Photoelectron spectroscopic assignment of symmetry to the ground state and first excited state of the 1,4-cyclohexadiene radical cation

Journa! of Electron Spectroscopy and Related Phenomena, 9 (1976) 227-239

@ Elsevier Scientific Publishing

Company,

Amsterdam

PHOTOELECTRON SPECTROSCOPIC THE GROUND STATE AND FIRST HEXADIENE RADICAL CATION

E. HEILBRONNER

- Printed in The Netherlands

ASSIGNMENT OF SYMMETRY TO EXCITED STATE OF THE l,$-CYCLO-

and F. BROGLI

Physikalisch-chentischesInstitut der UniversitiitBasel, Klingelbergstrasse 80, CH-4056 Basel (Switzerland)

E. VOGEL Institut fiir Organische Chemie der Universitiit K&t,

(Received

3 December

Ziilpicherstrasse 47, D-5 Ktiln ( W. Germany)

1975)

ABSTRACT

The empirical correlation of the photoelectron spectra of 1,Ccyclohexadiene (molecule 4), 1,4, 5,%tetrahydronaphthalene (molecule 5), 1,4,5,6,9, IO-hexahydroanthracene (molecule 6), and 1, 4, 5, 6, 7, 10, 11, 12-octahydronaphthacene (molecule 6) proves that the electronic ground state of these molecules is *Blu, assuming that they have &,, sy mmetry. In particular this confirms previous predictions for 1,4cyclohexadiene (molecule 4), for which the “inverted” orbital sequence 2blu(n) above lb,,(n) had been proposed under the assumption that hyperconjugative “through-bond” interaction dominates the “through-space” interaction of the two semi-localized 7c-orbitals. Consideration of the interplay of “through-space” and “through-bond” interactions1 between the localized x-orbitals in the series of bicyclodienes, molecules I(n), of (assumed) symmetry CZV leads one to expect that the sequence of their two highest occupied r-orbitals is b2(n) above ar(7t) for n = 1 (norbornadiene) but a,(x) above b,(x) for large values of n (ref. 2). Assuming the validity of Koopmans’ approximation3 this implies that the ground and first excited states of the radical cations l(n)* are 2B, and 2A1 respectively for n = 1 and the reverse order for large n. By applying a correlation technique proposed some time ago4 it was possible to prove experimentally that the two lowest states of the radical cation of molecule l(l), norbornadiene [5], and molecule l(2), bicyclo[2.2.2]octadiene6, are ‘B, and ‘A, (in this order) and the reverse for molecule l(3), bicyclo[3.2.2]nona-6, 8-diene’, and molecule l(4), bicyclo[4.2.2]deca-7, 9-diene *. These findings are summarized in Fig. 1 where the spectroscopically determined photoelectron vertical state energies of radical cations l(n)+, relative to the singlet ground state of closed shell molecules

228

Et2A,)+(‘A,)

eV

E(2B2)-E('A,>

6

1 I,

100"

1lcP

120=’

13~'

140'

160"

160'

170°

160D

1. Energies (in eV) of the ground and first excited states of the radical cations l(n)+ of the 1,4-bridged bicyclo[rz. 2. 2]dienes, molecules I(R), as a function of the dihedral angle w. The labels refer to an (assumed) symmetry CS. Figure

I(n),

are plotted

state energies

vs. the dihedral

and the symmetry

angles

o of the latter,

assignments

for n = 14.

are experimental

Note that the

results.

n

H 2

l(n)

3

of molecules l(n) with n = 0 and benzene, molecule l(0) = molecule 2, lacks a bridging alkyl moiety, i.e. its centres 1 and 4 are connected by a rather long and weak CC a-bond’; it has recently been shown 1’ that here the radical cation states A ‘ , and B ‘ , are almost degenerate. On the other hand the hypothetical compound l(m) can be regarded as a 1,4-cyclohexadiene, molecule 3, the two groups R being (infinitely) long n-alkyl side-chains. According to previous experiencell their influence on the n-ionization energies consists in shifting them towards lower values, relative to those observed for the parent hydrocarbon 1,4-cyclohexadiene, molecule 4, i.e. molecule 3 with R = H; however they are not expected to cause a switch in the orbital sequence, as long as one assumes that the conformation of the six-membered ring in the hypothetical systems, molecule l(co) and molecule 3, is the same as that of molecule 4 (ref. 12). Thus the assignment of irreducible representations to the two The

n =

two limiting

co are of course

members

special

cases.

