— —< <

C —M Bond Contribution to3 a3 ( )Polarizability Tensor and J C M1 a

NMR Coupling Constant in[ ]1-X-3-M-Bicyclo 1.1.1 Pentanes

1* ´ 1* ´ 1†C. G. GIRIBET M. C. RUIZ DE AZUA, S. B. GOMEZ,E. L. BOTEK,1‡ R. H. CONTRERAS,1* W. ADCOCK,2 E. W. DELLA,2

A. R. KRSTIC,2 I. J. LOCHERT 2

1Departamento de Fısica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires,´( )Ciudad Universitaria, Pab. I 1428 Buenos Aires, Argentina

2Department of Chemistry, Flinders University, Bedford Park, SA 5042, Australia

Received 4 June 1997; accepted 24 July 1997

ABSTRACT: In the present work, the relationship between the large substituent3 Ž . w xeffects on J C H in 1-X-3-M-bicyclo 1.1.1 pentanes, I, and the polarizability of1

the bridgehead C —M bond is investigated. The existence of such a relationship3 a

is suggested by the finding that the effect of an electronegative substituent X on3 Ž . Ž .J C M couplings in I M s H is due to a distortion of the C —H bond1 a 3toward the C center, which enhances the Fermi contact interaction. If such1distortion originates in an electrostatic effect, then in other members of this

3 Ž .series it can be expected that the substituent effects on J C M couplings1 a

should depend strongly on the C —M bond polarizability. Two approaches3 a

are followed. First, the ab initio CLOPPA-IPPP method is applied to study theC —M bond contribution to the molecular static polarizability tensor in I3 aŽ .M s H, F, CH . Such bond polarizabilities are found to follow the same trend3as calculated as well as experimentally determined substituent effects on3 Ž . wJ C M couplings, which were measured as part of this work in I X s H, Cl;1 a

Ž . x 3 Ž .M s F, CH and X s OCH ; M s Sn CH . Second, J C M couplings3 3 3 3 1 a

*Member of the Argentine National Research CouncilŽ .CONICET

† ŽFellow of the Argentine National Research Council CON-.ICET

‡ Ž .Fellow of SECyT UNNECorrespondence to: R. H. Contreras; e-mail: contrera@

df.uba.arContractrgrant sponsors: UBACYT and CONICET

( )Journal of Computational Chemistry, Vol. 19, No. 2, 181]188 1998Q 1998 John Wiley & Sons, Inc. CCC 0192-8651 / 98 / 020181-08

GIRIBET ET AL.

Ž .M s H, CH are calculated at an ab initio level for X s H, F, and they are3Ž .compared with those obtained in the parent compound X s H if the calculation

is carried out in the presence of an inhomogeneous electric field. Q 1998 JohnWiley & Sons, Inc. J Comput Chem 19: 181]188, 1998

w xKeywords: bond polarizabilities; localized orbitals; bicyclo 1.1.1 pentanes; NMRcoupling constants; CLOPPA-IPPP

Introduction

ubstituent effects on NMR spin]spin cou-S pling constants in a family of compounds areusually small compared with the correspondingcoupling in the reference compound. If the sub-stituent effect on a given coupling constant isdefined by the ratio between that coupling in thesubstituted compound and the corresponding onein the parent compound, E s J Ž X .rJ , thenŽ X . NM NME is a number close to one.Ž X .

A conspicuous exception to this rule wasdetermined in a previous study 1 for 1-X-

w x Ž .bicyclo 1.1.1 pentanes, I M s H , Scheme 1,3 Ž .where, for J C H couplings surprisingly large1

values of E were measured for some sub-Ž X .stituents. For instance, for X s Br a value of EŽ X .as large as 3.8 was obtained. Such extremely large

Ž .substituent effects were found: a to correlaterather well with the substituent electronegativity;

Ž .and b that theoretical bond contribution analyses

SCHEME 1.

of such effects carried out with the CLOPPA-IPPP2, 3 Žmethod Contributions from Localized Orbitals

within the Polarization Propagator Approach]In-.ner Projections of the Polarization Propagator

yielded, as the main factor defining such EŽ X .values, an enlargement of the rear lobe of the LMOcorresponding to the bridgehead C —H bond, ow-3

ing to the electronegativity of the X substituent.The increase in size of that rear lobe, yielded anincrease in the electronic density associated withthat bond at the site of C which, in turn, rendered1

an increase in the Fermi contact interaction that3 Ž .defines that J C H coupling constant.1

Ž .Point a suggests the following rationalization:Owing to the electronegativity of the X group, the

Ž . Ž .X—C bond is polarized as X y —C q , which1 1

creates an electric field along the C —H bond.3

This electric field renders a displacement of theelectronic density associated with this bond, yield-

Ž .ing the effect described in b .Should this be a sound rationalization, then the

E substituent effect for coupling constants ofŽ X .3 Ž .type J C M in families with different M sub-1 a

stituents in I, would depend on the polarizabilityof the C —M bond, that is, E would depend3 a Ž X .on the response of this bond to the electric fieldquoted above.

In the present study, such a conclusion is ana-lyzed from theoretical and experimental points ofview. Theoretically, two approaches are used. Onthe one hand, bond contributions to the molecularstatic polarizability tensor are compared for differ-

Ž .ent C —M bonds in I M s H, F, CH . These3 a 3

are calculated with the CLOPPA-IPPP method re-cently extended to include this response property.4

3 Ž .On the other hand, the J C M couplings in I1 a

Ž .X s H, F; M s H, CH , are calculated at the ab3

initio level, and then they are compared with thoseobtained in the same compound if the calculationis performed in presence of an inhomogeneouselectric field which intends to mimic that pro-

Ž . Ž .duced by the X y —C q polar bond. Experi-13 Ž .mentally, J C M coupling constants are mea-1 a

VOL. 19, NO. 2182

C —M BOND CONTRIBUTION3 a

sured in I for X s H, Cl, or OCH ; and M s F,3Ž .CH , and Sn CH .3 3 3

Methods

The CLOPPA-IPPP method for calculating localpolarizabilities was implemented in the System

Ž . 5 ] 7Modena SYSMO program, which is based onthe equation of motion method8 for computing the

Ž .polarization propagator PP at the RPA level. In-direct nuclear spin]spin coupling constants werealso calculated with the SYSMO program. Molecu-lar geometries were optimized with the GAMESSprogram.9 In all cases, calculations were carriedout using the 6-31G** basis set.10

LOCAL POLARIZABILITIES AND OCCUPIEDLMO POLARIZABILITIES

As the CLOPPA-IPPP method for analyzingmolecular polarizabilities is described elsewhere,4

only a brief overview of its main features is givenhere. This approach is intended for decomposingmolecular polarizabilities into ‘‘local’’ contribu-tions starting from a unideterminantal ground-statewave function. Within this approach, the ‘‘localpolarizability,’’ that is, the polarizability of amolecular fragment, is calculated within the PP

Žapproach at the RPA random phase approxima-.tion level, by means of the inner-projections tech-

nique11 applied onto the molecular fragment ofinterest. The electronic structure of the molecularfragment is represented in terms of orthogonal

Ž .localized molecular orbitals LMOs which closelyresemble the chemical notions of bonds, lone pairs,cores, and antibonding orbitals. Occupied and va-cant LMOs are obtained by means of unitary trans-formations applied separately to canonical occu-

Ž .pied and vacant molecular orbitals MOs . VacantLMOs can be localized on one or several centers.Those vacant LMOs localized on two centers in-volved in a chemical bond can be ascribed toantibonding LMOs. The localization procedureused in this work is Engelmann’s,2, 12 which is ageneralization of Verwoerd’s,13 used in a repeti-tive fashion.

