1 - introduction to organic spectroscopy · 2017-03-06 · chem*3750 course notes | 1 of 26 1 -...

CHEM*3750 COURSE NOTES | 1 of 26

1 - INTRODUCTION TO ORGANIC SPECTROSCOPY

SF: 10th EDITION; SFS: 11th EDITION

1. 1H Nuclear Magnetic Resonance Spectroscopy (SF 9.2-9.9; SFS 9.2-9.9)

Atomic nuclei with odd mass numbers have angular momentum and behave as though they were spinning on an axis, that is, they behave as a tiny magnet. In the absence of an applied field, these tiny magnets are randomly oriented, but when they are put in the presence of an applied magnetic field (Ho), they will align with or against

that field. A slight excess will align with the field (-spin) while the others will be anti

parallel (-spin).

The spin nuclei are higher in energy and irradiation of light can induce the

flipping of-spins to -spins. Such a process is called resonance.

These concepts apply to 1H, 13C, and 19F nuclei which have spin quantum

numbers of ±1/2. i.e., two spin states. Many other atoms are also NMR active and they

may have positive integers as spin states. Atoms with spin quantum numbers of zero are not NMR active.

Alignment in the presenceof a strong magnetic field

In the absence of anapplied magnetic field

Ho

-spin -spin


The two energetically different states afford the necessary condition for resonance. Energy differences can be quantitated to be

Ho is the applied field strength at the nuclei

is a constant of the nucleus, the magnetogyric ratio. The frequency has units of Hertz (Hz) or cycles per second. One can irradiate the nuclei and achieve the 'flips' or resonance. The irradiation is in the radio frequency region of the electromagnetic spectrum. The rf is varied and when the light energy matches the spin state energy difference, then the signal (a resonance) occurs. An important element of nuclear magnetic resonance is that the energy difference is proportional to the strength of the applied magnetic field.

a) Chemical Shift

Not all protons (for 1H NMR spectroscopy) in a given molecule absorb the same energy. The position of an NMR signal depends on the nucleus's electronic environment. In a covalently bound organic molecule, different hydrogens are under the influence of a number of electronic factors. The principal factors include the hybridization of the attached atom, the polarity of the bond and the presence of electron withdrawing or electron donating groups nearby. The applied magnetic field induces movement of the electrons in the bond, which in turn induces smaller magnetic fields. One may view the extremes of electronic environment as a hydrogen cation and a

=Ho

2

energy

external magnetic field strength, Ho

magnetic moment of

nucleus is opposed to Ho

magnetic moment of

nucleus is aligned with Ho


hydrogen anion. On the 1H NMR scale the free proton would resonate at the extreme left while the free hydride would be observed at the extreme right.

When hydrogens are surrounded by electron density, they shift upfield and are said to be shielded. Conversely, when their environment is lacking in electron density, the hydrogens are deshielded and move downfield. For example, consider the following trends based on electronegativity;

The scale is based on the resonance position of tetramethylsilane ((CH3)4Si, TMS)

which is observed as a singlet at 0.00 ppm. Any organic compounds that you will be seeing can be found downfield from TMS. Some organometallic compounds may have hydrogens that resonate upfield from TMS. Most hydrogens of organic compounds will

be found between 0 and 10 on the scale, although the hydrogens of carboxylic acids

are often observed farther downfield. The value can be calculated as follows:

upfielddownfield

Increasing radio frequency (rf), (Ho constant)

Increasing chemical shift ( scale

Increasing applied magnetic field (Ho), (rf constant)

typical 1H region since proton and hydride are the extremes

H-H

+

CH3I

2.10iodomethane

CH3Br

2.70bromomethane

CH3Cl

3.05chloromethane

CH3F

4.30fluoromethane

= (sample) - (TMS)

of the instrument ( Ho)X 106


A chemical shift expressed in has units of parts per million (ppm). All hydrogens

that are not chemically equivalent will afford a unique resonance in the 1H NMR

spectrum, since each is in a unique environment in the molecule. Table 9.1 on SF p.

387 (SFS p. 393 Table 9.1) gives a listing of typical peak positions.

For typical chemical shifts, some important beginning points are 0.9 ppm for a methyl group, 5.25 ppm for a hydrogen on a double bond and 7.27 ppm for aromatic hydrogens. Some examples: 1,4 Dimethoxybenzene


Methyl propiolate (Methyl propynoate)

Succinimide

NH

O

O


b) Signal Areas The area under the curve of a given signal is proportional to the number of hydrogens responsible for the signal. Hence if one can integrate the signal, one would have the relative number of hydrogens for each resonance.

So to summarize, to this point, 1H NMR tells what sort of electronic environment that the individual hydrogen or groups of hydrogens are in while the integration tells us the number of hydrogens at a given resonance. 1,4-Dimethoxybenzene


Methyl Propiolate

Succinimide


c) Signal Splitting (Coupling) When hydrogens are close to one another in a molecule, they usually exert a slight chemical shift influence on one another. Consider two non-equivalent hydrogens as shown below:

The chemical shift of Ha is influenced by whether the spin of Hb is aligned with or against the applied field. If the spin of Hb is aligned with the applied field, it adds to the applied field and a smaller external strength is required than that necessary for Ha in the absence of any perturbations. Hence a peak at lower field is observed. However this absorption size is only half of the Ha protons since only half of the Hb protons are in the

spin state. The other half of Hb are in the spin state and since those are aligned against the external field, the local field strength around Ha in this case is diminished. To achieve resonance, Ho has to be increased and an upfield shift is observed. The resonance of Ha is said to be split into a doublet. Note that one hydrogen has induced the appearance of a doublet and also note that the doublet peaks are of equal height. The same arguments apply to the influence of Ha on the appearance of the Hb resonance are applicable. Indeed the coupling constant, J , is defined as the distance between the two split peaks. It is the same for both Hb's effect on Ha and Ha's effect on Hb.

C C

Ha Hb

Ha without the

influence of Hb

Hb is aligned

with theapplied field

Hb is aligned

against theapplied field

Ha is split into a doublet


When Ha is near two Hb's (equivalent H's) then the Hb resonance will again be a doublet as it is affected by a lone hydrogen. However, Ha will now feel the effect of two hydrogen's spin states. The various combinations will afford a triplet (three peaks) in a 1:2:1 ratio. Note that two hydrogens have induced a triplet.

Similarly if Ha is near a methyl group, i.e., three equivalent Hb's, then Ha will appear as a quartet due to 8 possible spin orientations and their relative ratio is 1:3:3:1. Note that three hydrogens have induced splitting and the appearance of a quartet. The number of lines that appear for a given resonance is the multiplicity.

Ha

spin of Hb:

Jab

Jab = coupling constant

Hb

spin of Ha:

Jab

1a

2b

1c

1a

3b

3c

1d


The lines that appear are all equally spaced, by a distance that is the Jab coupling

constant. The coupling constant may vary and is on the order of 0 - 20 Hz. In freely rotating systems, H-C-C-H, the four bond coupling constant is ca. 6-8 Hz. There are other molecular subunits for which the coupling constant may be known or accurately predicted.

As an example n-propyl chloride with three different types of hydrogens would afford the following types of peaks.

Note that the central methylene group is coupled to two sets of different hydrogens. The resonance appears as a sextet because the coupling constant are essentially the same to each of the other hydrogens. That is the central methylene group is coupled to five hydrogens with J = 7 Hz. Many coupling constants can be predicted (or expected) base on a significant compilation of data from millions of compounds. Typical values are listed in a table on the next page. This information is a useful reference.

Jab Jab Jab Jab Jab

ca. 3.5 ppm (triplet, J = ca. 7 Hz, 2H)

ca. 1.7 ppm (sextet, J = ca. 7 Hz, 2H)

ca. 1.1 ppm (triplet, J = ca. 7 Hz, 3H)

CH2Cl

CH2

CH3


Representative Coupling Constants

Coupling arrangement

J (Hz)


J (Hz)


J (Hz)

6 to 8

6 to 10 (ortho)

a,a: 8 to 14 a,e: 0 to 7 e,e: 0 to 5

11 to 18

8 to 11

cis: 6 to 12 trans: 4 to 8

6 to 15

5 to 7

cis: 2 to 5 trans: 1 to 3

4 to10

0 to 5 (often 0)

0 to 1.5 (4 bond!)

There are typical and recognizable coupling patterns that are indicative and rather diagnostic of certain fragments of molecules. The table on the following page. There are times when a given hydrogen is surrounded by a number of different hydrogens and they all couple with a different coupling constant. Then there can be a series of splittings that are superpositioned on one another. This type of resonance is sometimes simply called a multiplet.

C C

HHH

H

H

H

H

H H

H

H

H

HH

H

H

O

H

H

H

CH R R'

HHH C

H


Commonly Observed Splitting Patterns

appearance situation appearance

Some examples: trans-3-phenylpropenal

CH CH

YXX Y

CH2 CH

CH2X CH2 Y

X Y

CH3 CH

CH3 CH2

CH

CH3

CH3


3-methyl-1-butanol

borneol

Me

MeMe

OH


2. 13C NMR Spectroscopy (SF 9.11; SFS 9.11)

The major isotope of carbon (12) does not does not have magnetic spin and cannot produce NMR signals. However, carbon-13 which is 1.1% naturally abundant has a spin number of ½. Using a pulsed NMR instrument so that numerous scans can

be accumulated, 13C NMR resonances can be observed and are very useful. Some of the initial points to be mentioned are that coupling is usually not

observed in 13C NMR spectra. First of all, having two 13C atoms side by side is

statistically rare and so most often 13C atom is beside a 12C atom and there is no

coupling. 1H-13C coupling can be readily observed but in most cases, the NMR

acquisition is carried out so that no coupling is observed and the 13C signals appear

simply as single lines. The 1H-13C coupling is eliminated by broad band proton

decoupling, a process that is achieved by continuously irradiating the 1H NMR

frequency range with another radio frequency source. The result is that -spin and -spin energy levels do not hold any protons very long and thus they cannot be observed

by the 13C atoms. This makes the protons essentially invisible.

A broad band decoupled 1H NMR spectrum will show a number of vertical lines. There will be one line for each carbon in the molecule, once symmetry considerations have been taken into account. Solvent peaks are also evident. a) Symmetry

Gaining information from a 13C NMR spectrum is often simply an exercise in symmetry. The spectra of two isomeric trimethylcyclohexanes on the following page illustrate this point.

b) 13C Chemical Shifts

The 13C NMR scale is much wider than that for the proton. A typical ppm range is

-15 to +240 ppm, with most peaks occurring between -5 and +215 ppm. Again the 0.0 point on the scale is defined by the position of the peak of TMS, but in this instance, it is

a 13C resonance. TMS has only one peak because all the methyl carbons in the molecule are identical due to symmetry.

There can be less predictability in the 13C NMR spectrum, but there are

nevertheless expected ranges of chemical shifts for functional groups. In Table 9.2 SF p. 418 (SFS p. 424), a listing is provided. The trends in 13C chemical shifts essentially

follow those of 1H chemical shifts. That is, electronegative atoms push the chemical shift downfield, and the more of those atoms directly attached, the farther the resonance is shifted.


On the next page the 13C NMR spectra of three isomeric dichlorobenzenes can be solved on the basis of symmetry and intensity of signals.

0 ppm20406080

0 ppm20406080


Some examples: Succinimide

o-dichlorobenzene

m-dichlorobenzene

o-dichlorobenzene

140 130 120 140 130 120 140 130 120


Ethyl oxirane (ethyl epoxide, 1,2 epoxybutane)

1-decene


expansion of a portion of 1-decene


3. Infrared (IR) Spectroscopy (SF 2.15-2.16; SFS 2.15-2.16)

The relative movements of covalently bonded atoms within a molecule are usually called vibrations. The vibrational levels of atoms are quantized and the energy levels for typical molecules cover the range from 2 to 10 kcal/mol. The vibrations can be induced by irradiation of light in the infrared region of the electromagnetic spectrum. In relation to one another, atoms can undergo a collection of different vibrations. For these vibrations to be measurable in the IR spectra, two criteria must be met. 1. the frequency of the applied IR radiation must match that of the vibration and 2. the bond(s) undergoing the vibration must have a dipole moment. That is, it must be a polar bond and furthermore, the greater the polarity, the more intense the absorption. If these two criteria are met, then the vibration is said to be infrared-active. Symmetric bonds such

as Br-Br, C=C or CC do not have dipole moments, although the carbon-carbon bonds can be easily influenced by substituents. Stretching Vibrations:

Bending Vibrations: (in-plane) (out-of-plane)

C

R R

H H

C

R R

H H

a) symmetric b) asymmetric

a) scissoring b) rocking

C

R R

H H

C

R R

H H

c) wagging d) twisting

C

H H

R R

C

H H

R R


Stretching has different modes and can also be observed for two-atom frameworks such as carbonyls. In fact the simple stretching of a two atom system is modeled by Hooke's law for a harmonic oscillator;

where:

A sample calculation for a 12C-1H stretch gives a frequency prediction of 3032

cm-1 while the usual range for these systems is 3060-2850 cm-1. So the model is fairly good. Note that the most common isotopes were used in this case. The use of "wavenumbers" as the units in this spectroscopy allows the chemist to relate the spectrum directly to energy in the bonds of the molecule since wavenumber

(frequency/speed of light) is directly proportional to energy. Wavelength (= 1/) is directly related to the reciprocal of energy and is a less preferred term. An IR frequency

range of 4000 - 600 cm-1 is typical for an organic chemist and valuable information about the presence and absence of functional groups can be gained from the analysis of an IR spectrum of a compound. Typical values for stretching frequencies of some functional groups are listed in the table below

BOND ABSORPTION REGION (CM-1)

C-C, C-N, C-O 1300-800 C=C, C=N, C=O 1870-1500

CC, CN 2300-2000

C-H, N-H, O-H 3650-2850

Note some of the trends in the table. Going from single to multiple bonds requires more energy. In fact single bonds of carbon and another non-hydrogen atom can be difficult to assign since they occur in a crowded region of the spectrum. However, multiple bonds are usually easy to assign and indeed can provide more diagnostic information.