of the series Dewar

229

lowest states of the radical cations of molecule l(co) and molecule 3 can be reduced to an assignment for radical cation 4+. Although there seems to be hardly any doubt that the ground state of radical catlon 4+ (assuming C,, sy mmetry in analogy to molecules l(n)) and thus of molecule l(o0) is 2A, (cf. Fig. l), this has not yet been proven experimentally. For obvious reasons the procedure applied to the series of molecules l(n) with n = 1-4 cannot be used in this instance. Therefore in this paper a different type of approach is described, namely the correlation of the photoelectron spectroscopic data obtained for the higher analogues of molecule 4, i.e. for 1, 4, 5, Wetrahydronaphthalene, molecule 5, 1, 4, 5, 6, 9, lo-hexahydroanthracene, molecule 6, and for 1, 4, 5, 6, 7, 10, 11, 12octahydronaphthacene, molecule 7, which possess N = 3, 4, and 5 parallel double bonds respectively. It will emerge that the observed z-band positions are only compatible with the orbital sequence 2b,,(7r) or 8a,(7c) above lb,,(n) or 5b,(n) in molecule 4, which makes 2B1u or ‘A1 the electronic ground state of radical cation 4+, under Da,, or C, LIsymmetry respectively13.

4

6

5

7

In Fig. 2 are shown the He(I) photoelectron spectra of molecules 4 to 7. The vertical ionization energies collected in Table I are assumed to be equal to the positions of the band maxima, in a first approximation: Iu,j w Imax,j. Using Koopmans’ theorem in reverse, i.e. transforming the n-ionization energies 1,,j(7r) into “observed” orbital energies EJ~L) = -rv,j(~), the correlation diagram of Fig. 3 is derived for the n-dominated orbitals of molecules 4 to 7 and of ethylene (I,(n) = 10.5 eV) [14], to facilitate comparison with the results of the following theoretical discussion. From a purely empirical point of view, it is remarkable that the n-band system does not spread out with increasing number N of interacting double bonds for N 2 3. The ranges are 1.06 eV (N = 2), 1.33 eV (N = 3), 1.34 eV (N = 4) and 1.36 eV (N = 5), if characterized by the difference between the first and last fu,j(n). In addition the mean n-ionization energy

(1) decreases with 8.80 eV (N = controlled by orbitals acting

increasing N: 9.34 eV (N = 2), 8.97 eV (N = 3), 8.84 eV (N = 4), 5). This by itself suggests that the pattern of the n-bands is primarily hyperconjugative “through-bond” interaction, the pseudo-n CH,as “relay” orbitals.

6

7

8

9

10

11

12

13

14

15

16

ir

18

1s

20 IS?

Figure 2. He(I) photoektron

21

(eV)

spectra of the hydrocarbon molecules 4, 5, 6 and 7.

8.83 9.80

E3’

14.70

-

lbzg

2blu lbs, 3blg

Orb. 8.27b 9.03c 9.60 10.35 11.35 12.35

I

I

14.70

8.24 9.24 9.66

I

co

lbzp, lau

14.70

2bzg, lbzg lau

4blu 3b3g 3blu 2bsg 8bl,

8.08 8.59 9.40 9.60

3blu 2b3g 2blu 6blg

8.16d 8.55 9.15 9.50 9.98 11.15

Orb.

Orb.

__....

8.14e 8.55 9.25 (9.50) 9.80 10.92

I *,j

Molecule 7

14.70

8.01 8.33, 8.82 9.46 9.57

-&3

2beg, lbzg 2au, la,

5blu 4bsg, 4blu 3b3g 3blu llbls

Orb.

(5).

-

-

a, b, C, d Spacing of the dominant vibrational fine structure: aC = 1750 cm-l, 640 cm-l; b G = 1450 cm-l; c P = 1370 cm-l; d a = 1600 cm-l. e The peak at 8.44 eV could be due to a vibrational progression (fi = 1600 cm-l) of the first band or to electron ejection from the 4bsg orbital.

