13.9 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 Quintet Doublet Septet Triplet Octet Quartet Nonet This happens because of interaction with neighboring hydrogens and is called SPIN-SPIN SPLITTING.

1,1,2-Trichloroethane integral = 2 integral = 1 triplet doublet

n + 1 RULE

singlet doublet triplet quartet quintet sextet septet MULTIPLETS this hydrogen’s peak is split by its two neighbors these hydrogens are split by their single neighbor MULTIPLETS singlet doublet triplet quartet quintet sextet septet two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet

Some Common Patterns

SOME COMMON SPLITTING PATTERNS CH-CH3 ( x = y ) CH-CH2 -CH2-CH3 CH3 ( x = y ) CH CH3

tert-butyl group

EXCEPTIONS TO THE N+1 RULE IMPORTANT ! 1) Protons that are equivalent by symmetry usually do not split one another no splitting if x=y no splitting if x=y 2) Protons in the same group usually do not split one another or

SOME EXAMPLE SPECTRA WITH SPLITTING

NMR Spectrum of Bromoethane

NMR Spectrum of 2-Nitropropane 1:6:15:20:16:6:1 in higher multiplets the outer peaks are often nearly lost in the baseline

NMR Spectrum of Acetaldehyde offset = 2.0 ppm

The propyl group

INTENSITIES OF MULTIPLET PEAKS PASCAL’S TRIANGLE

PASCAL’S TRIANGLE Intensities of multiplet peaks 1 singlet 1 1 The interior entries are the sums of the two numbers immediately above. doublet 1 2 1 triplet 1 3 3 1 quartet 1 4 6 4 1 quintet 1 5 10 10 5 1 sextet 1 6 15 20 15 6 1 septet 1 7 21 35 35 21 7 1 octet

THE ORIGIN OF SPIN-SPIN SPLITTING HOW IT HAPPENS

THE CHEMICAL SHIFT OF PROTON HA IS AFFECTED BY THE SPIN OF ITS NEIGHBORS aligned with Bo opposed to Bo +1/2 -1/2 50 % of molecules 50 % of molecules H H H H A A C C C C Bo downfield upfield neighbor aligned neighbor opposed At any given time about half of the molecules in solution will have spin +1/2 and the other half will have spin -1/2.

SPIN ARRANGEMENTS H H H H C C C C one neighbor n+1 = 2 doublet yellow spins blue spins The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.

SPIN ARRANGEMENTS two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet methine spins methylene spins

SPIN ARRANGEMENTS C H C H three neighbors n+1 = 4 quartet two neighbors n+1 = 3 triplet C H C H methylene spins methyl spins

13.10 The Coupling Constant

THE COUPLING CONSTANT J J J J J The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet. J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet.

100 MHz 6 5 4 3 2 1 200 MHz 3 2 1 FIELD COMPARISON 200 Hz 100 Hz Coupling constants are constant - they do not change at different field strengths 7.5 Hz J = 7.5 Hz 6 5 4 3 2 1 200 MHz 400 Hz Separation is larger 200 Hz 7.5 Hz The shift is dependant on the field J = 7.5 Hz 3 2 1 ppm

50 MHz J = 7.5 Hz Why buy a higher field instrument? 3 2 1 Spectra are simplified! 100 MHz J = 7.5 Hz Overlapping multiplets are separated. 3 2 1 200 MHz J = 7.5 Hz Second-order effects are minimized. 3 2 1

NOTATION FOR COUPLING CONSTANTS The most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms. This is sometimes called vicinal coupling. It is designated 3J since three bonds intervene between the two hydrogens. 3J Another type of coupling that can also occur in special cases is 2J or geminal coupling ( most often 2J = 0 ) Geminal coupling does not occur when the two hydrogens are equivalent due to rotations around the other two bonds. 2J

LONG RANGE COUPLINGS Couplings larger than 2J or 3J also exist, but operate only in special situations, especially in unsaturated systems. Couplings larger than 3J (e.g., 4J, 5J, etc) are usually called “long-range coupling.”

SOME REPRESENTATIVE COUPLING CONSTANTS 6 to 8 Hz three bond 3J vicinal 11 to 18 Hz three bond 3J trans cis 6 to 15 Hz three bond 3J geminal 0 to 5 Hz two bond 2J

cis 6 to 12 Hz three bond 3J trans 4 to 8 Hz 4 to 10 Hz three bond 3J four bond 4J 0 to 3 Hz four bond 4J Couplings that occur at distances greater than three bonds are called long-range couplings and they are usually small (<3 Hz)

13.11 NMR Spectra of Carbonyl Compounds Anisotropy in carbonyl compounds Anisotropy deshields C-H on aldehydes: 9-10 ppm Anisotropy also deshields methylene and methyl groups next to C=O: 2.0 - 2.5 ppm Methylene groups directly attached to oxygen appear near 4.0 ppm

1 2-Butanone (Methyl Ethyl Ketone) 60 MHz Spectrum

2-butanone, 300 MHz spectrum WWU Chemistry

2 Ethyl Acetate Compare the methylene shift to that of Methyl Ethyl Ketone (previous slide).

3 t-Butyl Methyl Ketone (3,3-dimethyl-2-butanone)

4 Phenylethyl Acetate

5 Ethyl Succinate

a-Chloropropionic Acid 6

13.12 and 13.13 Alkenes, Alkynes and Aromatic Compounds

Alkenes and alkynes CHEMICAL SHIFTS vinyl protons appear between 5 to 6.5 ppm (anisotropy) methylene and methyl groups next to a double bond appear at about 1.5 to 2.0 ppm for terminal alkynes, proton appears near 2 ppm

BENZENE RING HYDROGENS Ring current causes protons attached to the ring to appear in the range of 7 to 8 ppm. Protons in a methyl or methylene group attached to the ring appear in the range of 2 to 2.5 ppm.

