Vol. 10, 1977
EPR of Triplet States
27
Electron Paramagnetic Resonance of Triplet States: Cyclic 4~-ElectronSystems, CH2, and Environmental Effects E. Wasserman* a n d R. S. H u t t o n Bell Laboratories, Murray Hill, New Jersey 07974, a n d School of Chemistry, Rutgers University, New Brunswick, N e & Jersey 08903 Receiued October 23, 1975
commonly found in organic systems due to small hyElectron paramagnetic resonance is a particularly perfine interactions and g shifts. satisfactory spectroscopic tool for the study of triplet For the triplet the dominant consideration is the states.l The transitions observed are between the three presence of the second unpaired electron with its atmagnetic sublevels which arise from the two unpaired tendant magnetic field. This field may be thousands electrons and require microwave energies of 0.1-1.0 of gauss. We may view the resonance process as occm-l. With such a gentle probe, deleterious photocurring when- electron 1 experiences a total field chemistry associated with high-energy visible or ulequivalent to Ho. In a triplet this total_fieldis composed traviolet radiation can be avoided, particularly with of the external field of the magnet, H , 5nd ;he intraground-state triplets. With mixtures, only paramagmolecular field arigng from electron 2, Hi. Hi adds to netic species are observed, and within that family or subtracts from H so that smaller or larger external triplets are readily separated from doublet radicals. fields are required for resonance. In measuring these From these measurements we obtain a comparatively external fields we indirectly obtain the internal fields direct look at a most significant aspect of the triplet and information on the distribution of the unpaired state, since the spectral parameters are largely deterelectrons in the triplet. mined by the distribution of the two unpaired electrons. The effective internal field from the magnetic dipole As these are the two electrons of highest energy, they is are also those of greatest chemical interest. In this Account we review the determination of the EPR spectrum of triplet states, with particular application to randomly oriented samples. By eliminating the need for single crystals a large variety of species may whe_e GH is the magnetic moment of the electron along be examined; among these are highly reactive systems H, z is the vector, fixed in the molecule, betwee? the which have only been available in dilute powders a t two unpaired electrons, and 6’ is the angle between-z and cryogenic temperatures. H (Figure la).9 Due to the cos2 0 dependence, Hi will Two species examined recently, the cyclic 47r-electron change magnitude and sign with variations in oriensystems, hexachlorobenzene dipositive ion2 (1) and tation. cyclopentadienyl cation3 (2), are found to be groundTwo extremes of orientation are of greatest signifistate triplets as expected from the Huckel molecular cance, 6’ = 0’ and 6’ = 90’ (Figure lb). For 6’ = 0”, cos orbitals and Hund’s multiplicity rule. Another triplet 0 = 1and Hi = 3 p ~ / = 2 D’, ~ the largest possible internal ground state is CH2. Although its EPR is complicated field. An external field of Ho - Hi or Ho - D’ is required by the rotations and oscillations associated with a small for resonance. For 6’ = go”, cos 8 = 0 and Hi = m o l e c ~ l e ,the ~ , ~spectra of isotopic derivatives allow a -3/2(pH/z3)= -D’/2. Resonance will occur at an exdetermination of the geometry of the ground ~ t a t e . ~ - ~ternal field of Ho = +0’/2. If the triplet system is Finally, we note the interaction of triplet states with tuqbling freely, as in a liquid, 3 cos2 6’ - 1averages to their environment. Experiments demonstrate that in 0, Hi vanishes, and resonances may be observed near condensed media partial charge transfer between solHO in favorable systems.1° If the molecules have a vent and unfilled orbitals of the triplet is a general common fixed orientation in a solid, Hi varies with 8, phenomenon.
