PHYS BRITTON Flashcards
if a question asks for non active nmr nuclei, what are we looking for?
even numbers because to be spin active we need odd numbers of electrons. HOWEVER 2h is an exception as this can be NMR active
magnetic moment of spinning nucleus
u = yP
mag moment = magnetogyric ratio x angular momentum
nb p = I
formula for orientations of spin states in a mag field
2I +1
Energy associated with each state =
- (mhyB0) / (2pi)
where m = state eg 1/2 or -1/2
Energy ( that we need to apply to cause NMR signal) =
delta E = hv = hbar x y x Bo = hyBo/2pi
order of energy levels in nmr dependent on … (2)
m and y
larmor freq is defined as ….
frequency at which a nucleus precesses
larmor frequency dependent on (2)
strength of magnetic field Bo
and magnetogyric ratio y
larmor freq (rad s-1) =
w = -y Bo
larmor freq (Hz)
v = -yBo/ 2pi
why does H nmr have higher signal and senistivity than c nmr?
because magnetogyric ratio of H (yH) > yC
also signal is effected by natural abundance and 1H = 100% 13C=1%
why is NMR a sensitive technique compared to other soectroscopies?
because the gap between the energy levels is small
spin up nuclei have m =
1/2
spin down nuclei have m =
-1/2
what has higher/ lower energy, spin up or spin down nuclei?
spin up lower (alpha)
spin down higher (beta)
eqn for boltzmann distribution of spin up ans spin down nuclei =
N1/2
——– = e ^(deltaE/KT)
N -1/2
eqn for contribution to nmr =
dif in population of spin up spin down / total polulation
how do stronger magents affect population difference and thus intensity of signal?
Stronger magnets , bigger population difference, more signal, more intense signal
how do stronger magnets effect resolution?
higher field strength gives better resolution
eigenvalues for hamiltonian operator for ψ-1/2?
opposite sign because hamiltonian operator is negative
eigenvalue = 1/2yHbarBo
eigenvalues for hamiltonian operator for ψ1/2?
opposite sign because hamiltonian operator is negative
eigenvalue = -1/2yHbarBo
how can population of energy levels be changed?
by applying larmor frequency
what effect does a 90 degree pulse have on spin populations?
there is no difference in spin populations after a 90 pulse - populations of spin up and spin down are equal
what effect does a 180 pulse have on spin populations?
spin populations are inverted , so there are now more spin down than spin up
Mo now points down -z axis
Angle of an Rf pulse determined by (2)
its power (B1)
its duration (tp)
equation for angle of rf pulse =
theta (angle) = (360y/2pi) x B1Tp
what effect does doubling the duration of a 90 degree pulse have?
you get a 180 degree pulse
where does magnetisation point after a 90 y pulse
along x axis (Mx)
where does magnetisation point after a 90 x pulse
down the y axis (My)
what is the reference point for nmr ?
TMS usually at 0
what is free induction decay
the oscillating voltage induced by the magnetisation vector the coil
what effect does amount of magnetisation in transverse axis (x) have on intensity of nmr signal
they are linked. the more magnetisation in the transverse axis(x) , the stronger the nmr signal
maxiumum intensity is when the magnetisation vector is where?
on the x axis
The chemical shift, , for a peak in a 1H NMR spectrum is 3.5 ppm. What is the
difference in frequency between this peak and TMS (at 0 ppm) on a 500.4 MHz
spectrometer?
omega = v-vo (Hz)
delta = (v-vo)/vo x 10^6
delta = omega/vo x 10^6
delta x vo = omega x 10^6
(delta x vo) / 10^6 = omega
3.5 x 500.4 / 10^6 = omega mhz
we need to consider changing 500.4MHz to Hz
(3.5 x 500.4 x 10^6 )/ 10^6 = omega Hz = 1751.4 Hz
A: 1751.4 Hz
The chemical shift, , for a peak in a 1H NMR spectrum is 8.1 ppm. If the transmitter
frequency is set to 0 = 400.4 MHz, what is the offset frequency, , for this peak?
omega = v-vo (Hz)
delta = (v-vo)/vo x 10^6
delta = omega/vo x 10^6
delta x vo = omega x 10^6
(delta x vo) / 10^6 = omega
8.1 x 400.4 / 10^6 = omega mhz
we need to consider changing 400.4MHz to Hz
(8.1 x 400.4 x 10^6 )/ 10^6 = omega Hz = 3243.2 Hz
treu or false
The Larmor frequency for a nucleus is proportional to the strength of the static
magnetic field B0.
true
true or flase
NMR signal intensity is dependent on population diff between spin states
true
true or false
j cpupling is dependent on field strength
false
true or false
line width of a peak in a nmr spectrum is dependent on the t2 relaxation time of the protons accociated with that peak
true
true or false
spin spin coupling is repsonsible for peak splitting in nmr spectrum
true
photons from which region of EM spectrum will most liekly induce a rottional transition ?
microwave
List, in order of energy (highest to lowest), the energy levels for the 17O nucleus in a
static magnetic field.
Y (
17O) = −36.26 106
rad s−1 T−1
I = 5/2
amount of levels = 2i+1 = 6
usually we have positive values of gamma and lowest energy is positive eg 1/2m lowest then -1/2 m for I=1/2
however, here we have a rare case where gamma is positive so this trend is flipped
therefore lowest energy = -5/2 for I=5/2
therefore highest to lowest energy =======
m5/2, m3/2, m1/2, m–1/2, m–3/2 m–5/2
true or false
The Larmor frequency is in the MHz range for superconducting NMR
magnets.
true
true or flase
the nmr signal is stored in KHZ
true
true or false
multiple aquisitions are not required for quadrature detection
true
the line widths of hnmr peaks are dependent on transverse relaxation time of photons?
true or false
true
true or false
NMR spectroscopy measures the absorption and emission of radio frequency
radiation.
false - it measures the precession of a magnetisation vector
what factors will effect t1 relactaion time in water ?
A: Adding 1 mM GdCl3.
B: Decreasing the temperature.
C: Measuring the T1 relaxation time on a 600 MHz instrument.
D: Shimming the magnet.
E: Removing dissolved gaseous oxygen from the water.
all apart from D (shimming the magnet) will effect t1.
true or false
In pulsed gradient spin echo NMR spectroscopy, the normalised signal, (S(G)/S(0)), is dependent on the strength of the PGSE gradients.
true
true or flalse
In pulsed gradient spin echo NMR spectroscopy, the normalised signal, (S(G)/S(0)), is dependent on the temperature of the
sample.
true
true or flalse
In pulsed gradient spin echo NMR spectroscopy, the normalised signal, (S(G)/S(0)), is dependent on the magnetic field strength of the spectrometer
false
true or flalse
In pulsed gradient spin echo NMR spectroscopy, the normalised signal, (S(G)/S(0)), is dependent on the separation between the PGSE gradients
true
true or flalse
In pulsed gradient spin echo NMR spectroscopy, the diffusion co-efficient of a molecule can be measured.
true
Which of the following cannot affect the line width of a peak in a NMR spectrum?
A: the temperature of the sample
B: chemical exchange
C: the homogeneity of B0
D: the tip angle of the radio frequency pulse
E: the presence of a paramagnetic salt.
D the tip angle of the radiofrequency pulse
Which of the following will affect the line width of a peak in a NMR spectrum?
A: the temperature of the sample
B: chemical exchange
C: the homogeneity of B0
D: the tip angle of the radio frequency pulse
E: the presence of a paramagnetic salt.
all but D
A: the temperature of the sample
B: chemical exchange
C: the homogeneity of B0
NOT D: the tip angle of the radio frequency pulse
E: the presence of a paramagnetic salt.
will this improve the resolution of the sample in nmr?
increasing temp of sample
yes
will this improve the resolution of the sample in nmr?
aquiring the NMR spextrum on a spectrometer with higher frequency proton resonance frequency
eg 600 MHZ instead of 400 MHZ
yes
will this improve the resolution of the sample in nmr?
changing the solvent to one with a lower viscocity
yes
will this improve the resolution of the sample in nmr?
removing dissolved gaseous 02 from sample
yes
will this improve the resolution of the sample in nmr?
increasing the number of signal averages
no
true or false
the NMR signal arises from the decay of maganetisation as the system returns to thermal equilibrium
false
true or flase
the line widths of 1H NMR are dependent on dipolar coupling
true
A 1 H NMR spectrum is collected on a 400 MHz spectrometer, using four signal
averages, a spectral width of 10 kHz and a repetition time of 1 s. The T1 relaxation
time for protons in the sample is 1.45 s. What pulse angle will result in maximum
signal for the experimental parameters used?
cos(theta) = exp^(-TR/T1)
cos(theta) = exp^(-1/1.45)
cos(theta) = 0.50175
cos(0.50175) = 60
therefore 60 degrees
will this increase S/N ratio of peaks in NMR?
increase signal averages
yes
will this increase S/N ratio of peaks in NMR?
add 10mM of Gd3+ to the sample
yes
dipolar coupling effected by paramagnetic ions in the sample
will increase s/n ratio
note: will reduce t1 relaxation times of molecules around it
will this increase S/N ratio of peaks in NMR?
adding paramagentic substance to sample?
yes
will this increase S/N ratio of peaks in NMR?
increase repeptition time
yes
will this increase S/N ratio of peaks in NMR?
collect data from a spectrometer with higher MHZ
yes
Which of the following cannot affect the integral of a peak in a NMR spectrum?
