A contribution to the structure of the van-der-Waals complex phenylacetylene-argon by microwave spectroscopy and quantum chemistry

Continuing earlier work on phenylacetylene (PhAc), we have measured the molecular rotational spectra of 24 isotopologues of the van-der-Waals complex phenylacetylene-argon (PhAc-Ar) by molecular beam Fourier transform microwave spectroscopy. All atomic positions could be singly substituted or in combination with others. The inertial moments were employed to calculate the molecular structure by recent methods (rm, rs). In contrast to PhAc, the complex PhAc-Ar is an unfavorable object for this type of structure determination: (1) several atoms near the center of mass have very small coordinates in the principal inertial axis system (PAS), (2) obviously neither the simple treatment of the rovib contributions to the inertial moments by the rs-method (by the dominance of inertial moment differences) nor the more sophisticated one by the rm-method(s) can take proper account of the influence of the large amplitude vibrational motion of the weakly bound argon. Detailed quantum-chemical calculations (optimizing the full complex) succeeded in getting a better molecular structure although different methods yielded an argon position with a somewhat larger than usual range of uncertainty. Most interestingly, the results hinted at a very slight deformation of the PhAc moiety. However, with the methods presently available, a possible non-planarity of the PhAc ligand can neither be proved nor disproved. Borrowing a few elements from the quantum-chemically obtained structure and applying them as constraints within the rm(1) method eventually lead to a tolerable molecular structure also by the method of fitting inertial moments.