GPAT 360

In NMR chapter, from the GPAT point of view, the quantum numbers, their role and chemical shift phenomenon are the important points.  so, here I have given details about the same! View other instrumental analysis notes on the homepage ⇢ www.gpat360.blogspot.com  Nuclear Magnetic Resonance (NMR) - Absorption of radio frequency (electromagnetic radiation) waves induce transition in the molecules. - Radiofrequency region: 4 MHz to 750 MHz Principle: - It is a branch of absorption spectroscopy in which radio frequency waves induce transitions between magnetic energy levels of nuclei of a molecule. - The nucleus or proton behaves as a spinning magnet and it can align itself or oppose to an external magnetic field . - It has precessional motion around itself. - When external magnetic field is applied, this spinning proton either align (lower energy) or oppose (higher energy) to the field. Precessional frequency (ν)  ∝  Strength of external magnetic field (B

Mass Spectrometry Spectrometry = use of no radia tions. Compound under investigation is bombarded with a beam of electrons to produce the ionic fragments or an ionic molecule of the original species. Resulting charged particles are then separated according to their masses . The spectrum produced shows information abound various masses produced and their relative abundance. PRINCIPLE OF MASS SPECTROMETRY This technique is based on the principle of separation of individual atom or molecule according to difference in their masses. When molecule M is bombarded with a beam of electrons, M + e - à M + + 2e - Resulting ionized molecule M + is then accelerated in an electric field at voltage V. In this condition, the energy given to the particle is eV and this is equal to the kinetic energy (1/2mv 2 ). When there are multiple particles, all the particles will possess the same energy eV and also the same kinetic energy. As the value of ‘m’ in kinetics e

INFRARED SPECTROSCOPY: BASICS This technique is used for the  structural analysis of molecule.   After absorption of IR radiations, molecules vibrate at many rates of vibrations, which shows various characteristic peaks in the spectrum. Various wavelength ranges (IR): REGION WAVELENGTH (μ) WAVE NUMBER (cm -1 ) Photographic region Visible to 1.2 Visible to 8333.33 Very near IR (Overtone) region 1.2 to 2.5 8300 to 4000 Near IR (Vibration) region 2.5 to 25 4000 to 400 Far IR (Rotation) region 25 to 300-400 400 to 33.33-25 Conversion of wavelength into wavenumber and vice versa: Remember, 1μ =  10 -6 m,  and  cm -1  = 1/cm,  Simply, multiply (1/wavelength) or (1/wavenumber) with  10 4 . Principle of IR spectroscopy: 1. Correct wavelength of radiation: Natural frequency of vibration of a molecule should match with the incident IR radiation.  Example:

Ultraviolet Spectroscopy In this anaytical technique, ultraviolet radiations are used. Wavelength ranges of radiations: Visible light : 400 - 800 nm Near UV radiations : 200 - 400 nm Far/ Vacuum UV : below 200 nm Generally, we carry out UV analysis in near UV region of radiations. PRINCIPLE : Electronic transitions When a molecule absorbs UV radiations, the electronic excitation occur where electrons go from lower to higher energy state.  Electronic excitation causes the electron to go from electron bonding orbital to antibonding orbital. After some time electron returns to its original state (stable). While returning to its stable form (into bonding orbital) it releases excess energy. That energy is measured by the detector to access the wavelength (λmax) /absorption pattern. TRANSITIONS: σ → σ* (126-135 nm) n → σ* (180-200 nm) π → π* n → π* ALLOWED TRANSITION:  π → π* FORBIDDEN TRANSITIONS:    n → π* ENERGY OF VARIOUS TRANSITIONS: σ