JACQUES LEMAIRE
612
should be totally excluded from the effects that tend to stabilize the low-pH state in PMA. The stability of this compact form seems, however, to be due primarily to a direct stabilization of certain conformations of the chain by interactions between its nearest neighbors rather than through the intermediary of a change in the structure of the surrounding solvent. This conclusion, of course, does not exclude the possibility of establishing important hydrophobic interactions between different PIMA molecules as suggested recently by E l i a ~ s a f These . ~ ~ should, however, become important at polyelectrolyte concentrations higher than those used in the present investigation. Finally, it may be pointed out that the spectroscopic results give some additional information about the a state of PMA. In the previous work of this laboratory21it has been shown that the infrared C=O stretching frequency of the -COOH group in PMA shifts gradually without splitting during the conformational
transition in DzO. Thus it is not likely that the vibrational states of the carboxylic acid group differ in the a and b forms of PMA. As in the latter case solvation of -COOH by water certainly occurs, the same solvation for most of the carboxylic groups in the a state may be expected. The absence of any influence on v(C=O) on the addition of methanol (up to 40%) shows that there is no infrared evidence for the replacement of water by methanol molecules at least in the first solvation layer of the -COOH group. The small changes observed for €AH' in the ultraviolet region on the addition of methanol may be attributed to long-range effects involving further solvation layers. Acknowledgment. The authors wish to thank Miss M. van Reisen for her help with the infrared measurements. (24) J. Eliassaf, Polymer Letters., 3, 767 (1965).
The Pyridine-Sensitized Isomerization of cis-2-Butene
by Jacques Lemairel Department o j Chemistry, University o j Texas, Austin, Texas
(Received August 19, 1966)
The yields of isomerization of cis-2-butene sensitized by pyridine have been measured. The yield is higher in the n --t -T* than in the -T --t -T* region, but both yields are low. Neither fluorescence nor phosphorescence of pyridine vapor is observed. The possible methods of energy degradation are discussed.
Introduction The electronic structure of the pyridine molecule has been extensively investigated. In the nearultraviolet absorption spectrum of pyridine there occurs an n--T* transition as well as a AT* transition.a The most successful treatment of n--T* transition uses simple molecular orbital theory in the one-electron approximation. The various singlet and triplet levels are predicted in reasonably good agreement with the data.3 Transition energies and intensities of the P - T * bands of pyridine have been ~ a l c u l a t e d . ~ ~ ~ The Journal of Physical Chemistry
Evanseobserved an S --+ T absorption band of pyridine in the presence of high pressures of oxygen. He con~~
(1) To whom all correspondence should be addressed at Laboretoire de Chimie Generale, University of Nancy, France. (2) (a) M. Kasha, DisCuSsiOns Faraday SOC.,9, 14 (1950); (b) H. P. Stephenson, J. Chem. Phys., 2 2 , 1077 (1954). (3) L. Goodman and R. W. Harrell, ibid., 30, 1131 (1959). (4) R.Pariser and R. G. Parr, ibid., 21, 466,767 (1953). (5) G.Favini, I. Vandoni, and M. Simonetta, Theoret. Chim. Acta, 3,45,418 (1965). (6) D.F.Evans, J . Chem. SOC.,3885 (1957).
PYRIDINE-SENSITIZED ISOMERIZATION OF CZ'S-2-BUTENE
613
cluded that the S T transition was H-a* since it showed a very small solvent shift. The enhancement of the S --+ T transition has been discussed theoretically by several authors.'-'O No fluorescence and no phosphorescence of pyridine has been observed either in the gas phase or in a solid matrix at low temperatures.'1*12 The sensitized isomerizations of the 2-butenes have been used to determine singlet-triplet crossover yields for certain m~lecules.'~The general validity of this method has never been fully proven even when energy relationships seem to be satisfactory. The reported identity of crossover yields in benzene and in pyridine seemed surprising. First results cast doubt on this identity, and after the completion of this work and personal communication of our results to Cundall,I4 a reinvestigation by him gave results in agreement with those reported below.
