SPECTROSCOPIC METHODS OF ORGANIC MOLECULES. Basics... · 2019-02-12 · SPECTROSCOPIC METHODS OF ORGANIC MOLECULES Professor Ioannis P. Gerothanassis Department of Chemistry, University

SPECTROSCOPIC METHODS

OF ORGANIC MOLECULES

Professor Ioannis P. GerothanassisDepartment of Chemistry, University of Ioannina

FEBRUARY 2018

SPECTROSCOPIC METHODS

OF ORGANIC COMPOUNDS

Professor Ioannis P. GerothanassisDepartment of Chemistry, University of Ioannina

Concepts and Applications of NMR

What is NMR ?

• A spectroscopic technique that gives information aboutthe number and types of atoms in a molecule.

• Nuclear magnetic resonance spectroscopy is a powerfulanalytical technique used to characterize molecules byidentifying e.g. hydrogen-carbon frameworks withinmolecules.

MagneticNuclear Resonance

In the Nucleus Involves

MagnetsAt the Nucleus

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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/

Nuclear Spin• A nucleus with an odd atomic number or an odd

mass number has a nuclear spin.

MagneticNuclear

• The spinning charged nucleus generates a magnetic field e.g. acts like a bar magnets.

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Precessional Motion:Consider a spinning top, in which the spinning axis of the top moves slowly around the vertical. This is precessional motion and the top is said to be precessing around the vertical axis of earth’s gravitational field.

Precessional Frequency:Likewise the proton (tiny magnet) precesses about the axis of the external magnetic field in the same manner as above.


The spinning proton in an external magnetic field

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The nuclear magnetic resonance process; absorption occurs

when ν =ω

ΒΟ


Two Energy States in Quantum Chemical

Terms

The magnetic fields of the spinning nuclei will align either with the external field, or against the field.

An r.f. with the right amount of energy can be absorbed and cause the spinning proton to flip.

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E, Magnet Strength and

The energy difference ΔΕ is proportional to the magnetic field strength B0 and .

E = h = h B0

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• Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1gauss-1 for 1H).

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E

Bo

500 MHz

11.74 T2.35 T

100 MHz

60 MHz

1.41 T

E = h/2(Bo)

Rf Field vs. Magnetic Field for a Proton

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Rf / values for Some Nuclei

/2 (MHz/T)Bo (T)Rf (MHz)Nuclei

500.00 11.74 42.58

125.74 11.74 10.71

76.78 11.74 6.54

470.54 11.74 40.08

202.51 11.74 17.25

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1H

13C

2H

19F

31P


Where Nuclei Resonate at 11.74T

470.54

~220 ppm

19F (CFCl3)

12 ppm1H (TMS)

500.00

76.78

12 ppm2H

220 ppm

13C

125.74

500 MHz400300200100

RCO2H

CH4

-SO2F -CF2-CF2-

202.51

~380 ppm

31P (H3PO4)

CPH2PX3

��RCH3R2C=O

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http://textlab.io/doc/9993536/the-basics-of-nmr--powerpoint-

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Magnetic Shielding

• If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained.

• But protons are surrounded by electrons that shield them from the external field.

• Circulating electrons create an induced magnetic field that opposes the external magnetic field.

Beff=Bexternal-Bshielding

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http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_13/Wade13.ppt

• Such shifting in position of NMR absorption signals whicharise due to the shielding or deshielding of proton bysurrounding electrons is called the Chemical shift.

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Shielding or Deshielding Protons in a Molecule

• Depending on electronic environment protons inmolecules are shielded or deshielded by differentamounts.

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• Shielded proton shows absorption of signals to right side anddeshielded protons at left side of spectrum.

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NMR Signals

• The number of signals shows how many different kinds of protons are present.

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• The location of the signals shows how shielded or deshielded the proton is.


The NMR Graph

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Integral: 1 3


NMR Signals


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• The intensity of the signal shows the number of protons of that type.


IR spectrum of methanol

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• The value of δ for a substance relative to TMS can be obtainedby Chemical shift, ppm.

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δ =

• Any proton or set of protons which absorb at higher frequencythan TMS is given a positive value for (δ).

• The value of chemical shift, δ expressed in ppm and their valueis between 0 to 10 in the δ scale.

