straight line formed by A and B.24 This is true both in the solid state (where X-ray
crystallography and neutron diffraction have been used to determine structures),25
and in solution.26 It is significant that the vast majority of intramolecular hydrogen
bonding occurs where six-membered rings (counting the hydrogen as one of the six)
can be formed, in which linearity of the hydrogen bond is geometrically favorable,
while five-membered rings, where linearity is usually not favored (though it is
known), are much rarer. A novel nine-membered intramolecular hydrogen bond
has been reported.27