102
J. Phys. Chem. 1988, 92, 102-106
of the Langevin capture cross section. Perhaps the primary effect of increasing the total energy of the complex is to diminish the time available to carry out difficult bond insertion steps. In the context of Figure 4, this would mean that k, increases more rapidly with total energy than does k,,,,.
Summary and Conclusion The study of reaction rates in 0.75 Torr of He provides a different perspective on M+ + alkane reactivity than previous single-collision experiments. The association efficiencies k / k L measured at fixed H e density are sensitive primarily to the duration of binary M+-alkane collision. It appears that long-lived M+-alkane collisions are a necessary but not sufficient condition for the occurrence of single-collision elimination chemistry. The M+ ions that react the slowest at 0.75 Torr of He (Cr', Mn', Zn') yield no single-collision elimination products at thermal collision energy, but some of the fastest reactions at 0.75 Torr (Cu+ with C2H6 and C3H8;Co' and Ni+ with C,H,) also fail to eliminate. The branching between redissociation, elimination, and collisional quenching gives important new insights into the time scale of these steps for each M+-alkane pair. In order to understand the detailed pattern of reaction rates across the first transition series, we must invoke both metal-dependent size effects on long-range repulsive forces and metaldependent chemical interactions. The Sc', Ti', and Fe' results highlight the crucial role of potential energy surface intersections in determining the outcome of long-lived M+-alkane collisions. When all of the important orbitals lie at sufficiently low energies, the availability of a low-lying state that can preserve total spin during bond insertion may well be the most important factor governing singlecollision elimination cross sections at low kinetic energy. Orbital selection rules seem less important for the exothermic alkane reactions than for endothermic reactions with H2,6 4s doin accord with the lower symmetry. Favorable uCH nor-acceptor interactions may provide the dominant long-range
-
attractive force between linear alkanes and transition metal ions, including the inert ions Cr+, Mn', Cu', and Zn'. The richness and selectivity of the M' reactions are related to subtle changes in the size and relative energy of the important 3d and 4s orbitals across the first transition-metal ion series. We emphasize that the similarity of the rate constant pattern across the metal series for CH,, C2H6, and C3H8argues against direct insertion of M+ into C-C bonds as the initial step of the complicated M+-alkane interaction, even in those cases of eventual CH4 elimination. However, we still have little insight into why Ti+ and V+ preferentially activate C-H bonds while Sc+, Fe+, Co+, and Ni' activate both C-H and C-C bonds. When interpreted with the simple quenching model, the 0.75-Torr product branching ratios and rate constants indicate that elimination steps occur much more slowly for the latter ions. Our qualitative arguments about the nature of M+-alkane interactions have necessarily been speculative. However, chemical, photochemical, and spectroscopic experiments will test these ideas in the near future. Further ab initio calculations on the nature of low-energy paths to H-H and C-H bond insertion will provide important guidance as our understanding of metal ion chemistry develops further.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the National Science Foundation, Grants No. CHE-8302856 and CHE-8703076, for partial support of this research. R.T. thanks the Wisconsin Alumni Research Foundation for graduate fellowship support. We thank Professors Peter Armentrout and Jack Beauchamp for communication of their results prior to publication. Registry No. CH,, 74-82-8; C2H6, 74-84-0; C,Hs, 74-98-6; Sc', 14336-93-7;Ti', 14067-04-0;V', 14782-33-3;Cr', 14067-03-9;Mn*, 14127-69-6;Fe', 14067-02-8;Co', 16610-75-6;Ni', 14903-34-5;Cu', 17493-86-6;Z n ' , 15 176-26-8;He', 7440-59-7.
Infrared Laser Induced Multiphoton Dissociation of Decafluorocyclopentane in a Concerted Pathway: Time-Resolved Evidence of :CF, Formation P. K. Chowdhury, K. V. S . Rama Rao, and J. P. Mittal* Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 08.5, India (Received: January 6, 1987; In Final Form: July 1.5, 1987)
The infrared multiphoton dissociation (IRMPD) of decafluorocyclopentane (DFCP) generates tetrafluoroethylene and difluorocarbeneas the primary products. The :CFzdimerizes to form C2F4;the kinetics of the reaction is followed by monitoring the disappearance of :CFz absorption at 249 nm in real time after the COz laser pulse, with a rate constant k2 = 4.35 X lo7 M-' s-'. The vibrational temperature associated with the nascent :CF2is found to be 1100 K. The MPD yield of DFCP shows a strong fluence dependence, with a threshold of -0.5 J/cmZ for the 10 R(40) C 0 2 laser line. The MPD spectra reveal two peaks, one 22 cm-' red-shifted from the 989 cm-' strong IR absorption band. Addition of SF6 decreases the MPD yield.
