PowerPoint Presentation: SPIN-SPIN SPLITTING
PowerPoint Presentation: Often a group of hydrogens will appear as a multiplet rather than as a single peak. SPIN-SPIN SPLITTING 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 .
PowerPoint Presentation: integral = 2 integral = 1 triplet doublet 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.
PowerPoint Presentation: n + 1 RULE
1,1,2-Trichloroethane: 1,1,2-Trichloroethane integral = 2 integral = 1 Where do these multiplets come from ? ….. interaction with neighbors
PowerPoint Presentation: 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
PowerPoint Presentation: EXCEPTIONS TO THE N+1 RULE IMPORTANT ! Protons that are equivalent by symmetry usually do not split one another no splitting if x=y no splitting if x=y 1) 2) Protons in the same group usually do not split one another or more detail later
PowerPoint Presentation: 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 YES YES
PowerPoint Presentation: SOME COMMON PATTERNS
PowerPoint Presentation: SOME COMMON SPLITTING PATTERNS ( x = y ) ( x = y )
PowerPoint Presentation: SOME EXAMPLE SPECTRA WITH SPLITTING
NMR Spectrum of Bromoethane: NMR Spectrum of Bromoethane
NMR Spectrum of 2-Nitropropane: 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: NMR Spectrum of Acetaldehyde offset = 2.0 ppm
PowerPoint Presentation: INTENSITIES OF MULTIPLET PEAKS PASCAL’S TRIANGLE
PowerPoint Presentation: 1 2 1 PASCAL’S TRIANGLE 1 1 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1 1 6 15 20 15 6 1 1 7 21 35 35 21 7 1 singlet doublet triplet quartet quintet sextet septet octet The interior entries are the sums of the two numbers immediately above. Intensities of multiplet peaks
PowerPoint Presentation: THE ORIGIN OF SPIN-SPIN SPLITTING HOW IT HAPPENS
PowerPoint Presentation: C C H H C C H H A A upfield downfield B o 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 aligned neighbor opposed +1/2 -1/2
PowerPoint Presentation: C C H H C C H H 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.
PowerPoint Presentation: two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet SPIN ARRANGEMENTS methylene spins methine spins
PowerPoint Presentation: three neighbors n+1 = 4 quartet two neighbors n+1 = 3 triplet SPIN ARRANGEMENTS C C H H H H H C C H H H H H methyl spins methylene spins
PowerPoint Presentation: THE COUPLING CONSTANT
PowerPoint Presentation: J J J J J THE COUPLING CONSTANT 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
PowerPoint Presentation: 100 MHz 200 MHz 1 2 3 4 5 6 1 2 3 100 Hz 200 Hz 200 Hz 400 Hz J = 7.5 Hz J = 7.5 Hz 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
PowerPoint Presentation: 100 MHz 200 MHz 1 2 3 4 5 6 1 2 3 100 Hz 200 Hz J = 7.5 Hz J = 7.5 Hz ppm 4 200 Hz 400 Hz 5 6 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
PowerPoint Presentation: 1 2 3 1 2 3 100 MHz 200 MHz Why buy a higher field instrument? Spectra are simplified! Overlapping multiplets are separated. Second-order effects are minimized. 1 2 3 50 MHz J = 7.5 Hz J = 7.5 Hz J = 7.5 Hz
PowerPoint Presentation: 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 ) 3 J 2 J
PowerPoint Presentation: 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 H H 4 J , for instance, occurs mainly when the hydrogens are forced to adopt this “W” conformation (as in bicyclic compounds). LONG RANGE COUPLINGS
PowerPoint Presentation: 6 to 8 Hz 11 to 18 Hz 6 to 15 Hz 0 to 5 Hz three bond 3 J two bond 2 J three bond 3 J three bond 3 J 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 3 J trans cis geminal vicinal
PowerPoint Presentation: 4 to 10 Hz 0 to 3 Hz four bond 4 J three bond 3 J 0 to 3 Hz four bond 4 J cis trans 6 to 12 Hz 4 to 8 Hz three bond 3 J 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).
PowerPoint Presentation: OVERVIEW
PowerPoint Presentation: 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 ( d, 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.
PowerPoint Presentation: 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
PowerPoint Presentation: 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