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Back to the Structure of Benzene.

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Back to the Structure of Benzene
By R . Janoschek*
In a communication entitled “On the Structure of Benzene”, Ermer“] recently demonstrated in an impressive way
that still questionable properties of the benzene molecule
trace back to seemingly well-founded fundaments such as
the Dbh symmetry of the structure. In particular, neither
our present experimental nor theoretical knowledge can
precisely exclude a structure with C C bond alternation
(D3hsymmetry), he notes.
The term “Back” in the above title should, however, be
understood in another sense-namely as referring to a supplement to Ermer’s review. Obviously, certain much discussed topics make a periodical appearence, possibly reflecting a change of generation and a changing consciousness with time. For, in the mid-sixties, it was Rogowski
whose papers and lecturesL2]on the &h-D3h problem were
the subject of heated discussions, although the corresponding electron diffraction experiments or at least their assignments were widely unaccepted. At the time also Rogowski
turned to quantum chemistry for help. Relevant computations were performed and p~blished.‘~]
A D3,? structure
could certainly not be found, but the smooth increase in
energy of only 4.6 kcal-mol-’ on eoing from Dbh (1.39/
1.39 A) to D3h symmetry (1.3511.43 A) was remarkable.
A renewed call for quantum chemical calculations o n
the structure of benzene is not unwarranted, for such calProf. Dr. R. Janoschek
lnstrtut fur Theoretische Chemie der Universitat
Mozartgasse 14, A-8010 Graz (Austria)
culations can nowadays be performed more accurately
and, therefore, more reliably than 20 years ago. The new
calculations have been investigated in the following way:
starting from the optimized Dbh structure, geometries with
C C bond alternation have been generated by choosing a
fixed length for three short C C bonds and a complete optimization of all other structural parameters. This procedure yields the optimal energy curve for a D6h+D3hdistortion which is much more informative than the calculation
of the complete set of harmonic force constants,141in particular that of the BZuvibration, which unequivocally indicates a Dbh structure, but cannot exclude a D3,, structure.
C-C,,,,(optim.) [A]
These a b initio calculations at the S C F level with the 321G* basis set (spd) indicate again no energy minimum
with D3,, symmetry. The use of an ethylene C = C bond
length *(1.339 A) and a butadiene C-C bond length
(1.483 A) for a D3h structure causes a surprisingly slight
destabilization of only 8 kcal- mol- relative to Ddh
(1.396 A).
[I] 0. Ermer, Angew. Chem. 99 (1987) 791; Angew. Chem. Int. Ed. Engl. 26
(1987) 782.
121 F. Rogowski, Dtsch. Apoth.-Zfg. 39 (1965) 1334: U. Kriiger, F. Rogowski,
Z . Nu/urfOsch. B19 (1964) 1157.
131 R. Janoschek, H. Preuss, G. Diercksen, In/. J. Quantum Chem. I S (1967)
[4] P. Pulay, G. Fogarasi, J. E. Boggs, J . Chem Phys. 74 (1981) 3999.
The manuscript of the Nobel Lecture “Concepts in
Reaction Dynamics” by John C . Polanyi (Angew. Chern.
Int. Ed. Engl. 26 (1987) 952) was inadvertently incomplete. The third full paragraph on p. 960 should read:
Turning to the experimental findings of Section 2.1, it
was evident that the moderate mean fractional conversion
of reaction energy into product vibration, (fv,), could be
explained either by a highly attractive interaction leading
to secondary encounters, or by predominantly repulsive
energy release. The first of these alternatives appeared implausible since it implied a substantially broader distribution over product vibrational and also rotational states
than was
Instead the evidence favored a
strongly repulsive PES, with the light-atom anomaly explaining the markedly reduced ( f ; ) for H + X 2 as cornpared with X + H2 o r X + HY, and the lower barrier leading to a slightly increased HA in H Br2 as compared with
H + CI, (cf. the correlation noted above) accounting for the
greater ( f ; ) observed for H Br, than for H CI2.[lb1
The success of a strongly repulsive PES in accounting
for the general form of the triangle plot for
CI HI-HCI + I is illustrated, by way of an example, in
Figure 15,I6’l which should be compared with Figure 9
above. Similar success has been obtained for the reaction
F + H,+HF+H
(Fig. 8) using a strongly repulsive
PES.f62-641In this case there is now dependable evidence
from ab initio variational treatments of FHHL65,661
that the
energy release is indeed substantially repulsive. The success of a repulsive PES in describing L + HH dynamics
will be demonstrated (for H + F 2 + H F + F ; cf. Fig. 7) in
Section 3.2.
The discussion of the previous paragraphs as it relates to
repulsive energy release is summarized pictorially in Figure 16. Though visualization of reaction dynamics is generally, and often adequately, based o n the collinear PES to
which the sliding-mass analysis applies, tests of the validity of PESs are made by 3 D trajectories.
Product rotational excitation is eliminated from the picture in the collinear world. Though this is a minor constituent of the product energy, it is revealing of the dynamics.
In the visualization of Figure 16, we have included the effect of repulsive energy release in bent configurations as
one source of product rotation. The experimental data in
the triangle plots of Figure 1 1 give persuasive evidence of
the significance of this effect in the important reaction
F + H,. Product repulsion should be expe~ted’~’.~’]
on the
basis of momentum conservation to give rise to decreasing
( f R r ) in the series F H . D > F H . H > F D - D > F D - H (the dot
indicated the locus of the repulsion); this is found theoretically in 3 D trajectory studies, and also experimentally
(see Fig. 15). The triangle ptots in Figure 1 1 correspond to
the extremes of this range of isotopic mass combinations;
( f R . )for the F H .D pathway substantially exceeds that for
Angew. Chem. I n t . Ed. Engl. 26 (1987) No. 12
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