Microwave spectra Stark effect

Unsubstituted 1.2 0.3 Microwave spectra, Stark effect 76ACH(20)65... 

One of the earliest physical methods employed in the study of heteroaromatic systems was the determination of dipole moments. Table 7 lists the dipole moments of a number of simple derivatives in addition to those reported in CHEC-I. The experimental values for the furan derivatives are those utilized in conformational analysis and discussed in Section 2.05.3. For hydrofuran derivatives the dipole moments reported are those determined from microwave spectra (Stark effect) for the ground state of the ring-puckering mode. 

From a study of the microwave spectrum of 2-methylselenophene, the second-order Stark effect in the ground state was determined.11 The technique used was double radiofrequency-microwave resonance. For the identification by the double resonance method transitions of chiefly the A-state were chosen. From these observations the components of the dipole moment of 2-methylselenophene and the total dipole moment were determined. 

Breslow etal.133 investigated the microwave spectrum of cyclopropenone and determined data for bond lengths, bond angles, dipole moment (4.39 D from the molecular Stark effect), and magnetic susceptibility anisotropy (Ax) as seen in Table 5 in comparison with cyclopropene5 3 ... 

The rotational spectrum of 1,2-dithiin was measured using a pulsed-beam microwave spectrometer in the 8-18 GHz range <1996JSP(180)139> by Stark effect measurements, the electric dipole moment was also determined (/ta = 1.85 D). The molecule proved to be of C2 symmetry with a twisted conformation about the S-S bond and a C-S-S-C dihedral angle of 53.9... 

The experimental dipole moment of COF j, as measured from the Stark effect upon the microwave spectrum, is 0.951 D (3.17 x 10 3° C m) [1215]. Many theoretical dipole moments for COF have been reported [463,472,486,513,673,921,1414,1532,2037,2234], and the range of values is commensurate with the range of techniques used to calculate them. 

Integrated i.r. intensity data have been used to calculate bond moments and their derivatives [453] the calculated dipole moment was -0.93 D (3.10 x 10" ° C m), compared with an experimental value (Stark effect upon the microwave spectrum) of -0.95 D (3.17 x 10-30 c m) [1215]. The original vibrational intensity data for COFj [981] has now been corrected [1363], and the CNDO/2 calculations [1827] of the dipole moment derivatives have been revised [289,292,1363,1417] many of the bond moment derivatives (the squares of which are approximately proportional to band intensities) were shown to be transferable [1675]. Bond dipole moments have also been calculated by ab initio methods (with a STO 5-31G basis set) [1941]. In addition, the dipole moment derivatives have been calculated under the MINDO/3 formalisms [1586], but little reliance can be placed on the results obtained. 

Fig. 1.3. This figure shows a small section of the rotational spectrum of ethyleneoxide in the presence of a magnetic field of 25.672 kG. A Stark effect modulated microwave spectrometer operated with AM = 0 selection rule was used for this recording, which actually consists of two superimposed absorption spectra. One of these spectra is observed in the absence of the modulating Stark field (above the horizontal line) and the other is observed during the periods when the modulating field is switched on (below the horizontal line). In most investigations only the upper part (pure Zeeman effect) is used for the analysis, since calibration uncertainties and the inhomogoneity of the modulating Stark-field lead to a reduced accuracy of Zeeman data derived from the splittings observed in the simultaneous presence of both fields...

Experimental value (1) 1.4 D (from dielectric measurements).411 (2) 1.50 0.1 D (from Stark-effect measurements of its microwave spectrum).433... 

A final point worth noting is that most steady-state spectrometers use Stark modulation. This modulation gives rise to Stark lobes, which are very helpful for assignments and dipole moment measurements. However, Stark lobes complicate the Zeeman spectrum and can lead to distortion of the Zeeman transitions and the baseline. The Fourier transform spectrometer eliminates this complication. The microwave molecular Zeeman effect in trans-crotonaldehyde has also been reported.16... 

We have already discussed the use of electric field modulation as a means of providing the selective detection of those molecular absorption lines having the strongest Stark effects, using tunable lasers as sources ( 2.4). As in microwave spectroscopy, where field modulation is routine, magnetic fields may also be used for this purpose, as was demonstrated by Urban and Herrmann (1978). The spectrum of NO was recorded with extremely high sensitivity using Zeeman modulation with a spin-flip Raman tunable infrared laser. 

Microwave spectra of (52) and three isotopic species (1- C, 2- C, and 5- C) have been obtained and analysed. Stark effect measurements yield a dipole moment H = 0.299 0.008 D. The i.r. spectra of gaseous, liquid, and polycrystalline bicyclo-[2,2,2]octa-2,5,7-triene, and the Raman spectrum of the liquid, have been obtained. The observed frequencies are reproduced with an average error of +2.0 cm by a 37-parameter potential function. 

Dipole Moments. On the basis of Stark effect measurements on the / = Ipi Ooo transition in the microwave spectrum of monomeric thio-formaldehyde, Johnson and his co-workers have calculated the dipole moment p = 1.6474(14) D] of the thioaldehyde in question. For similar X (X = O, S, and Se), the dipole moments of the phthalides (379), (380), and (381) were found, increasing in that order. The conformations... 

The Stark effect in the microwave (MW) spectrum was used to obtain fx = ( )0.297 0.005 [1], see also the tables of selected electric dipole moments [2] and of constants from the MW spectrum [3, 4]. The even lower moment = ( )0.1759 0.0010 from similar measurements [6] has been criticized [1] on the grounds of an erroneous assignment of spectral lines. The moment n = ( )0.4 0.1 follows from the temperature dependence of the dielectric polarization, which was measured for OF2 at atmospheric pressure between -78 and +80°C [7]. Compare also the tables [5] of experimental dipole moments. 

For cis-N2F2, fx = 0.16 0.01 D was determined from the Stark effect of the 2ii -2o,2 transition in the rotational microwave spectrum. As in the case of NF3 or N2F4, the small value of the molecular dipole moment is attributed to the hybridized lone pair of the nitrogen atoms counteracting the NF bond moments [8]. For trans-N2F2, = 0 for symmetry reasons. 

Dipole Moments. The dipole moment of thioformamide (p, = 4.01 D) has been calculated from Stark effect measurements on its microwave spectrum. Dipole-moment measurements of simple aliphatic and alicyclic dithio-acids indicated that these exist mainly in the s-cts-form (530) rather than the s-trans (531). The conformations of a series of thio-oxamides, dithio-oxamides, and ethyl thio-oxamates have been determined on the... 

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