The polarizability tensor can be written withinthe PP approach as14 :

1l ª ªŽ < . Ž < . Ž .a s 2 r Q P Q r 1

where 1P is the singlet part of the PP matrix:Qqs aqi are excitation operators of one particle,

where i stands for an occupied HF spin orbital andª ªqŽ < . ² < w x < :a for a vacant one; and r Q s 0 r , Q 0 is

written in superoperator formalism.14

The ‘‘local’’ contribution to the rs-CartesianŽcomponent of the polarizability tensor r, s s

.x, y, z ; that is, the contribution due to a molecularL, r s Ž .fragment, a , is defined by rewriting eq. 1 in

the LMO basis:

localL , r s ² < < : ² < < :a s y2 a x i W b x jŽÝ r i a , jb s

iaFjb

² < < : ² < < :q b x j W a x i .r i a , jb s

localL , r s Ž . Ž .s a r , s s x , y , z 2Ý i a , jb

iaFjb

where i and j are occupied LMOs and a and b areŽ .vacant LMOs. The sum in eq. 2 is restricted to the

subset of LMOs that define the chosen molecularfragment. W is the element of the singlet PPi a, jbmatrix block which corresponds to a real perturba-tion, inner-projected onto the subset of LMOs thatdefine the chosen fragment.

Ž . L, r sEach term of the sum in eq. 2 , a , involvesi a, jbat most two occupied and two vacant LMOs andindicates to what extent the a-vacant LMO con-tributes to the polarization induced in the i-oc-cupied LMO by the effect of intramolecular inter-actions, as the j-occupied LMO is coupled with theb-vacant LMO by the external perturbation; that is,the electric field. However, it must be noted thatW depends on all LMOs belonging to the locali a, jbfragment and, thus, each a L, r s contains the influ-i a, jbence of the whole fragment under consideration.

Contributions to the polarizability tensor, whichdepend only on occupied LMOs, can be defined by

Ž .summing in eq. 2 over the vacant ones belongingto the local fragment:

localL , r s L , r s Ž .a s a 3Ýi j i a , jb

a; b

These terms can be interpreted as follows.4 Anª

external electric field, E s E x , considered as aˆr r< :perturbation connects the occupied LMO j with

the vacant LMOs. Owing to such a perturbation,< :another occupied LMO i is modified and the

s-component of its dipole moment results:

˜ ˜ 1² < < : ² < < : ² < < : ² < < :i x i s i x i q 2 E i x a P b x jÝj s j s r s i a , jb rab

Ž .4

JOURNAL OF COMPUTATIONAL CHEMISTRY 183

GIRIBET ET AL.

˜< : < :where i is the corrected i LMO up to the firstj< :order owing to the perturbation on j . From eq.

Ž .4 , it follows that:

1L , r s ˜ ˜² < < : ² < < : Ž .a s i x i y i x i 5i j j s j sEr

Thus, the a L, r s terms can be interpreted as thei js-component of the induced dipole moment on the< : Ž .i -occupied LMO per unit field due to the polar-

< :ization of the j LMO in the presence of theª

external field, E. These types of terms are consid-ered ‘‘mutual’’ polarizabilities.4

In the same way, if the perturbation connects< :the i -occupied LMO with the vacant LMOs of

the local subspace:

1L , r s ˜ ˜² < < : ² < < : Ž .a s i x i y i x i 6i i s sEr

Žis obtained, which is the dipole moment per unit. < :field of the LMO i induced by the external field;

< :that is, the polarizability of the occupied LMO i< :on the fragment considered. If the i -occupied

LMO represents a bond, a L, r s is referred to as thei i‘‘bond’’ polarizability of the i bond. It is notewor-thy that the a L, r s terms depend on the subset ofi ivacant LMOs, which are included in the chosenfragment, and thus it yields an idea on how vacantLMOs of different molecular regions contribute to

< :the polarization of the i -occupied LMO, because< :the external electric field connects i with those

vacant LMOs.

Results and Discussion

The component of the C —M bond polariz-3 a

ability tensors along the C —M direction in I3 a

Ž .X s H; M s H, F, and CH are displayed in the3first row of Table I. These bond polarizabilities aredefined using the following subspace of occupiedand vacant LMOs: occupied LMO—that of theC —M bond; vacant LMOs—all those localized3 a

in the C and M atoms. In this way, only the3 a

local effect on this bond is taken into account. Aconspicuous difference for the C —H, C —F, and3 3C C bond polarizabilities is observed, the3largest being that of the C —H bond. This trend is3thought to originate mainly in the different excita-tion energies of these three compounds in theconsidered subspace; that is, it should be easier toobtain excitations within the C—H electronic sys-

TABLE I.Bond Polarizabilities of Bridgehead C —M Bond3 a

XXXXX( )M = H, F, CH , a and a , and of C—H3 C — M C — M( )Bond, a , in I X = H and M = H, F, CHC — H 3

( )All Values in a.u. .

M = H M = F M = C

( ) [ ]a 1-X-bicyclo 1.1.1 pentanea

a 1.66 1.38 0.94C— MX b

a 1.70 1.40 0.99C— Mc

a 1.66 1.65 1.67C— H

( )b CH M3a

a 2.70 1.62 1.09C— M

aCalculated considering C—M-occupied and vacant LMOsas local subspace.bCalculated considering C—M-occupied and vacant C—Hvacant LMOs as local subspace.cCalculated considering C—H-occupied and vacant LMOsas local subspace.

tem than in the C—F or C—C ones. It is of interestto note that the respective optimized bond lengthsfollow the opposite trend than that of the bondpolarizability; that is, the largest bond polarizabil-ity corresponds to the shortest bond length. Asfound previously1 the substituent effect, E , onŽ X .3 Ž . Ž .J C H coupling constants in I M s H is mainly1originated in distortions of the C —H bond elec-3tronic distribution owing to the electronegativityof the X substituent. For other M groups, thisdistortion should follow the C —M bond polar-3 a

izabilities. Therefore, it can be expected that, inwthese compounds, the E i.e., the substituentŽ X .

3 Ž . 3 Ž . 3 Ž .effects on J C H , J C F , and J C C coupling1 1 1 Mexconstants should follow a similar trend. However,

a direct proportionality between E and the CŽ X . 3—M bond polarizability should not be expecteda

because other factors are also affecting these cou-pling constants.

To assess to what extent the C —M bond3 a

polarizability is influenced by excitations involv-ing the vacant LMOs localized on the C —X1 a

bond, values obtained when considering all excita-tions from the C —M -occupied LMO to all the3 a

vacant LMOs localized either on the C —M or3 a

the C —X bond are displayed in the second row1of Table I. It is observed that only minor correc-tions to values shown in the first row are obtained.Thus, it is concluded that the main features ofbond polarizabilities are adequately taken into ac-count when considering only the subset of vacantLMOs considered in the first row.

VOL. 19, NO. 2184

C —M BOND CONTRIBUTION3 a

In compounds I with X s H and M s H, F,and CH , the C —H bond polarizabilities were3 1also calculated to determine if they depend on thesubstitution at the other bridgehead carbon atom.The corresponding values are displayed in thethird row of Table I and, when compared with thatof the first row, only minor variations are ob-served.

It is known15 that the strain of the cage sub-strate in compounds of type I is very important.To assess how such strain affects bond polarizabil-ities shown in the first row of Table I, C—Ma

Žbond polarizabilities calculated in CH M M s H,3.F, CH compounds are displayed. A notably lower3

value of bond polarizability is observed in thestrained compounds than in the unstrained ones,especially for the C—H bond. This large differencecan be rationalized in terms of the different hy-bridization at the C atom. In fact, the conspicu-

1 Ž .ously larger value of the bridgehead J CH cou-w x 1 Ž .pling constant in bicyclo 1.1.1 pentane, than J CH

in methane, 167.8 Hz16 and 125.306 Hz,17 respec-tively, is an indication that the s character on the Catom of the bridgehead C—H bond in the formeris notably larger than that in the methane C—Hbond.18 A larger s character implies a smaller pcharacter, and it is known19 that a bond polariz-ability is larger the larger the p character of thatbond.

