NMR spectroscopy Flashcards
what does NMR stand for?
Nuclear Magnetic Resonance
what does the spinning charge of a nuclei generate?
spinning charge generates a magnetic dipole
what does nuclei possess?
nucleic possess charge and can spin
how are magnetic dipoles characterized?
by nuclear magnetic spin quantum number “I”
what are the 3 “cases” or values “I” can be?
- I = 0
- I = 1.2
- I > 1/2
what does I = 0 indicate?
- nuclei has a EVEN number of protons and EVEN number of neutrons
- the nuclei does NOT spin –> does not interact with applied magnetic field
what are 2 examples of atoms that have magnetic dipoles of I = 0?
12C
16O
what does I = 1/2 indicate?
nuclei are NMR VISIBLE samples
give 2 examples of I = 1/2 nuclei
1H
13C (the non-radioactive isotope of C, 1.06% natural abundance)
what are 2 examples of nuclei with magnetic dipole of I > 1/2
2H
14N –> difficult to observe
more nuclei are aligned with what?
more nuclei are aligned with the externally applied magnetic field OR the lower energy alpha-spin state
what direction does the arrow point in for the beta-spin state, the alpha-spin state, and the no field state
beta-spin (higher energy): arrow points DOWN
alpha-spin (lower energy): arrow points UP
no field: arrow points in all directions
what happens to the spins of nuclei when a magnetic field is applied?
when there is no magnetic field applied, the nuclei spin in ALL directions and there are NO energy differences
when there is a magnetic field applied, the nuclei spins ALIGN WITH or AGAINST the magnetic field (hence the up and down arrows)
is there an energy difference between the nuclei when no magnetic field is applied?
NO, there is no energy difference
–> energy difference increases as magnetic field applied increases though
what is deltaE?
the energy difference between alpha and beta-spin states
what is the relationship between applied magnetic field and deltaE?
increasing applied magnetic field = increasing deltaE between spin states
what does it mean when it is said that the “nucleus is in RESONANCE with the applied magnetic field”?
when the nucleus ABSORBS ENERGY and flips from the alpha-spin state to the beta-spin state
what is the relation between the energy difference deltaE and frequency of the EM spectrum?
The difference in energy ΔE between the two spin states corresponds to a frequency in the Radiofrequency region of the EM spectrum
what is Planck’s equation?
∆E = hν
∆E = γBoh/2π = hν
ν = γBo/2π
γ = gyromagnetic ratio; a constant
that depends on the magnetic
moment of the nucleus
what is the gyromagnetic ratio γ?
a constant that depends on the magnetic moment of the nucleus
–> it is unique for each nucleus
which equation describes the condition for resonance?
Larmor equation: relates the precession frequency of nuclear spin (w) and the external magnetic field (Bo)
what is the Larmor equation and what does it relate together?
Relates the precession frequency of nuclear spin (w) and the external magnetic field (Bo)
Larmor equation: ν = γ Bo /2π or ω = γ Bo
–> where ν= ω/ 2π
ν is in hertz and ω is in radians/second
Γ is the gyromagnetic ratio (unique for each
nucleus
draw the diagrams/pictures describing nuclei spin and magnetic field
describe the x-axis and y-axis of an NMR spectrum
x-axis: frequency (increases to the LEFT - different from norm)
Y-AXIS: intensity (increases UPWARD)
compare deshielded vs shielded nuclei
deshielded nuclei: these protons sense a LARGER effective magnetic field –> come into resonance at a HIGHER frequency
shielded nuclei: these protons sense a SMALLER effective magnetic field –> come into resonance at a LOWER frequency
relation between shielding and frequency
deshielded = HIGHER frequency = more DOWNFIELD
shielded = LOWER frequency = more UPFIELD
upfield and downfield relation to frequency
upfield = to the RIGHT and LOWER frequency
downfield = to the LEFT and HIGHER frequency
describe where CH3Cl and CH4 NMR peaks will look
CH3Cl - electronegative CL withdraws electron density from the C and H atoms –> thus DESHIELDING the C and H atoms –> peak is more DOWNFIELD (to the left, higher frequency)
what does deshielding mean?
deshielding = lesser shielding = mose of the applied magnetic field strength is experienced = higher frequency absorbed (downfield)
draw the schematic of an NMR spectrometer
what are the components of an NMR spectrometer?
- superconducting magnet
- NMR console
- NMR spectrum
- workstation
- sample tube
describe how an NMR spectrometer works
the sample tube is dissolved in solvent in a thin NMR tube and placed in a magnetic field –> in the NMR probe, the sample is rotated in a magnetic field and irradiated with a short pulse of RF radiation
definition of NMR spectrum
a plot of the intensity of an observed NMR signal vs the frequency measured relative to a reference compound (which is typically TMS - tetramethylsilane)
what is TMS and its function in NMR spectrum?
