Sequential Capture of O(P) and HCN by Helium Nanodroplets: Infrared Spectroscopy and ab Initio Computations of the Σ O-HCN Complex.

Catalytic thermal cracking of O is employed to dope helium droplets with O(P) atoms. Mass spectrometry of the doped droplet beam reveals an O dissociation efficiency larger than 60%; approximately 26% of the droplet ensemble is doped with single oxygen atoms. Sequential capture of O(P) and HCN leads to the production of a hydrogen-bound O-HCN complex in a Σ electronic state, as determined via comparisons of experimental and theoretical rovibrational Stark spectroscopy. Ab initio computations of the three lowest lying intermolecular potential energy surfaces reveal two isomers, the hydrogen-bound (Σ) O-HCN complex and a nitrogen-bound (Π) HCN-O complex, lying 323 cm higher in energy. The HCN-O to O-HCN interconversion barrier is predicted to be 42 cm. Consistent with this relatively small interconversion barrier, there is no experimental evidence for the production of the nitrogen-bound species upon sequential capture of O(P) and HCN.