NUCLEAR MAGNETIC RESONANCE II Instrumental Analysis NUCLEAR MAGNETIC RESONANCE II Dr. Nermin Salah 12th Lecture

Objectives Magnetic Shielding Chemical Shift Factors Affecting Chemical Shift Inductive Effect by Electronegative Groups s-character (Hybridization Effect) Magnetic Anisotropic Effect Hydrogen Bonding Equivalent and Nonequivalent Protons Integration of NMR Signal Spin-Spin Splitting (Spin-Spin Coupling)

MAGNETIC SHIELDING NMR would not be very valuable if all protons absorbed at the same frequency. Different protons usually absorb different radiofrequencies (ν ). The electrons in a bond shield the nuclei from the magnetic field. A moving charge (electron) creates a magnetic field, and the field created by the moving electrons opposes the applied magnetic field Bo. The electrons in a bond shield the nuclei from the magnetic field.

The more electron density around a proton, the more the shield, the lower magnetic field affecting the proton , The less electron density , the less the shield , the higher magnetic field.

Shielded Protons The effective magnetic field, therefore, what the hydrogen nuclei actually “sense” through the surrounding electronic environment is always less than the actual applied magnetic field B0. Beffective = Bapplied  Blocal 1 Gauss= 0.001 Tesla In the classical NMR experiment, magnetic field strength must be increased for a shielded proton to flip at the same frequency.

Low external applied field Higher external applied field

Protons in a molecule upfield Lower frequency downfield higher frequency

The CHEMICAL SHIFT shift in Hz chemical = d = = ppm shift The “chemical shift” is a field independent value. Chemical shift is : the difference in frequency between the sample and the standard over the operation frequency. chemical shift = d = shift in Hz spectrometer frequency in MHz = ppm This division gives a number independent of the instrument used. parts per million A particular proton in a given molecule will always come at the same chemical shift (constant value). Of course, we don’t do any of this, it’s all done automatically by the NMR machine.

Tetramethylsilane (TMS) TMS is added to the sample as internal standard. TMS protons are all identical, highly shielded providing a single sharp peak always isolated from peaks of interest. The TMS was assigned d = 0.00. Organic protons absorb downfield (to the left) of the TMS signal. TMS is inert , highly soluble in organic liquids and easily removed from samples by distillation.

FACTORS AFFECTING CHEMICAL SHIFT Four major factors account for the resonance positions (on the ppm scale) of most protons Deshielding by electronegative elements Inductive effect by electronegative groups s-character (hybridization effect) Magnetic Anisotropic effect (magnetic fields usually due to -bonded electrons in the molecule( Hydrogen bonding.

C H Cl d- d+ 1. DESHIELDING BY ELECTRONEGATIVE ELEMENTS Chlorine “deshields” the proton, that is, it takes valence electron density away from carbon, which in turn takes more density from hydrogen deshielding the proton. “highly shielded” protons appear at upfield (lower ) “deshielded“ at downfield (higher ) deshielding moves proton resonance to lower field and higher  NMR CHART

ELECTRONEGATIVITY DEPENDENCE OF CHEMICAL SHIFT Dependence of the Chemical Shift of CH3X on the Element X Compound CH3X CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si Element X F O Cl Br I H Si Electronegativity of X 4.0 3.5 3.1 2.8 2.5 2.1 1.8 Chemical shift d 4.26 3.40 3.05 2.68 2.16 0.23 0 most deshielded TMS deshielding increases with the electronegativity of atom X

SUBSTITUTION EFFECTS ON CHEMICAL SHIFT Cumulative Effect most deshielded The effect increases with greater numbers of electronegative atoms. CHCl3 CH2Cl2 CH3Cl 7.27 5.30 3.05 ppm CH2Br CH2CH2Br  CH2CH2CH2Br 3.30 1.69 1.25 ppm most deshielded The effect decreases with increasing distance from the electronegative atom. The effect completely vanished at the fourth bond from the electronegative atom.  X

2. s-CHARACTER (HYBRIDIZTION OF CARBON ATOM)

But in fact, we have the following order: As s-character of carbon atom increases, the electronic cloud is held more closer to the carbon and provides less electron density for shielding of protons, and thus the chemical shift, δ, increases (shifted downfield). According to the above reasoning, the following trend for the chemical shift is expected: expected to be observed at  > 7 (down field - higher ) But in fact, we have the following order: actually observed at  = 2-3 This suggested that other factors than the sp character of carbon might affect the chemical shift in this case. The above discrepancy can be explained by what is called Magnetic Anisotropic Effect.

3. MAGNETIC ANISOTROPIC FIELDS DUE TO THE PRESENCE OF -BONDS The presence of a nearby pi bond or pi system greatly affects the chemical shift. Induced magnetic fields due to the  - electrons have greatest effect.

Aromatic protons  = 7-8 ppm Prediction of direction of magnetic field using right hand rule.

