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.
-
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
-
-
(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+)
-
-
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;
02,
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) N S F 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 , ,
I
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
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5701
Deuterium Effect on T,-State Lifetimes of Propanal
TABLE I: Kinetics Data for Propanals compd
CzH&HO C2 H 5 C D 0
k9, M-I
TCI, S-l
s-l
(1.3 f 0.3) X 10" (1.2 f 0.3) X 1OO '
(9.0 f 1.0) x 104 (2.1 f 0.2) x 104
T~
(deuterated)/To
klo, M-I
s-l
(3.5 f 0.3) x 107 (2.1 0.2) x 107
4.4 f 1.0
support the existence of the three classes. Studies of propanal and propenal seemed promising for two reasons. First, propanal and propenal have a three-carbon-atom similarity with propynal; as a result, correlation of differences in molecular geometry with the magnitude of the deuterium effect was expected to be straightforward. Second, a b initio calculations have shown the out-of-plane angle of T1-stateacetaldehyde and propanal to be similar? In contrast, propenal is believed to be more nearly planar in the TI state. Thus, it should be expected that propanal will provide a second example of a compound with an intermediate deuterium effect, whereas propenal will have a small deuterium effect.
of time. The following mechanism has been found to be consistent with this behavior:
Experimental Section Chemicals. Propanal with stated purity of 97% was obtained from Aldrich and was twice distilled. Hexane and propenal from Aldrich, with stated purity of 99%, and biacetyl from Fluka, with stated purity of >99.5%, were used without further purification. Propanal-dl and pr0pena1-d~(both stabilized with hydroquinone) with isotopic purities of 99.4 and 97.7 atom %, respectively, were obtained from Merck and were used without further purification. All chemicals were stored in the dark, and all samples were trap-to-trap distilled several times just before use. An early fraction of the melting deuterated aldehydes was collected in each experiment in order to preclude contamination of samples by hydroquinone. Gas pressures were established with a Wallace and Tiernan pressure gauge as described previously.1° Instrumentation. The vacuum system has been described previously.1's12 Excited-state propanal was produced by emission from either a frequency-doubled Phase-R Model DL-1400 flash-lamppumped dye laser or a repetitively pulsed Xenon Corp. flash lamp (Model N-734C). Excited states of propanal-dl, propenal, and propenal-d, were produced exclusively by the flash lamp. The laser emission pulses were centered at about 308 nm and had an energy of less than 1 mJ, a bandwidth of -4.0 A, and a duration of 700 ns (fwhm). The flash-lamp pulses had an energy of 10 J and a duration of 1.7 p s (fwhm) and were filtered through a Corning C S 7-54 filter to produce a band-pass of excitation that coincides with the So SI absorption spectrum of propanal between 250 and 350 nm. Similar results were obtained with either the laser or broad-band-pass flash used as excitation source. In the cases of propenal and propenal-d4, a combination of Corning C S 7-54and C S 0-52 filters produced an excitation bandwidth of 350-400 nm. Phosphorescence was focused into a 1/4-m Jarrell-Ash grating monochromator, which was operated in first order with a band-pass of 20 nm and was centered at 51 5 nm, the wavelength for maximum phosphorescence intensity of biacetyl. The phosphorescence detection system has been described p r e v i ~ u s l y .The ~ only difference between previous experiments and the current experiments was that about 300 oscilloscope traces were averaged to produce one experimental record of biacetyl phosphorescence intensity vs. time. Kinetic Model and Procedure. The phosphorescence of flash-excited propanal and propenal is too weak to permit easy detection. Therefore, the collisionally sensitized biacetyl phosphorescence method of Parmenter and Ring was used.13 The kinetic model, procedure, and method of data analysis have and only a summary is presented here. been described before,11s12 The sensitized biacetyl phosphorescence signal increases, reaches a maximum at time t,,,, and decays exponentially as a function
3P P 3P B P 3B 3P + P 2P
-
-
(9) Peterson, M. R.; DeMare, G. R.; Csiznadia, I. G.; Strausz, 0. P. J. Mol. Struct. 1981, 76, 131. (10) Beck, W. F.; Schuh, M. D. Williams, I. R.Chem. Phys. Lett. 1981, 81, 435. (11) Schuh, M. D. J . Phys. Chem. 1978.82, 1861. (12) Holtzclaw, K. W.; Schuh, M. D. Chem. Phys. 1981, 56, 219. (13) Parmenter, C. S.; Ring, B. L. J. Chem. Phys. 1967, 46, 1998.
P
-+
+ hv
'P hvf
+ -+ + -. +
'P
P
'P 3P 'P-P 'P P 2P 'P+B-P+'B 3P P hv,
-+ -
3B -* B 3B
B
hv,
(1) (2) (3) (4) (5) (6) (7)
(8) (9)
(10) (1 1) (12)
3B+P-? (13) P designates propanal or propenal, and B designates biacetyl. At high experimental pressures, vibrational relaxation within the triplet-state manifold is assumed to be complete. For instance, during a triplet-state lifetime of 1 ps, approximately 100 hardsphere collisions occur at a pressure of 10 Torr [kh(P) = lo7(10) s-l]. The use of low biacetyl pressure relative to the aldehyde pressure ensures that little excited-state biacetyl is formed by direct absorption of the flash pulse and that little 'B or 3B is formed as a result of reaction 6 . The intensity of phosphorescence was measured for a biacetyl blank that consisted of hexane, used as a collisional buffer gas a t the same pressures as were used for propanal and propenal, and biacetyl, used at the same pressures as were employed in the aldehyde/biacetyl mixtures. In the cases of propanal and propanaldl the phosphorescence of the blank was negligible compared to that for collisionally sensitized biacetyl, and no subtraction of phosphorescence for the blank was necessary. The rate equations for the concentrations of 'P, 3P, and 3B are solved analytically, and the time-integrated equation for (3B) is used with the experimental parameters t,, and T ~ the , lifetime of biacetyl phosphorescence, to determine 7,the lifetime of 3Pas a function of both biacetyl and aldehyde pressures. 7 - I = k7 + ks + k9(B) + klo(P) = ~ 0 - l+ k9(B) + klo(P) (14) The above method was used to determine T for propanal and propanal-dl. However, the small value of T~ for propenal and propenal-d4 made the biacetyl phosphorescence intensity too low Attempts to determine T for the to permit resolution of t,,,. propenals with a steady-state kinetics method, in which the relative quantum yield of biacetyl phosphorescence equals the area under the phosphorescence intensity vs. time profile, also failed. The areas for propenal-sensitized biacetyl phosphorescence were only slightly larger than the areas for the corresponding blanks, and reliable relative quantum yields of sensitized phosphorescence could not be obtained. The cause of the very short value for T for propenal is not known. Results All plots of i1 vs. (B) were linear for all experimental pressures of propanal. The slopes of these plots were the same within experimental error for propanal and for propanal-d,, and the average value of these slopes, k9, is given in Table I. The intercepts of the plots of T-' vs. (B) are plotted vs. (P) in Figure 1 for propanal and propanal-dl. The intercepts, T ~ - and ~ , slopes, klo, from Figure 1 are given in Table I. 7,varied little with pressure of propanal, which indicated a value for k13 of C20 Torr-' s-'.
5702 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Longmire et al.
TABLE 11: Comparison of T1-State Lifetimes and Out-of-Plane Angle of Alkyl Aldehydes compd formaldehyde acetaldehyde propanal glyoxal propynal
s-1 >2.5 X IO7 2.9 x 104 9.0 x 104 300 3100 TOJ-I:
s-1 5.9 x 105 5.1 x 103 2.0 x 104 160 1700
T0,$-l:
T0,D/70,H
ref
>42 5.7 4.4 1.9 1.8
8 4 this work 22 Sb
a ~ o , Hrefers to the T,-state lifetime of the perprotonated compounds. bValues of propanal, and propynal and to the perdeuterated form of formaldehyde and glyoxal.
-
0
10 20 30 40 Propanal pressure (Torr)
0
Figure 1. Plots of the reciprocal TI-state lifetime vs. propanal pressure. The upper and lower plots are for C 2 H 5 C H 0 and C2H5CD0, respectively.
Discussion Table I shows that the rate constant for self-quenching, k,,, is reduced by a factor of 1.7 upon deuteration of the aldehyde group in propanal. As in ref 4, this reduction is consistent with a suggested hydrogen abstraction reaction.I4J5 In view of the low quantum yield of phosphorescence for propanal and propanal-dl, it seems apparent that (as is the case for most aldehydes and ketones) the rate constant for radiative decay is small and the sum of rate constants for nonradiative decay channels dominates the value of T ~ - I . Photodissociation is not included in the proposed mechanism because the activation barrier is expected to be high (e.g., 14 kcal/mol for acetaldehyde16 and >9 kcal/mol for formaldehydeI7J8). Moreover, the experimental pressures produce several thousand hard-sphere collisions between TI-state and ground-state molecules during the lifetime of TI-state propanal molecules. Hence, for those TI-state molecules that escape photodissociation only decay channels from the vibrationally relaxed TI state should contribute to the lifetime. Furthermore, (14) Turro, N . J . Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, CA, 1980; pp 367, 393. (15) Blank, B.; Fisher, H. Helu. Chim. Acta 1973, 56, 506. (16) (a) Horowitz, A,; Kershner, C. J.; Calvert, J. G. J. Phys. Chem. 1982, 86, 3094. (b) Horowitz, A,; Calvert, J. G. Ibid. 1982, 86, 3105. (17) McQuigg, R. G . ;Calvert, J. G . J . Am. Chem. Soe. 1969, 91, 1590. (18) Hayes, D. M.; Morokuma, K. Chem. Phys. Lett. 1972, I2, 539.
T
out-of-plane angle 40.9O 39.1 39.50 0 0 ~ refer , ~ to
ref 7 7 7 2s 23, 24
classificatn large intermediate intermediate small small
the aldehyde-deuterated form of acetaldehyde,
since the biacetyl pressure was kept at least 50 times smaller than that of the propanals and since the rate of vibrational relaxation within the T1-state manifold of propanal is about 10 times larger than the rate of energy transfer to biacetyl, it is expected that energy transfer to biacetyl occurs predominantly from vibrationally relaxed levels of the TI-state propanals. By analogy to the behavior of acetaldehyde: the different values of T { I in Table I are assumed to be due primarily to the change in ICTs, which equals k8 in the mechanism. Similarly, Icn is assumed to be the dominant term in T ~ - I for all alkyl aldehydes that are compared in Table 11. Comparison , ~ propanal suggests that propanal is an of T , , ~ / T ~ for “intermediate” molecule. On the basis of arguments presented in ref 4, the nonplanarity of T,-state propanal is consistent with the intermediate deuterium effect and is not inconsistent with the relatively large value of kls. Additional support for assigning propanal to the “intermediate” class comes from a comparison of the fundamental frequencies of the out-of-plane vibration and the fact that the Franck-Condon factor associated with k , increases as the fundamental frequency of the acceptor mode increases. The similar frequency for the out-of-plane aldehyde C-H wag of propanal (791 cm-I, ref 19) and acetaldehyde (764 cm-I, ref 20) is qualitatively consistent with the similar deuterium effect for these molecules. Furthermore, the higher frequency for the out-of-plane wag for formaldehyde (1 167.2 cm-I, ref 21) is qualitatively consistent with its larger deuterium effect.
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We also acknowledge Research Corp., the National Science Foundation, and the Apple Education Foundation for partial support of this research. Registry No. C2H5CH0, 123-38-6; C2H5CD0, 5972-03-2; D,, 7782-39-0; biacetyl, 43 1-03-8. (19) Worden, E. F., Jr. Spectrochim. Acta 1962, 18, 1121. (20) Hollenstein, H.; Gunthard, Hs. H.; Spectrochim. Acta, Part A 1971,
27, 2027. (21) Herzberg, G. Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand: New York, 1966; p 612. (22) Yardley, J. T. J . Chem. Phys. 1972, 56, 6192. (23) Brand, J. C. D.; Callomon, J. H.; Watson, J. K. G.Discuss. Faraday Soc. 1963, 35, 175. (24) (a) Lin, C.T.; Moule, D. C. J . Mol. Spectrosc. 1971, 38, 136. (b) Lin, C. T.;Moule, D. C. Ibid. 1971, 37, 280. (25) Spectroscopic evidence in ref 26 shows that the S, state of glyoxal is planar and the geometry of the T,state is assumed to be planar. (26) King, G. W. J. Chem. SOC.1957, 5054. (27) Russegger, P.; Huber, J. R. Chem. Phys. 1984, 89, 33. (28) Russegger, P.; Lishka, H. Chem. Phys. 1984, 86, 3 1 .
