TYPES OF INFORMATION FROM NMR SPECTRUM

Size: px

Start display at page:

Download "TYPES OF INFORMATION FROM NMR SPECTRUM"

Roy Greene
7 years ago
Views:

1 TYPES OF INFORMATION FROM NMR SPETRUM 1. Each different type of hydrogen gives a peak or group of peaks (multiplet). 2. The chemical shift (δ, 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.

2 EMIAL SIFT

3 DIAMAGNETI ANISOTROPY SIELDING BY VALENE ELETRONS

4 Diamagnetic Anisotropy Applied magnetic field induces circulation of valence electrons - this generates a magnetic field that opposes the applied field. valence electrons shield the nucleus from the full effect of the applied field magnetic field lines B o applied B induced (opposes B o )

5 PROTONS DIFFER IN TEIR SIELDING All different types of protons in a molecule have a different amounts of shielding. They all respond differently to the applied magnetic field and appear at different places in the spectrum. DOWNFIELD deshielded protons appear here. SPETRUM UPFIELD shielded protons appear here.

6 PEAKS ARE MEASURED RELATIVE TO TMS Rather than measure exact position of a peak, we measure how far downfield it is shifted from TMS. 3 3 Si 3 3 reference compound tetramethylsilane TMS ighly shielded protons appear way upfield. n shift in z downfield TMS 0

7 TE EMIAL SIFT The shifts from TMS in z are bigger in higher field instruments (300 Mz, 500 Mz) than they are in the lower field instruments (100 Mz, 60 Mz). We can adjust the shift to a field-independent value, the chemical shift in the following way: parts per million chemical shift = δ = shift in z spectrometer frequency in Mz = ppm This division gives a number independent of the instrument used. A particular proton in a given molecule will always have the same chemical shift (constant value).

8 NMR orrelation hart DOWNFIELD -O -N UPFIELD DESIELDED l 3, SIELDED TMS δ (ppm) ROO RO = 2 F 2 l 2 Br 2 I 2 O 2 NO 2 2 Ar 2 NR 2 2 S - = O Ranges can be defined for different general types of protons. This chart is general, the next slide is more definite.

9 DESIELDING AND ANISOTROPY Three major factors account for the resonance positions (on the ppm scale) of most protons. 1. Deshielding by electronegative elements. 2. Anisotropic fields usually due to pi-bonded electrons in the molecule. 3. Deshielding due to hydrogen bonding.

10 DESIELDING BY ELETRONEGATIVE ELEMENTS

11 DESIELDING BY AN ELETRONEGATIVE ELEMENT δ- δ+ l electronegative element δ- δ+ NMR ART hlorine deshields the proton, it takes valence electron density away from carbon, which in turn takes more density from hydrogen deshielding the proton. deshielded protons appear downfield More shielded protons appear upfield deshielding moves proton resonance to lower field

12 Electronegativity Dependence of hemical Shift Dependence of the hemical Shift of 3 X on the Element X ompound 3 X Element X Electronegativity of X hemical shift δ 3 F 3 O 3 l 3 Br 3 I 4 ( 3 ) 4 Si F O l Br I Si most deshielded deshielding increases with the electronegativity of atom X TMS

13 Substitution Effects on hemical Shift most deshielded l 3 2 l 2 3 l ppm The effect increases with greater numbers of electronegative atoms. most deshielded - 2 -Br Br Br ppm The effect decreases with incresing distance.

14 ANISOTROPI FIELDS DUE TO TE PRESENE OF PI BONDS The presence of a nearby pi bond or pi system greatly affects the chemical shift. Benzene rings have the greatest effect.

15 Ring urrent in Benzene irculating π electrons Deshielded fields add together B o Secondary magnetic field generated by circulating π electrons deshields aromatic protons

16 ANISOTROPI FIELD IN AN ALKENE Deshielded fields add protons are deshielded shifted downfield = B o secondary magnetic (anisotropic) field lines

17 YDROGEN BONDING

18 YDROGEN BONDING DESIELDS PROTONS R O O R O R The chemical shift depends on how much hydrogen bonding is taking place. Alcohols vary in chemical shift from 0.5 ppm (free O) to about 5.0 ppm (lots of bonding). ydrogen bonding lengthens the O- bond and reduces the valence electron density around the proton - it is deshielded and shifted downfield in the NMR spectrum.

19 SOME MORE EXTREME EXAMPLES R O O O O R arboxylic acids have strong hydrogen bonding - they form dimers. With carboxylic acids the O- absorptions are found between 10 and 12 ppm very far downfield. 3 O O O In methyl salicylate, which has strong internal hydrogen bonding, the NMR absortion for O- is at about 14 ppm, way, way downfield. Notice that a 6-membered ring is formed.

20 SPIN-SPIN SPLITTING

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

22 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 l l integral = 1 l triplet doublet The subpeaks are due to spin-spin splitting and are predicted by the n+1 rule.

23 n + 1 RULE

24 1,1,2-Trichloroethane integral = 2 l l integral = 1 l Where do these multiplets come from? Ö.. interaction with neighbors

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

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

27 SOME OMMON PATTERNS

28 SOME OMMON SPLITTING PATTERNS X Y 3 ( x = y ) X 2 2 Y 3 ( x = y ) 3

29 SOME EXAMPLE SPETRA WIT SPLITTING

30 NMR Spectrum of Bromoethane Br 2 3

31 NMR Spectrum of 2-Nitropropane 3 3 O N+ O - 1:6:15:20:16:6:1 in higher multiplets the outer peaks are often nearly lost in the baseline

32 NMR Spectrum of Acetaldehyde 3 O offset = 2.0 ppm

33 INTENSITIES OF MULTIPLET PEAKS PASAL S TRIANGLE

34 PASALíS TRIANGLE Intensities of multiplet peaks 1 singlet The interior entries are the sums of the two numbers immediately above doublet triplet quartet quintet sextet septet octet

35 TE OUPLING ONSTANT

36 TE OUPLING ONSTANT J J J J J J The coupling constant is the distance J (measured in z) between the peaks in a multiplet. J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet.

37 NOTATION FOR OUPLING ONSTANTS The most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms. 3 J 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 ( most often 2 J = 0 ) Geminal coupling does not occur when the two hydrogens are equivalent due to rotations around the other two bonds. 2 J

38 SOME REPRESENTATIVE OUPLING ONSTANTS vicinal 6 to 8 z three bond 3 J trans 11 to 18 z three bond 3 J cis geminal 6 to 15 z 0 to 5 z three bond two bond 3 J 2 J ax ax,ax = 8 to 14 eq ax,eq = 0 to 7 three bond 3 J eq ax eq,eq = 0 to 5

39 OVERVIEW

40 TYPES OF INFORMATION FROM TE NMR SPETRUM 1. Each different type of hydrogen gives a peak or group of peaks (multiplet). 2. The chemical shift (δ, 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.

Used to determine relative location of atoms within a molecule Most helpful spectroscopic technique in organic chemistry Related to MRI in medicine

Structure Determination: Nuclear Magnetic Resonance CHEM 241 UNIT 5C 1 The Use of NMR Spectroscopy Used to determine relative location of atoms within a molecule Most helpful spectroscopic technique in