Тема: FTIR Spectra, FTIR Spectral Library. FT-IR Database

V ibrational modes within a molecule can be described using the anharmonic oscillator model. This model assumes that the two masses (with known weight) are connected with a spring (with known strength). With the help of quantum mechanical calculations (Schroedinger equation) you can find the frequencies of basic stretching and bending modes:

Ring strain usually increases the C=O stretching frequency. Conjugation of teh carbonyl group with other double bonds (aromatics, alkenes) or the formation of hydrogen bonds decreases the bond strength (shift: 15-60 cm -1 to lower wavenumbers) as the following examples of cyclic ketones demonstrate.

Electromagnetic Spectrum! All light travels at a constant speed! The difference is the wavelength of the light! (which also determines the energy of the light)!

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b) Appearance of a C=O peak at about 1700 cm^-1. c) 1,3-butadiene as it has conjugated double bonds. 1-butene has only a single double bond and would absorb at a lower lambda max value. e) racemic mixture is a mixture of enantiomers (non-superimposable mirror images) in equal proportions. Meso form of a compound is a compound that would be expected to be chiral although since it has an internal symmetry axis it doesn t have a non-superimposable mirror image. See the following site for a diagram: http://members.optushome.com.au/scottsoftb/enant3.htm f) Make up a known concentration of the substance you want to determine the molar extinction coefficient for. Run a spectrum scan on a UV-visible spectrophotometer and observe where lambda max (maximum absorbance) occurs. Assuming you use a standard UV-Vis the cell you will use is 1cm and the molar extinction coefficient can be determined by: A/cl (where A=absorbance value at lambda max, c=concentration in mol/L, l = cuvette path length in cm and molar extinction coefficient has units of mol^-1 cm^-1 L

V ibrational modes within a molecule can be described using the anharmonic oscillator model. This model assumes that the two masses (with known weight) are connected with a spring (with known strength). With the help of quantum mechanical calculations (Schroedinger equation) you can find the frequencies of basic stretching and bending modes:

From this equation, one can deduce some basic trends can be deducted:

a. If the force constant F (= bond strength) increases, the stretching frequency will increase as well (in cm -1 )

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Compound A: You probably have an alcohol. Aliphatic. The apparent m/e of 43 should really ring a bell as to what group that is. (C3H7 = 43) And from the NMR, consider an isopropyl group. Don t try to over interpret the IR, which is common for starting students. Look for the obvious peaks, like carbonyl, alcohol, amine, etc. Not worrying about C-H wagging, etc. Hope this helps steer you. Compound B: Aromatic, and you have a carbonyl. It is substituted, and if you had a better IR, you could actually determine the substitution pattern. You have a C2H5- group attached. From the splitting pattern of the aromatic protons, you can put together the substitution. I do wonder if the singlet is due to chloroform used in the NMR. If it is at 7.23, then it is chloroform. The strong loss of m/e 45 could be C2H5O- If there is a loss of m/e 29 that is an aldehyde. So I think I know the structure, just not the substitution pattern.