J. Phys. Chem. 1993,97, 43314334
4331
Infrared Spectroscopy of Low-Temperature Matrix-Isolated NFCl Radicals Ann M. Zarubaiko and Jdanna V. Gilbert’ Department of Chemistry, University of Denver, Denver, Colorado 80208 Received: January 8, 1993
NFCl radicals were isolated in low-temperature argon, krypton, and xenon matrices. The matrices were produced by passing the NFClz/rare gas mixture through a stainless steel metering valve and depositing the effluent from the valve onto a cold (10 K) KC1 window. Infrared spectra of the matrices were measured, and absorbances at 720 and 917 cm-l were assigned to the N-C1 and N-F stretches of NFCl, respectively. The N-Cl peak was split into two peaks, with a 3:l ratio as expected for the 35C1 and 3’Cl isotopic components. The normalcoordinate analysis for NFCl is also reported.
Introduction Free radicals are important intermediates in a variety of chemical processes. Spectroscopy is the usual method of choice for following their formation and destruction in reactive systems but is, of course, only feasible if spectroscopic information is availablefor the free radicals. The halogen aminyl radicals (NXZ and NXY, X,Y = halogen atoms) are a particularly interesting class of free radicals, but very little spectroscopic information is available for them. The reaction of NFz with hydrogen atoms is strongly constrained by rules governing the conversion of spin angular momentum and generates NF(alA) with near unit efficiency.’ This specificity may also exist for other aminyl radicals, as suggested by several recent s t u d i e ~ , and ~ , ~ understanding such state-specific reaction systems is of fundamental interest to chemists. Halogen aminyl radicals have been postulated as important species in a variety of processes involving the halogen amines, for example, in their sometimes explosive decomposition,4.5 in their reactions with atomic specie^,^^^^^ and as products of halogen amine The dihalo carbenes CC12, CFz, and CFCl have been isolated in low-temperature matrices where they were generated photolytically from the parent compounds which had been trapped in the matri~.~-IzIn earlier studies carried out in our laboratory, however, we had observed that photolysis of NFClz and of NC13 isolated in low-temperature argon matrices generated NF and NC1, respectively, and no NFCl or NClZ was observed.13J4 Consequently, a different technique for isolating these radicals was required. Low-temperature matrices containing NClz were successfullyproduced by passing NC13in argon through a heated stainless steel metering valve and depositing the effluent on a cold (10 K) KCl window.15 Infrared and UV absorption spectra were obtained for the matrix-isolated NC12. For the experiments presented and discussed in this paper, the same general technique was employed in the expectation that NFCl would be generated upon passing NFClz in argon through a heated stainless steel valve. A new species, identified as NFC1, was deposited in the matrix using this technique, and its infrared spectrum and normalcoordinate analysis are reported.
Experimental Section The synthesis of NFC12has been published.I6 Briefly, a 30.5cm by 1.5-cm (id.) stainless steel reactor tube filled with clean, dry copper shot, NH4C1, and NaCl is heated to 60 OC. A 20% mixture of Fz in helium (Spectra Gases) is passed through the reactor, and the effluent from the reactor is passed through a Teflon trap immersed in liquid Nz. The NFC12 is separated from the other products of the synthesis (NFzCl, HF, and Clz) by vacuum distillation techniques.) A clean 2-L bulb is filled with a few tenths of a Torr of NFClZ and either argon (99.999%, 0022-365419312097-4331%O4.OO/O
General Air Service and Supply), krypton 99.997%, Spectra Gases), or xenon (99.996%, Spectra Gases). NFClz to inert gas ratios were 1:1000for theseexperiments. UV and infrared spectra of the gas mixtures were taken to verify the purity of the samples, and the only measurable impurity in some samples was SiF4, as indicated by its IR absorbance at 1030 cm-I.l7 Matrices containing NFCl and NFClz (and presumably C1 atoms) were prepared by passing NFClz in Ar, Kr, or Xe through a stainless steel metering valve (Whitey, SS-21RS4). The valve was heated by wrapping heating tape around the valve, and the temperature of the valve was monitored with a J-type thermocouple. The effluent from the valve was deposited on a cold KCI window mounted in a RMC Cryosystem Model LTS-22 cold head system. The temperature of the KCl window mount was maintained at 10 K, and the pressure in the cold head was (3.0 f 1) X lod Torr. FTIR spectra of the matrices were obtained with a Bomem Model MBlOO (resolution 1 cm-I). The rate of deposition was monitored by following the rate of increase of the strong NFClz peaks. Matrices with the highest NFCl to NFC12 ratios were obtained when the NFClz peak grew in at about 0.01 absorbance unit per 5 min. We found that the amount of NFCl deposited in these matrices was very sensitive to the condition of the metering valve and that cleaning the valve between runs ensured that significant amounts of the NFCl would be generated. A normal-coordinate analysis program called “Norcord” was supplied to us by Professor Stewart J. Strickler, Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO. This program is a version of the normal-coordinate analysis programNORCRD QCPE PROGRAM 176 written by William D. Gwinn, Department of Chemistry, University of California, Berkley,California, that had been modified by Professor Strickler for use on an IBM-PC computer. The loosely focused output from a Xe lamp (PTI, 150 W) was used to photolyze the matrix-isolated species. The light from the Xe lamp was filteredwith a water-filled cell (10 cm long) equipped with quartz windows.
Results Infrared Spectra. From the results of our previous experiments in which NClz was deposited in an argon matrix by passing NC1) in argon through a stainless steel metering valve,15 it was anticipated that NFCl would likewise be generated from NFClz at the metal valve and deposited in the matrix. Although no information exists for NFC1, the N-F and N-Cl stretching modes in NFCl were predicted to lie between the known asymmetric stretching vibrational frequencies in NC12 (679 cm-1)15-18 and NFz (930 cm-1).19920Therefore, the infrared spectra of the matrices generated were examined for any new spectral features in this energy range. 0 1993 American Chemical Society
Zarubaiko and Gilbert
4332 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993
I
i
I IB
.3 1
I
I
I
I
I 1
1
900 800 700 GOO Waveiiuinbers (cm? Figure 1. FTIR absorptionspectra of low-temperature matrices prepared by passing NFCI2 in an inert gas through a heated metering valve: (A) NFCI/NFCl2 in argon matrix, T,,I,, = 150 OC; (B) NFCl/NFCI2 in krypton matrix, T,,I,, = 130 "C; (C) NFCI/NFC12 in xenon matrix, Tvaive= 130 "C.
1000
Infrared spectra of matrices prepared by passing a mixture of NFC12 in argon, krypton, and xenon through the heated metal metering valve are shown in Figure 1, A, B, and C, respectively. The presenceof NFCl2 is apparent in the three spectra by infrared peaks at 8 16 ( u I ) , 686 (ug), and 612 cm-1(v2).I3 The spectra are, however, dominated by peaks at 917 and 720 cm-I. The NFClz peaks and the new peaks appeared at the same frequencies in the argon, krypton, and xenon matrices. Infrared spectra of "blank" matrices which had been prepared by passing pure argon through the heated valve contained no peaks and verified that the species responsible for the new peaks required the presence of NFClz as well as a heated valve. The new peaks could not be assigned to any known candidates that may have been formed at the metal valve or that may have been present as contaminants in the gas samples (for examples, NF,21NC1,21ClF,22N2F2,23324 N2F4,25 NF2Cl,13J6 and NF327). Therefore, the possibility that the 917-cm-1 peak was the N-F stretch in NFCl and the 720-cm-1 peak was the N-Cl stretch in NFCl was examined. Because of the difficulties associated with the synthesis of I5NFCl, its spectrum was not measured. If, however, the 720-cm-I feature is the N-Cl stretch, then 35C1-37C1isotope splitting should be observed for this peak, and according to normal-coordinate calculations, the splitting should be 2-3 cm-I. Upon expanding the 70&750-~m-~region, a second peak at 717 cm-1 becomes obvious. This expanded region is shown for an argon matrix in Figure 2A and for a krypton matrix in Figure 2B. The intensity ratioof the 720-cm-I peak to the 717-cm-I peakis 3:1, as expected for the chlorineisotopes. The 720-cm-l peakis, therefore, assigned to the N-Cl stretch, and the 917-cm-I peak is assigned to the N-F stretch in NFC1. These assignments are consistent with the normal-mode analysis discussed below. Even with the metal metering valve at room temperature, some NFCl was deposited in the matrix for all of the matrix gases used. After a few minutes of deposition time, however, the NFCl peaks stopped growing in, and the NFCl2 peaks began to dominate the IR spectrum. This same effect was observed in the NC12matrix experiments and implies that the valve becomes passivated. A
750 740
730 720
710 700
Wavenumbers (cm.') Figure 2. FTIR absorption spectra of the chlorine isotopic splitting in the N-CI stretching frequency for the NFCl radical in (A) an argon matrix and (B) a krypton matrix.
significant increase in the NFCl to NFCl2 ratio was observed upon heating the valve, and 130-150 OC was found to be the optimum temperature range for the generation of NFCl. At temperatures lower than 130 OC, the NFCl to NFClz ratio decreased, and at temperatures above 150 OC, lower amounts of both NFCl and NFCl2 were deposited. Several experiments were carried out to examine the effect of the NFCl2 to argon ratio and of the deposition rate on the decomposition. These studies showed that the higher the NFCl2 concentration, or the higher the deposition rate, the lower the NFCl to NFC12 ratio deposited in the matrix. Consequently, the concentration, the deposition rate, and the valve temperature are all important in determining the optimum conditions for generating the aminyl radicals. Broad-band photolysis experiments were carried out on argon matricescontainingNFClandNFC12. No changes wereobserved in the NFCl peakintensities, even after several hours of irradiation. The NFC12 peaks, however, decreased as a result of the irradiation, and a peak at 1113 cm-I (NF) appeared. Normal-Mode Analysis of NFCl. The general valence bond (GVB) force constants (for an asymmetric triatomic system) were computed, and with an assumed geometry, used to calculate the Urey-Bradley force field (UBFF)28 force constants. A reiterative process was implemented in which the UBFF force constants were recalculated from the difference between the calculated and observed normal-modefrequenicesuntil agreement was reached between the calculated and observed values. The initial geometry of the NFCl radical was chosen on the basis of the related species. The average of the published bond angles in NCl2 and NF218v20was used as the initial NFCl bond angle. The NFCl N-Cl bond length was calculated from the N-Cl bond lengths in NOCl, NOC12, and NC13 by multiplying the N-Cl bond length in NC13 by the ratio of the N-Cl bond lengths in NOCl and NOC12.29,30The N-F bond length was assumed to be the bond length in NF3.Z7 The initial N-Cl and N-F force constants (&XI and KN-F) were calculated from the IR absorbances at 720 and 917 cm-I. Since no absorbance corresponding to the NFCl bend has been
Infrared Spectroscopy of NFCl Radicals observed, the average of the known u2 for NF2 of 573 cm-l and an estimated u2 for NCl2 of 320 cm-I was used to compute u2 for NFCl and the bending force constant (Il).25J5 The NFCl u2 was treated as an adjustable parameter, and the value that gave the best agreement between the observed and the calculated frequencies was 341 cm-I. The dependence of the calculated frequencies on the bond angle was also investigated, and the best agreement between calculated and observed frequencies was obtained with a bond angle of 114 f 2O. The validity of the final values was investigated by testing the sensitivityof the NFCl u2 frequency to small changes in the UBFF force constants K N ~and I F N Z ~ ,Small ~ . changes in these force constantsshould cause smallchangesin both u3 (the N-Cl stretch) and u2, and this was observed. The validity was also verified via a symmetry analysis of the normal modes as discussed in ref 3 1. The final values for the bond lengths, the bond angle, the vibrational frequencies, and the force constants for N F W l and NF3'C1 are listed in Table I. It is of interest to note that the chlorine isotope shift in NFCl is less than what would be expected for a diatomic N-Cl in this frequency region and demonstrates that the fluorine atom is active in the v3 vibrational mode.
Discussion In the low-temperature argon matrix studies with NC13 and NFC12, a Teflon metering valve had been used to admit the gases to the cold head vacuum chamber, and no indication of any decomposition was ~ b s e r v e d . ~Consequently, ~J~ even though the process taking place at the metal metering valve is not understood, it is clear that for both NC13 and NFC12 a valve with metal surfaces is required for the formation of the aminyl radicals. In both the NC13 and NFCl2 experiments, warming the valve increased the amount of aminyl radical deposited, but more rigorous heating is required in theNFC12experiments. Matrices containingNFC1 with no NFC12, however, have not been achieved, whereas in the NCl3 experimentsmatrices with NC12and no NC13 weregenerated at relatively mild temperatures (75 "C). These observations are all indicative of the fact that the NFC12 is more stable than NC13 and is, therefore, less likely to undergo decompositionat the metal valve than NC13. In addition to the N-F and N-C1 stretching frequencies observed in the infrared spectrum of NFCl, several other features appeared consistently and can be identified as belonging to NFC1. The small peak at 903 cm-I is assigned to an alternate matrix site for the N-F stretch in NFCl. This assignment is made since although its height relative to the 917-cm-' peakvaried when the deposition conditions were changed, the ratio of the sum of the peak heights for the 903- and 917-cm-I relative to the 720-cm-1 peak height remained constant. When the region between 700 and 750 cm-I is expanded, in addition to the 35C1-37C1isotope splitting observed for the N-Cl stretch of NFC1, several peaks appear to the blue of the 720-cm-I peak. Of these, the peak at 728 cm-I is assigned to NFCl aggregates since this feature dominated the 720-cm-l peak under conditions for which aggregation would be expected (fast deposition, high NFCl2 concentrations, warming of the matrix, etc.). The results of the photolysis experiments on the NFCl/NFC12/ Ar matrices reported here are consistent with the results observed in NFC12/Ar photolysis studies, NClJAr photolysis studies, and NC12/Ar photolysis studies.'3-'5 No change was observed in the NC12peaks upon photolysis of NCl2 in argon matrices, whereas photolysis of NC13/Ar and NFC12/Ar matrices generated NCl and NF, respectively. We assume that if NFCl has dissociated or predissociated states in the ultraviolet as has been indicated for both NClz and NF2,14J2then the matrix cage effect prevents photofragmentation. Bond lengths listed for several N-F and N-Cl containing compounds in Table I1 show little variation, with an average N-C1 bond length of 1.74 and an average N-F bond length of
The Journal of Physical Chemistry, Vol. 97, No. 17. 1993 4333 TABLE I: Geometry, Force Constants, and Calculated Frequencies for NFCl angle = 114O KNXI= 3.48 mdyn/A H(knd)= 1.00 (mdyn A)/rad2 KN-F= 3.89 mdyn/A N F W NF3'CI
YI YI
= 918.2 cm-I, = 917.6 cm-I,
FN-F,knd FN-Cl,knd F(N-F)(N-cI) u2 u2
= 1.33 (mdyn rad)/A = -0.09 (mdyn rad)/A = 0.10 mdynlA
= 341.4 cm-I, = 336.9 cm-I,
u3 u3
= 720.2 cm-I = 717.8 cm-I
TABLE Ik Bond Lengths, Selected Force Constants, and Stretching Frequencies for Fluorine/Chlorine Containing Radicals and Molecules compound NClj"*' NC1zd NFClf NF2CIu