Shift high frequency from TMS (in Hz)

Spectrometer frequency (in MHz)


Delta Scale

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Hydrogen Chemical Shifts

ppm

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http://rd.nctu.edu.tw/web.case/nctu-rd-2/upload/ckeditor/20150410151405.pdf

Classification of the Chemical Shift of the -OH Functional Groups2424

Siskos et al., Org. Biomol. Chem, 2013, 11, 7400-74011

Kontogianni et al., Org. Biomol. Chem, 2013, 11, 2013-2025

O

O

2

3

4

5

H

O O H

5 4

A C

17.5-20 ppm

12-13 ppm

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14.75 ppm 14.17 ppm

Effect of Strength of Hydrogen Bond on 1H Chemical Shift26

http://www.google.gr/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=S2ei9ua3g8uN7M&tbnid=iVPQvENYnEeH8M:&ved=0CAUQjRw&url=http%3A%2F%2Fwww.consumerhealthanswers.com%2Fhypericin-reviewed.html&ei=SMmhUrOKOMSUtQaf1YGYAQ&bvm=bv.57752919,d.bGQ&psig=AFQjCNEyAQMguilF69kWt-OLiaxw19I7zA&ust=1386420927956401

http://www.google.gr/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=S2ei9ua3g8uN7M&tbnid=iVPQvENYnEeH8M:&ved=0CAUQjRw&url=http%3A%2F%2Fwww.consumerhealthanswers.com%2Fhypericin-reviewed.html&ei=SMmhUrOKOMSUtQaf1YGYAQ&bvm=bv.57752919,d.bGQ&psig=AFQjCNEyAQMguilF69kWt-OLiaxw19I7zA&ust=1386420927956401

http://www.authorstream.com/Presentation/knowledgexplorers-2245188-anwar-nmr/

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most deshielded


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Hydrogen Chemical Shifts

ppm

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Aromatic Protons, 7-8 ppm

The aromatic protonsare deshielded!


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Aldehyde Proton, 9-10 ppm


Electronegative

oxygen atom

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Vinyl Protons, 5-6 ppm


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Acetylenic Protons, 2.5 ppm


NMR Signals


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• The intensity of the signal shows the number of protons of that type.

• Signal splitting shows the number of protons on adjacent atoms.


1,1,2-Tribromoethane

Nonequivalent protons on adjacent carbons.

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Spin-Spin Splitting


Doublet: 1 Adjacent Proton

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Triplet: 2 Adjacent Protons

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Spin-Spin Coupling Constants

• Distance between the peaks of multiplet.

• Measured in Hz.

• Not dependent on strength of the external field.

• Multiplets with the same coupling constants may come from adjacent groups of protons that split each other.

• Protons separated by four or more bonds will not couple.

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?

Values for

Coupling Constants

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Splitting Tree

C C

H

H

Ha

b

c

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http:///http://rd.nctu.edu.tw/web.case/nctu-rd-2/upload/ckeditor/20150410151405.pdf

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https://translate.google.gr/translate?hl=el&sl=en&u=http://bionmr.unl.edu/courses/chem991a_-_intro_to_nmr/lectures/chapter-3-NMR-coupling.ppt&prev=search

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Hydrogen

Chemical Shifts

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and Carbon


Carbon-13

• 12C has no magnetic spin.

• 13C has a magnetic spin, but is only 1% of the carbon in a sample.

• The gyromagnetic ratio of 13C is one-fourth of that of 1H.

• Signals are weak, getting lost in noise.

• Hundreds of spectra are taken, averaged.

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The NMR Spectrometer

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continuous change in frequency


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Periodic variation of experimental nuclear

shielding ranges

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Fourier Transform NMR

• Nuclei in a magnetic field are given a radio-frequency pulse close to their resonance frequency

• The nuclei absorb energy and process (spin) like little tops

• A complex time domain signal is produced, then decays as the nuclei lose energy

• Free induction decay is converted to spectrum

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http://www.chemistry-solutions.com/wp-content/uploads/2015/07/CHE-242-Unit-5-Chapter-13.ppt

excitation pulse (μs)

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http://rutchem.rutgers.edu/~nmurali/nmr_course/Chem_542_Spring2010_Lecture_3.pdf

1H NMR spectra of caffeine

8 scans ~12 secs

13C NMR spectra of caffeine

8 scans ~12 secs

13C NMR spectra of caffeine

10,000 scans ~4.2 hours

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Spin population NMR - Sensitivity

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Nuclear Spins in Bo

• In an applied field strength of 7.05T the difference

in energy between nuclear spin states for

– 1H is approximately 0.120 J (0.0286 cal)/mol, which

corresponds to a frequency of 300 MHz (300,000,000

Hz).

– The population difference of the two nuclear spin

states is only ∼10-5 and, therefore, has meaning only

at the Avogadro number!

http://www.authorstream.com/Presentation/kesarwaniarti-1755809-nmr-spectroscopy/

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- intrinsic property of nucleus can not be changed

C)3 for 13C is 64x N)3 for 15N is 1000x

1H is ~ 64x as sensitive as 13C and 1000x as sensitive as 15N !

NMR Sensitivity

• Relative sensitivity of 1H, 13C, 15N and other nuclei NMR spectra depend on

Gyromagnetic ratio ()

Natural abundance of the isotope

Consider that the natural abundance of 13C is 1.1% and 15N is 0.37%relative sensitivity increases to ~6,400x and ~2.7x105x !!

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http://www.authorstream.com/Presentation/solairajan87-1493928-nmr-spectroscopy/

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N.O.E. Transient NOE and Distances

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Prof. Claudio Santi Dipartment of Chemistry and Technology of Drugs University of Perugia

N.O.E. Example “steady-state”

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Ha Hb

NMR and Chemical Exchange

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What information can NMR spectroscopy provide

Structural Theory NMR equivalent

Qualitative composition Choice of nucleus / isotope

Quantitative composition Signal intensities / integration

Functional groups Chemical shifts

Connectivity J (scalar) couplings

Spatial relationship of atoms NOE (distance)

J couplings (dihedral angle)

Dynamics Chemical exchange, lineshape

broadening

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Books in NMR - Web Sites

Solving Problems with NMR Spectroscopy by Atta-ur-Rahman and Muhammad Iqbal Choudhary

http://www.elsevier.com/books/modern-nmr-techniques-for-chemistry-research/derome/978-0-08-

032513-2

Modern NMR Techniques for Chemistry Research by A.E. Derome, University of Oxford, UK

http://www.elsevier.com/books/modern-nmr-techniques-for-chemistry-research/derome/978-0-08-

032513-2

Modern NMR spectroscopy, a guide for chemists, by J. K. M. Sanders and B. K. Hunter

http://onlinelibrary.wiley.com/doi/10.1002/mrc.1260251217/abstract

Nuclear Magnetic Resonance : Volume 3 by R K Harris

http://pubs.rsc.org/en/Content/eBook/978-0-85186-272-9#!divbookcontent

NMR Spectroscopy: Basic Principles, Concepts and Applications in Chemistry, by H. Gunther

http://www.wiley-vch.de/publish/en/books/ISBN978-3-527-33000-3/

Carbon 13 NMR Spectroscopy, by Hans-Otto Kalinowski, Stefan Berger and Siegmar Braun

http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0471913065.html

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http://www.elsevier.com/books/modern-nmr-techniques-for-chemistry-research/derome/978-0-08-032513-2

http://www.elsevier.com/books/modern-nmr-techniques-for-chemistry-research/derome/978-0-08-032513-2

http://onlinelibrary.wiley.com/doi/10.1002/mrc.1260251217/abstract

http://pubs.rsc.org/en/Content/eBook/978-0-85186-272-9#!divbookcontent

http://www.wiley-vch.de/publish/en/books/ISBN978-3-527-33000-3/

http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0471913065.html

Some NMR Web Sites

The Basics of NMR by J.P. Hornak Hypertext based NMR course

http://www.cis.rit.edu/htbooks/nmr/nmr-main.htm

Integrated Spectral Data Base System for Organic Compounds

http://www.aist.go.jp/RIODB/SDBS/menu-e.html

Educational NMR Software All kinds of NMR software

http://www.york.ac.uk/depts/chem/services/nmr/edusoft.html

NMR Knowledge Base A lot of useful NMR links

http://www.spectroscopynow.com/

NMR Information Server News, Links, Conferences, Jobs

http://www.spincore.com/nmrinfo/

Technical Tidbits Useful source for the art of shimming

http://www.acornnmr.com/nmr_topics.htm

BMRB (BioMagResBank) Database of NMR resonance assignments

http://www.bmrb.wisc.edu/

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http://www.cis.rit.edu/htbooks/nmr/nmr-main.htm

http://www.aist.go.jp/RIODB/SDBS/menu-e.html

http://www.york.ac.uk/depts/chem/services/nmr/edusoft.html

http://www.spectroscopynow.com/

http://www.spincore.com/nmrinfo/

http://www.acornnmr.com/nmr_topics.htm

http://www.bmrb.wisc.edu/

STRUCTURE EUCIDATION USING NMR

SPECTROSCOPY – AN EXAMPLE

THE CASE OF THE FLAVONOID QUERCETIN

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2.2 Hz 2.02 Hz 2.02 Hz8.5Hz, 2.2 Hz70

TWO DIMENSIONAL SPECTRATHE 1H-1H COSY EXAMPLE

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Jacobsen, NMR Spectroscopy Explained, Wiley, 2007

Homonuclear 2 Dimensional Experiment: 1H – 1H COSY (COrrelated SpectroscopY)

1H

1H

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Homonuclear 2 Dimensional Experiment: 1H – 1H TOCSY (TOtal Correlated SpectroscopY)

1H

1H

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Heteronuclear 2 Dimensional Experiment: 1H – 13C HSQC (Heteronuclear Single Quantum Spectroscopy)

13C

1H

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Heteronuclear 2 Dimensional Experiment: 1H – 13C HMBC (Heteronuclear Multiple Bond Correlation)

13C

1H

C-OH

C=O

A

B

C

H6H8H5’

C-OH

6

8

2’

1

4

5’

6’

H6’H7’

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SPECTROSCOPIC METHODS OF ORGANIC MOLECULES. Basics... · 2019-02-12 · SPECTROSCOPIC METHODS OF ORGANIC MOLECULES Professor Ioannis P. Gerothanassis Department of Chemistry, University