656
J. Phys. Chem. 1988, 92, 656-660
Overtone Vibrational Photochemistry of Quadricyclane David G. Lishan,*t K. V. €teddy,* George S. Hammond, and Jack E. Leonard! Allied-Signal Corporation, Morristown, New Jersey 07960 (Received: December 15. 1986; I n Final Form: August 26, 1987)
The photochemistry of quadricyclane (Q) was explored by single-photon excitation to high vibrational levels. Spectra of the u = 4-7 carbon-hydrogen overtones were recorded by using intracavity absorption and photoacoustic detection. These spectra were compared to the infrared fundamental spectrum and assigned. Excitation of the v = 5 and v = 6 bands of cyclopropanoid and methylenic hydrogens leads to reaction. At least one intermediate, probably a vibrationally excited form of norbornadiene (N), must be involved because partitioning among various reaction channels is pressure dependent. Apparent rate constants were measured and correlated with variations in pressure according to the Stem-Volmer relationship. Experimental values were compared with rate constants calculated by RRKM theory. Although there is a modest amount of Uexcess" reaction observed for the lowest excitation energies above threshold, overall evaluation provides no significant evidence for concentration of energy in localized modes for times that are long compared with reaction times.
SCHEME 1 Introduction An important question in reaction rate theory asks the extent to which reaction rates can be influenced by preparation of molecules with a nonrandom distribution of internal energy among /\ 'n / CPD A their vibrational modes. The question is only meaningful when the molecular energies are above the threshold for some reaction. Such nonrandomly excited molecules may be prepared by multiphoton excitation, by internal conversion of electronic to vibrational excitation, or by chemical activation. None of these 1 methods deposits vibrational energy in precisely known patterns. Berry and Reddy' observed that samples of compounds containing T C H bonds could be excited in a one-photon intracavity laser process to relatively high vibrational levels. The narrowness of norbornadiene (N) are shown in Scheme I. The isomerization the absorption 6ands indicates that the excitation is deposited in of Q to N and the isomerization and fragmentation of N are all relatively pure, localized modes. substantially exothermic processes. Yet, examination of highly vibrationally excited polyatomic There are three kinds of C H bonds in Q which are disposed molecules indicates that, even between such "local" modes, courather differently with respect to the bonds in the ring-opening pling can strongly affect the intramolecular energy dynamic^.^^^ reaction which produces N. In particular, stretching of the four More recent spectroscopy of C H fundamental and overtone transitons have provided additional insights into intramolecular (1) Reddy, K. V.; Berry, M. J. Chem. Phys. Lett. 1977, 52, 111. rovibrational redistrib~tion.~-~ They have described resonant (2) Heller, D. F.; Mukamel, S. J . Chem. Phys. 1979, 70, 463. coupling between modes and a relaxation time of the C H (3) Sage, M. L.; Jortner, J. Adu. Chem. Phys. 1981, 47, 293. stretching overtones (from bandwidth studies) of less than (4) Perry, J. W.; Moll, D. J.; Kupperman, A,; Zewail, A. H . J . Chem. s.4 Phys. 1985, 82, 1195. (5) Dubal, H.-R.; Quack, M. J . Chem. Phys. 1984, 81, 3779. Still, a natural question is whether or not such relatively (6) Baggott, J. E.; Chuang, M.-C.; Zare, R. N.; Dubal, H. R.; Quack, M. mode-specific excitation energy can be selectively coupled to J . Chem. Phys. 1985,82, 1186. molecular reaction coordinates with a subsequent enhancement (7) Peyerimhoff, S.; Lewerenz, M.; Quack, M. Chem. Phys. Lett. 1984, of rates of molecular reaction.+" A more subtle, but equivalent, 109, 563. (8) Sibert, E. L.; Hynes, J. T.;Reinhardt, W. P. J . Chem. Phys. 1984,81, question is whether or not the selectively deposited energy may 1135. be exceptionally unavailable to the reaction coordinate. (9) Baggott, J. E.; Law, D. W.; Lightfoot, P. D.; Mills, I. M. J . Chem. Reported photochemical reactions12 induced by vibrational Phys. 1985,85, 5414. overtone excitation include isomerizations (methyl'J3 and ally114 (101 Uzer, T.;Hvnes, J. T.Chem. Phvs. Lett. 1985, 113, 483. isocyanides to nitriles, cyclobutene to 1 , 3 - b ~ t a d i e n e , ~1~ ' ~ ~ ' ~ (11) Holme, T. A.; Hutchinson, J. S: J . Chem. Phys. 1985, 83, 2860. (12) Crim, F. F. Annu. Reu. Phys. Chem. 1984, 35, 657. and cyclopropylcyclobutene to 2-cyclopropyl-1,3-butadiene,17 (13) Reddy, K. V.; Berry, M. J. Faraday Discuss. Chem. Soc. 1979, 67, 2-methylcyclopentadiene to 1-methylcyclopentadiene'*)and 188. tert-butyl hydroperfragmentations (tetramethyldi~xetane,~~ (14) Reddy, K. V.; Berry, M. J. Chem. Phys. Lett. 1979, 66, 223. (15) Jasinski, J. M.; Frisoli, J. K.; Moore, C. B. J . Chem. Phys. 1983, 79, oxide,20-22and hydrogen p e r ~ x i d e ~ ~The , ~ ~results ). are generally 1312. close to predictions from ergodic behavior. However, Berry and (16) Baggott, J. E. Chem. Phys. Lett. 1985, 119, 47. ReddyI4 noted that there was evidence of nonequivalence among (17) Jasinski, J. M.; Frisoli, J. K.; Moore, C. B. J . Phys. Chem. 1983, 87, the three kinds of C H bonds in the in the photoinduced isomer3826. (18) Jasinski, J. M.; Frisoli, J. K.; Moore, C. B. J . Phys. Chem. 1983, 87, ization of allyl isocyanide. Zare and co-workers2' concluded that 2209. the rate of redistribution of OH vibrational energy in excited (19) Canon, B. D.; Crim, F. F. J . Chem. Phys. 1980, 73, 3013. West, G. tert-butyl hydroperoxide molecules is only competitive with the A.; Mariella, R. P.; Pete, J. A,; Hammond, W. B.; Heller, D. F . J . Chem. rate of 0-0 bond breaking. Phys. 1981, 75, 2006. (20) Rizzo, T.R.; Crim, F. F . J . Chem. Phys. 1982, 76, 2754. We have chosen to study the photochemistry of quadricyclane (21) Chandler, D. W.; Farneth, W. E.; Zare, R. N. J . Chem. Phys. 1982, (Q; Figure 1). The thermal reactions of Q and the isomeric
g3
~~~~
'Current address: ECE Department, University of California at Santa Barbara, Santa Barbara, CA 93106 'Current address, AMOCO Research, Napierville, IL 60566 *Current address: Indianapolis Center for Advanced Research, Inc , Indianapolis, IN 46204
0022-3654/88/2092-0656!$01 S O / O
77, 4447. (22) Chuang, M.-C.; Baggott, J. E.; Chandler, D. W.; Farneth, W. E.; Zare, R. N. Faraday Discuss. Chem. SOC.1983, 75, 301. (23) Rizzo, R.; Hayden, C. C.; Crim, F. F. J . Chem. Phys. 1984, 81, 4501. (24) Scherer, N. F.; Doany, F. E.; Zewail, A. H.; Perry, J. W. J . Chem. Phys. 1986, 84, 1932.
r.
0 1988 American Chemical Society
Photochemistry of Quadricyclane HC
\
/HC
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 657 TABLE I: Assignments of Quadricyclane CH Overtone Spectra' obsd freq
=5
bond
u
=O
u=4
u
CHa CHb CHC
3087 3065 2949
11613 11492 10985
14237 14070 13434
mech u
=6
16688 16574 15689
freq (A)
anharmonicity ( B )
3149 3123 3082
-6 1 -6 1 -66
'All data are in wavenumbers, cm-I.
Quadricyclane
Figure 1. Quadricyclane structure and C H labeling sequence.
basal CH bonds will move the hydrogen atoms in the general direction in which they are displaced in the ring-opening reaction. Primitive physical intuition suggests that such a motion would be more likely to couple directly with the reaction coordinate than would the other CH stretches.
Experimental Section Materials. Quadricyclane was synthesized by the triplet.sensitization method of Hammond, Turro, and F i ~ c h e r *and ~ purified by repeated bulb-to-bulb distillation. Methane and Gold Label argon were used as supplied by Matheson Chemical Co. Spectroscopy. The gas-phase fundamental vibrational spectrum was recorded with a Perkin-Elmer Model 283 infrared spectrophotometer. The C H overtone spectra for v = 4-7 (resolution < O S A) were obtained by photoacoustic detection of the absorption of dye laser radiation, using the intracavity mode described by Reddy and Berry.I3 Peak absorption cross sections for the v = 5 and v = 6 bands were measured with methane as an internal standard to calibrate the photoacoustic detector response?6 The contribution of reaction exothermicity to the acoustic signal was neglected because negligible isomerization occurs at the pressures used for these measurements (about 100 Torr). Zrradiationr. Irradiations were performed in a Pyrex cell (length = 26.3 cm, volume = 32.2 cm3) fitted with quartz Brewster angle windows and placed in the dye laser cavity. An MKS capacitance manometer with a 10-Torr differential pressure head was used to measure sample pressures (fO.O1 Torr). Irradiations varied in duration between 0.25 and 2.5 h, depending on the absorption cross section and the intracavity photon intensity. Conversions ranged from 3% to 48%, but most were about 10%. Products were analyzed by direct gas injection into a Hewlett-Packard Model 5840A gas chromatograph fitted with a 3 ft X 0.25 in. bentone-packed glass column at 35 O C . Peak areas (f5%) were obtained by digitizing the strip-chart record with a Hewlett-Packard Model 9830A desk computer equipped with a Model 9864A digitizer. The response of the flame ionization detector was assumed to be proportional to the number of carbon atoms. Thus, the ratio of peak area to moles was assumed to be larger by a factor of 3.5 for A than for the C7products or starting material. Since the CPD:A ratio was consistently less than the expected stoichiometric ratio of unity (possibly due to cyclopentadiene dimerization), the amount of retro-Diels-Alder fragmentation was monitored by the amount of A alone. It is conceivable that Q might isomerize to N on the chromatographic column, even at the low temperature used, so each product analysis was paired with an analysis of an unreacted sample of Q, and the relative quantity of Q was used. No evidence for this information was found, even during repeat injections of t h e same sample.
RRKM Calculation. A general R R K M program developed by Hase and Bunker2' was used to calculate the statistical rate constants. A cyclopropanoid C-C stretching frequency (925 cm-l) (25) Hammond, G. S.; Turro, N. J.; Fischer, A. J. Am. Chem. Soc. 1961, 83, 4674. (26) Giver, L. P. J. Quant. Spectrosc. Radiat. Transfer 1978, 19, 311. (27) Hase, W. L.; Bunker, D. L. Program QCPE-234, Quantum Chemistry Program Exchange, Bloomington, IN, 1968. Sums and densities of states were calculated with the Whitten-Rabinovitch semiclassical algorithm.
n
vCH=6
VCH
vo = 16688
=5
vo=14237)
4 Frequency ("1)
Figure 2. C H overtone spectra of quadricyclane.
was chosen as the critical frequency to simulate a "norbornadiene-like" transition state. The reported thermal activation parameters (E, = 33.5 kcal/mol, A = s-l) were used.% The peaks observed in the fundamental infrared spectrum (cm-I, s = strong absorption) are 3098, 3087 (s), 3078, 2949 (s), 2876 (s), 1255, 1247 (s), 1235, 955, 922, 912 (s), 900, 850, 811, 801 (s), 787, 770 (s), and 760.29 The RRKM calculation used the following distribution of frequencies (cm-I) for the ground state: 4 at 3087, 2 at 2949, 2 at 2876, 10 at 1250, 6 at 925, 4 at 775, 4 at 600, 2 at 500, and 1 at 450. For the transition state, two of the 1250-cm-' bands were raised to 1350 cm-I and two more lowered to 800 cm-'.
Results Overtone Spectroscopy. The infrared spectrum in the C H region for the fundamental and the third through sixth overtones are shown in Figure 2 and summarized in Table I. The peaks in the fundamental spectrum are centered at 3087,2949, and 2876 cm-ISz9 The highest energy peak is assigned to the six cyclopropanoid hydrogens (CH" and CHb in Figure 1) while the other two peaks are the asymmetric and symmetric stretches of the methylene group (CHC).30*31 (28) Frey, H.M. J . Chem. SOC.,Faraday Trans. 1964, 3 6 5 .
(29) The C H frequencies do not include the 3050-cm-l band reported by Dauben and Cargill [Dauben, W. G.; Cargill, R. L. Tetrahedron 1961, 15, 1971 which has been used by other groups for the identification of this compound. Dr. C. Pouchert of the Aldrich Chemical Co. reports (personal communication) that the bands which they use for Q identification include the CH stretching frequencies (3096, 2950, and 2874 cm-l) and a skeletal frequency (1244 cm-I). These bands are in good agreement with the values we found but do not agree with the earlier workers. (30) Bellamy, L. J. The Infra-Red Spectra of Complex Molecules, 2nd ed.; London: Methuen, 1958; p 13. (31) Gordon, A. J.; Ford, R. A. The Chemist's Companion; New York: Wiley, 1972; p 187.
658 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Lishan et al.
TABLE II: Ouadricyclane Kinetic Data" slope x loi2, transition CH", u = 5 CHa, u = 6 CHC,u = 5 CHC,u = 6
wavelength, nm 702.0 598.9 744.4 637.4
intercept, s 710 f 290 5960 i 620 3750 f 670 10800 f 3400
k(E)/k,
X lO-I4, molecules cm-3 5.7 f 2.4 55.2 i 5.8 7.9 f 1.5 11.8 i 4.4
s cm3 molecule-l
1.24 i 0.06 1.08 f 0.01 4.77 i 0.39 9.12 & 0.92
OError limits shown are one standard deviation. bObtained by k(E)/kd assuming kd = 4.2
The assignment of the two types of cyclopropanoid carbonhydrogen stretches was not possible in the fundamental because of the high degree of overlap. However, they were better separated in the overtone spectra. Their assignment was made by comparison with the fourth overtone spectrum of norbornadiene in which the observed absorptions are centered at 13 540, 13 820, and 14 800 cm-I and are assigned to the methylene, bridgehead, and olefinic hydrogens, r e s p e c t i ~ e l y . ~The ~ methylenic absorption in the spectrum of Q is slightly lower (13 430 cm-I) while the bridgehead (CHb) has moved higher to 14070 cm-I. The assignment of the large "local-mode" cyclopropanoid peak to the basal hydrogens (CHa) is consistent with the ratio of basal to bridgehead hydrogens. The overtone spectra are not as clean as those observed in some molecules.33 There is considerable substructure in each of the assigned groups, and the spectra are broader than in earlier examples. The structure of the band groups probably contains information about the coupling of C H stretching motions with skeletal vibrations: but the data are insufficient for full analysis. For each C H absorption, we have assigned the energy maximum in each overtone spectrum as E,. These values were fitted to the Birge-Sponer equation [E, = v ( A + vB)] to give the mechanical frequencies ( A ) and anharmonicity constants ( B ) listed in Table I. Photochemistry. The v = 5 and v = 6 transitions for the CHa and CH' stretches involve excitation energies greater than the activation energy for the ring-opening reaction, Q N.34 The thermodynamics of this reaction are well-known, and the kinetics of the thermal cracking (retro-Diels-Alder) and isomerization are also known.35336 The potential surface for these reactions is shown schematically in Figure 3. An isolated molecule with enough internal energy to surmount the first barrier would also be above the second barrier, so the cracking and isomerization products from N should also be observed. Because the bond-breaking and -making processes required to convert N to the ultimate products are fairly complex, we might expect that vibrationally excited N would retain its identity long enough to be quenched to the ground state by collision, and such is the case. Thus, an increase in reaction pressure decreases the total product yield by quenching Q* and increases the fraction of N in the product mix by quenching N * . The reaction kinetics for Q are derived from
-
k(E)
2.4 23.2 3.3 5.0
cm3 molecule-l
X
X 10-5.bSKI
quantum vield (6) 1.8 9.7 1.5 10.8
s-l,
-
300 6
r
-
u -
E
-7
5
iIf
VCH
Products
9-
-
m
7-
599 nm
200-
z
$ w
100
0
I
CPD+A
-
i
CHT+T
1
Reaction Coordinate
Figure 3. Energetics of quadricyclane reactions. AH, and E, are the reaction enthalpy and activation energy, respectively. AHR,Q = -105 kJ mol-', E,,Q = 140 kJ mol-', = -54 kJ mol-', and Ea,N= 212 kJ mol-'. C H stretch vibrational ladders for Q and N are shown. Also portrayed is the absorption of a 598.8-nm laser photon which excites the C H a bonds. Not shown is E, = 218 kJ mol" for fragmentation of N to CPD and A.
r 0
1x1016
2x1016
[ Quadricyclane] (molecules c ~ n - ~ )
Q-Q* Q*
k(E)
P
Figure 4. Reciprocal rate constant vs quenching pressure for total product formation from quadricyclane at 598.9 nm (CHa, v = 6).
(3) where P is the products (N, A + CPD, CHT, and T), CT = absorption cross section, Io = input photon intensity, and M is a quencher of vibrational excitation. Application of the steady-state assumption to [Q*] gives the rate eq 4 for the formation of products, where 6 is the quantum
yield in the absence of a quencher. We fitted the experimental rate data to eq 4 using the initial pressure as the measure of [MI. This treats all C7Hs species as equivalent and neglects the fragmentation to CPD A which produces two smaller molecules from one C7 species. Since the fragmentation products never constituted more than a few percent, this approximation produces no significant error. With very low absorption cross sections, the reaction becomes first order in Q.
(32) Reddy, K. V.; Lishan, D. G. unpublished spectra. (33) Wong, J. S.; Moore, C. B. J . Chem. Phys. 1982, 77, 603. (34) Wiberg, K. B.; Connon, H. A. J . Am. Chem. SOC.1976,98, 541 1 and references therein. (35) Woods, W. G. J . Org. Chem. 1958, 23, 110. (36) Rocquitte, B. C. Can. J . Chem. 1964, 42, 2134.
Figure 4 shows a typical plot of eq 7, the Stern-Volmer relationship, and Table I1 lists the intercepts, slopes, derived rate constants, and quantum yields. The latter were estimated by using
Q*
+ M .A. Q + M
+
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 659
Photochemistry of Quadricyclane /
TABLE 111: Rate Constant Ratios for the Appearance of Products and the Ratio of Observed Products k ’ ( ~ ) / k , ’x 10-15, x,/(x, transition wavelength, nm molecules cm-3 XCHT)’ CHa, u = 5 702.0 1.05 h 0.07 1.9 i 0.5 CH”, u = 6 598.9 8.0 + 1.0 1.9 f 0.7 CHC,u = 5 744.4 0.62 f 0.07 1.1 i 0.5 CHc, = 6 637.4 3.0 f 0.2 2.5 f 0.5
+
N P’
i!i222I
0
1 .ox 10’6
“X,= mole fraction of y. SCHEME I1
2.0x 10’6
[ Quadricyclane] (molecules ~ m - ~ )
Figure 5. Pressure and wavelength effects of quadricyclane photochemical branching ratio, NIP’.