1. Introduction In recent papers, we have reported concerted retro-Diels-Alder reaction of decafluorocyclohexene' and :CF2 elimination from octafluorocyclopentene.Z These unimolecular reactions of the above perfluoro cyclic olefins are examples of WoodwardHoffmann's generalized pericyclic r e a ~ t i o n . ~ Sigmatropic reaction ( 1 ) Chowdhury, P. K.; Mittal, J. P.; Rama Rao, K. V . S . J . Photochem. 1984, 24, 373. ( 2 ) Chowdhury, P. K.; Rama Rao, K. V. S.; Mittal, J. P. J . Phys. Chem. 1986, 90, 2877. ( 3 ) Woodward, R. B.; Hoffmann, R. The Conservarion of Orbital Symmetry; Academic: Yew York, 1970.
0022-3654/88/2092-0102$01.50/0
is a class of pericyclic reaction where a u-bond participation occurs in a concerted transformation process. In order to see whether similar behavior is followed in case of saturated cyclic perfluorocarbon, the IR laser chemistry of decafluorocyclopentane (DFCP) was studied and the results are reported in this paper. Perfluorinated compounds are chosen in the studies since the energy required to break a C-F bond (- 145 kcal/mol) is much higher than that required for the dissociation of a primary C-H bond (-92 kcal/mol). The multiphoton dissociation of neat decafluorocyclopentane (DFCP) in a C 0 2 laser field gives C2F4(see below) as the lone final product. By adding a suitable scavenger such as chlorine during IRMPD of DFCP, CF,CI2 was obtained as an additional 0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 1, 1988 103
IRMPD of Decafluorocyclopentane
r---7 0
0
90
X
9 W
v) J
2
a a W
a 0 1
w
* z 0
Figure 1. Schematic diagram of the experimental setup for the transient absorption studies: 1, TEA C02 laser; 2, beam splitter; 3, attenuator; 4, BaF2 lens; 5 , stainless steel cell; 6 , energy meter; 7, xenon lamp; 8, monochromator; 9, PMT; 10, Biomation digitizer/averager; 11, Y-T plotter; 12, photon drag detector.
product, indicating that :CF2 was generated as a primary product which subsequently dimerizes to give CzF4 in absence of any reactive quencher. No C3 or C, product could be identified in the irradiated sample mixture and from mass balance, >95% of the dissociated DFCP appeared as CZF4. The :CF2 formation as a product in the IRMPD of DFCP was studied by real time observation of :CF2formation and decay by its transient absorption at X = 249 nm using kinetic spectroscopy. The decay kinetics of :CF2 is in conformity with its dimerization to form C2F4as the end product.
2. Experimental Section A grating-tuned multimode TEA pulsed COz laser (Lambda Physik EMG-201E-C02) was used. A typical pulse consisted of a 100-ns spike followed by a tail of 1 ks. The experimental details are as described in ref 2. A nearly parallel beam geometry was maintained in the sample cell by condensing the laser beam with a f = 100 cm BaF2 lens and positioning the 5 cm optical path length cell intermediate between the lens and its focal point. The stainless steel sample cell employed has a volume of about 35 cm3 and was provided with end KCI windows. A conventional greaseless glass vacuum system was used for sample preparation. All the fluoro compounds used in this work were supplied by PCR Research Chemicals Inc. (Gainesville, FL). An indigenously built gas chromatograph1 with a flame ionization detector was employed for the gas analysis. The progress of the laser-induced decomposition was monitored with a Perkin-Elmer Model 577 infrared grating spectrophotometer at the DFCP absorption at 989 cm-I. The time-resolved experiments for real time observation of :CF2 species generated by COz laser irradiation were performed as described earlier., A C W spectroscopic UV-probe beam from a xenon lamp (1 50 W) with associated optics, monochromator (f= 3 . 9 , fast P M T (1P 28), and signal processing system (Biomation 4500) was part of a computer (LS1-11/23) controlled kinetic spectrometer (Applied Photophysics, UK). The gould Biomation 4500 transient digitizer was triggered by the photon drag detector (Rofin Model 7415) signal generated by the partially reflected laser beam. In our parallel beam irradiation, the UV-monitoring beam and the C 0 2 laser beam pass orthogonally (Figure 1). The geometry of excitation and monitoring beams for deriving actual optical absorption in a crossed beam arrangement has been analyzed in detaiL5s6 Since the monitoring beam diameter is somewhat larger than the IR beam dimension in the present experiments, overlap correction was made to evaluate the actual optical density of the transient species. A specially designed stainless steel cell has been made for these double beam on line IR-UV experiments. Two KCI windows were positioned 3 cm apart for IRMPE and two ~~
~
(4) Sarkar, S.K.; Palit, D. K.; Rama Rao, K. Phys. Lett. 1986, 131, 303. ( 5 ) Bebelaar, D. Chem. Phys. 1974, 3, 205.
