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Theoretical structures for protonated methane and protonated ethane
Tetrahedron Letters No. 31, pp. 2699-2701, '970.
pergamon Press.
TRROP.RTICAL STRUCTURRS FORPROTONATEDMRTRANR
Printed in Great Britain.
ANDPROTONATEDETRANR
W. A. Lathan, W. J. Hehre and J. A. Pople Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213 (Received in USA 4 May 1970; received in UK for publication 8 June 1970) Protonated methane (CR,+) and protonated ethane (C2H,+) are of interest ae simple models for electrophilic
attack on alkanes.
We wish to report theoretical structures for
these species using _ab initio molecular orbital theory with a minimal basis set of Slatertype atomic orbitals. The actual basis functions are linear combinations of three gaussian functions which are fitted to Slater orbital6 by least-squares methods (STO-SC).' exponents
(S-values) were used throughout.
Standard molecular
All geometrical parameters were varied (subject
only to certain symmetry constraints) until the calculated total energy was minimized. For CR3+, we considered a trigonal bipyramid
(D3h), a square pyramid (C4v) and . (I). The optimum energies were
the less symmetric (Cs) structure auown in the figure found to be ordered E(D3h) > E(C4V) > E(Cs).
For the Cs structure, the geometrical
parameters are r' r3
= 1.098 i
r2
= 1.370 ;
r4
CL = 140.00 9 This structure corresponds molecule.
- 37.2'
- 1.106 ; - 1.367 1
8
- 03.8"
5
= 117.7O
to a rather loose complex between a CR3+ ion and a hydrogen
The energy of the C4v structure is about 2 kcal/mole higher.
tion energy for hydrogen rearrangement within the ion. other workers. 2,3
2699
This is an activa-
These conclusions parallel those of
No. 31
2700
+
For C2H, , there is a previoussemi-empiricalstudy by Olah, Klopman and Schlosbarg,4but without full geometricalvariation. We have made a carefulstudy of the two structuresillustratedin the figures (II and III). StructureII (syrmnetry C2v) correspondsto a protonatedC-C bond. StructureIII is analogousto the lowest energy form of cli5+and can be consideredas a complexbetween an ethyl cation and a hydrogenmolecule (protons6 and 7). III correspondato protonationof a C-H bond. Toe calculationsshow that the protonatedC-C structureII has the lowest energy (11 kcal/molebelow the lowest energy for III). The geometricalparametersfor II are r, - 2.362 i r3 = 1.097 i CL = 52.7O
r2 * 1.251 ; r4
= 1.094 i
B - 09.5O
8 - 115.6' The bridging proton H7 is at the vertex of a very flat iioscelestrianglewith a long C-C distance. This suggests that protonationof alkanes can easily lead to carbon-carbon cleavage. The optimum structurefor III is a very loose C2H5+-H2 complex,the distance r5 being 2.746 i.
It appears, therefore,that C2H5' has a smallerhydrogenmolecule affinity
+ than CH3 , presumablybecause the additionalmethyl group donates electronsinto the vacant orbital on the carboniumcenter,making it less availablefor the electronsof an approachinghydrogenmolecule. A number of other structuresincludingmethylatedversions + of the D3h form of CH5 were tested,but not refined. These all led to higher energies. Thus, although the completepotentialsurface has not been explored,these studies do
+
indicate that the C-C bridged form II is the lowest energy configurationof =ZH7 ' Acknowledgments This researchwas supportedin part by National Science FoundationGrant GP-9338. References 1. W. J. Hehre, R. F. Stewart and J. A. Pople, J.Chem.Phys.2,
2657 (1969).
2. W. Th. A. M. Van der Lugt and P. Ros, Chem.Phys.Ltrs. 4, 389 (1969). 3. H. Kollmar and B.*O. Smith, Chem.Phys.Ltrs. 5, 7 (1970). 4. G. A. Olah, G. Klopman and R. H. Schlosberg,J.Am.Chem.Soc.9l, 3261 (1969).
2701
No. 31
I
"5
"6
I\
/\
H,C,H2=HsC2H4=8
Hz”, “,
Cl",
=C '9
pergamon Press.
TRROP.RTICAL STRUCTURRS FORPROTONATEDMRTRANR
Printed in Great Britain.
ANDPROTONATEDETRANR
W. A. Lathan, W. J. Hehre and J. A. Pople Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213 (Received in USA 4 May 1970; received in UK for publication 8 June 1970) Protonated methane (CR,+) and protonated ethane (C2H,+) are of interest ae simple models for electrophilic
attack on alkanes.
We wish to report theoretical structures for
these species using _ab initio molecular orbital theory with a minimal basis set of Slatertype atomic orbitals. The actual basis functions are linear combinations of three gaussian functions which are fitted to Slater orbital6 by least-squares methods (STO-SC).' exponents
(S-values) were used throughout.
Standard molecular
All geometrical parameters were varied (subject
only to certain symmetry constraints) until the calculated total energy was minimized. For CR3+, we considered a trigonal bipyramid
(D3h), a square pyramid (C4v) and . (I). The optimum energies were
the less symmetric (Cs) structure auown in the figure found to be ordered E(D3h) > E(C4V) > E(Cs).
For the Cs structure, the geometrical
parameters are r' r3
= 1.098 i
r2
= 1.370 ;
r4
CL = 140.00 9 This structure corresponds molecule.
- 37.2'
- 1.106 ; - 1.367 1
8
- 03.8"
5
= 117.7O
to a rather loose complex between a CR3+ ion and a hydrogen
The energy of the C4v structure is about 2 kcal/mole higher.
tion energy for hydrogen rearrangement within the ion. other workers. 2,3
2699
This is an activa-
These conclusions parallel those of
No. 31
2700
+
For C2H, , there is a previoussemi-empiricalstudy by Olah, Klopman and Schlosbarg,4but without full geometricalvariation. We have made a carefulstudy of the two structuresillustratedin the figures (II and III). StructureII (syrmnetry C2v) correspondsto a protonatedC-C bond. StructureIII is analogousto the lowest energy form of cli5+and can be consideredas a complexbetween an ethyl cation and a hydrogenmolecule (protons6 and 7). III correspondato protonationof a C-H bond. Toe calculationsshow that the protonatedC-C structureII has the lowest energy (11 kcal/molebelow the lowest energy for III). The geometricalparametersfor II are r, - 2.362 i r3 = 1.097 i CL = 52.7O
r2 * 1.251 ; r4
= 1.094 i
B - 09.5O
8 - 115.6' The bridging proton H7 is at the vertex of a very flat iioscelestrianglewith a long C-C distance. This suggests that protonationof alkanes can easily lead to carbon-carbon cleavage. The optimum structurefor III is a very loose C2H5+-H2 complex,the distance r5 being 2.746 i.
It appears, therefore,that C2H5' has a smallerhydrogenmolecule affinity
+ than CH3 , presumablybecause the additionalmethyl group donates electronsinto the vacant orbital on the carboniumcenter,making it less availablefor the electronsof an approachinghydrogenmolecule. A number of other structuresincludingmethylatedversions + of the D3h form of CH5 were tested,but not refined. These all led to higher energies. Thus, although the completepotentialsurface has not been explored,these studies do
+
indicate that the C-C bridged form II is the lowest energy configurationof =ZH7 ' Acknowledgments This researchwas supportedin part by National Science FoundationGrant GP-9338. References 1. W. J. Hehre, R. F. Stewart and J. A. Pople, J.Chem.Phys.2,
2657 (1969).
2. W. Th. A. M. Van der Lugt and P. Ros, Chem.Phys.Ltrs. 4, 389 (1969). 3. H. Kollmar and B.*O. Smith, Chem.Phys.Ltrs. 5, 7 (1970). 4. G. A. Olah, G. Klopman and R. H. Schlosberg,J.Am.Chem.Soc.9l, 3261 (1969).
2701
No. 31
I
"5
"6
I\
/\
H,C,H2=HsC2H4=8
Hz”, “,
Cl",
=C '9