Acetylene⋅⋅⋅Furan Trimer Formation at 0.37 K as a Model for Ultracold Aggregation of Non‐ and Weakly Polar Molecules

Abstract

We have studied the aggregation process of (C(2)H(2))⋅⋅⋅furan trimers at ultracold temperatures (0.37 K) in helium nanodroplets. Computational sampling of the potential energy surface using the multiple-minima-hypersurface (MMH) approach yielded seven possible minimum structures, optimized at the MP2 level of theory with the cc-pVTZ and 6-311++G(d,p) basis sets. Experimentally, we could assign five transitions in the IR spectrum of acetylene-furan aggregates in the acetylene C-H(asym) stretch region between 3240 and 3300 cm(-1) to vibrational bands of the 2:1 acetylene-furan trimer. The transitions were assigned to three ring structures that all contain the T-shaped acetylene dimer as structural sub-motif. Two of the structures form a nonplanar ring involving a C-H(Ac) ⋅⋅⋅π(Fu) bond, the third is a nearly planar ring containing a C-H(Ac) ⋅⋅⋅O(Fu) bond. This assignment was corroborated by quantum mechanical/molecular dynamics (QM/MD) simulations mimicking in detail the aggregation process of precooled monomers. The simulations provided evidence for a transition from a higher level local minimum to the global minimum state over a small barrier during the aggregation process. The experimentally observed structures can be explained by a step-by-step aggregation of moieties pre-cooled to 0.37 K that are steered by intermediate and short-range electrostatic interactions. Thus, we are able to unravel a special aggregation mechanism which differs from aggregation of molecules with large dipole moments where this aggregation process is dominated by long range 1/r(3) dipole-dipole interaction ("electrostatic steering"). This mechanism is expected to be a general mechanism in ultracold chemistry.

Helium droplet infrared spectroscopy of glycine and glycine–water aggregates

“…the acetylene-furan trimer. 33 Therefore, we suggest that structures which correspond to a global minimum on the potential energy surface are formed preferentially, since a rotational realignment subsequent to cooling is feasible.…”

Infrared laser spectroscopy of the helium-solvated allyl and allyl peroxy radicals

“…43 In these cases, the intact gas-phase molecules were picked-up by the droplets, and the equilibrium conformer distribution at the vapor temperature was largely preserved upon cooling to 0.4 K. 44,45 Moreover, as molecular cluster systems condense within He droplets, metastable structures are often kinetically trapped behind small isomerization barriers (greater than about 100 cm −1 ), 39,40,[46][47][48] although there are notable exceptions. 49,50 For these systems, the sequential pick-up process results in cold monomers that condense and produce relatively small amounts of intermolecular vibrational kinetic energy, which can be effectively dissipated prior to cluster rearrangement.…”

Sequential Capture of O(3P) and HCN by Helium Nanodroplets: Infrared Spectroscopy and ab Initio Computations of the 3Σ O–HCN Complex

“…We are left to conclude either that zero-point/nonadiabatic effects render the interconversion of N-bound isomers barrierless or that, upon complexation in the vicinity of the N-bound potential well, the helium droplet is not capable of quenching the kinetic energy gain prior to its rearrangement over (or through) the ∼40 cm –1 barrier. This is not without precedent; previous work on (HF) n clusters and acetylene/furan clusters (for example) showed that barriers of similar magnitudes could be overcome during the aggregation process within the helium droplet. Future computations of the O–HCN potential surfaces, including spin–orbit coupling and nonadiabatic effects, , may be capable of providing further insight into this issue.…”