A: the number of signal averages
B: the repetition time
C: the homogeneity of B0
D: the tip angle of the radio frequency pulse
E: the concentration of the sample
C the homogeneity of B0
can the integral of a peak in NMR be affected by :
number of signal averages
yes
can the integral of a peak in NMR be affected by : the repetition time
yes
can the integral of a peak in NMR be affected by : the homogeneity of BO
no
can the integral of a peak in NMR be affected by : the tip angle of the radiofrequency pulse
yes - maximum peak height / intensity is at a specific tip angle
cos(theta)= exp^(-tr/t1)
can the integral of a peak in NMR be affected by : the cocnetration of the sample
yes
Radiation of which wavelength is most likely to induce a purely rotational transition
in a molecule?
A: 1 m
B: 1 cm
C: 1 mm
D: 1 microm
E: 1 nm
B 1cm
microwaves between 1mm and 30cm
what properties of an NMR spectrum does relaxation effect ?
width and intensity of peaks
does chemical exchnage affect the appearence of an NMR spectrum?
yes
nuclei near electronegative species will experience a stronger or weaker mag field than those that are near e donating species?
stronger
Nmr spectra has ppm on the axis , but what are we actually plotting?
frequency
true or false t1 and t2 relaxtation times originate from dipole dipole relaxation
true
less neighbouring spins nearby will mean it relaxes more slowly.tr
true or flase
t1 and t2 relaxation times are influenced by paramagnetic species.
true
dipolar interactions between the spins on neighbouting nuclei influence t1 and t2 relaxation . these interactions are influenced by paramagnetic species.
they will reduce the t1 relaxation times of moleules around them
t1 is the ……. relaxation time
spin lattice relaxation
treue or false
in liquids there is no splitting
true - due to the rapid motion of molecules, the distnace and angle between molecules averages so there is no splitting.
what is tc
rotational correlation time
tc is dependent on what (3)
temp size of molecule and viscocity of solvent
eg small molecule will have lower tc as its faster
true or false
spin lattice relaxtaion time is depdndent on rotational correlation time
true
true or false
spin spin relaxation only occurs in the transverse plane
true
what effect does t2 have on lifetime of a spin state?
reduces lifetime
what effect does t2 relaxation have on shape of peak?
causes line broadening
which statements are correct ??
spin spin relaxation (t2) occurs through
1) inter and intra molecular relaxation
2) magnetic filed inhomogeneity
both are correct
a broad peak shows what
fast decay (short T2)
a narrow peak shows what
slow decay (long t2)
true or false
t2 can be longer than t1
false . it can never be longer than t1
true or false
at fast motion t1 and t2 have the same value
true
true or false
multiple aquisitions can affect signal intensity
true
in an inversion recovery experiment for T1 relaxation time , what relationship is present between the intenisties of the peaks in the spectrum and Mz(tao)?
proportional
Tnull=
T1ln2
true or false, in CPMG experiment echoes are repeatedly refocused , this minimised the effct of the inhomogenous field
true
NMR signal in measured in…..
MHz
NMR signal stored in
KHz
Nyquist theorem suggests data should be sampled how many times to correctly assign a frequency?
twice per wavlength
sample rate (delta t) =
1/SW
1/ maximum frequency
quadrature detenction decreases signal to noise
true or false
false - it INCREASES signal to noise
signal averaging / multiple aqusitions allows signal to noise to be improved, true or false
true
true or false
signal adds coherently
true
true or false
noise adds coherently
false, noise adds more slowly than signal (which adds coherntly) if n is the number of averages, noise adds as rootN akaN^1/2
organise these 2d NMR techniques into through bond and through space interactions
COSY NOSEY HMQC
COSY HMQC through bond interactions
NOESY throigh space interactions
what 2d nmr technique is used for analysing molecular dynamics / chemical exchange ?
EXSY
4 steps for a 2d pulse sequence
preparation
eveolution
mixing
acquisition
true or false
magnetisation transfer through scalar coupling can be meausreed using a two pulse sequence and will result in diagonal cross peaks
false - will result in off diagonal cross peaks becsuse the larmor freq of the first and second pulse will be different
REVISE FROM NUCLEAR OVERHAUSER EFFECT IN TOPIC 3
