Nuclear magnetic resonance spectroscopy (NMR)
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules. When placed in a magnetic field, NMR active nuclei (such as 1H or 13C) absorb electromagnetic radiation at a frequency characteristic of the isotope. The resonant frequency, energy of the absorption, and the intensity of the signal are proportional to the strength of the magnetic field. For example, in a 21 Tesla magnetic field, protons resonate at 900 MHz. It is common to refer to a 21 T magnet as a 900 MHz magnet, although different nuclei resonate at a different frequency at this field strength in proportion to their nuclear magnetic moments.
Solving H-NMR problems
Ok, it’s a puzzle. You have to keep few things in your head and solve the puzzle step by step.
a) How many different sets of signal appear in the spectrum? (This can be determined by the
number of integrated peaks.)
b) What is the relative ratio of all the different hydrogen peaks present? Do they add to the total
number of hydrogen atoms in the molecular formula?
c) The chemical shift provides information into the chemical environment of the different
hydrogen atoms in the molecule. As a rule of thumb, a chemical shift seen at ≤1.0 ppm and an
integration of 3.0 most possibly indicates an alkyl group whose terminal is a –CH3. An aldehyde
proton will show at ~9.0 ppm and a carboxylic acid ~11.0 ppm. Remember that electronegative
atoms tend to deshield hydrogen atoms moving their chemical shifts downfield by the inductive
effect.
d) The splitting patterns are very important because they can help us understand the hydrogen
atoms attached to adjacent carbon atoms.
1. Remember the n+1 rule for the splitting pattern. (n = number of neighboring hydrogen. Neighboring H are located on immediately next carbon atom)
click on the image to enlarge
2. Remember characteristic splitting patterns. An isolated ethyl group (-CH2CH3) will show a particular pattern. A triplet from the –CH3 (the neighbor is a -CH2-) and a quartet from the –CH2- An isolated isopropyl group (-CH(CH3)2) will show a doublet from the methyl groups
and a septet from the methyne (-CH-).
click on the image to enlarge
e). After assigning partial structures on the 1H-NMR spectrum try to place them together to form the compound. Next, try to generate a spectrum from the proposed structure and compare it to the spectrum provided.
Please assign structure for the compounds represented by the following NMR spectra.
CH3Br
Hint: singlet- indicate no neighboring H
click on the image to enlarge
C3H6O
Hint: indicate no neighboring H
C2H6O
Hint: indicate no neighboring H
C2H5Br
Hint: triplet and quartet together- indicate ethyl group
C3H6O2
Hint: indicate no neighboring H. Look at a chemical shifts table.
C3H7Cl
Hint: septet together with doublet-indicates presence of isopropyl group
C4H8O2
Hint: triplet together with quartet- indicate ethyl group, singlet indicate H without neighboring H. Look at chemical shifts.
C4H8O2
Hint: triplet together with quartet- indicate ethyl group, singlet indicate H without neighboring H. Look at chemical shifts.
Posts
