Skip to main content

Hooke’s Law for IR spectrscopy

Hooke's Law is an equation which helps in interpreting the molecular vibrations by assuming bonds as spring having masses (atoms) at two ends. When a molecule is exposed to infrared light, the bonds between atoms can absorb specific frequencies of that light, causing the bonds to stretch, compress or bend, just like a spring. This characteristic absorption of light corresponding to certain specific vibrational frequencies helps in identifying the functional groups in that molecule and thus adding to the characterization of molecular structures.  The mathematical expression of Hooke’s law is given as follows:

ν = 1/2π√k/µ

ῡ = 1/2πc√k/µ

where:

· ν = the frequency of vibration

· ῡ = Wave number (reciprocal of wavelength)

· k = the force constant

· c = Velocity of light

· Π = Constant

· μ = reduced mass

The reduced mass of the two atoms involved in the bond is calculated as follows:

1/µ = 1/m1 +1/m2;

                                               µ = m1.m2/m1+m2

Streching frequency is directly proportional to force constant. So, Stronger bonds (multiple bonds) absorb at higher frequencies, while weaker bonds (single bonds) absorb at lower frequencies.

Streching frequency is inversely proportional to reduced mass. So, Bonds between lighter atoms (when one of the atoms is hydrogen) absorb at higher frequencies, while bonds between heavier atoms (when no hydrogen is there and both the atoms are heavier atoms) absorb at lower frequencies. 

Labels:

Comments

Post a Comment

Popular posts from this blog

Atomic Absorption Spectrometry

  Atomic Absorption Spectrometry is an absorption spectroscopic technique in which radiation of a particular frequency from a source is absorbed by non-excited neutral gaseous atoms  generated in an atomizer in their ground state. The light is absorbed in the UV-visible region and makes transitions to higher electronic energy levels. The amount  of light absorbed is quantified and this amount of absorption helps in determining the analyte concentration. (It follows Beer's Law) The concentration is measured by drawing a calibration curve  after calibrating instrument with a  standard of known concentration. Factors affecting  the amount of light absorbed Length of path transversed Concentration of absorbing atoms in the vapour state. The diagrammatic representation of Atomic Absorption Spectrometer are shown in the below diagram:  Components Used in Atomic Absorbance Spectrometer: 1. Hollow Cathode Lamp: Acts as source of radiation  It is a sha...
Post a Comment
Read more

Updated list of questions after deletion of some topics for organic spectroscopy -II

  Q 1: Explain the effect of conjugation, hydrogen bonding and steric factor on vibrational frequency? Q 2: Application of Far, Near and Mid-IR spectroscopy? Q 3: Application of NMR Spetcroscopy? Q 4: Applications of Mass Spectrometry or its advantages over other spectroscopic techniques? Q 5: Why Carbon NMR is less sensitive than proton NMR or disadvantages of Carbon NMR over proton NMR Q 6: Advantages of FT-NMR over CW-NMR or difference between FT-NMR and CW -NMR Q 7: Short Notes on Following: a.  McLafferty rearrangement b.  Magnetic Anisotropy (for alkenes, alkynes, aromatic compounds and aldehydes with diagrams) c.  Mechanism of spin-spin interaction or splitting of signals d.  Coupling constant and factors affecting it e.  Long range coupling f.  Simplification of complex spectra (High Field strength, Double irradiation or spin-spin decoupling, Use of Shift Reagents) g.  Nitrogen rule. h.  EI (Electron Impact Ionization) i.  CI (Ch...
Post a Comment
Read more

Gross and Specific Selection rules for different molecular spectroscopies

Gross selection rule   tells whether a molecule will be active or inactive in a particular Spectroscopy.   Specific selection rule tells about the allowed changes in the magnitude of energy when it is active in a particular Spectroscopy.     Rotational or Microwave Spectroscopy Gross selection rule: permanent dipole moment  Specific selection rule: ∆J = +1 or -1 ( for rigid rotor) 2. Vibrational or IR Spectroscopy Gross selection rule: change in dipole moment during vibrations Specific selection rule: ∆v = +1 or -1 ( for harmonic oscillator) 3. Raman Spectroscopy Gross selection rule: change in polarizability  Specific selection rule:   ∆J = 0, +2 or -2   ∆v = +1 or -1  4. Electronic or UV-Visible Spectroscopy Gross selection rule: No stringent rule Specific selection rule: ∆s = 0  and ∆l = +1 or -1 5. Nuclear Magnetic  Resonance or Radio waves Spectroscopy Gross selection rule: nuclear spin should be no zero  Specific se...
Post a Comment
Read more