J. Phys. Chem. 1991, 95, 2692-2696
2692
it is very difficult to identify a new product species on the basis of one infrared absorption, and the above discussion of a type I1 complex (complex C) must be regarded as tentative. For the F/CF$CH system, several absorptions were observed in the region 2050-2100 cm-I and are assignable to the triple-bond stretching modes of complexes A, B, and C. However, to uniquely identify which band is associated with which complex is very difficult, and the assignments indicated in Table I1 must be regarded as very tentative. CsHloReactions. tert-Butylacetylene was codeposited with CsF in several experiments, and one weak feature was observed at 2075 cm-l. This is undoubtedly due to a perturbed triplebond stretching vibration. The tert-butyl group is electron-donating in nature, which should lower susceptibility to nucleophilic attack, as well as hydrogen-bonding ability. Earlier studies of tert-butylacetylene with neutral bases have identified hydrogen-bonded complexes with shifts of us less than for the corresponding acetylene complexes. If the product complex involved nucleophilic attack of the triple bond, then a slightly red-shifted alkynic C-H stretch would be anticipated, as for species A, above. This should fall around 100 cm-' to the red of the parent mode and was not observed. If, instead, the complex were hydrogen bonded as in species B, then a shift of some 300-400 cm-I would be anticipated (somewhat less than the 425cm-' shift for the C2H2-F complex). This would shift the alkynic C-H stretching mode to between 2900
and 3000 cm-I, a region obscured by the stretching modes of the three methyl groups of the parent alkyne. Consequently, assignment to a hydrogen-bonded complex fits the available data more effectively and is so tentatively assigned. Summary
The codeposition of CsF, CsCI, and CsI with several alkynes has led to the formation of molecular complexes. For the simplest alkyne, acetylene, a hydrogen-bonded complex was observed for each halide anion, with shifts that were consistent with the basicity of the halide anion. The complexes of CH2CICCH, in contrast, appeared to involve nucleophilic interaction of the halide anion with the carbon of the chloromethyl group. The interaction of CsCl and CsI with CF3CCH also led to a nucleophilic interaction, while the interaction of F with CF3CCH led to hydrogen-bonded and nucleophilic complexes. Acknowledgment. The authors gratefully acknowledge support of this research by the National Science Foundation, under Grant C H E 87-21969. Most importantly, the interest in hydrogen bonding and enthusiasm for chemistry of George Pimentel, which he shared with us, is fondly remembered. Registry No. HCECH, 74-86-2; (CH,),CC=CH, 917-92-0 F3CC e H , 661-54-1; CICH2*CH, 624-65-7; CSF, 13400-13-0; CSCI, 7647-17-8; CSI, 7789-17-5.
Generation of Excited NCI by the Reaction of Hydrogen Atoms with NC13 D.B. Exton, J. V. Gilbert, and R. D. Coombe* Department of Chemistry, University of Denver, Denver, Colorado 80208 (Received: August 8, 1990)
Discharge-flow methods were used to study the reaction of gaseous NC13 with e x w s hydrogen atoms. The system produces electronically excited NCl(alA,b'Z+) by a two-step mechanism in which the hydrogen atoms first react with NClp to produce HCI and NCI2 and then with NC12to produce HC1 and excited NCI. From measurements of the time dependence of NCI b ' F X3Z- chemiluminescence in the system, the two rate constants were found to be (9 f 4) X IO-" cm3 s-I and (4.0 f 0.4) X cm3s-I, both at 300 K. Based on the rates of analogous reactions with NF3 and NF2, the larger rate is tentatively assigned to the H + NC12 reaction. The branching fraction for production of NCl(aIA) by H + NCI2 has a lower limit of 0.15. The propensity for production of NCl(alA) by H + NC12is considered in terms of an addition-dimination mechanism similar to that operative in the analogous H + NF2 reaction.
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Introduction The production of excited singlet N F by the reaction of hydrogen atoms with NF2 has been studied in many previous experiments." Such processes are of interest because of the very strong dynamic constraints on the distribution of product states and because NF(a'A) has potential utility as an energy carrier in high-energy laser systems. The branching fraction to NF(a) in the H + NF2 reaction has been determined to be in excess of 0.90.a8 The product-state selectivity in this case is thought to arise from an addition-elimination mechanism involving attack of the hydrogen atom on the nitrogen atom of NF2to produce an excited singlet difluoramine intermediate, which eliminates singlet H F to leave excited singlet NF.3*4 This process has been used' to ( I ) Clyne, M. A. A.; White, 1. F. Chem. Phys. Lett. 1970, 6, 465. (2) Malins, R. J.; Setser, D. W. J . Phys. Chem. 1981, 85, 1342. (3) Herbelin, J. M. Chem. Phys. Lett. 1976, 42, 367. (4) Herbelin, J. M.; Cohen, N. Chem. Phys. Lett. 1973, 20, 605. (5) Cheah, C. T.; Clyne, M. A. A.; Whitefield, P. D. J . Chem. SOC., Faraday Trans. 2 1980. 76,711. Cheah, C. T.; Clyne, M. A. A. J . Chem. Soc.. Faraday Trans. 2 1900, 74, 1543. (6) Koffend, J. B.; Gardner, C. E.; Heidner, R. F. J . Chem. Phys. 1985, 83, 2904. (7) Herbelin, J. M.; Spencer, D. J.; Kwok, M. A. J . Appl. Phys. 1977,48, 3050. (8) Heidner, R. F.; Helvajian, H.; Holloway, J. S.; Koffend, J. B. J . Phys. Chem. 1989, 93, 7818.
