Indian Journal of Chemistry
Vol. 43A, March 2004, pp. 455-457
Rapid Communication
Design of neutral hydrocarbons
having a planar tetracoordinate carbon
U Deva Priyakumar & G Narahari Sastry*
Molecular Modelling Group, Organic Chemical Sciences,
Indian Institute of Chemical Technology,
Hyderabad 500 007, India
Received 4 November 2003
Cyclopropyne (C 3H 2) and related spiro bicyclic structures show
unusual structural preference for the planar tetracoordinate structure
over the conventional tetrahedra l arrangement. C 5H 4 has been
proposed as the smallest neutral hydrocarbon with plan ar
tetracoordinate carbon . NICS values indicate substanti al
stabilization of the planar arrangements over the tetrahedral
counterparts .
The recognition of the preference of tetrahedral form
for tetracoordinate carbon atoms by van't Hoff and Le
Bel provided the basis for structural organic chemistry
and laid foundations for the third dimension in
chemical structures and bonding 1• Nearly about a
century later, Hoffmann's group using the orbital
symmetry principles critically analyzed the causative
factors for the stability of the tetrahedral arrangement
for a tetracoordinated carbon2•3 • Importantly, they put
forth strategies to stabilize the planar tetracoordinate
carbon (ptC). Over the past three decades or so, a
number of elegant computational and experimental
strategies towards stabilizing the planar tetracoordinate
carbon (ptC) have been put forward 4· 8 • Two different
approaches can be adopted to achieve to stabilize the
ptC: the first one is to force the central carbon atom to
be planar by structural constraints 9 and the next is by
electronic stabilization 10 •
Stanton et al. have reported that the planar form
of 1 (Fig. 1) is about 7 kcallmol more stable than the
tetrahedral form 11 • While such unusual structural
preference is exciting from theoretical point of view,
it failed to enthuse the chemistry community since the
planar structure, as well as the tetrahedral structure, is
not a minima on the potential energy surface. In the
present communication we venture to design, through
structural, modifications so as to obtain a viable
structure with planar tetracoordinate carbon (Fig.l ).
Hybrid density functional theory B3LYP calculations
with 6-31 G * basis set were employed using the
Gaussian 98 program package 12 •
The relative energies of the planar and tetrahedral
forms , and the number of imaginary frequencies
obtained at the B3LYP/6-31 G* level are given in
Fig. 1. Encouragingly, in all the ten cases, the planar
forms are more stable than the corresponding
tetrahedral counterparts. While most structures are not
minima on the potential energy surface, structure 3P
is a minimum. To our knowledge, this is the smallest
pure neutral hydrocarbon skeleton containing ptC,
which is characterized as a minimum on the potential
energy surface. The energy difference between the
planar and tetrahedral forms of 3 is computed to be
higher than that found in case of 1. We have examined
the shapes of the frontier molecular orbitals to
understand the unusual stability(Fig.2). Clearly, the
unusual stability of the planar arrangement is traced
to the unique bonding mode exhibited by the C 3-unit,
with a highly delocalized n:-framework which is the
case in all the structures considered. The planar
molecules exhibit substantial HOMO-LUMO energy
gap compared to the tetrahedral ones indicating higher
stability of the planar molecules except in 4 1+.
Similarly, the aromatic stabilization in this class of
compounds is demonstrated using the nucleus
independent chemical shift (NICS) values 13 • NICS
criterion has become very useful to gauge the aromatic
stabilization of conjugated sy ~ te ms in recent years. All
the planar forms have high negative NICS values
indicating aromaticity, whereas the tetrahedral forms
possess nonaromatic/antiaromatic character.
In summary, the present study reports a novel class
of ptC containing neutral hydrocarbons. All the spiro
compounds considered here exhibit higher stability for
the planar structural arrangement compared to the
traditional tetrahedral forms. The planar form of 3 is
computed to be a minimum on the potential energy
surface and is more stable than the tetrahedral structure
by about 74 kJ/mol. To our knowledge, this is the
smallest neutral hydrocarbon skeleton proposed so far
INDIAN J CHEM, SEC A, MARCH 2004
456
C><H
L><ll
H
H
lP, 0.0 (1); -18.1; 4.48
IT, 51.7 (2); 23.3; 2.78
H
2P, 0.0 (! ); -16.9; 4.54
H
2T, 30.2 (2); 0.3; 3.66
H
H
L><t:.
H
''' ...
I
I-1
,.
H
3P, 0.0 (0); -18.0 ; 4.69
4P
+,
H
0.0 (3); ·-14.4; 1.45
H
1-1
H
1
4P, 0.0 (1) ; -16.0
II
H
4T
1
+,
I-1
4T, 75.6 (1); 15.3
H
3T, 74.0 (2); 43.3 ; 2.75
II
23 .8 (3); 14.9 ; 1.48
1-l
4P
1
-,
H
0.0 (4); -26.7 ; 1.96
I-1
268.6 (3); -1.2; 0.33
I-1
H
H
H
I-I
5T, 249.3 (3); 23.6; 1.63
4T
1
-,
H
H
5P, 0.0 (I); -15.8; 4.47
I-1
H
H
6P, 0.0 (1) ; -17.5; 3.08
6T, 33.5 (2); 22.9; 2.18
SP, 0.0 (3); -17.0; 4.28
8T, 18.8 (4); 12.7; 2.89
HI-I
H
H
7P, 0.0 (2); -16.7; 4.33
H
H
7T, 21.8 (2); 13.9; 2.88
Fig. 1- The relati ve energies (kJ/mol), number of imagi nary frequencies (in parentheses), NICS values in ppm (bold face) and 1-IOMO-
LUMO energy gap in eV (i talici zed) obtained at the 133LYP/6-3JG* level.
RAPID COMMUNICATION
(a)
(b)
457
(c)
(d)
Fig. 2- The isosurfaces (obtained using MOLDEN package 14 ) of the occupied molecular orbitals corresponding to bonding (a) and
anti bonding (b) combinations of the lone pairs on the two acetylenic carbons, 7t-bonding (c) and the valence bond structure proposed (d)
for the C 3-unit. The pattern is qualitatively identical in all the structures considered.
in the literature. The high stability for the planar form
is traced to the delocalization in the C 3-ring, which is
demonstrated using the shapes of molecular orbitals,
NICS and HOMO-LUMO energy gap.
8
Choukroun R & Cassoux P, Ace Chem Res, 32 (1999) 494.
9
(a) Rasmussen D R & Radom L, Angew Chem, Int Ed, 38
(1999) 2877. (b) Wang Z- W & Schleyer P v R, JAm Chern
Soc, 124 (2002) 11979.
10
(a) Wang Z- X & Schleyer P v R, Science, 292 (2001) 2465.
(b) Exner K & Schleyer P v R, Science, 290 (2000) 1937. (c)
Merino G, Mendez-Rojas M A & Vela A, JAm Chem Soc,
125 (2003) 6026.
Acknowledgement
UDP thanks UGC, New Delhi for a senior research
fellowship. We thank Dr J S Yadav, Director, IICT
for support and encouragement.
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