Alkyl-substituted phenolic pyrazoles such as 4-methyl-2-[5-(n-octyl)-1H-pyrazol-3-yl]phenol (L2H) are shown to function as Cu-extractants, having similar strength and selectivity over Fe(III) to 5-nonylsalicylaldoxime which is a component of the commercially used ACORGA® solvent extraction reagents. Substitution in the phenol ring of the new extractants has a major effect on their strength, e.g. 2-nitro-4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L4H) which has a nitro group ortho to the phenolic hydroxyl group unit and has an extraction distribution coefficient for Cu nearly three orders of magnitude higher than its unsubstituted analogue 4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L8H). X-ray structure determinations and density functional theory (DFT) calculations confirm that inter-ligand hydrogen bonding between the pyrazole NH group and the phenolate oxygen atom stabilise the Cu-complexes, giving pseudomacrocyclic structures. Electron-accepting groups ortho to the phenol oxygen atoms buttress the inter-ligand H-bonding, enhancing extractant strength but the effectiveness of this is very dependent on steric factors. The correlation between the calculated energies of formation of copper complexes in the gas phase and the observed strength of comparably substituted reagents in solvent extraction experiments is remarkable. Analysis of the energies of formation suggests that big differences in strength of extractants arise principally from a combination of the effects of the substituents on the ease of deprotonation of the proligands and, for the ortho-substituted ligands, their propensity to buttress inter-ligand hydrogen bonding.

This benchmark study documents a combined computational and experimental investigation into the binding of three commercial dithio collector ligands used in industrial froth flotation processes to separate high-value minerals from lower-value materials. First-principles condensed-matter simulations showed that ethyl xanthate, N,N-diethyl dithiocarbamate, and diisobutyl dithiophospinate anions all bind least strongly to the [100] Miller index plane of the platinum-containing mineral sperrylite, followed by the [111] MI surface of the mixed nickel/iron sulfide mineral pentlandite, whereas all ligands showed the strongest binding affinity to the [111] MI surface model of pure platinum. Calculations also support experimental observations that neutral ethyl xanthogen disulfide formed upon oxidation of ethyl xanthate binds much more weakly than the monomer. A monolayer of water molecules would easily be displaced from all surfaces by any of the collector ligands. The hydroxide anion was found to have binding energies with magnitudes comparable to those of the collector ligands on all surfaces. Cyclic voltammetry measurements performed on working electrodes constructed from sperrylite, pentlandite, and platinum permitted measurement of the mixed oxidation potential associated with the surface dimerization reaction for all three collector ligands, although the data obtained for diisobutyl dithiophospinate were less clear-cut than those obtained for the other ligands. Comparison of the relative ordering of the mixed potentials for the three ligands gave a favorable match with the predicted outcome of binding energy strengths obtained from the modeling study. This study demonstrates that first-principles simulations can be used to predict the binding energies of collector ligands to mineral and metallic surfaces.

High anion selectivity for PtCl62– over Cl– is shown by a series of amidoamines, R1R2NCOCH2CH2NR3R4 (L1 with R1 = R4 = benzyl and R2 = R3 = phenyl and L3 with R1 = H, R2 = 2-ethylhexyl, R3 = phenyl and R4 = methyl), and amidoethers, R1R2NCOCH2CH2OR3 (L5 with R1 = H, R2 = 2-ethylhexyl and R3 = phenyl), which provide receptor sites which extract PtCl62– preferentially over Cl– in extractions from 6 M HCl solutions. The amidoether receptor L5 was found to be a much weaker extractant for PtCl62– than its amidoamine analogues. Density functional theory calculations indicate that this is due to the difficulty in protonating the amidoether to generate a cationic receptor, LH+, rather than the latter showing weaker binding to PtCl62–. The most stable forms of the receptors, LH+, contain a tautomer in which the added proton forms an intramolecular hydrogen bond to the amide oxygen atom to give a six-membered proton chelate. Dispersion-corrected DFT calculations appear to suggest a switch in ligand conformation for the amidoamine ligands to an open tautomer state in the complex, such that the cationic N–H or O–H groups are also readily available to form hydrogen bonds to the PtCl62– ion, in addition to the array of polarized C–H bonds. The predicted difference in energies between the proton chelate and nonchelated tautomer states for L1 is small, however, and the former is found in the X-ray crystal structure of the assembly [(L1H)2PtCl6]. The DFT calculations and the X-ray structure indicate that all LH+ receptors present an array of polarized C–H groups to the large, charge diffuse PtCl62– anion resulting in high selectivity of extraction of PtCl62– over the large excess of chloride.

We present a first principles static and dynamical study of the transition metal hydride series MH4L3 (M = Fe, Ru and Os; L = NH3, PH3 and PF3), with a view to arriving at an understanding of how the variation in the electronic properties of the metal sites and ligands can influence the dynamics of the resulting complexes. A broad range of behaviour was observed, encompassing stable classical minima (M = Os, L = NH3 and M = Ru, L = PH3) to stable η2-H2 non-classical minima (M = Fe, L = PF3 and M = Ru, L = PH3 or PF3), with the other structures exhibiting dynamical behaviour that spontaneously converted between the classical and non-classical states during the molecular dynamics simulations. The importance of a small Laxial–M–Laxial angle in stabilising the non-classical state is highlighted, as is a short η2-H2⋯Hcis distance in non-classical complexes that spontaneously convert to the classical form. We also investigated the changes in the electronic structure of the complex FeH4(PH3)3 during a η2-H2 bond breaking/bond making reaction and observed direct evidence of the ‘cis effect’, whereby a neighbouring hydride ligand acts to stabilise the intermediate classical state.

Density functional theory, in conjunction with a cluster expansion model, has been used to study the structure and stability of the positionally disordered iron–nickel sulfide mineral pentlandite (Pn), (Fe,Ni)9S8, with results indicating heterogeneous nearest neighbor metal contacts are more energetically favorable than homogeneous contacts. The virtual crystal approximation was also explored as a means to address positional disorder, but while reliable results could be obtained for the bulk model, the same was not true for the surface, as local distortions which affected the surface model energies could not be reproduced. We also address the binding of ethyl xanthate (CH3CH2OCS2–), water, and hydroxide to the [111] Pn surface to understand the mode of action of industrial xanthate flotation agents better. In order to model anionic ligands bound to a periodic boundary condition surface we propose applying a correction derived from the surface work function to remove the additional charge introduced by the ligand. The results obtained from the ligand binding studies indicate that while ethyl xanthate could readily displace up to a full monolayer of water per unit cell it is likely that Fe-enriched surfaces will bind xanthate in competition with the hydroxide anion, while a Ni-enriched surface will preferentially bind hydroxide anions over xanthate.

A subtractive implementation of the QM/QM hybrid method for the description of photochemical reactions occurring in molecular crystals is presented and tested by applying it in a simulation study of the ultrafast intramolecular excited-state proton transfer reaction in the crystal form of 7-(2-pyridyl)-indole, an organic compound featuring an intramolecular hydrogen bond within a six-membered ring. By propagating molecular dynamics on the excited-state potential energy surface, a mean proton transfer time was calculated as 80 fs. The reaction mechanism is discussed in terms of three-dimensional reaction coordinate diagrams. Proton transfer was found to be barrierless and to be strongly coupled to vibrational modes of the photoexcited molecule that modulate the proton donor–acceptor distance. Some 300 fs after the initial photoexcitation, the excited state molecule reached an S1/S0 conical intersection through the mutual twist of the pyridyl and indolyl moieties.

In this paper, we report the application of the QM/QM hybrid simulation technique to the photoisomerisation reactions of anils (i.e., Schiff bases of salicylaldehyde with aniline derivatives) in the solid state, on the example of the photochromic polymorph of N-salicylidene-2-chloroaniline. By propagating molecular dynamics on a potential energy surface constructed using a combination of time-dependent DFT and ground-state DFT calculations, two reaction pathways of the cis-enol isomer were observed, which occur with approximately equal probability. In the first pathway, the photoexcited molecule undergoes an intramolecular proton transfer reaction on average 25 fs after photoexcitation. It then persists in the cis-keto form for a few hundred femtoseconds before undergoing a pedal motion through which it reaches an S1/S0 conical intersection. This pathway, whose existence has previously been proposed in the literature to rationalize the feasibility of the photoisomerisation reaction in the confined environment of the crystal lattice, is predicted to lead to the formation of the trans-keto form. The second pathway is nonreactive and is analogous to a previously characterised radiationless de-excitation pathway of the isolated molecule. The cis-enol to trans-keto photoisomerisation is reversible. Following the photoexcitation of a trans-keto molecule, it persists in a largely unchanged geometry for a period of time ranging from a few hundred femtoseconds to over a picosecond, and subsequently undergoes a pedal motion in the same direction as the one involved in the cis-enol to trans-keto photoisomerisation, leading to the cis-keto isomer through another S1/S0 conical intersection.

