J. Phys. Chem. 1996, 100, 12305-12310
12305
Direct Measurements of Removal Rates of CFCl(X ˜ 1A′(0,0,0)) and CFCl(X ˜ 1A′(0,1,0)) by Simple Alkenes Jose´ A. Ferna´ ndez, Roberto Martinez, Marı´a N. Sa´ nchez Rayo, and Fernando Castan˜ o* Departamento de Quı´mica Fı´sica, UniVersidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain ReceiVed: August 7, 1995; In Final Form: March 22, 1996X
Direct measurements of the rate constants for the collisional removal of CFCl(X ˜ 1A′(0,1,0)) and CFCl1 (X ˜ A′(0,0,0)) by ethylene (C2H4), propene (C3H6), 1-butene (1-C4H8), isobutene (i-C4H8), and 1,3-butadiene ˜ 1A′(0,n,0)) species were prepared by infrared multiple photon dissociation, (C4H6) are presented. CFCl(X IRMPD, of the CHFCl2 precursor and their population time evolution followed by probing the transition ˜ 1A′(0,n,0)) with a tunable dye laser. Removal rate constants of CFClCFCl(A ˜ 1A′′(0,1,0)) r CFCl(X (X ˜ 1A′(0,1,0)) by ethylene and propene, independent of CO2-laser fluence, have been measured as follows: 0.85 ( 0.06 and 1.18 ( 0.09 (in units of 10-12 cm3 molecule-1 s-1). Removal rate constants by 1-butene, isobutene, and 1,3-butadiene are found to be fluence dependent with threshold fluence limits of 1.30 ( 0.17, ˜ 1A′(0,0,0)) the removal 1.60 ( 0.18, and 0.70 ( 0.07 (in units of 10-12 cm3 molecule-1 s-1). For CFCl(X rate constants for ethylene and propene, and those dependent on CO2-laser fluence, namely, 1-butene, isobutene, and 1,3-butadiene, are found to be as follows (in units of 10-13 cm3 molecule-1 s-1): 3.4 ( 0.5, 2.2 ( 0.3, 1.6 ( 0.4, 0.9 ( 0.2, and 2.5 ( 0.5. A discussion of the influence of the intermolecular potential well depths of the reactants and their vibrational spacing on the removal rates is also presented.
Introduction Singlet carbenes undergo bond insertion and cycloaddition1,2 on reaction with various molecules. Kinetic experiments of broadband photochemically produced CTF (T ) tritium) with olefins leading to stereospecific addition and with hydrogen halides where bond insertion was established were reported in past years.3-6 In particular, reactions of CHF with alkenes were shown to proceed through the cycloaddition channel.7 Empirical correlations based on thermochemical properties have helped to characterize the philicity (electrophilic, ambiphilic, and nucleophilic) behavior of carbenes, while frontier molecular orbital theory provided the framework for rationalizing and predicting the experimental kinetic results. Developments in preparative organic chemistry, computational facilities, and progress in instrumentation, including the extensive use of lasers together with modern electronics and stimulated by a wealth of applications to atmospheric chemistry, astrophysics, and dry semiconductor etching, have sustained and enhanced the interest in carbene chemistry in the past few decades.1-4,8-15 The preparation and measurement of relative kinetic removal rates of CFCl in its ground state were first made by reacting CF2dCFCl with atomic oxygen.16 Improvement in state selective ground state measurements was accomplished by using infrared laser-induced multiple photon dissociation (IRMPD) of precursors such as CF2dCFCl, CFCldCFCl, and CHFCl2.17-20 Vibrational, rotational, and translational energies of the fragment CFCl have been characterized recently.21,22 Direct measurement of removal rates of CFCl(X ˜ 1A′(0,0,0)), referred to henceforth as either CFCl(X ˜ (0,0,0)) or CFCl(X ˜ ) according to convenience, with other species17-20,22 have been reported in the past few years. In this paper a study of the removal of vibrationally excited transient species CFCl(X ˜ (0,1,0)) with some selected simple alkenes and Ar is presented. The steady increase of rate constants with molecular size and, more specifically, with the * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
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potential energy well depth suggests cycloaddition as the preferred reaction channel. The large differences between the CFCl bending mode energy and the normal vibrational modes of the alkene indicate V-V energy transfer to be very inefficient. Reactions of ground CFCl(X ˜ (0,0,0)) follows a similar pattern of behavior although with lower rate constants. ParmenterSeaver plots and the empirical vibrational efficiency parameter have been used to rationalize the results. Experimental Section The experimental arrangement to characterize the stateselected fragments and to measure the removal rates has been described previously22-24 and is briefly summarized here. CHFCl2 dissociation was effected by means of a TEA-CO2 laser (Lumonics K-103) set at the line P(10) of the (0,2°,0)-(0,0°,1) band of CO2 (λ ) 9.47 µm). The beam was focused to a ≈1.5 mm average diameter spot into the stainless steel sample cell center. The average fluences employed were varied in the range from 80 to 43 (or 36) J cm-2. These values should be taken as relative averaged fluences to allow the determination of the rate constant at threshold. The sample cell was filled with the precursor CHFCl2 (26.7 µbar), buffer gas (Ar, 1.33 mbar), and the selected reactant alkene partner at variable pressures (usually in the range up to 267 µbar). The precursor pressure was selected as a compromise between obtaining sizable fluorescence signals and low noise background due to luminescence (caused either by collisional dissociation of the precursor molecules following infrared laser multiple photon absorption or by reaction/energy transfer of MPD transients with or to the added gases). State-selected CFCl(X ˜ (0,1,0)) was probed by a Nd:YAG/dye laser coupled system (Quantel 581 and Datachrom) using Exalite 398 and Exalite 389 dye to pump the CFCl(A ˜ (0,n,0)-X ˜ (0,1,0)) transition, where the experimental intensity factor is high.21,22 Spectra were recorded with a delay between the photodissociation and the probe lasers of 3 and 10 µs. Observation of the emission was made through filters GG435 (6 mm) and GG400 (3 mm) with a fast side-on photomultiplier (Hamamatsu R928). © 1996 American Chemical Society
12306 J. Phys. Chem., Vol. 100, No. 30, 1996
Ferna´ndez et al. diffusion method27 and found to be close to 300 K, rather smaller than the vibrational temperature (750 K). Results and Discussion Removal rates for ground and first vibrationally excited CFCl in buffer Ar were determined by following the time evolution of the initially generated population at 389.5 (A ˜ (0,1,0)X ˜ (0,0,0)) and 397.45 nm (A ˜ (0,1,0)-X ˜ (0,1,0)). The removal rates at buffer pressures up to 8 mbar were fitted to the following equation:28
Figure 1. Relative population yields of the CFCl(X ˜ (0,0,0)) fragment in the IRMP dissociation of the precursor CHFCl2 as a function of the CO2-laser fluence (λ ) 9.47 µm) and the argon pressure, PAr. The fluence threshold for production of CFCl is 33 ( 3 J cm-2. Similar behavior is found for vibrational states (0,1,0) and (0,0,1).
Crossed laser beams (in ethylene and propene samples) and collinear configurations were used to improve the fluorescence signal-to-noise level. Time-dependent signals were captured with a digital oscilloscope (Tektronix TDS 520) IEEE interfaced with a PC computer, where time-resolved removal rates were obtained by nonlinear fitting and, alternatively, integrated between selected times to simulate a boxcar integrator, so that the laser induced fluorescence (LIF) spectra were obtained by scanning the laser wavelength. In both cases, care was taken in the subtraction of background luminescence signals. The laser resolution used for either still or time-resolved measurements was ∆λ ) 0.16 cm-1. Alkenes were subject to freeze-pump-thaw cycles in a vacuum manifold to remove impurities. CHFCl2 was used as commercially available. Mixtures of each precursor, buffer gas, and the alkene partner at selected pressures (as measured with a capacitance manometer, Datametrics 10-0.001 Torr (1 Torr ) 1.3332 mbar)) were homogenized in a spherical bulb by magnetic stirring for a minimum of 30 min before flowing into the reaction vessel. All experiments were carried out at room temperature. Chemicals used were as follows: CHFCl2 (>98.0 vol %, Aldrich), Ar (99,9995%, SEO), C2H4 (99.7%, Matheson), C3H6 (99.5%, Matheson), 1-butene (99.0%, Matheson), isobutene (99.0%, Matheson), and 1,3-butadiene (99.7%, Matheson). Preparation and Identification of the Fragments The LIF spectrum of transient molecules generated by high intensity IRMPD of precursor CHFCl2 in argon (26.7 µbar/1.33 mbar ratio) at room temperature and 10 µs delay between dissociation and probe lasers, at wavelengths in the range from 389.5 to 397.5 nm, has been reported elsewhere.21 The spectrum ˜ (0,1,0)-X ˜ (0,1,0) transition of is readily assigned21,22 to the A the CFCl transient. CFCl(X ˜ ) is a bent molecule (101.6°) with a CF stretch vibrational mode at 1181.5 cm-1 (in Ar) and a bending mode in Ar of 448 cm-1.25 The triplet state of CFCl (a˜ 3A′′) has an electronic energy determined as T0 < 5140 cm-1 above the ground level,26 and, presumably, it does not affect or perturb the first (and second) vibrational state. The presence of Ar buffer gas in the photolyzed sample influences strongly the population of ground and first vibrational states of CFCl created in the IRMPD process. As an example, Figure 1 shows the behavior of the ground vibrational state of CFCl. The threshold for dissociation in our system appears at a fluence of about 33 J cm-2. The translational temperature of the fragments in the IRMPD has been determined by the
k ) β′/PAr + k1[CHFCl2] + kQ[Ar] + k0
(1)
where the first term on the right hand side corresponds to diffusional loss, k1 and kQ are the overall absolute second-order rates for the removal of CFCl by CHFCl2 and by Ar, and k0 ()1/τ) is the inverse lifetime of the state considered, which is negligible for CFCl(X ˜ (0,0,0)). CHFCl2 removes CFCl22 (bond insertion) relatively efficiently and, in the plot of k versus [Ar], shifts the rate constant upward by a constant quantity for a fixed pressure of precursor. Finally, β′ includes the diffusion coefficient21,22 of CFCl in Ar together with the appropriate boundary conditions of the diffusion equation. Removal rate constants and diffusion coefficients for CFCl(X ˜ (0,1,0)) have been reported elsewhere22 (kQ ) (1.73 ( 0.22) × 10-14 cm3 molecule-1 s-1; β′ ) 4.2 ( 0.5 bar s-1). Accurate determination of removal rates and diffusion coefficients for ground state CFCl(X ˜ (0,0,0)) in Ar are experimentally difficult on account of the luminescence generated in samples with Ar pressures higher than 13.3 mbar. A rough comparative estimation yields kQ )
