Changes

One of the molecular orbitals is bonding, one almost nonbonding, and the third antibonding.  These three orbitals need to accompodate four electrons (two from the orginal A-H nond and two from the lone pair of B), so two enter the bonding orbital and two enter the nonbonding orbital.  Because the antibonding orbital remains empty, the net effect-depending on the precise location of the almost nonbonding orrbital-may be a lowering of energy.

One of the molecular orbitals is bonding, one almost nonbonding, and the third antibonding.  These three orbitals need to accompodate four electrons (two from the orginal A-H nond and two from the lone pair of B), so two enter the bonding orbital and two enter the nonbonding orbital.  Because the antibonding orbital remains empty, the net effect-depending on the precise location of the almost nonbonding orrbital-may be a lowering of energy.

In practice, the strength of the bond is found to be about 20kJ mol^-1.  Because the bonding depends on orbital overlap, it is virtually a contact-like interaction that is turned on when AH touches B and is zero as soon as the contact is broken.  If hydrogen bonding is present, it dominates the other intermolecular interactions.  The properties of liquid and solid water, for example, are dominated by the hydrogen bonding between H₂O molecules.  The structure of DNA and hence the transmission of genetic information is crucially dependent on the strength of hydrogen bonds between base pairs.  The structural evidence for hydrogen bonding comes from noting that the internuclear distance between formally non-bonded atoms is less than their van der Waals contact distance, which suggests that a dominating attractive interaction is present.  For example, the O-O distance in O-H^...O is expected to be 280 pm on the basis of van der Waals radii, but is found to be 270 pm in typical compounds.  Moreover, the H^...O distance is expected to be 260 pm but is found to be only 170 pm.

In practice, the strength of the bond is found to be about 20kJ mol⁻¹.  Because the bonding depends on orbital overlap, it is virtually a contact-like interaction that is turned on when AH touches B and is zero as soon as the contact is broken.  If hydrogen bonding is present, it dominates the other intermolecular interactions.  The properties of liquid and solid water, for example, are dominated by the hydrogen bonding between H₂O molecules.  The structure of DNA and hence the transmission of genetic information is crucially dependent on the strength of hydrogen bonds between base pairs.  The structural evidence for hydrogen bonding comes from noting that the internuclear distance between formally non-bonded atoms is less than their van der Waals contact distance, which suggests that a dominating attractive interaction is present.  For example, the O-O distance in O-H^...O is expected to be 280 pm on the basis of van der Waals radii, but is found to be 270 pm in typical compounds.  Moreover, the H^...O distance is expected to be 260 pm but is found to be only 170 pm.

Hydrogen bonds may be either symmetric or unsymmetric.  In a symmetric hydrogen bond, the H atoms lies midway between the two other atoms.  This arrangement is rare, but occurs in F-H^...F^-, where both bond lengths are 120 pm.  more common is the unsymmetrical arrangement, where the A-H bond is shorter than the H^...B bond.  Simple electrostatic arguments, treating A-H^...B as an array of point charges (partial negative charges on A and B, partial positive on H) suggest that the lowest energy is achieved when the bond is linear, because then the two partial negative charges are furthest apart.  The experimental evidence from structural studies support a linear or near-linear arrangement.

Hydrogen bonds may be either symmetric or unsymmetric.  In a symmetric hydrogen bond, the H atoms lies midway between the two other atoms.  This arrangement is rare, but occurs in F-H^...F^-, where both bond lengths are 120 pm.  more common is the unsymmetrical arrangement, where the A-H bond is shorter than the H^...B bond.  Simple electrostatic arguments, treating A-H^...B as an array of point charges (partial negative charges on A and B, partial positive on H) suggest that the lowest energy is achieved when the bond is linear, because then the two partial negative charges are furthest apart.  The experimental evidence from structural studies support a linear or near-linear arrangement.