8817
J. Phys. Chem. 1992, 96,8817-8820
(2) Rabolt, J. F.;Swalen, D. J. In Spectroscopy ojSurface; Clark, R. J. H., Hester, R. E., Eds.; John Wiley and Sons: 1988; Chapter 1. (3) Swalen, J. D.; Schlotter, N. E.; Rabolt, J. F. J. Adhesion 1981, 13, 189. 414 (4) 1 Rabolt, J. F.; Schlotter, N. E.; Swalen, J. D. J . Phys. Chem. 1981,85,
OWG-R techniques cannot be applied to the thin film because of large attenuation. (2) The OWG-R spectra established the difference in structure between the interface and surface regions of the iron phosphate thin film. (3) The interface region assumes an amorphous state, giving rise to a P-O stretching band near 1060 cm-I, while the surface region contains a crystalline part, which gives a P-0 stretching band near 1030 cm-I. (4) The IR spectra suggest that the crystalline part contains Fe2(HPO,) and/or Fe(H2P0J3. Refereaces and Notes ( I ) Lavy, Y.;Imbert, C.; Cipriani, J.; Racine, S.;Dupeyrat, R. Opt.
._
Miller, D. R.; Han, 0. H.; Bohn, P. W. Appl. Spectrosc. 1987, 41,249. Tien, P. K. Appl. Opt. 1971, 10, 1207. Milton, A. F.; Burns, W.K. Appl. Opr. 1975, 14, 1207. Itoh, K.; Madou, M. J . Appl. Phys. 1991, 69, 7425. Rothon, R. N.; Ashley, R. J. Chem. Ind. 1975, 15, 976. (10) Sloper, A. N.; Flaganan, M. T.Elec. Left. 1988, 24, 353. (1 . 1), Preston. C. M.: Adams. W. A. J . Phvs. Chem. 1979.83. 814. (12) Simon, Von A.; Richter, H. Z . Anorg. Allg. Chem. 1960, 304, 1. (13) Toy, D. F. Comprehensive Inorganic Chemistry; Pergamon Press: Oxford, 1973; Vol. 2, p 481. (5) (6) (7) (8) (9)
Commun. 1974, 1 1 , 66.
Far-Infrared Spectra and Pseudorotational Potential Energy Function of l,3-Oxathiolane-2,2-d2 Sarah J. Leibowitz, Jaan Laane,* Department of Chemistry, Texas A&M University, College Station, Texas 77843
Ruben Verastegui, Jr., and John R. Villarreal Department of Chemistry, University of Texas- Pan American, Edinburg, Texas 78539 (Received: June 15, 1992)
The far-infrared spectra of 1,3-oxathiolane-2,2-d2 showing pseudorotational single and double quantum jump transitions, radial transitions, and pseudorotational-radial difference bands have been recorded and analyzed. Fourteen pseudorotational (ring bending) transition frequencies were observed in the 30-1 IO-cm-' region and were fit with the onedimensional potential function 2V (cm-I) = -577( 1 - cos 24) + 104(1 - cos 44). The molecule is twisted with a barrier to pseudorotation of 577 f 20 cm-'(1.65 f 0.06 kcallmol). The bamer height agrees well with the 541 f 20 an-'value determined for the undeuterated species. As was the case for the parent molecule, the spectra recorded include transition frequencies occurring above the barrier. In addition, several radial transitions arising from different pseudorotational states of 1,3-oxathiolane-2,2-d2have been observed in the 280-310-cm-I region. Those in the pseudorotational ground state can be fit with a one-dimensional double-minimumfunction with a bamer to planarity of 3000 cm-I. However, this value from the onedimensional approximation is believed to be too high. Difference bands in the 170-230-cm-' region and double quantum transitions in the 140-2OO-cm-' region were also detected and used to confirm the assignments.
Introduction
.
