Tetraethynylethylene, a molecule with four very short CC single bonds. Interpretation of the infrared spectrum

Volume 191, number 6

CHEMICAL PHYSICS LETTERS

17 April 1992

Tetraethynylethylene, a molecule with four very short C-C single bonds. Interpretation of the infrared spectrum Buyong Ma, Yaoming Xie and Henry F. Schaefer III Centerfor Computational Quantum Chemistry, Universityof Georgia, Athens, GA 30602, USA Received 30 December 1991; in final form 7 February 1992 The (HC---C)2C=C(C-CH)2 m••ecu•erecent•ysynthesizedbyRubin•Kn•b•er•andDiederichhasbeenstudiedusingabiniti• molecular quantum mechanics. The experimental crystal structure shows significant distortions from D2h symmetry, so the equilibrium geometry has been determined via the self-consistent-field method using a double zeta plus polarization basis set. The structure of the isolated molecule displays perfect D2h symmetry. Theoretical vibrational frequencies and infrared intensities make possible an interpretation of the observed IR spectrum. Some attention is given to the molecular orbitais of this the first C]oH4 isomer to be prepared in the laboratory.

1. Introduction The " e n e d i y n e s " H

"c II

,c H have been known for m a n y years (see, for example, ref. [ 1 ] ) and are i m p o r t a n t species in chemistry a n d b i o c h e m i s t r y [2]. However, the a t t a c h m e n t o f additional acetylene substituents to the C=C double b o n d in 1 has p r o v e n to be challenging [ 3 ]. Nevertheless, the parent cross-conjugated n-system tetraethynylethylene H

xC

C/

H

\/ II

H Elsevier Science Publishers B.V.

H

has finally been synthesized [ 4 ]. Tetraethynylethylene is fairly soluble in pentane a n d crystallizes out o f this solvent at - 10 °C as white plates. Tetraethynylethylene is also the first C~oH4 isomer to be prep a r e d in the laboratory. The tetra-substituted molecule with all four hydrogens replaced by trimethylsilyl ( T M S ) groups (CH3)3St was also subjected to a crystallographic study by the U C L A group [4 ]. Unfortunately, from the chemistry p o i n t o f view, the structure appears to be very strongly influenced by crystal packing forces #~. Specifically two o f the C - C single b o n d distances are 1.424 A, while the other two are 1.479 A. Similarly, two o f the C ~ C triple b o n d distances are 1.163 A, while the other two are 1.207 A. Presumably all four C - C distances are equivalent in the iso-

Dr. Knobler has supplied the following further information concerning the distortion of the C~o structure from D2h symmetry: There is no simple explanation for the differences found, although they may arise from packing effects, from an underestimation of the standard deviations (the differences in bond lengths are about 5a), or simply from some non-random effect on the data. These crystals are very thin square plates. This is a room temperature measurement and relatively few reflections were considered to be "observed". Also, as can be noted from the ellipsoids, distances and angles involving the TMS groups will be affected by the "wagging" of the molecule. 521

Volume 191, number 6

CHEMICALPHYSICSLETTERS

lated molecule, as are all four C---C distances. The solid state deviations of 0.055 A (single bonds) and 0.044 A (triple bonds) are seen to be extraordinarily large. Also very pertinent to the present research is the identification by Rubin, Knobler, and Diederich [ 4 ] of four fundamental vibrational frequencies from the infrared spectrum. Three other features in the infrared spectrum (at roughly 1300, 1200, and 1000 cm- 1) were not identified. Motivated by the molecular elegance of 2 and its expected future importance [3,4] in organic chemistry, we have carried out a theoretical study of tetraethynylethylene. Our immediate goals were the consideration of the molecular structure (severely distorted from D2h symmetry in the crystal structure [4] (see footnote 1 ) ) and the interpretation of the infrared spectrum.

2. Theoretical details

The ab initio single configuration self-consistentfield (SCF) method was used here in conjunction with a double zeta plus polarization (DZP) basis set [ 5 ], designated C (9s5pld/4s2pld), H (4slp/2slp). Polarization function orbital exponents were Old(C)=0.75 and Otp~H)=0.75. Pure spherical d functions were used, yielding a total of 170 contracted Gaussian basis functions. The computations were carried out using the TURBOMOLE program developed by Ahlrichs group [6] in Karlsruhe. The DZP SCF total energy at the DEh equilibrium geometry is -380.77234 hartree. Less reliable theoretical structures were also obtained using the smaller STO-3G minimum basis [7] and the DZ basis set (same as D Z + P described above, but without the polarization functions).

3. Results and discussion

The theoretical equilibrium geometries for tetraethynylethylene are shown in fig. 1. As demonstrated by the subsequent vibrational analysis (see below), the equilibrium geometry is of perfect D2h symmetry. This is consistent with Knobler's remarks (footnote 1 ) concerning the experimental crystal structure [4 ] for the TMS tetra-substituted molecule. 522

17 April 1992

We expect the predicted DZP SCF bond lengths to be reliable [8] to within +0.02 A. The most interesting feature of the C toH4 molecular structure is the very short C-C single bond distance, 1.441 A. The origin of this short internuclear separation, of course, is the presence of adjacent double and triple bonds [9 ]. The extreme limit of this type of bond distancq behavior is the diacetylene molecule [ 10 ] H C - C C---CH, for which the C-C separation is only 1.384 A. As discussed above, the crystal structure of tetraethynylethylene has two distinct C-C single bond distances 1.424 and 1.479 ,~. Our best opportunity for comparison with experiment is thus to take the average C-C distance, namely 1.451 A. We find the agreement between this distance and the DZP SCF value (1.441 A) bo be quite satisfactory under the circumstances. The central C=C double bond distance is predicted to be 1.347/~ for the isolated molecule with the DZP SCF method. This result is 0.023 A longer than the value 1.324/k found in the crystal structure [ 4]. Both results are quite close to the standard 1.34 /k carbon-carbon double bond distance. The four C-=C triple bond distances are predicted to be 1.191 A (DZP SCF), i.e. a bit longer than the average value 1.182/k from 350 known crystal structures. In the crystal structure [4] for 2, the two different C----C distances are 1.163 and 1.207 A, which average to 1.185/k. The agreement (0.006 A) between the average experimental distance in tetraethynylethylene (1.185 ,~) and the DZP SCF theoretical prediction ( 1.191 A) is entirely satisfactory. Another example of the distortion seen in the crystal structure is the presence of two distinct C=C-C bond angles [4], 125.3 ° and 118.2 °. The average value is 121.7 °, in close agreement with the DZP SCF value 0e(C=C-C)=I21.9 °. The four equivalent C - C - C bond angles predicted here (DZP SCF) are 178.6 ° , in close agreement with the two distinct experimental values 178.8 ° and 179.1 ° . The four C---C-H angles are even closer to linearity, namely 179.6 ° . Table 1 reports the harmonic vibrational frequencies, infrared intensities, and potential energy distributions (PEDs) for tetraethynylethylene at the DZP SCF level of theory. The DZ SCF results were qualitatively similar. The PEDs require the assumption of a set of symmetrized internal coordinates, and

Volume 19 l, number 6

CHEMICAL PHYSICS LETTERS

17 April 1992

D2h

H13~

1.066A 1.053/~ 1.173~ STO-3G 1.199~ DZ ~91,~ DZP

~ ' C -

~ ' 8

.,H12

1.o6o~ / / r , /

1.347]~ i

1.35s.A II 121.7° 1.347AI1~ 121.S° 11 "~ 121.9° -CLJ

.

C/~"

179.4 1177 : 0:

u / ' '11

~Clo .0° 179.6°

~ .

H14

Fig. 1. Theoretical equilibrium geometries for the tetraethynylethylene molecule, CtoHa. The atoms are numbered to allow definitions of internal coordinates in table 2. these are presented in table 2. An important part of this research is the attempt to relate the experimental infrared spectrum (fig. l of ref. [4] ) to the theoretical vibrational frequencies and IR intensities. For this purpose, the DZP SCF harmonic vibrational frequencies have been multiplied [ 8 ] by the factor 0.9, and these results are presented in table 1 as "scaled". The fundamental vibrational frequency with highest IR intensity is the C - H stretch (B3u symmetry) estimated at 3271 cm -1. Also of strong intensity is the lEu mode predicted at the same frequency. In fact, all four C - H stretches are estimated to be the same(3271 cm -~) to within one cm -1. It is encouraging that the strongest IR band is observed at 3307 cm -1 only 38 cm - I or 1.1% higher. The theoretical vibrational frequencies with the second and third highest IR intensities lie quite close

