Microwave Spectrum, Dipole Moment, and Internal Dynamics of the Methyl Fluoride−Carbonyl Sulfide Weakly Bound Complex

Abstract

Rotational spectra for the normal and four isotopically substituted species of the 1:1 complex between methyl fluoride (H3CF) and carbonyl sulfide (OCS) have been measured using Fourier-transform microwave spectroscopy in the 5-16 GHz frequency region. The observed spectra fit well to a semirigid Watson Hamiltonian, and an analysis of the rotational constants has allowed a structure to be determined for this complex. The dipole moment vectors of the H3CF and OCS monomers are aligned approximately antiparallel with a C...C separation of 3.75(3) A and with an ab plane of symmetry. The values of the Pcc planar moments were found to be considerably different from the expected rigid values for all isotopologues. An estimate of approximately 14.5(50) cm-1 for the internal rotation barrier of the CH3 group with respect to the framework of the complex has been made using the Pcc values for the H3CF-OCS and D3CF-OCS isotopic species. Two structures, very close in energy and approximately related by a 60 degrees rotation about the C3 axis of the methyl fluoride, were identified by ab initio calculations at the MP2/6-311++G(2d,2p) level and provide reasonable agreement with the experimental rotational constants and dipole moment components.

Internal rotation effects in the pulsed jet rotational spectrum of the trifluoromethane–carbon dioxide dimer

“…The two lowest energy structures (Fig. 2) were similar to those observed for the TFM-OCS [1], DFM-OCS [2], and FM-OCS [3] complexes, with a CHÁ Á ÁO contact between the methane subunit and the O atom of the OCS or CO 2 . In the minimum energy structure (Fig.…”

“…In fact, the planar moment of the monomer is 44.62 uÅ 2 [6], differing from the dimer value of 44.9903(2) uÅ 2 by only about 0.37 uÅ 2 . The small difference between the values can be attributed to the large amplitude motions within the complex, which are likely to slightly contaminate the derived moments of inertia [3]. Unfortunately, the planar moment of the complex provides no information on whether the equilibrium structure more closely resembles ab initio structures I or II (Fig.…”

“…Two possible fixed values of the TFM angle were used: first, it was fixed to the angle obtained Table 5 Definitions of symbols used in the internal rotation (H ir ) and internal rotationdistortion (H ird ) terms of the Hamiltonian for the TFM-CO 2 complex Symbol Definition F 0 Rotational constant of the internal rotor; F 0 = ⁄ 2 /2I a F Reduced rotational constant of the internal rotor; F = ⁄ 2 /2I r P a Angular momentum operator of the internal rotor P r Angular momentum operator along the principal (rho) axis of the top P Total angular momentum operator P g Angular momentum operators about each of the principal axes of the complex; g = a, b, c V 3 Three-fold barrier to internal rotation a Torsional angle of the internal rotor I a Moment of inertia of the internal rotor I r (1 À Rk g q g )I a q…”

“…Trifluoromethane-CO 2 (TFM-CO 2 ) is the latest in our series of systematic studies involving fluorinated methanes complexed with OCS and CO 2 (TFM-OCS [1], difluoromethane-OCS (DFM-OCS) [2], fluoromethane-OCS (FM-OCS) [3] and TFM-CO 2 ). These very near-prolate dimers are characterized by an intermolecular CHÁ Á ÁO interaction, and typically also show a secondary interaction between the fluorine atom(s) and the carbon atom of the OCS or CO 2 .…”

“…of up to 70 cm À1 . TFM-CO 2 is the first complex in the TFM-OCS, TFM-CO 2 , DFM-OCS, FM-OCS series, however, to exhibit clearly resolved internal rotation doublets in its spectrum, with typical A-E splittings on the order of a few hundred kilohertz (the FM-OCS spectrum contains perturbations of the moments of inertia that were attributed to a low barrier internal rotation of the methyl top, but so far only A state transitions have been located [3]). …”