Complexation of Allyl Radicals and Hydrochloric Acid in Helium Nanodroplets

Infrared spectra of the allyl radical–HCl complex in superfluid helium nanodroplets have been recorded in the IR frequency range of 2750–3120 cm–1. Six fundamental bands were observed, five of which have been assigned to the C–H stretch vibrations of the allyl radical. No additional CH bands were observed upon the binding of HCl. The band at 2800.3 cm–1 can unambiguously be assigned to the bound HCl stretch, which is red-shifted by 106 cm–1 compared to that of the free HCl. Stark spectra and pickup curves were recorded and support our assignment. In accompanying ab initio calculations, we found four equivalent minima and computed a two-dimensional potential energy surface for the HCl positioning on the allyl radical plane at the CCSD­(T)/TZVPP level. Based on our findings, we conclude that the ground-state structure of the complex shows two energetically equivalent T-shaped minimum structures. Because of small barriers between the two minima, a delocalization of the HCl is anticipated.