J . Phys. Chem. 1990, 94, 2313-2316
2313
Observation of Low-Lying Electronic States of (C02)2+by Using Photoelectron-Photoion Coincidence Measurements Koichiro Mitsuke,* Koichi Ohno, and Shigeki Kat0 Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro- ku, Tokyo 153, Japan (Received: July 12, 1989; In Final Form: October 13, 1989)
The photoelectron-photoion coincidence (PEPICO) spectra of (CO,), have been measured in the ionization energy range of 13.300-14.125 eV. Three broad peaks are observed at the lower energy side of the monomer band of COz+(~211,,v’=C!,0,0). They are_assigned as resulting from transitions to four ionic states of (C02)2+which asymptotically dissociate into COz+(XzlI,) + COz(X’Zg+).The identification of the peaks is based on the results of ab initio calculations including configuration interaction (CI) for the ground state of (C02)z and the ground and excited states o-f (COz)z+2Vertical ionization potentials are estimated to be 13,53 0.03 eV for the X2B, state, 13.61 0.03 eV for the A2B, and B2A, states (unresolved), and 13.712b0,SeV for the C2A, state.
*
Introduction The ionic ground state of the COz dimer has been investigated by a number of authors using high-pressure mass spectrometry’4 and photoionization efficiency measurement^.^ Illies et al. have suggested that an equilibrium nuclear configuration of (CO2),+ is a slipped parallel geometry or a T-shape geometry.6 The dissociation energy of (C02)2+ is reported to be 11.8-16.2 kcal/mol.’” Despite the scatter of the data, experimental and theoretical results show a larger dissociation energy and a shorter equilibrium intermolecular distance for the ion than those for the neutral (CO,),. These findings can be interpreted as that a dimer composed of two closed-shell molecules is ionized by the removal of an electron from an antibonding orbital to form the lowest ionic state.’ In contrast, very little is known about the electronically excited states of (C02),+. As far as we know, the excited states have only been observed by photodissociation spectroscopy at visible wavelength.8-i0 It is well documented that photoelectron spectroscopy of van der Waals clusters provides direct information on their ground and excited ionic states. Values of the vertical ionization potential I, have been determined for rare-gas dimers, (H20),, and (N2)2.11-13 Identification of the photoelectron bands of the dimer, however, is usually accompanied by some difficulties, since the bands are entirely or partly hidden by intense monomer bands due to the much lower concentration of the dimer in a supersonic molecular beam than that of the monomer. Recently, Cordis et al. and Norwood et al. have applied the technique of photoelectron-photoion coincidence (PEPICO) measurements to Ar, Kr, Xe, CO, and N2 dimers and higher cluster^.'"'^ They detected (1) Illies, A. J. J . Phys. Chem. 1988, 92, 2889. (2) Keesee, R. G.; Castleman, A. W., Jr. J . Phys. Chem. ReJ Data 1986, 15, 1011. (3) Meot-Ner, M.; Field, F. H. J . Chem. Phys. 1977, 66, 4527. (4) Headley, J. V.; Mason, R. S.; Jennings, K. R. J . Chem. SOC.,Faraday Trans. 1 1982, 78, 933. ( 5 ) Linn, S. H.; Ng, C. Y. J . Chem. Phys. 1981, 75, 4921. (6) Illies, A. J.; McKee, M. L.; Schlegel, H. B. J . Phys. Chem. 1987, 91, 3489. ( 7 ) Haberland, H. Physics and Chemistry of Small Clusters; NATO AS1 Series; Jena, P., Rao, B. K., Khanna, S. N., Eds.; Plenum: New York, 1987; p 667. (8) Smith, G. P.; Lee, L. C. J . Chem. Phys. 1978, 69, 5393. (9) Vestal, M. L.; Mauclaire, G. H. Chem. Phys. Lett. 1976, 43, 499. (IO) Illies, A. J.; Jarrold, M. F.; Wagner-Redeker, W.; Bowers, M. T. J . Phys. Chem. 1984, 88, 5204. ( 1 I ) Dehmer, P. M.; Dehmer, J. L. J . Chem. Phys. 1978, 69, 125. (12) Tomoda, S.; Achiba, Y . ;Kimura, K. Chem. Phys. Lett. 1982,87, 197. ( I 3) Carnovale, F.; Peel, J. B.; Rothwell, R. G. Physics and Chemistry of Small Clusters; NATO AS1 Series; Jena, P., Rao, B. K., Khanna, S. N., Eds.; Plenum: New York, 1987; p 595. (14) Cordis, L.; Gantefor, G.; Hesslich, J.; Ding,A. Z . Phys. D 1986, 3, 323. ( I S ) Norwood, K.; Guo. J.-H.; Luo, G.;Ng, C. Y . Chem. Phys. 1989,129, 109.
