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4. Freshman Organic Chemistry: Coping with Smallness and Scanning Probe Microscopy
Poziom:
Temat: Edukacja
Prof: Okay,
so we saw last time that Lewis
wasn't so dumb.
He knew about Earnshaw's
theorem, but he still thought
there was an octet of electrons
around the nucleus.
How could he think such a thing?
Because if you have
inverse-square force laws you
can't have static positions of
charged things,
unless they're right on top of
one another or blown apart.
So what did he think?
Student: It's not an
inverse-square.
Prof: That it's not an
inverse-square force law,
that Coulomb's law breaks down
when you get to a very small
scale.
And J.J. Thomson thought the
same thing, and wrote here these
two terms here.
The first one is Coulomb's law
indeed, but there's this other
thing, c/r.
So as long as r is very
big you don't compare -- you
don't care about that term,
as long as it's very big
compared to c.
And c is a distance
that's something like the size
of atoms.
So once r -- once the
distance of the things that are
interacting with one another,
the electron and the nucleus
say, once they get near the
distance c,
then this c/r thing
becomes significant,
if r becomes very small,
and it even changes the sign.
So what was attractive becomes
repulsive.
So that would do okay then for
a structure.
That was in 1923.
In 1926, just three years
later, quantum mechanics came
along and showed that Coulomb's
law was just fine,
nothing wrong with Coulomb's
law.
It goes to much,
much, much, 10^20th,
smaller distances than the size
of atoms.
Coulomb's law still works.
What was wrong was the way they
treated kinetic energy,
because kinetic energy quantum
mechanics reformulates.
So it gives you electron clouds.
And so what's a cloud is not
the positive charge -- remember,
J.J. Thomson had a positive
sphere of electricity in which
he embedded the electrons --
what's a cloud is the
electrons, and nuclei are put in
it,
to make molecules.
So it really is -- he was right
about plum-puddings,
he just had the charges
backwards.
Not many people give him credit
for that.
So cubic octets of Lewis and
ad hoc electrostatic
force laws,
like this c/r term in
there,
soon disappeared from
conventional chemistry and
physics;
immediately in fact,
I mean within a month say.
But the idea of shared-pairs,
and lone-pairs,
that Lewis came up with,
remained useful tools for
discussing structure and
bonding.
So we still do it,
and that's why you did problems
today that had to do with that.
Yes Niko?
Student: Yes.
Can you explain the kinetic
energy theorem again?
Prof: Yes,
but I won't do it for a week
still, because it takes a little
while.
Okay, so Earnshaw said there's
no structure of minimum energy
for point charges --
that is, no classical one --
but if you come to quantum
mechanics and fiddle around with
kinetic energy,
then you can get it.
But despite Earnshaw,
might Lewis have been right?
Might there still be
shared-pair bonds and
lone-pairs?
What are bonds?
And that's what we're going to
try to find out.
And how do you know,
or how do you know?
How are we going to know
whether there are electron pairs
that are shared between atoms
and lone-pairs on atoms?
How can we find out?
Student: Experiment.
Prof: Right.
You've got to do an experiment.
So what kind of experiment?
Now these experiments are tough.
Why are the experiments tough?
Student: It's so small.
Student: It's really
small.
Prof: Because things are
-- well we could see,
or we could feel whether these
electron pairs are there.
But there's a problem,
as you say, that it's
inconceivably small,
the thing we're looking for,
to see or to feel.
Okay?
But is it really inconceivable
how small they are?
When I was in your place
everybody thought yes,
they're inconceivably small,
we'll forget it.
Right?
But there are more recent
techniques that suggest that you
can really see things that
small;
at least you can think about
them clearly.
Now, the first idea of seeing
them came from this book here.
Actually this book was
published -- this is a Third
Edition of the book.
The First Edition was 1909 and
the Second was 1919,
and this is the Third Edition,
which is still in print.
You can go on the web and buy
this baby,
Occult Chemistry,
A Series of Clairvoyant
Observations on the Chemical
Elements,
by Annie Besant and Charles
Leadbeater.
So these are the occult
chemists who started practicing
their trade in 1895.
"Bishop"
Charles Webster Leadbeater;
he was a bishop of a church he
founded that had -- he was the
head bishop;
there were like 100 bishops and
500 members of the religion.
This is Annie Besant,
who is one of the most
interesting people in all of
history.
They closed the stock exchange
in India the day she died,
for example.
This is in her -- the outfit
she designed to lead the Boy
Scouts in India.
And she also proposed marriage
to George Bernard Shaw.
There are all sorts of
interesting things about Annie
Besant.
But one of the things we're
interested in is that she could
see atoms.
And then their helper,
who became leader of the
operation later on,
was Jinarajadasa.
Okay, so this is a page from
this book,
from the paper that became the
book in 1895,
which shows what they could see
for hydrogen,
oxygen and nitrogen.
Now there are a number of
levels here.
The very lowest phase is solid,
H O N, then liquid,
then gas.
So this is what,
when they went into sort of a
trance, they could see.
This is the gas.
But then there's Ethereal 4,
Ethereal 3, Ethereal 2 and
Ethereal 1 phases as well,
with higher and higher
resolution.
You can see these.
So these, for example,
these little dots that you see
here are what get blown up here,
and the little things inside
this are what get blown up here,
and the things inside this get
blown up here,
and the things inside this are
this;
and that's the same for all
atoms, that's the fundamental
unit.
Okay, so here's hydrogen at the
gas resolution level.
So there are wheels in wheels
in wheels here.
And here's what oxygen looks
like;
it's these little helices.
And now that's the thing you
finally get to,
the thing called anu.
And that they actually ripped
off from this book,
The Principles of Color,
which was published in America
in 1878 by Edwin D. Babbitt.
And here's his picture of this
thing.
And lo and behold,
that's what they saw as the
fundamental constituent of
matter.
It's fun to look at in detail.
You can look at it at the Web,
if you want to.
It also has pictures of the
Angel of Innocence here,
and the psycho-magnetic curves
surrounding your head.
Okay, now lo and behold the
real confirmation of this thing
was that if you counted up the
anu in the different
elements,
there were eighteen anu
in hydrogen,
290 in oxygen and 261 in
nitrogen, and if you divide that
by eighteen,
you get the atomic weights,
Q.E.D. Okay,
so they have pictures in this
book of a bunch of the atoms.
This is helium,
which has seventy-two
anu.
This is lithium with 127
anu.
Not the bottom part;
this is a model and this is the
wooden base it sits on.
Here's iron, 1008 anu.
Here's neon, 360 anu.
Now look at neon,
and that is a 4f orbital
electron density,
calculated by quantum
mechanics.
Now the question is,
why would you believe me,
or believe a textbook,
or believe a quantum
mechanician, and not believe
them?
Because they said they saw it
too.
Okay?
Well it takes time.
We've talked about this before,
the basis for scientific
belief: evidence,
you always cross-examine an
assertion; logic;
and taste, but taste matures
with experience and at the
beginning you don't know which
people to believe and which not
to believe.
It's a problem in politics too.
Okay, there's sodium, 418.
They not only said they saw
atoms, they also saw molecules.
For example,
here is sodium carbonate.
What do you see in there?
Two sodiums, right?
Okay?
In the back are pieces of a
carbon atom.
What else do you see?
Students: Oxygen.
Prof: Ah,
there are three oxygen helices
wrapped around the sodium and
the carbon.
And they wrote:
"Note that this triangular
arrangement of O_3 has just been
deduced by Bragg from his X-ray
analysis of Calcite."
We'll talk about Bragg and what
he was doing next time.
But they have experimental
evidence that supports what
they're reporting,
independent experimental
evidence.
Here's their model of benzene,
and they say about that,
"Each of the three
valencies of each Carbon are
satisfied by Hydrogen,
and the fourth valency,
which some have postulated as
going to the interior of the
molecule,
does actually do so."
In fact, Annie Besant was the
first woman to get a degree in
Chemistry at the University of
London;
but that's another story.
Her teacher was the guy who
translated Das Kapital
into English,
and had a lot of other
nefarious things that he did.
Question six will have to do
with that.
Okay, so here's benzene and its
resonance.
These were structures that were
first early proposed;
we'll talk about that later on.
But some people didn't like
this idea of going back and
forth between two things,
and they thought that you just
have the fourth valence of every
carbon going into the middle,
all satisfying one another.
So this was what they said that
their model confirmed,
that the fourth valence of
every carbon goes into the
middle.
Okay, needless to say most
people nowadays don't think too
much of occult chemistry,
even if you can still buy the
book online.
This portal you came through
this morning.
It says "Erected
1921".
But here's the same place,
in 1923, when the building was
dedicated, and you notice those
plaques weren't carved yet.
It didn't say erected 1921.
They were still finishing the
building at the time the
American Chemical Society had
their national meeting here to
celebrate the biggest academic
building for chemistry in the
world.
Okay?
So the slates were clean.
Now if you go down into the
crowd here, you see some
interesting people.
For example,
there's G.N. Lewis with his
Philippines' cigar in his hand.