ments. Thus the wavelength range under the most favorable conditions exceeded 100 A although most of the intensity was in a 60-A range. The light transmitted through the cell was measured with an RCA 935 phototube connected to a Keithley micro-microammeter Model 410. The emission intensity was measured by an RCA 1P28 photomultiplier placed opposite the side arm of the cell. cis- and trans-2-butene were analyzed on a Wilkens Hy-Fi Aerograph chromatograph, Model 600C, equipped with a 7.6-m dibenzyl ether (20% on firebrick 60-80 mesh) column followed by a 3.05-m silicone gum rubber column acting as a scrubber. At room temperature, the two peaks of cis- and trans2-butene, respectively, were well separated. Reproducibility was improved by heating the sample to about 50" in the injector.
Experimental Section
Emission of Pyridine. Emission by pyridine, either fluorescence or phosphorescence, in the gas phase could not be observed either by use of the 1P28 tube or by use of an Aminco-Bowman spectrofluorimeter. Attempts to excite emission by wavelengths of 2400-3000 A failed. Experiments with the 1P28 tube equipped either with a Corning filter 754 (no. 9863) or with a filter 053 (no. 7740) were unsuccessful. Photochemistry of Pyridine. Long irradiation of pure pyridine in the gas phase by either T-H* (2480 i 128 A) or n--R* excitation (2650 f 64 A), did not produce gaseous products which could be detected either by the mass spectrograph or by gas chromatography. Long irradiations at 2480 =t 128 A and at 2650 f 64 A with 6 and 3 X l O I 3 photons absorbed per second gave negative results. For example, pyridine a t a pressure of 10 torr was irradiated in a cell 6 cm in length as follows: (a) X = 2480 128 A; 6 X 1013photons sec-' absorbed in the entire cell of 50-ml volume; time = 4 hr; T = 90"; (b) = 2650 f 64 A ; 3 X lOI3 photons sec-' absorbed in the entire cell of 50-ml volume; time = 23 hr; room temperature. Pyridine-Sensitized Isomerization of &-&Butene.
--+
Materials. Two samples of pyridine were used: Mallinckrodt, analytical grade, and United States Testing Co., spectrophotometric grade. A very careful search for impurities was made in view of effects reported by Cundall, et a l l 3 Both mass spectrometry and gas chromatography were used and the following impurities were found: (a) Mallinckrodt pyridine: 0.02570 benzene (not detectable by mass spectrometry since less than 0.1% benzene in a mixture of pyridine-benzene could not be determined) ; 0.1% HzO. (b) United States Testing Co. pyridine: no benzene; far less than 0.1% of an impurity X, not detectable by mass spectrometry and absent in pyridine Mallinckrodt; 0.001% H2O. Both pyridines were subjected to bulb-to-bulb distillation and different fractions behaved similarly. The benzene was Matheson Coleman and Bell fluorometric or chromatographic grade. cis-2-Butene (Phillips Petroleum Co., research grade) had as its only impurity 0.06% trans-Zbutene. A conventional high-vacuum, grease-free line was empl~yed.'~All experiments were carried out in a 50-ml T-shaped cell. The cell was encased in an aluminum block which reduced stray light and could be used as a furnace. The light sources were a Hanovia 5-100 medium-pressure mercury arc and an Osram HBO 500W super-pressure mercury arc. The grating monochromator (Bausch and Lomb, Model 33-86-45) has 16 A/mm as reciprocal linear dispersion. Since quantum yields were low, relatively large slit widths were used, 10 and 4 mm, respectively, for the entrance slit and 4 mm for the exit slit in most of the experi-
Results
(7) M. Tsubomura and R. S. Mulliken, J . A m . Chem. Sac., 8 2 , 5966 (1960). (8) G. W. King and E. H. Pinnington, J . M o l . Spectry., 15, 394 (1965). (9) G.J. Hoijtink, Mol. Phys., 3 , 67 (1960) (10) F.A. Matsen, J . Phys. Chem., 68, 3282 (1964). (11) C.Reid, J . Chem. Phys., 18, 1673 (1950). (12) G.J. Brealey, ibid., 24, 571 (1956). (13) R.B.Cundall, F. J. Fletcher, and D. G. Milne, Trans. Faraday Sac., 60, 1146 (1964). (14) R. B. Cundall, private communication. (16) Cf.1, Unger, J . Phys. Chem., 69, 4284 (1965).
Volume 71, Number 3 February 1087
JACQUESLEMAIRE
614
Table I : Pyridine-Sensitized Isomerization of cis-2-Butene Entrance X, A
slit, mm
Exit slit, mm
2400 2480 2537 2537 2650 2650 2650 2750 2780
10 10 10 4 10 10 4 10 4
4 4 4 4 10 4 4 4 4
Light absorbed, [photon aec-1 (60 ml) -11
8.96 X 8.37 X 11.1 x 3.27 X 28 X 15.7 X 4.60 X 2.71 X 1.65 X
Table I presents the data on numbers of molecules of trans-2-butene formed per photon absorbed. It should be noted that neither cis-2-butene nor trans-2-butene absorbs in the spectral ranges studied. Photochemical reaction between benzene and the 2butenes to give an adduct has been reported by Srinivasanl6 and confirmed by Wilzbach and K a ~ 1 a n . l ~The adduct is really one involving a benzene isomer formed photochemically. Evidence for a t least a trace of such reaction in the gas phase has been found. In the case of pyridine no evidence for a similar adduct was found. Possible Influence of Impurities. An efficient quenching impurity might be responsible for these low-quantum yields of sensitized isomerization. The amount of "quenching" impurity cannot exceed 0.1% of the pyridine. From experimental values of the oscillator strengths2bobserved in the liquid phase we can estimate the mean lifetimes of both the singlet states rn-fi E lo-' sec;
r,,*
S 10" sec
If these values have the same magnitude in the vapor phase, we may calculate the quenching cross section an impurity must have to produce the observed effect and compare it to the cross section of the cis-%butene. For example, in a mixture of 12 mm of pyridine, 20 mm of cis-2-butene1 and 0.01 mm of an impurity X, the number of collisions of one molecule of pyridine with the impurity and with the cis-2-butene are, respectively
zx= 2.3 x
2-butene formed
15.60 22.75 12.80 25.67 8.75 10.79 18 14.43 14.58
0.08 0.14 0.10 0.05 0.41 0.34 0.18 0.35 0.21
10" 1018
lo'* 10"
lo'* 1018 1018 10" 10'8
frons2-Butane quantum yields
0.005 0.006 0.006 0.006 0.014 0.018
0.020 0.081 0.078
state of lo-' sec, we may calculate ax2 = 4300 A2 and U'cts-butme = 2 A2. Quenching of the singlet state of pyridine by an impurity may be excluded. Unfortunately, we cannot estimate the lifetime of pyridine triplet. If we assume a mean lifetime of sec, then ax2 = 4.3-43 A2 and U2c(a--butene = to 0.002-0.02 A2. In the presence of cis-2-butene1 we must assume a high stereospecificity of the interaction between pyridine and an impurity to give high quenching of the triplet state. We may add that two samples of pyridine of different origins containing different impurities (benzene and X) gave the same experimental results. Moreover, the effect of small quantities of benzene was checked by addition of increasing quantities of benzene in the pyridine (0.025-0.5%). The results were not altered by these additions. Therefore, we feel that a quenching by impurities will not explain the low yields of triplet formation.
Discussion The quantum yields of the pyridine-sensitized isomerization of cis-Zbutene as given in Table I are very low, although they are somewhat higher in the n -t A* region than in the A -t A* region. Cundall and his co-workers have used the isomerization of the butenes as a means of determining triplet state yields. The preferential excitation of triplet emission from biacetyl has also been used for certain molecule^.^^ For benzene, fluorobenzene,l 6 ~ 2 1 13418--20
~
1 0 1 9 ~ ~ 2
where ux is the collision diameter in centimeters, and (we assume the same molecular mass for the impurity as for pyridine). From the mean life of the singlet The J o u d of Physical Chsmietry
% trana-
Irradiation time, hr
~-
(16) R. Srinivasan and K. A. Hill, J. Am. Chem. S O ~ .87, , 4663 (1965). (17) K. E. Wilxbach and L. Kaplan, &id., 88, 2066 (1966). (18) R. B. Cundall and A. S. Davies, Trans. Faraday Soc., 62, 1151 (1966). (19) H. Ishikttwa and W. A. Noves. Jr.. J . Am. Chem. SOC..84. 1602 (1962); H. Ishikawa and W. A.-Noyes, Jr., J . Chem. Phys., 37, 583 (1962).
CZ'S-8-BUTENE
615
and perfluorobenzene,22 agreement is good within a rather large experimental error between the two methods. I n the case of pyridine, the biacetyl method gave no positive results and the Cundall method as described above shows very little triplet formation. The question now arises as to whether triplet yields are in fact very small or whether the methods themselves are fallacious. The triplet-state yields based on the assumptions made by Cundall, et al., are given below. These are obtained by dividing the yields of trans-2-butene by 0.578, the fraction of excited cis-2-butene which becomes trans.13 Quantum Yield of Triplet-State Formation: (24006537 A, 0.010 f 0.001; 6650 A, 0.030 i 0.003; and 6750-6780 A, 0.14 f 0.01). Quite evidently, the yield is higher in the n -P T * than in the T + T * region. Nevertheless, the yields in both cases are very low and there is no emission. Thus if one accepts these values for the triplet-state yields, either products are formed which have so far escaped detection or there are means of energy degradation which account for nearly all of the energy. There are reasons based on analogy with related molecules for believing that triplet yields should be small for pyridine. The pyridazine (1,2-diazine) does not phosphoresce, even in hydrogen and rare-gas matrices, a t 4.2"K1 and the intersystem crossover was s h o ~ n to ~ be ~ pl o~3 to ~ 106 slower a t low temperatures than in pyrazine or pyrimidine, which strongly phosphoresce. The survey of the three diazines may allow us to point out that a lack of phosphorescence in azines a t low temperatures seems to be more a consequence of an inefficient crossover than a consequence of rapid, radiationless deactivation processes from the lowest triplet state. It seems reasonable to assume that radiationless deactivation from the lowest triplet state has a similar importance for all these mono- and diazines.
Therefore, the lack of phosphorescence of pyridine a t low temperatures may reflect the inefficient crossovers our results show. Nevertheless, the formation of an adduct with benzene under certain experimental conditions raises a reasonable doubt about the universal validity of the isomerization technique for triplet-state measurement. An excited singlet state of pyridine in the spectral region used could not transfer electronic energy directly to cis-Pbutene (unless spin conservation were violated). Nevertheless, an intermediate complex between excited pyridine and the double bond of an olefin is possible. This complex could dissociate to give two normal molecules. It is wise not to pursue a discussion of these possibilities further. There may be methods for identifying such complexes. Isomerization of benzene and of benzene derivatives a t wavelengths around 2500 A is firmly established. The possibility of similar isomerizations with pyridine certainly exists, but no evidence for them has been reported. Such isomerizations might provide a pathway for energy degradations since the isomers would be formed endothermically from normal molecules to the extent of 2 or 3 ev. Attempts by others to find pyridine isomers have failed.
PYRIDINE-SENSITIZED ISOMERIZATION O F
Acknowledgments. The author is greatly indebted to Professor W. A. Noyes, Jr., for his help and encouragement throughout the course of this work. He also wishes to thank colleagues for many helpful discussions. Financial support from the Robert A. Welch Foundation is also gratefully acknowledged. (20) W. A. Noyes, Jr., W. A. Mulac, and D. A. Harter, J. Chem. Phys., 44, 2100 (1966). (21) D. Phillips, private communication. (22) D. Phillips, in press. (23) M . A. El Sayed, J. Chem. P h y s . , 36,573 (1962). (24) M. A. El Sayed, ibid., 38, 2834 (1963).
Volume 71 I Number 9 February 1967
