Microwave and ab initio studies of the interal rotation of ethylene in the Ar-ethylene and Ne-ethylene van der Waals complexes

摘要

Rotational spectraof the weakly bound complexes Ar-ethylene and Ne-ethylene were measured with a pulsed molecular beam Fourier transform microwave spectrometer in the range from 3.5 to 26 GHz.Spectra of five isotoponmers of Ar-ethylene,namely Ar-C_2H_4,Ar_~(13)C_2H_4,Ar_C_2D_4,Ar-trans-1,2-C_2D_2H_2,andAr-cis-1,2-C_2D_2H_2,and of eight isotopomers ofNe-thylene,namely ~(20)Ne-C_2H_4,~(20)Ne-C_2D_4,~(20)Ne-trans-1,2-C_2D_2H_2,~(20)Ne-cis-1,2-C_2D_2H_2,~(22)Ne-C_2H_4,~(22)Ne-C_2D_4,~(22)Ne-trans-1,2-C_2D_2H_2,and ~(22)Ne-cis-1,2-C_2D_2H_2,were assigned and analyzed.The spectra are in accord with T-shaped,planar structurs,where the rage gas atoms are located on the b-principal inertial axis of the ethylene monome.rFor isotopomers containign C_2H_4,~(14)C_2H_4,C_2D_2,and trans-1,20C_2D_2H_2,all observed transitions are doubled due to an internal rotation motion of the ethylene subunit within the complexes.The observed transition intensities are in agreement with nuclear spin statistical weights obtained from molecular symmetry group analyses nder the assumption of an internal rotation of the ethylene unt about the C=C bond,i.e.,the out-of-plane motion.The observation of K_a=1,m=0 transitions in Ne-trans-1,2-C_2D_2H_2 provides further proof that the out-of-plane motion is responsible for the observed tuneling splittings.Information about the energy level ordering of the K_a=1,m=0 and K_a=0,m=1 states was obtained from the rotational spectra of the Ne-trans-1,2-C_2D_2H_2 isotopomers.Electronic structure calculations of Ne_C_2H_4 were done at the CCSD(T)level of theory with the aug-cc-pVTZ basis set for all atoms,supplemented with bond functions.The global minimum is at the T-shaped,planar configuration,with a distance of R=3.55 A between the Ne atom and the center-of-mass of ethylene and a well depth of -81.5 cm~(-1).One-dimensional minimum potential energy paths for possible internal rotation motions were determined.The rusults confirm that the out-of-plane motion is the preferred internal motion.The out-of-plane minimum potential energy path was used to determine the energy difference between the two lowest tunneling states using the one-dimensional fiexible model by Meyer [R.Meyer,J.Mol.Spectrosc.76,266 (1979)].