8.82a 9.88 11.00 12.0” 13.30 13.75

Molecule 4

01 I

VERTICAL IONIZATION ENERGIES & OF MOLECULES 4 TO 7 AND ORBITAL ENERGIES ~j OF MODEL (6) WITH PARAMETERS ALL VALUES IN eV. THE ORBITAL ASSIGNMENT “Orb.” IS BASED ON THE RESULTS OF THE MODEL CALCULATION

TABLE 1

232

!+ -8-

b3,

b 1U -9-

Figure

3. Correlation diagram of the “observed” n-orbital energies ~5 = -IV+ Assumed symmetry

DZh.

Recent experimental results indicate that 1,4-cyclohexadiene, molecule 4, belongs to D,, [12] in contrast to previous findings which favoured a CZu structure with a dihedral angle of w sz 165’ [13]. As is evident from Fig. 1 the presence of a slight bend (o < 180 “) in molecule 4 would not affect our arguments. Therefore we shall assume that molecule 4 and consequently molecules 5, 6 and 7 all belong to DZh, the z-axis being perpendicular to the plane containing the carbon atoms. Under this assumption the N highest occupied 7c-orbitals of molecules 4 to 7 can be described as linear combinations of the N two-centre localized ;rr-orbitals G,,+, bb,+, 7c09 rcb, .._ 7cN and the N- 1 symmetry adapted in-phase combinations . . . D~_~,+ of the pseudo 7~CH,-orbitals, i.e. CT

r,+

=

(0, -t urp)/J2 ;

r

a, b, ___N-

=

1

which are symmetric with respect to a reflection in the z, y-plane. x1

w

ub’

u(N-l)’

The relevant matrix elements are the following

and cross-terms:

a, b, . . . N

A* = <~rI~I%>; A, = :

s = a, 6, . . . N -

B nR = <%Wl~,+J; B ILo= <%-I*Ifl,>;

r = a, b, . . . N s = r, r - 1, r’, (r -

r =

self-energies

1, a’, . . . (N 1 1)’

1)’ (4)

233 From previous experience the size of these matrix elements is expected to be of the order A, z -1Oto -lleV,A, z -14to -15eV,B,, z -1/2eVandB,, w -2 to - 3 eV. A trial calculation based on these values reveals that as a necessary consequence of the relationships A, - A, z 4 eV and B,, - B,, x 2 eV, one has to expect that the order of the molecular orbitals dominated by n,, 7~~, . . . JT~ is an “inverted” one, meaning that in the highest occupied orbital all the n, are in-phase and in the lowest (Nth from the top) orbital out-of-phase. The rc-ionization energy of ethylene is 1(7cn)= 10.5 eV [14]. Replacement of two in-plane CH o-bonds by CC a-bonds tends to destabilize the n-orbital by ca. + 0.2 eV, yielding A, = - 10.3 eV. (Note that this destabilization is not the one due to hyperconjugation with a-positioned alkyl groups.) The value of A, is not critical. For symmetry reasons the linear combinations G,,_ = (a, - a,,)/,,/2 cannot interact with the 7~~. Furthermore interaction between the linear combinations a,,_ with s = a, b . . . N - 1 is presumably slight, so that one expects a set of N - 1 molecular orbitals with energies close to A,. Indeed, in all photoelectron spectra of molecules 4 to 7 one observes a strong, prominent band near 14.7 eV. This is also in agreement with the results derived from a SPINDO [15] calculation. Therefore the value A, = - 14.7 eV has been adopted for the ensuing discussion. Once the self-energies A, and A, have been fixed, it is a simple matter to derive estimates for the cross-terms B,, and B,, from the experimental data, e.g. from those of the diene, molecule 4. Assuming that the n-orbital sequence is indeed the inverted one, i.e. 2b,,(x) above lb3Jn), the cross-term B,, is given by the relation= A, + I,, 2 which yields B,, = - 10.3 + 9.9 = - 0.4eV. ship B,, = A, - .s(lb3Jr)) can be obtained by solving the equation which connects The last unknown, B,,, A,, A,, B,, and B,, with s(2b,,(n)) = -Io,1 = -8.8 eV. This leads to B,, = - 1.65 eV. Slightly different values are obtained if the experimental data for molecule 5 are used. In the following we have used the values: A, B,,

= - 10.3 eV; = - 0.5 eV;

A, B,,

= - 14.7 eV = - 1.7 eV

(5)

with which the orbital energies given in Table 1 and in Fig. 4 have been calculated. Although the graph representing the orbital interactions (4) in the n/pseudo-n system (3) is rather simple, i.e.

-B ---

KU B,,

(6)

there is no formula aj = f(A,, A,, B,,, B,,, j) which would yield the orbital energies .sj of the system in closed form. However, if the “through-space” cross-terms B,, are set equal to zero, i.e. if the dashed lines are removed from the graph (6), then the

234

I

I

I (cl

1

I

I 8

9

2

II

3

I

4

I

5

10

I,(&)

Figure 4. Comparison of observed and calculated z-band positions. N refers to the number of double bonds in the system. For the assignment of orbital labels see Table 1. (a) Observed band positions, taken from Fig. 2 and Table 1. The shaded region in the spectrum of molecule 7 (N = 5) indicates the width of the double band near 8.5 eV, the dashed line marking its maximum. (b) Values calculated by solving the secular determinant corresponding to the graph (6) with the parameters listed in (5). The numerical values are given in Table 1. (c) Results of a test calculation assuming a natural order of orbitals in l,Ccyclohexadiene, molecule 4, and thus in the molecules 5, 6 and 7. See remark (5).

energies E; of those n-orbitals

which are dominated

by TV, znb . . . nN are given by

(7) j = 1, 2, .._ N The E; reflect the pure “through-bond” interaction of the 7c, values. One is now in a position to test how well the parameters given in (5) reproduce the spectra of the molecules 4, 5, 6 and 7. This is shown in Fig. 4 where the results obtained by solving the secular determinant corresponding to the graph (6) are shown side by side with the experimentally determined band positions, given as sj = - lU,j. It is obvious that the agreement is surprisingly good both qualitatively and quantitatively. We shall now investigate how “through-bond” and “through-space” interactions combine to yield the “observed” orbital scheme shown in Fig. 3 and 4. If for a start one neglects “through-space” interactions between the two-centre 7c-orbitals n (1, rb . . . =‘V (IL = 0), one obtains from formula (7) the sets of orbital energies ~4 which are represented in Fig. 5 by dashed lines. The corresponding molecular orbitals

235

-7-

Ej (PV)

- B-

- 9-

44 “1,

-11

1

1

2

3

4

5

N

Figure 5. Diagram showing the interplay of “through-bond” and “through-space” interactions between the z-orbitals z~, ~0 . . . ZN in molecules 4 to 7. The dashed levels are those obtained from the closed formula (7) B,, = 0, the solid ones those derived by solving the secular determinant corresponding to the graph (6), using the parameters (5). N = number of double bonds in the molecule.

are $9. Note that these are only the N highest occupied ones, out of the (3N - 2) n-orbitals of the system, assuming D,, symmetry. The orbital I& is the highest occupied (HOMO), t&, the lowest n-orbital of this set. The molecular orbital t& has N - 1 nodal planes containing the N - 1 pairs of para-positioned methylene groups, so that its orbital energy is necessarily E; = A,, independent of N (see expression (7)). Thus any two consecutive basis orbitals z,, z,+~ are out-of-phase in the linear reintroducing “through-space” interaction combination I++;. As a consequence between consecutive basis orbitals 7t,, 7cP+1, i.e. setting B,, = -0.5 eV, will have a strong destabilizing effect on I& and strong stabilizing effect on @ as indicated by the arrows in Fig. 5, leading from E: and E; to the levels Ed and Ed, represented by solid lines, which have been obtained by solving the secular determinant corresponding to the complete graph (6). Depending on the phase relationship of n,, Q . . . 7cNin $7, interaction are observed. As a result of intermediate shifts due to “through-space” this partial compensation of the shifts due to the two postulated mechanisms, the orbital diagram of the upper occupied 7-c-orbitals tij is compressed within the narrow energy range extending from - 8 to - 10 eV, even for larger values of N. Assuming the validity of Koopmans’ approximation, this explains why the ranges I,, N(~) -I,, 1(n) and the mean ionization energies I,(Z), defined in expression (l), are practically independent of N for N > 3.

236

Thus the analysis by correlation of the spectra shown in Fig. 2 constitutes “through-bond” interaction dominates the “throughproof that hyperconjugative space” interactions between consecutive basis orbitals rc,, rep+1 in the molecules 4 to 7. In particular it confirms unequivocally that the ground state of the radical cation 4+ is ‘BIU under D,,, (or ‘A, if the slightly bent molecule 4 has C,, symmetry). REMARKS We shall conclude this analysis with a few remarks: (1) SPINDO models [ 151 have proved to be rather reliable for the interpretation of saturated and unsaturated hydrocarbons. Application of this SCF technique to molecules 4 and 5 yields the following results if D,, symmetry and standard bond-lengths or angles are assumed (values in eV): Molecule 4

Molecule 5 _

Orb.

ESPINDO L

2bluCn)

lb&) 3b, p(b)

9.48 10.29 11.06

i

8.83 9.80 11.00

Orb.

_

3b,,(x) 2b&) 2b,,(x) 6b>1&)

9.;5 9.87 10.28 10.67

$PlNDO Iv,

j

8.27 9.03 9.60 10.35

(8)

Whereas the numerical agreement is rather good for the o-orbitals, one finds that the n-orbitals are predicted at too low an energy. This is due to the fact that the SPINDO treatment definitely underestimates hyperconjugative destabilization of n-orbitals, as has been observed before [16]. However, the relative spacings of the rc-orbitals and thus of the x-bands are well reproduced and confirm the above assignment. (2) It has been shown by Koenig and Longmaid [17] that the photoelectron spectra of 1 ,Cdihydropyridine, molecule 8, and of N-methyl-l,Cdihydropyridine, molecule 9, also support the assignment proposed for the lowest states of radical cation 4’ [a]. Both these spectra show a second band near 1,. 2 = 9.8 eV, which the

8

9

IO

authors associate with the lowest ‘B3, state of molecules 8 and 9 respectively. (Symmetry label relative to the axes used in this work.) Consequently the 9%eV band in the photoelectron spectrum of molecule 4 should also correspond to a 2B,, state, which makes ‘BIU the ground state of radical cation 4+. This type of correlation, i.e. of molecule 4 with molecules 8 and 9, although convincing is less straightforward than the one given in this paper because of the difficulty in assigning self-energies to the basis lone-pair sc-orbitals. This is exemplified by the photoelectron spectrum of 1,4-

237 dioxacyclohexa-2,5-diene, molecule 10, the first three bands of which are found at Z0. 1 = 8.13 eV, Z,, 2 = 10.7 eV, Z,, s = 12.3 eV [ 181. Although the oxygen atoms lie on the nodal plane, the energy of the ‘Bs, state of radical cation lO+ is shifted by almost 1 eV relative to that of radical cations 4+, 8+ or 9+. However, it is true that replacement of a methylene group by a nitrogen atom constitutes less of a perturbation than by an oxygen atom. (3) In another important contribution, Koenig et al. [19] have succeeded in recording the photoelectron spectrum of p-quinodimethane, molecule 11, which shows two bands at Z,, 1 = 7.9 eV and Z,, 2 = 9.7 eV. The latter has been interpreted as being associated with two states close in energy, namely ‘B3, and ‘Bzr. (Symmetry labels with respect to our choice of axes.) If one calculates estimates for the band positions of molecule 11 using the parameters of (5) and in addition the value of B’ = - 1.25 eV for the resonance integral between two-centre sr-orbitals [l l] one Z,, 3 = -10.30 eV, Z,, 4 = - 13.06 eV. In fizs Z,, 1 = -8.04 eV, Z,, 2 = -9_80eV, view of the crudeness of the model the agreement is rather good. Thus the photoelectron spectrum of molecule 11 provides additional support for the sequence of states in radical cation 4+.

11

12

13

(4) In the course of a reinvestigation of the “perfluoro-effect” [20] we have recorded the photoelectron spectrum of perfluoro- 1,Ccyclohexadiene, molecule 12 [21]. The two n-bands are found at Z,, i(n) = 11 .l eV and Z,, z(x) = 12.0 eV. The interpretation of this spectrum is ambiguous. From the photoelectron spectroscopic data on perfluorocycloalkenes, compounds 13 (n = 2-4), it is known that Z”(X) = 11.6 eV, which happens to be exactly the mean of Z,, 1(~) and Z,, 2(rc) of molecule 12: Z,(x) = 11.5, eV. If one assumes that the localized orbitals of a CF,-group are much less readily available as relay-orbitals for “through-bound” interaction between the two z-orbitals (for an example see ref. 22), and that the value B,, = -0.5 eV given in (5) still accounts for their “through-space” interaction, then I,, 1(rc) and I”, JTC) of molecule 12 are exactly reproduced if Ai = - 11.6 eV (from the spectrum of molecule 13) is used for the self-energy of the two basis-orbitals. If this analysis is true in principle, the remarkable consequence of perfluorination would be that in contrast to molecule 4 the orbital sequence in molecule 12 is the natural one, which makes ‘B3, the ground state and ‘BIu the first excited state of radical cation 12+. On the other hand, if one begins the analysis by postulating that the x-orbita sequence in molecule 12 is still the inverted one, i.e. b,,(x) above bJo(lt), then one has to assume that A,’ = - 12.5 eV which is 1 eV lower than Ai - 11.6 eV, in the monoenes 13, although

238 a bigger perfluorinated alkyl moiety exists in the latter, which makes the necessary assumption of A; lower than A, rather unreasonable. Therefore one can assume as a working hypothesis that in molecule 12 the n-orbital sequence is the natural one. However, this hypothesis is still in need of further experimental and/or theoretical support. (5) To conclude, we shall briefly comment on the results that would have been obtained, had one assumed the natural sequence b,,(x) above blU(x) for molecule 4. One would have obtained B,, = - 1.5 eV and B,, = - 1.8 5 eV instead of the values given in (5). Whereas B,, is still reasonable, B,, is evidently much too large for two rc-orbitals separated by 2.5 A. This in itself makes the assumption of a natural n-orbital ordering in molecule 4 more than unlikely. In addition, using the above parameters for the higher members of the series of molecules 4 to 7 leads to a much less satisfactory agreement with the experimental data, although the discrepancy is perhaps not as large as one might have expected. This is also shown in Fig. 4. In particular the rather critical value I,, 1(rc) for the position of the first band in the photoelectron spectra is predicted to occur at much too low energies in the cases of molecules 6 and 7. In addition the relative spacings of the bands in the spectra of molecules 5 and 6 are quite at variance with the observed pattern. ACKNOWLEDGEMENTS

This work is part 91 of project no. 2.159.74 of the Schweizerischer Nationalfonds zur Fiirderung der wissenschaftlichen Forschung; for part 90 see ref. 23. We thank Ciba-Geigy S.A., F. Hoffmann-La Roche & Cie. S.A. and Sandoz S.A. for their financial support.

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6 7 8 9

10

R. Hoffmann, Accounts C/rem. Res., 4 (1971) 1; R. Gleiter, Angew. Chem., 86 (1974) 770 and refs. therein. R. Hoffmann, E. Heilbronner and R. Gleiter, J. Amer. Chem. Sot., 92 (1970) 706. T. C. Koopmans, P&sica (Utrecht), 1 (1934) 104. E. Heilbronner, Israel J. Chem., 10 (1972) 143. E. Heilbronner and H.-D. Martin, Helv. Chim. Acta, 55 (1972) 1490; R. W. Hoffmann, R. Schiittler, W. SchSifer and A. Schweig, Angew. Chem., 84 (1972) 533. E. Haselbach, E. Heilbronner and G. Schrijder, Hetv. Chim. Acta, 54 (1971) 153. M. J. Goldstein, S. Natowsky, E. Heilbronner and V. Hornung, nelv. Chim. Acta, 56 (1973) 294. E. Heilbronner, XXZZZrd Iaternatioltal Congress of Pure and Applied Chemistry, Vol. 7, Butterworths, London, 1971, p. 9. E. A. McNeil1 and F. R. Scholer, J. Mol. Strut., 31 (1976) 65; D. Bougeard, B. Schrader, P. Bleckmann and T. Plesser, Justus Liebigs Ann. Chem., (1974) 137; M. J. Cardillo and S. H. Bauer, .7. Amer. Chem. Sot., 92 (1970) 2399; K. L. Gallaher, Y. C. Wang and S. H. Bauer, J. Mol. Struct., 25 (1975) 35; cf. B. Andersen, H. M. Seip and B. Beagley, Acfa Chem. Sand., 23 (1969) 1837; B. Ahlquist, B. Andersen and H. M. Seip, J. Mol. Struct., 22 (1974) 141. G. Bieri, E. Heilbronner, M. J. Goldstein, R. S. Leight and M. S. Lipton, Tetrahedron Lett.,

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