NMR and HNMR Flashcards
what is nuclear magnetic resonance
the absorbance of radio-frequency radiation by nuclei in a magnetic field
what is the frequency and wavelength range for nmr
v=4 to 900 MHz, Wavelength = 0.6 to 10 m
what happens when a nuclei absorbs radiation
change in nuclear spin
4 basic applications of nmr
structure, molecular dynamics, concentration, and diagnostics
p is
angular momentum
what creates the magnetic field
the circular movement of charge
each nucleus is a what
magnetic body
magnetic moment is proportional to what
angular momentum
what is the magnetogyric ratio
how sensitive a magnetic unit will respond to a change in the magnetic field
what are fermions
odd mass. nuclei with an odd number nucleons (fraction spins)
what are bosons
even mass nuclei with odd number of protons and neutrons
even mass nuclei with even p and n have what spin quantum number
0
fermions have what spin quantum number
fractional
bosons have what spin quantum number
integral
if a needle is not spinning along its axis and it is bent 90 degrees then released what happens
oscillation of the needle until it returns to its normal frequency
if a needle is spinning along its axis and it is bent and then released what happens
the needle precesses around the axis of the magnetic field.
if a needle is spinning along its axis, bent, then released what is the restoring force perpendicular to
the original position causing a gyroscopic motion
the angular frequency of precession of magnetic dipole around the applied magnetic field is proportional to what
the strength of the magnetic field
the lamar frequency is
the frequency of precision, and the “resonance” frequency of the nucleus
the nucleus can assume how many discrete states
2I+1
what is the magnetic quantum number
+I to -I
what 4 nuclei are of greatest interest for NMR
1H, 13C, 19F, and 31P
in the absence of a magnetic field, the energies of m= +1/2, and -1/2 are equal, meaning
equal number of atoms in each state
what is the zeeman effect
in the presence of a magnetic field the energy levels split, causing atoms with their magnetic moments aligned more with the field are lower in energy than those aligned more against it.
what can be used to calculate the relative number of nuclei in each state
boltzmann distribution
what can be provided to cause nuclei to transition from one state to another
thermal energy
what are two types of nmr
continuous wave, and fourier transform
frequency of the continuous wave instrument
60, 90, 120 MHz
frequency of the fourier transform nmr
200-1000 MHz
magnet of the continuous wave instrument
permanent
magnet of the fourier transform instrument
superconducting magnet of liquid nitrogen
three things required to get a nmr spectrum
- nuclear spins in zero field 2. magnetic field 2. RF source
in zero field, how are the nuclear spins of 1H and 13C oriented
randomly
in zero field does the number of allowed nuclear spin states change
no
in the presence of an external magnetic field, what orientation of nuclear magnetic moments are allowed
only certain orientations
what is resonance
the absorbance of EMR by a nucleus which causes the nuclear spin to flip from a low energy state to a high energy state
what is the source, detector, monochromate of a nmr
coil, coil, none
what is the uncertainty principle of the continuous wave nmr
1 sec/Hz of the sweep width to measure the frequency with a precision of 1 Hz
how long does it take for the CW nmr to make one scan at 300 MHz with a poor resolution of 1 Hz
4500seconds per scan at
how long does it take for the FT NMR to make one scan at 300 MHz for a poor resolution of 1 Hz
1 second/ scan
In FT NMR how is a strong magnetic field applied
brief pulses of intense RF (100-1000 MHz) to nuclei for 1 to 10 microseconds
what is the free induction decay signal (FID)
RF signal emitted over time
Source, detector, monochromate of FT nmr
coil, same coil, none
FT NMR is in what domain
frequency
what are the 4 types of available nmr spectra
chemical shift, multiplicities for 1H spectra (proton NMR), peak areas, and coupling constants
what is chemical shift
the small difference in absorbance frequency from normal value
what causes chemical shift
reduced shielding of nuclei from the applied magnetic field
what are electron withdrawing groups that cause chemical shifts
nearby groups within the molecule that decrease the electron density around the nucleus causing a greater chemical shift
typical range of chemical shifts for 1H, and 13C
0-13 ppm, and 0-220 ppm
what standard is the chemical shift measured relative to
TMS
high field is high or low frequency
low
what is the effect of hetero atoms on chemical shifts
electronegativity deshields 1H nuclei and increases the chemical shift
is chemical shift dependent or independent of magnetic field strength
independent
what does resolution do with field strength and why
increases because lower peak widths
does lamar frequency increase or decrease with field strength
increase
what is the relationship between frequency and field strength
proportional
nuclear spin on what 1H nuclei gives rise to multiplied peaks
adjacent
the spin of neighboring 1H nuclei adds to or subtracts from what, causing what?
the applied magnetic field causing the effective field to be greater than the applied field
whether the effective field is greater or less than the applied field is dependent on what
whether the adjacent nucleus’ spin is aligned with or against the applied field
in the peak integral spectra, what is the area under the peak proportional to
the number of protons of a particular type that are absorbing EMR
JAB is what
the coupling constant
splitting almost always results from what
1H on adjacent C
what is the minimum sample size for 1H analysis
0.01 to 1 mg
what is the minimum sample size for 13C analysis
1 to 5 mg
solutions must be soluble in what
a denatured solvent such as CDCl3 or D2O
what can be done is a denatured solvent cannot be used
use a solvent that does not interfere the chemical shift of the analyte
why do the samples have to have a low concentration
so viscosity does not become significant
what do particulates in the samples do
lowers the resolutions
all samples must contain what
tms