1. Fitting the high-resolution spectroscopic data for NCNCS Zbigniew Kisiel, a Brenda P. Winnewisser, b Manfred Winnewisser, b Frank C. De Lucia, b Dennis W. Tokaryk, c Steven C. Ross, c Brant E. Billinghurst d a Institute of Physics, Polish Academy of Sciences, Warszawa, Poland b Department of Physics, The Ohio State University, Columbus, USA c Department of Physics, University of New Brunswick, New Brunswick, Canada d Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Canada 69th International Symposium on Molecular Spectroscopy RJ15

2. Ground breaking studies: Winnewisser et al.Phys.Rev.Lett. 95, 243002 (2005) J.Mol.Struct. 798, 1 (2006) PCCP 12, 8158 (2010) Complete mm-wave spectrum was recorded with the FASSST technique and rotationally resolved spectra of all fundamentals recorded at the CLS (HR125 Bruker+synchrotron) The problem: even for the ground state only a small subset of the FASSST transitions was fitted with a standard asymmetric rotor Hamiltonian, and even then only at 10 . The general semi-rigid bender Hamiltonian (GSRB) provided satisfactory reproduction of the global picture but at a level greater than 10 3 . Furthermore, specific vibration-rotation perturbations have been identified but not yet made use of. Background: NCNCS is a quasilinear molecule with intriguing barrier crossing behaviour described by quantum monodromy.

3. Precision ? Signs ? Deviation ? A good effective fit may not be the best one: Previously reported fit to only the ground state K a =0,1,2 transition sequences (five sequences) Octic level constants required!

4. The SPFIT traps for unwary users: See: Crib-sheet to SPFIT/SPCAT on the PROSPE website at http://info.ifpan.edu.pl/~kisiel/prospe.htm TRAP 1 : printed errors have a specific meaning and will normally not be standard errors TRAP 2 : the fit may not be converged TRAP 3 : derived parameter values may be constrained by too small a priori uncertainties TRAP 4 : almost all quartic centrifugals have reversed sign relative to convention TRAP 5 : improper use of the DIAG parameter affecting state labelling

5. Towards an improved effective fit: … fewer adjustable parameters, better deviation of fit, more reasonable values of quartic constants

6. Assignment using Loomis-Wood type plots: The K a =0 R-branch transition sequence for NCNC 34 S

7. Assignment using Loomis-Wood type plots: The K a =2 l R-branch transition sequence for NCNC 34 S

8. Assignment using Loomis-Wood type plots: The K a =0 R-branch transition sequence for NCN 13 CS

9. Spectroscopic constants for the rare isotopic species of NCNCS:

10. Assignment of the ground state K a =3 transitions for NCNC 34 S Prediction from the effective fit assignment

11. Differences between experiment and GSRB: The ground state K a =0 transition sequence NCNCS (parent) NCNC 34 S The current GSRB fit is based on reproducing constants in the J(J+1) power series approximation using a small low-J subset of eigenvalues. Advances in computer power and isotopic information allow for a considerable update.

12. Selective perturbations in transition sequences in different vibrational states: Fig.14, B.P.Winnewisser et al., PCCP 12, 8158 (2010)

13. v=0: 266030.29 MHz +153.58 from the series v=0: 269043.24 MHz -98.04 from the series Resonances also affect the ground state: Resonance in the K a =8 transition sequence, maximum at J”= 80,81

14. v=0: 269043.24 MHz -98.04 from the series v=0: 266030.29 MHz +153.58 from the series v=1: 264010.03 MHz -153.56 from the series v=1: 264505.16 MHz +97.95 from the series The matching resonance in v b =1 : The resonance is now in the K a =5 transition sequence

15. The neighbouring K a =6 sequence for the ground state: v b =2, K a =4

16.  The effective fit of the rotational Hamiltonian has been improved and reproduces the five lowest K a transition frequencies to within experimental accuracy (but with no predictive properties outside this data range)  The effective constants allowed making precise predictions of spectra for the singly substituted isotopic species of NCNCS  The spectra of the three strongest naturally abundant isotopic species of NCNCS: 34 S, 13 C 1, 13 C 2 have all been assigned and validity of the derived spectroscopic constants confirmed  The prediction of isotopic species with GSRB is found to be at a very systematic deviation relative to experiment, and may be refined with a suitable correction  The recent observation of pure rotational b-type transitions with the Bruker HR125 interferometer at the CLS provides a much stronger constraint on the effective fit. Reliable estimate of v b should now be possible from perturbations in the pure rotation spectrum (providing a double check and complementing the results from the vibration-rotation spectrum…) CONCLUSIONS: