Pronounced conformational preferences in 1,4-disubstituted but-2-ynes

Structure, 443 (1978) 259-270 @Elsevier Scientific Publishing Company, Amsterdam -

Jounzal 0fMolecukrr

PRONOUNCED 1,4-D~~ST~~D

LEO RADOM,

CONFOBJklATIONAL BUT-Z-YNES

PETER J. STILES and MARK

Research School of C~e~~~~, (Australia)

Australian

Printed in The Netherlands

PREFERENCES

IN

A. VINCENT

National

University.

Canberra, AX-T.

2600

(Received 17 .November 1977)

ABSTRACT Internal rotation in 1,4_disubstituted but-2-ynes XCH,C=CCH,Y (X, Y=H, CH,, Li, F) has been studied with the aid of ab initio molecular orbital theory. When either or both of X and Y are approximately eiectroneutral (H or CH,), the barrier to internal rotation is very small. On the other hand, when X and Y are both electropositive (LiCH,C=CCH,Li) or both electronegative (FCH,CkCCH,F), there are pronounced preferences for conformations in which the dihedrat angle XC- - mCY is slightly greater than 90”. When X is electropositive and Y electronegative (LiCH,C=CCH,F), the stable rotational isomers are cis and trams with the former being preferred_ For all molecules examined here the threefold component of the torsional potential function is very small. INTRODUCTION

Internal rotation about the carbon--carbon triple bond in but-2-yne (dimethylacetylene) and its mono- and di-substituted derivatives is poorly characterized experimentally. The early view [I] that internal rotation in but-2-yne is completely free can be discounted in the light of present knowledge concerning long-range interactions. However, the upper limit to the threefold barrier of CH3CSCCH3 is only about 0.05 kJ mol-‘. This estimate is based on data from high resolution infrared spectra [Z--4]. Microwave measurements 151 on the monosubstituted derivative I-chlorobut-2-yne, C1CH2C~CCH3, indicate a threefold barrier smaller than 0.4 kJ mol-I. On the other hand, Raman and infrared spectra of this molecule appear to be consistent with a free rotation model 161. Gas phase dipole moment [7] and electron diffraction analyses [S] for 1,4_dichlorobut-2-yne, ClCH$Z CCH,C!l, have suggested that barriers in this species are also less than 0.4 kJ mol-’ but a recent electron diffraction study [9] of 1,4-dibromobut-2-yne, BrCH&= CCH,Br, revealed a potential barrier of roughly 3 kJ mol-’ at the trans conformation, with respect to the preferred conformation in which the dihedral angle BrC -0 - CBr lies between 90” and 1009 Theoretical analyses [lo-121 of internal rotation in but-2-yne have also been carried put with the aid of ab initio molecular orbital calculations.

The threefold barrieris found to be very small and within the range O-0.03 kJ mol-‘. In this paper, we use similar theoretical techniques to study internal rotation in a set of simply substituted but-2ynes XCHzCSCHzY (X, Y=H, F, Li, CH3). We rationalize the calculated conformational preferences in terms of a qualitative theory for the interactions across the acetylenic bond. In addition, we compare our potential functions for the butS-ynes with those for the corresponding ethanes XCHzCHzY described in a companion paper 113, see also 143. METHOD

Ab initio LCAO SCF molecular orbital calculations were carried out using a modified version of the Gaussian 70 system of programs [15] and the 4-31G (for H, C, F [ 161) and 5-21G (for Li [17]) basis sets. Bond lengths and bond angles were taken to have standard values [ 18,191. For monosubstituted but-2-ynes, CH3C=CCH2Y, calculations were performed for eclipsed (@ = 0”) and staggered (@J= 60”) conformations. For the disubstituted systems, XCH,CSCCH,Y, calculations were performed on conformations defined by XC -. - CY dihedral angles, 0, of O”, 60°, 120”and 180”. For these species, we have taken 4 = 0” to be the eclipsed conformation in which the bonds XC and CY are cis. The computed energies V(G)were then fitted to the truncated Fourier expansion (1) [20] V(G) = $*(l-

cos@)+

~v,(l-cos2@)+

~v,(1-cos3~)

(1)

of the potential function describing internal rotation about the C-C=C--C axis. With the aid of (1), stationary points in the potential function were determined and additional calculations were performed on the more interesting of these. Calculated energies are listed in Table 1 and the potential constants V,., areinTable 2. Included in Table 2 are the potential constants for the corresponding substituted ethanes. Potential functions for 1,4difluorobut-2-yne, 1,4dilithiobut-2-yne and l-fluoro-4~lithiobut-2-yne are displayed in Figs. 1-3 together with their onefold and twofold Fourier components. The threefold components are omitted because their magnitudes (cf. Table 2) are insignificant relative to those of the other two terms. DISCUSSION

Prom eqn. (1) it is easy to show that the individual n-fold potential constants V, of the disubstituted but-2-ynes XCH2CSCCHzY can be regarded as linear combinations of the torsional energies at selected conformations. In particular, the V, constant can be related to the difference between the sum of the energies of the three staggered conformations and the sum of the energies of the three eclipsed conformations

,=$I

1.23 v[@i (staggered)] 1-l

i i=l

V[4$(echpsed)]j

(2)

261 TABLE

1

Calculated total energies (hartrees)a and relative energies (kJ mol-‘) substituted but-2-ynes, XCH,+C-CH,Y Molecule XCH,--C=C--CH,Y

XCCY dihedral angle cpb

CH,C=CCH, fit: CH,C=CCH,CH,, CH,C=CCH,F CH,C%CCH,Li CH,CH,C=CCH,CH,

CH,CH,C=CCH,F

CH,CH,C&CH,Li

FCIi,C!=CCH,F

LiCH,C=CCH,Li

LiCH,C=CCH,F

al hat-tree = 2625.5

0:o

60.0 0.0 60.0 0.0 60.0 0.0 60.0 120.0 180.0 0.0 60.0 120.0 180.0 0.0 60.0 120.0 180.0 0.0 60.0 103.7 120.0 180.0 0.0 60.0 113.9 120.0 180.0 0.0 60.0 102.4 120.0 180.0

for conformations of

Energy TOtal

-154.68866 -154.68865 -193.66574 -193.66572 -253.40793 -253.40792 -161.52117 -161.52116 -232.64281 -232.64281 -232.64284 -232.64283 -292.38512 -292.38507 -292.38503 -292.38499 -200.49840 -200.49835 -200.49843 -200.49848 -352.12338 -352.12468 -352.12541 -352.12531 -352.12461 -168.33997 -168.34353 -168.34598 -168.34596 -168.34497 -260.24854 -260.24585 -260.24445 -260.24467 -260.24607

Relative

0 + 0.03 0 -I-0.05 0 + 0.03 0 -I-0.03 0 0.00 -0.08 -0.05 0 +0.13 -I-0.24 i-o.34 0 to.13 -0.08 -0.21 0.0 -3.41 -5.33 -5.07 -3.23 0.00 -9.35 -15.78 -15.73 -13.13 0.0 +7.06 + 10.74 + 10.16 + 6.48

kJ nt01-~. SDegrees.

Similarly, the twofold constant V, is given b;

i.e. half the difference between the total energy of conformations with CX and CY orthogonal and the total energy of conformations with CX and CY

262 TABLE

2

Calculated but-2-ynes

potential constants V, (kJ mol-* ) describing and the corresponding ethanes

Molecule

v,

V,

CH,C=CCH, CH,-CH,* CH,C=CCH,CH,

0 0

0

0

0

CH,--CH,CH,* CH,C=CCH,F CH,-CH,F * CH,CZCCH,Li CH,-CH,Li b CH,CH,C!=CCH,C!H, CH,CH,-CH,CH,= CH,CH,C=CCH,F CH,CH,-CH,F = CH,CH,CkCCH,Li CH,CH,-CH,Li b FCH,CkCCH,F FCH,--CH,F = LiCH,CkCCH,Li LiCH,-CH,Li b LiCH,C=CCH,F LiCH,-CH,F b

0 0 0

0

+ 0.03 -13.63 + 0.04 -15.46 + 0.04 -15.18 +0.03 -13.78 + 0.04 -16.15 + 0.03 -19.54 + 0.08 -13.72 + 0.02 -17.12

0 0 0 0

-0.07 -13.34 +0.31 -2.01 -0.29 t2.12 -3.25 -19.59 -13.01 -54.47 + 6.40 + 31.88

in substituted

V,

0

0 0

internal rotation

-0.02 -5.99 + 0.03 -2.54 +0.17 + 2.61 -3.50 -11.39 -7.95

-0.12

-19.89 +7.15 + 28.84

-16.63 +O.lO -12.47

*From ref. [20]. bFrom ref. [13]. =From ref. [141-

1 0'

90' FC

CF

160' OlHEORAL

ANGLE

I

270’

3600

00

I+1

Fig. 1. Potential energy function, V(Q),and Fourier describing internal rotation in FCH,C=CCH,F.

Fig. 2. Potential energy function, describing

coplank.

internal rotation

-

V(0) -

CLi

DIHEDRAL

components,

2700 ANGLE

I+1

V, (6)= kVn (1 -

cos

n&),

V(o),and Fourier comgonents, V, (0)= kVn (1 - cosn~),

in LiCH,C=CCH,Li.

Finally, the one-fold

v, = V(180)

l60-

900 LIC.

potential constant

v,

may be expressed as the energy difference between the trans and cis conformations with a correction for the V, coefficient given by (2). In this

(4)

263

I

00

90Lit-.CF

180 DIHEDRAL

270e ANGLE

i:

? iO0

(#I

Fig. 3. Potentialenergyfunction,V(Q), andFouriercomponents,Vn (c$)= +V, (1 - cosn@), describing internalrotationin LiCH,C=CCH,F. section, we aim to investigate the principal factors influencing these potential constants which define the overall potential function V(4).The forces determining the potential constants Vn can be considered to include: (i) longrange through-space interactions between the XCH, and CHzY groups, and (ii) overlap dependent forces between orbitals localized on adjacent functional groups. Electrostatic dipolar interactions between the local electric dipole moments of the XCHl and C&Y groups provide the most important through-space contribution to the Vi coefficients of the disubstituted but-2ynes XCH2Cz CCHzY. This contribution takes the form VpiP

= _

&XCH,) &CH,Y) 2xeOR3

(5)

where pl is the component of the electric dipole of the substituted methyl group perpendicular to the torsional axis of the molecule and R is the separation between the XCH, and CHzY groups. Evidence for this mechanism m-2 ratio of the V, parameters for is provided by the approximately 4 :1. LiCH&=CCH2Li, FCHzC=CCHIF and LiCH&=CCHzF taken together with the datum that the calculated dipole moment of CH3Li is more than twice as large as and is opposite in sign to that of CH3F. In the corresponding ethanes XCH2CH2Y the terminal groups are much closer together and the dipolar expression (5) must be regarded as the leading term in a more slowly converging multipolar expansion [Zl, 221 for the through-space interaction. In addition to the through-space efectrostatic interactions, forces related to orbital overlap play a significant role in determining the V, coefficients of the disubstituted ethanes, though not of the corresponding but-2-ynes. In the former the overlap dependent forces between the two substituents are clearly most important in the cis conformation. Such forces are dominated by exchange repulsions when they occur between filled orbitals of identical sub~ituents (as in FCH&H2F) and

264

in such cases give a negative contribution to VI.In molecules such as LiCH2CHIF, donor-acceptor attractions outweigh exchange repulsions and the overlap contribution to VI is positive [ 131.

