What Happens When a Molecule Absorbs Light

19-A What Happens When a Molecule Absorbs Light  

The lowest energy excited singlet and triplet states are called Si and T]. In general, Ti has lower energy than Si. In formaldehyde, the weakly absorbing transition n — 7T (Ti) requires visible light with a wavelength of 397 nm. The more intense n — tt (Si) transition takes 355-nm ultraviolet radiation. 

C—0 stretching is reduced from 1 746 cm in the So state to 1 183 cm in the i state because the strength of the C—0 bond decreases when the antibonding tt orbital is populated. 

Internal conversion radiationless transition between states with the same spin (e.g., Sj So) Intersystem crossing radiationless transition between states with different spin (e.g.,Ti So) 

The terms singlet and triplet are used because a triplet state splits into three slightly different energy levels in a magnetic field, but a singlet state is not split. 

Absorption of visible or ultraviolet radiation promotes electrons to higher-energy orbitals in formaldehyde. Infrared and microwave radiation are not energetic enough to induce electronic transitions, but they can change the vibrational or rotational motion of the molecule. 

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What happens when light is absorbed by a pigment molecule such as chlorophyll The energy from the light excites an electron from its ground... 

At last we come to the remaining question. Why should the source of energy make a difference in the mode of migration Remember, the common, thermally induced shifts are [1,5], whereas the photochemical shifts are [1,3]. A molecule absorbs a photon of light and an electron is promoted from the HOMO to the LUMO, producing a new HOMO and changing the symmetry of the lobes at the ends of the n system. What happens when the symmetry of the HOMO changes ... 

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