13C nuclear magnetic resonance spectra—VI

‘I’ahlc

1. “C

chemical \hifi\ of l~~ub~litutcd adamantane$ and rnono~ub~~ltutcd horno~da~nre~~ ~mono~darn~nfenc~~ wh&uenr chemical shafts (S’S1 in parcnrhcac’ Q

b

&’

Y

Y’

28.5

37.8

--

28.5

*-

and

b’

I

c

--

**

__

(32:;) -*

__

__

35.6 (-2.2)

_-

b

37.8

--

me”

-.. “.”

lT8.1

we

725

09

128.1 C 0.01

Me.? (-1.4)

(xi,

??R.3 ( 0.“)

‘zC.6 ( 3.7)

128.? ( 3.C)

‘.x.5 ( 0.6)

128.0 f-o.11

IX.5 ( 0.6)

128.1 ( 0.0)

126.4 f. 0.5)

??8.1 ( 0.0)

126.4 ( 0.5)

129.3

125.6

7”s

126.1 ( 0.5)

IT??.? (

0.0)

129.0 C-0.3)

126.1 ( 0.5)

128.9

q26.0 ( 0.4)

(-0.4)

The chemical \hiRs arc given relative 10 telramerh~lsilam?. Yfl; J?.Bppm. ‘CH, L.7 ppm: C = 0: 170 Oppm

sion. This interpretation, however. should be avoided, because there is evidence that there ic no essential steric repulsion between a monoatomic suhstituent and a hydrogen both being in 13-diaxial configuration, e.g. an X-ray study of 2-i~oadamant~‘* showed that there is no significant distortion of the molecular geomcrry of the adamantanc framework. Very recentiy.‘b Beierbeck and Saunders presented a new inte~rerat~n of the y,,. &ccl which denies that steric interaction between a substituent X and a y hydrogen is the reason for the upiicld shift at all. Other possibilities of configurations, e.g. the substftuenf being antj~ripla~~ to the y carbon. arc not well investigated. Uniy fur the second-row subst~tuents 6, 0, F) not fixed at a bridgehead carbon is there a detailed studyP showing that a substituent of this type induces an upheld shift at a antiperiplanar y carbon which is even larger than at a carresponding gauche carbon. Often however, and particularly in cases excluded in

Po\irw

\akx

126.2 ( 0.6)

iorrcspond

to dcrhielding

Ref. 21, downfield SCS at y carbons are observed. The y increments of the compounds presented in this study are mostly positive or about zero. Astonichingl~, the average of the two different y shifts at the homoadamantene derivative\ matches nearly exactly the corresponding y shift at the adamantanes for all different cubstitucnts, the y’ SCS being larger and the y SC’S being smaller. Only the y’ shifts of rhe homonoradamantcnes (series 3) are slightly negative (- I .Qto - 1.9ppm). Indeed. this is the only arrangemem which differs significantI) from an anIi~ripla~r cl~n~~u~tit)n. and a rtcric in~eracrion between the substituent and the hydrogen at the y’ carbon is at least conceivable. In Fig. I the y SCS of the various hromo derivatives arc plotted versus the distance I between the bromine and the next y hydrogen nuclei in each case. For comparison reasons the y,,. SCS of 2.bromoadamantanc” was added. Although this compound i\ not a tertiary but a secondary bromide this increment gives a idea of a y

“C nuclear

magnfllc

resonance

1973

spectra--VI

20

30

50

CO

P’I~. I. Substiruent effects upn the 7 positioned cartrons (y .SCS) wrsus the distances berween the bromine atom and thcncxl hydrogennucki(0. r’SCSof 2d;Cv 7 KSof Id: -: y SCSof Zd; x: 7 SCSof3d;W: y’.SCSof Marxi?: y..SC’S of !~bromoadamantanc).

shift induced by a gauche bromine suhrtituent. The internuclear distances in Fig. I are estimated from IXciding models. The other substituents (OH. Cl and 1) show similar figures. The diamagnetic SCS decrease with incrcasitg internuclear distance. At large distances (4.7-5.0 A) the increments become positive and cover a range of about 5 ppm. This points IO the fact that there are at least two interaction mechanisms leading to diamagnetic and to paramagnetic y effects. I.ooking at the models it can be realized that several of the X-C’-C’-C’ arrangements show deviations from coplanarity (torsion angle T = MY). There are configurations with the substitucnt X distorted from the plane formed by the three carbons. thus decreasing the torsion angles:

of the homoadamantenes this angle is about 170”. and for the homonoradamantenes angles of 145” for the y carbons and 125” for the y’ carbons are determined. All these values arc roughly estimated. because the model cannot reflect the true molecular geometry exactly. Therefore a deviation of a few degrees must be accepted. An inspectrion of the various y increments shows that the smaller the torsion angle T. i.e. the larger the deviation from coplanarity, the smaller arc the paramagnetic shifts of the signals in question for an individual substituent. In Fig. 3 the y SCS of the various substituted compounds of the series 1.2 and 3 are plotted versus the sine function of the torsion angle I.+ WC suppose that the angular dependence of the downfield y SCS in antiperiplanar contigurdtion can he described by the follow-

These angles are llup for the adamantanec and the y’ c&on\ of the homoadamantenes. of course, because in these cases all atoms concerned are located in a symmetry plane of the molecule. However, for the y carbons 0 +One referee pointed OUI that the bmc i\ an unusual function in thls conrcxt. Of course. we arc aware [ha1 ever) MO. rhcor) rclate$ whsrltuenr efTec15 IO cosine dcpcndcnccx HUI neverthelew. we want IO report thus surprising tin 0 dependence by whrch rhrs y_, efTecr can be dcwxibcd. although UC cannot give lhcorcflcal crplanarmns so far.

leoo

05 1700

1.0

, 1450

t 123*

Rp. 3. 7 Substirutenr chemical shifts of rhc various adaman. lanes. bomoadamanlencs and homonora.damanlcncs (see 1~x1) versus lhc sine of the lorsioa an&es (sin 7).

ing expression: y SCS = -.4 sin T - B.

(II

The good accordance of the experimental data for X = I with eqn (1) is becoming worse with the decreasing sire of the substituents. The curves bend towards the horizontal direction when the increments are approaching the value zero for the y SCS. This indicates that at least in this region the y effect can no longer be described by only one mechanism. With small deviations from coplanarity the slopes are approximately the same for all four substituents. Thus A in cqn (II seems to be an intrinsic factor of the downfield effect. more or less independent of the substituents’ character. B, however, is associated with the substituents themselves and decreases in the series I. Br. Cl and OH, i.e. downfield shifts induced by iodo substituents arc larger than those induced by bromo substitucnts. etc. Comparing the y SCS of l-substituted adamantanes with those of l-substituted bicyclo[2.!.2]octanes’ (X = OH: -0.2; X = Cl: 1.0; X = Hr: 1.9; X = I: 2.5 ppm) it is observed that the downfield shifts of the latter compounds arc about 2-2.5 ppm smaller than the corresponding ones of the adamantanes. This seems lo contradict the description of the y effects outlined above. since the conformation of the bicyclooctanes with Dw-symmetry is totally eclipsed and the four atoms X, C’. C” and C’ arc coplanar again. However. the bicyclooctanes cannot be regarded as rigid but as swinging molecules.” They arc mixtures of both cnantiomers with D,-symmetry which arc separated by a small energy harrier (Fig. 4).

D,

D,*

Ds

FIB. 4. Even I)reiding models reveal the flexibility of these molecules, and it was reported” that they can bc distorted up IO 12” without any significant changes of bond lengths. This indicates that the weighted average of all conformations has a certain distortion angle 0, thus producing a deviation from coplanarity of the atoms of interest and giving reduced downfield shifts. The concept of an angular dependent downfield y SCS. of course, cannot explain discrepancies of ymU cffcc~s occurring at different molecular arrangements with coplanar atoms X. C”. C” and C’ in all cases. e.g. Fig. 3 shows that the y’ signals of the homoadamantencs appear 1.5-Z ppm at lower field than the corresponding y signals of the adamantanes. although in both cases the torsion angles arc equal (180”). This led us IO the conclusion that there must be an additional mechanism affecting downfield y-, SCS. If WC include the IWO different y SCS of I-substituted 2heteroadamantanes given in Table 2. WC find lhal the y(. SCS agree with the y SCS of the corresponding adamantanes rather well. whereas the yr SCS are significantly larger. Similarly. an enhancement of the y’ SCS of bromo-homoadamantcnc 2d from 5.3 lo 7.5 ppm is observed. if the C-8’ methylcnc group in this molcculc

-@

@ ,

Y= O,NH,CH2 Scheme ! Tabk

!. “C 7 substiruent chemical shifts of I.,ubbtitutcd hclcroadamantane\ ’ L

I Y-O

Y -

Y -

Nil

C1f2

I

x -

011

x . Cl

Y*:

3.9

6.1

YC:

z.7

3.7

Yy:

3.6

‘b. 5

Yc:

2.9

5.6

‘?.3

3.2

Y

:

Z-

‘D-dla laken from Ref. 27 “The substiluenl chemical thifls are given m ppm; positive values correspond IO dounficld shifts. is replaced by oxygen IO form the corresponding oxahomoadamantene derivative. This y’ SCS was again obtained by comparison with the corresponding parent compound oxa-homoadamantcnc. looking at these hetcroadamantanes it is found that the shorter the internuclear distances d between the substitucnts X and the respective y carbons the larger the downfield increment. Figure 5 shows this graphically. The distances d are again estimated from Drciding models, and Fig. 5 can only give a tentative description. Although the d values are only roughly estimated their trend is evident. Furthermore, it should bc well noted that the distance d is only a gross parameter depicting a general change in bond lengths and bond angles. An explanation for the different y SCS in totally coplanar arrangements may be suggested: Replacing a methylcnc group by oxygen or a NH group in the adamantane framework the bond lengths and angles are dccrcaxd, thus giving smaller d values. By this change of the molecular geometry the hybridiration states of the yv carbons are changed slightly causing different y SCS: in other words, by compressing the molecular moiety in question within the plane like a concertina. the downfield y SCS can be enhanced remarkably. For both substituents (X = OH and Cl) the d values of the y’ SCS of the homoadamantcnes (0 in Fig. 5) do not fir the correlation. This is exactly what we would expect if WC consider that there should be a considerable thrust upon the Q and the y’ carbons towards each other in the homoadamantcnc framework (Fig. 6: see also next seelion). This should result in a shortening of the parameters d compared with that of the adamantancs. On the other hand. the approach of these IWO carbons cannot be reproduced by Dreiding models which arc loo rigid.

wls

“C nuckar magneticresonancespectra--Vi

3-

LO 37 3-6 3-9 3-6 35 Fig. Ir. Substituentedcc~supon I positioned carbons (y SCS) versus the internuclear distances d between the substitucntr X and the rcspcctivc y carbons (0: y‘ .SCS of 2b and 2~; 3: y SCS of lb and Ic; -: yr SCS of the oxaadamantancs; X: yI .SCS of the oxaadamantancs; I: yr SC‘S of the azaadamantancs; L;: yC SCS of the ~~~d~rn~~cs).

co Fig.

b.

To summarize, we obtain from the presented data an insight into the influences of substituents upon y positioned carbons in b~dge~ad-substituted molecuIes. The totaf y..* effect is composed of at least two cons t~buti~ns: y SCS = f(r, d).

30

20

(21

The first parameter describes the angular-dependent downfield y effect which apparently is operating through the a bond framework and is only effective, if the torsional angle is relatively large (about 1~1800). The second parameter represents a mechanism producing changes of yu*, effects by compressing the atomic ~ngements X-c”-C6-CY without changing the torsion angle. This effect could only be observed un-_ ambigu~)usiy for I = 180” giving enhancements of the downfield SCS. Al the present slate of our knowledge it cannot be ascertained whether this term is effective at other torsion an&s also, and if it is so, which sign it has. a-Efecr. There are several approaches to jnte~rc~tion of substituent effects on Q carbon shieldings in terms of clcctronegativity. intramolecular dispersion effects. neigh~ur-~nis~~tropy effects and others.‘ AJth(~ugh a general concept able to explain these effects on carbons in Q position relative to the substitucnt for a variety of aliphatic compounds does not exist. it is commonly agreed that one major contribution to the aSCS is con trolled by the electronegativity of the substituent’s atom directly attached to the a carbon-“ However, it can be seen clearly from Fig. 7 that ekctronegativity cannot be the only major contribution. P~i~ularly in the case of the l-substituted adamantancs

10

0

I

Br

Cl

0

25

28

30

35

*r

Fig. 7. Substitucnt ctkts upon u positioned carbons (d SO) versus Pauling’s clectronqativitrcs; -: I-substituted adamantants; .D: ~nnsubstrtut~ homoadamantcncs: 8~ monosuhstituted homonor~da~ntcnc~.

