A hydrogen bond is a weak interaction between a hydrogen attached to an electronegative atom and another electronegative atom. An example is between an NH group and a carbonyl:

The energy of the bond depends on the distance and angle between the three atoms, θ (e.g. N-H..O). You will explore the distance dependence in this experiment. The expermental measurement is the energy of the interaction, negative means energy was released. This exercise is based on H-bond lengths in helicies and is an estimation of hydrogen bond strength in vacuum.

The energy for H-bond formation in solution is lower, because it is the difference between the energy of an H-bond with water versus the H-bond within the protein/nucleic acid.

1. Use Excel to make a plot of energy versus d for values from 2.5 to 6 Å, using 0.5 increments.
2. What distance gives the lowest energy?
3. Why does your plot have regions of unfavorable (positive) energy?

Obtain the energy versus d by entering a value for d in this box:

d =	Å
For the above value of d, Calculate E
E =	kJ/mol