Lecture 13 Saturday February 7 2015 11 07 AM In a molecule atoms tend to constantly vibrate In a simple diatomic molecule A B the vibration is along the A B axis around the lowest energy position Bond length A B distance Lowest vibrational level If we bring a proper quant of energy in a form of electromagnetic radiation A B will absorb it and will populate a higher energy level The energy difference between the levels is determined by a type of a bond For stretching vibrations it correlates with the bond strength The energy difference determines the frequency of the electromagnetic radiation that A B will absorb when it goes from one energy level to the next IR Quantized molecular vibrations bending and stretching Every type of a covalent bond has characteristic vibrations and gives a characteristic peak in IR spectrum IR spectrum tells us how our compound interacts with electromagnetic radiation in IR part of the spectrum That part of the s pectrum carries energy to promote vibrations in a molecule to a higher energy level X axis tells us the energy of the radiation and is given in inverse wavelength in invers cm Inverse in proportional to the energy of radiation see below Y axis tells us what portion of the radiation was absorbed by the sample In this case it is given as absorbance Sometimes it is given as transmittance wha t got through vs what got absorbed For export Page 1 Lecture 13 Saturday February 7 2015 11 28 AM Given that the energy absorbed by a specific molecule depends on the types and strengths of chemical bonds in it it is not surprising that IR can be used to determine the presence of speciufic functional groups We can see signals from different C H bonds depending on their strengths sp2C H is stronger than sp3C H bonds We can see different carbonyl compounds C O bond and so on So IR can help us figure out functional groups in a molecule below ids the list of important signals in IR Stretching C H vibrations 3000 sp2 sp C H bonds 3000 sp3 C H bonds 2700 2900 aldehydes C H stretch Broad over 3000 are X H bonds X heteroatom 2000 2500 triple bonds alkynes and nitriles 1550 1800 carbonyl C O stretch X OR 1730 1750 X H 1710 1730 X R 1710 1730 X OH 1700 1725 X NR2 1650 1700 1050 1300 C O stretch of alcohols ethers and esters Here are few examples that we ll discuss next lecture For export Page 2
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