High-resolution rotational spectroscopy of iminosilylene, HNSi

By means of Fourier transform microwave spectroscopy of a supersonic beam, the fundamental rotational transition of isotopic and vibrationally excited iminosilylene, HNSi, has been detected. In addition to seven isotopic species, vibrational satellite transitions from more than 30 vibrationally excited states, including the three fundamental modes, have been detected. Those from ν2 are particularly intense, enabling detection of transitions from as high as (0,220,0) (i.e. ∼10,000 cm−1 above ground). At high spectral resolution, well-resolved nitrogen quadrupole structure has been observed in nearly every transition. Excitation of ν1 or ν3 changes eQq(N) little, but eQq(N) systematically decreases with increasing excitation of the ν2 bend, from a value of 0.376(5) MHz for (0,00,0) to −2.257(5) MHz for (0,200,0). With the large amount of new data in hand, it has also been possible to determine the leading vibration–rotation constants (αi and γi) for ν2 or ν3 to high precision, and derive a revised semi-empirical equilibrium structure for this fundamental triatomic molecule. Various electronic and molecular properties of iminosilylene have been calculated at the coupled cluster level of theory, and these generally agree well with experiment and previous calculations. An unsuccessful search for HSiN, a highly polar isomer calculated to lie nearly 3 eV above HNSi, is also reported.