The rotational spectrum and structure for the argon-cyclopentadienyl thallium van der Waals complex: Experimental and computational studies of noncovalent bonding in an organometallic π -complex

The rotational spectrum of a noble gas-organometallic complex was measured using a pulse molecular beam Fourier transform microwave spectrometer. Rotational transitions for the neutral argon-cyclopentadienyl thallium weakly bound complex were measured in the 4-9 GHz range. Analysis of the spectrum showed that the complex is a prolate symmetric-top rotor with C₅V symmetry. The experimentally determined molecular parameters for Ar- C ₅H₅ ²⁰⁵Tl are B=372.4479 (3) MHz, D _J=0.123 (2) kHz, and D_JK =0.45 (2) kHz. For Ar-C ₅H₅ T 203 l, B=373.3478 (5) MHz, D_J=0.113 (3) kHz, and D_JK=0.37 (3) kHz. Using a pseudodiatomic model with Lennard-Jones potential yields an approximate binding energy of 339 cm ^-1. The argon atom is located on the a -axis of the C ₅H₅Tl monomer, directly opposite from the thallium metal atom. The measured separation distance between argon and the cyclopentadienyl ring is R=3.56 Å. The overall size of the cluster is about 6 Å, measuring from argon to thallium. Relatively small DJ and DJK centrifugal distortion constants were observed for the complex, indicating that the structure of Ar- C₅H₅Tl is somewhat rigid. MP2 calculations were used to investigate the possible structures and binding energies of the argon-cyclopentadienyl thallium complex. Calculated, counterpoise corrected binding energies are evaluated at R=3.56 Å for Ar- C₅H₅Tl range from 334 to 418 cm^-1. The experimental binding energy ε=339 cm^-1 for Ar-C ₅H₅Tl falls within this range. The higher-level MP2/aug-cc-pVTZ-PP (thallium)/aug-cc-pVTZ(Ar, C, H) calculation with variable R yielded Re =3.46 Å and binding energy of 535 cm^-1. Our estimated binding energy for argon-cyclopentadienyl thallium is very similar to the binding energy of argon-benzene. Calculations for the new van der Waals complexes, Ar (C₅H₅Tl)₂ and (C ₅H₅Tl)₂, have been obtained, providing further information on the structures and bonding properties of previously observed cyclopentadienyl thallium polymer chains. The calculated intermolecular distance RTl-Cp =3.05 Å for the (CpTl)₂ chain subunit (Cp is cyclopentadienyl, C₅H₅) is slightly longer than the measured x-ray value RM-Cp (M=Tl) =2.75 Å. The x-ray distance RTl-Tl =5.5 Å for the chain structure is almost identical to the calculated R _Tl-Tl =5.51 Å for the (C₅H₅Tl) ₂ dimer.