# Massimo Mella Università degli Studi dell..., Varese

Università degli Studi dell..., Varese

## Theoretical Chemistry, Spectroscopy, Physical Chemistry

Ph. D.

## Publications

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**ABSTRACT:**The mechanism for vibrational inelastic excitation during the collision between Li$_2$($\nu=0$) and Li was investigated exploiting classical trajectory simulations over a potential energy surface generated by fitting valence full configuration interaction calculations employing a large basis set. From the trajectory results, it emerges that the vibrational excitation in non--capture collisions presents uniquely a forward--scattered projectile for the highest levels of excitation ($\Delta$E($0\rightarrow \nu'$)$\simeq E_{\mathrm{coll}}$). For lower $\nu'$, a minor contribution presenting a backward scattered projectile appears, which, however, has its major contribution coming from a ``slingshot''--like (orbiting) mechanism exploiting the attractive features of the Li$_3$ potential energy surface rather than a direct recoil. -
##### Conference Paper: Ionization properties of weak polyelectrolytes: the effects of chemically specific interactions and restricted geometries

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**ABSTRACT:**Isolated weak polyelectolytic chains were studied employing Monte Carlo simulations and coarse grained potential models focusing onto the effects that a strong hydrogen bond interaction between ionized and neutral acid groups, and/or the constraints imposed by reduced geometries (spherical cavities, cylindrical tubes and planar slits) may have on the structural and ionization properties of the polymeric species as a function of pH. Simulation results showed that the former always increases chain ionization compared to species without such specific chemical interactions, even though the extent of such increase is controlled by the chain stiffness and the topological peculiarity of the restricting geometries. As an example, it is mentioned that a cylindrical confinement substantially decreases chain ionization when the formation of strong hydrogen bond is included in the polymer model compared to a free chain. The opposite, instead, happens for species without chemical interactions. All nuances in the ionization behaviour are rationalized in terms of the impact that confining geometries have onto the structure of a chain. -
##### Conference Paper: Out of equilibrium self-assembly of Janus nanoparticles: steering it from nanostructures to 2D patterns

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**ABSTRACT:**Recent theoretical and experimental studies have investigated the formation of useful structures via spontaneous self-assembly of Janus particles, whose halves are characterized by different physico-chemical properties. To extend our understanding of such process, we implemented a 2-dimensional coarse-grained lattice gas model to study the self-organization of two-faced Janus nanoparticles from a thin film of their solution while the solvent irreversibly evaporates. We investigated how emerging structures depends on the evaporation rate, nanoparticles’ size and coverage fraction, rate of translational and rotational diffusion and, especially, interaction strength between the halves of two different nanoparticles and the halves and the solvent. Despite the simplicity of the model, we found that nanoparticle self-organization may give rise to a very rich array of structural features, forming micelles, vescicles and stripe or chessboard-like patterns as a function of evaporation rate and interaction parameters exploiting the fact that system far from equilibrium often exhibits complex transitory phases. The simulation results suggest that structural patterns obtained via energy optimization of off-lattice models may be obtained also under far from equilibrium conditions with the appropriate choice of dynamical and energetic parameters; we thus hope that our work has helped in elucidating the role played by Janus nanoparticle features on their self-assembly and in providing design rules to produce useful template surface structures. - [Show abstract] [Hide abstract]

**ABSTRACT:**Several Hybrid Monte Carlo-replica exchange (HMC) simulations are carried out to obtain the classical thermodynamic behavior of the ammonia dodecamer and hexadecamer. We find evidence of solid to solid phase changes that we characterize using the structures of low lying minima. In a recent publication [40], we introduce a new integrator for clusters of rigid tops that works directly with non-Cartesian coordinates. This letter contains the first application of the integrator to enhance the efficiency of the Hybrid Monte Carlo simulation of clusters of rigid tops. - [Show abstract] [Hide abstract]

**ABSTRACT:**We test the second order Milstein method adapted to simulate diffusion in general compact Riemann manifolds on a number of systems characterized by nonconfining potential energy surfaces of increasing complexity. For the 2-sphere and more complex spaces derived from it, we compare the Milstein method with a number of other first and second order approaches. In each case tested, we find evidence that demonstrate the versatility and relative ease of implementation of the Milstein method derived in Part I. - [Show abstract] [Hide abstract]

**ABSTRACT:**We develop a set of diffusion Monte Carlo algorithms for general compactly supported Riemannian manifolds that converge weakly to second order with respect to the time step. The approaches are designed to work for cases that include non-orthogonal coordinate systems, nonuniform metric tensors, manifold boundaries, and multiply connected spaces. The methods do not require specially designed coordinate charts and can in principle work with atlases of charts. Several numerical tests for free diffusion in compactly supported Riemannian manifolds are carried out for spaces relevant to the chemical physics community. These include the circle, the 2-sphere, and the ellipsoid of inertia mapped with traditional angles. In all cases, we observe second order convergence, and in the case of the sphere, we gain insight into the function of the advection term that is generated by the curved nature of the space. - [Show abstract] [Hide abstract]

**ABSTRACT:**Intrinsic antimicrobial thermoplastic A(BC)n copolymers (n = 1, 2, 4), where A was poly(ethylene glycol) (PEG), BC was a random chain of methylmethacrylate (MMA), and alkyl-aminoethyl methacrylate (AAEMA), were synthesized and the antimicrobial activity and hemolyticity were evaluated on plaques obtained by casting as a function of the architecture, the N-substituent groups of the AAEMAs (methyl, ethyl, isopropyl, and tert-butyl groups) and the hydrophobic/charge density balance. Antimicrobial effectiveness and efficiency is controlled by the surface charge density and by the influence of N-alkyl groups on the surface morphology. Also interestingly, it is the absence of hemolitytic activity in all copolymers. In presence of Escherichia coli, the A(BC)2 copolymer with 40% of N-methyl groups is the most efficient, killing 91% of the bacteria already after 1.5 h. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. - [Show abstract] [Hide abstract]

**ABSTRACT:**For weak polyelectrolytes, the interplay between pH, solvent properties, and polymer structure affects the amount of charges, their distribution, and hence their conformations via Coloumb repulsion. Attractive interactions can also develop between charged and neutral sites counteracting the expected Coulomb-induced expansion. To gauge how such competition affects polyelectrolyte structure and ionization, the titration of a single polyelectrolyte chain, isolated or close to a charged sphere, mimicked with a novel many-body potential model is simulated with Monte Carlo. Apart from showing a 10-fold higher ionization than isolated monomers at low pH, interacting species contracted forming short-range clusters of charged and neutral ionizable groups. The presence of a charged sphere synergically boosted both effects due to monomer interactions, forcing the chains to condense onto its surface at much lower pH. Structural properties, however, seem to be controlled only by the ionization degree despite the presence of the topological restraint represented by the spherical surface. Using Monte Carlo titration results, the equilibrium ionization of isolated chains is also estimated; the results evidence that even weak interactions can easily lead to a doubling of the total charge. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 - [Show abstract] [Hide abstract]

**ABSTRACT:**A common way to convert ethanol into chemicals is by upgrading it over oxide catalysts with basic features; this method makes it possible to obtain important chemicals such as 1-butanol (Guerbet reaction) and 1,3-butadiene (Lebedev reaction). Despite their long history in chemistry, the details of the close inter-relationship of these reactions have yet to be discussed properly. Our present study focuses on reactivity tests, in situ diffuse reflectance infrared Fourier transform spectroscopy, MS analysis, and theoretical modeling. We used MgO as a reference catalyst with pure basic features to explore ethanol conversion from its very early stages. Based on the obtained results, we formulate a new mechanistic theory able to explain not only our results but also most of the scientific literature on Lebedev and Guerbet chemistry. This provides a rational description of the intermediates shared by the two reaction pathways as well as an innovative perspective on the catalyst requirements to direct the reaction pathway toward 1-butanol or butadiene. - [Show abstract] [Hide abstract]

