Microwave spectroscopy centrifugal distortion constants

Microwave spectroscopy has also supplied useful information on the centrifugal distortion constants (section II.E) and nuclear quadrupole... 

Linear ONN-HF was first detected by Lovejoy and Nesbitt using absorption spectroscopy and the HF chromophore [164], with NNO isotopic substitution verifying the structure. Subsequent microwave experiments confirmed the presence of both isomers [165], and infrared studies were also able to detect both isomers simultaneously [162, 163]. Upon vibrational excitation of HF, the rotational constant, increases while the centrifugal distortion constant, decreases, and this was attributed to enhanced attraction between N2O and HF as a result of the increased dipole moment of vibrationally excited HF [164]. Interestingly, excitation of the N2O asymmetric stretch vibration results in a decrease in and an increase in D, [158]. 

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. 

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). 

Higher order effects introduce additional distortion terms and also distortion terms that can give rise to splittings of certain -levels. The effects of centrifugal distortion on the observation of forbidden h.K = 3 transitions have already been mentioned. Induced dipole moments also allow the observation of pure rotational spectra of spherical tops which, because they have no permanent dipole moment, would otherwise have no rotational spectra. For CH4, the distortion moment is on the order of 5 X 10 D. Both J J +l and J J transitions have been observed. The leading terms in the frequency equation for the 7 y -I-1 transitions are like those for a linear molecule, Eq. (51) however, the molecular distortion in such molecules is more complicated, and additional terms are required to adequately characterize the rotational spectrum. Such observations have provided the rotation and distortion constants. Some examples of nonpolar molecular studies via microwave spectroscopy are spherical tops with Td symmetry like CH4, SiHd, and GeITt and those with Dsh symmetry like BF3 and SO3. For SO3, a planar molecule, the centrilugally induced rotational spectrum provides = 1.4175 A. 

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