Vinyl (Olefinic) protons,  = 5-6 ppm

Acetylene protons ̃ ≈ 2.5 ppm

Aldehyde proton = 9-10 ppm Electronegative oxygen atom

HYDROGEN BONDING DESHIELDS PROTONS O-H and N-H Signals HYDROGEN BONDING DESHIELDS PROTONS The chemical shift depends on how much hydrogen bonding is taking place (observed in high concentrated solutions). O H R Hydrogen bonding lengthens the O-H bond and reduces the valence electron density around the proton it is deshielded and shifted downfield in the NMR spectrum. Alcohols vary in chemical shift from 0.5 ppm (free OH) to about 5.0 ppm (lots of H bonding). D2O-exchangeable (peak for OH proton in alcohol and NH in amines disappears upon shaking with D2O)

SOME MORE EXTREME EXAMPLES δ- δ+ -I Carboxylic acids have strong hydrogen bonding - they form dimers. Resonance, electronegativity of oxygen and the formation of hydrogen bonding withdraw electron cloud from the acid protons. Thus, protons attached to carboxylic acids are the least shielded protons and have a chemical shift of 10-12 ppm. In methyl salicylate, which has strong internal hydrogen bonding, the NMR absorption for O-H is at about 14 ppm, (highly downfield( Notice that a stable 6-membered ring is formed

disappears upon shaking NMR CORRELATION CHART Chemical shift gives the electronic environment of protons (Shielding and Deshielding) PROTON IN ELECTRON-POOR ENVIRONMENTS DESHIELDED DOWNFIELD HIGHFREQUENCY LARGE  PROTON IN ELECTRON-RICH ENVIRONMENTS SHIELDED UPFIELD LOWFREQUENCY SMALL  disappears upon shaking with D2O 7-8 CHCl3 , aromatic -OH -NH 0 - 5 3 - 5 10-12 RCOOH acid 1.5 - 3 TMS 9-10 RCHO aldehyde 4.5 - 7 0 – 2 d (ppm) 12 11 10 9 8 7 6 5 4 3 2 1 H CH2F CH2Cl CH2Br CH2I CH2O CH2NO2 sp3 CH2Ar CH2NR2 CH2S C C-H C=C-CH2 CH2-C- O sp3 C-CH-C C C-CH2-C C-CH3 sp3 C=C olefinic sp2 sp

EQUIVALENT AND NONEQUIVALENT PROTONS All of the protons in a molecule which are in chemically identical environments will often exhibits the same chemical shift i.e., shows one signal in NMR spectrum at the same value of . The protons in this case are said to be chemically equivalent On the other hand, molecules which have sets of protons which are chemically distinct (have different chemical environments) from one another give rise to different absorption signals from each other. Chemically nonequivalent protons

INTEGRATION OF A PEAK Integration = determination of the area Not only does each different type of hydrogen give a distinct peak in the NMR spectrum, but we can also tell the relative numbers of each type of hydrogen by a process called integration. Integration = determination of the area under a peak

The integrated area measured by a ruler are 5 : 2.5 : 22 In the NMR spectrum, the area under each peak is proportional to the number of hydrogens generating that peak. The NMR spectrometer has the capability to electronically integrate the area under each peak. The integrated area measured by a ruler are 5 : 2.5 : 22 Divide by the smallest number give us the simplest ratio of 2 : 1 : 9 Note that the integration gives only ratios, not absolute values for the number of the hydrogen present in the sample 2,2-dimethyl-1-propanol (C4H12O)

Modern spectrometers automatically print out the integrals as numbers on the spectrum NMR spectrum can reveal the number of protons assigned for each signal Integral lines : no. of protons in each signal

SPIN-SPIN SPLITTING

SPIN-SPIN SPLITTING Often a group of hydrogens will appear as a multiplet rather than as a single peak. Multiplets are named as follows: Singlet (s) Quintet (quin) Doublet (d) Sixtet (six) Triplet (t) Septet (sept) Quartet (q) Multiplet (m) This happens because of interaction with neighboring hydrogens and is called SPIN-SPIN SPLITTING Spin-spin coupling

Nonequivalent protons on adjacent carbons. 1,1,2-Tribromoethane Nonequivalent protons on adjacent carbons. Chapter 13

THE ORIGIN OF SPIN-SPIN SPLITTING HOW IT HAPPENS Spin-spin splitting arises because hydrogen on adjacent carbon atoms can “sense” one another

Doublet 1 adjacent proton Chapter 13

Triplet 2 Adjacent Protons Chapter 13

n + 1 RULE singlet doublet triplet quartet quintet sixtet septet MULTIPLETS this hydrogen’s peak is split by its two neighbors these hydrogens are split by their single neighbor singlet doublet triplet quartet quintet sixtet septet two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet Where n is the number of EQUIVALENT protons on Adjacent carbon atoms

EXCEPTIONS TO THE n+1 RULE IMPORTANT ! 1) Protons that are equivalent by symmetry usually do not split one another splitting if x  y two triplets no splitting if x = y 2) Protons in the same group usually do not split one another or 3) Splitting is not observed if the protons are separated by more than three  bonds

1 1 1 1 2 1 1 3 3 1 INTENSITIES OF MULTIPLET PEAKS PASCAL’S TRIANGLE singlet 1 Intensities of multiplet peaks 1 1 doublet 1 2 1 triplet 1 3 3 1 quartet

NMR spectrum indicates the carbon skeleton Spin-spin splitting gives the number of equivalent protons on adjacent carbon atoms

INFORMATION WE CAN GET FROM NMR SPECTRUM Summary INFORMATION WE CAN GET FROM NMR SPECTRUM Number of 1H-NMR signals = Number of kinds of protons. Chemical shift gives the electronic environment of protons (Shielding and Deshielding). NMR spectrum can reveal the number of protons assigned for each signal (integral lines : no. of protons in each signal). NMR spectrum indicates the carbon skeleton Spin-spin splitting gives the number of equivalent protons on adjacent carbon atoms.

Typical Values Note that these are typical values and that there are lots of exceptions!

Resources and references Textbook: Principles of Instrumental Analysis, Skoog, Holler, Nieman Recommended further reading: “Principles of instrumental analysis, 5th ed. by Skoog, Holler, Nieman” Chapter 19. Extra resources are available on the intranet. Relevant web sites http://www.chemguide.co.uk/analysismenu.html