and is believed to correspond to removal of NFC12 by the waxed walls of the flow reactor. NF(b) Formation. In addition to the direct formation process reaction 2a and the energy-pooling mechanism reaction 4, NF(b) might also be formed in our system from NF(a) via resonant V-E energy transfer involving HF:”,20 HF(uL2) + NF(a) NF(b) + H F (1 1)
+
-
In order to investigate the role of these processes in our system, H2was replaced by D, as a reagent, such that reaction 1 1 did not occur. Thorough passivation of the flow system with Dz was verified by the absence of HF(v) emission. The visible (Figure 2C) and near-1R (Figure 3B) emission spectra from the D2 system display the same emission features which are apparent in the H2 system. Within expetimental error, there is no change observed in the relative intensities of NF(a) and NF(b), which would indicate that reaction 1 1 plays a minimal role in the formation of NF(b) in this system. The NCl(b) emission intensity decreases significantly upon the substitution of D2 for Ha, possibly attrib-
H + N F q + HI
c
30 20
I
500
550
600
650
700
Wavelength, ma
Figure 6. Visible emission spectra from the H + NFCl2system without HI (A) and with added HI (B), displaying a 4-fold increase in the NF(b) signal at 528 nm following the addition of HI.
utable to resonant energy transfer with DF. Also seen in the spectra is much stronger emission from N 2 than was observed in the H2 system, as is observed in Figure 2, B and C, which were recorded with the same NFCl, and H/D atom densities. Although the reason for this is not readily apparent, the results suggest possible enhancement of the rate of reaction 7,the rate-limiting step in this mechanism, in the deuterated system. A plot of emission intensities from the D + NFCl, reaction vs initial NFC12 density is shown in Figure 5. The NCl(b) intensity is seen to rise linearly with increasing NFCI,, as is predicted by eq 10 above. The emission intensity of NF(b) varies as the NFClz density raised to the 1.6 power, as determined by a least-squares fit to the data. This fit is represented by the dashed line in Figure 5 . The greater than linear dependence surely indicates the operation of second-order processes in the formation of NF(b). Since there is no HF(u) present, this process is thought to be the NCl(a)/NF(a) reaction (eq 4) noted above. The deviation from a precisely linear or square dependence indicates the combined action of both first-order and second-order processes in the formation of NF(b), Le., reactions 3b and 4. It is likely that the rate of NF(b) quenching in the system is constant (invariant with NFCI2) and is dominated by collisions with DF (present in large excess over NFCI,). The catalytic role of I*(2PI12)atoms in the energy pooling process to form NF(b) according to reactions 5 and 6 was investigated by adding HI to the flow system for subsequent reaction with F or H atoms:
F + HI H
+ HI
-
HF + I ( ~ P ~ / ~ )
(12)
H~ + 1(2p3I2)
(13)
Reaction 12 is very rapid36and is expected to dominate3’*”’over reaction 13, since k I 2is: 2 k , 3 and HI is admitted to the system with H2. When a small flow of HI (-2 sccm, or 10%of the F atom flow) was added to the flow system, the NCl(a) emission disappeared and the NF(a) signal decreased by approximately 15%. This was accompanied by a 4-fold increase in NF(b) emission, as is shown ( 3 6 ) Jonathan, N.;Melliar-Smith, C. M.;Okuda, S.; Slater, D. H.;Timlin, K.Mol. Phys. 1971, 22, 561.
(37) Cadman. P.; Polanyi, J . C.; Smith, 1. W . M . J . Chem. Phys. 196’1, 64;l 1 I . (38) Sullivan, J . H. J . Chem. Phys. 1959, 30, 1292, 1577; 1962, 36, 1925.
