[Enthalpy]

Hello dear friends!

Making radicals by light is known and used. A next step could be to create two pending bonds near to an other so they couple:
- At one single molecule, for internal coupling;
- Or maybe at two nearby molecules, for cross-coupling.

This would need enough light power density to abstract a second atom or group before the first one has reacted elsewhere. If this can be done, a part of the trick would be a small pressure, an other part short and concentrated laser pulses - they can already ionize nitrogen with several photons having too little energy individually, which seems more difficult than abstracting two groups over more time.

The simplest example could be Br-C₃H₆-Br making cyclopropane without any metal.

Different groups react to different maximum wavelengths: OH, F, Cl, Br, CF₃, C₆Cl₅... so varied groups and successive wavelengths would permit selective couplings - or light and metal at different steps. Cage molecules, ladderanes...

Before I spend hours looking for optical densities, would you tell me if it's already done?
Thank you!
Marc Schaefer, aka Enthalpy

[discodermolide]

Tuning lasers to different frequencies of functional groups to create radicals, I'm sure this has been done.
Anyway is it not the basis of spectroscopy?

edit: I found this http://www.ncbi.nlm.nih.gov/pubmed/12597716 it may be what you are looking for?

[Enthalpy]

Thanks for your interest!

Yes, tuning light can target specific groups to separate them, that's known. What may be new is to create several pending bonds close enough to an other, and within a short time, so that the radicals recombine in the intended way to synthesize the desired compound.

I'll have to check the necessary peak power for that. Nitrogen ionization must be more difficult than groups abstraction, but the laser that cracks nitrogen is sort of extreme, so I hope something more practical suffices for chemical synthesis.

Spectroscopy... To my limited knowledge, the photon energy used to break bonds exceeds the bond energy (often in the UV), which isn't very selective. I often dreamt of exciting the selective mechanical resonance of bonds (usually in the IR) to break them, but this needs to absorb many photons at one bond, despite the energy will spread elsewhere and this bond will radiate the light away. Worse, the mechanical frequency will drift as the amplitude increases. That must be difficult.

[curiouscat]

Empirically how would you distinguish @disco's described mode from yours? All you'll see is that the reaction happened.

[Enthalpy]

If abstracting groups with a low power density of light, the scarce radicals will react with untreated molecules in a chain reaction or with other radicals when the chain terminates.

With the huge power density of a concentrated pulsed laser, I expect to favour reactions between radicals since these get close to an other, and even reactions within a molecule where several groups have been abstracted - internal coupling.

See as an example the "esquema 3.8" gratefully pinched from Carles Ayats Rius' thesis, "Sintesis i reactivitat de derivats del triciclo[3.3.0.0^3,7]octà":
from 272 to 252, light instead of potassium vapour would abstract two iodine atoms, snappily enough that no reaction occurs with neighbour molecules.

[Irlanur]

I don't know any details about that at all, but I know Prof. Martin Quack who told us something about "mode-selective chemistry", maybe you find something with that keyword? He is concerned about energy transfer inside molecules upon excitation of one bond/one part of the molecule. so I guess this is going in this direction...

[discodermolide]

Hello Mr Enthalpy,
I knew I had seen something recently,
Short-lived Phenoxyl Radicals Formed from Green-Tea Polyphenols and Highly Reactive Oxygen Species: An Investigation by Time- Resolved EPR Spectroscopy**
Dmytro Neshchadin, Stephen N. Batchelor, Itzhak Bilkis, and Georg Gescheidt*
In Angewandte International edition, DOI: 10.1002/anie.201407995
Abstract: Polyphenols are effective antioxidants and their behavior has been studied in depth. However, a structural characterization of the species formed immediately upon hydrogen-atom transfer (HAT), a key reaction of oxidative stress, has not been achieved. The reaction of catechin and green-tea polyphenols with highly reactive O-centered H- abstracting species was studied at the molecular level and in real time by using time-resolved electron paramagnetic reso- nance (EPR) spectroscopy. This mirrors the reaction of highly reactive oxygen species with polyphenols. The results show that all phenolic OH groups display essentially identical reactivity. Accordingly, there is no site specificity for HAT and initial antioxidative events are demonstrated to be largely ruled by statistical (entropic) factors.

[Enthalpy]

Thanks Irlanur! There are two different attempts, both with ultrashort pulses, but one typically with IR and the other with UV.

With IR, "mode-selective chemistry" uses the mechanical resonance of specific bonds to energize them, possibly break them. One example, though in software, there
http://chem.wayne.edu/schlegel/Pub_folder/359.pdf
That would be very nice since such resonances are selective. It looks very difficult because the molecule redistributes the vibration quickly over many bonds, and - the cited paper doesn't mention it - because of stimulated emission, which lets an already excited mode lose energy when a new photon arrives, and also because the resonance must drift with the amplitude.

Obviously I wasn't the only one dreaming of that one. By the way, this process would distinguish isotopes. Maybe the secret-held US patent by Australian researchers for uranium enrichment relates with that.

With UV, one won't get the strong selectivity of IR; it's more like "break the weakest bond". But splitting molecules with UV has been common use for decades, say to halogenate alkanes. The only impovement I seek is to abstract two groups close and snappily enough that the two pending bonds react immediately - possibly within one single molecule. Less accurate, less ambitious than the "mode-selective chemistry", but hopefully easier. It seems easier than the already observed multiphoton ionization of nitrogen.

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Thanks Discodermolide! I'll invest more time in this.

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Would someone propose a link to the cross section, optical density, extinction coefficents (or similar!) that result from photodissociation? Something like haloalkanes would be fine.

Up to now (hours...) I've found cross sections towards the antibonding state of C-Br and many more. Is it correct that photodissociation always results from one electron in a (single) bond absorbing light, getting in the antibonding state, and then the two atoms don't attract an other any more? Then I'd have the necessary data. Or are there different processes? Thanks!

[Enthalpy]

Finally, I've found one good data source that isn't behind a paywall, thanks JPL:
http://jpldataeval.jpl.nasa.gov/pdf/JPL%2010-6%20Final%2015June2011.pdf (9MB)
which gives absorption cross-sections for freons that release halogens in the atmosphere under ultraviolet light.
Data for more gases and wavelengths there, free, thanks MPI!
http://satellite.mpic.de/spectral_atlas

• Photolysis needs well over one bond energy per photon. Never trust a textbook...
• For halohydrocarbons between 200nm and 400nm, each absorbed photon abstracts one haloatom and leaves the carbons and hydrogens, with few exceptions.
• Nitrites and nitrates are other candidates, acid dimers and esters maybe, nitriles, aldehydes and cetones little so.
• Adequate light sources for UV aren't really ripe but they progress quickly - I plan indications. They permit to abstract I and Br, but Cl and F improbably.
• The absorption and best wavelength for a haloatom depend much on the haloatom, on its geminal polarizing atoms, far less on the neighbour carbons and the atoms they carry.
• The cross-sections seem to permit intramolecular coupling through the abstraction of two haloatoms per molecule - under optimized conditions, where the wavelength demands exotic light sources, which I believe can be purposely made. This suggests the method isn't common right now.
  

Cross-couplings, CO₂ eliminations, and other steps where one absorbed photon per molecule suffices, accept light less concentrated for which sources are easier and may already exist, especially excimer lamps
http://goldbook.iupac.org/PDF/ET07372.pdf
I plan to concentrate on intramolecular coupling.

Marc Schaefer, aka Enthalpy

[Enthalpy]

If we suppose (found no data) that abstracting a second haloatom from a different location of a molecule is about as difficult as abstracting the first one, then "remove most often a second haloatom if one is removed" needs as much absorbed energy density as "remove a haloatom from most target molecules" (the chosen wavelength shall favour the second abstraction if it differs). This is a big energy density, which for instance exceeds the vaporization energy. Some pulsed focussed lasers achieve it.

The numerical example takes C₃H₇-I as a model for I-C₃H₆-I to be cyclized. Its absorption cross-section at 255nm is 140*10^-20cm² per molecule. 170g/mol and 1743kg/m³ let the liquid or solid absorb 63% of the energy within 1.2µm distance. The short depth permits to focus the light to D=2µm and deposit the energy in 2.5µm³ or 15*10⁹ molecules. With 2 photons per molecule, that's 40nJ light and, at 10GHz repetition rate, 400W to process 2.5*10^-4 moles per second, or 150g per hour and per beam. 1000 beams process 45t in a 300h month.

A gas would absorb light over a longer distance, like 0.3mm at 1bar partial pressure and 300K, over which the light spot is necessarily broad. The absorption volume contains more molecules, which would demand more energy per pulse.

The pulses of a modelocked laser are often 1ps long, equalling the free flight time in a liquid, so bonds are clear to rearrange before meeting many other species. The irradiated liquid expands in 1ns during which the collision frequency drops. I find useful to irradiate the liquid or solid near its surface, so the products expand freely:

• It limits the explosion overpressure;
• Hydrocarbons supposed to survive these conditions are often gases, but their halides liquid or solid;
• The haloatom flies away from the hydrocarbon, reducing the risk of further reactions;
• The gas can contain a quencher. While iodine isn't very reactive, that's useful with bromine. Scavenging the haloatom is also good.

The left variant on the sketch makes a film of reactants at the optics' surface where light converges. Silica is transparent to 200nm; one lens might concentrate several beams, and the reactor can have several lenses. In all variants, the focal area must wobble a bit so the reactants have time to replenish.

The right variant on the sketch lets light cross a short distance of gas at low partial pressure. This preserves the optics, but may require to chill the reactants, and demands to adjust the focus to the surface - DVD burners do it also, and more accurately.

Bringing light through a narrow fibre (not sketched) to the reactants without a lens there looks interesting. Maybe capillarity can replenish a thin reactant film at the fibre's tip.

More about the wavelengths and the light sources should come.
Marc Schaefer, aka Enthalpy

[curiouscat]

The left variant on the sketch makes a film of reactants at the optics' surface where light converges. Silica is transparent to 200nm; one lens might concentrate several beams, and the reactor can have several lenses. In all variants, the focal area must wobble a bit so the reactants have time to replenish.

Do you actually build any of this?

[Enthalpy]

The idea is available to all here.

The difficult part is the UV laser, and I did build semiconductor components. Well, that was during the paleomonolithic era...

[Enthalpy]

Illuminating a gas instead spreads the energy among more molecules, needing more energy per pulse, but this is accessible to many lasers.

ArF exciplex lasers at 193nm for semiconductor lithography can deliver 15mJ, while an inertial fusion setup obtained 10kJ from KrF, and one could also multiply the frequency of a solid laser. C₂H₅Br molecules for instance absorb 193nm with 6*10^-19cm² cross section (and BrC₂H₄Br 3*10^-18cm²).

At 2mbar, C₂H₅Br (for want of *C₂H₄Br data) absorbs 63% of the light within 0.34m. A light path 0.4mm wide can converge from and diverge to 0.6mm within 0.17+0.17m, optimally and with constant index in the gas. The 96mm³ path contains 4.7*10¹⁵ molecules; each absorbing two photons takes 63% of 15mJ.

At 2mbar without a carrier gas, the mean free flight nears 60ns, so a 20ns light pulse from ArF has decent chances to abstract two Br from each molecule that hopefully cyclizes. This can improve with shorter laser pulses, stronger pulses, or with less pressure - additional lenses or concave mirrors, possibly using internal reflection, can the keep the light bundled and the reactor short.

Ammonia masers show that 1ps pulses don't leave time for hydrogen atoms to tunnel in a molecule, but 20ns do. Iodine atoms abstracted with 0.1eV excess energy move already 400pm away in 1ps. This speaks for short pulses. A lithography laser isn't a natural choice neither. I'll give the laser more thoughts later.

Marc Schaefer, aka Enthalpy

[Enthalpy]

This digression considers reactions sparked by one photon per coupling. They get the unconcentrated light from lamps or semiconductor light-emitting diodes, both more mature than lasers of same wavelength:

• ArF, KrCl, KrF, XeI lamps for 193nm, 222nm, 248nm, 253nm;
• AlN diodes for 210nm, AlGaN for 245nm-400nm approximately;
• Hg lamps for 254nm.

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Cyclobutanes from illuminated substituted ethylenes are known. The "Mono-enes" diagram shows that 193nm, 210nm and 222nm, maybe 248nm and 254nm, discriminate enough between the ethylenics to favour the desired product, with a gas pressure adapted to the absorption.

For instance beta-pinene absorbs 100 to 10,000 times more than pentene and ethylene, so a 1:10 to 1:100 dilution hopefully lets convert its double bond to one cyclobutane more. This is slightly better than hydrogenating it to cis-pinane, as a rocke... oh, you guessed.

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Polyethylenics show an even better constrast to ethylene on the "poly-enes" diagram. Again, a discriminating wavelength and proper dilution shall excite the diene to react with the ene.

Take as an example the product cyclobutyl-cyclobutane, which is... a compound that burns well with oxygen and doesn't polymerize in a cooling jacket. I suggested there to make it from butadiene and ethylene:
http://www.chemicalforums.com/index.php?topic=50579.msg233808#msg233808
and at least the UV cross-sections support this scenario, the orientations of Lumo and Homo pi as well if textbooks don't cheat. The lamps were affordable to buy and run when I checked.

Better: the cross-section at 193nm of methylbutene C=CC(C)C, taken for want of vinyl-cyclobutane C=CC1CCC1, suggests a one-pot one-lamp synthesis, where butadiene diluted in ethylene becomes vinyl-cyclobutane and then cyclobutyl-cyclobutane.

I've also put propadiene and isobutene (for want of methylenecyclobutane) on the diagram, if someone wants to try his big luck at spiroheptane...

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More exotic: the cited JPL's Pdf suggests on page 4G-24 that tribromomethane can absorb one photon to convert in carbene CHBr by expelling Br₂ when it doesn't split in CHBr₂ and Br. The purported photon-to-carbene yield is 0.16 at 267nm and 0.26 at 234nm - and at 222nm we have strong KrCl lamps.

Such carbenes diluted in alkenes may lead to cyclopropanes without using active metals, where the remaining haloatom is useful or easy to remove. Tribromomethane absorbs 222nm light better than alkenes do (diagram), including beta-pinene (consider also alpha-pinene, carene...); longer waves improve this selectivity.

Avoiding active metals eases the cyclopropanation in big amounts considered there
http://www.chemicalforums.com/index.php?topic=65186.0

I haven't seen carbenes cited for longer molecules in the Pdf, nor for triiodomethane, and for chlorodibromo neither. If iodides make carbenes at longer waves than bromides do, the selectivity over alkenes would improve (diagram).

Marc Schaefer, aka Enthalpy

[Enthalpy]

I've suggested to irradiate butadiene diluted in ethylene to obtain first vinylcyclobutane, then cyclobutylcyclobutane. Worry: butadiene is known to make cyclobutene with light. Though, the carbon movement must take time, so I hope that ethylene dense enough, possibly liquid, reacts with the excited butadiene first. Anyway, catalysts are known to make this first step without light, as an excellent alternative.