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. DOUGLASSNational 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 66th International Symposium on Molecular Spectroscopy, June 23rd, 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

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

1.981 million averages, 40 psi, 50°C, neon carrier gas

Noise Floor <200 nV

Formic Acid Trimer

13C’s and 18O’s in natural abundanceaccompanying isotopic information:Ddouble and triple 13Cdouble and triple DD and 13C

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 lines3 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.

Formic Acid Trimer Stark

μa μb θ†

EXP 1.18(6) 0.995(12) 40.1(15)

B3LYP/6-311++G(d,p) 1.43 1.13 38.3

MP2/6-311++G(d,p) 1.05 1.36 52.3

† 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

11-10 AB quartet

Complex PES

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

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

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

Formic Acid PentamerIsomer Energy (cm-1)

F540 1167

F5181 1062

F5192 200

3g 32

3b 0

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

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 trimerDoes pentamer reach a minimum?

Formic Acid Pentamer Parameters

IsomerA

(MHz)B

(MHz)C

(MHz)μA (D) μB (D) μC (D)

F540 1622 110 103 3.9 0.5 1.3

F5181 533 222 157 1.7 0.03 0.3

F192 692 285 257 1.3 2.0 0.1

3g 638 381 330 1.0 0.6 1.2

EXP 642 375 318 3.0*X 1.0*X 2.0*X

Relative Dipoles

3g MP2/6-31++G(d,p)

Calculated 10+ structures with similar rotational constants

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 lines8 kHz rms

Experimental dipole green, Ab inito dipole blue

Formic Acid Trimer + Water97 cm-1

ZPEC

MP2/6-31++G(d,p)

A. Allouche. J. Chem. Phys., 122, (2005), 234703

Formic Acid Trimer + WaterA0 (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

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

Acknowledgments

Award Number CHE-0960074