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?
 

Topic 22: 1H-NMR: Integration

22.1What is integration in the context of NMR spectroscopy (and in mathematics), and why is it valuable?
 
22.2What do we mean when we say that the numbers given for integration are relative numbers?
 
22.3How are integration values in 1H NMR useful?
 

Topic 23: 1H-NMR: Splitting

23.1How does proton coupling lead to observable splitting of NMR signals?
 
23.2Where does the rule that a set of n equivalent protons splits the signal of a nearby nonequivalent proton into n + 1 peaks arise from? What if the n protons are not equivalent?
 
23.3Can you fix this statement: When two sets of adjacent protons are different from each other (n protons on one adjacent carbon and m protons on the other), the number of peaks in an NMR signal equals (n + 1)(m + 1) by changing just one word?
 
23.4How many lines are there in an HMR signal when a proton is split with a coupling constant of 8 Hz by one proton a coupling of 4 Hz by two others? [Answer: 5 lines. But why? That‘s not (1+1)(2+1).] Can you show that their relative areas within the split signal will be in the ratios 1:2:2:2:1?
 
23.5Why do the two methylene (CH2) group hydrogens of phenylsuccinic acid (shown below) split each other and also have different coupling constants with the adjacent methine (CH) hydrogen?


 
23.6 What is special about OH protons in 1H NMR spectra (three things)?
 
23.7What is the simple NMR experiment that can be done see whether an NMR signal is due to an OH or a CH?
 

Topic 24: 13C NMR

24.1In what four ways is 13C NMR significantly different from 1H NMR?
 
24.2Why is 13C-13C splitting not observed? [Hint: What is the natural abundance of carbon-13?]
 
24.3Can 1H-13C splitting be observed on a standard 13C NMR spectrum?
 
24.4Can 1H-13C splitting be observed on a standard 1H NMR spectrum? (Look carefully at an NMR spectrum you may have taken. What would you be looking for?)
 
24.5How could you distinguish between a carboxylic acid and an aldehyde using 13C NMR? Using 1H NMR?
 
24.6Why don‘t we integrate 13C NMR spectra? [Hint: Find out what relaxation time is in NMR spectroscopy.]
 
24.713C NMR spectra often have three tall lines, all equal height, at ? 77 ppm. What are they from? Why three equal-height lines?
 

Topic 25: Infrared Spectroscopy

25.1Why are IR absorptions generally quite broad?
 
25.2What sort of IR absorptions are extraordinarily broad? Why?
 
25.3What is wavenumber, and what are its units as used in IR spectroscopy?
 
25.4Does increasing wavenumber correspond to increasing or decreasing energy of absorption?
 
25.5What is the overall range in wavenumbers of a typical IR spectrum?
 
25.6What is the range in wavenumbers of the functional group region of an IR spectrum? Why is it called that?
 
25.7What is the range in wavenumbers of the fingerprint region of an IR spectrum? Why is it called that? Why do we generally not look for functional groups there?
 
25.8Why do C=C bonds vibrate at higher energy than C-C bonds?
 
25.9Why do C-H bonds vibrate at FAR higher energy than C-C bonds?
 
25.10What is reduced mass, and how does that play into the difference in vibrational energies of C-H and C-C bonds?
 
25.11How can we tell from an IR spectrum that a compound is probably an alcohol? a ketone? an aldehyde? a carboxylic acid? an ester? a primary amine? a secondary amine? an alkyne? a terminal alkyne? an aromatic comound?
 
25.12How can we use IR to see if our product in lab that we have recrystallized from water is dry and so also ready for us to take its melting point?
 

Topic 26: Mass Spectrometry - Theory

26.1What is the principle behind mass spectrometry?
 
26.2What is a radical in the context of chemistry?
 
26.3What are molecular fragments in the context of mass spectrometry, and why do we see them?
 
26.4What usually is the charge of the observed species in mass spectrometry? How are these charges produced?
 
26.5What does probability have to do with mass spectrometry?
 

Topic 27: Mass Spectrometry - Practice

27.1What is a molecular ion?
 
27.2Where do we find the molecular ion on a mass spec?
 
27.3What is the base peak? Use the language of probability.
 
27.4Why do we always either use the mass of a fragment itself or measure back from the molecular ion when considering fragmentation (never ever measuring between two fragments)?
 
27.5Why are the fragments of a compound with the formula CxHyOz generally odd mass, such as 79, 87, or 111, not even?
 
27.6How can we tell using mass spec if our compound has a chlorine in it?
 
27.7How can we tell using mass spec if our compound has a bromine in it?
 
27.8What happens quite generally with alcohols that causes trouble for us in analyzing mass spectra?
 
27.9What are three ways that mass spectrometry can help us identify a compound if we already have NMR and IR spectra?
 
27.10What is high resolution mass spectrometry? Why is this sooooo useful in organic synthesis?
 
62 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)