Hydrogen Bond

• The attraction of the hydrogen bond permits a closer approach of B to A than would have been expected based on the sizes of sigma for these three atoms.
• A and B tend to be electronegative atoms (such as F, O, N) and not too large (Cl)
• The strength of the hydrogen bond can be in the range 1-10 kcal/mol
  • Roughly an order of magnitude weaker than covalent bonds.
  • Roughly an order of magnitude stronger than van der Waals forces.
• Pairs of functional groups that contain both a potential hydrogen bond donor (A-H) and a potential hydrogen bond acceptor (B) often interact by forming a cyclic structure with two hydrogen bonds. Examples
  • Carboxylic acid dimer, O-H donor and C=O acceptor, eight atoms in the cycle.
  • Urazole dimer, N-H donor and C=O acceptor, eight atoms in the cycle.

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