NMR Spectrum of Toluene 5 3

THE EFFECT OF CARBONYL SUBSTITUENTS When a carbonyl group is attached to the ring the o- and p- protons are deshielded by the anisotropic field of C=O Only the o- protons are in range for this effect.

Acetophenone (90 MHz) 3 2 3 deshielded

NMR Spectrum of 1-iodo-4-methoxybenzene 3 CHCl3 impurity 2 2

NMR Spectrum of 1-bromo-4-ethoxybenzene 3 4 2

THE p-DISUBSTITUTED PATTERN CHANGES AS THE TWO GROUPS BECOME MORE AND MORE SIMILAR All peaks move closer. Outer peaks get smaller …………………..… and finally disappear. Inner peaks get taller…………………………. and finally merge. all H equivalent X = X X = Y X ~ X’ same groups

NMR Spectrum of 1-amino-4-ethoxybenzene 3 4 2 2

NMR Spectrum of p-Xylene (1,4-dimethylbenzene) 6 4

13.14 Hydroxyl and Amino Protons

Hydroxyl and Amino Protons Hydroxyl and amino protons can appear almost anywhere in the spectrum (H-bonding). These absorptions are usually broader than other proton peaks and can often be identified because of this fact. Carboxylic acid protons generally appear far downfield near 11 to 12 ppm.

C O H H SPIN-SPIN DECOUPLING BY EXCHANGE In alcohols coupling between the O-H hydrogen and those on adjacent carbon atoms is usually not seen. This is due to rapid exchange of OH protons between the various alcohol molecules in the solution. The OH peak is usually broad. C O H H In ultrapure alcohols, however, coupling will sometimes be seen.

NMR Spectrum of Ethanol 3 2 1

1-propanol

13.16 Unequal Couplings Tree Diagrams

WHERE DOES THE N+1 RULE WORK ? The n+1 rule works only for protons in aliphatic chains and rings, and then under special conditions. There are two requirements for the n+1 rule to work: 1) All 3J values must be the same all along the chain. 2) There must be free rotation or inversion (rings) to make all of the hydrogens on a single carbon be nearly equivalent. C H 3Ja = 3Jb Hydrogens can interchange their positions by rotations about the C-C bonds. The typical situation where the n+1 rule applies.

WHAT HAPPENS WHEN THE J VALUES ARE NOT EQUAL ? 3Ja = 3Jb C C C H H H 3Ja 3Jb In this situation each coupling must be considered independently of the other. A “splitting tree” is constructed as shown on the next slide.

CONSTRUCTING A TREE DIAGRAM ( SUPPOSE 3Ja = 7 Hz and 3Jb = 3 Hz ) The largest J value is usually used first. H H H -CH2-CH2-CH2- C C C C H 3Jb = 3 triplet of triplets H H H 3Ja = 7

WHEN BOTH 3J VALUES ARE THE SAME The n+1 rule is followed -CH2-CH2-CH2- n+1 = (4 + 1) = 5 ….. because of overlapping legs you get the quintet predicted by the n+1 rule.

2-PHENYLPROPANAL A case where there are unequal J values.

Spectrum of 2-Phenylpropanal b d J = 7 Hz c TMS a c J = 2 Hz d b

3J1 = 7 Hz 3J2 = 2 Hz ANALYSIS OF METHINE HYDROGEN’S SPLITTING 7 Hz Rather than the expected quintet ….. the methine hydrogen is split by two different 3J values. quartet by -CH3 3J1 = 7 Hz 3J2 = 2 Hz doublet by -CHO ANALYSIS OF METHINE HYDROGEN’S SPLITTING quartet of doublets

2-PHENYLPROPANAL Adjacent protons are three bonds away from each other: 3J, often = 7 Hz The aldehyde proton d has a 3J = 2 Hz coupling to the single proton b the methyl protons a have a 3J = 7 Hz coupling to proton b proton b is a quartet of doublets

VINYL ACETATE

PROTONS ON C=C DOUBLE BONDS COUPLING CONSTANTS PROTONS ON C=C DOUBLE BONDS PROTONS ON C=C DOUBLE BONDS In alkenes, 3J-cis = 8 Hz In alkenes, 3J-trans = 16 Hz In alkenes, when protons are on the same carbon, 2J-geminal = 0-2 Hz

NMR Spectrum of Vinyl Acetate

Analysis of Vinyl Acetate H 3 O HC HA HB 3J-trans > 3J-cis > 2J-gem HC HB HA 3JAC 3JBC 3JBC cis trans trans 3JAC 2JAB 2JAB cis gem gem

OVERVIEW

TYPES OF INFORMATION FROM THE NMR SPECTRUM 1. Each different type of hydrogen gives a peak or group of peaks (multiplet). 2. The chemical shift (d, in ppm) gives a clue as to the type of hydrogen generating the peak (alkane, alkene, benzene, aldehyde, etc.) 3. The integral gives the relative numbers of each type of hydrogen. 4. Spin-spin splitting gives the number of hydrogens on adjacent carbons. 5. The coupling constant J also gives information about the arrangement of the atoms involved.

SPECTROSCOPY IS A POWERFUL TOOL Generally, with only three pieces of data 1) empirical formula (or % composition) 2) infrared spectrum 3) NMR spectrum a chemist can often figure out the complete structure of an unknown molecule.

EACH TECHNIQUE YIELDS VALUABLE DATA FORMULA Gives the relative numbers of C and H and other atoms INFRARED SPECTRUM Reveals the types of bonds that are present. NMR SPECTRUM Reveals the environment of each hydrogen and the relative numbers of each type.