7278
J. Phys. Chem. 1991, 95, 7278-7281
Photolysls of NCI, in Low-Temperature Argon Matrices Julanna V. Gilbert* and Londa J. Smith Chemistry Department, University of Denver, Denver, Colorado 80208 (Received: January 14, 1991; In Final Form: March 21, 1991)
NCI, has been photolyzed in low-temperature argon matrices and the products identified via the IR spectra of the matrices. For matrices in which the NCI, was not aggregated with C12,NCI was produced at all photolysis wavelengths used (220, 248,280, and 330 nm, selected on the basis of the UV absorption spectrum of NCll and CI2). In matrices in which significant amounts of NCI,/CI, aggregates were deposited, NCI2 was also produced. The rate of production of NCI as function of photolysis time tracks the NCl, UV absorbance and is proportional to the photolysis light intensity.
Introduction Experimental studies of the halogen amines have been motivated by their ability to store and release energy and to form interesting excited-state fragments.'V2 Although NF3 is a stable molecule, the NX3 get progressively less stable as the halogen atoms get larger, until NI,,which has never been isolated in a pure condensed state., To study the energetics of these systems, we have undertaken an investigation of their photoprocesses and reactions, starting with NCI, and the fluorochloramines,NF2Cl and N F Q . An important consequence of these studies has been the generation of excited-state nitrenes produced from the reaction of hydrogen atoms with NCI, and with NFC12.2.4 These long-lived excitedstate species have been researched extensively as energy carriers to support lasing, but the only reviously known precursors were the halogen azides and N2F4,>' compounds which are extremely difficult to work with. In order to understand how the halogen amines may be utilized for this and other applications, detailed information concerning their spectroscopy and dynamics is necessary. Several early investigations of NCI, have been reported, and include somewhat qualitative studies of the decomposition9J0and studies of the Cl + NC13 reaction.'' More recently, pulsed laser (248 nm) photolysis experimentsof gaseous NCI, in helium have been completed in our laboratory.' Two types of visible emission were observed in these experiments, one consisting of a vibrational progression, and the other of a broad continuum. The vibrational progression was assigned to NCI, and the continuum to NCI2. Because of the lack of detailed information for the NCI, system, these assignments are tentative and have raised interesting questions about the spectroscopy of the NCI, species. In order to isolate and spectroscopically identify the species present in the halogen amine systems, we have begun photolysis experiments of the low temperature matrix isolated amines. The UV absorption spectra of the gas-phase halogen amines generally consist of broad, structureless features, suggesting that the excited states in this region of the spectrum are not bound states, and that photolysis of the amines in low-temperature matrices may produce ~~~~
( I ) Gilbert, J. V.; Wu, X. L.; Stedman, D. H.; Coombe, R. D. J . Phys. Chem. 1981, 91, 4265. ( 2 ) Exton, D. B.; Gilbert, J. V.;Coombe, R. D. J . Phys. Chem. 1991, 95, 2692. (3) Jander, J.; Englehardt, U. In Deuelopmenrs in Inorganic Nitrogen Chemistry; Colburn, C. B., Ed.; Elsevier: Amsterdam, 1973; Vol. 2. (4)Exton, D. H.; Gilbert, J. V.; Coombe, R . D. J . Phys. Chem., to be published. ( 5 ) Benard, D. J.; Winker, B. K.;M e r , T. A,; Cohn, R.H. J . Phys. Chem. 1989, 93, 4790. (6) Coombe, R. D.; Patel, D.; Pritt, Jr., A. T.; Wodarczyk, F. J. J. Chem. Phys. 1981, 75, 2177. (7) Coombe, R. D. J . Chem. Phys. 1983, 79, 254. (8) Herbelin, J. M.; Cohen, N. Chem. Phys. Lert. 19l3,20,605. Herbelin, J. M. Chem. Phys. Lett. 1916, 42, 367. ( 9 ) Briggs, A. G.; Norrish, R. G . W. Proc. R . Soc. 1964, A278, 27. (IO) Markevich, E. A. Kine?. Katal. 1986. 27. 729. Markevich. E. A.: Aratyan, V. V. Klner. Karal. 1986, 27, 726 and references therein. ( I I ) Clark, T. C.; Clyne, M.A. A. Trans. Faraday. Soc. 1%9,65,2994. Clark, T.C.; Clyne, M. A. A. Trans. Faraday. Soc. 1970, 66. 372.
0022-3654/91/2095-7278S02.50/0 -~ .~ , I
I
stable fragments. The identification of the photofragments can then provide information about the excited states. In the low temperature matrix isolation studies carried out with NFCI2,I2 we observed the clear formation of N F upon irradiation of the matrix isolated amine at 270 nm, the maximum of the UV absorption spectrum for NFCI2. For NF2CI, on the other hand, no evidence of the formation of stable photofragments was observed, presumably due to a reaction of the fragments which regenerated the parent NF2CI. This paper reports the results of photolysis studies performed on NC13 in low-temperatureargon matrices. The photoproducts NCI and NC12 were observed in these experiments and mechanisms for their production are discussed. Experimental Section The NCI, synthesis, described in detail in ref 1, is based on the method developed by Clark and Clyne." Briefly, C12 in argon is bubbled through a solution of (NH4)2S04in 1.0 M H,S04 contained in a 500-mL Nalgene bottle. The product NCl, and the excess CI2 are swept out of the solution in the argon stream, through a drying tube, and to a trap maintained at -80 OC (dry ice/methanol). When a small amount of NCI, accumulates in the bottom of the cold trap, the synthesis is stopped. While continuously passing argon over the NCI,, it is warmed to room temperatureto allow the trapped C12to escape and then is recooled to -80 OC. If the synthesis proceeds too rapidly, liquid NCl, forms in the bottom of the (NH4)2S04solution. This can result in explosions due to the shock-sensitive nature of NCl, and the turbulence in the solution. The NC13 is held in the trap at -80 OC and the vapor is entrained in a flow of argon (U.H.P.) at atmospheric pressure which carries it through 1/4 in. (0.d.) Teflon lines to the deposition apparatus. At the deposition apparatus, a Teflon metering valve (Gilmont, GM-7302-A) allows a small amount of the flow to enter the reduced pressure region, and the remainder of the NCI3/argon flow is returned to the hood. From the reported extinction coefficient^^*'^ and the measured absorbances of NCI, at 220 nm and of C12 at 330 nm in the gas flow just before the Teflon metering valve, the NC13:Ar ratio is estimated to be less than 1:lOOO and the NCI3:Cl2ratio is about 1 5 . (The C12 is due to the decomposition of NC13 as it passes from the cold trap to the deposition apparatus.) In our early experiments, the Teflon metering valve was the only device used to control the deposition rate. Because of the near impossibility of maintaining a slow, constant gas flow through the valve, it was very difficult to achieve good matrices and NC13/C12aggregates sometimes formed during the deposition. A second method was developed in which the Teflon valve admitted the NCI3/Ar to a region maintained at about 500-1000 mTorr by a vacuum pump and throttle valve. A piece of 1 /4 in. (0.d.) glass tube with one end almost closed served as the connection of this intermediate region to the cold head region where (12) Conklin,
R. A.; Gilbert, J. V. J . Phys. Chem. 1990, 94, 3027.
(13) Gibson, G . E.;Bayliss, N.
S.Phys. Reo. 1933, 44, 188.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 1219
Photolysis of NC13 in Argon Matrices TABLE I: IR Frequencies (cm-I) for Various Species Observed before and after Photolysis of NCls in Low-Temperature Argon Matrices and Comparison to Previously Published Values this work N3sCIC N”CI N”CI NClz NClj NClo 3 5 ~ 1 1
’5CI”Cld Wl2d
83 1 824 817 619 649 644 546 539 53 1
Kohlmiller-Andrews’ 823.5 817.5 679
MilliganJacoxb
oo.F
824 818
646
’Reference 15. bReference 18. CFrequenciesshifted due to the presence of NCI,/CIz aggregate structures. IR activity induced by molecular aggregates.
