NMR2 Flashcards
(48 cards)
Comparing NMR data can be difficult, but can you compare data in a reasonable way?
By using the ppm-scale, where the difference in frequency is converted to ppm. This conversion is called the chemical shift.
δ=((v_sample−v_standard)/v_standard) ∗10^6
What is the chemical shift standard for H1 and C13?
TMS (tetramethylsiliane) it is a silicon atom bound to four methyl groups (Si(CH₃)₄). TMS is not water soluble. It has a sharp peak at 0.00 ppm.
What is the chemical shift standard for water?
DSS (4,4-dimethyl-4-silapentane-1-sulfonic acid) and it is commonly used as a chemical shift reference in aqueous (water-based) NMR solutions, especially for ¹H NMR. It is water soluble and provides a sharp signal at 0.00 ppm just like TMS
What is the old ppm scale?
Tau where it is given at:
τ[ppm]=10−δ
What are the chemical shift ranges for H1 and C13?
- H1=0-10 ppm
- C13= 0-230 ppm
What is PPM an abbreviation for.
Project portfolio management
Particles per milliliter
Parts per million
Parts per million
Which compounds are used to standardize the PPM scale, and therefore have a δ=0
DSS
PPM
TMS
TSP
OBH
DSB
DSS, TMS and TSP
A compound resonating at 5.24 PPM using a 6.345 T magnet. What PPM will the compound have using a 37.6 T magnet?
5.24 because the ppm-scale is used and therefore the two different magnets are comparable
Using a 7.05 T magnet, you record a proton spectrum where your standard resonates at ν=300.1712236 MHz. The proton of your compound resonates at ν=300.1724993 MHz. Calculate δ
(300.1724993 - 300.1712236)/300.1712236*10^6 =4.25
What is the mesomeric effect or resonans effect?
The mesomeric effect is the delocalization of electrons through π bonds in a conjugated system, caused by electron-donating or electron-withdrawing groups via resonance.
Rank the chemical shift of the meta, para and ortho position
δ_meta>δ_para>δ_orto
Give examples of when the meta position of the chemical shift is not the highest.
In compounds like benzoic acid, the ortho and para protons show higher chemical shifts than the meta proton. This is because the –COOH group is electron-withdrawing, which causes deshielding (especially via resonance) at the ortho and para positions.
Order of chemical shift:
δ(ortho)>δ(para)>δ(meta)
Consider the following substituents on benzen. Do they donate or remove electrons from the aromatic system?
-NH2
-OH
-COOH
do they: select for each
-donate electrons
- remove electrons
-NH2:donates electrons
-OH:donates electrons
-COOH: removes electrons
At which position in a benzene ring is the electron density affected the least by a substituent?
Ortho, Meta or Para
meta
If a benzen ring is substituted with an electron donating group (e.g. -OH), then you would expect the chemical shift of the Ortho position to be __________ the Meta position.
Higher than, lower than or the same as
lower than
Describe the effect of magnetic anisotropy
Magnetic anisotropy is the directional variation of the magnetic field experienced by a nucleus due to nearby π systems (like double bonds, aromatic rings, or carbonyls).
These π systems create induced local magnetic fields that can shield (+) or deshield (-) nearby nuclei depending on their position.
What are the chemical shift ranges for different types of molecules?
- 0–2 ppm → Saturated hydrocarbons (alkyl)
- 4.5–6.5 ppm → Alkenes (vinyl)
- ~7 ppm → Aromatic protons
- 9–10 ppm → Aldehydes
- 10–13 ppm → Carboxylic acids and H-bonded protons
What is scalar coupling (j-coupling)?
Scalar coupling occurs when nuclear spins influence each other through bonds. For hydrogen atoms, this means that the spin state (α or β) of a neighboring hydrogen affects the local magnetic field, leading to signal splitting in the NMR spectrum.
Describe the formula for mulitplicity ( m,the number of signals the molecules are split into)
m=2∗n∗I+1
- n, the mumber of equvalent coupling partners
- I, the nuclear spin quantum number of the coupling partner
How do you determine the couplings constant?
You determine the coupling constant (J) by measuring the distance between split peaks (multiplets) in a signal, expressed in hertz (Hz:
J=Δv
Name the different splitting patterns
- Singlet (s) only one peak
- Doublet (d) two peaks
- Triplet (t) split into three peaks
- Quartet (q) split into four peaks
- Quintet split into 5
- Sextets split into 6
Describe the pascal triangle
It is a pyramidal row of numbers that predicts the relative intensities of peaks in a multiplet due to spin-spin coupling in NMR.
what is I for hydrogen?
0,5
In 1H NMR, the formula for multiplicity can be simplified. Write the formula for calculating multiplicity in 1H NMR m=__________
n+1
