First Laboratory Detection of N13CO- and Semiexperimental Equilibrium Structure of the NCO- Anion.

The cyanate anion (NCO-) is a species of considerable astrophysical relevance. It is widely believed to be embedded in interstellar ices present in young stellar objects but has not yet been detected in the dense gas of the interstellar medium. Here we report highly accurate laboratory measurements of the rotational spectrum of the N13CO- isotopologue at submillimeter wavelengths along with the detection of three additional lines of the parent isotopologue up to 437.4 GHz. With this new data, the rotational spectrum of both isotopologues can be predicted to better 0.25 km s-1 in equivalent radial velocity up to 1 THz, more than adequate for an astronomical search in any source. Moreover, a semiexperimental equilibrium structure of the anion is derived by combining the experimental ground-state rotational constants of the two isotopologues with theoretical vibrational corrections, obtained by using the coupled-cluster method with inclusion of single and double excitations and perturbative inclusion of triple excitations (CCSD(T)). The estimated accuracy of the two bond distances is on the order of 5 × 10-4 Å: a comparison to the values obtained by geometry optimization with the CCSD(T) method and the use of a composite scheme, including additivity and basis-set extrapolation techniques, reveals that this theoretical procedure is very accurate.

The Submillimeter Rotational Spectrum of Ethylene Glycol up to 890 GHz and Application to ALMA Band 10 Spectral Line Data of NGC 6334I.

The rotational spectrum of the most stable conformer of ethylene glycol (HO(CH2)2OH) has been recorded between 360-890 GHz using a frequency-modulation submillimeter spectrometer. The refinement and extension of the spectroscopic parameters over previous efforts provide predicted catalog frequencies for ethylene glycol with sufficient accuracy for comparison to high-frequency astronomical data. The improvement in the cataloged line positions, and the need for improved accuracy enabled by high-frequency laboratory work, is demonstrated by an analysis of ethylene glycol emission at 890 GHz in the high-mass star-forming region NGC 6334I in ALMA Band 10 observations. The need for accurate rotational spectra at submillimeter wavelengths/THz frequencies is discussed.

The rotational spectrum of 15ND. Isotopic-independent Dunham-type analysis of the imidogen radical.

The rotational spectrum of 15ND in its ground electronic X3Σ- state has been observed for the first time. Forty-three hyperfine-structure components belonging to the ground and ν = 1 vibrational states have been recorded with a frequency-modulation millimeter-/submillimeter-wave spectrometer. These new measurements, together with the ones available for the other isotopologues NH, ND, and 15NH, have been simultaneously analysed using the Dunham model to represent the ro-vibrational, fine, and hyperfine energy contributions. The least-squares fit of more than 1500 transitions yielded an extensive set of isotopically independent Ulm parameters plus 13 Born-Oppenheimer Breakdown coefficients Δlm. As an alternative approach, we performed a Dunham analysis in terms of the most abundant isotopologue coefficients Ylm and some isotopically dependent Born-Oppenheimer Breakdown constants δlm [R. J. Le Roy, J. Mol. Spectrosc., 1999, 194, 189]. The two fits provide results of equivalent quality. The Born-Oppenheimer equilibrium bond distance for the imidogen radical has been calculated [rBOe = 103.606721(13) pm] and zero-point energies have been derived for all the isotopologues.

The spectroscopic analysis of the v2=1, v5=1, and v3=v6=1 infrared vibration system of H3SiI.

The ν2 (A1)/ν5 (E)/ν3+ν6 (E) band system of H3(28)SiI was investigated using Fourier transform infrared spectra recorded from 820 to 1100 cm(-1) at a resolution of 2.0×10(-3) cm(-1). In total, 11,903 transitions were assigned. Additional 1466 transitions reaching the v3=v6=1 state were obtained from the ν3+ν6-ν6 and ν3+ν6-ν3 hot bands near 360 and 590 cm(-1), respectively. Moreover, 30 highly accurate CO2 laser sideband transitions of the (r)Q0 branch of ν5 (J.M. Frye, W. Schupita, and G. Magerl, J. Mol. Spectrosc. 128, 427 (1988)) were implemented in the data set with J(max)″=140 and K(max)″=21. To adequately reproduce the complex pattern of interacting levels the Hamiltonian employed included 14 off-diagonal terms. These comprise x,y Coriolis ro-vibration resonances, between ν2/ν5, ν2/ν3+ν6 and ν5/ν3+ν6, and the anharmonic Fermi resonance between ν5/ν3+ν6. All these resonances strongly perturb the v2=1, v5=1, and v3=v6=1 excited states whose rounded deperturbed vibrational term values are 904.5, 941.1, and 953.7 cm(-1), respectively. In addition, the Δl=Δk=±2 l-resonance was found to be active within the v3=v6=1 state and between v5=1 and v3=v6=1; the Δl=±2,Δk=∓1 l-resonance within the v5=1 state and between v5=1 and v3=v6=1 was established, as well as the Δl=±1,Δk=∓2 α resonance between v2=1 and v5=1. A standard deviation of the fit, 0.48×10(-3) cm(-1), resulted which is ca. three times the estimated precision of experimental wavenumbers. Improved J-dependent ground state parameters of H3SiI were obtained by fitting 5420 combination differences, σ(fit)=0.22×10(-3) cm(-1).

The experimental equilibrium structure of acetylene.

The empirical equilibrium structure of acetylene has been derived by exploiting the very precise experimental rotational constants available in the literature for the 10 isotopologues relative to all the possible combinations of H, D, (12)C and (13)C atoms. The geometry obtained when data for all species are fitted together is: re(CH) = 106.167(14) pm and re(CC) = 120.2866(72) pm. This determination shows some systematic residuals due to the singly D-substituted isotopologues. If we exclude such species from the fit, we obtain our most precise evaluation: re(CH) = 106.1689(23) pm and re(CC) = 120.2817(12) pm. The possibility of a breakdown of the Born-Oppenheimer approximation has also been tested.

The infrared spectrum of (12)C2D2: the stretching-bending band system up to 5500 cm(-1).

The infrared spectrum of the perdeuterated acetylene, (12)C2D2, has been recorded from 900 cm(-1) to 5500 cm(-1) by Fourier transform spectroscopy at a resolution ranging between 0.004 and 0.009 cm(-1). Ninety-two bands involving the ν1, ν2, and ν3 stretching modes, also associated with the ν4 and ν5 bending vibrations and 9 bands involving pure bending transitions have been observed and analysed. In total, 8345 transitions for the stretching-bending, and 862 for the pure bending modes have been assigned in the investigated spectral region. All the transitions relative to each stretching mode, i.e. the fundamental, its first overtone, and associated hot and combination bands involving bending states up to v4 + v5 = 2, were fitted simultaneously. The Hamiltonian adopted for the analysis is that appropriate to a linear molecule and includes vibration and rotation l-type interactions. The Darling-Dennison interaction between v4 = 2 and v5 = 2 levels associated with the various stretching states was also considered. The standard deviation for each global fit is smaller than 0.0006 cm(-1), of the same order of magnitude of the measurement precision.

Silicon Oxysulfide, OSiS: Rotational Spectrum, Quantum-Chemical Calculations, and Equilibrium Structure.

Silicon oxysulfide, OSiS, and seven of its minor isotopic species have been characterized for the first time in the gas phase at high spectral resolution by means of Fourier transform microwave spectroscopy. The equilibrium structure of OSiS has been determined from the experimental data using calculated vibration-rotation interaction constants. The structural parameters (rO-Si = 1.5064 Å and rSi-S = 1.9133 Å) are in very good agreement with values from high-level quantum chemical calculations using coupled-cluster techniques together with sophisticated additivity and extrapolation schemes. The bond distances in OSiS are very short in comparison with those in SiO and SiS. This unexpected finding is explained by the partial charges calculated for OSiS via a natural population analysis. The results suggest that electrostatic effects rather than multiple bonding are the key factors in determining bonding in this triatomic molecule. The data presented provide the spectroscopic information needed for radio astronomical searches for OSiS.

A frequency-modulated quantum-cascade laser for spectroscopy of CH4 and N2O isotopomers.

We report the development of a novel laser spectrometer for high-sensitivity detection of methane and nitrous oxide. The system relies on a quantum-cascade laser source emitting wavelength of around 8.06 microm, where strong fundamental absorption bands occur for the considered species and their isotopomers. The detection technique is based on audio-frequency and radio-frequency modulation of laser radiation. First experimental tests have been performed to estimate the achievable detection limits and the signal reproducibility levels in view of possible measurements of (13)C/(12)C, (18)O/(16)O, (17)O/(16)O and (15)N/(14)N isotope ratios.