0022-3654/9l/2095-2692$02.50/0
generate NF(a) densities in excess of 1OIs ~ m - ~In. principle, similar reactions of H atoms with other NX2 or NXY radicals (X,Y = halogen) might also produce high yields of excited singlet N X species. Such reactions have not been explored, however, in large measure because chlorinated or brominated analogues of N2F4 (the normal precursor of NF2) are unknown. Recent work in our laboratory has involved studies of the spectroscopy and reactions of fully halogenated amines NX3 or NX2Y, and techniques for the safe synthesis and manipulation of these energetic compounds have been developed."' Like N2F4, these species can be used as precursors of NX2 or NXY radicals and hence of excited NX nitrenes by reactions with H atoms. This paper presents the results of experiments involving the reaction of NCI3 with excess H atoms. Excited NCI can be produced in this system by a two-step mechanism as follows: H + NCI3 NC12 HCI (1)
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+ NCl* + HCI
H + NC12 (2) To the extent that reaction 2 proceeds by the addition-elimination (9) Gilbert, J. V.; Wu, X. L.; Stedman, D. H.; Coombe, R. D. J . Phys. Chem. 1987, 91, 4265. (10) Conklin, R. A.; Gilbert, J. V. J . Phys. Chem. 1990, 94, 3027. (11) Gilbert, J. V.; Conklin, R. A.; Wilson, R. D.; Christe, K. 0. J . Fhorine Chem. 1990, 48, 361.
0 1991 American Chemical Society
Generation of Excited NCI
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2693
mechanism, excited singlet NCI should be produced. By use of discharge-flow methods, rate constants for reactions 1 and 2 have been determined, and lower limits on the branching fractions to NCl(b'Z+) and NCl(alA) have been measured. The results of these experiments indicate that halogen amines are indeed useful reagents for the generation of excited N X species.
Experimental Section The experiments were performed with a 2.54-cm4.d. m e x flow reactor equipped with a double concentric sliding injector for the admission of reagent gases. This type of apparatus has been used extensively in our laboratory and has been described previously.12 Hydrogen atoms were produced either by a microwave discharge through H2/Ar mixtures or by the reaction of H2 with fluorine atoms. The latter method was found to be a more efficient H atom source and was used in the measurements presented below. For this purpose F atoms were produced by a microwave discharge (2.45 GHz, 50 W) through F2/Ar mixtures. The discharge was located on a side arm 47 cm upstream of a fixed observation window. It was assumed that the F atoms were completely removed by H2 (admitted to the flow through the outer tube of the sliding injector), such that the subsequent H atom flow was equivalent to the original F atom flow. Initial F atom flow rates were determined by chemiluminescent titrations with CI2.l3 During NCl(b) experiments, these titrations indicated that the F2 flow was completely dissociated in the microwave discharge, over a wide range of F2 flow rates. Therefore, complete dissociation was assumed for measurements at very low F (hence H) atom flows, for which titrations were not possible. Titrations performed during subsequent NCl(a) experiments indicated that the assumption of full dissociation was no longer valid, and individual titrations were done with Clz in order to determine the H atom concentration. NC13 was synthesized according to the method originally described by Clark and ClyneI4 and subsequently developed in our l a b o r a t ~ r y .