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.
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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.
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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 C h e n * ? l Mallinckrodt Chemical Laboratory, Harvard University, Cambridge, Massachusetts 02138 (Received: November 26, 1991; In Final Form: January 8, 1992)
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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.
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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
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(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
2026 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
Letters n
exparimental 238
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Figure 1. Survey scan of the 238-250-nm region by mass-resolved, 1 + 1 resonant multiphoton ionization for m / e = 41. Allyl radicals, C3H5, are produced by supersonic jet flash pyrolysis of allyl iodide.
trometer. The jet was skimmed, photoionized, and mass-analyzed by time-of-flight. One-photon vacuum-W photoionization (1 18.2 nm, 10.49 eV) of the pyrolysate showed exclusively two peaks: m / e = 41 for C3H5and m / e = 127 for 1'. An injection-seeded Nd3+:YAG laser (Spectra-Physics GCR-3, Model 6300 seeder) was used to pump a dye laser (Spectra-Physics PDL-3, DCM dye) at 20 Hz. The dye laser output was frequency-doubled and mixed with the YAG fundamental (SpectraPhysics WEX-1) to produce tunable far-UV radiation in the 238-250-nm wavelength region with a pulse energy of approximately 1 mJ. The far-UV laser radiation was isolated from the other frequencies using an Inrad four-prism filter. The far-UV spectral line width is estimated to be 0.1 cm-I. The far-UV radiation was focused into the ionization region of the spectrometer using a 350 nm focal length fused silica spherical lens. Spectra were Obtained by scanning the dye laser and monitoring the integrated area of the m / e = 41 signal as a function of wavelength. The light was slightly defocused in order to avoid fragmentation of the radicals or radical cations; under tightly focused conditions, C,H, (x = 1-3) fragments with m / e < 41 were observed. Allyl iodide-d5was prepared from allyl alcohol-d6 (MSD Isotopes) by the method of Landauer and Rydon.* The 118.2-nm photoionization mass spectrum of pyrolyzed allyl iodided, showed only two peaks: m / e = 46 for C3D5 and m / e = 127 for 1'.
Results and Discussion Figure 1 shows the 1 1 resonant MPI spectrum of allyl radical in the 238-250-nm region. The origin band of the transition, with a maximum at 248.15 nm, is detailed in Figure 2. To the blue of 238 nm, we observe additional bands that are progressively weaker and broader. We assign the 248.15-nm band to the transition from the vibrationless ground state to a vibrationless excited state based on three lines of argument: (i) No large bands are observed to the red of the 248.15-nm band. We expect cooling of the radical by supersonic expansion subsequent to pyrolysis to have suppressed hot bands in the spectrum. (ii) We find that the 248.15-nm band maximum exhibits only a small spectral shift of 14 an-I to the red upon complete deuteration of the radical. Other bands in Figure 1 shift to the red by much larger amounts. (iii) A time-of-flight photoelectron spectrum of allyl radical obtained in our lab by 1 1 resonant MPI through the 248.15-nm band shows only a single, sharp peak corresponding to the adiabatic ionization potential of the radical. We note that the one-photon vacuum-UV photoelectron spectrum of allyl radical reported by Houle et aL9 (He I) or recorded in our laboratory (1 18.2 nm) shows vertical and adiabatic ionization potentials differing by 50 meV, indicative of a geometry change upon ionization. Our current 1 + 1 MPI photoelectron results, and the relative intensities among the bands in Figure 1, suggest that the equilibrium geometry of the excited state resembles that of allyl cation. Having established that the 248.15-nm band corresponds to the origin for the observed transition, we assigned the upper-state electronic symmetry by simulationlo of the partially resolved
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Figure 2. The 248.15-nm origin band of the e[22B,] k[l2A2]transition, scanned at a laser bandwidth of 0.1 cm-' from 40363.7 to 40214.9 cm-l. Each "line" is an unresolved P-branch subbandhead. The Rbranches, to the blue of the origin, while less intense than in the simulation, are nevertheless present in the spectrum above and can be seen clearly in the more compressed spectrum of Figure 1.
structure in that band, in conjunction with simple molecular orbital arguments. One-electron excitations from the principa! config uration of the ground state, (core)(6aJ2(4bJ2( lbJ2( la$X[ 12A2], lead to low-lying excited states with principal electronic configurations and symmetries of (core)(6a1)2(4b2)2(1bl)1(la2)2[ l2BI], (core)(6a1)2(4b2)2(1bl)2(2bl)1[22B11,(~ore)(6a,)~(4b,)~(5b~)l(lb1I2[12B2],( ~ o r e ) ( 6 a , ) ~ ( 4 b ~ ) ~2...(7a13s)1[12Al(3s)], (lb,) and (core) (6a 1)2(4b2)2(1bl)2...(8a14s)I [22A1(4s)l. Ab initio calculations, which we use only as approximate guides, by H a et al." at the CI level give vertical energies for those excited states below 6 eV (207 nm) of 3.13 eV [12Bl], 5.26 eV I12B2],5.33 eV [12Al], 5.52 eV [22Bl],and 5.86 eV [22A1]. The A[l2BI] state has been observed by Currie and RamsayI2 in a cry0 enic matrix at 404 nm (3.07 eV) as a broad absorption. The [12A1(3s)]state is forbidden in one-photon absorption from the ground state but has been seen as a two-photon resonance by Hudgens et aLI3 Sappey and WeisshaarI4 recorded the same spectrum with much higher resolution, identifying several vibronic bands and setting the origin of the B[12A1(3s)] state at 4.97 eV, only 249 cm-l to the red of our 248.15-nm band origin. The upper state of the 248.15-nm (5.00-eV) band in the present work can therefore be plausibly assigned to either the 12B2or 22B1excited (valence) states on energetic grounds. Inspection of a C, character table indicates that a B2 A2 transition would have its transition moment aligned along the inertial axis perpendicular to the molecular plane, giving a type C band profile. Similarly, a B, A2 transition would have its transition moment aligned along the long axis of the molecule, giving a type A band profile. Allyl radical is a near-prolate asymmetric top ( K = -0.9) for which the type C and type A bands should resemble perpendicular and parallel bands, respectively, in the prolate symmetric top limit. Simulation as a type A band is most consistent with the observed structure of the 248.1 5-nm band. We accordingly assign that band to the origin of the c[22Bl] X[ 12AJ transition. The simulated band is shown below the observed spectrum in Figure 2. While the spectrum is insufficiently resolved for a detailed fit of the band contour to a full set of rotational constants, a reasonable model can be used for the simulation. Because the gross rotational structure in the observed spectrum is most sensitive to changes in the CCC bending angle, we take, as a model, an excited-state radical for which all bond lengths and angles, except
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(10) Spectral band simulations were performed with the ASYROT-PC program: Judge, R. H. Compur. Phys. Commun. 1987, 47, 361. (11) Ha, T. K.; Baumann, H.; Oth, J. F. M.J . Chem. Phys. 1986, 85,
1438.
(8) Landauer, S. R.; Rydon, H. N.J . Chem. Soc. 1953, 2224. (9) Houle, F. A.; Beauchamp, J. L. J . Am. Chem. SOC.1978, 100, 3290.
(12) Currie, C. L.; Ramsay, D. A. J . Chem. Phys. 1966,45,488. (13) Hudgens, J . W.; Dulcey, C. S. J . Phys. Chem. 1985,89, 1505. (14) Sappey, A. D.; Weisshaar, J. C. J . Phys. Chem. 1987, 91, 3731.
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J. Phys. Chem. 1992, 96, 2027-2029 the CCC bending angle, are fixed at those in the experimental geometry of the ground-state allyl radical.’ An upper state angle of 1 17.5’, rotational temperature of 150 K, and line width of 1.0 cm-’ yield a close match with the rotational structure in Figure 2. The rotational constants used for the simulation are A” = 1.803, E N = 0.328, C N =0.278, A ‘ = 1.619, E’= 0.351, and C‘= 0.288 cm-I. Because allyl is so close to the prolate limit, only AK, = 0 subbands were included in the simulation. We note that a CCC bond angle of 117S0 is quite close to the 118’ predicted for allyl cation by ab initio calculation^^^ and is therefore consistent with our photoelectron results above. The large decrease in A displaces each successive subband origin to the red. In addition, the increase in both B and C causes each subband to shade to the blue. Each of the features to the red of the band maximum is the unresolved P-branch head for a subband with a single value for K,. The band maximum itself consists of several unresolved low-K, P-branch subbandheads. Because of the large change in A, the subband Q-branches do not overlap to produce a central maximum as is typical for asymmetric rotor type A bands. The R-branches produce a broad feature to the blue of the band maximum which, while much less prominent in the spectrum than would be expected from the simulation, is nevertheless present in all scans over the 248.15-nm band. We can speculate that the relative weakness of the R-branches with respect to the P-branches may be due to an electronic transition moment that is not constant over the band. Such behavior would not be surprising for a state that is heavily mixed with other nearby excited Etates. The vibrational structure of the C[22Bl] R[12A2]transition evident in Figure 1 affords no simple interpretation. The gross spacings of 1.390 cm-I can be attributed to excitation of the totally symmetric u7 CCC bending vibration in the excited state.I6 A long progression in that mode is consistent with the decrease in equilibrium CCC bond angle, upon electronic excitation, that comes out of the simulation of the rotational structure of the 248.15-nm band. At a more detailed level of analysis, the richness of the spectrum in Figure 1 is puzzling, especially because the absence of hot bands suggests that sequence bands should also be weak. We are currently considering two possibilities: (i) nonplanarity in the excited state would introduce more allowed bands and further complicate the spectrum with a double-well
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(15) Wiberg, K. B. Private communication. Calculations were at the HF/6-31G* level. (16) This value for u, for the C[2*B excited state would be similar to that reported for the same vibration in the [12A1(3s)]excited state (ref 14) and the cation (ref 9).
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potential problem, or (ii) vibronic coupling may allow levels of the 8[l2AI(3s)]state, the onephoton forbidden 3s RydbeJg state whose origin is only 249 cm-l to the red of the C[22Bl] X[12A2] origin, to borrow intensity from allowed transitions to levels in c[22Bl] and appear as extra bands. The coupling could be promoted by one quantum of the uI2CH2symmetric twisting mode of bl symmetry. Callear and Lee2 reported a strongly broadened series of absorpLions centered at 224.9 nm which they assigned to the C[22Bl] X[12A2] transition. None of the bands were rotationally resolved, and no vibronic assignments of individual bands were given. Interestingly, most of the bands in Figure 1 were reported i, that work as “weak bands, almost line-like features” but not as part of the ‘main system”. No regularities were reported in the spacings. We suggest a tentative interpretation for the far-UV spectrum of allyl radical. The C[22Bl] X[12A2] transition, whose origin band lies at 248.15 nm, shows an extended Franck-Condon envelope due, at least in part, to a substantial change in the equilibrium CCC bond angle upon excitation. Approximately 3500 cm-l above the origin, a fast radiationless process markedly shortens the excited-state lifetime, broadening the spectrum and introducing irregularities to the band spacings. This is consistent with our observation that the bands to the blue of 238 nm are progressively weaker and broader in 1 + 1 resonant MPI despite their increasing intensity in the absorption measurements by Callear and Lee. While the spectroscopic studies presently favor no one radiationless process over another, ab initio calculation^^^ have suggested that a disrotatory closure of allyl radical to cyclopropyl radical is favorable for the C[22Bl]state. If so, time-delayed pumpprobe 1 1 resonant MPI spectroscopy of allyl radical may prove to be an interesting kinetic probe of excited-state dynamics. Further wGrk is undezway to identify and label the vibronic bands of the C[22Bl] X[12A2]transition using selective isotopic substitution, photoelectron spectroscopy, and ab initio calculations.
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Acknowledgment. The authors acknowledge helpful discussions with Professors P. B. Kelly (UC Davis) and J. C. Weisshaar (U. of Wisconsin, Madison). Support from the Department of Energy and the Exxon E d ~ c a t i oFoundation ~l is gratefully acknowledged. Funding from the National Science Foundation for the purchase of the lasers used in this work is also acknowledged. (17) Merlet, P.; Peyerimhoff,S. D.; Buenker, R. J.; Shih, S. J. Am. Chem. SOC.1974, 96, 959. Farnell, L.; Richards, W. G. J. Chem. Soc., Chem. Commun. 1973, 334.
Modeling the Orientational Ordering Transition in Solid Cs0 Michiel Sprik,* Zurich Research Laboratory, IBM Research Division, 8803 Riischlikon, Switzerland
Ailan Cheng, and Michael I;. Klein Department of Chemistry and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 (Received: December 6, 1991) We propose a new intermolecular potential for Ca molecules that not only reproduces the correct low-temperature structure but also correlates a wide range of experimental properties including the molecular reorientational time in the room temperature rotator phase, the volume change at the orientational ordering transition, and the librational frequencies in the low-temperature phase. Recent structural studies have shown that at low temperatures The the molecules in crystalline Cso are orientationally observed Pa5 structure has been rationalized by noting that *Corresponding author. BITNET address: SPR at ZURLVMI.
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electron-rich short interpentagon bonds face electron-poor pentagon centers of adjacent molecules.2 To date, static structure ( 1 ) Heiney, P. A.; Fischer, J. E.; McGhie, A. R.; Romanow, W. J.; Denenstein, A. M.; McCauley, J. P., Jr.; Smith, A. B., III; Cox, D. E. Phys. Rev. Leu. 1991.66, 291 1-2914.
0 1992 American Chemical Society
