J. Phys. Chem. 1995, 99, 11365- 11369
11365
Laser-Induced Emission from NFCl Radicals Isolated in Low-Temperature Argon Matrices Julanna V. Gilbert" and Gregory S. Okin Department of Chemistry, University of Denver, Denver, Colorado 80208-0179 Received: March 8, 1995; In Final Form: May 11, 1995@
The spectrum and lifetime of laser-induced emission from NFCl radicals isolated in low-temperature argon matrices are reported. The spectrum is compared to emission observed following the photolysis of gas phase NFClz, and the tentative assignment of the gas phase emission to NFCl is confirmed. The lifetime of the NFCl emission in the matrix is 610 f 40 ns. The difference in lifetimes observed in the gas phase emission and the matrix emission is discussed in terms of a mechanism involving the ability of different vibrational modes of predissociated electronic states to couple in the low-temperature matrix environment. Gaussian 92 calculations of the ground state vibrational frequencies are presented, and there is good agreement between these values and the experimental values. The N-Cl bond energy for ground state NFCl is computed from a G-1 calculation of the Mf,,of NFC1.
Introduction Halogen aminyl radicals have been proposed to be important reactive species in processes involving halogen amines. For example, when hydrogen atoms are reacted with NCl3, NC1(a'Z) and NCl(b'A) are produced, implying the presence of NC12 as a reactive intermediate.] Similarly, when hydrogen atoms are reacted with NFClz, the observed products NCl(a'Z), NC1(bIA), NF(alC), and NF(blA) strongly suggest that NFCl is involved in the mechanism.2 In another set of experiments, visible emission was tentatively assigned to NFCl and NClz following excimer laser photolysis of NFClz and NC13, respect i ~ e l y . ~In. ~the NFClz photolysis study, strong ultraviolet emission from Clz and ClF was also observed, indicating that in this molecule the photolytic process had more than a single branch, one leading to the formation of Cl2 NF and the other to the formation of C1F NCl. If the weak visible emission was indeed from NFC1, then a third branch of the photolytic process would be identified. The general lack of spectral information for these radicals has made it impossible to unequivocally confirm their presence in these systems and has been one of the motivations for the ongoing spectroscopic investigation of the aminyl radicals. A method of depositing aminyl radicals in low-temperature matrices was developed in our laboratory, and the infrared and W l v i s absorption spectra were subsequently The W / v i s absorption spectra of NFCl and NC12 display diffuse band structure with an underlying continuum, suggesting that the excited states are predissociated. The emission observed following the excimer laser photolysis of the parent amines in the gas phase was tentatively assigned to the aminyl radicals and also shows banded structure, which is much more pronounced for NCl2 than for NFCl. The emission assigned to NClz is, however, shifted approximately 16 000 cm-I to the red of the UV and is surely not from the same state as seen in the matrix absorption studies. On the other hand, the emission assigned to NFCl may well be from the same state seen in matrix absorption experments, and if so, then laser excitation into the absorption band should result in emission from NFCl radicals isolated in low-temperature matrices. The results of our LIF studies of NFCVAr matrices are presented and discussed in this paper, as well as results from Gaussian 92 frequency and AHf,,, c a l ~ u l a t i o n s .A ~ ~picture ~ of the low-lying electronic states of
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Abstract published in Advance ACS Abstracts, June 15, 1995.
0022-365419512099-11365$09.00/0
the aminyl radicals is beginning to emerge from the body of information that is now available for these interesting radicals.
Experimental Section Since the synthesis of NFCl;! and the preparation of the lowtemperature NFCl argon matrices have been reported previously in detai1,'O only a brief description of these methods is included here. To prepare NFC12, NH&1 and NaCl salts were mixed with copper shot in a 12 in. by 1 in. stainless steel tube (heated to 60 "C), and a 20% mixture of F2 in He (Spectra Gases) was passed through the tube. The effluent from the tube was passed through a Teflon U-tube submerged in liquid nitrogen, and the products of the synthesis were condensed in the U-tube. The NFClp was separated from the other products (primarily NFzC1 and Clz) via vacuum distillation. Gas bulbs containing from 0.5% to 1.0% NFC12 in argon were prepared for the matrix deposition experiments. Low-temperature matrices containing NFCl radicals were generated by passing the NFClz in argon through a heated stainless steel metering valve (200 "C) and depositing the effluent onto the cold (10 K) KCl window contained in a lowtemperature matrix isolation apparatus (Rh4C-CryosystemsLTS22 closed cycle system). The cold head was positioned in the sample compartment of a Nicolet Model 5DXC FTIR spectrometer (2 cm-' resolution) to monitor the matrix during the deposition and identify species present in the matrix. The infrared spectrum of NFCl in low-temperature matrices has been reported,6 and the presence of NFCl in the matrices prepared for this investigation was verified via the FTIR spectra. The vacuum shroud of the cold head was equipped with two 50 mm diameter KC1 windows on parallel sides through which the infrared beam traveled while the deposition was in progress. Quartz windows were installed in the remaining two sides of the vacuum shroud. After the matrix was deposited, the cold head assembly was moved to the LIF apparatus and oriented so that the laser excitation beam traveled through one of the KCl windows, and the visible emission was collected through one of the quartz windows. This arrangement was decided on because the emission was weak compared to the laser beam and the quartz windows are of higher optical quality than the KC1 windows. The matrix was irradiated with an excimer pumped dye laser (Lambda Physik, Model LTSlOOFL3001) with DMQ (Exciton) in 1P-dioxane as the laser dye. The emission was collected 0 1995 American Chemical Society
Gilbert and Okin
11366 J. Phys. Chem., Vol. 99, No. 29, 1995 by placing the entrance slit of a monochromator (PTI, 0.25 m) as close as possible to the quartz window of the vacuum shroud and coupling the exit slit of the monochromator to a cooled GaAs photomultiplier tube (Hammamatsu, R943-02). The bandwidth of the emission monochromator was 1 or 2 nm, depending on the intensity of the emission. The signal from the photomultiplier tube was amplified when necessary (Stanford Research Systems amplifier, Model SRS445) and sent to a boxcar integrator (Standford Research Systems, Model SR250) which was interfaced to a 486-DX computer. The data collection system was triggered externally with an RCA 31034 PMT which detected the laser flash. To collect time profiles of the emission, the boxcar integrator and the amplifier were replaced by a LaCroy Model TR8828C transient recorder, and the monochromator was replaced with an interference filter (A,,,= = 460 nm, fwhm = 10 nm). The time profile data were digitally stored on the 486-DX computer.
Results The UV absorption spectra of both NFCl and NFCl2 in lowtemperature argon matrices have been reported. The NFCl spectrum consists of several partially resolved bands with an average band separation of 340 cm-' and a maximum at 365 nm.' The NFCl2 absorption spectrum (which displays no vibrational structure) is shifted with respect to the NFCl spectrum with a maximum at 275 Since there is no overlap between the two absorption spectra, NFCl can be excited in the presence of the parent amine without exciting the parent. This was an important consideration since small amounts of the parent amine in addition to NFCl were sometimes present in the argon matrices (as demonstrated by the FTIR matrix spectra). For the data presented here, the excitation wavelength was 365 nm, which corresponds to both the maximum of the laser dye and of the NFCl absorption spectrum. Excitation of the matrices containing NFCl resulted in bright blue emission that was easily visible to the eye. This emission was assigned to NFCl since it was only observed when NFCl was present in the matrix, as determined by the FTIR spectrum of the matrix.6 Although the emission was easily visible to the eye and the deposition window was at 45" to both the excitation light and the emission monochromator, the matrices caused considerable scattering, and only a small portion of the emission passed through the quartz window of the vacuum shroud to the entrance slit of the monochromator. A representative spectrum of the NFCl emission is shown in Figure 1A. In contrast to the gas phase emission observed following the photolysis of NFC12 (Figure lB), the matrix emission is shifted to lower energies, and no vibrational structure is present in the matrix spectrum. The majority of the matrices studied for this investigation yielded spectra similar to the spectrum of Figure lA, with some variation as to the extent of the red tail. On a few occasions, however, the emission spectrum consisted of several resolved bands, with a band separation of 390 cm-', in agreement with the band separation of the gas phase emission. The banded matrix spectrum, shown in Figure lC, was quite weak compared to the more often observed continuous matrix spectrum, and it could not be correlated with the presence of any unusual species or aggregates in the matrix or with the manner in which the matrix was generated. Since the infrared spectra of these matrices indicated that NFCl was indeed present, it was assumed that the banded matrix emission was also from NFC1. The lack of vibrational structure in the more frequently observed spectrum of Figure 1A may be due to the fact that, in most of the matrices prepared for this study, the NFCl radicals occupy a variety of sites in the matrix with an accompanying distribution of matrix shifts.
19000 20000 21000 22000 23000 24600 25000 2 00
Wavenumber (cm-1) Figure 1. (A) Emission spectrum from NFCl radicals isolated in a low-temperature argon matrix, A,, = 365 nm. (B) Emission observed following the 249 nm photolysis of NFCll in the gas phase (from ref 7). (C) Banded emission spectrum from NFCl radicals isolated in a low-temperature argon matrix, observed on occasion, lex = 365 nm.
18000 19000 20000 21000 22000 23000 24000 25000
Wavenumber (cm-1) Figure 2. (A) Emission spectrum from NFCl radicals isolated in a low-temperature argon matrix, Lex = 365 nm. (B) Emission observed following the 249 nm photolysis of NFCll in the gas phase (from ref 7), shifted 1600 cm-' to the red. (C) Banded emission spectrum from NFCl radicals isolated in a low-temperature argon matrix, observed on occasion, Le,= 365 nm.
When the gas phase spectrum is shifted 1600 cm-' to the red, the similarity between the spectral shapes of the underlying continuum of the gas phase spectrum and the continuous matrix spectrum becomes obvious. This is shown in Figure 2A,B. In addition, there is agreement between the positions of the vibrational bands in the gas phase spectrum and the banded matrix spectrum (Figure 2B,C). The lack of structure on the red side of the maximum observed in both the gas phase and the continuous matrix spectra suggests that there is a considerable Franck-Condon shift for the excited state and that the region of the lower state accessed during the transition is close to the dissociation limit. The reason that the weak banded
NFCl Radicals Isolated in Ar Matrices
J. Phys. Chem., Vol. 99, No. 29, 1995 11367
TABLE 1: Frequencies (cm-') and Geometries (A, deg) for NFCl experimental VI
v2 v3
4N-F) r(N-C1) angle
I
a 0
300
600
900
1200
1500
1800
Time (ns) Figure 3. (A) Time profile of the emission from a low-temperature NFCVAr matrix. (B) Time profile of the light scattered from the KCl window in the cold head. (C) Time profile of light scattered from a low-temperature NFClZ/Ar matrix.
917a 390b 720"
normal-mode calculationb
Gaussian 92 calculationc
918.2 341.4 720.2 1.37 1.76 114
935.5 416.1 771.5 1.40 1.71 105.7
Reference 6. Reference 7. This work and ref 8.
The difference in the lifetime reported for the gas phase emission (