*
cluster ions correlated to threshold energy electrons while scanning the wavelength of the vacuum-ultraviolet light. By such a method, autoionization via Rydberg states as well as direct ionization can be observed. In order to study the direct ionization alone, we have constructed an apparatus for the PEPICO experiment that uses a light source with a fixed photon energy (He I resonance light).M In the present study, the PEPICO spectrum of (CO,), is measured near the ionization potential for the ground R2n,state of C02+. If Dmh symmetry of CO2+(X2II,)is degraded by forming a dimer, the removal of the degeneracy results in four elestronic states for (C02)2+correlating to the C02+(X211,) C02(X’Zg+)asymptote. Hence, photoelectron bands arising from transitions to these ionic states may appear in the PEPICO spectrum of (CO,),.
+
Experimental Section Neutral clusters are formed in an adiabatic expansion of a CO, gas into a vacuum chamber from a sonic nozzle of 50-pm diameter and nozzle temperature of 298 f 3 K. The stagnation pressure Po is adjusted to 650 f 5 Torr by regulating the gas flow through a needle valve. The chamber is evacuated by a 6-in. oil diffusion pump (Nihon Shinku, ULK-06), and the pressure is maintained below 1 X Torr. The cluster beam is sampled through a conical skimmer into a chamber which is differentially pumped by two oil diffusion pumps (Nihon Shinku, ULK-04; Edwards High Vacuum, Diffstak 100/300M). The pressure is 4 X IO-’ and 4.2 X 10” Torr when the C 0 2 beam is off and on, respectively. The schematic diagram of an ionization cell and its surroundings is illustrated in Figure 1. The cell is composed of a cupronickel cylinder of 22-mm length and 16-mm diameter and is usually grounded. The cluster beam passes through holes of 6-mm diameter on the side of the cell. The beam intersects at 90” with a He I photon beam (wavelength 584 A) in the central region of the cell. Emitted photoelectrons are collected by a molybdenum slit of 5-mm diameter and focused onto the entrance hole of the analyzer of a hemispherical electrostatic deflection type. The analyzer is made of cupronickel and is placed in a differentially pumped chamber which is evacuated by a 2.5411. oil diffusion pump (Nihon Shinku, U-250). The mean radius of the electron orbit and the spacing between the two spherical surfaces are 30 and 10 mm, respectively. The potentials applied to the two hemispheres are so fixed that electrons with a desired kinetic energy can pass through the analyzer. The energy resolution of 5120 meV (fwhm) can be obtained with a pass energy of 0.72 eV. The energy scale is calibrated by reference to the photo(16) Norwood, K.; Guo, J.-H.; Ng, C. Y. J . Chem. Phys. 1989.90, 2995. (17) Norwood, K.; Luo, G.; Ng, C. Y . J. Chem. Phys. 1989, 90, 4689. (18) Norwood, K.; Guo, J.-H.; Luo, G.; Ng, C. Y. J . Chem. Phys. 1989, 90, 6026. (19) Norwood, K.; Luo, G.; Ng, C. Y . J . Chem. Phys. 1989, 91, 849. (20) Mitsuke, K.; Ohno, K. J . Phys. Chem. 1989, 93, 501.
0022-3654/90/2094-23 13$02.50/0 0 1990 American Chemical Society
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Mitsuke et al.
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
n
n
(b) C02 Monomer
,--x2ng(o,o,o)
> -,
......................................
"
Cluster Beam
MS
Figure 1. Schematic diagram of the apparatus. Neutral clusters formed in a supersonic cluster beam are photoionized by a H e I resonance line at the ionization cell (IC). Emitted photoelectrons pass through an electron energy analyzer (AN) and are detected by a channel electron multiplier (CEM), while photoions are mass-analyzed by a quadrupole mass filter (MS).
14.10
14.00
13.90
13.80
13.70
13.80
13.50
13.40
13.50
Ionization Energy /eV Figure 3. PEPICO spectrum of (CO,), (panel a) and photoelectron spectrum of C 0 2 (panel b). The PEPICO spectrum is obtained by plotting the true coincidence counts determined from the TOF spectra as a function of the ionization energy. The energy resolution of the . analyzer is 5120 meV (fwhm).
C02
Dimer
(high resolution)
i-i -
31.50
34.65
37.80
Time Of Flight
40.95
/
44.1 0
I I . . I . . I . . I . . I
ps
Figure 2. Typical coincidence time-of-flight (TOF) spectrum of (C02),+ at the ionization energy of 13.60 eV. A peak at 37.2 ps results from the true coincidence signal counts.
electron band for the ionic and vibrational ground state of C02+. Upon detection of an electron, a pulse voltage (duration 12 ps; amplitude 26.0 V) is applied to the cell, and photoions are pushed out of the cell. Ions are then extracted by a series of ion lenses and mass analyzed by a quadrupole mass filter (Nihon Shinku, MSQ-400). A time lag from the ionization event to triggering the pulse is estimated to be less than 500 ns from the flight time of the electron and the delay of the electronic circuit. The electron signal is fed into the start input and the ion signal into the stop input of a time-to-amplitude converter (ORTEC, 467). A coincidence time-of-flight (TOF) spectrum of the photoion is accumulated in a multichannel analyzer (Laboratory Equippment, MCA-48F) operating in a pulse-height-analysis mode. Figure 2 shows a coincidence TOF spectrum of (C02),+ at the ionization energy IE of 13.60 eV. A peak observed at -37.2 ps results from (C02),+ ions produced in single ionization events with the energy-selected electrons (true coincidence). The width of the peak is about 1.2 ps. Uniform background counts between 33 and 42 ps are due to (CO,),+ ions which have no correlation with the electrons valse coincidence), since the instrumental ion collection efficiency is exceedingly higher during the application of the pulse voltage than when the cell is grounded. The rate RFC of the false coincidence counts is given by RFC= vRERIT,
(1)
where T, is the effective period of the pulse voltage by which the photoions are pushed out, RE is the count rate of photoelectrons, and R, is the count rate of (C02),+ions measured with a continuous application of a 26.0-V potential to the ionization cell. The constant v is found to be about 0.8-0.9. At IE = 13.78 eV,
13.58
13.55
13.52
13.49
13.46
Ionization Energy /eV
Figure 4. PEPICO spectrum of (CO,), with the energy resolution of 90 meV (fwhm).
i.e., the ionization potential for CO2+(w2II,,u'=0,O,0), RFC is estimated to be 1.3-1.5 cps from T,,, = 9 ps, RE = 450 cps, and RI = 400 cps. This high false coincidence rate leads to large errors in evaluating the true coincidence counts. On the other hand, R E at IE = 13.60 eV is less than 0.1 cps and is comparable to the rate RTc of the true coincidence counts (0.1-0.2 cps). Results Measurements of coincidence TOF spectra of (C02),+ are carried out from IE = 13.300 eV to IE= 14.125 eV in steps of 25 meV. Coincidence counts are accumulated at each IEposition for about an hour. The true coincidence peaks emerge in the TOF spectra at IE = 13.325-13.800 eV. The PEPICO spectrum of, (CO,), in Figure 3a is obtained by plotting the integrated intensity of the true coincidence peak as a function of IE. The rate RTC increases with Po in the range of 300-2000 Torr. The shape of the PEPICO spectrum is, however, found to be essentially the same for Po = 450-1200 Torr. The PEPICO intensity starts to appear at 13.325 f 0.04 eV. There exist three peaks in Figur? 3a at the lower IE side of the intense monomer band of C02+(X211,,u'=0,0,0) with the vertical ionization potential I,(CO,) of 13.78 eV (see Figure 3b).21 Peak 1, which is almost hidden by the overlapping of peak 2 in Figure 3a, can be resolved in Figure 4, the PEPICO spectrum measured with a better energy resolution (fwhm = 90 meV). A sharp drop in the PEPICO intensity is observed at 13.75 eV.
-
(21) Potts, A. W.;Fattahallah, G . H. J. Phys. B 1980, 13, 2545.
