J. Phys. Chem. 1996, 100, 15539-15550
15539
Spectra and Photochemistry of Trifluoronitrosomethane Adsorbed on Alkali Halide Films Leanna C. Giancarlo,† Brendan C. Haynie,‡ Kevin M. Miller,§ James M. Reynolds,⊥ Janine M. Rusnock, and Christopher A. Baumann* Department of Chemistry, The UniVersity of Scranton, Scranton, PennsylVania 18510-4626 ReceiVed: April 8, 1996; In Final Form: July 15, 1996X
Trifluoronitrosomethane (CF3NO), which is readily photolyzed in the gas phase when exposed to light in the 600-700 nm portion of the spectrum, does not undergo significant photolysis when irradiated while adsorbed on sublimated films of alkali halides (NaCl, NaBr, KCl, KBr) at temperatures of 10-100 K. This quenching does not occur when the molecule is irradiated while adsorbed on argon films (thickness > 0.1 µm) or in multilayer coverage. Comparison of the quantum efficiencies for photolysis indicates that vibrational relaxation induced by surface phonons is primarily responsible for the alkali halide quenching, but lateral diffusion of the photofragments and/or excited-state molecules may also play a role in the kinetic scheme. The photochemistry was followed using infrared spectroscopy, providing the first vibrational characterization of adsorbed sub-monolayer CF3NO. A photodimer of CF3NO was also observed and vibrationally characterized on the argon films and on the overlayers. The NO stretching frequency for the dimer is significantly lower than had been previously reported for the N-nitritoamine photodimer {(CF3)2NONO} and may indicate the existence of a second photodimer species. Adsorption potentials were calculated for various sites on the four alkali halide films. The lowest energy orientation in each case had the C-N bond parallel to the surface, aligned in the [110] direction. The vibrational spectra and thermal desorption curves are consistent with the calculated energies.
Introduction The photochemistry of molecules possessing the trifluoromethyl (CF3) moiety has been the topic of much study over the past 40 years. The role of the CF3• radical in the depletion of the ozone layer has promoted renewed activity toward understanding the mechanisms of the formation of this radical and its subsequent reactions. The contribution of airborne sodium chloride particles to the atmospheric content of chlorine has been debated heatedly.1 These particles are unlikely to find their way into the stratosphere, and represent an unlikely source of chlorine for the production of compounds which may find their way into the stratosphere, but might have an effect on the atmospheric lifetimes of important species which may be adsorbed onto the particle surfaces, by altering photochemical pathways in an active (photoreaction of substrate and adsorbate) or a passive sense (altering photophysical rate constants).2 The electronic and vibrational spectra of trifluoronitrosomethane (CF3NO) have been characterized in the gas and condensed phases3-9 and in inert gas matrices.10-14 The blue color of the gas is attributed to a weakly allowed transition to the A1A′′(nπ*) state at 690 nm.6 Facile decomposition of the compound occurs from this state in the gas phase,5 presumably through intersystem crossing to the corresponding triplet state (1130 nm),7 although there has been indication that internal conversion into the ground state may also play a role.15 The energy distribution into the CF3 and NO photoproducts has been well *Corresponding author. E-mail:
[email protected]. FAX: (717) 941-7510. † Present address: Department of Chemistry, Columbia University, New York, NY 10027. ‡ Present address: Department of Chemistry, University of Virginia, Charlottesville, VA 22901. § Present address: A.E. Kirby Memorial Health Center, Wilkes-Barre, PA 18701. ⊥ Present address: McNeil Consumer Products, Fort Washington, PA 19034-2292. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01051-9 CCC: $12.00
characterized.16-19 Kinetics of the recombination of CF3 and NO have been studied as well.20-22 The technique of salt film spectroscopy, where a sublimated film of an alkali or alkaline earth halide is used as a substrate for the spectroscopic study of an adsorbed species, has its roots in experiments performed by de Boer over 60 years ago.23 Advances in the method have been effected more recently by Folman and co-workers,24-26 Heidberg, Hartmann, and coworkers,27,28 and Ewing and co-workers.29,30 Advantages found in this method are the high surface area of the sublimated film and the transparence of the film throughout a large spectral range. Disadvantages in the method result from the inhomogeneities and corrugation of these films, broadening spectral features to ∼10 cm-1 width at half-height. The process of annealing these films helps to overcome this, by allowing the high energy sites to relax to more stable, hence less perturbing, arrangements. Annealing has also been shown to reduce film porosity, which can produce hysteresis in observed isotherms.26 Annealed films have been shown to consist largely of cubic crystals 100 nm on an edge, with the exposed faces being the (100) plane.31 The photochemistry of molecules adsorbed onto such films has been studied, yielding a variety of results. Dunn and Ewing observed that energy transfer from a triplet self-trapped exciton in NaCl crystallites may be efficiently transferred to a nearresonant triplet state of adsorbed acetylene.32 Berg and Ewing found that ketene’s photodissociation may be enhanced or quenched upon adsorption, depending on the electronic state into which the molecule is excited.33 Richardson was able to observe secondary reaction products as a result of photolysis of adsorbed acetone,34 demonstrating the mobility of adsorbed fragments. In this paper, we report the results of our investigations into the photochemistry of CF3NO adsorbed onto sublimated films of NaCl, NaBr, KCl, and KBr. The lack of observed S1 photochemistry is indicative of surface-induced quenching, © 1996 American Chemical Society
15540 J. Phys. Chem., Vol. 100, No. 38, 1996
Figure 1. Schematic of the experimental apparatus. The external NaCl windows constitute the infrared optical path, and the internal window contains the sublimated film.
caused by relaxation of the S1 species to highly excited vibrational levels of the ground- or triplet-state species, followed by rapid vibrational relaxation mediated by the surface. To determine the degree to which the phonons of the substrate affect the quenching process, S1 irradiation of CF3NO adsorbed onto multilayers of argon deposited on the alkali halide surfaces was attempted, leading to the observation of an adsorbed photodimer of CF3NO.35 Experimental Section The cryostat used in these experiments was a Displex CS202 closed-cycle cryogenic refrigerator (APD Cryogenics, Inc.). The temperature was monitored and controlled by a Scientific Instruments, Inc. 9600-1 temperature controller, using a tipmounted silicon diode as a sensor. The temperature was controllable to (0.1 K throughout the region of interest (10150 K). A cylindrical alkali halide window (Harshaw 1904) was held in an OFHC copper holder (an indium wire gasket between the holder and window improved the thermal contact), mounted onto the OFHC copper tip. The adsorption chamber is shown schematically in Figure 1. The film was sublimated from a chromel coil contained in a furnace attached to the cryostat. Crystals of the alkali halide, either cleaved from used windows or grown from aqueous solution and dried, were placed in the chromel coil, which was held in place with ceramic supports. External windows on the adsorption chamber were NaCl cylindrical windows (Harshaw 2504), held in place with a layer of high vacuum resin sealant (Huntington Laboratories, Inc., VS-101). Vacuum was maintained in the system by means of an oil diffusion pump (CVC VMF 10), trapped with liquid nitrogen and backed with a two-stage rotary roughing pump (Welch 1405). The adsorption chamber was maintained at 310 K when not refrigerated, to minimize adsorbed condensables and to keep the external windows from clouding. As the film deposition began, the coil was resistively heated using a potentiostatic constant voltage source (McKee-Pederson MP 1026). As the voltage was slowly increased, pressure bursts arising from the outgassing of the crystals were noted. To minimize the trapping of these impurities under or inside the film, the refrigerator was not turned on until the outgassing was unobservable. The refrigerator was then set to 77 K for the film deposition. The system pressure was measured to be about 2 × 10-6 Torr, as measured by a Bayard-Alpert-type ionization gauge (Granville-Phillips 260 with 274-002 tube), throughout the deposition. The voltage across the coil was gradually (over several hours) increased to a terminal voltage of 6.0 V. After
Giancarlo et al. the deposition was complete, the cryostat shroud was rotated 90° so that the inner window was perpendicular to the optical path of the spectrometer in use (this is the orientation shown in Figure 1). The film was annealed by setting the temperature to 270 K, allowing it to stabilize at that temperature for a few minutes, then bringing it back down to the predetermined temperature for adsorption. The films produced were transparent, with a slight bluish haze. Spectra in the infrared were acquired on a Perkin-Elmer model 1310 spectrometer, subsequently on a Mattson Galaxy 5022 Fourier transform infrared spectrometer (DTGS detector). Background spectra of a clean film (after annealing, before adsorption) were acquired in the 4000-600 cm-1 range with the 1310, 4000-500 cm-1 for the 5022 (all FTIR spectra were obtained as 32 scans at 1 cm-1 resolution). The CF3NO was obtained from SCM Corp. Specialty Chemicals, Gainesville, FL. The sample of gas to be used in a particular experiment was withdrawn from the lecture bottle and retained in an opaquely coated glass vacuum flask. Samples stored in the flask over longer periods developed NO impurities, which were removed by condensation of the CF3NO in the coldfinger of the flask and pumping on the condensate. The normally 0.1 µm) to shield the adsorbate from the underlying NaCl. The quantum efficiency on argon at 12 K (Φ ) 0.18) indicates that the relaxation rate is significantly lower, and the surface mobility significantly higher, on argon as compared with the alkali halides. Even at the lowest temperatures (12 K), the photodimer production efficiency still exceeds that of the high-temperature alkali halide surface. Given the small inherent energy of the argon phonon, surface-induced relaxation processes should be slower than their alkali halide counterparts and comparable to those found in argon matrices (lifetimes less than 1 µs12). The quantum efficiency (Φ′) is given by
Φ′ )
kp′kf kp′kf + kp′kr′ + kr′kg
(15)
where the primed quantities represent those found on the argon surface (the fragmentation rate constants should change negligibly going from one surface to another, and the recombination rate constant should follow the functional form described in the alkali halide case). The photochemical rate constant (incorporating the surface concentration of CF3NO, which still remains relatively constant under low fluence and high coverage conditions) then becomes
kp′ )
kr′kgΦ′ kf(1 - Φ′) - kr′Φ′
(16)
Using the 12 and 28 K data from the argon surface and the above parameters in eq 16, kp′ was found to range from 6 × 108 to 2 × 1010 s-1. A forthcoming paper will examine the temperature dependence of the photochemistry of CF3NO adsorbed on argon and other rare gas surfaces.50 Since the argon data show no readily discernible temperature dependence, an average argon value (7 × 109 s-1) was used for kp in the alkali halide calculations. Another approach to estimating kp on the alkali halide surfaces would be to calculate the barrier to intersite translation of the
fragments, assuming that an Arrhenius-type expression governs fragment mobility:32
kp ) νe-(Ea/RT)
(17)
where ν is a calculated external mode frequency for motion along the unit cell diagonal, serving as the pre-exponential term, and Ea is the calculated activation energy for leaving the site. Our calculations for CF3NO on NaCl yield a barrier of 2.7 kJ mol-1 and a pre-exponential factor of 3 × 1011 s-1 . From these, kp should vary from less than 1 s-1 at the lowest temperatures to over 1010 s-1 at 100 K, assuming the radical fragments translate in a manner similar to that of the parent species. Thus, the transfer of the argon kp average value to the alkali halide case is reasonable at the higher temperatures and probably results in an overestimation of kp at the lower temperatures. In the alkali halide and argon substrate studies, kp should be less than kg, since it is much easier for the radicals to combine at the site than it is for them to leave the site for reaction. Another attempt to pin down the contributions of the various steps of the process involved the irradiation of overlayers of CF3NO on a NaCl window without a film. The lack of a film left little discernible sub-monolayer species, and the difference in the spectrum going from sub-monolayer to overlayer made it relatively easy to ascertain that the species undergoing irradiation was overlayer CF3NO. This scenario should maximize the rate constant for product formation, since each set of radicals is already on top of an unreacted, most likely groundstate, CF3NO. Assuming that the activation energy to product formation is zero (the fragments do not have to leave the site to find a CF3NO), the pre-exponential term may be estimated from the corresponding kg value. There are 35 different collisions a CF3• can make with a CF3NONO radical neighbor, but only one of these collisions will produce the dimer directly. We would expect the ratio kg/kp to be approximately 0.03, making kp equal to 2 × 1010 s-1, and the parameters governing fragmentation to remain unchanged. The rate constants for relaxation at 633 and 670 nm are then calculated from the observed quantum efficiencies to be (0.5-2) × 107 s-1, within the range established for the minimum rate constants for relaxation on the alkali halide substrates. The photophysical parameters derived using the above equations may be found in Table 10. The possibility of forming the classical C-nitroso photodimer38 opens the door to a simpler kinetic scheme: ke
kp
} CF3NO* 98 products CF3NO {\ k
(18)
r
since the formation of this photodimer does not necessarily depend on fragmentation. Equation 12 would simplify to
kr ) kp
1-Φ Φ
(19)
where kp, the rate constant for product formation, will incorporate the probability that an excited-state molecule will find an adjacent ground-state neighbor. We see no evidence of van der Waals surface dimerization, leading us to presume that the dimerization occurs after excitation, rather than before. In a similar fashion, eq 16 becomes
Φ′ kp′ ) kr′ 1 - Φ′
(20)
Spectra and Photochemistry of Trifluoronitrosomethane
J. Phys. Chem., Vol. 100, No. 38, 1996 15549
TABLE 10: Derived Photophysical Parameters for Adsorbed CF3NO: Fragmentation Model substrate
temp (K)
NaCl
wavelength (nm)
Φ
kfa (s-1)
kg (s-1)
kp (s-1)
kr (min) (s-1)
670 633 670 633 670 633 633 633 633 633 670 633 670 633 670 633
