Spectra and Photochemistry of Trifluoronitrosomethane Adsorbed on Alkali Halide Films

Abstract

Trifluoronitrosomethane (CF 3 NO), 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 CF 3 NO. A photodimer of CF 3 NO 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 {(CF 3 ) 2 NONO} 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.

Spectra and Photochemistry of Trifluoronitromethane Adsorbed on Alkali Halide Films

“…The apparent irreversibility beyond fragmentation allows the scheme to be simplified: where k p represents a second-order rate constant for product formation. This equation is similar in form to that derived in this group to describe the photochemistry of adsorbed CF 3 NO, where, once the second-order rate constants have been converted to their first-order equivalents, the quantum efficiency may be expressed as The value of k p may be obtained from the second-order constant obtained by Sugawara and co-workers 20 ( k = 2.5 × 10 -11 cm 3 molecule -1 s -1 ): taking the reaction volume to be that of one adsorption cell (2.24 × 10 -23 cm 3 molecule -1 ), yielding a k p of 1.1 × 10 12 s -1 . We may use the values of k g from ref , (0.3−3) × 10 12 s -1 , since the rate of recombination should not differ much for CF 3 + NO or NO 2 .…”

“…The experimental setup has been described in a previous paper . The salt film was deposited on a window of the same alkali halide (NaBr was deposited on a NaCl window), maintained at 77 K via a Displex closed-cycle refrigerator.…”

“…Alkali halide surfaces have been among the most widely studied of the dielectric substrates, owing to their transparence over a large portion of the spectrum, their well-characterized structures, and their ability to physisorb molecules without undergoing chemical reaction. Much of the experimental , and theoretical − emphasis has been on molecules adsorbed onto LiF (001), although sublimated films of alkali halides have also been used. − Previous studies from this laboratory have demonstrated an alkali halide film's ability to quench the 600−700 nm photolysis of adsorbed trifluoronitrosomethane (CF 3 NO) …”

“…Much of the experimental 1,2 and theoretical [3][4][5] emphasis has been on molecules adsorbed onto LiF (001), although sublimated films of alkali halides have also been used. [6][7][8][9][10] Previous studies from this laboratory have demonstrated an alkali halide film's ability to quench the 600-700 nm photolysis of adsorbed trifluoronitrosomethane (CF 3 NO). 10 The trifluoronitromethane (CF 3 NO 2 ) molecule has not received the same degree of attention as its nitroso analogue.…”

“…[6][7][8][9][10] Previous studies from this laboratory have demonstrated an alkali halide film's ability to quench the 600-700 nm photolysis of adsorbed trifluoronitrosomethane (CF 3 NO). 10 The trifluoronitromethane (CF 3 NO 2 ) molecule has not received the same degree of attention as its nitroso analogue. The molecule's infrared spectrum has been observed and characterized, 11,12 although there have been differing estimates of the barrier to internal rotation, and hence, torsional frequency, arising from a coupling between the torsional mode and other low-frequency vibrations, presenting different apparent barriers to microwave and electron diffraction measurements.…”