We have recently reported the far-infrared spectra of 1,3-oxathiolane,' CH2CH20CH2S,and have determined that the molecule has a moderate barrier to pseudorotation of 541 f 20 cm-l (1.54 f 0.06 kcallmol). This value represents the energy difference between the lowest energy twisted conformation and the highest energy bent form. The planar form has an energy more than 2000 m-'higher than the lowest energy structure. This study represents the only case where the transitions between the energy levels for a hindered pseudorotation could be observed both below and above the barrier to pseudorotation. Free or nearly free pseudorotation has been observed for ~yclopentane,~-~ tetrahydrof~ran,~-'and 1,3-dio~olane.~-~ Substantial barriers to pseudorotation, ranging from 773 to 2043 m-I,have been reported for thiacyclopentane,9 silacyclopentane,lOJlselenacyclopentane,12 and germacyclopentane.13 In order to verify our unusual results and in order to seek a second case for which data above a moderate pseudorotational barrier could be observed, we have prepared and studied 1,3r oxathiolane-2.2-d2, CH2CH20CD2S. The far-infrared spectra and pudorotational potential energy function for this isotopomer are presented here.
.
0022-365419212096-88 17$03.00/0
Theory Details of the theory applicable to 1,3-oxathiolane have been presented previously.' The wave equation for hindered pseudorotation is -B d2$/d4' V$ = E$ (1)
+
where
v = '/ZV2(1 - cos 24) + f/2V4(1- cos 44)
(2) and where V2and V, are potential energy constants. The pseudorotational constant B is related inversely to the masses and out-of-plane amplitudes of the ring atoms, and I$ represents the phase angle for pseudorotation. At 4 = 0 the molecule is bent with carbon atom 2 out-of-plane relative to the other four atoms. At 4 = u/2 the ring is twisted about an axis connecting carbon atom 2 to the midpoint of the C(4)-C(5) bond. The radial vibration, which for the molecule in its lowest energy twisted conformation is essentially a twisting motion, can be approximated by a one-dimensional potential energy function of the form
- bq2
(3) where q is the radial coordinate, and a and b are potential energy constants. V(q) = a d
0 1992 American Chemical Society
jjL;;x
1 fl s: 4
L)
t 8 e
P)
a
2
>rJ
TABLE I: Observed rad Calculated Frequencies (cm-l) for the Pseudorotation of 1,3-0uthid.m-Z,Z-d* frequency intensities transition" obsd calcd diff calc obsd 0-2 103.4 103.0 1.00 1.00 0.4
310
300 290 Wavenumber (cm")
2-4 4-6 6-8 8-10 10-12 10-13 12-14 12-15 13-14 15-17 15-18 16-17 18-20
280
Figure 2. Far-infrared spectrum of the radial bands of 1,3-0xathiolane2,2-d2 ( 5 Torr, 7.5-mpath, 0.25-cm-l res).
The kinetic energy calculationswere carried out as previously d e ~ c r i b e d . ~ The . ' ~ bending and twisting reduced masses were calculated to be 133.47 and 30.38 amu, respectively.
