Category Archives: Chemistry

Aurides Chemistry – New Paper in Organometallics


Compound 2 represents the first structural example of a 12 e− auride complex, with a pseudohalide/hydride nature in bonding. According to our NBO calculations, this electron deficient gold center is stabilized by weak intramolecular interactions between Au p orbitals and σC−C and σC−H bonds of adjacent aromatic rings together with a Ga−Au−Ga 3 centers−2 electrons bond (I like the term ‘banana bond‘, don’t you?).

Fig. 1 Crystal structure for Compound 2. Au in the center is effectively an auride.

I was invited to participate in this wonderful venture by my good friend and colleague Dr. José Oscar Carlos Jiménez-Halla, from the University of Guanajuato, Mexico, with whom we’re now working with Prof. Rong Shang at the Hiroshima University. Prof. Shang has synthesized this portentous Auride complex and over the last year, Leonardo “Leo” Lugo has worked with Oscar and I in calculating their electronic structure and bonding properties.

Gold catalysis is an active area of research but low valent Au compounds are electron deficient and therefore highly reactive and elusive; that’s why researchers prefer to synthesize these compounds in situ, to harness their catalytic properties before they’re lost. Power’s digalladeltacyclane was used as a ligand framework to bind to a Au(I) center, which became reduced after the addition and breaking of the Ga−Ga bond while the opposite face of the metallic center became blocked by the bulky aromatic groups on the main ligand. NBO calculations at the M05-2X/[LANL2TZ(f),6-311G(d,p)] and QTAIM BCP analysis show the main features of Au bonding in 2, noteworthy features are the 3c−2e bond (banana) and the σC−C and σC−H donations (See figure 2).

Fig.2 Natural Hybrid Composition for the Ga−Au−Ga ‘banana‘ bond (left). Bond Critical Points (BCPs) for Au in 2 (right).

One of the most interesting features of this compound is the fact that Au(PPh3)Cl reacts differently to the digallane ligand than it does to analogous B−B, Si−Si, or Sn−Sn bonds. The Au−Cl bond does not undergo metathesis as with B−B, nor does it undergo an oxidative addition, so to further understand the chemistry of−and leading to−compound 2, the reaction mechanism energy profile was calculated in a rather painstakingly effort (Kudos, Leo, and a big shoutout to my friend Dr. Jacinto Sandoval for his one on one assistance). Figure 3 shows the energy profile for the reaction mechanism for the formation of 2 from Power’s digallane reagent and Au(PPh3)Cl.

Fig. 3 Free Energy profile for the formation of 2. All values, kcal/mol

You can read more details about this research in Organometallics DOI:10.1021/acs.organomet.0c00557. Thanks again to Profs. Rong Shang and Óscar Jiménez-Halla for bringing me on board of this project and to Leo for his relentless work getting those NBO calculations done; this is certainly the beginning of a golden opportunity for us to collaborate on a remarkable field of chemistry, it has certainly made me go bananas over Aurides chemistry. OK I’ll see myself out.

Mario Molina, Nobel Laureate. Rest In Peace


Prof. Mario Molina was awarded the Nobel Prize in Chemistry in 1995, the same year I started my chemistry education at the chemistry school from the National Autonomous University of Mexico, UNAM, the same school from where he got his undergraduate diploma. To be a chemistry student in the late nineties in Mexico had Prof. Molina as a sort of mythical reference, something to aspire to, a role model, the sort of representation the Latinx and other underrepresented communities still require and seldom get.

I saw him several times at UNAM, where he’d pack any auditorium almost once a year to talk about various research topics, but I remember distinctly the first time I sort of interacted with him. It was 1997 and I attended my first congress, the 5th North America Chemistry Congress. Minutes before the official inauguration which he was supposed to preside, I caught a glimpse of him in the hallways near the main conference room. Being only 19 years old, I thought it’d be a good idea to chase him, ask for his autograph and a picture. He was kind enough not to brush me off and took just a minute to shake my hand, sign my book of abstracts, and get his picture taken with me. But cameras back then relied on the user to place a roll of film correctly. I did not; so the picture, although it happened, it doesn’t exist. Because of this and other anecdotes, that congress cemented my love for chemistry. I never asked for a second picture in the few subsequent occasions I had the pleasure to hear him talk.

Prof. Molina was an advocate of green and sustainable sources of energies. His work predicted the existence of a hole in the ozone layer and his struggle brought change into the banning of CFCs and other substances which interfere with the replenishment of ozone in the sub-stratosphere. Today, his legacy remains but also do his pending battles in the quest for new policies that favor the use of green alternative forms of energy. May he rest in peace and may we continue his example.

The #LatinxChem Twitter Poster Contest


For the past few weeks, some chemists of the worldwide Latinx community have been cooking an online project devoted to showcase the important contributions to chemistry made by workers, students, and researchers from Latinamerican origin.

The result is the #LatinXChem Twitter Poster Contest which will take place 7th September during a 24 hour span and the corresponding Twitter account @latinxchem (go follow it now! I’ll wait right here.)

All chemists from Latinx origin are called to participate by registering their posters in our website latinxchem.org before August 25th. Upon registration, each poster should be classified into one of the eleven categories available and use the corresponding hashtag during the event (e.g. #LatinxchemTheo for the readers of this blog), in which prominent Latinx chemist will serve as reviewers and cast their votes for the best one in each category. Some prizes will be available, thanks to our kind sponsors (RSC, Chemical Science, ACS, Carbomex, The Brazilian Chemical Society, and more to come), but just for those registered works; if anyone wishes to present a poster without being registered at the website they can do so but eligibility for prizes remain for those who complete the register. Official languages for the poster are Spanish, Portuguese, and English.

Each category is organized by young prominent Latinx chemists; for the particular case of Computational Chemistry –the recurring theme of this blog– Prof. Fernanda Duarte (@fjduarteg) from Chile now working at Oxford University in the UK and yours truly (@joaquinbarroso) will be in charge of the #LatinXChemTheo section. Please check the website to learn about the other sections and the wonderful people working hard in the organizing committee (see below for the full list of the organizers and their Twitter handles).

The main goal of the event is to celebrate and showcase the espectacular research, education, and innovation brought to chemistry by a large and vibrant community dispersed throughout the globe of Latinx identification. We want to celebrate diversity by showcasing our contributions in the context of a global science interconnected with people from other groups.

So please visit our website, help us spread the word and get those posters ready, we’re eager to read, comment, Tweet and Retweet your work and show the world the drive and passion of Latinxs for chemistry, knowledge, and the betterment of the world through science.

Go follow us all and of course @LatinXchem too!

¡Gracias! Obrigado! Thank you!

Gabriel Merino Cinvestav Mérida, México @theochemmerida
Miguel A. Méndez-Rojas UDLAP, México @nanoprofe
Joaquín Barroso UNAM, México @joaquinbarroso
Javier Vela Iowa State University, USA @vela_group
Diego Solís-Ibarra UNAM, México @piketin
Braulio Rodríguez-Molina UNAM, México @MolinaGroup
Paula X. García-Reynaldos Science Communicator, México @paux_gr
Liliana Quintanar Cinvestav Zacatenco, México @lilquintanar
María Gallardo-Williams North Carolina State University, USA @Teachforaliving
Fernanda Duarte University of Oxford, UK @fjduarteg
Yadira Vega Tec de Monterrey, México @yivega
Gabriel Gomes University of Toronto, Canadá @gpassosgomes
Luciana Oliveira UNICAMP, Brasil @LuBruGonzaga
Cesar A. Urbina-Blanco Ghent University, Belgium @cesapo
Ariane Nunes HITS, Germany @anunesalves
Walter Waldman Brazil, @waldmanlab

DFT Estimation of pKb Values – New Paper in JCIM


As a continuation of our previous work on estimating pKa values from DFT calculations for carboxylic acids, we now present the complementary pKb values for amino groups by the same method, and the coupling of both methodologies for predicting the isoelectric point -pI- values of amino acids as a proof of concept.

Analogously to our work on pKa, we now used the Minimum Surface Electrostatic Potentia, VS,min, as a descriptor of the availability of Nitrogen’s lone pair and correlated it with the experimental basicity of a large number of amines, separated into three groups: primary, secondary and tertiary amines.

Interestingly, the correlation coefficient between experimental and calculated pKb values decreases in the following order: primary (R2 = 0.9519) > secondary (R2 = 0.9112) > tertiary (R2 = 0.8172). This could be due to steric effects, the change in s-character of the lone pair or just plain old selection bias. Nevertheless, there is a good correlation between both values and the resulting equations can predict the pKb value of an amino group within less of a unit, which is very good for a statistical method that does not require the calculation of a full thermodynamic cycle.

We then took thirteen amino acids (those without titratable side chains) and calculated simultaneously VS,min and VS,max for the amino and the carboxyl group (this latter with the use of equation 2 from our previous work published in Molecules MDPI) and the arithmetical average of both gave us their corresponding pI values with an agreement of less than one unit.

This work is now available at the Journal of Chemical Information and Modeling (DOI: 10.1021/acs.jcim.9b01173); as always a shoutout is due to the people working on it: Leonardo “Leo” Lugo, Gustavo “Gus” Mondragón and leading the charge Dr. Jacinto Sandoval-Lira.

Estimation of pKa Values through Local Electrostatic Potential Calculations


Calculating the pKa value for a Brønsted acid is very hard, like really hard. A full thermodynamic cycle (fig. 1) needs to be calculated along with the high-accuracy solvation free energy for each of the species under consideration, not to mention the use of expensive methods which will be reviewed here in another post in two weeks time.

Thermodynamic_Cycle
Fig 1. Thermodynamic Cycle for the pKa calculation of any given Bronsted acid, HA

Finding descriptors that help us circumvent the need for such sophisticated calculations can help great deal in estimating the pKa value of any given acid. We’ve been interested in the reactivity of σ-hole bearing groups in the past and just like Halogen, Tetrel, Pnicogen and Chalcogen bonds, Hydrogen bonds are highly directional and their strength depends on the polarization of the O-H bond. Therefore, we suggested the use of the maximum surface electrostatic potential (VS,max) on the acid hydrogen atom of carboxylic acids as a descriptor for the strength of their interaction with water, the first step  in the deprotonation process. 

We selected six basis sets; five density functionals; the MP2 method for a total of thirty-six levels of theory to optimize and calculate VS,max on thirty carboxylic acids for a grand total of 1,080 wavefunctions, which were later passed onto MultiWFN (all calculations were taken with PCM = water). Correlation with the experimental pKa values showed a great correlation across the levels of theory (R2 > 0.9), except for B3LYP. Still, the best correlations were obtained with LC-wPBE/cc-pVDZ and wB97XD/cc-pVDZ. From this latter level of theory the linear correlation yielded the following equation:

pKa = -0.2185(VS,max) + 16.1879

Differences in pKa turned out to be less than 0.5 units, which is remarkable for such a straightforward method; bear in mind that calculation of full thermodynamic cycles above chemical accuracy (1.0 kcal/mol) yields pKa differences above 1.0 units.

We then took this equation for a test with 10 different carboxylic acids and the prediction had a correlation of 98% (fig. 2)

47051619_1824157374360101_2244437569725005824_n
fig 2. calculated v experimental pKa values for a test set of 10 carboxylic acids from equation above

I think this method can really catch on for a quick way to predict the pKa values of any carboxylic acid imaginable. We’re now working on the model extension to other groups (i.e. Bronsted bases) and putting together a black-box workflow so as to make it even more accessible and straightforward to use. 

We’ve recently published this work in the journal Molecules, an open access publication. Thanks to Prof. Steve Scheiner for inviting us to participate in the special issue devoted to tetrel bonding. Thanks to Guillermo Caballero for the inception of this project and to Dr. Jacinto Sandoval for taking the time from his research in photosynthesis to work on this pet project of ours and of course the rest of the students (Gustavo Mondragón, Marco Diaz, Raúl Torres) whose hard work produced this work.

To Chem, or not “Too Chem”? That is the #ChemNobel Question


To chem or not -quite- too chem, that is the ChemNobel question:
Whether ’tis Nobeler in the mind to suffer
The curly arrows of organic fortune
Or to take rays against a sea of crystals
And by diffracting end them.

Me (With sincere apologies to WS)

Every year, in late September -like most chemists- I try to guess who will become the next Nobel Laureate in Chemistry. Also, every year, in early October -like most chemists- I participate in the awkward and pointless discussion of whether the prize was actually awarded to chemistry or not. Indeed, the Nobel prize for chemistry commonly stirs a conversation of whether the accomplishments being recognized lie within the realm of chemistry or biology whenever biochemistry shows its head, however shyly; but the task of dividing chemistry into sub-disciplines raises an even deeper question about the current validity of dividing science into broad branches in the first place and then further into narrower sub-disciplines.