V. S.; Mittal, J. P. Chem.
(6) Goldschmidt, C. R. In Lasers in Physical Chemisrry and Biophysics; Joussot Dubien, J., Ed.; Elsevier: New York, 1975; pp 499.
L
0
;405 0
I
I
1400
1200
1000
1
800
WAVENUMBER (Cm-')
Figure 2. (A) Infrared absorption spectra of 1.5 Torr of decafluorocyclopentane (DFCP) in the region of 800-1500 cm-' (---). (B) Infrared spectra after irradiation of 2 Torr of DFCP at 10 R(40) C02 laser line (-). (C) Multiphoton dissociation yield of DFCP per pulse ( a ) as a function of C02 laser frequency used for the irradiation (-O-O-).
quartz windows 9 cm apart were used for monitoring the UV beam as shown in Figure 1.
3. Results The infrared absorption spectrum of decafluorocyclopentane (DFCP) between 900 and 1500 cm-' is shown in Figure 2a. DFCP has a strong absorption band at 989 cm-' corresponding to the output of the C 0 2 laser. The infrared spectrum of the gas after irradiation with 10 R(40) C 0 2 laser line is shown in Figure 2b. The new absorption peaks at 1186, 1332, and 1342 cm-' are due to tetrafluoroethylene (TFE) formed as a result of laser irradiation of DFCP. No product other than TFE could be detected in these experiments. However, when the C 0 2 laser irradiation of DFCP was carried out in the presence of excess of chlorine, CF2C12was detected as a product. The details of the product identification and analysis are given in our earlier paper.2 The decomposition of DFCP was studied as a function of number of laser pulses, n. The decomposition yield per pulse, a, is given by the expression a = ( l / n ) In ([DFCP],/[DFCP],] (1) where [DFCPIo and [DFCP], are the decafluorocyclopentane concentrations before and after irradiation. 3.1. Frequency Dependence. The DFCP was photolyzed at different laser lines close to the absorption band of DFCP at 989 cm-I, at a fixed fluence of 1 J/cm2. From the results shown in Figure 2c, we find that the most efficient laser lines for inducing IRMPD of DFCP are 10 R(44) and 10 R(6). The latter is 22 cm-' red-shifted from the absorption maximum of DFCP at 989 cm-I. 3.2. Fluence Dependence. The extent of DFCP dissociation is very much dependent on laser fluence with an apparent dissociation threshold at 4 0.5 J/cm2 as shown in Figure 3. The initial steep rise of a with fluence follows a power law for a higher @, the order of the multiphoton order multiphoton process, a process n being 4.5 for this compound. At higher fluence the dissociation yield dependence on fluence is less pronounced. 3.3. Effect of Added Buffer Gas. Addition of a buffer gas (SF,) or Ar to a fixed 0.5 Torr of DFCP at the same laser fluence of 2 J/cm2 decreases the dissociation yield. TFE was observed as the lone final product even in the presence of added buffer gas. Plot of In a vs partial pressure of SF, gives a straight line as shown in Figure 4. 3.4. Time-Resolved On-Line UV Optical Probe. On laser pulse irradiation of DFCP, a long-lived transient species is generated
-
-
Chowdhury et al. - -
10-2
f
d
--
t z
W
a
J
4
-
3
a
w* !?
10
m
v)
a
__1
-
8
A 4
z 0 m
‘0-3k I
z
0 Ih
E 0 v)
m 4
13-4
II t
I
0
I
4
t
d
/ 0
i
1
6
2
FLUENCE ( J / m 2 )
1
2
3
4
5
TIME
(lis)
+
6
7
Figure 5. Growth of the optical absorption signal at 249 nm due to :CF2 produced in the IRMPD of 4 Torr of DFCP at a laser energy fluence of 2 J/cm2.
Figure 3. Dissociation yield per pulse (a)in the IRMPD of 0.5 Torr of DFCP vs energy fluence (J/cm2) at the 10 R(40) C 0 2 laser parallel
beam irradiation.
K
w
20
v)
J
4
3
a
I 0
1
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
TIME ( m S )
a w
9
---)
a
Figure 6. Decay of the optical absorption signal at 249 nm due to :CF2
n
dimerization to form tetrafluoroethylene.
J
w>
is not sufficient to completely resolve the spectrum but the general shape of the spectrum and the oscillator strength (cf. section 4.4) are in good agreement with the l i t e r a t ~ r e . ~ J ~ A few time-resolved measurements were carried out at different energy fluences of the C 0 2 laser and the dissociation yields were measured for each experiment. The concentration of :CF2in terms of pressure in Torr was calculated from the measured a as
P(SF6) / TORR
---t
Figure 4. Dissociation yield per pulse (a)in the MPD of DFCP as a function of added foreign gas (SF,) pressure at a laser energy fluence of 2 J/cm2 The partial pressure of DFCP was 0.5 Torr
which has an optical absorption in the UV region with the maximum at X = 249 nm. As shown in Figure 5, the rise of the optical absorption signal at 249 nm reaches its maximum within 5 p s after the C 0 2 laser pulse. The much slower decay of the signal at 249 nm is shown in Figure 6 at the millisecond time scale. The absorption spectrum of the transient agcees with the wellknown absorption spectrum of :CF2 A ‘B, X ‘A, band system at X = 230-275 nm.’-15 The present resolution of about 2 nm +-
where V, is the volume of the irradiation beam geometry. This was calculated by considering the beam path length within the cell as parallel when f = 100 cm BaF, lens was used and deter(7) Mathews, C. W. Can. J . Phys. 1967, 45, 2355. (8) Laird, R. K.; Andrews, E. G.; Barrow, R. F. Trans. Faraday Soc. 1950, 46, 803. (9) Tyerman, J. R. Trans. Faraday Soc. 1969, 65, 1188; 1969, 65, 163. (10) Modica, A. P. J . Phys. Chem. 1968, 72, 4594. (11) Bass, M.; Mann, D. E. J . Chem. Phys. 1962, 36, 3501. (12) Mann, D. E.; Thrush, B. A. J . Phys. Chem. 1960, 33, 1732. (13) Thrush, B. A,; Zwolenik, J. J. Trans. Faraday SOC.1963, 59, 582. (14) Dalby, F. W. J . Chem. Phys. 1964, 41, 2291 ( 1 5 ) Duperrex, R.; Van den Bergh, H J . Chem. Phys. 1979, 71, 3613.
The Journal of Physical Chemistry, Vol. 92, No. 1, 1988 105
IRMPD of Decafluorocyclopentane
SCHEME I1
/cFf
CF2-CFz
JF\
CF2
CF3
\ C2F4
+ :CFz
be expected to have low and comparable activation energies. Kinetic analysis for the competitive processes PLCF2) / T O R R
4
Figure 7. Dependence of the optical absorption at 249 nm on the concentration of :CF, radicals produced in the IRMPD of DFCP.
