Hydrogen bonds (HBs) are the most important 'weak' interactions encoun
tered in solid, liquid and gas phases, The HE can be defined as an att
ractive interaction between two molecular moieties in which at least o
ne of them contains a hydrogen atom that plays a fundamental role, Cla
ssical HBs correspond to those formed by two heteroatoms, A and B, wit
h a hydrogen atom bonded to one of them and located approximately in b
etween (A-H ... B). Recently, knowledge of the number of functional gr
oups which act as hydrogen bond donors or accepters has increased cons
iderably and most of these new groups are discussed.