65 th OSU International Symposium on Molecular Spectroscopy

65 th OSU International Symposium on Molecular Spectroscopy TH 03 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 Patec a. Institute of Physics, Polish Academy of Sciences, Warszawa, Poland b. Departamento de Quimica Fisica e Quimica Inorganica, Universidad de Valladolid, Spain c. Department of Chemistry, University of Virginia, Charlottesville, Virginia, USA

Clusters observable above aqueous hydrochloric acid: Relative intensity + Ar Ar-HCl 1000 Ar 2 HCl 100 Ar 3 HCl 50 (H 2 O)2 400 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-HCl HClaq 1000 (H 2 O-HCl)-Ar 100 (H 2 O)2 HCl 300 Larger clusters ?

The path to H 2 O(HCl)2: ð 2007: First survey measurements at 8 -18 GHz (Alberto Lesarri + Ne carrier gas) ð 2008: More measurements targeted at maximising the (H 2 O)2 HCl signal (still Ne carrier) ð 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 ð H 2 O(HCl)2 assigned (removal of the many known species crucial) ð Targeted J =1 0 measurements at 6 GHz Deuterated species observed by using D 2 O ð 2010: 18 O species by using H 218 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 ! New options: - Several nozzles - Multiple chirps on one gas pulse • Rev. Sci. Instrum. 79, 053103 (2008)

Chirped pulse spectrum of Ar + HCl + H 2 O: H 2 O…HCl Ar…HCl (H 2 O)2 H 35 Cl, 211 110

Chirped pulse spectrum of Ar + HCl + H 2 O: 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, 303 202 Ar 2 H 35 Cl, 541 440 (H 2 OHCl)Ar (H 2 O)2

Analysis tools: AABS package + SPFIT/SPCAT:

Subtraction of transitions for known clusters: The region of the 221 110 transition of H 2 O(HCl)2 Raw spectrum: Residual: parent Isotopic satellites ?

Key properties of H 2 O(HCl)2: Cyclic global minimum MP 2/aug-cc-p. VDZ calculation. Note that at lower levels of calculation the global minimum is a chain structure. ma = 0. 64 D mb = 2. 04 D

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

Double chlorine nuclear quadrupole hyperfine structure: Lines in the spectrum but not in prediction Lines predicted but not in the spectrum 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: 9 111 000 transition (a simple triplet) The 111 000 transition of (H 2 O)2 H 35 Cl: B 2+ B 1+ A 2+ A 1+

The 111 000 transition of H 2 O(H 35 Cl)2: Obs. Calc. W = 3 state W = 1 state

Spectroscopic constants for H 2 O(H 35 Cl)2: W = 3 state: sfit = 8. 16 k. Hz 388 distinct hyperfine components 36 different rotational transitions: b. R-, b. Q-, a. R-type + 5 other isotopic species MP 2/aug-cc-p. VDZ

Heavy atom rs distances in (H 2 O)2 HCl and H 2 O(HCl)2: Cl. O 1 3. 083(5) Å Cl 1 O 3. 391(4) Å Cl 2 O 3. 126(3) Å Cl. O 2 3. 402(3) Å Cl 1 Cl 2 3. 695(1) Å In H 2 O. . HCl: Cl. O = 3. 218(2) Å rvd. W(O) = 1. 52 Å rvd. W(Cl) = 1. 75 Å rvd. W(O+Cl) = 3. 27 Å rvd. W(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: 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 (MP 2/aug-cc-p. VDZ) Hyperfine: 21. 1(3) o 73. 2(5) o Ab initio: 21. 4 o 71. 4 o

SUMMARY: ð The rotational spectrum of the cyclic H 2 O(HCl)2 cluster was assigned for the first time. ð Spectroscopic constants for 6 different isotopic species have (so far) been determined. ð The experimental geometry, centrifugal distortion constants, and hyperfine coupling constants are in good semi-quantitative agreement with results of ab initio calculations. ð 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. ð There are still unassigned, complex multiplets in the spectrum that give hope for identifying further clusters in the Ar+H 2 O+HCl system.