[Note: ~ Persons working with NC13 are advised that this compound is dangerously explosive, and due caution should be exercised.] For the purpose of these experiments, it was held in a trap at 193 K, and the small vapor pressure at this temperature was entrained in a flow of Ar at atmospheric pressure. A small portion of this NC13/Ar flow was bled into the discharge-flow reactor via a stainless steel metering valve, and the remainder of the flow was returned to the fume hood. The NC13/Ar that passed through the valve entered the H atom flow through the inner tube of a sliding concentric injector. For photon yield measurements, absolute concentrations of NC13 in the flow upstream of the metering valve (Le., on the high-pressure side) were determined by optical absorption measurements. For this purpose, a 41-cm cell equipped with quartz windows was inserted in the flow, and absorbance was measured either at 253.7 nm with a Hg lamp or a t 220 nm with a Xe lamp. The detector was a solar-blind photodiode. NC13 concentrations were determined by using extinction coefficients of 400 L mol-' cm-' at 254 nm or 2000 L mol-' cm-I at 220 nm.I4 The amount of the known NC13/Ar mixture entering the discharge-flow reactor was determined by measurements with a calibrated mass flowmeter (Tylan FM360). In order to minimize loss due to decomposition of NC13 on metal surfaces, the flowmeter was removed from the system following the flow determinations. The total pressure of the flowing gases in the flow reactor was typically 800 mTorr for NCl(b) experiments and 1.6 Torr for NCl(a) studies, with a linear velocity of approximately 1600 cm s-l. The pressure in the system was measured with a capacitance manometer (MKS Baratron). The pressure of the NC13/Ar mixture upstream of the metering valve was measured with an inductance manometer (Validyne). Chemiluminescence produced in the flow reactor was viewed through a 2.5-cm quartz window and dispersed by 0.25-m (12) David, S. J.; Coombe, R. D. J . f h y s . Chem. 1985, 89, 5206. (13) Ganguli, P. S.: Kaufman, M.Chem. fhys. Lctr. 1974, 25, 221. (14) Clark, T. C.; Clyne, M.A. A. Trans. Faraday Soc. 1%9,65,2994.
z
i l L+-U-&bl
AV=+I
610 620 630 640 650 660 670 680 690 700
710 720
WAVELENGTH l n m l
Spectrum of visible emissions produced by the reaction of NCI, with excess hydrogen atoms. The strong band centered at 665 nm is identified as the NCI bizt X3Z- transition. Figure 1.
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t
n
AV:O
0 WAVELENGTH lnml
Figure 2. Near-IR emission spectrum produced by the reaction of NCI, with excess hydrogen atoms. The band centered at 1080 nm is identified as the NCI a'A X3Z- transition.
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(NCl(b)) and 0.275-m (NCl(a)) monochromators. Visible emission was detected by a cooled GaAs photomultiplier tube, whose response was converted to a voltage signal with an electrometer, and recorded with a strip-chart recorder. Emission in the near-IRregion was chopped at 310 Hz and detected by a liquid nitrogen cooled Ge detector. The detector signal was transformed by a lock-in amplifier and recorded on a strip chart recorder. For experiments in which very low reagent densities were employed (as in the determinations of rate constants), the monochromator was replaced by filters whose transmission bands were centered at 665 nm (10 nm fwhm) and 1.08 pm (20 nm fwhm).
Results and Discussion Chemiluminescence. When a small flow of the NC13/Ar mixture was admitted to a stream containing H atoms (from a discharge through H2 or from the F + H2 reaction as above), a red flame clearly visible to the eye was produced. Production of the flame required both the NC13and H atom flows; Le., admission of NC13 to a stream of F atoms alone produced no red chemiluminescence. Figure 1 shows a spectrum of the red emission, which is readily identified as the b'Z+ X3Z- transition in NCI.IS From the clear delineation of the Av = +1, Au = 0, and Av = -1 sequences of this band system, it is evident that the NCl(b) does not have significant vibrational excitation, as would be shown by --+
(15) Colin, R.; Jones, W. E. Con. J . Phys. 1967, 45, 301.
