J. Phys. Chem. 1995, 99, 13392-13396
13392
Single Crystal Study of C100. Matrix Effects in the ESR and IR Spectra of ClOO Trapped in KC104 J. R. Byberg Department of Chemistry, Aarhus Univeristy, Langelandsgade 140, DK-8000 Aarhus C, Denmark Received: May 25, 1995; In Final Form: July 5, 1 9 9 9
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Photolysis at 6 K of OClO trapped in a single crystal of KC104 produces the isomer ClOO in three inequivalent configurations A, B, and C. A and B are metastable, the conversions A B and B C occumng at -35 and -60 K, respectively. Comparison of observed values of the 0-0 stretching frequency V I with those reported for ClOO matrix-isolated in argon and the geometry deduced from the spin Hamiltonians both suggest that ClOO in the stable configuration C is only slightly perturbed by the KC104 lattice while stronger perturbations occur in configurations A and B. Hence, the spin Hamiltonian obtained for C should represent the properties of the free ClOO molecule to a good approximation. The matrix effects observed for A and B are attributed to polarization of C100, transfemng electronic charge from C1 to 0 2 .
Introduction Owing to the very short lifetime of the radical ClOO in the gas phase’ as well as in liquid s ~ l u t i o n ,the ~ . ~principal source of structural information has so far been the IR spectrum of ClOO isolated at low temperature in noble-gas mat rice^.^-^ In spite of its extremely weak C1-0 bond, ClOO apparently suffers only a rather limited distortion in such matrices. Thus, the highlevel ab initio calculation on the free ClOO by Peterson and Werner7 reproduces satisfactorily the vibrational frequencies and anharmonicity constants measured for ClOO in neon and argon, while the calculated geometry is compatible with that deduced from a detailed IR studye5 The IR spectrum of ClOO being well accounted for, accurate measurements of other properties seem desirable to develop further the understanding of this intriguing molecule. Such properties could be the electron spin distribution and the electric field gradient at the chlorine nucleus, both encoded in the hyperfine structure of the ESR spectrum. ESR spectra of ClOO trapped in various glassy and polycrystalline matrices including argon were reported earlier.8-13 In fact, the first spectroscopic detection of ClOO (at the time incorrectly denoted C10) was made by ESR.8 Also, ClOO has been produced at 93 or 77 K by photolysis of the symmetric isomer OClO in single crystals of KC104.9-” The spin Hamiltonians reported for ClOO all feature a near-axial hyperfine tensor with IAlll 50 MHz, whereas there is little agreement as to the magnitude or relative sign of the remaining hyperfine parameters. This is not too surprising considering that the ESR spectra of ClOO in polycrystalline matrices have so far defied complete analysis because as many as seven independent spin Hamiltonian parameters must be derived from the observed line shape in a narrow, poorly resolved region, and that the fully analyzed spectrum of ClOO in Kc104 was recorded at 93 K9 at which temperature large-amplitude motion of ClOO could strongly modify the spin Hamiltonian. Hence, the apparent discrepancies between the spin Hamiltonians may indicate experimental flaws as well as real matrix effects. Seeking a reliable spin Hamiltonian for ClOO with the correct signs of the hyperfine and quadrupole terms and the orientations of the principal axes with respect to the molecular frame unambiguously established, we have reexamined the ESR
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‘Abstract published in Advance ACS Abstracts, August 1, 1995.
0022-3654/95/2099-13392$09.00/0
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spectrum of ClOO trapped in single crystals of KC104 (which provides the only known oriented sample of ClOO), this time measuring the spectrum at 6 K and using also crystals tagged with I7O. KC104 has in earlier investigations proved a useful, passive host for small paramagnetic molecules. Thus, a comparison with gas-phase data for OClOI4 shows that the matrix effects on hyperfine and quadrupole parameters for this molecule are small in KC104 at 26 K,I5 in fact amounting to only 30-40% of those observed in noble-gas matrices.I6 Photolysis at 6 K of OClO in KC104 yields three inequivalent configurations of ClOO with spin Hamiltonians that reflect configuration-specific perturbations. To gauge these against the perturbations experienced by ClOO in frozen-gas matrices, we have measured also the 0-0 stretching frequency V I ,which is expected to be sensitive to environment.’ The structural clues conveyed by the spin Hamiltonians combined with the nominal geometry of the clod- site occupied by ClOO are used to construct models of the configurations, which serve to rationalize the observed variations of the spectroscopic properties. Experimental Section Single crystals of KC104 were obtained by evaporation at room temperature of aqueous solutions made with analytical grade KC104 and triply distilled water. KC104 tagged with I7O was prepared by electrolysis with platinum electrodes of NaC103 dissolved in H20 enriched to 48% I7O and subsequent addition of the equivalent amount of KCl to precipitate KC104, which was recrystallized twice to remove the residual KCl and KC103. The IR measurement described below indicates that the enriched water contained also about 25% I8O and that some C104- were labeled at more than one oxygen position. Irradiations with X-rays were made with a Machlett OEG 60 tube with tungsten target operated at 50 kV and 32 mA. The sample was placed 30 mm from the beryllium end window of the tube and was shielded by 1 mm quartz. W irradiations were made with an Osram HQL 80 W mercury lamp from which the glass envelope had been removed. A band-pass filter of width 40 nm centered at 2 = 380 nm isolated the group of mercury lines around 365 nm, and a pair of quartz lenses were used to project a 1:1 image of the arc of the lamp on the sample. IR spectra were obtained with a Bruker 113v FTIR instrument in conjunction with an APC Heli-Tran cryostat. The vacuum 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 36, 1995 13393
Single Crystal Study of ClOO
TABLE 1: Parameters in the Spin Hamiltonian of C100; Principal Values of Hyperfine and Quadrupole Tensors (in MHz) Refer to_ 35Cl ~ _ config. T(K) Aexp
Bexp Cexp cap
Ccal$
6 6 6 61 (61)
g,
g,
gz
(4,ew
1.9970 1.9992 1.9950 2.0040 2.0051
2.0065 2.0042 2.0046 1.9966 1.9954
2.0148' 2.0113' 2.0112e 2.0103' 2.0102
(68,86,152) (0,84.4,0) (25.6,63.1,-115.6) (OS5.8.0) (0,57.4,0)
Ax
A,
16 5 12.1 8.4 15.0 6.0 6.4 13.6 5.5 14.3
Az
(wwo
-46d (95,48,141) -46.7f (0,57.2,0) -52.0' (-10.0,54.4,-98.2) -50.0" (0,53.8,0) -50.8 (0.54.3,O)
Qx
Qy
Qz
1.8 7.5 7.0 6.4 6.8
7.8 7.3 7.5 7.2 7.2
-15Sd 14% -' -14.9 -13.6' -14.0
(wtw
(95.47,-) (0,56.3,0) (-11.0,54.4,-98)g (0,53.8,0) (0,54.0,0)
ab(deg)
45 27 32 2 -
"The Eulerian angles (q5,B.v)(ref 19) specify the orientation of the principal axes for one of the equivalent orientations with respect to a reference frame defined by the axes (b,a,c)of the orthorhombic KC104 crystal. a is the angle between the principal axes Z(A) and Z(g). Standard deviation 0.0001; estimated limit of error f0.0002. Standard deviation 0.4 MHz. e Standard deviation 0.00002; estimated limit of error fO.OOO1. fStandard deviation 0.2 MHz. Qx and Q,calculated with the constraint V ( Q ) = V(A). Algebraic mean of the spin Hamiltonians at 6 K for a pair of orientations related through reflection in the a-c plane. f
shroud of the cryostat was equipped with a pair of KCl windows and (perpendicularly) with a pair of Spectrosil windows. The sample crystal measuring approximately 3.5 x 3 x 1 mm3 was glued with epoxy resin to a mask made from indium foil. The mask was attached with a spring to a sample holder made from silver which was screwed into the cold block of the cryostat. Rotation of the cryostat with respect to the vacuum shroud allowed X and UV irradiation of the sample through the Spectrosil windows and subsequentrecording of the IR spectrum through the KCl windows. The temperature of the sample was taken as that of the cold block, which was monitored by germanium (4-30 K) and platinum (30-300 K) resistance thermometers. ESR spectra were recorded with a Varian E-15 spectrometer operated at 35 and 9.3 GHz in conjunction with the Heli-Tran cryostat. The static magnetic field Bo and the microwave frequency were monitored by a Bruker ER-35 NMR gaussmeter and an EIP 548 electronic counter, respectively. Simultaneous field modulation at 33 and 100 kHz was employed to enhance the spectral re~olution.'~ The resulting peak-to-peak line width was 0.6 G at 6 K. The spin Hamiltonians reported below were derived from series of spectra corresponding to rotations of BO about the crystallographic axes a, b, and c of the orthorhombic KC104 crystal (space group Pnma). The parameters of the spin Hamiltonians were calculated from the positions of 36- 105 selected ESR transitions by means of the iterative numerical procedure outlined earlier.I8 Results ClOO was produced in KC104 crystals by photolysis of the symmetric isomer OC10. Hence, samples of OClO in KC104 were first prepared by irradiation with X-rays at T 5 100 K, which generates the complex [OC10,02],'5 and subsequent annealing for 20 min at 355 K to remove 0 2 from the complex. Irradiation of the crystals with UV (A 365 nm) at or below 20 K leads to decay of the ESR signal from OClO and to emergence of three signals which may be assigned to ClOO in three inequivalent configurations A, B, and C. The signals from the configurations A and C each represent four equivalent, distinct orientations of ClOO in the lattice, pairwise related by reflection in the a-c plane, whereas the signal from configuration B represents only two distinct orientations. These observations show that C100(A) and ClOO(C) both have an unsymmetrical position in the host lattice, so that their spin Hamiltonians each contain 17 independent parameters, while C100(B) apparently conforms to the mirror symmetry of the c104- site occupied by C100, which reduces the number of independent parameters to 11. C100(A) is metastable with respect to conversion into C100(B), which in tum is metastable with respect to ClOO(C), the conversions taking place at -35 and -60 K, respectively. ClOO(C) is stable for months at room temperature contrary to
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earlier claims." C100(A) and C100(B) may be regenerated from ClOO(C) by renewed irradiation at low temperature with near-ultraviolet or visible light, the photolytic interconversions of the three configurations of ClOO eventually producing a stationary mixture that depends somewhat on the wavelength and polarization of the photolyzing light. The ESR signal from ClOO(C) undergoes a reversible change with temperature in the range 20-30 K; above 30 K the signal represents two distinct orientations rather than four, and the spin Hamiltonian now displays the mirror symmetry of the C104site. The parameters in the spin Hamiltonians of C100(A), C100(B), and ClOO(C) obtained at 6 K are listed in Table 1. The parameters appropriate to ClOO(C) at 61 K are also included in the table, along with the algebraic sum of the spin Hamiltonians at 6 K for the members of a reflection-related pair of orientations of ClOO(C). In Table 1 we have for each configuration chosen Qz < 0, from which the signs of all other hyperfine and quadrupole parameters follow without ambiguity because of the asymmetry of the hyperfine pattern. As the spin Hamiltonian for ClOO(C) could be determined after C100(A) and C100(B) had decayed, a good accuracy was attained: 105 line positions were reproduced with a rms deviation of 0.08 G. Even so, the data did not allow a reliable simultaneous determination of the asymmetry of the near-axial quadrupole tensor and the Eulerian angle qj specifying the orientation of the XY axes. The values of Qx and Q, shown in Table 1 were calculated with the symmetry-based constraint W(Q) = W(A). The less complete data obtained for C100(A) failed to yield a reliable value of the asymmetry of Q. 1 7 0 Splittings. The ESR signal from ClOO(C) produced in KC104 tagged with I7Odisplays large hyperfine splittings from I7O nuclei in two inequivalent positions. The I7O hyperfine lines may be observed for Bo in the mirror plane (a-c) at any temperaturebelow 130 K as well as for Bo lying within extended angular regions in the a-b and b-c planes both below and above the temperature of the transition from four to two equivalent sites. The observable sections of the angular variation of the splittings, which were measured also at 35 GHz, can be represented in terms of the axial hyperfine tensors given in Table 2. We assign the larger hyperfine tensor A('0) to the terminal oxygen of ClOO in accordance with the calculated shape of the singly occupied x orbital2' and with results for the analogous species FO0.22 The sign of AL could not be determined. However, taking A1 > 0 for both tensors, we obtain the values of the isotropic hyperfine constants 'a(C) = -62 f 6 MHz and %(C) = -38 f 6 MHz, which agree with values reported for F00:22 llal = 62 MHz and 12al = 40.6 MHz. A pair of corresponding but substantially smaller " 0 splittings were observed for C100(B). These splittings attain their maximum values for Bo1 Ib (Table 2). The I7O hyperfine lines of C100(B) are broadened even at 6 K and cannot be traced
Byberg
13394 J. Phys. Chem., Vol. 99, No. 36, 1995 TABLE 2: I7O Hyperfine Tensors (in MHz) of ClOO(C) and ClOO(B) config C
01‘ 0 2
B
IAll
A,
28 & 10 3 0 f 10
-235 -170
01‘
-184
0 2
-133
(W3VY (160.5.36.6,-) (158.7,37.5,-)
ezpb(Al>O)
0.58 0.44
(90,90,-) (90,90,-)
a Eulerian angles refer to the orientation for which 97%is given in Table 1. Population of unpaired electron in 2p orbital on oxygen calculated by means of atomic hyperfine data in ref 20 assuming A1 > 0. 01 is taken as the terminal oxygen atom of ClOO (ref 21).