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2315
Low-Lying Electronic States of (C02)2+
rc-o
TABLE I: Vertical Ionization Potentials and Dissociation Energies for the Electronic States of (Cod,+ (All Entries in eV)
;c
p)
d
C2A,
B2Au
21 (unresolved) 3
13.53
* 0.03
13.61 f 0.03 13.712$
0.31
0-c-0
I
* 0.05
0.23 f 0.05
0.13!$0":
,/
Vertical ionization potential of (CO,), determined from Figures 3 and 4. bDissociation energy for (C02),+ in the equilibrium nuclear configuration of the neutral (CO,),.
RCC
a
Table I summarizes values of the vertical ionization potential IV[(CO2),]for the observed ionic states of (C02)2+. From the reported binding energy for the neutral dimer,5*6D[(CO,),] = 1.3-1.4 kcal/mol, we deduce the dissociation energy Dv[(C0,),+] for (COZ),+ in the equilibrium geometry of the neutral dimer by using the relation ~v[(c02),+1 = I"(C02) - ~"[(CO,)Zl+ D[(COZ),l
/
0-c
y;
c-o Figure 5. Equilibrium nuclear configuration of the C 0 2 dimer (point group C2b).24Structural parameters are R C . - = ~ 3.599 A, rc-0 = 1.162 A, and Bc...C-o= 58.2'.
(2)
The resultant values are also listed in Table I. In several experimental runs, a weak maximum is additionally observed at IE i= 13.4 eV, where RTC is affected by the stagnation conditions of the nozzle beam expansion. The reproducibility of RTc is poor. There may be some contributions to this feature from ionization of trimer or larger clusters followed by dissociation into (CO2),+ neutral fragment(s).
+
Discussion A . PEPICOSpectrum of (COZ)Z.The onset of the PEPICO intensity, 13.325 f 0.04 eV, is in good agreement with that of the photoionization efficiency curve5 of (COZ)Z+,13.32 f 0.02 eV but is 0.18 eV higher than the adiabatic ionization threshold of (CO,), estimated by high-pressure mass spectrometry.' This suggests that the equilibrium geometry of (COZ)Z+is substantially different from that of (CO,),. Small Franck-Condon factors for adiabatic transitions do not exclude the formation of (C02)?+at bU the adiabatic ionization threshold. Nevertheless, the photoioni' ~ ~ ~ ~ Figure ~ 6. Schematic shape of orbitals of (CO,), derived from llr, orbitals zation cross section near the threshold may be so ~ m a 1 1that of two CO, monomer units. In-plane 1~~ orbitals are shown by pairs of the PEPICO intensity of (C02)2t is beyond the detection limit ellipses, while out-of-plane IT, orbitals are shown by circles. Signs of of our apparatus. orbital coefficients are indicated by the thickness of the curves: thick The sharp drop in the PEPICO intensity at I , 13.75 eV is curves are for a positive sign and thin curves are for a negative sign. possibly related to the short lifetime of (C02)2t with respzct to dissociatjon. If (COZ)Z+ions are formed above the C02+(X2n,) atoms and the molecular axis is 58.2". + CO,(X'Z,+) asymptote, the ions efficiently undergo dissociation This geometry suggests that in-phase overlap of the two COZ on the way from the ionization cell to the ion detector. The l a g orbitals located within the molecular plane of (CO,), gives thermochemical threshold for the dissociative ionization is estirise to a bonding orbital $(a ) with a, symmetry as shown in Figure mated to be about 13.83 eV from the sum of D[(CO,),] and the 6. An antibonding orbitaf $(bJ with b, symmetry arises from adiabatic ionization potential of COZ, 13.773 eV, reported by out-of-phase overlap of the in-plane l a , orbitals. In this case, M ~ C u l l o h . The ~ ~ calculated threshold is somewhat higher than a nodal plane exists between two adjacent oxygen atoms. Apthe ionization energies where the intensity drop is observed, yet parently, $(b,) is higher in energy than $(a,). The out-of-plane the possibility of this dissociation effect cannot be ruled out judging orbitals arising from the two COZ l a , orbitals belong to a, and from the large errors inherent in the PEPICO measurements at bg symmetry. Because of a small overlap between the out-of-plane energies close to IV(CO2)and from the relatively low energy COZ la, orbitals, the orbital energy of $(a,) is very close to that resolution of our spectrometer. of $(b,). Thus, removal of an electron in turn from $(bJ, $(b,), B . Electronic States of (C02)2+.l3y forming a dimer ion, $(a,),and $(a,) leaves the (CO,),+ ion in the grojnd ionic_state removal of the degeneracy of CO2'(X2n,) gives rise to-four of XZB,, the first and the second excited states of A2Bgand !,A,,, electronic states which asymptotically dissociate into C02+(X2n,) which are almost degenerated, and the third excited state of CZA,. + C02(X1Z,+). The ordering of the ionic states of (C02),+ Provided that IyIs for these four states are lower than IV(CO2), crucially depends on the interaction between la, orbitals of two one may assign peaks l,?, and 3 as res_ulting from vzrtical ionC 0 2 monomer units. Assuming that vertical transitions charization transitions to the X2B, state, the A2B, and the B2A, states, acterize photoionization processes, we will first make reference and the C2A, state, respectively, as shown in Table I. to the equilibrium nuclear configuration of the neutral dimer. C. Ab Initio Calculations for I v [ ( C 0 2 ) 2 ]We . performed ab Recent studies on free-jet infrared absorption spectroscopy predict initio calculations to give more definite identification of the peaks that neutral ( C 0 2 ) 2has an equilibrium nuclear configuration of in the (CO,), PEPICO spectrum. Single-configuration Hara slipped parallel geometry with C2, symmetry as depicted in tree-Fock (HF) wave functions with the split-valence MIDI-4 Figure 5.24 The distance Rc-c between two carbon atoms is 3.599 basis setZ5were first employed. The experimentally determined A, and the acute angle Oc...c-o between the line joining carbon geometry for the neutral dimer was used for all the states considered here (Figure 5).24 It was found that the H F wave functions (22) Ng, C. Y . Adu. Chem. Phys. 1983, 52, 263. do not provide proper description on the vertically ionized states
-
(23) McCulloh, K . E. J . Chem. Phys. 1973, 59, 4250. (24) Jucks, K. W.; Huang, Z. S.; Miller, R. E.; Fraser, G . T.; Pine, A. S . ; Lafferty, W. J. J. Chem. Phys. 1988, 88, 2185.
( 2 5 ) Tatewaki, M.; Huzinaga, S . J. Comput. Chem. 1980, I , 205.
2316
J . Phys. Chem. 1990, 94, 2316-2320
weights of reference configurations in the total wave functions were more than 88% and 85% for the neutral and ionic states, respectively. For the CI calculations, the table-MRDCI program developed by Buenker et al. was ~ s e d . ~ ~ J ~ Table I1 shows total electronic energies of the ground state of band state E,,* 4[(C02)2lC ~[(co2)2+ld (C02)2and the ground and first three excited states of (C02)2+. Calculated I,[ (C02),1value~are givenin the fifth column. Ex(CO,), %'A, -375.0627 citation _energies for A2B,, B2A,, and C2A, with respect to the (COJ2+ a %,Bu -374.5643 13.562 0.0 ground X2B, state are listed in the sixth column. The ordering b A2B. -374.5608 13.659 0.097 of the (C02)2+states is certainly in accord with that predicted 13.677 c B2A: -374.5601 0.1 I5 13.771 d (?'A, -374.5567 0.209 from the correlation of the orbitals between (CO,), and two C 0 2 molecules (see section B). The first and the second excited states "The nuclear configuration is fixed to the equilibrium geometry of are found to be essentially degenerated as expected from a weak the neutral dimer in Figure 5. Energy determined by the MRSD-CI interaction between the out-of-plane C 0 2 1mg orbitsls. The X'B, method. Vertical ionization potential calculated from the difference state lies -0.1 eV b$ow the A2B, state, and the C2Agstate lies between the total energy of (CO,), and that of (COz)z+.dExcitation -0.1 eV above the B2A, state. Ab initio calculations of (CO,), energy calculated from the difference between the total energy of (CO,),+(%zBu)and that of the excited state of (CO,),'. and ( C 0 2 ) 2 +thus support our assignment in Table I made for the three peaks which appear to have -0.1 eV energy spacings in of (CO,),+ because of their instability to a spacial symmetry the PEPICO spectrum. breaking. That is, the wave functions representing the asymmetric It should be noted that there remains uncertainty with regard (CO,-CO,+) electronic configurations gave lower energies than to the ionization potential for the C2A state, since peak 3 could the symmetry-adapted wave functions. This finding shows that be the result of the dissociation of (CQ2),+ instead of reflecting at least two electronic configurations are required to describe the the vertical transitions to this ionic state (see section A). There electronic states of (CO,),+. is a need for further PEPICO studies under improved experimental We therefore carried out the multireference configuration inconditions, i.e., better energy resolutions and/or greater RTc- R, teraction (CI) calculations to obtain reliable energies of (CO,), ratios, in order to obtain a more reliable I,[(C02)2] value. and (C02)2+. We constructed the CI wave functions which involve all single and double excitations from the reference configurations Acknowledgment. We thank Dr. Y. Ohshima for valuable for respective states (MRSD-CI method). Here, the lowest 10 discussions and suggestions. The present work has been supported orbitals were fixed to be doubly occupied. We chose the HF by a Grant-in-Aid for Scientific Research from the Japanese configuration as the reference for the neutral state. For the ionic Ministry of Education, Science and Culture. states, two configurations were taken as discussed above. The one-particle basis functions for the CI wave functions were ob(26) Buenker, R. J. Proceedings of the Workshop on Quantum Chemistry tained by the 17 configuration pair type MCSCF method for the and Molecular Physics in Wollongong, Australia; Carbo, R.,Ed.; Elsevier: neutral state. The Ak method with the threshold value of 10 Amsterdam, 1980; p 17. phartrees was applied to reduce the number of configurations in (27) Buenker, R. J.; Phillips, R. A. J . Mol. Struct. (Theochem) 1985, 123, 291. the CI wave functions and the extrapolation was performed. The
TABLE 11: Total Energies (in hartrees) of the Ground State of (C02)2and the Ground and Excited States of (C02)2+,Vertical Ionization Potentials (in eV) for the States of (C02)2+,and Excitation Energies (inpV) for the Excited States of (C02)2+with Respect to the Ground X2B. State"
Stereochemical Consequences of Halogen Atom Substitution. 1. Rotational Conformer Eftects in Gaseous Diastereomeric 2,3-Dihalobutanes Ram B. Sharma,+ Richard A. Ferrieri,*Q*Richard J. Meyer,+ Edward P. Rack,*,+ and Alfred P. Wolft Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0304, Medical Research, Veteran's Administration Medical Center, Omaha, Nebraska 68105, and Chemistry Department, Brookhaven National Laboratory, Upton, New York 1 1 973 (Received: July 13, 1989; I n Final Form: October 3, 1989)
The stereochemical consequences of translationally excited 38C1-for-X(X = F, C1) substitution in (2S,3R)-meso-and (2S,3S)-dl-difluorobutane, (2S,3R)-dl-and (2S,3S)-dl-~hlorofluorobutane,and (2S,3R)-meso-and (2S,3S)-d/-dichlorobutane were studied in the gas phase. Although retention of configuration was determined to be the dominant substitution pathway, substantial inversion product yields were observed in all cases. A comparison of these yields revealed that the (S,R)configuration within each set always gave a smaller yield of the inverted product than the (S,S) configuration. This observation was consistent with the hypothesis that back-side substitution was a direct mechanism and that steric hindrance to such attack was subject to differences in the rotational conformer populations within each substrate configuration. In contrast, the measured yields of retention products were insensitive to substrate configuration.
Introduction
Several studies performed in the gas phase can be found in the literature dealing with the subject of the stereochemical consequences of hot ( S H H 2homolytic ) bimolecular halogen-for-halogen substitutions in molecules possessing either or singles-12 studies Of asymmetric centers' Noteworthy are the
' University of Nebraska-Lincoln and VA Medical Center. * Brookhaven National Laboratory. 0022-3654/90/2094-2316$02.50/0
and c o - w o r k e r ~ ~on- ~halogen-for-halogen substitution in mesoand dl-l,2-dichloro-1,2-difluoroethanesand in meso- and dl( 1 ) Tang, Y. N.; Ting, C. T.; Rowland, F. S. J . Am. Chem. SOC.1964,86,
2525.
(2) Rowland, F. S.; Wai, C. M.; Ting, C. T.;Miller, G. Chemical Effects ofhuclear Transformations; International Atomic Energy Agency: Vienna, 1965; Vol. I , p 333. ( 3 ) Wai, C. M.; Rowland, F. S. J .Phys. Chem. 1967, 71, 2752 (4) Wai. C. M.; Rowland, F. S. J . Phys. Chem. 1970, 74, 434.
0 1990 American Chemical Society
