Galen Sedo, Jamie Doran, Jane Curtis, Kenneth R. Leopold Department of Chemistry, University of Minnesota A Microwave Study of the HNO 3 -(H 2 O) 3 Tetramer

Fundamentally… We know that for a strong acid, like HNO 3, the following is true. The question becomes: How much water does it take to ionize one HNO 3 molecule? HNO 3 (g) + H 2 O( )H 3 O + (aq) + NO 3  (aq) H 2 O( ) Atmosphere… Troposphere -- acid rain, and ammonium nitrate aerosol -- reservoir for NO x and HO x species Stratosphere -- Polar Stratospheric Clouds (PSC’s) -- polar ozone depletion Nitric Acid Hydrates

(1) HNO 3 -H 2 O (2) HNO 3 -(H 2 O) 2 Nitric Acid Hydrates 1.Canagaratna, M.; Ott, M.E.; Leopold, K.R. "The Nitric Acid - Water Complex: Microwave Spectrum, Structure, and Tunneling.“ J. Phys. Chem. A 1998, 102, Craddock, M. B.; Brauer, C. S.; Leopold, K. R. “Microwave Spectrum, Structure, and Internal Dynamics of the Nitric Acid Dihydrate Complex” manuscript in preparation. Complete experimental gas-phase structure Experimental 14 N Quadrapole Coupling Constants Insight into the internal dynamics of the water sub-units

Nitric Acid Tri-hydrate, HNO 3 – (H 2 O) 3 1.Taesler, I.; Delaplane, R. G.; Olovsson, I. Acta Cryst. 1975, B31, Refined the crystal structure of Luzzati et al. (1953). 2.Ritzhaupt, G.; Devlin, J. P. J. Phys. Chem. 1991, 95, 90. FTIR investigations of thin crystalline films, HNO 3 -(H 2 O) n n = McCurdy, P. R.; Hess, W. P.; Xantheas, S. S. J. Phys.Chem. A 2002, 106, MP2/aug-cc-pVDZ geometry optimization of the 10-Member Ring confirmation Fourier transform infrared (FTIR) spectra for smaller nitric acid complexes 4.Escribano, R.; Couceiro, M.; Gomez, P. C.; Carrasco, E. Moreno, M. A.; Herrero, V. J. J. Phys. Chem. A 2003, 107, B3LYP/aug-cc-pVTZ geometry optimizations of both the 10- and 8-Member Ring confirmations Reflection-absorption infrared (RAIR) spectra 5.Scott, J. R.; Wright, J. B. J. Phys. Chem. A 2004, 108, MP2 and B3LYP geometry optimizations using the G(2d,p) basis for both the 10- and 8-Member Ring confirmations

10-Member Ring: Top View 10-Member Ring: Side View 8-Member Ring: Top View 8-Member Ring: Side View E binding [kcal/mol] Method/Basis E binding [kcal/mol] -22.7MP2/ G(2df,2pd) *this work* -22.4(+0.3) -31.6(+0.4)MP2/ G(2d,p) Scott et al B3LYP/ G(2d,p) Scott et al (+0.1) -20.5B3LYP/aug-cc-pVTZ Escribano et al (+0.8) Theoretical Structures of the HNO 3 -(H 2 O) 3 Tetramer

MirrorMirror Antenna Argon bubbled through a sample of 90% HNO 3 Backing Pressure 2–3 atm Microwave Electronics Computer Spectrum Fabry-Perot Cavity Diffusion Pump Pulsed Nozzle MirrorMirror The Pulsed Nozzle FTMW Spectrometer

MirrorMirror Antenna Argon bubbled through a sample of 90% HNO 3 Backing Pressure 2–3 atm Microwave Electronics Computer Spectrum Fabry-Perot Cavity Diffusion Pump Pulsed Nozzle MirrorMirror The Pulsed Nozzle FTMW Spectrometer Series 9 Pulsed Solenoid Valve Needle Adaptor Stainless Steal Needle Dimensions ID = 0.016"Length = 0.205" Argon bubbled through H 2 O at a rate of 1 sccm.

