The Rotational Zeeman Effect in Fluorobenzene and the Molecular Quadrupole Moment in Benzene

Abstract

The rotational Zeeman effect in fluorobenzene is reinvestigated with a resolution improved by a factor of almost five to give more accurate g-tensor elements, magnetic susceptibility anisotropics and molecular electric quadrupole moments. The results fit into the pictures of a linear dependence of the out of plane molecular electric quadrupole moment, Qcc, on the number of fluorine substituents and of a linear correlation between the nonlocal (ring current) susceptibility, Χccnonlocal, and the CNDO/2-π-electron density alternation. They lead to a gasphase molecular electric quadrupole moment in benzene, Qcc,benzene = - (28.4 ± 4.7) · 10-40 Cm2 which is slightly less negative than the value deduced from electric field-gradient birefringence experiments on dilute benzene solutions with carbon tetrachloride as solvent. A detailed description of the high resolution microwave spectrometer is given in the appendix.

The Rotational Zeeman Effect of Pyrazole and Imidazole

“…Where ever possible the spectra were recorded in the superheterodyne bridge spectrometer with additional 5 kHz Stark effect square wave modulation described earlier [9]. Only two Q-band transitions of imidazole, the 1| 0 -2 U transition at 32893.824 MHz and the 2 02 -3o3 transition at 33416.088 MHz, are recorded with a conventional Stark effect moduled spectrometer (30 kHz square wave modulation, compare too [10,11]).…”

The Rotational Zeeman Effect of 1,2,4-Trifluorobenzene

“…Above 12.4 GHz we have used our superheterodyne bridge spectrometer [3,9], Phase stabilized Klystrons and, above 12.4 GHz, phase sta bilized backward wave oscillators were used as radia tion sources. In order to reduce collision broadening, sample pressure were kept below 0.9 Pa (6.8 mTorr) at cell temperatures close to 220 K. Typical experimental linewidths were 100 to 150 kHz full width at half height.…”

“…It forms part of a systematic investigation of substitution effects on the magnetic properties of fluorinated benzenes [1][2][3][4][5] and pyridines [4][5][6], It was initiated to check a linear corre lation postulated earlier to hold between the CNDO/2-electron charge density alternations at the ring atoms and the nonlocal 7i-electron contribution to the mag netic susceptibility perpendicular to the plane of the ring [4]. The latter serves as a quantitative criterion for the aromaticity of ring compounds with (2n + 2) 7i-electrons (compare Refs.…”

The Molecular 0-Tensor, the Magnetic Susceptibility Anisotropy, and the Molecular Electric Quadrupole Moment Tensor of Monofluoroacetonitrile, a Rotational Zeeman Effect Study

“…At frequen cies below 19 GHz the spectra were recorded with our Stark-effect modulated superheterodyne bridge spec trometer [7]. At higher frequencies, i.e.…”