NMR - Spectropy II

Question 1

Infrared Spectroscopy (IR)

Answer 1

Measures vibrations of bonds to identify functional groups

Question 2

Ultraviolet Spectroscopy (UV

Answer 2

Measures electronic transitions to provide information on the electronic bonding in the sample

Question 3

Mass Spectrometry (MS):

Answer 3

Breaks the molecule into fragments (destructive) and measures the MW of fragments to give information to the structure and functional groups present

Question 4

Nuclear Magnetic Resonance (NMR)

Answer 4

Uses electromagnetic fields to measure spinning of nuclei to identify chemical environments leading to identification of functional groups and structure of the sample

Question 5

Which wave in electromagnetic is NMR?

Answer 5

Radio (type of wave)

Question 6

What is Carbon-13 (13C) in NMR?

Answer 6

Determines the magnetic environments of the carbon atoms instead of protons

12C (99%) has no magnetic spin

13C (1%) does have a magnetic spin – but sensitivity is decreased by x100

The resonance frequency of 13C is only one fourth of that for 1H NMR (gyromagnetic ratio)

Hundred of spectra were taken and then averaged (old technique)

Now we have Fourier-Transform

Question 7

Fourier Transform NMR Spectroscopy

Answer 7

Mathematical technique used to compute a spectrum from the FID

A pulse is used to introduce non-equilibrium nuclear spin magnetization

The relaxation of spins to their resonance is time dependent and measured to create the spectrum

Question 8

Carbon Chemical Shifts

Answer 8

13C signals are also deshielded by EWG (similarly to 1H NMR)

The carbon atom is one atom closer to the shielding or deshielding group than the attached hydrogen atom

Therefore, carbon chemical shifts are usually 15-20 times larger than proton shifts

Question 9

Comparison of 13C and 1H NMR

Answer 9

1H Spectrum = 10ppm, 13C Spectrum = 200 ppm (20x)

Deuterated Chloroform (CDCl3) used as common solvent for 13C NMR

Question 10

What are the differences between 1H and 13C NMR?

Answer 10

Operating frequency

Peak Areas

Spin-Spin Splitting

Question 11

Operating frequency:

Answer 11

Gyromagnetic ration and resonance frequency of 13C is about ¼ of that for 1H NMR

Question 12

Peak Areas:

Answer 12

The areas of the peaks of 13C are not necessarily proportional to the number of carbons giving rise to those peaks – therefore can be used to identify the type of chemical environment but may not relate directly to the number of carbons present

Carbon atoms with 2 or 3 protons attached usually give the strongest absorptions while carbon atoms with no protons attached usually give the weakest absorptions

Question 13

Spin-Spin Splitting:

Answer 13

Since only 1% of Carbon atoms are the 13C isotope, carbon-carbon splitting is ignored

Carbon-Hydrogen splitting patterns can be complicated

Question 14

What is Proton Spin Decoupling??

Answer 14

The splitting of a resonance for a 13C atom by hydrogen can be eliminated to generate a singlet by a technique called proton decoupling.

= spectrum is called a proton-decoupled NMR spectrum

Question 15

What happens when Proton Spin Decoupling:

Answer 15

H are continuously in resonance (rapidly flip spins) due to being irradiated by a broadband proton transmitter

The C nuclei see an average of the possible proton spin states

Each C signal appears as a single unsplit peak because any C-H splitting has been eliminated

Question 16

Off-Resonance Decoupling

Answer 16

Simplification of the spectrum that still allows some of the splitting information to be retained
13C nuclei are only split by the protons directly bonded to them

N+1 Rule Applies: C with 1H appears as doublet, C with 2H gives a triplet etc.

TMS is a quartet at 0 ppm (split by 3 protons of each methyl group)

Question 17

How many times is the spectrum run?
What happens each time?

Answer 17

TWICE

1st: Broadband-Decoupled Spectrum (indicated number of non-equivalent environments of C and their chemical shifts)

2nd: Off-Resonance-Decoupled Spectrum (multiplicities of signals indicate the number of H bonded to each C)

Question 18

DEPT 13C NMR

Answer 18

DEPT: Distortionless Enhanced Polarization Transfer

Question 19

How do peaks remain in spectrum? (after splitting)

Answer 19

All peaks remain as decoupled singlets

Question 20

Dept 13C NMR - explain process and the results;

Answer 20

All peaks remain as decoupled singlets

Has better sensitivity and avoids overlapping multiplets

Each 13C nucleus is magnetically coupled to the protons bonded to it

There is transfer of polarization from the protons to the carbon nucleus

How the polarization transfer occurs is determined by the number of protons bonded to the 13C nucleus

Question 21

Normal decoupled scan, in which each type of 13C nucleus appears as a ______

Answer 21

singlets

Question 22

Only the CH carbons bonded to exactly ____ proton appear

Answer 22

one

CH3 and CH up, and the CH2 down. C with no H do not appear

Question 23

more protons the CH is attached to…
angles?

Answer 23

the more it effects of the angles

Question 24

Interpreting 13C NMR Spectra

Answer 24

1. The number of different signals implies how many different types of carbons are present
2. The chemical shifts of those signals suggest what types of functional groups those carbon atoms belong to
3. The splitting of signals in the Off-Resonance-Decoupled, DEPT-90 and DEPT-135 spectra indicate how many protons are bonded to each carbon atom