Interconversions between oxalic acid monoamide rotamers: Photochemical process versus tunneling

UV irradiation converts matrix isolated (E,E)-oxalic acid monoamide (4) to its energetically less favorable (E,Z)-rotamer 5. The back-reaction 5 → 4 seems to be governed by [H]tunneling. Matrix IR spectroscopy in combination with density functional computations is the method of choice to identify the bridged and unbridged rotamers 4 and 5.