17.2 Infrared spectroscopy

why are atoms in molecules in constant motion

atoms in molecules are joined by covalent bonds

tese bonds possess energy and vibrate naturally about a central point

these molecules can absorb IR which makes them bend or stretch more

stretch

a rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases

bend

results in change in bond angle

2 things the amount that a bond stretches or bends depends on

mass of the atoms in the bond

strength of the bond

steps of global warming

1) radiation passes through the atmosphere to the Earth’s surface where most of it is absorbed

2) some radiation is re-emitted from Earth’s surface in the form of longer-wavelength IR radiation

2) water vapour, CO₂, methane absorb this longer-wavelength IR

4) the vibrating bonds in these molecules re-emit this energy as radiation

5) this increases the temperature of the atmosphere close to Earth’s surface

steps of infrared spectroscopy

1) sample is placed inside an IR spectrometer

2) a beam of IR radiation n the range 200-4000cm^-1 is passed through the sample

3) the molecule absorbs some of the IR frequencies

4) the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample

5) the IR spectrometer is connected to a computer that plots a graph of transmittance against wavenumber

O-H alcohols

3200-3600

C=O

1630-1820

O-H carboxylic acid

2500-3300

fingerprint reigon

the peaks below 1500cm^-1

why d all organic compounds produce a peak between 2850-3100cm^-1

presence of C-H bonds

infrared spectroscopy

use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule