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forget everything you know about
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chemistry we have finally found real
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evidence of a carboncarbon sigma Bond
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based on not two but just one single
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electron will this truly rewrite the
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textbooks though and inflict more
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despair on poor students in this video
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we'll learn about the concept of one
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electron bonds and the details behind
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this new discovery especially the design
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and synthesis of this strange molecule
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will surprise you so what is a bond in
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organic chemistry we focus on calent
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bonds as they are much more common than
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ionic bonds in calent bonds two atoms
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share two electrons of their outer
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shells indicated by two dots in their
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leis structure in the case of chlorine
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this completes its octet of eight veence
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electrons which lowers its energy by the
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way it was Gilbert Lewis who proposed
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this model of bonding already more than
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a century ago in 1916 the concept of
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electro negativity did not formally
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exist but it's interesting that Louis
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mentioned that we might draw the
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electron as being closer to certain
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atoms indicating Bond polarization again
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something students are still drawing
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today to understand the basics of
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bonding not all models from back in the
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day survived for example the cube model
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to explain electron sharing but look at
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this we have some nice foreshadowing of
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our topic structure B which LS just
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threw theoretically for completeness
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would correspond to sharing of a a
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single electron an explicit theory on
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single electron bonds was provided by
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lonus polling in
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1931 this was rather bold because at the
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time chemists were unable to directly
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observe these very weak bonds the
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simplest and quite obvious example is
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the dihydrogen cation this is H2 plus
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consisting of two protons that are
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sharing a single electron decades after
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pauling's Theory chemists started to
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actually characterize compound with
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single electron bonds check out this
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strange phosphorous structure which
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mind-blowingly can be isolated forming
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red crystals it's a dimer of two
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radicals linked by single electron bonds
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between the two freem membered rings you
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might have figured with just a single
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electron such a bond is relatively weak
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and accordingly becomes longer that's
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cool but no offense to phosphorus
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organic chemistry is really all about
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that carbon one electron Sigma bonds of
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carbon have been postulated in exotic
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mechanisms involving electron transfer
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reactions such as the cattin radical
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cope rearrangement this means that until
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recently we had no clue whether they
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really exist as a side note one electron
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bonds are not as special as you might
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expect just like two electron calent
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bonds they predominantly arise due to
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Quantum interference phenomena if you're
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interested you can pause and read
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through these texts on the situation of
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the dihydrogen cat
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as mentioned this is the simplest case
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where one electron fluctuates between
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two protons the theory is pretty
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complicated and also our understanding
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is still evolving as it seems that the
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specific impact of interference changes
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if we go from hydrogen two heavier
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elements I didn't bother trying to
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understand everything here as I'm really
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happy with not being a physical chemist
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so how did chemists manage to isolate a
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carbon compound with such an elusive
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Bond well the first design idea was to
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use a hexa fenel Fane derivative by
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attaching aromatic substituents to a
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carbon carbon Bond we could stabilize
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the potential radical and catin
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characters that would develop so far so
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good the most obvious way to generate a
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one electron Sigma bond is to Simply
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remove an electron out of an existing
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one the ease of this oxidation is given
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by the oxidation potential E1 but here's
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the issue despite the attempted
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stabilization the radical cation would
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immediately fragment and simply give off
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another electron E2 the oxidation
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potential of radical cattin is less
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positive than E1 so the key task is to
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find a system where the second oxidation
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is more controlled and proceeds in a
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stepwise manner conveniently the
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research team had studied some strange
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hexenal F derivatives before they all
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have a shell of aromatic Rings which act
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to stabilize and protect the weak
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central CC Bond interestingly tweaking
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the bottom half slightly changes the
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oxidation Behavior completely while
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compounds 1 a and 1B undergo a onestep
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two electron oxidation the compound 1 C
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shows a stepwise one electron oxidation
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you can see in the cyclic volog that
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instead of the continuous rise that we
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see for 1 a and 1 B we have two little
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bumps for 1 C corresponding to the
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individual one electron
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oxidations but what makes this
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particular system special well it boils
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down to the length of the bond that we
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are oxidizing compared to Fane which has
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a bond length of 1.54 angstrom the bonds
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in our series get longer and longer this
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is driven by the steric repulsion of the
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Rings as the two huge groups simply
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don't want to get too close to each
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other if the bottom half becomes even
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more locked up the central Bond
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elongates further at the time of its