0080 0 0900
the pressure is about 5 X 10” Torr. This method made it fairly easy to produce clean, reproducible matrices. The matrices were deposited on a KCI window at 8.9 K. The deposition of NCI, in the matrix was monitored by following the growth of the NCl, IR absorbance a t 644 cm-l. The optimum matrix was achieved by allowing this peak to grow in a t about 0.014.02 absorbance units per 10 min. Under these conditions, a transparent matrix with an absorbance of approximately 0.10 a t 644 cm-l took about 1 h to produce. Photolysis of matrices made this way gave very reproducible results. Longer deposition times frequently resulted in opaque matrices even when the rate of deposition was slower. This opacity tended to inhibit the photolysis, possibly due to scattering of the photolysis light. Long deposition times were also impractical since the Teflon valve tended to slowly close and required constant readjustment during the deposition. A Nicolet Model 5DXC FTIR (resolution *2 cm-l) was used to monitor the species present in the matrix. The FTIR,the cryostat, and the photolysis source are described in ref 12. For the broad-band photolysis experiments, the full output of the xenon lamp was sent through a water-filled 10-cm cell (with quartz windows) to filter out the infrared light, and the monochromator was set to zero order. For the single-wavelength experiments, the bandwidth was about 6 nm. A quartz lens was used to condense the diverging UV light beam on to the matrix so that the UV beam was approximately the same size as the IR beam (-8 mm diameter). Overlap with the IR beam was achieved by centering the UV light around the HeNe laser beam that is coincident with the IR light; however, there was always a small region in the center of the UV beam with no UV light because of the geometry of the reflector behind the Xe lamp. The lamp intensity was varied by changing the current through the lamp, and the relative lamp intensities were calculated from the values measured with a Scientech Model 362 power meter. Curve-fitting routines contained in the RS/l data-analysis software package (BBN Software Products Corp., Cambridge, MA) were used to fit the decay of NCI, and the rise of NCI absorbances as a function of photolysis time. Results
NC13 UV Spectrum. A UV absorption spectrum was taken of the NC13/Ar flow on the high-pressure side of the Teflon metering valve. The spectrum consists of three broad structureless features, at 220, 260, and 330 nm, and is consistent with the spectrum published by Clark and Clyne.lI The maxima at 220 and 260 nm are assigned to NCI,, whereas the 330-nm peak is assigned to C12. The UV spectrum served as a guide for selecting the photolysis wavelengths: 220 nm (the maximum in the spectrum), 280 nm (the low-energy side of the 260 nm peak), and 330 nm (the CI2 absorption). We used 248 nm for comparison of the matrix photolysis with the gas-phase laser photolysis experiments mentioned above. Infrared Spectra. The assignments for the 1R peaks observed in the matrix spectra taken before and after photolysis are sum-
I “ ‘
perature argon matrix.
9 0 060
f
O‘O
L 0.020
-
I
ii
Figure 2. IR spectrum from 500 to IO00 cm-*of an NCldargon matrix before (--) and after (-) photolysis at 248 nm.
marized and compared with data from other researchers in Table I. Matrices prepared with no or very few molecular aggregates displayed IR spectra consisting of a narrow feature at 644 cm-I (fwhm < 4 cm-I) as shown in Figure 1. On the basis of IR spectrum of NC13 in liquid CCI4l4and assignments made in the matrix experiments by Kohlmiller and Andrews (see Table I),ls the single W m - ’ feature is assigned to the asymmetric Cl stretch of NC13 (v,). The presence of NCI,/Cl2 aggregates was detected by CI2 absorbances just below 550 cm-I1I6and by the presence of a shoulder on the NCI, peak at 649 cm-’. In cases where the aggregate concentration was extremely high, three C12peaks were clearly observed a t 546, 539, and 531 cm-’ (for 3sC12, 3sC137CI, and 37C12,respectively) with an intensity ratio of about 9:6:1, and the 649-cm-’ NCI3 aggregate feature dominated the spectrum (fwhm 20 cm-I). No additional features were observed between 200 and 400 cm-’ when the KCl windows in the cold head were replaced by CsI windows. v2, the “umbrella” mode of NCI,, appears at 349 cm-’ in CCI, solution” but is apparently too weak to see in these dilute
Matrix Pbotolysis. Broadband photolysis and photolysis at 220, 248,280, and 330 nm of matrices in which no or only very weak aggregate features were observed resulted in the appearance of a doublet with peaks at 824 and 817 cm-I and a decrease in the 644-cm-’ NCl, peak. Figure 2 shows the IR spectra of such a matrix before and after photolysis a t 248 nm. The 824-cm-l/ 817-cm-’ doublet is assigned to the 3sCl and 37CIisotopes of NCl.15,18During the photolysis experiments, FTIR spectra were collected as a function of time, and plots of the absorbance of NC13 and of NCI vs time were made. The equations used to fit the decay of the NCI3 and the rise of the NCI absorbances are A(NC1,) = (14) Bayersdorfer, L.; Engelhardt, U.;Fischer, J.; Hohne, K.;Jander, J. 2.Anorg. Allg. Chem. 1969, 366, 169. (15) Kohlmiller, C. K.;Andrews, L. Inorg. Chem. 1982, 21, 1519. (16) Machara, N. P.;Auk, B. S. Inorg. Chem. 1988, 27, 2383.
