Structure Study of Formic Acid Clusters By ChirpedPulse

Structure Study of Formic Acid Clusters By Chirped-Pulse FTMW Spectroscopy 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, Mc. Cormick Rd. , P. O. Box 400319, Charlottesville, VA 22904 KEVIN O. DOUGLASS National Institute of Standards and Technology, Optical Technology Division, Gaithersburg, MD 20899 The Ohio State 66 th International Symposium on Molecular Spectroscopy, June 23 rd, 2011.

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

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.

Experimental Reduced Bandwidth Higher Throughput: 24 Gs/s AWG x 3 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 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).

Noise Floor <200 n. V 1. 981 million averages, 40 psi, 50°C, neon carrier gas

Formic Acid Trimer B 3 LYP/6 -31++G(d, p) 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 (k. Hz) 0. 07676(24) ΔJK (k. Hz) -0. 28380(85) ΔK (k. Hz) 4. 560(34) δJ (k. Hz) 0. 2925(35) δK (k. Hz) 0. 016740(55) 136 lines 3 k. Hz rms Conformational Studies in Formic Acid Oligimers. Richard D. Suenram, Pam L. Crum, Kevin O. Douglass, and Brooks H. Pate. The Ohio State 59 th International Symposium on Molecular Spectroscopy.

Formic Acid Trimer Stark Experimental dipole green, Ab inito dipole blue Emilsson, T. , Gutowsky, H. S. , de Oliveira, G. , Dykstra, C. E. J. Chem. Phys. 112, 1287, (2000). μa μb θ† EXP 1. 18(6) 0. 995(12) 40. 1(15) B 3 LYP/6 -311++G(d, p) 1. 43 1. 13 38. 3 MP 2/6 -311++G(d, p) 1. 05 1. 36 52. 3 † angle between the dipole moment vector and the a principle axis

11 -10 AB quartet

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

2. 16 million averages, 13 C-enriched FA introduced 1: 4, ~0. 5 m. L sample!

4. 5 million averages, d-enriched FA introduced 1: 4, ~0. 5 m. L sample!

Formic Acid Pentamer Isomer Energy (cm-1) F 540 1167 F 5181 1062 F 5192 200 3 g 32 3 b 0 MP 2/6 -31++G(d, p) § Notice the structures are all dominated by hydrogen bonding, B 3 LYP study § True for trimer, but there is theoretical evidence for pi-stacking interactions in tetramer 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), 66246627.

Sister Structures 32 cm-1 3 b 3 g 0 cm-1 Issues with pulsed-jet: Large amounts of dimer and trimer Does pentamer reach a minimum? MP 2/6 -31++G(d, p)

Formic Acid Pentamer Parameters Isomer A (MHz) B (MHz) C (MHz) μA (D) μB (D) μC (D) F 540 1622 110 103 3. 9 0. 5 1. 3 F 5181 533 222 157 1. 7 0. 03 0. 3 F 192 692 285 257 1. 3 2. 0 0. 1 3 g 638 381 330 1. 0 0. 6 1. 2 EXP 642 375 318 3. 0*X 1. 0*X 2. 0*X Relative Dipoles Calculated 10+ structures with similar rotational constants 3 g MP 2/6 -31++G(d, p)

Formic Acid Pentamer Experimental dipole green, Ab inito dipole blue A (MHz) 642. 23341(15) B (MHz) 375. 924663(68) C (MHz) 318. 329871(74) ΔJ (k. Hz) 0. 03918(15) ΔJK (k. Hz) 0. 31960(61) ΔK (k. Hz) 0. 0023(17) δJ (k. Hz) 0. 1317(16) δK (k. Hz) 0. 002571(72) 362 lines 8 k. Hz rms

Formic Acid Trimer + Water 97 cm-1 ZPEC A. Allouche. J. Chem. Phys. , 122, (2005), 234703 MP 2/6 -31++G(d, p)

Formic Acid Trimer + Water MP 2/6 -31++G(d, p) A 0 (MHz) A 1 (MHz) 1326. 61613(40) 1326. 58144(40) B 0 (MHz) B 1 (MHz) 588. 85651(18) 588. 87434(19) C 0 (MHz) C 1 (MHz) 416. 19911(21) 416. 21038(21) ΔJ 0 (k. Hz) 0. 16540(71) ΔJK 0 (k. Hz) -0. 4077(35) ΔK 0 (k. Hz) 1. 6759(96) δJ 0 (k. Hz) 0. 04621(27) δK 0 (k. Hz) 0. 2165(54) E 1 (MHz) 178. 8329(33) Fbc (MHz) 0. 4581(94) 315 lines 15 k. Hz rms

Conclusions § Kraitchman substitution structures 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

Acknowledgments Award Number CHE-0960074