hey it's professor Dave let's talk about
IR spectroscopy he knows a lot about the
science stuff professor Dave explains so
a lot of times when we do chemistry we
face a very big challenge and that is
that we cannot see molecules so we might
do some chemical reaction get a product
and we need some way to be sure that
what we got is what we think it is
because we can't see them we can't see
what the structure of the molecule is so
we have an we have a number of
techniques that fall under the umbrella
term of spectroscopy spectroscopy is the
study of the interaction of light and
matter and so there's a lot of forms of
spectroscopy that we can use to gather
data about what the structure of a
molecule might be and so right now we're
going to look at IR spectroscopy IR
stands for infrared as an infrared light
so what we're going to do is say we do a
reaction we have some molecule we want
to figure out the molecule is we
irradiated a sample with infrared
infrared light that light is going to
interact in some way with the with the
compounds in sample and then it'll
eventually reach a detector so some
light will be absorbed by the sample
some will not some will pass through and
this is information that we can use to
figure out something about the structure
of the molecule the reason that we use
infrared light is because different
molecules we know we already know that
molecules move around they have
translational motion right we know that
heat energy is converted into kinetic
energy of motion so molecules let's say
in a gas or even in a liquid are moving
around with respect to one another and
through conformational analysis we also
know that bonds are rotating all the
time so there there is rotational motion
for all of all of the Sigma bonds but
chemical bonds are also doing other
things as it happens they can do these
other things that that occur
specifically when they are irradiated
with a very specific wavelength of
infrared light they can do something
like a symmetric or an asymmetric
stretch so let's say my body is the
central atom and these are two atoms
that it is bound to and so my arms are
covalent bonds and so this would be a
symmetric stretch the covalent bonds can
actually be contracting and expanding at
a particular vibration depending on
whether it has absorbed this
photon of UV light so this is a
symmetric stretch this would be an
asymmetric stretch okay and we can also
have a symmetric Bend or an asymmetric
Bend in plane so that would look like
this this would be symmetric or we could
have a symmetric and we can actually
have twisting we can have it like this
which would be symmetric or it would be
I can't even do it it's very difficult
to do but the point is we have all of
these different kinds of vibrational
motions happening in a molecule and they
occur at very particular energies so
they will occur when a wavelength of a
very specific infrared light is absorbed
and it will vary depending on the
functional group so the identity of
these atoms will will affect the photon
of absorption so this is information
that we're going to use to generate an
IR spectrum so let's look at what that
looks like right now okay so what I've
done here is I've drawn a sample IR
spectrum so let's take a look at what
these look like and and try to
understand what we can use them for so
let's look at the axes first on the
bottom here we have the wave number the
wave number is telling us about the very
particular energy of infrared light so
we infrared light is a certain band
right it's a certain part of the
electromagnetic spectrum all the way
from this wavelength to this wavelength
so there are many energies of infrared
light in between and so we're irradiated
our sample with all of this infrared
light of all of those different
wavelengths and so the wave numbers here
are the way we differentiate between
those individual energies of IR
radiation on this axis we have the
transmittance or the percentage of that
particular wavelength or that particular
wave number rather that reaches the
detector so if we get if we have
information at a hundred percent or near
a hundred percent transmittance that
means that IR light that corresponds to
that wave number is all getting through
it's all getting through the sample it
is not being absorbed by the sample it's
going to make it to the detector and
then where we have peaks here where we
have information so let's look at this
wave number right here whatever is going
on with the with IR light of this
particular wave number it is not making
it to the detector we can see that it
has a
very low percent transmittance and if it
is not making it to the tech detector is
being absorbed by something in the
sample so there is a particular
functional group in the molecule that is
in the sample that absorbs IR light of
that wave number and that is information
that tells us about what kind of bonds
are in that compound so looking across
here we can see that here's some
important data here's some important
data here's some important data we
should note that this region here around
1500 and below is called the fingerprint
region there's a very delicate
information there but it's very
complicated to analyze and it goes
beyond the scope of this tutorial so we
won't talk about the fingerprint
fingerprint region right now but what we
want to understand is that we have
tables now I've only drawn I've only
written three things up here but there's
long tables of every functional group
that we can imagine in a molecule and we
can talk about the particular wave
number of IR light that is absorbed by
that functional group so for example we
have the Oh H we have the Oh H Sigma
bond right here the Oh H stretch occurs
at this wave number so if that oxygen
hydrogen covalent bond absorbs infrared
light of that particular wave number it
will begin to stretch like this and if
it does if it is irradiated with light
of that wave number it is being absorbed
which means it does not get to the
detector and that's why we see something
like this so we call this the Oh H
stretch and so what this means in terms
of data that we can interpret from an IR
spectrum if there is an O h stretch
there is an O H group in the molecule
that means there is a hydroxyl group
somewhere in the molecule we can look at
the spectrum and be very confident that
there is a hydroxyl group in this
molecule now there's a couple others
we'll just look at the three most basic
ones the next one is called the
saturated CH stretch so the saturated CH
stretch means that there is fully
saturated carbon bound to a hydrogen
meaning this is not an sp2 carbon it is
not an SP carbon this is a this is a
carbon this is a fully saturated carbon
so it has
maxximum bonds to other carbons and
hydrogens and so this carbon hydrogen
bond if it absorbs IR light of that wave
number it will begin to stretch and so
it's not all making it to the detector
and so that's there and most organic
compounds will give you a saturated CH
stretch because most organic compounds
have some saturated carbon hydrogen bond
in it okay and then lastly let's look at
the carbonyl stretch now this may vary
depending on whether it is a carbonyl
from an aldehyde ketone ester amat or
what have you but it will generally
occur somewhere in this region and it
will look somewhat like this and so if
we have that stretch we know that there
is a carbonyl now the last thing that we
want point out a very common error is
that people will say well there's a
saturated CH stretch to that that
dominate that's the only functional
group present in a molecule or they will
say there's a some other functional
group all we are trying to understand is
that every functional group in the
molecule will be represented on the IR
spectrum so let's just say
hypothetically we had a molecule that
gave us the spectrum we would know that
there is saturated carbon hydrogen bonds
we know that there's a hydroxyl group
and we know that there's a carbonyl of
some kind so that could look like
anything it doesn't tell us the precise
structure of the molecule but it could
look say like this it could have a
hydroxyl there and it could have a
carbonyl there we're not sure right it's
not telling us the precise structure but
the reason this could be useful is that
let's say we do some we do some reaction
and maybe we even know the empirical
formula of our product and we know that
our we know that our compound has an
oxygen atom in it well the reason IR
would be useful for that is we would say
what kind of oxygen is it is it a
hydroxyl oxygen is that an ether oxygen
is it a carbonyl oxygen and we might get
an IR spectrum and know just from that
at least what kind of functional group
the oxygen is participating in if we saw
an O H stretch we would say that oxygen
must be a hydroxyl oxygen if that was
not present we would know that it is not
that is not a hydroxyl oxygen so the
main thing that we want to get from this
very basic introduction to IR
spectroscopy is that functional groups
that absorb IR light at specific wave
numbers will provide very specific Peaks
not only are they occurring at specific
wave numbers but they have
characteristic shapes the O H has a nice
curvy dip like this the carbonyl is thin
and long and so we can take an IR
spectrum and we can know roughly what
kind of functional groups are in a
molecule thanks for watching guys
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