(17) Hendra, P. J.; MacKenzie, J. R. Chem. Commun. 1968, 760. (18) Milligan, D. E.; Jacox, M. E. J . Chem. fhys. 1964, 40, 2461.
7280 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 0lOl
,
,
,
,
,
,
,
,
,
,
,
,
,
Gilbert and Smith
0 010
0lli
0 10
,
,
,
,
,
,
,
,
,
,
-
,
,
,
,
,
u+Qooooo o o o o
,
,
-
0 011
0 010 0 - 0 018
O
ow008-
:
001-
H
o o r 0
'
IO
'
10
'
I
I
u1
I
I
'
I
'
10
'
M
'
90
I
IDJ
I
I10
I
110
I
00 130
00000 O
005-
Two Different Photolysis Lamp Intensities for Each Wavelength, and
the Ratios (R(1)) of the Photolvsis Lamp Intensities 220 248 280
330
+
1.7 f 0.3 2.5 2.4 1.7
1.5 k 0.3 2.4 1.8 2.1
B C[exp(-Dt)] and A(NCI) = F[1 - exp(-Dt)], respectively. In these equations A is absorbance, E is the absorbance of NC13 at t = -, E C is the initial absorbance of NCI,, D is the decay (rise) rate, and F is the absorbance of NCI at t = -. Figure 3 shows the plot from photolysis 280 nm. As can be seen by the computer fits to the data, the rate of production of NCI is equal to the rate of destruction of NCI, as expected. Once the final NC13 and NCI absorbances were reached for photolysis at 220, 248, and 280 nm, changing the wavelength caused no further changes. For photolysis at 330 nm, however, the final NCI, absorbance was never less than about 40% of the initial absorbance. When the photolysis wavelength was subsequently changed to 220, 248, or 280 nm, however, the NC13 absorbance immediately decreased and the NCI absorbance increased. This is shown in Figure 4 where the matrix was photolyzed for 90 min at 330 nm and then for 90 min at 220 nm. To examine the dependence of the rates on the lamp intensity, different lamp intensities were used on matrices which were otherwise the same. The ratio of the rate of production of NCI at the high intensity to that at the low intensity was compared with the ratio of the lamp intensities. Table I1 shows these ratios, and within the experimental error, a linear relationship exists. The rates also varied with photolysis wavelength. When the ratios of the rate of destruction of NCI, to the lamp intensity at each wavelength are calculated, the values generally tracked the NCl, UV absorption spectrum reported by Clark and Clyne." Photolysis of matrices with strong NC13/C12 aggregate absorbances gave variable results depending upon the transparency of the matrix. Very foggy matrices frequently showed no photolysis, presumably because the photolysis light was scattered. For the clearer matrices, NCI2 was frequently observed at 679 cm-I, and NCI at 831 cm-I. The NCI peak was blueshifted and broader than in the matrices with no aggregates, phenomena that can both be ascribed to the presence of aggregates. Apparently, the conditions in the matrix cage have a profound affect on the photolysis, and a particular aggregate structure is necessary for the production and stabilization of NC12after photolysis. The products observed in these matrices were presumably due to chain mechanisms involving the molecules in the aggregates and led to different results depending on their relative positions and numbers. Photolysis of these matrices at 330 nm (the C12absorption band) could initiate the chain as could photolysis at one of the NC13 absorption wavelengths. An apparent disagreement exists between our photolysis results and those reported by Kohlmiller and Andrews.IS Upon broadband photolysis, they saw an increase in the NCI, IR band (646
+
-0 O M
IO
-0010
00
0
x)
Y)
k0
50
60
70
10 iilr
TABLE 11: Ratios (R(NC1,)) of the NC13 Decay Rates Collected at
- 0 016
0 NC1
-0012
riur I M ~ I , ~ ~
Figure 3. Changes in the IR absorbances at 644 cm-I (NC13) and 824 cm-' (NCI) for photolysis at 280 nm as a function of time. The lines through the points are the computer tits to these data: (--) A(NC13) = [0.009+ 0.09(exp(4).04t))];(-) A(NCI) = (0.02(1 - exp(4.04t))l.
0 YC1,
w0
ON-
0
oo ooo
O0
IC0
90
110
110
?I
140
1y1
160
E 5
1
110 180
imllym
Figure 4. Changes in the IR absorbances at 644 cm-' (NC13)and 824 cm-l (NCI) for photolysis at 330 nm for 90 min followed by photolysis at 220 nm for 90 min.
cm-I). However, this discrepancy is understandable since, judging from the appearance of the IR spectrum shown in ref 15, their matrix conditions were very different from ours. They produced NC13 in an argon matrix by passing a mixture of C12and N2 in Ar through a microwave discharge and depositing the effluent on a cold window. A large number of species were identified in the resulting matrix via the IR spectrum (e.g., NCl, NC12, NCl,, CCI4, HCly, CI2C0, unidentified aggregates, etc.). The fact that irradiation caused a completely different result simply verifies that specific photoprocesses are very dependent upon matrix conditions.
Discussion The production of NCI following photolysis of matrices with no aggregates at 220,248, and 280 nm is analogous to the photolysis experiments of matrix isolated NFC12 in which N F was produced.I2 Similarly, three mechanisms can be proposed. mechanism 1: NCI3 hu NCl2 + C1
+
NC12
+
NCl
+ CI2
NCI
+ C12
+ C1+
mechanism 2: NC13 + hu mechanism 3:
+ hu NCl2 + hu
NC13
+
+
+ Cl NCl + CI
NC12
All of these mechanisms assume that the photolytically produced CI atoms are unable to escape from the argon matrix cage, a reasonable assumption considering the available energy. Kunz, et al. reported that CI atoms generated from the photolysis of CI2 require a minimum of 6.5 eV above the gas-phase dissociation limit in order to escape from the Ar matrix cage.I9 The maximum energy available in our expkriments was only 5.6 eV (the photon energy at 220 nm), not enough energy for escape, even if the CI2N-CI bond energy (which is not known) is zero. The fact that NC12 was never observed in the matrices in which there were no CI2/NCl3aggregates implies that, although NCI2 may form at 248 nm as suggested by the gas-phase laser photolysis experiments, it is not stable in a matrix cage with a CI atom. Mechanisms 1 and 2 are in fact not distinguishablein the matrix environment under our experimental conditions since the photofragments are prevented from moving freely away from each other. No measure of bond strengths have been made for NC12; however, the fact that it is not observed following photolysis of NCl, suggests that the CI abstraction procedes with ease. This is in contrast to an ESR study in which evidence was presented for the stable formation of PC12 and a CI atom in a matrix cage following photolysis of PC13.20 If the CIP-CI bond is stronger than the (19) Kunz, H.;McCaffrey,J. G.; Schriever, R.;Schwentner. N. J . Chcm. Phys. 1991, 94, 1039. (20) Burdett, J.; Turner, J. J. In Cryochcmlsfry;Ozin, G. A,, Moskovits, M.,Eds.; Wiley: New York, 1976.
