TRAVISTRA

 NMR SPECTROSCOPY  NMR depends on the absorption of energy when the nucleus of an atom is excited from its lowest energy nuclear spin state to the next higher one.  The nuclear energy levels are produced by keeping nuclei in a magnetic field. The NMR spectrum is produced by EM radiation in the radio wave frequency region (60-500MHz).  NMR spectroscopy deals with nuclei having I=1/2. The spin quantum number (I) is related to the atomic number and mass number of the nucleus. Elements with either odd mass numbers or add atomic numbers have the property of nuclear spin. Radio waves are the lowest energy form of EM radiation. This energy is too small to vibrate, rotate or excite an atom or molecule. It is sufficient to influence the nuclear spin of the atoms of a molecule.  Principle of NMR The nucleus of an atom is positively charged, when it spins it develops a magnetic field and acts like a tiny bar magnet with a magnetic moment (µ) µ= I(I+1)1/2 h/2 π  The sp...

 IR VIBRATIONAL SPECTROSCOPY  It involves the transition between vibrational energy levels of a molecule. It arises due to the change in dipole moment with the vibration of a bond.  The vibrational spectra are given by the EM radiations in the IR region of range 500-4000cm-1  This type of spectrum is given by diatomic molecules with permanent dipole moments like HCl, H2O, and molecules with fluctuating dipole moments like CO2. These are IR-active molecules. Compounds which are IR inactive are H2 , O2, N2, Cl2  It is also called vibrational rotational spectrum since the rotational energies are smaller than vibrational energies so both vibration and rotation occur simultaneously.  It is used to establish the identity of a compound or determine the structure of a new compound. It can also predict the functional groups which absorb at definite frequencies. PRINCIPLE The movement of atoms and the chemical bonds (vibration) is like a spring and ball. The charact...

 ELECTRONIC TRANSITION  1. σ-------> σ* transitions       An electron is excited from the bonding σ orbital to the corresponding σ* orbital. The transition energy required is very high. This type of transition occurs below 150nm. Therefore the absorption occurs in the vacuum UV region. This type of transition occurs in saturated hydrocarbons. Thus saturated hydrocarbons do not show any absorption in the UV, visible region. Therefore they are colorless and known as UV transparent compounds. 2. π------->   π* transitions     The unsaturated hydrocarbons containing double or triple bond shows this type of transition. The π ---->π* bands appear at 180-190nm in the case of aliphatic compounds and at 200-210nm in the case of simple aromatic compounds.  Eg: Ethene-190nm, 1,3-butadiene – 217nm, 1,3,5-hexatriene – 247nm, Benzene- 255nm 3. n------->    π* transitions These transitions are observed in aldehydes and ketones whi...

 U-V VISIBLE SPECTROSCOPY  UV-Vis spectrum results from the interaction of EM radiation in the UV-Vis region with atoms and molecules. The UV radiation region extends from 200nm to 400nm and the visible region extends from 500nm to 800nm. Organic compounds, inorganic compounds, and coordination compounds fall in the UV-Vis spectrum. THEORY The absorption of radiation in the UV-Vis region of the spectrum is dependent on the electronic structure of the absorbing species.  When a photon of a given wavelength interacts with a molecule it may cause a transition among the electronic energy levels if its energy matches with the difference in the energies of these levels. A given electronic energy level has a number of vibrational energy levels in it and each of the vibrational energy levels has a number of rotational energy levels in it. So along with the electronic transition, vibrational and rotational transitions also occur simultaneously. So electronic spectrum appears broad...

BEER LAMBERTS LAW   When a beam of monochromatic EM radiation is passed through an Absorbing solution of concentration (c), the rate of decrease in intensity(-dI) of radiation with a thickness of the solution (dx) is proportional to the intensity of the radiation (I) and concentration (c) of the solution. -dI/dx ∝ Ic  -dI/dx = kIc  -dI/I =kcdx Beer Lambert's law gives the linear relationship between the absorbance of EM radiation and the concentration of an absorber. A = εcL A is the absorbance c is the concentration L is the path length Ε is the molar extinction coefficient