4/11/2023 0 Comments Ir spectra chartThe circle inside the ring represents that the electrons are delocalized which is illustrated in Figure 2.įigure 2: Top: The P orbitals on each of the six carbon atoms in benzene that contribute an electron to the ring. A carbon atom is located at each vertex of the hexagon and a hydrogen atom is attached to each carbon, although it is not written in. The drawing in Figure 1 is that of a six-membered ring or hexagon. The prototypical aromatic molecule is benzene, C 6H 6, whose structure is represented in Figure 1.įigure 1: The chemical structure of the benzene molecule, C 6H 6. It has been found that the bonding in aromatic molecules is unique. However, if you have ever smelled pyridine you know that not all aromatic molecules do smell nice. In today’s world many people interpreting spectra do not have this background, so I have and will continue to discuss the structure and nomenclature of functional groups whose spectra we will discuss.Īromatic molecules were originally named because some of them smell nice. To be able to interpret IR spectra, one must have a nodding familiarity with the nomenclature and structures of organic chemistry. The purpose of these columns is to give you the knowledge you need to interpret spectra and the problem spectra give you the opportunity to practice what you have learned. Similarly, you can’t just walk up to a spectrum and pull information out of it, you have to practice first. You can’t just walk up to a Steinway and play a Beethoven sonata, you have to practice first. One final thought: I compare interpreting IR spectra to playing a piano. Many of the upcoming columns will be devoted to the useful group wavenumbers of economically important functional groups. For example, a peak may be too weak to be seen reliably, may appear in a region where lots of other peaks appear, or move around a lot from molecule to molecule. To be clear, not all of the peaks from a given functional group will be useful group wavenumbers. Now, we have given these peak patterns a proper name. For example, the methyl and methylene C-H stretches that were introduced previously (1). We have talked about group wavenumbers and peak patterns in previous articles without realizing it. It will fall in a narrow wavenumber range regardless of what molecule the functional group appears in.It will appear in a unique wavenumber region where no other functional groups absorb.It will be intense so it is easy to see.A good group wavenumber peak has three useful properties: The x-axes of IR spectra are plotted in wavenumber, not frequency. Some people call these “group frequencies,” but this is a misnomer. The peaks that define a functional group are what I will call its group wavenumbers. My approach going forward will be to emphasize the pattern of peaks that defines the presence of a functional group in a sample rather than throwing hundreds of peak positions at you. Since humans are good pattern recognizers, I believe most people can learn to interpret IR spectra. A computer’s inability to interpret spectra is part of why there is a need for a column series like this one. Computers are good at many things except pattern recognition, hence the need to use our eyeballs and brains to interpret infrared (IR) spectra. The human brain has evolved to be great at pattern recognition. Thus, interpreting spectra is not about memorizing peak positions, but is instead an exercise in pattern recognition. It is this pattern of peaks that defines the presence of a specific functional group in a sample, not one specific peak. The IR spectrum of benzene is analyzed in detail as a prototype example of an aromatic hydrocarbon.Ī look at the spectrum of any pure molecule will disclose that many functional groups have multiple peaks. Continuing our theme of investigating the IR spectra of hydrocarbons, we look at the nature of aromatic bonding and why aromatic rings have unique structures, bonding, and IR spectra. The concept of group wavenumbers is defined, and the importance of recognizing patterns in infrared (IR) spectra is discussed.
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