Knowledge Bank

Vinyl isocyanate (CH$2$=CH--N=C=O) can exist in two stable conformations, having a \textit{cis} or \textit{trans} arrangement about the C-N single bond. Electronic structure calculations indicate that the \textit{trans} conformer is about 250 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ lower in energy. There is a low barrier to isomerization of the \textit{trans} conformer calculated to be about 580 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. These energetics are supported by the pure rotational spectrum of vinyl isocyanate in a molecular beam, where transitions of the \textit{trans} conformer are about 300 times stronger than those of the \textit{cis} conformer due to conformational relaxation in the free-jet expansion. The dominance of the \textit{trans} conformer guarantees that vibrational excited states prepared by pulsed infrared laser excitation are effectively conformer-selective. Dynamic rotational spectra of laser-prepared excited states in the 3000 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ region of the spectrum show evidence of conformational isomerization through coalescence of the overall line shape. Strong mode-specific reaction yields are observed from different vibrational bands where, in some frequency regions, there is no evidence of conformational isomerization despite the molecule having a total vibrational energy greatly exceeding the barrier to reaction. The nuclear quadrupole hyperfine structure of the dynamic rotational spectra shows that isomerization, when it occurs, conserves the $Ka$ angular momentum projection quantum number.