95.0 87.0 78.2 66.6 53.3 55.7 34.3 49.9 31.5 42.4 43.6 33.6 50.8
.99 0.84 0.63 0.44 0.14 0.14 0.03 0.03 0.05 0.04 0.02 0.03 0.04
-0.3 0.0 0.3 -0.7 0.0 -0.2 0.8 1.4 0.6 -1.5 -0.5 -1.5 1.1
1.09 0.96 0.78 0.54 0.13 0.15 0.04 0.05 0.06 0.07 0.06 0.04 0.04
" Where the pseudorotational levels given by no are doubly degenerate, only the even quantum number is indicated. TABLE II: Pseudorotational Frequencies (em-') in Radial Ground and Excited States
Experimental Section 1,3-Oxathiolane-2,2-d2was synthesized at the University of Texas-Pan American using the procedure of Gokel, Gerdes, and D ~ h 0 n g . lIn ~ order to introduce the deuteration into the molecule, paraformaldehyde-d2(Merck, 98% isotopic purity) was used as one of the reactants. The chemical and isotopic purity of the product was verified using nuclear magnetic resonance spectroscopy. The spectroscopic equipment and experimental conditions were similar to those described for 1,3-oxathiolane.l
~~
ns = 1
ns = 0
transition"
obsd
calcd
obsd
Calcd
0-2 2-4 4-6 6-8 8-10
103.4 95.0 87.0 78.2 66.6
103.0 95.3 87.0 77.8 67.3 3.06 -571 104
105.1 93.4 84.9 76.1 65.0
103.1 94.9 86.0 76.1 64.2 3.24 -550 99
B v2
v4
Results and Discussion Figure 1 shows the far-infrared spectrum of 1,3-0xathiolane2,2-d2with the pseudorotational bands in the 30-1 10-cm-I region, Figure 2 shows the radial bands between 280 and 310 cm-l, and Figure 3 shows both the pseudorotational double quantum jump transitions and pudorotational-radial bands in the 160-22O-cm-l region. Figure 4 shows the energy-level diagram determined from these spectra. The pseudorotational data for the lowest radial state can be fit nicely using B = 3.06 cm-l and 2 V (cm-I) = -577(1 - cos 24) + 104(1 - cos 44) (4) The frequencies calculated for this potential energy function are compared to the observed values in Table I. The potential function with its 577 f 20 cm-' (1.65 f 0.06 kcal/mol) barrier is shown in Figure 5. As was the case for the undeuterated species, several transitions above the barrier were observed. As shown in Figures 1 and 4, pseudorotational transitions in the first excited radial state (nq = 1) can also be observed. These 'side bands" are unusual in that they begin on the high frequency
95.3 87.0 77.9 67.3 53.3 55.9 33.5 48.5 30.9 43.9 44.1 35.1 49.7
"The transitions indicated by no only show the even quantum numbers for each of the doubly degenerate levels.
TABLE 111: Panreeters for tbe Hindered Pseudorotatioo of 1,3-0wthidrae-Z,Z-d2 CH~CH~OCH~S
o 1 2
3.25 3.31 3.38
-541 -578 -596
139 127 126
3.06 3.24
-577 -550
104 99
I
CH2CH20CD2S
0 1
side of the main band series but then move to the low-frequency side. Table I1 compares these frequencies to those in the nq = 0 state. The calculated frequencies and potential energy parameters for both states are also compared in the table. As e x p t e d , ' B, = 3.24 cm-' for the radial excited state is larger than BO = 3.06 cm-I. The V, = -550 cm-l and V, = 99 cm-I values for nq = 1 are both decreased slightly relative to the radial ground state.
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8819
Far-IR Spectra of 1,3-0xathiolane-2,2-d2
n,
n, = 2
=1
transition frequencies appear in Table I.)
TABLE IV: Observed and Calculated Frequencies (em-') of the Difference Bands and Pseudorotational Double Quantum Jumps of 1,3-0xathiolane-2,2-d2 (n,,n,) (ng/,n;) obsd calcd' Difference Bands (0,2)-(1 ,O) 194.5 194.4 (0,4)-(1a 204.6 204.5 (0,6)-(194) 210.9 210.9 (08-(1.6) 217.6 217.6 (0,10)-(13) 227.2 226.5 (1,2)-(2,0) 186.7 186.6 ( I ,4)-(2,2) 198.2 198.9
-
0.0
0.2
0.4
0.6
0.8
1.0
PHI (CIRCLES)
Figure 5. Onedimensional pseudorotational potential energy surface of 1,3-oxathiolane-2,2-d2. The minima correspond to the lowest energy twisted forms; the maxima correspond to the higher energy bent forms.
Table I11 compares the B, V2,and V4 values for the undeuterated and deuterated 1,3-oxathiolane for the different states. It should be noted that in our previous publication on t e undeuterated spccies,I the factor of 2 before the potential energy expression V was inadvertentlyomitted in a few cases. For the radial ground state for 1,3-oxathiolanethe correct expression is 2V (cm-I) = -541(1 - cos 24) 139(1 - cos 44) ( 5 ) Table IV lists the observed difference bands and pseudorotational double quantum jumps for the d2species and compares these values to those expected from single quantum jump pseudorotational and radial transitions. The observation of these bands has helped to confirm the assignments shown in Figure 4. The pure radial transitions can be fit with the potential function in cq 3 using a = 3.06 X lo4 cm-I/A4 and b = -2.08 x lo4
1
+
Pseudorotational Double Quantum Jumps (0,0)-(0,4) 198.2 198.4 (0,2)-(0,6) 182.0 181.8 (0,4)-(0,8) 165.2 165.0 (0,6)-(0,10) 144.8 144.6 Frequencies are calculated from observed pseudorotational and radial transitions.
TABLE V Observed and Calculated Frequencies for the Radial Transitions of 1,3-Oxathiolane-2.2-d, frequency (cm-') re1 int transition4a obsd calcdb obsd calcd* no = 0.1 0-1 297.8 297.9 (1.0) (1.0) 1-2 291.7 291.6 0.3 0.3 2-3 284.8 284.8 0.1 0.1 no
0- 1
299.6
= 2,3 0.4
"Quantum numbers for transitions refer to nq (see text). bFor V(q) = 3.60 x IO4 q4 - 2.08 X lo4 q2 and pT = 30.38 amu for nQ = 0,l
J. Phys. Chem. 1992, 96,8820-8821
8820
cm-I/A2 and a reduced mass value of pT = 30.38 amu. Table V presents the calculated frequencies for this potential function and compares these to the observed values. This potential function has a barrier of 3000 cm-I, a value, which because of the onedimensional approximation, is most likely somewhat of an overestimation.' For the undeuterated species, the twisting barrier calculated in this manner was 2720 cm-I. In order to determine the barrier to planarity more accurately, we will carry out a two-dimensional potential energy surface calculation which simultaneously considers the ring-twisting and ring-bending motions (which can be transformed to radial and twisting motions).
Conclusion The barrier of 577 f 20 cm-' determined for the d2 species compares favorably with the 541 f 20 cm-' value we reported for the undeuterated 1,3-oxathiolane, especially in view of the fact that both calculations assume the validity of the one-dimensional approximation. Some coupling with other vibrations is expected to result in at least a small perturbation on the calculation. It was very satisfying that we were once again able to observe pseudorotational transitions both below and above the pseudorotational barrier and to accurately calculate the frequencies in a region where the energy level diagram is very complex and sensitive to even minor changes in the potential function. The twisting barrier (barrier to planarity) for the d2 molecule was calculated to be 3000 cm-l using the one-dimensional approximation. A similar calculation resulted in a value of 2720 cm-' for the undeuterated molecule. A somewhat lower value is
expected when an improved two-dimensional potential energy surface is calculated for both isotopic species.
Acknowledgment. The authors thank the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Research Program for financial assistance. R.V. and J.V. acknowledge assistance from the NIH-NIGMS.
