Knowledge Bank

The methyl cyanide molecule was studied in the $v8=2$ and 3 vibrations, for rotational energy levels with $1⩽J⩽5$ to establish the trend in the energy level in the frequency range 17 to 95 gHz. Anomalous behaviour in the $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH3\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C15N$ rotational spectrum of the $V8=3$ vibration for the $(K=\pm 1)(\ell =\pm 1)$ components with large frequency departures, $=6(J+1)MHz$, from a standard model were investigated to determine what interactions were at play which had been ignorable in $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH3\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C15N$ molecule. The method of isotopic substitution enabled the molecular system to be ``tuned'' so that resonant interactions could be explored in more detail for each of the molecules. These large departures are explained using a basic molecular model in which accidentally strong resonances (ASR) are at play.