Pure rotational transitions of silicon monosulfide ((28)Si(32)S) and its rare isotopic species have been observed in their ground as well as vibrationally excited states by employing Fourier transform microwave (FTMW) spectroscopy of a supersonic molecular beam at centimetre wavelengths (13-37 GHz) and by using long-path absorption spectroscopy at millimetre and submillimetre wavelengths (127-925 GHz). The latter measurements include 91 transition frequencies for (28)Si(32)S, (28)Si(33)S, (28)Si(34)S, (29)Si(32)S and (30)Si(32)S in upsilon = 0, as well as 5 lines for (28)Si(32)S in upsilon = 1, with rotational quantum numbers J''< or = 52. The centimetre-wave measurements include more than 300 newly recorded lines. Together with previous data they result in almost 600 transitions (J'' = 0 and 1) from all twelve possible isotopic species, including (29)Si(36)S and (30)Si(36)S, which have fractional abundances of about 7 x 10(-6) and 4.5 x 10(-6), respectively. Rotational transitions were observed from upsilon = 0 for the least abundant isotopic species to as high as upsilon = 51 for the main species. Owing to the high spectral resolution of the FTMW spectrometer, hyperfine structure from the nuclear electric quadrupole moment of (33)S was resolved for species containing this isotope, as was much smaller nuclear spin-rotation splitting for isotopic species involving (29)Si. By combining the measurements here with previously published microwave and infrared data in one global fit, an improved set of spectroscopic parameters for SiS has been derived which include several terms describing the breakdown of the Born-Oppenheimer approximation. With this parameter set, highly accurate rotational frequencies for this important astronomical molecule can now be predicted well into the terahertz region.

The millimeter-wave spectra of the unstable halofulminates BrCNO and ClCNO were recorded at room temperature in several frequency intervals between 52 and 230 GHz. Besides rotational transitions in the vibrational ground state, transitions in numerous thermally excited states of the XCN bending mode (X ) Br or Cl) could be observed. The irregular sequence of these satellites indicated that in both molecules, the XCN bending mode is highly anharmonic. Indeed, the XCNO molecules exhibit truly quasilinear behavior. From semirigid bender analyses of the rotational data, the effective potential functions and their barriers to linearity were determined. The barrier heights were found to be 130.82 (56) cm -1 for BrCNO and 166.86 (84) cm -1 for ClCNO, resulting in quasilinearity parameters γ 0 of +0.362 and +0.416, respectively.

The low-lying CCN bending mode of cyanofulminate, NCCNO, was characterized by rotational spectroscopy in the millimeter-wave and submillimeter-wave range as well as by rovibrational spectroscopy in the farinfrared range. The spectra exhibit the gross features of a linear molecule. However, a closer qualitative analysis regarding the low-lying CCN bending mode revealed significant deviations from a harmonic bending mode that is normally found in a linear molecule. This result was confirmed by a quantitative analysis of the combined data with an effective Hamiltonian for a linear molecule. In linear molecule notation, the term value of the first excited state ν 7 is 80.524 182 (10) cm -1 , and the term values for the l 7 ) 0 and l 7 ) 2 levels of the second excited state 2ν 7 are 166.118 254 (16) and 164.604 243 (22) cm -1 . A semirigid bender analysis of our data, including rotational transitions of the isotopomers 15 NCCNO, N 13 CCNO, NCC 15 NO, and NCCN 18 O observed in natural abundance, yielded a considerable quartic contribution to the effective CCN bending potential function V(F)/cm -1 ) 747.40 (81) × (F/rad) 2 + 959.2 (24) × (F/rad) 4 .

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