Chemical Shift

Question 1

what energy level transition is NMR concerned with

Answer 1

nuclear spin energy levels

Question 2

what is the nuclear spin quantum number

Answer 2

I

Question 3

what is the formula for calculating I

Answer 3

I = h/2pi + root(I(I+1))

Question 4

what values can I take

Answer 4

0, 1/2, 1, 3/2, 2, 5/2 etc.

Question 5

what values of I are NMR active

Answer 5

nuclei in which I does not = 0 are NMR active

Question 6

how is the value of I calculated

Answer 6

protons / neutrons / I
even      / even       / 0
odd       / odd         / 1
even     / odd      / 1/2, 3/2 etc
odd      / even / 1/2, 3/2 etc

Question 7

give examples of 1=1/2 nuclei

Answer 7

1H 13C 15N 19F 29S

Question 8

what is a nucleus with a I value greater than 1/2 known as

Answer 8

quadrupolar

Question 9

give examples of nuclei with I=1

Answer 9

2H 14N

Question 10

give examples of I=3/2 nuclei

Answer 10

7Li 23Na

Question 11

give examples of I=5/2 nuclei

Answer 11

27Al

Question 12

describe the physical properties of spin 1/2 nuclei

Answer 12

spherical charge distribution

electric energy is independent of orientation (acts as point charge)

Question 13

how many projections does a nucleus have on the z axis

Answer 13

(2I+1) projections of Iz

Question 14

what is the formula for Iz

Answer 14

Iz = (h/2pi) x m

Question 15

what is the formula for the magnetic quantum number and what values can it take

Answer 15

m = 2m + 1 and can take values of 1 - -1

Question 16

what is the magnetic moment of a nucleus proportional to, and what is the constant of proportionality

Answer 16

mu is proportional to I

the constant of proportionality is the gyromagnetic ratio (gamma)

Question 17

what are the units of the gyromagnetic ratio

Answer 17

radT-1s-1

Question 18

what is the energy of a magentic moment (mu) in electric field (B)

Answer 18

E = -mu x B

Question 19

how can the energy of a magnetic moment in I also be written

Answer 19

E = -m(h/2pi) x nu B

Question 20

how is the energy of a transition calculated

Answer 20

E(m=1/2) - E(m=-1/2)

Question 21

what is the formula for angular frequency wrt. Bo

Answer 21

W = gamma x B0

Question 22

what is the formula for angular frequency wrt nu

Answer 22

w = 2pi nu

Question 23

what is formula for the Larmor frequency

Answer 23

nu = -gammaBo/2pi

Question 24

what is the typical field strength of an NMR machine

Answer 24

9.4 T = Bo

Question 25

what is the formula for the population of energy levels

Answer 25

n(upper)/n(lower) = e^deltaE/kT | difference is always very small

Question 26

other than the energy levels population what else does nmr intensity depend on

Answer 26

abundance of the NMR active isotope

Question 27

what is the formula for the frequency of an NMR nucleus

Answer 27

nu = gammaBo/2pi

Question 28

what is the strength of the induced field proportional to

Answer 28

- sigma Bo

Question 29

what does sigma represent

Answer 29

the extent of chemical sheilding

Question 30

why is the induced field inversely proportional to the applied field

Answer 30

the induced field acts in the opposite direction to the applied field

Question 31

what is the formula for localised magnetic field B(loc)

Answer 31

B(loc) = Bo + Binduced | =Bo(1-sigma)

Question 32

what is the formula for the NMR frequency of B(loc)

Answer 32

nu = gammaBo/2pi (1-sigma)

Question 33

what is the formula for chemical shift

Answer 33

shift (delta) = 10^6 (nu-nuref)/nuref

Question 34

what is the formula for chemical shift involving sigma

Answer 34

shift (delta) = 10^6 (sigmaref - sigma)/(1-sigma ref)

Question 35

what are the 2 main components of chemical shielding

Answer 35

``` sigma p (paramagnet shielding) sigma d (diamagnetic shielding) ```

Question 36

what effect does paramagnetic shielding have

Answer 36

augments Bo - deshields

Question 37

what effect does diamagnetic shielding have

Answer 37

opposes Bo - shields

Question 38

what is the size of sigma p in different nuclei

Answer 38

in spherical nuclei sigma p =0 | in heavy nuclei sigma p is large

Question 39

what is the effect of sigma d and sigma p in 1H nuclei

Answer 39

minimal

Question 40

what are the 4 effects that make up total sigma

Answer 40

sigma = sigma dloc + sigma ploc + sigma ring current + sigma other

Question 41

what does local diamagnetic shielding depend on

Answer 41

number of electrons in an atom | higher e- density = greater shielding = smaller chemical shift

Question 42

what does local paramagnetic shielding depend on

Answer 42

mixing of ground states and excited states | extent of mixing depends on energy gap between homo and lumo and distance between nucleus and electrons

Question 43

how does the interaction of Bo and magnetic flux effect shielding

Answer 43

when Bo is in the same direction as flux, the proton in the flux will be deshielded and vice versa

Question 44

describe flux about an alkyne bond

Answer 44

deshielding occurs around the bond | - effects all pi systems

Question 45

describe flux around an aromatic ring

Answer 45

shielding occurs in the centre of the ring | desheilding around the around the outside of the ring

Question 46

what effect does H-bonding have on shielding

Answer 46

large deshielding effect as e- density is removed from protons

Question 47

give an example where h bonding increases chemical shift

Answer 47

with increasing ethanol conc the position of the OH peak increases frequency

Question 48

how does a positive charge effect shielding

Answer 48

deshields

Question 49

how does a negative charge effect shielding

Answer 49

increases shielding

Question 50

what is the effect of a paramagnetic compound (unpaired e-)

Answer 50

substantial shielding

Question 51

why do unpaired electrons give rise to such large magnetic field

Answer 51

gyromagnetic ratio for an electron is 660 times greater than that of a proton

Question 52

what is the role of a shift reagent

Answer 52

speads out peaks of similar frequency to increase resolution

Question 53

how do solvents effect the NMR

Answer 53

different solvents have a different affinities for different molecules

Question 54

what effect does temperature effect an NMR spectrum

Answer 54

can be used to view thermodynamic parameters