Observation of the weakly bound (HCl) 2 H 2 O cluster by chirped-pulse FTMW spectroscopy Zbigniew Kisiel, a Alberto Lesarri, b Justin Neill, c Matt Muckle, c Brooks Pate c a Institute of Physics, Polish Academy of Sciences, Warszawa, Poland b Departamento de Quimica Fisica e Quimica Inorganica, Universidad de Valladolid, Valladolid, Spain c Department of Chemistry, University of Virginia, Charlottesville, Virginia, USA 65th OSU International Symposium on Molecular Spectroscopy TH03

Relative intensity Ar-HCl1000 Ar 2 HCl 100 Ar 3 HCl 50 (H 2 O) Ar-H 2 O 200 Ar(H 2 O) 2 Ar(H 2 O) 3 Ar 2 (H 2 O) Ar 3 (H 2 O) N 2 -HCl 200 N 2 -H 2 O 50 H 2 O-HCl1000 (H 2 O-HCl)-Ar 100 (H 2 O) 2 HCl Ar HCl aq Clusters observable above aqueous hydrochloric acid: Larger clusters ?

The path to H 2 O(HCl) 2 :  2007: First survey measurements at 8-18 GHz (Alberto Lesarri + Ne carrier gas)  2008: More measurements targeted at maximising the (H 2 O) 2 HCl signal (still Ne carrier)  2009:Switch to Ar carrier: many complex multiplets seen at successively better S/N by bubbling an HCl rich mixture in Ar through an H 2 O trap  H 2 O(HCl) 2 assigned (removal of the many known species crucial)  Targeted J =1  0 measurements at 6 GHz Deuterated species observed by using D 2 O  2010: 18 O species by using H 2 18 O (three weeks ago)

Chirped-pulse Fourier transform microwave spectrometer with supersonic expansion: Developed in the Pate group, U of. Virginia, USA: Up to 10 GHz in a single shot ! Rev.Sci.Instrum. 79, (2008) New options: - Several nozzles - Multiple chirps on one gas pulse

Chirped pulse spectrum of Ar + HCl + H 2 O: H 2 O … HCl (H 2 O) 2 H 35 Cl, 2 11  1 10 Ar … HCl

The lines of many different known clusters have to be accounted for. Practically all known Ar/HCl/H 2 O combinations are visible in this spectrum and the 35 Cl/ 37 Cl abundance further increases the number of species (H 2 O) 2 H 35 Cl, 3 03  2 02 (H 2 OHCl)Ar Chirped pulse spectrum of Ar + HCl + H 2 O: Ar 2 H 35 Cl, 5 41  4 40 (H 2 O) 2

Analysis tools: AABS package + SPFIT/SPCAT:

Subtraction of transitions for known clusters: The region of the 2 21  1 10 transition of H 2 O(HCl) 2 Residual: parent Raw spectrum: Isotopic satellites ?

Key properties of H 2 O(HCl) 2 : Cyclic global minimum MP2/aug-cc-pVDZ calculation. Note that at lower levels of calculation the global minimum is a chain structure.  a = 0.64 D  b = 2.04 D

Understanding of the spectrum of H 2 O(HCl) 2 : parent 37 Cl Cl 2 37 Cl 1 37 Cl 2 Obs. Calc.

Double chlorine nuclear quadrupole hyperfine structure: Lines predicted but not in the spectrum Lines in the spectrum but not in prediction Obs. Calc.

Additional satellites due to H 2 O dynamics in the cluster: (H 2 O) 2 (HCl):G 8 symmetry group four low energy species with statistical weights of 1:3:3:  0 00 transition The 1 11  0 00 transition of (H 2 O) 2 H 35 Cl: B2+B2+B2+B2+ B1+B1+B1+B1+ A2+A2+A2+A2+ A1+A1+A1+A1+ (a simple triplet)

The 1 11  0 00 transition of H 2 O(H 35 Cl) 2 : W = 1 state W = 3 state Obs. Calc.

Spectroscopic constants for H 2 O(H 35 Cl) 2 : MP2/aug-cc-pVDZ W = 3 state:  fit = 8.16 kHz 388distinct hyperfine components 36different rotational transitions: b R-, b Q-, a R-type + 5 other isotopic species

Heavy atom r s distances in (H 2 O) 2 HCl and H 2 O(HCl) 2 : Cl 1 Cl (1)  Å Cl 1 O 3.391(4)  Å ClO (3)  Å ClO (5)  Å Cl 2 O 3.126(3)  Å In H 2 O..HCl: ClO = 3.218(2) Å r vdW (O) = 1.52 Å r vdW (Cl) = 1.75 Å r vdW (O+Cl) = 3.27 Å r vdW (Cl+Cl) = 3.50 Å H 2 O..HCl:J.Phys.Chem. A 104, 6970 (2000) (H 2 O) 2 HCl:J.Chem.Phys. 112, 5767 (2000)

Orientation of the two HCl subunits: Hyperfine: 21.1(3) o Ab initio: 21.4 o Hyperfine: 73.2(5) o Ab initio: 71.4 o The complete nuclear quadrupole tensors for both chlorine nuclei are available. The quadrupolar angle resulting from tensor diagonalisation will be close to the orientation of the HCl bond, and can be compared against ab initio (MP2/aug-cc-pVDZ)

 The rotational spectrum of the cyclic H 2 O(HCl) 2 cluster was assigned for the first time.  Spectroscopic constants for 6 different isotopic species have (so far) been determined.  The experimental geometry, centrifugal distortion constants, and hyperfine coupling constants are in good semi-quantitative agreement with results of ab initio calculations.  The results, in comparison with those for H 2 O … HCl and (H 2 O) 2 HCl provide detailed insight into the balance of intermolecular interactions in these clusters, in particular, into the evolution of the properties of the H 2 O … HCl subunit.  There are still unassigned, complex multiplets in the spectrum that give hope for identifying further clusters in the Ar+H 2 O+HCl system.SUMMARY: