Conformations of highly hindered aryl ethers

Journal of Molecular Structure

ElsevierPublishingCompany, Amsterdam. Printed in the Netherlands

CONFORtiATIONS IX.

OF HIGHLY

DJIPOLE MOMENT

DONOR-ACCEPTOR D. M. MCEACHERN

HWDERED

EVIDENCE

FOR

AN

ARYL

ETHERS.

INTRAMOLECULAR

COMPLEX*

B. AND

P. A. LEHMANN

F.

Department of Chemistry, Center for ResearcJz and Adcanced Institute, A.P. M-740, Mexico 14, D-F- (Mexico)

Studies, National Polytechnic

(ReceivedAugust 3rd, 1971)

ABSTRACT

In a search for evidence that a n cloud proximal halogen interaction is of importance to the mode of action of the thyroid hormones, the dipole moments of the l’-naphthyl (I), (4.29 D), 4’-carboxymethylphenyl (II), (2.90 D), 2’-bromo4’-carboxymethylphenyl (III), (3.56 D), and methyl (IV), (3.24 D) ethers of 2,4dinitro-6-bromophenol were determined in benzene at 25 “C. The moments of (I), (II) and (III) are approximated best by those calculated for the twisted conformation in which the bromine closely approaches the face of the other ring. That of the naphthyl ether (I) can, however, be explained only by inclusion of an intramolecular

electron donor-acceptor

charge-transfer

moment

of 1.6 D directed

from the z cloud of the naphthyl ring to the proximal nitro group of the other ring, or of 2.8 D to the proximal bromine of that ring. In the preferred conformation of the methyl ether (IV), the methoxy group is coplanar with the ring while the Znitro group is twisted 90” from coplanarity to accomodate it. A mesomeric moment of 2.3 D is estimated from this for the 2,4-dinitro-6bromophenoxy moiety and is used in calculating the expected moments of the other ethers. INTRODUCTION

Earlier work in this series on the dipole moments (DM)lm3 and nuclear magnetic resonance (NMR)~~ 5 of hindered diphenyl ethers established that they exist in preferred conformations in which the o&o substituent of one ring approaches very closely the face of the other ring. This suggested the possibility of a direct interaction between the substituent and the n cloud which would be of great theoretical importance. In particular it prompted the suggestion6 that such an *

Presented at the XI Latinamerican Congress of Chemistry, 1972. Abstract No. W-71. For Part VIII see ref. 55.

J. Mol. Structure, 11 (1972)

Santiago

de ChiIe,

January

5-11,

127

interaction between the x cloud of the outer ring and the iodine atoms of the inner ring of the thyroid hormones’ might be indispensable to their mode of action. This interaction is envisaged as the formation of an intramolecular electron donoracceptor (EDA) complex8-‘1 with an accompanying charge-transfer (CT) from the ring (D) to the proximal iodine (A):

1 R =_

I

or

H

Theory predicts* and experiments have established’2-16 that such interactions in intermolecular complexes are accompanied by a charge-transfer moment of considerable magnitude. This suggested that a corresponding intranzoZecuZarCT might be present and detectable by DM measurements on appropriate examples. Of all the-ethers avaiIable to us, the most likely to exibit this effect was 2,4dinitro-6-bromophenyl (DNB) I’-naphthyl ether (I) since it includes (a) ortho substituents of appropriate steric hindrance to aid adoption of the twisted conformation3, (b) a donating moiety (naphthyl) of very low ionization potential”, which favors EDA complex formation, and (c) an acceptor moiety (bromine or nitro) whose CT accepting propensity should be increased by the inductive effect of two electron withdrawing groups nzeta to it. For comparison, two other ethers were included in which the same acceptor moiety is present, but in which the good donor moiety has been replaced by a poor one (p-carboxymethylphenyl), without (II) and with (III) the corresponding steric hindrance. Since a key quantity determining the DM’s of such dinitro ethers is the mesomeric moment (MM) of the dinitro ring’, the corresponding anisole (IV) was synthesized and its DM measured.

02Nqp&

02N~~~coocH3 2

cc)

(II)

OCH3 02N~~~coocH3

a~$/02

Br-

NO2

OlT)

128

NO2

un, J. Mol. Strrrcture,11 (1972)

EXPERIMENTAL

The diary1 ethers (I), (II) and (IQ

synthesized as described in Fart iY4,

were thoroughly purified, and their purity ascertained to be at least 99.95 ‘A by DSC as described in Part 1’. The ethers (II) and (III) are colorless, whereas (I) is an intense orange, although it gives light yellow solutions in the usual organic solvents. The 2,4-dinitro-6-bromoanisole (IV), prepared from the phenol in 29 y0 yieId with CH,1 and Ag,O in CHCI,, after repeated recrystallizations from aq. ethanol, gave very light yellow crystals, m.p. 45-46 “C (Iiteraturel’ gives 48 “C) The DM’s were determined by the procedure of Halverstadt and KumIer’g as describeti in Part I. Melting points were determined on a Kofler hot plate and eIectronic spectra on a Unicam SP-800 spectrophotometer. RESULTS

The data from the DM measurements (Table I) were used to calculate the results (Table 2) by the procedure reported previously’. Table 3 presents a comparison of the measured DM’s with those calculated for the most probable conformations. TABLE DIPOLE

1 MOMENT

MEASUREMENTS:

MOLE

FRACTION

2,4-Dinitro-6-bromophenyl I’-naphthyl ether (I) 0.000 0.336 0.705 1.082 1.485 1.827 2.07 1 2.515

(2.277)* 2.277 2.278 2.280 2.282 2.284 2.286 (2.286)*

2.277 2.279 2.279 2.281 2.282 2.283 2.284 2.285

DIELECTRIC

CONSTANT

2,4-Dinitro-6-bromophenyI 4’-carboxymethylphenyl (l.t4511)* 1.14484 1.14457 (1.14399)* 1.14425 I.14405 1.14380 (l-14365)*

2,4-Dinitro-6-bromophenyl .?‘-bromo4’-carboxymethyfphenyl 0.000 0.451 0.694 1.088 1.478 1.839 2.152 2.494

versus

1.14512 1.14499 I.14459 1.14438 1.14414. 1.14385 1.14366 1.14342

0.000 0.338 0.683 1.084 1.480 1.832 2.101 2.555

2.280 2.282 2.28 1 2.282 2.284 (2.287)* 2.285 2.286

2,4-Dinitro-6-bromoanisole

AND

etlter

SPECIFIC

VOLUME?

(II)

1.14500 1.14472 1.14459 1.14433 1.14416 1.14394 I.14369 1.14361 (IV)

ether (ZIZ) 0.000 0.330 0.681 1.076 1.473 1.819 2.143 2.497

2.276 2.278 2.279 2.281 2.282 2.284 2.285 2.287

1.14491 1.14479 1.14436 1.14422 1.14394 1.14371 1.14355 1.14331

* These points were not used in computing p. J. Mol.

Structure,

11 (1972)

129

TABLE DIPOLE

2 MOMENT

RESULTS

El

dl

aa

P

P 203

MIZ

P(D)

(I)

2.274

0.87336

27.00

2.096

465.34

87.98

4.29f0.17

(II)

2.280

0.87341

12.60

2.148

254.78

82.07

2.90*0.17

011)

2.277

0.87331

19.19

3.008

349.16

89.78

3.56&0.10

(IV)

2.276

0.87344

15.38

1.750

266.85

51.69

3.24&O-05

il a and /3 are the slopes of the dielectric

TABLE

constant

and density respectively

us. mole fraction.

3

COhIPARISON

OF MEASURED

OF 2.‘+-DINITRO-6-BROMO

AND ARYL

CALCULATED

DIPOLE

MOMENTS

FOR THE PROBABLE

CONFORMATIONS

ETHERS

COOCH3

COOCH,

Measured

DM

1

0 =

2 3 4 5 6*

8 8 0 0 6

370, 9’ =

370

= 217”, 8’ = 37” = O”, 0’ = 90” = 180”, 8’ = 90” = 900, 0’ = 0” = &j-90”

* Approximate

2.90*0.17

4.29+0.17

(0)

concerted

3.56*0.10

3.66

(-15

%)

3.14

(ts

2.56 4.28 3.41 2.44 3.14

(-40 (0 %) (-21 (-43 (-27

%)

2.93 4.06 3.64 2.74 3.29

(; 1 %) (+40 %) (f26 %) (-6 %) (+I4 %)

rotation;

%) %) %)

not depicted

in Fig.

%)

3.64

(+2

2.82 4.95 4.09 3.47 3.92

(-21 %) (t39 %) (+I5 %) (-3 %) (f 10 %)

%)

1.

Assumptions

In order to estimate the DM for a given conformation, in addition to the assumptions stated previously1 - 3, the following were made: acts along the 0-Cl-C4 axis in the 2,4-dinitro-6-bromo(i) the MM’

phenoxy moiety. (ii) the COOMe has a static group moment of 1.83 D at an angle of 70” from the C,,-C(OOMe) bond, but in these compounds it is assumed to be rotating freely. Its contribution to the total molecular moment was calculated using the formula given by Minkin, Osipov and Zhdanovzl. (iii) The total available MM of 2.3 .D (vide infra) is partitioned between the rings depending on their substitution pattern: in (I) the total. amount is directed into the.DNB ring, while in (II) and (III) the maximum MM into. the 4-carboxy130

.k Mol. Structure,

11. (1972)

methylphenyl moiety is 0.5 D, thus reducing-the maximum MM into the DNB ring to 1.8 D. In all conformations the MM values are reduced by the cos2 of the twist angles 8 and 8’. (iv) The MM of 0.5 D f or ap-carboxymethylphenyl group was obtained by assuming it to be equal to that of ap-acety122 group (-0.46 D). No direct estimate exists and it could not be deduced from the reported DM of methyl-p-hydroxybenzoate23 ( 2.60 D) because the geometry causes the resultant to be very insensitive to the value of the MM. Conformation of the anisole (IV) In order to calculate the DM’s for different conformations of the diary1 ethers, it is necessary to know the magnitude of the MM in the dinitro moiety since this is quite large’. This can be estimated from the measured DM of the corresponding methyl ether. In the case of (IV) three conformations were considered probable for it:

In conformation A the methoxy group is coplanar with the ring, but, as spacefilling molecular models24 show, this is possible only if the 2-nitro group is twisted 90” out of coplanarity with the ring. For this structure a MM of 2.30 D is required in order to match the measured value of 3.24 D. In conformations B and C the methoxy group is twisted out of coplanarity with the ring by 60” away from the nitro and bromine substituents respectively. This is the minimum angle from coplanarity permitted by van der Waals’ radii as estimated from models. The MM necessary to make the calculated and measured DM’s agree are 1.2 D and 2.0 D respectively, which represent total (coplanar maximum) MM’s of 8.0 D and 4.8 D because of the COS%,~, dependence’. Such high values for a MM are very improbable, and it can therefore be assumed that (IV) exists primarily in conformation A. This is in good agreement with the finding that similar conformations are preferred by 2,6-dinitroanisole and 2,6-dinitro-4-chloroanisole’. On the other hand the MM found for DNB is larger than for these (1.2 .D) and is probably due to the large inductive effect of the bromine on the Cr carbon ortho to it. Alternatively, conformation A could be invoked to explain the measured DM by assuming a MM of only 1.5 D from the methoxy group and one of 1.0 D from the bromine. That L Mol. Structure,11 (1972)

131

R’

The results of calculations for these conformations are listed in Table -3 along with the measured DM’s. It is assumed that (I), (II) and (III) exist in equivalent conformations; therefore poor agreement for one member .of the series excludes this conformation for all of them. Comparison between them leads to the following conclusions: conformations between calculated and measured moments compounds (II) or (Ill) and can, therefore, of conformation 6 is regarded as fortuitous, found in (I) where a CT interaction cannot (see below). The remaining conformations

3, 4 and 6 show poor agreement (discrepancies greater than 10 %) for be excluded. The marginal agreement especially because of poor agreement be invoked to explain the discrepancy which show good agreement and are,

therefore, the most probable for this kind of ether are the twisted ones (1and 2) and the sterically least hindered one (5). The first column of Table 3 shows that the calculated moments for (I) differ considerably from the measured one for these conformations_

Intranrolecrrlar CT moment It is believed that the best explanation for this discrepancy is that in (I) a strong intramolecular donor-acceptor complex is formed with which is associated an interaction moment directed from the naphthyl moiety (D) toward the DNB moiety (A). This complex is envisaged such that (D) and (A) interact directly, i.e. by the shortest distance through space and not through bonds. Regardless of whether this interaction moment is of aCTnature or the result of dipole anddipoleinduced dipole interactions (see Discussion), its existence is not unexpected in this molecule. Although the necessary data for deciding the exact direction of this moment are not available, a reasonable assumption is that it is directed from the point of closest approach of the naphthyl 7~ cloud to the proximal ortho substituent, a separation of only 2.8 A (as estimated from Dreiding models). Molecular models also show that this direction coincides with the direction of the p orbital above C, and has the direction cosines, cos a = 0.80, cos j? = 0.31 and cos y = 0.59, with respect to the coordinate axes shown in Fig. 2. If this direction is used, calculations show that a CT moment of 2.8 D in conformation 1 for (I) is required for perfect agretiment with the measured value (4.29 D). The invocation of a similar CT moment is not possible in conformation 5 because the D and A moieties are much too far apart for effective interaction. Thus 5 may be excluded as a probable conformation for DNEI ethers because of the poor agreement in (I). The X-ray diffraction structure determination of compound (I), completed recently*, showed that in the solid-state it adopts a twisted conformation similar to 2 (0 = 248”, 6’ .= 2S”), with one oxygen of the o-nitro group approaching to * Determination of the conformation of (I) in the crystal state by X-ray diffraction analysis has been completed in co-operation with Prof. Eli Shefter and submitted for publication. 1. Mol. Structure, 11 (1972)

133

Fig. 2. Drawing of (I) in its twisted conformation 2 (8 = 217”, 8’ = 37’) in the chosen Cartesian coordinate system; the arrow represents the charge-transfer moment which makes angles of 37’, 72” and 54” with the X, Y, and Z axes respectively.

within 2.9 A of Cl’. The DM calculated for (I) in this conformation requires a CTM of 2.2 D to the nitro group in order to match the measured DM. Since there are no compelling reasons for supposing it to be very different in solution, this allows a choice to be made in favor of 2. Probably in solution the greater rotational freedom allows (I> to adopt the slightly fess constrained ~onfo~ation 2, which would make 1.6 D the best estimate for the CTM. Compound (IL) clearly also prefers conformation 2, whereas compound (III) prefers the alternate twisted conformation 1 due to dipole-dipole repulsion of the two bromines. The exchrsion of conformation 3 is more clear-cut than for 2 and 4: not only are (II) and (III) in poor agreement, but the absence of a CT moment in (1) where one would be expected, serves as a further basis for its exclusion_ The marginal agreement for (II) and (III) in conformation 6 is not considered significant, especially in view of the improbability of any CT interaction because of the changing distances between D and A. Thus, the large discrepancy for (I) in conformation 6 cannot be brought into agreement by invocation of complex formation. Therefore, the previous conclusion that the most likely conformation for these compounds are the twisted on& and that there is present in (I) a CTM of

1’6 D (in the directions shown in Fig. 2) is still the most reasonable one.

DISCUSSION

Intramolecular* and demonstrated

charge-transfer (or EDA) complexes have been postulated

in a variety of compounds 28-38,

but in no case was the ensuing

* The term ‘intramokzcular charge-transfer complex’ has been used by Mulliken to denote systems in which charge is transferred from one portion to another &the same moleculez6. However, in all his examples the charge transfer is between atoms directly bonded together. Our use of the term;although formally similar, implies the transfer of charge between atoms not bonded directly i.e. ‘acrossspace’27.

I34

.L Mol. Structure,

1I (1972)

