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39 Cards in this Set
- Front
- Back
Properties of light are explained by two theories
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Wave theory – Light travels in the form of waves.
Corpuscular Theory – |
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Nature of EMR and how they are produced
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by oscillation of electric charge and magnetic field residing on the atom.
characterized by their wavelengths/wavenumbers or frequencies |
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The energy carried by an electromagnetic radiation is directly proportional to
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frequency
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emission and absorption is quantified by
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a photon (quantum of radiation)
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Can EMRs travel through vacuum
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they can travel through vacuum.
All EMRs travel with same velocity |
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Visible light passes through a prism – it splits to give how many colours?
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Seven colors at definite wavelengths – dispersion.
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compound absorbs certain wavelength is based on
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the compound and/or the functional groups attached to the compound.
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Planes of EMR waves are
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X and Y with Z as midline
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Wave length definition
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distance between adjuscent crests or troughs
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Visible light range
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3800A (violet) - 7600A (Red)
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wave number definition
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reciprocal of wave length
It is the number of waves that can pass through a space of 1cm Expressed as per cm |
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molecule absorb a part of that EMR due to
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vibrational or rotational energy levels permitted for its atoms
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What happens to electronic energy levels when molecule absorbs EMR in UV
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Electrons (σ, π and n*) are promoted /excited from ground level to higher level (energy state)
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Why does only an EMR of a specific frequencycause the excitation
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Energy level of a specific molecule are quantified hence the energy required to cause this excitation is also of a fixed quantity
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Why are Ultraviolet region below 200 mμ is not generally worked on
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oxygen and nitrogen absorb so they have to be carried out under vacuum
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Beer Lambert law state
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When beam of radiation pass through absorbing medium decrease of intensity of light and thickness of absorbing medium is directly proportional to intensity of incident light as well as concentration
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Beer Lambert law equation
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-dI / dx = KIc
dI / dx decrease in of intensity of light and thickness of absorbing medium K proportionality constant I intensity of light passing through medium c concentration of solution in mol/l |
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Beer Lambert law derived equation
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log I0/I = A = ε.b.c
Io = intensity of incident radiation, I = intensity of transmitted radiation; A is the absorbance ); ε = molar extinction coefficient, b = pathlenght of the cell (1 cm) and c = concentration of the analyte in moles/liter. |
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Spectrophotometer definition
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an instrument that measures the transmittance of light radiation when a specific intensity and frequency range is passed through a sample solution.
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Spectrophotometer consists of following parts
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Light source; Monochromator; Detector; Amplifier; Recorders
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double beam Spectrophotometer
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beam is split into two equal intensities using- dispersion grating . One beam is passed through sample solution and the other is passed through the reference solvent.The photoelectric cells receive a strong beam from reference and a weak beam from sample cell resulting in generation of impulse from amplifier causing recorder to record the signal
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Light source:
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Hydrogen-deuterium discharge lamp - UV region
Tungsten filament lamp – Visible region |
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Compounds that absorb between 400 – 800 nm will appear
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collared to human eye
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Chromophores grouping according to electrons they contain and their exaples
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Chromophores groups that contain π electrons and that undergo π π* transitions (ethylenes, acetylenes…)
Chromophore groups that contain both π and n (non bonding) electrons and undergo both π π* and n π* (carbonyls, nitriles…) |
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Auxochrome
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not a chromophore but its presence effects a shift of the absorption band towards the longer wavelength (red end of the spectrum)
also known as color enhancing group. e.g., -OH, -NH2, -SH. … |
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Bathochromic shift or Red shift:
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shift in absorption maximum towards longer wavelength due to presence of auxochrome or by change in solvent.
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Hypsochromic effect or Blue shift:
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shift in absorption maximum towards shorter wavelength due to removal of conjugation and also change in polarity of solvent
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Hyperchromic effect:
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intensity of absorption maximum increases due to introduction of auxochrome…
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Hyporchromic effect:
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intensity of absorption maximum decreases due introduction of a group that causes distortion of geometry of the molecule
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electronic transitions diagram
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σ --> σ* > n ---> σ* > π ---> π* > n ---> π*
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σ ---> σ*
Very high energy process - |
Bonds are very strong –. E.g., saturated hydrocarbons (C – C )
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n ---> σ*
Comparatively less energy |
in molecules containing hetero atom with unshared pair of electrons
E.g., alcohols, ethers, ketones etc. |
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π ---> π*
Excitation requires less energy |
occurs in molecules containing unsaturated centers
E.g., double and triple bond molecules. Alkenes, alkynes, azo, carbonyls etc. |
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n ---> π*
Requires least amount of energy |
when an electron of unshared electron pair on a hetero atom gets excited. And occurs at longer wavelengths
E.g., saturated carbonyl compounds that undergo two type of transitions |
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Potassium dichromate solution is used for
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calibration of absorbance scale
The A (1%, 1 cm) values at specified wavelengths have to be within the limits. e.g Potassium dichromate in Suphuric acid |
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Control of wavelength:
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5% w/v solution of holmium perchlorate is scanned between 400 – 200 nm. wave length may not differ by 1nm
based on spectral lines of Deuterium or mercury discharge lamps (most instruments have auto diagnostics) |
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Control of resolution:
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controlled by – slit width settings
assessed by 0.02% w/v soultion of toulene in hexane. The ratio of the absorbance at 269 nm compared to 266 nm should be 1.5 |
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Determination of stray light
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due to light scattering within or outside light entering the instrument.
checked using 1.2% solution of KCl in water against water a blank at 200nm. If the absorbance of the sample is higher that 2 – the instrument performance is not optimum. |
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Applications of UV-Vis Spectroscopy:
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information on the functional groups
Structural elucidation of certain vitamins (A and K) Elucidation of polyneuclear hydrocarbons Detection of tautomeric forms Configurations of geometric isomers Axial confirmations of molecules Quantitative and quantitative analysis |