|Posted on May 1, 2015 at 2:00 PM|
Our article with Dr. Óscar Jiménez-Halla (Univ. Guanajuato, Mexico) was selected as one of the six hot articles published in Dalton Transactions in December 2014. In this paper, we performed a theoretical study of the aromaticity in the neutral an anionic borole systems synthesized in the group of Prof. Holger Braunschweig. The results show that the neutral borole structures with four π electrons are antiaromatic and become increasingly more aromatic by addition of one and two electrons, in agreement with Hückel’s rule. While the uptake of one electron to the borole leads to a nonaromatic system, addition of a second electron fully aromatizes the ring and reliefs the system from its inherent electron deficiency. It is also shown that the exocyclic substituent at the boron atom has a considerable influence on the degree of antiaromaticity in the borole ring. Substituents with π-donating abilities, such as an amino or thiophene group, seem to mitigate the destabilizing electron delocalization in the ring, whereas π-accepting groups result in an enhanced antiaromatic destabilization.
(Artwork by J. Óscar C. Jiménez Halla)
|Posted on April 1, 2015 at 4:30 PM|
At the end of 2013 I was awarded an ikerbasque fellowship. The position includes a five-year contract in the University of the Basque Country (Donostia delegation) and a very small startup budget. As a hosting group I chose the group of Jesus Ugalde, currently hosted at the Donostia International Physics Center (DIPC). By the beginning of 2015, some crew from Girona already joined the group: Mauricio Rodriguez (holding a FPU grant) and Eloy Ramos (postdoctoral position paid by the DIPC). By mid March I joined them and, so far, I have only found facilities to settle down.
By September, we will have a new team member, Mr. Sebastian Sitkiewicz, who has been awarded a scolarship to take the European Master of Theoretical Chemistry and Computational Modelling (TCCM). Irene Casademont, who had been working with Eloy and myself for the past two years, also agreed to undertake her PhD studies in Donostia. In 2016, after the courses of the Master in Advanced Catalysis and Molecular Modelling (MACMoM) finish, Irene will join our group as well.
The five of ous, Irene, Sebastian, Mauricio, Eloy and myself will work on the next years on the development of density functional theory (DFT) and other related topics such as reduced density matrices, density matrix functional theory and time-dependent processes.
|Posted on March 25, 2015 at 5:05 PM|
A few weeks ago we published our work on molecular electrides in Chemical Communications. The work has received some additional attention by Chemistry World, the blog from the Royal Society of Chemistry. In this post I will try to summarize the contents of our recently published work.
Ionic compounds are chemical compounds consisting of positive and negative ions held together by electrostatic forces. For instance, in sodium chloride ⎯commonly known as table salt⎯ the anionic part (negative ion) is Cl- and the cationic part (positive ion) is Na+. Electrides are unique ionic compounds where the anionic part is constituted only by isolated electrons. This feature grants electrides many different properties. Thus far, only solid-state electrides have been reported and despite there have been some suggestions of molecular electrides in the literature, their electronic structure has not been confirmed as a true electride one.
James L. Dye is the father of electrides: he postulated its existence in 1960s, synthesized and characterized the first electride in 1980s, and it was the first to produce a room-temperature-stable organic electride in 2005. The first room-temperature-stable electride was inorganic and due to Prof. Hosono, see below).
Barely a handful of electrides have been synthesized and only three of them are stable at room temperature. Electrides show particular magnetic (exalted susceptibilities), chemical (organic synthesis, preparation of nanoscale metal and alloy particles), electric (an ideal electride should be a (Mott) insulator) and optical properties (low optical spectra peaks as compared to alkali anions; large nonlinear optical properties). Large second hyperpolarizabilities make electrides of high interest due to their potential utilization in optical and opto-electronic devices. Indeed, electrides have found a plethora of diverse applications, including the catalysis of the ammonia synthesis, its usage as reversible H2 storage devise, electron emitters and chemical reagents —to mention a few. All these applications are due to the group of Prof. Hosono, who synthesized the last two electrides: [Ca24Al28O68]·4e− and [Ca2N]+·e−.
Unfortunately, electrides are difficult to synthesize and identify because their experimental characterization is only possible by indirect means. The density of a free electron (or a handful of them) is not large enough to be located in the X-ray of a crystal structure. As a consequence, the presence of isolated electrons in electrides always comes from indirect evidences such as the similarity of this structure with analog alkalides, the chemical shift of the corresponding cation (133Cs), EPR studies, magnetic susceptibilities, electrical resistivity or optical reflectance experiments. These evidences merely suggest the presence of an electride; they do not guarantee its existence.
In our paper we provide an unambiguous computational means to distinguish electrides from similar species, proving the existence of some electrides in gas phase. In contrast with solid state, we use the term molecular electrides for the gas-phase species. The molecular electrides studied in this work were previously characterized by frontier molecular orbital analysis and large nonlinear optical properties. Namely, these studies found an occupied orbital with large density values in the vicinity of the position where one would expect the isolated electron of the electride. Some works also included large second hyperpolarizabilities to support the discovery of a molecular electride. However, neither of these criteria are enough to assess the existence of an electride, as we have also proved in this work.
The electron localization function (ELF) and the non-nuclear attractors of the electron density were used to characterize solid-state electrides. Our study proves that these properties are actually necessary conditions for the existence of electrides. However, these features also show in molecules that do not have an electride structure such as acetylene. In our work we show that large nonlinear optical properties can be used in conjunction with the latter techniques to unambiguously characterize electrides.
The electronic structure of electrides shows an important signature: a maximum of the electron density in a non-nuclear position. This rare feature opens a new route towards the design of new electrides. We currently study the possibility to enforce non-nuclear maxima of the electron density (NNA). Since NNA are not a frequent feature of molecular densities, its mere existence increases the probability of having an electride. We believe that learning how to construct molecules with NNAs could pave the way towards the design of new electrides.
|Posted on March 20, 2015 at 5:30 PM|
Eloy and I have been working together for the past two years. At some point, it occurred to us studying the first stages of the physical processes taking place in dye-sensitized solar cells (DSSC). It all started as a mere simple exercise to motivate young students to try computational chemistry research. Little by little, we have familiarized ourselves with the computational techniques used (there is still a long way to go) and that's how we started working on charge-transfer and time-dependent processes. The natural tool to study time-dependent processes in large molecules is the well-known time-dependent density funcional theory (TDDFT). Since I was already working on the design of new DFT methods, it was only natural to apply the same strategy to time-dependent DFT. Merging all these pieces is how AccuCT (after the development of an accurate charge-transfer computational method), Eloy's postdoctoral subject, was born.
The project was then put down into words and submitted to the cruelly competitive Marie Curie Global Fellowship application. The good news came at the beginning of this year: his project was granted. This is awesome news for Eloy's research career and it also guarantees the funds to carry out AccuCT. The project will start on 2016, when Eloy will move to Martin Head-Gordon's group in Berkeley and it will finish in 2018, a year we will spend working together at the DIPC.
Some statistics about the MSCA-IF-2014-GF have been leaked. The % success is 11.2% (all proposals) and the Spanish success is 7.6% (only 14 proposals granted).
|Posted on February 2, 2015 at 3:25 PM|
|Posted on September 8, 2014 at 7:05 PM|
My talk 'New Stringent Conditions for the Cumulant Matrix' was selected for 10th edition of the World Association of Theoretically-Oriented Chemists (WATOC) that will be celebrated in Santiago de Chile. In addition, I was invited to give a talk in the satellite meeting "New Approaches in Theoretical Chemistry" that will be held in the University of Chile in Santiago two days before WATOC.
|Posted on July 19, 2014 at 6:55 PM|
Last week the European Young Chemist Award Organization notified me that I had been selected among the finalists of the EYCA2014. The final step of this competition is a 10min talk that takes places at the 5th European Chemistry Congress that I will attend in Istanbul. As far as I remember there are eight candidates in the postdoc category; one is chosen for the gold medal and two for the silver one. Needless to say, I'm very happy with the nomination and I will do my best at Istanbul. :-)
|Posted on June 29, 2014 at 1:00 PM|
This summer the group will enjoy the visit of a few researchers. Some of them will come next week to attend the XI edition of the Girona Seminar and the others will arrive during the following weeks and stay for a longer time.
The Girona Seminar will bring a few collaborators: Dr. J. Óscar C. Jiménez-Halla (alumni currently in Univ. Guanajuato), Prof. Jesus Ugalde (Univ. Basque Country), Prof. Gabriel Merino (Univ. Mérida) and Prof. Patrick Bultinck (Univ. Ghent). The visit of Óscar will help to push forward a current research project about borole aromaticity with Prof. Braunschweig. Patrick's student, Jelle Vekeman, whose master thesis I have co-promoted, will defend his thesis after the closure of the Girona Seminar.
