J . Phys. Chem. 1986,90, 5695-5700 benzophenone or 2-methylbenzophenone. Thus, one cannot evaluate the values of k, and k i , too. Figure 3b indicates that the rate constant (k,) for intramolecular hydrogen abstraction decreases with the increase of u+. This is in great contrast to a similar Hammett plot for the intermolecular hydrogen abstraction of unhindered benzophenones with positive p values (-0.6),15 and Ito et al.3hgave the following interpretations. (1) The rate of intramolecular hydrogen abstraction in TIB-X is determined by the restricted rotation around the C1-C7 bond in the TIstate, and once the coplanar structure is achieved in the transition state the hydrogen abstraction will occur very easily. (2) The TI state can be stabilized by the electron-withdrawing substituent, whereas the coplanar transition state will be less affected by the substituent, being due to the deconjugation between the carbonyl group and the substituent. Therefore, the energy barrier for the hydrogen abstraction should be increased by the substituent with an electron-withdrawing character. Thus, the larger triplet decay constant of 2-methylbenzophenone (3.57 X lo8 s-’ in ethanol2) than those of TIB-X may be ascribed to the less steric hindrance to the bond rotation, as a result of less bulkiness of the methyl group than the isopropyl group. Actually, we observed the formation of the dienols upon the steady-state photolysis of 2-methylbenzophenone even at 77 K in EPA,2 while no photochemical reaction was observed upon the photolysis of TIB-X a t 77 K. Except for TIB-H, the values of kd are nearly constant (cf. Table I). Ito et interpreted this in terms of the small temperature dependence on kdrbeing due to the nonrequirement of CI-C7 bond rotation. For the compounds with type I substituents, no transient absorptions due to the 1,4-biradicals were observed. This may indicate that the intersystem crossing from the triplet TIB-X to (15) Yang, N. C.;Dusenbery, R. L. Mol. Phorochem. 1969, 1, 159. Wagner, P.J.; Thomas, M. J.; Haris, E. J . Am. Chem. SOC.1976,98,7675. Wagner, P. J.; Lam, H.M. H. J. Am. Chem. SOC.1980,102,4167.Wagner, P. J.; Truman, R. J.; Scaiano, J. C. J . Am. Chem. SOC.1985, 107, 7093.
5695
the ground state ones competes with the 1,Cbiradical formation, in accordance with the relatively larger values of k d / k ,and the relatively smaller values of @cB compared with those for the compounds with types I1 and I11 substituents. Reactivities of the 1 ,4diradicals. Table 11 indicates that the decay constants (k2)of the 1P-biradicals produced from TIB-X are much smaller than that produced from 2-methylben~ophenone~ (3.84 X lo7s-l), and the electron-withdrawing substituents appear to decrease the values of k2 one-half in going from TIB-4’-OCH3 However, their overall change to TIB-3’-C0(2,4,6-(i-Pr)3C6H2). is small compared with those produced from various types of o-alkylphenyl ketones, where the values of k2 varied by nearly 2 orders of magnitude depending on the molecular structure.’s Although the Hammett plot of k2 against a+ shows a relatively good correlation as shown in Figure 8, the effects of the substituents on these rate constants are smaller than those on the rate constants for the intramolecular hydrogen abstraction. Ito et al.3h proposed that the respective values of k2 will be of approximately the same magnitude based on the fact that the steric environment around the hydroxy group of the 1,4-biradical is considered to be the same for all TIB-X. In conclusion, we have presented that triplet 2,4,6-triisopropylbenzophenones give the 1,4-biradicals, followed by the formation of the benzocyclobutenols and the dienols by analogy to the Norrish type I1 reaction. The dienols probably reketonize to the original ketones with the rate constant of kE. For the compounds with the small quantum yields of benzocyclobutenol formation, the intersystem crossing from triplet ketones to the ground-state ones may also participate in the nonradiative processes of the lowest triplet states.
Acknowledgment. W e express our sincere thanks to Dr. Yoshikatsu Itb of Kyoto University for his gift of TIB-X. Registry No. 1, 76893-84-0;2, 84369-66-4;3, 76893-85-1;4, 33574-11-7;5,84369-67-5; 6,84369-68-6; 7,74766-25-9; 8,76893-81-7; 9. 76893-80-6.
Photolytic Production of C2H: Collisional Quenching of Emission and the Removal of Excited C2Ht
i2rI
-
S2Z+ Infrared
F. Shokoohi,*T. A. Watson,* H. Reisler, F. Kong,l A. M. Renlund,”and C. Wittig* Chemistry Department, University of Southern California, Los Angeles, California 90089-0484 (Received: January 21, 1986; In Final Form: April 10, 1986)
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We report the observation of time-resolved C2H A211 %*Z+infrared emission (1-5 pm) following the 193-nm photolyses of C2H2and C2HBr. Quenching of this emission by numerous collision partners (M) under pseudo-first-order conditions leads to large bimo)ecuiar ”rate coefficients” (e.g. >lo-” cm3 molecule-’ s-I, except when M is a rare gas or N2). Although such “rate coefficients” can be assigned to the quenching of fluorescence, they do not represent state-to-state processes, since quenching is due to an intricate combination of reactive, radiative, and energy-transfer processes. In separate experiments, rate coefficients are determined by monitoring the time-resolved CH AZA X211 chemiluminescence which is produced directly by the reaption of C2H with 02,and the C2H species responsible for the CH emissions is identified as electronically and/or vibrationally excited C2H. The above results are in agreement with recent molecular beam experiments that show that nascent C2H contains considerable internal energy following the 193-nm photolysis of C2H2.
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I. Introduction The gas-phase ethynyl radical, C2H, is an important chemical entity. This ubiquitous intermediate has provoked much study, ‘Research supported by the Chemical Science Division of the Office of Basic Energy Sciences, U S . Department of Energy. $Present address: Bell Communications Research Inc., MH7D-318, Murray Hill, N J 07974. Present address: Hughes Aircraft Co., Industrial Products Division, Carlsbad, CA 92008. Present address: Department of Chemistry, Chinese University of Science and Technology, Hefei, Anhui, People’s Republic of China. 1 Present address: Sandia National Laboratories, Division 2516, Albuquerque, N M 87175.
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even though the absence of UV, visible, and near-IR absorptions makes it difficult to directly monitor this species spectroscopically. Its presence is well-documented in several astrophysical environments,1-2 and it is an intermediate in various combustion systems and a product of the UV photolysis of acetylene and acetylenic precursor^.^" Numerous kinetics studies have relied (1) Tucker, K. D.;Kutner, M. L.; Thaddeus, P. Astrophys. J . 1974,193, L115. (2) Ziurys, L. M.; Saykally, R. J.; Plambeck, R. L.; Erickson, N. R. Astrophys. J . 1982, 254, 94. (3) Tanzawa, T.; Gardiner, W. C., Jr. J . Phys. Chem. 1980, 84, 236. (4) Irion, M. P.; Kompa, K. L. Appl. Phys. 1982, 827, 183.
0 1986 American Chemical Society
5696
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
on the detection of products from the reaction of C2H with various collision partners,"" and in recent reports,I2>l3 we discussed the X211 reaction of CzH with 02,in which nascent C H A2A emission was used to study the collisional behavior of the species responsible for the emission. In_itially,12we attributed our observation? to thermalized C2H(X2Z+),since we were confident that the X22+ state was much more populated than the A211state (whose location had not been determined experimentally at the time) under our experimental conditions. However, the removal rate coefficients obtained in these experiments were uniformly larger than those reported by other workers, whose measurements clearly involved thermalized C2H,*-l suggesting that our observations may have been due to long-lived electronically and/or vibrationally excited C2H. Ab initio calculations by Shih et a1.14J5predicted that the lowest electronically excited sta_teof CzH would be of 211 symmetry and lie -0.5 eV above the X2Z+ ground state. This state was subsequently detected by Carrick et a1.,16J7who used high-resolution color center laser spectroscopy, with magnetic rotaticn sensitivity enhancement, to assign several bands of the A211 XzZ+ system in the region 3600-4200 cm-'. They placed the electronic ozigin at 3693 cm-' and noted perturbations between the A211and X2Z+ zeroth order states: Subsequent ab initio calculations by Fogarasi et a1.18 placed the A211state -2000 cm-' above the ground state, in qualitative agreement with the previously predic_ted14s15 and low-lying excited state. Clearly, the A211 state is energetically accessible via the 193-nm photolysis of several CzH precursors. Using molecular beam methods, Wodtke and Lee have recently shown that C2H internal excitation peaks at -3700 cm-' following the 193-nm photodissociation of C2H2,l9confirming the efficient photolytic production of excited C2H. Thus, it is possible to make enough of the A211state, and/or X2Z+state with a similar energy, to account for the C H chemiluminescence that we detect in our experiments. Therefore, we now believe that our initial assignment of the CH(AZA)chemiluminescent product to reactions of thermalized C2H(X2Z+)was ! i error and that CzH(A211) and/or vibrationally excited CzH(X2Z+)are the reactive species X211 chemiluthat we monitor when detecting C H A2A minescence. In this paper, the detection of C2H A211 g2Z+ infrared spontaneous emission is reported. Time-resolved fluorescence is monitored, in order to obtain information pertinent to the removal of the excited radical by several collision partners. We have carefully established C2H(A211)as the emitting species and have prepared CzH via 193-nm photolysis using different precursors, none of which form fragments that interfere with the measurements. Adequate time and wavelength resolution eliminates the possibility of interference from reaction products. In addition, rate coefficients were also measured by detecting the time-resolved
Shokoohi et al.
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(5) Okabe, H. J. Chem. Phys. 1983,78, 1312.
(6)Reisler, H.; Mangir, M.; Wittig, C. Chem. Phys. 1980, 47, 49. (7) Cullis, C. F.; Hucknall, D. J.; Shepherd, J. V. Proc. R. SOC.London, A. 1973, 335, 523. (8) Laufer, A. H.; Bass, A. M. J. Phys. Chem. 1979, 83, 310. (9) (a) Laufer, A. H. J. Phys. Chem. 1981,85, 3828. (b) Laufer, A. H.; Lechleider, R. J . Phys. Chem. 1984, 88, 66. (10) Okabe, H. J. Chem. Phys. 1981, 73, 2772. (1 1) Lange, W.; Wagner, H. G. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 165. (12) Renlund, A. M.; Shokoohi, F.; Reisler, H.; Wittig, C. Chem. Phys. Lett. 1981, 84, 293. (13) Renlund, A. M.; Shokoohi, F.; Reisler, H.; Wittig, C. J . Phys. Chem. 1982,86, 4165. (14) Shih, S. K.; Peyerimhoff, S. D.; Buenker, R. J. J. Mol. Spectrosc. 1977, 64, 164. (15) Shih, S. K.; Peyerimhoff, S.D.; Buenker, R. J. J . Mol. Spectrosc. 1979, 74, 124. (16) Carrick, P. G.; Pfeiffer, J.; Curl, R. F., Jr.; Koester, E.; Tittel, F. K.; Kasper, J. V. V. J. Chem. Phys. 1982,76, 3336. (17) (a) Carrick, P. G.; Merer, A. J.; Curl, R. F., Jr. J . Chem. Phys. 1983, 78, 3652. (b) Curl, R. F.; Carrick, P. G.; Merer, A. J. J . Chem. Phys. 1985, 82, 3479. (18) Fogarasi, G.; Boggs, J. E.; Pulay, P. Mol. Phys. 1983,50, 139. (19) Wcdtke, A.; Lee, Y . T. J. Phys. Chem. 1985, 89, 4744.
\
BAFFLED ARJIS LESSES, FILTERS
A
PJIT o; InSh DETECTOR
h,lAf: CHAMBER
)ybFL( JlOSITOR
GAS ISLET
I
,
I
Figure 1. Schematic drawing of the experimental arrangement.
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CH AZA X2Z chemiluminescence which derives from the reaction of electronically and/or vibrationally excited C2H with O2 From the above measurements, we conclude that nascent C2H excitations are degraded efficiently by reactive and/or inelastic processes. 11. Experimental Section The experimental arrangement is shown schematically in Figure 1. Briefly, unfocused excimer laser outputs (Lumonics TE 8618-2 or Lambda Physik EMG 50, -20 m J cm-2, IO-ns fwhm) were used to phtolyze the C2H precursors in the reaction chamber; baffled arms minimized effects due to scattered laser radiation. Fluorescence passed through a CaF, window and was detected at right angles to the photolysis beam. Interference and cut-off filters provided a means of isolating specific fluorescence spectral regions. A GaAs photomultiplier tube (PMT, RCA 3 1034) was used to detect emission in the 200-900-nm region, and emission in the 1-5-hm region was monitored with a LN2 cooled InSb detector (Spectronics, photovoltaic, 1.2 cmz). The outputs from the detectors were amplified, digitized, (100 ns/channel), summed over 64-5 12 laser firings in order to achieve suitable signal-to-noise ratio (S/N), and finally plotted on an XY recorder. CzH2 (Airco), C2HBr, and C 2 H C H 0 were used as the precursors for C2H. C2HBr and C,HCHO were synthesized as described previously.12 Ethylene (Airco,>99.99%), propyne (CPL, 97%), ethane (Matheson, 99.96%), propane (Matheson, >99.96%), C02 (Airco, >99.8%), benzene (Mallinckrodt, >99%), and the C2H precursors were purified by repeated trap-to-trap distillation. O2 (Airco, 99.95%), H2 (Airco, 99.999%), CHI (Baker, 99.99%), Ar (Airco, 99.998%), N2 (Matheson, 99.99%), and He (Airco, 99.999%) were used without purification. In experiments where >10 Torr of buffer gas was used, a catalyst purifier (Matheson, G P 6406) was used. Doubly distilled H20was subjected to several freeze-pump-thaw cycles. Sample purities were periodically checked by IR spectrophotometry. Mixtures containing several gases were carefully prepared in a large Pyrex storage vessel and were passed slowly through the reaction chamber. Precursor concentrations were kept sufficiently low that pseudo-first-order kinetics conditions prevailed at all times. Experiments were usually performed at different total pressures to ensure that the measured rate coefficients are independent of the total pressure and the nature of the C2Hprecursor. Buffer gas was used to minimize diffusion of the emitting species out of the detector viewing region. Special care was taken when measuring the rate coefficient for removal by H 2 0 ; Le., the gas handling system as well as the reaction chamber were "seasoned" with H 2 0vapor prior to these measurements. 111. Results
-xZ+
C2H A211 Infrared Emission. Infrared spontaneous emission is detected following the 193-nm photolysis of CzHBr, CzH2,and CzHCHO, and the signal rise time is limited by the
Photolytic Production of C2H TABLE I: Rate Coefficients for the Quenching of C2H A211
quenching species
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5697
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g22+Emission and the Production of CH A2A
C2HBr
C2HBr
photolysis wavelength 193 nm
C2H2
C2HBr
193 nm
C2H(A211)
C2H2 C2H2 H2
C2H2 C2HCHOb C2HBr
193 nm 10.6 pm 193 nm
C2H(A211) CH(A2A) C2H(A211)
H2
C2H2
193 nm
C2H(A211)
H2 CH4
C2HZC C2HBr
193 nm 193 nm
CH(~~A) C2H(A211)
CH4
C2H2
193 nm
C2H(A211)
CH4
C2H2c C2HBr
193 nm 193 nm
CH(k2A) C2H(A211)
C2HBrC C2H2d C2HBr C2H2 C2H2 C2HBr C2HBr
193 nm 193 nm 193 nm 193 nm 193 nm 193 nm 193 nm
CH(A~A)
0 2 0 2
02
Ar Are Arf He N2
precursor
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X211 Chemiluminescence
4,"
species monitored
emission region, pm
molecule-l s-l
C2H(A211)
1.5-3.4 2.2-2.7 5 1.0-5.0 1.5-3.4 2.2-2.7 5 1 .O-5.0 0.43 1.0-5.0 2.2-2.75 1.5-3.4 1 .0-5.0 1 .O-2.75 0.43 1 .O-5.0 1.5-3.4 2.2-2.7 5 1.0-1.2 1.2-2.75 1 .O-5.0 1.2-2.75 0.43 1.0-1.2 1.2-2.75 0.43 0.43 1 .0-5.0 0.43 0.43 1 .0-5.0 1 .O-5.0
6.4 9.7 8.4 8.1 11.0 8.0 10.0 2.0 2.2 2.2 2.7 1.7 1.2 2.4 2.2 2.9 2.1 2.7 2.7 3.0 0.5 1.2 1 .o 2.2 2.4 0.5 0.1 0.002 0.4 0.8
CH (k2a) C2H(A211) CH(A~A) CH(~~A) C2H(A2n) C2H(A211)
lo-" cm3
'Uncertainties (2u) are always 3 pm, where the emission in our experiments is weak. These transitions have relatively long spontaneous emission lifetimes because of the v3 frequency dependence of the Einstein coefficient. Thus, in detecting- IR spontaneous emission, we mainly monitor emissions from AZII vibrationally excited levels. C2H is isoelectronic with C N , and the ground and first electronically excited states are analogous. Similar bond lengths and transition moments provide qualitative guidance for the case of C2H, and we calculate that the measured spontaneous emission lifetimes -can be_reconciled by assuming that the C N A211-X2Z+ and C2H A211-X22+ transition moments are equal, while taking into account the u3 frequency dependence of the Einstein coefficients. "Rate Coefficients" for Quenching of C2H A211 RZ+ Emission and the Production of CH A2A P'II Chemiluminescence. Rate coefficients pertinent to CzH(A211)removal were obtained from two different sets of measurements: timeresolved observations of C2H A211 X2Z+ spontaneous emission and C H A2A X211chemiluminescence from the reaction of C2H with 02.In a previous publication,l2 we assigned the observed CH A2A X211 chemiluminescence to the C2H(X2Z+) O2 reaction. This hasty assjgnment was later modified to include the possible role of the A211 state.I3 We believe that the removal rate coefficient for the species responsible for the observed C H A2A X211chemiluminescence is not affected significantly by vibrational excitation above the reaction threshold, since the rate coefficient is independent of Ar pressure (0.1-40 Torr) and C2H precursor. For example, the 193-nm photolyses of C2HBr and C2H2are 200 and 65 kJ mol-' exoergic, respectively,20 and these energies appear as product E,
+
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-
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+
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(25) Katayama, D. H.; Miller, T. A,; Bondybey, V. E. J . Chem. Phys. 1979, 71, 1662.
V, R, T excitations. Nevertheless, the rate coefficients obtained when monitoring C H chemiluminescence are the same for these two pcecursors (i.e., an order of magnitude larger than for the C2H(X2Z+) O2reaction9). Thus, we conclude that-an excited C2H species is removed very rapidly relative to CzH(X2Z+),and the removal of this species does not depend on Vibrational excitation above reaction threshold. This implicates the A211state, whose open T-electron system may also be conducive to the formation of CH(A2A). However, H C C 0 2 is strongly bound relative to C2H + O2 _and it is quite possible that distinctions between the A211and XzZ+ states of the C2H re_actant will not be carried through to products, except thaj the A211 state may store energy more efficiently than nearby X2Z+ states, as in the case of deactivation by Ar discussed below. Thus, we tentatively XzII assign the rate coefficient determined with_ C H A2A chemiluminescence to_the removal of C2H(A211)and/or vibrationally excited C2H(X2Z+)by 02,and this includes reactive as and varying the well as deactivation channels. By fixing [02] concentration of a collision partner, M, rate coefficients for the removal of excited C2H by M are obtained. In measurements of the quenching of IR fluorescence, it is mainly vibrationally excited C2H(A211) that is monitored, as mentioned above. These species are removed by relaxation to lower A211vibrational levels, i,n addition to chemical reaction and/or electronic quenching to X2Z+ Because of the complex interplay between reaction, vibrational relaxation, and spontaneous emission, the fluorescence quenching "rate coefficients" are phenomenological and do not involve specific C2H(A211)states and processes. For example, efficient vibrational relaxation w$hin the A211 manifold, with relatively inefficient quenching of A211electronic excitation, would yield a large rate coefficient for IR fluorescence X211 chemiluquenching, whereas monitoring CH A2A minescence would yield a smaller :ate coefficient. Conversely, efficient and rapid removal of C2H(A211)via the CH A2A X211 chemiluminescent channel will result in both rate coefficients (IR emission quenching and C H chemiluminescence) being similar. These considerations suggest that on average the "rate coefficients" obtained by monitoring IR emission will be at least as large as those obtained with C H chemiluminescence. This qualitative conclusion is in agreement with the data given in Tables I and 11, except for the case of 02. Finally, the physical processes responsible for the quenching of IR fluorescence merit f_urthercomment. Collisional relaxction of vibrationally excited A211can occur (i) entirely _within A211, (ii) by transfer to X2Z+and (iii) by a combination of A211 X2Z+ and vibrational energy exchanges, similar to the case of CN.25 The IR fluorescence measurements alone ca_nnot distinguish between these possibilities, since vibrationless A211emits weakly at wavelengths longer than 3 pm because of Franck-Condon factors and is therefore just as "dark" as X2Z+ Carrick et $.I7 show many perturbations in their-spectra, which suggests that A211levels may be easily coupled to X22+ levels via collisions. A collision partner such as 02,which readily forms a, peroxy intermediate, can promote mixing between A211 and X2Z+,and the removal rate may not be sensitive to the C2H energy content above reaction threshold. This is in accord with our experimental results, where we find that excess vibrational excitation does not influence the rate of removal by OF With thermalized C2H,the rate coefficient for reaction with O2was measured only for its lowest vibrational statesgb-" and the rate coefficient is much smaller than the quenching rate coefficients of its excited C2H counterpart. Even collisions with nonreactive gases may influence the net removal rate of excited C2H (monitored via decay of the C H chemiluminescence, as described above) in a complex way. Such collisions may cause rotational and vibrational r_elaxationsp d state-specific mixings between the zeroth order X2Z+ and A211 states. In a small polyatomic molecule like C2H, where the A211 state shows perturbations in numerous vibrational the interplay among the different collisional phenomena may lead to a complex, non-Stern-Volmer pressure dependence of the decay rate, as shown in Figure 3 for the case of Ar. Here, the initial rapid quenching (0-1 Torr) is followed by a much slower quenching rate (5-40 Torr). Similar behavior has been observed
+
-+
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J . Phys. Chem. 1986, 90, 5700-5702
5700
with other small molecules with coupled electronic states (e.g., quenching of S02(3B,)by N, and CO?6 and C H 2 0 ( S l ) by Ar and CH3FZ7). An attempt to explain such behavior in terms of fast rovibrational relaxations within each electronic state, collisional line broadenings, the densities of the coupled states, and the mixing between the two states has been given by Freed.28 -However, in our experiments, the initial C2H(A211)and C2H(X2Z’) distributions are unknown and probably rather broad and the experimental results are not accurate enough to warrant a quantitative explanation. This can cnly evolve_subsequent to careful spectroscopic studies of the X2Z+ and A211states and single-vibronic-level excitation of C2H A211 R2Z+ transitions.
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V. Summary Infrared spontaneous emission in the wavelength region 1.O-2.75 pm, following the 123-nm pJotolyses of C2H2 and C2HBr, is assigned to the C2H AZII X2Z+system. Vibrationally excited A211 levels _are responsible for much of this emission, since vibrationless A211 levels have szall vertical Franck-Condon fastors due to the different A211and X2Z+C C bond lengths. C2H(A211) is produced directly by photolysis and can also be obtained by collision-induced A211 X22+ internal conversion. This production mechanism is consistent with recent molecular beam
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(26) Strickler, S . J.; Rudolph, R. N. J . Am. Chem. SOC.1978, 100, 3326. (27) Weisshaar, J. C.; Bamford, D. J.; Sprecht, E.; Moore, C. B. J . Chem. Phys. 1981, 74, 226. (28) Freed, K. F. Adv. Chem. Phys. 1981, 47, part 2, 297.
experiments, which show a peak in the C2H internal energy distribution at the energy of the A211origin following the 193-nm photodissociation of C2H2. Although the authors interpret that peak ic the nascent TOF spectra to be due to vibrationally excited C2H(X2Z’), collisions can cause mixing with th_e A 2 g state (as indicated by the numerous perturbations in the A X spectra) and lead to the observed emission. Collisional quenching of the IR emission can be described phenomenologically using pseudofirst-order kinetics, and the corresponding bimolecular “rate coefficients” for quenching of the emission by several molec_ules are Leported. Such rate coefficients involve a combination of A211 9 X2Z+ exchanges, vibrational relaxations, reactions, and spontaneous emission and are expected to be comparable or greater than those obtained from measurements of C H A2A X211chemiluminescence, since the latter measures the net removal rate of excited C2H. For most cases where comparisons are available, this trend is verified. The quenching of excited C2H by Ar in the presence of O2suggests a complex interplay between z k2Z+ rovibrational relaxation within each manifold, rapid internal conversion, and reactive removal of C2H(A211e W2Zf and/or vibrationally excited C2H(k2Z+)
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Acknowledgment. We acknowledge the very helpful suggestions of S.Benson, D. Golden, and R. F. Curl and thank A. Laufer and J. E. Boggs for providing results prior to publication. Registry No. C2H2,74-86-2; C,HBr, 593-61-3; C,HCHO, 624-67-9; C,H, 2122-48-7; Ar, 7440-37-1; He, 7440-59-7; N,, 7727-37-9; H,, 1333-74-0; CH,, 74-82-8; 0 2 , 7782-44-7; C H , 3315-37-5.
