The Microwave Spectrum of Silylisothiocyanate, SiH3NCS

The microwave spectra of 15N-and 13C-substituted SiH3NCS were recorded in the frequency region between 8 and 40 GHz. Combining the resulting rotational constants with values obtained previously for other isotopic species, the complete restructure of the heavy atom chain could be determined. This leads to the following rs-bond distances: rC-S = l-5745 Å, rN-C = 1.2208 Å, and rSi-N = 1.6725 Å. From Stark effect splittings the electric dipole moment of the most abun­dant species was determined for the ground vibrational state and for the first excited state of the lowest frequency bending vibration ν10 . The values are <ν10=0 | μz | ν10=0> = 2.38 + 0.02 D and <ν10=1, l=1| μz | ν10=1, l=1>=2.36 ±0.02D. The direction of the dipole moment is discussed. From the quadrupole hyperfinestructure of the J = 2→J′=3 rotational transition the 14N-quadrupole coupling constant could be determined as Xzz=0.75 MHz. The experimental results are compared to CNDO/2 calculations.