Relationship between Infrared Peak Maximum Position and Molecular Interactions

Abstract

We explored the interpretation of the well-accepted correlation between the apparent peak maximum position shift and extent of molecular interactions, like hydrogen bonding and dipole-dipole interactions, based on the overlapped multiple band model. The simulation of two overlapped Lorentzian bands was carried out to interpret how the maximum position of a composite peak relates to the relative contributions of two species representing the different levels of molecular interactions, i.e., free (or very weekly bound) vs. strongly bound. To demonstrate the validity of our interpretation of the origin of the peak position shift, the temperaturedependent IR spectra of ethylene glycol were also analyzed. It was found through the analysis of simulated and experimental spectra that the apparent peak shift in certain case can be safely interpreted as the measure of the strength of hydrogen bonding. The result of this study gives a new insight to interpret molecular interactions probed by vibrational spectroscopy.Key Words : Frequency shifts, Overlapped bands, Self-modeling curve resolution (SMCR), Hydrogen bonding, Molecular interaction IntroductionThe extent of bathochromic shift of certain IR peaks under the influence of temperature or solution composition has been historically used as a convenient metric to characterize the degree or strength of specific molecular interactions, such as hydrogen bonding and dipole-dipole interactions. Pimentel and Sederholm published a classical paper on the "empirical correlation" between shift of stretching frequencies and hydrogen bond distances in solid crystals. 1Similarly, the energy of hydrogen bonds has been estimated with the equation developed by Struszczyk based on the extent of peak position shift.2 The success of such empirical correlation provided the support for the so-called single band position shift model, where the position of the vibrational frequency of a single absorption band associated with a specific species under the influence of molecular interaction gradually changes as temperature or concentration of a mixture is changed.Recently, we have reported that many, if not most, of the bathochromic shift of IR peaks of liquid samples may be explained by the relative intensity changes of closely overlapped multiple bands, representing the relative population of discrete species in different state of interactions, which contribute to the absorption at fixed vibrational frequencies. [3][4][5][6] In this model, the individual vibrational frequency does not gradually shift with molecular environment, but their relative contribution changes as environmental factors, like temperature or concentration of a mixture is varied.In this study, we explore the interpretation of the wellaccepted correlation between the apparent peak maximum position shift and the extent of molecular interactions, like hydrogen bonding and dipole-dipole interactions, based on the overlapped multiple band model. To see how the maximum position of a composite peak relates to the relative contributio...

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