C--H HYDROGEN BONDING INTERACTIONS IN THE CARBONYL SULFIDE--FLUOROFORM DIMER

Author Institution: Department of Chemistry, Eastern Illinois University, 600 Lincoln Ave., Charleston, IL 61920 USAThe rotational spectrum of the weakly bound dimer of carbonyl sulfide (OCS) and fluoroform (HCF$_3$) has been assigned using Fourier-transform microwave spectroscopy. The rotational constants for the normal isotopomer are $A = 4745.7157(25)$ MHz, $B = 813.9222(27)$ MHz, and $C = 790.9238(27)$ MHz, and dipole moment components are $\mu_a = 0.828(4)$ D and $\mu_b = 0.858(7)$ D. The experimental rotational constants are found to be in very good agreement with ab initio optimizations at the MP2/6-311++G(2d,2p) level of calculation. This $C_s$ symmetry structure has the OCS and HCF$_3$ aligned such that there is a C--H...O and C--F...C interaction, with an approximate C...C intermolecular distance of 3.60 \AA

CHARACTERIZATION OF WEAKLY BOUND HCF3_3--OCS, H2_2CF2_2--OCS, H3_3CF--OCS, AND HCF3_3--CO2_2 VAN DER WAALS COMPLEXES BY AB INITIO CALCULATIONS AND MICROWAVE SPECTROSCOPY

Author Institution: Department of Chemistry, Eastern Illinois University, 600; Lincoln Ave., Charleston, IL 61920 USAHigh resolution Fourier-transform microwave spectroscopy has been utilized in a systematic study of the structural and dynamic properties of a series of fluorinated methane molecules complexed to carbonyl sulfide and carbon dioxide. Theoretical data from \textit{ab initio} calculations at the MP2/6-311++G(2d, 2p) level have provided good agreement with the observed experimental values. The structures of HCF$_3$--OCS and H$_2$CF$_2$--OCS have been determined from experimentally obtained rotational spectra fit to a Watson \textit{A} reduction Hamiltonian to within 4 kHz. The \textit{a}-type rotational spectrum of H$_3$CF--OCS has been assigned, however, \textit{b}-type transitions still need to be located. The HCF$_3$--CO$_2$ spectrum showed a doubling of transitions into \textit{A} and \textit{E} states (by up to 1.5 MHz), and is currently fit to approximately 20 kHz using XIAM} \underline{\textbf{51a}},\ (1996), 923.}. The ab initio structures of all the complexes within the series are very similar with O$\cdots$H distances consistently between 2.57-2.65 \AA\ and O=C$\cdots$C bond angles of 61-64$^{irc}$, although with a wider range of H--C$\cdots$C angles (\textit{ca.} 60$^{irc}$ to 75$^{irc}$). Results for all members of the series will be presented and compared to theoretical data

C--H...π\pi and C--F...H--C INTERACTIONS IN THE ACETYLENE--FLUOROFORM DIMER

Author Institution: Department of Chemistry, Eastern Illinois University, 600 Lincoln Ave.,; Charleston, IL 61920 USARotational spectra for four isotopomers of the HCCH--HCF$_3$ complex have been identified by Fourier-transform microwave spectroscopy. The spectra exhibit considerable fine splittings, presumably arising from the internal motion of one or both subunits; both first- and second-order Stark effects have been observed. A fit of selected $K$=0 and 1 components for the normal isotopomer (assigned using second-order Stark effect data) gave an rms of around 4 kHz and provided rotational constants $A = 5450.236(4)$ MHz, $B = 1381.3361(9)$ MHz, $C = 1375.1333(12)$ MHz. These rotational constants and the planar moments are consistent with a structure of $C_s$ symmetry exhibiting C--H...$\pi$ and C--F...H--C interactions and are in excellent agreement with a structure obtained from optimizations at the MP2/6-311++G(2d,2p) level

STRUCTURE AND INVERSION MOTIONS OF THE WEAKLY BOUND CH2_2F2⋯_2 \cdots CO2_2 DIMER

Author Institution: Department of Chemistry, Eastern Illinois University, 600 Lincoln; Ave., Charleston, IL 61920The rotational spectrum of the CH$_2$F$_2 \cdots$CO$_2$ dimer has been measured using chirped-pulse and resonant cavity Fourier-transform microwave (FTMW) spectroscopy, with the broadband spectrum playing an essential role in allowing identification of the tunneling splittings. The observed $a$-type transitions were doubled by a few megahertz, while $c$-type transitions were split by around 200 MHz, suggesting a tunneling motion that inverts the $\mu_c$ dipole moment component. A Pickett-type coupled Hamiltonian has been used to analyze the spectra, giving an energy difference ($\Delta E$) between the tunneling states for the normal isotopologue of $115.140(2)$ MHz, and rotational constants of $A_0 = 5567.8604(27)$ MHz, $B_0 = 1832.9676(5)$ MHz, $C_0 = 1828.6132(5)$ MHz, $A_1 = 5567.8540(26)$ MHz, $B_1 = 1831.7711(4)$ MHz, $C_1 = 1828.6106(4)$ MHz. It was also necessary to include a Coriolis coupling term ($G_b = 7.740(6)$ MHz) as well as fourth and sixth order centrifugal distortion constants to obtain a satisfactory spectroscopic fit. The rotational constants and planar moments are consistent with a $C_s$ symmetry structure in which the $C_2$ axis of CH$_2$F$_2$ makes an angle of roughly $23^irc$ with the axis of the CO$_2$, with the fluorine atoms of CH$_2$F$_2$ straddling the CO$_2$ carbon atom. The spectra of five additional isotopologues were analyzed, providing detailed structural information, and all except the mixed C$^{18}$O$^{16}$O species showed inversion splittings similar to the normal species. The observation of unsplit spectra for two distinct C$^{18}$O$^{16}$O isotopologues confirms that the internal motion involves movement of the CH$_2$F$_2$ subunit between the two ends of the CO$_2$ molecule

EFFECTIVE C2vC_{2v} SYMMETRY IN THE DIMETHYL ETHER--ACETYLENE DIMER

Author Institution: Department of Chemistry, Eastern Illinois University, 600 Lincoln Ave.,; Charleston, IL 61920 USARotational spectra for five isotopomers of the dimethyl ether--acetylene complex have been measured by Fourier-transform microwave spectroscopy and the rotational constants and planar moments are consistent with a structure of effective $C_{2v}$ symmetry in which the HCCH lies along the $C_2$ axis of the dimethyl ether (DME). The C--H...O intermolecular distance is determined to be 2.08(3) \AA. Measurement of the dipole moment gives a value of $\mu_a = \mu_{total} = 1.79(4)$D, also consistent with a $C_{2v}$ symmetry geometry. \vspace{1em} {\it Ab initio} calculations at the MP2/6-311++G(2d,2p) level reveal a very flat potential energy surface around the $C_{2v}$ geometry; basis set superposition error and zero point energy corrections are found to be crucial in predicting the correct order of stabilities of the minima located on the potential energy surface. Less than 24 cm$^{-1}$ separates the structure in which the HCCH is coordinated to the lone pair of electrons on the DME from the structure with the HCCH lying along the $C_2$ axis of the DME. The structure and binding of the DME--HCCH complex will be described and compared to similar complexes

The Pure Rotational Spectrum of Pivaloyl Chloride, (CH₃)₃CCOCl, between 800 and 18 800 MHz

A search accelerated correct intensity Fourier transform microwave (SACI-FTMW) spectrometer has been used to rapidly record the rotational spectrum of pivaloyl chloride between 8 and 18 GHz. Both a radio frequency and regular cavity-based instrument have then been used to obtain high resolution measurements below 4 GHz and in the 4-18 GHz regions, respectively. Rotational constants, centrifugal distortion constants, and chlorine nuclear quadrupole coupling constants have been determined for both the 35Cl and 37Cl isotopologues. the spectrum provides no evidence concerning internal rotation in the molecule

Concerning the Electron Density at the Pb Nucleus in PbO as a Function of Bond Length

High resolution measurements on the J=1←0 transitions of PbO have been performed using a Fourier transform microwave spectrometer. the available transition frequency for the 208Pb18O isotopomer has allowed the rotational molecular field shift parameter for lead, VPb01, in PbO to be determined for the first time. the parameter gives unique insight concerning how the electron density at the Pb nucleus alters as a function of the Pb-O internuclear separation