= 1

2cf(m1 + m2)

m1m2

= vibrational frequency in wavenumbers (cm-1)

c = velocity of light in cm/sec

f = force constant in g/sec2

m1 = mass of atom 1 in gm2 = mass of atom 2 in g


The other trend to notice is that smaller atoms give higher energy vibrations. Hence, it important to look for and assign C-H, N-H, O-H and S-H bonds. Similarly, C-2H stretching frequencies can readily be observed. Analysis of the harmonic oscillator equation indicates that vibrations involving smaller atoms should occur at higher energy. A view of a typical IR spectrum helps to put things in perspective.

Region A: -usually X-H single bonds -diagnostic region of spectrum -C-H stretching peaks will virtually always be present in this area

Region B: -triple bond range (i.e., -CC-, -CN) -also X=Y=Z region where X, Y, Z= C, O or N (called a cumulene) -usually devoid of peaks if triple bond or cumulene is not present in the

molecule Region C: -area for the observation of double bonds involving carbon, nitrogen and

oxygen. -area includes aromatic multiple bonds -very diagnostic area Region D: -least diagnostic area in terms of individual peaks

-but as a whole, this section is called the "fingerprint region" of the spectrum since no two compounds will be exactly alike throughout this region.

4000 6003000 2000 1000

A B C D

Frequency (wavenumbers (cm-1))


-contains many C-O, C-C and C-N single bonds whose existence may have been determined by other methods and which are present in many organic molecules

SF p. 86 (SFS p. 89) provides a good table of IR band values for a collection of

functional groups. Spectra can be obtained by a variety of methods. The sample is usually held in place by salt* plates of some sort. These may be the "windows" on a solution cell or they may act as a surface on which one can spread a thin film of the compound. If the material is a crystalline solid, then a mull is sometimes prepared to make the sample more manageable. The mull is prepared with nujol which is a mixture of thick, high boiling alkanes. Hence your spectrum will have interference: the peaks of the nujol. Another possibility when working with a solid is to grind it together with a salt such as KBr. The result is a pellet comprised of finely divided salt and substrate. The resulting "KBr pellet" can then be placed in the spectrometer. The C=O stretching frequency of carbonyl compounds can often be quite informative about the type of carbonyl compound under scrutiny. Note the table and stretching values.

C=O of Saturated Unsaturated

aldehyde 1725 cm-1 1690 cm-1

ketone 1715 1680

carboxylic acid 1730-1700 1680

ester 1735 1710

amide 1680-1640

acid chloride 1800 1780

anhydride 1820, 1740 --

* Many salts made from a halogen and an alkali metal are transparent in the practical infrared regions of the electromagnetic spectrum.


Cycloheptanone

4-cyanobenzaldehyde


As stated, spectra are sometimes obtained as a nujol mull. That means the IR spectrum of your substrate will be contaminated (masked) by the following peaks.

#peak wavenumber intensity 1 1376.15 .390 2 1457.35 .264 3 2857.57 0.092 4 2925.86 0.042 The latter two peaks are approximate but are characteristic what is observed when there are many aliphatic C-H bonds in the molecule. Nujol mull of 4-phenyl-3-buten-2-one

Ph Me

O


peaks detected by instrument: #peak wavenumber intensity 1 745.01 .605 2 977.51 .638 3 1075.59 .731 4 1176.50 .678 5 1253.97 .654 6 1370.03 .552 (nujol 1376.15) 7 1455.38 .538 (nujol 1457.35) 8 1609.16 .622 9 1677.40 .597 10 2908.65 .663 (also nujol)


4. Other Spectroscopic Methods -each of these concepts will be addressed in more detail in CHEM*3760.

a) Mass Spectrometry (SF 9.13-9.17; SFS 9.13-9.17)

The chapter in SF/SFS outlines the principles behind acquisition and

interpretation of mass spectra. The result for an organic chemist is that one can determine the molecular weight of a compound and can usually also get information regarding type of functional group and its position of attachment in the molecule. More intricate information such as that regarding stereochemistry may also be possible, but it requires good familiarity with the mass spectrometer and with the family of compounds that are being studied. Rarely is mass spectrometry used exclusively as a structure determination tool, but it is a valuable complement to IR and NMR.

b) Ultraviolet and Visible Spectroscopy (SF 13.9; SFS 13.8)

Transitions between electronic energy levels of a molecule give rise to this field of spectroscopy. Absorption spectroscopy can provide information regarding functional groups and conjugation close to those functional groups. Again, it is not valuable for structural identification by itself, but is useful in tandem with other methods. A handout with more information is available for interested individuals.

1 - introduction to organic spectroscopy · 2017-03-06 · chem*3750 course notes | 1 of 26 1 -...

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