1. DANIEL P. ZALESKI, JUSTIN L. NEILL, MATTHEW T. MUCKLE, AMANDA L. STEBER, NATHAN A. SEIFERT, AND BROOKS H. PATE Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904 KEVIN O. DOUGLASS National Institute of Standards and Technology, Optical Technology Division, Gaithersburg, MD 20899 Structure Study of Formic Acid Clusters By Chirped-Pulse FTMW Spectroscopy The Ohio State 66 th International Symposium on Molecular Spectroscopy, June 23 rd, 2011.

2. MW Spectroscopy and Clusters  Has played a role in studying intermolecular forces in clusters  But mostly limited to dimers  Goal here is to push MW spectroscopy to larger clusters  Push limits of theory and experiment  Imperative broadband spectroscopy  Complicated PES  No real target to go for  Ultimately need atom positions  Measure first, see what’s present, then get out the structure

3. Introduction D. Priem, T.-K. Ha, A. Bauder. J. Chem. Phys., 113, (2000), 169-175. S. T. Shipman, J. L. Neill, R. D. Suenram, M. T. Muckle, and B. H. Pate. J. Phys. Chem. Lett., 2, (2011), 443-448.

4. Experimental Gordon G. Brown, Brian C. Dian, Kevin O. Douglass, Scott M. Geyer, Steven T. Shipman, and Brooks H. Pate. Rev. Sci. Instrum. 79, 053103, (2008). Reduced Bandwidth Higher Throughput: 7-9 GHz – mix down with 9.9 GHz PDRO, filter as necessary 9-13 GHz – mix down with 14040 MHz PDRO, filter as necessary Allows digitization at lower sampling rates – faster averaging High purity formic acid (98%), lower purity has too much water Going for Kraitchman  Need Speed x3 24 Gs/s AWG

5. 1.981 million averages, 40 psi, 50°C, neon carrier gas Noise Floor <200 nV

6. Formic Acid Trimer 13 C’s and 18 O’s in natural abundance accompanying isotopic information: D double and triple 13 C double and triple D D and 13 C A (MHz)2936.5115(23) B (MHz)595.07077(69) C (MHz)495.25988(58) ΔJ (kHz)0.07676(24) ΔJK (kHz)-0.28380(85) ΔK (kHz)4.560(34) δJ (kHz)0.2925(35) δK (kHz)0.016740(55) B3LYP/6-31++G(d,p) 136 lines 3 kHz rms Conformational Studies in Formic Acid Oligimers. Richard D. Suenram, Pam L. Crum, Kevin O. Douglass, and Brooks H. Pate. The Ohio State 59th International Symposium on Molecular Spectroscopy.

7. Formic Acid Trimer Stark μaμa μbμb θ†θ† EXP1.18(6)0.995(12)40.1(15) B3LYP/6-311++G(d,p)1.431.1338.3 MP2/6-311++G(d,p)1.051.3652.3 † angle between the dipole moment vector and the a principle axis Emilsson, T., Gutowsky, H. S., de Oliveira, G., Dykstra, C. E. J. Chem. Phys. 112, 1287, (2000). Experimental dipole green, Ab inito dipole blue

9. 11-10 AB quartet

10. Complex PES A. K. Roy and A. J. Thakkar. Chem. Phys., 312, (2005), 119-126.

11. 2.16 million averages, 13 C-enriched FA introduced 1:4, ~0.5 mL sample!

12. 4.5 million averages, d-enriched FA introduced 1:4, ~0.5 mL sample!

13. Formic Acid Pentamer IsomerEnergy (cm -1 ) F5401167 F51811062 F5192200 3g32 3b0 MP2/6-31++G(d,p) A. K. Roy and A. J. Thakkar. Chem. Phys., 312, (2005), 119-126. Y. Z. and D. G. Truhlar. J. Phys. Chem. A. 109, (2005), 6624- 6627.  Notice the structures are all dominated by hydrogen bonding, B3LYP study  True for trimer, but there is theoretical evidence for pi-stacking interactions in tetramer

14. Sister Structures 0 cm -1 32 cm -1 3b 3g MP2/6-31++G(d,p) Issues with pulsed-jet: Large amounts of dimer and trimer Does pentamer reach a minimum?

15. Formic Acid Pentamer Parameters Isomer A (MHz) B (MHz) C (MHz) μA (D)μB (D)μC (D) F54016221101033.90.51.3 F51815332221571.70.030.3 F1926922852571.32.00.1 3g6383813301.00.61.2 EXP6423753183.0*X1.0*X2.0*X Relative Dipoles 3g MP2/6-31++G(d,p) Calculated 10+ structures with similar rotational constants

16. Formic Acid Pentamer A (MHz)642.23341(15) B (MHz)375.924663(68) C (MHz)318.329871(74) ΔJ (kHz)0.03918(15) ΔJK (kHz)0.31960(61) ΔK (kHz)0.0023(17) δJ (kHz)0.1317(16) δK (kHz)0.002571(72) 362 lines 8 kHz rms Experimental dipole green, Ab inito dipole blue

17. Formic Acid Trimer + Water 97 cm -1 ZPEC MP2/6-31++G(d,p) A. Allouche. J. Chem. Phys., 122, (2005), 234703

18. Formic Acid Trimer + Water A0 (MHz) A1 (MHz) 1326.61613(40) 1326.58144(40) B0 (MHz) B1 (MHz) 588.85651(18) 588.87434(19) C0 (MHz) C1 (MHz) 416.19911(21) 416.21038(21) ΔJ0 (kHz)0.16540(71) ΔJK0 (kHz)-0.4077(35) ΔK0 (kHz)1.6759(96) δJ0 (kHz)0.04621(27) δK0 (kHz)0.2165(54) E1 (MHz)178.8329(33) Fbc (MHz)0.4581(94) MP2/6-31++G(d,p) 315 lines 15 kHz rms

19. Conclusions  Kraitchman substitution structures for formic acid trimer, pentamer, and trimer+water  Shown that using the tools of microwave spectroscopy, and with isotopic information, for an assigned spectrum with an unknown carrier, the structure can be backed out  Even with isoptopic labeling, only the magnitudes are directly known, not the signs – leading to a daunting amount of potential structures

20. Acknowledgments Award Number CHE-0960074