I made a very lazy histogram of all the 178 Laureates since 1904 to 2017 based on subjective and personal categories (figure 1), and the creation of those categories was in itself an exercise in science contemplation. My criteria for some of the tough ones was the following: For instance, if it dealt with phenomena of atomic or sub-molecular properties (Rutherford 1908, Hahn 1944, Zewail 1999) then I placed it in the Chemical Physics category but if it dealt with an ensemble of molecules (Arrhenius 1903, Langmuir 1932, Molina 1995) then Physical Chemistry was chosen. Some achievements were about generating an analysis technique which then became essential to the development of chemistry or any of its branches but not for a chemical process per se, those I placed into the Analytical Chemistry box, like last year’s 2017 prize for electron cryo-microscopy (Dubochet, Frank, Henerson) or like 1923 prize to Fritz Pregl for “the invention of the method of microanalysis of organic substances” for which the then head of the Swedish Academy of Sciences, O. Hammarsten, pointed out that the prize was awarded not for a discovery but for modifying existing methods (which sounds a lot like a chemistry disclaimer to me). One of the things I learnt from this  exercise is that subdividing chemistry became harder as the time moved forward which is a natural consequence of a more complex multi- and interdisciplinary environment that impacts more than one field. Take for instance the 2014 (Super Resolved Fluorescence Microscopy) and 2017 (Cryo-Electron Microscopy) prizes; out of the six laureates, only William Moerner has a chemistry related background a fact that was probably spotted by Milhouse Van Houten (vide infra).

Some of the ones that gave me the harder time: 1980, Gilbert and Sanger are doing structural chemistry by means of developing analytical techniques but their work on sequencing is highly influential in biochemistry that they went to the latter box; The same problem arose with Klug (1982) and the Mullis-Smith duo (1993). In 1987, the Nobel citation for Supramolecular Chemistry (Lehn-Cram-Pedersen) reads “for their development and use of molecules with structure-specific interactions of high selectivity.”, but I asked myself, are these non-covalent-bond-forming reactions still considered chemical reactions? I want to say yes, so placed the Lehn-Cram-Pedersen trio in the Synthesis category. For the 1975 prize I was split so I split the prizes and thus Prelog (stereochemistry of molecules) went into the Synthesis category (although I was thinking  in terms of organic chemistry synthesis) and Cornforth (stereochemical control of enzymatic reactions) went into biochem. So, long story short, chemistry’s impact in biology has always had a preponderant position for the selection of the Nobel Prize in Chemistry, although if we fuse the Synthesis and Inorganic Chemistry columns we get a fairly even number of synthesis v biochemistry prizes.

Hard as it may be to fit a Laureate into a category, trying to predict the winners and even bet on it adds a lot of fun to the science being recognized. Hey! even The Simpsons did it with a pretty good record as shown below. Just last week, there was a very interesting and amusing ACS Webinar where the panelist shared their insights on the nomination and selection process inside the Swedish Academy; some of their picks were: Christopher Walsh (antibiotics); Karl Deisseroth (optogenetics); Horwich and Hartl (chaperon proteins); Robert Bergman (C-H activation); and John Goodenough (Li-ion batteries). Arguably, the first three of those five could fit the biochem profile. From those picks the feel-good prize and my personal favorite is John Goodenough not only because Li-ion batteries have shaped the modern world but because Prof. Goodenough is 96 years old and still very actively working  in his lab at UT-Austin (Texas, US) #WeAreAllGoodEnough. Another personal favorite of mine is Omar Yaghi not only for the development of Metal-Organic-Frameworks (MOFs) but for a personal interaction we had twenty years ago that maybe one day I’ll recount here but for now I’ll just state the obvious: MOFs have shown a great potential for applications in various fields of chemistry and engineering but perhaps they should first become highly commercial for Yaghi to get the Nobel Prize.

simpson_betting_poll_-_h_-_2016

W.E. Moerner and B.L. Feringa are now Nobel Laureates. Zare and Moerner have worked in spectroscopy whereas Feringa and Sonogashira are deep into synthesis

Some curiosities and useless trivia: Fred Sanger is the only person to have been awarded the Nobel Prize in Chemistry twice. Marie Curie is the only person to have been awarded two Nobel Prizes in different scientific categories (Physics and Chemistry) and Linus Pauling was awarded two distinct Nobel Prizes (Chemistry and Peace). Hence, three out of the four persons ever to have been awarded two Nobel Prizes did it at least once in chemistry – the fourth is John Bardeen two times recipient of the Nobel Prize in Physics.

