Galen Sedo, Jamie L. Doran, Shenghai Wu, Kenneth R. Leopold Department of Chemistry, University of Minnesota A Microwave Determination of the Barrier to Internal Methyl Rotation in Acetic Acid Monohydrate

Small Cluster Formation and Microsolvation Types of molecular interactions and bond formation in small reactive molecular clusters – Molecular and electronic structure changes upon complexation between water and a strong acid

Small Cluster Formation and Microsolvation

H 2 O-HF Z. Kisiel, A. C. Legon, D. J. Millen, J. Mol. Struct. 1984, 112, 1-8. H 2 O-HCl & (H 2 O) 2 -HCl A. C. Legon, L. C. Willoughby, Chem. Phys. Lett. 1983, 95, Z. Kisiel et al., J. Phys. Chem. A 2000, 104, Z. Kisiel et al., J. Chem. Phys. 2000, 112, H 2 O-HBr & (H 2 O) 2 -HBr A. C. Legon, A. P. Suckley, Chem. Phys. Lett. 1998, 150, Z. Kisiel et al., J. Chem. Phys. 2003, 119, HCOOH-H 2 O, HCOOH-(H 2 O) 2 & (HCOOH) 2 -H 2 O D. Priem, T.-K. Ha, A. Bauder, J. Chem. Phys. 2000, 113, CF 3 COOH-H 2 O, CF 3 COOH-(H 2 O) 2 & CF 3 COOH-(H 2 O) 3 B. Ouyang, T. G. Starkey, B. J. Howard, J. Phys. Chem. A 2007, 111, Microwave Investigations of Acid Hydrates

W. J. Tabor, J. Chem. Phys. 1957, 27, CH 3 COOH – Three A and E state transitions, V 3 = 174 cm -1 Observed large differences between obs and rigid L. C. Krisher, E. Saegebarth, J. Chem. Phys. 1971, 54, CH 3 COOH – 39 A and 38 E state transitions, V 3 = cm -1 Determined the barrier height with the principal axis method (PAM) B. P. van Eijck et al., J. Mol. Spectrosc. 1981, 86, Determined the barrier height with the principal axis method (PAM), V 3 = cm -1 Determined the barrier height with the internal axis method (IAM), V 3 = cm -1 Acetic Acid Monomer: CH 3 COOH V. V. Ilyushin et al., J. Mol. Spectrosc. 2001, 205, V. V. Ilyushin et al., J. Mol. Spectrosc. 2003, 220, V. V. Ilyushin et al., J. Phys. Chem. Ref. Data 2008, 37, Global Fit of transitions in = 0, 1, and 2. Determined the barrier height with the rho axis method (RAM) V 3 = (17) cm -1 V 6 = (13) cm -1 V 9 = (70) cm -1

The Pulsed Nozzle FTMW Spectrometer MirrorMirror Antenna Argon bubbled through a sample of Glacial Acetic Acid: Backing Pressure 2.25 atm Microwave Electronics Computer Spectrum Fabry-Perot Cavity Diffusion Pump Pulsed Nozzle MirrorMirror

E StateA State 1 11 ←0 00 CH 3 COOH Frequency [MHz] 1,000 pulses 10,000 FID’s a V. V. Ilyushin et al., J. Mol. Spectrosc. 2001, 205, a

CH 3 COOH

CH 3 COOH XIAM a,b Fit Rotational Constants Centrifugal Distortion Constants a) H.Hartwig and H.Dreizler, Z. Naturforsch 51a, (1996). b) Available for download from the Programs for Rotational Spectroscopy website:

a-axis CH 3 COOH XIAM a,b Fit Internal Rotation Barrier Inverse of the Rotor Moment of Inertia: h 2 /8  2 I  Angle between the Internal Rotor Axis and the Principle a-axis a) H.Hartwig and H.Dreizler, Z. Naturforsch 51a, (1996). b) Available for download from the Programs for Rotational Spectroscopy website: Internal Rotation – Overall Distortion

CH 3 COOH

Møller-Plesset Second-Order Perturbation (MP2) Theory G(d,p) G(2df,2pd) 3.aug-cc-pVDZ 4.aug-cc-pVTZ Ab Initio Barrier Determination CH 3 COOH Monomer HO-C-C-H Dihedral Angle [º] HO-C-C-H Dihedral Angle Møller-Plesset Second-Order Perturbation (MP2) Theory G(d,p) G(2df,2pd) 3.aug-cc-pVDZ 4.aug-cc-pVTZ Experimental Monomer Barrier S. Bell, Spectrochimica Acta Part A 2005, 61,

HO-C-C-H Dihedral Angle [º]  E [cm -1 ] Ab Initio Barrier Determination CH 3 COOH Monomer Percent Difference ~10.5% Experimental Monomer Barrier MP2/ G(d,p) MP2/aug-cc-pVDZ

Ab Initio Barrier Determination CH 3 COOH-H 2 O P. R. Rablen, J. W. Lockman, W. L. Jorgensen, J. Phys. Chem. A 1998, Q. Gao, K. T. Leung, J. Chem. Phys. 2005, 123, † Counter-poise corrected geometries calculated with MP2/ G(2df,2pd) E bind = kcal/mol E bind = kcal/mol

Ab Initio Barrier Determination CH 3 COOH-H 2 O Conformer A HO-C-C-H Dihedral Angle [º]  E [cm -1 ] Experimental Monomer Barrier MP2/ G(d,p) MP2/aug-cc-pVDZ

The Pulsed Nozzle FTMW Spectrometer MirrorMirror Antenna Argon bubbled through a sample of Glacial Acetic Acid: Backing Pressure 2.25 atm Microwave Electronics Computer Spectrum Fabry-Perot Cavity Diffusion Pump Pulsed Nozzle MirrorMirror Series 9 Pulsed Solenoid Valve Needle Adaptor Stainless Steal Needle Dimensions Argon bubbled through H 2 O at a rate of 1 sccm.

The Pulsed Nozzle FTMW Spectrometer MirrorMirror Antenna Argon bubbled through a sample of 60% Acetic Acid Solution: Backing Pressure 2.25 atm Microwave Electronics Computer Spectrum Fabry-Perot Cavity Diffusion Pump Pulsed Nozzle MirrorMirror Intensities x 10

Frequency [MHz] E State A State 2 02 ←1 01 CH 3 COOH-H 2 O 2,000 pulses 20,000 FID’s

Evidence of Internal H 2 O Motion Frequency [MHz] 2,000 pulses 20,000 FID’s 3 12 ←2 11 CH 3 COOH-H 2 O A State 4 04 ←3 13 CH 3 COOH-H 2 O A State

CH 3 COOH-H 2 O

† Counter-poise corrected geometries calculated with MP2/ G(2df,2pd) a-axis  = 4.52º  = 74.84º

Frequency [MHz] E State A State 2 02 ← CH 3 COOH-H 2 O 500 pulses 5,000 FID’s

13 CH 3 COOH-H 2 O

Fit Parameters for the Two Isotopic Forms of Acetic Acid Monohydrate

V 3 eff = cm -1 F eff = 5.3 cm -1 V 3 eff = cm -1 → V 3 actual = 357 cm -1 G. T. Fraser, F. J. Lovas, R. D. Suenram, J. Mol. Spectrosc. 1994, 167, Noted the apparent reduction in barrier height in weakly bound methanol complexes. Attributed the reduction to the neglected OH large amplitude motion Free methanol V 3 = 373 cm -1 F = cm -1 R. L. Kuczkowski et al., J. Mol. Spectrosc. 1995, 171, Noted the apparent V 3 decreased more in Ar-CH 3 OH than in other more strongly bond methanol complexes Observed Barrier Decrease Upon Complexation F 0 = 5.263(1) cm -1 Monomer F 0 = 5.246(1) cm -1 Hydrate

14 A State and 13 E State transitions were observed for the CH 3 COOH-H 2 O and 13 CH 3 COOH-H 2 O complexes. The rotational constants, along with the value of the angle , strongly suggest a structure with the acidic proton of acetic acid forming a primary hydrogen bond with the water molecule. The three fold barrier, V 3, to internal methyl rotation was determined to decrease by 17.8 % upon formation of the monohydrate. Conclusions

Funding National Science Foundation (NSF) Petroleum Research Fund (PRF) Minnesota Supercomputing Institute (MSI) Dr. Kenneth Leopold Dr. Shenghai Wu Jamie L. Doran Acknowledgements

124.7º111.6º 126.3º111.1º Å1.360 Å Å Å Å1.344 Å Å Å

R. L. Kuczkowski et al., J. Mol. Spectrosc. 1995, 171, Noted the apparent V 3 decreased more in Ar-CH 3 OH than in other more strongly bond methanol complexes Scaling the barrier with the F value of free Methanol corrected for the neglected secondary motion. Observed Barrier Decrease Upon Complexation V 3 = 373 cm -1 (Free Methanol)

CH 3 COOH XIAM a-c Fit a) H.Hartwig and H.Dreizler, Z. Naturforsch 51a, (1996). b) Available for download from the Programs for Rotational Spectroscopy website: c) N.Hansen, H.Mader and T.Bruhn, Molec. Phys. 97, (1999). Internal Rotation – Overall Rotation Distrotion c