I expected to see a question like the following example on the exam:
A 1H NMR signal is observed at 157.0-Hz (downfeild of TMS) on a 135-MHz instrument.
a.) What is the observed chemical shift in ppm? 1.16 ppm
The chemical shift is found by dividing the downfield signal (Hz) by the spectrometer frequency (MHz). (157.0-Hz/135-MHz=1.16 ppm)
b.)What is the chemical shift in ppm, if the sample is analyzed with a 300.0 MHz instrument? 1.16 ppm
Chemical shift is a ratio, the value in hertz increases proportionally to the value in megahertz. The shift observed at 135-MHz will be 1.16 ppm at any other operating frequency.
c.)Where will the proton signal appear, in hertz, if the sample is analyzed with a 300.0-MHz instrument? 349 Hz
The downfield signal (Hz) is the chemical shift (ppm) multiplied by the spectrometer frequency (MHz). (1.16 ppm*300-MHz=300-Hz)
This type of problem was assigned for the Sapling Homework, and we discussed this topic in class. Dr. Mullins even spent time with an example, although there was no question even similar to this on the exam.
I felt mislead after taking the exam. In the Monday-Wednesday lab section, we did an example where we had to draw an IR spectrum for a compound provided by Dr. Mullins. I specifically remember asking Dr. Mullins aloud, if we were going to be required to draw an IR or MS spectrum on the test and his answer to my question was NO. There were multiple problems on the exam where we had to draw the spectrum for a compound. I am not implying whatsoever that we should know specifics about our exam prior to the test date. I am however saying that when ANY teacher says that something will not be on the exam, they should hold to their word. Because I know I am not alone when I say, I don't even bother to study material that is not going to be on the exam.
I am sorry you feel this way Laura, but I never meant to mislead you. We should talk about your frustration sometime...outside of class.
ReplyDeleteI agree that this would have been a good question to have had on the exam, it wouldn't have requiered as much time and would still show that we understood the material.
ReplyDeleteThis question could have related to a variety of subjects. It would have required for us to know further knowledge about the 1H NMR spectrum.
For example protons in different environments give different NMR signals, likewise equivalent protons give the same NMR signals. Also looking into shielding shifts and how protons near electronegative atoms are deshielded, so they absorb downfield.
The book goes into further details about this subject on pages 499 to 502.
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