Topic 20: Nuclear Magnetic Resonance

20.1Why is the NMR technique given this particular name?
 
20.2Roughly what percent of 1H nuclei (we will just call them “protons”, since that‘s what they are) are excited at equilibrium in a 9.4-tesla (400 MHz) magnet at room temperature? (based on ?E = hv, n1/n0 = e–?E/kT, and v = 400 x 106 sec–1)? How much different is this for an 800 MHz?
 
20.3How many Hz is 1 ppm on a 60 MHz NMR spectrometer? 400 MHz? What are the implications of this difference? (That is, why would we pay $2,000,000 for an 800-MHz spectrometer, when we could get five 400 MHz instrument for that price?)
 
20.4What are three common isotopes that are observable using NMR spectroscopy?
 
20.5What does “FT” in “FT-NMR” mean? Why was it a huge improvement in technology? (See Nobel Prize in Chemistry 1991 - Richard Ernst.)
 
20.6What is needed to establish a “frequency lock” on an NMR spectrometer, and why is that important to do?
 

Topic 21: Chemical Shift and Nuclear Magnetic Environments

21.1What does it mean for a proton in a compound to be deshielded? (What shields protons from the magnetic field they are placed into in an NMR spectrometer?)
 
21.2How does NMR shielding relate to acidity, roughly?
 
21.3What is chemical shift in NMR spectroscopy?
 
21.4If a signal is “shifted downfield”, is it absorbing at higher or lower energy? Higher or lower frequency? Does it appear at higher or lower chemical shift? Is this to the left or to the right on an NMR spectrum?
 
21.5Why do the terms upfield and downfield make no sense at all, given your author‘s description of an NMR spectrometer?
 
21.6What direction is in the direction of higher energy in just about all spectra from any instrument -- left to right, or right to left?
 
21.7What is the name and structure of the compound that used to be added to NMR samples as a chemical shift reference, arbitrarily given the value of ? 0.0 ppm? (It is not necessary to add this compound anymore, because modern FT NMR spectrometers like ours are extraordinarily stable - their accuracy is on the order of 400,000,000.0 +/- 0.1 Hz.)
 
21.8If a signal appears at 2.1 ppm on a 60 MHz NMR spectrometer, where will it appear on a 400 MHz spectrometer? How is this useful?
 
21.9What is the approximate chemical shift of each of the following protons, in bold?



 
21.10What other factors besides electron density play roles in understanding the origins of chemical shift?
 
21.11What role does symmetry play in relation to chemical shift and the number of signals in an NMR spectrum?
 
21.12How can you tell if two protons are in different magnetic environments, such that they will appear at two different places in an NMR spectrum?
 
18 questions
364 questions for Organic I

364 Questions for Organic I

Dearest Student. This web page is a set of questions I call Detailed Intended Learning Outcomes For Organic Chemistry (Part I). Some time ago I asked myself, "Bob, what do you think are the key questions that you hope students will be able to answer after taking Chemistry 247 at St. Olaf College?" So here you have them. Some are very simple; some require more thought. All are good questions to be thinking about as you read, discuss, and master the practical aspects of organic chemistry. They are discussion starters and review tips. There are no answers here (though there are some clues). The questions are here to help you focus on what's important (to me, at least!) and organize your learning process. The questions are grouped into topics. Bite-size chunks. Take them a few at a time. Don't feast on them all at one sitting!

(preliminary version 2021.01.15 feeback: Bob Hanson)