ORGANIC CHEMISTRY : Infra Red Spectroscopy
BOND POLARIZATION occurs from stretching and vibrational motions when molecules absorb and emit IR radiation of certain frequencies and this correlates with the strength of chemical bonds that holds them together. Thus, the absorbed or emitted energy radiation is measured as IR frequencies in hertz or wavenumbers (cm-1) in the IR spectrum. The magnitude of the IR frequency is inversely proportional to the bond distance between two atoms. It requires more energy to stretch or vibrate a stronger bond compared to a weaker bond. Bonds that have higher value of IR stretching frequency are generally stronger, rigid, shorter or more polarized.
High value means shorter bond distance; weaker value implies longer bond distance.
BONDS BETWEEN TWO ATOMS OF THE SAME ELEMENT BUT DIFFERENT HYBRIDIZATION.
EXAMPLE.
CARBON-CARBON BONDS
Alkane C-C sigma is longer than alkene C=C pi-bond
alkane C-C stretch 2890 cm-1
alkene C=C stretch 3000cm-1
alkyne C=C stretch 3300cm-1
Similar atoms sizes mean good orbital overlap to form chemical bond, and this leads to higher bond strength and shorter bond distance and high IR signal peak values because the energy needed to stretch the bonds correlate with IR stretching bond frequencies.
BONDS OF SAME HYBRIDIZATION BUT DIFFERENT ELEMENT
Electronegative heteroatoms smaller than carbon can form shorter, stronger and more polarized bonds; therefore, have higher IR stretch frequencies in spectrum.
Electrons are more stabilized when closer to the nucleus of the atom. The most de-shielded bond has more bonding electrons exposed to the nuclei and this causes increased bond strength and decreased bond distance and higher IR values.
Hydrogen bonding with water impurities slightly weakens the O-H bonds leading to broad slightly lower IR peak frequency signal in the IR spectrum.
The C = C sp carbons are more de-shielded than in sp2 of C=C which are more de-shielded than sp3 of C-C. Therefore, electrons are more stable in the space between sp carbons than sp2 or sp3 carbon
In amides and esters, the carbonyl C=O is weaker and longer in amides because nitrogen is less electronegative than oxygen and is less de-shielding or shields and thus pushes more electron cloud onto the carbonyl carbon. Thus C=O ester is 1725cm-1 but C=O amide is 1670cm-1.
The C=C alkene is slightly less that C=C aromatic because of extra strength from electron cloud delocalization in aromatized structure. The same reason why =C-H aromatic is higher than =C-H alkene.
There are several other types of molecular vibrational motions besides stretching motion and they also follow the same rules for correlating IR frequencies with bond strength and bond distance.
Examples are these motions include TWISTING, WAGGING and SCISSORING motions.
In linear molecules, the SYMMETRICAL stretching vibrational motions do not cause bond polarization and are not detected in IR spectroscopy but are identified in RAMAN spectroscopy.
