Analysis of the phosphorus lone-pair orientational effect on 31P13C couplings in 7-(phosphamethyl)norbornene

Journal of Molecular Structure (Theochem), 164 (1988) l-15 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ANALYSIS OF THE PHOSPHORUS LONE-PAIR ORIENTATIONAL EFFECT ON 31P-13C COUPLINGS IN 7-(PHOSPHAMETHYL)NORBORNENE

G. A. AUCAR” Departamento de Fisica, Facultad de Ciencias Exactas y Naturales y Agrimensura, UNNE, (3600) Corrientes (Argentina) C. G. GIRIBET*,

M. C. RUIZ DE AZUA*, A. C. DIZ and R. H. CONTRERAS**

Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, UBA, Ciudad Universitaria, Pa b. 1, (1428) Buenos Aires (Argentina) (Received 30 March 1987)

ABSTRACT An IPPP-INDO analysis of P-C couplings is carried out in 7-(phosphamethyl) norbornene for the anti and syn conformations. A strong orientational effect of the P lone pair is expected to be responsible for the differences in the coupling values between conformations. Total couplings are decomposed into through-space and through-bond components. Further analysis of the through-space Fermi contact term is carried out by resolving it into contributions given by different “through-space coupling pathways”, each pathway being defined by excitations which are amenable to direct analysis using the inner-projected polarization propagator technique. The experimental trends of P-C couplings in both conformations are closely reproduced. The differences between both conformations are found to be ruled by: (i) the orientation of the P lone pair, which is shown to be an efficient means of transmitting the spin information; (ii) the transmission through the C-H rear lobe; and (iii) the orientational effect of the P-C antibonding molecular orbital. From (iii) the important role played by vacant molecular orbitals is also brought to attention. INTRODUCTION

The orientational effect of the P lone pair on P-X couplings is very well documented [ 1,2] , and is presently recognized as an adequate tool for analyzing molecular conformations. From an empirical point of view, the importance of the P lone pair orientation in defining these couplings was demonstrated by analyzing similar couplings in pentacoordinated phosphorus compounds [l]. Further evidence for this effect was obtained from the following observations: when X = H large differences were found for geminal couplings *With a Fellowship from CONICET. **Member of the Carrera de1 Investigador of CONICET, and author to whom correspondence should be addressed. 0166-1280/88/.$03.50

o 1988 Elsevier Science Publishers B.V.

2

whether the H atom is placed cis or truns to the P lone pair [ 3-51. For X = C the P lone pair orientational effect was observed for geminal and uicinal couplings [l] . Some attempts were made to relate uicinal J(PC) couplings to structural parameters, especially in Karplus-like curves. However, anomalies in this type of curve were observed when the P lone pair is close to the interacting C nucleus [6, 71. This anomaly can easily be attributed to a throughspace contribution which is operating in this conformation. As studied for other types of couplings [8], such a through-space transmission should be greatly magnified when a C-H bond is placed close to the P lone pair. When X = P, a surprisingly large orientational lone pair effect was reported both for uicinal and geminal couplings [9]. Several authors have studied the relationship between P-P couplings and the P lone pair orientation from a theoretical point of view [lo, 111. Theoretical calculations where this effect becomes apparent in P-C couplings have been carried out by Galasso [12]. The IPPP method [13] allows the resolution of calculated spin-spin coupling constants into contributions transmitted through different mechanisms by the use of inner projections of the polarization propagator (PP). Its general features as well as its abilities and shortcomings have recently been critically discussed [l].At present, it has been applied to the study of the o--n decomposition [13], the through-space transmission of J(CSe) [14 ] and J(SeSe) [15] couplings, and the analysis of the multipath transmission in multicyclic compounds [16]. It was also used to decompose PP couplings in through-bond and through-space components for the geminal coupling of bis(difluoro)phosphinoamine [17] and for the uicinal coupling of cisdiphosphinoethene [18]. A strong lone pair orientational effect was found for the through-space component which permitted an explanation of several experimentally observed trends. Recently, the IPPP method was modified to allow the description of the Fermi-contact contribution due to a given molecular fragment JL as a sum of individual terms J, jb (eqn. (1)) each of them involving only two vacant and two occupied localized molecular orbitals (LMOs) which represent chemical functions as bonds, lone pairs and antibonding MOs J= =

C i
Jti,jb

(1)

where the summation is extended to all occupied i, j and vacant a, b LMOs that belong to the given molecular fragment. Each term constitutes a given “coupling pathway”. In this way the relative importance of the different LMOs in the coupling transmission is given through the Jia,lbvalues, yielding a better understanding of the transmission mechanisms giving rise to JL. This procedure was first used to analyze the through-space transmission of J(FF) couplings [19], and is used here to analyze the case of P-C couplings and the explicit influence of the P lone pair on this coupling. In a recent paper by Quin et al. [20] a series of substituted 7-phosphanorbornenes were synthesized and the corresponding 31P NMR spectra were

3

measured. In these compounds, the spatial proximity of the P and C atoms hints that a large orientational effect of the P lone pair is to be expected in the transmission of 2*3J( PC) couplings. Scheme 1 shows the basic skeleton

5

2

2

5

6 (a)

(c)

I

R2

which is expected to be important in defining the P,--C, coupling for (a) the anti and (b) the syn conformations. This coupling was measured for different substituents R1 and R, [20], with Rz syn or anti with respect to the C=C double bond. Large differences between the measured PC couplings for R2 in each position were found. For example, the measured value for J(P,-C,) when R1 = R2 = CH3 (Scheme l(c)) is 19.5 Hz for the anti conformation while for the syn conformation it is only 4.9 Hz. This seems to indicate that an important through-space component is transmitted by the P lone pair in the anti conformation. The same trend is observed in all cases where the syn and anti PC couplings are reported. In other work by the same group [ 211, a similar compound with R1 = CH3 and R2 = Cl was synthesized. Prom the 31P NMR spectrum, the measured value of J(P,-C2) in the anti conformation is 24.2 Hz. In this paper a theoretical analysis is made of the way in which the efficiency of the coupling transmission is increased in the anti conformation by the orientation of the P lone pair. An IPPP INDO analysis of the J(P,-C,) coupling is carried out in the model compound shown in Scheme 1, with R1 = H, Rz = CH3, for the anti and syn conformations. Total couplings are split into through-space (TS) and through-bond (TB) components. Further analysis of the through-space Fermi contact (TS FC) component is carried out by resolving it into contributions through different coupling pathways (see eqn. (l)), in order to analyze the importance of the P lone pair in this transmission. Plots of the localized molecular orbitals involved in the main

4

coupling pathways yield an intuitive vision of the coupling mechanism. The importance of other bonding and antibonding MOs is also analyzed. METHODOF CALCULATION Since the fundamental aspects of the IPPP method are given in detail elsewhere [13, 221, only a brief summary will be given here. The IPPP method allows the calculation of the Fermi contact (FC), paramagnetic spin-orbital (SO) and spin-dipolar (SD) contributions to the total isotropic coupling constant J(NN’) between nuclei N and N’, and also the resolution of each one of these terms into contributions transmitted through different electronic pathways. This is done by means of the polarization propagator (PP) technique used to evaluate second order corrections to the electronic energy of the molecule involved [23] . In this paper the electronic ground state is calculated at the INDO level of approximation [ 241, and the RPA approximation is used to evaluate the PP [ 231. When total couplings are calculated, the IPPP method at the RPA level is equivalent to the FPT [25] and SCPT [26] schemes. Therefore, all three approaches yield exactly the same total values, provided the same ground state wavefunction is used. Separation into different transmitted components is achieved by innerprojecting the PP onto the subspace spanned by a subset of occupied and vacant localized molecular orbitals (LMOs), considered to properly describe the molecular fragment whose contribution is sought. These LMOs are obtained from the canonical ones by applying the localization technique of Engelmann and Contreras [ 131: a projector is constructed with an appropriate basis of atomic orbit& (AOs), which spans the subspace of interest. The canonical MOs are then unitarily transformed to obtain the eigenvectors of the mentioned projector. Eigenvectors with eigenvalues close to unity are considered to be properly localized on the molecular fragment, and eigenvectors with eigenvalues close to zero are considered to be orthogonal to it. This procedure is applied separately to both occupied and vacant MOs. Usually, the number of LMOs can be predicted intuitively: when a minimal valence basis set is used, the number of occupied LMOs is that of the bonding and non-bonding (lone pairs) orbitals in the chosen fragment, and the number of vacant ones is that of the antibonding orbitals in it. However, occupied as well as vacant canonical MOs are spread over the whole molecular fragment and they do not represent chemical functions. A representation of this type can be obtained by properly applying this technique repeatedly. Once this type of molecular orbital is obtained, an intuitive insight on coupling transmission pathways can be achieved. In all couplings with a substantial through-space component studied so far [l],it was found that the FC term is the main one by far. In such cases further analysis is possible [19], owing to the simplicity of the perturbative Hamiltonian which describes it. Its “local” contribution, i.e. that given by the relevant molecular fragments, may be written in terms of the LMOs and

5

the inner-projected triplet PP, Ph,ib, as 1 JL(NN’) = k C U& Phjb UgFNj = i
Jbjb

(2)

with

where the sum runs over occupied LMOs i, j and vacant LMOs a, b; 7N and TN’ are the magnetogyric ratios of nuclei N and N’; ugcN is the “perturbator” element corresponding to the FC interaction u&‘N = (iI6 (i’ -%!N)b> = J/ i(2N)J/a(3N’)

(3)

where cm is the coefficient of the ith MO corresponding to the s orbital of atom N, and S&(O) is the semi-empirical factor representing the electron density at the site of nucleus N [24], when U&‘N is evaluated within the IPPPINDO method. As shown in eqn. (2), the expression for a coupling constant may be separated into a sum of individual terms involving only four LMOs. The relative importance of the individual terms Jhjb in the sum of eqn. (2) gives an idea of the efficiency of the i-a-b-j pathway in transmitting the spin formation. So, each term Jhjb will be referred to as a “coupling pathway”. The analysis of these coupling pathways gives a deeper and intuitive insight into the main mechanisms of the coupling transmission. This analysis is complemented with plots of the LMOs which are shown to play an important role in the transmission of the TS FC term. To this end, a modified version of the QCPE program MOMAP is used [27]. RESULTS

AND DISCUSSION

Calculations were carried out in the model compound of Scheme 1, with R1 = H and R2 = CHB, for the anti and syn conformations. The geometric structure was taken as follows: the experimental structure of norbomene [28] was used as the basic compound, replacing C, (at the top of the bridge) by the P-CH3 moiety. The P-C bond lengths were taken to be the typical empirical lengths found in ref. 29. The C1-P-CHJ angle was chosen as 117”. The total J(P,-C,) values, as well as the FC contributions are shown in Table 1 for both conformations. In all cases the main contribution comes from the FC interaction. Although the calculated values are somewhat smaller than the experimental ones [ 201, the similarity between them and the correct way in which the experimental trend is reproduced (signs are not specified in the experimental work) allows one to believe that the main features of the coupling transmission are properly taken into account by these theoretical

6 TABLE 1 Calculated J(P,-C,) conformations

couplings (Hz) in compound I (R, = H; R, = CH,) for anti and syn

anti

FC FC+SO+SD ExD.~ aExperimental

wn

TS

Total

TS

7.32 7.54

17.09 17.92 19.5

0.23 0.25

Total -2.35 -2.34 4.9

values in compound I(c) (R, = R, = CH,) taken from ref. 20; signs not given.

calculations. An analysis of the TS and TB transmitted components of that coupling is considered to be meaningful and therefore is performed with the IPPP-INDO method. The local subspace which is considered to yield the TS component is built up from 9 occupied and 8 vacant LMOs, namely: (i) the C2-H, C&-H, CzCJ, P-C(Me) and the three methyl C-H bonds, the Cz-C3 s system and the P lone pair are the occupied LMOs; (ii) the antibonding LMOs corresponding to the bonding ones mentioned above. In Table 1 the calculated TS contributions for the anti and syn conformations are also shown. It is seen that non-contact terms are ca. 2% of the FC one. Besides, the SO term is mainly transmitted through the bonds. Due to its relative magnitude, only the FC contributions are hereafter taken into account. The TS FC term is decomposed into its individual “coupling pathways” Jhjb In Table 2, the main Jaja terms are listed for both conformations, as well as the corresponding perturbators and inner-projected PP elements. The LMOs playing a significant role in the TS transmission are: (i) the P lone pair, which will be called LP(P) (plotted in Fig. 1 and identified as orbital number 5 in Table 2); (ii) the P-C(Me) bond, which will be called B(PC) (LMO 4, shown in Fig. 2); (iii) the Cz-C3 0 and C2-H bonds identified as B(CC) (LMO 2) and B(CH) (LMO 1) respectively; (iv) the main vacant MOs are the P-C(Me) antibonding orbital, called AB(PC) (LMO 7, plotted in Fig. 3), and the C2-H antibonding orbital, identified as AB(CH) (LMO 6). Fig. 4 shows the Cz-C3 R system (LMO 3) for both conformations. The MO plots deserve some qualitative comments: (1) In the anti conformation, LP(P) stands nearly perpendicular to the plane that contains the C2-H and the C2-CJ bonds. A steric effect due to the proximity of the a system and the CH3 moiety may be important in defining this orientation. As a result, it does not extend itself towards the Cz position. In the syn conformation, the position of the CHJ group forces the LP(P) to be oriented with its rear lobe facing the Cz atom.

7 TABLE 2 Main coupling pathways which define the TS FC component of J(P,-Cl)a*b iaj

b

u.l&P

QlT c

%fb,P

anti Conformation 0.199 5757 0.199 5727 0.100 4747 0.001 1616 -0.027 5656 -0.027 5626 0.001 2626 5717 0.199 -0.014 4646 0.199 5747

-0.001 -0.001 0.001 0.147 0.007 0.007 -0.141 -0.001 -0.011 -0.001

0.199 -0.009 0.100 0.001 -0.027 0.001 0.001 -0.008 -0.014 0.100

-0.001 0.018 0.001 0.147 0.007 -0.141 -0.141 -0.018 -0.011 0.001

3.258 -0.119 2.913 2.088 1.659 -0.059 1.118 -0.049 1.105 -0.866

3.73 2.92 -2.85 -2.21 2.07 1.58 1.32 -1.24 -1.18 1.18

syn Conformation 5757 0.207 5656 0.020 0.207 5717 0.107 4747 -0.023 5 6’ 5 6’ 2626 0.001 0.000 1616 4646 0.011 4727 0.107 5616 0.020

-0.001 0.019 -0.001 0.000 0.003 -0.141 0.145 -0.006 0.000 0.019

0.207 0.020 0.003 0.107 -0.023 0.001 0.000 0.011 0.006 0.000

-0.001 0.019 -0.010 0.000 0.003 -0.141 0.145 -0.006 0.009 0.145

3.266 1.643 -0.079 2.823 1.665 1.113 2.085 1.096 0.082 -0.020

-4.43 -1.11 -0.89 -0.72 0.67 -0.56 0.52 -0.58 0.43

*ib,C

‘iqjb

Jia,jb(HZ)

5.90

aOnly the first ten main through-space coupling pathways are listed. bThe numeration of the LMCs used is as follows: 1, B(CH); 2, B(CC); 3, n system; 4, B(PC); 5, LP(P); 6, AB(CH); 6’, AB(C,H); 7, AB(PC).

(2) Unlike the syn conformation, in the anti conformation AB(PC) fully reaches the CZposition. As will be shown, this fact is crucial in assuring a good P-C coupling transmission in the anti case. (3) The n system is somewhat delocalized over the phosphorus envhonment, mainly in the anti conformation. This could be reproducing a physical fact: a charge transfer from the II system to the phosphorus electronic surroundings could be one of the reasons explaining the great difference between the phosphorus chemical shift in both conformations (experimental values are SF:“= 26.5 ppm and a$?‘”= 100.8 ppm [20]). A detailed discussion of the coupling mechanisms as given by the Jhjb terms is carried out for the anti conformation. They are shown in Table 2, where it is seen that major contributions to the TS FC coupling come from the JS7.27, J57,57 and J47,47terms. These terms involve only the vacant MO that corresponds to the P-C(Me) antibonding orbital (AB(PC)). The largest two terms involve LP(P), showing that the P lone pair is an efficient means by which to transmit the spin formation, in spite of its unfavourable orientation, as was previously pointed out by Goldwhite et al. [ 31. J57.27 also involves

_--

_-11111~~~~~~~_______--_____~~~ 11~1___________________11

_ _ __ ~~~1~~~~~~________--____~~,~

____------_________________________

__________-____________----________

_________________-______----________

____

I I----_______-----

ti

t

________

_________

_________

__________

-__---_-__

-___--_-_

H

______________________.

---_____~_________________--____

I ~...............................t.....................................................................X

Fig. 1. Plot of the P lone pair (LP(P)) MO amplitude for compounds (a) Ia and (b) Ib, at the symmetry plane of the molecule. Symbols used are: +, 0, 2, 4, 6, 8,* for positive amplitudes in order of increasing value; -, 1, 3, 5, 7,9, = for negative amplitudes in order of increasing absolute value.

H

. ... . . .. .. . .. .. .. . . .. .. . .. ...c.........-------------------------.....................x

_____-______________--________--______

_______-_____-______----_________________

-___----________1 B

--------------

+**++t*+ ++tt*t+*+*++ __________ ++++++++++++t+*+

l t**++*+*+**.+*

rrr+++*+,r+~+**+++

. .

z

(a)

l **~+++*80000000~**~++*‘~+.

.____________ _________--______ tt+t+** _-_--_______________ ++++*+tt++t+*+++ _____________________-+,+++.+t+++++tt+++tt* ______________-_______--,++++t*t++++++++*~+.++** _______________________--++++tt++t++t**++ttt+++++++

L

(b)

Fig. 2. Plot of the P,-C, bond (B(PC)) MO amplitude for compounds (a) Ia and (b) Ib at the symmetry plane of the molecule. See caption to Fig. 1 for symbol convention.

. . . .

*.++t+*++**+ ++*t++t+++++t+**t* +**~~*+~+t+t**++*+*+++ ++~+t~+~*+++t+~+*,++**~**+ +~*~+++,+*+~+*+t*+t***+,++ ++*++++t*+++*+++t.t+*++++*

W

II

1

. .. . .

.

.x

l

+++++*+*++

+tt*+++++++++

I H

-

ttk+++*++++t+t+r++.+‘

+++t+++t,

++t+t++r.++++t+++*+* ++++r++t++*,++.t++ +t+r++r++t+<+++

-___-_ ++ttt*+*++++t

Fig. 3. Plot of the P,-C, antibonding MO amplitude (AB(PC)) for compounds (a) Ia and (b) Ib at the symmetry plane of the molecule. See caption to Fig. 1 for symbol convention.

H

+*tt++

++*:+++ ++t**+

... .. .. .. . .. ... .. .. .. .. ... ...c..----.......................................

I

L

lb)

I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..C.......................................................................~

-:\r::r:_+*++++++t+++++++* *,+++*+t++*

l

_1__ +++*+.~.t+t~+**++~~+**++ ++<.t+++*++++tt+++++++

___ --_________---_ ____-______-_----__-_____-______-_---------_______________________--___ . __-_____--___-__-----------__--

L

(a)

11 e~

U

N ÷..÷÷÷.**~:÷~

÷ ++ ÷÷ ~

÷÷+

:÷÷÷÷~.÷ i

++÷+

÷++

÷+÷

..÷

**÷

:

* * *+ +* + * * * ÷ * + * ~ * o * o *o *o *÷ *÷ *+ * * * * ÷* + *÷ +

/ " " +++

\ ....... .......

I~ . . . . . . . . fIN . . . . . . . .

.......

11%

iii

A

I

/

l

l

l

l

= l

~

eg$~$gg gg ....

e~

. . . . . . . . . . g . . . . . . . . . .

....................

, l * r * r II

~

.....................

+**+'+OO~NNNNNNNOOO+++*++

~

÷ + ÷ ÷ o o o = o o o ~ = + + + ÷ + I

t

l

l

l

l

l

l

l

l

l

N

I

I

l

l

l

T

I I I I I I l l l ~ l i l l l l l l l l l l l l I

I

I

I

I

I

I

I I I I I I I l l l I I J I I I I I I I I I I I I I I I I

I

I

I

I

I

I

I

I

l

l

I I I I I I

l

l

( l l l I I I I I t l l l I I I I I l i l

I

i

I

I

I

~

l

l

l

l

l

i

l

=

=

0

2

=

e~

N ÷ ÷ + ÷ ÷ ÷

÷ + + ÷ + + ÷ + ÷ ÷ ÷

+

+ + + + + + o o o o o o o o o + * + + + + ÷ + . . . . . . . . . . . . i

i

~

N N O 0 ~ + +

......... I ii

G~

NNNN~NOO+~+

,

÷ ÷ o o Q o o o o o ~ + + ÷ * +

i

. . . . . . . . . . . . .

•

'i . . . . . . . . . . . . . . . . . . . =

/

~

,I , ,I , ,I, , I, , ,I

I

I

I

I

I

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f

l

I I I l l ~ l l l l I I ~ 1 1 1 i 1 1 1 1 1 1 1 I I I I I I I 1 ( I

I

~

12

B(CC). The efficiency of this pathway is mainly determined by the large overlap of AB(PC) and B(CC) at the C nucleus, due to the favourable orientation of the antibonding MO, as well as the large superposition of AB(PC) and LP(P) at the P centre thus providing large perturbators in the corresponding J&j* term. As has been observed [19], this last kind-of coupling pathway is characteristic of a TS transmission mechanism and, in general, its ability to transmit the Fermi contact interaction seems to be due to two main factors: (i) virtual excitations occur from different occupied LMOs to the same vacant MO; (ii) one occupied MO belongs to the electronic subsystem surrounding one of the interacting nuclei and the other one belongs to the electronic subsystem of the other nucleus, thus yielding large perturbators. On the other hand, all MOs involved in J 5,,5, and J4,,4, belong to the phosphorus electronic environment and their large values are mainly due to a very large associated propagator. This fact is characteristic of “diagonal” polarization propagator matrix elements P,, [19]. It can be interpreted as meaning that second order energy effects are most important when the perturbation induces virtual excitations from only one occupied MO to only one vacant MO. J16,+ and J56,56 involve the CZ-H LMOs. It is worth mentioning that besides AB(PC), AB(CH) is the only vacant MO that yields contributions to the TS coupling over the 10% limit. This means that this vacant MO extends itself up to the phosphorus position. J56,56 constitutes a coupling pathway involving AB(CH) and LP(P), while J 16,16involves B(CH). This shows the existence of a coupling pathway through the rear lobe of the C-H bond, as could be expected from an intuitive viewpoint. It is also interesting to analyze the influence of the CZ-C3 71system in the coupling transmission. It might be expected to play a significant role due to its proximity to the phosphorus nucleus. However, when a J calculation is performed excluding this MO from the LMOs set, a TS FC coupling of 6.42 Hz is found. This means that the n system contributes only 0.90 Hz. As is well known, owing to the nature of the 71orbital, the overlap between it and any other MO at the C position is zero, yielding nil-associated perturbators. There are two ways in which the 71orbitals may manifest themselves in these couplings: (i) through the u-71 exchange terms in the polarization propagator calculation; (ii) through the overlap of the R occupied or vacant MO with another MO at the P position, where the perturbator may not be zero. The influence of the first mechanism is in fact the most important one in this case. J 3,,1, could be mentioned to illustrate the second mechanism, but the magnitudes of all terms of this kind are negligible. In a similar way, the main coupling pathways for the syn conformation could be discussed. Nearly the same mechanisms are found to be important in defining the coupling in this case also, as seen in Table 2. However, the relative importance of each Jbjb term differs from one conformation to the other, depending on the orientation of each LMO involved. As will be shown, not only the orientation of the P lone pair but also the orientation of AB(PC) is crucial in defining the TS coupling in both cases.

13

The main contribution for both conformations is J5,,5,. In this coupling pathway two major factors are in competition for conformations syn and anti, namely: (i) orientation of AB(PC), which extends itself up to the C2 nucleus in the anti conformation, providing large perturbators at this center; and (ii) the orientation of LP(P) which favours the syn conformation. This later fact results in a larger J57,57 value for the syn conformation. The drop of the TS value for the syn conformation arises mainly from its second most important coupling pathway: J56,56 consitutes a negative contribution for the syn conformation, but is positive for the anti conformation. This change of sign can be attributed to an orientational effect of the P lone pair, which is expressed as a difference in the corresponding perturbator sign. When the rear lobe of the P lone pair is oriented towards the C2 site (as in the syn conformation) it is seen that the 56 perturbator has the same sign over both P and C nuclei. This comes from the fact that the s coefficients of the LCAO expansion of LP(P) are positive in both cases, generating a Js6,+ term which is negative. The opposite is observed for the anti conformation, with the LP(P) main lobe closer to the CZ position. In that case 56 perturbators over both nuclei are of different sign, due to different signs in the corresponding s coefficients. It is the phosphorus s coefficient which changes sign from one conformation to the other. Since LP(P) is expressed as an LCAO, for its orientation to change it is necessary that the coefficients describing it change. This is achieved mainly through a change in the sign of the s coefficient and a switch to different p orbitals which are important in its description. This can be confirmed by comparing Figs. l(a) and l(b), and noting the change in sign of the main lobe. Moreover, the second most important term in defining the syn-unti difference in the coupling value is J5,,2,, which falls off drastically from the anti to the syn conformation. This is again due to the orientation of AB(PC) in the former which provides a large overlap of this antibonding MO with B(CC) at the C2 nucleus site and increases the efficiency of this coupling pathway. This can be seen by comparing the corresponding polarization propagator elements. The TB components follow the same trends as the TS ones: they are drastically reduced in the syn conformation, making the total coupling small and negative. This behaviour would indicate that similar mechanisms to the TS ones could be responsible for the difference in flB(P,-C,) between both conformations. CONCLUSIONS

As the experimental trend of J(P,-C$) in the anti and syn conformations is closely reproduced by the theoretical approach employed here, the chosen model compound is adequate for a deeper analysis of the factors influencing that stereochemical behaviour. This deeper analysis involved the decomposition of that coupling in through-bond and through-space contributions. As in

14

this coupling constant, the Fermi contact interaction was found to be the main one by far; non-contact contributions were not further analyzed. Besides, the through-space component of the Fermi contact terms was analyzed in terms of eqn. (l), which involves two occupied and two vacant localized molecular orbitals. Each term of the equation was considered to be a through-space coupling pathway. The following results are worthy of being summarized. For both syn and anti conformations, those through-space coupling pathways involving the phosphorus lone pair were found to be the most important ones. Equation (1) explictly shows the important role played by the vacant MOs in the spin-spin coupling transmission. This importance is seldom stressed with enough emphasis. In the case under consideration the orientation of the antibonding P-C(Me) orbital (AB(PC)) is fundamental in defining the throughspace transmission of the J(P,+&) coupling constant. The only other vacant LMO involved in a through-space coupling pathway, with a contribution of more than 10% of the total TS component, is the antibonding C&-H orbital (AB(CH)). This contribution can be interpreted as a coupling pathway through the rear lobe of the C-H bond. Nearly the same mechanisms operate for Gg(P7-C2) in both conformations, although the relative importance of each Jhja term changes from one conformation to the other, depending on the orientation of the LMOs involved. When going from the anti to the syn conformation, two main effects are present: the transmission through the B(CH) rear lobe is negative and compensates the positive component yielded by J57,5,, and the unfavourable orientation of AB(PC) inhibits the J57,27pathway. TB components show a trend similar to the TS ones, being drastically reduced for the syn conformation. This behaviour finally makes the total coupling small and negative. It would be interesting to test this prediction by experimentally determining the sign of the couplings. ACKNOWLEDGMENTS

A grant from the Argentine National Research Council (CONICET) is acknowledged. The authors would also like to thank IBM Argentina for the computational time provided at the Centro de Computes Buenos Aires. REFERENCES 1 R. H. Contreras, M. A. Natiello and G. E. Scuseria, Magn. Reson. Rev., 9 (1985) 239. 2 D. G. Gorenstein, Prog. Nucl. Magn. Reson. Spectrosc., 16 (1983) 1. 3 H. Goldwhite, D. Rowsell, L. E. Vertal, M. T. Bowers, M. A. Cooper and S. L. Manatt, Org. Magn. Reson., 21 (1983) 494. 4A. Zchynke, C. Miigge, H. Meyer, A. Tzschach and K., Jurkschat, Org. Magn. Reson., 21 (1983) 315. 5 R. A. Palmer and D. R. Whitcomb, J. Magn. Reson., 39 (1980) 371.

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