[6] His work was interrupted in October, 1943 when he and other university staff members were arrested by the Nasjonal Samling and handed over to the occupation authorities.

He introduced the Norwegian scientific community to the concepts of the electric dipole moments and electron diffraction.

He focused his research on ring-shaped carbon molecules, which he suspected filled three dimensions instead of two, the common belief of the time.

By using the number of bonds between the carbon and hydrogen atoms, Hassel demonstrated the impossibility of the molecules existing on only one plane.

Hassel was awarded the Nobel Prize in Chemistry in 1969, shared with English chemist Derek Barton.