EPR of Triplet States An unpaired electron in a magnetic field, fi, undergoes resonance when the difference insnergy of spin “up” and spin “down” with respect to H corresponds to the microwave energy impinging on the sample. For the common microwav: source of -0.3 cm-l, transitions occur near 3000 G, Ho. Variations of -100 G are Edel Wasserman obtained his undergraduate education from Corneil University and his graduate studies were done at Harvard under P. D. Bartlett and the late W. E. Moffitt. In 1957 he joined the Bell Telephone Laboratories in Murray Hill, N.J., where h e remained until 1976 as a Member of the Technical Staff. In addition, beginning in 1967 he was Professor of Chemistry at Rutgers University. Dr. Wasserman is now Director of the Chemical Research Center at Allied Chemical Corporation. Richard S. Hutton is a native New Jerseyan. He received his B.S. from Stevens Institute of Technology and his M.S. from the State University of New York at Stony Brook. Since 1970 he has been an Associate Member of the Technical Staff at Bell Laboratories.
(1) See, e.g., S. P. McGlynn, T. Azumi, and M. Knoshita, “The Triplet State”, Prentice-Hall, Englewood Cliffs, N.J., particularly Chapters 2, 9, 10, and 11. (2) E. Wasserman, R. S. Hutton, V. 3. Kuck, and E. A. Chandross, J . Am. Chem. SOC.,96, 1965 (1974). (3) M. Saunders, R. Berger, A. Jaffe, J. M. McBride, J. O’Neill, R. Breslow, J. M. Hoffman, Jr., C. Perchonock, E. Wasserman, R. S. Hutton, and V. J. Kuck. J. Am. Chem. SOC..95. 3017 (1973). (4) E. Wasserman, W. A. Yager, andV. J. Kuck, Chem. Phys. Lett., 7, 409 (1970). (5) E. Wasserman, V. J. Kuck, R. S. Hutton, and W. A. Yager, J . Am. Chem. Soc., 92, 7491 (1970). (6) G. Herzberg and J. W. C. Johns, J. Chem. Phys., 54, 2276 (1971). (7) R. A. Bernheim, H. W. Bernard, P. S. Wang, L. S. Wood, and P. S. Skell, J . Chem. Phys., 54, 3223 (1971). (8) E. Wasserman, V. J. Kuck, R. S. Hutton, E. D Anderson, and W. A. Yager, J . Chem. Phys., 54, 4120 (1971). (9) Equation 1includes interaction of electron 1with components of H , parallel and perpendicular to R. The perpendicular interaction which is not shown in Figure 1 may be viewed as the origin of the factor 3/2: E. Wasserman, Prog. Phys. Org. Chem., 8, 322 (1971). (10) H. R. Falle, G. R. Luckhurst, H. Lemaire, Y. Marechal, A. Rassat, and P. Rey, Mol. Phys., 11, 49 (1966).
Wasserman and Hutton
28
Accounts of Chemical Research
tH H~-D'
Z
Hg
Hg+D'/2
t
(b)
(a)
Figure 1. (a) Magnetic field from electron 2 at the site of electron 1.9 (b) Extreme resonance fields due to algebraic limits of Hi.
Figure 3. EPR absorption and first derivative curves for randomly oriented triplets with less than threefold symmetry. Derivative curve is computed with nonzero line width.13
Figure 2. E P R absorption and first derivative curves for randomly oriented triplets with threefold or higher symmetry. The dashed curves refer to the contributions of the up and down orientation of the spin of electron 2. Derivative curve is computed with nonzero line width.13
the orientation of the ensemble with respect to the external field. Such spectra were first observed in single-crystal studies by Hutchison." In our investigations, single crystals are usually not available. We use powders in which all values of 0 occur simultaneously, but a given molecule assumes a single, fixed value. A range of resonant fields is observed in which the intensity of absorption is not uniform but rises with increasing 8. The variation is due to the larger probability of orientations with larger values of 0 in the can-domly oriented systems. The orientation 0 = O", i.e. zllH, is only found if the second spin is directly above or below the first. In the 0 = 90" orientation, which correspond: to a plane in which the z axis is perpendicular to H (Figure lb), the second spin can be front or back, right or left, with respect to t$e first. As seen from the axes fixed in the molecule, H lies in the x,y plane. The probability is infinitely greater that absorption occurs corresponding to a 0 = 90" orientation than to 0 = 0" (Figure 2).'2313
(11) C. A. Hutchison, Jr., and B. W. Mangum, J. Chem. Phys., 34,908 (1961). (12) For a more detailed treatment of the line shape following this argument, see: R. Breslow, H. W. Chang, R. Hill, and E. Wasserman, J . Am. Chem. SOC.,89, 1112 (1967). (13) The quantum mechanical treatment is found in E. Wasserman, L. C. Snyder, and W. A. Yager, J . Chem. Phys., 41, 1763 (1964).
The curves in Figure 2 (and Figure 3 below) are symmetrical about Ho. The doubling of the characteristic features is a consequence of the spin of the second electron being up or down with essentially equal probability, and the envelope is the sum of the absorptions associated with each direction. The two orientations correspond to opposite signs of p H so that *Hi appear with the same intensity. Viewed from Hot the spectra provide a measure of Hi. The characteristic shape in Figure 2 requires a magnetically isotropic plane, the r , y plane, in the triplet. Perpendicular to the plane would be an axis of threefold or higher symmetry. If only lower symmetry is present, each of the two large features due to the isotropic plane in Figure 2 split into two discontinuities, a cusp and a step, characteristic of the x and y axes (Figure 3). The separation between these two features is designated 3E' and is a measure of the anisotropy of the plane.13 The overall spread of the spectrum may be thousands of gauss for randomly oriented samples. Nevertheless, the sudden changes in intensity remain, corresponding to an orientation of one of the thr_eeprincipal molecular axes of the triplet parallel to PI. Because the first derivative is the usual means of presenting EPR absorptions, these infinities dominate the spectrum. The discontinuities select just those orientations in the randomly oriented sample which are of greatest utility in determining the internal fields. Rather than specifying D' and E' in reporting the triplet interaction, the standard designation is in terms of the energies of one unpaired electron in the field of the second, D and E. The conversion factor is 1070 G = 0.1000 cm-l.14 Since D and E measure the magnetic dipole interaction of the unpaired electrons in the absence of an external field, they are known as the zero-field parameters. Cyclic 4 ~ E l e c t r o nTriplets An orbital view of cyclic 4ir electron systems implies that the two electrons of highest ener'gy are to be as(14) The two different conversion factors in ref 9 and 12 are both i n ~ o r r e c tbut , ~ the values of D and E stated there were obtained using the correct relation.
EPR of Triplet
Vol. 10, 1977
li
\I'
x 1000
29
States
i/ x 250
x500
Figure 4. First derivative of the EPR absorption of the dipositive ion of hexachlorobenzene.2
signed to a degenerate pair of orbitals. Following Hund's multiplicity rule the species are expected to be ground-state triplets. CI
2
1
While detailed considerations below indicate a more complex tale, this is a useful first approach. The six-member benzenoid ring was prepared from hexachlorobenzene by route 2.2 Because of the affinity C,Cl,
- Cl,,SbF
25 OC
C,Cl,+
Cl,,SbF
hv, 4-77 K
c,c1,2+
of SbF5for halide anions, C12-SbF5 was expected to be a strong oxidizing agent. The more difficult second oxidation was accomplished by irradiation; presumably the excited monopositive ion loses an electron to a low-temperature C12-SbF5 trap. The EPR spectrum clearly demonstrates the presence of a triplet state (Figure 4). The intense absorption at 3283 G arises from the monopositive doublet ion. The triplet absorption at 1534 G occurs when both spins flip simultaneously; such Am = 2 transitions were first observed by van der Waals and de Groot in metastable tri~1ets.l~The shape of the triplet lines (E = 0 corresponding to Figure 2) indicates a species with at least threefold, and probably sixfold symmetry. The hexagonal form is expected as the equal population of the degenerate levels ensures the same symmetry as that of the parent benzene; an unequal population as in a monopositive ion can lead to a Jahn-Teller distortion and twofold symmetry.16 The dipositive ion thus provides us with our most direct look a t the highest occupied orbitals of a benzene, a benzene with its electronic skin partially removed but without distortion from the sixfold symmetry of the parent. Implicit is the a-orbital array and the associated aromaticity. The assignment to 1 is based on a detailed examination of the parameters obtained from the EPR spectrum.2 Here we concentrate on D. The deviation of the weak ( z ) lines from Ho implies D' = 1080 G and D = 0.101 cm-l, corresponding to an average distance, (l/r3)-1/3, between the two unpaired electrons of -2.5 (15) J. H. van der Waals and M. S. de Groot, Mol. Phys., 2,333 (1959). (16) M. K. Carter and G. Vincow, J. Chem. Phys., 47, 292 (1967).
A. For comparison, the metastable triplet state of benzene has D = 0.157 cm-l,17 the greater value implying -20% closer approach of the spins. The spins in phosphorescent benzene are largely confined to the carbon 7r orbitals. The greater average separation for 1 indicates some delocalization onto the chlorines. The positive charge on the ring is stabilized by one-electron donation from C1 to C, placing both charge and spin on the halogen. The simplest 47r triplet which has been observed is C5H5+,2. Its synthesis was dependent on the formation of 5-halocyclopentadienesls and the vacuum, lowtemperature technique for the formation of carbonium ions.lg 5-Chloro- or 5-bromocyclopentadiene was condensed with SbF5 directly from the vapor onto a surface cooled to 77 K (eq 3). The solid was transferred
9 9 NXS
TI
H
SURFACE
@
SbF5X-
(3)
X
X = Br, C1
to an ESR tube without warming, and a spectrum similar to that of Figure 4, but expanded to 5300 G, was observed. D = 0.1844 cm-l corresponding to an average separation of -2 A, a reasonable value for a fivemembered ring. With E = 0, triplet I1 must have fivefold symmetry. Both 1 and 2 are unstable a t dry ice temperatures. Part of their reactivity is undoubtedly that of any small carbonium ion, but these are unusually unstable species. The instability is compatible with the antiaromatic character of cyclic 4a systems. The temperature dependence of the EPR spectra indicates that 1 and 2 are ground-state triplets. With 1 there is no evidence of the singlet state, but with 2 line-width variations with temperature indicate that the lowest singlet state is -0.5 kcal/mol above the ground state. This conclusion assumes that an excited vibrational level of the triplet reversibly crosses over to the ground vibrational state of the singlet, limits the triplet lifetime, and is responsible for the increase in the line widths. The approximate degeneracy of the singlet and triplet state continues a pattern seen in other cyclopentadienyl cations. Although the pentachloro has a triplet ground (17) M. S. de Groot, I. A. M. Hesselman, and J. H. van der Waals,Mol. Phys., 13, 583 (1967). (18) R. Breslow and J. M. Hoffman, Jr., J . Am. Chem. SOC.,94, 2110 (1972). ~ -. -,_ -
(19) M. Saunders and D. Cox, J . Am. Chem. SOC.,95, 3017 (1973).
Wasserman and Hutton
30
state, pentaphenyl and simple derivatives of it are ground-state singlets with the first triplet 0.5-4 kcal above the ground state.12 Attempts to observe triplet ESR spectra associated with cyclobutadiene20,21 have not been successful.Z2 The species is most likely a ground-state singlet with the triplet energy >2-3 kcal/mol higher. Thus, within the 47r systems there appears to be a distinction between the four-membered ring with the singlet lowest and the five- and six-membered systems where the triplet is lowest or approximately degenerate with the singlet. Examination of the molecular orbitals shows that the unpaired electrons have a different relationship in a four-membered ring from that in the five-, six-, or seven-carbon system^.^^^^^ In the 4-ring, the spins occupy two orbitals which do not have any atoms in common (3). In a simple picture which only treats
4
3
one-center interactions, the singlet and triplet states would be degenerate. With the five-, six-, or sevenmembered rings, the half-filled orbitals have atoms in common, as in 4. For these triplet states the Pauli exclusion principle precludes two electrons with parallel spins occupying the same 2p-7r atomic orbital simultaneously. When one-center, two-electron interactions are included, the triplet state will have a larger average separation for the unpaired electrons and a lower energy. This orbital picture is only a first step. On including distortion from a regular polygon and the effect of interaction of the lower states with higher configurations, the singlet is stabilized more than the triplet.25 For cyclobutadiene the original S-T degeneracy is split; the singlet becomes the ground state as it is depressed below the t r i ~ l e t . ~For ~ , the ~ ~ larger rings a singlet is lowered so that it is now approximately degenerate with the triplet; the particular ordering of the states depends on ring size and substituents. 4-ring before CI and distortion -S -T after --
-- T
s
5 - , 6-, and 7-rings
before CI and distortion -S
-T after -S
-T
(20) C. Y. Lin and A. Krantz, J . Chem. Soc., Chem. Commun., 1111 (1972). (21) 0. L. Chapman, C. L. McIntosh, and J. Pacansky, J . Am. Chem. Soc., 95, 614 (1973). (22) Unpublished results. Also, S. Masamune, N. Nakamura, M. Suda, and H. Ona, J. Am. Chem. Soc., 95, 8481 (1973). (23) H. C. Longuet-Higgins and K. L. McEwen, J . Chem. Phys., 26, 719 (1957). (24) W. T. Borden, Chem. Commun., 881 (1969). (25) R. J. Buenker and S. D. Peyerimhoff, J. Chem. Phys., 48,354 (1968); L. C. Snyder, J. Phys. Chem., 66, 2299 (1962). (26) The square geometry assigned to cyclobutadiene on the basis of the infrared absorptionm,?'might be alternatively assigned to a rectangular form which is slightly lower in energy than the square. One would then have to consider the vibrational levels of a nonrigid species.
Accounts of Chemical Research
In closing this section we note that a conclusion from the experiments is a limitation in the applicability of iso-a-electronic arguments. They are of considerable utility in dealing with closed shells such as the ground states of the 6-electron aromatics. However, they must be applied with care in the case of open-shell, antiaromatic 4-electron systems where several states may be approximately degenerate.
Methylene Herzberg's observation of the singlet and triplet electronic spectra provided the first physical evidence for CH2.27 Geometrical conclusions were drawn for several states. The time development of the spectra demonstrated that the lowest state was a triplet. EPR spectroscopy has allowed a determination of the structure of the ground The resulting geometry closely approximates that predicted by Foster and Boys in their 15-year-old a priori calculation.28 Recent computations of higher accuracy by Bender and SchaeferZ9and by Harrison3@ have supported the earlier conclusion. To prepare methylene suitable for EPR examination, solid solutions of diazomethane or diazirene in any of a variety of hosts are irradiated. The procedure is analogous to that used earlier for a number of divalent carbon derivative^.^^ The identification as methylene follows from the EPR spectra which were obtained on isotopic substitution of I3C and one or two deuteriums4 and which demonstrated the presence of one carbon, two hydrogens, and no other atom of mass greater than or equal to that of carbon. Given the elemental composition of the irradiated mixture, we may conclude that methylene was the species observed. The zero-field parameters were DcH = 0.68 cm-l, E C H=~0.0034 cm-l and DCDz= 0.76 cm-1, EcD2 = 0.0046 ~ m - l . ~ * For a linear structure E = 0, as the molecular axis would be an axis of cylindrical symmetry and the perpendicular plane would be isotropic. On bending, IEI increases and is expected to become a reasonable fraction of D for a 90' g e ~ m e t r y . ~The , ~ " small values of E for methylene might imply a slightly bent geometry33or a linear structure in an asymmetric environment. However, the substantial difference between DCHz and D C ~indicates z a more complex ~ y s t e m . ~ D measures an average distance between the two unpaired electrons, so we do not expect a variation between immobile isotopic derivatives of more than -0.01 cm-l as their spin distributions are essentially the same. The observed change of 0.08 cm-I arises from differences in the motion of the molecules. Above we noted that internal fields are reduced if 0 can vary for an individual molecule. A wobble of the z axis partially averages 8. Such an oscillation primarily involves the hydrogens and will be reduced with the more massive deuteriums leading to D C D>~D c H ~Assuming . that the (27) G. Herzberg, Proc. R. SOC.London, Ser. A , 262, 291 (1961). (28) J. M. Foster and S. F. Boys, Reb. Mod. Phys., 32, 305 (1960). (29) C. F. Bender and H. F. Schaefer, HI. J. Am. Chem. Soc., 92,4984 (1970). (30) J. F. Harrison, J . Chem. P h p . . 54, 5413 (1971). (31) A. M. Trozzolo, R. W. Murray, and E. Wasserman, J . Am. Chem. SOC.,84, 4990 (1962). (32) These values are for a xenon matrix. Other environments can lead to variation of up to 40%. (33) Such was the conclusion of the first report of the EPR of methylene by R. A. Bernheim, H. W. Bernard, P. S.U'ang, L. S. U'ood, and P. S. Skell, J . Chem. Phys., 53, 1280 (1970).
EPR of Triplet States
Vol. 10, 1977
I 5
I
6
7
motion occurs in a harmonic well, we find that CH2 swings through an arc of 48' ( 5 ) and CDz through 40'. At 4 K, the molecule is in the lowest motional state; we are dealing with a zero-point oscillation of the triplet. The large amplitudes demonstrated for the z wobble imply that rotation about the z axis is essentially free. Because of the wobble, each bond to hydrogen sweeps out a cone with a base -0.8 A in diameter, and within that cone motion is largely unhindered. Rotation about z , even for a substantially bent methylene, involves hydrogen displacements which would lie entirely inside the cone. There is only a small barrier to the rotation. The x and y axes of the methylene readily interchange so that the molecule approximates cylindrical symmetry. In fact, the E observed is the residue of a much larger E (- 0.05 cm-') which has been largely averaged due to in the matrix. To determine the HCH angle, a, we first approximate the motion about z by assuming it can be represented as a perturbed, cylindrically symmetric free rotator. Using a first-order treatment? we find E o h d = EfV/ w2, where Eobsd is the zero-field parameter determined from experiment, Ef is the value expected for a fixed methylene, V is the twofold barrier for rotation about z, and Wz is the energy of the second rotational state of the free rotator. All three quantities on the right are functions of cy, For cy = 180' both E f and V = 0, Ef = 0 by symmetry, and there is no barrier to rotation about the unique axis of a linear molecule. On bending, Ef and V increase. Wz is infinite for a linear molecule, decreasing to finite values on bending; with Ef and V in the numerator and wzin the denominator, &bsd is a rapidly increasing function of bending. E,, V , and W2 may be expressed as functions of a and of known theoretical or experimental quantities. The Eohdyields a = 136' with no adjustable parameters. A direct indication of the importance of rotation about z occurs with CHD. This species was examined because the inequivalence of the H and D masses turns the axis of rotation toward D and away from H (6). Here H swings through a larger arc than in CH2or than D in CD2. The greater excursion implies a larger interaction with the environment (larger V) and a larger Eobsd. For CHD, Eobsd = 0.0064 cm-l, greater than that of CH2 and CD2. An angle of 136' is in remarkably good agreement with the best theoretical calculations available.35 However, an objection could be raised that the matrix is distorting the geometry of an isolated methylene. As an answer, the larger amount of motion, both of the z axis and about that axis, implies that the molecule is (34) The "motion" is the averaging over the cylindrically symmetric ground state of the rotator. There is no rotational energy in that state. The situation is analogous to a hydrogen s orbital which is spherically symmetric but does not have any energy associated with angular motion. (35) S. V. ONeil, H. F. Schaefer, 111,and C. F. Bender, J . Chem. Phys., 55, 162 (1971).
31
largely free and probably has the geometry it would assume in the gas phase.5 Alternatively phrased, the barriers to rotation are too small to be responsible for substantial distortions. A better check on the absence of matrix distortions is available from the 13C hyperfine interactions. The 13Cnucleus has a magnetic moment and is capable of interacting with unpaired electrons at the nucleus. The p orbitals of a linear methylene have nodes a t the nucleus and give a small, second-order interaction. On bending, the s portion of the resulting in-plane sp hybrid can yield a larger interaction with the nucleus as the s orbital does not vanish there (7). We thus have an independent measure of angle-one which does not depend on the motional behavior. A value of 137' results from this d e t e r m i n a t i ~ n . ~ ~ ~ Of particular importance is the observation of the 13C interaction for methylene in two different sites in xenon. In these two sites the molecules differ greatly in D and E and thus in motional freedom and barriers to rotation. Nevertheless, the HCH angles are the samewithin -0.5°.8 Distortions by the matrix are most unlikely, and we should be observing the geometry of free methylene. Herzberg and Johns have shown that, if one assumes a heterogeneous predissociation of methylene in the excited triplet state, the electronic spectrum indicates a ground-state angle of 136°.6 The close agreement of this conclusion and the EPR results provides strong indirect support for the process. The two experimental determinations and the best theoretical calculations are now in agreement, pointing to an angle of -136'. With the angle of the ground state of methylene determined, the geometries of the known states of the series BH2, CHz, NH2, and OHz fall into a simple pattern. The critical variable is the population of the sp hybrid orbital which appears in the molecular plane on bending (7). As the deviation from linearity increases, so does the s character of the orbital. Because 2s is more stable than 2p, the central atom hybrid becomes more stable so that increasing electron occupancy decreases the HAH angle. Population of the out-of-plane orbital, which remains 2p-T regardless of angle, is largely irrelevant. If the hybrid has no electrons, the angle is 180'; if one electron, 131-144'; and if two, 102-105'. Thus, the ground state of CHz with one electron in the hybrid has an angle of 136'. The singlet state with the same orbital population has 140°.27 For the lowest singlet which has two electrons in the hybrid the angle is 102°.27This value is similar to that of water, which also has two electrons in the hybrid orbital. This simple but intuitively appealing picture is a quantitative statement of Walsh's rules, which were formulated qualitatively before most of these species were known.36 The AHz states provide the simplest series for the correlation of geometry and orbital population and show that the approach is of considerable ~ t i l i t y . ~ ~ , ~ ~ The study of motion in triplet methylenes has extended to derivatives of the prrent. Cyanomethylene, which was originally assigned a linear s t r ~ c t u r e , ~ is' (36) A. D. Walsh, J. Chem. Soc., 2260 (1953). (37) G. Herzberg, Science, 177, 123 (1972). (38) E. Wasserman, Chem. Phys. Lett., 24, 18 (1974).
Wasserman and Hutton
32
apparently bent with an HCC angle of about 150’. Its spectrum, which is characteristic of a linear species, arises from the essentially free rotation of the hydrogen. Also CF&H shows an abnormally small E , because of intramolecular rotation about the CC bond. The barrier is -20 cal/mol, small but typical of systems where two groups, one of threefold and the other of twofold symmetry, rotate past each other.
Environmental Effects In this final section we consider the interaction of triplets with their environments. With the zero-field parameters providing a distance between unpaired electrons, we have a sensitive function of the distribution of the spin density. The high accuracy attainable in the EPR measurements allows comparatively small changes in spin distribution to be detected. In particular, intermolecular transfer of electrons involving unfilled orbitals of host and guest will place some spin on the solvent, increase the average distance between the unpaired electrons, and reduce D. If an electron moves from a filled orbital of solvent to a half-filled orbital of the triplet, spin moves in the opposite direction from charge, 8 9. If an unpaired triplet electron moves into a vacant solvent orbital, spin and charge move in the same direction, 8 10. __ 4.-
-
*
+
-+ triplet
44-
-4.k
solvent 8
+ 9
-
f 4+ 10
% 4!+11
An examination of the zero-field parameters of a metastable triplet (naphthalene) and a ground-state triplet (phenylnitrene from photolysis of phenylazide) in a variety of matrices indicates parallel variations. The decrease of D and the increase in the average distance between the spins follows the ease with which charge transfer is expected to occur. The strongest correlation is with the electronic structure of the host. There is no observed dependence on the host’s dipole moment or molecular shape, be it linear, cyclic, or branched. The lack of influence of molecular dipoles indicates that the changes of spin distribution in the triplets are not due to extramolecular electric fields. The solvents studied were composed solely of first-row elements, so that spin-orbit effects from heavy atoms were avoided. Such contributions to the zero-field splittings were previously observed with methylene in xenon.*O D decreases in the series of solvents: saturated fluorocarbons, saturated hydrocarbons, benzene derivatives, and naphthalene. By examining benzenes with electron donor and electron acceptor substituents, (39) R. A. Bernheim, R. J. Kempf, J. V. Gramas, and P. S. Skell, J . Chem. Phys., 43, 196 (1965). (40) E. Wasserman, R. S. Hutton, V J. Kuck, and W. A. Yager, J . Chem. Phys., 5 5 , 2693 (1971).
Accounts of Chemical Research
we find that both groups appear to increase charge transfer with the metastable triplet naphthalene. As the phosphorescent triplets have one low-lying and one high-lying half-filled orbital, the triplet can act as an acceptor for the electrons of the solvent or as a donor int,o the solvent’s high-lying vacant orbitals. Quantitatively, spin delocalization onto solvent from naphthalene is about 1% in fluorocarbons, increasing to about 8% in anisole or dimethylaniline. Delocalization in fluorocarbons was demonstrated by comparing D for the ground-state triplet .NCN in CF4, after correction for zero-point motion, with the value previously observed in the gas phase.41 The decrease in D is about twice as large for metastable triplets as for ground-state triplets in the same solvent. The reduced effectiveness for ground-state triplets is explained by the greater energy separation expected between triplet and solvent orbitals (11). The phenomenon of charge transfer is a useful means of describing the changes in spin distribution. Other factors which are not orthogonal to charge transfer might also be introduced, such as orbitals of different sizes for different spins. That charge transfer occurs between triplets and solvent a t the van der Waals distances is not unreasonable. The distances are largely controlled by Pauli exclusion acting on electrons with the same spin in the different molecules. This “contact” between solute and solvent electrons implies that donation of charge can occur if an unfilled orbital is of energy not much higher than a nearby electron. The partially transferred electron does not encounter the repulsion of an electron of the same spin in the orbital entered. The triplet state has been the focus of our attention because the zero-field parameters allow a direct measurement of a distance between the spins. Chargetransfer effects of similar magnitudes are expected with excited singlet states and their unfilled orbitals. Again the excited state could act as donor and/or acceptor. As a photochemical reaction proceeds along its reaction coordinate, regardless of multiplicity, charge-transfer effects should be considered. Doublet free radicals are also capable of charge transfer. In ion pairs, with their strong coulombic attraction, such effects are well known.42 They have also been observed in neutral ~ystems~~~,~~ Charge transfer should be considered a general phenomenon of open shell systems in condensed media. Transfers of the order of a few per cent are to be expected at van der Waals distances even with neutral molecules in saturated environments. W e t h a n k our colleagues for their collaboration in t h e various aspects of t h e work reported here, particularly F. B. Bramwell, E. A. Chandross, V . J . K u c k , and W . A . Yager. (41) G. Herzberg and D. N. Travis, Can. J . Phys., 42, 1658 (1964). (42) F. C. Adam and S. I. Weissman, J . i l m . Chem. Soc., 80,1518 (1958). (43) A. S. Kabankin, G. M. Zhidomirov. and A. L. Buchachenko, J . Magn. Reson., 9, 199 (1973), and references cited therein. (44) I. Morishima. K. Ishihara, K. Tomishima, T. Inubushi, and T. Yonezawa, @J.Am. Chem. Soc., 97, 2749 (1975), and references cited therein.
