J. Phys. Chem. 1992, 96,2023-2025 aaa*
(C-C)
I
no*
(C-GI)
\
I
\.
cn,coci C-C -Resetion fission
F
Coordinate ___t C-CI flsrlon
i 3. Schematic reaction coordinates for C C and CC1 fission upon
l(n,x*) excitation at 248 nm.
The C-C bond fwion to form ground-state products can only occur via intersystem crossing to the triplet surface (shown for nonplanar geometries), while C-Cl fission can occur directly on the singlet A” electronic state. The dotted lines show the electronic states before configuration mixing. The schematic C-C reaction coordinate is shown for dissociation to a bent COCl radical in analogy with calculations on acetaldehyde.2 Subpicosecond selective C-Cl fission can proceed directly on the resulting singlet A” electronic state, shown schematically in Figure 3, which evolves adiabatically from ‘nr*(C=O) character in the Franck-Condon region to np(Cl)u*(C-Cl) character beyond the barrier along the C-Cl reaction coordinate. A full report1*on (17) Robin, M. B. Higher Excited States of Polyatomic Molecules III; Academic Press: New York, 1985; p 26. (18) Person, M. D.; Kash, P. W.; Butler, L. J. Submitted to J . Chem. Phys.
2023
this work together with related work on the competition between a-and @-bondfission in bromoacetyl chloride19 is forthcoming. Although these authors could not find any published electronic structure calculations of the C-Cl reaction coordinate on the ‘A” excited electronic state of acetyl chloride (only calculations of the vertical transition energy, etc.), Chandra3 has previously considered the possibility of fission of the C-CI bond on the triplet surface. His MND0/3 calculations identify a 3(n~0*) configuration which increases in energy as the C-C bond lengthens but decreases in energy as the C-Cl bond lengthens, so an avoided crming of this state with the 3n1r* state could result in fission of the C-Cl bond over the C-C bond on the triplet surface. We should note that although C-C bond cleavage upon Inas excitation usually occurs via internal conversion or, more often, intersystem crossing as described above, there is strong evidence that terr-butyl alkyl ketones can undergo a C-C bond cleavage via the singlet excited state. Yang and Feit’szo early product quantum yield measurements (as a function of triplet quencher) on t-C,H9COCH3 in hexane at 3 130 A suggest a quantum yield of 0.22 for type I cleavage via the singlet excited state. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-9007769 and the Office of Naval Research under Grant N00014-87-K-0546. L.J.B. gratefully acknowledges the support of a Camille and Henry Dreyfus Foundation Teacher-Scholar Award. We thank Prof. N. C. Yang for bringing the work on tert-butyl alkyl ketones to our attention. (19) See the short communication of the bromoacetyl chloride results in: Person, M. D.; Kash, P. W.; Schofield, S. A.; Butler, L. J . Chem. Phys. 1991, 95, 3843. (20) Yang, N. C.; Feit, E. D. J. Am. Chem. Soc. 1968,90, 504.
Dynamic Demonstration of Volume Phase Transition in Polyelectrolyte Gels Seiji Katayama,* Shunsuke Kazama, and Hisashi Yoshioka University of Shizuoka, School of Pharmaceutical Sciences, 395 Yada Shizuoka, 422 Japan (Received: November 7, 1991; In Final Form: January 14, 1992)
The motion of spin-labeled network polymers in a sodium acrylate gel was examined by electron paramagnetic resonance during volume phase transition. The discontinuous mobility change at transition may be considered consistent with the discontinuous volume change at transition. This is the first, strong evidence for a reversible, discontinuous volume phase transition at the molecular level.
Introduction First referred to by Flory,’ the tremendous swelling behavior of polyelectrolyte gels was studied by a number of investigators in the 1950s. Later, the swelling behavior was extended to studies concerning the volume phase transition of polymer g e l ~ . ~ J Polymer gels with an ionizable group undergo reversible, discontinuous volume changes upon changes in solvent composition, salt, pH, and temperature. This is the volume phase transition occurring between a swollen gel and a collapsed gel and nowadays is taken as a typical example of fmt-order phase transition. Since volume phase transition was successfully theorized on the basis of a mean field theory by Tanaka? volume phase transition has
gained wider acoeptance with various findings of novel phenomena and theories. In recent years, scientific and technological aspects of volume phase transition have become increasingly important, because of the mechanochemical properties of ionic gels. Recently, particular attention has been given to the microscopic environment of an ionic gel before and after its volume phase transition and especially at transition. A number of conventional approaches to volume phase transition have been based mainly on macroscopic observations of the equilibrium volume of gels and rarely on their elasticity.5-6 The only microscopic approaches performed have been by examining the critical fluctuation of gel networks by laser light scatterings.’+ The features of the mi-
( I ) Flory,P.J. Principles ofpolymer Chemistry;Come11 University Press: Ithaca, NY, 1953; Chapter 9. (2) Tanaka, T. Sci. Am. 1981, 244, 110. (3) Katayama, S. In Mechaochemisrry; Sasabe, H., Ed.; Marzen Press: Tokyo, 1989; pp 55-118.
(4) Tanaka, T.; Fillmore, D. J.; Sun, S.-T.;Nishio, I.; Swislow, G.; Shah, A. Phys. Reo. Lerr. 1980, 45, 1636. ( 5 ) Ilavsky, M.; Hrouz, J.; Havlicck, I. Polymer 1985, 26, 1512. ( 6 ) Ilavsky, M.; Hrouz, J.; Bouchal, K. Polym. Bull. 1985, I4,301. (7) Tanaka, T.; Ishiwata, S.; Ishimoto, C. Phys. Reu. Lerr. 1977,38,771.
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0 1992 American Chemical Society
2024 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
Letters
100
IO
OBSERVED
11 \
I
SSIMULATED IMU LATED
0.1
0.1
20
40
60
80
100
( V / V K)
I
1mT
ACETONE CONTENT IN ACETONE-WATER MIXTURES
Figure 1. The right-side Y axis shows the equilibrium swelling ratio (V/ V,) of a spin-labeled gel immersed in acetone-water mixtures, where Vand V, are initial and final equilibrium volumes of the gel, respectively. The black circles indicate volume phase transition. The left side shows the correlation time, T ~of, a spin-labeled gel versus the acetone concentration in the acetone-water mixtures. The white circles indicate the . correlation times were estimated by simulation of the change in T ~ The EPR spectra observed for the spin-labeled gel based on Freed's line shape theory.
Figure 2. Curve A is an EPR spectrum for the collapsed state of a spin-labeled gel immersed in acetonewater mixture solvent (61% acetone composition). Curve B is an optimized spectrum estimated by simulation based on Freed's line shape theory. The optimized rotational diffusion constants are R,, = 1.0 X lo8 rad/s and R , = 0.07 X lo8 rad/s. The other parameters are described in the text. X
croscopic environment of polymer gels that become apparent during volume phase transition remain to be clarified. The present study was conducted to clarify the microscopic environment of an ionic gel by examining the mobility of the gel network by electron paramagnetic resonance (EPR). A spin-label method1+13 is particularly useful for investigating the microscopic environment of polymer gels because the EPR spectra of a spin-labeled polymer in a gel can provide information on the local mobility of the gel polymer at the molecular level. EPR spectra of spin-labeled gels immersed in acetonewater mixtures were observed for thermally equilibrated swollen and shrunken states formed during the volume phase transition. The correlation times, T ~of, a spin-label bound to the gel network polymer were estimated by a fitting technique based on Freed's line shape theory."-'*
Experimental Section It is generally difficult to make spin-labeled gels, because the spin-label acts as an inhibitor for radical polymerization. Synthesis of a spin-labeled gel is an important part of the present study. Samples of the spin-labeled gel were prepared as follows.lo A pregel was first made by copolymerization of an aqueous solution which was prepared by dissolving acrylamide (1 .O g), sodium acrylate (500.0 mg), N,N'-methylenebis(acry1amide) (26.7 mg), and radical precursor (200.0 mg) in water to a final volume of 20 mL. The radical precursor was previously produced by the equimolar reaction of acryloyl chloride with 4-amino-2,2,6,6tetramethylpiperidine. The pregel was neutralized under mild alkaline conditions (0.1 N, NaOH solution) and then oxidized by an aqueous solution containing HzOz,EDTA, and tungstic acid to afford the final spin-labeled gel. Preparation of the pregel was conducted by solution copolymerization in micropipets (diameter 1.6 mm) at 50 OC for 1 h after the addition of ammonium persulfate (40 mg). The spin-labeled gel was cut into 10-mm-long cylindrical pieces, which were immersed in acetonewater mixtures of various compositions. After equilibrium was reached, the diameter of each cylindrical gel sample was measured and each (8) Nishio, I.; Sun, S.-T.; Swislow, G.; Tanaka, T.Nature 1979, 281, 208. (9) Nishio, I.; Swi$low,G.; Sun, S.-T.;Tanaka, T.Nature 1982, 300, 243. (10) Gaffney, B. J. In Spin Labeling; Berliner, L. J., Ed.; Academic Press: New York, 1976; p 214. ( 1 1) Freed, J. H. In Spin Labelina; Berliner, L. J., Ed.; Academic Press: New York, 1976; Part 1;pp 53, 1327 (12) Hwang, J. S.; Mason, R.; Hwang, L. P.; Freed,J. H. J . Phys. Chem. 1975 -_._,.79~,AR9 (13) Pilar, S.; Labsky, J. J . Phys, Chem. 1986, 90, 6038.
