La gota de ácido más pequeña del mundo/The world's smallest droplet of cid

Exactamente cuatro moléculas de agua y una de cloruro de hidrógeno se necesitan para formar la gota de ácido más pequeña. Este es el resultado del trabajo de los grupos de la Prof. Dra. Martina Havenith (Química-Física) y del Prof. Dr. Domink Marx (Química Teórica) del grupo de investigación FOR 618. Para ello han llevado a cabo experiencias a temperaturas ultrafrías cercanas al cero absoluto, utilizando espectroscopía láser infrarroja para monitorizar las moléculas.
Todo ello ha venido acompañado de simulaciones teóricas ab initio. Según los cálculos, la reacción a estas temperaturas tan extremadamente bajas, es solo posible si las moléculas se agregan una detrás de otra.
Al poner en agua un ácido como el cloruro de hidróegno, las moléculas de ácido pierden un protón (H+) y la disolución se vuelve ácida, formándose iones hidronio (H3O+) al protonarse el agua. A pesar de que esta es una de las reacciones más fundamentales, hasta ahora no estaba claro cuantas moléculas de agua se necesitaban para formar un anión cloruro Cl- y un catión H3O+.
La combinación de cálculos y de experimentos han demostrado que la reacción de protonación del agua es solo posible por un proceso de adición/agregación sucesiva y que se necesitan exactamente cuatro moléculas de agua para formar la gota de ácido más pequeña:(H3O)+(H2O)3Cl-. En lugar de poner juntas y de forma simultánea 4 moléculas de agua y una de HCl y esperar a que se produzca el proceso de disociación,se ha demostrado que alaañidir moléculas de agua una tras otra, la transferencia del protón tiene lugar exactamente al añadir la cuarta molécula de agua

Exactly four water molecules and one hydrogen chloride molecule are necessary to form the smallest droplet of acid. This was the result of work by the groups of Prof. Dr. Martina Havenith (physical chemistry) and Prof. Dr. Dominik Marx (theoretical chemistry) within the research group FOR 618. They have carried out experiments at ultracold temperatures close to absolute zero temperature using infrared laser spectroscopy to monitor the molecules.
This has been accompanied by theoretical ab initio simulations. According to their calculations, the reaction at these extremely cold temperatures is only possible if the molecules are aggregating one after the other.
If you put a classical acid, for example hydrogen chloride in water, the acid molecules will preferentially lose a proton (H+) and the solution becomes acidic and hydronium ions (H3O+) are formed by protonated water molecules. Despite of the fact that this is one of the most fundamental reactions, it was not clear until now how many water molecules are actually required in order to form a charge separated negative Cl- ion and a positive H3O+ ion.
The calculations, in combination with experiment, showed that the reaction is only possible by a successive aggregation process and that exactly four water molecules are required to form the smallest droplet of acid: (H3O)+(H2O)3Cl-. Instead of putting together 4 water molecules and an HCl molecule simultanesously at the beginning and the waiting for a dissociation process to occur, they found in their simulations that when adding the water molecules step by step, a proton is transferred exactly when adding the fourth water molecule.

Tomado de /Taken from Science Daily

Resumen de la publicación/Abstract of the research paper
Anna Gutberlet, Gerhard Schwaab, Özgür Birer, Marco Masia,Anna Kaczmarek, Harald Forbert, Martina Havenith, Dominik Marx. "Aggregation-Induced Dissociation of HCl(H2O)4 Below 1 K: The Smallest Droplet of Acid". Science, 324, 1545-1548 (2009)
DOI: 10.1126/science.1171753
Acid dissociation and the subsequent solvation of the charged fragments at ultracold temperatures in nanoenvironments, as distinct from ambient bulk water, are relevant to atmospheric and interstellar chemistry but remain poorly understood. Here we report the experimental observation of a nanoscopic aqueous droplet of acid formed within a superfluid helium cluster at 0.37 kelvin. High-resolution mass-selective infrared laser spectroscopy reveals that successive aggregation of the acid HCl with water molecules, HCl(H2O)n, readily results in the formation of hydronium at n = 4. Accompanying ab initio simulations show that undissociated clusters assemble by stepwise water molecule addition in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ringlike undissociated HCl(H2O)3 then spontaneously yields the compact dissociated H3O+(H2O)3Cl– ion pair. This aggregation mechanism bypasses deep local energy minima on the n = 4 potential energy surface and offers a general paradigm for reactivity at ultracold temperatures.