15min:

SEAN A. PEEBLES, DANIEL A. OBENCHAIN AND REBECCA A. PEEBLES, Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, IL 61920; MICHAEL H. PALMER, School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK; GAMIL A. GUIRGIS, ZICHAO WANG AND JONEL LIRJONI, Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424.

The rotational spectrum for the normal isotopologue of difluorosilyl isocyanate, HF₂SiNCO, 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 ¹⁴N nucleus are consistent with the predicted hyperfine patterns, and indicate near-cylindrical symmetry around the nitrogen atom ( _bb- _cc = -0.0214(58) MHz).

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<sup>o</sup> or ``trans'' =180^o). 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^-1) 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 HF₂SiNCO will be compared to related molecules.