7762 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 in Figure 6. Further addition of HI did not change the intensities, until, a t a much larger flow (-8 sccm), all of the emission was quenched, likely by loss of H atoms due to reactions 12 and 13. Assuming unit efficiency3’ for the production of I*(2P1/2)b y NCl(a), the initial decrease in NF(a) suggests a branching fraction to NCl(a) of 1596, contrasting with the 5% NCl(a)/NF(a) relative density figure measured previously. The difference is attributable to the fact that quenching of NCl(a) by I(2P3,2) atoms present in our system is more efficient than a X radiation. Hence the larger percentage (1 5%) is likely closer to the true branching fraction in reaction 3. In any case, these numbers are surely comparable and clearly indicate that NF(a) is the favored reaction product. From the 4-fold increase in NF(b) relative to the 15% decrease in NF(a), it can be deduced that the branching fraction to NF(b) is about 4% of that to NF(a) in reaction 3a.
-
Summary and Conclusions
These results clearly indicate that the H + NFCI, system presents an interesting alternative for the generation of excited metastable NF(a) and NCl(a) in a single reaction system. The relative densities of these species in the system indicate that the dominant reaction pathway in this process favors the formation of NF(a) and HCI. Addition of an I atom source to this system efficiently catalyzes the upconversion of NF(a) to NF(b) through resonant energy pooling with NCl(a). Clearly, these findings are of interest with respect to possible utilization of NFCI, as a fuel in laser systems. Apart from their significance with respect to laser applications, comparison of the present data with that from the H + NC13/NC1, and H NF3/NF2 systems reveals a number of interesting trends. Foremost among these is the enhancement of the rates of these processes upon replacement of F atoms by CI atoms. For the reactions of the amines with H atoms, the rate constants fall in the order H + NCIJ > H + NFCI, >> H + NF,. A corollary to this trend is the fact that in H + NFCI,, the path leading to HCI + NFCI (i.e., N-CI bond scission) is strongly favored over
+
(39) Bower,
R. D.;Yang, T.T.J . Opt. SOC.Am. B
1991.8, 1583.
Exton et al. the path leading to H F + NCl, (N-F bond scission). Although the N-C1 bonds in NFCI2 are likely to be weaker than the N-F bond, H F and NC12 are still the thermodynamically favored products. Another difference between the N-CI and N-F bonds in these compounds is in their polarity. Since CI is less electronegative than either N or F atoms, the polarity of the N-CI bonds will be reversed from that in N-F bonds, the C1 atoms having a partial positive charge. The strength and polarity of the N-Cl bonds would be expected to have an effect on the rates if the reactions were indirect processes involving an intermediate. We note, however, that recent work by Gilbert, Setser, et al.” has suggested that these reactions are in fact direct abstractions, in which case these factors are unlikely to have an effect. A similar trend is found in the subsequent reactions of aminyls (NX,), with the rate constants in the order H + NCI2 > H NF2 Although the rate constant for H + NFCI was not measured in the present experiements due to the rapidity of the process, it too is surely greater than that for H + NF2 Here again we find that in H + NFCI, N-CI bond scission is strongly preferred over N-F bond scission. Since these processes are thought to be additionelimination mechanisms,lO-” this preference can be understood in terms of the weaker N-C1 bond in the intermediate HNFCI relative to the N-F bond. The trend in rates may also be linked to the electronegativity difference noted above. Substitution of a C1 atom for F will increase the electron density on the N atom, making it more attractive to attack by the H atoms. It is clear that there is much yet to be understood about these interesting reactions, which are potentially useful for lasers and also pose an interesting problem in the dynamics of abstraction and unimolecular decomposition mechanisms. These issues are being pursued in our laboratory as well as others.31
+
Acknowledgment. R.D.C. is grateful to the Air Force Office of Scientific Research for partial support of this work under grants no. AFOSR-90-0259 and AFOSR-90-0296. J.V.G.gratefully acknowledgessupport from the National Science Foundation under grant no. CHE-8910143 and from the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. H,12385-13-6; NFC12, 17417-38-8; HI,10034-85-2.
