Hi all, Say I have two atoms A--B. Now, Iunderstand that several factors will af

Question

Hi all,    Say I have two atoms A--B. Now, Iunderstand that several factors will affect the wavenumberabsorption, such as hydrogen bonding, mass of the molecules, andbond order. Why doesn't increasing the electronegativity of oneatom increase the wavenumber absorption? Wouldn't increasing theelectronegativity create a greater dipole moment, thus resulting ina higher energy bond? Furthermore, why would increasing theelectronegativity increase the molar absorptioninstead? Thanks in advance!    Say I have two atoms A--B. Now, Iunderstand that several factors will affect the wavenumberabsorption, such as hydrogen bonding, mass of the molecules, andbond order. Why doesn't increasing the electronegativity of oneatom increase the wavenumber absorption? Wouldn't increasing theelectronegativity create a greater dipole moment, thus resulting ina higher energy bond? Furthermore, why would increasing theelectronegativity increase the molar absorptioninstead? Thanks in advance!

Explanation / Answer

      The increase inelectronegativity will not increase the IR streaching frequency.The electronegativity difference between the two atoms willincrease the polar nature i.e the electrons drawn towards themore electronegative atom . So the force constant decrease whichdecrease the streaching frequency of the bond .                C-H    2980 - 2860 cm-                  C-N   1200 cm-    The increase in electronegativity increase themolar absorption i.e more intense peaks are observed . The IR peaks are observed if there is a change indipolemoment during the vibration . So the more electronegativeatoms will cause large dipole moment change during thevibration . So the peaks are more intense .                  C-N   1200 cm-    The increase in electronegativity increase themolar absorption i.e more intense peaks are observed . The IR peaks are observed if there is a change indipolemoment during the vibration . So the more electronegativeatoms will cause large dipole moment change during thevibration . So the peaks are more intense .

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