The linear combination of the (ψ3 = C1ψ1 + C2ψ2) is normally the third
quantum state |ψ3>, which is described by a wave
function ψ3 and will describes the actual benzene molecule (resonance
hybrid). The coefficients C1 and C2 will specify the contributions of
resonance structures (Kekule structures, for our case C1/C2 = 1,
|С1|^2 = |С2|^2 = 0.5) in a
real molecule of benzene (resonance hybrid), which can not be described
by separate resonance structures. And now it is important to note that
the real molecule of benzene it really real and unique, and can not have
a discrete description, ie the measurements we will never ”see” the one
Kekule molecule (resonance structure 1), then another (resonance
structure 2), and this is directly contrary to the principle of quantum
superposition. Moreover, adopted in theory of resonance Kekule
structure, ie resonance structures are ideal structures that do not
exist in reality, because they have all the bonds are equal, and this is
despite the alternation of single and double C-C bonds, which in reality
are different length. Therefore, the principle of quantum superposition
(for resonance theory) is not executed, even for quantum states
|ψ1> and |ψ2>, because as
resonance structures is not reality, and any physical quantity (real) we
can not measure.
If, however, as the resonance structures we take the real ”curved”
Kekule structures with ties that have different lengths, then the
resonance theory does not make sense, since the transition from one
structure to the other will vary internuclear distance. But then the
principle of quantum superposition is applicable to all three quantum
states (for |ψ1> and |ψ2> it
is obvious, and for |ψ3> will be the one ”twisted”
Kekule structure, then the other, at 50 : 50). In fact, no change bond
lengths have therefore chemistry is not applicable.
Especially notice, that resonance structures (Kekule structures with the
same C-C bonds) is not electronic tautomers. Real benzene molecule is
unique and no tautomerism exist. Incorrect represent benzene molecule as
transitions between electronic tautomers (electrons in benzene does not
migrate from one bond to another). There is a single benzene molecule,
which has a constant electron distribution in time and which can not be
represented by a discrete description. Therefore, it is accepted in
resonance theory that every resonance structure contributes to the a
real molecule (resonance hybrid) (usually chemists determined the
contribution by the ”eye”). And the third quantum state
|ψ3> describes the real benzene molecule, which
represents something between a resonance structures (remember the
contribution of each resonance structure to the real structure
(resonance hybrid)).