FORBIDDEN TRANSITIONS IN THE VERY RICH PURE ROTATIONAL SPECTRUM OF TRANS-1-IODOPERFLUOROPROPANE

Abstract

Using both chirped pulse-, and low frequency cavity-based Fourier transform spectroscopy over 700 transitions have been recorded for the title molecule for the first time. The C$_1$, C$_2$ and C$_3$ carbon-13 species have been observed in natural abundance allowing a substitution structure for the CCC backbone to be determined. Nearly all the transitions observed were either $a$-type $R$ branches or $b$-type $Q$ branches. No $c$-type transitions were observed. The $\chi_{aa}$, $\chi_{bb}$, $\chi_{cc}$ and $\chi_{ab}$ components of the iodine nuclear quadrupole coupling tensor have been determined. Of note, several forbidden transitions were also observed, such as $J_{k_{-1},k_{+1}}$ $F$ = 14$_{4,10}$ 23/2 $\leftarrow$ 12$_{5,7}$ 21/2. In this particular case it seems that the upper energy level is nearly degenerate with a level connected to the lower energy level by a $\mu_a$ dipole component term, but the lower energy level is also nearly degenerate with a second level connected to the upper energy level, again by a $\mu_a$ dipole component term. The two intermediate energy levels are themselves connected by a $\mu_b$ dipole component term. Near degeneracies such as these, together with a large $\chi_{ab}$ value ($\approx$ 1 GHz) mean that certain $\Delta J = 2$ transitions become allowed. This phenomena has been previously observed for 1-iodopropane by Fujitake and Hayashi (J. Mol. Spect, 127, (1988), 112-124).