The molecular structure of difluoroisocyanato silane: A combined microwave spectral and theoretical study

Abstract

The microwave (MW) spectrum of HF 2SiNCO (1) has been obtained and analyzed in the I r representation for C S symmetry. The rotational constants (RC) are: A 7111.28104(179), B 1565.77581(49) and C 1347.52275(77) MHz; the centrifugal distortion constants are: Δ J 2.661(11), Δ JK 455.44(25), δ J 0.4237(51), ϕ K −54.96(29) kHz; the 14N nuclear quadrupole coupling constants are: χ aa +1.8833(27) and ( χ bb − χ cc ) −0.0214(58) MHz. The low value for ( χ bb − χ cc ) implies a nearly linear SiNC structure, while the experimental A value is only consistent with cis-HSiNC and trans-SiNCO orientations. The MW analysis was assisted by calculations of the equilibrium structure. Whilst small bases give a linear SiNCO skeleton, a 6-311G ++ (3df,3pd) basis set calculation shows that the molecule is quasi-linear for all four methodologies: DFT, MP2, MP4 or CCSD(T). However, all these methods find the lowest energy conformer has trans-HSiNC and trans-SiNCO dihedral angles, although the energy difference is very small. The internal rotation barrier for the HF 2Si group is less than 1 cm −1, with the lower energy conformer having a trans-HSiNC moiety. The calculated SiNC angles for the cis-HSiNC conformer are: 171.2 (B3LYP), 167.7 (MP2), 154.9 (MP4) and 154.6° (CCSD(T)), with differences up to 9° (MP2) in the trans-series. The potential energy (PE) surface for SiNC bending ( x) is unsymmetrical, but the differences from a symmetric form are very small. A B3LYP study leads to a polynomial fit of the SiNC angle ( x, radians) with the energy ( E, cm −1), where E = 1993(536) x 2 − 422(134) x 4 + 27(10) x 6 − 0.5(2) x 8; the alternating signs indicate a double minimum potential. We have re-determined the PE surfaces for silyl group rotation for several other isocyanates using similar methods.