He always had a cigar;
he smoked seventeen cigars a
day.
>
Groan.
Okay, but they were trying to
decide how to decorate all these
plaques around the building,
and they wanted to put the
names of distinguished American
chemists.
Had they done that,
it would be pretty much a
disaster because the people who
were thought to be distinguished
American chemists in those days
were not so hot actually in
retrospect.
But they chose a wonderful
group, and the idea came from
Giuseppe Bruni from Bologna who
had come to the meeting to
speak.
And he said,
"Don't put living chemists
there, put dead chemists."
And that's what they did;
in almost all cases they were
dead.
And it's a wonderful group;
as we've already used Faraday
over here.
Okay, here's one of them that's
over in that corner of the
building.
And van't Hoff,
have you heard of van't Hoff?
You'll have heard a lot of him
by the end of this semester.
And Gibbs?
Students: Yes.
Prof: Yes Gibbs,
we'll talk about him too.
And Mendeleev you've heard of.
But I suspect that Crookes is
not so familiar to you.
This is Sir William Crookes.
He was a Fellow of the Royal
Society, and FTS.
In 1861 he discovered the
element thallium.
He also developed the cathode
ray tube, which he's holding
there, which became the x-ray
tube, and he invented the
Cooke's radiometer.
I bet every one of you has seen
a Cooke's radiometer.
Do you know?
There's a picture of a Cooke's
radiometer, to measure the
intensity of light by how fast
it spins.
You've seen these things you
buy in novelty stores that sit
there and spin in the light.
Okay?
From 1913 to 1916 he was
President of the Royal Society,
the same thing that was founded
back by Boyle -- remember?
-- and Robert Hooke and those
guys.
In 1898 he was President of the
British Association for the
Advancement of Science,
and he was also,
in that year,
President of the Society for
Psychical Research.
And FTS is Fellow of the
Theosophical Society.
In his presidential lecture to
the British Association for the
Advancement of Science,
one of the things he said was,
"Telepathic research does
not yet enlist the interest of
the majority of my scientific
brethren."
But he's the guy who supplied
the samples for Leadbeater and
Besant to look at.
He supplied them with lithium,
chromium,
selenium, titanium,
vanadium, boron and beryllium
samples so that they could draw
these pictures of what they saw
by clairvoyance.
So you can read more on the Web
if you want to about him,
he's an interesting character.
And there were a lot of semi-,
well really serious scientists.
Oliver Lodge was another one
who invented radio and was head
of the Physics Department at
Bristol,
I think , who was into
communicating with the dead and
stuff like that.
So it wasn't obvious at the
beginning who you should believe
and who you shouldn't believe.
But to get back to the point,
and forget clairvoyance for
awhile,
which I don't believe in,
the question is,
are molecules unobservably
small for what Newton called
"vulgar eyes"?
Is there no way we can see
something and measure it if it's
that small?
So consider water.
A cubic centimeter of water is
1/18th of a mole,
since the molecular weight is
eighteen.
That means it's 6/18^th times
10^23rd molecules,
in a cc of water.
Now that's a really,
really big number and it sounds
like they must be just
impossibly small,
hopeless to try to see such a
thing.
But the difference is between
the cube and the cube root,
because when we measure
distance we measure linear,
not volume.
So to get something about
distances you take the cube root
of the volume.
So if we take the cube root of
that, it's about three times
10^7.
Now that's still a very big
number, 30,000,000.
Right?
But it's not impossibly small.
It's about -- 3 angstroms is
this dimension,
the size of a water molecule.
Remember, we'll talk a lot
about a carbon bond --
carbon-carbon bond,
being about one and a half
angstroms.
Okay, now think about 10^5.^(
)So this lecture room is about
ten meters wide.
One of my hairs is order of
magnitude 100 microns in
diameter, and that's a ratio of
10^5.
Right?
So 10^5 of my hairs sideways
would go across this room.
So that's a lot.
Right?
But it's not impossible to
think of lining up hairs to go
across the room,
or putting them side by side.
It'd take a while,
but it's not impossible to
think about that,
a hair compared to the room.
Right?
But a molecule is about a
nanometer, and a small atom is
about 1/10 of a nanometer,
or an angstrom.
Right?
And that ratio is 10^5.
So the room to my hair is like
my hair to a molecule,
or even an atom,
a big atom.
So it's not impossibly small,
it's just very,
very small.
In fact, a nucleus is 10
femtometers, which is another
10^5 down.
So that means if the lecture
room were an atom,
the nucleus would be the
diameter of a hair;
very small, but not impossibly
small to conceive of.
Okay?
Now Newton, in Opticks,
the book we looked at before,
in 1717 wrote on page 369:
"The thickness of the
Plate where it appears very
black,
is three eights of the ten
hundred thousandth part of an
Inch."
"Is"
- that means he knew the size
of it.
Now how big is three-eighths of
a ten-hundred-thousandth part of
an inch?
Well ten-hundred-thousand is a
million;
three-eights of a millionth of
an inch is thirty water
molecules.
So somehow Newton was able to
measure something,
in 1717, that was the size of
thirty water molecules.
Now what tool could Newton
possibly have had in 1717 that
allowed him to measure something
so very small?
What?
Student: Glass ring.
Prof: Can't hear very
well.
Student: Glass ring.
Prof: Glass springs?
Student: Ring,
like you would --
Prof: That's right.
Okay, so here it is,
Newton's Rings,
which really should be called
Hooke's Rings,
because he's the guy that in
Micrographia published
how they worked.
But remember,
they had different light
theories.
So here are two pieces of
glass, two disks of glass.
And I'll change the lights so
we see light through them.
Now I can't remember which is
which, but let me guess that if
I put this one over here,
having turned it over -- okay.
Oh I can see, yes you can see.
See those patterns?
You've seen things like that
with microscope slides,
colors.
Okay?
So these are two flats against
one another, they get very close
to one another.
But if I turn the top one over,
you see that it's a little bit
-- let me -- so that's what we
just saw.
But notice that if I turn it
over there's a little --
there turns out here at the
edge to be a little bit of a
gap,
because the top one is just
very slightly curved.
And now let me see if I can see
something.
I can't see it yet,
let me zoom in.
We're going to need to focus on
that.
Ah, see that thing, see that.
<<Technical
adjustment>>
There.
Okay, so here's what you're
seeing.
In the middle is what's called
Newton's Rings.
They look like that.
Different colors, right?
And Newton could measure the
thickness of the air gap that
caused different colors.
The colors repeat in higher
orders.
Okay?
Now how could he know the
distance?
So he could associate every
color with a distance.
How could he know the distance?
Because he could measure the
diameter of the rings.
And he said this:
"Observation Six:
The Diameter of the fixth
Ring" (Right?
One, two, three,
four, five, six) "at the
most lucid part of its Orbit was
58/100ths of an inch,
and the Diameter of the Sphere
on which the double convex
Object-glass was ground was
about 102 Feet;
hence I gathered the thickness
of the Air or Aereal Interval of
the Glasses in that Ring."
So in other words,
here's the air gap he's trying
to measure, which is the sixth
ring out.
He knows the diameter of that
ring.
He knows that they touch in the
middle, the two glasses,
and he knows that it's a sphere
and it has a radius of fifty-one
feet.
So he can just use trigonometry
to figure out that the air gap,
if he knows that angle there,
the air gap is 1.8 microns,
right?
So that's a lot bigger than an
atom;
but if you go into the first
ring, or even closer,
then you can measure it.
Right?
So all he needed was a
spherical glass with a very
large radius that he could
measure his distances from.
Okay, but here's a simpler
measure of an even smaller
distance, about 100 years later,
and here's the guy who did it.
You know who that is?
<<Students speak over one
another>>
Prof: Benjamin Franklin
of course;
we'll get back to the artist
later on.
So he published in the
Philosophical Transactions of
the Royal Society in 1774 --
the Society is now
110-years-old,
114-years-old --
this article on the stilling of
waves by means of oil.
Can you see what that means?
What's that related to,
a common saying?
Student: Separation?
Prof: Pouring oil on
troubled waters;
have you heard of that one?
So the stilling of waves by
means of oil.
Extracted from sundry letters
between Franklin and Brownrigg.
So he wrote to Brownrigg,
1773: "I had,
when a youth,
read and smiled at Pliny's
account of a practice among the
seamen of his time,
to still the waves in a storm
by pouring oil into the sea;
as well as the use made of oil
by the divers…."
Pearl divers would fill their
mouth with oil and when they
went down,
if a breeze came up and ruffled
the surface,
which made it hard to see
because the light wouldn't come
through clearly,
then they'd let oil out of
their mouths that would float to
the surface,
stop the ripples,
and they could see to get their
pearls.
This is what Pliny said.
Right?
But he said he smiled at that,
what a quaint thing to think;
it's like the occult chemists.
"I think that it has been
of late too much the mode to
slight the learning of the
ancients.
The learned,
too, are apt to slight too much
the knowledge of the vulgar.
In 1757, being at sea in a
fleet of ninety-six sail bound
against Louisbourg,"
(off Cape Breton Island)
"I observed the wakes of
two of the ships to be
remarkably smooth,
while all the others were
ruffled by the wind,
which blew fresh.
Being puzzled with the
differing appearance,
I at last pointed it out to our
captain and asked him the
meaning of it.
'The cooks,' said he,
'have I suppose just been
emptying their greasy water
through the scuppers,
which has greased the sides of
those ships a little.'"
(So there's experimental
evidence for what Pliny said.)
"Recollecting what I had
formerly read in Pliny,
I resolved to make some
experiments of the effect of oil
on water when I should have the
opportunity."
And when he had the opportunity
was when he was in London.
So here's London,
and his experiment in 1762 was
in that place,
on Clapham Common in South
London.
"At length being at
Clapham, where there is on the
common a large pond which I
observed one day to be very
rough with the wind,
I fetched out a cruet of oil
and dropped a little of it on
the water.
I saw it spread itself with
surprising swiftness upon the
surface;
but the effect of smoothing the
waves was not produced."
(So the experiment was a
failure.)
"For I had applied it
first on the leeward side of the
pond where the waves were
greatest;
and the wind drove my oil back
along the shore."
So it didn't spread on the
surface.
So what did he do?
Students:
Go to the other side.
Prof: Go the other side,
right?
"I then went to the
windward side where the waves
began to form and there the oil,
though not more than a teaspoon
full,
produced an instant calm over a
space several yards square which
spread amazingly and extended
itself gradually till it reached
the lee side,
making all that quarter of the
pond,
perhaps half an acre,
as smooth as a looking
glass."
So a teaspoon is five cubic
centimeters,
and he stilled half an acre,
which is 2000 square meters,
which means the thickness of
this layer,
to spread that many cubic
centimeters over 2000 square
meters,
would be twenty-five angstroms,
which is the length of a
molecule of the fat that's in
the oil.
So Franklin had in fact
measured the length of an oil
molecule, in this way.
Now he didn't think he had
measured it and didn't claim to
have measured it.
He said, "When put on the
water it spreads instantly many
feet around,
becoming so thin as to produce
the prismatic colours"
(the Newton Ring's colors,
right?)
"for a considerable space,
and beyond them so much
thinner"
(it became so thin that you
didn't even get Newton's colors,
right?)
"so much thinner as to be
invisible except in its
effect of smoothing the waves at
much greater distance."
(So the technique was stilling
the water;
allowed you to measure how big
the area was.
Isn't that cool?)
"It seems as if a mutual
repulsion between the particles
took place as soon as it touched
the water."
That is, they pressed one
another apart and moved.
So what he wouldn't have known
was that they pushed one another
apart but they stayed in contact
with one another,
to become a monomolecular thick
layer over the water.
So he didn't claim to have
measured it, but indeed he did
measure the size of an oil
molecule.
So molecules are very,
very, very small,
but they're not inconceivably
small and there are ways you can
measure them.
So we have this question,
are there electron pairs?
So let's try first feeling,
with Scanning Probe Microscopy,
or SPM.
Can we feel individual
molecules?
Can we feel individual atoms?
Can we see what bonds are by
feeling them?
So Scanning Tunneling
Microscopy was invented in 1981
by these guys in Zurich,
Gerd Binnig and Heinrich
Rohrer.
And here's the first
publication.
They worked at IBM and here's
the first publication on the
cover of their journal about a
meeting about this new
technique,
Scanning Tunneling Microscopy.
And they got the Nobel Prize
within five years,
or maybe it was even less;
in very short order, 1986.
Now how does that Scanning
Tunneling Microscopy,
which is one type of SPM
scanning probe --
Scanning Tunneling is one way,
and I'll mention that shortly.
But first I'm going to talk
about Atomic Force Microscopy.
And for this you need a little
chip, and I'll show you.
The chip is in here,
in this little bug box.
Ah you can't see it here.
Well there's the chip in the
middle, right?
And I'll pass it around so you
can look at it.
And if you tilt it so that the
light from the ceiling glints
off the gold,
and you see it really shining,
you may be able to see at the
thin end these little v's;
you can see here at the bottom
little v's.
The chip is 1.4 millimeters
wide.
It's a gold-coated silicon chip.
And if you look at those with
even higher magnification you
see this -- and there's the size
of a hair.
You can actually see -- did you
see them?
They're not easy to see and
maybe you will and maybe you
won't, but they're there.
Okay, so there's the size of a
hair.
And in any given experiment you
have a choice of five different
cantilevers, as they're called,
that you can use.
Here's a higher resolution
electron micrograph that shows
the tip of one of these v's,
and you see a little bulge
here, at the bottom,
and if you zoom in on that you
see this;
it's a little pyramid.
And so at the bottom of that
pyramid the radius of curvature
of this tip is only about twenty
nanometers, so about the size of
twenty molecules.
Right?
It's round on the tip,
it's not truly pointed and come
to nothing;
it's a little bit rounded.
You can buy ones that are a
third that size,
their radius of curvature.
Okay, and then you have this
piece of glass.
I said, "Do not
touch" when you came in
because that piece of glass
costs $2000.
Okay, so there's this piece of
glass, and if I hold it up here
there's a little chip on the
bottom of the piece of glass.
I know what I'm looking for.
I don't know if you can see it
or not.
But anyhow it's held into this
piece of stuff here.
And those little tips are
pointing up here,
but in fact the thing is used
upside down so they point down.
So here's that thing mounted in
an instrument,
and you see there's a red light
glowing in the glass,
and the reason is it's being
irradiated by --
so there's a blown up picture
of one of these tips,
pointing down.
And laser light comes and
bounces off the shiny gold on
this tip, and gets reflected up
to a detector.
Now what would happen if you
pushed up on the tip?
It would deviate the
reflection, right?
It would go like this.
And you have a very precise
position detector for the laser
light up where you see the light
glowing.
So you can tell the deviation
of that tip, moving up and down,
by a size that's less than an
atom.
Isn't that neat?
So now what you do in this is
move this back and forth over
the sample and watch the little
needle go up and down by the
reflection of the light.
So if you had,
if the surface was like this,
they'll click,
click, click,
click, click,
click, up, up,
up, and just scan across.
And it's sensitive,
as I say, to less than one
molecule change in height;
although it doesn't feel an
individual molecule because the
tip, remember is twenty
nanometers wide.
So the width of the tip is like
100 or so atoms.
Right?
So it's touching a bunch of
atoms at any one time.
But if you come to a ledge,
unlike a crystal ,
it would be going at a certain
height,
touching a bunch,
and then it would move up and
touch a bunch more,
and you'd be able to tell if it
went up and down,
if that distance was only one
molecule; easy.
Here's, in fact,
a picture taken by an
undergraduate.
In fact, it's a movie.
So it's AFM traces.
So what you do,
it's like a TV where you go
zoom, zoom, zoom,
zoom, zoom, zoom,
zoom,
zoom, and then you go back to
the top,
zoom, zoom, zoom,
zoom, zoom, say how high it is
at every point along the scan,
and then color code it to show
how high.
So the width of this is about
600 molecules,
right?
But the lines you see,
which are ledges that go up and
down;
each of those ledges is one
unit cell, 1.7 nanometers,
one set of molecules.
So these are -- and that,
the thing at the top surface
where it's flat,
is absolutely flat,
except for there's little holes
there.
And those holes were made by
taking this tip and pressing
down and scratching a little bit
to knock some of the molecules
away.
So the larger pit is five
nanometers deep,
which means it's four layers of
molecules that've been knocked
off in there.
And now this is underwater,
or underwater in propanol.
So the crystal is dissolving
slowly.
So those pits open up,
as molecules come away.
So watch.
Right?
Those are at one-minute
intervals.
So we're actually feeling
individual molecular heights
with this thing.
There's an interesting point
here that I'm not going to dwell
on, but the smallest one did not
lose any molecules.
If I back up,
watch the smallest one.
It always stays the same size
as the others dissolve.
That's an interesting puzzle
isn't it?
But at any rate, you can do it.
And you can measure the rate at
which different ledges dissolve.
Some dissolve much faster than
others.
Here's scanning tunneling
microscopy.
This is done in a different way.
It's not -- you don't reflect
the laser light.
What you do is detect the
electrical conductivity through
a layer.
And if you have an atom there
that is a good conductor of
electricity, you get more
current through and less and so
on.
So as you scan a real -- now
this could be a really,
really sharp tip where the tip
is just one atom or a couple of
atoms.
So now as you go across you can
feel things that are much
smaller laterally.
Okay?
So this picture here,
that thing right there,
which is as you see a repeated
pattern,
that's one of those molecules,
with 12 carbons and a bromine
on the end.
And here's a model that shows
what the molecule looks like.
The yellow is bromine,
the brown are the two oxygens,
and the green are carbon and
the grey are hydrogens.
They don't come colored that
way, they come looking like
this, as to conductivity.
But you can see individual
atoms.
Right?
Or one of the more dramatic
early examples,
in 1993, was at the IBM labs at
Almaden in California,
where they worked on iron atoms
on a surface of copper,
and they used this tip and
changing the voltage to pick up
atoms and move them where they
wanted,
then change the voltage and
drop them there,
then go back and get another
one and do it.
So they made this thing they
called a quantum corral of
whatever, fifty-eight or
something iron atoms.
Yes?
Student: Is that along
the lines of when they moved the
atoms --
Prof: Pardon me?
Student: Is this along
the lines of when they moved the
atoms to spell out IBM?
Prof: Oh yes,
they also spelled out IBM.
They knew which side their
bread was buttered on too,
just like the Royal Society
did.
Okay, so that's neat.
So clearly you can see
individual, or feel individual
atoms, but you don't see the
bonds.
The bonds are smaller.
It just looks like a bunch of
balls, right?
You don't see the electron
pairs between them.
Here's SNOM.
SNOM stands for Scanning
Near-Field Optical Microscope.
So this is in a sense seeing,
but actually it's scanning.
It's like feeling.
So what you have here is a
glass fiber that's drawn down to
a very sharp point and coated
with aluminum,
and the hole at the bottom is
just 100 nanometers;
that's like 100 molecules,
not really, really tiny like
the scanning tunneling
microscope.
So what you do is you have
something on the surface of a
slide here that you want to
probe with this,
and you send light down the
thing, and if there is a
molecule where the light's
coming out,
through this tiny hole,
if there's a molecule there,
that will take the blue light
and emit red light.
Then you know when it's under
the tip.
Suppose there was just one
molecule on the whole slide.
You scan it around until
suddenly red light comes out.
Then you know that tip is
pointed toward that particular
molecule.
Okay?
So red light comes out and you
have a detector that detects it,
and you scan the sample back
and forth,
like a TV, and you find out
where such molecules are.
And here's a picture taken in
that way.
So this is a scale of microns,
2 x 2 microns;
in fact it's a distorted scale,
as I see here.
Okay?
And that arrow,
doubled-headed arrow,
is the wavelength of red light.
So you're seeing things
significantly smaller than the
wavelength of the light,
having done it in this way.
You're not really looking with
your eye, you're doing this
scanning trick.
Okay, so scanning probe
microscopies,
like atomic force microscopy,
scanning tunneling microscopy,
scanning near-field optical
microscopy,
are really powerful,
and you can see individual
atoms.
Right?
The sharp points can resolve
individual molecules,
and even atoms,
but not bonds.
So it's not doing what we need
to do for our purposes.
Now, that's -- I'm going to
spend the last few minutes going
over the problem that we were
looking at with --
remember you were supposed to
draw all the resonance
structures for H,
C, O, N isomers.
I think you remember that.
Okay, so let's just look at it.
So what we're going to do is
compare what we see with Lewis
theory with things that have
been calculated by reasonably
good quantum mechanics;
not the very,
very highest level,
but a pretty good level.
Okay, so we want to try to make
all the possible arrangements
here.
So we can put the three atoms,
O, C, N, with any one of them
in the middle,
and then we can put hydrogen on
either end.
So then we'll do it with
nitrogen in the middle and then
with oxygen in the middle,
so we'll have done all
possibilities there.
So here are two possibilities.
Now are these good structures
is what we have to decide?
Okay, now it would be possible
to shift electrons here and draw
a different Lewis structure.
Which of those two do you think
is better?
Student: Probably the
top one.
Prof: Why?
<<Students speak over one
another>>
Prof: Okay,
there's separation of charge on
the bottom.
Okay?
So there's bad charge
separation.
The most electronegative atom,
oxygen, in fact has a positive
charge not a negative charge.
Right?
So that's -- not only is there
a charge separation,
it's in the wrong direction.
But they're all octets that
we've drawn in both structures.
All atoms have octets.
Now let's try the same trick
over here.
We can do it that way.
How about that?
Which of those is better?
In both structures we could go
through and count and see that
they're all octets.
Would you like me to do that,
or is that something that's
become second nature to you by
now?
Anybody want me to count them?
Okay, you've got that.
But now which of those two
structures do you think is
better?
<<Students speak over one
another>>
Prof: So you say the top
one's better,
you say, because it doesn't
have charge separation.
Now, as compared to the case on
the left, which do you think --
if you have to have charge
separation, which one is better?
Student: The right.
Prof: The one on the
right because at least you have
it with the right atom,
the oxygen being negative.
You could've drawn the one on
the top too, right?
And that goes the other way.
Notice that I've introduced
something here that I haven't
really talked about enough,
the idea of curved arrows.
Curved arrows show a shift of
electrons, an electron pair.
What would you draw if you
wanted to show just one electron
shifting, if you had to think up
the notation?
Students:
A dotted curve.
Prof: A dotted curve
would be one possibility.
Another possibility?
Sophie?
Student: Half a head
instead of a full head.
Prof: Ah,
like a fishhook instead of the
double arrow.
And that's in fact what's used.
If you want to shift just one
electron you show a single barb,
and if you do a pair of
electrons you show a double
barb, a curved arrow.
So curved arrows don't show
atoms moving.
They don't show this atom goes
to there.
They show an electron pair
moving, or sometimes a single
electron moving,
and you can use a double or a
single barb to show which one.
Okay, so get that straight,
because that's often a problem.
Okay, so the one on top is much
worse than the one on the
bottom.
Okay, so there are our
possibilities with C in the
middle.
Now how about if you put N in
the middle?
Here are two structures with
octets.
Now -- oh pardon me,
they don't all have octets,
that one on the right has a
sestet, because the carbon's
making two bonds;
so it gets three electrons out
of those two bonds --
pardon me, it gets two
electrons out of those two
bonds,
one from each bond,
that's its share from the
bookkeeping,
plus an unshared pair.
Right?
That's only -- what's
associated with it,
it has only three…
pardon me, I'm saying the wrong
thing here.
It has four electrons that it
calls its own,
from a bookkeeping point of
view, one from each bond and two
in the unshared pair.
So it's not charged.
It came in with four electrons,
it's still got four electrons,
but participates with only
three pairs of electrons;
so it's a sestet.
So that's not so good on the
right.
The one on the left is not so
good because it's
charge-separated.
At least it's on oxygen,
the negative charge.
We could draw this other one,
which puts the negative charge
on carbon.
That's, we would say,
not so good.
Right?
So that's a bad charge.
Or we could do it that way.
That looks like the best charge
probably;
better to put positive on
carbon than on nitrogen.
Okay, but that carbon has a
sestet;
the others are octet ones.
Okay, we can do this other one
over here.
Now we've got an octet on the
carbon, but we've got negative
charge on the carbon.
That's not such a great charge
separation thing.
Or we can do it this way,
but again we got bad charge.
Okay, so now if we put O in the
middle we can do these
structures, both of which you
notice have -- what's bad about
them?
<<Students speak over one
another>>
Prof: Sestet there,
sestet on the right too.
There's a plus charge on oxygen;
that's not so great.
You can draw curved arrows and
shift the electron pairs around.
You've now got an octet on
carbon but you got a sestet on
nitrogen, and bad charge,
negative on carbon;
not so great.
And over here you could do that.
Sestet again, bad charge.
Or you can make the three heavy
atoms into a ring,
and then you could put the
hydrogen on any one of the --
coming out of any one of them.
Now the one on the left,
with my computer,
I can't calculate it.
I can't find an energy minimum
that has that structure.
So that one is very bad;
which doesn't surprise anyone.
It's got horrid angles for the
bonds, although we haven't
really said that we prefer one
angle over another.
But it also has bad charge
separation.
Okay?
That one's a bad charge.
So we could draw this other one
here, but it's still got a
positive on oxygen,
a sestet.
What do we have here?
A sestet on carbon.
Okay, so here are the bunch of
isomers we've just gone through.
Now a couple of years ago,
when I first talked about this
in the class,
and when these things had been
accurately calc…
or as accurately as people are
likely to do,
to calculate their true energy
and structure --
I tested my colleagues in the
department,
asked them if they could rank
these things as to what's the
lowest and what's the highest
energy and so on.
And no one succeeded;
in fact, only a few people get
the -- know which the lowest one
is, on the basis of their
familiarity with Lewis theory.
So the point is,
don't be depressed when you are
trying to do this.
It takes a lot of lore,
and even with all of the lore
that's been gathered over many
years,
people can't use Lewis theory
for this purpose because it's
just not that good.
Okay, but in fact the way I've
drawn them there is their
energy, according to these
pretty good calculations.
The very lowest energy is the C
in the middle and H on the
nitrogen, and then C in the
middle and H on the oxygen.
Now you can go back and look at
what we said was good and bad
about these things and convince
yourself that "Ah!
now you understand why those are
on the bottom and these are on
the top."
But if it had come out
differently you would have
convinced yourself differently.
Okay?
So that's enough for today.
Actually, there's a reference
for this if you want to see it.
The energies were published in
the Journal of Chemical
Physics in 2004.