outside a narrow angular region around b. No attempt was made to study I7O splittings of the signal from C100(A). Assignment of Axes. The directions of the principal axes X for the g and A tensors coincide within the limit of error for all three configurations, whereas the Z axes deviate by 27-45”. Hence, we take X to represent the normal to the molecular plane, in accordance with observation that the unique axes of ‘A(I7O) and 2A(170)both have this direction too. The common Z axis of A(C1) and Q(C1) is taken to represent the direction of the C1-0 bond, and Z(g), chosen as the axis corresponding to the largest principal g value, is assumed to be roughly perpendicular to the 0-0 bond in the molecular plane. Spin Populations. Representing within the conventional LCAO picture the I7O splittings for ClOO(C) in terms of spin populations 4 on the oxygen atoms, we obtain ~ 2 ~ , ( = ~ 00.58 ) and Q2p,(20) = 0.44, using the atomic hyperfine constant20 for I7O and assuming as above A1 > 0. This result indicates that the x orbital holding the unpaired electron is strongly localized on the oxygen atoms. However, a small contribution from the 3p, orbital on chlorine must be expected. Interpreting the nonaxial component of A(C1) as arising from this contribution, we find by means of the atomic hyperfine constant of chlorine20 the spin populations ~ 3 ~ ~ ( C0.02 l ) and &:(Cl) -0.10, the latter value indicating a very efficient spin polarization in C100. Incidentally, the small value of ~ 3 suggests ~ , that the 3px orbital on C1 is almost completely filled because the remaining two x orbitals of the system, which are both doubly occupied, must have large amplitudes on C1 in order to reserve the 2pn* orbital on 0 2 almost exclusively to the unpaired electron. This observation may serve to justify our choice of QL(C1)< 0, which corresponds to placing a hole in the 3pz(C1) orbital. Shape and Location of ClOO in the KC104 Lattice. The calculated geometry of ClOO (rclo = 2.14 A, roo = 1.20 A, and 8 = 11.5’) combined with the van der Waals radii of oxygen and chlorine (1.4 and 1.8 A, respectively) gives ClOO a total “length” of 6.06 A, which exceeds by -0.2 the maximum dimension of the vacant c104- site as estimated from the nominal geometry of the latticez3and the crystal radii of 0 and K+ (1.6 and 1.25 A, respectively). Considering the extreme weakness of the C1-0 bond in C100, we therefore assume the values rclo = 2.0 8, and 8 = 112”, thus allowing a distortion which would increase the energy of the free ClOO molecule by 3-400 cm-’ only.’ With this geometry, the orientations of Z(A) and X ( g ) (representing the C10 bond and the normal to the molecular plane, respectively) are in each configuration fully consistent with a position of ClOO determined by minimum electronic overlap with the atoms of the lattice, considered as spheres having the crystal radii. This is illustrated in Figure 1. All configurations have the chlorine atom located in the mirror plane through the site, in B and C occupying either of the inplane oxygen positions of C104- and in A lying halfway between these positions. Both oxygens of A are located near one of the pair of reflection-related oxygen positions of C104-. This applies also to the terminal oxygen of C, while the central
-
-
Figure 1. Proposed shape and location of ClOO superimposed on the structure of KC104. (A-C): Projections of the configurations A, B, and C along b on the mirror plane through the C104- site. For compactness the three configurations have been made to occupy adjacent Clod- sites along c. (A’-C’): Schematic representations of the overlap between ClOO and the pertinent atoms of the lattice shown as “projections” on planes normal to the mirror plane and intersecting this plane at the lines indicated in (A)-(C). The atoms of the host lattice are represented as circles showing their contact radii, whereas the atoms of ClOO are circles (in heavy line) with the van der Waals radii (rcl = 1.80 A, ro = 1.40 A). Cations are marked +. The position of ClOO is in each configuration chosen to yield the observed directions of the C1-0 bond (along Z(A)), the normal to the molecul?r plane (aloag X(g)),and the assumed geometry of ClOO (roo = 1.2 A, rclo = 2.0 A, and 0 = 112”),while minimizing the overlap between ClOO and the atoms of the lattice. oxygen lies near the mirror plane. Hence, the motion of C seen above 20 K may be visualized as jumps of the terminal oxygen between a pair of equivalent positions lying on either side of the mirror plane. In the symmetrical configuration B both oxygens would appear to lie in the mirror plane. However, the observation of the much reduced I7O hyperfine splittings of B compared to those of C suggests that some large-amplitude motion of the 0 2 moiety of B persists even at 6 K. Hence, the observed ESR signal might correspond to a motional average of pairs of asymmetric, equivalent configurations analogous to the merged pairs of C seen above 20 K. IR Spectra. Figure 2 shows the absorptions in the region 1350-1450 cm-’ produced by photolysis at 1 365 nm and subsequent annealings in a KC104 crystal that was first X-irradiated at 20 K and annealed at 360 K to maximize the concentration of OC10. The spectra were recorded at 8 K. The growth and decay of the absorptions at 1369, 1374, and 1426 cm-’ during annealings at 40 and 60 K (curves b and c) closely follow the changes of the ESR signals from C100(A), C100(B), and ClOO(C), respectively, from which we assign the IR absorptions to the 0-0 stretch of the three configurations of C100. This assignment is corroborated by the IR spectrum of a KC104 crystal tagged with I7O and ‘*O (curve d), obtained after the same sequence of irradiations and annealings as led to curve c: isotope shifts of the 1426 cm-I line corresponding to two slightly inequivalent oxygen atoms are clearly visible; the shifts produced by I7O andI80 may be accounted for in terms of the diatomic molecule approximation as shown in Table 3. The absorption at 1374 cm-I assigned to C100(B) has at 6 K an asymmetric shape with a pronounced shoulder at 1372 cm-I, which gradually merges with the main line as the temperature is raised: at 20 K an almost symmetrical line of width 3 cm-I centered at 1374 cm-I is observed; at 30 K the
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Single Crystal Study of ClOO
J. Phys. Chem., Vol. 99, No. 36, 1995 13395
1
17-17 17-18
1360
\
16-16
1380
16-17
1400
1420
16-16
1440
Wavenumber / cm -1
-
Figure 2. IR absorptions assigned to the 0-0 stretch V I of ClOO in KC101. Curve a: spectrum produced at 8 K by photolysis (1 365 nm) of an X-irradiated KC104 crystal. Curves b and c: spectra recorded at 8 K after annealing at 40 and 60 K, respectively. Curve d: spectrum corresponding to that shown in curve c but obtained from a KC104 crystal tagged with I7O and I8O. Lines from various isotopomers are indicated. The spectral resolution was 0.2 cm-' for all curves.
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TABLE 3: Spectral Positions (in cm-') of the Absorptions Produced during Photolysis (A 365 nm) of OClO in KC104 at 8 K Vexp
vcalca
1426.1 1374.0 1371.9 1368.7 1406.4 1405.0 1388.7 1386.0 1385.0 1366.7 1365.4
} 1405.9
} 1385.8
assignmentb
C1l60I60(A) c1~70~60(c) c1160~70(c) C1'80'60(C) C1~60~80(C1
1383.3
} 1364.0
a Calculated with the diatomic molecule approximation. Assignment of isotopomers corresponds to model I of ref 5.