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Discovery this third compound featured
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the longest CC Bond ever found in
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neutral
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hydrocarbons you might wonder how
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exactly Bond length explains the
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oxidation potential oxidations depend on
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the energy of the electrons that are
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removed so unfortunately we do have to
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look at some orbital Wizardry called
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through Bond interaction many viewers
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will know that orbitals can interact
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when directly next to each other but
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this also works through space or through
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other bonds this diagram shows the basic
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logic for a simpler dienel Fane system
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assuming a parallel alignment our CC
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Sigma Bond can interact with the pi
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system of the neighboring fennel rings
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as usual this stabilizes the symmetric
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orbital combination while destabilizing
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the anti-symmetric one if the
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destabilization is high enough the anti-
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symmetric combination becomes the new
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highest occupied molecular orbital or
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home
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this is what happens in our more
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complicated system at hand so what is it
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with the bond length a longer bond is a
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weaker bond with a higher energy level
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of the original Sigma orbital you might
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know that orbital interactions get
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stronger the closer the two orbitals are
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in energy lengthening the bond and
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increasing the sigma energy gets it
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closer to the pi orbital intensifying
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the destabilization of the homo given
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enhanced through Bond coupling in our
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compound it turns out this is just
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enough to put some brakes on The
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Unwanted second oxidation this only
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works because the single occupied
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molecular orbital or SoMo of the radical
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catin is less destabilized than the homo
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of the neutral compound you can call it
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interesting strange or complicated
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beyond belief the important thing is
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that the chemists expected that with
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this specific molecule they might be
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able to isolate a one electron carbon
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carbon Bond before we get to that you
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might wonder how the framework is
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synthesized it's four steps in total we
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start with the bottom half bearing two
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bromo groups upon lithiation they turn
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into nucleophiles that can attack the
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carbonal groups of two equivalents of
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dibenzo subone now creating the ultra
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long Bond makes use of the same factor
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that we need to stabilize the radical
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cation the aromatic rings by adding
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protons or a Louis acid here TMS Plus
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the hydroxy groups dissociate leaving
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back a stabilized DCA the two carbon
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centers are linked via reduction with
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Zinc powder and the product can be
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purified by column chromatography
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without further issues so what happens
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when we try to oxidize this Bond when
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using free equivalents of iodine at room
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temperature we basically reverse the
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last synthetic step giving the die catin
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due to two electron oxidation they were
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able to rec crystallized Dark Violet
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crystals containing triiodide as a
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counter anine and confirmed the
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structure but by just using one and a
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half equivalent of iodine they
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suppressed the over oxidation and struck
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gold or carbon you might ask how can we
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be sure what this second product really
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is the first information comes from
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electron spin resonance spectroscopy
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which can detect the unpaired electrons
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we really are dealing with a highly
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stabilized radical here more important
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is the X-ray analysis of their nice
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crystals you don't see the packing of
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the counter but given its presence we
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know we really are looking at a radical
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cattin our Central Sigma bond which was
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already Ultra long before is stretched
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out to an amazing 2.9 angstrom obviously
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with just one electron being shared the
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bond is weak and does very long what you
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don't see when drawing this on paper is
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the geometry of the top two rings the
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one on the bottom here is slightly bent
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while the one on the top is as flat as a
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pancake despite the slight Distortion it
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looks like the two carbons are both SP2
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hybridized just what you would expect
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without the bond they have one
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unhybridized p orbital each between
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which the single electron is moving
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despite this asymmetrical structure the
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bond lengths within the two rings are
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extremely similar indicating a strong
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delocalization of the electron Spin and
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positive charge you can even see the one
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electron bond in the electron density
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map of the xray data the blue color
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indicates positive electron density
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outside of the two carbon atoms again
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showing that the unpaired electron is
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sitting between them they did a few
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other things like Raman spectroscopy and
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orbital computation which I will skip
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over the point is we are sure this is a
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one electron Bond overall this is a
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break break through for carbon but as we
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said initially it's not like one
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electron bonds or some special magic so
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will this discovery be in the textbooks
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it might be but it will definitely not
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rewrite them high school and undergrad
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students can be relieved still pretty
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awesome stuff and I hope this video
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expanded your Chemistry Horizon thank
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you so much for watching and the special
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Thanks goes to my channel supporters
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here on YouTube and on patreon if you
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like the topic please leave a like
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comment or subscribe I hope to catch you
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again in the next one
