The rotational spectrum of acrylonitrile to 1.67 THz Zbigniew Kisiel, Lech Pszczółkowski Institute of Physics, Polish Academy of Sciences Brian J. Drouin, Carolyn S. Brauer, Shanshan Yu, John C. Pearson Jet Propulsion Laboratory, California Institute of Technology, 64th OSU International Symposium on Molecular Spectroscopy WH10

Rotational spectroscopy of acrylonitrile: review:Gerry et al., J.Phys.Chem.Ref.Data A 8, 107 (1979)  :Stolze+Sutter, Z.Naturforsch. 40a, 998 (1985) satellites:Cazzoli+Kisiel, J.Mol.Spectrosc. 130, 303 (1988) smm+struct.:Demaison et al., J.Mol.Spectrosc. 167, 400 (1994) mmw:Baskakov et al., J.Mol.Spectrosc. 179, 94 (1996) isotopic:Colmont et al., J.Mol.Spectrosc. 181, 330 (1997) first astro:Gardner+Winnewisser, Astrophys.J.. 195, L127 (1975) isotopes+astro:Muller et al., J.Mol.Spectrosc. 251, 319 (2008) Planar (C s ), relatively rigid molecule, positioned in the ab inertial plane  a = 3.815(12) D  b = 0.894(68) D

Temperature dependence of the acrylonitrile rotational spectrum:  a type transitions  b type transitions spectra measured in this work

Spectra measured with the cascaded frequency multiplication spectrometer at jpl: Broadband coverage possible well into the THz region with single scans reaching frequency spans of 100 GHz. Drouin, Mailwald, Pearson, Rev.Sci.Instr. 76, (2005)

All spectra were merged into a single file: JPL spectra/MHz Span/GHz TOTAL = Gb 500 GHz: n = 30 (6  5) 1600 GHz: n = 108 (6  2  3  3)

AABS AABS has been applied to many different types of broadband spectra: FASSST, cascaded multiplication THz, chirped pulse FTMW, Bruker FTIR..

Good visibility of high-J, a R-type transitions In this Loomis-Wood type display spectral strips are aligned on frequencies K a = 0 transitions for successive values of J. At the same time abundant spectra rapidly produce various surprises. This approach allows rapid assignment and data file construction.

Understanding of the 1.0 THz spectrum: obs. calc. b Q, K a =12←11 The majority of the visible transitions are b-type, and the strongest a-type transitions are indicated by 

Understanding of the 1.6 THz spectrum: obs. calc.

Perturbations in the lowest vibrational states in acrylonitrile: Notation used for identified perturbations, in this case between: K a = 18 in  11 =1 and K a = 22 in g.s.

Principal perturbations identified in a R-type g.s. transitions: Broadband coverage possible well into the THz region with single scans reaching frequency spans of 100 GHz. g.s. lines  11 = 1 lines Differences relative to effective single state fits are plotted.  K a = 6  K a = 4

The Hamiltonian: The g.s. and  11 =1 both belong to the A’ representation of the C s point group. These states can thus be connected by Fermi resonance and c-axis Coriolis interactions. The Hamiltonian is in 2  2 block form, where for the diagonal blocks we used Watson’s Hamiltonian in both S- and A-reduction, and vibrational energy separation  E in the  11 =1 block. The dominant off-diagonal contribution between g.s. and  11 =1 turns out to come from Fermi resonance : H F (i, j) = W F + W F J P 2 + W F K P z 2 + …, while for the c-axis Coriolis interaction it is possible to use : H c (i, j) = (G c + G c J + G c K + …) P c + (F ab + F ab J + G ab K + …) (P a P b + P b P a ) + …, although only F ab proved to be determinable. Fits and predictions were made with the SPFIT/SPCAT package of H.M.Pickett.

The fitted spectroscopic constants for g.s.   11 =1 coupling: Broadband coverage possible well into the THz region with single scans reaching frequency spans of 100 GHz.

The progress in measurements of the g.s. : Symbol size proportional to ( obs - calc )/  Red symbols for ( obs - calc ) > 3  previous: 602 lines,   fit = 94 kHz this work: 3145 lines,  fit = 143 kHz

g.s. lines can also be affected by very specific perturbations: Example here shows successive g.s. a R-type doublets for K a = 12. The two components should in all cases be degenerate, but one is shifted by perturbation with K a = 2 of  11 =1. The effect is accounted for in the fit, and note that the perturbation is for  K a =10 !

Nominal interstate transitions involving the g.s.: Transitions are the result of strong mixing between But the mixing is at J =102 and it is necessary measure the spectrum at 960 GHz ! Current work leads to  E = (2) cm -1 from the rotational spectrum To compare with  E = (18) cm -1 from the gas-phase fir spectrum (Cole+Green, J.Mol.Spectrosc. 48,246(1973). K a = 16 levels of the g.s. K a = 10 levels of  11 =1 {

Comparison of some observables with calculation: a – calculated using the 6-31G(d,p) basis, GAMESS and VIBCA.

 Extensive measurements of the rotational spectrum of acrylonitrile have been made, at frequencies up to 1.67 THz, and covering a total of more than 540 GHz.  The data set for the ground state has been extended by a factor of 5 in the number of measured lines. Coverage of quantum number values is now up to J = 129 and K a = 30.  Multiple perturbations affecting ground state lines were identified and successfully fitted in terms of coupling with  11 =1, even though that state differs in vibrational energy by (2) cm -1 (determined in this work).  The most spectacular perturbations are at J >100 and THz frequencies.  New results for all 13 C and for the 15 N species have also been obtained.  A ladder of perturbations extending from the ground state upwards has been identified, and it is possible that precise energies of all low lying vibrational states may eventually be determinable from a global analysis of the rotational spectrum.SUMMARY: