The Millimeter-Wave Rotational Spectrum of Phenylacetylene

65th OSU International Symposium on Molecular Spectroscopy RH1465th OSU International Symposium on Molecular Spectroscopy RH14

Zbigniew Kisiel, Adam Kraśnicki

Institute of Physics, Polish Academy of Sciences,

Warszawa, Poland

Background information:

a = 0.656(5) D

= 0.375(10) D ( toluene )

v24 = 140 cm-1 v36 = 151.9 cm-1 Gauche

cmw, first: Zeil + Winnewisser +…, Z.Naturforsch. 15a, 1011 (1960)cmw, details: Cox et al., JCS Farad.Trans. II 71, 93 (1975)FTMW, geometry: Dreizler et al., J.Mol. Struct. 698, 1 (2004)

ir, normal modes: King + So, J.Mol. Struct. 36, 468 (1970)LIF, v24,v36: Bacon + Hollas + Ridley, Can.J.Phys. 62, 1254 (1984)

ab-initio: Csaszar + Fogarasi + Boggs, J.Phys.Chem. 93, 7644 (1989)

astro related: made from benzene, acetylene, vinylacetylene…

Marker of aromatic chemistry? Titan?

Assignment of the phenylacetylene ground state:

J ”Spectrum dominated by specific bands formed by high-J, aR-type transitions [ type-II+ n=2 classification and properties in Kisiel, Pszczolkowski, J.Mol.Spectrosc. 178, 125 (1996) ]

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

Quantum number coverage for the ground state:

Symbol size is proportional to (nobs-ncalc)/dn

STARK = Zeil et al.Cox et al.

FTMW = Dreizler et al.+ this work

Spectroscopic constants for the ground state ofphenylacetylene:

Rather rudimentary HF/4-21G calculation with scaling,Csaszar et al , J.Phys.Chem. 93, 7644 (1989).

n 340 GHzJ 140Ka 59

Excited state type-II+ bands in phenylacetylene:

Changes in excited state line patterns relative to g.s. are usually moderate and dependant on changes in inertia defect. In this case the changes are much greater requiring the interstate perturbation treatment.

Lowest normal modes in phenylacetylene:

140 cm-1

151.9 cm-1

These are the out-of-plane and in-plane distortion of –CCH relative to the phenyl ring.

The next higher mode is n23(B1)=349 cm-1.

The Hamiltonian:

The symmetry point group for phenylacetylene is C2v and 24=1 (B1) and 36=1 (B2) can couple around the axis, which transforms as:

B1 B2 = A2,This is the z-axis and, since phenylacetylene is prolate, the two states can thus perturb through a-axis Coriolis interactions.

The Hamiltonian is set up in 22 block form, where the diagonal blocks are Watson’s rotational terms for each state, the 36=1 block is augmented by the vibrational energy separation DE, and the off-diagonal terms are:

Hc(24 ,36) = i (Ga + Ga

JP2 + GaKPz

2 + …) Pz + (Fbc + Fbc

JP2 + FbcKPz

2 + …) (Px Py + Py Px ) + …,

The major coupling constant Ga is related to the Coriolis coefficient za24,36 by:

Ga = A za24,36 [ (w24/w36)1/2 + (w36/w24)1/2 ]

Fits and predictions were made with the SPFIT/SPCAT package of H.M.Pickett.

Nuclear spin statistical weights in phenylacetylene:

24 = 1 36 = 1g.s.

5:3 weights arise from the presence of two pairs of symmetry-equivalent protons. weights will reverse between g.s. and B-symmetry excited states.

each doublet below consists of 444,41 434,40 transition (left) and 443,41 433,40 transition (right)

The Coriolis fit for (24=1 36=1) in phenylacetylene:

za24,36=0.8393(3)

Calculated:za

24,36=0.84

Two alternative fits of interstate interaction:

Discrimination on the basis of standard deviation or values of other constants not very sharp.

Clearest distinction is provided by calculation of Av-A0 made with the CFOUR package.

Solution I: A24 < A0 < A36 sfit=36.1 kHz

Solution II: A24 > A0 > A36 sfit=36.8

kHz

Vibrational changes in rotational constants (MHz):

MP2/6-31G(d,p)

Solution I

Deperturbed values obtained from the effective values:

A24 - A0 = -173.45 MHz and A36 - A0 = 172.09 MHz

calculated by CFOUR.

The perturbation contribution subtracted from the effective values is given by:

J.Mol.Spectrosc. in press

Room temperature rotational spectrum of phenylacetylene was studied up to 340 GHz.

Ground state transitions were measured up to J =140 and Ka=59.

Transitions in the two lowest vibrationally excited states have been assigned and the strong a-axis Coriolis resonance between the two states was successfully fitted (sfit=36 kHz).

All excited state lines that are strong enough for confident assignment are in the fits, including several nominal interstate transitions, but all of these lines are almost equally well fitted with two different solutions.

Anharmonic force field calculations of vibrational changes in rotational constants allowed unambiguous discrimination between alternative solutions.

Strongest lines in the rotational spectrum can be predicted well into the submillimeter.

SUMMARY: