NMR Video Flashcards
(23 cards)
What are the 3 components of NMR?
Chemical Shift, Integration, and J-coupling
What is the entire concept of NMR based on?
Each nucleus has a specific spin. When you apply a magnetic field to the nucleus, it can either be aligned with or against the field. Essentially, a radio frequency pulse is applied and the nuclei absorb the energy. When talking about NMR, we’re dealing with hydrogens.
Define Chemical Shift
Different protons absorb at different frequencies; it’s a measure of the magnetic field the hydrogens experience.
When the value of a chemical shift is 0, what do we call it?
Low Shift/Upfield/Shieded
When the value of a chemical shift is high, what do we call it?
High Shift/Downfield/Deshielded
What is the relationship between the frequency of light absorbed by hydrogens and the amount of magnetic field that they experience?
Direct; it’s proportional.
What do the terms “shielded’ and “deshielded” refer to respectively?
The amount of electron density surrounding the hydrogens; shielded means that there is a high electron density and deshielded means that there’s a low electron density.
What does Integration cover?
The amount of hydrogens are measured by measuring the area under each peak.
How can the area under each peak be determined if numerical values are not provided?
Take a ruler, measure the height of each curve provided, and compare them in length. From there, you determine the ratio, giving you the # of hydrogens.
What do the height of curves correspond to?
the # of hydrogens in the respective peaks.
What is the standard used for chemical shift?
Trimethylsilane, located at 0 ppm.
What are the units of chemical shift?
ppm
What does J coupling involve?
Hydrogens adjacent to other hydrogens can be split into multiple peaks.
How can each hydrogen be regarded?
As a magnet; it is either aligned or against the magnetic field.
How do you know how many peaks will arise based on the number of adjacent hydrogens?
n+1 (n being the # of hydrogens that another hydrogen is adjacent to).
What are the ratios based on the # of adjacent H?
0- Singlet- 1
1- Doublet- 1:1
2- Triplet- 1:2:1
3- Quartet- 1:3:3:1
4- Quintet- 1:4:6:4:1
5- Sextet - 1:5:10:10:5:1
Can chemically identical nuclei couple to one another?
No, they cannot.
What unit is the spacing between peaks in?
Hertz.
In what cases is the coupling constant 7Hz?
In acyclic alkanes, CH’s have a coupling constant of 7Hz.
In what cases is the coupling constant different for protons?
In Alkenes; cis protons have a coupling constant of 10Hz and trans protons have a coupling constant of 18Hz.
When do you get a doublet of doublets? What’s an example?
When a proton is coupled with 2 different protons with 2 different coupling constants. For example, a hydrogen in an alkene coupling with the cis and then trans hydrogen.
What is the ratio of a doublet of doublets and how many peaks do you get?
4 peaks. 1:1:1:1
What is the relationship between electron density and chemical shift?
The higher the electron density, the lower the chemical shift. INVERSE.
