all blogs

Germanane in photocatalysis

22 August 2014 - Chemical Zoo

germanium hydride photocatalysis.PNGNew in the family of graphene / graphane like materials is germanane (GeH). It was invented in 2013 by Goldberger et al. (DOI). Synthesis of precursor CaGe2 is accomplished by heating elemental calcium and germane at 1000°C for 18 hours. This compound is a typical Zintl compound with the calcium atoms in between 2D germanium layers. When treated with HCl for an extended time (-40°C / 8 days), hydrogen replaces calcium and germanane is formed. The ion-exchange expands a layer from 5 to 5.5 Angstrom. Properties reported in 2013: surface stable over months, loses hydrogen at 200°C and can be exfoliated with scotch tape to single layers on silicon. The band gap is 1.58 eV.
A recent publication by Zhenhua Liu et al. (DOI) has more on properties. One change was made in the synthetic procedure , mercury was used as a flux in the CaGe2 reaction and later removed by ultracentrifugation. Compared to nitrogen doped (ammonia exposed) titanium oxide oxidation of Rhodamine B by visible light by germanane (in water) is 6 times as fast. Photocatalytic water splitting was also tested. A solution of germanane in water under irradiation did nothing but adding methanol as a so-called hole-scavenger saved the day with lots of hydrogen generation. These hole scavengers prevent the recombination of the electron/hole pair formed after a photon strikes the catalyst surface, increasing the catalyst efficiency. No reference catalyst was tested making for certain the finding cannot be graded against other catalysts that exist in the literature.

Chematica on linguistics

08 August 2014 - The CRD

Chematica caused quite a stir back in 2012 with their announcement of a "chemical brain" and a "Google on steroids". Chematica is software that promises to design synthetic pathways by computer algorithms and contains a database of 10 million compounds interconnected by reaction vectors. It would make the retrosynthetic chemist obsolete. Chematica is associated with Northwestern University, Illinois but it is also a commercial product. The website does not have a lot to offer. How was the chemical reaction database compiled? A licence from Scifinder or Reaxys would cost a fortune. So far all claims made by Chematica are untested by independent sources and remain unchallenged. Is this network really only second in size to the World-Wide Web as the website claims? Computer-generated brains must exist larger than that.

Chematica is back in the news with an article in Angewandte Chemie on organic chemistry as a language. Too bad Angewandte has a crappy disclosure policy: the last entry in the references. Other publications exist in the chemical realm that position disclosures more prominently. The article describes the relationship between organic molecules and the English language. In linguistic terms the latter follows Zipf's law and it would be interesting to see if the same holds for the first. Key finding: it does, but not based on functional groups but on molecular fragments in general.
In linguistics two sentences are compared from a set of sentences and the maximum common substrings (MCS) are recorded. In a ranking of MCS's a given entry is always twice as populous as the one immediately below. In the Chematica research the same is done with sets of molecules. Now a MSC can be the isopropylbenzene fragment or a specific carbon-carbon bond in a larger frame. Fragments such as the methyl group that are likely to show up in every molecule are filtered out. Again the ranking follows Zipf's Law but the actual ranking is not disclosed in the article. The research program also had a second goal: find out if in a molecule the "most characteristic bond" (one with highest fragment frequency) would also be also the prime target for retrosynthetic disconnection. The article does not state a rationale for this hypothesis and this blog is clueless. Disconnections are about chemical opportunity and not about graph theory. In any event, a panel of pH.D.-level chemists was set to work and "In about 97% of cases, at least one chemist suggested one of the top-three computer-chosen bonds".

See previous update on chemical reaction databases here.