Towards understanding quantum monodromy in quasi-symmetric molecules: FASSST rotational spectra of CH 3 NCO and CH 3 NCS Zbigniew Kisiel, a Sarah Fortman, b Ivan R. Medvedev, b Manfred Winnewisser, b Frank C. De Lucia, b Jacek Koput c a Institute of Physics, Polish Academy of Sciences, Warszawa, Poland b Department of Physics, The Ohio State University, Columbus, Ohio, USA c Department of Chemistry, Adam Mickiewicz University, Poznan, Poland 65th OSU International Symposium on Molecular Spectroscopy RC13

From quasi-linearity to quantum monodromy: B.P.Winnewisser, Chapter 6 of ”Molecular Spectroscopy: Modern Research” Vol. III, (1985) 00 bent molecule limit linear molecule limit B.P.Winnewisser et al., Phys.Rev.Lett. 95,243002(2005) M.Winnewisser, et al., J.Mol.Struct. 798,1(2006) B.P.Winnewisser et al., PCCP (2010), doi: /B922023B Have you been to MF12 ?

Quasi-symmetry: 00 Asymmetric rotor Symmetric rotor OCurl et al., J.Chem.Phys. 39,3335(1963)  = 2.81(6)D O + SLett+Flygare, J.Chem.Phys. 47,4730(1967)V 3 (CH 3 NCO)=29(5) cm -1 SS.Cradock, J.Mol.Spectrosc. 92,170(1982)V bend  160 cm -1 SM.Kreglewski, CPL 112,275(1984) V 3  1 cm -1 OJ.Koput, J.Mol.Spectrosc. 106,12(1984) OJ.Koput, J.Mol.Spectrosc. 115,131(1986)V bend =928(5) cm -1, V 3 =20.7(1) cm -1 SJ.Koput, J.Mol.Spectrosc. 118,189(1986)V bend =192.9(6) cm -1, V 3 =1.5(2) cm -1 O + SKasten+Dreizler, Z.Naturf. A 41,637(1986)    D,    S  D O + SJ.Koput, J.Mol.Spectrosc. 127,51(1988)  

CNC bending potentials: cm  CH 3 NCS, Koput, J.Mol.Spectrosc. 118,189 (1986) CH 3 NCO, Koput, J.Mol.Spectrosc. 115,131 (1986) v b =  /deg   = 180 –  (CNC)

 FASSST spectra of both molecules were recorded: almost continuous GHz coverage at estimated frequency accuracy of 50 kHz.  Spectra were reduced into transition series described by effective, linear molecule, expansion of rotational energy: E = B J(J+1) – D J 2 (J+1) 2 + H J 3 (J+1) 3 + L J 4 (J+1) 4 + P J 5 (J+1) 5 + … The SPFIT/SPCAT package was used for the calculations and the AABS package was used as the front end for graphical assignment and building up the datasets. This project stimulated modification of AABS to deal with up to 350 datasets simultaneously and improvement of mechanisms for navigating between datasets. Mechanisms for eliminating known lines from the spectrum and from peakfinder lists are also available. AABS = A ssignment and A nalysis of B roadband S pectra, available at: This work:

FASSST = FA st S canning S ubmillimeter S pectroscopic T echnique Coverage: GHz Petkie et al., Rev.Sci.Instrum. 68, 1675 (1997) Improvements: U/D averaging, correction for effect of H 2 O lines on calibration

The FASSST spectrum of CH 3 NCO: MHz

FASSST spectra of CH 3 NCO and CH 3 NCS: CH 3 NCS CH 3 NCO J = 27  26 J = 16  15 J = 28  27 J = 26  25 J = 15  14 ?

CH 3 NCS: tentative = 0 region and correlation with cmw lines: CH 3 NCS: tentative v b = 0 region and correlation with cmw lines: J” = 0 J” = 70 J” = 22

CH 3 NCS: tentative = 1 region and resonances: CH 3 NCS: tentative v b = 1 region and resonances: J” = 0 K = 0

Reduction of the spectrum to transition sequences: Spectral strips in the FASSST spectrum of CH 3 NCS plotted from: J = 23  22 at GHz (bottom) to: J = 71  70 at GHz (top) Spectrum Residual

Reproduction of the FASSST spectrum by assigned sequences: The J = 24  23 transition of CH 3 NCS Spectrum Sequences

The numerical form of the reduction to sequences (CH 3 NCS): Each of >220 datasets consists of SNNN.PAR,.LIN,.INT, and.CAT files Relative intensity of a sequence derived automatically from the spectrum  b = 0, m = 0, K = 0

CH 3 NCO CH 3 NCS / 2(J”+1) (MHz) J” Reduced Fortrat diagrams for CH 3 NCS and CH 3 NCO: K a assignment is based on Stark spectra at < 40 GHz K a = 1 K a = 0 K a = 2  b = 0, m = 0

Are the effective constants meaningful ? D J CH 3 NCSCH 3 NCO calc: kHz histogram:0.65(5) kHz Comparison of values estimated from histograms for ca 220 empirical sequences and B3LYP/6-31G(d,p) calculation calc: 1.98 kHz histogram:2.15(10) kHz

Are the effective constants meaningful ? H J histogram:0.0013(2) Hz histogram:|H J | < Hz ? CH 3 NCSCH 3 NCO

 FASSST spectra of CH 3 NCO and CH 3 NCS have been recorded, providing almost continuous coverage of GHz.  Each spectrum was reduced to about 220 different line sequences described by effective expansion of rotational energy in power series of J(J+1).  Each sequence can be reconstructed from fitted values of B eff, D eff, H eff, … augmented by its relative intensity determined from the spectrum.  The sequences in the FASSST spectrum could be unambiguously connected with many centimeter-wave, low-J sequences assigned by Stark spectroscopy.  The next step in this work is to use this experimental information to extend the techniques that were successfully applied to quasi-linearity in NCNCS. The further steps are far from trivial as the additional dimension necessary to deal with the nearly free internal rotation is still to be added.SUMMARY: