SPIN-SPIN SPLITTING

Often a group of hydrogens will appear as a multiplet rather than as a single peak. SPIN-SPIN SPLITTING Multiplets are named as follows: SingletQuintet DoubletSeptet TripletOctet QuartetNonet This happens because of interaction with neighboring hydrogens and is called SPIN-SPIN SPLITTING.

integral = 2 integral = 1 tripletdoublet 1,1,2-Trichloroethane The two kinds of hydrogens do not appear as single peaks, rather there is a “triplet” and a “doublet”. The subpeaks are due to spin-spin splitting and are predicted by the n+1 rule.

n + 1 RULE

1,1,2-Trichloroethane integral = 2 integral = 1 Where do these multiplets come from ? ….. interaction with neighbors

two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet 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

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

3) The n+1 rule applies principally to protons in aliphatic (saturated) chains or on saturated rings. EXCEPTIONS TO THE N+1 RULE or but does not apply (in the simple way shown here) to protons on double bonds or on benzene rings. NO NO YESYES

SOME COMMON PATTERNS

SOME COMMON SPLITTING PATTERNS ( x = y )

SOME EXAMPLE SPECTRA WITH SPLITTING 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

INTENSITIES OF INTENSITIES OF MULTIPLET PEAKS PASCAL’S TRIANGLE

1 2 1 PASCAL’S TRIANGLE singlet doublet triplet quartet quintet sextet septet octet The interior entries are the sums of the two numbers immediately above. Intensities of multiplet peaks

THE ORIGIN OF THE ORIGIN OF SPIN-SPIN SPLITTING HOW IT HAPPENS

CC HH CC HH AA upfielddownfield BoBo THE CHEMICAL SHIFT OF PROTON H A IS AFFECTED BY THE SPIN OF ITS NEIGHBORS 50 % of molecules 50 % of molecules 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. aligned with B o opposed to B o neighbor alignedneighbor opposed +1/2-1/2

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

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

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

THE COUPLING CONSTANT

JJ J JJ 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. J

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

100 MHz 200 MHz Hz 200 Hz J = 7.5 Hz J = 7.5 Hz ppm Hz 400 Hz 56 J = 7.5 Hz Note the compression of multiplets in the 200 MHz spectrum when it is plotted on the same scale as the 100 MHz spectrum instead of on a chart which is twice as wide. Separation is larger

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

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 3 J since three bonds intervene between the two hydrogens. Another type of coupling that can also occur in special cases is 2 J or geminal coupling Geminal coupling does not occur when the two hydrogens are equivalent due to rotations around the other two bonds. ( most often 2 J = 0 ) 3J3J 2J2J

Couplings larger than 2 J or 3 J also exist, but operate only in special situations. Couplings larger than 3 J (e.g., 4 J, 5 J, etc) are usually called “long-range coupling.” C C C HH 4 J, for instance, occurs mainly when the hydrogens are forced to adopt this “W” conformation (as in bicyclic compounds). LONG RANGE COUPLINGS

6 to 8 Hz 11 to 18 Hz 6 to 15 Hz 0 to 5 Hz three bond 3J3J two bond 2J2J three bond 3J3J 3J3J SOME REPRESENTATIVE COUPLING CONSTANTS H ax,H ax = 8 to 14 H ax,H eq = 0 to 7 H eq,H eq = 0 to 5 three bond 3J3J trans cis geminal vicinal

4 to 10 Hz 0 to 3 Hzfour bond 4J4J three bond 3J3J 0 to 3 Hzfour bond 4J4J cis trans 6 to 12 Hz 4 to 8 Hz three bond 3J3J Couplings that occur at distances greater than three bonds are called long-range couplings and they are usually small (<3 Hz) and frequently nonexistent (0 Hz).

OVERVIEW

TYPES OF INFORMATION TYPES OF INFORMATION FROM THE NMR SPECTRUM 1. Each different type of hydrogen gives a peak or group of peaks (multiplet). 3. The integral gives the relative numbers of each type of hydrogen. 2. The chemical shift (  in ppm) gives a clue as to the type of hydrogen generating the peak (alkane, alkene, benzene, aldehyde, etc.) 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.

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. SPECTROSCOPY IS A POWERFUL TOOL

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 enviroment of each hydrogen and the relative numbers of each type. EACH TECHNIQUE YIELDS VALUABLE DATA