to each other, at 721 cm -] (B2u) and 718 cm - l (Bju). These two fundamentals appear to correspond to the IR bands observed at 653 and 632 c m - t. Rubin, Knobler, and Diederich [ 4 ] ascribe these two strong absorptions to the coupled C - H bending modes, which appear to be broadly consistent with the more detailed PEDs reported in table 1. The final IR feature assigned by the UCLA group is a weak C-=C stretching band at 2102 c m - t . The agreement with theory is very good in the sense that we predict weak C=-C stretching fundamentals at 2156 and 2148 cm -j. Note that the other C - C stretching fundamentals are forbidden in the infrared. The only very strong theoretical IR fundamental not assigned by Rubin, Knober, and Diederich is the B3u mode v~] (a C - C single bond stretching frequency) estimated at I 106 c m - t . The IR intensity of this fundamental is 31% of the strongest IR the523

Volume 191, number 6

17 April 1992

CHEMICAL PHYSICS LETTERS

Table 1 Harmonic vibrational frequencies, infrared intensities and potential energy distributions (DZP SCF) Symmetry

Description a~

co (cm- ~)

to (cm- ~) (scaled)

IR relative intensity

As B2u B~u Big B2u B3u

1 2 3 4 5 6

C-H stretch 95% C-H stretch 95% C-H stretch 95% C-H stretch 95% C---C stretch 83% + C-C stretch 12% C-=C stretch 81% + C-C stretch 14% C-=C stretch 81% + C-C stretch 14% C---C stretch 83% + C-C stretch 12% C=C stretch 75% + C-C stretch 13% C-C stretch 54% + C=C-C bend 37% C-C stretch 85% + C---C stretch 12% C-C stretch 63% + C=C-C bend 23% C-=C-H bend 93% C---C-H bend 96% C---C-H bend94% C---C-H bend 96% H-C-~C-C torsion96% H-C-~C-C torsion96% H-C---C-C torsion 97% H-C-~C-C torsion 95% out-of-plane 77% + C-C-C=C torsion 23% C-C stretch 54% + C=C-C bend 24% + C---C-C bend 15% out-of-plane47% + C---C-C=C torsion 43% C=C-C bend 40% + C---C-C bend 28% + C-C stretch 28% C-C stretch 29% + C=C-C bend 27% + C-=C-C bend 25% + C=C stretch 16% C=C-C bend 40% + C---C-C bend 28% + C-C stretch 28% C=-C-C=C torsion 74% + C-C=C-C torsion 23% C-~C-C bend 64% + C=C-C bend 35% C=-C-C=C torsion 97% C---C-C=C torsion 74% + out-of-plane 24% C - C - C bend 72% + C=C-C bend 25% out-of-plane 53% + C---C-C=C torsion 37% C---C-C bend 62% + C=C-C bend 37% C---C-C bend 58% + C=C-C bend 41% C=C-Cbend 64% + C - C - C bend 36% C-C=C-C torsion 71% + C-C-C=C torsion 29%

3635 3634 3634 3634 2395 2387 2385 2376 1773 1387 1229 1041 805 802 801 799 798 796 796 795 790 647 633 627 563

3272 3271 3271 3271 2156 2148 2147 2138 1596 1248 1106 937 725 722 721 719 718 716 716 716 711 582 570 564 507

0 0.46 1.00 0 0.002 0.06 0 0 0 0 0.31 0.12 0 0.08 0.59 0 0.79 0 0 0 0 0 0.08 0.06 0

552 482 463 395 285 274 163 155 132 123 86

497 434 417 356 257 247 147 140 119 111 77

0 0 0.10 0 0 0 0.05 0.02 0 0.02 0

Btg

7

Ag Ag B~s B3u B2u As B3u B2u Big B~u B2s B3s Au B3s As B~u B2u As

8 9 l0 II 12 13 14 15 16 17 18 19 20 21 22 23 24 25

B~s Au B~ B2s B3s Bts B~u B2u As B3u Au

26 27 28 29 30 31 32 33 34 35 36

a) The PEDs reported here are defined following ref. [ 11 ].

oretical feature, t h e B3u C - H s t r e t c h o b s e r v e d a t 3307 c m -1. A m o n g t h e f e a t u r e s a p p e a r i n g t h e F T - I R s p e c t r u m o f t e t r a e t h y n y l e t h y l e n e i n d i l u t e CCI4 sol u t i o n , t h e m o s t p l a u s i b l e m a t c h is a s h a r p l i n e at a b o u t 1000 c m - ~ . T h e o n l y o t h e r p o s s i b l e f u n d a m e n t a l o f ( H C - - - C ) 2 C = C ( C - - - C H ) 2 t h a t c o u l d corr e s p o n d t o t h e l a t t e r e x p e r i m e n t a l I R p e a k is t h e B2u mode estimated at 937 cm-~.

524

I n a d d i t i o n to t h e a b o v e - d i s c u s s e d I R f e a t u r e n e a r 1000 c m - 1, t h e s p e c t r u m i n fig. 1 o f ref. [ 4 ] i n c l u d e s p e a k s a t ~, 1300 a n d ~ 1200 c m - ~. T h e o r y is q u i t e clear that there are only two IR allowed fundamental vibrational frequencies of 2 in the range 750-2000 c m - 1. T h u s a t least o n e o f t h e s e t h r e e o b s e r v e d p e a k s ( 1 3 0 0 , 1200, a n d 1000 c m -~ ) is n o t a f u n d a m e n t a l o f t e t r a e t h y n y l e t h y l e n e . T h e s i t u a t i o n is p o t e n t i a l l y

Volume 191, number 6

CHEMICAL PHYSICS LETTERS

17 April 1992

Table 2 Internal coordinate system for the tetraethynylethylene molecule Internal coordinates a)

Description

qi=R(1,2) q2...q5=R(2, 5), R(2, 4), R( 1, 3), R( 1, 6) qr...qg=R(3, 7), R(6, 10), R(5, 9), R(4, 8) q~o...q~=R(7, 11),R(8, 12),R(9, 13),R(10, 14) q14...q17=Ot(2, 1, 3), ta(2, 1, 6), Or( 1, 2, 4), ~X(1, 2, 6) qla'"q2t =Or( 1, 3, 7), ta(2, 4, 8), a( 1, 6, 10), a(2, 5, 9) q::...q2~=ot(3, 7, l 1), a(4, 8, 12), or(5, 9, 13), a(6, 10, 14) q:6=r(3, 1,2,4)+r(6, 1,2,4)+z(3, 1,2, 5)+r(6, 1,2, 5) q27...q3o=r(ll, 7, 3, 1), r(12, 8, 4, 2), r(13, 9, 5, 2), r(14, 10,6, l) q~...q34=r(7, 3, l, 2), z(8, 4, 2, l), r(9, 5,2, 1), z(10, 6, 1,2) qas...qa6=atom( 1) out of the plane of atoms(2, 4, 5); atom(2) out of the plane of atoms( l, 3, 6)

C=C stretch C-C stretch C---C stretch C-H stretch C=C-C bend C-C-C bend C=-C-H bend C-C=C-C torsion H-C---C-C torsion C-C-C=C torsion out-of-plane

a) q~...q:are symmetry-adapted linear combinations of the simple internal coordinates listed in the same line. R stands for bond length, ct for bond angle and r for torsional angle. Atom numbering is that provided in fig. 1.

c o m p l i c a t e d b y the fact that l i q u i d CC14 itself also has I R features i n this region #2, b u t the a u t h o r s o f ref. [4] have assured us that all three peaks are due to t e t r a e t h y n y l e t h y l e n e a3. Finally, s o m e c o m m e n t s o n the electronic structure o f this cross-conjugated n-electron system are in order. T a b l e 3 gives the s y m m e t r i e s a n d o r b i t a l energies o f the twelve highest-lying o c c u p i e d D Z P S C F ~2 See, for example fig. 11, p. 28 of ref. [ 12 ]. #a Professor Diederich has suggested that the IR feature near 1300 cm-t could be an overtone of the fundamental at 653 cm-~.

m o l e c u l a r orbitals. N o t e that the H O M O lies significantly higher energetically (0.07 h a r t r e e ) t h a n the other o c c u p i e d MOs. N o t u n e x p e c t e d l y b o t h the H O M O (2b~u) a n d L U M O (2b3g) are n orbitals. C o n t o u r m a p s o f the H O M O a n d L U M O are seen o n figs. 2 a n d 3. T h e H O M O is seen to have o n e n o d e across the four C - C single b o n d s . T h e L U M O has a

/ i ,."..,~;."--("-,~'~,,

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Table 3 DZP SCF valence electron orbital energies for tetraethynylethylene Symmetry

Energy (hartree)

5b3u 7as 5bis lbl, lb3g 8as 7b2u lb2s 6b3u lau 6bts 2biu 2b3~

-0.700 - 0.689 -0.638 -0.550 -0.453 -0.434 -0.425 -0.421 -0.417 -0.410 -0.391 -0.321 (HOMO) 0.037 (LUMO)

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

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

,,:-,, i I

i

l

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j

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.

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.....

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Fig. 2. The highest occupied molecular orbital ( HOMO (2bt u) ) of tetraethylyethylene, C,oH4. Heavy solid dots indicate atomic positions. Solid lines indicate a positive sign for the orbital, while dotted lines indicate a negative sign. The dashed line designates the node that divides all four single bonds. 525

Volume 191, number 6

CHEMICAL PHYSICS LETTERS

/ //

/

/

"x X X

/

\

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17 April 1992

ferent C-=-C b o n d distances. The theoretical I R spectrum provides a satisfactory interpretation o f the four f u n d a m e n t a l s assigned by Rubin, Knobler, and Diederich. T h e H O M O a n d L U M O o f 2 are both n orbitals.

Acknowledgement This research was s u p p o r t e d by the U n i t e d States D e p a r t m e n t o f Energy, Office o f Basic Energy Sciences, Division o f Chemical Sciences, F u n d a m e n t a l Interactions Branch, Grant No. DE-FG0987ER 13811. We thank Dr. Carolyn Knobler for much helpful correspondence.

References xX N

/

1t

Fig. 3. The lowest unoccupied molecular orbital (LUMO (2b3g)) of tetraethynytethytene C~oH4. Heavy solid dots indicate atomic positions.

node across the C=C double b o n d and another node across the four C---C triple bonds.

4. Concluding remarks The synthesis o f tetraethynylethylene b y Rubin, Knobler, a n d Diederich completes an i m p o r t a n t chapter in the history o f organic chemistry. The adjacency o f the four triple b o n d s to the central double b o n d creates a measure o f cross conjugation hitherto unknown in h y d r o c a r b o n chemistry. The present theoretical studies show that the isolated molecule has Dzh symmetry, in contrast to the crystal structure, which displays two different C - C a n d two dif-

526

[ 1] R.G. Bergman, Accounts Chem. Res. 6 (1973) 25. [2] K.C. Nicolau and W.-M. Dai, Angew. Chem. Intern. Ed. Engl. 30 (1991) 1387. [ 3 ] H. Hopf and M. Kreutzer, Angew. Chem. Intern. Ed. EngL 29 (1990) 393. [4] Y. Rubin, C.B. Knobler and F. Diederich, Angew. Chem. Intern. Ed. Engl. 30 (1991) 698. [5] S. Huzinaga, J. Chem. Phys. 42 (1965) 1293; T.H. Dunning, J. Chem. Phys. 53 (1970) 2823. [6] R. Ahlrichs, M. B~ir,M. H~iserand H. Horn, Chem. Phys. Letters 162 (1989) 165. [7] W.J. Hehre, L. Radom, P. von R. Schleyer and J.A. Pople, Ab initio molecular orbital theory" (Wiley-lnterscience, New York, 1986). [8 ] H.F. Schaefer III, in: Critical evaluation of chemical and physical structural information, eds. D.R. Lide and M.A. Paul (National Academy of Sciences, Washington, 1974) pp. 591-602. [ 9 ] M.J.S+Dewar and H.N. Schmeising, Tetrahedron 5 ( 1959 ) 166. [ 10 ] M. Tanimoto, K. Kuchitsu and Y. Morino, Bull. Chem. Soc. Japan 44 ( 1971 ) 386. [ 11 ] Y. Morino and K. Kuchitsu, J. Chem. Phys. 20 (1952) 1809; J.H. Schachtschneider and R.G. Snyder, Spectrochim. Acta 19 (1963) 117. [ 12] J.W. Robinson, Handbook of spectroscopy, Vol. II (CRC Press, Boca Raton, 1974).