Deuterlum Effect on the T,-State Lifetime of Propanal Vapor: Support for Three Classes of Alkyl Aldehydes Michael L. Longmire,’ Lynmarie A. Posey,2 and Merlyn D. S c h ~ h * ~ Department of Chemistry, Davidson College, Davidson, North Carolina 28036 (Received: February 10, 1986; In Final Form: June 17, 1986) Reciprocal lifetimes, extrapolated to zero pressure (T ~ - ’ ) and , rate constants of self-quenching (k,) for propanal and propanal-d, and kqdecrease with deuteration. have been measured with a flash (laser)-sensitized biacetyl phosphorescence technique. Both ~ in size and is consistent with a previously proposed classification scheme for The 4.4-fold reduction in T ~ is- intermediate alkyl aldehydes and ketones Introduction Radiationless transitions are generally better understood in aromatic hydrocarbons than in aldehydes and ketones, and the TI So intersystem crossing process in the latter molecules has In ref recently been studied experimentally and theoretically.’s 4 it was suggested that the out-of-plane C-H aldehyde wag is an efficient acceptor mode during radiationless transitions of TI-state alkyl aldehydes. It was also observed that alkyl aldehydes and ketones seem to belong to three classes based on the magnitude So intersystem of the reduction in the rate constant of TI crossing, km, caused by deuteration. The following characteristics were noted for molecules in the three classes:
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(1) Present address: Department of Chemistry, University of North Carolina, Chapel Hill, N C 27514. (2) NSF Graduate Fellow at present address: Department of Chemistry, Yale University, New Haven, CT 06520. (3) Author to whom correspondence should be addressed. (4) Beck, W. F.; Schuh, M. D., Thomas, M. P.; Trout, T. J. J . Phys. Chem. 1984.84, 3431. ( 5 ) (a) Russegger, P.; Huber, J. R. Chem. Phys. Lett. 1982, 92, 38. (b) Bruhlmann, U.; Russegger, P.; Huber, J. R. Ibid. 1980, 75, 179. (c) Russegger, P.; Huber, J. R. Chem. Phys. 1981, 61, 205. (6) Bruhlmann, U.; Nonella, M.; Russegger, P.; Huber,J. R. Chem. Phys. 1983, 81, 439. (7) (a) Hirata, Y.; Lim, E. C. Chem. Phys. Lett. 1980,71, 167. (b) Hirata, Y.; Lim, E. C. J. Chem. Phys. 1980, 72, 5505. (8) Luntz, A. C.; Maxson, R. T. Chem. Phys. Lett. 1974, 26, 553.
0022-3654186 12090-5700$01.50/0 , ,
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1. Molecules with a small deuterium effect have a TI-state geometry that is either planar or pyramidal about the carbonyl group. Planar molecules have only a single-minimum potential energy surface with respect to inversion through the molecular plane, which should produce a relatively small Franck-Condon overlap for coupling the lowest vibrational level of the T, state and the isoenergetic vibrational level of the ground state and thereby reduce the enhancement of intersystem crossing by out-of-plane modes. Pyramidal molecules with a small deuterium effect have a smaller out-of-plane deformation angle than do molecules with intermediate and large deuterium effects. 2. Molecules with an intermediate deuterium effect have a pyramidal configuration that produces a double-minimum potential energy surface with respect to inversion through the molecular plane and increases the enhancement of intersystem crossing by out-of-plane modes. The presence of one small carbonyl substituent and one large carbonyl substituent produces an intermediate-sized damping of out-of-plane modes and an intermediate-sized deuterium effect. 3. Molecules with a large deuterium effect have a pyramidal TI-state geometry. Both substituents are small, making the out-of-plane deformation angle the largest possible and reducing the damping of the out-of-plane mode. Both effects make k,, very large and very sensitive to deuteration. Only a limited number of alkyl aldehydes and ketones have been studied, and studies of more alkyl aldehydes are needed to further
0 1986 American Chemical Society