lb-lc and the homoadamantcnes 2C2e there is only a very rough correlation between the it effects and the substituents’ eiectrontgativities. whereas the corresponding correlation for the homonoradamantenes 3h3e is much better. Grutzner PI ut* established a r&e to predict differing Q and fi effects, as well, by considering the number of substituents other than hydrogen at the u and the fl carbons in addition to the inductive effect exerted by the character of the substituent. For hydroxyl and methyl

groups they reported a linear correlation between the a shifts and the number n of hydrogens replaced by other substituents at the u and fl carbon(s) showing a decrease of the a effects with increasing n in the ma8nitudc of

approximately 3 ppm per hydrogen replaced. HUI already Dam” has pointed out that this concept cannot be generalized to other bicyclic systems and other substituents and that for a given substituent the incremental enhancements of the shielding of the Q carbons with increasing n can vary remarkably. In the present study we calculate n = S for the homoadamantcnes and n = 6 for the homonoradamantenes. Rut only for the halogen substituents we found larger Q increments for the homoadamantenes than for the homonoradamantencs. For the hydroxy compounds, however, the a SCS is smaller. Darn-” derived from his results that there must be more parameters for explaining the Q effects. at least for bicyclic compounds, and suggested taking into account the strain within the molecular framework. He tcntatively presented a rough correlation depicting a decay of the a effects with increasing strain estimated from thermodynamic data.” But this assumption. if valid at all. cannot be applied to the molecular systems presented in this communication. The three skeletons of the adamantanes. the homoadamantenes and the homonoradamantenes differ significantly in strain. Whereas the adamantanes can be regarded as nearly free of strain. the widening of one bmembered ring lo a cycloheptcnc-type ring in the homoand homonoradamantene series is expected to cause a considerable thrust upon the other bridgehead carbons towards each other (see Fig. 6) which has to be counteracted by the j?’ carbon to form the homoadamantenes. To form the homonoradamantenes the approach of fhe bridgehead carbons has even IO be reinforced. Furthermore. in both frameworks the arrangements of rhe sub stituents. carbons and hydrogens are distorted from the energetically favoured staggered configurations existing in the adamantane framework, thus causing additional strain due lo steric interaction. Therefore. accepting the tentative model of straindependent a SCS one would expect that the Q increments of both the homo- and the homonoradamantents are different from those of the adamantanes for a given substituent. Fig. 7. however clearly demonstrates that at least for the homoadamantenes strain play no larger part affecting the a shifts. They differ from those of the corresponding adamantancs only by +0.6 +!.4ppm. On the contrary, the (I effects of the homonoradamantenes show a completely different be-

haviour with differences of - 17.7 for X = 1. -10.3 for X = Br. - 3.7 for X z Cl and +7.4 ppm for X 3 OH. For that reason we believe that the concept of straindependent Q SCS cannot be adopted generally. and further investigations are necessary to explain their absolute values and their behaviour within certain series of various substitucnts in different molecular frameworks. B Eflec-1s. Similar IO the Q effects the p cffec~s arc interpreted in terms of electronegativify and other influences enumerated in the previous section.“’ Again one main contribution seems lo be the cleclronegativity. hut here the trend is reversed compared with that for the a effects: With increasing Ck!CtrOne&hity a decrease of the /I increments is observed. This trend is verified by our resulls. This sequence has been rationalircd by a hypothesis”-?’ suggesting an alternating, decaying charge distribution at the hydrocarbon chain induced by the substituent X: X-C’-Cd-CT-... s-s-&Y-

6664

Grutzner et al.’ also extended their rule of .SCS dcpendence on the substitution pattern at the Q and the B carbons to the B effects and found a steady decay of the fi SCS up to n = M. With larger n a constant B SCS is reached. According IO this explanation similar values for the @ effects in the series 1. 2 and 3 are expected. Indeed. for a certain substituent all f3 .SCS are the same within a range of 0.7 ppm for the halogens and 1.4 ppm for the hydroxy group. Long-rongr efecrs. All 6 SCS of all series and suhstituents are negative. II is surprising that all 6 SCS of halogen substitucnts are nearly identical (-!.I 2 0.3 ppm) whatever the molecule and the halogen atom is (chlorine. bromine or iodine). II is even insignificant whether the 6 carbon is aliphatis (C-6 of I and C-6’ of 2) or aromatic (C-6 of 2 and 3). The hydroxy substituents only differ from the halogen substitucnts by their absolute values of the 6 SCS (- I.5 ~0.2 ppm). but they show the same behaviour. The c effects are very small as they are expected to be (-0.4 to rO.l ppm), whereas the l effects arc a little bit larger (0.4-0.7ppm). Both effects are loo small lo he interpreted reasonably.

Table 3. “C chemical shifts (6)of dirubstitutcd homoadamantcncs (series 4).“lIw vatuec in parentheses arc deviations of these experimental vah~er from those calculated by the formula gi\cn m the text fM = 6 .&,I

1

0

28.8

68.8

e 35.6 43.9

6’

36.4

c

3

a

41.6

148.1

128.1

125.9

144.0

128.3

127.2

( 0.1)

( 0.0)

(-0.1)

143.9

'28.2

127.1

v

56.0

42.5

(-6.0)

( 0.1)

(-1.4)

(-1.9)

64.2

44.9

58.2

44.6

(-9.2)

( 0.0)

(-1.8)

C-1.4)

( 0.2)

C-0.1)

( 0.0)

( 47.8 0.0)

(-2.4) 63.4

(-0.2) 47.6

144.4 ( 0.7)

128.2 ( 0.3)

12'1.3 ( 0.1)

,-z,

7?1e chemical chills are relatwe IO ktramcthylsllane. deshlclding.

poGti\e

values correqwnd

IO

“C nuckar

Disrtibured compounds f series 1) The calculated chemical shifts are ohtained

magnetic resonance \pcctra--VI

with

the

help of eqn (3): 6:,

= .A, + AS,’ + As,:

(3)

with i. j, k = Q. 8. @‘. y. 6. t and {. The A, arc the experimental basic chemical shifts taken from the unsuhstitutcd homoadamantenc h. and the A&’ and A6,’ arc the SCS of the IWO suhstituents atl the (I IO IJ positioned carbons u hich have been ohtained h) comparison of the data of the monosuhstitutcd homoadamantenes Zc-h and those of homoadamantenc 2a itself (see previous section). The deviations, A& of the experimental and the calculated chemical shifts for the 8. 8’. y. S, e and { carbons are rather small; onl) for the 8’ and the y carbons small negative discrepancies are observed, i.e. these carbons arc slightly more shielded than expected. Only for the Q carbons do considerably larger deviation\ occur which arc also negative and increase with increasing atomic wcrght of the halogen suhctituents. Similar results were reported very recently by Perkins and Pincock” who presented the spectra of 1.3.dihaloadamantanes among other data.

ExRRtWXTAt. The “t’SMK

\pccrra were ohramcd al natural abundance m the pulsed Pourrcr rrawform mode a~ 22.63 MHs using a Rrukcr WH-90 spcctromcrcr ‘The wmpk\ wcrc run 31 a conccnrration of ahour I 5 \I in deutcratcd chloroform wrrh rctramcthylsilam a\ internal standard Posiribc saluc\ of chcmrcal thrfr\ corrc\pond IO downfield shift\ The accuracy of the \rgnal\ IS ahoul z0 05 ppm. The compound\ la and lb arc commercially abadablc: the other rdamantanc\ arc tardy prepared from lb b) ~onvcntmnal mcrhod\ The prcparatron of the home- and rhc homonoradamanlcnc\ will hc reported In a xparak communicatmn x

‘For Parr V. H f)uddcck and P. Wolff. 0rg Magn. Hesonanre in prc\\ ‘1. 8. Stothcr\. Curbon. 13 :V.UN Specfroscrrpy .&cadcmrc Prc\s. Kcu York (197!) ‘(i. (‘. Levy and G I. Sclwn. Corhon~l3 Surlear MunnrncRtsonunrr /or Oryanrr Chemists Wile). New York (1972). ‘1 7’. Clcrc. E. Prctsch und S. Stcrnhcll. “C’~Kemresonun:~ spekwskopir .Akadcmi\chc Verlagsgcscllschaft Frankfurt/ Main (19i3r.

1977

‘E Brcrtmamr and W. Vocltcr. “C 8~V.\fR-.Sperfnwop~ Vcrlag Chcmrc GmbH. WcmhcimlRcrg~tr. (1974) “S. K Wrl$on and 1. H Stothcr\. Sferrorhemical rlsprcfs oj “c’S.WH Sperrrorcopy In Topies of Sfeerorhemisrr-v (Edrtcd by E. I.. EM and !i 1. .AllmPerr. Vol ft. n I. Wile\. Sew York . f 1974). ‘ti Ii. Macrcl. Subsiriruenr Efftc~s on “C (‘hemteal Shifrs in 7’optrs m (‘u&on. I3 NMR .Sper/msropy (Edited h> G C I.c\)J. Vol. I. p (3 Wdc). Scu York 11974) ‘I H Grurcncr. 5t Jaurcla~. I. H. Dcncc. R. .A. Smith and I. I) Rohcrt\. J Am. Chrm Sor 92. 7107 (1970) ‘ft. I. .Sohrwdcr and H’. Rrcmw. 7irrahtdnm I.clrm 5197 (19701. .‘(a) E Lrppmaa. T Pchk. I. PaaGirta. S Hclikova and A. PM. Org. Bfacn Reconunrc 2. tgl (1970). fhJ E. Lippmaa. ‘I’. Pchk. N. A Rclrkova. A. A. Rohylc\a. A N Kalinichcnko. ht. D Ordubadr and .A. F PM. 0~. .Magn Rrsonanrr 8. 74 f 1976) “‘I’ Pchk. El Lrppmaa. V V. Scvo\lJano~a. M !.i(. Krayushkm and A. I Tarawva. Org. .Wugn Rcronunc-t 3. 7X3 (1971) ‘%i E Mac~cl. H C I)orn. R I. Greene. W. A. Klc\chick. M R Pcrcr\on. Jr. and fi VI Wahl. Jr.. Org. 3fafn Rtsonunre 6. I78

(IY?J) “H. Duddeck. Org .Wugn Hesonanre 7. 1’1 (1975) “R. R. Pcrkm\ and R E Pincock. Ory Muon. Hrsonanre II. 165 (19761 “It I. Rcrch. $1. Jautclar. 51. T Sh\c. F J Wcrgcfl and 1. D. Robert\. J .Am (‘hem .Sor 91. 7565 (1969). ‘“far H. Bcrcrhcck and J K Saundcr\. (‘on 1. (‘hem. 53. 1.30; (IT!): fb) Ibrd. 54. 632 (1976). fc) Ibid 54. 2985 (1976). “far H Gunther. H Schmrcklcr and G. Jikch. J 3fugn Nesonanre II. 344 (1973): (br G Jikch. W Hcrrrg and H. fiiinrhcr. J .4m f‘hrm .Sor. %. 33 (1974). “tar W. R. Woolfcndcn and I) 31 Grant. J .4m (‘hem. .Soc. 1. 14% (19661. fbl 1). M Grant and R. V. Chcnc). J .4m (‘hem .Sur 89. 531’ fIW71. “fi H Wahl. Jr.. R I.. firccnc and 1. Rordner. 1. Chrm .Sor (‘hem (‘ommun. 93 (1973) “J< I.. El~cl.W F. Bade). I. D Kopp. R. I. Wrllcr. D M.Granr. R. Bcrrrand. K A Chrrstcnwr. D K. DaJhng. ht. W. L)uch. E. Wcnkcrr. F \t Sihcll and I). u’ Csh-hran. J .4m (‘hem .Soc-. 97. i!? (19’5) “0 Ermcr and J. D. I)unitr. Htk (‘him. .Wu 52. I%1 (1969). “It Sprcwckc and W G Sshncidcr. J (‘hem Phxr. 3. 722 (IWIJ “ft. C Darn. The\)\. trnwr\ir~ of Califorma. Ihtls (1974). :. P s R. Schlcycr. J. E William\ and K R. Rlanchard. J .Am. (‘hem. .SOC- 92 . -‘377 (1970). ‘“(al J .A Popk and f; h. Scgal. J (‘hem. Phy 43. I36 fI%Cr; rhJ I. .A. Poplc. 1) P Sanrry and (i .4 .Scgal. J (‘hem. Phys. 44. 3x9 (I9MJ. “H. Klcm. IO bc puhh\hcd “H Duddcck and I’. Wolff. 0rg. .tfagn. Rtsonunrr 8. ,591 (1976)