Rotational Spectrum and Structure of the Carbonyl Sulfide−Trifluoromethane Weakly Bound Dimer

Abstract

Pure rotational spectra of five isotopomers of the 1:1 weakly bound complex formed between carbonyl sulfide and trifluoromethane (TFM) have been measured using Fourier transform microwave spectroscopy. The experimental rotational constants and dipole moment components are consistent with a structure of C(s) symmetry in which the dipole moment vectors of OCS and HCF(3) are aligned antiparallel and at an angle of about 40 degrees and with a center of mass separation of 3.965(26) A. The derived H...O distance is 2.90(5) A, which is up to 0.6 A longer than is seen in other similar TFM complexes exhibiting C-H...O interactions. Ab initio calculations at the MP2/6-311++G(2d,2p) level give a structure with rotational constants that are in reasonable agreement with those of the normal isotopic species.

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

“…The experimental values are nearly identical to those calculated for the TFM-OCS dimer (k s = 1.2(1) N m À1 and E B = 1.6(1) kJ mol À1 ) [1], indicating that the switch of partner from a polar OCS molecule to non-polar CO 2 has little effect on the strength of the intermolecular interaction (although it is important to keep in mind that these values are a rough estimate, especially considering the approximate nature of the experimental structure in the case of TFM-CO 2 ). The force constant and binding energy estimated by the same method for the FM-OCS [3] complex is slightly larger (k s = 3.25 (7) and E B = 3.5(1) kJ mol À1 ) [3], and this can likely be attributed to the difference in dipole moment between the TFM and FM monomers.…”

“…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 all the other members of this series studied so far (TFM-OCS [1], DFM-OCS [2], FM-OCS [3]) there has been no direct indication of splittings into A and E states due to internal rotation of the fluorinated subunit, although FM-OCS shows a large discrepancy between observed and predicted planar moments, which can most likely be attributed to a very low barrier internal rotation of the methyl group which would shift many of the E-state transitions out of the frequency range of the spectrometer [3]. Calculations using the difference between observed and expected planar moments have given an estimate of the reduced barrier (s = 4V 3 /9F) in FM-OCS of 1.1 [3], compared to 45 (from the XIAM fit) for TFM-CO 2 .…”

“…The R CÁ Á ÁC distance, h O-CÁ Á ÁC angle and h H-CÁ Á ÁC angle for TFM-OCS are 3.642(17) Å, 60.2(1)°and 81.1(26)°, respectively [1], using the inertial fit values. The angles are essentially the same as the inertial fit angles for TFM-CO 2 (62(3)°and 80(7)°), while the distance in TFM-OCS is only about 0.07 Å longer than in TFM-CO 2 .…”

“…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 .…”