References rad Notes (1) Leibowitz, S.J.; Laane, J.; Verastegui, Jr.; Villarreal, J. R. J . Chem. Phys. 1992, 96, 7298. (2) Durig, J. R.; Wertz, D. W. J . Chem. Phys. 1968, 49, 2118. (3) Bauman, L. E.; Laane, J. J . Phys. Chem. 1988,92, 1040. (4) Carreira, L. A.; Jiang, G. J.; Person, W. B.; Willis, J. N . J . Chem. Phys. 1972,56, 1440. (5) Lafferty, W. J.; Robinson, D. W.; St. Louis, R. V.; Russell, J. W.; Strauss, H. L. J . Chem. Phys. 1965,42, 2915. (6) Engerholm, B. G.; Luntz, A. C.; Gwinn, W. D.; Hams, D. 0.J. Chem. Phys. 1969, 50, 2446. (7) Greenhouse, J. A.; Strauss. H. L. J. Chem. Phys. 1969, 50, 124. (8) Durig, J. R.; Wertz, D. W. J. Chem. Phys. 1968, 49, 679. (9) Wertz, D. W. J . Chem. Phys. 1969, 51, 2133. (10) Laane, J. J . Chem. Phys. 1969, 50. 1946. (11) Colegrove, L. F.;Wells, J. C.; Laane, J. J. Chem. Phys. 1990, 93, 6291. (12) Green, W. H.; Harvey, A. B.;Greenhouse. J. A. J . Chem. Phys. 1971, 54, 850. (13) Durig, J. R.; Willis, J. N. J . Chem. Phys. 1970, 52, 6108. (14) Schmude, R. W.. Jr.; Harthcock, M.A.; Kelly, M.B.; Laane, J. J . Mol. Spectrosc. 1987, 124, 369. (15) Gokel, G. N.; Gerdes, H. M.;Dishong, D. M.J . Org. Chem. 1980, 45, 18.
Fourier Transform Electron Paramagnetic Resonance Study of the Photoreduction of Anthraquinone with 4-Methyl-2,6-dEtert-butylphenol In Alcoholic Solutions M. Pliiscbau, G.Kroll, Institut fur Physik, Universitdt Dortmund, Otto-Hahn-Strasse, W-4600 Dortmund, Germany
K.-P. Dinse,* Physikalische Chemie III, TH Darmstadt, Petersenstrasse 20, W-6100 Darmstadt, Germany
and D. Beckert Max- Planck- Arbeitsgruppe 'Zeitaufgeloste Spektroskopie" an der Universitdt Leipzig, Permoserstrasse 15, 0-7050 Leipzig, Germany (Received: June 16, 1992)
Using FT-EPRfollowing laser excitation, the primary photochemical process in the photoreduction of anthraquinone with 4-methyl-2,6-di-tert-butylphenoi was investigated. Highly-resolved spin-polarized EPR spectra taken with nanosecond time resolution gave unambiguous evidence for a two-step hydrogen abstraction reaction, consisting of a primary electron transfer followed by proton abstraction with a time delay, which allows for a noticeable escape probability of the initially generated anthrasemiquinone radical anion (AQ-). The time dependence of the EPR intensities of the neutral 10-hydroxyanthroxyl-9 radical (AQH') as well as of AQ'- could be simulated for the full experimentally accessible time interval of 10 ns to 100 ps. The kinetic model used invokes optical spin polarization, spin-lattice relaxation, radical generation, and AQH'/AQ'interconversion. In addition, from an analysis of the highly-resolved FT-EPR spectra a complete set of AQH' hyperfine splitting (hfs) constants could be measured in two different alcohols for the first time.
1. Introduction The photoreduction of aromatic ketones and quinones with different donors has been investigated extensively,'-1othe primary electron-transfer involved being one of the most important processes in photochemistry and in photosynthesis reaction centers. Several reaction intermediates, like exiplexes and/or complexes of triplet ketones (quinones) with ground-state donors (amines, phenols) have been proposed. However, it should be expected that the mechanism for the formation of ionic or neutral radical species 0022-3654/92/2096-8820$03.00/0
and their interconversion will generally be dependent on the specific combination of donor and acceptor molecules as well as on the nature of the solvent. The distinction of the different intermediates like contact ion pairs, solvent-separated ion pairs, exciplexes, and/or free radicals by optical methods is Micult and not always unambiguous, although optical methods excel in time resolution. The recent development of timeresolved EPR, resulting in the improvement of the time resolution of Fourier transform EPR (FT-EPR) into the range of 10 ns, renders it possible, however, 0 1992 American Chemical Society
