Chapter 29: Chromotography and Spectroscopy

Question 1

how does NMR spectroscopy work?

Answer 1

Nuclear Magnetic Resonance (NMR) spectroscopy uses a combination of strong magnetic field and radio frequency radiation. The nuclei of some atoms absorb and release the radiation repeatedly in a process called nuclear magnetic resonance

Question 2

what is used as the standard for chemical shift values?

Answer 2

Tetramethylsilane (TMS)

(CH3)4Si

Question 3

what effects something’s chemical shift?

Answer 3

chemical shift depends on the chemical environment of atoms and is greatly influenced by the presence of nearby electronegative atoms or pi-bonds

Question 4

why do we use deuterated solvents to dissolve substances for NM?

Answer 4

Deuterium is the 2H isotope of hydrogen, and therefore D2O (heavy water) doesn’t produce either H+ or C-13 peaks

Question 5

what does the analysis of a C-13 NMR give us?

Answer 5

number of carbon environments (number of peaks)

types of carbon environments present (chemical shift)

Question 6

equivalent carbon atoms

Answer 6

have the same chemical environment and therefore the same chemical shift

Question 7

non-equivalent carbon atoms

Answer 7

have different environments and therefore different chemical shifts

Question 8

carbon atoms that are positioned symmetrically within a molecule:

Answer 8

• are equivalent and have the same chemical environment

- have the same chemical shift and contribute to the same peak

Question 9

what does analysis of a proton NMR spectrum give us?

Answer 9

• numer of proton environments (number of peaks)
• types of proton environment (chemical shift)
• relative numbers of each type of molecule (relative peak areas)
• number of adjacent non-equivalent protons (spin-spin splitting pattern)

Question 10

splitting pattern rule

Answer 10

n + 1
i.e. for n protons (H+) on an adjacent carbon atom, the peak will be split into n + 1

-OH and -NH are always singlets

Question 11

issues with identifying O-H and N-H peaks

Answer 11

the NMR peaks are:
often broad (from hydrogen bonding) and do not have any splitting
of variable chemical shift

Question 12

proton exchange using D2O

Answer 12

this is for identifying O-H and N-H peaks

1. a H+ NMR spectrum is ran as normal
2. a few drops of D2O are added, the mixture is shaken, and a second spectrum is run
3. deuterium exchanges with any OH or NH protons, removing their peaks from the second NMR so that they can be identified

Question 13

spectroscopy table headings

Answer 13

peak number
chemical shift
environment
splitting 
no. of adjacent hydrogens
relative peak areas
no. of atoms in environment 
conclusion