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The neutral and ionic vinylidene—acetylene rearrangement
Volume
100, number
6
THE NEUTRAL AND ICNIC VINYLIDENE-ACETYLENE
Gemot
30 September 1983
CHEMICAL PHYSICS LETTERS
REARRANGEMENT
FRENKING
Instirut fir Urganiscire Ci~ernie. Techniscize Universihit Berlin. Straw des 1% Juni 135. D-1000 Berlin 12, West Germatlv
Kscei\ed 12 June 19133: in final form S August
1983
Tr.xnKition sfd%c structures for the neutral. ationic and anionic vinylidene-acetylene rearrangement are calculated by nb initlo methods_ While rhc bxriers for the neutrA and cationic E-shift arc found to be low or even zero involving a planar structure. rr.uxtngement of the \inylidenc anion proceeds with high activation encr~~ possibly via a perpendicutar transition
1. Introducr~on
In recent years quite a few highly accurate, theoretical ixlsestigations on the ~nylidene-acetylene isomers have been published [I-G] _For the neutral species, calculations indicate a very low activation barrier for the singlet state [l-3] casting some doubt on the experimental results which gave rise to the suggestion that ‘At HzCC may esist (71. However, for the triplet states ‘Bz vinylidene was calculated to be a long-living species [S], separated from ‘B1 acetylene by a substantial barrier [G] _ For the cations only the minima have been insestigated showing that the acetylene structure is much more stable [9] _There is no experimental proof for the e_xistence of H,CC+. The situation is different for the anions. Here, the existence of a stable C2Hr species is experimentahy proven [lo] and it was suggested that it is the radical anion of vinylidene and not acetylene [5,l O,l I] _The electron aftinity was calculated to be positive for vinylidene but strongly negative for acetylene IS] in agreement with results produced by electron transmission spectroscopy [ 121. However, the total energy of the acetylene anion was stih lower than the totd ener,q of the vinylidene anion [S J . _Apossible reason for the esistence of the stable HZCC- structure could be a large activation barrier for rearrangement to the acetylene anion_ In this paper we report the results of a comparative study for the neutral, oationic and anionic vinylidene-acetylene rearrangement using ab 4s4
initio calculations
including
2. computations
details
electron
correlation.
Geometry optimizations have been carried out using GRADSCF [ 131 with a 4-3 1G basis set. The unrestricted Hartree-Fock method was used for open shell systems. Transition state structures were located as saddle point on the ener7 hyperface with one negative eigenvalue of the hessian matrix. Single point calculations were added using a modified version of GAUSSIAN 76 [ 141 which includes the Mater-Plesse~ perturbation method for correlation effects terminated at second order [ 15]_ A 6-31+ G” basis set was used which employs polarization functions and diffuse orbit& (exponent 0.04) [I 61 on carbon. The notation for these calculations is MP2/6-31 + G*//4-31G. The diffuse orbitals are necessary for an adequate calculation of the anion [ 16,17 ] and has been used throughout for reasons of comparison. The Is functions on carbon have been frozen in the ME calculations.
3. Results and discussion The calculated
energy values are listed in table 1,
stationary point geometries in table 2_ Our results for the neutral rearrangement are restricted to the energetically lower singlet states and
0 OOP-_,614/S3/0000-0000/S
03.00 0 1983 North-Holland
30 September
CHEMICAL PHYSICS LETTERS
Volume 100. number 6
1983
Table 1 Total and relative energies a) for 4-31G optimized geometries 4-31G acetylene TS vinylidene acetylene TS vinylidene acetylene TS planar TS perp. vinylidcne acetylene TS plandr TS perp. vinylidene
neutral neutral neutral cation cation cation anion b, anion anion anion neutral b-c) neutral c) neutral c) neutral ‘1
-76.7114 -76.6100 -76.6517 -76.3498 -76.2603 -76.2942 -76.5925 -76.5093 -765125 -76.6181 -
MP2/6-31
6-31 -t G* 0.0 63.6 37.5 0.0 56.2 34.9 0.0 52-2 50.2 -16.1 -
-76.8230 -76.7431 -76.7695 -76.4632 -76.3853 -76.4002 -76.7463 -766709 -76.6754 -76.7661 -76.7499 -76.7100 -76.6969 -76.7633
0.0 50.1 33.6 0.0 48.8 39.5 o-0 473 44.5 -12.4 -2.3 22.8 31.0 -12.4
+ G’
-77.0711 -76-9888 -76.9941 -76.6636 -76.5822 -76.5813 -77.0071 -16.9340 -76.9220 -77.0010 -77.0302 -76.9628 -76.9455 -76.9911
0.0 51.6 48.3 0.0 51.1 51.6 0.0 45.9 53.4 3.8 -14.5 27.8 38.7 10.0
a) Total energies in au, relative energies in kcal/mol. b, Only the tram isomer is listed since it is more stable than the cis structure. c) Calcukted at the geometries of the anions_ Relative energies are given with respect to the acetylene anion.