width of this line has attained a minimum value (2.6 cm-I) and increases to 3.2 cm-' at 40 K. Combined with the reduced I7O splittings in the ESR signal from C100(B), these reversible changes of the shape of the IR absorption suggest a motion of C100(B) in the lattice with a frequency w that is high enough at 6 K (w 1 100 MHz) to produce a motionally averaged ESR signal, whereas a motionally averaged IR absorption, which requires w 100 GHz, is observed above 20 K only. N
Discussion The spin Hamiltonians for the three configurations of ClOO in KC104 (Tables 1 and 2) correspond closely to the model developed originally by Adrian for the analogous species F W Z 4 and later confirmed by ab initio calculations for C100:792'the main spin density resides in a 2pn* orbital on the oxygen atoms, while the near-axial hyperfine interaction with the chlorine nucleus, directed along the C1-0 bond, reflects a substantial spin polarization of a o orbital located in part on the chlorine atom. As the axial component Qz(Cl)of the nuclear quadrupole interaction, similarly directed along the C1-0 bond, has the
same sign as A,(Cl), this interpretation implies that both quantities are negative. The negative spin density at the chlorine atom in conjunction with the large spin-orbit constant of the 3p(C1) orbital may contribute to the substantial negative g shift observed perpendicular to the molecular plane, which likewise cannot be accounted for within a one-electron description of a n radical. We note that the axis Y(g) does not follow the 0-0 bond direction but is deflected by 10-20" toward the C1-0 bond, thus again suggesting a significant contribution to the g tensor from the chlorine atom. The spin Hamiltonian obtained at 61 K agrees with that reported earlier9 and is well represented in terms of the average of a pair of low-temperature spin Hamiltonians of ClOO(C) (Table 1). The g and A tensors of the high-temperature spin Hamiltonian are much distorted: the X and Y components seem interchanged, and the axes have come deceptively close to each other. As the assumed motion of C100(B) could affect the observed spin Hamiltonian in a similar way, an interpretation of the parameters obtained for this configuration is unwarranted. Lattice Effects on the Spectroscopic Properties. Comparing the values of the 0-0 stretching frequency V I observed for ClOO in KC104 with values reported for ClOO trapped in two inequivalent sites in argon (1415 and 1442 cm-'):s6 we find that YI(C)= 1426 cm-' suggests a lattice perturbation of the stable configuration in KC104 which is similar to that experienced by ClOO in argon, while the values YI(A)= 1369 cm-' and VI(B) = 1374 cm-' suggest that the metastable configurations are somewhat more perturbed by the KC104 lattice. We may note that the reduction of V I observed when going from C to A is accompanied by a 10% reduction of the negative spin density at the chlorine atom and by a significant increase of all g values. The structure of configuration B suggests a simple interpretation of the reduction of q ( A ) and q ( B ) with respect to YI(C): as may be seen from Figure lB, the oxygen atoms of B have little overlap with the lattice atoms. Hence, the perturbation that shifts V I would seem to act primarily on the chlorine atom. In fact, this atom sits squeezed between three (negatively charged) oxygen atoms of adjacent clod- ions (Figure 1B'). The electronic overlap between chlorine and these oxygens will not change appreciably if the C1-0 bond length or the bond angle of ClOO is changed. Therefore, rather than distorting the nuclear geometry, this overlap tends to polarize ClOO by reducing the amplitude of the molecular orbitals on the chlorine atom, thus counteracting the charge transfer from 0 2 to C1 seen in the free m ~ l e c u l e .As ~ the value of V I depends on the amount of charge transfer, a reduction is expected. In configuration A a large overlap of the chlorine atom with two oxygen atoms of the lattice apparently outweighs the overlap of the oxygens of ClOO with the surroundings (Figure lA'), producing a net transfer of charge from C1 to 0 2 similar to that occurring in B. In contrast, the atoms of the stable configuration C have little overlap with the atoms of the lattice (Figure lC,C') so that ClOO(C) should be much less polarized by external perturbations than are C100(A) and C100(B). Hence, rather than being a mere coincidence, the similarity of the value of YI(C)with those measured for ClOO in argon could reflect the absence of strong perturbations in both environments. To test the implied similarity of the spin Hamiltonians, we attempted to simulate the published ESR spectrum of ClOO in argon'* using exact diagonalization of the spin Hamiltonian of C100(C) (Table 1). As expected from the reported values of gx and gY,l2 a substantial adjustment of the g tensor was required; moreover, a slight increase of JAJproved useful. A reasonable fit was obtained with g = (1.9918, 2.0025, 2.0100), a = 28",
13396 J. Phys. Chem., Vol. 99, No. 36,1995 and A, = -54 MHz, and with the remaining components of A and Q taken as those of ClOO(C). The spectrum calculated from this spin Hamiltonian displays correctly all the “peaks” of the observed curve, but the line shape of the rather poorly resolved high-field end is not faithfully reproduced, which could suggest an anisotropic broadening of the spectrum. We note that the pertinent values of gz, a, and Ai could also be obtained from those of C100(A) and ClOO(C) by linear extrapolation to V I = 1442 cm-I. The resemblance between the simulated and observed curves indicates that the hyperfine and quadrupole tensors of C100(C) and of ClOO in argon are consistent as suggested by the similar values of V I ,while the g tensors are sensitive to subtle differences in the two environments that hardly affect V I . Conclusions The stable configuration of ClOO trapped in KC104 avoids significant electronic overlap with the atoms of the host lattice and is therefore the best available approximation to “free” ClOO that lends itself to single crystal study. Accordingly, the hyperfine and quadrupole tensors of this configuration, which appear to be consistent also with the ESR spectrum of ClOO in an argon matrix, may be taken to represent the spin distribution and electric field gradient of the free ClOO molecule to a good approximation. References and Notes (1) Johnston, H. S.; Moms, E. D.; Van den Bogaerde, J. J. Am. Chem. SOC. 1969,91, 7712.
Byberg (2) Dunn, R. C.; Richard, E. C.; Vaida, V.; Simon, J. D. J. Phys. Chem. 1991,95, 6060. (3) Dunn, R. C.; Simon, J. D. J. Am. Chem. SOC.1992,114, 4856. (4) Arkell, A,; Schwager, I. J. Am. Chem. SOC.1967,89, 5999. (5) Muller, H. S. P.; Willner, H. J. Phys. Chem. 1993,97, 10589. (6) Johnsson, K.; Engdahl, A.; Nelander, B. J. Phys. Chem. 1993,97, 9603. (7) Peterson, K. A.; Werner, H.4. J. Chem. Phys. 1992,96, 8948. (8) Atkins, P. W.; Brivati, J. A.; Keen, N.; Symons, M. C. R.; Trevalion, P. A. J. Chem. SOC. 1962,4785. (9) Byberg, J. R. J. Chem. Phys. 1967,47, 861. (10) Eachus, R. S.; Edwards, P. R.; Subramanian, S.; Symons, M. C. R. Chem. Commun. 1967,1036. (11) Eachus, R. S.; Edwards, P. R.; Subramanian, S . ; Symons, M. C. R. J. Chem. SOC.A 1968,1704. (12) Adrian, F.J.; Cochran, E. L.; Bowers, V. A. J. Chem. Phys. 1972, 56, 625 1. (13) Raghunathan, P.; Sur, S. K. J. Am. Chem. Soc 1984,106, 8014. (14) Curl, R. F. J. Chem. Phys. 1962,37, 779. (15) Byberg, J. R.; Linderberg, J. Chem. Phys. Lett. 1975,33, 612. (16) McDowell, C. A.; Raghunathan, P.; Tait, J. C. J. Chem. Phys. 1W3, 59, 5858. (17) Glarum, S. H. Rev. Sci. Instrum. 1965,36, 771. (18) Byberg, J. R.; Jensen, S. J. K.; Muus, L. T. J. Chem. Phys. 1966, 46, 131. (19) Goldstein, H.Classical Mechanics; Addison-Wesley: Cambridge, MA, 1950; p 109. (20) Dalgaard, E. Proc. R. SOC.London 1978,A361, 487. (21) Jafri, J. A.; Lengsfield, B. H.; Bauschlicher, C. W.; Phillips, D. H. J. Chem. Phys. 1985,83, 1693. (22) Fessenden, R. W.; Schuler, R. H. J. Chem. Phys. 1966,44, 434. (23) Bats, J. W.; Fuess, H. Acta Crystallogr. 1982,838, 2116. (24) Adrian, F. J. J. Chem. Phys. 1967,46, 1543.
JF95 1462F
