Understanding the Bohr Model does not really
help you for this concept. You rather need to know
about different kinds of covalent bonds, some
organic chemistry and physics.
Molecules with C-C single bonds have their
electrons fixed in the molecule and these
molecules absorb in the ultraviolet and are
colorless. Molecules with double bonds often
can spread the electrons over the molecule and
they absorb in the visible light and have a
Now, the plant pigment molecule cyanidin
is a quite complicated organic molecule. All you
have to understand, however, is which form has
the electrons in a more fixed stage and in which
form can the electrons move more freely.
Normally you have to look at resonance
structures. For cyaniding, the acid form is
the one where the electrons are fixed (high energy
form) and the basic form is the one where the
electrons can move (low energy form).
You also need to know the visible part of the
electromagnetic spectrum. Going from low
energy to high energy the colors appear red,
orange, green, blue, violet and then ultra violet.
So a red shift means a shift to lower energy
and a blue shift means a shift to higher
energy. If a molecule goes from a high energy
form (fixed electrons) to a lower energy form
(mobile electrons) the molecule does a red shift.
When the molecule goes from low energy form
(mobile electrons) to high energy form
(fixed electrons) it does a blue shift. Now lets
look at the colors:
I already said the acidic form of cyanidin is
the high energy form with the electrons more
fixed. The molecule is red, because it absorbs
violet, blue, green light. Now when the molecule
is in a basic solution it undergoes a red shift in
absorption. The plant now absorbs in the yellow,
orange, red area and this makes the molecule
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