Magnetic Nuclei other than 1 H

Transcription

1 Magnetic Nuclei other than 1 H 2 H (Deuterium): I = 1 H,D-Exchange might be used to simplify 1 H-NMR spectra since H-D couplings are generally small; O D 2 -O- triplet of triplets slightly broadened triplet 31 P: I = 1/2 (100% natural abundance) hemical shift range between 270 to -480 ppm; Large P-H ( 1 J) coupling constants between Hz; 19 F: I = 1/2 (100% natural abundance) hemical shift range between 276 to -280 ppm; coupling constants F-H = Hz; 29 Si: I = 1/2 (100% Natural abundance) Si-H coupling constant is about 6 Hz; only low intensity (satellites) (Si-H is about 215 Hz) 13 : I = 1/2 (1.1% Natural abundance) -H coupling (about Hz) is not resolved unless the molecule is enriched with F,H coupling (I = ½) Fluoroacetone, H 3 O F 2 J 4 J

2 13 -NMR Spectroscopy Some Facts: 12 is not magnetically active but 13 is (I = ½); its natural abundance is 1.1%; The low abundance of 13 causes a sensitivity problem (only 1/5700 of 1 H), which has been overcome with the development of Fourier Transform (FT) NMR instrumentation in the 1970 s; However, higher concentrations are usually used for solution NMR (10 mg in 0.5 ml of solvent for a 5 mm tube); 13 chemical shifts are reported relative to TMS; 300 MHz for 1 H-NMR equals 75.5 MHz for 13 -NMR; Peak splittings due to couplings with protons are usually removed by broadband decoupling in a double resonance experiment; Broadband decoupling can also enhance the 13 signal intensity caused by the Nuclear Overhauser Effect (NOE); The range of chemical shift values is much wider than for 1 H (typically between ppm); Therefore, chemical shift is of high analytical value in 13 -NMR; Peak Intensities in 13 -NMR Relaxation times in 13 -NMR vary over a wide range so that peak areas do not integrate for the correct number of nuclei; Long delays between each acquisition would resolve this problem but the required measurement time is prohibitive; NOE response is not uniform for all atom environments; arbon atoms without protons attached to them have low intensities because of the missing NOE and for other reasons; Substitution of H by D results in decreased intensity of the 13 signal; Deuterium has I = 1 so that a 13 signal is split into 3 lines ratio 1:1:1 when coupled to one deuterium (possible spin states of D are -1, 0, +1; D 3 exhibits a 1:1:1 triplet in 13 -NMR!) Double Resonance: Spin-Spin Decoupling irradiate H 3 triplet - sextet - triplet H 3 triplet - quartet H 3 irradiate H 3 singlet - singlet H 3 irradiate H 3 triplet - triplet Protons can be readily decoupled individually if their deferens in resonance frequency (chemical shift) is 100 Hz;

3 13 -NMR of diethylphthalate proton coupled q of t (q not resolved) t of q (t not resolved) why triplet? 13 { 1 H} NMR of diethylphthalate proton decoupled why low intensity? 13 { 1 H}-NMR of diethylphthalate proton decoupled with 10 seconds delay

4 hemical Shifts in 13 -NMR arbon chemical shifts parallel (generally) proton shifts but with a much broader range Number of different aromatic 13 resonances in substituted benzene molecules Br Br 6 Br Group Specific hemical Shifts in 13 -NMR Diamagnetic shielding (electrons in s- and p-orbitals) and paramagnetic shielding (electrons in p-orbitals with angular momentum) contribute to the shift of -atoms. Well defined for acyclic, saturated hydrocarbons alculation of 13 shifts δ = A i n i Replacement of hydrogen causes a relative constant shift that depends primarily on the electronegativity of X. corrections for branching δ Methane = -2.5 ppm replacement of H by (H 3,, H, ) causes a shift of +9.1 ppm in the α- position, +9.4 ppm in the β-position, and -2.5 ppm in the γ-position;

5 Functional group X attached to internal carbon X β γ α All tables from Silverstein & Webster Functional group X attached to terminal carbon β γ α X t-butyl alcohol

6 2,2,4-trimethyl-1,3-pentanediol ycloalkanes The equation for alkanes is based on a weighted average for open chain conformers. A complete new set of parameters is necessary for cyclic alkanes as the average conformation is different. yclopropane = -2.6, cyclobutane = 23.3, and cyclohexane = 27.7; all other rings have 27.7 ± 2 ppm H 3 H 3 β-ax = 5.2 β-gem = -1.2; extra correction factor for branching at β- (2 methyl groups); H 3 β-eq = (2 x 8.9) = 49.5 (obs. 49.9) Alkenes hemical shift ranges of alkene carbon atoms strongly depend on their degree of substitution = = ppm; =HR = ppm; =R 2 = ppm; α,β,γ represent substituents on the same end of the double bond while α, β, γ are on the far side. Example:

7 Starting with benzene = omprehensive Table of aromatic 13 -NMR chemical shift increments Example alculations (Predictions) δ 1 = (-2.5x2)+0.3 = 11.3 [1α,1β,2γ,1δ] δ 2 = -2.5+(9.1x2)+(9.4x2)+(-2.5)+(-2.5) = 29.5 [2α,2β,1γ,2º(3º)] δ 3 = -2.5+(9.1x3)+(9.4x2)+(-3.7x2) = 36.2 [3α,2β,3º(2º)] δ = = 43 (obs. 42) δ = = 35 (obs. 34) -M effect O NO M effect 84.2 O Starting with ethene = Starting with benzene = or (depending on table)

8 Distortionless Enhancement by Polarization Transfer (DEPT) H H, H 3 onventional 13 -NMR α-terpinene