Ab initio MD simulations on a polyglycine helix and water-wire expressed under periodic boundary conditions have created a channel that supports proton transfer up to distances of 10.5 Å. The effect of varying the density of water molecules in the channel has been investigated. A range of cationic states are identified with widely varying lifetimes. The mechanism of proton transport in this model shares some features with the simulations reported for bulk water, with, e.g., the hydrogen bond distance shortening in the time period leading up to successful proton transfer. However, there are also some important differences such as the observation of a heightened number of proton rattling events. We also observe that the helix plays an important role in directing the behavior of the water wire: the most active proton transport regions of the water-wire are found in areas where the helix is most tightly coiled. Finally, we report on the effects of different DFT functionals to model a water-wire and on the importance of including dispersion corrections to stabilize the α-helical structure.

A series of ab initio calculations have been carried out to determine why the a,b- and c,c-isomers are the most commonly observed mono-oxides of C70 in ozonolysis reactions, when existing calculations in the literature report that these structures are not the most stable conformations. We show that the a,b- and c,c-isomers are the two most stable structures on the C70O3 potential energy surface, which suggests that the reaction pathway toward oxide formation must proceed via the corresponding ozonide structure. From our calculations, we offer a mechanism for the thermally induced dissociation of C70O3 that share the first two steps with the general mechanism for ozonolysis of alkenes proposed by Criegee. We suggest further steps that involve C70O3 losing O2 in its triplet or singlet state, thus leaving C70O in its triplet or singlet state, respectively. A pair of products in their singlet states seems to be more likely for the decomposition of a,b-C70O3, which ultimately leads to the closed a,b-C70O epoxide structure. For c,c-C70O3, the more thermodynamically favorable route is the triplet channel, resulting in the triplet open c,c-C70O oxidoannulene structure, which may subsequently decay to the singlet ground state c,c-C70O epoxide form. This finding offers an alternative interpretation of the experimental observations which reported an open d,d-C70O oxidoannulene structure as the metastable intermediate.

As part of an on-going programme to study the high pressure structural behaviour of hydrated small molecular systems, sodium formate dihydrate has been studied using high pressure single crystal X-ray and neutron powder diffraction methods. A new phase was initially identified at 17 kbar by X-ray diffraction and high level quantum mechanical calculations completed the structure, allowing definitive hydrogen atom positioning. The resulting structure compared favourably with that found subsequently by high pressure neutron diffraction. The neutron diffraction study also revealed that the deuterated form, NaDCO2·2D2O, is stable in a different structural form to that of the non-deuterated material at ambient pressure. The structure of this phase is related to that of the high pressure phase via a simple translation of the molecular layers.

The molecular structures of CH(SiMe2H)3 and CH(SiMe2Br)3 have been determined by gas-phase electron diffraction, ab initio calculations, and, for CH(SiMe2Br)3, X-ray diffraction. In each case, 11 distinct conformations, with energies lying within a range of 8.5 kJ mol−1 for CH(SiMe2H)3 and 26 kJ mol−1 for CH(SiMe2Br)3, were calculated and thus many conformers for both compounds are likely to exist in the gas phase. The structures are compared with those of related alkylsilanes to assess the changes in molecular geometry resulting from crowding at the carbon centre.

