Ion-molecule chemistry within boron tribromide clusters: experiment and theory

Molecular clusters of BBr3 were subjected to electron ionization and mass analysis in a reflectron time-of-flight mass spectrometer. Five series of cluster ions were observed, with formulas corresponding to each of the possible fragment ions of BBr3 being solvated by neutral BBr3 molecules. Geometry optimizations on the observed cluster ions using density functional theory (B3LYP/6-31G*) predict that fragment ions smaller than BBr3+ undergo reactions with neutral BBr3 molecules to form covalently bound adduct species that function as core ions within the clusters. Once all boron atoms are saturated, the reactions cease, and larger cluster ions consist of BBr3 molecules loosely bound to the core ions. Divalent bromine atoms are present in at least three of the cluster ions, and most of the intermolecular contact within the clusters is between Br atoms. Enthalpies of formation, addition reactions, and BBr3 elimination from the cluster ions were derived from B3LYP and MP2 calculations at the B3LYP/6-31G* geometries using both the 6-31G* and the 6-311++G(2df,2p) basis sets. The results are compared to limiting expectations based on known bulk thermochemistry.     

Density functional study of the magnetic properties of Bi4Mn clusters: discrepancy between theory and experiment

We have performed collinear and noncollinear calculations on neutral Bi(4)Mn and collinear ones on ionized Bi(4)Mn with charges +1 and -1 to find out why theoretical calculations will not predict the magnetic state found in the experiment. We have used the density functional theory to find a fit between the theoretical prediction of the magnetic moment and the experimental value. Our calculations have consisted in a structural search of local energy minima, and the lowest energy magnetic state for each resulting isomer. The geometry optimization found three local minima whose fundamental state is the doublet spin state. These isomers could not be found in previous theoretical works, but they are higher in energy than the lowest-lying isomer by ≈1.75 eV. This magnetic state could help understand the experiment. Calculations of noncollinear magnetic states for the Bi(4)Mn do not lower the total magnetic moment. We conclude arguing how the three isomers with doublet state could actually be the ones measured in the experiment.     

Superhalogen and Superacid

A superhalogen F@C₂₀(CN)₂₀ and a corresponding Brønsted superacid were designed and investigated on DFT and DLPNO-CCSD(T) levels of theory. Calculated compounds have outstanding electron affinity and deprotonation energy, respectively. We consider superacid H[F@C₂₀(CN)₂₀] to be able to protonate molecular nitrogen. The stability of these structures is discussed, while some of the previous predictions concerning neutral Brønsted superacids of record strength are doubted. © 2019 Wiley Periodicals, Inc.     

Photocatalytic properties of graphene-supported titania clusters from density-functional theory

Density-functional theory calculations of (TiO₂)_n clusters (n = 1–5) in the gas phase and adsorbed on pristine graphene as well as graphene quantum dots are presented. The cluster adsorption is found to be dominated by van der Waals forces. The electronic structure and in particular the excitation energies of the bare clusters and the TiO₂/graphene composites are found to vary largely in dependence on the size of the respective constituents. This holds in particular for the energy and the spatial localization of the highest occupied and lowest unoccupied molecular orbitals. In addition to a substantial gap narrowing, a pronounced separation of photoexcited electrons and holes is predicted in some instances. This is expected to prolong the lifetime of photoexcited carriers. Altogether, TiO₂/graphene composites are predicted to be promising photocatalysts with improved electronic and photocatalytic properties compared to bulk TiO₂.     

Kinetic energy release of protonated methanol clusters using the low-temperature fast-atom bombardment: experiment and theory combined

Low-temperature fast-atom bombardment was found to be an excellent method for generating large protonated methanol clusters, (CH(3)OH)(n)H(+) (n = 2 to 15). Metastable dissociations of these clusters, involving elimination of one methanol molecule, were studied using mass-analyzed ion kinetic energy spectra (MIKES). From metastable peak profiles kinetic energy release (KER) distributions were obtained, even for clusters as large as (CH(3)OH)(15)H(+). The results were analyzed by a simple thermal model, by the finite heat bath theory (FHBT) and by the RRKM-based MassKinetics algorithm. The KER distribution was shown to correspond to a three-dimensional translational energy distribution, implying statistical energy partitioning in the transition state. The mean KER values and transition state temperatures were found to increase with cluster size, reaching 25 meV and approximately 210 K for large clusters (n = 10).     

Connecting theory with experiment to understand the initial nucleation steps of heteropolyoxometalate clusters

A complimentary combination of Density Functional Theory (DFT) methodology and Electrospray Ionization-Mass Spectrometry (ESI-MS) has been utilized to increase our limited understanding of the first nucleation steps in the formation of the [XM(12)O(40)](n-) Keggin polyoxometalates (POMs) (where addenda metal atom M = W or Mo, and the heteroatom X = P or As). We postulate that the first key steps of nucleation into discrete, high nuclearity heteropolyanions proceed via the formation of isodinuclear species (e.g. [M(2)O(7)](2-)), which undergo successive steps of protonation and water condensation to form a heterotrinuclear fragment, which acts as a template for the constituent parts required for subsequent aggregation and formation of the plenary Keggin heteropolyanion. The stability of calculated structures of the numerous postulated intermediates has been analysed and discussed in detail, and these results complemented using experimental mass spectrometry, using an assembly (reaction solution analysis) and disassembly (fragmentation of single crystals) approach. Overall, no significant differences between the Keggin POMs were found when changing the addenda metal atom (W or Mo) or the heteroatom (P or As); although small differences among the lowest-energy structures were detected.     

Dielectric properties of water clusters containing CO and NO molecules. Computational experiment

The absorption of CO and NO molecules by (H₂O)₂₀ clusters was studied by the method of molecular dynamics. In general, the clusters containing CO molecules are more stable mechanically, while the clusters with NO molecules are more stable against heating. The mobility of NO molecules in such clusters is higher than that of CO molecules. The total dipole moment, the static dielectric permeability, the number of active electrons in the clusters, and the specific number of hydrogen bonds between water molecules possess peak values when the number of doping molecules i = 6. IR absorption spectra mostly acquire a smooth shape at i > 6. Capture of CO and NO molecules by water cluster operates as anti-greenhouse effect.     

ThH5: An Actinide-Containing Superhalogen Molecule

Thorium and its compounds have been widely investigated as important nuclear materials. Previous research focused on the potential use of thorium hydrides, such as ThH₂, ThH₄, and Th₄H₁₅, as nuclear fuels. Here, we report studies of the anion, ThH₅⁻, by anion photoelectron spectroscopy and computations. The resulting experimental and theoretical vertical detachment energies (VDE) for ThH₅⁻ are 4.09 eV and 4.11 eV, respectively. These values and the agreement between theory and experiment facilitated the characterization of the structure of the ThH₅⁻ anion and showed its neutral counterpart, ThH₅ to be a superhalogen. ThH₅⁻, which exhibits a C_4v structure with five Th−H single bonds, possesses the largest known H/M ratio among the actinide elements, M. The adaptive natural density partitioning (AdNDP) method was used to further analyze the chemical bonding of ThH₅⁻ and to confirm the existence of five Th−H single bonds in the ThH₅⁻ molecular anion.     

Phosphorus carbides: theory and experiment

The recent finding that radio frequency plasma activation of CH(4)/PH(3) gas mixtures can yield films with P : C ratios < or = 3 has served to trigger further research into new 'phosphorus carbide' materials. Theoretical and experimental results relating to periodic and amorphous materials, respectively, are presented here: (i) The electronic structure and stability of different crystalline phosphorus carbide P(x)C(y) phases have been studied using first-principles density-functional theory. Calculations have been carried out for P(4)C(3+8 n) (n= 0-4), PC, and PC(3) and the most likely periodic structures examined in detail. Particular attention is paid to the composition PC(3), for which there are several possibilities of similar energy. (ii) Recent experimental efforts have involved use of pulsed laser ablation methods to produce hydrogen-free phosphorus carbide thin films. Mechanically hard, electrically conducting diamond like carbon films containing 0- approximately 26 at.% P have been deposited on both Si and quartz substrates by 193 nm PLA of graphite/phosphorus targets (containing varying percentages of phosphorus), at a range of substrate temperatures (T(sub)= 298-700 K), in vacuum, and analysed via laser Raman and X-ray photoelectron spectroscopy.     

Tetrafluorobenzyne thermochemistry: experiment and theory

Gas-phase thermodynamic properties of 1,2,3,4-tetrafluorobenzyne (1 H-(2)) were determined by Fourier transform mass spectrometry and ab initio and density functional theory methods. 1,2,3,4-Tetrafluorobenzyne radical anion was generated by abstraction of a proton and a hydrogen atom upon reaction of 1,2,3,4-tetrafluorobenzene (1) with O(-.). The resulting structure was confirmed by converting it to a species which could be independently prepared. Bracketing results provided the proton affinity of 1,2,3,4-tetrafluorobenzyne radical anion and the electron affinities of 1,2,3,4-tetrafluorobenzyne and 1,2,3,4-tetrafluorophenyl radical. These measured values were combined in a thermodynamic cycle to provide the heat of hydrogenation of 1 H(2) (DeltaH degree (hyd) = 367 +/- 18 kJ mol(-1)) and the first and second CH bond dissociation energies of 1 (481 +/- 11 and 321 +/- 13 kJ mol(-1)). The same approach failed for the meta and para isomers, but their energetics were examined using B3LYP and CCSD(T) computations.     

Organocatalytic claisen rearrangement: theory and experiment

A combined computational and experimental study on the Claisen rearrangement of a 2-alkoxycarbonyl-substituted allyl vinyl ether in the presence of thioureas as potential noncovalent organocatalysts has been performed. DFT calculations employing different basis sets were utilized to predict a catalytic cycle for the thiourea-catalyzed Claisen rearrangement. The nature of the transition state in the presence and absence of thioureas was studied in detail. Critical geometrical data of the transition state that are indicators for the relative barrier height of the Claisen rearrangement are discussed. Although we did observe a significant transition state stabilization, due to endergonic conformational changes and endergonic complexation the overall effect on the barrier is small, in accordance with experimental results.     

Silver nanoparticles electrooxidation: theory and experiment

This article presents the findings of microscopic and electrochemical studies of electrooxidation of silver nanoparticles of varying sizes in comparison with “bulk” silver. Silver particles were immobilized on the surface of indifferent carbon-containing screen-printed electrodes. Vacuum-deposited silver represented the “bulk” electrode. The calculations and experimental studies demonstrated that the transition from macro- to nanostructural electrodes is followed by a shift of the maximum current potential of metal oxidation into the area with more negative potentials. A positive correlation between experimental and calculated data confirms once again a relevant application of the earlier proposed mathematical model and the possible use of the shift of the maximum current potential of electrooxidation to describe the electrochemical activity and surface energy properties of metal nanoparticles.     

Efficient fluoride-selective receptor: experiment and theory

A novel artificial receptor, (3′-nitrobenzo)[2,3-d]-(3′′-nitrobenzo)[9,10-d]-1,4,8,11-tetraazacyclotetradecane-5,7,12,14-tetraone, has been synthesized and shows high selective and recognitive ability for F^- among F⁻, Cl⁻, Br⁻, AcO⁻, H₂PO₄⁻ by UV-vis and ¹H NMR titration experiments. Theoretical investigations suggest that the fluoride selectivity among various anions comes from the fact that the fluoride approaches much closer to the amide protons than other anions located above the cavity. The interaction energies support the large binding ability difference between F⁻ and Cl⁻/Br⁻/AcO⁻/H₂PO₄⁻.     

Bismuth nanoparticles electrooxidation: theory and experiment

The article presents the findings of microscopic and electrochemical studies of electrooxidation of bismuth particle of varying sizes. Bismuth particles were immobilized on the surface of indifferent carbon containing screen-printed electrodes. The calculations and experimental studies demonstrated that the transition from macroparticles to nanoparticles caused a shift of the maximum current potential of bismuth oxidation into the area with more negative potentials. A positive correlation between experimental and calculated data confirms once again a relevant application of the earlier proposed mathematical model and the possible use of the shift of the maximum current potential of electrooxidation to describe electrochemical activity and energy properties of metal nanoparticles.

     

Bismuth nanoparticles electrooxidation: theory and experiment

The article presents the findings of microscopic and electrochemical studies of electrooxidation of bismuth particle of varying sizes. Bismuth particles were immobilized on the surface of indifferent carbon containing screen-printed electrodes. The calculations and experimental studies demonstrated that the transition from macroparticles to nanoparticles caused a shift of the maximum current potential of bismuth oxidation into the area with more negative potentials. A positive correlation between experimental and calculated data confirms once again a relevant application of the earlier proposed mathematical model and the possible use of the shift of the maximum current potential of electrooxidation to describe electrochemical activity and energy properties of metal nanoparticles.     

Enzyme mechanisms: interplay of theory and experiment

An overview is given on experimental and theoretical methods applied to the study of enzyme mechanisms. While experiments provide more or less precise data on realistic systems, the obtained information often overlap and a mixture of observations has to be resolved in order to appropriately understand enzymatic processes at the molecular level. On the other hand, computations to be done on adequate models reflecting all important properties of the system, may not provide results that are accurate enough to be compared with experiments. It is stressed that in a computational study the level of sophistication of the model and the method applied to it should be about the same in order to obtain sound results. A case study, the catalytic mechanism of serine proteases, is discussed in detail calling attention to the problems where the interplay between computations and experimental studies was necessary to understand mechanistic details. It is computational chemistry that lead to the discovery of the crucial role of the protein electrostatic field in the acceleration of the enzyme reaction as well as protonation state of the side chains of the active site. We stress that computational studies are especially important in the right interpretation of experimental observations and should replace speculations based on textbook chemistry.     

Effect of end-group sticking energy on the properties of polymer brushes: comparing experiment and theory

Using surface force balance measurements we have established that polystyrene chains bearing three zwitterionic groups have a higher end-group sticking energy than equivalent chains bearing a single zwitterionic group. In a good solvent, polystyrene chains end-functionalized with three zwitterionic groups form brushes of a higher surface coverage than those bearing a single zwitterion. The increase in surface coverage is slow compared with the initial formation of the brush. Measurements of the refractive index allow us to directly quantify the variation of surface coverage, permitting comparison with models for the kinetics of brush formation based on scaling theory and an analytical self-consistent field. We find qualitative support for associating the kinetic barrier with the energy required for an incoming chain to stretch as it penetrates the existing brush.