1618
Vaclav Zabransky and Robert W. Carr
Photodissociation of Ketene at 313 nm' Vaclav Zabransky and Robert W. Carr, Jr.' Department of Chemical Engineering and Msterials Sclence, University of Minnesota, Minneapolis, Minnesota 55455 (Received January 14, 1975) Publication costs assisted by the University of Minnesota
The yields of products resulting from reactions of CH#A1) and CH2(3B1)in 313-nm photolysis of ketenepropane mixtures, and ketene-n-pentane mixtures, were determined at pressures between 1 and 709 Torr. Products resulting from C H Z ( ~ Breactions ~) account for 30 f 3% of the methylenes, with the balance attributed to CH#Al) reactions. The relative singlet and triplet methylene yields are independent of pressure in directly in the ketene primary process, and the range investigated. About 85% of the C H Z ( ~ Bis~formed ) the remaining 15% is produced by collisional deactivation of CH2('A1). A model for electronic relaxation and photodissociation of ketene at 313 nm in which the initially excited CH&O('A'') state rapidly internally converts to the ground state, CH&O('A1), is shown to be consistent with the data, and to also explain other features of ketene photochemistry and spectroscopy.
Introduction Although the photodissociation of ketene to methylene and carbon monoxide is well known, there is little information about the details of electronic relaxation processes occurring after excitation. A t least part of the difficulty in understanding the primary process lies in the fact that light emission from ketene has never been observed. Although it has been possible to study excited state behavior by luminescence methods, there is an alternative possibility which can be applied to this relatively small and intrinsically interesting molecule. At 300 nm and longer wavelengths both CH2(3B1) and CH&A1), the ground and first excited states of methylene, respectively, are the only two states energetically accessible, and both are present in ketene photolysis in this range. Moreover, these two spin states undergo distinguishably different reactions with hydrocarbons. Measurements of products arising through CH2('AI) reactions, and those arising through CH2(3B1) reactions, were undertaken to determine if information on the excited states of ketene which dissociate to these methylene states could be obtained. Several investigations have differed on the pressure dependence of the relative proportions of products resulting from C H Z ( ~ Band ~ ) CH2(lA1) reactions in the vicinity of 313 nm. Rabinovitch and Simons2* reported that the products resulting from C H Z ( ~ Breactions ~) increased with increasing pressure, relative to products of CHz(lA1) reactions, in the 1 Torr to 1 atm range, and Rabinovitch, Watkins, and Ring2b reported that the proportion of products attributable to CH2(3B1) reactions went through a maximum near 1 atm, but Ring and Rabinovitch suggested that the maximum may have been due to a pressure effect on the 1,2-dimethyltrimethylene biradical.2c Both investigations used ketene photolysis at -320 nm in the presence of cis-2-butene. In contrast, Voisey3 reported that the proportion of triplet products from photolysis of ketene in the 'esence of methyl ethyl ether at wavelengths 2320 nm decreased with increasing pressure. Finally, Carr and Kistiakowsky4 found no evidence for a pressure dependence of relative singlet and triplet product yields from 313-nm photolysis of ketene in the presence of trans-%butene between 162 and 532 Torr, and Rowland, McKnight, and Lee5 The Journal of Physical Chemistry, Vol. 79. No. 15, 1975
reported that of the methylenes produced during photolysis of CHTCO-CzH4 mixtures between 10 and 300 Torr, 29% (independent of pressure) triplet CHT addition to the double bond occurred. Since the pressure dependence of singlet and triplet products is diagnostic of whether or not CHz(lA1) and C H Z ( ~ Bare ~ ) formed by competing first-order and secondorder processes, and hence to the details of excited state behavior, we have reinvestigated this aspect of ketene photolysis a t 313 nm. In all of the above mentioned work, the yields of singlet and triplet products were obtained from relative product yields. Since many of the reactions and products may be difficult to determine. In this work we have used a pairwise photolysis technique, which we have previously demonstrated to be reliable, to obtain relative have used a pairwise photolysis technique, which we have previously demonstrated to be reliable, to obtain relative total singlet and triplet product yields without having to separate individual products into the singlet and triplet components.6
Experimental Section Steady photolysis experiments were done on a conventional vacuum apparatus routinely evacuated to approximately Torr or better. The optical train consisted of an Osram HBO 500 high-pressure Hg arc, quartz lens, light stop, chemical filter for the 321.6-313.2-nm region: 2-cm i.d. (141.6 cm3) cylindrical reactor, and an RCA 935 phototube. Ketene was prepared and stored as previously described.8 Mixtures of ketene (-6%) with either propane and 0.48 mol % n-pentane (internal standard), or n-pentane and 0.32 mol % propane (internal standard), were photolyzed both with and without oxygen (-5%) by a pairwise technique.6 Product yields were measured relative to the accurately known amount of internal standard. Gas chromatographic analyses were done on a Barber-Colman Series 5000 chromatograph using either 3% squalane on firebrick or Porapak Q columns, and a flame ionization detector. Phillips research grade propane, n-pentane, or n-heptane were used after degassing and gas chromatography impurity check, and Airco 0 2 was used without treatment.
1619
Photodissociation of Ketene
Results Method of Determining Methylene Yields. It is well known that in ketene photolysis at 313 nm both CH2(1A1) and CHz(3B1) (the only two accessible states of CH2 at this energy) are both formed. Furthermore, it is generally agreed that these two states react with alkanes (R-H) via two different mechanisms, as outlined in reactions l-lo. CHz('Al) + CHzCO = C2H4 + CO (l) CHz('Al) CH,(3Bi)
+
+
R-H = RCH:,
CHZCO = CzH4 +
CHz(3Bl) + R-H = CH3* CH:,.
+
+
co R-
R * = RCH3
CH3* + R * = CH, CH3*
( 2)
+
olefin
RH = CHd + R * 2R. = Rz
2R. = RH
+
olefin
2CH3 = CzH,
( 3)
( 4) ( 5) ( 6)
(7) ( 8)
( 9)
(10)
chemically activated butanes formed in reaction 2, which is more than 100 kcal/mol exothermic. Several photolyses of ketene in mixtures of n-pentane and n-heptane ( 0 2 scavenged) were done at pressures from l.o to looTorr. The ratio of the sum of the hexanes, which were formed uia reaction 2, to the sum of the octanes, also produced by (2), remained constant over this pressure range. If chemically activated hexanes were dissociating at the lower pressures, the hexane to octane ratio would be exuected to decrease since the octane dissociation would occur to a substantially lesser extent at an arbitrary pressure. Thus "loss" of pentanes by a hot dissociation path did not occur. Table I contains selected data from runs with n-pentane and propane. The yields of CH4, C2H4, C2H6, and either ZCsH14 (n-hexane, 2-methylpentane, and 3-methylpentane or ZC4H10 (n-butane and isobutane) are given, along with (YT,the fractional yield of CH2(3B1), where LYT= 1 - as. Oxygen suppresses CzH6, reduces CH4 by about 90%, and substantially reduces the yields of C2H4, CC6H14, and ZCIH10, as expected. The experimentally determined values of CUTare plotted in Figure 1. These are independent of pressure from 1.0 to 700 Torr, and have an average value of 0.30 f 0.03. A linear regression analysis of the data in Figure 1,fitted by y = a + bx yielded a = 0.29 f 0.05 (standard deviation) and b = 0.00 with a standard deviation of 0.00. The HP 65 program used rounded off to two significant figures. Mechanism of Methylene Formation. Although CHZ(lA1) must be formed only in the ketene primary process, there are two possible mechanisms for CH2(3B1) production. These are (1) the primary process via dissociation of electronically excited, presumably triplet CHzCO, and (2) collision-induced intersystem crossing, reaction 12. As-
Since CHZ('A1) and C H Z ( ~ Beach ~ ) react with R-H to form products that are in many cases identical, we rely upon the selective removal of C H Z ( ~ B by ~ ) Oz9 to determine the amounts of CH&A1) and CH2(3B1) formed during photolysis. That is, in ketene-alkane mixtures without 0 2 , the stoichiometry of (1-10) shows that measurements of CHI, CzH4, C2H6, and those alkanes one CH2 group larger than the reactant counts the total yields of CHz(lA1) and C H Z ( ~ B ~Also, ). since 5% 0 2 is sufficient to scavenge CHZ('Ai) + M = CHz(3B1)+ M (12) C H Z ( ~ B(but ~ ) not CH&A1)), in an experiment identical in every respect except that 5% 0 2 is included, measurement suming that all of the CHz(lA1) produced in the primary of CzH4 and those alkanes one CH2 group larger than the process either reacts with CHzCO and R-H, or is deactivatreactant counts the CHz(lA1) produced. The C H Z ( ~ B ~ ) - O ~ed to C H Z ( ~ B ~and ) , that all of the C H P ( ~ Bproduced ~) in reaction yields CO, Con, H, and probably H20 and OHa9 02-free experiments reacts with either CH2CO or R-H, the These products will not change the stated stoichiometry of rates of formation of CHZ(lA1) and C H Z ( ~ Bin~ )the prithe CHZ(lA1) reactions. We have previously shown that a mary process, Y , and Yt, respectively, are given by (13) and small amount of 0 2 does not change the relative amounts of (141, where R , and Rt are the overall rates at which singlet CHZ(lA1) and C H Z ( ~ Bin ~ ) the ketene primary process.1° ys = R, + ki,('CH,)(M) (13) Also, accuracy of gas chromatography analysis dictated measurement of methane and ethane, rather than Rz and Yt = R, - kl,('CHJ(M) (14) olefin to monitor the radical-radical reactions. and triplet methylene react with CH2CO and R-H, and Equation 11 follows from the stoichiometry of reactions ('CHz) E [CHzWi)] [CZHd] + [R-CHs] [C2H4] + [CH,] + 2[C,Hg] + [R-CH,] = as (11) R, = k1('CH,)(CH,CO) + k,('CHz)(R-H) (15)
1-10, where as is the fractional yield of singlet methylene, the numerator results from 02-containing experiments, and the denominator results from 02-free experiments. The concentrations in eq 11 were calculated from product peak areas relative to the peak area of an internal standard, accurately which was deliberately added in small (el%), measured amounts. Corrections were made for differences in gas chromatography sensitivity among all of the peaks measured, and individual product yields for each photolysis pair were put on an equal percent conversion (of ketene) by making small corrections for differences in reaction time and light intensity. Propane was the reactant (R-H) at higher pressures, 200 to 700 Torr, but was replaced by n-pentane in experiments from 1 to 370 Torr because of appreciable decomposition of
(CH,CO)/(R-H) = J ki/k, = g The quantity Yt/k12(1CH2)(RH) in eq 16 is the ratio of the rate of primary process C H Z ( ~ B production I) to the rate of C H Z ( ~ Bproduction ~) by (12). To evaluate this quantity values of kdk2 = ( C Z H ~ ) ( R H ) / ( C H ~ C O ) ~ ( Rwere CH~) computed from the data for 02-scavenged experiments, and R J R , = aJa,, which holds for small conversions. Finally kizlk2 was computed from a previously published correlation of this quantity with polarizability of the collision partner, M,l0 yielding k1Zlk2 = 0.06 for propane and klz/kz = 0.09 for pentane. Data for computation of k12/k2 The Journal of Physical Chemistry, Vol. 79, No. 75,1975
1620
Vaclav Zabransky and Robert W. Carr
TABLE I: Product Yields Relative to Internal Standard for 313-nm Photolysis of Ketene-Alkane Mixtures Pressure. Torr CH,
C2H,
C2H,
SC,H1,
CjH,? 0.29 0.41 0.14 0.090 0.17 0.064
Discussion
at
71-
6.5
0.090 0.009 0.077 0.003
6.7’
253 253’
Pressure. Torr CH, 198 211b
0.098
C?H,
C,H,
0.30
3‘69 3.28 1.98 1.67
o.31
SCAHl,
at
C,H, 0.289 0.218 0.186 0.125
02.
O6
t I
I
was restricted to that obtained from ketene photolysis in the middle uv absorption band, that is, restricted to conditions commensurate with the present experiments. Using Rt/R, = 0.48 for propane and 0.42 for n-pentane yielded Y J ~ I ~ ( ~ C H ~ ) ( R=- H7.5 ) and 3.7 for propane and n-pentane systems, respectively, or -12 to -20% of the total CH#B1) results from (12), the remaining 88 to 80% being formed in the primary process. The value of K12/k2 would have to be a factor of 6 larger if Yt = 0, which seems highly unlikely. Thus significant amounts of C H Z ( ~ Bare ~ ) produced in the ketene primary process. Substituting the stationary state (1CHz) into (16) yields eq 17. Since R J R , is
(17) independent of pressure (Figure l ) , and (RH)/(M) and (CHzCO)/(R-H) were constant in these experiments Yt/Y, is pressure independent. As a consequence Yt and Y , are either both pressure independent or both have the same pressure dependence, which cancels in the ratio YJY,. It The Journal of Physical Chemistry, Vol. 79, No. 15, 1975
The electronic absorption spectrum of ketene in the middle and near-uv consists of a series of diffuse bands between 473.5 and about 240 nm.11J2 Although there is disagreement whether the bands belong to one electronic state12 or to two electronic states,11J3J4 with the second starting near 370 nm, there is general agreement that the transition in the 370-240-nm region is lA2 + lA1 if the upper state is described in C2, symmetry, or lA” lA1 if the upper state is in-plane bent, (C, symmetry) as indicated by Del Bene’s13 calculations. The diffuse spectrum is interesting since a molecule the size and rigidity of ketene would be expected to have detectable rotational structure if the excited states were unperturbed, but no evidence of structure was found a t high resolution, even a t -780.11 Another feature of ketene is the failure to detect emission following excitation to IA”. A recent attempt to detect emission with to a sensitive apparatus has permitted the limit & < be placed on the fluorescence quantum yield.15 From the integrated intensity of the 380-240-nm interval, the radiative lifetime of the lA” state, TO = 4 x low5sec, can be calculated and a limit placed on the excited state lifetime, r = &To. This yields r < 4 X 10-losec, which is consistent with the lack of rotational structure. However since the bands are completely diffuse even at energies considerably below the dissociation limit, which is in the vicinity of 350 nm,16 true predissociation is not the cause. Thus electronic relaxation, as described by Byrne and must be invoked as the reason for diffuseness, a t least below the dissociation limit. Near the dissociation limit, and a t shorter wavelengths dissociation to CO and CH2 occurs, and dissociative quantum yields are pressure dependent up to about 300 nm. The primary dissociative quantum yield extrapolated to p = 0 is unity a t 313 nm18J9 and slightly less than 1 at 334 nm,18 but at 366 nm18919 & extrapolates to considerably less than 1 a t p = 0. Values of the overall first-order kd-l, where kd is the photodissociation lifetimes, 7d overall photodissociation rate coefficient, obtained from the Stern-Volmer plots, are considerably longer than the sec, a t 36618J9 computed limit of the lA” lifetime, 4 X and 334 nm.18 At 313 nm Td values spanning the range from 5X to 0.9 X sec19 have been reported. Thus a t 366 and 334 nm the state that i s collisionally deactivated is not IA”, but some other state reached by rapid electronic relaxation. At 313 nm the situation is not as clear. Although Td is either about equal to or shorter than 4 X sec, this figure is a lower limit and Td may in fact be longer than the actual lifetime of lAff.Also, there is no evidence for a change in lA” behavior between 334 and 313 nm, and it is most likely that rapid electronic relaxation to another longer-lived state occurs a t 313 nm as well as a t longer wavelengths. The electronic relaxation cannot be photodissociation because lA” correlates with higher excited states of the CH2 and CO photofragments,20 which are not accessible at 313 nm. Photodissociation mechanisms can be most easily visualized with reference to Figure 2, in which potential energy is plotted as a function of r(C-C). It should be noted at the outset that the locations of the minima of these curves are only approximate with respect to both energy and r(C-C), since these (particularly the energies) are either poorly +
0.341 3.72 o.32 0.02 3.04 706 0.073 0.161 1.72 o.32 70gb 0.004 0.03 1.37 0.32 mol ’70 C3H8 in n-CsH12-CHzCO photolyses; 0.48 mol % C5H12 in C3Hs-CH2CO photolyses. Experiments with 5 mol % 0.004
should be emphasized that although (12) is bimolecular it does not put any pressure dependence into RJR, since the “branching ratio”, k2Ik12, is pressure independent.
