Ch 15 - NMR Spectroscopy

B Spin state

opposing B₀, higher energy spin state

a spin

aligned with B_0, lower energy spin state

Shielded atom

feels the B₀ less thus requires less energy to spin

high electron density around the nucleus =

more shielding

What does it mean for an atom to be in resonance in NMR?

the atom flipped from an a spin state → b spin state.

absorbed energy

Homotopic Protons

if the molecule has an axis of rotational symmetry that allows one proton to be rotated onto the other without changing the molecule

• chemically equivalent

Enantiotopic Protons

if the molecule has a plane of reflection that makes one proton the mirror image of the other

• chemically equivalent

Diastereotopic protons

do not have any kind of symmetry → not chemically equivalent

• 2+ chiral centers = diastereomers

Finding signals Shortcut #1

the 2 protons on a CH₂ group will be equivalent if there are no chiral centers in the molecule

Finding signals Shortcut #2

The 2 protons on a CH₂ group will not be equivalent if there is a chiral center in the molecule

Finding signals Shortcut #3

the 3 protons on a methyl group will always be equivalent to each other

Finding Signals Shortcut #4

multiple protons are equivalent if they can be interchanged through rotational or reflectional symmetry

NMR spectra characteristics

1. Number of signals

2. signal location (chemical shift)

3. signal intensity

4. signal shape (splitting pattern)

Downfield

left side of NMR spectra. low field strength

• unshielded protons

Upfield

right side of NMR spectra. high filed strength

• shielded protons

What NMR signal does this compound give?

1.0ppm

What NMR signal does this compound give?

2.2 ppm

What NMR signal does this compound give?

2.7ppm

What NMR signal does this compound give?

3.1ppm

What NMR signal does this compound give?

4.3 ppm

What NMR signal does this compound give?

7.3

What NMR signal does this compound give?

5.3 ppm

What NMR signal does this compound give?

3.1ppm

The Oxygen of an alcohol or ether has how much effect on Alpha protons?

+2.5ppm

The Oxygen of an ester has how much effect on Alpha protons?

+3ppm

A standard Methyl group gives what signal?

~0.9ppm

A standard Methylene group gives what signal?

~1.2ppm

A standard Methine group gives what signal?

~1.7ppm

The effect on beta protons is _______ the effect on alpha protons.

1/5

Diamagnetic ansiotropy

different regions of localized space have different magnetic strengths

what is the chemical shift

~2ppm

what is the chemical shift

~2.5 ppm

what is the chemical shift

~2.5ppm

what is the chemical shift

~2- 4ppm

what is the chemical shift

~2 - 5ppm

what is the chemical shift

~4.5-6.5ppm

what is the chemical shift

6.5-8ppm

what is the chemical shift

~10ppm

What is the chemical shift

~12ppm