Chapter 1. (COE) Spectroscopy

8/3/2019 Chapter 1. (COE) Spectroscopy

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Chem. 36 Ch1. MMUy 01

CHEM 36ORGANIC CHEMISTRY II

I. Spectroscopy and Structure of Organic Compounds

Reference: Organic Chemistry. Solomons and Fryhle. 8th Edition


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Spectroscopy: the study of the interaction of quantized

energy (electromagnetic energy) with matter

Energy applied to matter can be absorbed, emitted, cause achemical change, or be transmitted.

In organic chemistry, we typically deal with molecularspectroscopy particularly absorption spectroscopy.

Introduction



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Examples of Spectroscopy

Ultraviolet Spectroscopy

Absorption of ultraviolet energy results to electronic

transitions in molecules.

UV spectroscopy indicates the presence ofchromophores

in a molecule particularly unsaturation, conjugatedsystems, carbonyl and aromatic systems.

Infrared (IR) Spectroscopy

Infrared energy causes bonds to stretch and bend.

IR is useful for identifying functional groups in a molecule.



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Nuclear Magnetic Resonance (NMR) Spectroscopy

Energy applied in the presence of a strong magnetic fieldcauses absorption by the nuclei of some elements (most

importantly, hydrogen and carbon nuclei).

NMR is used to identify connectivity of atoms in a molecule.

Mass Spectrometry (MS)

Molecules are converted to ions by one of several methods (including

bombardment by a beam of electrons).

The ions formed may remain intact (as molecular ions, M+), or they may

fragment.

The resulting mixture of ions is sorted by mass/charge (m/z) ratio, and

detected.

Molecular weight and chemical formula may be derived from the M+ and

M+1 ions.

Molecular structure may be deduced from the distribution of fragment ions.



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Schematic absorption spectrum:

An absorption band is characterized primarily by two parameters:

(a) the wavelength at which maximum absorption occurs(b) the intensity of absorption at this wavelength compared to

baseline or background absorption

wavelength

frequency

change in energy



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Spectroscopic transition: takes a molecule from one stateto a state of a higher energy and its change in energy is

given by E:

where h = Plancks constant

The energy gap of a transition is a molecular

property and is a characteristic of a

molecular structure.



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Electromagnetic radiation has the characteristics of both waves

and particles.

The wave nature of electromagnetic radiation is described bywavelength () or frequency ().

The relationship between wavelength (or frequency) andenergy (E) is well defined.

The Electromagnetic Spectrum



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Wavelength and frequency are inversely proportional (=c/).

The higher the frequency, the greater the energy of thewave.

The shorter the wavelength, the greater the energy of thewave.



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I. Ultraviolet Spectroscopy

generally refers to electronic transitions occurring in theultraviolet ( range 200-380 nm) region of the electromagneticspectrum

Electronic transitions are also responsible in the visible region

( range 380-800 nm) which is easily accessible instrumentallybut are ofless importance in the solution of structural problems

since most organic compounds are colorless.

An extensive region at wavelengths shorter than ~200 nm

(vacuum UV) also corresponds to electronic transitions

but this region is not accessible with standard instruments.



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types of electronic transitions:

the transitions are brought about by the presence of

chromophores in the molecule ( a part of the molecule which

can be a functional group or a single atom or group of atoms

in a molecule which may not be associated with chemical

functionality)

n to *

to *

to *

n to *



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Basic Instrumentation



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In practice, double-beam instruments are used where the

absorption of a reference cell, containing only solvent, issubtracted from the absorption of the sample cell.

The energy source, the materials from which the dispersing

device and detector are constructed must be appropriate forthe range of wavelength scanned as transparent as possible

to the radiation.

For UV measurements, the cells and optical components are

typically made ofquartz and ethanol, hexane, water or

dioxan are usually chosen as solvents.



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Quantitative Aspects

The y-axis of a UV spectrum may be calibrated in terms of the

intensity of transmitted light (% transmission or absorption)

or on a logarithmic scale i.e., in terms ofAbsorbance A:



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BeerLambert law: Absorbance is proportional to the concentration

and path length; intensity is usually expressed as molar

absorbance or molar extinction coefficient :

UV absorption bands are characterized by the wavelength ofmaximum absorption (max) and . The values of vary from 10 and 105 and are conveniently

tabulated as log10().

The presence of small amounts of strongly absorbing impuritiesmay lead to errors in the interpretation of UV data.

Where M = molecular weight

C = concentration (g/L)

l = path length (cm)



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Classification of UV Bands

UV absorption bands have fine structures due to the presence ofvibrational sub-levels but are rarely observed in solution due to

collisional broadening.

As transitions are associated with changes of electron orbitals,

they are often described in terms of the orbitals involved or by

another classification method:



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A molecule may give rise to more than one band because it

contains more than one chromophore or because more than

one transition of a single chromophore is observed.

UV spectra typically contain far fewer features than IR, NMR orMS spectra and thus have a lower information content.



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Special Terms in UV Spectroscopy

Auxochromes (auxillary chromophores): groups which have

little UV absorption by themselves, but which often havesignificant effects on the absorption (both max and ) of a

chromophore to which they are attached; are atoms with one

or more lone pairs ( -OH, -OR, -NR2, -halogen)

bathochromic shift: shift of absorption maximum to longer

wavelength

hypsochromic shift: shift of absorption maximum to shorter

wavelength

Hyperchromic effect: an increase in absorption intensity

Hypochromic effect: a decrease in absorption intensity



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Important UV Chromophores

Most of the reliable and useful data is due to relatively

strongly absorbing chromophores ( > 200) which are mainlyindicative ofconjugated or aromatic systems.

(1) Dienes and Polyenes

Extension of conjugation in

C chain is always associated

with a pronounced shift

towards longer wavelength

(bathochromic shift) and

towards greater intensity

(hyperchromic shift).



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Bicyclic diene



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Table 1. The Effect of Conjugation on UV Absorption

When there are more than 8 conjugated double bonds, the absorption

maximum of polyenes is such that they absorb light strongly in the visible

region.

Empirical rules (Woodward Rules) of good predictive value are available to

estimate the positions ofmax in conjugated polyenes and conjugated

carbonyl compounds.



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Woodward-Fieser Rules for Calculating the max of Conjugated

Dienes and Polyenes

Core Chromophore Substituent and Influence

Transoid Diene

215 nm

R- (Alkyl Group) .... +5 nm

RO- (Alkoxy Group) .. +6

X- (Cl- or Br-) ......... +10RCO2- (Acyl Group) .... 0

RS- (Sulfide Group) .. +30

R2N- (Amino Group) .. +60

Further -Conjugation

C=C (Double Bond) ... +30

C6H5 (Phenyl Group) ... +60

Cyclohexadiene*260 nm



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(i) Each exocyclic double bond adds 5 nm. In the example on

the right, there are two exo-double bond components: one

to ring A and the other to ring B.

(ii) Solvent effects are minor.

* When a homoannular (same ring) cyclohexadiene chromophore is present, abase value of 260 nm should be chosen. This includes the ring substituents.

Rings of other size have a lesser influence.

max (calculated) = Base (215 or 260) + Substituent Contributions



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(2) Carbonyl compounds

All carbonyl derivatives exhibit weak ( < 100) absorptionbetween 250 and 350 nm, and this is only of marginal use

in determining structure.

Conjugated carbonyl derivatives

always exhibit strong

absorption.



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Table 2. UV Absorption Bands in Common Carbonyl Compounds



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Woodward-Fieser Rules for Calculating the __> * max of

Conjugated Carbonyl Compounds

Core Chromophore Substituent and Influence

R = Alkyl 215 nm

R = H 210 nm

R = OR' 195 nm

- Substituent

R- (Alkyl Group) +10 nm

Cl- (Chloro Group) +15Br- (Chloro Group) +25

HO- (Hydroxyl Group) +35

RO- (Alkoxyl Group) +35

RCO2- (Acyl Group) +6

Cyclopentenone

202 nm



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- Substituent

R- (Alkyl Group) +12 nm

Cl- (Chloro Group) +12

Br- (Chloro Group) +30HO- (Hydroxyl Group) +30

RO- (Alkoxyl Group) +30

RCO2- (Acyl Group) +6

RS- (Sulfide Group) +85

R2N- (Amino Group) +95

& - Substituents

R- (Alkyl Group) +18 nm (both & )

HO- (Hydroxyl Group) +50 nm ()

RO- (Alkoxyl Group) +30 nm ()

Further -Conjugation

C=C (Double Bond) ... +30

C6H5 (Phenyl Group) ... +60



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(i) Each exocyclic double bond adds 5 nm. In the example on

the right, there are two exo-double bond components: one

to ring A and the other to ring B.

(ii) Homoannular cyclohexadiene component adds +35 nm

(ring atoms must be counted separately as substituents)

(iii) Solvent Correction: water =8; methanol/ethanol = 0;

ether = +7; hexane/cyclohexane = +11

max (calculated) = Base + Substituent Contributions and Corrections



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(3) Benzene derivatives

exhibit medium to strong absorption with bands havingcharacteristic fine structure and the intensity of the

absorption being strongly influenced by substituents:

Weak auxochromes: -CH3, -Cl, -OCH3

Groups which increase conjugation: -CH=CH2,

-C(=O)-R, -NO2

Auxochromes whose absorption is pH dependent:-NH2 and -OH



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Table 3. UV Absorption Bands in Common Benzene Derivatives



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Table 3. cont.



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The added conjugation in naphthalene, anthracene and

tetracene causes bathochromic shifts of these absorption

bands.



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Electromagnetic radiation in the infrared (IR) frequencyrange is absorbed by a molecule at certain characteristicfrequencies.

Energy is absorbed by the bonds in the molecule and they

vibrate faster.

The bonds behave like tiny springs connecting the atoms.

The bonds can absorb energy and vibrate faster only when

the added energy is of a particular resonant frequency. The frequencies of absorption are very characteristic of the

type of bonds contained in the sample molecule.

II. Infrared Spectroscopy: An Instrumental Method

For Detecting Functional Groups



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The type of bonds present are directly related to thefunctional groups present.

A plot of these absorbed frequencies is called an IRspectrum.

Infrared Spectrometer An infrared spectrometer detects the frequencies absorbed

by the sample molecule.

Light of all the various IR frequencies is transmitted to themolecule and the frequencies absorbed are recorded.

The absorption frequencies are specified as wavenumbers inunits of reciprocal centimeters (cm-1).



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Alternatively the wavelength () in units of microns (m)can be specified.

The spectrum is a plot of frequency on the horizontal axisversus strength of absorption on the vertical axis.


There are different types of stretching and bending vibrations


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There are different types ofstretching and bending vibrationsinduced by the absorption of infrared energy:



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The actual relative frequency of vibration can be predicted:

Bonds with lighter atoms vibrate faster than those with heavier atoms.

Triple bonds (which are stiffer and stronger) vibrate athigher frequencies than double bonds:

Double bonds in turn vibrate at higher frequencies than single bonds.



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The IR spectrum of a molecule usually contains many peaks.

These peaks are due to the various types of vibrationsavailable to each of the different bonds.

Additional peaks result from overtone (harmonic) peakswhich are weaker and of lower frequency.

The IR is a fingerprint of the molecule because of theunique and large number of peaks seen for a particularmolecule.



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Generally only certain peaks are interpreted in the IR.

Those peaks that are large and above 1400 cm-1 are mostvaluable.

Hydrocarbons

The C-H stretching regions from 2800-3300 cm-1 ischaracteristic of the type of carbon the hydrogen is attachedto.

C-H bonds where the carbon has more s character are shorter,stronger and stiffer and thus vibrate at higher frequency.

Interpreting IR Spectra



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C-H bonds at sp centers appear at 3000-3100 cm-1

C-H bonds at sp2 centers appear at about 3080 cm-1

C-H bonds at sp3 centers appear at about 2800-3000 cm-1

C-C bond stretching frequencies are only useful for multiple

bonds:

C-C double bonds give peaks at 1620-1680 cm-1

C-C triple bonds give peaks at 2100-2260 cm-1

These peaks are absent in symmetrical double and triple

bonds.



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Example: octane


E l 1 h


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Example: 1- hexyne



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Alkenes

The C-H bending vibration peaks located at 600-1000 cm-1

can be used to determine the substitution pattern of the

double bond:



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Example: 1-hexene


Aromatic Compounds


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Aromatic Compounds

The C-C bond stretching gives a set of characteristic sharppeaks between 1450-1600 cm -1

Example: Methyl benzene


Other F nctional Gro ps


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Other Functional Groups

Carbonyl Functional Groups

Generally the carbonyl group gives a strong peak which occurs

at 1630-1780 cm-1. The exact location depends on the actual functional group present:


Al h l d Ph l


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Alcohols and Phenols

The O-H stretching absorption is very characteristic:

In very dilute solutions,hydrogen bonding is absent

and there is a very sharp

peak at 3590-3650 cm-1.

In more concentrated

solutions, the hydroxyl

groups hydrogen bond to

each other and a very broad

and large peak occurs at

3200-3550 cm-1.

A phenol has a hydroxyl

group directly bonded to an

aromatic ring.


Carboxylic Acids


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Carboxylic Acids

The carbonyl peak at 1710-1780 cm-1 is very characteristic

The presence ofboth carbonyl and O-H stretching peaks is a

good proof of the presence of a carboxylic acid Example: propanoic acid



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Amines

Very dilute solution of 1o and 2o amines give sharp peaks

at 3300-3500 cm-1

for the N-H stretching.

1o amines give two peaks and 2o amines give one peak.

3o have no N-H bonds and do not absorb in this region.

More concentrated solutions of amines have broaderpeaks.

Amides have amine N-H stretching peaks and a

carbonyl peak.



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NMR involves absorption of energy in the

radiofrequency range of the electromagneticspectrum.

The nuclei of protons (1H) and carbon-13 (13C), and certainother elements and isotopes, behave as if they were tiny

bar magnets.

When placed in a magnetic field and irradiated with radiofrequency energy, these nuclei absorb energy at

frequencies based on their chemical environments.

NMR spectrometers are used to measure these

absorptions.

Nuclear Magnetic Resonance Spectroscopy



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Continuous-Wave (CW) NMR Spectrometers

the oldest type of NMR spectrometer

The magnetic field is varied as the electromagneticradiation is kept at a constant frequency.

Different nuclei absorb the electromagnetic energy basedon their chemical environment and produce peaks indifferent regions of the spectrum.



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Fourier Transform (FT) NMR Spectrometers

The sample is placed in a constant (and usually very strong)

magnetic field. The sample is irradiated with a short pulse of radio frequency

energy that excites nuclei in different environments all at once.

The resulting signal contains information about all of the absorbing

nuclei at once. This signal is converted to a spectrum by a Fourier transformation.

FT-NMR allows signal-averaging, which leads to enhancement ofreal spectral signals versus noise.

The strong, superconducting magnets used in FT-NMRspectrometers lead to greater sensitivity and much higherresolution than continuous wave instruments.



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Diagram of a FT NMR Spectrometer



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Chemical Shift: Peak Position in an NMR Spectrum

Nuclei in different chemical environments in a molecule

will absorb at slightly different frequencies.

The position of the signals in the spectrum is called thechemical shift.

There are two reasons for differences in the magneticenvironment for a proton:

1) The magnetic field generated by electrons circulatingaround the nucleus giving the signal

2) Local magnetic fields generated by electrons elsewhere inthe molecule


Example: 1,4-dimethylbenzene


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p y

The spectrum is measured on a delta () scale in units of partsper million (ppm).

Lower frequency is to the left in the spectrum; these absorptionsare said to be downfield.




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Example: 1,4 dimethylbenzene

Higher frequency is to the right in the spectrum: these absorptionsare said to be upfield.

The small signal at 0 corresponds to an internal standard calledtetramethylsilane (TMS) used to calibrate the chemical shift scale.


Example: 1 4-dimethylbenzene


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The number of signals in the spectrum corresponds to the numberof unique sets of protons.

1,4-dimethylbenzene has protons in two unique environments and soshows two signals.


Integration of Peak Areas. The Integral Curve


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Integration of Peak Areas. The Integral Curve The area under each signal corresponds to the relative number of

hydrogen atoms in each unique environment within a molecule.

The height of each step in the integral curve is proportional to thearea of the signal underneath the step.

1,1,2-trichloroethane


Si l S litti


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Signal Splitting The signal from a given proton will be split by the effect of

magnetic fields associated with protons on adjacent carbons.

Characteristic peak patterns result from signal splitting that arerelated to the number of protons on adjacent carbons.

1,1,2-trichloroethane


Nuclear Spin: The Origin of the Signal


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The nuclei of certain elements and

isotopes have spin states that are

quantized.

1H has a spin quantum number

I = 1/2 and has allowed spin states of

+1/2 or -1/2.

Other nuclei with I = 1/2 are 13C, 19F and 31P and these alsorespond to an external magnetic field.

Nuclei with I = 0 do not have spin (12C and 16O) and do notrespond to an external magnetic field.





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The nuclei of NMR-active nuclei behave like tiny bar magnets.

In the absence of an external magnetic field these barmagnets are randomly orientated (a).

In an external magnetic field they orient either with ( spinstate) or against ( spin state) the magnetic field (b).




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Nuclei aligned with the magnetic field are lower in energy thanthose aligned against the field.

The nuclei aligned with the magnetic field can be flipped to alignagainst it if the right amount of energy is added (E).

The amount of energy required depends on the strength of theexternal magnetic field:

The stronger the external magnetic field, the higher the radiofrequency energy required to flip the nuclear spin.



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At (a) there is no external magnetic field and therefore no energydifference between the two states.

At (b) the external magnetic field is 1.41 Tesla and energy

corresponding to a frequency of about 60MHz is needed to flipbetween the spin states.

At (c) the external magnetic field is 7.04 Tesla energy corresponding toa frequency of about 300MHz is needed to flip between the spin states.


Shi ldi d D hi ldi f P t


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Protons in an external magnetic field absorb at differentfrequencies depending on the electron density around that

proton.

Shielding and Deshielding of Protons



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High electron density around a nucleus shields the nucleusfrom the external magnetic field.

Shielding causes absorption of energy at higher frequencies(more energy is required for this nucleus to flip between spinstates) - the signals are upfield in the NMR spectrum.



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Lower electron density around a nucleus deshields thenucleus from the external magnetic field.

Deshielding causes absorption of energy at lower frequencies(less energy is required for this nucleus to flip between spinstates) - the signals are downfield in the NMR spectrum.


Electronegative atoms draw electron density away from nearby


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g y y yprotons and therefore deshield them.

Circulation of electrons leads to a local induced magnetic field: The induced field can reinforce or diminish the external field sensed by a

proton (depending on the location of the proton), causing deshielding orshielding, respectively.

Alkene and aromatic ring hydrogens are deshielded by the circulation of electrons.

A terminal alkyne hydrogen is shielded by the circulation of electrons.


Ch i l Shif


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Chemical shifts are measured in relation to the

internal reference tetramethylsilane (TMS).

The protons of TMS are highly shielded because of the strongelectron donating capability of silicon.

The signal for TMS is well away from most other proton

absorptions.

The scale for chemical shifts is independent of the magneticfield strength of the instrument (whereas the absolutefrequency depends on field strength).

Chemical Shift


Thus the chemical shift in units for protons on benzene is


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Thus, the chemical shift in units for protons on benzene isthe same whether a 60 MHz or 300 MHz instrument is used.



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To predict the number of signals to expect in an NMRspectrum it is necessary to determine how many sets ofprotons are in unique environments.

Chemically equivalent protons are in the same environmentand will produce only one signal.

Chemical Shift Equivalent and Nonequivalent Protons



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Homotopic Hydrogens

Hydrogens are chemically equivalent or homotopic if

replacing each one in turn by the same group would lead toan identical compound.


Enantiotopic and Diastereotopic Hydrogen Atoms


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If replacement of each of two hydrogens by some groupleads to enantiomers, those hydrogens are enantiotopic.

In the absence of a chiral influence, enantiotopic hydrogenshave the same chemical shift and appear in the samesignal.



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If replacement of each of two hydrogens by some groupleads to diastereomers, the hydrogens are diastereotopic.

Diastereotopic hydrogens have different chemical shifts andwill give different signals.


Signal Splitting: Spin spin Coupling


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The signal from a given proton will be split by the effect ofmagnetic fields associated with protons on adjacent carbons.

Characteristic peak patterns result from signal splitting thatare related to the number of protons on adjacent carbons.

The effect of signal splitting is greatestbetween atoms separated by 3 or fewer

bonds.

Signal Splitting: Spin-spin Coupling



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Signal splitting is not observed between homotopic orenantiotopic protons.

Signal splitting occurs only when two sets of protons havedifferent chemical shifts (i.e., are not chemical shiftequivalent).


The magnetic field sensed by a proton (Ha) being observed isff t d b th ti t f dj t t (H )


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affected by the magnetic moment of an adjacent proton (Hb):

A proton (Hb

) can be aligned with

the magnetic field or against the

magnetic field, resulting in two

energy states for Hb.

The observed proton (Ha) sensesthe two different magnetic

moments of Hb as a slight change

in the magnetic field; one

magnetic moment reinforces

the external field and one

substracts from it.



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The signal for Ha is split intoa doublet with a 1:1 ratio

of peak areas.

The magnitude of thesplitting is called the

coupling constant Jab

and is measured inHertz (Hz).


When two adjacent protons Hb are coupled to Ha, there are fourpossible combinations of the magnetic moments for the two


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possible combinations of the magnetic moments for the twoHbs:

Two of these combinations involvepairings of magnetic moments that

cancel each other, causing no net

displacement of signal.

One combination of magneticmoments reinforces and another

subtracts from the applied

magnetic field.

Ha is split into a triplet havinga 1:2:1 ratio of signal areas.



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When three adjacent protons are coupled to H there are 10


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When three adjacent protons are coupled to Ha, there are 10possible combinations of the magnetic moments for the Hbs:

Four unique orientationsexist and so Ha is split

into a quartet with

intensities 1:4:4:1.



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The general rule for splitting is that if there are n equivalentprotons on adjacent atoms, these will split a signal into n + 1

peaks. Coupled peaks have the same coupling constantsJ.

Comparison of coupling constants can help with the analysis of

complex spectra.

Several factors complicate analysis of NMR spectra:

Peaks may overlap. Spin-spin coupling can be long-range (i.e., more than 3 bonds).


Splitting patterns in aromatic groups can be confusing:


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A monosubstituted aromatic ring can appear as an apparentsinglet or a complex pattern of peaks.


Much more complex splitting can occur when two sets of adjacentprotons split a particular set of protons.


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protons split a particular set of protons.

In the system below, Hb is split by two different sets of hydrogens :Ha and Hc

Theortically Hb could be split into a triplet of quartets (12 peaks) butthis complexity is rarely seen.

The spectrum of 1-nitropropane shows splitting of Hb into only 6 peaks.



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Proton NMR and Rate Processes


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An NMR spectrometer is like a camera with a slow shutterspeed.

The NMR spectrometer will observe rapid processes as if theywere a blur, i.e., only an average of the changes will be seen.

When a 1H NMR spectrum ofvery pure ethanol is taken, thehydroxyl proton is split into a triplet by the two adjacent

hydrogens. When an 1H NMR ofregular ethanol is taken the hydroxyl

proton is a singlet.

Impure ethanol contains acid and base impurities which catalyze

the exchange of hydroxyl protons.

This rapid exchange is so fast that coupling to the adjacent CH2 isnot observed.

This process is called spin decoupling.

Proton NMR and Rate Processes



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Spin decoupling is typical in the 1H NMR spectra of alcohols,

amines and carboxylic acids.

The proton attached to the oxygen or nitrogen normally appearsas a singlet because ofrapid exchange processes.


Carbon-13 NMR Spectroscopy


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13C accounts for only 1.1% of naturally occurring carbon:

12C has no magnetic spin and produces no NMR signal.

One Peak for Each Unique Carbon Atom:

Since the 13C isotope of carbon is present in only 1.1%natural abundance, there is only a 1 in 10,000 chancethat two 13C atoms will occur next to each other in amolecule.

The low probability of adjacent13

C atoms leads to nodetectable carbon-carbon splitting.

Carbon 13 NMR Spectroscopy


1H and 13C do split each other, but this splitting is usually


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H and C do split each other, but this splitting is usuallyeliminated by adjusting the NMR spectrophotometeraccordingly.

The process of removing the coupling of1H to an attachedcarbon is called broadband (BB) proton decoupling.

Most 13C NMR, therefore, consist of a single peak for each

unique carbon.


13C Chemical Shifts


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Just as in 1H NMR spectroscopy, chemical shifts in 13C NMRdepend on the electron density around the carbon nucleus:

Decreased electron density causes the signal to movedownfield (desheilding).

Increased electron density causes the signal to moveupfield (sheilding).

Because of the wide range of chemical shifts, it is rare tohave two 13C peaks coincidentally overlap.

A group of 3 peaks at 77 comes from the common NMR

solvent deuteriochloroform and can be ignored.



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Off-Resonance Decoupled Spectra

Broad-band decoupling removes all information aboutthe number of hydrogens attached to each carbon.

Off-resonance decoupling removes some of the coupling

of carbons to hydrogens so that the coupled peaks willnot overlap.

Use of off-resonance decoupled spectra has beenreplaced by use of DEPT 13C NMR.


DEPT 13C NMR


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DEPT (Distortionless Enhanced Polarization Transfer)

spectra are created by mathematically combiningseveral individual spectra taken under specialconditions.

The final DEPT spectra explicitly show C, CH, CH2

, andCH3 carbons.

To simplify the presentation of DEPT data, thebroadband decoupled spectrum is annotated with the

results of the DEPT experiments using the labels C, CH,CH2 and CH3 above the appropriate peaks .


Example: 1-chloro-2-propanol (a) The broadband decoupled spectrum and (b) a set of DEPT spectra

h i th t CH CH d CH i l


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showing the separate CH, CH2, and CH3 signals


Introduction to Mass Spectrometry (MS)


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A mass spectrometer produces a spectrum of masses

based on the structure of a molecule.

A mass spectrum is a plot of the distribution of ion massescorresponding to the formula weight of a molecule and/orfragments derived from it.

The x-axis of a mass spectrum represents the masses ofions produced.

The y-axis represents the relative abundance of each ionproduced.

The pattern of ions obtained and their abundance ischaracteristic of the structure of a particular molecule.



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The Mass Spectrometer


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One common type is the Electron Impact Mass

Spectrometer (EI MS).

Ionization (the formation of ions)

A molecule is bombarded with a beam of high energyelectrons.

An electron may be dislodged from the molecule by theimpact, leaving a positively charged ion with an unpairedelectron (a radical cation).

This initial ion is called the molecular ion (M+.) because it hasthe same molecular weight as the analyte.


Fragmentation


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Excess vibrational energy is imparted to the molecularion by collision with the electron beam - this causes

fragmentation.

The fragmentation pattern is highly characteristic of thestructure of the molecule.



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Ion Sorting

The fragments are sorted according to their mass tocharge ratio, (m/z).

Most of the fragments detected have charge +1; the neteffect is sorting of the ions by mass (m/z, where z = +1).

The charged molecular ion (M+) and fragments passthrough an analyzer that sorts the ions according to m/z.


One method of sorting involves directing the ions through acurved tube that passes through a magnetic field; as the


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curved tube that passes through a magnetic field; as themagnetic field is varied, ions of different m/z valuessuccessfully traverse the tube and reach the detector.


The Mass Spectrum


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After ion sorting the results are plotted as a spectrumwith m/z on the horizontal axis and relative abundance of

each ion on the vertical axis.

Data from a mass spectrometer can be represented as agraph or table.

The most abundant (intense) peak in the spectrum iscalled the base peak and is assigned a normalizedintensity of 100%.

The masses are based on rounding of atom masses to the

nearest whole number (in low resolution massspectroscopy).


The data and fragmentation patterns for ammonia are asfollows:


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The base peak for ammonia is the molecular ion, but this is oftennot the case.



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The small peak at m/z 18 comes from the small amount of15

N1

H3because of the small natural abundance of15N compared to 14N.

This peak is called an M+1 peak.



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Determination of Molecular Formulas and


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The Molecular Ion and Isotopic Peaks

The presence of heavier isotopes one or two mass unitsabove the common isotope yields small peaks at M+.+1and M+.+2.

The intensity of the M+.+1 and M+.+2 peaks relative to theM peak can be used to confirm a molecular formula.

Molecular Weights


Predicted ratios of


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molecular ions for

compounds containing

various ratios ofchlorine and bromine,

based on natural

isotopic abundance.


Example: In the spectrum of methane one expects anM+ 1 k f 1 17% b d 1 11% l b d


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M+.+1 peak of 1.17% based on a 1.11% natural abundanceof13C and a 0.016% natural abundance of2H:


High-Resolution Mass Spectrometry


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Low-resolution mass spectrometers measure m/z values tothe nearest whole number.

High-resolution mass spectrometers measure m/z valuesto three or four decimal places.

The high accuracy of the molecular weight calculationallows accurate determination of the molecular formula of

a fragment. Example: One can accurately pick the molecular formula of a

fragment with a nominal molecular weight of 32 using high-resolution MS:


The exact mass of certain nuclides is shown below:


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Fragmentation


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In EI mass spectrometry the molecular ion is highly

energetic and can break apart (fragment).

Fragmentation pathways are predictable and can be usedto determine the structure of a molecule.

The processes that cause fragmentation are unimolecular.

The relative ion abundance is extremely important in

predicting structures of fragments.

g


Fragmentation by Cleavage at a Single Bond


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Cleavage of a radical cation occurs to give a radical and a

cation but only the cation is observable by MS. In general the fragmentation proceeds to give mainly the

most stable carbocation.

In the spectrum of propane the peak at 29 is the base peak

(most abundant) 100% and the peak at 15 is 5.6%:


Fragmentation Equations

The M+ Ion is formed by loss of one of its most loosely held


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The M+. Ion is formed by loss of one of its most loosely heldelectrons .

If nonbonding electron pairs or pi electrons are present, an electronfrom one of these locations is usually lost by electron impact to formM+.

Loosely held nonbonding electrons on nitrogen and oxygen, and electrons in double bonds are common locations for an electron tobe lost (i.e., where the remaining unshared electron in M+. resides).

In molecules with only C-C and C-H bonds, the location ofthe lone electron cannot be predicted and the formula iswritten to reflect this using brackets.


Example: The spectrum of hexane


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Example: spectrum of neopentane


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Fragmentation of neopentane shows the

propensity of cleavage to occur at a branch

point leading to a relatively stable

carbocation. The formation of the 3o carbocation is so

favored that almost no molecular ion is

detected.


Carbocations stabilized by resonance are also formedpreferentially.


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p y

Alkenes fragment to give resonance-stabilized allylic carbocations.

Carbon-carbon bonds next to an atom with an unsharedelectron pair break readily to yield a resonance stabilizedcarbocation.


Carbon-carbon bonds next to carbonyl groups fragmentreadily to yield resonance stabilized acylium ions


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readily to yield resonance stabilized acylium ions.


Alkyl substituted benzenes often lose a hydrogen or alkylgroup to yield the relatively stable tropylium ion


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group to yield the relatively stable tropylium ion.


Other substituted benzenes usually lose their substitutentsto yield a phenyl cation


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to yield a phenyl cation.


Fragmentation by Cleavage of 2 Bonds


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The products are a new radical cation and a neutral

molecule.

Alcohols usually show an M+.-18 peak from loss of water.


Cycloalkenes can undergo a retro-Diels Alder reaction to yield

an alkadienyl radical cation.


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Carbonyl compounds can undergo a McLafferty Rearrangement:

here Y ma be R H OH OR etc

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Chapter 1. (COE) Spectroscopy

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