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

• Published in: Chemphyschem 2011-07, Vol.12 (10), p.2009-2017
• Main Authors: Metzelthin, Anja, Sánchez-García, Elsa, Birer, Özgür, Schwaab, Gerhard, Thiel, Walter, Sander, Wolfram, Havenith, Martina
• Format: Article
• Language: English
• Subjects: Ab initio calculations; aggregation; Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; hydrogen bonds; Infrared spectra; molecular dynamics; Molecular dynamics and other numerical methods; Molecular properties and interactions with photons; Molecular spectra; Physics; quantum fluids
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201001040

We have studied the aggregation process of (C2H2)⋅⋅⋅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Hasym 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HAc⋅⋅⋅πFu bond, the third is a nearly planar ring containing a CHAc⋅⋅⋅OFu 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/r3 dipole–dipole interaction (”electrostatic steering”). This mechanism is expected to be a general mechanism in ultracold chemistry. Brrrr—cold! The aggregation process of (C2H2)⋅⋅⋅furan trimers at ultracold temperatures (0.37 K) in helium nanodroplets is studied by a combination of computational and spectroscopic techniques. The results unravel a special aggregation mechanism for non‐ and weakly polar molecules, which is expected to be generally applicable in ultracold chemistry.