Alkyl-substituted phenolic pyrazoles such as 4-methyl-2-[5-(n-octyl)-1H-pyrazol-3-yl]phenol (L2H) are shown to function as Cu-extractants, having similar strength and selectivity over Fe(III) to 5-nonylsalicylaldoxime which is a component of the commercially used ACORGA® solvent extraction reagents. Substitution in the phenol ring of the new extractants has a major effect on their strength, e.g. 2-nitro-4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L4H) which has a nitro group ortho to the phenolic hydroxyl group unit and has an extraction distribution coefficient for Cu nearly three orders of magnitude higher than its unsubstituted analogue 4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L8H). X-ray structure determinations and density functional theory (DFT) calculations confirm that inter-ligand hydrogen bonding between the pyrazole NH group and the phenolate oxygen atom stabilise the Cu-complexes, giving pseudomacrocyclic structures. Electron-accepting groups ortho to the phenol oxygen atoms buttress the inter-ligand H-bonding, enhancing extractant strength but the effectiveness of this is very dependent on steric factors. The correlation between the calculated energies of formation of copper complexes in the gas phase and the observed strength of comparably substituted reagents in solvent extraction experiments is remarkable. Analysis of the energies of formation suggests that big differences in strength of extractants arise principally from a combination of the effects of the substituents on the ease of deprotonation of the proligands and, for the ortho-substituted ligands, their propensity to buttress inter-ligand hydrogen bonding.

As part of an on-going programme to study the high pressure structural behaviour of hydrated small molecular systems, sodium formate dihydrate has been studied using high pressure single crystal X-ray and neutron powder diffraction methods. A new phase was initially identified at 17 kbar by X-ray diffraction and high level quantum mechanical calculations completed the structure, allowing definitive hydrogen atom positioning. The resulting structure compared favourably with that found subsequently by high pressure neutron diffraction. The neutron diffraction study also revealed that the deuterated form, NaDCO2·2D2O, is stable in a different structural form to that of the non-deuterated material at ambient pressure. The structure of this phase is related to that of the high pressure phase via a simple translation of the molecular layers.

In this paper, we report the application of the QM/QM hybrid simulation technique to the photoisomerisation reactions of anils (i.e., Schiff bases of salicylaldehyde with aniline derivatives) in the solid state, on the example of the photochromic polymorph of N-salicylidene-2-chloroaniline. By propagating molecular dynamics on a potential energy surface constructed using a combination of time-dependent DFT and ground-state DFT calculations, two reaction pathways of the cis-enol isomer were observed, which occur with approximately equal probability. In the first pathway, the photoexcited molecule undergoes an intramolecular proton transfer reaction on average 25 fs after photoexcitation. It then persists in the cis-keto form for a few hundred femtoseconds before undergoing a pedal motion through which it reaches an S1/S0 conical intersection. This pathway, whose existence has previously been proposed in the literature to rationalize the feasibility of the photoisomerisation reaction in the confined environment of the crystal lattice, is predicted to lead to the formation of the trans-keto form. The second pathway is nonreactive and is analogous to a previously characterised radiationless de-excitation pathway of the isolated molecule. The cis-enol to trans-keto photoisomerisation is reversible. Following the photoexcitation of a trans-keto molecule, it persists in a largely unchanged geometry for a period of time ranging from a few hundred femtoseconds to over a picosecond, and subsequently undergoes a pedal motion in the same direction as the one involved in the cis-enol to trans-keto photoisomerisation, leading to the cis-keto isomer through another S1/S0 conical intersection.

We present a first principles static and dynamical study of the transition metal hydride series MH4L3 (M = Fe, Ru and Os; L = NH3, PH3 and PF3), with a view to arriving at an understanding of how the variation in the electronic properties of the metal sites and ligands can influence the dynamics of the resulting complexes. A broad range of behaviour was observed, encompassing stable classical minima (M = Os, L = NH3 and M = Ru, L = PH3) to stable η2-H2 non-classical minima (M = Fe, L = PF3 and M = Ru, L = PH3 or PF3), with the other structures exhibiting dynamical behaviour that spontaneously converted between the classical and non-classical states during the molecular dynamics simulations. The importance of a small Laxial–M–Laxial angle in stabilising the non-classical state is highlighted, as is a short η2-H2⋯Hcis distance in non-classical complexes that spontaneously convert to the classical form. We also investigated the changes in the electronic structure of the complex FeH4(PH3)3 during a η2-H2 bond breaking/bond making reaction and observed direct evidence of the ‘cis effect’, whereby a neighbouring hydride ligand acts to stabilise the intermediate classical state.