Chemical Shift Flashcards
what energy level transition is NMR concerned with
nuclear spin energy levels
what is the nuclear spin quantum number
I
what is the formula for calculating I
I = h/2pi + root(I(I+1))
what values can I take
0, 1/2, 1, 3/2, 2, 5/2 etc.
what values of I are NMR active
nuclei in which I does not = 0 are NMR active
how is the value of I calculated
protons / neutrons / I even / even / 0 odd / odd / 1 even / odd / 1/2, 3/2 etc odd / even / 1/2, 3/2 etc
give examples of 1=1/2 nuclei
1H 13C 15N 19F 29S
what is a nucleus with a I value greater than 1/2 known as
quadrupolar
give examples of nuclei with I=1
2H 14N
give examples of I=3/2 nuclei
7Li 23Na
give examples of I=5/2 nuclei
27Al
describe the physical properties of spin 1/2 nuclei
spherical charge distribution
electric energy is independent of orientation (acts as point charge)
how many projections does a nucleus have on the z axis
(2I+1) projections of Iz
what is the formula for Iz
Iz = (h/2pi) x m
what is the formula for the magnetic quantum number and what values can it take
m = 2m + 1 and can take values of 1 - -1
what is the magnetic moment of a nucleus proportional to, and what is the constant of proportionality
mu is proportional to I
the constant of proportionality is the gyromagnetic ratio (gamma)
what are the units of the gyromagnetic ratio
radT-1s-1
what is the energy of a magentic moment (mu) in electric field (B)
E = -mu x B
how can the energy of a magnetic moment in I also be written
E = -m(h/2pi) x nu B
how is the energy of a transition calculated
E(m=1/2) - E(m=-1/2)
what is the formula for angular frequency wrt. Bo
W = gamma x B0
what is the formula for angular frequency wrt nu
w = 2pi nu
what is formula for the Larmor frequency
nu = -gammaBo/2pi
what is the typical field strength of an NMR machine
9.4 T = Bo
what is the formula for the population of energy levels
n(upper)/n(lower) = e^deltaE/kT
difference is always very small
other than the energy levels population what else does nmr intensity depend on
abundance of the NMR active isotope
what is the formula for the frequency of an NMR nucleus
nu = gammaBo/2pi
what is the strength of the induced field proportional to
- sigma Bo
what does sigma represent
the extent of chemical sheilding
why is the induced field inversely proportional to the applied field
the induced field acts in the opposite direction to the applied field
what is the formula for localised magnetic field B(loc)
B(loc) = Bo + Binduced
=Bo(1-sigma)
what is the formula for the NMR frequency of B(loc)
nu = gammaBo/2pi (1-sigma)
what is the formula for chemical shift
shift (delta) = 10^6 (nu-nuref)/nuref
what is the formula for chemical shift involving sigma
shift (delta) = 10^6 (sigmaref - sigma)/(1-sigma ref)
what are the 2 main components of chemical shielding
sigma p (paramagnet shielding) sigma d (diamagnetic shielding)
what effect does paramagnetic shielding have
augments Bo - deshields
what effect does diamagnetic shielding have
opposes Bo - shields
what is the size of sigma p in different nuclei
in spherical nuclei sigma p =0
in heavy nuclei sigma p is large
what is the effect of sigma d and sigma p in 1H nuclei
minimal
what are the 4 effects that make up total sigma
sigma = sigma dloc + sigma ploc + sigma ring current + sigma other
what does local diamagnetic shielding depend on
number of electrons in an atom
higher e- density = greater shielding = smaller chemical shift
what does local paramagnetic shielding depend on
mixing of ground states and excited states
extent of mixing depends on energy gap between homo and lumo and distance between nucleus and electrons
how does the interaction of Bo and magnetic flux effect shielding
when Bo is in the same direction as flux, the proton in the flux will be deshielded and vice versa
describe flux about an alkyne bond
deshielding occurs around the bond
- effects all pi systems
describe flux around an aromatic ring
shielding occurs in the centre of the ring
desheilding around the around the outside of the ring
what effect does H-bonding have on shielding
large deshielding effect as e- density is removed from protons
give an example where h bonding increases chemical shift
with increasing ethanol conc the position of the OH peak increases frequency
how does a positive charge effect shielding
deshields
how does a negative charge effect shielding
increases shielding
what is the effect of a paramagnetic compound (unpaired e-)
substantial shielding
why do unpaired electrons give rise to such large magnetic field
gyromagnetic ratio for an electron is 660 times greater than that of a proton
what is the role of a shift reagent
speads out peaks of similar frequency to increase resolution
how do solvents effect the NMR
different solvents have a different affinities for different molecules
what effect does temperature effect an NMR spectrum
can be used to view thermodynamic parameters