Effects of fluorination on conformation and bonding in diphenyl sulphides

Journal of Fluorine Chembtry, 48 (1990) 45 -62 Received:

45

July 28, 1989; accepted February 3, 1990

IN

E2 IN DIPHENYL

NEIL

SULPHIDES

GOODHAND

Department

of

AND

Chemistry,

THOMAS

A. HAMOR

University

of

Birmingham,

Birmingham

Bl52Tl’(LJ.K.)

A survey of the solid-state structures of substituted diphenyl sulphidu shows that when there are no more than three &

substituents the molecules predominantly adopt

a conformation in which the ring bearing the more electron-withdrawing substituents is oriented approximately parallel to the central C-S-C approximately perpendicular to this plane.

plane and the other ring is oriented

Theoretical calculations indicate that this is

due to an electronic effect involving some conjugation between sulphur and the more electron+thdrawing

ring.

In fluorinated compounds interaction of one of the e

fluoro mubstituentswith the r-electron

cloud and carbon 1 of the other ring, which

carries a small negative charge, precludes this conformation, the preferred conformation having both rings approximately equally inclined to the C-S-C 550.

plane at an angle of ca,

The bond angle at sulphur appears to be affected by the nature of the ring

substituents. electron-withdrawing the C-S-C

groups (F, NO2) causing a small decrease in the site of

angle.

0022-l 139/90/$3.50

0 Elsevier Sequoia/Printed

in The Netherlands

46 INTRODUCTION

The and

diphenyl

by theoretical

the overall with

et

the ring

Coulson,

relative

nearly

perpendicular

we found

as

of the nine the twist

the other

while

diphenyl

diphenyl

structure

of unsymmetrical

torsion

angle,

in most

skew conformers Since been

where

1979.

cases

determined

In general

previously,

these

[9] where ring)

both

and

of these

rings is

2).

Hamor

[6]

is the

angles

in each

the dihedral 31°

carrying

plane.

68 and

angles

27O

with respect

(electron-withdrawing

is quite

of some

Exceptiow

small,

fifteen

compounds

or quite

additional

derivatives

conform adopting

to

ring).

an electron-withdrawing

If the dihedral

angles

case by the sign of the smaller

new structures

fluorinated

when

conformation

[7] (see Fig.

[8], dihedral

bis(tetrafluoro-4-nitrophenyl)

these

This

by Goodhand One

and

the ring

of the fluorinated

and

the above [7] even

the two phenyl

plane. et

listed

signs are the same.

structures

those

sulphide

established

given

one of the angles

tetrafluorophenyl) [2].

are

adopted Gal.,

wherein

skew.

sulphide

maleate

substitution,

the crystal

[l ] and

reported

sulphides

at < 310 to the C-S-C signs

3dxy

oriented

of N-(3dimethylammoniopropyl)-

(electron-donating

are 660

their

has,

1) , bis(2,3,6-trifluoro-

to the C-S-C

4,4’-diaminodiphenyl

the C-S-C

and

which

the empty

ring

generally

Fig.

towards

sulphide

be < 90°,

and

the interaction

ring

Further,

of the other

nearly

Vaciago

favour

of the parallel

of van der Heijden

but tending

is in the crystal

is oriented

would

conformation,

angle

2-amino-4-chlorodiphenyl

In all cases

factors,

electron-donating

bear

Domenicano,

sulphides

(see

a different

non-fluorinated

substituent

of steric

might

* by van der Heijden

as ‘skew

the same

conformation,

plane

group

by van der

is oriented

substituents.

symmetrically

adopted

symmetrically-substituted

C-S-C

substituents

conformation

with the II system

‘twist’ in the nomenclature

have

thii

[I,21

to study

It was noted

in the absence

to this plane.

that

has been

of the central

atom.

which

with the II system

[6] that

at approximately

Two

ring,

crystallography

plane.

substituted

sulphide

oriented

ethers,

electron-withdrawing

interact

described

were

4-nitrophenyl)

denoted

ring,

conformation,

the compounds

and

perpendicular

to the C-S-C

In 1979 mentioned

the geometry

of the sulphur

with the other

of sulphur

could

by X-ray investigations

electron-withdrawing

suggested

3px orbital

studied

of these

and

state

more

plane,

oriented

of sulphur

object

like the corresponding

the relatively

to the other

orbital

[3,4].

[3] independently,

of the filled

extensively

The

for the bonding

to the C-S-C

substituents,

been

of the molecules

[5] that,

bearing

parallel

have

methods

conformation

its implications

Heijden

are

sulphides

occur close

are taken C-C-S-C

only

in a few

to 90°.

diary1 sulphides

have

bis(4+ifluoromethylsulphide

have

been

to the conformational twist conformations.

patterns

to

Fig.1. Stereoscopic view of the skew (90,O) conformation of a model of the diphenyl sulphide molecule viewed in a direction rotated 10° from the perpendicular to the Cl -S-Cl ’ plane.

. 2

S

S

Fig .2 .I Stereoscopic view of the twist (61.61) conformation of the bii(2,3,6-trifluoro-4-nitrophenyl) sulphide molecule viewed in a direction perpendicular the Cl-S-Cl’ plane; both rings are inclined at 610 to the Cl-S-Cl’ plane.

to

In the present paper the results of analysing the energetica of diphenyl sulphide conformation

by molecular obital (MNDO). [lo] molecular mechanics 111] and simple van

der Waala non-bonded

interaction

[ll] calculations are reported,

to the effects of fluorination on bonding and conformation.

with particular reference

48

RESULTS AND DISCUSSION (a)

The experimental

data

In Tables la and 1b are listed the 28 diary1 sulphide systems used in the present analysis together with the references (l)-(18)

to the crystal structure determinations.

Compounds

(Table la) are unsymmetrically substituted compounds, so that a distinction can

be made between rings bearing electron-withdrawing

and electron-donating

substituents; or

one ring may be more electron withdrawing or more electron donating than the other ring.

Under Q and /3 are listed the angles (defined to be < 9Oo) which the rings

bearing respectively the relatively more electron-donating withdrawing substituents make with the C-S-C by the respective C-C-S-C in Fig.

3.

torsion angles.

The corresponding

plane.

and the more electronThe signs of o and 6 are given

Compound (18). Q = 90, fl = Oo is depicted

C-S bond lengths and C-S-C

angles are also listed.

Compounds (19)-(28) (Table lb) are symmetrically substituted, (26)-(28) having fluorine substituents in the four ortho positions.

Here there is no distinction betwen the rings

and the dihedral angles are listed in order of size.

Stereoscopic view of the skew (90,O) conformation of 4-nitro-4’-aminodiphenyl Fig. 3. sulphide [compound (18)] viewed in a direction rotated 100 from the perpendicular to the CI -S-Cl ’ plane.

The dihedral angles for these 28 compounds are plotted in Fig. 4. horizontal axis represents angle.

The

the larger of the two angles and the vertical axis the smaller

It can be seen that the unsymmetrically-substituted

compounds fall predominantly

into the skew classification; only five are *twist* and four of these are fairly close (movements of o and /3 < 100) to the twist/skew boundary.

In all cases except five,

49

TABLE 1 Selected structural parameters of diphenyl sulphides as determined by X-ray crystallography. Bond lengths are in A. angles in degrees. Listed are the dihedral angles between the C-S-C plane and the two phenyl rings, and the C-S bond lengths. The dihedral angles are taken to be. < 900 with sign defined by the relevant C-C-S-C torsion angle. Multiple entries occur when the crystal contains more than one independent

molecule.

(a) Unsvrnmetricallv substituted co Doundg. plane and the relatively more elect&-donating angle involving the other ring is denoted 6.

The angle between the central C-S-C ring is denoted a. The corresponding

a

6

s-CB

s-c& c&-s-co

12

75

6

1.781

1.771

104

(2)

13

74

-2

1.784

1.777

103

N-(3-dimethylammoniopropyl)-2amino-4-chlorodiphenyl S maleate

(3)

9

66

31

1.786

1.767

103

1,4-bis(phenylthio)benene

(4)

14

57

14

1.777

1.783

105

(3)

15

a7 79

-6 -14

1.778 1.768

1.781 1.774

102 105

2-(2-nitrophenylthio) e

(6)

16

82

4

1.757

1.782

103

Methyl 2-(2-nitrophenylthio) phenylacetate

(7)

16

68

16

1.768

1.785

103

Compound

(S denotes

’ sulphide

‘)

Ref.

(1)

N-(3-dimethylannnoniopropyl)-2,amino-2’ ,4-dichlorodiphenyl sulphide maleate

N-(3-dimethylanreoniopropyl)-2amino-2’-chlorodiphenyl maleate

S

2,4-bis(phenylthio)nitrobenzene

Methyl benzoat

(continued)

TABLE 1 (cont.> 2-nitro-1,3-bis(phenylthio) benzene

(8)

17

85 83

-13 14

1.776 1.777

1.786 1.781

102 101

Methyl 2-(4-nitrophenylthio) benzoat e

(9)

18

6

80

1.780

1.783

102

2-diazoacetyl-4’-nitrodiphenyl S

(10)

18

32

46

1.766

1.777

102

hexakis(phenylthio)benzene S

(11)

19

56

25

1.772

1.769

103

(12)

4

85

-5

1.769

1.786

104

4-nitro-4’-dimethylaminodiphenyl S

(13)

20

69

2

1.774

1.791

106

2,4-dinitrodiphenyl

(14)

21s

77 86

4 12

1.756 1.751

1.787 1.784

103 103

2lk

76 86

4 12

1.758 1.743

1.775 1.785

103 102

(15)

5

87

4

1.758

1.784

103

(16)

20

84

-2

1.778

1.780

104

(17)

22

67

39

1.784

1.775

101

(18)

23

90

0

1.771

1.780

104

4-dimethylaminodiphenyl

S

S

2-(4’-carbomethoxy-2’-nltrothiophenyl)-1,3,5-trimathylbenzene

4-nitrodtphenyl

S

N-(3-diethylammoniopropyl)-2amino-4-chlorodiphenyl sulphide oxalate

4-nitro-4’-aminodiphenyl

S

(continued)

51

TABLE 1 (cont.> (b) Svmmetricallv substituted cornDow&. The angles batwan phenyl riqa are liited in order of magnitude.

Ref.

2,2’-thiobis(4-methyl)-6-tbutylphenol

Dihedral

the C-SC

plane and the

angles

C-S Bond lengths

c-s-c Angle

(19)

24

68

68

1.780

1.780

105

4,4’-diaminodiphenyl

S

(20)

8

27

68

1.808

1.785

104

2,2’-dinitrodiphenyl

S

(21)

16

20

65

1.768

1.777

101

(22)

25

4

89

1.778

1.764

103

(23)

26

7

79

1.776

1.776

103

(24)

16

22

82

1.785

1.787

102

(25)

27

32

39

1.75

1.74

110

(26)

6

61

61

1.772

1.772

100

(27)

2

47

50

1.757

1.768

102

(28)

2

54

61

1.768

1.773

100

bis(3,4-dimethoxyphenyl) s

2,2’-dimethyldiphenyl

S

dimethyl-2,2’-thlodibenzoate

4,4’-dlmethyldiphenyl

S

bis(2,3,6-trifluoro-4nitrophenyl) S

bis(4-trifluoromethyltetrafluorophenyl)

bis(tetrafluoro-4nit rophenyl) S

S

0

Unsymmetrically

*

Symmetrically

*

Symmetrically substituted fluorinated compounds

Larger

Fig.

I.

substituted substituted

interplanar

compounds compounds

angle

(degrees)

Conformations of diphenyl sulphides. Plot of the dihedral angles listed in Table 1. The numbers refer to those in Table 1. With the exception of compound (8), multiple entries in Table 1 are represented by mean values.

53

the signs of (Y and 6 are the same. quite small, maximum 14.00.

In all of these five compounds one of the angles is

Of the ten symmetrically-substituted

compounds (19)-(28),

three are skew, one lies on the twist/skew boundary, five are twist and one has a steep twist conformation, conformation.

described by van der Heijden et al. [5,7] as the ‘butterfly’

Thus for the symmetrically-substituted

towards a twist conformation, compounds.

rather than the skew conformation

of the unsymmetrical

It is noteworthy that all three fluorinated compounds have a near ideal

twist conformation, the C-S-C

diphenyl sulphides the trend is

with the two phenyl rings inclined at approximately

equal angles to

plane.

Considering the unsymmetrically substituted compounds,

in every case except two

[compounds (9) and (lo)], the angle CY,defined above, is larger than 6. the 18 compounds

/I is less than 200 so that the electron-withdrawing

to be nearly parallel to the C-S-C further four have (Y > 650. the orientation

In 13 out of

ring can be said

plane, and in 10 of these 01 is greater than 70°; a

The prediction of van der Heijden et al. [5,7] regarding

of the rings in unsymmetrical diphenyl sulphides, as exemplified by

compound (18) (Fig,. 3), is therefore,

confirmed.

In the two exceptions to this pattern of orientation, electron-withdrawing

substituents.

both rings bear

Compound (9) has a nitro group in one ring

competing with an almost-as-strong

electron-withdrawing

carboxylic ester group in the

other ring, while (10) has one ring bearing a nitro group and the other ring a diaxoacetyl group. electron-withdrawing

(b)

The distinction between the rings regarding their relative power is, therefore,

rather finely balanced in these compounds.

Theoretical considerations In terms of simple valence bond theory one can picture the electron distribution in

e.g. 4aitro4’-aminodiphenyl

sulphide as involving conjugation between the sulphur lone

pair and the oxygen atoms of the electron-withdrawing bond (S-C6 in our convention)

The SC(PhNO2)

attains partial double bond character thus forcing the ring

to orient parallel or nearly parallel to the C-S-C the other rhtg to a near-perpendicular amino-substituent

nitro group.

orientation.

plane.

Steric effects would then drive

The presence of an electron-releasing

in this ring, however, enhances the electronic effect of the nitro group.

Inspection of S-C bond lengths (Table la) shows that in 13 out of the 18 compounds,

the bond to the more electron-withdrawing

to the other ring.

ring, S-CS, is shorter than that

Mean values are S-C6 1.773(2)A, S-C,

1.78O(l)A*, a small but

* Values in parenthesis are the estimated standard deviations in the mean.

54

significant Four

difference

pointing

of the five exceptions

compounds with

(l)-(3)

respect

cationic

and

(17),

to the C-S-C

compounds

to the relevance to this pattern

are

despite plane

excluded

S-C,

become

1.769(2)

and

case

of S-C6

> S-Co

occurs

is only

2 shows

MNDO

diphenyl

The

the known

molecular

geometry

TABLE

postulated are

the mean

of the phenyl

respectively,

a much

(ll),

values

clearer

above.

in the cationic

with S + C6 conjugation.

rings

If the four

for S-Q

and

distinction.

The

between

the lengths

but the difference

la,

[lo]

calculations bond

at sulphur,

on amino

lengths (C-S,

sulphides. conformation

see specifically

mechanics

[ll 1, molecular

of van der Waals

of diphenyl

and

fifth

(18)

electronic

group

(12)].

the same

Van der Wash for the

factors

a greater

which effect

2 parameters Energies

for aminoand are in It.7 mol-l

nitro-substituted

4-NO2-4’

a-00,

sulphide

4-NO2

model

4-NH2

p-900

Van der Waals energy Molecular mechanics energy MNBO enthalpy of formation Bond order S-C6 Bond order S-Co

(b) a-900,

-NH2

diphenyl

1144 137 349 0.972 0.973

1023 148 324 0.972 0.978

1144 136 341 0.994 0.977

1023 147 319 0.993 0.975

940 109 267 0.973 0.975

p-00

Van der Waals energy Molecular mechanics energy MNBO enthalpy of formation Bond order S-C@ Bond order S-C,

method

a = 900,

energies

steric

having

from

by the MNDO

calculated

than

the

103O) is derived

(16) and

rather

the nitro

[I1 ]

nitro-substituted

in the solid state,

give essentially

of the rings,

mechanics

in modelling

C-S-C,

Enthalpies found

and

angles

1.774A.

compounds

calculations

It is, therefore,

the orientation

Calculated molecules.

(4

effect

differences

the orientations

the averaging,

in compound

standard

the predominant

two conformations. influence

orbital

using

structures

favour

6 = O” [Table and

that

are consistent from

1.782(1)A,

the results

molecular

sulphides

molecules.

clearly

the fact

length

0.003A.

Table and

of the conjugative

of bond

940 110 265 0.981 0.975

than

55 the

amino group.

in the preferred

Thii is further indicated by the higher bond order of the S-C6 bond conformation.

All three types of energy calculation, however, show

lower energies for twist or butterfly conformations 4-nitro4’-aminodiphenyl

with cr = (3. For

sulphide, van der Waals energy calculations show a minimum of

1138 kJ when OL= 6 = 65O, the molecular mechanics energy reaches a minimum of 115 k.J when OL= 6 = 32O and the MNDO enthalpy is at a minimum of 329 kJ at Q = 6 = 900.

In agreement

with experimental

results, conformations

of type a = -6, when (3 is

in the range 20-700 have higher energies. Figure 5 shows the conformational

enthalpy space with respect to dihedral angles

between the aromatic rings and the C-S-C

plane calculated for an idea&d

diphenyl

sulphide molecule by the MNDO method.

Axes are as for Fig. 1; contours are drawn

at 8 kJ intervals with the lowest enthalpy taken to be zero (both dihedral angles 900). The experimental

values for the symmetrically substituted diphenyl sulphides liited in

Table 1 are plotted on thii diagram using the same conventions as for Fig. 1. the experimental

All

points lie no more than 24 k.J above the global minimum, 225 k.J*.

There are, however no experimental

points close to the minimum, the closest being

compound (19) (dihedral angles both 680). The conformational

behaviour of the three fluorinated compounds (26)~(28) differs

from those of the other compounds in that none of them adopt the skew conformation, and all are close to an ideal twist conformation

with equal dihedral angles.

Figure 6 is

analogous to Fig. 5, but the MNDO molecular orbital enthalpy calculations are based on decafluorodiphenyl

sulphide.

Here the global minimum is displaced from 90.90 to

approximately

70.70 and the enthalpy rises very much more rapidly away from the

twist/butterfly

equal angles region, but the overall picture is quite similar.

lines representing

The contour

the 80 and 120 kJ enthalpy levels have been included to emphasize the

qualitative similarity between Fig#s.5 and 6. Thus the enthalpy at 90,O is 88 k.J above the global minimum, compared with only 16 kJ for the unfluorinated

analogue.

The same trend appears also in simple van der

Waals energy calculations, the energy at 90.0 being repectively 5 W and above the global minimum for the hydrocarbon and the fluorocarbon. a conformation

and 30 kJ

It is evident that

close to 90.0 adopted by many diphenyl sulphides (see Table 1 and

Figs. 4 and 5) would be energetically unfavourable for fluorinated diphenyl sulphides, even if one ring is strongly electron-withdrawing

relative to the other ring.

* This value may be compared with the experimental [28] enthalpy of formation of diphenyl sulphide, 231 f 3 kJ mol -1. The structure of diphenyl sulphide has been studied by gas-phase electron diffraction. Two twist conformations with C2 symmetry and dihedral angles of 43 and 56O fit the experimental data best [29]. No crystal structure analysis for diphenyl sulphide is, as yet, available [l].

56

60-

30

Fig. 5.

60

Conformational enthalpy contour map for diphenyl sulpbide. Conformational parameters are the two dihedral angles defined in Table 1. Enthalpies calculated by the MNDO molecular orbital method. Contours at 8. 16, 24, 32 and 40 kJ above global minimum.

90

57

Fig. 6.

Conformational enthalpy contour as for map for 5. decafluorodiphenyl sulphide. parameters Fig. Contours at 8, 16, 24, 32, 40, 80 and 120 id above global minimum.

58

That

the skew conformation

compounds

does

substituents. both

not appear

The

closest

is energetically

F . . . F distances

A, and at least 0.25 A greater

3.25

unfavourable

to be due to repulsive

for the fluorinated

interactions

between

are F2 . . . F2’ and than

the &

F2 . . . F6’ (see Fig. 7).

the sum of the van der Waals

Fig. 7. The skew (90.0) conformation of a model of the decafluorodiphenyl molecule showing the close contact between F2 and Cl ‘.

Of significance atom

is the m which

F2 lies above

the

the interaction 0.18

e) with

0.26

e, which

fluorine

longer

plane

the

of the in-plane

svnoeriolanar

of thii

r-electron

makes

ring

at 2.37 close

show

a positive

A.

Cl’.

The

minimum,

charge

MNDO

difference

with Cl’,

The

here

of 0.08

the

e on the hydrogen

enthalpy

for this (90.0) conformation

found

(Cl’

charge

carrles

to the

in Fig. 7). It is presumably

calculated

a negative

by MNDO,

charge

of

To confirm

the role of this

in our model

of the fluorinated

HZ . . . Cl

the favourable

and

the butterfly to that

short

we have cloud

atom

ring

A from Cl’.

(negative

unfavourable.

corresponding

*-electron

of 2.24

which

sulphide

(F2 in Fig. 7) lied

of the other

atom

it with a hydrogen

However, b(-)

fluorine and

ring

to Cl

at a distance

conformation

system.

to the

global

thii

cloud

we replaced

sulphide

atom

enthalpy

F atom

is oriented

of the electronegative

atom,

diphenyl

radii [30].

F6

!=6

carbon

fluoro

a(-) and

’ distance

situation

Cl’.

is only

of a a(+)

The

MNDO

a negative

charge

slightly hydrogen

calculations of 0.27

skew conformation

is only

17 kJ above

with both

angles

700,

for diphenyl

sulphide

dihedral itself.

e on

a similar

the

59

Studies

by Gould

interactions

between

conformation

of diphenyl

be noted

and

have

additional

r-electron

cloud

to non-bonded

derived isolated

only

only gas-phase

[29].

The

and

postulated

both

in

and

with

theoretical

(c)

The

bond

whose and

crystal

accuracy] angle

angle

study

paper

this angle

effect

(43,43)

were

known

ranged

from

la

sites may

forces

may

6(+) hydrogen

to predict

enthalpy

above

would

of the observed conformation.

are

undoubtedly

that

the experimentally

by theoretical be considered

[33] is that

as

of diphenyl

for

on the

‘noise ‘, probably

(see footnote

results

affected

calculations

As far as we are

conformations

the

In the

sulphides.

(90,90)

..,

method,

an interaction

however,

rationaliied

directed

in a smaller

of the additional are

now known

that

whilst

aware,

the

sulphide on page 55)

related

molecules

inducing

towards

Cl and

angle

mean

unfluorinated

(3),

parameters

to 105.60,

[6] in terms

bond

(l),(2),

structural

rings

in bis(2.3,6-trifluoro-4-nitrophenyl)

in the nine

[compounds

102.9

orbitals

structures

fact,

was 99.7O.

(25) for which

sulphur

Inclusion

om

of this type,

butterfly

solid-state

we noted

of the fluorinated

crystal

atom.

in Tables

of the MNDO

2). such

discussed

The

or (56.56)

at sulphur

40 was rationallied

resulting

listed

diphenyl

on conformation.

with

electronegatively

sulphides,

by a hydrogen

the other

the calculated

of a diary1 sulphide

[6],

angle

structures

bonding

to the

at sulphur

but excluding

by some

energy

8 kJ mol-*

calculations.

the bond

(23).

by which

packing

agreement

In our earlier sulphide,

that

twist

good

in the skew

interaction

due to the

for most

of the predicted

be largely

random

structural

are

although

(see Table

forces.

can

indicates

a small

that

packing

conformations

molecules,

having

occupied

interactions

of the diary1 sulphides

by crystal

crystal

that

as in the skew conformation

for the failure

attractive

sulphide

exceeds

conformations extent

position,

conformation

for ~g 8 of the 12 kJ mol-l

The

21$& position

be the reason

as the preferred

[23] skew conformation

to some

are oriented

of the skew conformation may

of 4-nitro4’-aminodiphenyl

account

sulphides

Non-bonded

with the skew conformation

at this m

stabiliition

is not sensitive

case

of diphenyl

shown

atoms.

interaction

skew conformation

[32] have

of approximately

rings

&l the compounds

by other

Petsko

structures.

contribution

the aromatic

atom

and

to those

crystal

with the significant

that

a hydrogen

be occupied

which

similar

in protein

when

sulphides,

It may

The

rings

by Burley

1321 lead to an enthalpic

of the system

lb

[31] and

aromatic

are common

calculations stability

et al.

(12), are

103.70.

The effects

degree

Cl’, compared

(15).

of relatively

of rehybridixation a greater

(13).

diary1 sulphides (16),

(20)

low

closing [34],

up of this the greater

of p-character

in the

with the unfluorinated

at sulphur.

two fluorinated

and fifteen

gives for the angles

unfluorinated

at sulphur

a range

compounds of 99.7

whose -

60

102.0,

mean

103.2

100.5

(8)o in the fluorinated

(3)o in compounds

(l)-(24),

compounds

have

that

in the fluorinated

found

angles

one or two &

sulphur

This

as was postulated be expected

pattern

angle

calculations.

Thus

(70,70)

conformation,

the angle

in the butterfly and

There mean

(70.70) lOS.Oo,

angle

a smaller

compounds

mechanism

and

another an

therefore,

so that

than

either

contains

may,

compounds,

angle

contain

be

the angle

at

sulphide

refines

(9090)

is, however,

not replicated

in its optimum

to 106.80,

whilst

conformations,

by

butterfly

for diphenyl

the angle

sulphide

at sulphur

refines

respectively.

is no correlation

C-SC

at sulphur

at sulphur

optimum

-i.e.

small.

for decafluorodiphenyl

and

mean

- 105.6,

Five unfluorinated

in one ring

of these

[6] for the fluorinated

variation

100.8

however,

substituent

None

rehybridixation

to be relatively

of bond

of these,

nitro

ring.

A similar

MNDO

to 105.6

or a a

and

difference. - 101.80,

100.8 Four

(27).

in the other

substituent.

here, might

substituents

(26)-(28)

less clear-cut

in the range

compound

group

electron-donating operative

at sulphur

nitro

electron-withdrawing

compounds

a much

between

is 1030

for both

conformation

and

the angle

at sulphur.

Thus

the

the twist and the skew compounds.

ACKNOWLEDGEMENT We thank

the University

of Birmingham

for the award

of a Research

Studentship

to

N.G.

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