Chemical shift scale Interpretation of H NMR spectrum

Chemical shift scale

Interpretation of 1H NMR spectrum

11/20/2012Organic Spectroscopy 1

Lecture 820th November 2012

Chemical shift is the frequency of signal on

an NMR spectrum where the peak occurs.

Atoms in different chemical environment will

resonate at different frequency and will

appear at different chemical shifts.

The chemical shift scale is calibrated such

that the frequency point is tetramethyl saline

(TMS).


Since silicon is less electronegative than

carbon, TMS protons are highly shielded and

its signal defined as zero.

Organic protons absorb downfield (to the left)

of the TMS signal

It’s is inert to most organic samples


Si

CH3

H3C CH3

CH3


TMS

Chemical shift can be expressed in terms of Hz by

setting the TMS peak at 0 Hz. The magnetic field

decreases toward left.

When chemical shifts are given in Hz the applied

magnetic frequency must be specified

i.e 60, 90, MHz because the chemical shift in Hz is directly proportional to

the strength of the applied field, Bo and therefore to the applied frequency.

The value of chemical shift, n in Hz is;

n (in Hz)= nsample - nreference


The number of Hz shift from TMS for a given

proton (or nuclei) will depend on the strength

of the applied magnetic field.

Thus, the resonance proton in an applied

field of 14,100 G is at approx. 60 MHz where

100 MHz is at field strength of 23,500 G.


The ratio of the resonance frequency is the

same as the ratio of the two field strength.

Note:

In 100 MHz the shift in Hz from TMS is 5/3 larger than at 60MHz.

The SI unit Telsa (T) is the unit of measurement for B,

replacing the term Gauss (G). i.e 1T = 104 G.


100 MHz

60 MHz

23,500

14,100

5

3= =

Usually chemical shifts are expressed in d

unit, a form that is independent of the field

strength (instrument used i.e 60, 90, 300,

MHz).

It is a proportionality and thus dimensionless.


The factor 106 is included in the equation to

avoid fractional values, since d (which is

dimensionless) is expressed in part per

million (ppm).


Observed shift from TMS in Hz Spectrometer frequency in Hz

x 106Chemical shift, dor ppm) =

On a delta scale in ppm, the frequency of aresonating proton is recorded such that thevalue of chemical shift increases from right toleft.

Different protons have different resonancebecause protons surrounded by differentelectronic environments


Increasing chemical shift from 0 to 12 ppm theproton is said to be Downfield or Deshielded.

Decreasing chemical shift from 10 to 0 ppm,proton is said to be Upfield or Shielded

Protons with high electron density are said to beshielded whereas those with low electron densityare said to be deshielded.



A peak of proton X at 60 Hz (n 60) from TMS

at an applied frequency 60 MHz would be at

d1.00 or 1.00 ppm.

The same peak of the same proton at 100

MHz would be at 100 Hz but would still be at

d1.00 or 1.00 ppm.


d (or ppm)60

60

106= = 1.00 d (or ppm)

100

100

106= = 1.00106 106

Solution:

The chemical shift in d unit express the amount bywhich a proton resonance is shifted from TMS inppm of the spectrometer basic operatingfrequency. It is independent of the field strength

Hence the value of d for a given proton will alwaysbe the same irrespective of whether themeasurement was made at 60 MHz or at 90 MHz.

So the peak of that proton will be observed at 3.4ppm.


What would be the chemical shift of a peak

that occur 655.2 Hz downfield of TMS on a

spectrum recorded using a 90 MHz

spectrometer?

Solution:

d (or ppm) = [655.2 Hz/90x106 Hz]×106

= 7.28 ppm


1) At what frequency would the chemical shift

of CHCl3, δ = 7.28 ppm occur relative to

TMS on a spectrum recorded on a 300 MHz?

2) Why we measure the resonance of nuclei in

ppm and not in Hz?

Solution:

ν= 7.28 ppm x 300 MHz = 2184 Hz


1. How many type of H’s?

This indicated by how many groups of signals are there in a spectrum

2. What type of H’s?

Indicated by the chemical shift of each group e.g shielded or deshielded, CH,

CH2, CH3 with respect to chemical environment.

3. How many H’s of each type are there?

Indicated by integration (relative area of signal for each group)

4. What is the connectivity?

Look at the coupling patterns. This tells you what is next to each group



Both phenyl acetone and

benzyl acetate have δ = 7.3

ppm.

Methyl groups attached

directly to a carbonyl have

resonance at δ = 2.1 ppm.

Aromatic protons characteris-

tically have resonance near 7-

8 ppm

Acetyl groups (methyl group of

this type) resonance ~2 ppm.


01234567PPM

CO

OCH2

CH3H

H

H H

H (a)

(b)

(c)

(a)

012345678PPM

CH2

CH3

O

HH

H

H H

Resonance of the benzyl (-

CH2-) protons comes at higher

value of δ = 5.1 ppm in benzyl

acetate than phenyl acetone δ

= 3.6 ppm. Reason?

Being attached to an

electronegative element

(oxygen atom) these electrons

are more deshielding than

those in phenyl acetone


01234567PPM

CO

OCH2

CH3H

H

H H

H (a)

(b)

(c)

(a)

012345678PPM

CH2

CH3

O

HH

H

H H

The higher the electron density the higher the

shielding hence the slow the resonance.

The higher the shielding the more the

external energy is required.


The intensity of an 1H-NMR signal is proportional to the

number of proton of each type in the molecule.

The integral measures the area of the peak and gives the

relative ratio of the number of H’s for each peak.

An FT-NMR instrument (to be discussed later) digitally

integrates each signal area and provides ratios of number of

hydrogen for each signal.


1. For a known molecular formula

Proton per unit is determined by taking total number of proton divide by total units.


# of Hs in signal =total heights of all integrals

total # of Hsheight of integral

x

2. For unknown molecular formulaIntegrate by determine the ratio between thesignals and round off to a nearest wholenumber or multiply by a factor to produce awhole number



Given formula C5H10OSolution

Signal units (units ÷ total units)× Total Hs # of Hs

(1) 2 (2 /10) x 10 = 2H

(2) 3 (3 /10) x 10 = 3H

(3) 2 (2 /10) x 10 = 2H

(4) 3 (3 /10) x 10 = 3H

Total 10 10 H

Calculating DBE = [(2C+2+N)-(H + h)]/2 =1; May be 1 double bond or a ring.

H3C

H2C

CH2

O

CH3(2)

(1)(4)

(3)

Possible structure pentan-2-one


Qn 1. Dispersion is a term used to express thenotion of how well resonances in an NMR spectrumare separated from one another (in Hz) - it is aqualitative term expressing the ease with whichsignals can be distinguished. Two signals occur at2.1 and 2.3 ppm in the proton spectrum in aspectrometer operating at 200 MHz for 1H.

i) What is the frequency difference between theresonances in Hz?

ii) What is their frequency difference (in Hz) in aspectrometer operating at 600 MHz for 1Hobservation?


Qn2. In order to acquire a good spectrumappropriate filling of the sample tube withrespect to coil is the main concern. Thefollowing figures are spectrum A though Dacquired from the same sample but withsample tube filled differently (sample tube 1to 4).

Giving reasons, assign each spectrum withsample responsible for.



1. Stronger magnetic fields Bo cause the instrument to operateat higher frequency (v)

2. NMR field strength vs 1H operating frequency

1.41T => 60 MHz; 2.36T => 100MHz; 7.06T => 300 MHz

3. The d unit has been criticized because d values increased inthe downfield direction. These are really negative numbers.Other scales are expressed in t values; t = 10.00 - d.

4. d unit is treated as positive number. Shifts at higher fieldsthan TMS are rare, that is, if d = -1.00. Then, t = 10.00- (-1.00) = 11.00.

5. Each ppm unit represents either a 1 ppm change in Bo

(magnetic field strength, Tesla) or a 1 ppm change in theprocessional frequency in (MHz)


6. The shift observed for a given proton inHz also depends on the frequency of theinstrument used. Higher frequencieslarger shifts in Hz

i.e 60, 100 and 300 MHz( Operating frequency) is 60,100 and 300 Hz (equivalent to 1 ppm), respectively.

n MHz x 10-6 = n Hz


Larmor frequency is dependent upon MF strength, and

the use of Hz position of the peaks are also dependent uponMF strength


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Chemical shift scale Interpretation of H NMR spectrum