H Nuclear Magnetic Resonance - Chemistrychemistry.syr.edu/totah/che575/support/3a1/4-1.HNMR.pdf ·...

1H Nuclear Magnetic Resonance

x-rays ultraviolet(UV) visible Infrared

(IR) microwaves radiowaves

High Frequency Low FrequencyMiddle Frequency

Frquency (MHz)

Sample Nuclei 1H, 19F 31P, 11B, 13C 2H, 14N

energy

frequency

wavelength


Atomic Mass, Number and Spin Quantum Number 1s

2p

2p


Atomic Mass, Number and Spin Quantum Number

nucleus qantum number, I

# of spin states

resonance at 100MHz

natural abundance

1H 1/2 2 100.0 99.98%

2H 1 3 15.5 0.02%

12C 0 0 0 98.9%

13C 1/2 2 25.3 1.1%

19F 1/2 2 94.1 100%

31P 1/2 2 40.5 100%

number of spin states allowed = 2I +1 +

not NMR active


Nuclei in a Magnetic Field

+

+

+

+

+

+

+

+

+

+

++

+

+

B0

AppliedMagneticField


Nuclei in Magnetic Field

+

++

+

+

+

ΔE = hv ΔE = hv

magnetic field (B0)

spin -1/2

spin + 1/2

ΔE = (hγ/2π)B0 = hv h = Planks constant γ = magnetogyric ratio



+ E1

E2

+

ΔE = (hγ/2π)B0 = hv


Highest Field NMR (2015)

!

A part of the recently developed 1,020 MHz-NMR system equipped with superconducting magnets (about 5 m high and weighing about 15 tons). This part contains coils made of a high-temperature superconductor. Liquid helium is used for cooling.

http://www.nims.go.jp/eng/news/press/2015/07/201507010.html




+ E1

E2

+

spin state populations 60MHz upper: 1,000,000

lower: 1,000,009 300MHz upper: 1,000,000

lower: 1,000,048

relaxation modes: 1. spin lattice-relaxation (T1): through bond thermal energy transfer. Usually between a C-H bond (most impt here)

2. transferal-relaxation (spin-spin relaxation) (T2): through space energy transfer of spin information. Usually H H.

no signal if # spin +1/2 = # spin -1/2 (saturation)! nuclei (to avoid, nuclei must return to ground state between Rf pulses)


Chemical


Chemical


Pulsed FT Spectrometer

4.73 Τ magnet


1H NMR Pulse Sequence

• A short (µs), powerful rf pulse (B1) of frequency ν1 is applied along x axis • Tips M0 (bulk magnitization vector) into xy plane for all protons at the same time • The detector is in the xy plane, so the “angle of the pulse” is important • Has implications for 2D NMR and other experiments


What Happens During the Pulse?

• Consider an M0 for three individual net magnetizations (e.g., 3 protons), each with their own Lamor frequency (ω) • The 90º pulse tips all into xy plane, but each proton precesses at its own ω relative to ν1. This can be slower or faster than ν1. • Resulting raw data known as the FID (free induction decay)


What Happens During the Pulse?

ω3

ω1

ω2


The FID


Signal to Noise

!f = frequency n = # scans

How To Increase Sensitivity/Signal

• use larger sample, increase conc. (increase # of spins, N)

• increase # of scans (ns)

• ↑B0 (increases ν)


Sample Preparation

• 5–40 mg typical → only µg needed for 1H NMR

• solvents: typically deuterated to avoid overlap with sample peaks - CCl4, CDCl3, C6D6, acetone-d6, DMSO-d6, CD2Cl2, CD3CN, etc. - CDCl3 has become the “standard” solvent

- single peak observed, - relatively far away from other peaks (δ 7.27 ppm)

- best solvents are those that are volatile and not very hygroscopic - No-D NMR, see: Hoye Org. Lett. 2004, 6, 953.

• standard: varies with nucleus of interest - for 1H and 13C: SiMe4 (TMS) = 0 ppm - solvent peak

Collecting Spectra


Interpretation of 1H NMR Spectra

important features:

chemical shift: describes position of the absorption (aka: peak, resonance) provides information on the chemical environment

integration: indicates how many protons are represented by each signal multiplicity: aka spin-spin splitting

indicates how many protons are on neighboring carbons

H3C SiCH3

CH3

CH3

tetramethylsilane (TMS)


Chemical Shift

Effect of Field Strength on Signal


Chemical Shift and the Delta Scale (ppm)

Effects of External Magnetic Field (MHz)


Chemical Shift and the Delta Scale (ppm)