Photolysis of NC13 in Argon Matrices CIN-Cl bond, formation of P-Cl and CI2 would not necessarily be the energetically favored process. Mechanism 3 requires that NCI, absorb efficiently at the same wavelengths that NC13 absorbs to produce NCI. This is inconsistent with our matrix experiments in which NCll was produced and persisted even under extended photolysis times with broadband radiation. Both Clark and Clyne" and Briggs and Norrishg reported weak transient spectra from 285 to 315 nm which they tentatively ascribed to NC12. Except for these early reports, however, the spectroscopy of NC12 is virtually unknown, and techniques for isolating NC12 in a matrix are being developed in our laboratory. (These include the gas-phase reaction of fluorine atoms with NCI, before the deposition, and the use of a metal needle valve for catalyzing the NCI, decomposition prior to deposition.) The production of NCl after 330-nm photolysis was initially somewhat surprising since in our NFC12 matrix experiments, in which C12 was also present, no N F production was observed following photolysis at 330 nm.I2 We had assumed that as long as the IR spectrum gave no indication of CI2/NCI3aggregates in the matrix, that photolysis of C12would similarly have no affect on the NC13 The production of NCI via an energy transfer from CI2 in one cage to NCI, in another cage seems unlikely. NCI, has only a very small absorbance at A 1 330 nm, suggesting that transitions to states in this energy range are improbable. The production of NCI following the direct absorbance of 330-11171light by NCI, is doubtful since no more than about 60% of the NCl, a u l d be removed at this wavelength, and changing the wavelength caused more NC13 photolysis, phenomena that were not observed for any of the other wavelengths used. It is very unlikely that two photons were absorbed, one by NCl, and one by CI2, since this would have been apparent by a square dependence of the NCI production on the light intensity. Consideration of the matrix structure, however, gives a possible explanation. The probability that a molecule is isolated in a matrix is strongly dependent on the number of lattice sites occupied by the molecule and on the matrix to guest ratio. Given the size of NCI, (from the crystal structure determination2'),and the matrix to guest (C12plus NCl,) ratio between 1OO:l and 1000:1, only about half of the NCh molecules would be isolated.22 In other words, it is possible thai (21) Burgi, H. B.;Stcdman, D.; Bartell, L. S. J . Mol. Srruct. 1971, 10, 31.
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7231 about half of the NCh molecules may have another guest molecule as a member of the matrix cage (a nearest neighbor). These guests are not distinctly aggregated, however. With a CI2:NCl3 ratio of 5 1 , the nonisolated NCI, molecules would be more likely to have a CI2 rather than an NCl, molecule as a nearest neighbor. The CI atoms produced photolytically at 330 nm from the nearest-neighbor CI2 molecules would then react with the NCI3 molecules. Our observations for the 330-nm photolysis are consistent with this picture and imply that 60% of the NC13 occupy sites with a Clz nearest neighbor.
Conclusions Argon matrices containing only NC13 and C12 were prepared and photolyzed at wavelengths selected from the NCI3/Cl2UV absorption spectrum. Broad-band photolysis and photolysis at 220,248, and 280 nm of these matrices resulted in the formation of NCl. The rate of appearance of NCl (and disappearance of NCI,) was followed as a function of photolysis time, and the rate at which the NCI was produced tracked the NCI, UV absorbance and was proportional to the light intensity. The probable mechanisms are either a concerted process in which molecular CI2 is eliminated, or a process in which first one N-CI bond is broken followed by abstraction of a second CI atom leaving NCl and C12. NCI was also produced when the matrix was photolyzed at 330 nm, the maximum of the C12 absorption band. The data suggest that about 60% of the NCI, have CI2 nearest neighbors and that photolysis of these CI2produces C1 atoms that react with the NCI,. When the nearest-neighbor C4 molecules are "bleached" out no further reduction in NCI, is observed for 330-nm photolysis. When significant numbers of NCI3/CI2 aggregates are present in the matrix, NC12 was also produced during photolysis and persisted even under broad-band irradiation. Acknowledgmenr. L.J.S.was supported by the National Science Foundation, Research Experience for Undergraduates Grant CHE9000781. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the National Science Foundation under Grant CHE-8910143. (22) Cradock, S.;Hinchcliffe, A. J. In Matrix Isolation; Cambridge University Press: Cambridge, UK, 1975. (23) Sawodny, W.; HHrtner, H.; Minkwitz, R.; Bernstein, D. J. Mol. Struct. 1989, 213, 145.
