Lecture 12 - NMR Spectroscopy

Question 1

NMR and how it differs to MS & IR:

Answer 1

probes molecular structure in greater detail than IR or MS

Question 2

what can be determined from NMR spectroscopy:

Answer 2

a complete molecular structure can oftentimes be determined by NMR alone

Question 3

NMR spectroscopy has revolutionized organic chemistry since its introduction in the 1950’s

Before the advent of NMR, structure elucidation may have taken months (or years!)

Question 4

NMR is based on:

Answer 4

nuclear excitation

Question 5

nuclei can only be observed when:

Answer 5

nuclei posses the magnetic property of spin

Question 6

what nuclei are of the most important spin:

Answer 6

1H & 13C

Question 7

natural abundance of - 1H, 13C & 12C:

Answer 7

1H - 99.9%

13C - 1.10%

12C - 98.90%

Question 8

what is the large TMS shift at the end of the NMR spectra?

Answer 8

the large TMS shift is a standard control molecule to compare to

Question 9

in 1H-NMR the absorptions detect:

Answer 9

the protons in the molecule

Question 10

the numbers on the ppm scale (delta) and the frequency are:

Answer 10

proportional - related by the operating frequency of the NMR spectrometer, Vo (in MHz)

Question 11

peaks are referred to as:

Answer 11

resonances, absorptions or lines - all are used and accepted as correct

Question 12

the position of the adsorption is called:

Answer 12

the chemical shift (ppm)

Question 13

___________ is added to each sample as an internal reference (d 0.0 ppm)

Answer 13

tetramethylsilane (TMS)

Question 14

TMS properties:

Answer 14

TMS has a strong absorption, is chemically inert and can be easily removed (volatile)

Question 15

organic compounds generate a separate resonance for:

Answer 15

each chemically non-equivalent set of nuclei

Question 16

the chemical shift is determined by:

Answer 16

the nature of nearby groups (the chemical environment) in a predictable way

Question 17

the size (area under, or integration) of a peak is proportional to:

Answer 17

the number of contributing protons

Question 18

what can also be determined from the 1H-NMR spectra?

Answer 18

the number of protons on adjacent carbons can also be determined

Question 19

nuclear spin causes the nucleus to behave like:

Answer 19

a tiny magnet

Question 20

the tiny magnet can either:

Answer 20

align with the field or go against the field

Question 21

the 1H (and 13C) nucleus can have one of two:

Answer 21

spin states (quantum numbers +½ or -½)

Question 22

(YH) gyromagnetic ratio:

Answer 22

constant that is different for different nuclei

Question 23

how do the tiny nuclear magnets align with a magnetic field?

Answer 23

initially the nuclei exist as randomly oriented spins of equal energy → then you add a magnetic field → before the lower energy state has an excess of spins, however after radiation + energy you have half -1/2 spins and half +1/2 spins

Question 24

what actually is the absorption?

Answer 24

this absorption is nuclear magnetic resonance and is detected by an NMR spectrometer

Question 25

the local magnetic field sensed by a proton is different than the applied magnetic field, what is this due to?

Answer 25

due to the electrons around the proton which oppose the external field

Question 26

reduction of the local field is called:

Answer 26

shielding, hence the electronegativity of nearby atoms will affect the shielding around a proton

Question 27

one of the most important factors in NMR:

Answer 27

electronegativities of nearby groups

Question 28

proton chemical shift is increased by:

Answer 28

•Increasing electronegativity
•Increasing number
•Decreasing distance
•or nearby electronegative groups

Question 29

how will alkyl substitution affect chemical shift?

Answer 29

the more R groups surrounding a proton, the higher the ppm delta values (chemical shift)

Question 30

Protons have different chemical shifts when they are in:

Answer 30

different chemical environments

Question 31

predicting chemical shift non-equivalence is usually the same as:

Answer 31

predicting chemical non-equivalence

Question 32

chemically equivalent protons have:

Answer 32

identical chemical shifts

Question 33

Groups are constitutionally equivalent when:

Answer 33

they have the same connectivity relationship

Question 34

In general, constitutionally nonequivalent groups are:

Answer 34

chemically nonequivalent

Question 35

why might Constitutionally equivalent groups may be chemically nonequivalent?

Answer 35

Chemical equivalency between two groups depends on their stereochemical relationship

Question 36

Equivalency is revealed by:

Answer 36

a substitution test

Question 37

substitution tests to look for equivalency:

Answer 37

(1) substitute each constitutionally equivalent group with a fictitious group and compare

(2) if identical molecules are obtained, the groups
are homotopic and are chemically equivalent

(3) If enantiomers are obtained,
the groups are enantiotopic

(4) If diastereomers are obtained, the groups are diastereotopic

Question 38

enantiotopic groups are:

Answer 38

chemically nonequivalent toward chiral reagents, but are chemically equivalent to achiral reagents

Question 39

Diastereotopic groups are:

Answer 39

chemically nonequivalent under all conditions