Ionisation energies and electronic structures of the phenyl 1,3-dipoles

and Related Phenomena, 9 (1976) 307-3 16 Company, Amsterdam - Printed in The Netherlands

Journal of Electron Spectroscopy @ Elsevier Scientific Publishing

IONISATION ENERGIES PHENYL 1,3-DIPOLES

J. BASTIDE*

AND ELECTRONIC

STRUCTURES

OF THE

and J. P. MAIER

Physikuiisch-chemisches

lnstitut

der Universitiit

BaseI, KIinge lbergstr.

80, CH-4056

Base1 (Switzerland)

T. KUBOTA Shionogi

Research

Laboratory,

(First received 18 December

Shionogi

and Co. Ltd.,

1975; 6 February

Fukushima-ku,

Osaka,

553 (Japan)

1976)

ABSTRACT

The He(I) photoelectron spectra of phenyldiazomethane (molecule l), phenyl azide (molecule 2), benzonitrile-N-oxide (molecuIe 3) and C-phenyl N-methylnitrone (molecule 4), as well as four derivatives of the latter two molecules, have been obtained and the inferred ionisation energies are reported. The bands in the spectra are assigned in terms of the valence electronic structure and from the variation of the ionisation energy of the x bands of the benzene moiety, the inductive effects of the 1,3-dipole substituents on the aromatic rc system are deduced. The determined first ionisation energies are considered in conjunction with the frontier orbital concepts to describe the 1,3-dipolar cycloaddition reactions. The interpretation of the electronic absorption spectra of molecules 3, 4 and their derivatives are in agreement with the photoelectron spectral data. Finally the intermolecular charge-transfer behaviour of the complexes of molecules 3, 4, their derivatives, as well as pyridine-N-oxide and trimethylamine-N-oxide, with iodine are discussed in relation to the ionisation-energy data. INTRODUCTION

Photoelectron spectroscopic studies of benzenes have now included many substituent groups since the first compilation of ionisation-energy data of a variety of benzene derivatives’. However, no ionisation-energy data at all are available for the phenyl 1,3-dipoles. Recently we have studied the valence electronic states of the radical cations of the 16 valence electron 1,3-dipoles-fulminic acid (HCNO), diazomethane (CH2N2), and hydrazoic acid (N,H)-by photoelectron spectroscopy * Permanent address: Centre Universitaire,

66500 Perpignan,

France.

308 and discussed their electronic structures2. In this paper we present the ionisation energies and photoelectron spectra of the corresponding phenyl-substituted 1,3dipoles-phenyldiazomethane (molecule I), phenyl azide (molecule 2), benzonitrileN-oxide (molecule 3), and also C-phenyl N-methylnitrone (molecule 4).

o-

CHN,

o--

%

/” CHN,

CNO

o-

CH3

1

2

3

4

The derivatives of molecules 3 and 4, 2,Gdimethylbenzonitrile-N-oxide (molecule 5), 2,4,6-trimethylbenzonitrile-N-oxide (molecule 6), C-(4-cyanophenyl)-N-methylnitrone (molecule 7), and C-(4-hydroxyphenyl)-N-methyl nitrone (molecule 8) are included to realise the assignment of the spectra of molecules 3 and 4 and to compare the ionisation-energy data with previous physicochemical studies3. Molecules with the nucleus of species 1 to 4 have constituted several studies of 1,3-dipolar cycloaddition reactions4, and the rates as well as the regioseleclivity of the reactions have been interpreted by the frontier orbital theory’. The first ionisation energies determined in the present work are considered in view of this I

8

9

10

14

14

15

16

15

16

IONISATION

ENERGY

I

,

I

eV

Figure 1. He(I) photoelectron spectra of phenyldiazomethane(molecule l), phenyl azide(molecule benzonitrile-N-oxide (molecu!e 31, and C-phenyl N-methylnitrone (molecule 4).

21,

309

0 J

0

1

I

y_kL7

8

NC-

9

1

T

‘CH,

7

a

”

i

a

9 IONISATION

ENERGY

9

to

11

eV

Figure 2. He(I)

photoelectron spectra of 2,ddimethylbenzonitrile-N-oxide trimethylbenzonitrile-N-oxide (molecule 6), C-(Ccyanophenyl)-N-methylnitrone C-(4-hydroxyphenyl)-N-methylnitrone (molecule 8).

concept.

Finally,

charge-transfer results obtained

electrostatic

electronic

absorption

2,4,6-

spectroscopy’,

and

of the species of type 3 and 4 are discussed in view of the

AND RESULTS

The He( spectrometers

of

5),

(molecule 7) and

in the present work.

EXPERIMENTAL

are reproduced

the interpretations

behaviour7

(molecule

(2 1.22-eV)

excited photoelectron

(PE)

spectra of molecules

1 to 8

in Figs. 1 and 2. The spectra were obtained with two photoelectron a n/,/2 (IO-cm and 5-cm radii) type’, incorporating

of the Turner cylindrical

condenser

analyser. The ionisation

energies (IE) were inferred

310 TABLE

1

IONISATION

ENERGIES

(eV) OF THE PHENYL

Phenyl 1,3-dipole

Molecule

CsH5N3 CaHsCNO GHKH-N

(Me)-0

2,6-(Me)KGH&N

0

2,4,6-(MeWsHaCNO p-CNGHKH-N (Me)0 p-OHGHKH-N (Me)0

c

hb 9.23 9.53 9.80 8.95 9.28 9.05 9.40 8.95

7.72 8.72 8.96 8.01 8.62 8.35 8.35 7.76

CfiHsCHNz

1,3-DIPOLESa,

11.75 11.35 10.84 9.91 10.55 10.26 10.2 9.5

10.12 11.00 10.0 9.3 9.62 9.50 9.82 9.26

12.3 12.3 11.55 11.8 11.6 11.6 11.4

Ionisation energiesgiven are the vertical values taken either from the most intense vibrational peak or from the maximum of the band. Values to.02 eV when the vibrational fine structure is resolved, f 0.05 eV when the broad band is separated. In case of overlap of bands, the values given are the estimates. to the band number i shown in Figs. 1 and 2.

The index i of the ionisation

energies (1;) corresponds

IE values of (CH&CCNO:

11 = 9.57, IZ = ca. I2 eV.

IE/eV 7

t

‘\

I

,’

‘\

zb!,’

‘\ ‘\

‘\

‘,

r[3b,,’

14 i N,H (C2”)

(C2”)

C&h)

(C,,)

CC,)

HCNO

DCNO

CC,,)

0

@CHNIl

CH2=N
(CZ”)

(D6,,)

CC,,)

Figure 3. Correlation diagrams of the ionisation energies of the 1,3-dipoles and the phenyl 1,31 to 4. The symmetry la&Is shown refer to the adopted point group given in dipoles, molecules parentheses below the formulae. symmetry (see text).

The symmetry

imposed

does not necessarily

represent

the true

the internal calibrations with Xe and Ar, as well as the first ionisation energy of benzene (9.25 eV)9, excited by the He( and He(I)/? photon radiation, and are collected in Table 1. In the legend to Table 1 details regarding the uncertainty of the from

311

values are given. The samples were prepared according to the descriptions given in the literature: molecule Ilo@) 210(b) 3”(‘) 51o(b), 61o(b), 46(a), 76(a), and 8’@) . In the discussion and in the correlation iagrams of Fig. 3, a pseudo CZUsymmetry is assumed for molecules 1 to 4. Though the true symmetries are C,, classification within CZv provides a convenient distinction between the symmetric (b,) and antisymmetric (a,) components of the ne, g orbitals of benzene in order to discuss the effect of substituents on these orbitals. DISCUSSION

Whereas the photoelectron spectra of the unsubstituted 1,3-dipoles can be reasonably analysed over the whole photoionisation-energy region accessible2, in the phenyl-substituted derivatives only the bands lying in the low ionisation-energy region, ca. -=z 12 eV can be discussed, owing to the conglomeration of bands in the higher-energy region. In the PE spectra of the isoelectronic phenyI 1,3-dipoles, molecules 1, 2 and 3, three or four bands are expected in the low 1E region (< 12 eV). This follows from the L!Z of the benzene ne Ig pair (IE = 9.25 eV) and the outermost orbitals of the 1,3-dipoles. The latter are degenerate in HCNO (1E = 10.83 eV) but split into the out-of-plane 2b, (IE = 9.00 eV) and 2a” (IE = 10.74 eV), and in-plane 2b, (IE = 14.13 eV) and 9a’ (IE = 12.25 eV) components in CH,N, (ref. 2) and N,H (refs. 2, 11) respectively (Fig. 3). Indeed the spectra shown in Fig. 1 confirm this. The relatively large IE of the 2b, band (14.13 eV) in the PE spectrum of CH,N, results in the location of the corresponding band in the spectrum of molecule 1 within the profusely overlapping bands (Fig. 1). In the PE spectra of molecules 1 and 2 (Fig. 1) bands (1) and (3) are associated with the photoionisation of electrons from the molecular orbitals of b, symmetry (in C,,) which result from mixing of the nel,(s) component of the benzene moiety and the out-of-plane n orbital of the substituent chain (Fig. 3). Bands (2) are associated with the 7za2 orbitals, localised on the benzene ring. Band (4) in the spectrum of molecule 2 is associated with the orbital derived from the 9a’ orbital of N,H (IE = 12.25 eV)‘y ‘I. The latter orbital is of nitrogen lone-pair character (cf. ammonia) and appreciable interaction with the benzene CJ orbitals is expected. In the PE spectrum of N3H the band is broad and spread over ca. 2 eV (refs. 2, 11) and in the spectrum of molecule 2 the Franck-Condon profile of band (4) is also relatively broad, in contrast to the nbl bands which are usually sharper (cf. Fig. 1). The PE spectra of molecules 3 and 4 also show three well separated bands in the low L!? region. However, the second band is now composed of two closely overlapping bands. The IE region suggests that the two ionisation processes responsible are the electron ejection from the 7caZ benzene orbital, and the in-plane component of the degenerate pair of the CNO moiety (IE = 10.83 eV in HCNO)’ in molecule 3, or the in-plane a0: “lone-pair” orbital (1E = 9.07 eV) in N-tert-butylmethylene

312 nitrone”. The JE of the ~0: “lone-pair” orbital can be compared with the IE region of the in-plane 00: lone-pair in heterocyclic-N-oxidesI (cf. pyridine-N-oxide 9.22 The bands (1) and (4) are again sharp, and are assigned to the 7c4b, and 7c3b, eV)14. orbitals. In the case of molecule 4, the highest occupied 7~ orbital (4b,) is largely localised on the oxygen atom (cf. Fig. 3). The PE spectra of the 2,6-dimethyl (molecule 5), and 2,4,6-trimethyl (molecule 6) derivatives of molecule 3 andp-CN (molecule 7) andp-OH (molecule 8) derivatives of molecule 4 allow confirmation of the proposed assignments. These substituents do not introduce additional bands below L!? of 11 eV in the PE spectra (Fig. 2). In the case of molecules 5 and 6, the overlapping bands are now well separated (Fig. 2), and the four bands are well defined, for both molecules. The bands (1) and (4) of molecule 5 are shifted by ca. 0.3 eV to lower 1E and the overlapping bands, (2), (3) of molecule 3 now have their counterparts at 9.28 eV and 9.62 eV. In comparison to m-xyleneL5@), where the nel B (A) b enzene band is moved by 0.75 eV, and the ne, B (S) by 0.3 eV to lower L??, the IE of the naZ band of molecule 3 is estimated as 10.03 eV in accordance with the experiment (Fig. 1). In molecule 6, the additional methyl group in the pava-position is expected to shift the 7ra2 band of molecule 5 by 0.25 eV, to lower I,!?, by reference to methyl-substituted benzenes15(“), whereas the effect on the in-plane nb, band (of the CNO group) is probably rather remote. The spectrum of molecule 6 indicates that bands (2) and (3) shift by 0.23 eV and 0.12 eV to lower IE respectively with respect to the L!? values of molecule 5. These relative changes are well defined as the bands in question are sharp (Fig. 2). Bands (1) and (4) again shift by 0.3 eV (relative to those of molecule 5) to lower I,!?_ In the case of molecule 7, the four bands are now apparent. The LY data on benzonitrilel 7 ’ shows that the na2 band has 1E 0.9 eV higher than benzene. If band (2) of molecule 4 and band (3) of molecule 7 are associated with the ionisation of electrons stemming from the 7ca2 orbital, the shift is again ca. 0.9 eV. In contrast bands (1) and (4) are shifted by only ca. 0.3 eV to higher IE. This assignment is supported by the data of molecule 8. The stabilization of the na, orbital by the hydroxy group is small (ca. 0.2 eV)l”(b) and consequently the corresponding bands in the PE spectra of molecules 4 and 8 are expected to be close in IE. This is precisely the observation; bands (2) both correspond to an IE of around 8.9 eY. The nbl bands, on the other hand, are displaced towards lower ZYE,as is observed in phenolsi5(b), and the 7c3b, band coalesces with the 00: band, at ca. 9.3 eV. The good linear correlation, previously noted, between the IE (eV) of the 7raZ band of substituted benzenes (4X) and the dipole moment p(D) of the derivative14 IE na,(+X) = 0.19,~ + 9.35 leads to corroboration of the assignment made. The known dipole moments of molecules 2 and 3 (1.56 D16@) and 4.0 DL6cb) respectively), yield the following predicted IE (naZ), with the assigned experimental values given in parentheses: for molecule 2,9.64 eV (9.53 eV) and for molecule 3, 10.11 eV (10.00 eV). The agreement is very good. In the case of molecule 1, the dipole moment is not given in the literature;

313 TABLE 2 COMPARISON OF THE CNDO/2 EEGENVALUES FOR THE PHENYL 1,3-DIPOLES, MOLECULES 1,2,3 AND 4 WITH THE EXPERTMENTAL IONISATION ENERGIES GIVEN IN PARENTHESES” MoIecule nbl

naz

nb2 (in-plane) nbl

1

- 9.53 ( 7.72) -13.27 ( 9.23) -16.83 -15.0 (10.12)

Molecule ~ 9.78 --13.24 -12.30 -15.25

2 ( 8.72) ( 9.53) (11.35) (11.00)

Molecufe -11.11 - 13.99 -12.05 -15.28

3 ( 8.96) (10.00) ( 9.80) (10.84)

~_

Molecule 4 -~~. -l.“- 9.72 -13.10 -11.85 - 14.48

(8.01) (8.95) (9.3 )b (9.91)

a All values in eV. The symmetry labels refer to an assumed CZ, point group for the species 1 to 4. b This 7E value refers to the CTorbital, which is predominantly the in-plane 00: lone-pair (see text).

however the IE data (Table 1) and the above relationship, suggest only a small dipole moment (cf. molecule 2). The substituent of molecule 4 can no longer be regarded as simple, and reasonable agreement is not attained. Nevertheless, the trend in the 1E of the 7ra2 band reflects the inductive eflect of the introduced substituent’4. The changes of the L!Z values of the rca2 in species 1 to 4 (Table 1) then lead to the inference that the XHN, and the N-methyl-nitrone groups have a feeble donor inductive effect on the n system. On the other hand the shift to higher IE induced by the -N, group (0.28 eV) is much alike in magnitude to the effect of a hydroxy group’S(b) (0.2 eV> and the inductive effect of the -CNO group (shift of rca2 LK = 0.75 eV) is not as large as that of a strongly electron-withdrawing group such as a cyano (ca. 0.9 eV shift)‘. In Table 2 are summarised the orbital energies of molecules 1, 2, 3 and 4 quantitative agreement is poor, as is obtained by CND0/2 calculations ” , Although usually the case with this method, the order of the 7c levels inferred from the PE spectra is in all cases in accord with the calculations_ The relative high energy value of the in-plane 7~ orbital of the substituent, is a well known artefact of the method. In Fig. 3, the assignments discussed in the preceding paragraphs are summarised in the form of correlation diagrams. When the frontier orbital theory has been applied to rationalise reaction rates and regioselectivity of 1,3-dipolar cycloaddition reactions, the orbital energies or the experimental ionisation energies were calculated by the CND0/2 procedure5, and electron affinities were utilised in conjunction with Koopmans’ theoremI’. In the case of the phenyl 1,3-dipoles, molecules 1, 2, 3, the IEvalues were not known. The required IE values were consequently estimated from known IE data of related molecules12p I8 and now one can compare the present IE data with the estimated values and in the instance of considerable discrepancies to consider the consequences. For phenyl azide (molecule 2) Houk et al.12 estimated the value of the eV which differs appreciably from the highest occupied MO (HOMO) as -9.5 Koopmans’ value of the first L?Z, -8.72 eV. However the difference arises from the

314 use of an incorrect first IE of hydrazoic acid, and consequently the estimated value of the lowest unoccupied MO (LUMO), based on the revised HOMO value, - 8.72 eV, should be +0.6 eV. The interaction HOMOdipole-LUMOdipolarophile is therefore greater than that originally postulated and explains more clearly the results obtained where the two interactions favouring different paths are with phenylacetylene”, almost equivalent. In the case of benzonitrile-N-oxide, the displacement of the HOMO value by 1 eV, to - 8.96 eV, with respect to the estimated value, again augments the importance of the HOMOdipole-LUMOdipolarophile. When the substituents on the dipolarophile are electron-attracting, this interaction becomes the important one, but owing to the feeble variation of the coefficients for the HOMO dipole, its influence does not become important on the orientation except for the dipolarophiles possessing a high value of For 2,4,6-trimethyl-benzonitrile-N-oxide, HOMO, such as methyl propiolate. molecule 6, the lowering of the first IE to 8.35 eV further augments this interaction”. For phenyl diazomethane, molecule 1, the decrease (1.3 eV) of the first IE compared to that of diazomethane2 reinforces the HOMO-LUMO interaction but does not modify in essence the arguments proposed for diazomethane. Finally, in the case of molecule 4 the value utilised by Houk et al.” was the experimental first ZE given as 7.89 eV compared to a value of 8.01 eV in the present work. The UV absorption spectra and their interpretations by means of MO calculations have aheady been reported by Kubota and co-workers3* ’ for molecules 3, 4 and their derivatives. Although molecules 3 and 4 are isoelectronic from the viewpoint of the out-of-plane rc electronic system, the main difference in the UV spectra is that the strong IA, band (lL, with charge-transfer character from the N-oxide group oxygen atom to the residual x electronic system) of molecule 3 appears at a considerably shorter wavelength than the corresponding band of molecule 4. Consequently the weak benzenoid lLb band occurs in the longest wavelength region for molecule 3 but is obscured by the strong IA, band in the case of molecule 4. Previous MO calculations3’ ’ suggested that one of the important reasons was that the HOMO energy of molecule 3 is relatively stabilised in comparison with that of molecule 46. This is now confirmed by the present PE spectral data (Table 1). Also the sequences of 7c orbital energies of molecules 3 and 4 obtained in the previous MO studies coincide completely with those deduced from the PE spectra6. That the oxygen lone-pair orbital (00:) of molecule 4 lies quite deep in energy, suggests that the n-rc* transition of molecule 4 may be hidden under the n-n* transitions. Therefore the PE spectral data support the interpretation of UV spectra of molecules 3, 4 and their derivative@. Finally we shall discuss the intermolecular charge-transfer (CT) spectra of some basic aromatic N-oxides with iodine7, since vertical first IE values (Ix) of pyridine N-oxide, molecule 3, molecule 4, and trimethylamine-N-oxide have now been reported’ 4. The first IE values of pyridine-N-oxide, molecule 3 and molecule 4, are associated with rc molecular orbitals whose nature have been established, and

315 therefore these compounds are bx donors. As the iodine complexes of pyridineN-oxide, molecule 3 and molecule 4, are not strong, (the complexes of molecules 3, 5 and 6 are especially weak’(b)), the formula’ 1-23 - 5.2)] hvc, = 1; - 5.2 + [l.S/(I; is applicable to predict the CT band position. Introducing the values of 1: = 8 .38 eV, 8.96 eV, and 8.01 eV for pyridine-N-oxide, molecule 3 and molecule 4, respectively, one obtains hv,, = 3.65 eV (339 nm) for pyridine-N-oxide, 4.16 eV (298 nm) for molecule 3 and 3.34 eV (371 nm) for molecule 4. Alternatively when one applies the equation” hV CT = 0.87 1; - 3.6 the values of hvc, = 3.69 eV (336 nm), 4.20 eV (295 nm), and 3.37 eV (368 nm) are obtained for pyridine-N-oxide, molecule 3 and molecule 4, respectively. These values are in quite good agreement with the observed values of 3.89 eV (318.4 nm) for pyridine-N-oxide and 3.62 eV (342.5 nm) for molecule 4 in Ccl, solvent’@), although the observed values are 0.25 eV blue-shifted relative to the calculated ones. Unfortunately, the charge-transfer band of the iodine complexes of molecule 3 and its methyl derivatives, molecules 5, 6, could not be accurately recorded as the complexes were very weak 7(b) . On the other hand the complexing ability of trimethylamine-Noxide, which is an n donor, with iodine is strong (one of the strongest complexes among oxo-compounds). This was verified from the data of equilibrium constant, heat of formation, and the blue-shifted iodine band’(“). The same treatment as in the case of the weaker brt donors is thus not apphcable. Therefore, it is reasonable that the CT band observed clearly at 257 nm, in CH,Cl,, for the trimethylamine-Noxide-iodine complex is at much shorter wavelength than that estimated using 1: = 8.25 eV and the equations given above. The observed CT band (257 nm) falls in the region of the CT bands of aliphatic amine-iodine complexes. The CT band of the latter complexes appear at much shorter wavelength21’ 22Y 24 than those of the bx donor iodine complexes when the comparison is made for the same 1: value. ACKNOWLEDGEMENTS

We wish to thank Dr. Jean-Francois Muller, Universite de Metz, for his help in the preparation of benzonitrile-N-oxide and the sample of (CH 3)3CCN0, and F. Texier, Universite d’Oran, for the gift of phenyl azide. This work is part 93 of project no. 2.159.74 of the Schweizerischer Nationalfonds zur Fiirderung der wissenschaftlichen Forschung. Part 92 is ref. 25. We thank Ciba-Geigy SA, F. HoffmannLa Roche & Cie. SA, and Sandoz SA, for their financial support.

REFERENCES 1 2

A. D. Baker, D. P. May and D. W. Turner, J. Chem. Sot. J. Bastide and J. P. Maier, Chem. Whys., 12 (1976) 177.

23, (1968) 22.

316 3 4 5

6

7 8 9 10

11

12 13 14 15 16

17 18 19 20 21 22 23 24 25

M. Yamakawa, T. Kubota and H. Akazawa, Theor. Chim. Acta, 15 (1969) 244. J. Bastide, J. Hamelin, F. Texier and Y. Vo-Quang, Bull. Sot. Chim. Fr., 7-8, 2555; 9-10 (1973) 2871; R. Huisgen, Angew. Chem., Int. Ed. Engl., 2 (1963) 633. R. Sustmann and H. Trill, Angew. Chem., Znt. Ed. Engl., 11 (1972) 838; J. Bastide, N. El GhanTetrahedron Left., 44 (1972) 4225; K. N. Houk, J. Amer. Gem. dour and 0. Henri-Rousseau, Sot., 94 (1972) 8953; K. Bast, M. Christl, R. Huisgen and W. Mack, Chem. Ber., 106 (1973) 3312. (a) T. Kubota, M. Yamakawa and Y. Mori, B&l. Chem. Sac. Jup., 36 (1963) 1552; (b) T. Kubota and M. Yamakawa, Bull. Chem. Sot. Jup., 36 (1963) 1564; M. Yamakawa, T. Kubota and H. Akazawa, Bulk Chem. Sot. Jap., 40 (1967) 1600; M. Yamakawa, T. Kubota, H. Akazawa and I. Tanaka, Bull. Chem. Sot. Jap., 41 (1968) 1046. (a) T. Kubota, J. Amer. Chem. Sot., 87 (1965) 458; (b) T. Kubota, M. Yamakawa, M. Takasuka, K. Iwatani, H. Akazawa and I. Tanaka, J. Phys. Chem., 71 (1967) 3597. D. W. Turner, Proc. Roy. Sot., Ser. A, 307 (1968) 15. D. W. Turner, A. D. Baker, C. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy, Wiley-Interscience, London, 1970. (a) D. G. Farnum, J. Org. Chem., 28 (1963) 870; fb) R. 0. Lindsay and C. F. H. Allen, Organic Syntheses, Vol. 22, Wiley, New York, 1942, p. 96; (c) l-l. Wieland, Ber. Bunsenges. Phys. Chem., 40 (1907) 1667. J. H. D. Eland, Philas. Trans. Roy. Sot. London, Ser. A, 268 (1970) 87: S. Cradock, E. A. V. Ebsworth and J. D. Murdoch, J. Chem. Sot. Faraday Trans. 2, 68 (1972) 86; T. H. Lee, R. J. Colton, M. G. White and J. W. Rabalais, J. Amer. Chem. Sot., 97 (1975) 4845. K. N. Houk, 5. Sims, R. E. Duke, Jr., R. W. Strozier and J. K. George, J. Amer. Chem. Sot., 95 (1973) 7287. T. Kubota, J. P. Maier, J.-F. Muller and M. Yamakawa, Nelv. Chim. Acta, 58 (1975) 1634; T. Kubota, J. P. Maier and J.-F. Muller, nelv. Chim. Acta, 58 (1975) 1644. J. P. Maier and J.-F. Muller, J. Chem. Sot. Faraday Trans. 2, 70 (1974) 1991. (a) J. P. Maier and D. W. Turner, J. Chem. Sot. Faraday Trans. 2, 69 (1973) 196; (b) J. P. Maier and D. W. Turner, J. Chem. Sac. Faraday Trans. 2, 69 (1973) 521. (a) N. V. Sidgwick, L. E. Sutton and W. Thomas, J. Chem. Sot., London, (1933) 406; (b) G. Del Re, Atti Accad. Naz. Lincei, Cf. Sci. Fis. Mat. Natur., Rend., 22 (1957) 481; Chem. Ah&r., 51 (1957) 17399a. J. A. Pople and G. A. Segal, J. Chem. Phys., 44 (1966) 3289. K. N. Houk, J. Sims, C. R. Watts and L. J. Luskus, J. Amer. Chem. Sot., 95 (1973) 7301. W. Kirmse and L. Horner, Justus Liebigs Arm. Gem., 614 (1958) 1. M. Christ1 and R. Huisgen, Chem. Ber., 106 (1973) 3345. R. S. Mulliken and W. B. Person, Molecular Complexes, Wiley, New York, 1969, pp. 143-151, 481. R. S. Mulliken and W. Person, Ann. Rev. Phys. Chem., 13 (1962) 107. G. Briegleb, Electronen-Donor-Acceptor-Komplexe, Springer Verlag, Berlin, 1961, p. 78. H. Yada, J. Tanaka and S. Nagakura, Bull, Chem. Sot. Jup., 33 (1960) 1660. D. A. Sweigart and J. P. Maier, Znorg. Chem., in press.