**ABSTRACT:**The previously unexplored use of methanol as a H-transfer agent for the Meerwein-Ponndorf-Verley reduction of aromatic aldehydes and aryl ketones is described. Furfural, 5-hydroxymethylfurfural, benzaldehyde, and acetophenone were selectively reduced to the corresponding alcohols in mild conditions. The reaction mechanism was elucidated by means of reactivity tests and DFT calculations. It was found to include the highly efficient H-transfer with the formation of formaldehyde, which further reacted with excess methanol to generate the adsorbed hemiacetal. In turn, the latter reduced carbonyl, with the formation of methylformate, which further decomposed into CO, CH4, and CO2. Compared to the alcohols typically used for carbonyl reductions, methanol showed the advantage of producing gaseous components as the only co-products, which are easily separated from the reaction medium. In the case of furfural, a 100% yield to furfuryl alcohol was obtained, using the high-surface area MgO as the easily recoverable and reusable catalyst. - [Show abstract] [Hide abstract]

**ABSTRACT:**M$^+(^2P)$$^4$He$_n$ species, possibly involved in the post $^2P \leftarrow ^2S$ excitation dynamics of Sr$^+$ and Ba$^+$ in cold $^4$He gas or droplets, are studied employing both high level {\it ab initio} calculations to determine the potential energy curves (PEC), and diffusion Monte Carlo (DMC) to obtain information on their ground state structure and energetics. PEC for the excited M$^+(^2P)$He dimers were obtained using MRCI calculations with extended basis sets. Potential energy surfaces (PES) for M$^+(^2P)$He$_n$ were built with the DIM model including spin--orbit coupling via a perturbative procedure. DMC simulations indicated several exciplexes ($n>2$) to be stable against He dissociation whatever the ion state, a finding that is at variance with what previously suggested for Ba$^+(^2P_{1/2})$ due to the repulsive nature of the interaction potential obtained in [Phys. Rev. A {\bf 2004}, {\it 69}, 042505]. Our results, instead, support the suggestion made in [J. Chem. Phys. {\bf 2012}, {\it 137}, 051102] for the existence of Ba$^+(^2P_{1/2})$He$_n$ exciplexes emitted following the excitation of the barium cation solvated into He droplets. In the $^2P_{1/2}$ state, the Ba ion also shows a peculiar behavior as a function of $n$ with respect to the location and binding strength of the attached He atoms compared to Sr$^+$. While the latter forms the usual equatorial He ring, Ba$^+$ deviates from this geometry for $1\leq n \leq 4$, with the radial distribution functions strongly depending on the number of solvent atoms. Finally, a putative species is proposed to explain the emission band at 523 nm that follows D1 or D2 excitation of Ba$^+$ in superfluid bulk helium. - [Show abstract] [Hide abstract]

**ABSTRACT:**The torsional ground state for ethane, the torsional, rotational, and mixed torsional and rotational ground state of propane are computed with a version of diffusion Monte Carlo adapted to handle the geometric complexity of curved spaces such as the Ramachandra space. The quantum NVT ensemble average for the mixed torsional and rotational degrees of freedom of propane is computed, using a version of Monte Carlo path integral, also adapted to handle curved spaces. These three problems are selected to demonstrate the generality and the applicability of the approaches described. The spaces of coordinates can be best constructed from the parameters of continuous Lie groups, and alternative methods based on vector spaces, where extended Lagrangian terms would be too cumbersome to implement. We note that the geometric coupling between the torsions and the rotations of propane produces a substantial effect on the ground state energy of propane, and that the quantum effects on the energy of propane are quite large even well above room temperature. © 2014 Wiley Periodicals, Inc. - [Show abstract] [Hide abstract]

**ABSTRACT:**Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using time-dependent density functional simulations that the solvation of a Ba(+) ion created by photoionization of neutral Ba at the surface of a (4)He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba(+) ion. The process is expected to be quite general and very efficient under standard experimental conditions. -
##### Article: Infinite swapping in curved spaces

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**ABSTRACT:**We develop an extension of the infinite swapping and partial infinite swapping techniques [N. Plattner, J. D. Doll, P. Dupuis, H. Wang, Y. Liu, and J. E. Gubernatis, J. Chem. Phys. 135, 134111 (2011)] to curved spaces. Furthermore, we test the performance of infinite swapping and partial infinite swapping in a series of flat spaces characterized by the same potential energy surface model. We develop a second order variational algorithm for general curved spaces without the extended Lagrangian formalism to include holonomic constraints. We test the new methods by carrying out NVT classical ensemble simulations on a set of multidimensional toroids mapped by stereographic projections and characterized by a potential energy surface built from a linear combination of decoupled double wells shaped purposely to create rare events over a range of temperatures. - [Show abstract] [Hide abstract]

**ABSTRACT:**Several Smart Monte Carlo (SMC) and Hybrid Monte Carlo (HMC) simulations coupled with the Replica Exchange (RE) strategy are compared in multidimensional flat and curved manifolds characterized by extremely rugged potential energy surfaces, to quantify their convergence properties with respect to walk length and overall cost. We learn that the HMC coupled with a sampling enhancing method is much more efficient in manifolds mapped with unconventional coordinates than SMC. This is due to an inherent difficulty in conserving energy in curved spaces directly mapped, and the lack of such strict requirement for HMC. - [Show abstract] [Hide abstract]

**ABSTRACT:**Mixed ammonia–hydrogen molecule clusters [H 2 –(NH 3) n ] have been studied with the aim of explor-ing the quantitative importance of the H 2 quantum motion in defining their structure and energetics. Minimum energy structures have been obtained employing genetic algorithm-based optimization methods in conjunction with accurate pair potentials for NH 3 –NH 3 and H 2 –NH 3 . These include both a full 5D potential and a spherically averaged reduced surface mimicking the presence of a para-H 2 . All the putative global minima for n ≥ 7 are characterized by H 2 being adsorbed onto a rhomboidal ammonia tetramer motif formed by two double donor and two double acceptor ammonia molecules. In a few cases, the choice of specific rhombus seems to be directed by the vicinity of an ammo-nia ad-molecule. Diffusion Monte Carlo simulations on a subset of the species obtained highlighted important quantum effects in defining the H 2 surface distribution, often resulting in populating rhom-boidal sites different from the global minimum one, and showing a compelling correlation between local geometrical features and the relative stability of surface H 2 . Clathrate-like species have also been studied and suggested to be metastable over a broad range of conditions if formed. - [Show abstract] [Hide abstract]

**ABSTRACT:**The structure and energetics of exciplexes M*((2)L)Hen (M = Cu, Ag and Au; L = P and D) in their vibrational ground state are studied by employing diffusion Monte Carlo (DMC). Interaction potentials between the excited coinage metals and He atoms are built using the Diatomics-in-Molecule (DIM) approach and ab initio potential curves for the M((2)L)-He dimers. Extending our previous work [Cargnoni et al., J. Phys. Chem. A, 2011, 115, 7141], we computed the dimer potential for Au in the (2)P and (2)D states, as well for Cu and Ag in the (2)D state, employing basis set superposition error-corrected Configuration Interaction calculations. We found that the (2)Π potential correlating with the (2)P state of Au is substantially less binding than for Ag and Cu, a trend well supported by the M(+) ionic radiuses. Conversely, the interaction potentials between a (n - 1)d(9)ns(2 2)D metal and He present a very weak dependency on M itself or the projection of the angular momentum along the dimer axis. This is due to the screening exerted by the ns(2) electrons on the hole in the (n - 1)d shell. Including the spin-orbit coupling perturbatively in the DIM energy matrix has a major effect on the lowest potential energy surface of the (2)P manifold, the one for Cu allowing the formation of a "belt" of five He atoms while the one for Au being completely repulsive. Conversely, spin-orbit coupling has only a weak effect on the (2)D manifold due to the nearly degenerate nature of the diatomic potentials. Structural and energetic results from DMC have been used to support experimental indications for the formation of metastable exciplexes or the opening of non-radiative depopulation channels in bulk and cold gaseous He. - [Show abstract] [Hide abstract]

**ABSTRACT:**A possible rationale for the different catalytic behaviors of systems based on rac-(ethylenebis(1-indenyl))zirconium dichloride (rac-EBIZrCl2), rac-(ethylenebis(1-indenyl))hafnium dichloride (rac-EBIHfCl2), and rac-(isopropylidenebis(1-indenyl))zirconium dichloride (rac-iPrBIZrCl2) toward ethene–styrene copolymerization has been sought by studying related active systems. For this purpose, the metallocene ion pairs (IPs) rac-EBIZrMe—MeB(C6F5)3, rac-EBIHfMe—MeB(C6F5)3, and rac-iPrBIZrMe—MeB(C6F5)3 have been synthesized and their structures in solution explored with ROESY and pulsed gradient NMR spectroscopy. The energetics of dynamical processes relevant for catalysis that can be used as indicators of the cation acidity have been studied with variable-temperature NMR experiments and density functional theory (DFT). NMR experiments successfully provided IP structural details in solution and also indicated the presence of an intricate dynamic behavior for all the IPs. DFT results, instead, indicated quantitatively how changing the metal and/or the ancillary ligand bridge influences the energetics of the active species and modifies the reaction energy profile. The theoretical results also drew attention to the fact that finding a rationale for the ligand influence on the catalytic behavior of rac-EBIZrCl2/MAO and rac-iPrBIZrCl2/MAO in ethene–styrene copolymerization requires not only considering the steric effects but also determining how subtle changes in the ligand sphere affect the capability of the metal center to accept electrons from the counteranion or the olefins. - [Show abstract] [Hide abstract]

**ABSTRACT:**Interactions between bare and tri-solvated (Eigen) hydronium with water and methanol is investigated with ab initio methods. Unexplored configurations on the oxygen side of H3O+ are covered to improve our understanding of the solvation energetics. Results indicate that the interaction between the charged and polar species, with oxygen atoms oriented toward H3O+ oxygen (‘back to back’ or ‘O-side’ to ‘O-side’ interaction), is similar to or stronger than a hydrogen bond. The repulsion along the O–O directrix onset in the range 2.2–2.7 Å, indicating H2O short-range O–O contacts with (H3O+) as possible despite water coordination. - [Show abstract] [Hide abstract]

**ABSTRACT:**The effect of quantum mechanics (QM) on the details of the nucleation process is explored employing Ne clusters as test cases due to their semi-quantal nature. In particular, we investigate the impact of quantum mechanics on both condensation and dissociation rates in the framework of the microcanonical ensemble. Using both classical trajectories and two semi-quantal approaches (zero point averaged dynamics, ZPAD, and Gaussian-based time dependent Hartree, G-TDH) to model cluster and collision dynamics, we simulate the dissociation and monomer capture for Ne(8) as a function of the cluster internal energy, impact parameter and collision speed. The results for the capture probability P(s)(b) as a function of the impact parameter suggest that classical trajectories always underestimate capture probabilities with respect to ZPAD, albeit at most by 15%-20% in the cases we studied. They also do so in some important situations when using G-TDH. More interestingly, dissociation rates k(diss) are grossly overestimated by classical mechanics, at least by one order of magnitude. We interpret both behaviours as mainly due to the reduced amount of kinetic energy available to a quantum cluster for a chosen total internal energy. We also find that the decrease in monomer dissociation energy due to zero point energy effects plays a key role in defining dissociation rates. In fact, semi-quantal and classical results for k(diss) seem to follow a common "corresponding states" behaviour when the proper definition of internal and dissociation energies are used in a transition state model estimation of the evaporation rate constants.

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