Rotational Spectrum of the Dimethyl Ether−Acetylene Complex:  Evidence for an Effective C 2 v Geometry

The rotational spectra for five isotopomers of the 1:1 weakly bound complex formed between dimethyl ether (DME) and acetylene (HCCH) have been measured by Fourier transform microwave spectroscopy. The experimental rotational constants, planar moments, and dipole moment components are consistent with a floppy complex possessing an effective C 2 v structure in which the hydrogen atom of acetylene is hydrogen bonded to the oxygen atom of dimethyl ether with an intermolecular H···O separation of 2.08(3) Å. Experimental rotational constants for the normal isotopic species are A = 10382.5(17) MHz, B = 1535.7187(18) MHz, and C = 1328.3990(17) MHz and the dipole moment components are μ a = μtotal = 1.91(10) D. Ab initio calculations at the MP2/6-311++G(2d,2p) level indicate that the energy barrier for motion of the HCCH subunit between the lone pairs of the DME, via a C 2 v intermediate structure, is very low (∼0.29 kJ mol-1). Inclusion of basis set superposition error and zero point energy corrections to the energies of four stationary points located on the potential energy surface shows that the relative stabilities are particularly sensitive to these corrections. The ab initio optimizations give rotational constants for the C 2 v structure of A = 10066 MHz, B = 1496 MHz, and C = 1324 MHz, and a dipole moment of μ a = μtotal = 2.12 D, in reasonable agreement with the experimentally determined values. The structural parameters and energetics of the DME−HCCH complex will be discussed and compared to similar complexes such as H2O−HCCH.