the extracavity laser power, output coupler transmission, and absorption cross sections. The estimated values are all greater than 1, the expected value for a unimolecular reaction under collision-free condition. Because of experimental uncertainties, especially in estimating cross sections by comparison with methane, the values obtained in excitation to the two u = 5 levels are certainly within experimental error of the expected results. The same may be true for measurements with u = 6 because the absorption intensities are very low, increasing the uncertainty in the cross sections. However, the values of about 10 are large enough to evoke some reservation in the interpretation of the results. Conceivable explanations might attribute the excess rate to a thermal reaction or to a photoinitiated chain process. The former explanation is rendered highly improbable by numerous control experiments showing that no detectable reaction occurred when the laser was off. Although zero-order chain reactions are known, they are not common, and the rates of mast chain processes increase with increasing pressure, whereas we observed linear quenching behavior. Estimates of the relative and absolute values of the first-order reaction rate constant k ( E ) are derived by dividing the slopes of the Stern-Volmer plots by the intercepts, and these should be independent of the quantum yields and of any pressure-independent nonphotochemical reaction. The value of kd was taken to be the calculated gas kinetic collision rate at 298 K for the purposes of calculating k(E).j7 Estimation of the product distribution as a function of pressure showed that the fraction of N in the products increases regularly as the pressure is increased. This suggests that N* be treated as a second, discrete, quenchable intermediate, as described by
Q
N
P’
isomerization averaged over a series of pressures at fixed wavelength. This ratio is the photochemical branching ratio, and within the experimental error,38 there is no signficant variation as a function of excitation wavelength. This is consistent with, but does not compel, the conclusion that it is a single distinguishable excited species, P*, that partitions among the various secondary reactions by processes not influenced by quenching. Quenching by Inert Gas. We attempted to quench the reaction with added argon to explore the possibility of a selective quenching mechanism. As explained above, we expect that Q and the other seven-carbon products would behave simply as a collisional quencher without preferential enhancement or deactivation. By running the reaction under conditions where the principal quenching molecule was argon, we were able to test this expectation. These experiments were done at one wavelength, 598.9 nm, and at a constant Q pressure of 0.05 Torr. At least a tenfold excess pressure of argon was employed in each experiment to compensate for its relatively lower vibrational quenching efficiency. Modification of the previous mechanism yields
where kk is the deactivation rate constant for argon quenching. A linear least-squares fit of the plot of l/kapp against [MI (=[Qo] [Ar]) yielded a value for (kd[Qo] k A , ) / k ( E ) . From this we calculate a relative quenching effectiveness, kA,/kd= 0.1 f 0.05. This value is consistent with other studies examining the quenching of vibrationally excited species with inert gases.39 For the argon-quenched system, the branching ratio value [A]/( [TI + [CHT]) is within the range of values observed for the system when Q is the quencher. Both of these results suggest that Q is not imparting any selectivity on the quenching process.
+
+
Plots of the data are shown in Figure 5, and the values of k i / k ’ ( E ) calculated by linear regression analysis are summarized in Table 111. Equation 8 indicates an excited species, P*, decaying to products other than N via fragmentation (appearance of A) and isomerization (appearance of T .and CHT) reactions. This mechanism implies a constant ratio for [A]/([T] + [CHT]). The last column of Table I11 shows the ratio of fragmentation. to
Discussion Product Distribution. We find that vibrationally excited quadricyclane maintains its identity long enough to be intercepted by quencher molecules and deactivated to ground-state quadricyclane. There is evidence that this hot quadricyclane decays to an excited, norbornadiene-like species, as indicated in Scheme 11. A kinetic scheme that includes this intermediate explains the increasing preponderance of norbornadiene in the product mix at high pressure. In the limit of zero quencher concentration, the Q P conversion is a maximum, yet no N is produced (Figure 5). This result indicates the importance of the hot norbornadiene intermediate in controlling the overall gas-phase reaction. The negligible intercepts in Figure 5 reveal that the unimolecular decay rate for the norbornadiene-like intermediate t o ground-state
(37) The Stern-Volmer plots of l/kappvs [ Q ] are linear in the pressure range studied. We calculate a collision rate of 4.2 X cm3 molecule-’ s-l. At the highest pressure used (1 Torr) in the pure quadricyclane experiments, this translates to a collision time of approximately 75 ps. This is considerably slower than the predicted
