J. Phys. Chem. 1980, 84,2135-2141
2135
Electron Donor-Acceptor Complexes R. Foster Chemistry Department, University of Dundee, Dundee, DD 1 4HN, Scotland (Received: September 4, 1979)
Among Robert Mulliken’s many original contributions to chemistry have been his incisive ideas regarding molecular complex formation between electron donors and electron acceptors (EDA complexes). The growth of interest in this field reflects very largely the stimulus and the framework of understanding which Robert Mulliken has provided. This present contribution describes a few of the many achievements, some outstanding problems, and some possible developments involving EDA complexes.
It is not easy to define the term “complex”. LonguetHigginsl suggesteld that a molecule might be defined as a group of atoms (or a single atom) with a binding energy which is large in clomparison with the thermal energy kT. This means that a molecule can interact with its environment without losing its identity. We might define a complex as a group of atoms or molecules with a binding energy that is not large compared with kT. In other words it is a system whoive identity might be severely shaken, if not lost, by interaction with its environment. Such a definition would at least highlight one of the problems and one of the interests in molecular complexes, namely, the perturbations of the complex by the next sphere of influence of its molecular environment. A more accustomed use of the term is in its application to interactions greater than those which give rise only to deviations from ideal gas behavior and explained in terms of van der Waals forces, but less than those which yield new chemical species in which covalent bonds are formed and/or broken. It was already apparent by the middle of the last century that there were many examples of stable molecules interacting with other stable molecules where the essential identity of the component molecules was preserved within the adduct, whiclh nevertheless showed characteristics separate from its components: for example, solubility, crystal structure, and, often most strikingly, color. Thus a Scottish chemistry textbook2 of the 1850’s dkscribes such a complex (quinhydrone) prepared by Wohler3 in 1844 as “long prisms of the most brilliant gold-green metallic lustre, surpassing murexide in beauty”. The historical development of ideas of complex formation between what were later to be identified as electron donors (D) and electron acceptors (A) has been well deThe most tantalizing observation was the color which generally characterized the complex not only in the solid state but often persisted, a t least to a degree, on dissolution of the complex. Indeed, the intensity of the optical absorption of the D-A systems has been the dominant property used to gain information about the degree of association of the components in solution. It was in fact the publication of such studies on the interaction of molecular iodine with benzene and with alkylbenzenes by Benesi and Hildeb~rand~ which attracted Robert Mulliken’s attention and led him to suggest in a celebrated “note added in proof’10 that electronic absorption bands in the complexes extra tlo the absorptions which appear in the separated components were the result of intermolecular charge-transfer tramsitions. Two years later he4 provided a quantum-mechanical resonance structure description of the ground state ($N) of such complexes in terms of a “no-bond’’ structure $,(D,A) and a dative structure $z(D+-A-): 0022-3654/80/2084-2135$01 .OO/O
(for the case of an even-electron donor and even-electron acceptor). Corresponding to the state $N, there will be an excited state $E such that For weak interactions between A and D, a >> b and a x a*, b = b* so that in such cases the contribution from the dative structure to the ground state is small. The characteristic absorption of the complex corresponds to the $N. The details of this simplified allowed transition $E resonance structure theory and the development of a generalized resonance structure theory have been described in detail by Mulliken and P e r ~ o n .The ~ latter theory extends the two-term expression in eq 1 to involve other dative states, back-donation, and locally excited states, thus
-
Their monograph7also provides the most complete account of the relation of these descriptions with the experimentally observed properties of EDA complexes. The original publications of the theory in 1950 and 1952 gave coherence to many of the observations that had puzzled chemists for a century and which had found only partial rationalization through the contributions of Briegleb,ll Pauling,12 Weiss,13Woodward,14and Brackman.15 It also gave a massive stimulus to new work, which is still burgeoning. Some of these developments are described along with some problems which are still not wholly resolved.
Experimental Evaluation of Thermodynamic Parameters In the liquid and gas phases, the component species are normally in equilibrium with the complex(es). For a wide range of interactions the forward and back reactions are fast. Apart from the intrinsic interest in the values of the thermodynamic parameters themselves, knowledge of the values of the formation constants is necessary for the evaluation of “secondary” data, as was emphasized by Hanna and L i p p e r P (Table I). The experimental evaluation of formation constants has often proved remarkably troublesome. Although as early as 1952 McConnell and his c o - ~ o r k e r s suggested ’~ that complexes with stoichiometries other than 1:l might be coexisting with the DA complex in equilibrium mixtures, nearly all analyses were made ignoring this possibility. There is now evidence that, for many r-donor-r*-acceptor 0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vo/. 84, No. 17, 1980
TABLE I : Available Types of Experimental Data on Electron Donor-Acceptor Complexes together with a Designation Indicating Whether I t Is Separated from Raw Experimental Measurements b y an Interpretive Process That is Reasonably Certain (Primary) or Uncertain ( Secondary)a data frequencies of charge-transfer absorption bands geometry of solid complexes NMR studies of motion in solid complexes nuclear quadrupole resonance measurements o n solid complexes association constants molar absorptivity or other measures of absorption intensity of electronic transitions enthalpy changes upon association entropy changes upon association dipole moments infrared intensity changes NMR chemical shifts of magnetic nuclei in complexes
type primary
Foster
Scheme I. AG” and, in Parentheses, AU“ Values (in kcal mol-’) [from Ref 261 NMe3(g)
t
( - 4 8)
(-9 I )
( - 3 8) 40
57
NMe3(heptane)
-3 5 5
SO,(g)
1-1
t
SO,(heptane)
- 4 73
-
h‘Je,.SO,(j)
NMe3-SO,(
heptarc)
(-11 0 )
primary primary pimary secondary secondary secondary secondary secondary secondary secondary
a Based o n ref 1 6 .
TABLE 11: Experimental Values for the Association Constant (K,) of Tetracyanoethylene with Various Donors in Dichloromethane
K J L mol-’ calorimetrya (25 “C)
spectroscopyb ( 2 2 ”)
RamanC ( 2 1 “C)
0.1076 I 0.0003 0.5990 i 0,0010 1.150 I 0.0005 11.998 t 0.0008
0.128 1.11 3.4 16.8
1.6 k 0.6 3.3 i 0.4 17i: 2
donor benzene mesitylene durene hexamethylbenzene a
Reference 22.
Reference 24.
210
I
I
220
230
2 0
h (nml Figure 1. Vapor-phase contact charge transfer of cyclohexane-I, at 110 OC in a 75.0-cm cell: [I2] = 6.50 X M and [cyclohexane] (CXA) = 3.50 X IO-‘ M. These are absorbance difference curves between (4) CXA-I, and I,; (3) CXA-I, and He-I,; (2) CXA-I, and n-pentane-I,; (1) CXA-I, and n-hexane-&. [Reproduced with the permission from ref 27. Copyright 1971 American Chemical Society.]
Reference 23.
systems, higher-order complexes are indeed present.18J9 That the 2:l complex, D2A, has the structure D-A-D rather than D-D-A derives from the low (? zero) experimental dipole moment of D2A for hexamethylbenzenetetracyanoethylene (0 f 1.5 D).20 Further support for the structure D-A-D comes from the similarity in energy, and the nearly twofold ratio of intensity, of the intermolecular charge-transfer transition for the D2A complex of hexamethylbenzene-fluoranil compared with the DA complex.21 However, for many other complexes the stoichiometry is clearly 1:1,for example, the iodine-monoamine complexes. Here complexing between the u* acceptor and the n donor is relatively strong and the simple stoichiometry is reflected in well-defined isosbestic points. There remain many inconsistencies. To cite but one example: there is poor agreement between the association constants for alkylbenzenq-tetracyanobenzene systems obtained from calorimetric measurements22and those from resonance Raman spectraz3under the condition [D], = [A], (where effects of higher order complexes should be minimal). Yet the same values from the resonance Raman experiments agree well with values derived from spectrophotometric measurementsz4obtained under conditions where [D], >> [A], and disagreement would be expected because of the presence of significant amounts of D2A (Table 11). Experimental methods will not normally distinguish between the formation of a single 1:l complex and the formation of two or more isomeric 1:l c~rnplexes.~~ Such isomerism, while providing a possible explanation for anomalous variations in estimated secondary data (Table I), cannot account for the anomalies referred to in this paragraph.
