CHEM2 Practical Course Spectroscopy Supplement

Lab Supplement: Fundamentals of Organic Spectroscopy

A. UV Spectroscopy

The -electrons of a molecule may be promoted to a higher energy level with absorption of light in the ultra-violet (200-400 nm) or visible (400-800 nm) region of the spectrum. Such an absorption appears as a broad band in the spectrum and indicates the presence of two or more conjugated double bonds (i.e. alternating double and single bonds). Figure 1 shows examples of some conjugated systems.

O

O

Figure 1: Some conjugated molecules

Figure 2 shows the UV spectrum of ethyl p-aminobenzoate (benzocaine) in ethanol solution, a plot of absorbance (A) versus wavelength (). The wavelength of maximum absorbance (max) is read directly from the spectrum and the molar extinction coefficient (or molar absorptivity) at max is calculated from the formula:

A = cl where c is the concentration of the solute in moll-1 and l is the pathlength in centimetres of the cell in which the sample is measured. The data from a UV spectrum is recorded in the following way:

Compound name: max (solvent) wavelength () nm

So for this example: Ethyl p-aminobenzoate: max (EtOH) 293 (18000) nm

The position of the absorption maximum and its intensity (which sometimes is recorded as its logarithm log ) gives information about the constitution of the organic molecule. In the case of ethyl p-aminobenzoate, the strong absorption confirms the presence of the aromatic ring.

Figure 2: UV spectrum of ethyl p-aminobenzoate.

O OCH2CH3

NH2

CHEM2 Practical Course Spectroscopy Supplement

B. IR Spectroscopy Absorption of infrared radiation is associated with vibrations within a molecule. Different functional groups absorb radiation of different energies. Not all peaks in the IR spectrum provide useful information. For example peaks in the region 14001000 cm-1 (called the fingerprint region) arise mainly from complex interactions involving many atoms and bonds. Table 1: Absorption regions for common functional groups

Functional Groups Chromophore (bond absorbing radiation)

Absorption Region

alcohols, phenols, amines, amides all organic compounds* aldehydes, ketones, esters, amides, carboxylic acids

OH, NH

CH*

C=O

33003600

29003000*

16301850

* Not diagnostic: present in all organic compounds

Example: Infrared spectrum of ethyl p-aminobenzoate (Figure 3). Useful frequencies in the IR spectrum are:

3410 cm-1 NH 3327 cm-1 NH 1680 cm-1 C=O

Note: 1280 cm-1 absorption is not useful as it lies in the fingerprint region.

Figure 3: IR spectrum of ethyl p-aminobenzoate in a KBr disc.

O OCH2CH3

NH2

3410

3327

1680 1280

CHEM2 Practical Course Spectroscopy Supplement

C. NMR Spectroscopy Nuclear magnetic resonance (NMR) spectroscopy is the most valuable spectroscopic technique available to organic chemists. NMR spectroscopy can be used to provide a map of the carbonhydrogen framework of an organic molecule. More specifically, NMR spectroscopy is concerned with the spin-state transitions of particular atomic nuclei (of non-zero spin quantum number) upon absorption of radio frequency energy, when placed in a strong magnetic field. All nuclei with an odd number of protons (e.g. 1H, 19F, 31P) or an odd number of neutrons (e.g. 13C) show magnetic properties. Only nuclei with an even number of both protons and neutrons (e.g. 12C, 16O) do not have magnetic properties. In structure determination of organic molecules 1H NMR spectroscopy and 13C NMR spectroscopy are the most valuable tools. 13C NMR Spectroscopy The frequency (measured in MHz) at which 13C nuclei absorb radiofrequency radiation depends mainly on the strength of the applied magnetic. The magnetic field experienced by each nucleus, however, is not exactly the same as the applied magnetic field because the electrons in the bonds very slightly alter the magnetic field in their immediate vicinity. Nuclei in different environments thus absorb at different frequencies an effect which is measured in Hz. Atoms (and therefore their nuclei) in exactly the same surroundings (by symmetry analysis) are called chemically equivalent.

All chemically equivalent atoms absorb at the same frequency in an NMR spectrum. In reporting the frequencies where groups of chemically equivalent carbons absorb, all signals are reported in terms of how far they are shifted from a reference signal. The difference between the reference signal and the signal of interest is termed the chemical shift and given the symbol . For 13C NMR spectroscopy, the reference signal is usually the single absorption of tetramethylsilane (TMS, Si(CH3)4) which is defined as 0.00 ppm Chemical shifts are reported in dimensionless units of ppm (parts per million) which are independent of the magnetic field strength at which the spectrum is obtained. In 13C NMR spectra there is one absorption for each different carbon environment in a molecule. If more than one carbon is in an identical environment within the molecule, the signals in the spectrum are coincident. For example, in the spectrum of propane (CH3CH2CH3), the two CH3 carbons appear as one line which is in a different place to the signal from the CH2. In the 13C NMR spectrum the frequency of absorption is represented by the scale which runs from 0 to 200 with units of parts per million (ppm). A particular carbon environment results in a signal in a characteristic region of the spectrum, summarised below:

200 150 100 50 0

C O C C

(ppm)

C C(alkenes)(aromatic

rings)

C O C CC H(alkanes)

TMS

Figure 4: Approximate positions of common signals in a 13C NMR spectrum.

CHEM2 Practical Course Spectroscopy Supplement

1H NMR Spectroscopy

A 1H NMR spectrum provides four kinds of chemical information:

i. the position of signals along the spectrum (the chemical shift) is sensitive to the environment of the hydrogen atoms, i.e. it reveals the different types of hydrogen present:

aromatic vinylic Hydrogens (Hs on unsaturated Cs) ~ 510 ppm

aliphatic Hydrogens (Hs on saturated Cs) ~ 05 ppm

Hydrogens adjacent to oxygen (eg OCH2) ~ 3.54.0 ppm

Hydrogens adjacent to carbonyl groups ~ 2.02.5 ppm

ii. the area under the signals (as determined by integration of the peak) is proportional to the relative numbers of hydrogen atoms giving rise to each signal.

iii. the fine structure of signals (the splitting pattern or multiplicity) reveals the number of hydrogens on adjacent carbon atoms. The number of lines that a signal is split into is equal to (n + 1) where n is the number of hydrogens on the adjacent carbon atom(s).

iv. OH, CO2H, and NH hydrogen atoms are readily exchanged by deuterium atoms when the solution is treated with D2O, this means that such signals disappear from the spectrum, as deuterium is not detected in 1H NMR spectroscopy.

Putting that all into a handy table

Observation Information

Number of signals: Position of signal (chemical shift): Area of signal: Splitting of signals: Exchangable with D2O:

Number of H environments Type of H environment Number of Hs of each type Number of adjacent hydrogens H of OH, CO2H or NH

Table 2: Information that can be obtained from a 1H NMR spectrum

CHEM2 Practical Course Spectroscopy Supplement

Example: 1H NMR spectrum of ethyl p-aminobenzoate (Figure 5).

Figure 5: 1H NMR spectrum of ethyl p-aminobenzoate in CDCl3 solution; the broad

resonance near 4 ppm disappears when a drop of D2O is added.

Analysis of above spectrum:

Number of signals in the spectrum (5) = number of different H environments. Signal at 1.30: is a triplet from the (n + 1) rule it must have 2 neighbouring

hydrogens in a different chemical environment.

this is the methyl group CH3 which is next to the CH2 Signal at 4.27: is a quartet from the (n + 1) rule it must have 3 neighbouring

hydrogens in a different chemical environment.

this is the CH2 group which is next to the CH3 Signal at 4.13: is a broad singlet which disappears on adding a drop of D2O.

this is the NH2 Signals at 6.57 and 7.80 are both doublets and are due to

the aromatic hydrogens. Because of free rotation around the bonds as shown on the right, the pair of hydrogens Ha are in identical chemical environments and appear as one signal. And the pair of hydrogens Hb are also in identical chemical environments and hence appear as one signal. Each Ha has one neighbour (Hb) hence by the (n + 1) rule appears as a doublet.

