Chapter 13

Question 1

A nucleus with an _______ atomic number or mass number has a nuclear spin that can be seen by NMR

Answer 1

odd

Question 2

1H and 13C have two spin states called _______ and ________

Answer 2

alpha and beta

Question 3

in the absence of an external magnetic field. spins of the magnetic nuclei are oriented _______ and are all _______ in energy

Answer 3

randomly
equal

Question 4

when nuclei are placed in a external magnetic field (B0), the nucleus can align with the magnetic field in a way that complements (_____) or opposes (_____) that external magnetic field

Answer 4

alpha
beta

Question 5

Nuclei oriented in the same direction as the external magnetic field are slightly _______ in energy than those that oppose the external magnetic field.

Answer 5

lower
alpha lower than beta

Question 6

When are magnetic nuclei said to be in resonance?

Answer 6

when radio frequency energy is absorbed and the lower energy spin state flips to the higher energy spin state (alpha to beta)

Question 7

In 1H NMR spectrum, NMR can only see _______

Answer 7

hydrogens

Question 8

an NMR spectrum has the _______ region at the left (10) and the _______ region to the right (0)

Answer 8

downfield
upfield

Question 9

What is the solvent used to dissolve an organic sample in NMR?

Answer 9

CDCl3
deuterium is invisible in NMR

Question 10

What are the 3 basics for interpreting NMR?

Answer 10

1) chemical shift
2) integration
3) splitting

Question 11

The chemical shift or position of an NMR absorption will depend on the density of the H atom: affected by
1)
2)
3)

Answer 11

1) inductive effect
- EN of atoms
2) hybridization
- pi effects
3) H-bonding

Question 12

What is the formula for ppm?

Answer 12

observed peak (Hz) over operating frequency (MHz)

Question 13

If a H is closer to more electronegative groups = _______ = _______

Answer 13

deshielded = downfield

Question 14

Around what range do H connected to aromatic rings show up? (ppm)

Answer 14

7-8 ppm

Question 15

Around what range do H connected to carbonyls show up? (O=C-CH3) (ppm)

Answer 15

2-3 ppm

Question 16

around what range do H connected to O show up? (O-CH3) (ppm)

Answer 16

3-4 ppm

Question 17

EN effects are _______, meaning more EN atoms will make the chemical shift more downfield

Answer 17

additive

Question 18

Chemical shifts are affected by the _______ from an electronegative atom

Answer 18

distance

Question 19

explain the pi effect

Answer 19

pi electrons move more easily than electrons in a sigma bond

Question 20

Explain how the pi effect differs between double bonds and triple bonds

Answer 20

db pi e’s cause a downfield shift while tb pi e’s cause an upfield shift

Question 21

H-bonding often reveals _______ signals because of constant breaking and reforming

Answer 21

broad

Question 22

Different types of _____ = # of signals that will appear in 1H NMR

Answer 22

H
- draw lines of symmetry and count different H’s

Question 23

Enantiotopic: H’s on carbon atoms are electronically equivalent but chemically distinct. These H will likely show _______ peak(s) in the 1H NMR

Answer 23

a single

Question 24

Diastereotopic: H’s on carbon atom are neither chemically or electronically equivalent. These H are likely to show _______ peak(s) in the 1H NMR

Answer 24

2 different
- even though they are on the same carbon

Question 25

What is the difference between enantiotopic and diastereomeric, and how are you supposed to tell the difference?

Answer 25

enenatiotopic = 1 peak diastereotopic = 2 peaks diastereotopic will have a chiral center on OG molecule

Question 26

Integration: the relative _______ of the integral is proportional to the # of _______ contributing to the peak

Answer 26

height H's (hydrogens)

Question 27

it is important to keep in mind that integration values are relative and that they represent the _______ of protons not absolute values

Answer 27

ratio

Question 28

Multiplicity or spin-spin splitting results when _______ types of _______ are close enough that their magnetic fields influence each other

Answer 28

2 different protons - need to be H's on neighboring Carbon

Question 29

N+1 rule: # of peaks a H produces = (# of _______ H) +1

Answer 29

neighboring

Question 30

What is the exception to the N+1 rule?

Answer 30

OH and NH ignore neighbors

Question 31

Most spin-spin splitting is between protons on _______ carbon atoms. (equivalent protons do not split each other)

Answer 31

adjacent

Question 32

Splitting tree diagrams: What is the ratio for a doublet (d)?

Answer 32

1:1

Question 33

Splitting tree diagrams: What is the ratio for a triplet (t)?

Answer 33

1:2:1

Question 34

Splitting tree diagrams: What is the ratio for a quartet (q)?

Answer 34

1:3:3:1

Question 35

Protons that split each other are said to be _______

Answer 35

coupled

Question 36

The distance between adjacent lines of a splitting pattern is called the coupling constant (___) measured in Hz

Answer 36

J

Question 37

Coupled protons will have the same value of _____

Answer 37

J

Question 38

Using J values is most useful to distinguish between possible _______ of a compound or _______

Answer 38

isomers stereoisomers - Distinguish between cis and trans bonds

Question 39

_______ provides a reference singlet at 0ppm or delta

Answer 39

TMS

Question 40

describe the common splitting pattern for ethyl groups

Answer 40

2H quartet to 3H triplet

Question 41

describe the common splitting pattern for an isopropyl group

Answer 41

1H septet to a 6H doublet

Question 42

describe the common splitting pattern of a PARA-substituted aromatic ring

Answer 42

leaning doublets

Question 43

Leaning doublets is common splitting pattern of _______

Answer 43

PARA-substituted aromatic ring

Question 44

_______ results from different types of H with the same chemical shift

Answer 44

multiplets - overlapping signals

Question 45

more complex patterns can arise when a signal is split by two or more _______ proton types

Answer 45

non-equivalent - ex styrene doublet of doublets

Question 46

The doublet of doublets is common splitting pattern of _______

Answer 46

monostubstitued alkene

Question 47

If H are rapidly moving in 3D space, the NMR signal will appear at the position where the _______ H would be detected = _______ signal

Answer 47

average single

Question 48

Any process faster than ________ will be recorded as an average by NMR spectroscopy.

Answer 48

0.01 sec

Question 49

What are 2 ways that NMR can show two non-equivalent types of protons that split each other?

Answer 49

1) low temperatures 2) add big group (tBu)

Question 50

if H are lost/gained rapidly because ______ or _______ is present, no neighbor information is retained

Answer 50

acid or base - this is why OH and NH ignore neighbors

Question 51

What is the only situation where OH splitting will appear on an NMR?

Answer 51

ultrapure ethanol

Question 52

In what situation will the 3.5 peak disappear?

Answer 52

when OD is used

Question 53

Proton exchange an be used to identify OH and NH NMR signals by shaking the sample with _______. Any exchangeable protons are quickly replaced by deuterium atoms, which are invisible to NMR

Answer 53

D2O

Question 54

for 13C NMR, most important is the _______ of the individual arbon resonances

Answer 54

chemical shift

Question 55

Values for 13C NMR are about ____ times the chemical shift of 1H NMR

Answer 55

20

Question 56

13C NMR shows _______ carbon types

Answer 56

all

Question 57

DEPT-90 shows signals for ______ carbons only

Answer 57

C-H

Question 58

DEPT-135 shows Positive: _______ Negative: _______

Answer 58

postive: CH, CH3 negative: CH2