Studies of systems in the vapor phase are of particular interest because of the close resemblance to the theoretical model of an isolated donor-acceptor pair. Comparisons of thermodynamic parameters for the system NMe3-S02 in the vapor phase and in solution (Scheme I) emphasize the influence of an even relatively “inert” solvent.26 Extension of studies of EDA systems in the vapor phase, NMR measurements for example, should be encouraged. Tamres and GrundnesZ7have shown that not only are electronic absorptions of stable complexes observable in the vapor phase but so also are absorptions which can be assigned to “contact charge transfer” (Figure 1). Mulliken and OrgeP proposed such a concept to explain absorptions which appear in some iodine solutions, extra to the absorptions of the components, but for which there is no evidence for complex formation, e.g., in cyclohexane solutions. It was suggested that there is sufficient overlap between the HOMO of the donor and the LUMO of the acceptor, even a t van der Waals distance, to permit a significant intermolecular electron-transfer transition. In such cases the normal experimental optical methods yield zero formation constants and apparent E values of infinity. This contact charge transfer may also provide a contribution to the optical absorption in systems where complexes of fini.te stability are simultaneously formed. In many systems it is difficult to decide whether an equilibrium quotient is in fact zero or just very small. J t has been suggested that some “operational” definition is necessary.* The concept of “contact charge transfer” has not found complete aceeptance and consequently provides a field for further investigation. In the now “classical” experiments of Benesi and Hildebrand, the activity coefficients of the species were con-
Electron Donor--Acceptor Complexes
The Journal of Physical Chemistry, Vol. 84, No. 17, 7980 2137
TABLE 111: Contributions of Electrostatic ( A E E ) , Charge Transfer ( A E ~ T )and , Repulsion ( A E R ) t o the Total Stability of Some Donor-Acceptor Complexesh ___ complex donor benzene
acceptor
c1 Br,
:[
p-xylene
'I'CNE IC1* 'I'CNE (0" )" 'TCNE (30")a 'TCNE (60")a 'TCNE
-AEE
-AECT
AER
- A h
exptl
18.5 22.4 29.0 37.9 20.0 37.2 38.2 '37.7 36.6
2.2-6.5b 3.4-10.0b
7.9-1 1.5d 10.1-15.4d
13.2-13.9 15.7-17.0
-4.2e -4.F
C
C
7.5 1.5-4.3b 10.5 12.5 9.7 9.4
17.7 7.9-10.4 21.4 38.4 37.6 23.1
26.0 13.2-1 3.4 26.2 12.2 9.8 22.9
-9.6f
I
-14.11 or -30.8g
A range is given corresponding a 0" as such a t the TCNE C=C axis is perpendicular to the CH,C...CCH, axis of p-xylene. t o using different semiempirical constants in calculating the charge-transfer contribution. Not calculated because of the lack of available iodine wave functions. The range comes about because of the two different charge-transfer constants. e Calculated from -RT In K assuming the TAS" term was identical with that for benzene-iodine. f Solution value (ref 5 ) . Some experimental values are included to show reasona Gas-phase value ( M . Kroll, J. A m . Chem. SOC.,90, 1097 (1968)). able order of magnitude agreement. Energies are in kJ mol-'. Based on ref 16.
creasing concentration of pyrene and a new, red-shifted, sidered. In the intervening years this question was largely fluorescence band appears. This was attributed to the ignored, to be revived only relatively recently. These are formation of a complex between an excited molecule of undoubtedly very significant in many systems because of pyrene with a second molecule of pyrene in the ground (a) the large ranges of donor concentrations often used state. (sometimes up to rnole fraction l),(b) the strong interIn 1963 Leonhardt and Weller35observed the quenching action by many solvents (e.g., CHCl,, CH3CN),and (c) the of the fluorescence of perylene by ground-state diinteractions between the D and A molecules which are so methylaniline and proposed that a complex had been weak that other solvent-solute interactions are comparable or greater. However, there are other systems where these formed between an excited state perylene and dimethylconditions do not obtain for which there appears no need aniline (so called exciplex or heteroexcimer) in an essentially dative structure l(A-D+)*. to have recourse to postulate significant departure from In the cases cited, the ground states of the systems are ideality. A very recent interesting treatment of the problem of weakly interacting systems in nondilute solurepulsive states. However, as in other areas, this distinction has been given lby Nagy, Muanda, and NagyZ9in which tion may not be substantial in that it would be difficult they use the concept of microscopic partitioning of a solute to differentiate between a repulsive ground state and one where the bonding is very weak. In the limiting case we between two miscible solvents, a principle originated by Purnell and his c o - ~ o r k e r swho , ~ ~ have applied it in gasmay be forced to an operational definition determined by liquid chromatographic studies of EDA e q ~ i l i b r i a . ~ ~ our ability or otherwise to detect complexes in the ground Through the stimulus of this work we may see considerable state. developments in the application of this hitherto underuObservations on excited states of EDA complexes have tilized technique. led to a further vindication of the Mulliken description. From his resonance structure representation, the first Forces Involved i n t h e Ground States of EDA excited state from two singlet neutral components would Complexes be expected to be a singlet, strongly dative structure. This is confirmed from studies36 of S, S1 absorption of a The range of interactions that may be considered under complex such as TCNB-durene which closely represents the general heading of EDA complexes is so wide that no the absorption spectrum of TCNB- (TCNB = 1,2,4,5generalizations are possible. At one end of the scale we tetracyanobenzene). could include such interactions as argon-argon dimers, at The polar nature of exciplexes originating from neutral the other end, species such as NMe3-BF3. (Do we classify species has been demonstrated by the expected relationthe latter as a complex or a compound?) ship of V F for the complex and (ID - EA) for the complex As indicated above, for relatively weak interactions, the (where ID is the ionization potential of the donor and EA contribution of dative structures to the stabilization of the is the electron affinity of the acceptor); also a relationship ground state is small, although the term "charge-transfer of VF with solvent polarity has been observed.37 complex" is still sometimes used instead of the generally Very elegant studies of picosecond time-resolved spectra preferred expression "electron donor-acceptor complex." showing the formation of intermolecular exciplexes have Estimates by Hanna and LipperP of various contribubeen made. For example E i ~ e n t h a has l ~ ~compared the tions to the ground-state stabilization of some weak complexes are given in Table 111. More recently M ~ r o k u m a ~ ~ rate of development of the exciplex derived from anthracene and diethylaniline with that formed intramolecularly has calculated the various energies involved in some mofrom I (Figures 3 and 4). The slower rate of the latter lecular complexes, using energy decomposition analysis. Resulting electron diensity maps for the complex OC-BH3 are shown in Figure 2. We may soon expect to see extensions of this method to complexes between larger molecules.
-
Excited-State Complexes33 Under this heading come excimers first observed by Forster and K a ~ p e in r ~ 1954. ~ The intensity of the fluorescence of the pyrene monomer decreases with in-
66
I
process reflects the time taken for rotations about the CH2-CH2 bonds.
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The Journal of Physical Chemistry, Vol. 84, No, 17, 1980
-1.0
1.0
0.0
2.0
Foster
3.0
Time Ipsec)
Figure 3. Complex formation FCT (normalized population) vs. time for anthracene plus N,N-diethylaniline in hexane. The solld line is the theoretical curve. [Reproduced with the permission from ref 38b. Copyright 1975 American Chemical Society.] I
-1.0
1.0
0.0
2.0
T
I
3.0
1.0
0.0
., -
-1.0
-0.2 -200 -1.0
1.0
0.0
I 2.0
I 3.0
/
I
I
I
0
200
LOO
I
600 Time ( p s e c )
I
I
800
1000
Flgure 4. Complex formation FCT (population normalized)vs. time for the compound with structure I in hexane. The soli line is the calculated exponential formation function. [Reproduced with the permission from ref 38b. Copyright 1975 American Chemical Society.
comparison of the behavior of the fluorescence in the systems IVa-d. In IVa, IVb, and IVd there is a large red
0.0
I
-1.0
I 1.0
0.0
I
I
2.0
3.0
Flgure 2. Component electron density maps for the complex OC-BH, at R(C-B) = 1.57 A in the C, approach. Full lines indicate density increases and dotted lines indicate decreases. Contours are, successively, f l X lo-,, f 5 X f 9 X IO-,, f13 X IO-, (bohr radius)3. The coordinates are in angstroms relative to the boron atom, and the plotting is made for an HBCO plane. [Reproduced with the permission from ref 32.Copyright 1977 American Chemical Society.]
There is evidence that for many exciplex systems termolecular complexes are present. Here the structures appear to be DDA rather than DAD,39 e.g., 11, and 111.
b
a
II
d
C
IV
Ill
Evidence for the DDA-type structure has also come from
shift with increase in solvent polarity, whereas in IVc, which corresponds to DAD-type complex, little change is observed.*O In contrast to excimers the relative geometry of the acceptor and donor moieties in exciplexes does not
The Journal of Physical Chemistry, Vol. 84, No. 17, 1980 2139
Electron Donor-Acceptor Complexes
appear to be too critical. Thus in intramolecular complexes of the type I and in the cup-dipyrenylalkanes, Py(CH,),Py, where n = 2-16, interaction is observed in all cases.41 Photophysical aspects of exciplexes and photochemistry involving exciplexes will continue to be areas of high activity.
Orientation of Donor and Acceptor in EDA Complexes In the early papers on EDA complexes Mulliken argued that maximization of the overlap between the HOMO of the donor and the LUMO of the acceptor might largely determine the relative configuration of the two moieties in the complex. It is clear from the large number of crystal structures now known that this principle is not always an overriding factor. For a number of complexes Mayoh and P r o ~ were t ~ ~able to rationalize the configurations to a degree by considering contributions from a number of excited states. Attempts to understand the influence of the relative orientations on the charge-transfer transition have been made from studies of the optical absorption of paracyclophanes which lhave incorporated donor and acceptor moieties as in V and VI.43 The stronger absorption of VI
compared with V (Figure 5) has been taken as a measure of the cross-space interactions between the D and A moieties within each molecule. However, the rather similar interactions shown by VI1 and VI11 suggest44that inter-
VI I
VI I f
action transmitted through the annelated [2.2]paracyclophane system rnay be more important than the direct D-A “through-space” interaction. Although the [3.3]paracyclophanes are preferred to the [2.2] compounds because of tho distortion of the aromatic rings of the latter, the former allow a significant degree of conformational change. Fyfe has suggested that it would be of interest to study the possible molecular motion in crystals of this type of paracyclophane (see below). Obviously most of the information on D-A orientation has come from studies on solid crystals although this may not represent the situation in solution. Indeed, the results of X-ray diffraction studies on solids can themselves be misleading. Historically, X-ray studies are of importance because, through such work, Huse and showed that D and were still A molecules in p-iodoaniline-1,3,5-trinitrobenzene a t little less than vam der Waals bonding distance in the complex, thus ruling out covalent interaction. As an example of a very precise X-ray diffraction study, Herbstein and S n ~ m a have n ~ ~determined the structure of pyrene-pyromellitic dianhydride (PMDA) at 110 K and a t room temperature. The basic structure is typical of EDA complexes, namely, stacks of alternate A and D
300
I
I
I
100
500
600
‘1
1 lnml Figure 5. Electronic spectra of the [3.31paracyclophane quinhydrones V and V I (in dioxane). [Reproduced with the permission from ref 43. Copyright 1977 Verlag Chemie.]
