The Growth of HF Polymers in Helium Nanodroplets:  Probing the Barriers to Ring Insertion

Abstract

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.

The isomers of HF–HCN formed in helium nanodroplets: Infrared spectroscopy and ab initio calculations

“…The static electric field was calibrated using the method discussed in detail elsewhere. 38 For the studies reported here, a tunable F-center laser was used to excite both the C-H and H-F stretches of the complexes. The vibrational energy imparted to the molecules is rapidly relaxed to the helium, resulting in the evaporation of several hundred helium atoms.…”

Spectroscopy and dynamics in helium nanodroplets

“…Such barriers are particularly important when preformed hydrogen bonded rings must be broken up to reach the minimum energy structure for the next larger complex. In a study of the growth of HF polymers in helium, it was found that a cyclic tetramer is formed (which requires insertion into the cyclic trimer), but that the fifth HF is not able to enter the ring and thus leads to a polar pentamer [226]. The formation of cyclic water complexes has also been studied [227,228].…”

Spectroscopy of free radicals and radical containing entrance-channel complexes in superfluid helium nanodroplets

“…With this theoretical work supporting the existence of a barrier between the hydrogen bonded and the chemical bonded minima, there is good reason to think that the pre-reactive dimer, as well as larger nanoclusters, can be stabilized by following the minimum energy approach path analogous to the production of HCN chains [60], cyclic water hexamer [61], and polymers of HF [203][204][205][206]. (H 2 ) n -HF clusters, which are a good model for the OH/H 2 metastable system, have also been studied in helium droplets [207][208][209][210].…”