Infrared Spectroscopy

IR eradiation refers to that part of the electromagnetic spectrum between visible and microwave regions. Frequency, n in Hz ~1019 ~1017 ~1015 ~1013 ~1010 ~105 Wavelength, l ~.0001 nm ~0.01 nm 10 nm 1000 nm 0.01 cm 100 m Energy (kcal/mol) > 300 300-30 300-30 ~10-4 ~10-6 g-rays X-rays UV IR Microwave Radio nuclear excitation core electron excitation (X-ray cryst.) electronic excitation (p to p*) molecular vibration molecular rotation Nuclear Magnetic Resonance NMR (MRI) Visible

The IR wavenumber are divided into three regions : The far IR : 40-400 cm-1 The mid IR : 400-4000 cm-1 The near IR :4000-12000 cm-1 - The units used in IR spectroscopy are (cm-1) . - The most of organic compounds absorbs in mid-IR region because the stretching vibrations occur in this region . - The absorption appears as bands rather than lines .

Wavelength (λ) : is defined as the distance between two consecutive crests C or troughs T. measured in micrometer (µm) or micron (µ), 1 µm =1 µ =10-6 m nanometer (nm) or millimicron (m µ) 1 nm = 1 m µ =10-9 m , angstrom (A0) (l A0 = 10-10m) Wavenumber (υ) : the total number of waves which can pass in one cm . expressed in centimeter (cm-1) Frequency (v) : is defined as the number of waves which can pass in a point in one second, measured in cycles per second (cps) or hertz (Hz) (1Hz= 1 cps).

where c is velocity of light (2.998 x 1010 cm/s). By their definitions, frequency and wavelength are inversely proportional, where c is velocity of light (2.998 x 1010 cm/s). Energy E where h is Planck's constant (6.626 x 10-27 erg s). The higher the frequency ( or the shorter the wavelength) of the radiation, the greater is its energy.

Are all the vibrations IR-active?! For a molecule to show infrared absorptions it must possess a specific feature, i.e. an electric dipole moment of the molecule must change during the vibration. This is the selection rule for infrared spectroscopy. ‘infrared-active’ molecule, a heteronuclear diatomic molecule. The dipole moment of such a molecule changes as the bond expands and contracts ‘infrared-inactive’ molecule is a homonuclear diatomic molecule because its dipole moment remains zero no matter how long the bond. Change in the dipole moment of a heteronuclear diatomic molecule.

So, what is the IR-inactive vibration?! If a bond is symmetrical and has zero dipole moment, the electric field does not interact with the bond. e.g. The triple bond of the acetylene molecule: H-C≡C-H which has zero dipole moment (whether the bond is stretched or compressed), so the vibration does not produce any change in the dipole moment and consequently there is no energy absorption. This kind of vibrations said to be IR-inactive because it produces no absorption in the IR spectrum. Q/ some molecules such as N2 , Cl2 , H2 do not appear band in IR spectroscopy? Ans : Because they have symmetrically and have zero dipole moment .

Types of basic vibrations : There are two types of vibrations in IR spectroscopy : (1) Stretching vibrations (υ) : the distance between the two atoms increases or decreases but the atoms remain in the same bond axis . There are two types of Stretching vibrations in IR spectroscopy : (a) Symmetric Stretching (υ sym.) : the movement of the atom with respect to central atom in molecule is in the same direction . (b) Asymmetric Stretching (υ asym.) : one atom approaches central atom while the other departs from it.

(2) Bending vibrations (δ) : The positions of the atoms change with respect to the original bond axis . There are four types of Bending vibrations in IR spectroscopy : (a) Scissoring : two atoms approach each other . (b) Rocking : the movement of the atoms in the same direction . (c) Wagging : two atoms move up and down of the plane with respect to the central atom . (d) Twisting : one of the two atoms moves up of the plane while the other moves down of the plane with respect to the central atom .

Vibrational frequency : Hooks law shows relationship between the wavenumber or frequency with force constant (K) and reduce mass (μ) . υ=1/λ , υ = C/ λ = C υ υ = frequency υ = wavenumber C = velocity of light μ = reduce mass K = force constant of the bond υ/ = υ / C