J . Phys. Chem. 1989, 93, 7670-7677
7670
Stereocontrol of the Red Light Induced Photoepoxidation of 2-Butenes by Nitrogen Dioxide in Solid Ar Munetaka Nakatat and Heinz Frei* Chemical Biodynamics Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received: May 23, 1989)
-
Photooxidation of cis-2-butene w_as initiated in an inert gas matrix by exciting cis-2-butene-N02pairs at red, yellow, and green wavelengths (NO2 2B2 X2AI). Chemical reaction was monitored by FT-IR spectroscopy, and emission from an Ar ion or a tuned CW dye laser was used for photolysis. As in the case of the tram-2-butene + NO2reaction reported earlier, 2,3-epoxybutane was the only final oxidation product observed upon direct photolysis of reactant pairs. While in the case of the tram-2-butene reaction stereochemical retention was complete, we found in the cis case 85% of the epoxide with retained configuration when conducting the reaction at low matrix concentration. This fraction decreased with increasing reactant to matrix ratio. Infrared bands of two conformers of a butyl nitrite radical were observed concurrently with the epoxide, one syn, the other anti with respect to conformation (CH3 groups) about the central CC bond. A correlation was found between the syn to anti nitrite radical and the cis to trans epoxide ratios, suggesting a common transient precursor. It is most probably an oxirane biradical, whose conformation determines the stereochemistry of the epoxide product. The photolysis wavelength dependence of the product growth kinetics was studied, and relative reaction efficiencies so obtained are shown to give insight into aspects of the dynamics of the reaction that relate to the observed product and stereospecificity. The two trapped butyl nitrite radical conformers were found to photodissociate under exposure to long-wavelength visible light with complete conformer specificity. The anti conformer gave trans-2-butene oxide and NO at a threshold wavelength of 613 nm, while the syn form was found to decompose to 2-methylpropanal and NO upon 573 nm and shorter wavelength irradiation.
I. Introduction Product-specific catalytic oxidation of hydrocarbons is currently a very active research area, with many approaches based on catalysis by metal comp1exes.l Epoxidations of small alkenes are of special interest since small epoxides are among the large volume petrochemicals that form starting materials for many important synthetic proce~ses.~-~ Particularly desirable are product-specific epoxidations that would utilize molecular oxygen as terminal oxidant under mild conditions (for example, conditions avoiding elevated reaction temperatures). We have recently reported high product specificity, including diastereospecificity, upon oxidation of trans-Zbutene by NO2 excited at red or yellow wavelengths in an Ar m a t r i ~ . ~Such long-wavelength photons excite NO2 to vibronic levels that lie tens of kilocalories below the dissociation threshold of the reactant. 2,3-Epoxybutane (2-butene oxide) was formed under complete stereochemical retention, and no other oxidation products (e.g., carbonyl compounds) were observed. NO was coproduct with the oxidized hydrocarbon. Since the latter can easily be reoxidized to NO2 by molecular oxygen, a photoassisted, catalytic cycle for alkene epoxidation can be envisioned that is based on N O as catalyst and O2 as the terminal oxidant. We report the results of the study of the red light induced photooxidation of cis-2-butene by NO2 at cryogenic temperature. Analysis of the photolysis wavelength dependence of this reaction allowed us to test the interpretation proposed in the case of the photooxidation of the trans isomer4 and, in comparing the results of the two reactions, learn more details about the origin of the observed high product specificity. Reaction was studied by exciting cis-2-butene-N02reactant pairs, isolated in solid Ar at 12 K, with tuned radiation from a C W dye laser, and monitoring the chemistry by FT-infrared spectroscopy. 11. Experimental Section
The matrix isolation equipment, infrared spectrometer, and photolysis laser system have been described in detail in our previous rep01-t.~ Briefly, matrix suspensions of cis-2-butene and NO2were prepared by slowly codepositing rare gas mixtures of the two reactants, N02/Ar and cis-2-butene/Ar, through separate stainless steel deposition lines onto a 12 K cooled CsI window. Chemical On leave from the Faculty of Science, Hiroshima University, Hiroshima, Japan.
reaction was monitored with an IBM-Bruker FT-IR instrument Model IR-97, and spectra were taken at 0.5 cm-' resolution. For irradiation with an Ar ion laser pumped C W dye laser (or directly with emission from the Ar ion laser), the cold window was rotated by 90' to expose the matrix to the photolysis light entering the vacuum shroud of the cryostat through a quartz window. cis-2-Butene (Matheson, 95%) and argon (Matheson, 99.998%) were used without further purification. 2-Methylpropanal (Aldrich, 99%) was used as received. Nitric oxide impurity was removed from nitrogen dioxide (Matheson, 99.5%) by adding oxygen and condensing NO2 at 77 K. A sample of N'60180/ Nf802= 7/93 (by infrared) was obtained by oxidizing N180 (Icon Services, Inc., 94.5 atom 3'% I8O) by l8O2(MSD Isotopes, 98.2 atom % 180). The mixture was purified by trap to trap distillation (LN,) before use. 111. Results
After presentation of the reactant infrared spectra in subsection 1, spectra of trapped intermediate and final oxidation products
observed upon irradiation of cis-2-butene/N02/Ar matrices with red and shorter wavelength visible light will be given (subsection 2). In subsection 3, the photolysis wavelength dependence of the product growth kinetics will be presented and a kinetic model will be introduced. I . Reactant Spectra. Since all infrared absorptions necessary to identify intermediate and products lie between 2000 and 400 cm-l, we will restrict the report of infrared spectral data to this range. Before photolysis, spectra of matrices cis-2-butenel N 0 2 / A r = 2.5/1/100 show bands of the reactant cis-2-butene (1669, 1459, 1456, 1444, 1429, 1426, 1423, 1408, 1383, 1357, 1290, 1136, 1037, 1010, 971, 685, 681, and 566 ~ m - ' )and ~ those of NO, (1609, 1593 (NI60I8O,natural abundance), and 749 cm-').6 Small amounts of N204isomers, together with traces of NO and N203, are also trapped in the matrix. Frequencies (1) Sheldon, R. A,; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (2) Nogradi, M. Stereoselectiue Synthesis; VCH Verlagsgesellschaft: Weinheim, FRG, 1987. ( 3 ) March, J. Advanced Organic Chemistry, 3rd ed.; Wiley: New York, 1985. (4) Nakata, M.; Frei, H. J. Am. Chem. SOC.1989, 1 1 1 , 5240-5247. ( 5 ) McKean, D. C.; Mackenzie, M. W.; Morrison, A. R.; Lavalley, J. C.; Janin, A,; Fawcett, V.; Edwards, H. G. M. Spectrochim. Acta 1985, 41A, 435-450. (6) Fateley, W. G.; Brent, H. A.; Crawford, Jr., B. J . Chem. Phys. 1959, 31. 204-217.
0022-3654/89/2093-7670$01.50/0 0 1989 American Chemical Society
?'he Journal of Physical Chemistry, Vol. 93, No. 22, 1989 7671
Photoepoxidation of 2-Butenes by Nitrogen Dioxide
TABLE I: Infrared Spectrum of Trapped Nitrite Radical in a cis-2-Butene/N02/Ar = 2.5/1/400 Matrix freuuency, cm-' absorbance
~~
8
-0.050
t
t
I
0
I/I
0.050
2000
.
15'00
'
1000
'
500
an-' Figure 1. Infrared difference spectra. (a) Growth of absorptions in the spectral range 2000-500 cm-l after 6.5-h irradiation of a matrix cis-2butene/N02/Ar = 2.5/1/400 at 590 nm, 446 mW cm-2. (b) Absorption changes upon 5-min photolysis of a matrix cis-2-butene/N02/Ar = 2.5/1/400 by 458-514-nm Ar ion laser emission at 98 mW cm-2 following accumulation of intermediates by 6-h irradiation at 590 nm. Growing bands are due to photolysis products NO, tram-2-butene oxide, and 2-methylpropanal (see text). A few small absorbance changes due to N,O, photoisomerization are also observed (see ref 4, Table I).
of NxOyspecies and their photoisomerization behavior under visible light irradiation have been reported in Table I of ref 4. 2. Spectra of Trapped Intermediate and Final Products. When irradiating matrices cis-2-butene/N02/Ar at various wavelengths in the range 625-600 nm with the C W dye laser, we found a threshold of 61 3 nm for infrared absorbance changes that are not merely due to N,O photoisomerizations. The infrared difference spectrum obtained upon 590-nm irradiation of a matrix cis-2butene/N02/Ar = 2.511 1400 at 446 mW cmd2for 6.5 h, Figure la, shows the loss of cis-2-butene and NO2 reactant absorptions, and concurrent growth of new bands not assignable to N,Oy species. Most easily detected are new bands a t 1652.0, 1645.5, 837.4, 771.9, and 754.6 cm-'. A number of additional, weaker bands could be discerned from expanded spectra. Frequencies and relative intensities of these new absorptions, together with their l 8 0 isotope shifts, are given in Table 1. Five of these infrared absorptions, assigned to a species labeled I, in Table I, are identical in frequency, l 8 0 isotope shift, and relative intensity with those of the trapped butyl nitrite radical which we have observed upon photolysis of trans-2-butene.NOz pairs in solid Ar.4 The other seven new bands grew at constant relative intensities, which suggest assignment to a single species, labeled I, in Table I. Further evidence that these additional seven absorptions belong to a single molecule was obtained by subsequent irradiation with shorter wavelength, 458-514-nm emission of the Ar ion laser (all lines bluegreen). The resulting infrared difference spectrum, Figure 1b, showed that these bands decreased a t intensity ratios that were identical with those observed during growth when irradiating at 590 nm (cf. columns 3 and 4 of Table I). The two most intense absorptions of I,, those of 1652 and 772 cm-l, are close in frequency to those of the u ( N 4 ) and v(N-0) modes of I, (1646 and 755 cm-', respectively) and exhibit similar l 8 0 isotope shifts. This strongly indicates that I,, like I,, has a nitrite functional group.4 In fact, warming of an Ar matrix containing I,, like the one shown in Figure la, from 12 to 22 K (in the absence of light) led to complete conversion of I,to I,. This indicates that I, is a less stable conformer of the butyl nitrite radical trapped upon red light induced trans-2-butene + NOz r e a ~ t i o n .Unfortunately, ~ annealing of the matrix also caused thermal isomerization of NxOy species, resulting in numerous
~
1652.0 1645.5 1373.5 1366.7 1051.6 1035.0 986.8 920.5 837.4 771.9 754.6 651.2
1.00 1.00 0.08 0.05 0.03 0.04 0.06 0.09 0.23 (1.00) (1.00) 0.09
1610.0 1605.0 1373.6 1366.7 1047.0 1030.3 981.2 901.2 834.4 755.0 735.8 639.7
1.01 1.13 0.08 0.07 0.03
0.05 0.04 0.11 0.22 (1.00) (1.00) 0.09
I, I, I,
v(N4) v(N4) B(CH3) 6(CH3)
1,
I, I, I, I, I, I, I, I,
z:k-c) vSy
v(C-0) vsu
v(O-N) v(O-N) vstr
#Observed in a matrix containing I8O labeled nitrogen dioxide (N160180/N1802 = 7/93). bPeak absorbance growth upon 590-nm irradiation of a matrix cis-2-butene/N02/Ar = 2.5/1/100 at 446 mW cm-* for 30 min. 'Peak absorbance depletion upon blue-green Ar ion laser irradiation at 98 mW c d of a matrix cis-2-butene/N02/Ar = 2.5/1/100 for 5 min, after species were accumulated by 590 nm over a period of 10 h. "I, is identical with the butyl nitrite radical observed upon photolysis of trans-2-butene/N02/Ar matrices reported in ref 4.
t
I '
m
4 -0.06 It
2000
1500
1000
1 500
cm-1 Figure 2. Thermal interconversion of nitrite radical conformers. Infrared difference spectrum obtained upon 5-min photolysis of a matrix cis-2butene/NO,/Ar = 2.5/1/400 with 458-514-nm Ar ion laser emission after accumulation of intermediates by 590-nm irradiation for 6 h, followed by warm up of the matrix to 22 K for 5 min. Increasing bands are due to photolysis products NO and tram-2-butene oxide. A few small absorbance changes due to N,O, photoisomerization are also observed (see ref 4, Table I).
spectral changes that made a clear display of I,to I, interconversion difficult. Therefore, we show the effect of matrix annealing on the spectra of I, and I, by comparing the result of subsequent photolysis with blue-green Ar ion laser light '(which causes destruction of both forms, see below) of two identical matrices, one without (Figure lb), the other with prior annealing to 22 K (Figure 2). Comparison of the two infrared difference spectra clearly shows that warming of the matrix results in complete conversion of I,to I,. , We conclude that the species I, is an unstable conformer of CH,-CH-CH(CH,)-ONO. Upon prolonged irradiation of a matrix cis-2-6utene/N02/Ar = 2.5/1/400 at 590 nm, product bands appeared in addition to those already assigned to I, and I, which did not erode upon irradiation of the matrix with blue-green Ar ion laser light. One of these additional absorptions lies a t 1872 cm-' and is readily assigned to NO! The next easily observed ones are at 1102 and 997 cm-' and agree in position and relative intensity with bands of cis-2-butene oxide, an expected oxidation product. Infrared spectra of the latter have been reported in gas phase' and Ar matrixes This assignment was confirmed when conducting the 590-nm photolysis with a more concentrated matrix, cis-2-butene/N02/Ar = 2.5/1/100 where a more complete spectrum of (7) Kirchner, H. H. Z. Phys. Chem. N.F.1963, 39,273-305. (8) Kuehne, H.; Forster, M.;Hulliger, J.; Ruprecht, H. Bauder, A,; Gunthard, H. H. Helv. Chim. Acta 1980,63, 1971-1999.
Nakata and Frei
7672 The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 TABLE II: Infrared Spectra of Final Oxidation Products in a cis-Z-ButenelNOJAr = 2.5/1/100 Matrix (Frequency, c d ) C4H80n cis-2-butene trans-2-butene oxide oxide 2-methylpropanal C4H8180b 1745 (7.85) 1695 1492' 1492 1487 I469 (0.90) I469 1464 (0.51) 1464 1460d 1459 1394 1398 (1.00) 1393 (0.64) 1393 1383'~~ 1383d I383 1371 (0.80) 1371 1155 1157c 1157 1118 1114 1113 1113 1102 (1.00) 1097 1026 1026 1015 1015 997 ( 1 .OO) 996 963 (0.35) 963 958' 958 958 933 (0.80) 933 877 891' 89 1 812 804 790 (0.61) e 726 72 1 Numbers in parentheses give peak absorbances normalized to the absorbance 997 cm-I (cis-2-butene oxide) and 1398 cm-' (2-methylpropanal). Relative intensities of trans-2-butene oxide absorptions were reported in ref 4. bobserved in matrices containing '*O-enriched NO2 reactant of composition N'60180/N1802 = 7/93. cBand overlaps with rrans-2-butene oxide absorption. dBand overlaps with cis-2-butene absorption. CBandoverlaps with asym N204 absorption.
cis-2-butene oxide was observed. Frequencies and relative intensities, summarized in the first column of Table 11, are in agreement with those reported in the literature.8 For some bands intensity measurement was rendered difficult because of overlap with absorptions of other products or with those of cis-2-butene. I80isotope shifts of the cis-2-butene oxide bands, obtained by 590-nm irradiation of matrices cis-2-butene/NOZ/Ar = 2.5/1/ 100 = 7/93 are also given with isotopic composition N16N180/N180z in Table II. In matrices cis-2-butene/N02/Ar = 2.51 1/400 no infrared absorptions of the trans diastereomer of 2-butene oxide were observed upon 590-nm irradiation at 450 mW cm-z over a period as long as 10 h. The latter has an intense band at 11 18 cm-I, whose extinction coefficient is about the same as that of the 1102-cm-' band of cis-2-butene oxide.4 With a measured absorbance of 0.0018 for the 1102-cm-' cis-2-butene oxide band, and a peak to peak noise of 0.0006 at 11 18 cm-I where trans-2butene oxide would absorb, we calculate a lower limit of 3 for the cis to trans 2-butene oxide product branching ratio in the case of these dilute matrices. As previously reported, we did not observe any loss of stereochemical retention in the epoxide product, at any matrix concentration, in the case of the trans-2-butene + NO2 reaction in solid Ar. The sensitivity of our experiments allowed us to put a lower limit of 50 on the trans- to cis-2-butene oxide product branching ratio of the trans-2-butene photo~xidation.~ It is interesting to compare these estimates of the specificity in terms of epoxide stereochemistry with the branching among the two conformations of the trapped butyl nitrite radical, I, and I,. For cis-2-butene/N02/Ar = 2.51 11400, we measure absorbance growth of 0.0566 at 772 cm-I (I,) and 0.0098 at 755 cm-l (I,) upon 6.5 h irradiation at 590 nm, as shown in Figure 3a. Assuming equal extinction coefficients for the v(N-0) absorptions of the two conformers at 772 and 755 cm-I, we obtain a branching ratio IJI, of 6. The assumption of equal extinction coefficients for the two bands is based on the observation that the ratio of their intensities is close to that of their u(N=O) absorptions at 1652 cm-' (I,) and 1646 cm-' (I,) (Figure l), two bands which we expect to have equal extinction coefficients. No I, absorption was ever
0.OF.
I
1
800
n
750
7 0 0 cm-l
Figure 3. Branching between nitrite radical conformers I, and I,. (a) cis-2-Butene/N02/Ar = 2.5/1/400, irradiated for 6.5 h at 590 nm (446 mW (b) rrans-2-Butene/N02/Ar = 2.5/1/400,6.0-h photolysis at 590 nm (446 mW cm-*). The small negative feature at 771 cm-' in both spectra is due to asymmetrical N 2 0 , (ref 4, Table I).
observed in matrices trans-2-butene/NOZ/Ar at any concentrat i ~ n .The ~ N-0 stretching region of a matrix trans-2-butene/ NOZ/Ar = 2.5/1/400, irradiated for 6 h at 590 nm a t 446 mW cm-2, displayed in Figure 3b, shows that the IJI, ratio is at least 50. Based on these estimates we conclude that in diluted matrices (2-butene/NOZ/Ar = 2.5/1/400), the branching ratio It/& between trans-2-butene + NO2 and cis-2-butene + NO2 changes by a t least a factor of 300, while that of trans- to cis-2-butene oxide changes by at least a factor of 150. In contrast to the dilute matrix case just discussed, trans-2butene oxide product growth was observed when more concentrated matrices cis-2-butene/N02/Ar = 2.5/1/100 were irradiated a t 590 nm. Table 11, column 2, shows frequencies and relative intensities of product bands that appeared in addition to those listed in column 1, already assigned to cis-2-butene oxide, and the I, and I, bands shown in Table I. They agree completely with the trans-2-butene oxide spectrum reported earlier? Using again the intensities of the 1102-cm-' (cis) and the 1118-cm-' band (trans) of 2-butene oxide products to determine the stereochemical branching, we found in matrices cis-2-butene/NOz/Ar = 2.5/ 1/100 a cis/trans 2-butene oxide ratio of 0.8 f 0.3.9 In the same matrix, a branching ratio I& of 1.I f 0.39 for the two conformations of trapped butyl nitrite radical was found by comparing, as before, intensities of the corresponding N-0 stretching absorptions at 772 and 755 cm-'. Clearly, in the case of the cis2-butene + NOz reaction the branching of the two forms of the radical and the cis/trans branching of the epoxide product both decrease in parallel with increasing reactant concentration, from 6 and 2 3 at 2.5/1/400, to about 1 in both cases at 2.5/1/100. In the trans-2-butene + NOz study we found that the trapped nitrite radical I, photodissociates to trans-2-butene oxide and NO at wavelengths as long as 613 nm. Consistent with this, we observed upon 613-nm irradiation of a matrix cis-2-butene/ NOZ/Ar = 2.5/1/100 an induction period for the trans-2-butene oxide growth. Secondary photolysis of I, was even more efficient (9) In order to obtain the branching ratio that reflects the stereochemical outcome of the one-photon path of the cis-2-butene + NO2 reaction, product buildup had to be kept small; hence uncertainties are correspondingly large (red and shorter wavelength light induces photodissociation of I, to trons-2butene oxide and NO').
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 1613
Photoepoxidation of 2-Butenes by Nitrogen Dioxide TABLE III: Photolysis of Butyl Nitrite Radical'
absorbance change AA nitrite radical expt A
B C
cis-2-butene/N02/Ar 2.5/ 1/ l o 0 2.5/l/lOOb (annealed) 2.5/1/400
IC
It
837 cm-l -0.0104 (5) -0.0027 (10) -0.0067 (5)
920 cm-l -0.0047 (5) -0.0098 (8) -0.0012 (5)
CBO 997 cm-' 0.0000 (5) 0.0000 (8) 0.0000 (5)
products MP 1745 cm-' 0.0241 (13) 0.0081 (18) 0.0163 (8)
ratio AA(812)/ AA(1745)l AA(920) AA(837) 0.87 (14) 2.32 (17) 0.81 (12) 3 (1) 2.43 (22) 0.83 (54)
TBO 812 cm-' 0.0041 (5) 0.0079 (10) 0.0010 (5)
'Irradiation by blue-green Ar ion laser emission for 5 min, 98 mW crK2, after accumulating nitrite radicals by prolonged 590-nm illumination. bPrior to photolysis, matrix was annealed at 22 K, causing almost complete I, to I, conversion.
when illuminating a t 590 nm, as indicated by more rapid attainment of a concentration maximum. In contrast, no secondary photolysis of I, was detected a t 613 or 590 nm. However, when cis-2-butene/N02/Ar matrices were photolyzed at 573 nm or shorter, green or blue wavelengths, an additional set of product bands appeared which is displayed in Table 11, column 3. At the same time, the I, absorptions reached a maximum and ultimately started to decrease. The intense 1745-cm-' band of the new product with its 50-cm-I I8O isotope shift (column 4) indicates that the molecule has an aldehyde functional group.I0 Indeed, the spectrum of column 3, Table I1 agrees both in band positions and relative intensities with the matrix infrared spectrum of an authentic sample of 2-methylpropanal (isobutyraldehyde) obtained in our laboratory (2-methylpropanal/Ar = 1/350). In order to establish more firmly the reaction path (or paths) that lead to 2-methylpropanal and to determine conclusively the photodissociation channel@) of I,, nitrite radical was accumulated in matrices cis-2-butene/N02/Ar by prolonged irradiation at 590 nm. Subsequent brief ( 5 min) exposure of the matrix to 458514-nm light of the Ar ion laser led to final oxidation product growth that originated almost entirely from secondary photolysis of accumulated I, and I, since at these wavelengths the trapped radicals photoreact much more readily than the remaining unreacted cis-2-butene.N02 pairs. Results are summarized in Table 111. The table shows that a close to 8-fold decrease (from experiment A to B) and a 2.5-fold increase (from experiment A to C) of the ratio I$, of photolyzed nitrite radical does not affect the ratio of trans-Zbutene oxide (TBO) growth to I, depletion. That ratio is indicated by the intensity ratio of the 812-cm-' (TBO) and 920-cm-' (I,) bands in the second column from the right. This confirms that secondary photolysis of I, gives the trans diastereomer of 2-butene oxide. A similar comparison of the ratio of 2-methylpropanal (MP) growth to I, depletion, measured by absorbance changes at 1745 cm-' (MP) and 837 cm-' (I& between experiments A, B, and C indicates that it is not influenced by changes in IJI, (Table 111, last column). We conclude that the aldehyde is produced by secondary photolysis of I,. Entries in column 4 of Table 111 show that no cis-2-butene oxide (CBO) is produced by secondary nitrite radical photolysis. Hence we find not only distinctly different photodissociation thresholds for I, and I,, but the dissociation products of the two nitrite radical conformers are completely different as well. 3. Wavelength Dependence of Kinetics. In order to understand the detailed mechanism of the photoinduced cis-2-butene + NO2 reaction, and to get the wavelength dependence of yields, the kinetic behavior of nitrite radical and final product absorptions was measured as a function of photolysis wavelength and laser power. Measured bandwidths were found to be constant even upon prolonged photolysis, so that peak absorbances in place of integrated absorbances could be used. Figure 4 shows the photolysis wavelength dependence of the growth of I,, I,, cis- and trans-2-butene oxide, and 2-methylpropanal in matrices cis-2-butene/N02/Ar = 2.5/ 1/loo. Rates of formation of nitrite radicals I, and I, increase as photolysis wavelength gets shorter, with I, attaining a maximum buildup faster than I, due to its lower threshold to secondary photolysis. Upon 514-nm irradiation, even at a laser intensity as low as 50 (10) Bellamy, L. J. The Injrared Spectra of Complex Molecules; Chapman and Hall: London, 1975; Vol. 1, p 151.
(b) 590 nm
( a ) 613 nm
0.02
c: ::
i
(c) 573
c
s
s
F
1
3
5
m
382 mWUn-*
4 4 6 mWUnS
382 m w m - 2
P
1
3
5
Irradiation Time (hours)
Figure 4. Absorbance growth behavior of nitrite radical bands at 837 cm-' (I,) and 921 cm-I (I,), and final oxidation product absorptions at 997 cm-I (cis-2-butene oxide (CBO)), 812 cm-' (trans-2-butene oxide (TBO)), and 933 cm-' (2-methylpropanal (MP)) upon irradiation of matrices cis-2-butene/N02/Ar = 2.5/1/100: (a) 613 nm, 382 mW c d ; (b) 590 nm, 446 mW c d ; (c) 573 nm, 382 m W c d .
mW cm-2, photodissociation is too fast for I, absorptions to be observed. Consistent with the 613-nm photodissociation threshold, the trans-Zbutene oxide product exhibits a clear induction period at that wavelength. Contribution of trans-2-butene oxide growth from secondary photolysis of I, is also indicated in the 590-nm growth curve by its sigmoidal shape. Upon 540- and 514-nm excitation this product exhibits single exponential growth, apparently in part because of very efficient photolysis of I,. 2Methylpropanal growth shows a clear induction period a t its 573-nm threshold as well as at all shorter photolysis wavelengths (540 and 514 nm). This suggests that the aldehyde is exclusively generated by secondary photolysis of the nitrite radical conformer I,. In contrast, cis-2-butene oxide exhibits single-exponential growth at all orange, yellow, and green photolysis wavelengths, including its 590-nm threshold. This is in agreement with the conclusion from the data of Table 111 that the cis isomer of 2-butene oxide is not produced by secondary photolysis of either I, or I,. The laser power dependence of the cis-2-butene oxide and methylpropanal product growth supports our initial conclusion that these molecules are produced by a one- and a two-photon path, respectively. When a matrix cis-2-butene/N02/Ar = 2.5/1/100 was irradiated with 514-nm light at 55 mW cm-2 for 25 min, methylpropanal absorbance growth AA at 1745 cm-' was 0.0059 and that of cis-2-butene oxide at 997 cm-' was 0.0013. Illuminating a new matrix with the same concentration again for 25 min at 514 nm,but now at a laser intensity of 110 mW cm-2, gave AA(1745 cm-I) = 0.0193 and AA(997 cm-I) = 0.0027, up by factors of 3.3 and 2.1, respectively. This is consistent with 2-methylpropanal being produced by a two-photon and cis-2butene by a one-photon process. As expected, the growth of I, and I, was found to be linear in laser power at all photolysis wavelengths, confirming that both conformers of the butyl nitrite radical are produced by a one-photon event. Hence these laser power dependences, combined with the kinetic curves of Figure 4, and the secondary photolysis behavior of I, and I, suggest the photochemical reaction paths shown in the kinetic Scheme I. Integrated forms of the corresponding rate equations are derived in the Appendix. Seven parameters were used for fitting eq 8-12 to I,, I,, CBO, TBO, and M P absorbance growth, respectively,
7674
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989
Nakata and Frei
TABLE I V Photolysis Wavelength (A) Dependence of First-Order Rate Constants Obtained from Integrated Equations (cis-2-Butene/N02/Ar = 2.5/1/100) Adjusted Values, h-' 613 590 573 514
0.01 f 0.01 0.05 f 0.01 0.06 f 0.01
382 446 382 53
110 613 590 573 514
0.00
0.04 f 0.02 0.10 f 0.02
0.04 f 0.03 0.22 f 0.03 5.4 f 0.4 12.0 f 0.4
0.01 f 0.01 0.04 f 0.01 0.07 f 0.01 0.38 f 0.14
0.00 0.03 f 0.03 0.23 f 0.03 48.0 f 2.80
0.00 0.04 0.05 0.03 0.07
f 0.01
f 0.01 f 0.01 f 0.01
0.03 f 0.01 0.1 1 f 0.02 0.12 f 0.02
0.11 f 0.03 0.18 f 0.03 0.31 f 0.03
f 0.01
Normalized Values,' h-' 0.00 f 0.01 0.03 f 0.01 0.06 f 0.01 0.27 f 0.07
0.03 f 0.01 0.10 f 0.02 0.13 f 0.02
0.11 f 0.03 0.16 f 0.03 0.34 f 0.03
'Rate constants normalized to the laser photon intensity of 2 X 10" mol cm-* s-' used in the 613-nm irradiation experiment. approximation for I, and I,; only the sum k l , + k3, could be determined.
0.00 0.05 0.09 0.07 0.14
f 0.01 f 0.02 f 0.02 f 0.02b
0.00 0.04 0. IO 0.59
f 0.01 f 0.02 f 0.02 f 0.14b
f 0.02b
Steady-state
Two observations point to the conformation about the central
SCHEME I
< = 'A k3c
It K2t
k3 t
CBO+NO M P + NO TBO -+ NO
1
R:cis 2.butene.NOz reactant pair 1t:trans butyl nitrite radical Ic:cis butyl nitrite radical T-B0:trans 2-butene oxide C-B0:cis 2-butene oxide MP:2-methyl propanal
using the iterative approximation by the Gauss-Newton procedure." These were the six rate constants of Scheme I plus the parameter AoR/eR (proportional to the initial reactant pair concentration), as discussed in the Appendix. Values for rate constants at various photolysis wavelengths are displayed in Table IV, and fit curves for irradiation experiments at 613, 590, and 573 nm are shown as solid curves in Figure 4. NOz or cis-2-butene infrared absorbance decays (eq 7) were not used for parameter fitting since reactant pair absorptions were overlapped by much more intense absorptions of isolated species, adding large uncertainties to absorbance difference measurements. Rate constants of Scheme I are defined as kl, = ehNQ4lJ;kz, = cvis1c42J; k3, = evisN0243cl; k l t = evisNoVltl; kzt = cvis1t4z,l; and k3, = eVirN0~$3tI (evis, decadic extinction coefficients of NOz and nitrite radical intermediate, respectively, at visible photon wavelength; 4, reaction quantum efficiency; I , laser photon intensity). These expressions hold in the weak absorption limit, which in our experiments applies both for NOz and the nitrite radical conformers. Rows 5-8 of Table IV exhibit first-order rate constants normalized to the laser photon intensity used in the 613-nm irradiation experiment, 2 X 10" mol cm-2 s-l. Ratios of these normalized rate constants give relative values of the products of extinction coefficient and reaction quantum efficiency,
€4. IV. Discussion 1 . Trapped Nitrite Radical Conformers. The infrared spectrum of the trapped intermediate I, has been discussed in ref 4 and could unequivocally be assigned to a butyl nitrite radical. Interestingly, the frequencies associated with the N = O and N-O stretching modes were found to be very close to those of closed-shell alkyl nitrites, implying little interaction of the nitrite group with the unpaired carbon electron. Furthermore, comparison of u(N=O) of I, at 1646 cm-I with the corresponding absorption of alkyl nitrites revealed that the conformation about the N-O bond is antiperiplanar (trans). The closeness of the two intense absorptions Of the second trapped form IC at 1652 and 772 cm-' to the corresponding I, bands not only indicates that it too has a nitrite group but moreover suggests that it has the same anti conformation with respect to rotation around the N-O bond (u(N-0) of cis-(N-0) nitrites lie at 1625-1605 cm-1).32 ( 1 1) Walsh, G.R. Methods of Optimization;Wiley: New York, 1975.
C-C bond as the main structural difference between butyl nitrite radical forms I, and I,. First, the fact that I, can be thermally converted to I, implies that I, is a less stable form than I,. This would be consistent with I, having a synperiplanar (or synclinal) arrangement of the methyl groups, while I, would have a more stable antiperiplanar CH3, CH3 arrangement. Assignment of I, to a nitrite radical with a syn and I, to one with an anti conformation with respect to the methyl groups is supported by infrared spectral evidence. For 4, we find four infrared absorptions between 2000 and 600 cm-l that can be associated with vibrations of the butyl fragment (1374, 1052, 837, and 651 cm-')," while for It there are only two (1367 and 1035 cm-', Table I). This would be consistent with I, having a butyl moiety with a local center of symmetry. In this case, half of the bands associated with vibrations of the C4 fragment are expected to be very weak because of g symmetry with respect to the local inversion center. The syn form lacks such a local center of symmetry; hence the cross section of most skeletal infrared absorptions would not be subject to such symmetry restriction. We conclude that spectroscopic evidence and thermal behavior suggest that I, and I, differ in their conformation around the central C-C bond, with I, being syn and It anti with respect to the methyl groups. Thus,for the cis-Zbutene NO2 reaction this assignment would imply that I, is the trapped nitrite radical formed under stereochemical retention, while I, would be the one formed under change of configuration. 2. Reaction Mechanism. Our quantitative study of product growth behavior allows us to discuss the photolysis wavelength dependence of the one-photon and the two-photon paths that lead to the final oxidation products separately. Mechanistic implications concerning the two paths will be discussed in turn. One-Photon Path. Our finding that the branching ratios Ic/It and cis-2-butene oxide/trans-2-butene oxide exhibit parallel behavior when we changed experimental conditions (reactant concentration) strongly suggests that butyl nitrite radical and epoxide have a common transient precursor whose stereochemistry with regard to the central CC bond of the C4 moiety determines that of the products. In our previous report on the trans-2-butene + NOz reaction, we have discussed two possible transient intermediates, a hot butyl nitrite radical and an oxirane b i r a d i ~ a l . ~ Either transient would be consistent with the results of the cis2-butene + NOz reaction as well. Nevertheless, interpretation in terms of the hot nitrite radical path has speculative aspects that vanish if we postulate an oxirane biradical as transient precursor. One such speculative aspect is an increase of the total reaction efficiency of 2-butene.NOz pairs with photolysis photon energy
+
(12) (a) Tarte, P. J . Chem.Phys. 1952.20, 1570-1575. (b) Colthrup, N. B.; Daly, L. H.; Wiberley, S.E. Introduction to Infrared and Ramon SpecIroscoDv~Academic Press: New York. 1975: DD 328-330. (ITAssignment of these four bands to C4H8modes,and the 987-cm-' band to v(C-0), is somewhat arbitrary in light of the heavy mixing of v(C-0) and u(C-C) modes, as manifested by the spread of the '*O isotope shift over all five bands (Table I). With regard to the argument presented here, however, only the number of infrared active modes associated with the butyl fragment is relevant.
The Journal of Physical Chemistry, Yol. 93, No. 22, 1989 7675
Photoepoxidation of 2-Butenes by Nitrogen Dioxide SCHEME I1
P
F)
N
H O H
F YCH3
H
CH3
CH3
H
CH3
+ NO
that exceeds what one would expect in the case of RRK behavior! Another one, which follows directly from the observed stereospecificity, is complete absence of internal rotation about the central CC single bond despite the fact that the hot nitrite radical would carry tens of kilocalories of vibrational energy prior to N O elimination. Hence we will restrict our discussion of the onephoton path to the more probable oxirane biradical mechanism. By this route, excited NO2 would transfer an 0 atom to the alkene double bond to form an oxirane biradical (BR). Product branching between epoxide and butyl nitrite radical would be determined by competition between ring closure and trapping of the transient biradical by addition to the N O cage neighbor, as depicted in Scheme 11. Red or shorter wavelength visible photons excite the ZB2 R2Al transition of NO2 to levels well below its 398-nm dissociation thresh01d.I~ The photolysis wavelength dependence of the first-order rate constants in Table IV confirms our conclusion from the trans-2-butene NO2 study that NO2 reacts from vibrationally excited levels. These are best described as having 2B2W2Al mixed character because of the strong coupling of the two electronic statese4 The sum k,,, of the normalized rate constants of kl,, k3,, klc, and k3, increases monotonically with decreasing photolysis wavelength, namely from 613 to 590 nm by a factor of 5.8, to 573 nm by a factor of 9, and to 514 nm by a factor of 32. Since the total rate constant is proportional to the quantum efficiency 4total= + 41c+ 43t+ 43cof cis-2-butene-NO2 photolysis, dtotalh= ktotalh/chlv the ratios ( k t o t a ~ ' / k t o t a f ~ ~ ) / (thN0z/~613N0') give the photon energy dependence of the cis-2butene NO2 reaction quantum efficiency, 4totalX/4tota1613. According to the extinction coefficient ratios of NO2 at these wavelengths given in ref 4, 4tota~/4totaf'13 is 2.6 at 590 nm, 3.3 at 573 nm, and 4.2 at 514 nm. This rapid increase of the reaction quantum efficiency with photolysis photon energy requires that reacting NO2 is vibrationally unrelaxed and is consistent with an RRK trend if we accept the estimate of AHo = 40-45 kcal mol-' for the 0 atom transfer step discussed previously4
-
+
+
CH3-CH=CH--CH3
-
+ NO,(RfAl)
+
CH3-CH-CH(CH,)-O
+ N O (I)
According to thermodynamic data derived earlier: 2-butene + NO2 butyl nitrite radical is approximately thermoneutral, while AHo is -14 kcal mol-l for formation of epoxide + N O from 2-butene and nitrogen dioxide. Hence with an estimated endothermicity of 40 kcal for reaction step 1, we calculate exothermicities for formation of butyl nitrite radical and epoxide N O from oxirane biradical of -40 and -54 kcal mol-',respectively,
+
~
(14) Hsu, D. K.; Monts, D. L.; Zare, R. N. Speciral Atlas of Nitrogen Dioxide; Academic Press: New York, 1978.
(neglecting differences due to stereochemistry). According to our proposed mechanism (Scheme II), the stereochemistry of an oxirane biradical precursor would dictate both the conformer distribution of the butyl nitrite radical and the diastereomer branching of the epoxide. Oxirane biradicals with syn conformation with respect to the CH3 groups (BR,) react under stereochemical retention with N O to give butyl nitrite radical in the I, conformation, or undergo ring closure to yield cis-2-butene oxide. Similarly, the oxirane intermediate in its anti conformation (BR,) forms nitrite radical in the I, form, and trans-2-butene oxide under retention of stereochemistry. The branching ratios &/I, and CBO/TBO of 6 and 1 3 , respectively, measured in dilute matrices suggest that about 85% of the oxirane biradicals formed upon 0 atom transfer undergo addition to NO/ring closure prior to internal rotation about the central CC bond, while 15% of the initially formed biradicals convert to the anti conformation prior to combination with NO/ring closure. That branching ratio is smaller a t the higher reactant concentration, cis-2-butene/N02/Ar = 2.5/1/ 100. Branching B%/BR, estimated from ratios (klc + k3,)/(klt + k3J, Table IV, are 0.3 f 0.4 (613 nm), 0.5 f 0.2 (590 nm), 0.6 f 0.1 (573 nm), and 1.1 f 0.4 (514 nm). Within uncertainties the branching ratio of retained ( B K ) to changed conformation (BR,) of the transient biradical stays fairly constant with photolysis wavelength, and the value estimated from the kinetic analysis is consistent with that obtained from incremental absorbance growth measurements (section 111.2). According to the rate constants given in Table IV, ring closure yields k3t/(kIt + k3,)of BR, increase with decreasing photolysis wavelength, from 0.0 to 613 nm to 0.28 f 0.15 at 590 nm to 0.43 f 0.10 at 573 nm. Within experimental uncertainty, this is the same steep increase we found for this ratio in the case of the trans-2-butene + NO2 reaction." As argued previously, this increase may manifest RRK behavior for epoxide formation (involving a barrier of a few kilocalories per mole)4 competing with barrier-free oxirane N O radical c~mbination.~~ The ring closure yield k3,/(kl, + k X ) of BR, also increases sharply from 0 at 613 nm to 0.43 f 0.17 at 590 nm to 0.46 f 0.09 at 573 nm. The initial increase of the ring closure yield of the syn oxirane biradical appears to be even steeper than in the case of the anti form, but speculation about this is not warranted because of too large experimental uncertainties. A possible implication of the increase of the oxirane biradical ring closure yields with increasing photolysis photon energy is that ring closure (or combination with NO) occurs at a point where BR, or BRt still carries excess vibrational energy acquired from photoexcited NO2. On the other hand, the fact that the BR, to BR, ratios changes very little as photolysis photon energy goes up may signal a lack of energy flow from initially excited stretching and bending vibrations of the oxirane biradical into overtones of the torsion around its central CC bond. This would imply an extremely short lifetime for the transient, which could readily be rationalized by the fact that ring closure can proceed under spin conservation4 (NO2 and N O have S = and hence the oxirane biradical can be produced in a singlet state along a path that conserves spin). The importance of singlet spin multiplicity of the oxirane biradical intermediate for product and stereospecificity is underlined by a comparison of the reaction presented here with oxidation of 2-butenes by O(3P). Most relevant are studies of reactions of O(3P>with cis- and trans-Zbutenes conducted in the solid at 90 K by Scheer and Klein.'"18 Oxidation of trans-Zbutene by O(3P) gave both carbonyl compounds and epoxides. 2-Butanone and isobutanal accounted for 44% of the total product yield, and 56% were 2-butene oxide with a trans to cis ratio of 8.'* In the case of cis-2-butene + OCP), the overall epoxide yield was 54%, with a cis to trans ratio of 0.8, while 46% were 2-butanone and iso-
+
~
~
(15) Gibian, M. J.; Corley, R. C. Chem. Reu. 1973, 73, 441-464. (16) Hughes, A. N.; Scheer, M. D.; Klein R. J . Phys. Chem. 1966, 70,
798-805. (17) Scheer, M. D.; Klein, R. J . Phys. Chem. 1969, 73, 597-601. (18) Scheer, M. D.; Klein, R. J . Phys. Chem. 1970, 74, 613-616.
7676 The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 butanal.l8 In contrast, we find no carbonyl compounds in cryogenic reactions of N02(2B2)with 2-butene, and stereospecificity for epoxides is substantially higher, particularly at low reactant concentration. We attribute this marked difference in product specificity mainly to the fact that in the case of the O(3P) reaction, spin conservation demands that the postulated oxirane biradical intermediatel93 is formed in a triplet state. Therefore, ring closure to epoxide requires intersystem crossing, a process that may be sufficiently slow to make H or CH3 group migration, and rotation around the central CC bond, competitive.21 In addition, the oxirane biradical formed upon 2-butene + O(3P) reaction carries 225 kcal mol-l more internal energy than the biradical ptoduced by 0 atom transfer from N02(2B2)excited at red wavelengths. With substantially more excess energy available, reaction paths (like migration) may open up to oxirane biradical intermediates formed by O(3P) reaction that are not accessible in the case of the much milder N02(2B2)reaction. An interesting point regarding the stereocontrol of the 2-butene + N02(2B2)system is the difference between the trans and cis2-butene reaction in terms of stereochemical integrity. As far as we can tell from our work, stereochemical retention is complete in the case of the trans-2-butene oxidation at any concentration (TBO/CBO 2 50). In contrast, 15% change of configuration in the epoxide is observed in the case of the cis-2-butene reaction at low reactant concentration, and that fraction goes up with increasing matrix concentration. We attribute this marked difference to the fact that 0 atom transfer to trans-2-butene does not require a change of the dihedral angle about the central CC bond of more than 10 or 20° to form the oxirane biradical in its most stable, antiperiplanar conformation. Hence there would not be sufficient energy available in the torsional coordinate of the biradical for conversion to the syn conformer. In contrast, 0 atom transfer to cis-Zbutene without adjustment of the central dihedral angle would yield an oxirane biradical whose conformation corresponds to the maximum of the potential to internal rotation around the CC bond. Hence enough energy would be available in the torsional coordinate for relaxation to the anti form to occur. That the extent to which relaxation to the most stable oxirane biradical conformation takes place depends on matrix conditions is not surprising since even small changes of the potential to internal rotation by intermolecular interactions are expected to influence the relaxation process. Secondary Photolysis. The finding that the photoinduced expulsion of N O from the syn conformer ICof the trapped butyl nitrite radical gives exclusively isobutanal, while elimination of N O from the excited anti conformer I, yields exclusively trans2-butene oxide is a startling case of conformer specific photochemistry. Spectroscopic studies are in progress to elucidate, in conjunction with the wavelength dependence of the dissociation rate constants k2c and kzt (Table IV), the origin of the distinct differences of the photodissociation thresholds (613 nm vs 573 nm). In order to obtain insight into the factor(s) responsible for this product specificity, it is important to find out about the excited electronic state(s) of the butyl nitrite radical that are involved in the photodissociation process.
Nakata and Frei dicate that the stereochemistry of the reaction is controlled by the conformation of a transient intermediate, most probably an oxirane biradical. Within the framework of the proposed mechanism, spectroscopically detected butyl nitrite radical, produced concurrently with epoxide and NO, constitutes chemically trapped biradical intermediate. It serves as a probe for the conformation of the transient biradical that determines the stereochemistry of the epoxide. Hence analysis of the conformer distribution of the trapped butyl nitrite radical gives insight into the factors that control the stereochemical outcome of the photooxidation. There is no doubt that, in the framework of the proposed mechanism, the availability of a spin conserving path for epoxidation by N02('B2) is a key factor for the observed product specificity, making ring closure competitive with conformational scrambling of the oxirane biradical intermediate. Aside from this electron spin factor, the increase of the ring closure yields with increasing photolysis enei-gy on the one hand but nearly constant product stereoisomer ratios on the other hand suggests, in addition, control of the stereochemical outcome by rates of intramolecular energy flow. Energy migration from initially excited vibrations of the biradical intermediate into internal rotation around the central CC bond appears to be slow on the time scale of ring closure/combination with NO. The complete conformer specificity of the photodissociation of the syn add anti form of the butyl nitrite radical constitutes an interesting case of stereoelectronic control of a dissociation which we are currently investigating further. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76 SF00098. M.N. was supported by a grant from the Japan-US Cooperative Photoconversion and Photosynthesis Research Program.
Appendix Starting with a fixed reservoir of cis-2-butene.N02 pairs, the kinetic behavior of reactants, trapped nitrite radicals, and final oxidation products according to Scheme I is described by the following set of equations (in terms of concentrations) d[Rl/dt = -(kit + k ~ c+ k3t + hc)[RI (1) (2) d[Icl/dt = kIC[Rl - k2AIC1 (3) d[Itl/dt = klt[Rl - kzt[I,I d[CBO]/dt = k,,[R] (4) (5) d[TBOI/dt = k,t[RI + k2l[It1 d[MPI/dt = k2c[Ic1 (6) Integration of these equations, using the integrating factor method in the case of eq 2 and 3,23gives (expressed in absorbances) ( k , = klt + klcr k3 = k3t + k3c) AR = AORe-(k~+kdl (7)
V. Conclusions The results of the reaction of cis-2-butene with NO2 reported here, combined with those of the trans-2-butene system presented earlier, show that 0 atom transfer from nitrogen dioxide excited at long visible wavelengths in solid Ar affords epoxide as the sole oxidation product. Stereochemical retention2 is complete (>96%) in the case of trans-2-butene, and 70% for the cis alkene when low reactant concentration is used. Infrared spectroscopic analysis of products and photolysis wavelength dependence of yields in(19) Cvetanovic, R. J. J . Phys. Chem. 1970, 74, 2730-2732.
(20) Scheer and Klein argued for intermediacy of a transition structure with loose 0.-H interactions. Scheer, M. D.; Klein, R. J . Phys. Chem. 1970, 74, 2732-2733. (21) Cvetanovic, R. J.; Singleton, D. L. Reu. Chem. Intermed. 1984, 5, 183-226. (22) Mislow, K. Inrroduction to Stereochemistry; Benjamin: New York, 1965; p I18
(23) Smirnov, W. I. Lehrgang der hoheren Mathematik, Part 11; VEB Deutscher Verlag der Wissenschaften: Berlin, 1964; p 211.
J . Phys. Chem. 1989, 93, 7677-7680 AMP
= A~R-
-
klk:,k3 1 klCe-hr- mklck2c +-('l+kd' k2c - (kl + k3)
[
+
I)
(12)
In order to reduce the number of adjustable parameters, extinction coefficients of product bands that were used for curve fitting were estimated relative to that of the 812-cm-' trans-2butene oxide band. This was done by assuming that the N=O stretching modes of I, and I, have identical extinction coefficients,
7677
and by taking eCBo (997 cm-I) = 0.8tTB0 (812 cm-l) from gasphase spectra of the two molecules.' Then, absorbance changes measured upon photolysis of ICand I,, displayed in Table 111, were combined with relative intensities of product bands given in Tables I and I1 to give ratios t11(921)/&(837)/cTBo(812)/cMP(933)/ tCB0(997cm-I) = 1.0/2.5/1.0/0.7/0.8. Hence along with the six first-order rate constants only the parameter AoR/cR had to be adjusted. Registry No. I,, 121675-47-6; TBO, 21490-63-1; CBO, 1758-33-4; MP, 78-84-2; (z)-H3CCH=CHCH3, 590-18-1; NO2, 10102-44-0; NO, 10102-43-9.
Physicochemical Properties of Decyldlmethylammonlum Propanesulfonate and Its Homologous Compounds in Aqueous Medium Bianca Sesta Dipartimento di Chimica, Universitci di Roma "La Sapienza", Piazzale A. Moro, 5, Roma 00185, Italy (Received: November 14, 1988; In Final Form: May 1 1 , 1989)
The micellar properties of decyldimethylammonium propanesulfonate and its higher homologues, dodecyl and tetradecyl, have been investigated in aqueous solutions by densimetric, cryoscopic, and viscometric methods. The results were analyzed to find the partial molal volume, the osmotic coefficient, and the relative viscosity. The sudden change in these properties allows us to assign the critical micellar concentration, cmc. Data on the solutesolvent interactions and on the aggregation mechanism were drawn from the behavior of solutions. below and above the cmc.
Introduction The alkylbetaines are well-known zwitterionic surfactants, widely employed for industrial purposes. Thus, scientific interest has been largely focused on the synthesis, on the analytical determination of purity, and on some useful properties, such as their compatibility with ionic and nonionic surfactants and their high solubility in water.'" Some contributionsb1' have been devoted to understand the thermodynamic behavior and the kinetic mechanism of aggregation in water and in organic solvents. The present research, concerning single dispersed and micellar solutions of alkylsulfobetaines, is on these lines. The investigations have been performed on aqueous solutions of decyldimethylammonium propanesulfonate, and its homologues, dodecyl- and tetradecyldimethylammonium propanesulfonates. The dependence of viscosity on surfactant concentration, the changes of the molar volume at the cmc and the deviations of the osmotic coefficients, above the cmc, have been analyzed to obtain information on the shape and the molecular conformation of micelles, the micellization process being interpreted on the basis of the mass-action model. Experimental Section ( A ) Materials. Calbiochem decyl-, dodecyl- and tetradecyldimethylammonium propanesulfonate, hereafter indicated as B10, B 12, and B14, respectively, were purified by crystallization from acetone and dried under vacuum at 70 OC. They have been (1) Ernst, R. US.Patent 3,280,179, 1966. (2) Konig, H. Fresenius' Z.Anal. Chem. 1972, 259, 191. (3) Ernest,R.; Miller, E. J., Jr. In Amphoteric Surfactants; Bluestein, B. R.;Hilton, C. L., Eds.; Dekker: New York, 1982; p 137. (4) Kato, K.; Kondo, H.; Morita, A.; Esumi, E.; Meguro, K. Colloid Polym. Sei. 1906, 264, 737. (5) Essaddam, H.; Pichot, C.; Guyot, A. Makromol. Chem. 1988, 189, 619. ( 6 ) Tori, K.; Nakagawa, T. Kolloid Z.Z . Polym. 1963, 188, 47. (7) Tori, K.; Kuriyama, K.; Nakagawa, T. KolloidZ. Z.Polym. 1964,191, 48. (8) Herrmann, K. W. J. Colloid Interface Sci. 1966, 22, 352. (9) Swarbrick, J.; Daruwala, J. J. Phys. Chem. 1969, 73, 2627. (10) Jansson, M.; hyong, L.; Stilbs, P. J. Phys. Chem. 1987, 91, 5279. (1 1) Marignan, J.; Gauthier-Fourmier, F.; Appel, J.; Koum, A.; Lang, J. J . Phys. Chem. 1988, 92,440.
checked by thermogravimetric methods to determine the water content. Negligible presence of ionic impurity was detected by conductometric analysis. The absence of organic nonionic impurities was ascertained by tensiometric'* and melting point measurements. Water was purified by Millipore columns and degassed under vacuum. Its conductivity was x = lo-' Q-' cm-' , at 25 OC. ( B ) Methods. Solutions were prepared by weighing or by dilution, starting from concentrated stock of solutions. The samples were recovered overnight before being used. Density measurements were carried out at 25 O C on an A. Paar Model 602 apparatus. Frequencies, 1/ T, were converted into density, d, through the equation The constants A and B were determined with water and NaCl solutions as calibrating 1iq~ids.I~ The densimetric cell was connected to a Heto thermostatic bath, assuring a constancy within 0.01 "C. The temperature was tested by a Paar probe just around the samples. The accuracy of density values was 10" g ~ m - ~ . Ubbelhode viscometers, placed into a Haake thermostatic bath, regulated at 25 f 0.02 OC, were used to measure the flow time of the solvent, to, and of the solutions, t. The relative viscosity, qr, was calculated by the equation assuming v0 = 0.8903 cP. d and do are the density of solution and solvent, respectively. The flow time for water was 185 f 0.2 S.
The lowering of the cryogenic temperature was measured on 0.15 cm3 of solutions by a Knauer osmometer, equipped with a digital display and a Leeds and Northrup recorder. The calibration was performed by NaCl aqueous solutions of known molality. Osmotic references have been obtained from Scatchard and (12) Harrold, S. P. J . Phys. Chem. 1959, 63, 317. (1 3) Wagenbreth, H.; Blanke, W. Die Dichte des Wasser im Internationalen Einheitenrystem und in der InternationalenPraktischen Temperatuskaka oon 1968; PTB-Mitteilungen 6/71; Vieweg: Wiesbaden, FRG,1968; p 412.
0022-3654/89/2093-7677$01 .50/0 0 1989 American Chemical Society
