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.