Resonance structure straight arrow

The compound above has two important resonance structures. Notice that we separate resonance structures with a straight, two-headed arrow, and we place brackets around the structures. The arrow and brackets indicate that they are resonance structures of one molecule. The molecule is not flipping back and forth between the different resonance structures. 

Benzene is described as a resonance hybrid of the two extreme forms which correspond, in terms of orbital interactions, to the two possible spin-coupled pairings of adjacent p electrons structures 1 and 2. These are known as resonance contributors , or mesomeric structures , have no existence in their own right, but serve to illustrate two extremes which contribute to the real structure of benzene. Note the standard use of a double-headed arrow to inter-relate resonance contributors. Such arrows must never be confused with the use of opposing straight fish-hook arrows that are used to designate an equilibrium... 

A double-headed straight arrow <—>) between two structures indicates that they are resonance structures. Such an arrow does not indicate the occurrence of a chemical reaction. The double-headed arrows between resonance structures (Sec. 1.12) for the C=0 bond are shown above. 

The nine chemically sensible resonance structures for a double bond are shown in Scheme 1. A straight line represents a covalent bond (i.e., one electron in an M-centered AO-like MO and one electron in the E-centered counterpart), an arrow pointing towards M represents a dative bond (both electrons on the E-centered AO-like MO) and an arrow pointing towards E is a back-bond (i.e., both electrons on the M-centered AO-like MO). The bottom line (or arrow) describes the cr-bond and the upper line describes the n component of the ME bond. A triple bond can be described in a similar manner, with two sets of 7T bonds.Note that the nine resonance structures shown in Scheme 1 are less than half of the 20 configurations generated from the MCSCF(4,4) wavefunction. The remaining contributions correspond to less sensible arrangements, such as much weaker one- or three-electron bonds. 

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