moment determined. Thus this measurement is the first of its kind. At present the terminology of molecular complexes is in a state of flux, and the meanings as well as the physical significance of the terms ‘donor-acceptor complex’ and ‘chargetransfer’ are in the process of redefinition 15039-442. The results obtained in the present work indicate only that a net electric moment is present but they cannot clarify the nature of the interaction. CT moments have been determined by Briegleb and his coworkers12-14 on a series of intermolecular complexes, including a few of the Z-R type. Specifically the three cases of interest in this context are the complexes of naphthalene with 1,3,5trinitrobenzene (0.69 D), with chloranil (0.9 D) and with tetracyanoethylene (2.28 D). The value of 1.6 D estimated in the present work for an intramolecular complex is thus quite reasonable in view of the increased probability of formation <, and reduced D to A distance. As with other complexesg, in our case it is not known exactly which atoms or moieties are interacting, and consequently the exact direction of the CT moment cannot be determined. The assumption that it is from the point of nearest approach of the naphthyl 71:cloud to the bromine, however, seems the most reasonable (Fig. 2). Determinations of the geometry of comparable intermolecular complexes in the solid state (by X-ray diffraction analyses43) and in solution (NMR studies44) have shown that the D and A portions are not always positioned in the most symmetrical fashion, but that a variety of arrangements, including “tilted.” ones, are encountered. The excellent donor properties of the naphthyl ring are well known, and are enhanced in naphthyl ethers as judged by the relative stabilities of their complexes with 2-bromonaphthalene-1,4,5&tetracarboxylic dianhydride45. Furthermore, the z cloud density over the C1, carbon may be slightly higher than at any other point in the naphthyl ring due to the mesomeric electron-donating properties of an ether oxygen. Alternatively, the CT interaction can be considered as having its D-origin at the center of the 71cloud of the inner ring of the naphthalene residue, without greatly altering the results. Although no complexes involving exactly the same type of bromine have been described, there is no reason to suppose that it is any different from that in bromani146n47 or other aryl halogen complex components (hexafIuorobenzene4*, and chloranilg). A bromonaphthalene derivative has been described as an A, but the bromine is not indispensable to DA complex formation45. In fact theoretical calculations4g indicated that a very stable cotiguration for the iodine-benzene complex is one in which the iodine’s long axis is tilted from the plane of the benzene ring, and points to one of its sides (Mulliken’s model Ox). Furthermore, its properties should be similar in this context to those involving alkyl halogens (e.g. CH13) or even from halogen molecules, provided a similar geometry can be adopted for complex formation. The presence of two nitro groups on the same ring nreta to the bromine, may be counted on to further increase its acceptor properties. The complex which seems most like the one’ under discussion, is the well-documented J. Mol. Structure, 11 (1972)

135

1: 1 carbon tetrabromide-p-xylene complex5’ in which the p-xylene molecules, arrayed in planes, are each interacting with the bromine atoms of two CBr, molecules in a ‘face-centered’ approach on either side. The distance from the bromine atom to the donor plane of 3.34 A is the same as that in the bromine-benzene complex5 ‘. In the twisted conformation 1 a Dreiding model of the naphthyl ether (I), shows that the proximal bromine is positioned almost exactly over the C,, carbon, and that they are separated by ca. 2.8 A. This is somewhat less than the intermolecular counterpart, and should lead to a stronger interaction. Van der Waals attractive forces may be operative at this distance and, in fact, may further stabilize this conformation. The alternate possibility that the proximal nitro group is the acceptors2 is more probable. The measured and calculated DM’s can be made to agree for (I) in conformation 2 by including a CTM of only 1.6 D. Although the measured DM’s of a large series of 2,6-dinitropheny1 and 2,4-dinitronaphthy1 ethers3 uncovered no evidence for a CTM, none of them have present in the molecule moieties with such marked D and A qualities as (I). A search for this intramolecular effect was made among the compounds whose DM’s were measured previously in this series. Only two (III-9” and IV-I 5&) were found in which DA behavior might be expected apriori.

In these no appreciable

CT moment

did improve agreement.

Two further ethers have been measured6:

ISZ) X=

a small one (to.5

D)

X

NO2

QDlX=I

was present, although

Br,

Y =

,

Br

Y=CH3

In them the same elements (i.e. halogen atoms and dinitro-ring) are present, the same conformation is preferred, but there is no evidence for a CT moment, since the measured and calculated moments (V), 4.74 D US. 5.18 D and (VI), 6.55 D UK 6.64 D) agree very closely. Since no CT moment is to be expected in these com136

J. Mol.

Srrucrure,

11 (1972)

pounds (neither ring has appropriate D or A character), this is in agreement with the present findings. The naphthyl ether (I), alone among nearly a hundred similar ethers prepared in these laboratoriess3s 54, is a light yellow color in the usual organic solvents but its crystals are a deep orange. A possible explanation for this is that the orange color is produced by a CT band due to the DA complex formed intramolecularly in the crystal. Upon dissolution competitive colorless intermolecular complexes are formed. At the very high dilutions used in the DM determinations the conditions are again propitious for formation of the intramolecuIar CT complex, Absorption spectra of the ether were obtained in methanol and chloroform, but unfortunately the UV and visible absorption bands of the compound are so strong that any CT band present was not discernible. An attempt was made to detect the intemzoleczrlar analog of the intranzolecztlar DA complex found in (1). I-Bromo-3,5-dinitrobenzene was not available, but the corresponding iodo compound* was tested for complex formation with naphthalene. Detection was attempted in two ways. Solutions were prepared containing D and A and their graphicaIly-summed UV-visible absorption was compared with that of a mixture of the appropriate concentration. No band ascribable to complex formation was observed in the region where it could be detected (375 to 700 nm). In another attempt, equimolar solutions of the separate components in chloroform were mixed and allowed to evaporate slowly. NO material different from the individual components could be isolated. An independent confirmation of the “through-space” interaction of an aromatic E cloud and a properly disposed halogen atom was obtained recently”. Both ‘H and lgF NMR measurements of various o&o-fluorine substituted ethers showed a coupling of the fluorine to the ortho proton on the other ring. This couId best be rationalized as mediated by the ioterveningp orbitals. The present work in which the existence of a considerable intramolecular CT moment has been demonstrated between an aromatic rc cloud of one ring and a substituent on the other ring of a diphenyl ether, thus constitutes evidence favor of a similar interaction which has been suggested as being of importance the mode of action of the thyroid hormones”.

in to

ACKNOWLEDGEMENTS

We thank P. Gonzklez and .J. Guerrero for synthesizing the compounds used, M. Rioja for the DM and DSC determinations and Prof. W. D. Kumler for helpful discussions. * The synthesis and DM series. J. Mol.

Stmcture,

of this compound will be reported in a future communication

11 (1972)

of this

137

REFERENCES &‘LEHMANN F. AND D. M. MCEACHERN B., J. Mol. Sirucrure, 7 (1971) 253. M. MCEACHERN B. AND P. A. LEHMAN F., f. Mol. Srrucruq 7 (1971) 267. A. LEHMAN F. AND D_ M. MCEACHERN B., J. Mol. Strclctrrre,7 (1971) 277. A. LEHMANN F., Org. Magn. Resonance, 2 (1970) 467. A. LEHMAMJ F., Rev- Sot. Q&m- Mex., 14 (1970) 122. Part X: P. A. LEHMA~F., submitted to J. Med. CJtem.; see Abstract No. 170, Abstracts of Papers, XXIII International Congress of Pure and Applied Chemistry, Boston, Mass.,July 1971_ in A. BURGER (Editor), Medicinal Chemisrry, Wiley-Interscience, New York, 7 E. C. JORGENSEN, 1970, Chap. 31. W. B. PERSON, Mo~e~aIar Complexes: a Lecrta-e and Reprint Volume, 8 R. S. MULLIKEN m Wiiey-Interscience, New York, 1969. 9 R. FOSTER,Organic Charge-?ransfer Complexes, Academic Press, London andNewYork, 1969. 10 J. ROSE, Molecular Complexes, Pergamon Press, Oxford, 1967. 11 L. J. ANDREWS AND R. M. KEEFER,Molecular Complexes in Organic Chemistry, Holden-Day, San Francisco, 1964. 12 G. BRIEGLEBAND J. CZEKALLA, 2. E~ektrockem. Ber- Bansezzges- Phys- Chem-, 58 (1954) 249. AND J. CZEKALLA, 2. Efektrochem. Ber. .&msenges- Phys. Chem., 59 (1955) 184. 13 G. BRIEGLEB 14 G. BR~EGLEB,J. CZEKALLA AND G. RJSJSS, Z. Hzys. Chem. (Frankfurt am Main), 30 (1961) 333. 15 R. J. W. LE F&:vRE,D. V. RADFORD AND P. J. STILIZ?,J. Chem. Sot., B, (1968) 1297. 16 W. A. DUNCAN, J. P. SHERIDAN AND F. L. SWINTON, Trans. Faraduy Sot., 62 (1966) 1090. 17 M. J. S. DEBAR AND S. D. WORLEY, J. C%etn_PJlys., 50 (1969) 654. 18 A. VAN ERP, Rec. Trau- Chim- Pays-Bas, 29 (1910) 239. 19 I. F_ HAL~ER~TADTAND W. D. KUMLER, J- Amer. Chem- Sot-, 64 (1942) 2988. 20 C. P; SMY~, Dielectric Behauior arzd Stractwe, McGraw-Hill, New York, 1955, p. 253. 21 V. I. MINKIN, 0. A. OSIPOV AND Y. A. ZHDANOV, Dipole Moments in Organic CJtemistry, Plenum Press, New York-London, 1970, p. 93. 22 V. 1. MINKIN, 0. A. OSIPOV AND Y. A. ZHDANOV, Dipole Momenrs in Organic Chemistry, Plenum Press, New York-London, 1970, p. 205. 23 C. S. COPELAND AND M. W. RIGG, J. Amer. Chem. Sot-, 73 (1951) 3584. 24 W. L. KOLTUN, Bjo~or~mers, 3 (1965) 665. 25 IX L~TGERT, 2. Pisys. CheJn., B17 (1932) 460. 26 R. S. MULLIKEN AND W. B. PERSON, MoIecaJar Complexes: a Lecture and Reprint Volume, Wiley-Interscience, New York, 1969, p. 291. 27 M. BARFIELD AND M. KARPLUS, J. Amer. CJzem. SW., 91 (1969) 1. 28 R. C. COOK~~N AND N- LEWIN, Chem. Ind- (London), (1956) 984. 29 W. N. W~TE, J. Amer. Chem. Sot., 81 (1959) 2912; K. MUTAI, TetraJiedronLerr., (1971) 1125. 30 V. A_ GLUSCHENKOV, V. A. ISMAIL’SKIIAND Y. S_ Mosarrovszu. DokL Akad- Nattk SSSR, 153 (1963) 1363; Dokt. Phys. Chem., 153 (1963) 1125. 31 S. SHIIFRIN,Biochim. Biophys- Acta, 81 (1964) 205. 32 S. SHIFRIN, Biochim. Biophys. Acta, 96 (1965) 173. 33 N. C. YANG AND Y. GAONI, J. Amer. Chem. Sot., 86 (1964) 5022. 34 % W. VERHOEVEN,I. P. DIRKX AND T. J. DE BOER, Tetrahedron Lett., (1966) 4399. 35 I). J. CRAM AND A..C. DAY, J. Org. Chem-, 31 (1966) 1227. .36 R. CARR~THERS,F. M-DEAN, L.E;HOUGHTON AND A_ LED~ITX, Cfzem. Commzm-, (1967) 1206. 37 R. L. HANSEN AND J. J_ NEUMAYER, J. Phys. Chem., 71 (1967) 3047. 38 J. T. D’AGOSTINO.ANV H. H. Jd, J. Amer. Chem. Sot., 91 (1969) 3383. 39 R. S. MULLIKEN, .T.,Amer. Chem. Sot.; 91 (1969) 3409. 40 M. W. ,HANNA, J. Amer.- Chem. Sac., 90 (1968) 265; M: W. HANNA AND D. 3% WILLIAM% J. Amer. Chem. Sac_,‘90 (1968) 5358; a&d J. Lt LIPPERT. M. W. HANNA-AND P. J. TROTZER, J. Ama_. Chem. Sot., 91. (1969) 403% 41 H. 0: HOOPER, J. Chem. P&s_, 41 (1964) 599.. 42 M. J. S. DEWAR AND C_ C. THOMPSON, JR., Teiraltedron, SippIemen; EJo: 7, (1966) 97. 43 ‘R. :I$~ER, Org&& Charge-transfqr. Cbtiplexes; Academic Press, London .and New York, . 1969, Chap. 8. P. D. P. P. P.

I38

J;

MO& Structure, 11. (1972)

44 R. FGSTER, OrrJniiic Clrturge-trunsfe Complexes, Academic Press, L&don and New York, 196?, Chap. 5. 45 P, JACQUIGNON, N. P. Bw-HOI, ANI> M. MARGRAVE, &I& Sot. Chinr. Fr., (1964) 2517. 46 M.NEPRA~ AND R. Z ~NIK, Cofiect, Czech. Cfienz..Cornmun., 29 (1964) 1X5. 47 M, Xbzosrrrra. Bull, Chem_ Sot. Jap_.,35 (1964) 1609. 48 C. R. PATRICK AND GT S. PIZOSSER,Narure (London), 187 (1960) lO?I. 49 R. S. MULLIKEN, .f. Anet. Chem. Sots, 64 (1952) 811. SO F-J. STRIETERANR D. 5;.TEMPLETON,J, Chem. Phys., 37(1962) 161. $1 0. I-IASSEL,ikfOI.Phys., 1 (1958)241. 52 C. E. BUGG ANI, U. THEWELT, Scferrce, 270 (1970) 852. 53 P. A. LIZ-ANN F., Reo. Latfnouner. Q&n., 1 (1970) 112. 54 P. A. LEHMANN F., Anal. Chim. Acta, 54 (1971) 321. 55 Part VIII: L. F. JOHNSON AND P. A. LEHMAIW F., manuscript in preparation.

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