NMR - Spectropy I

Question 1

isotopes

Answer 1

different numbers of protons
have spins

Question 2

no magnetic field=

Answer 2

no spin

Question 3

nuclei with magnetic field

Answer 3

spins = aligned same direction/ with or against the field

Question 4

the energy difference between two spin states is

Answer 4

small

Question 5

resonance = when nucleus is subjected to the right combination of magnetic filed and electromagnetic radiation the spin can flip

detected by what?

Answer 5

back and forth

detected by NMR

Question 6

after alignment what happens

Answer 6

the nuclei gets excited

Question 7

hv = AE

Question 8

^ energy ^ magnetic force
then radiation is introduced =

Answer 8

energy back and forth

Question 9

Simple H1 NMR

Answer 9

difference between where absorbance happens - different shielding depends on absorbance

Question 10

What is shielding?

Answer 10

magnetic forcefield exerted by electrons: e- circulating and generating small induced magnetic field that opposes the external field (B0)

Question 11

What happen if shielding is decreased?

Answer 11

felt by the nucleus - results in a stronger filed being NEEDED for resonance to occur

Question 12

Electronegative atoms (O2) ____ e- away from near atoms =

Answer 12

pull

deshielding them

Question 13

Shilded atoms need higher field energy to cause

Answer 13

resonance

Question 14

What is the chemical shift?

Answer 14

difference in magnetic filed strength between the resonance field of the proton being observed and that tetramethylsilane measured in parts per million (ppm)

Question 15

chemical shift definition

Answer 15

the resonant frequency of an atomic nucleus relative to a standard in a magnetic field.

Question 16

does the chemical shift stay in a given proton?

Answer 16

yes, regardless of operating field and frequency of the spectrometer

Question 17

chemical shift =

Answer 17

shift downfield from TMS (Hz) / spec frequency (MHz)

Question 18

1) electronegativity

Answer 18

can cause deshieling of neighbouring atoms as the - are drawn towards the MORE electronegative of that atom

Question 19

2) distance from e- withdrawing effect

Answer 19

further away
less deshielding

Question 20

3) addition of e- withdrawing groups

Answer 20

can decrease the electron density at the nucleus, deshielding the nucleus and result in a larger chemical shift

Question 21

4) other groups with deshielding effects

Answer 21

• aromatic rings
• aldehydes - carbonyl groups (deshielding)
• OH / NH - hydrogen bonding = broad peaks absorbed
• COOH - C=O and O are involved

Question 22

number of signals depends:

Answer 22

can figure out how many protons there are in each peak = proportional

Question 23

e.g. peak - 3.2 = 11.5 mm ( if other peak is 1/3 of original peak =

Answer 23

share with 3 = it is in proportion

measure peak to make sure to get ratio

Question 24

spin-spin splitting

Answer 24

protons from different environments can influence the magnetic filed of each other they are close to one and other

results in splitting of their peaks as seen in the NMR spectrum - chemically equivalent protons do not split each other

Question 25

1 protons next to 2 protons what will happen to splitting

Answer 25

n+1
1 will be 2 (1+1=2)
2 will be 3 (2+1=3)

Question 26

splitting will NOT be observed if more

Answer 26

that 3 bonds exist between protons

Question 27

why does signal appear thicker?

Answer 27

extra protons - opening and lengthening

Question 28

coupling constants

Answer 28

distance between the peaks of multiplets measured in Hz

do not vary with the field strength of spectrometer

Question 29

coupling constant are used to distinguish between the isomers of compounds incl.

Answer 29

stereoisomers

Question 30

are 2 signals equivalent or not?

Answer 30

homotropic
ditropic
heteroprotic

Question 31

READ

Answer 31

organic chemistry, Wade and Simek, 9th Ed - chapter 13

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