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 results of a high resolution microwave study of low-J rotational transitions of N-cyanomethanimine are reported. The 14N quadrupole coupling constants fitted to the observed hyper fine splittings are: xaa (imine) = 2.032(9) MHz, / fcfc(imine) = -4.586(7) MHz, and / aa(nitrile) = -3.254(8) MHz, / fcfc(nitrile) = 0.844(16) MHz. Knowledge of these constants together with the rotational constants and centrifugal distortion constants determined earlier provides the basis for accurate prediction of hitherto unmeasured rotational transitions for radioastronomy. Introduction ExperimentalBecause of its astrophysical interest, the rotational spectrum of N-cyanomethanimine has been studied extensively by and by Winnewisser et al. at Giessen [3,4]. The latter group has also successfully resolved and analysed the 14N hyperfine structure in the millimeter wave spectrum. The hfs structure is a valuable tool for fingerprint line identification in radioastronomy. However two sets of quadrupole coupling constants were compatible with the observed mm wave spectra, and the authors based the choice of the correct set on the comparison with ab initio field gradients. Now, it turned out recently that present day ab initio methods may still have problems with the calculation of field gradients, especially at a sp2 hybridized 14N nucleus. We therefore initiated a complementary study of the 14N hyperfine patterns of low-J rotational transitions. These low-J hfs patterns differ sufficiently for the above two sets of coupling constants. Thus an unambiguous experimental deter mination of the quadrupole coupling constants and the reliable prediction of the hfs patterns also of hith erto unmeasured rotational transitions becomes pos sible. Since N-cyanomethanimine is highly unstable in the absorption cell (polymerisation), a flow system was set up. The sample was produced immediately before the front end of the cell by flash pyrolysis of trimethylenetetrazole [6] in a quartz tube.Our quartz tube had an inner diameter of 1 cm. The length of the heated zone was 20 cm. In order to min imize wall collisions the pyrolysis products were fed into the cell coaxially.Typical total pressures at the front end of the 1.8 m X-band absorption cell were around 10 mTorr and at the rear end around 1 to 3 mTorr. Aluminum wave guide cells proved to be superior to our standard brass waveguide cells. Preconditioning of the system with CF3COOC2H 5 (as used by Bak and Svanholt in their preparation of the molecule from dimethylcyanamide [2]) improved the signal to noise ratio in the spectra at least by a factor of two.To record the spectrum, a micro-computer con trolled cw-microwave spectrometer with 30 kHz Stark-effect modulation and with phase stabilized backward wave oscillators as radiation sources was 0932-0784 / 89 / 0300-305 S 01.30/0. -Please order a reprint rather than making your own copy. used. The computer control system, which also pro vides averaging and data handling capabilities, is de scribed in detail in [7], Typical experimental linewidths we...

on the Occasion of his 65 th BirthdayThe rotational Zeeman effect of 1,2,4-trifluorobenzene has been studied for 8 low-J rotational transitions in magnetic fields between 1.9 and 2.4 Tesla. The observed susceptibility anisotropics and molecular ^/-values are: (2/aa-^b()-x cc) = 37.85(69) • 10~6 erg G~2 mole" \ (2xbb--/.cc-x aa) = 56.85(54)-IO'"6 erg G "2 mole"1, gaa= -0.0393(3), ^"=-0.0277(3), and gcc = 0.0042(2). The Zeeman parameters have been used to derive the molecular electric quadrupole moments and vibronic ground state expectation values for the electronic second moments. The observed out-ofplane quadrupole moment is discussed with reference to an additivity scheme proposed earlier. The observed out-of-plane component of the molecular magnetic susceptibility tensor is in excellent agreement with the value predicted earlier from the CNDO/2-7i-electron density alternation at the ring atoms.

The rotational Zeeman effect in the J -> J' = 0 1 and 1 -»■ 2 rotational transitions has been observed for H-C = C -C = I4N, D-C = C -C = 14N, D -C = C -C = 15N, and H -C^C -C = 15N using a microwave bridge superheterodyne spectrometer. From the experimental <7±-values, susceptibility anisotropies and rotational constants, the molecular electric quadrupole moment, which plays an important role for collisional excitation and relaxation, follows as Q =2.14(9) x 10~26esu cm2 (referred to the center of mass of the H-C = C -C= 14N species).Since the Deuterium quadrupole hyperfine splittings in the Zeeman spectra could be resolved, it was also possible to fit the Deuterium quadrupole coupling constant. Our value, eqQ (2H) = 198.2(46) kHz is about 10% smaller than the value derived from a recent high resolution micro wave Fourier transform study. The reasons for this discrepancy are discussed.All experimental molecular parameters are compared to the results of ab initio quantum chemical calculations and it is argued that the 14N nuclear electric quadrupole moment is possibly about 30% larger than presently assumed.

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