In addition to C-H stretching peaks, methyl groups exhibit what is called an umbrella mode C-H bend at 1375 ☑0 cm -1 (2,3). In this spectrum, the methyl group asymmetric and symmetric stretches fall at 29 cm -1, and the asymmetric and symmetric CH 2 stretching peaks are seen at 29 cm -1. As stated in a previous column (2), these are C-H stretching peaks, and the presence of four peaks in this region means there are both methyl (CH 3) and methylene (CH 2) groups present in the sample (2). The first features encountered in Figure 1 are a set of four peaks between 30 cm -1. I read spectra from left to right like reading a sentence in a book. The problem from the July 2015 installment (1) is seen in Figure 1. Be aware, however, that inclusion of a peak in this table does not necessarily mean that peak will be useful in solving the problem. Each problem will also include a table listing the peaks labeled for easier interpreting. The interpretation section concludes our discussion of the spectra of straight-chain alkanes by answering the following question: Can infrared spectra be used to determine alkane chain length? Solution to Last Column’s Problem Spectrum Going forward, each problem spectrum will have peaks labeled on the spectrum, a description of the state of the sample, and the sampling technique used because these things affect spectral appearance. In a process called identity testing, spectral comparisons are used to ensure product quality and safety, particularly in the pharmaceutical, dietary supplement, and cosmetic industries. In this fourth installment, the theory section discusses how to properly compare spectra to each other. Now that this column is fully up to speed, each installment will consist of the answer to the most recent problem spectrum, a furthering of our discussion on interpretation theory, introduction to the spectra of one or more functional groups, and the posing of the next spectral interpretation problem. In part two, we conclude our discussion of the spectra of straight-chain alkanes by answering the following question: Can infrared spectra be used to determine alkane chain length? When comparing spectra to ensure product quality and safety, one must determine if two spectra are the same.
0 Comments
Leave a Reply. |