At 60 MHz

At 300 MHz

δ =162 Hz60 MHz

= 2.70 ppm

δ =νH (Hz) - νTMS (Hz)ν applied field (MHz)

= shift in Hzfrequency in MHz

= ppm

1H scale: typically 0-10 ppm (some variation)

δ =810 Hz

300 MHz= 2.70 ppm


Chemical Shift

General Ranges

upfield (shielded)

downfield (deshielded)

increasing frequency

increasing field strength


Chemical Shift

Pavia Table 5.4

see also: Appendices 2, 3, and 4


Chemical Shift

Influences

proton chemical shift is influenced by: • bond polarity • hydbridization of the attached atom • presence of e- donating or e- withdrawing groups

diamagnetic shielding


Chemical Shift

electron density about a proton nucleus affects it's chemical shift

downfield upfield

nucleus has↓ e- density

larger effectivemagnetic field

nucleus has↑ e- density

smaller effectivemagnetic field

deshielded shielded

increasing field strength (Hz)

C HO H

requires moreenergy to cause

spin flip

3.5 2.1 2.5 2.1

increasing frequency

Bi Bi Heffective > H0 Heffective < H0


Chemical Shift

Electronegativity Effects

Compound CH3X CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si

Element X F O Cl Br I H Si

Electronegativity 4.0 3.5 3.1 2.8 2.5 2.1 1.8

Chemical Shift 4.26 3.40 3.05 2.68 2.16 0.23 0.0

Dependance of Chemical Shift of Element Electronegativity


Chemical Shift

Substituent Effects

Additivity of Effects (Not Exactly)

Molecule CH3Cl CH2Cl2 CHCl3

Chemical Shift 3.05 5.30 7.26

CH3O

CH3 Cl

CH3

δ 3.05

δ 3.24

CH2 ClCH3O

δ 3.51 δ 5.46


Chemical Shift

see also: Pavia, Appendix 6 calculation of chemical shifts


Chemical Shift

Electronegativity Effects

C C HC

CC C H

H

CC C H

H

HHH

3°(methine)

2°(methylene)

1°(methyl) cyclopropane

> > >

2 ppm 1 ppm 0 ppm

shift not due to electronegativity

Hybridization Effects

sp > sp2 > sp3

BUT! H

R> H > C C H

4.5 - 7 ppm 2 - 3 ppm 0 - 2 ppm


Chemical Shift

Resonance Effects

H

H

H

H

5.28 ppmOCH3

H4.1 ppm

H

H6.4 ppm

OCH3

H

H

H

H

7.8 ppmH

H3C5.8 ppm

OEtO

H

H

H3C

OEtO


Chemical Shift

Hydrogen Bonding

functional group proton chemical shift

carboxylic acid COOH 10.5-12.0 ppm

phenol ArOH 4-7 ppm

alcohol ROH 0.5-5 ppm

amine RNH2 0.5-5 ppm

amide CONH2 5-8 ppm

enol C=C-OH >15 ppm

R O H O H ORRH

δ+ δ+ δ+

O H

OR

OH

OR


Chemical Shift

systems not readily explained by prior discussion

aromatic rings

alkenes

aldehydes terminal alkynes

9.0 – 10.0 ppm 4.5 – 7.0 6.5 – 8.0 1.5 – 3.0

all have π-systems it's all about circulating electrons!


Chemical Shift

Magnetic Anisotropy

aromatic rings

H

7.3 ppm

circulating π electrons(ring current)

Binduced (Bi)induced magnetic field

B0

induced field reinforces theapplied field(deshielding)

shift influenced by: 1. applied magnetic field 2. valence electrons about H nucleus 3. anisotropy


Chemical Shift

Magnetic Anisotropy

+

+

( - )( - ) HH

shielded

deshielded

HHH

HHH

HH

H HH

H

H

HHH

H

H

H

H

H deshielded (8.9 ppm)H shielded (-1.8 ppm)

H

H deshielded (7.27 ppm)

HH

H deshielded (ca. 2 ppm)H shielded (-1.0 ppm)


Chemical Shift

Magnetic Anisotropy

Binduced (Bi)induced magnetic field

circulatingπ electrons

circulatingπ electrons

B0

alkenes and aldehydes

H

R H

O

R9-10 ppm4.5-7 ppm

HH

+

+

( - )( - ) ORH

+

+

( - )( - )


Chemical Shift

Magnetic Anisotropy 1.5-3 ppm

HR

alkynes

RH ++

( - )

( - )

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H Nuclear Magnetic Resonance - Chemistrychemistry.syr.edu/totah/che575/support/3a1/4-1.HNMR.pdf ·...

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