vinylidene structure is either a very shallow minimum or itself is a saddle point of the surface” [ 18]_ This may explain the uncertainty concerning the transition state structure_ Calculations at 6-31G* level [3] and even accounting of correlation effects using seif-consistent electron pairs [l] produced a geometry very similar to ours which is more acetylene-like, in disagreement with the Hammond postulate [ 19]_ However, inclusion of MP2/correlation at 6-31G* level changes the
are similar to the previous published ones [l--3,5]. The calculated barrier of 3.3 kcal/mole may be compared with the 2.2 kcal/mole yielded by a MP4/6-31 l**// MP2/6-31G* calculation [3], which reduces further to 09 kcal/mol when zero-point vibration is taken into account. MP2 is known to overestimate correlation ener,v which partly compensates our smaller basis set, producing a similar activation energy. In the light of the very small barrier it was concluded that “the Czv
Table 2 Stationary point geometries (4-31G) a)
acetylene TS vinylidenc acetylene TS vinylidene acetylene TS planar TS perp. vinylidene
neutral neutral neutral cdtion cation cation anion trdns anion anion anion
ClC2
GHI
ClH2
C2H2
HlClC2
HlClGH2
l-190 1.247 1.295 1.236 1.235 1.294 1.308 1.338 1.384 1.352
1.051 1.056 1.074 1.066 1.068 1.084 1.082 1.120 1.145 1.098
-
1.051 1.220 -
lso.o
-
1.425 1.074 1.294 1.084
1.066 1.327 -
177.2 120.8 180.0
180.0 -
176.3 120.2
180.0 -
1.253 1.413 1.098
l-082 1.365 1.264
117.8 134.5 116.3 124.2
180.0 180.0 108.4 -
a) Bond lengths AB in A, bond angles _4BC and dihedral ar@s
in degree.
485
Volume
100. number 6
CHE!MCAL
PHYSICS
C1-H, bond length from 1.170 to 1.87 .A to a structure which is now more on the vinylidene side of the reaction [3 1. In spite of the dramatic change in the geometry this has practically no effect on the activation energy and may be neglected in regard to the reaction profile. The reaction mechanism can be considered as migration of the C-H bond towards the empty p A0 of C, as shown in fig_ 1_ Removal of ;LII electron should not influence this reaction path very much since the lone-pair orbital on C1 is not directly involved in the migration_ The results in tables 1 and 2 support this espectation. The activation barrier for the cationic H-shift at 4-31G and 6-31 + G’ level is lower than for the neutral rearrangement. With inclusion of MP2 there is no barrier at all! This indicates that the HICC+ structure should rather be considered as transition species and not a minimum on the energy hyperface. The data for the calculsted cationic transition state have value only for the judgement of the basis set qualities. The results for the anionic rearrangement are completely different. Firstly. the reaction is calculated to proceed with a large activation barrier. being 45.9 kcal/mol at the MP2/6-3 1 + G* level. Comparing our data for the neutral reaction with higher level calculations [3] we do not think this result will significantly be influenced if a more sophisticated approach is used. The geometry of the transition state had first been searched for starting with a planar geometry similar to the neutral and cationic case. Inspection of the forceconstant matrk of the optimizing structure shows nlx2 negative eigenvalues: One belonging to the reaction coordinate (2670i cm-l), the second one with only a small (17% cm- ’ ) imaginary frequency pointing to the z-direction of the S-J* molecular plane. Finally the true transition state at the 4-3 1G level with only one negative eigenvalue was detemrined to be non-planar, very different to the previous ones as shown in fig. 2. The energies of these two structures are not very different. being bigber for the planar geometry at 431G 486
30 September
LETI-ERS
1983
and 6-31 + G’ (table 1). However, inclusion of correlation energy by MP2 reverses this, favouring the planar structure by 7-5 kcal/mol. The electronic structure of the anions is characterized by a singly occupied MO being empty in the neutral singlet state_ However, in the lowest triplet state jB, of vinylidene the very same p A0 of the o-carbon atom is singly occupied as it is in the anion. Therefore it is not surprising that our results for the anionic Hshift resemble the values calculated for the vinylideneacetylene triplet rearrangement by Conrad and Schaefer [6] _They also found a high barrier of 55 kcal/mol and a non-planar transition state structure_ The reaction mechanism of the anionic rearrangement can be understood by esamining fig. l_ The C-H bond migrates towards a p A0 at Cz which is now occupied by one electron. In the planar transition state, the single occtrpied h-IO extends in an antibonding way in the molecular plane, leading to longer C-C and C,-H, bonds compared to the neutral and cationic structures. In the perpendicular transition state, hydrogen migrates along the n bond weakening the C-C bonding further thus resulting an even more stretched bond length. The calculated structure is very similar to the published one for the triplet rearrangement [6]. One more point has to be mentioned. The calculated transition state connects the vinylidene anion, calculated with a positive electron affinity, and the acetylene anion, calculated with a negative electron affinity. It has already been pointed out that the calculation of a negative electron affinity is a methodical artifact [5] _A perfect treatment would result a zero activation energy_ However, such a conclusion is only valid if the calculations are performed at the same geometry * . We calculated the neutral species at the geometries of the anions. The results are shown in table 1_ Even at the optimized (bent) geometry of trans acetylene anion the neutral HCCH lies below HCCH-. Also the calculated transition states for the * The author to him.
is indebted
to the reftxee for pointing
this out
Volume100, number6
CHEMICALPHYSICSLETTERS
anionic rearrangement show lower values for the neutral species. Therefore, the anionic vinylidene rearrangement would not produce the acetylene anion but neutral acetylene loosing an electron along the reaction coordinate.
4. Conclusion The results indicate that the vinylidene-acetylene rearrangement has a low barrier for the neutral and cationic species proceeding via a planar transition state. The anionic rearrangement resembles the triplet Hshift, owing a substantiaI activation barrier which may involve a perpendicular structure. The reason for the existence of the stable vinylidene anion is a kinetic rather than thermodynamic one.
Acknowledgement The author is very indebted to Professor P. von R. Schleyer for helpful comments and criticism, and to him and Dr. A. Sawaryn for the modified GAUSSIAN
76 program. I thank the Fonds der Chemischen lndustrie for a Liebig Stipendium. Technical assistence by the computer centers ZRZ and WRB, Berlin, is acknowledged.
[6] M.P. Conradand H-F. Schaefer,J. Am. Chem. Sot. 100 (1978) 171
Res. 11 (1978) 107; P.S. Skell. J-J. Have1and M.J. McGlinchey, Accounrs Chem. Res. 6 (1973) 97. [S] Y. Osamura and H-F. Schaefer, Chem. Phys. Letters 79 (1981) 412; J-H. Davis, W-A. Goddard and L.B. Harding, J. Am. Chem. Sot. 99 (1977) 2919. f9] R.A. Whiteside, h1.J. Frisch, J.S. Binkley. D.J. DeFrees. H.B. Schlegel, I;. Raghavachari and J.A. Pople.CarnegicMellon Quantum Chemistry Archive. 2nd Ed.. Pittsburgh (1981). 1101 I.H.J. Dawson and N_N_M. Nibbering. J. Am. Chcm. Sot. 100 (1978) 1928, and references therein; S.M. Kalil. Egypt. J. Chem. 21 (1978) 465 [Chem. Abstr. 93 (1980) 192475y]: \V_Lindinger. D.L. Albritton. E.C. Febsenfeld and E.E. Ferhwson, J_ Chem. Phys. 63 (1975) 3238; I. DOtan, W. Lindinger and D.L. Albritten, J. Chem. Pbys. 64 (1976) 4544; D.J. DcFrces, R.T. Mclver Jr. and W.J. Hehre. J. .4m. Chem. Sot. 102 (1980) 3334. 1111Y.B. Tadrit. M.C.R. Symons and 4-J. Trench. J. Chem. Sot. Faraday I7i (1977) 1149; N-D_ Chyvylkin, G.&i_ Zbidomerov and V-B. Kazanskii. Kinet. Kataliz 20 (1979) 250 [English transl. Kinet. catal. 20 (1979) 202]_ 1121 6-D. Jordan and P.D. Burrow. Accounts Chem. Res. 11 (1978) 341. [I31 A. Komornicki, GRADSCF, Nat. Resource Comput. Chem. Software Cat. Vol. 1, Progam No. QHO4 (19SO). L141 J.S. Binkley, R.A. Whiteside, P-C. Hariharan, R. Seeser. J.A. Pople, W.J. Hehre and h1.D. Newton. QCPE 11 263.
1151 C. hl@ller and MS Plesset. Phys. Rev. 46 (1934)
[ 1I C-E. Dykstraand H-F. Schaefer.J. Am. Chem.Sot. 100 (1978) 137s. [2] Y. Owmura. H.F. Schaefer. SK. Gray and W-H. Miller. J. Am. Chem.Soc. 103 (1981) 1904. [3] R. Krishnan, h1.F. Frisch, J.A. Pople and P_ van R. Schleyer, Chem. Phys. Letters 79 (1981) 408. [4] P. Rosmus. P. Botschwina and J-P. Maier, Chem. Phys. Letters 84 (1981) 71. [S 1 J. Chandrasekhar, R.A. Khan and P_ von R. Schleyer. Chem. Phys. Letters 85 (1982) 493.
7820.
P-J. Smg, Chem. Rev-78 (1978) 383; Accounts Chem.
(1978)
References
30 September1983
1161
[171 1181 1191
618: J-S. Binkley and J-A. Pople. Intern. J. Quantum Chem. 9s (1975) 229. J. Chandrasekhar. J.J. Andrade and P_ xon R_ Schleyer. J. Am. Chcm. Sot. 103 (1981) 5609; G-W. Spitznagel, T_ Clark. J. Chandrasekhar and P. bon R. Schleyer, J. Comput. Chem. 3 (19S2) 363. P. C&sky and hf. Urban, Ab inito calculations, Lecture notes in chemistry. Vol. 16 (Springer. Berlin, 1980) p- 50. J-A. Pople. Pure Appl. Chem. 55 (1983) 343. G.S. Hammond, J. Am. Chem. Sot. 77 (1955) 334.
487
100, number
6
THE NEUTRAL AND ICNIC VINYLIDENE-ACETYLENE
Gemot
30 September 1983
CHEMICAL PHYSICS LETTERS
REARRANGEMENT
FRENKING
Instirut fir Urganiscire Ci~ernie. Techniscize Universihit Berlin. Straw des 1% Juni 135. D-1000 Berlin 12, West Germatlv
Kscei\ed 12 June 19133: in final form S August
1983
Tr.xnKition sfd%c structures for the neutral. ationic and anionic vinylidene-acetylene rearrangement are calculated by nb initlo methods_ While rhc bxriers for the neutrA and cationic E-shift arc found to be low or even zero involving a planar structure. rr.uxtngement of the \inylidenc anion proceeds with high activation encr~~ possibly via a perpendicutar transition
1. Introducr~on
In recent years quite a few highly accurate, theoretical ixlsestigations on the ~nylidene-acetylene isomers have been published [I-G] _For the neutral species, calculations indicate a very low activation barrier for the singlet state [l-3] casting some doubt on the experimental results which gave rise to the suggestion that ‘At HzCC may esist (71. However, for the triplet states ‘Bz vinylidene was calculated to be a long-living species [S], separated from ‘B1 acetylene by a substantial barrier [G] _ For the cations only the minima have been insestigated showing that the acetylene structure is much more stable [9] _There is no experimental proof for the e_xistence of H,CC+. The situation is different for the anions. Here, the existence of a stable C2Hr species is experimentahy proven [lo] and it was suggested that it is the radical anion of vinylidene and not acetylene [5,l O,l I] _The electron aftinity was calculated to be positive for vinylidene but strongly negative for acetylene IS] in agreement with results produced by electron transmission spectroscopy [ 121. However, the total energy of the acetylene anion was stih lower than the totd ener,q of the vinylidene anion [S J . _Apossible reason for the esistence of the stable HZCC- structure could be a large activation barrier for rearrangement to the acetylene anion_ In this paper we report the results of a comparative study for the neutral, oationic and anionic vinylidene-acetylene rearrangement using ab 4s4
initio calculations
including
2. computations
details
electron
correlation.
Geometry optimizations have been carried out using GRADSCF [ 131 with a 4-3 1G basis set. The unrestricted Hartree-Fock method was used for open shell systems. Transition state structures were located as saddle point on the ener7 hyperface with one negative eigenvalue of the hessian matrix. Single point calculations were added using a modified version of GAUSSIAN 76 [ 141 which includes the Mater-Plesse~ perturbation method for correlation effects terminated at second order [ 15]_ A 6-31+ G” basis set was used which employs polarization functions and diffuse orbit& (exponent 0.04) [I 61 on carbon. The notation for these calculations is MP2/6-31 + G*//4-31G. The diffuse orbitals are necessary for an adequate calculation of the anion [ 16,17 ] and has been used throughout for reasons of comparison. The Is functions on carbon have been frozen in the ME calculations.
3. Results and discussion The calculated
energy values are listed in table 1,
stationary point geometries in table 2_ Our results for the neutral rearrangement are restricted to the energetically lower singlet states and
0 OOP-_,614/S3/0000-0000/S
03.00 0 1983 North-Holland
30 September
CHEMICAL PHYSICS LETTERS
Volume 100. number 6
1983
Table 1 Total and relative energies a) for 4-31G optimized geometries 4-31G acetylene TS vinylidene acetylene TS vinylidene acetylene TS planar TS perp. vinylidcne acetylene TS plandr TS perp. vinylidene
neutral neutral neutral cation cation cation anion b, anion anion anion neutral b-c) neutral c) neutral c) neutral ‘1
-76.7114 -76.6100 -76.6517 -76.3498 -76.2603 -76.2942 -76.5925 -76.5093 -765125 -76.6181 -
MP2/6-31
6-31 -t G* 0.0 63.6 37.5 0.0 56.2 34.9 0.0 52-2 50.2 -16.1 -
-76.8230 -76.7431 -76.7695 -76.4632 -76.3853 -76.4002 -76.7463 -766709 -76.6754 -76.7661 -76.7499 -76.7100 -76.6969 -76.7633
0.0 50.1 33.6 0.0 48.8 39.5 o-0 473 44.5 -12.4 -2.3 22.8 31.0 -12.4
+ G’
-77.0711 -76-9888 -76.9941 -76.6636 -76.5822 -76.5813 -77.0071 -16.9340 -76.9220 -77.0010 -77.0302 -76.9628 -76.9455 -76.9911
0.0 51.6 48.3 0.0 51.1 51.6 0.0 45.9 53.4 3.8 -14.5 27.8 38.7 10.0
a) Total energies in au, relative energies in kcal/mol. b, Only the tram isomer is listed since it is more stable than the cis structure. c) Calcukted at the geometries of the anions_ Relative energies are given with respect to the acetylene anion.
vinylidene structure is either a very shallow minimum or itself is a saddle point of the surface” [ 18]_ This may explain the uncertainty concerning the transition state structure_ Calculations at 6-31G* level [3] and even accounting of correlation effects using seif-consistent electron pairs [l] produced a geometry very similar to ours which is more acetylene-like, in disagreement with the Hammond postulate [ 19]_ However, inclusion of MP2/correlation at 6-31G* level changes the
are similar to the previous published ones [l--3,5]. The calculated barrier of 3.3 kcal/mole may be compared with the 2.2 kcal/mole yielded by a MP4/6-31 l**// MP2/6-31G* calculation [3], which reduces further to 09 kcal/mol when zero-point vibration is taken into account. MP2 is known to overestimate correlation ener,v which partly compensates our smaller basis set, producing a similar activation energy. In the light of the very small barrier it was concluded that “the Czv
Table 2 Stationary point geometries (4-31G) a)
acetylene TS vinylidenc acetylene TS vinylidene acetylene TS planar TS perp. vinylidene
neutral neutral neutral cdtion cation cation anion trdns anion anion anion
ClC2
GHI
ClH2
C2H2
HlClC2
HlClGH2
l-190 1.247 1.295 1.236 1.235 1.294 1.308 1.338 1.384 1.352
1.051 1.056 1.074 1.066 1.068 1.084 1.082 1.120 1.145 1.098
-
1.051 1.220 -
lso.o
-
1.425 1.074 1.294 1.084
1.066 1.327 -
177.2 120.8 180.0
180.0 -
176.3 120.2
180.0 -
1.253 1.413 1.098
l-082 1.365 1.264
117.8 134.5 116.3 124.2
180.0 180.0 108.4 -
a) Bond lengths AB in A, bond angles _4BC and dihedral ar@s
in degree.
485
Volume
100. number 6
CHE!MCAL
PHYSICS
C1-H, bond length from 1.170 to 1.87 .A to a structure which is now more on the vinylidene side of the reaction [3 1. In spite of the dramatic change in the geometry this has practically no effect on the activation energy and may be neglected in regard to the reaction profile. The reaction mechanism can be considered as migration of the C-H bond towards the empty p A0 of C, as shown in fig_ 1_ Removal of ;LII electron should not influence this reaction path very much since the lone-pair orbital on C1 is not directly involved in the migration_ The results in tables 1 and 2 support this espectation. The activation barrier for the cationic H-shift at 4-31G and 6-31 + G’ level is lower than for the neutral rearrangement. With inclusion of MP2 there is no barrier at all! This indicates that the HICC+ structure should rather be considered as transition species and not a minimum on the energy hyperface. The data for the calculsted cationic transition state have value only for the judgement of the basis set qualities. The results for the anionic rearrangement are completely different. Firstly. the reaction is calculated to proceed with a large activation barrier. being 45.9 kcal/mol at the MP2/6-3 1 + G* level. Comparing our data for the neutral reaction with higher level calculations [3] we do not think this result will significantly be influenced if a more sophisticated approach is used. The geometry of the transition state had first been searched for starting with a planar geometry similar to the neutral and cationic case. Inspection of the forceconstant matrk of the optimizing structure shows nlx2 negative eigenvalues: One belonging to the reaction coordinate (2670i cm-l), the second one with only a small (17% cm- ’ ) imaginary frequency pointing to the z-direction of the S-J* molecular plane. Finally the true transition state at the 4-3 1G level with only one negative eigenvalue was detemrined to be non-planar, very different to the previous ones as shown in fig. 2. The energies of these two structures are not very different. being bigber for the planar geometry at 431G 486
30 September
LETI-ERS
1983
and 6-31 + G’ (table 1). However, inclusion of correlation energy by MP2 reverses this, favouring the planar structure by 7-5 kcal/mol. The electronic structure of the anions is characterized by a singly occupied MO being empty in the neutral singlet state_ However, in the lowest triplet state jB, of vinylidene the very same p A0 of the o-carbon atom is singly occupied as it is in the anion. Therefore it is not surprising that our results for the anionic Hshift resemble the values calculated for the vinylideneacetylene triplet rearrangement by Conrad and Schaefer [6] _They also found a high barrier of 55 kcal/mol and a non-planar transition state structure_ The reaction mechanism of the anionic rearrangement can be understood by esamining fig. l_ The C-H bond migrates towards a p A0 at Cz which is now occupied by one electron. In the planar transition state, the single occtrpied h-IO extends in an antibonding way in the molecular plane, leading to longer C-C and C,-H, bonds compared to the neutral and cationic structures. In the perpendicular transition state, hydrogen migrates along the n bond weakening the C-C bonding further thus resulting an even more stretched bond length. The calculated structure is very similar to the published one for the triplet rearrangement [6]. One more point has to be mentioned. The calculated transition state connects the vinylidene anion, calculated with a positive electron affinity, and the acetylene anion, calculated with a negative electron affinity. It has already been pointed out that the calculation of a negative electron affinity is a methodical artifact [5] _A perfect treatment would result a zero activation energy_ However, such a conclusion is only valid if the calculations are performed at the same geometry * . We calculated the neutral species at the geometries of the anions. The results are shown in table 1_ Even at the optimized (bent) geometry of trans acetylene anion the neutral HCCH lies below HCCH-. Also the calculated transition states for the * The author to him.
is indebted
to the reftxee for pointing
this out
Volume100, number6
CHEMICALPHYSICSLETTERS
anionic rearrangement show lower values for the neutral species. Therefore, the anionic vinylidene rearrangement would not produce the acetylene anion but neutral acetylene loosing an electron along the reaction coordinate.
4. Conclusion The results indicate that the vinylidene-acetylene rearrangement has a low barrier for the neutral and cationic species proceeding via a planar transition state. The anionic rearrangement resembles the triplet Hshift, owing a substantiaI activation barrier which may involve a perpendicular structure. The reason for the existence of the stable vinylidene anion is a kinetic rather than thermodynamic one.
Acknowledgement The author is very indebted to Professor P. von R. Schleyer for helpful comments and criticism, and to him and Dr. A. Sawaryn for the modified GAUSSIAN
76 program. I thank the Fonds der Chemischen lndustrie for a Liebig Stipendium. Technical assistence by the computer centers ZRZ and WRB, Berlin, is acknowledged.
[6] M.P. Conradand H-F. Schaefer,J. Am. Chem. Sot. 100 (1978) 171
Res. 11 (1978) 107; P.S. Skell. J-J. Have1and M.J. McGlinchey, Accounrs Chem. Res. 6 (1973) 97. [S] Y. Osamura and H-F. Schaefer, Chem. Phys. Letters 79 (1981) 412; J-H. Davis, W-A. Goddard and L.B. Harding, J. Am. Chem. Sot. 99 (1977) 2919. f9] R.A. Whiteside, h1.J. Frisch, J.S. Binkley. D.J. DeFrees. H.B. Schlegel, I;. Raghavachari and J.A. Pople.CarnegicMellon Quantum Chemistry Archive. 2nd Ed.. Pittsburgh (1981). 1101 I.H.J. Dawson and N_N_M. Nibbering. J. Am. Chcm. Sot. 100 (1978) 1928, and references therein; S.M. Kalil. Egypt. J. Chem. 21 (1978) 465 [Chem. Abstr. 93 (1980) 192475y]: \V_Lindinger. D.L. Albritton. E.C. Febsenfeld and E.E. Ferhwson, J_ Chem. Phys. 63 (1975) 3238; I. DOtan, W. Lindinger and D.L. Albritten, J. Chem. Pbys. 64 (1976) 4544; D.J. DcFrces, R.T. Mclver Jr. and W.J. Hehre. J. .4m. Chem. Sot. 102 (1980) 3334. 1111Y.B. Tadrit. M.C.R. Symons and 4-J. Trench. J. Chem. Sot. Faraday I7i (1977) 1149; N-D_ Chyvylkin, G.&i_ Zbidomerov and V-B. Kazanskii. Kinet. Kataliz 20 (1979) 250 [English transl. Kinet. catal. 20 (1979) 202]_ 1121 6-D. Jordan and P.D. Burrow. Accounts Chem. Res. 11 (1978) 341. [I31 A. Komornicki, GRADSCF, Nat. Resource Comput. Chem. Software Cat. Vol. 1, Progam No. QHO4 (19SO). L141 J.S. Binkley, R.A. Whiteside, P-C. Hariharan, R. Seeser. J.A. Pople, W.J. Hehre and h1.D. Newton. QCPE 11 263.
1151 C. hl@ller and MS Plesset. Phys. Rev. 46 (1934)
[ 1I C-E. Dykstraand H-F. Schaefer.J. Am. Chem.Sot. 100 (1978) 137s. [2] Y. Owmura. H.F. Schaefer. SK. Gray and W-H. Miller. J. Am. Chem.Soc. 103 (1981) 1904. [3] R. Krishnan, h1.F. Frisch, J.A. Pople and P_ van R. Schleyer, Chem. Phys. Letters 79 (1981) 408. [4] P. Rosmus. P. Botschwina and J-P. Maier, Chem. Phys. Letters 84 (1981) 71. [S 1 J. Chandrasekhar, R.A. Khan and P_ von R. Schleyer. Chem. Phys. Letters 85 (1982) 493.
7820.
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