The s and p characters at the C atom in the3< :C —M bond represented by the LMO i can be3 a

< :estimated as the mean value over i of the orthog-H Ž .onal projector P m s s, p on the subspacem

spanned by the m-type atomic orbitals centered onC ; that is,3

² < H < :C s i P im m

Values thus calculated for M s H, F, and CH in I3Ž .X s H are compared in Table II with those cal-culated in CH M. It is observed that, for a given3M, the decrease in the p character follows that ofthe bond polarizability.

The C —M bond polarizability yields a mea-3 a

sure of the C —M bond distortion due to a3 a

uniform electric field along the bond direction. Incompounds I the X substituent can be thought toprovide an electric field with this direction. Themore electronegative the X is, the larger the posi-tive charge at C . The distortion of the electronic1density of the C —M bond is taken into account3 a

in the corresponding occupied LMO obtained fromthe ground state wave function of the substitutedcompound. As described previously for M s H

TABLE II.( )s and p character at C Atom of C—M M = H, F, C

( )Bonds in I and in CH M M = H, F, CH3 3( )All Values in a.u.

M = H M = F M = C

( )a CH X3C 0.372 0.160 0.385sC 0.544 0.245 0.392p

( ) [ ]b 1-X-bicyclo 1.1.1 pentaneC 0.541 0.229 0.517sC 0.379 0.202 0.284p

within the INDO approach,1 such distortion mani-fests itself as an increase in the contribution of thes-type AO of the C atom to the LMO describing1the C —M bond when increasing the X elec-3 a

tronegativity. It was shown in that work that thisextension of the C —H bond toward the C atom3 1increases the Fermi contact interaction of electronsthat bond with the C nucleus, thus yielding1

3 Ž .a larger contribution to the J C H coupling1constant.

To verify if such an electrostatic effect can beconsidered the main one defining the large sub-

3 Ž .stituent effects on J C H couplings, the following1theoretical approach is used. In I, the electrostaticeffect on the C —M bond can be reproduced3 a

artificially by creating an electrostatic field whichattracts electrons to the X center. This can beaccomplished by taking X s H and placing anadditional positive charge at the site of the proton.Of course, this procedure will distort the C —H1bond in a different way from that of an electroneg-ative substituent, like, for instance, X s F. How-ever, it can be considered that the electrostaticeffect in the region of the C —M bond is similar3 a

to that of an electronegative substituent, providedan adequate value for that charge is chosen.

Ž .In Table III, the calculated Fermi contact FC3 Ž . Ž .contributions to J C M couplings M s H, CH1 a 3

Ž d . din I X s H; H , F are shown, where H standsfor an H atom such that, at the site of its nucleus, a

Žqd charge is placed noncontact contributions areassumed to be by far much smaller than the FC

.one . Experimental values are quoted in parenthe-ses. The cage geometry employed to carry out theX s H d calculation is that obtained when optimiz-ing the geometry for the X s F compound. It isfound that for d s q0.25 a.u. the calculated3 Ž .J C M couplings are similar to both the calcu-1 a

lated and the measured ones1 in I with X s F. Asmentioned previously, effects other than the elec-

JOURNAL OF COMPUTATIONAL CHEMISTRY 185

GIRIBET ET AL.