TMS - tetramethylsilane
the typical reference compound that is used in NMR
draw the structure of TMS
at what number of x-axis (frequency) is TMS reference peak at?
at 0 on the x-axis
what are 3 advantages of using TMS in NMR?
- TMS has 12 equivalent Hs
- NMR signal due to protons in TMS appears s a singlet at 0 ppm (way upfield from other signals)
- it is low-boiling and inert
what are 2 common NMR solvents?
CDCl3 - deuterated chloroform
–> typically used for nonpolar compounds
DMSO-d6 - deuterated dimethyl sulfoxide
–> typically used for polar compounds
what is the purpose of using solvent for NMR?
the solvent is used to dissolve the compound that you are trying to run the NMR analysis on (in the test tube)
what NMR solvent is used for NONPOLAR compounds?
CDCl3 - deuterated chloroform
what NMR solvent is used for POLAR compounds?
DMSO-d6 (deuterated dimethyl sulfoxide)
draw the structure of deuterated dimethyl sulfoxide (DMSO-d6) and deuterated chloroform CDCl3
what is the typical NMR sample prep? (how much compound and solvent)
10-20mg of compound + 0.5-0.6mL of CDCl3 (solvent)
what is chemical shift?
aka the position of the peak
chemical shift - the frequency different between the resonance observed for the nucleus and the resonance observed for the reference compound (ie. TMS)
what is the unit for chemical shift?
ppm
degree of unsaturation formula
[2 (# C atoms) + (# N atoms) – (# halogen atoms) - (# H atoms) + 2 ] / 2
what does a degree of unsaturation greater than or equal to 4 signify?
DU >= 4 means 3 double bonds + 1 ring –> signifies 1 AROMATIC RING
what is the formula for chemical shift?
(sample freq - reference freq) / spectrometer freq
what features of a 1H NMR spectrum provides information about a compound’s structure?
- size of/area under the peak (aka integration) = indicative of the NUMBER of IDENTICAL protons in a particular environment
- # of signals = corresponds to different groups of NON-EQUIVALENT protons
- position of signals = indicative of the KIND of proton(s) responsible for the signal
- spin-spin splitting of signals = indicates the # of NEIGHBORING protons (which is calculated using the N+1 rule)
what does the size of/ area under the peak of NMR signals indicate?
Size of/ area under the peak ((Integration) of NMR signals = is indicative of the relative number of identical protons in a particular environment
what does the number of signals correspond with?
Number of signals = corresponds to different groups of non-equivalent protons
what does the position of the signals indicate?
Position of signals = is indicative of the kind of proton/ s responsible for the signal
what does the spin-spin splitting of signals indicate?
Spin-spin splitting of signals = indicates the number of neighboring protons = calculated using the (N+1) rule
chemically equivalent protons definition
protons in the same environment
–> chemically equivalent protons do NOT split each other’s signals
–> chemically inequivalent protons produce more than one peak
what is the area under an NMR signal proportional to?
Area under an NMR signal is proportional to the number of absorbing protons
what is the height of each step proportional to?
Height of each step is proportional to the area under the peak which in turn is proportional to the number of absorbing protons
number of peaks and their given names?
1 - singlet (s)
2 - doublet (d)
3 - triplet (t)
4 - quartet (q)
5 - quintet
6 - sextet
7 - septet
> 7 - multiplet (m)
relation between signal multiplicities and their relative intensities of their peaks
singlet - 1
double - 1:1
triplet - 1:2:1
quartet - 1:3:3:1
quintet - 1:4:6:4:1
sextet - 1:5:10:10:5:1
septet - 1:6:15:20:15:6:1
when does spin-spin splitting occur?
spin-spin splitting only occurs between NONEQUIVALENT protons on the SAME carbon or ADJACENT carbon
what is the (n+1) rule for spin-spin splitting?
a set of “n” nonequivalent protons splits the signal of a neighboring proton into n+1 peaks
when is splitting not generally observed?
splitting is not generally observed for protons separated by more than 3 sigma bonds
splitting by “n” nuclei of spin quantum number “I” gives ___ how many lines?
Splitting by “n” nuclei of spin quantum number “I” gives (2nI + 1) lines
–> For protons where I= ½, this equation results in (n + 1) lines
how are coupled nuclei identified?
Coupled nuclei are identified by labels as subscripts after the symbol “J”
what does the superscript before the symbol “J” represent?
Superscript before the symbol “J” represents the number of inclusive bonds between the coupled nuclei
what is coupling constant J and what units is it in?
Spacing (in Hz) between the lines in the multiplet = Coupling Constant (J) (units Hz)
units Hz
describe how OH protons are split and what their NMR signal/peak looks like
OH protons are NOT split and they do NOT split the NMR signal of adjacent protons either
–> OH proton signal appear as a SINGLET
what are the steps for NMR spectroscopy?
Steps
1. Sample is dissolved in solvent in a thick NMR tube and placed in a magnetic field
2. In the NMR probe, the sample is rotated in a magnetic field and irradiated with a short pulse of RF radiation