TF\
CF,
CF,
1
+ YF1
t
CF2-CF2
Ar
3- stable C3 products
4 8-scission
SCHEME I :CF2
'2
1 0 . 6 p CO,
:CF2 :CF2
+ +
:CF2
LASER
:CF2 C12
-
CF2Cl2
4. Discussion 4.1. Laser-Induced Chemistry. It is clear from the above results that in the laser-induced decomposition of decafluorocyclopentane (DFCP), both :CF2 and C2F4 are formed as initial primary products. The former was characterized by its transient optical absorption at time scales which are commensurate with the unimolecular dissociation of the vibrationally excited parent DFCP molecule. From the observed decay of :CF2 radicals (cf. section 4.6) and the final material balance between the DFCP decomposition and CzF4 end product, DFCP/C2F4 1:2.5, it is concluded that each decomposed parent molecule gives :CF2 and 2 C2F4 as primary products. The observed reactions are given in Scheme I. Such a novel primary reaction in which three primary products are formed in a unimolecular decomposition process is perhaps one of the very few examples of pericyclic reactions. The strong C-F bond inhibits migration of F atoms and it is likely that the laser-induced reaction proceeds in a concerted manner in which three u-bonds are broken and two H-bonds are formed in the transition state as depicted in Scheme I. The IRMP decomposition of CF3-CF=CF2 to give C2F4and :CF2has been studied by Hackett et a1.I6 and a cyclic transition state has been proposed. The decomposition of perfluorocyclobutane has also been considered to occur by a concerted pathway." An alternate mechanism for the decomposition process is a biradical formation as the rate-determining step, which is accompanied by two sequential &scission steps as illustrated in Scheme 11. The biradical mechanism would also give the same stoichiometry of (C2F4 :CF2) if the ring closure and isomerization (by 1,2 F shift) channels are not competitive with the p-scission of C, biradical. While 1,2 F shift may have large activation energy, the ring closure and fl-scission pathways can
-
+
~~~
[C2Fd
~
(16) Hackett, P. A.; Willis, C.; Nip, W. S . J . Am. Chem. SOC.1981, 103, 682. (17) Quack, M.; Seyfang, G. Ber. Bunsen-Ges. Phys. Chem. 1982,86,504.
=1+
5kdk2 + k3 [Ar])
(3) [C, products] 3klk3 [Arl for the ratio of total C2F4(from both the @-scissionsand :CF2 dimerization) and any C3 product stabilized by added argon. For k4 and k2 k3[Ar], we expect C3 product to comparable kl be about 20 to 30% of the C2F4 formed. However, no detectable amount of C3 product was observed even in the presence of 30 3 X lo7 SKI.If the biradical Torr of argon, Le., k3[Ar] k2/k3* mechanism is operative, this would imply that (1 [Ar])k4/kl > 10. The evaluation of these rate constants in terms of RRKM unimolecular reaction rate theory requires detailed knowledge of the structure of the intermediate biradicals and their energy dynamics. With proper quantitative inputs RRKM computations would be meaningful and may be carried out, as a separate study. Lack of quantitative information on the necessary thermochemical data for these transformations imposes a serious limitation on resolving the much discussed question of concerted vs stepwise mechanism. However, the absence of any stabilized C3 product in presence of large amount of argon in the above experiment appears to favor the concerted route of DFCP dissociation. 4.2. Multiphoton Dissociation Spectra. It is seen that the MPD spectra show two prominent features (Figure 2) the frequencies of which correspond to 10 R(44) and 10 R(6) C 0 2 laser lines. While the former closely follows the small signal IR absorption of DFCP, also shown in Figure 2, the molecule has relatively low absorption at 10 R(6) frequency. Such efficient excitation peaks have been occasionally observed in the MPD of various polyatomic molecules'* and have often been attributed to structure in the quasi-contin~um'~ in the multiphoton-excited molecule. Further insight into such spectral features is becoming available by twofrequency experiments such as reported by Borsella and coworkers.20s21 They suggest that intensity borrowing by combination bands near the IR-active fundamentals may be responsible for the observed MPD spectral features. 4.3. Dependence of a on Pressure. The observed decrease in the decomposition yield, a , with SF6pressure can be ascribed to collisional deactivation of vibrationally excited DFCP which is in competition with the photoexcitation and unimolecular decomposition processes. As discussed in our earlier paper2
-
c2F4
mining the beam cross section from the burn pattern of a heatsensitive paper positioned at the midpoint of the cell. The maximum absorbance at 5 ys was calculated for X = 249 nm for different pressure of :CF2 (PcF2)and a plot was obtained for optical density vs PcF2in Torr as shown in Figure 7 .
~
C2F4
leads to the expression
, . c
