Spin-Spin Coupling. H b1 H 3 C C Br. Review: 1 H- 1 H Coupling

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1 Review: 1-1 Coupling b1 3 C C Br b2 multiplicity: n + 1 rule can determine peak intensities by considering nuclear spin probabilities on adjacent hydrogens or use Pascal's triangle Coupling Constants (J) singlet (s) doublet (d) triplet (t) quartet (q) quintet/pentet 1 : 2 : 1 # adj protons multiplicity ratio of intensities C 3 C 2 Br J = 7.2 z J = 7.2 z 3 J 2 J

2 Mechanism of Coupling Fermi Contact Interactions a C b C 1 nuclear magnetic moment polarizes 1s electron spin of 1 a parallel spin (Pauli exclusion principle and und's rule) C 1 b electron spin polarizes nucear spin C nuclear spin electron spin

3 Coupling Constants A Little Bit About J Coupling constant, J, is measured in z Coupling constant, J, is independent of magnetic strength! Coupling constant, J, is the same in both directions!

4 Coupling Constants A Little Bit About J Coupling is most often vicinal, but geminal and long range coupling may also be observed! Magnitude of coupling constant, J, depends in large part on distance between the two nuclei.!

5 Coupling Constants A Little Bit About J Coupling is only observed between protons that are magnetically non-equivalent WAT DES TAT MEAN??!!

6 Chemical Equivalence Features of Chemically Equivalent Nuclei Chemically Equivalent Nuclei: experience identical chemical environments. are related by symmetry e.g. a rotational axis or mirror plane (or by a rapid exchange mechanism) have identical chemical shifts in 1 and 13 C NMR do not split each other; e.g. do not couple (most of the time) 3 C C 3 Cl Cl tbu tbu 2 C C 2 plane of symmetry axis of symmetry

7 Chemical Equivalence acetone 1,2-dichloroethane 3 C C 3 Cl Cl trans-2,3-di-t-butylcyclopropanone fumaric acid tbu tbu 2 C C 2

8 Magnetic Equivalence Features of Magnetically Equivalent Nuclei Magnetically Equivalent Nuclei: experience identical magnetic environments must have same chemical shift (isochronous) must have same J values to all other nuclei in the molecule do not couple to each other a b F a F b a / b are chemically equivalent

9 Magnetic Equivalence Features of Magnetically Equivalent Nuclei Magnetically Equivalent Nuclei: experience identical magnetic environments must have same chemical shift (isochronous) must have same J values to all other nuclei in the molecule do not couple to each other a b F a F b a / b are magnetically non-equivalent

10 N 2 Magnetic Equivalence a a chloronitrobenzene b Cl b

11 Magnetic Equivalence a N 2 a' chloronitrobenzene b Cl b' a / a b / b

12 Spin-Spin Coupling Simple Systems (First rder) individual protons are coupled to neighbors; J values are the same Pavia Appendix 5: Typical Coupling Constants

13 Spin-Spin Coupling Some First rder Splitting Patterns Entry Structure Notation and Pattern for a Entry Structure Notation and Pattern for a Entry Structure Notation and Pattern for a

14 Chemical Equivalence c b a C 3 C 2 C 2 Cl Free Rotation Cl a a b b b b b b a a a Cl Cl a C 3 C 3 C 3 Cool to -100 C rotation slows b Cl b' a a' C 3

15 Chemical Equivalence Proton/Group Relationships omotopic: have same electronic environment; are always equivalent and have same chemical shift replacing either proton with another group (A) leads to an identical compound replace a replace b a b A b a A X X X X X X identical examples: 3 C C 3 Cl Cl

16 Chemical Equivalence Proton/Group Relationships Enantiotopic: equivalent in an achiral environment (e.g. in a simple solvent like CDCl 3 ) will have same chemical shift under "normal" conditions will not couple to each other under "normal" conditions coupling constants to other nuclei will be the same BUT: are not equivalent in a chiral environment (e.g when dissolved in a chiral solvent or in presence of chiral shift reagent) replacing either proton with another group (A) leads to enantiomers replace a replace b a b A b a A X Y (X Y) X Y X Y prochiral enantiomers examples: Cl Br 3 C C 3

17 Chemical Equivalence Enantiomers in a Chiral Environment N 2 (±)-t-leucine C 3 CF 3 (+)-MTPA α-methyl-α-(trifluoromethyl) phenylacetic acid F Me F F F F (+)-PFE 1-pentafluoro ethanol C 3 CF 3 N 2 C 3 CF 3 N 2

18 Chemical Equivalence Proton/Group Relationships Diastereotopic: not-equivalent cannot be exchanged by any symmetry element have different chemical shifts have different J values when couple to other nuclei will often couple to one another (geminal coupling) replacing either proton with another group (A) leads to a pair of diastereomers (different) replace a replace b a b A b a A X Y* (X Y) X Y* X Y* prochiral diastereomers * = stereocenter present examples: Cl * C 3 C 3 2 C * C 3 C 2 3 C C 3 C 3 *

19 Chemical Equivalence Proton/Group Relationships What about indicated protons in these molecules? Ph Ph??

20 Chemical Equivalence Proton/Group Relationships Group a / b Relationship Examples 1 NMR: Appearance in Achiral Solvent 1 NMR: Appearance in Chiral Solvent

21 Chemical Equivalence Proton/Group Relationships Group a / b Relationship Examples 1 NMR: Appearance in Achiral Solvent 1 NMR: Appearance in Chiral Solvent

22 Diastereotopic ydrogens a c d b f 2-methylbutyraldehyde c C C 3 C c' C C 3 c' c' c c C 3 C 3 C 3 C 3

23 Diastereotopic ydrogens a c d b f 2-methylbutyraldehyde c C C 3 C c' C C 3 c' c' c c C 3 C 3 C 3 C 3

24 Diastereotopic ydrogens 3-methyl-2-pentanone a c e b C 3 d

25 Diastereotopic Methyl Groups (S)-(+)-3-methyl-2-butanol a, b e f c d C 3 C 3 C 3 C 3 C 3 C 3 C 3 C 3 C 3

26 Diastereotopic Methyl Groups (S)-(+)-3-methyl-2-butanol a, b e f c d

27 Chemical Equivalence Proton/Group Relationships g h f k i e j d c b a