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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
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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 The two kinds of hydrogens do not appear as single peaks, rather there is a “triplet” and a “doublet”. integral = 2 integral = 1 The subpeaks are due to spin-spin splitting and are predicted by the n+1 rule. triplet doublet
1,1,2-Trichloroethane integral = 2 integral = 1 Where do these multiplets come from ? ….. interaction with neighbors
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
EXCEPTIONS TO THE N+1 RULE IMPORTANT ! Protons that are equivalent by symmetry usually do not split one another 1) no splitting if x=y no splitting if x=y Protons in the same group usually do not split one another 2) more detail later or
EXCEPTIONS TO THE N+1 RULE The n+1 rule applies principally to protons in aliphatic (saturated) chains or on saturated rings. 3) or YES YES but does not apply (in the simple way shown here) to protons on double bonds or on benzene rings. NO NO
SOME COMMON SPLITTING PATTERNS ( x = y ) ( x = y )
SOME EXAMPLE SPECTRA WITH SPLITTING
in higher multiplets the outer peaks are often nearly lost in the baseline 1:6:15:20:16:6:1 NMR Spectrum of 2-Nitropropane
NMR Spectrum of Acetaldehyde offset = 2.0 ppm
INTENSITIES OF MULTIPLET PEAKS PASCAL’S TRIANGLE
The interior entries are the sums of the two numbers immediately above. PASCAL’S TRIANGLE Intensities of multiplet peaks 1 singlet 1 1 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 one neighbor n+1 = 2 doublet one neighbor n+1 = 2 doublet H H H H C C C C 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
H H H H H H C C C C H H H H SPIN ARRANGEMENTS three neighbors n+1 = 4 quartet two neighbors n+1 = 3 triplet methylene spins methyl spins
THE COUPLING CONSTANT J 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.
Separation is larger FIELD COMPARISON 100 MHz 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 200 Hz 7.5 Hz The shift is dependant on the field J = 7.5 Hz ppm 3 2 1
100 MHz 200 Hz 100 Hz J = 7.5 Hz J = 7.5 Hz 6 5 4 3 2 1 200 MHz Separation is larger 400 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. 200 Hz J = 7.5 Hz ppm 6 5 4 3 2 1
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. C H H C C 4J , for instance, occurs mainly when the hydrogens are forced to adopt this “W” conformation (as in bicyclic compounds). 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 6 to 15 Hz three bond 3J cis 0 to 5 Hz two bond 2J geminal Hax,Hax = 8 to 14 Hax,Heq = 0 to 7 three bond 3J Heq,Heq = 0 to 5
cis 6 to 12 Hz three bond 3J trans 4 to 8 Hz 4 to 10 Hz three bond 3J 0 to 3 Hz 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) and frequently nonexistent (0 Hz).
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 enviroment of each hydrogen and the relative numbers of each type.