Next week, Prof. Jacek Styszynsky will stay in Girona and give a talk about our recently published work on metal carbonyl complexes. The same week, Sebastian Sitkiewicz will join the group for a month stay. Sebastian is an Erasmus student and he will be working on the developmet of DFT functionals. Hopefully, Sebastian's visit will be the first of many others visits to our group. The fourth week of July, Dr. Tomás Rocha (an old friend from my postdoctoral time in Denmark) will be paying us a visit. Tomás will stay for two weeks and we will start a collaboration project together.
|Posted on June 18, 2014 at 6:45 PM|
In the last years we have worked in the design of a new approximation to the third-order density that just got accepted for publication in J. Chem. Theory Comput. The nth order density is a well-known quantity in quantum mechanics that gives the probability of finding n particles (usually electrons) simultaneously in different location of the molecular space. The first and second-order densities are actually needed for the calculation of the electronic energy of a given molecular system. In principle, higher-order densities do not enter the energy expression, however, the calculations performed with the contracted Schrödinger Equation and its antiHermitian version do use three and four-order density. In addition, the probability of finding three and four electrons depends on these densities. The latter is main focus of our development, since we want to use these quantities to calculate three and four-center indices. In particular, in this paper we study systems with three-center bonds, such as diborane, which contains a three-center two-electron bond.
The approximations we suggest for the third-order densities can be casted in the following expression:
where gamma is the pure three-body part and depends on a parameter a:
This expression can be used with a=1 (HF-like approach) and a=1/2 and a=1/3, which are the approximations suggested in this work. The calculation on a series of molecules proves that a=1/3 works significantly better than other approximations to the third-order density such as Valdemoro's, Mazziotti's or Nakatsuji's. We are currently trying to understand why our approximations works so well and see if its applicability can be extended to other properties.
|Posted on June 12, 2014 at 4:25 AM|
This year research will take me to travel around the world. Literally :-) My trip starts on the 8th of August and I will be back in Girona the 7th of September. I will attend three conferences and visit a research group.
My journey starts in San Franciso. I will participate in the 248th ACS meeting in San Francisco where I will give two oral presentations:
The first talk summarizes our research on multicenter bond indices with Ferran Feixas and Miquel Solà: from the first works during Ferran's thesis, until our last papers on applications of these indices aimed to characterize agostic interactions and approximate multicenter indices. The second talk focuses on two new stringent conditions for the two-particle cumulant matrix. This work has been carried out with Eloy Ramos, Pedro Salvador and Mario Piris, and introduces two stringent conditions that the most sophisticated natural orbital functionals fail to attain. Interestingly, these conditions do not only reveal important limitations of current functionals but also suggest modifications to improve them.
After the ACS meeting I will fly to Chicago to visit Prof. David Mazziotti's group. Mazziotti is a renown specialist in the field of density matrices with whom I would like to discuss about our recent approximation for a third-order reduced density matrix. I will give a talk on Mazziotti's covering this topic. Our approximation works better than any other for the calculation of n-variate probabilities, but we have not used it yet in the calculation of electronic energies. It would be interesting to see whether our approximation can be used in this context as well.
My third stop takes me to Vietnam. The Current Topics in Theoretical Chemistry workshop organized by Julia Contreras-Garcia, Andreas Savin and Ramon Carbó takes places in the city of Nha Trang. I have been invited to give a 45min presentation on Aromaticity. This lecture is a rather introductory talk on the subject, which also covers a few glimpses of my research on the topic over the last years. This workshop includes other interesting topics, including method development lectures by well-known experts such as John Perdew, Trygve Helgaker, Gustavo Scuseria, Jeppe Olsen or Weitao Yang, among many others. It will be a great opportunity to make new scientific connections and meet some current collaborators.
The last station is Turkey. I have applied for a talk in the 5th EuCheMS conference held in Istanbul. This conference is less theoretically oriented and, if chosen for an oral presentation, I will present a recent work on a new kind of materials known as electrides.
Altogether it will be a month tour around the world including four to five lectures, the opportunity to meet many researchers and, hopefully, establishing new fruitful collaborations.
Update 29/06/2014. My talk "Characterization and Identification of Electrides" has been chosen for an oral presentation in the 5th EuCheMS conference.