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College Park, Maryland      June 6 - 10 , 2004

M3-D4 (2:30 PM): Proton dynamics in short N-H---O hydrogen bonds

M. Hartl, L.L. Daemen (Los Alamos National Laboratory), J. Eckert (Los Alamos National Laboratory; Materials Research Laboratory, University of California, Santa Barbara), D. Hadzi, J. Stare (National Institute of Chemistry, Hajdrihova 19, SI-1000, Ljubljana, Slovenia)

The role of proton dynamics in determining the physical properties and chemical reactivity of systems with very short hydrogen bonds continues to attract a great deal of theoretical and experimental efforts. In principle, infrared (IR) spectroscopy is useful to extract information on proton dynamics (particularly the X-H stretch). In practice considerable discrepancies exist between observed frequencies and intensities and the results of ab initio calculations. Significant broadening in short hydrogen bonds further complicates the observation of other modes and the interpretation of the spectrum. The incoherent inelastic neutron scattering (IINS) vibrational spectrum is governed by entirely different (and less complex) neutron-nucleus interactions rather than photon-electron interactions. The IINS spectrum is easier to predict quantitatively from ab initio calculations and does not suffer from the problems mentioned above for IR spectroscopy. Despite these fundamental advantages, vibrational spectroscopy using inelastic neutron scattering remains underutilized in the study of hydrogen bonding.

Our study focused on acid-base complexes involving N-H---O hydrogen bonds. The weak Lewis bases pyridine and 3,5-dimethylpyridine (3,5-lutidine) were complexed with a number of weak carboxylic acids: benzoic acid, 2,4-dinitrobenzoic acid, and 3,5-dinitrobenzoic acid. By varying the substituents on the pyridine or benzoic ring, we were able to tune the strength of the hydrogen bonding interaction and alter the proton dynamics in the bond in a systematic fashion. For example, in the 3,5-dinitrobenzoic acid + 3,5-lutidine complex, the proton in this very strong hydrogen bond is fully disordered between O and N with two preferred H locations each with average occupancy of 0.5 [d(N-H1)=0.82 and d(N-H2)=1.65 , d(N-HO)=2.56 ]. On the other hand in 2,4-dinitrobenzoic acid + pyridine, the hydrogen bond has the same length as in the preceding complex, but the proton is at a single location near N [d(N-H)=1.05 , d(N-HO)=2.56 ]. We used fully deuterated reagents to synthesize the complexes, then selectively deuterated and protonated the hydrogen bond. The difference between the spectrum of the protonated and deuterated materials facilitates mode assignments. All our data were collected on the Filter Difference Spectrometer at Los Alamos National Laboratory. We will present a systematic comparison between IR and IINS data, as well as a discussion of ab initio modeling results.

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