STRUCTURES OF TWO ISOMERS OF NITROUS OXIDE TETRAMER FROM THEIR INFRARED SPECTRA J. Norooz Oliaee, F. Mivehvar, M. Dehghany, N. Moazzen-Ahmadi Department of Physics and Astronomy University of Calgary A.R.W. McKellar Steacie Institute for Molecular Sciences National Research Council of Canada

Motivation Weakly bound complexes provide a convenient starting point for a detailed understanding of different pathways that can be taken between the gas and condensed phases of matter. It is of considerable interest to determine the number of isomers for a cluster size and if and how geometrical choices made in the early stages of condensation influence the growth of larger clusters. Although it is expected that the number of isomers grows rapidly with cluster size, in many cases only a single isomer is observed experimentally.

The N 2 O dimer was also observed in the regions of the 1 band (Qian, Herrebout, Howard, 1997) and the 3 band (Oshima, Matsumoto, Takami, Kuchitsu, 1988) Summary of past works on N 2 O clusters Non-polar dimer

Polar and non-polar isomers of (N 2 O) 2 Dehghany, et al., JCP, 130, (2007). Walker et al., JMS, 251, 153 (2008). MW spectrum Non-polar isomer Polar isomer

Torsional combination band gives a frequency of 27.3 cm -1 for the out of plane torsional mode. Simulation based on the results by Hecker et al.(2003) Results by NRC (2007) Results by our group (2007) Our simulation

Combination band involving the in-plane geared bend

N 2 O trimer

( 14 N 2 O) 3 ( 15 N 2 O) 3 Three bands for ( 14 N 2 O) 3 in the 1 region of N 2 O monomer Two bands for ( 14 N 2 O) 3 in the 3 region of N 2 O monomer (1280 cm -1 ) Analogous bands for ( 15 N 2 O) 3 The barrel-shaped N 2 O trimer

Assignment is complicated by the lack of symmetry and a large number of possible combination bands (27) in the 1 region of N 2 O monomer. Intermolecular frequency (in cm -1 ) In-plane geared bend Torsion of the dimer unit Torsion of the upper monomer Rocking of the upper monomer in a-c plane (N 2 O) ? ?30.58? (N 2 O) Chemical Physics Letters 476 (2009) 143–146 Combination bands involving the intermolecular modes

N 2 O tetramer

14 N 2 O tetramer with D 2d symmetry A parallel band showing no K structure. It is centered at cm -1.

15 N 2 O tetramer with D 2d symmetry The band due to ( 15 N 2 O) 4 does not show any K structure either. It is centered at cm -1

c-type band can be well simulated using C s symmetry. A = MHz B = MHz C = MHz Mixed isotopomer ( 15 N 2 O) 3 ( 14 N 2 O)

1. The spectra above are consistent with a structure having D 2d symmetry. 2. The monomers are more or less parallel to the symmetry axis (close to perfect barrel shape) because of the absence of a perpendicular band sharing the same lower state parameters. 3. Tetramer is an oblate symmetric top with A = B = MHz and C ~ 698 MHz. Structure of (N 2 O) 4 with D 2d symmetry 4. Adjacent monomers have structure very similar to non-polar dimer. 5. Three structural parameters are required to define the geometry of the complex completely. 6. This is the same species that was observed by Miller and Pederson.

A second N 2 O tetramer band: this one is perpendicular, and located close to the trimer band The 15 N 15 N 16 O isotopomer is shown here For the normal isotope, this band is centered at cm -1

N 2 O tetramer with S 4 symmetry Top view Side view symmetry axis Side view

N 2 O tetramer with S 4 symmetry 1.Spectra are consistent with a structure having S 4 symmetry. 2. Monomers are more or less perpendicular to the symmetry axis, because of the absence of a parallel band sharing the same lower state parameters. 3. Tetramer is a prolate symmetric top with A = MHz, B = C = MHz (for ( 14 N 2 O) 4 ). 4. Diagonally opposite monomers have structure very similar to nonpolar dimer with a dihedral angle of about 30º. Adjacent monomers have T- or L-shaped orientation. 5. Four structural parameters are required to define the geometry of the complex completely.

Experimental setup TDL Jet Trigger Ref. Gas 12 bit DAQ Card Timer Controller Card (CTR05) Laser Sweep Trigger DAQ Trigger Gas Supply Jet Signal Jet Controller (Iota One) Jet Controller IR Detectors TDL Controller (L5830) Etalon Monochromator Supersonic jet apparatus with a tunable diode laser probe