TABLE III.3 ( )Fermi Contact Contribution to J C M Couplings1 a

d( ) ( )M = H, CH in I X = H, F, H .3

3 3( ) ( )X J C H J C C1 1

H 11.74 11.38( ) ( )10.0 9.7

b( )F opt. 24.67 17.93( )27.4

d aH 28.82 19.22b( )F unopt. 31.08 19.73

Experimental values are indicated in brackets below thecorresponding calculated ones. All couplings in hertz.aHd stands for the H atom such that at the site of its nucleusa d = +0.25 a.u. charge is placed.bopt.: calculation carried out with the X = F optimized geom-etry; unopt.: calculation carried out with the optimized ge-

( )ometry of the parent compound X = H taking the standardvalue for the F—C bond length.

trostatic one are also defining the notorious sub-3 Ž .stituent effects observed on J C M couplings.1 a

They were discussed in some detail in ref. 1. Here,the effect of the cage geometry is also studiedusing a different approach, namely, using twodifferent cage geometries to carry out calculations

Ž . Ž .in I X s F, M s H, CH . These are: a the3Ž .respective fully optimized geometries; and b the

optimized geometry of the respective parent com-Ž .pound I X s H, M s H, CH . Calculations car-3

Ž . 3 Ž .ried out with the a geometries yield J C M1 a

couplings somewhat smaller than those obtainedŽ .with the b geometries. The geometry changes can

easily be rationalized with Bent’s rule. In fact,upon substitution by a more electronegativesubstituent, like, for instance, X s F, the C s-1character of the C —C cage bonds increases.1Such change is consistent with a decrease of theX—C —C angle as it is in fact obtained in the1optimized geometry, where the bridgehead C1??? C distance is ca. 0.077 a.u. shorter than in the3parent compounds. It is interesting to observe that

3 Ž .the geometry effect on the J C M coupling is1 a

noticeably larger for M s H than for M s CH .3As part of this work, a few members of series I

3 Ž .were synthesized and their respective J C M1 a

coupling constants measured. The correspondingexperimental details will be given in a forthcomingstudy.20 The measured couplings are collected inTable IV where the corresponding substituent ef-fects, E , are also shown. Values for I with X sŽ X .H, OCH , and Cl taken from ref. 1 are also dis-3played in Table IV. It is worthy of noting that, forM s H, F, and CH , the respective E sub-3 Ž X .

TABLE IV.3 ( )Experimental J C M NMR Spin]Spin Coupling1

( )Constants in I, M = H, F, CH and Sn CH and3 3 3X = H, Cl, or OCH .3

3 ( )X M J C M E1 ( X )

aH H 10.0aOCH H 20.8 2.083

Cl H 31.5 3.15H F 42.4Cl F 108.0 2.55H CH 9.73Cl CH 18.6 1.923

( )H Sn CH 134.93 3( )OCH Sn CH 201.0 1.493 3 3

The respective substituent effects, E , are also shown. All( X )couplings in hertz.aThis work.

stituent effects follow the same trend as the C—H,Ž .C—F, and C—C bond polarizabilities in I X s H

shown in Table I. The C—Sn bond polarizabilitycould not be calculated because heavy-atom-con-taining molecules cannot be studied with theSYSMO program. However, from values displayedin Table IV, it can be expected that the C—Sn

w Ž . xbond polarizability in I M s Sn CH is some-3 3where between those of the C—F and C—C bonds

Ž . Ž .in I X s H, M s F and in I X s H, M s CH3respectively. However, it should be closer to thatof the former than to that of the latter. This impliesthat the C—Sn bond polarizability is larger thanthat of the C—C bond. This result is in agreementwith assumptions commonly made about the po-larizability of a C—Y bond where Y is a Group IVelement,21 namely, that it increases when goingdown the Periodic Table.

Conclusions

Studies on the interactions transmitted throughthe bridgehead carbon atoms in compounds oftype I presented in this work strongly support therationalization made previously1 on the surpris-

3 Ž .ingly large substituent effects on J C M1 a

spin]spin coupling constants in series I withM s H.

Ž .The C —M M s H, F, C bond polarizabili-3 a a

Ž .ties calculated in I X s H; M s H, F, CH fol-3low a trend similar to that of the E substituentŽ X .

3 Ž .effects on J C M coupling constants measured1 a

Žin this work in I compounds X s Cl; M s H, F,

VOL. 19, NO. 2186

C —M BOND CONTRIBUTION3 a

.CH . These bond polarizabilities are conspicu-3ously different from those calculated in unstrainedcompounds. This indicates that the C —M bond3 a

polarizabilities depend strongly on the hybridiza-tion at the bridgehead C atom. This dependence issuch that the polarizability decreases when in-creasing the C s character of the C —M bond.3 3 a

Looking for experimental evidence supportingthis conclusion, it is of interest to compare the3 Ž . 3 Ž . ŽJ C H and J C C couplings in I X s H;1 1 Me

.M s H, CH with the analogous couplings in bi-3w x Ž .cyclo 2.1.1 hexanes, II, X s H, M s H and CH ;3

3, 4Ž Ž . 3, 4 Ž .that is, the respective J C H and J C C1 1 Mecouplings. This comparison is displayed in TableV, where the experimental bridgehead C ??? C dis-tances in the parent compounds are also com-pared. Both types of couplings show opposite

Ž .trends—that of the J C C couplings is in agree-1 Mement with what it could be expected.25 In fact, in I

Ž .the following points should be recalled: a thebridgehead C —H and C —C bonds are1 3 Me

Ž .collinear, and this does not hold in II; and b thebridgehead C ??? C distance is smaller in the formerthan in the latter. These two facts would renderlarger analogous couplings in I than in II. How-

Ž .ever, this is not the trend depicted by the J C H1couplings, and there should be a further effect onthem. This further effect seems to provide experi-mental support for the results presented in thisarticle. In fact, it is known that the C s character4

Ž .of the C —H bond in II M s H is smaller than4Žthe C s character of the C —H bond in I M s3 3

. 26H . Therefore, the bond polarizability effect on3, 4 Ž . Ž .the J C H coupling in II M s H should be1

3 Ž .larger than that in J C H in I, compensating the1Ž . Ž .two effects a and b , quoted previously. Accord-

ing to the values shown in Table I, the hybridiza-tion effect on a C—C bond polarizability is no-tably lower than in a C—H bond. For this reason

TABLE V.3 ( ) 3 ( )Comparison of J C H and J C C Couplings in I1 1 Me

3, 4 3, 4( ) ( )with J C H and J C C Couplings in II1 1 Mea( )X = H, M = H, CH .3

M I II

b c( ) ( )J C H Hz H 10.0 12.61d c( ) ( )J C C Hz CH 9.7 7.51 3

e f˚( )D A 1.844 2.172

aThe experimental bridgehead C ??? C distances, D, in theparent compounds are also displayed. b Taken from ref. 1.c Taken from ref. 2. dThis work. eTaken from ref. 23. f Takenfrom ref. 24.

the bond polarizability effect cannot overcome theŽ . Ž . 3, 4 Ž . 3 Ž .a and b effects on J C C and J C C1 Me 1 Mecouplings displayed in Table V.

Another interesting example is the experimental4 Ž .value of J CF couplings in 4-substituted fluo-

27 4 Ž .rocubanes, III, where J CF s 13.54 Hz in the4 Ž .parent compound and J CF s 23.47 Hz in 4-Cl

fluorocubane were measured. These values yield asubstituent effect E s 1.73, which, although it isŽCl.

Žsmaller than the E value in I displayed inŽCl..Table IV , it is a very large value taking into

account that the bridgehead C ??? C distance in the˚parent compound of III is 2.7 A, which is notably

larger than the bridgehead C ??? C distance in the˚ 23parent compound of I, 1.844 A.

Acknowledgments

A copy of the SYSMO program, kindly pro-Žvided to the authors by Professor Lazzeretti Uni-

.versity of Modena is much appreciated.

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