In their paper ‘Helium chemistry: theoretical predictions and experimental challenge’ (J. Am. Chem. Soc., 1987, 109, 5917), W. Koch and co-workers used quantum mechanical calculations to demonstrate that helium can form strong bonds with carbon in cations. 

(a) Give the range of bond lengths calculated for these He–C cations. 

(b) What are the requirements for an element to form a strong bond with He? 

(c) To which branch of science do the authors suggest that this work would be particularly relevant?