H -C I
- C O - NtiL 4
I
Y
P'd H
I
I - C - C O - NH,,
I
,
L
Figure 3. Schematic representation of the three modes of rotational R,,, and R,) along a molecular axis. R, and R,, are less diffusion (Rz, than R,, and therefore R, acts as the determining factor for the EPR line shape. A,, is the principal z component of the hyperfine coupling constant. The other parameters are described in the text.
gel volume (V) was estimated by cubing the diameter.
Results and Discussion Figure 1 shows the volume data of the spin-labeled gel immersed i n the acetonewater mixtures together with the correlation time ( T ~ )of the spin-label incorporated in the gel network. The spin-labeled gel held a swollen state when immersed in mixtures below 60% acetone and a collapsed state in mixtures above 60% acetone. A discontinuous volume change could be clearly observed at the 60% acetone composition. The observed volume phase transition characterized by a swollen-shrunken behavior with a discontinuous volume change was the same as previously reported.I4 EPR spectra were observed for the spin-labeled gel a t each equilibrium state during the volume phase transition. The swollen gel in mixtures below 60% gave EPR spectra with a sharp triplet. These were motional narrow spectra which could be specified by rapid motion of 10-9s/rad or less. Collapsed gels in mixtures above 60% gave EPR spectra with a broadened triplet, ranging from slow tumbling spectra to rigid limit spectra. From the observed spectra, the mobility of the gel network at each stage during the volume phase transition was estimated.
(14) Hirokawa, Y .; Tanaka, T.;Katayama, S. In Microbial Adhesion and Aggregafion; Marshall, K. C . , Ed.; Springer-Verlag: Berlin, 1984; p 177.
J. Phys. Chem. 1992, 96, 2025-2027 Figure 2 shows a spectrum of collapsed gel immersed in 61% acetone, together with a simulated curve. Parameters used for the simulation were A, = 5.1 G, A, = 5.1 G , A, = 37.6 G, g,, = 2.0086, gyy= 2.0066, g,, = 2.0032, and 6 = where A,, A?, and A, were hyperfine coupling constants for each principal axis, g,,, gyy,and g,, were g values for each principal axis, and 6 was the angle between the z axis of the molecular coordinate and the p z orbital of the nitrogen atom. The rotational diffusion constants optimized for the EPR spectrum of collapsed gel immersed in 61% acetone were RII= 1.0 X lo8 rad/s and R , = 0.07 X lo8 rad/s. Anisotropic motions can be generally analyzed by examining the mobility of a spin-label bound to the gel network polymer. The anisotropic motions can be actually represented by the three modes of rotational diffusion (Rx,R,,, RZ),I3as shown in Figure 3. R, and Ry are perpendicular terms (R,) to the molecular axis of the spin-label, and R, is a parallel term (R,,)to the molecular axis of the spin-label. The intrinsic motion of the spin-label can be estimated by time and ensemble averages of the rotational diffusions. Based on the line shape theory of Freed and Polnaszek,16 optimum rotational diffusions can be obtained by simulation of the observed EPR spectra. The average correlation time, T,, a measure of the intrinsic motion of a spin-label, can be calculated from the average rotational diffusion using the equation 7, = 1/6R, where R is (R,,R,)1/2. In the present case, R , could (15) Capiomont, A. Acta Crystullogr., Sect. E 1972, 28, 2298. (16) Polnaszek, C. F. Ph.D. Thesis, Cornell University, Ithaca, NY, 1974.
+
2025
act as the dominant factor for determining the EPR line shape. Based on this 7c estimation procedure, the T, values of spinlabeled gels immersed in acetone-water mixtures of various compasitions were estimated. The correlation time, r,, was plotted as a function of acetone composition in Figure 1. As the acetone composition increases, T, increased linearly in the region below 60% acetone and subsequently increased gradually in the region above 60% acetone. A discontinuous T, change definitely occurred precisely at 60% acetone. As a consequence, the motion ( T c ) of the spin-label was characterized as follows: a linear increase (from 1.1 X to 3 X 1O-Io s/rad), a discontinuous change (from 3 X to 2 X lo4 s/rad), and a gradual increase (from 2 X 10-9 to 5 X s/rad) with increases in acetone concentration. As a result of the present experiment, it was found that the T, behavior of the spin-label incorporated in the network polymer is similar in pattern to the volume behavior of the gel and, in particular, that the discontinuous T, change at 60% acetone is in fair agreement with the discontinuous volume change at the same acetone composition. It may therefore be concluded that the mobility of a spin-label bound to a network polymer could secondarily reflect the overall mobility of the gel network polymer, although the spin-label is primarily sensitive to its own local environment and the dynamical behavior of the gel polymer is consistent with any macroscopic volume behavior a t the time of the volume phase transition. This is the first observation that presents strong evidence for the volume phase transition of an ionic gel on a dynamic level. The present results should facilitate future microscopic studies on the volume phase transition of polymer gels.
-
1 1 Resonant Multiphoton Ionization Spectrum of the Allyl Radical. Rotational Structure in the E[22B,] %[12A2] Origin Band David W. Minsek, Joel A. Blush, and Peter Chen*?l Mallinckrodt Chemical Laboratory, Harvard University, Cambridge, Massachusetts 02138 (Received: November 26, 1991; In Final Form: January 8, 1992)
r
A mass-selected,partially rotationally resolved, resonant multiphoton ionization spectrum of the allyl radical, C3H5,is re rted. Phofoelectron spectroscopy,isotopic labeling, and rotational analysis establish that the band system corresponds to the [22Bl] X[12A2]transition, with an origin band at 248.15 nm. Spectral simulation indicates that the equilibrium CCC bond angle of the radical decreases from 124.6' in the ground state to 117.5' in the excited state.
-
We wish to report a partially rotationally resolved, mass-selected 1 1 resonant multiphoton ionization (MPI) spectrum of the allyl radical, C3H5. This is the first electronic spectrum for this simplest of all conjugated r-radicals to show any rotational structure. We assign the upper state of the transition to the e[2*B1]state with origin at 40 306 cm-I, which, while previously observed2 in absorption at low resolution, had neither been assigned nor analyzed. A preliminary rotational analysis yields a decrease in the CCC bond angle from 124.6', reported for the ground state,3 to approximately 117.5' in the excited state. Comparison of photoelectron spectra taken with 10.49-eV one-photon ionization and 1 1 MPI via the e[22B1]state indicates that, while the CCC bond angle of allyl radical decreases from 124.6' to 117.5' upon
+
+
(1) NSF Presidential Young Investigator, David and Lucile Packard Fellow, Camille and Henry Dreyfus Teacher-Scholar, Alfred P. Sloan Research Fellow. (2) Callear, A. B.;Lee,H. K. Truns. Furuduy SOC.1968, 64, 308. (3) Vajda et al. (Vajda, E.;Tremmel, J.; Rozsondai, B.; Hargittai, I.; Maltsev, A. K.; Kagramanov, N. D.; Nefedov, 0. M . J. Am. Chem. Soc. 1986, 108,4352) report CC = 1.428 A, CH = 1.069 A, CCC = 124.6', and CCH = 120.9' for the ground state of allyl radical by gas-phase electron diffraction.
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excitation, there is little or no change in bond angle going from the e[22B,] excited state of allyl radical to allyl cation. This structural similarity constitutes the only experimental information on the geometry of the allyl cation. Experimental Section Allyl radicals were produced in the nozzle of a supersonic jet expansion by the pyrolysis of allyl iodide. Both the molecular beam time-of-flight mass spectrometerd7 and the magnetic-focusing time-of-flight photoelectron ~ p e c t r o m e t e rhave ~ ~ been previously described. The nozzle consisted of an electrically heated 1.0mm4.d. silicon carbide tube with a heated zone of 15.0 mm extending to the sonic orifice, as detailed elsewhere.' Allyl iodide (1-Torr partial pressure) was seeded into 2 atm of helium and expanded via a pulsed valve (General Valves Series 9) at 20 Hz through the hot nozzle into the source region of the mass spec(4) Blush, J. A.; Park, J.; Chen, P. J. Am. Chem. SOC.1989, 1 1 1 , 8951. (5) Minsek, D. W.;Chen, P. J. Phys. Chem. 1990, 94, 8399. (6) Clauberg, H.; Chen, P. J . Am. Chem. Soc. 1991, 113, 1445. (7) Clauberg, H.; Minsek, D. W.; Chen, P . J . Am. Chem. Soc. 1992, 114, 99.
0 1992 American Chemical Society
