Lecture 3

NMR Spectroscopy:

• Spin-spin Splitting in 1H NMR• Integration• Coupling Constants• 13C NMR• Sample Preparation for NMR Analysis

Due:Lecture Problem 1

1H NMR Spectrum of Ethanol: Spin-Spin

Splitting

ppm ()

CH3CH2OH

TMS

a - tripletb - quartetc - singlet

a ab

b

c

c

downfield upfield

Spin-Spin Splitting

CH3CH2OH

a - tripletb - quartetc - singlet

a b c General rules:• Neighboring, non-equivalent protons split each other’s signals• Equivalent protons do not split each other’s signals• Use the n + 1 rule to predict the splitting pattern of a proton’s signal

n + 1 ruleThe signal of a proton with n equivalent neighboring protons is split into amultiplet of n + 1 peaks.

In ethanol, a neighbors b; they split each other’s peaks. Note that b neighbors c and no splitting occurs between the two; b is only affected by a.In general, protons that reside on heteroatoms (O, N) do not get involvedwith spin-spin splitting with neighboring protons. Thus, c appears as a singlet.

Spin-spin Splitting

Spin-Spin Splitting

Determine the splitting patterns for the signals in the 1H NMR spectra ofthe following compounds.

OH

Cl

O

H

NH2NH2

CH3

Complex Spin-Spin Splitting

Consider the 1H NMR spectrum of a substituted alkene:

Spin-Spin Splitting

Spin-Spin Splitting

Determine the splitting patterns for the signals in the 1H NMR spectra ofthe following compounds.

O

O

O

H3C

Cl

HH

Integration • Area underneath signal; NMR machine will give integrals• First, gives the relative ratio of different types of protons in compound• Second, allows determination of actual ratio of different types of protons

1. Measure the length of the integral with a ruler2. Establish a relative ratio of protons (divide each length by the lowest number)

Coupling Constants (J)

Protons that split each other’s peaks will have the same coupling constantor J value.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

1H NMR Spectrum of a Taxol Derivative

Taken from Erkan Baloglu’s Masters Thesis

Nuclear Magnetic Resonance

Information Gained:• Different chemical environments of nuclei being analyzed (1H

nuclei): chemical shift• The number of different types of H’s: number of signals in

spectrum• The numbers of protons with the same chemical environment:

integration• The number of protons are bonded to the same carbon:

integration• The number of protons that are adjacent to one another:

splitting patterns• The exact protons that are adjacent to one another: coupling

constants

13C NMR Spectroscopy

Information Gained:• Different chemical environments of carbons in molecule:

chemical shift• The number different types of C’s: number of signals in

spectrum

Differences from 1H NMR:• No splitting of signals (proton-decoupled); thus, only singlets• No integration• ppm scale ranges from 0 to 220 ppm

13C NMR Chemical Shifts

CarbonylCarbons

UnsaturatedCarbons C-X

SaturatedCarbons TMS

200 100 0

ppm

Like with 1H NMR, the more shielded the carbon nuclei, the more upfieldits signal will appear and vice versa.

downfield upfield

13C NMR Spectrum of Chlorohexane

13C NMR Correlation Chart

NMR Sample Preparation & CDCl3

Sample Prep:Dissolve ~32 mg of sample in CDCl3 in an NMR tube.

Why use CDCl3?

Deuterated solvents are necessary in NMR because deuterium is NOT NMR activeand will not interfere with your sample’s spectrum.

CDCl3 is 98-99% pure with a trace amount of CHCl3. You will see a small solventpeak at ~7.26 ppm due to CHCl3 (1H NMR); see a triplet at 77 ppm 13C NMR. This peak serves as a reference peak; DO NOTcount it as one of your sample’s signals!