Of course the first thing I’ll do next Wednesday right after waking up is checking who got the Nobel Prize in Chemistry 2018 and most likely the second thing will be going to my Twitter feed and react to it, hopefully the third will be to blog about it.

The announcement is only two days away, who is your favorite?

#WeAreAllGoodEnough

 

New paper in Tetrahedron #CompChem “Why U don’t React?”


Literature in synthetic chemistry is full of reactions that do occur but very little or no attention is payed to those that do not proceed. The question here is what can we learn from reactions that are not taking place even when our chemical intuition tells us they’re feasible? Is there valuable knowledge that can be acquired by studying the ‘anti-driving force’ that inhibits a reaction? This is the focus of a new manuscript recently published by our research group in Tetrahedron (DOI: 10.1016/j.tet.2016.05.058) which was the basis of Guillermo Caballero’s BSc thesis.

fig1

 

It is well known in organic chemistry that if a molecular structure has the possibility to be aromatic it can somehow undergo an aromatization process to achieve this more stable state. During some experimental efforts Guillermo Caballero found two compounds that could be easily regarded as non-aromatic tautomers of a substituted pyridine but which were not transformed into the aromatic compound by any means explored; whether by treatment with strong bases, or through thermal or photochemical reaction conditions.

fig2

These results led us to investigate the causes that inhibits these aromatization reactions to occur and here is where computational chemistry took over. As a first approach we proposed two plausible reaction mechanisms for the aromatization process and evaluated them with DFT transition state calculations at the M05-2x/6-31+G(d,p)//B3LYP/6-31+G(d,p) levels of theory. The results showed that despite the aromatic tautomers are indeed more stable than their corresponding non-aromatic ones, a high activation free energy is needed to reach the transition states. Thus, the barrier heights are the first reason why aromatization is being inhibited; there just isn’t enough thermal energy in the environment for the transformation to occur.

fig3

But this is only the proximal cause, we went then to search for the distal causes (i.e. the reasons behind the high energy of the barriers). The second part of the work was then the calculation of the delocalization energies and frontier molecular orbitals for the non-aromatic tautomers at the HF/cc-pVQZ level of theory to get insights for the large barrier heights. The energies showed a strong electron delocalization of the nitrogen’s lone pair to the oxygen atom in the carbonyl group. Such delocalization promoted the formation of an electron corridor formed with frontier and close-to-frontier molecular orbitals, resembling an extended push-pull effect. The hydrogen atoms that could promote the aromatization process are shown to be chemically inaccessible.

fig4

Further calculations for a series of analogous compounds showed that the dimethyl amino moiety plays a crucial role avoiding the aromatization process to occur. When this group was changed for a nitro group, theoretical calculations yielded a decrease in the barrier high, enough for the reaction to proceed. Electronically, the bonding electron corridor is interrupted due to a pull-pull effect that was assessed through the delocalization energies.

The identity of the compounds under study was assessed through 1H, 13C-NMR and 2D NMR experiments HMBC, HMQC so we had to dive head long into experimental techniques to back our calculations.

Elements4D – Exploring Chemistry with Augmented Reality


A bit outside the scope of this blog (maybe), but just too cool to overlook. Augmented reality in chemistry education.

Science and Stuff - Sciencebase.com

This is a guest post from Samantha Morra of EdTechTeacher.org, an advertiser on FreeTech4Teachers.com. 

Augmented Reality (AR) blurs the line between the physical and digital world. Using cues or triggers, apps and websites can “augment” the physical experience with digital content such as audio, video and simulations. There are many benefits to using AR in education such as giving students opportunities to interact with items in ways that spark inquiry, experimentation, and creativity. There are a quite a few apps and sites working on AR and its application in education.

Elements4D, an AR app from Daqri, allows students explore chemical elements in a fun way while learning about real-life chemistry. To get started, download Elements4D and print the cubes.

There are 6 physical paper cubes printed with different symbols from the periodic table. It takes a while to cut out and put together the cubes, but it…

View original post 475 more words

%d bloggers like this: