Infrared spectra of complexes containing

acetylene-d2

Clément Lauzin, J. Norooz Oliaee, N. Moazzen-AhmadiDepartment of Physics and Astronomy

University of Calgary

A.R.W. McKellar Steacie Institute for Molecular Sciences

National Research Council of Canada

TDL

Jet Trigger

Ref. Gas12 bit

DAQ Card

Timer Controller Card (CTR05)

LaserSweep

Trigger

DAQTrigger

Gas Supply

Jet Signal

Jet Controller (Iota One)

Jet Controller

IR DetectorsTDL Controller (L5830)

Etalon

Monochromator

pulsed supersonic jet / tunable diode laserapparatus at The University of Calgary

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G.T. Fraser, R.D. Suenram, F.J. Lovas, A.S. Pine, J.T. Hougen, W.J. Lafferty, and J.S. Muenter, J. Chem. Phys. 89, 6028 (1988).

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K. Didriche, C. Lauzin, T. Foldes, X. de Ghellick D’Elseghem Vaenerwijck, and M. Herman, Mol. Phys. 108, 2155 (2010).

Acetylene dimer

A landmark example in microwave and IR spectroscopy of van der Waals complexes.

Acetylene Dimer

The possibility of tunneling between equivalent T-shaped configurations splits

each rotational level into three states, which are labelled A1

+, B1+, and E+.

J, Ka, Kc

B1+

A1+

E+

(HCCH)2, ground state: B1+ - A1

+ = 2207 MHz

(DCCD)2, ground state: B1+ - A1

+ = 424 MHz

Acetylene Dimer

We expect two IR bands, each of which has three components A1

+, B1+, and E+.

Parallel band (Ka = 0), associated with stretch of this “stem” monomer

Perpendicular band (Ka = 1), associated with stretch of this “top” monomer

a - axis

b - axis

3 asymmetric C-H stretch fundamentalHCCH: 3295 cm-1 (and 3282 cm-1, Fermi resonance)

DCCD: 2439 cm-1

Acetylene dimer

Nuclear spin statistics

Ka, Kc

(HCCH)2 (DCCD)2

A1+ B1

+ E+ A1+ B1

+ E+

ee 1 0 3 21 15 18

eo 0 1 3 15 21 18

oe 6 3 3 6 3 18

oo 3 6 3 3 6 18

Wavenumber / cm-1

2434 2435 2436 2437

Ka = 0 - 1

Ka = 1 - 0 Overview of (DCCD)2 spectrum

Our laser coverage was limited: we observe these 2 important sub-bands, but miss higher K values.

We also miss the parallel band, which probably(?) lies at a lower frequency.

Wavenumber / cm-1

2436.8 2437.0 2437.2 2437.4 2437.6

observedspectrum

simulated sum

A - B band

E - E band

B - A band

(DCCD)2

This plot shows the Q- and R-branches of the K = 1 0 sub-band

The selection rules are:A1

+ B1+

E+ E+ So there are actually 3 bands, as shown.

T0 / cm-1 A or A / MHz

B or B / MHz

C or C / MHz

A1+ Ground 0.0 26026. 1752.013 1624.354

B1+ Ground 0.0141494 26006. 1751.482 1624.269

E+ Ground X 26007. 1751.748 1624.303

A1+ Excited 2435.7990(1) +585.41(253) +0.818(86) +0.238(63)

B1+ Excited 2435.8037(1) +479.77(241) +1.220(102) +0.095(52)

E+ Excited X + 2435.7917(1)

+613.19(647) +0.582(249) +0.352(154)

(DCCD)2 parameters

Ground state parameters are from:Matsumura, Lovas, and Suenram, J. Mol. Spectrosc. 150, 576 (1991).

X 0.12 cm-1, but the precise value is not known.

Note the large change in A value!

(DCCD)2 parameters

The excited state tunneling splitting (B1+ - A1

+) turns out to be 141 MHz, considerably smaller than the ground state value of 424 MHz. Comparable (3 perpendicular band) values for (HCCH)2 are 2207 and 572 MHz. The excited state splittings are reduced because tunneling is inhibited by the need to exchange the vibrational excitation when the “top” and “stem” monomers interchange.

Wavenumber / cm-1

2436.83 2436.84 2436.85 2436.94 2436.95 2436.96

2436.27 2436.28 2436.29 2436.15 2436.16 2436.17

212 - 101

212 - 303

313 - 202

313 - 404

The (DCCD)2 spectrum exhibits small perturbations affecting low-J levels of the upper state E+ component (examples shown on the right). But these are much less extensive than the mammoth perturbations noted in the IR spectrum of (HCCH)2.

Wavenumber / cm-1

2434.4 2434.6 2434.8 2435.0

observed

DCCD - HCCH

DCCD - DCCD

We also observe the mixed dimer HCCH – DCCD

Almost exactly the same location as (DCCD)2. It’s a perpendicular band, so we know DCCD has the “top” position, and HCCH the “stem” position.

First observation of this isomer, which has a slightly higher energy than the form with DCCD as the “stem” of the T.

This is the linear complex DCCD-NN, which has been studied in the microwave region by:

Legon, Wallwork, and Fowler, Chem. Phys. Lett. 184, 175 (1991).

Air in our gas sample ?!?!?

Wavenumber / cm-1

2436.8 2437.0 2437.2 2437.4 2437.6

observedspectrum

simulated sum

A - B band

E - E band

B - A band

N2 - C2D2 band

We also observe a “mystery” spectrum (with B" = 1427 MHz)

We observe another “mystery” spectrum in a different region

2418 cm-1 mystery spectrum

There are two simple bands here, with B" = 2419 and 2513 MHz.

The first B"-value agrees with that of C2D2-D2O.* The second could be that of C2D2-HDO (not previously studied).

But where is the K = 1 – 1 sub-band of C2D2-D2O?

* Peterson and Klemperer, J. Chem. Phys. 81, 3842 (1984).

Conclusions• The (DCCD)2 IR spectrum (3) is much less perturbed than that

of (HCCH)2.

• The large increase in A-value in going from the ground to excited state is difficult to understand.

• Is it possible that the accepted (indirect) microwave A-values for acetylene dimers do not reflect the true ground state K = 0 to 1 energy spacings?

• A wider laser scan to observe more perpendicular subbands would give us a handle on this elusive ground state A-value. Might also allow observation of the parallel band.

• Unfortunately, lasers in this region (2430 cm-1) are not so readily available these days.