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.
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 color.
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
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).
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 appear blue.
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