The area of each signal (indicated by the height of the integration curve above the signal) confirms these assignments:

each aromatic signal corresponding to 2 H the signal at 4.27 also corresponds 2 H and the signal at 1.30 corresponds to 3 H

O OCH2CH3

NH2

Ha

Hb

Ha

Hb

1H NMR Spectrum

exchanges with D2O

CHEM2 Practical Course Spectroscopy Supplement

D. Mass Spectrometry

A mass spectrum is produced by first ionising a molecule (by expelling an electron from the molecule at high energy or by protonating the molecule (both of which give positive ions), or by forcing the molecule to absorb an extra electron, to give a negative ion), and then passing the ions through a magnetic field which separates them on the basis of their mass:charge ratio (m/z).

The mass of the heaviest ion usually corresponds to the molecular weight of the compound. If so, this ion is referred to as the molecular ion, derived from the loss/gain of one electron from/by the original molecule. The molecular ion can also decompose to give different fragments, with different masses. The nature of the fragment ions formed depends on the original bond energies present in the original structure. The intensities of the major peaks in a mass spectrum are recorded relative to the biggest peak (called the base peak) which is nominally assigned an intensity of 100%.

When writing up a mass spectrum, precisely how many peaks are recorded depends upon the complexity of the spectrum and the significance of the peaks. Peaks of less than 10% intensity are not normally noted, unless they correspond to the molecular ion or other diagnostic fragments. Isotope peaks are ignored for C,H,N, and O, but are included for halogens.

Example: Mass spectrum of ethyl p-aminobenzoate (Figure 6).

Figure 6: Low resolution mass spectrum of ethyl p-aminobenzoate

This spectrum is recorded in text form as follows::

m/z 165 (35%, [M+]), 137 (12%), 120 (100%, [M OEt]+), 92 (24%), 65 (22%) Note: m/z is the mass/charge ratio. Single positively charged ions are detected (ie z= 1.

And the structures of the major ions are as follows:

O OCH2CH3

NH2

HO O

NH2

O

NH2NH2

m/ z = 165 m/ z = 137 m/ z = 120 m/ z = 92

Mass Spectrum 120

165

M+.

137 92

CHEM2 Practical Course Spectroscopy Supplement

E. References

Revision References First Year Chemistry Lecture Notes

First Year Chemistry Laboratory Manual E35 Spectroscopy Workshop

McMurry Chapters 12 (MS & IR), 13 (NMR) and 14 (UV/VIS)

Further Reading

Spectroscopic Methods in Organic Chemistry Dudley H. Williams, Ian Fleming. Madsen 547.346 8 B [Link to book in USyd Library]

Organic Structures from Spectra.

Leslie D. Field, Sever. Sternhell, J.R. Kalman. Madsen 547.30858 4 A [Link to book in USyd Library]

Introduction to Organic Spectroscopy

Laurence M. Harwood, Timothy D.W. Claridge Madsen 543.0858 67 [Link to book in USyd Library]

Basic 1H- and 13C-NMR Spectroscopy

Metin Balc. Madsen 543.66 6 [Link to book in USyd Library] [Link to e-Book]

Nuclear Magnetic Resonance Spectroscopy: an Introduction

Joseph B. Lambert, Eugene P. Mazzola. Health Sciences 543.0877 LAM [Link to book in USyd Library]

IR Spectroscopy: an Introduction

Helmut Gnzler, Hans-Ulrich Gremlich Madsen 543.08583 17 [Link to book in USyd Library]