Structure Determination:

12.5 Spectroscopy of the Electromagnetic Spectrum

Structure Determination:

„ Radiant energy is proportional to its frequency

Infrared Spectroscopy

„ Different types are classified by frequency or

(cycles/s = Hz) as a wave (Amplitude is its height) wavelength ranges

Chapter 12 - Part 2

Spectroscopy of the Electromagnetic Spectrum

Properties of Electromagnetic Radiation

„ γ-rays (gamma rays)- greatest energy and highest

„ The mode of propagation of electromagnetic

„

„ The wave is characterized by its wavelength

„ „ „ „ „

frequency. Emitted from some radioactive nuclei. Can cause biological damage. X-rays- lower in energy than γ -rays. Can cause biological damage in high doses. Ultraviolet (UV) light- can cause sunburn and even skin cancer. Visible light- speaks for itself ! Infrared radiation (IR) - heat! Microwaves- we cook with them and also used in radar. Radio waves- lowest frequency. Radio and TV transmissions and NMR spectroscopy.

radiation is the ______.

(__), frequency (__), and amplitude.

„ Frequency (ν) units: s-1 or ________ „ The intensity of a wave is proportional to the

square of its amplitude.

„ Electromagnetic radiation travels at constant

velocity in a vacuum: 3.00 x 1010 cm/s (speed of light).

Properties of Electromagnetic Radiation

Electromagnetic Energy

Wavelength x Frequency = Speed

„ Thanks to Max Planck

and Al Einstein: λ(cm) x ν(s-1) = c (cm/s)

E = Energy of 1 photon (1 quantum) h = Planck’s constant (6.62 x 10-34 J·s = 1.58 x 10-34cal·s) ν = Frequency (s-1) λ = Wavelength (cm) c = Speed of light (3.00 x 1010 cm/s)

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Electromagnetic Energy

Electromagnetic Energy

„ The energy of a photon varies directly with

„ Quick calculations:

the frequency ν and inversely with the wavelength λ „ High frequencies and short wavelengths = ________ „ Low frequencies and long wavelengths = ________

E=

N A hc

λ

=

Absorption Spectra „ Organic compound exposed to electromagnetic

radiation, can absorb energy of only certain wavelengths (unit of energy) „ Transmits, energy of other wavelengths. „ Changing wavelengths to determine which are absorbed and which are transmitted produces an ________ „ Energy absorbed is distributed internally in a distinct and reproducible way (See Figure 12-11)

12.6 Infrared Spectroscopy of Organic Molecules

12.6 Infrared Spectroscopy of Organic Molecules

„ IR region lower energy than visible light (below red –

„ The IR spectrum covers the range from

produces heating as with a heat lamp)

„ ________ to ________ region used by organic

chemists for structural analysis

„ IR energy in a spectrum is usually measured as

4000 cm-1 to 400 cm-1 „ This represents energy ranges from

48.0 - 4.80 kJ/mol (11.5 - 1.15 kcal/mol).

wavenumber (cm-1), the inverse of wavelength and proportional to frequency „ Specific IR absorbed by organic molecule related to its structure

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Convert the following: „ 3.10 µm to cm-1 „ 5.85 µm to

cm-1

„ 2250 cm-1 to µm „ 970 cm-1 to µm „ Calculate the energy (kJ)/mol for the first two

Infrared Energy Modes „ IR energy absorption corresponds to specific modes,

corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called “normal modes” „ Energy is characteristic of the atoms in the group and their bonding „ Corresponds to vibrations and rotations

wavelengths.

12.7 Interpreting Infrared Spectra „ Most functional groups absorb at about the same

energy and intensity independent of the molecule they are in „ Characteristic higher energy IR absorptions in Table 12.1 can be used to confirm the existence of the presence of a functional group in a molecule „ IR spectrum has lower energy region characteristic of molecule as a whole (“fingerprint” region; 1300 to 625 cm-1) „ See samples in Figure 12-13

Regions of the Infrared Spectrum „ 4000-2500 cm-1 N-H, C-

H, O-H (stretching) „ 3300-3600 N-H, O-H „ 3000 C-H „ 2500-2000 cm-1 C≡C and C ≡ N (stretching)

„ 2000-1500 cm-1 double

bonds (stretching) „ C=O 1680-1750 „ C=C 1640-1680 cm-1 „ Below 1500 cm-1 “fingerprint” region

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Differences in Infrared Absorptions

12.8 Infrared Spectra of Hydrocarbons

„ Molecules vibrate and rotate in normal modes, which

„ C-H, C-C, C=C, C

are combinations of motions (relates to force constants) „ Bond stretching dominates higher energy modes „ Light objects connected to heavy objects vibrate fastest: C-H, N-H, O-H „ For two heavy atoms, stronger bond requires more energy: C ≡ C, C ≡ N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen

„

≡ C have characteristic peaks absence helps rule out C=C or C ≡ C

12.9 Infrared Spectra of Some Common Functional Groups

IR: Alcohols and Amines

„ Spectroscopic behavior of functional group is

„ O–H 3400 to 3650 cm−1

discussed in later chapters „ Brief summaries presented here

„ N–H 3300 to 3500 cm−1

„

„

1-butanol

Usually broad and intense Sharper and less intense than an O–H

butylamine

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IR: Aromatic Compounds

IR: Carbonyl Compounds

„ Weak C–H stretch at 3030 cm−1

„ Strong, sharp C=O peak 1670 to 1780 cm−1

„ Weak absorptions 1660 - 2000 cm−1 range

„ Exact absorption characteristic of type of carbonyl

„ Medium-intensity absorptions 1450 to 1600 cm−1 „ See spectrum of phenylacetylene, Figure 12.15

compound „ 1730 cm−1 in saturated aldehydes „ 1705 cm−1 in aldehydes next to double bond or aromatic ring

C=O in Ketones „ 1715 cm−1 in six-membered ring and acyclic ketones „ 1750 cm−1 in 5-membered ring ketones „ 1690 cm−1 in ketones next to a double bond or an

aromatic ring

C=O in Esters

Carboxylic Acids

„ 1735 cm−1 in saturated esters

1700 - 1725 cm-1 (strong)

„ 1715 cm−1 in esters next to aromatic ring or a

O

double bond

R

OH 2500 - 3300 cm-1 (very broad and intense)

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Pentanoic acid

Identify the functional groups in compounds that are responsible for the following absorptions:

„ A compound with a strong absorption at 1710 cm-1 „ A compound with a strong absorption at 1540 cm-1 „ A compound with a strong absorption at 1720 cm-1 and at

2500-3100 cm-1

How might you use IR spectroscopy to distinguish between the following pairs of isomers?

Last Slide Chapter 12 Part 2

„ CH3CH2OH and CH3OCH3 „ Cyclohexane and 1- hexene

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