Michal M. Serafin

Rotational spectrum, structure and modeling of the OCS–CS2 van der Waals dimer

The rotational spectrum of a 1 : 1 weakly bound complex between OCS and CS2 has been measured by Fourier-transform microwave spectroscopy, giving ground state rotational constants of A = 2369.6942(9) MHz, B = 994.4467(6) MHz and C = 700.5137(3) MHz for the normal isotopic species. The experimental dipole moment components are μa = 0.2893(4) D and μb = 0.6364(27) D, with μtotal = 0.6991(25) D. The rotational constants and dipole moment components are consistent with a structure of Cs symmetry, in which the CS2 and OCS monomers are aligned almost parallel to one another, with a center of mass separation of 3.8017(2) A. This structure is in good agreement with the lowest energy geometry obtained from an ab initio calculation at the MP2/6-311++G(2d,2p) level which predicts rotational constants of A = 2322 MHz, B = 1036 MHz and C = 716 MHz and dipole moment components of μa = 0.32 D and μb = 0.69 D. Semi-empirical modeling using the ORIENT program gives similarly good agreement, although the predicted rotational constants and dipole moment are a little further from the experimental results (A = 2458 MHz, B = 1027 MHz and C = 725 MHz and μa = 0.34 D and μb = 0.68 D).

Dimers of fluorinated methanes with carbonyl sulfide: the rotational spectrum and structure of difluoromethane-OCS.

The pure rotational spectra of four isotopologues of the difluoromethane-carbonyl sulfide dimer have been measured in the 5-15 GHz region with use of pulsed-nozzle Fourier-transform microwave spectroscopy. The complex was determined to possess an ab plane of symmetry with a center of mass separation of 3.41(2) A and dipole moment components mu(a) = 1.1386(18) D, mu(b) = 0.4840(63) D, mu(total) = 1.2372(41) D. Experimental planar moments indicate that the two fluorine atoms straddle the symmetry plane while one of the C-H bonds of the difluoromethane monomer is aligned to interact with the oxygen atom of the OCS molecule. The assignment of the rotational spectrum for this dimer completes the experimental studies of the series of dimers involving fluorinated methanes (HCF(3), H(2)CF(2), and H(3)CF) complexed with OCS and makes possible a comparison of properties within this series.

Microwave spectra and barrier to internal rotation in cyclopropylmethylsilane.

Rotational spectra for 3 silicon isotopologues (28Si, 29Si, 30Si) of cyclopropylmethylsilane (c-C3H5SiH2CH3) have been observed in natural abundance using Fourier-transform microwave spectroscopy, and the dipole moment of the most abundant (28Si) isotopologue has been determined using the Stark effect. The observed rotational constants (A = 8800.5997(9) MHz; B = 2238.6011(3) MHz; C = 2001.0579(3) MHz) and dipole moment components (mu(a) = 0.195(2) D, mu(b) = 0.674(11) D, mu(c) = 0.362(19) D, mu(total) = 0.790(13) D) for the 28Si species are consistent with ab initio predictions (MP2/6-311+G(d)) for a gauche conformation about the Si-cyclopropyl bond. All of the observed transitions were split into doublets due to internal rotation of the methyl group, allowing a determination of the V3 barrier to internal rotation of 6.671(9) kJ mol(-1) for the most abundant isotopologue. This barrier will be compared to those for other Si-CH(3) containing compounds and will be related to a partial structure determination from the available microwave and ab initio data.

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

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.