Microwave spectra centrifugal distortion from

In a second investigation of the microwave spectrum of 1,3,4-thiadiazole, 14N quadrupole coupling and centrifugal distortion for lines with J = 5 and / 50, respectively, are determined. These are compared with results obtained from 1,3,4-oxadiazoles and pyridazine. Comprehensive tables showing microwave transitions, the microwave spectrum and the a and n population from 14N quadrupole coupling data for 1,3,4-thiadiazole are also given (71JST(9)163> (see also Sections 4.01.3.2, 4.01.4.2.2.(i) and (iii), and 4.01 Table 2). 

In order to assign the Zeeman patterns for the three lowest rotational levels quantitatively, one must determine the spacings between the rotational levels, and the values of g/and gr-In the simplest model which neglects centrifugal distortion, the rotation spacings are simply B0. /(./ + 1) this approximation was used by Brown and Uehara [10], who used the rotational constant B0 = 21295 MHz obtained by Saito [12] from pure microwave rotational spectroscopy (see later in the next chapter). The values of the g-factors were found to be g L = 0.999 82, gr = —(1.35) x 10-4. Note that because of the off-diagonal matrix elements (9.6), the Zeeman matrices (one for each value of Mj) are actually infinite in size and must be truncated at some point to achieve the desired level of accuracy. In subsequent work Miller [14] observed the spectrum of A33 SO in natural abundance 33 S has a nuclear spin of 3/2 and from the hyperfine structure Miller was able to determine the magnetic hyperfine constant a (see below for the definition of this constant). 

Detection of emission signals from interstellar formamide has created renewed interest in the laboratory microwave spectrum of this molecule. Measurements have been extended to account accurately for centrifugal distortion and a total of 22 new transitions involving / < 29 have been measured for for the ground vibrational state. 

Rotational constants and centrifugal distortion constants of the upper vibrational state 2 vg of H2B-NH2 have also been determined by microwave spectroscopy for details, see [3]. Also, the He(I) photoelectron spectrum of H2B-NH2 (produced by controlled thermal decomposition of H3N-BH3) has been measured [4]. The five ionization potentials observed up to 21.2 eV have been correlated with those of ethene. A good correspondence of the observed values was obtained with data from Koopmans theorem calculations for the ground state molecule (semiempirical MNDO and SCF ab initio calculations with 3-21G and 6-31G bases). Experimental ionization potentials (IP) and calculated orbital energies are given in Table 4/24, p. 222 [4]. A correlation of the IP data of H2B-NH2 and H2CCH2 is given for the five uppermost filled levels in Fig. 4-47, p. 222. 

For an asymmetric top, all three rotation constants can in principle be determined from pure rotation spectra or from vibration-rotation spectra, but the accuracy of some of the constants might be low even though they are derived from very precise frequencies measured in the microwave spectrum. This arises in part because of the problems of centrifugal distortion, but also because the measured line positions could well depend only weakly on one of the rotation constants, which is then correspondingly uncertain. 

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