<P> An accompanying detailed technical report provides the rationale behind the new definition . </P> <P> Recent work proves that the hydrogen bond (O: H-O) integrates both the inter-molecular O: H lone pair ":" nonbond and the intra-molecular H-O polar - covelent bond associated with O-O repulsive coupling . </P> <P> A hydrogen atom attached to a relatively electronegative atom will play the role of the hydrogen bond donor . This electronegative atom is usually fluorine, oxygen, or nitrogen . A hydrogen attached to carbon can also participate in hydrogen bonding when the carbon atom is bound to electronegative atoms, as is the case in chloroform, CHCl . An example of a hydrogen bond donor is the hydrogen from the hydroxyl group of ethanol, which is bonded to an oxygen . </P> <P> In a hydrogen bond, the electronegative atom not covalently attached to the hydrogen is named proton acceptor, whereas the one covalently bound to the hydrogen is named the proton donor . </P>

What is the physical nature of the hydrogen bond