Through-space multipolar interactions across the but-2ynes account fairly well for the VI coefficients but do not appear to make significant contri-

butions to the V, coefficients of these molecules. The angular dependences of dipole--dipole and dipole-quadrupole forces preclude any contribution to V, and, in view of the small quadrupole moment of methyl fluoride measured with respect to its centre of mass [ 23], it seems unlikely that quadrupole-quadrupole.forces are sufficiently large to account for the magnitudes of the V, barriersof species such as FCH@CCH2F and LiCH,CE CCH*F. The larger V, constants in Table 2 can, however, be interpreted if hyperconjugation between the XCHJSC and CH2Y groups is considered. Hyperconjugative interactions in the disubstituted but-2-ynes can be analysed along the lines employed for the corresponding ethanes [ 133 using perturbation molecular orbital theory (see, for example [24] ). We begin by generating the pseudo-z-orbitals of the CH&S! group from appropriate p~udo-~-orbital of the methyl subgroup and n-orbitals of the acetylenic bond. Orbitals with the subscript a contain carbon-centred p-orbitals which are symmetric with respect to the plane of the paper. Group orbitals labelled b contain carbon-centred p-orbitals which are antisymmetric to reflection in the plane of the paper. Because of the proximity of the p-orbit& at the interacting termini, two orbitals with the same label (a or b) interact strongly but those with different labels interact weakly. An additional parenthetic label (H) has been used to describe orbitals which include a contr~ution from the unique* hydrogen atom of Fig. 4. Orbitals lacking this component are labelled (g). In Fig. 5, we depict schematicaLlythe effect of monosubstitution of a methyl hydrogen atom in CH3eC by an electropositive substituent X and by an electronegative substituent Y. As in the case of the methyl group [13], the degeneracies of the pseudo-n-orbitals are lifted. To a first approximation, only those Ir-orbitals,designated n(X) and A(Y), which include atomic s-orbit& on the substituents X and Y, are affected. There is little change in the energies of the K(F) and n(Y) orbitals which we define to have no contribution from s-orbitals on atoms X and Y. The energies of the XCH&ZC group orbitals 8) (X) are raised and the energies of the orbitals z(i) (Y) in YCH,CS! are lowered relative to those of n(i)(%) and n(i) (y) respectively (cf. also refs. 13, 25, and 26). To systematize our discussion of hyperconjugative contributions to V,, we divide the 1,4disubstituted butynes into three classes. *For the unsubstituted methyl group, this choice is, of course, arbitrary. Monosubstitution is assumed to occur at the “unique” hydrogen atom.

265

CH3C=C

CH3

Fig. 4. Generation of CH,C=C the CH, and C=C moieties.

c=c

group orbitals From the combination

of n-type orbitais of

ELECTRONEGATIVE

ELECTROPOSITIVE UNSUBSTITUTEO

SUBSTITUENT

SUBSTITUENT

,

_--_

-

-(Xl

I

,’ I

,’

-

2rKl,

r(H) -‘._

.’

I’

w(1)

a(X) -___ --__ -.__ --__ ---___ --=________

CH$‘C

XCH&=C

-

r(Y)

YCH@C

Fig. 5. Influence of electropositive substituents X and electronegative the energies of n-type orbitak of the CH,C=C group.

substituents

Y on

Both substituents electronegatiue or electropositive. Molecules in this class, which includes 1,4difluorobut-2-ye, have characteristically large and negative twofold coefficients V,. Perturbation molecular orbital arguments strongly suggest (see Fig. 6) that whenever the electronegativity splittings in the constituent groups of molecules such as YCH,CECCH,Y are large, there is a pronounced contribution to a preference for conformations in which the local symmetry planes of the YCH* groups are orthogonal rather than coplanar. This arises through a reduced energy separation between the interacting orbitals and hence an increased stabilization. The large negative value of V, is consistent with this preference. Large negative values of V, can also be expected for but-Zynes YCH2CZCCHzY in which the electronegative

+=o-

lp=so-

Fig. 6. Orbital interaction scheme showing hyperconjugative tendency of the but-2-yne YC!H,C=CCH,Y to adopt a conformation with Q,= 90” resulting from reduced energy separations between the most strongly interacting orbitak

substituents are not identical. Analogous arguments can be used to interpret the huge negative Vz coefficients .of LiCH2C=CCH2Li. One substitruentelectropositive, the other efectronegative. A few molecules belong to this class which is exemplified by species such as LiCH2C%XH2F. The hyperconjugative interactions (Fig. 7) contribute to a preference for coplanar rather than orthogonal structures for molecuies of this type. This tendency is consistent with a large positive twofold coefficient V,. Either or both substituents electroneutral. For molecules such as CH3CH2C+SCCH2Fand CH3CH,CSCCH,CH3, the electronegativity splittings at either

“\

.,jc--=

/’

“H

#I=00

#=SO”

Fig. 7. Orbital interaction scheme showing hyperconjugative tendency of the but-2-yne XCH,CkCCH,Y to adopt a conformation with @ = 0” resulting from reduced energy separations between the most strongly interacting orbitals.

267

or both substituted methyl groups are very small. The tendency towards preferential stabilization of orthogonal structures at the expense of coplanar structures, or vice versa, is consequently very small also.

V, constants Even a cursory inspection of Table 2 reveals that the threefold potential constants V, are extremely small, rarely exceeding 0.1 kJ mol-‘. Furthermore, these threefold coefficients are between one hundred and one thousand times smaller than the threefold coefficients of the corresponding ethanes. These points appear to be a consequence of the fact that direct overlap between vicinal bond orbitals and the associated exchange repulsions in eclipsed conformations of the ethanes [27] is greatly diminished in the but-2-ynes. Long-range through-space interactions in the but-2-ynes appear to be large enough to account for the magnitude of the V, coefficients but agreement with the data in Table 2 is far from quantitative. In fact the signs are incorrectly predicted to be negative. The leading long-range electrostatic contribution to the threefold barrier of but-2-yne and ethane comes from the component Qs3 of the electric octupole moment* of the methyl group and takes the form GCt = -

(Q33

+

Qz33)*/7%~’

(6)

;t3falls away as the seventh power of the distance R between the methyl groups. Because this distance is at least twice as large in but-2-yne as in ethane, the direct electrostatic contribution to the rotational barrier is at least two orders of magnitude smaller for but-2-yne than for ethane. Note that the octupolar interaction (6) favours staggered conformations whereas the molecular orbital calculations indicate a slight bias favouring eclipsed conformations of but-2-yne. Through-space inductive and dispersive forces between the methyl groups of but-2-yne which fall away more rapidly than R’ can be expected to be less important than the octupolar interaction (6) in determining conformational preferences_ It appears that specific orbital interactions might therefore need to be invoked to account for the potential minima at the eclipsed positions. Indeed, Lowe [28] has drawn attention to positive overlap between the terminal hydrogens in the highest occupied molecular orbitals of the eclipsed conformations. A complementary rationalization of the preference for eclipsed geometry is provided by perturbation molecular orbital theory which suggests (see Fig. 8) that attractive interactions between the terminal hydrogens of the CH,CZ and CH3 groups are maximized in the eclipsed configuration. The two-electron stabilizing interactions (Fig. 8) all involve po$itive overlap between the terminal hydrogens and hence are diminished on rotation from the eclipsed form. Because the terminal hydrogen atoms are far apart, bonding overlap The

octupole

component

charges ei of a function

Q,,

= -

g & ei$ sin3 Oie3i@i is the sum over all molecular J-describing the third moment of the group’s charge distribution.

268 u-type

orbitols

.-

r~‘(Hl

:

CH3CsC

b-type

orbitok

‘77:,(HI

Cf-f3

Fig. 8. Predominant two-electron stabilizing interactions between the n-type orbitals of CH,CkC and CH, in the eclipsed (b, = 0”) conformation of CH,C~CCH,. Degenerate a and b sets of orbitaIs are shown.

between them is exceedingly small but is nevertheless apparently sufficient to overcome the octupolar repulsion. But-2-yne, pent-2-yne, I-fluorobut-2-yne and I-lithiobut-2-yne. Symmetry demands that the leading n-fold barrier to internal rotation in these molecules - cos3~). In general, the values for the threefold is the threefold term $ V3 (1. barriers favouring eclipsed conformations are so small that they he on the borderline of computational error. The slight preference for an eclipsed conformation can, however, be attributed to orbital interactions such as those depicted in Fig. 8 (see also [28]). Hex-3-yne, I-fluoropent-2-yne and I-lithtopent-2-yne. Because the methyl group has low polarity, the V1 parameters for these molecules are very small. The effective electroneutrality of the methyl group also ensures that the V, parameter is small. Thus all three molecules have rather insignificant VI, V, and V3 potential constants. Our results suggest that 1-fluoropent-2-yne has a preferred eis configuration and that the favoured conformations of l-lithiopent-2-yne are cis and trans. 1,4-Diflumobut-2-yne and 1,4diiithiubut-2-yne. The large negative onefold coefficients VI are consistent with the large dipole moments of the FCHz and LiCHz groups in the two molecules (see eqn. (5)). Hyperconjugative stabilization of the orthogonal conformations as indicated in Fig. 6 accounts for the negative signs of the twofold coefficients. As mentioned previously, the threefold coefficients V, are too small to influence the preferred conformations significantly.

269

Our results for these two molecules cast doubt on the widespread belief that barriersto internal rotation in 1,4disubstituted derivatives of dimethylacetylene are less than 0.1 kcal mol-I. They suggest that 1,4difluorobut-2yne has stable enantiomeric conformations with @ close to 100” and 260” and that these rotational isomers are separated by barriersof 2.1 kJ mol-’ at the frans conformation and 5.3 kJ mol-’ at the cis conformation. They also lend some support to the tentative analysis by Ellestad and Kveseth [9] of the conformational preference of 1,4dibromo-2-yne, but stand in contrast to the conclusions of Morino et al. [73 who analysecl internal rotation in 1,4dichlorobut-2-yne. The latter authors assumed that the constancy of the electric dipole moment of this molecule over the temperature range 365-433 K indicates an absence of significant conformational preference_ However, it is easy to show that if the VI and V2 coefficients take values in the neighbourhood of those reported in Table 2, the dipole moment would not be expected to increase by more than about 1% over this temperature range. Such a change in dipole moment is well within the bounds of experimental uncertainty. I-Fluoro-4lithiobut-2-yne. This molecule is the sole representative of the class containing one electropositive and one electronegative substituent. It accordingly has large positive V, and V2 barriers and, like the corresponding ethane LiCH$H,F [ 13,261 exhibits cis and tram rotational isomers and a strong preference for the former. CONCLUSIONS

The following important points emerge from this study. (i) Rotational barriers in monosubstituted but-2-ynes and 1,4disubstituted but-2-ynes are very small ( 90” and @I< 270”. (iii) But-2-ynes such as LiCH2CECCHzF in which one substituent is strongly electropositive and the other is electronegative have stable cis and trams isomers and reveal a pronounced preference for the eclipsed cis conformation. (iv) For all molecules investigated here the threefold component, 3 V, (1 - cos 3r#~), of the total potential function is very small. This result contrasts with that observed for the corresponding 1,2disubstituted ethanes but is consistent with the absence of significant overlap repulsion between bond orbit& of the distant terminal EI=OUDS.

270 ACKNOWLEDGEMENT

We thank Mr. Alan Hinde for assistance with the plotting routines. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

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