Microsolvation of  -propiolactone as revealed by Chirped-Pulse Fourier Transform Microwave Spectroscopy Justin L. Neill, Matt T. Muckle, Daniel P. Zaleski, Brooks H. Pate Department of Chemistry, University of Virginia, McCormick Rd, P.O. Box , Charlottesville, VA I. Peña, C. Perez, J.L. Alonso Grupo de Espectroscopía Molecular (GEM), Departamento de Química Física y Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid, E Valladolid, Spain

High-Resolution Spectroscopy of Solvated Organic Molecules (H 2 O) 3-6 : Saykally group (THz tunneling) C.J. Gruenloeh et al., Science 276, 1678 (1997). Ch. Janzen et al., J. Chem. Phys. 110, 9898 (1999). B. Ouyang, T.G. Starkley, B.J. Howard, J. Phys. Chem. A 111, 6165 (2007). D. Priem, T.-K. Ha, A. Bauder, J. Chem. Phys. 113, 169 (2000). S. E. Novick, Bibliography of Rotational Spectra of Weakly Bound Complexes, (2010). Electronic updates are available on the web at Water cubes: -benzene-(H 2 O) 8 : Gruenloeh et al. -phenol-(H 2 O) 7,8 : Janzen et al. (UV/IR) Microwave studies: -trifluoroacetic acid-(H 2 O) 3 : Ouyang et al. -formic acid-(H 2 O) 2 : Priem et al.

Experimental Methods: Chirped-Pulse FTMW Spectrometer 1.23 million averages (59 h), 300 W TWTA 2 nozzles, 10 FIDs per valve injection He or Ne backing gas (4 atm) G.G. Brown, B.C. Dian, K.O. Douglass, S.M. Geyer, S.T. Shipman, and B.H. Pate, Rev. Sci. Instrum. 79 (2008) L. Alvarez-Valtierra, S.T. Shipman, J.L. Neill, B.H. Pate, A. Lesarri, ISMS 2008, WF12. T. Emilsson, H.S. Gutowsky, G. de Oliveira, C.E. Dykstra, J. Chem. Phys. 112, 1287 (2000). Tools for structural analysis: -10% H 2 18 O sample to observe isotopically substituted species -CP-FTMW Stark effect measurement (60 V/cm) calibrated with trifluoropropyne 1 st -order shifts

x50 x500 x5000 CP-FTMW Spectrum Dense spectrum—microwave-microwave double resonance spectroscopy used extensively

1966 lines detected with signal to noise > 3: lines (61%) still unassigned x230 CP-FTMW Spectrum

 -propiolactone ExperimentalAb InitioPct. Error A (MHz) (13) % B (MHz) (5) % C (MHz) (4) %  A (D) 3.675(10) %  B (D) 2.01(5) %  C (D)  total (D) % Ab Initio calculations: Gaussian 03W, mp2/ g(d,p) (all structures) Spectral fits: SPFIT/SPCAT, PIFORM (PROSPE, Z. Kisiel, QSTARK (PROSPE) Figures: PMIFST (PROSPE) N. Kwak, J.H. Goldstein, J.W. Simmons, J. Chem. Phys. 25, 1203 (1956). Z. Chen and J. van Wijngaarden, J. Mol. Spectrosc. 257, 164 (2009). Large circles: ab initio structures Small circles: substitution coordinates Avg. deviation: Å CP-FTMWKwak et al.  A (D) 3.675(10)3.67(4)  B (D) 2.01(5)2.00(2)

 -propiolactone-H 2 O ExperimentalAb InitioPct. Error A (MHz) (20) % B (MHz) (5) % C (MHz) (5) %  A (D) (11) %  B (D) 2.543(23) %  C (D) [0] (fixed)  total (D) % O deviation: Å No c-type transitions observed; searched for tunneling gap <500 MHz

 -propiolactone-(H 2 O) 2 ExperimentalAb InitioPct. Error A (MHz) (3) % B (MHz) (4) % C (MHz) (3) %  A (D) 2.160(10) %  B (D) 1.544(23) %  C (D) 0.330(3) %  total (D) 2.676(25) % average O deviation: Å