molecules with the molecules lying near parallel in the stack though with their centers displaced. A t the lower temperature the pyrene molecules occupy different sites. Near 200 K there is an order-disorder transition in which there is an apparent halving of the c-axis length from 14.4 to 7.34 A and the pyrenes become randomly distributed. The question is whether this disorder is static or dynamic. From NMR line-width studies, and in particular the second-moment dependence on temperature, has observed a second moment of 4.7 G2at 77 K in close agreement with the value given by the van Vleck formula assuming no motion (4.8 G 2 ) . At higher temperatures the second moment falls (to -2.3 G2 at 400 K), Since the angle between the pyrenes is 16O at the low temperature, the amount of line narrowing with increased temperature is too large to be accounted for by jumping between the disorder sites. The process must therefore be dynamic. From the dependence of the spin-lattice relaxation times TI and T1, on temperature, an activation energy of -57 kJ mot1 has been estimated@for this process. In a similar study on naphthalene-tetracyanobenzene, two barriers at -9.7 and -42.7 kJ mol-l were observed. A recent development in NMR studies by has been the high-resolution 13CNMR measurements on solids, using magic-angle spinning, high-power decoupling of protons, and cross polarization. The spectrum of hexamethylbenzene-tetracyanoethylene indicates clearly the capabilities of the technique (Figure 6). We may expect to see many applications of the method, particularly when measurements can be made over a wide temperature range.56
Applications A few applications of EDA complexes will be mentioned. The most notable to date is probably the use of complexes in such as poly(N-vinylcarbazole)-2,4,7-trinitrofluorenone electrophotography (xerography) to replace selenium as the photosemiconductor.50 Iz-perylene5l and 12-poly(2-~inylpyridine)~~ have been used as cell electrodes. Much has been written on the so-called “organic metals”, e.g., TCNQ-TTF?3 It is a question of choice as to whether these molecular ion-radical salts are classified as EDA complexes or not. They followed from the synthesis of strong organic electron acceptors such as tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) which, post-dating Mulliken’s key publications on EDA complexes, made us conscious of the potential of electron acceptors with favorably low LUMO’s. There will doubtless be continued efforts by organic chemists to
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The Journal of Physical Chemistry, Vol. 84, No. 17, 1980 cd
Foster
o’ 0.3
C
\
\
I
- .1 ‘
2 kHz Figure 6. I3C {’HI NMR spectrum of the solid complex hexamethylbenzene-tetracyanoethylene obtained with magic-angle spinning [ref
491.
synthesize others and to create novel effective electron donors. We shall see more inorganic and organometallic compounds similarly designed. There are overlapping interests in molecules such as metalloporphyrins with, for example, oxygen binding. Likewise, certain “host-guest’’ complexes, in which the two components also fullfil the roles of electron donor and electron acceptor and in which the two components are suitably orientated to one another, show evidence for EDA interaction. For example, the host molecule IX with p-toluenediazonium tetrafluoroborate
IX
has an absorption band in the visible which may be assigned to an intermolecular charge-transfer transitiona5* Model complexes based on this principle could be used to investigate the claims made concerning EDA involvement in drug-protein receptor interactions. Among the examples which reflect the increasing interest in the electron-transfer processes of electron-donor-electron-acceptor systems is the elegant experiment55whereby tetramethylbenzidine (TMB), solubilized in 0.1 M sodium lauryl sulfate, is irradiated with a 347.1-nm laser pulse. The product is mainly TMB+ and a solvated electron (Figure 7). The latter transverses the Stern layer probably to form molecular hydrogen (Figure 8). The corresponding process of the transfer of electrons from a donor outside the micelle to a triplet acceptor (3duroquinone)within the micelle has also been observed. The potential of such systems as radiation energy traps is obvious. Robert Mulliken’s incisive description of EDA complexes was a watershed. He changed the interest in this area from a low level, steady-state situation to one of increasingly rapid growth which has involved many areas of pure and applied science. To quote Henry Eyring: “I think it is
M”*
.I
Figure 8. Schematic representation of photoionization and electrontransfer processes in solutions of surfactant micelles containing a solubilized photoactlve probe D. The electron acceptor is Mn+ located in the Stern layer of the micelle and the electron is transferred through the Stern layer from the triplet (Dr),[Reproduced with the permission from ref 55. Copyright 1977 Plenum Press.]
really true in chemistry that a model that is suggestive of experiments is often more productive than a more detailed, more precise one that is a little bit harder to use.” Professor Mulliken has not only provided such a model but has also supplied much of the detail.
References and Notes (1) H. C. Longuet-Higgins, Faraday SOC. Discuss., 40, 7 (1965). (2) W. Gsegory, “A Handbodc of Chemistry”, 4th ed, Watton and Maberly, London, 1856-7,p 333. (3) F. Wohler, Justus Liebigs Ann. Chem., 51, 145 (1844). (4) R. S.Mulliken, J. Am. Cbem. Soc., 74, 81 1 (1952);J. Pbys. Cbem., 56, 801 (1952). (5) G. Briegleb, “Elektronen-Donator-Acceptor-Komplexe”, SpringerVerlag, Berlin, 1961. (6) L. J. Andrews, Cbem. Rev., 54, 713 (1954). (7) R. S.Mulliken and W. B. Person, “Molecular Complexes-A Lecture and Reprlnt Volume”, Wiley, New York, 1969. (8) L. J. Andrews and R. M. Keefer, “Molecular Complexes in Organlc Chemistry”, Holden-Day, San Francisco, 1964. (9) H. A. Benesi and J. H. Hildebrand, J . Am. Cbem. Soc., 71, 2703 (1949). (IO) R. S. Mulliken, J . Am. Cbem. SOC.,72, 600 (1950). (1 1) G.Briegleb, Z.Pbys. Chem., B16, 249 (1932);“Zwischenmolekulare Krafte”, G. Braun, Karlsruhe, 1949. (12) L. Pauling, Proc. Natl. Acad. Sci. U.S.A., 25, 581 (1939). (13) J. J. Weiss, Nature(London), 147, 512 (1941);Trans. Faradaysoc., 37, 780 (1941);J. Chem. SOC.,245 (1942). (14) R . 6 . Woodward, J. Am. Cbem. Soc., 64, 3058 (1942). (15) W. Brackrnan, Red. Trav. Cbim., Pays-Bas, 68, 147 (1949).
(16) M. W. Hanna and J. L. Lippert. “Molecular Complexes”, Vol. 1, R. Foster, Ed. Elek Sclence, London; Crane Russak, New York, 1973, Chapter 1.
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Pyrolysis of Alkyl Benzenes. Relative Stabilities of Methyl-Substituted Benzyl Radicals Barrle D. Barton and Stephen E. Stein” Department of Chemistry, West Virgha University, Morgantown, West Virgina 26506 (Received: February 1 1, 1980)
Decomposition rates for 0 C-C bond scission in several methyl-substituted ethylbenzenes were measured in a very low pressure pyrolysis (VLPP) system from 1050 to 1200 K, and relative stabilities of the resulting substituted benzyl radicals were derived. Relative to ethylbenzene, rates were highest for o-methyl-substituted ethylbenzenes, and little affected by meta and para substitution. Activation energy differences were approximately separable into ortho and nonortho contributions, amounting to a lowering of the activation energy, relative to ethylbenzene, of 1.3-1.7 kJ mol-1 per m- or p-CH,, and 5.0-6.3 kJ mol-’ per o-CH3. Rate differences were explained in terms of a partial relaxation of steric interaction in the activated complex. Methyl inductive effects were found to be very small, amounting to a decrease of
