The growth of HF polymers in helium nanodroplets: Probing the barriers to ring insertion

We report the first experimental observation of (HF)(n) (n = 4,5,6) clusters, grown in helium nanodroplets. The structures of these complexes are determined from infrared laser spectroscopy, recorded in the spectral region from 3100 to 3900 cm(-1). Comparisons with previous gas-phase spectra and ab initio calculations indicate that the helium-solvated tetramer is cyclic, suggesting that the barrier to inserting an HF into a preformed cyclic trimer is small. In contrast, the cyclic pentamer is not observed. Instead, we observe four of the five H-F stretching vibrational bands of a new isomer of (HF)5, corresponding to a tetramer ring to which a fifth HF is hydrogen bonded. A large static electric field is used to establish that this (HF)5 isomer is indeed polar and to measure the angle between the permanent dipole moment and the transition dipole moments for these vibrations. These vibrational transition dipole moment angles (VTMAs) and the corresponding vibrational frequencies are also calculated at the MP2/6-311++G(3df,3pd) level of theory. Comparisons between the experimental and calculated VTMAs and the vibrational frequencies provide convincing evidence for the formation of this "4 + 1" isomer. The results are rationalized in terms of the available intermolecular potentials for these systems.