2,000 gas-pulses / 20,000 FID’s Intensity = ,000 gas-pulses / 6,000 FID’s Intensity = 0.06 HNO 3 -(H 2 O) 3 Spectra HNO 3 Intensity ≈ 18,000 HNO 3 –H 2 O Intensity ≈ 500 HNO 3 –(H 2 O) 2 Intensity ≈ 5.0 Cavity Frequency

HNO 3 -(H 2 O) 3 Spectra 2,000 gas-pulses / 20,000 FID’s Intensity = ,000 gas-pulses / 6,000 FID’s Intensity = 0.12 Cavity Frequency

Nitric Acid Tri-hydrate Molecular Constants H 14 NO 3 -(H 2 O) 3 : 74 Assigned Transitions, K -1 = 0 – 4 H 15 NO 3 -(H 2 O) 3 : 18 Assigned Transitions, K -1 = 0 – 2 DNO 3 -(H 2 O) 3 : 18 Assigned Transitions, K -1 = 0 – 2

Comparison of the Theoretical and Experimental Results 10-Member Ring: Top View 10-Member Ring: Side View 8-Member Ring: Top View 8-Member Ring: Side View

Comparison of the Theoretical and Experimental Results 14 N → 15 N Isotope Shifts 10-Member Ring: Top View8-Member Ring: Top View

Comparison of the Theoretical and Experimental Results DNO 3 Isotope Shifts 10-Member Ring: Top View8-Member Ring: Top View

14 N Quadrupole Coupling Constants Nitrate Ion eQq = MHz 1 1.Adachi, A.; Kiyoyama, H.; Nakahara, M.; Masuda, Y.; Yamatera, H.; Shimizu, A.; Taniguchi, Y. J. Chem. Phys. 1989, 90, 392.

14 N Quadrupole Coupling Constants Nitric Acid Hydrates eQq Nitrate Ion ↔  cc c a b HNO 3 -H 2 O a b c HNO 3 -(H 2 O) 2 a b c HNO 3 -(H 2 O) 3  aa +  bb +  cc = 0  cc = ־ ½[  aa + (  bb -  cc )]

HNO 3 -H 2 O HNO 3 -(H 2 O) 2 HNO 3 -(H 2 O) 3 Proton Transfer in Nitric Acid Systems

 r 1 (OH) –  r 2 (H···O) The parameter rho (  ) has been devised c to quantify proton transfer in hydrogen bonded systems.  r 1 (OH) = Stretch in O-H covalent bond relative to covalent bond in free HNO 3 monomer.  r 2 (H···O) = Stretch in hydrogen bond relative to O–H bond distance in hydronium ion (H 3 O + ).  > 0 indicates proton transfer.  = 0 indicates equal sharing of proton.  < 0 indicates neutral pair. Proton Transfer in HNO 3 Complexes c Kurnig, I. J.and Scheiner, S. Int. J. Quantum Chem., QBS 1987, 14, 47.

HNO 3 -H 2 O HNO 3 -(H 2 O) 2 HNO 3 -(H 2 O) 3 Proton Transfer in Nitric Acid Systems

(1) HNO 3 -H 2 O (2) HNO 3 -(H 2 O) 2 (3) HNO 3 -(H 2 O) Proton Transfer in Nitric Acid Systems

(4) HNO 3 -(H 2 O) 3 (5) HNO 3 -N(CH 3 ) 3 (1) HNO 3 -H 2 O (3) HNO 3 -(H 2 O) 2 (2) HNO 3 -NH Proton Transfer in Nitric Acid Systems

Conclusions 1.Spectra for three isotopes were observed Rotational Constants and the isotopic shifts indicate the spectra are those of the 10-Member Ring confirmation 2.The Potential Energy Surface of the HNO 3 -(H 2 O) 2 system has been performed using MP2/ G(2pd,2df) The Global Minimum was found to be a 10-Member Ring An 8-Member Ring local minimum was also calculated 3.The degree of proton transfer was assessed using the experimental 14 N Quadrupole coupling and the theoretical structure. Both methods suggest an increase in proton transfer when compared with the mono- and di-hydrate.

Funding National Science Foundation (NSF) Petroleum Research Fund (PRF) Minnesota Supercomputing Institute (MSI) Dr. Kenneth Leopold Acknowledgements Dr. Matthew Craddock Dr. Carolyn Brauer Jamie Doran Jane Curtis