Miscellaneous Multivariate Raman Spectroscopy Projects Including Hydrated Proton Characterization

Louis M Streacker, Purdue University

Abstract

A homemade micro-Raman spectrometer is applied to answer questions relating small molecule hydration and to report standardized water spectra. Multivariate curve resolution is applied to generate solute-correlated Raman spectra for in-depth high-resolution hydration-shell spectra. The following is a summary of miscellaneous projects done using Raman spectroscopy as the primary experimental tool. Since Freiherr Christian J. Theodor von Grotthuss's published his mechanism for protons diffusing through water in 1806, the structure and behavior of H+ in water, the hydrated proton remains an active topic of research among physicists and chemists, both theoreticians and experimentalists alike in the present day. Despite the extensive infrared spectroscopy and non-linear spectroscopy methods which were done to elucidate its structure, there does not exist any published Raman spectra of the hydrated proton alone in solution. Here, multivariate curve resolution Raman spectroscopy, via self-modeling curve resolution, is applied to elucidate the spectrum of a hydrated proton in water for the first time. The experimental results of Raman-MCR shown here, in conjunction with complimentary infrared absorption multivariate curve resolution spectra are used to substantiate the claims and structures produced by quantum calculations and vibrational spectroscopic calculations onto ab-initio molecular dynamics simulation results. In conclusion, the proton tends to adopt a two water molecule hydration-shell with various hydrogen bond lengths and asymmetries. This interpretation of structure is misaligned with the oversimplified motif of hydration-shell structure of three water molecules surrounding the proton which is often taught to students in general chemistry courses, but it is aligned with current research on proton symmetries. Future work on the topic would be to make temperature dependent studies and consider the effects of the hydrated deuteron, D+, for isotopic effects. Currently theoreticians are refining the calculation of the Raman spectra of H2O and D2O from molecular simulations to both accurate and feasible. However, the experimental standards which theoreticians need to compare their results theoretical results are lacking. Other standard spectra exist in the literature, but they often have a narrow, reported frequency or temperature range. Hence, a Raman spectrum of liquid water throughout the whole frequency and temperature range should be reported both graphically and in tabular form. Here, such Raman spectra are reported. Details in data standardization, how the data reported here can be applied to other Raman systems when taking certain factors into account, are also reported. Finally, several projects have been tried, but yielded no meaningful results or were simply abandoned. These projects are outlined at the end of the thesis.

Degree

M.S.

Advisors

Ben-Amotz, Purdue University.

Subject Area

Physical chemistry

COinS