Double-headed straight arrow

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. 

FIGURE 18.5 (a) The atoms in neighboring straight-chain alkanes, represented by the tubelike structures, can lie close together. Ibl Fewer of the atoms of neighboring branched alkane molecules can get so close together overall, and so the London forces (represented by double-headed arrows) are weaker and branched alkanes are more volatile. 

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... 

Figure 7 Depmrination of calf thymus DNA Obliterated dCMP (blacked rounded border star) and dlMP, alters the retention and diminishes dCMP and S-methyl-dCMP, signiFicantly increases (eighteen or greater) apurinic and 2 deoxyribose-phosphates moieties (double arrow and black star at front of chromatogram). dUMP is between two broad square parentheses. Another spot (black closed arrow heads) below dUMP s Rx, is also significant Rx. 13.1,4.4 Ambis, 3.4% densitometry, S%). The open arrow head represents unknown adducts. The closed straight arrow is dTMP the curved closed arrow is dAMP.

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