Ilona Merke

Microwave and theoretical investigation of the internal rotation in m-cresol

The microwave spectrum of m-cresol (3-methylphenol) has been investigated using a molecular beam Fourier transform microwave spectrometer in the frequency range from 3 to 26.5 GHz. The rotation of the hydroxy group into two different unequal energetic minima leads to different spectra for the syn- and anticonformers. Because of a high potential barrier both conformers can be analyzed independently. The methyl group is undergoing an almost free internal rotation which is only hindered by small barriers and splits the vibrational ground state in two states of internal rotation denoted as A and E species. The spacing between the species is found to be up to 10 GHz. The potential for the internal rotation can be determined from the spectra and analyzed in terms of the Fourier components V3 and V6. For syn-m-cresol these parameters were determined as V3=673(3) GHz and V6=-335(24) GHz and for anti-m-cresol V3=95(5) GHz and V6=-416(46) GHz. The barriers to internal rotation were furthermore calculated with second-order Moller-Plesset perturbation theory and second-order coupled-cluster singles- and-doubles model (CC2) in the electronic ground state and with CC2 in the first excited state. The CC2 method is found to be an appropriate method to calculate potential barriers in electronic excited states of such compounds.

ab initio Calculation of 33S Quadrupole Coupling Constants. Reanalysis of the 33S Hyperfine Structure in the Rotational Spectrum of Thiirane

Abstract We present quantum chemical calculations on the MP4(SDQ) level with basis sets of high local quality to determine the nuclear quadrupole coupling tensor of 33S in a series of molecules, which were investigated up to now by microwave spectroscopy. The analysis of the nuclear quadrupole coupling in the rotational spectra provided experimental information on the tensors. As an example for such an analysis, improved values for thiirane, C2H433S, are given: χaa = - 32.9425(78) MHz, χbb = -16.402(14) MHz, χcc = 49.345(14) MHz.

Molecular beam Fourier transform microwave spectra of (chloromethyl)cyclopropane and (chloromethyl)oxirane

Abstract Pulsed molecular beam Fourier transform microwave spectroscopy has been used in order to remove disagreements in previous assignments of the rotational spectra of (chloromethyl)cyclopropane and (chloromethyl)oxirane. For the latter, two rotameric forms have been detected in a supersonic jet. It has been possible to determine, in addition to the rotational and quartic centrifugal distortion constants, the complete chlorine quadrupole coupling tensors including all of their off-diagonal elements for the 35Cl and 37Cl isotopomers of all gauche conformers. From these data, modified plausible structures could be deduced. These findings are supported by the results of quantum chemical calculations.

Rotational spectra, molecular structure, and dipole moment of methane sulfonyl fluoride

Abstract The rotational spectra of CH 3 SO 2 F and its 13 C, 33 S, 34 S, and 18 O isotopomers in their vibrational ground states were investigated by molecular beam Fourier transform microwave spectroscopy and millimeter wave spectroscopy in natural isotopic abundance. Accurate rotational, centrifugal distortion, and 33 S quadrupole coupling constants have been derived. An effective molecular structure was determined from the experimental data and ab initio calculations. Also the dipole moment components have been determined. Moreover, some lines belonging to vibrationally excited states have been identified and analyzed.

The Microwave Spectrum of 1,1-Dichloro-2,2,2-Trifluoroethane

Abstract We assigned the microwave spectrum of 1,1-dichloro-2,2,2-trifluoroethane in its vibrational ground state using a pulsed molecular beam microwave Fourier transform (MB-MWFT) spectrometer. For the 35Cl, Cl-isotopomer the rotational constants were found to be A = 2105.7098(38) MHz, B = 1689.8304 (35) MHz, and C = 1135.6955 (23) MHz, the quadrupole coupling constants of the 35Cl-nuclei are χaa = 12.385 (35) MHz and χbb= -42.284(31) MHz. The rotational constants of the 35Cl,37Cl-isotopomer are A = 2071.5469 (33) MHz, B = 1670.7648 (36) MHz, and C = 1117.1755 (24) MHz, the corresponding quadrupole coupling constants are χaa = 17.639 (51) MHz and χbb= -47.670(67) MHz for the 35Cl-nucleus and χaa = 4.701 (54) MHz and χbb= -28.331 (83) MHz for the 37Cl-nucleus