1621
Photodissociation of Ketene known or in dispute. This deficiency will in no way affect the arguments presented below. The curves in Figure 2 were deduced from the following information. The ketene ground state, CH2CO(1A1), correlates with CHz(lA1) through an avoided crossing,20 the highest point of which cannot be greater than about 85 kcal mol-' for consistency with the observations a t 334 nm that Stern-Volmer plots extrapolate to a value for & slightly less than unity a t p = o,1s,21and a t 360-370 nm & extrapolates to values appreciably less than unity at p = 0.18319The enthalpy of dissociation to CO and C H Z ( ~ Bis~78 ) f 1 kcal mol-' a t 298"K, which we have used to place the CH2 ground state on Figure 2. The cross-hatched region reflects the uncertainty in the excitation energy of CH2(1A1).16p22i23 Two triplet states are predicted to lie in the energy region of i n t e r e ~ t . ' ~ The , ~ ~ ?3A' ~ ~ state leads to CO(IZ+) CH2(3B1), and although this is completely repulsive in a least-motion path,20 geometry o p t i m i ~ a t i o n lhas ~ shown it to have a minimum a t large r(C-C). The fragmentation products from 3A" are not established, but do lie a t a higher energy than the methylene ground state. Finally, Basch20 shows the lA2 state, which is the state of C2" symmetry corresponding to lA", correlating with higher energy fragments in a least-motion dissociation, while Del Bene's geometry optimized calculation suggests that lA" leads to CO('Z+) and CH&B1). Thus the products resulting from lA" and 3A'r dissociation are left unspecified in Figure 2. This uncertainty has no effect on arguments presented below since the methylene states involved are energetically inaccessible a t 313 nm. The following three mechanisms for production of CH&A1) and C H Z ( ~ Bafter ~ ) excitation to CH2CO(1A") can be postulated. (1)The lA" state undergoes intersystem crossing to 3A', either with or without the intermediacy of 3A",25 yielding CH2(3B1), and concurrently undergoes internal conversion to the ground state, followed by dissociation to CHZ('A1). (2) The lAr' state undergoes intersystem crossing to 3Af,either with or without the intermediacy of to 3A", and can then either dissociate to C H Z ( ~ B or ~ )cross , 'A1 and from there yield CHZ('A1). (3) All the lAf' undergoes internal conversion to the ground state, followed by dissociation to CH2( lAl), and competitive crossing to 3A', followed by dissociation to C H Z ( ~ B ~ ) . Assuming that the CH2CO lA" state does not live long enough to be deactivated, mechanism 1 can be ruled out. The triplet states, 3A" and 3A', which would be populated by intersystem crossing in (1) are presumably short-lived because of the failure of ketene to sensitize emission from biacetyLz6 Thus the state which is collisionally deactivated would be CH&O(lAl). However formation of C H Z ( ~ Bby~ ) a pressure-independent process via the ketene triplet states, and pressure-dependent formation of CHZ(lA1) from dissociation of CH2CO( lAl), predicts that a, should decrease with increasing pressure, contrary to the experiments reported here. Even if the ketene triplet state(s) is long-lived enough to be collisionally deactivated, its lifetime would have to match the lA1 lifetime, a highly unlikely situation, in order for a, to be pressure independent. Mechanism 2 seems unlikely for the same reasons. If both IA'' and the triplet state(s) are too short-lived to be collisionally deactivated, the pressure sensitive state must be lA1, and again a pressure dependence of a, is predicted. Alternatively, if 3A' and lA1 can both be collisionally deactivated, as would be pressure dependent unless their lifetimes are the same.
+
I'
PA" , 100 313 334 355
75 -
-W
E
50
C
E
-0 u
1
C 01 W
-
>
P
W
25
- 0 10
15
'P
R ( C - C..) , A Flgure 2. Approximate potential energy curves for dC-C)in CH2CO. Mechanism 3 can be kinetically represented by (18-23),
where OK, 'K, "K, kK represent ketene molecules in 0, i, rn, k vibrational levels of corresponding electronic states. The photoexcitation into 'K(IA'') is followed by an internal conversion (19) into the ground state (lA1) in the mth vibrational level. The vibrationally excited ground state can decompose into CH2(IA1), (20), or intersystem cross into the (3A') state, (21), or can undergo the collisional deactivation, (23). A steady-state analysis of the proposed mechanism yields pressure-dependent primary rates, Y , = k z d / ( k 2 0 k21 + k23 [MI) and Yt = k211(k20 1221 + k23[M]), so the constant ratio Y J Y , = k21/k20 is consistent with experimental data and (17). (I is absorbed light intensity.) The mechanism predicts a Stern-Volmer type expression for 214~0,with unit value a t p = 0. In order to account for qh-0 = 1 up to 250 Torr a t 313 nm, as found by Strachan and Thornton,lg additional decamposition and intersystem crossing steps from a t least one lower vibrational level of the 'A1 state would have to be included in the model. Such intermediate vibrational levels are reached by the stepwise collisional deactivation of the ground state. The mechanism is also capable of explaining the decrease of a, with increasing wavelength, and the effect of wavelength on $J& As wavelength is increased from 313 nrn the observed decrease of CHZ(lA1) relative to CH2(%1) is predicted because dissociation along CHZCO(*AI) and over
+
+
The Journal of Physical Chemistry, Vol. 79, No. 15. 1975
1622
the maximum corresponding to the avoided crossing becomes slower as it becomes energetically less favored, while crossing to 3A’, which occurs at lower energy, would not be retarded, but would become relatively faster as the classical velocity in the crossing region becomes slower. At high photon energies (A 5280 nm), where f$d = 1, independent of pressure, the CH2CO(1A1) lifetime must be very short. However as the CH2CO(lA1) energy approaches the top of the “barrier”, its lifetime is sufficiently long that it can be collisionally deactivated at pressures below 1 atm. However, a t 92 kcal/mol (313 nm), which must be above the “barrier”, the lifetime of CH2CO(’A1) obtained from SternVolmer plots is about 0.1 nsec, and a large number of vibrations occur before dissociation. Thus crossing to 3A’ does not have to be very efficient to account for about 25% C H Z ( ~ Bfrom ~ ) the primary process. Finally, a t 360 nm only a fraction of the excited molecules dissociate even a t zero pressure, consistent with the dissociation limit a t about 350 nm. The small activation energy for dissociation at this ~ a v e l e n g t h and , ~ ~the ~ ~large ~ amount of CH2(3B1) relative to CH&A1), are also explained by the “barrier” in Figure 2. Russell and R ~ w l a n dhave ~ ~ recently suggested that 14CO formation in photolysis of 14CH2C0 results from rearrangement of an electronically excited singlet state, probably via the intermediacy of oxirene. While the present discussion has implicitly assumed that internal conversion from the lA” state to the ground state is a physical process, isomerization, as suggested by Lamola, Hammond, and M a l l ~ r yand , ~ ~discussed by Phillips, Lemaire, Burton, and Noyes31 may also provide a route for the radiationless transition. The amount of scrambling, however, is less than 10% a t 313 nm, implying that isomerization via oxirene is the lesser route for the radiationless transition. Acknowledgment. This research was supported by the
Communications to
the Editor
US.Atomic Energy Commission under Contract No. AT(111)2026.
References and Notes (1) AEC Document No. COO-2026-18. (2) (a) J. W. Simons and B. S. Rabinovitch, J. Phys. Chem., 68, 1322 (1964); (b) B. S.Rabinovitch, K. W. Watkins, and D. F. Ring, J. Am. Chem. Soc., 87, 4960 (1965); (c) D. F. Ring and B. S.Rabinovitch, J. Phys. Chem., 72, 191 (1968). (3) M. A. Voisey, Trans. Faraday Soc., 64, 3058 (1968). (4) R. W. Carr, Jr., and G. B. Kistiakowsky, J. Phys. Chem., 70, 118 (1966). (5) F. S. Rowland, C. McKnight, and E. K. C. Lee, Ber. Bunsenaes. Phvs. Chem., 72,236 (1968). (6) T. W. Eder and R. W. Carr, Jr., J. Phys. Chem.. 73, 2074 (1969). (7) J. G. Calvert and J. N. Pitts, Jr., “Photochemlstry”, Wlley, New York, N. Y., 1966, p 732. (8) M. G. Topor and R. W. Carr, Jr.. J. Chem. Phys., 58, 757 (1973). (9) R. L. Russell and F. S. Rowland, J. Am. Chem. Soc., 90, 1671 (1968). (10) T. W. Eder and R. W. Carr, Jr., J. Chem. Phys., 53, 2258 (1970). (1 1) R. N. Dlxon and G. H. Kirby, Trans. Faraday Soc., 62, 1406 (1966). (12) A. H. Laufer and R. A. Kelier, J. Am. Chem. Soc.. 93, 61 (1971). (13) J. E. Del Bene, J. Am. Chem. Soc., 94, 3713 (1972). (14) J. S. E. Mcintosh and G. B. Porter, Can. J. Chem., 50, 2313 (1972). (15) J. R . McDonald, private communication. (16) R. W. Carr. Jr., T. W. Eder, and M. G. Topor, J. Chem. Phys., 53, 4716 (1970). (17) J. P. Byrneand I. G. Ross, Aust. J. Chem., 24, 1107 (1971). (18) G. A. Taylor and 0.6. Porter, J. Chem. Phys., 36, 1353 (1962). (19) A. N. Strachan and D. E. Thornton, Can. J. Chem., 46, 2353 (1968). (20) H. Basch, Theor. Chlm. Acta, 28, 151 (1973). (21) B. T. Conneliy and G. B. Porter, Can. J. Chem., 36, 1640 (1958). (22) C. F. Bender, H. F. Schaefer, ill, D. R. Francschetti, and L. C. Allen, J. Am. Chem. Soc., 94,6888 (1972). (23) V. Staemmler, Theor. Chim. Acta, 31, 49 (1973). (24) J. W. Rabalais, J. M. McDonald, V. Scherr, and S. P. McGlynn, Chem. Rev., 71, 73 (1971). (25) Whether or not CHZCO(~A”)is involved in intersystem crossing to CHzCOfA’) is unimportant here. However, the density of states is so low in ( A”) at the energy of interest that it is an unlikely intermediate. (26) M. Grossman, G. P. Semeluk. and I. Unger. Can. J. Chem., 47, 3079 (1969). (27) S.Y. Ho and W. A. Noyes, Jr., J. Am. Chem. Soc., 89, 5091 (1967). (28) A. N. Strachan and W. A. Noyes, Jr., J. Am. Chem. Soc., 76, 3258 (1954). (29) R. L. Russell and F. S.Rowland, J. Am. Chem. Soc., 92, 7508 (1970). (30) A. A. Lamola, G. S. Hamrnond, and F. B. Maiiory, Photochem. Photobiol., 4, 259 (1965). (31) D. Phillips, J. Lemaire, C. Burton, and W. A. Noyes, Jr., Adv. Photochem., 5, 329 (1968).
COMMUNICATIONS TO THE EDITOR
Influence of Lower Alcohols on the Pfeiffer Effect of Tris( 1,lO-phenanthroline)zinc(II) SulfateCinchonine Hydrochloride and -/-Strychnine Hydrosulfate Systems in Water
Sir: When a racemic mixture of a certain labile complex is added to a solution containing an optically active substance (often called an environment compound), additional optical activity is developed. T o this phenomenon was applied the term “Pfeiffer effect” by Dwyer and Brasted in 1954 and some excellent reviews on it have already appeared.14 The Journal of Physical Chemistry, Vol. 79, No. 15, 1975
Some authors attributed the Pfeiffer effect to any differential interaction of the dextro and levo enantiomers of the complex with the environment compound and to the resulting “displacement” of an equilibrium between the dextro and levo enantiomers (in favor of one of them).5 In fact, Kirschner and Ahmad6 succeeded in partial resolution of [Ni(phen)3]2+ and [Ni(bpy)3I2+ (phen = 1,lO-phenanthroline and bpy = 2,2’-bipyridine) through the Pfeiffer effect. However, little is known yet on the nature of the interaction. In our preceding papers,’~~ we proposed the hydrophobic bondingg between the racemic complex and the environ-