2694 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
longer progressions of emission bands from the higher vibrational levels. Apart from the NCl features shown in Figure 1, no other emission features were found over the full range of sensitivity of the photomultiplier tube, Le., 200-890 nm. Examination of the near-IR region revealed the presence of a strong band centered at 1080 nm, as shown in Figure 2. This emission was identified as the NCI alA X'Z- transitionI6 and was also dependent upon the presence of both NC13 and H atoms. The spectral region from lo00 to 1 150 nm was scanned, with no other emission features found to be present. For these experiments, a monochromator with a grating blazed at 2.0 pm was used in second order. These results are consistent with the occurrence of reactions 1 and 2. Under ordinary operating conditions, no N2 first positive bands (B'II, A3Z,+) were found in the visible or infrared regions. Under extraordinary operating conditions, with a large excess of F2, it was possible to generate these N2 bands, though this was largely an irreproducible phenomenon. In the analogous H + NF2 system, reaction of the product NF(alA) with the H atoms present generates excited N(2D), and the N(2D) reacts with NF(a'A) to generate N2(B) and hence the first positive bands.4J7J8 The analogous sequence of reactions apparently does not occur in the present system, perhaps because the reactions of H atoms with NCl(alA) and NCl(blZ+) are slow. The exothermicity of the H NCl(a'A) reaction is only marginally sufficient for production of HC1 + N(2D), whereas the H NF(alA) reaction is much more exothermic, yet has a rate constantSof only 2.5 X cm3 s-l. Since the lifetime of NCl(b) has an upper limit set by its 630-ps radiative lifetime,I9 this state is likely too short-lived to suffer the requisite number of collisions with the H atoms, present at densities on the order of lo1, ~ m - ~ . Kinetics. Considering only reactions 1 and 2 above, the time dependence of the density of the intermediate NC12 is given by
Exton et al. I
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2.0
4.0
6.0
8.0
10.0
12.0
140
16.0
18.0
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-
+
+
[NCIZ] = k1[NC1310[e-kl[H]~ - e-kl[H]~] (3) k2 - kl where it is assumed that H atoms are present in large excess over NC13 or NCI2. Apart from reactions 1 and 2, the time dependence of the excited NCl product must include the rates of loss by radiation and collisional quenching: NCI* NCI(X) + hu (4) NCI*
+ Q
NCl(X)
+Q
(5)
These processes combine to produce a total pseudo-first-order decay rate kd = k4 ks[Q]. The time dependence of the NCl* density is then given by
+
e-kilH11 - &I e-kdHlf - e-kdl [NCI*] = klk*~HI[Nc1310[ k2 - kl kd - ki[H] kd - k J H 1
1
(6) for excess H atoms. For NCl(b) studies, where kd can be much larger than kl [HI and k2[H], yet H atoms are still in large excess over NCI, and NC12, this expression reduces to
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Hence, under these conditions, the NCI b X emission intensity will track the density of the NCll intermediate (eq 3) and will appear as a sum of rising and falling exponential terms, the rise and fall rates corresponding to kl[H] and k,[H], or vice versa. Considerable effort was expended to achieve experimental conditions corresponding to kd >> kl[H] and k2[H], with H atoms still in pseudo-first-order excess. Since kd can be as small as 1580 (16) Pritt, A. T., Jr.; Patel, D.; Coombe, R. D. J . Mol. Spccrrosc. 1981, 87,401. (17) Davis, S.J.; Rawlins, W. T.; Piper, L. G. J . Phys. Chem. 1989, 93, 1078. (18) Davis, S. J.; Piper, L. G. J . Phys. Chem. 1990, 94, 4515. (19) Barnes, 1.: Becker. K. H.; Fink, E. H. G e m . Phys. Lerr. 1979,67, 314.
o
TIME (mrl
Figure 3. Time profile of NCl(b-.X) emission intensity measured at 665 nm vs time. Data points in the first millisecond are skewed by the finite mixing time in the flow tube and have been omitted. The solid line represents a nonlinear least-squares fit of the data to the function intensity = A(e+' - e-*X'). I
t t El 3
0
2.0
4.0
6.0
8.0
10.0
12.0
N U , DENSITY IlO"ci?)
Figure 4. Intensity of NCl(b) emission, measured at 665 nm, as a function of initial NCll density. The solid line represents a linear least-squares fit to the data. I
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s-I (the b X radiative rate), small H atom flows and very much smaller NC13 flows are required. Such conditions were achieved, however, and good signal-to-noise in the NCl b X emission was retained by replacing the monochromator with a narrow band-pass filter as described above. A typical NCl b X time profile is shown in Figure 3, and it exhibits the rise and fall behavior predicted by eq 7. Further verification of eq 7 was obtained by measuring NCl b X emission intensity as a function of initial NCI, density. As eq 7 predicts, a linear correlation was obtained, as is shown in Figure 4. Rate constants were measured by acquiring data such as that shown in Figure 3 for a range of H atom densities. Rise and decay rates were obtained from the data by curve-fitting routines (part of an RS-Idata analysis package operated on an IBM RT-6152 microcomputer). In view of the limited number of data points obtainable for the rapid rise of the emission (see Figure 3), it proved most practical to fit the well-resolved decays to a single exponential and then to use that result to fit the full time profiles to the sum of a rising and a falling exponential (eq 7). Figure 5 shows a plot of the exponential decay rate vs the initial H atom density. We believe the scatter in the data shown to result largely from difficulties in controlling the very small NCI3 flows. The slope of the plot indicates a rate constant k = (4.0 f 0.4) X cm3s-'. A least-squares fit of the data for the rise of the emission yields a rate constant k = (9 f 4) X lo-" cm3 s-I, though the scatter is such that only the order of magnitude of this result is likely to be significant. It is not possible to predict on the basis of these experiments alone which rate constant corresponds to H + NC13 and which
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to H NC12. If, however, these reactions follow the same trend as is reported for H NF, and H + NF2 (with rates of
