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The Blue Shift Of Unactivated Alkyl CH Stretching Modes And Intermolecular Interactions

Posted on:2008-03-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z XuFull Text:PDF
GTID:1101360215477413Subject:Chemical Biology
Abstract/Summary:PDF Full Text Request
The C-H…Y weak hydrogen bonds has recently been a matter of keen interest for its possible contribution to the stability of supermolecules and biological macromolecules. As the classical method for studying hydrogen bonding in the condensed phase, the frequency shifts of the C-H stretching modes in vibrational spectroscopy were considered as the most important experimental evidence for C-H…Y weak hydrogen bonds. However, the C-H frequentcy shifts do not necessarily only related to direct C-H…Y contacts.By a combination of infrared and Raman spectroscopy, two-dimentional correlation analysis technique and quantum chemical calculations, the relationships between the frequency shift of C-H stretching modes of inactiveatd alkyl groups and intermolecular interactions were systematically studied in this work. By comparison of the spectral evolution of eight model compounds diluted by inert, proton donating and proton accepting solvents, respectively, the effect of hydrogen bonds formed on proton accepting groups (-N-, -O-, -S-; S=O, C-O, C=N; N-C=O) to the frequency shift of the adjacent inactivated alkyl vC-H were discussed. Additionally, those effects of ionic bonds and ionic hydrogen bonds were also studied.It was found that the hydrogen bonds formed on the proton accepting groups could cause the adjacent inactivated alkyl vC-H blue shift. The scale of vC-H blue shift correlated with the conection modes of C-H groups with proton accepting groups, which was related to electron density mobility in the molecules, and the basidity of the proton accepting groups.Two-dimensional (2D) correlation spectroscopy was used to study the composition-dependent spectral variations of the CH-stretching bands of N,N-dimethylformamide(DMF)-water mixtures with xDMF ranging from 0.98 to 0.60. By a detailed correlation analysis of the spectral changes of the CH- and OH-stretching bands, it is found that the intensities of the CH and OH bands change in different ways when water content is increased. It is also found that two different regions of water content can be distinguished, in which the intensity changes have different signatures. A tentative explanation for how these phenomena might be related to structural changes in the mixture is proposed. The structural change of DMF induced by water hydrogen bond on the carbonyl group is supposed to be the possible origin of methyl C-H blue shift instead of the direct C-H—O interactions before hydrophobic hydration taking place. This result was helpful for further investigations of hydrophobic hydration of biomolecule by vibrational spectroscopy such as IR and Raman.
Keywords/Search Tags:infrared spectroscopy, Raman spectroscopy, blue shifting hydrogen bond, quantum chemical calculation, two-dimensional spectroscopy
PDF Full Text Request
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