Computational Chemistry

1. Compare the use as a reference of two ethene molecules and of butadiene:

ab

a

a

a

pppa p

b

b a a a

a a

bbb

b b b

a

a

b

ab

Comparing cyclobutadiene with two ethene molecules:

Stabilization energy¼E CBDðÞ#E 2 ethenesðÞ¼ðÞ# 4 aþ 4 b 22 ðÞ¼aþ 2 b 0

Comparing cyclobutadiene with butadiene:

Stabilization energy¼E CBDðÞ#E butadieneðÞ¼ðÞ# 4 aþ 4 b ðÞ 4 aþ 4 : 472 b

¼# 0 : 472 b

The energy of the CBDp-system is higher than that of the butadienep-system);

recall thatbis a negative energy quantity, so#0.472bis a positive quantity.

Thus the SHM says that a cyclic array of p atomic orbitals is destabilized by the

interactions of four electrons, compared to an acyclic unbranched array.

2. Antiaromaticity [1] is the phenomenon of destabilization of certain molecules by
   interelectronic interactions, that is, it is the opposite of aromaticity [2]. The
   SHM indicates that when thep-system of butadiene is closed the energy rises,
   i.e. that cyclobutadiene is antiaromatic with reference to butadiene. In a related
   approach, the perturbation molecular orbital (PMO) method of Dewar predicts
   that union of a C 3 and a C 1 unit to form cyclobutadiene is less favorable than
   union to form butadiene [3].


3. Is one reference system better than another? Cyclobutadiene is destabilized
   relative to a butadiene reference, but has the same energy as a reference system
   of two separated ethenes. Simply closing or opening one system to transform
   it into another (e.g. butadiene € cyclobutadiene) is a less disruptive

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