Observation of the weakly bound (HCl)2H2O cluster by chirped-pulse FTMW spectroscopy

Zbigniew Kisiel,a Alberto Lesarri,b

Justin Neill,c Matt Muckle,c Brooks Patec

aInstitute of Physics, Polish Academy of Sciences,Warszawa, Poland

bDepartamento de Quimica Fisica e Quimica Inorganica,Universidad de Valladolid, Valladolid, Spain

cDepartment of Chemistry, University of Virginia,

Charlottesville, Virginia, USA

65th OSU International Symposium on Molecular Spectroscopy TH0365th OSU International Symposium on Molecular Spectroscopy TH03

Relative intensity

Ar-HCl 1000

Ar2HCl 100

Ar3HCl 50

(H2O)2 400

Ar-H2O 200

Ar(H2O)2 Ar(H2O)3

Ar2(H2O) Ar3(H2O)

N2-HCl 200

N2-H2O 50

H2O-HCl 1000

(H2O-HCl)-Ar 100

(H2O)2HCl 300

+ Ar

HClaq

Clusters observable above aqueous hydrochloric acid:

Larger clusters ?

The path to H2O(HCl)2:

2007: First survey measurements at 8-18 GHz (Alberto Lesarri + Ne carrier gas)

2008: More measurements targeted at maximising the (H2O)2HCl 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 H2O

trap

H2O(HCl)2 assigned (removal of the many known species crucial)

Targeted J =10 measurements at 6 GHzDeuterated species observed by using D2O

2010: 18O species by using H218O (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, 053103 (2008)

New options:

- Several nozzles- Multiple chirps on

one gas pulse

Chirped pulse spectrum of Ar + HCl + H2O:

H2O…HClH2O…HCl

(H2O)2H35Cl, 211110

Ar…HCl Ar…HCl Ar…HCl

The lines of many different known clusters have to be accounted for.

Practically all known Ar/HCl/H2O combinations are visible in this spectrum and the 35Cl/37Cl abundance further increases the number of species

(H2O)2H35Cl, 303202

(H2OHCl)Ar

Chirped pulse spectrum of Ar + HCl + H2O:

Ar2H35Cl, 541440

(H2O)2

Analysis tools: AABS package + SPFIT/SPCAT:

Subtraction of transitions for known clusters:The region of the 221 110 transition of H2O(HCl)2

Residual: parent

Raw spectrum:

Isotopic satellites ?

Key properties of H2O(HCl)2:

Cyclic global minimum MP2/aug-cc-pVDZ calculation. Note that at lower levels of calculation the global minimum is a chain structure.

ma = 0.64 Dmb = 2.04 D

Understanding of the spectrum of H2O(HCl)2:

parent37Cl1 + 37Cl237Cl1

37Cl2

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 H2O dynamics in the cluster:

(H2O)2(HCl): G8 symmetry groupfour low energy species with statistical weights of 1:3:3:9

111 000 transition

The 111 000 transition of (H2O)2H35Cl: B2

+

B1+

A2+

A1+

(a simple triplet)

The 111 000 transition of H2O(H35Cl)2:

W = 1 stateW = 3 state

Obs.

Calc.

Spectroscopic constants for H2O(H35Cl)2:

MP2/aug-cc-pVDZ

W = 3 state: sfit = 8.16 kHz

388 distinct hyperfine components

36 different rotational transitions:

bR-, bQ-, aR-type

+ 5 other isotopic species

Heavy atom rs distances in (H2O)2HCl and H2O(HCl)2:

Cl1Cl2 3.695(1) Å

Cl1O 3.391(4) Å

ClO2 3.402(3) Å

ClO1 3.083(5) Å

Cl2O 3.126(3) Å

In H2O..HCl: ClO = 3.218(2) Å

rvdW(O) = 1.52 ÅrvdW(Cl) = 1.75 ÅrvdW(O+Cl) = 3.27 ÅrvdW(Cl+Cl) = 3.50 Å

H2O..HCl: J.Phys.Chem. A 104, 6970 (2000)(H2O)2HCl: 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 H2O(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 H2O…HCl and (H2O)2HCl provide detailed insight into the balance of intermolecular interactions in these clusters, in particular, into the evolution of the properties of the H2O…HCl subunit.

There are still unassigned, complex multiplets in the spectrum that give hope for identifying further clusters in the Ar+H2O+HCl system.

SUMMARY: