RESUMEN
Quantum monodromy has a dramatic and defining impact on all those physical properties of chain-molecules that depend on a large-amplitude bending coordinate, including in particular the distribution of the ro-vibrational energy levels. As revealed by its pure rotational (a-type) spectrum [B. P. Winnewisser et al., Phys. Chem. Chem. Phys., 2010, 12, 8158-8189] cyanogen iso-thiocyanate, NCNCS, is a particularly illuminating exemplar of quantum monodromy: it clearly shows the distinctive monodromy-induced dislocation of the ro-vibrational energy level pattern for its low-lying bending mode. This dislocation centers on a lattice defect in the energy vs. momentum map of the ro-vibrational levels at the top of the barrier to linearity, and represents an example of an excited state quantum phase transition [D. Larese and F. Iachello, J. Mol. Struct., 2011, 1006, 611-628]. To complete the data, so far limited to ΔJ = +1 transitions, we decided to measure the high-resolution far-infrared band of the large-amplitude bending vibration ν7, and, if possible, mid-infrared bands. This Perspectives article presents our ongoing progress towards this goal, beginning with the description of how to predict line positions and intensities of the a- and b-type bands of the large amplitude bending mode using the Generalized-SemiRigid-Bender (GSRB) Hamiltonian for NCNCS and ab initio dipole moment functions [B. P. Winnewisser et al., Phys. Chem. Chem. Phys., 2010, 12, 8158-8189]. We include background information about synchrotron physics to clarify the advantages and limitations of that radiation source for our experiments. Details of the chemical preparation and sample handling, leading to the realization that NCNCS is 50 kJ mol(-1) lower in energy than its isomer S(CN)2 [Z. Kisiel et al., J. Phys. Chem. A, 2013, 117, 13815-13824] are included. We present the far-infrared and mid-infrared spectrum of NCNCS obtained at the Canadian Light Source synchrotron, using the IFS 125HR Bruker Fourier transform spectrometer. Eight of the fundamental vibrational modes of NCNCS have now been observed at high resolution. Initial analyses of the data confirm band assignments and demonstrate the accuracy of the predictions.
RESUMEN
The high resolution Fourier transform spectrum of the chemically challenging sulfur dicyanide, S(CN)2, molecule was recorded at the far-infrared beamline of the synchrotron at the Canadian Light Source. The spectrum covered 50-350 cm(-1), and transitions in three fundamentals, ν4, ν7, and ν8, as well as in the hot-band sequence (n + 1)ν4 - nν4, n = 1-4, have been assigned and measured. Global analysis of over 21,300 pure rotation and rotation vibration transitions allowed determination of precise energies for 12 of the lowest vibrationally excited states of S(CN)2, including the five lowest fundamentals. These results constitute an extensive set of benchmarks for ab initio anharmonic force field calculations and the observed and calculated vibration-rotation constants and anharmonic frequencies are compared. The semiexperimental equilibrium, r(e)(SE), geometry of S(CN)2 has also been evaluated. In the course of the measurements, new information concerning the physical chemistry of S(CN)2 has been obtained.