The vibrational spectrum of a molecule is considered to be a unique physical property and is characteristic of the molecule. As such, the infrared spectrum can be used as a fingerprint for identification by the comparison of the spectrum from an “unknown” with previously recorded reference spectra. This is the basis of computer-based spectral searching. In the absence of a suitable reference database, it is possible to effect a basic interpretation of the spectrum from first principles, leading to characterization, and possibly even identification of an unknown sample. This first principles approach is based on the fact that structural features of the molecule, whether they are the backbone of the molecule or the functional groups attached to the molecule, produce characteristic and reproducible absorptions in the spectrum. This information can indicate whether there is backbone to the structure and, if so, whether the backbone consists of linear or branched chains. Next it is possible to determine if there is unsaturation and/or aromatic rings in the structure. Finally, it is possible to deduce whether specific functional groups are present. If detected, one is also able to determine local orientation of the group and its local environment and/or location in the structure. The origins of the sample, its prehistory, and the manner in which the sample is handled all have impact on the final result. Basic rules of interpretation exist and, if followed, a simple, first-pass interpretation leading to material characterization is possible. This article addresses these issues in a simple, logical fashion. Practical examples are included to help guide the reader through the basic concepts of infrared spectral interpretation.

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Interpretation of Infrared Spectra, A Practical Approach

Synth. React. Inorg. Met-Org. Chem

We report data from infrared absorption (FTIR) and X-ray photoelectron spectroscopies that correlate the molecular conformation of oligo(ethylene glycol) (OEG)-terminated self-assembled alkanethiolate monolayers (SAMs) with the ability of these films to resist protein adsorption. We studied three different SAMs of alkanethiolates on both evaporated Au and Ag surfaces. The SAMs were formed from substituted 1-undecanethiols with either a hydroxyl-terminated hexa(ethylene glycol) (EG6-OH) or a methoxy-terminated tri(ethylene glycol) (EG3-OMe) end group, or a substituted 1-tridecanethiol chain with a methoxy-terminated tri(ethylene glycol) end group and a −CH2OCH3 side chain at the C-12 atom (EG[3,1]-OMe). The infrared data of EG6-OH-terminated SAMs on both Au and Ag surfaces reveal the presence of a crystalline helical OEG phase, coexisting with amorphous OEG moieties; the EG[3,1]-OMe-terminated alkanethiolates on Au and Ag show a lower absolute coverage and greater disorder than the two other compounds. The...

Molecular Conformation in Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers on Gold and Silver Surfaces Determines Their Ability To Resist Protein Adsorption

Salts and cocrystals are multicomponent crystals that can be distinguished by the location of the proton between an acid and a base. At the salt end of the spectrum proton transfer is complete, and on the opposite end proton transfer is absent in cocrystals. However, for acid−base complexes with similar pKa values, the extent of proton transfer in the solid state is not predictable and a continuum exists between the two extremes. For these systems, both the ΔpKa value (pKa of base − pKa of acid) and the crystalline environment determine the extent of proton transfer. A total of 20 complexes containing theophylline and guest molecules with ΔpKa values less than 3 have been prepared, resulting in 13 cocrystals, five salts, and two complexes with mixed ionization states based on IR spectroscopy and single-crystal diffraction data. We propose modifications to the ΔpKa rule for selecting salt screen counterions that focus on the discovery of solid forms with useful physical properties rather than an arbitrary ...

http://pubs.acs.org/doi/pdf/10.1021/mp0601345

The Salt−Cocrystal Continuum: The Influence of Crystal Structure on Ionization State

"Infrared and Raman Spectroscopy: Principles and Spectral Interpretation" explains the background, core principles and tests the readers understanding of the important techniques of Infrared and Raman Spectroscopy. These techniques are used by chemists, environmental scientists, forensic scientists etc to identify unknown chemicals. In the case of an organic chemist these tools are part of an armory of techniques that enable them to conclusively prove what compound they have made, which is essential for those being used in medical applications. The book reviews basic principles, instrumentation, sampling methods, quantitative analysis, origin of group frequencies and qualitative interpretation using generalized Infrared (IR) and Raman spectra. An extensive use of graphics is used to describe the basic principles of vibrational spectroscopy and the origins of group frequencies, with over 100 fully interpreted FT-IR and FT-Raman spectra included and indexed to the relevant qualitative interpretation chapter. A final chapter with forty four unknown spectra and with a corresponding answer key is included to test the readers understanding. Tables of frequencies (peaks) for both infrared and Raman spectra are provided at key points in the book and will act as a useful reference resource for those involve interpreting spectra. This book provides a solid introduction to vibrational spectroscopy with an emphasis placed upon developing critical interpretation skills. It is ideal for those using and analyzing IR and Raman spectra in their laboratories as well as those using the techniques in the field. It uniquely integrates discussion of IR and Raman spectra. Theory illustrated and explained with over 100 fully interpreted high quality FT-IR and FT-Raman spectra (4 cm-1 resolution). There are selected problems at the end of chapters and 44 unknown IR and Raman spectra to test readers understanding (with a corresponding answer key).

L. J. Bellamy

Papers

The Infrared Spectra of Complex Molecules