CHM 501 Lecture

Hydrogen Bonding

A special case of dipole-dipole (maybe), but is relatively common and has large energies (approaching covalent bond energies)

Characteristics:

1. Directional

2. Linear or near linear bond angles

3. Covalent-like bond energies 10-15 kcal/mole, 40-60 kJ/mole

4. Occurs between H⁺ and elements more electronegative than H⁺ (O, N, F)

liquid water (E. Grunwald, J. Am. Chem. Soc., 1986, 108, 5719 and 5726)

Coordination Number = 4.2-4.5 !

HF has very unusual properties because of strong H-bonding

1. high bp

	hydrogen halide	bp(oC,1atm)
	HF	+9
	HCl	-87
	HBr	-67
	HI	-35

2. oligomeric in the gas phase (HF)₆ cyclic hexamer

3. weak acid in water, K_a ~ 10^-4

HF(aq) + H₂O(l) H₃O⁺(aq) + F^-(aq)

2HF(aq)H₂F⁺(aq) + F^-(aq)

HF(aq) + F^-(aq) H₂F^-(aq)

multiple competing equilibria

Are VBT and weak interactions adequate for description of the chemical and physical properties in simple molecules?

molecule	bond	bp	bond length	bond energy	reactivity	magnetic properties
N2	triple bond	77 K	110 pm	942 kJ/mol	unreactive	diamagnetic
O2	double bond	81 K	121 pm	494 kJ/mol	moderately reactive	paramagnetic
F2	single bond	85 K	141 pm	155 kJ/mol	very reactive	diamagnetic

VBT helps explain everything except the paramagnetism of oxygen - this requires a different model.