N. Moazzenahmadi et al., MILLIMETER-WAVE SPECTRUM OF CH3CD3 IN THE 3 LOWEST TORSIONAL STATES, The Journal of chemical physics, 99(4), 1993, pp. 2429-2438
The pure rotational spectra of CH3CD3 in the three lowest torsional st
ates has been observed using a mm-wave spectrometer. A total of 87 rot
ational frequencies were measured between 230-363 GHz for J=7<--6 to 1
1<--10 in v6=0, 1, and 2, where V6 is the torsional quantum number. Fo
r the lowest two torsional states, the spectra have the classic form e
xpected for a symmetric top (with no internal rotation) in the ground
vibrational state. For V6=2 and for a given (J+1)<--J, a markedly diff
erent splitting pattern is observed as a result of the (K,sigma) depen
dence of the effective rotational constant B, where sigma labels the t
orsional sublevels. In order to identify the individual features in th
e (V6=2) spectrum, an assignment procedure was developed which is base
d on the fact that the ratio of moment of inertia of the top about the
molecular symmetry axis to that of the whole molecule about the same
axis is to a very good approximation 1/3. The torsion-rotation Hamilto
nian discussed earlier in connection with CH3SiH3 [N. Moazzen-Ahmadi e
t al J. Mol. Spectrosc. 119, 299 (1986)] was used to analyze the rotat
ional frequencies along with the molecular beam anticrossing data and
the origin of the torsional fundamental. Several constants which chara
cterize the J-dependence of the energy levels were determined. Effecti
ve values for the barrier height V3 and the shape parameter V6 associa
ted with the first-order correction in the Fourier expansion of the po
tential function were obtained. The effect of redundancies on the inte
rpretation of the measurements is discussed.