Question

Basic understanding how double bonds absorb light. Overall chemical reaction of ‘fixing’ carbon dioxide.

Answer 1

Dear friend

Basic understanding how double bonds absorb light:

Absorption or emission of ultraviolet or visible light by a molecule depends on electron transitions between molecular orbital energy levels, just as absorption or emission of electromagnetic radiation by an atom is determined by electron transitions between different energy levels in the atom. Molecular spectra follow rules analogous to the rules for atomic spectra: energy is absorbed only when the amount of energy provided matches the difference in energy When an electron goes from a higher to a lower energy state, a photon of definite wavelength and frequency is emitted. Every atom or molecule has a characteristic electronic spectrum . Molecular orbital theory provides a model for the way electromagnetic radiation interacts with molecules. For example, an electron in the pi bonding molecular orbital (MO) of an alkene can be excited to a pi antibonding MO. This is described as a π -> π* transition.

The diagram is not intended to be to scale - it just shows the relative placing of the different orbitals. When light passes through the compound, energy from the light is used to promote an electron from a bonding or non-bonding orbital into one of the empty anti-bonding orbitals.

In saturated compounds (non double bond containg compounds) only σ bonds are there σ -> anti bonding σ* or π* requires large amount of energy corresponds to lower wavenumber or higher frequency light or radiation required. Visible light cannot be useful.

In unsaturated comounds (double bond containing compounds) there is a possibility between π -> π* and n -> π* transitions possible. These transitions require comparatively very less energy radiations. Remember that a non-bonding orbital is a lone pair ( n ) on, say, oxygen, nitrogen or a halogen groups in a molecule which absorb light are known as chromophores.

Overall chemical reaction of ‘fixing’ carbon dioxide:

Photoautotrophic organisms, such as algae, cyanobacteria, and plants, all contain chloropyhll-a and obtain energy by a process known as oxygenic photosynthesis. The overall chemical reaction of this process is
CO₂ + H₂O --> (CH₂O))_n + O₂
where (CH₂O))n refers to organic material. In this reaction, the oxygen is derived from water. The sequence of reactions by which plants, algae, and cyanobacteria fix carbon dioxide is referred as the Calvin-Benson cycle. Carbon dioxide can also be fixed by bacteria that use inorganic chemicals as an energy source, a process called chemautotrophy. For example sulfur-oxidising bacteria obtain energy by oxidizing H₂S or elemental sufur as an energy source and produce sulfate as an end product. They fix carbon dioxide using the Calvin-Benson Cycle.

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