The diagram shows a triangular prism with a horizontal rectangular base ADFC, where CF = 12 units and DF = 6 units. The vertical ends ABC and DEF are isosceles triangles with AB = BC = 5 units. The mid-points of BE and DF are M and N respectively. The origin O is at the mid-point of AC.

Unit vectors i, j and k are parallel to OC, ON and OB respectively.

1. Find the length of OB.
2. Express each of the vectors \(\overrightarrow{MC}\) and \(\overrightarrow{MN}\) in terms of i, j and k.
3. Evaluate \(\overrightarrow{MC} \cdot \overrightarrow{MN}\) and hence find angle CMN, giving your answer correct to the nearest degree.