Proton NMR Spectroscopy

The NMR Phenomenon Most nuclei possess an intrinsic angular momentum, P. Any spinning charged particle generates a magnetic field. P = [I(I+1)] 1/2 h/2  where  spin quantum # I = 0, 1/2, 1, 3/2, 2, …

Which nuclei have a “spin”? If mass # and atomic # are both even, I = 0 and the nucleus has no spin. e.g. Carbon-12, Oxygen-16 For each nucleus with a spin, the # of allowed spin states can be quantized: For a nucleus with I, there are 2I + 1 allowed spin states. 1 H, 13 C, 19 F, 31 P all have I = 1/2  E =  h/2  )Bo

Spin states split in the presence of B 0

When a nucleus aligned with a magnetic field, B 0, absorbs radiation frequency (Rf), it can change spin orientation to a higher energy spin state. By relaxing back to the parallel (+1/2) spin state, the nucleus is said to be in resonance. Hence, NMR

Presence of Magnetic Field

NMR instruments typically have a constant Rf and a variable B 0. A proton should absorb Rf of 60 MHz in a field of 14,093 Gauss ( T). Each unique probe nucleus ( 1 H perhaps) will come into resonance at a slightly different - and a very small percentage of - the Rf. All protons come into resonance between 0 and 12/1,000,000 (0 – 12 ppm) of the R f.

Energy Difference (  E) Between Two Different Spin States of a Nucleus With I=1/2

What Does an NMR Spectrum Tell You? # of chemically unique H’s in the molecule # of signals The types of H’s that are present e.g. aromatic, vinyl, aldehyde … chemical shift The number of each chemically unique H integration The H’s proximity to eachother spin-spin splitting

How many signals in the Proton NMR Spectrum?

© 2013 Pearson Education, Inc. Chapter 1312 Shielded Protons A naked proton will absorb at 70,459 gauss. A shielded proton will not absorb at 70,459 gauss so the magnetic field must be increased slightly to achieve resonance.

Chemical Shift The variations in the positions of NMR absorptions, arising from electronic shielding and deshielding, are called chemical shifts. The chemical shift (in ppm) is independent of the spectrometer used. Most common scale is the  (delta) scale.

Typical 1H NMR Scale is 0-10 ppm

The  Scale

Tetramethylsilane (TMS)

© 2013 Pearson Education, Inc. Chapter 1322 Electronegative Atoms More electronegative atoms deshield more and give larger shift values. Additional electronegative atoms cause an increase in chemical shift.

© 2013 Pearson Education, Inc. Chapter 1323 Location of Electronegative Atoms The deshielding effect of an electronegative substituent drops off rapidly with distance.

Chemical Shift Ranges, ppm

Diamagnetic Anisotropy Shielding and Deshielding

Deshielding in Alkenes

Shielding in Alkynes

Chemical Shift is Affected by Electron Density Around Nucleus

from chemistry.msu.edu

© 2013 Pearson Education, Inc. Chapter 1331 Number of Signals Methyl tert-butyl ether has two types of protons, giving two NMR signals. Chemically equivalent hydrogens have the same chemical shift. All the methyl groups of the tert-butyl group are equivalent, and they produce only one signal.

© 2013 Pearson Education, Inc. Chapter 1332 Intensity of Signals: Integration The amount the integral trace rises is proportional to the area of that peak. The integration will have a trace for the tert-butyl hydrogens that is three times as large as the trace for the methyl hydrogens. The relative area for methyl and tert- butyl hydrogens is 1:3.

© 2013 Pearson Education, Inc. Chapter 1333 tert-Butyl Acetoacetate The spectrum of tert-butyl acetoacetate has only three signals. The most shielded protons are the methyl groups of the tert-butyl. The most deshielded signal is the methylene (CH 2 ) because it is in between two carbonyl groups.

Toluene at Higher Field

Integral Trace How many protons give rise to each signal?

Spin-Spin Splitting

The Doublet in 1 H NMR

H b in 1,1,2-Tribromoethane

The Triplet in 1 H NMR

H a in 1,1,2-Tribromoethane

1,1,2-Tribromoethane

The Quartet in 1 HMR

The N + 1 Rule If a proton is coupling to N equivalent protons, (on adjacent atoms) it is split into N + 1 peaks.

Equivalent protons do not split each other. Protons bonded to the same carbon will split each other if they are nonequivalent. Protons on adjacent carbons normally will split each other. Protons separated by four or more bonds will rarely split each other. Spin-Spin Splitting Distance

Predict Splitting

1,1-Dichloroethane

Ethyl benzene

Methyl Isopropyl Ketone

1-Nitropropane

Ethyl, propyl and isopropyl groups are common. Learn to recognize them from their splitting patterns.

2-Methyl-1-propanol

Propose a structure for the compound of molecular formula C 4 H 10 O whose proton NMR spectrum follows. Solved Problem 3 Solution

Solved Problem 3 (Continued) Solution (Continued)

Para Nitrotoluene

C-13 NMR Spectroscopy

C-13 chemical shifts

Coupling in C-13 NMR

Butanone - Coupled and Decoupled

1,2,2-Trichloropropane 1 H and 13 C NMR Spectra

Coupled C-13 NMR Spectrum