Chapter 17: Structural Determination of Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance Spectroscopy

An experimental process used for structure determination in whcih radio frequency radiation is used to cause spin flips in nuclei of atoms that have odd number of protons, neutrons or both

Radio Frequency

The region of the electromagnetic spectrum with wavelengths between -1000 to 1000m. This radiation is used in NMR spectroscopy to cause spin flips among nuclei

External Magnetic Field

A magnetic field applied to a sample by a magnet, such as a super conducting magnet in NMR spectroscopy

Free Induction Decay

In NMR spectroscopy, a digitized record of amplitude vs time for radio frequency radiation re emitted by a sample

Fourier Transform

Mathmatical algorithm used in NMR spectroscopy that decomposes a free induction decay into its individual frequencies or signals

Signals

In NMR spectroscopy, the frequencies of radiation emitted by NMR active nuclei, which are converted into peaks in the NMR spectrum amplitude of each peak is proportional to amount of radiation originally absorbed

Chemical Shift

A measure of the extent to which a signals frequency differs from that of a reference compound

Nuclear Spin

Property of some atomic nuclei, which generates small magnetic field, an outcome expected of a spinning charge

A spin state

Describes a nucleus that has a spin of ½ au

B spin state

Describes a nucleus that has a spin of -1/2 au

Spin Flip

A change from one spin state to another

Proton NMR Spectroscopy

A method of generating a spectrum whereby radio frequency electromagnetic radiation is used to cause spin flips int eh nuclei of H atoms

NMR active nucleus

A nucleus that posses spin and can be studied with NMR spectroscopy

Effective Magnettic Field

The net magnetic field that is felt by the nucleus, the sum of external and local magnetic fields

Shielded

In NMR spectroscopy, describes a nuclei in a molecule that have a local magnetic field that opposes an external magnetic field. Shielded nuclei have smaller chemical shifts

Chemical Environment

The electron distribution surrounding a particular location in space, governing the chemical behavior of an atom

Homotropic/Chemically Equivalent

Describes nuclei surrounded by identical electron distributions

Heterotropic/Chemically Distinct

Describe nuclei surrounded by different electron distributions

TMS

commonly added to samples to provide reference signal in NMR spectroscopy

Chemical distinction test

A protocol used to determine whether atoms in a molecule are chemically distinct

Deshielded

Nuclei whose shielding becomes diminished by nearby electronegative atoms or by magnetic anisotropy. Deshielded nuclei have higher signal frequencies and thus larger chemical shifts

Downfield

In an NMR spectrum appearing in a region of higher chemical shift

Upfield

In an NMR spectrum appearing in a region of lower chemical shift

Magnetic Anisotropy

The phenomenon resulting form ring current in double bonds and aromatic rings that contributes to the local magnetic field of nearby nuclei. It causes substantial deshielding f hydrogen and carbon nuclei associated with simple alkanes aromatic rings and aldehydes

Ring Current

The motion of pi electrons along a circular path parallel to the plane of the pi system to which they belong, giving rise to magnetic anisotropy, the motion is driven by forces from an external magnetic field.

Tesla

The SI unit for strength of a magnetic field

Gyromagnetic Ratio

A value that relates the energy separation between spin states of nucleus and applied magnetic field

Operating Frequency

The frequency at which a bare proton absorbs electromagnetic radiation when subjected to an NMR spectrometer’s magnetic field

Integration

The operation of calculating the area under a curve, such as a peak in a spectrum, used to generate an integral trace in an NMR spectrum

Integral Trace

The line overlaid on an NMR spectrum that corresponds to the cumaltive area under the peaks in the spectrum. The size of a stairstep in an integral trace is proportional to the number of nuclei that give rise to the corresponding peak.

Signal Splitting

The separation of an NMR signal into multiple peaks due to spin spin coupling between the nucleus giving rise to the signal and nuclei of near by atoms

Spin Spin Coupling

Interaction between nearby nuclei that have spin, the cause of signal splitting in NMR spectrum

Coupled

Describes two or more atoms that exhibit spin spin coupling in NMR spectrum

The N + 1 Rule for Signal Splitting

If proton A is coupled only to proton B and there are N equivalent protons B, then Proton A has N coupled protons, and the signal from proton A will be split into N-H peaks

Singlet

Splitting pattern of NMR signal when there are no coupled protons - single peak

Doublet

The splitting pattern of an NMR signal into two peaks of essentially equal height, when N=1

Triplet

The splitting pattern of an NMR signal into three peaks in a 1.:2:1 height ratio, N=2

Quartet

The splitting pattern of NMR signal into four peaks with a 1:3:3:1 height ratio, N=3

Quintet

Splitting pattern of NMR signal into 5 peaks, when N=4

Multiplet

The splitting pattern of NMR signal when there are large number of coupled protons

Pascal’s Triangle

A pattern that descries the ratio of the relative peak heights in splitting patterns of NMR signals

Coupling Constant

In NMR spectroscopy, the frequency difference between peaks of a split NMR signal, this property is independent of external magnetic field

Long Range Coupling

In NMR spectroscopy, the situation in which nuclei separated by more than three bonds exhibit weak coupling

Complex Splitting

In NMR spectroscopy, splitting that arises when a nucleus is coupled to mroe than one distinct type of nuclei

Splitting Diagram

A schematic that illustrates the complex splitting of an NMR signal in stepwise fashion

Doublet of Doublets

The splitting pattern of an NMR signal when each peak is doublet is split again into a double by a second coupling, resulting in a total of four peaks

Triplet of Doublets

The splitting pattern of an NMR signal when each peak of a triplet is split into a doublet by a second coupling, resulting in total of six peaks

13C NMR Spectroscopy

A method in which radio frequency electromagnetic radiation is used to cause spin flips int he nuclei of C 13 atoms.

Broadband Decoupling

A technique used to turn off the coupling between C13 and H nuclei so that carbon signals are not split by attached protons in NMR spectroscopy. In this method, radio frequency radiation forces protons to rapidly swithc between a and b spin states, resulting in a zero average magnetic field produced by each proton

Distortionless Enhancement by Polarization Transfer (DEPT)

Technique used in 13C NMR spectroscopy to determine type of carbon ( CH3, CH2, CH or C) responsible for producing a particular signal