Knowledge Bank

The rotational spectrum for the normal isotopologue of difluorosilyl isocyanate, HF$2$SiNCO, has been measured in the 5--15 GHz range using a 480 MHz bandwidth chirped-pulse Fourier-transform (CP-FTMW) microwave spectrometer and a Balle-Flygare resonant cavity FT-microwave spectrometer. Clearly resolved nuclear quadrupole hyperfine splittings arising from the $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}$N nucleus are consistent with the predicted hyperfine patterns, and indicate near-cylindrical symmetry around the nitrogen atom ($\chi bb-\chi cc=-0.0214(58)$ MHz). \vspace{1 em} Ab initio optimizations at several levels of theory (up to CCSD(T)/6-311++G(3df,3pd)) yielded two energetically very similar structures which differ predominantly in the value of the H--Si--N=C dihedral angle (cis'' $=0^irc$ or trans'' $=180irc$). Both structures have rotational constants similar to the observed values ($A=7111.2810(18)$ MHz, $B=1565.7758(5)$ MHz and $C=1347.5228(8)$ MHz), although the experimental constants, particularly $A$, seem to point to the slightly higher energy ``cis'' structure as the observed species. However, at every level of theory the difference in energy between the two forms is small (less than 0.45 kJ mol$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) and a variation of the $A$ rotational constant with the chosen level is observed, since this constant is especially sensitive to the Si--N=C angle and Si--N=C=O dihedral. Details of the two possible structures and their stabilities will be presented based on the computational and experimental data, and the results for HF$2$SiNCO will be compared to related molecules.