Nuclear Shielding and 1. H Chemical Shifts. 1 H NMR Spectroscopy Nuclear Magnetic Resonance

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1 NMR Spectroscopy Nuclear Magnetic Resonance Nuclear Shielding and hemical Shifts What do we mean by "shielding?" What do we mean by "chemical shift?" The electrons surrounding a nucleus affect the effective magnetic field sensed by the nucleus Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. 0 Shielding Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. The direction of the induced magnetic field is opposite to that of the applied field. The induced field shields the nuclei (in this case, and ) from the applied field. A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. 0 0

2 hemical Shift hemical Shift hemical shift is a measure of the degree to which a nucleus in a molecule is shielded. Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. hemical shifts (δ)( ) are measured relative to the protons in tetramethylsilane (TMS) as a standard. The reference point is defined by the position of TMS (δ) as zero δ = 3 3 Si position of signal - position of TMS peak spectrometer frequency 3 3 x Downfield Decreased shielding Upfield Increased shielding δ 7.28 l l l ( 3 ) 4 Si (TMS) hemical shift (δ,(, ) measured relative to TMS hemical shift (δ,(, ) Effects of Molecular Structure on hemical Shifts protons in different environments experience different degrees of shielding and have different chemical shifts The chemical shift is independent of the operating frequency of the spectrometer

3 hemical Shift Example: The signal for the proton in chloroform (l 3 ) appears 456 z downfield from TMS at a spectrometer frequency of 200 Mz. δ = δ = position of signal - position of TMS peak 456 z - 0 z δ = x 0 6 x spectrometer frequency x 0 6 x 0 6 Question What is the chemical shift of a proton that appears at 2065 z when recorded on a 300 Mz NMR spectrometer? A) 6.88 B) 0.4 ) 2.07 D).40 Electron withdrawal produces NMR signals downfield at higher frequency (at larger δ values) Electronegative substituents decrease the shielding of methyl groups 3 F N( 3 ) Si( 3 ) 3 δ 4.3 δ 3.2 δ 2.2 δ 0.9 δ 0.0 Electronegative substituents decrease the shielding of methyl groups Effect is cumulative 3 F N( 3 ) Si( 3 ) 3 δ 4.3 least shielded δ 3.2 δ 2.2 δ 0.9 δ 0.0 most shielded l l 3 2 l 2 3 l δ 7.3 δ 5.3 δ 3.

4 Question Which proton is most shielded? A) l 3 Methyl, Methylene, and Methine 3 more shielded than 2 ; 2 more shielded than B) 2 l 2 ) Br 3 D) Br 4 3 δ δ.6 3 δ δ.2 2 δ Question 3 Select the most shielded proton in,,2- trichlorobutane. A) B) 2 ) 3 D) 4 Protons attached to sp 2 hybridized carbon are less shielded than those attached to sp 3 hybridized carbon 3 3 δ 7.3 δ 5.3 δ 0.9 Diamagnetic Anisotropy The π electrons are less tightly held by the nuclei than are σ electrons; therefore they are more free to move in response to a magnetic field But Protons Attached to sp ybridized arbon are More Shielded than those Attached to sp 2 ybridized arbon δ 5.3 δ auses unusual chemical shifts for hydrogen bonded to carbons that form π bonds

5 Protons Attached to Benzylic and Allylic arbons are Somewhat Less Shielded than Usual Question 3 3 δ.5 δ 0.8 δ.2 3 δ δ 0.9 δ δ 0.9 Assign the chemical shifts δ.6, δ 2.2, and δ 4.8 to the appropriate protons of methylene cyclopentane. A) x =.6; y = 2.2; z = 4.8 B) x = 4.8; y =.6; z = 2.2 ) x =.6; y = 4.8; z = 2.2 D) x = 2.2; y =.6; z = 4.8 Proton Attached to = of Aldehyde is Most Deshielded Question 3 δ δ. δ 9.7 Assign the chemical shifts δ., δ2.4, and δ9.7 to the appropriate protons of 2-methylpropanal. A) x = 2.4; y =.; z = 9.7 B) x =.; y = 9.7; z = 2.4 ) x = 9.7; y = 2.4; z =. D) x =.; y = 2.4; z = 9.7 hemical Shift Table hemical Shift Table Type of proton hemical shift (δ),( Type of proton hemical shift (δ),( Type of proton hemical shift (δ),( Type of proton hemical shift (δ),( R Ar l Ar Br NR

6 hemical Shift Table Type of proton hemical shift (δ),( NR R Ar haracteristic Values of hemical Shifts NMR spectrum / Integration -bromo-2,2-dimethylpropane

7 Integration NMR spectrum / Integration -bromo-2,2-dimethylpropane The area under each signal is proportional to the number of protons that give rise to that signal. The height of each integration step is proportional to the area under a specific signal. (2) 4.5 (9) The integration tells us the relative number of protons that give rise to each signal, not absolute number.

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