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Novel photochemical ammonia synthesis

22 April 2016 - The biohybrids are coming

AmmoniaSynthBrown2016.PNGIn nature, bacteria convert nitrogen to ammonia using a iron-molybdenum cofactor (FeMoCo) and a lot of ATP. As ammonia is valuable, chemists have been working on a synthetic ammonia synthesis process for ages that thus far has only yielded the brute-force high-temperature high pressure Haber process. A new effort by the King team at the National Renewable Energy Laboratory reported in Science (Brown et al. DOI) describes a photochemical version. In it FeMoCo stays but electron-donor ATP is replaced by cadmium sulfide nanocrystals. When exposed to light this material can deliver electrons just as well.

For this study the bacterium Azotobacter vinelandii produced the required quantities of FeMoco. CdS nanorods were produced in a complex solid-state high temperature synthesis with calcium oxide, octadecylphosphonic acid, trioctylphosphine oxide, hexamethyldisilathiane and tributylphosphine. A mixture of CdS nanocrystals and FeMoCo with add buffer HEPES was irradiated at 405 nm and ammonia generation measured: TOF = 75 per minute. This compares well with the biochemical ATP process with a TOF of 119 per minute. The reported quantum yield is 3.3%. The usual FeMoCo inhibitors (CO, acetylene) also inhibited the photochemical process. Downside is that HEPES is a sacrificial catalyst in this system. The authors note that other more qualified reducing agents exist but the CdS nanorods deliver electrons more rapidly. This prevents the competing process of hydrogen generation before nitrogen binds.


08 April 2016 - Chemical Zoo

boron sheets Feng 2016.PNGIf you want to read up on the new two-dimensional boron sheets unrolled by Baojie Feng et al. you have options: choose either 'Experimental Realization of Two-Dimensional Boron Sheets' as published on arxiv (link, free-access) submitted 16 december 2015 or choose "Experimental realization of two-dimensional boron sheets" as published in Nature Chemistry (doi), paywall / :-) submitted 17 july 2015. Somehow Feng & associates were in a hurry to get their research published and the Nature Chemistry editors / reviewers were not. So what was all the fuzz about? The authors of either article note that while carbon is more than fond of flat sheets, boron prefers cages as in borospherene or if it must, lousy 36-atom not-even-flat sheets of borophene. The B40 anion was featured earlier in this blog. Still a far cry from 2-dimensional sheets. The Feng team evaporated 99.9999% boron on silver at 570K / 6x10-11 torr. The formation of boron islands is all in the STM evidence. The layers are of the type of hexagonal boron rows. Annealing at 650K produced a second type of sheet with a different hole / vacancy configuration. Interestingly both layer types were not among the various computationally predicted types. The formation of 3D-structures at higher boron loading hampers the development of larger sheets. On the other hand the boron layer is glued to the silver surface with the same strength as graphite to copper so 2D-boron should be able to detach itself.


Gas-phase SN2 rethink

02 April 2016 - Chemical modelling

Interesting perspective by Xie & Hase in this week's Science on the topic of bimolecular nucleophilic substitution (SN2), The title "Rethinking the SN2 reaction" sounds alarming but only the gas-phase variety is in need of a reconsideration. So what is going on: the textbooks all present SN2 (X` + CH3Y -> XH3C + Y`) as a classic and clean second order reaction with a single transition state and with inversion of configuration. Add more detail and attractive ion dipole pair interactions appear resulting in two wells flanking the TS. But Xie and Hase detect more issues standing in the way of accurately modelling this reaction type. It seems more like modelling a car crash. The approach of the anion is not limited to the backside of the halide: the anion can approach head-on or it can even fly-by. There is also a complex roundabout dance and what seems like a case of retention of configuration can also be a double inversion. Of course changing one halide for another or introducing solvent molecules complicate things further. By now you are looking at an army of modellers and experimental physical chemists to get things sorted out. Main conclusion: more work needs to be done. In this way the perspective seems more directed at influential people with grant money to distribute than at fellow scientists.