 -propiolactone-(H 2 O) 3 ExperimentalAb InitioPct. Error A (MHz) (6) % B (MHz) (6) % C (MHz) (5) %  A (D) 2.357(5) %  B (D) 0.60(13) %  C (D) 0.12(8) %  total (D) 2.44(15) %  14.28°10.3°-4.0°  87.18°81.2°-5.9° Experimental A (MHz) (18) B (MHz) (12) C (MHz) (12)  A (D) (24)  B (D) ---  C (D) 1.773(8)  total (D) 1.975(8) ~5x weaker than above spectrum No structural assignment average O deviation: Å

 -propiolactone-(H 2 O) 4 ExperimentalAb InitioPct. Error A (MHz) (7) % B (MHz) (4) % C (MHz) (5) %  A (D) (9) %  B (D) 3.785(11) %  C (D) 2.667(7) %  total (D) 4.657(13) % ExperimentalAb InitioPct. Error A (MHz) (10) % B (MHz) (9) % C (MHz) (8) %  A (D) (15) %  B (D) 4.215(19) %  C (D) 2.009(7) %  total (D) 4.772(20) % average O deviation: Å 2:1 intensity ratio E rel = 70.4 cm -1

 -propiolactone-(H 2 O) 4 -(H 2 O) 4 mininum: free protons are “udud” around the ring -BPL-(H 2 O) 4 : free protons are “uudd” around the ring--less stable by 325 cm -1 tunneling quenched by complexation -two structures differ only by the direction of H-bonding around the ring J.D. Cruzan, M.R. Viant, M.G. Brown, R.J. Saykally, J. Phys. Chem. A 101, 9022 (1997). M. Schütz, W. Klopper, H.-P. Lüthi, J. Chem. Phys. 103, 6114 (1995).

 -propiolactone-(H 2 O) 5 ExperimentalAb InitioPct. Error A (MHz) (8) % B (MHz) (5) % C (MHz) (5) % (B/A)2(B/A) (C/A)2(C/A) Discrepancy in relative b/c dipoles

Conclusions All observed structures cool to minimum-energy configurations (rather than sequential addition) Competition/compromise between water-molecule and water-water interactions Importance of isotopic substitution, Stark effect measurements in structure determination of large water clusters -substitution structures and dipole moments agree very well with ab initio values -Use of isotopic assignments to drive ab initio (rather than the reverse)

Acknowledgements Funding National Science Foundation Chemistry CHE CRIF:ID CHE Miniesterio de Ciencia y Tecnología (Grant CTQ ) Junta de Castilla y León, Fondo Social Europeo (Grant VA012C05)

ExperimentalAb InitioError  (D) %  28.7°29.0°0.3°  90° 0°  PL Dipole Moment Errors ExperimentalAb InitioError  (D) %  68.57°73.2°4.6°  90°76.2°-13.8°  PL-H 2 O ExperimentalAb InitioError  (D) %  35.56°33.8°-1.7°  82.92°82.5°-0.4°  PL-(H 2 O) 2 ExperimentalAb InitioError  (D) %  14.28°10.3°-4.0°  87.18°81.2°-5.9°  PL-(H 2 O) 3 ExperimentalAb InitioError  (D) %  82.44°82.1°-0.3°  55.07°53.3°-1.8°  PL-(H 2 O) 4 (stronger) ExperimentalAb InitioError  (D) %  76.85°73.5°-3.3°  65.10°65.0°-0.1°  PL-(H 2 O) 4 (weaker)

Experimental Methods: MW-MW Double Resonance Spectroscopy CP-FTMW-MW Double ResonanceCavity FTMW-MW Double Resonance

 -propiolactone-(H 2 O) 5 (H 2 O) 5 : slightly puckered, has an “uudud” orientation of the water pentamer BPL-(H 2 O) 5 : similar structure; internal tunneling/pseudorotation quenched by complexation K. Liu, M.G. Brown, J.D. Cruzan, R.J. Saykally, J. Phys. Chem. A 101, 9011 (1997).