•A potential energy surface was calculated for the 5-HT molecule in aqueous media.•Minima and TS were located using fully relaxed optimization at different LOT.•5-HT conformational and lattice energies of serotonin salts were calculated.•Comparison of conformations in crystals and crystallisation media were preformed.•New approach for crystal structure prediction using conformational study is suggested.A potential energy surface (PES) has been calculated for the protonated serotonin (5-HT) molecule in aqueous media. Three pairs of symmetrically equivalent minima were located using fully relaxed optimization at B3LYP/6-31G(d,p) level of theory. Insensitivity to basis set was verified over a range of different functionals and levels of theory. All 9 transition structures were found using the QST3 method with ansatz structures taken directly from the result of the calculated PES. Energies associated with conformational changes of protonated 5-HT in aqueous media were calculated and found to vary between 6 and 23 kJ mol−1. Lattice energies of serotonin picrate monohydrate and serotonin adipate were calculated as −741.7 and −716.9 kJ mol−1, respectively, and compared to conformational energies to consider the relative importance of each interaction type. The study contributes to achieving better insight into the importance of molecular conformations in solution as a precursor for the formation of the final crystal structure.

We have performed simulations utilizing the dispersion-corrected density functional theory method (DFT-D) as parametrized by Grimme on selected polymorphs of RDX (cyclotrimethylenetrinitramine). Additionally, we present the first experimental determination of the enthalpy of fusion (ΔHfus) of the highly metastable β-form of RDX. The characteristics of fusion for β-RDX were determined to be 186.7 ± 0.8 °C, 188.5 ± 0.4 °C, and 12.63 ± 0.28 kJ mol–1 for the onset temperature, peak temperature (or melting point), and ΔHfus, respectively. The difference in experimental ΔHfus for the α- and β-forms of RDX is 20.46 ± 0.92 kJ mol–1. Ambient-pressure lattice energies (EL) of the α- and β-forms of RDX have been calculated and are in excellent agreement with experiment. In addition the computationally predicted difference in EL (20.35 kJ mol–1) between the α- and β-forms is in excellent agreement with the experimental difference in ΔHfus. The response of the lattice parameters and unit-cell volumes to pressure for the α- and γ-forms have been investigated, in addition to the first high-pressure computational study of the ε-form of RDX—these results are in very good agreement with experimental data. Phonon calculations provide good agreement for vibrational frequencies obtained from Raman spectroscopy, and a predicted inelastic neutron scattering (INS) spectrum of α-RDX shows excellent agreement with experimental INS data determined in this study. The transition energies and intensities are reproduced, confirming that both the eigenvalues and the eigenvectors of the vibrations are correctly described by the DFT-D method. The results of the high-pressure phonon calculations have been used to show that the heat capacities of the α-, γ-, and ε-forms of RDX are only weakly affected by pressure.

We have obtained detailed structural information for the energetic salt ammonium perchlorate (AP) at pressures up to ∼8 GPa through a combination of X-ray and neutron diffraction. Under hydrostatic conditions, AP undergoes a first-order phase transition at 3.98(5) GPa, broadly consistent with results from previous studies. We have successfully solved and refined the structure of the new orthorhombic phase (phase II, space group Pnma), which features a more close-packed structure with more extensive hydrogen bonding than the polymorph obtained at ambient pressure (phase I). Equations of state have been obtained for phase I from 0 to 3.5 GPa and for the new phase 4 to 8.1 GPa. To complement these experimental studies, we have also performed density functional theory (DFT) calculations of the hydrostatic compression of AP in the region of 0.0–3.5 GPa. A comparison of the performance of different pseudopotentials and DFT dispersion correction schemes in calculating crystal geometries at high pressure has been performed. The results highlight the fact that care must be taken when choosing pseudopotentials for high-pressure studies and that no significant improvements in the calculation of crystal geometries of AP are obtained by employing DFT-D corrections.

The high-pressure, high-temperature ε-form of the widely used explosive RDX has been structurally characterised using a combination of diffraction techniques, and a sample of this form has been successfully recovered to ambient pressure.

The crystal structure of the highly metastable β-form of RDX shows that the molecules adopt different conformations compared to the α-form and that, contrary to previous reports, the β-form obtained at ambient pressure is not the same form as that obtained at elevated temperatures and pressures.

The reproducible crystallisation of elusive polymorphs and solvates of molecular compounds at high pressure has been demonstrated through studies on maleic acid, malonamide, and paracetamol. These high-pressure methods can be scaled-up to produce ‘bulk’ quantities of metastable forms that can be recovered to ambient pressure for subsequent seeding experiments. This has been demonstrated for paracetamol form II and paracetamol monohydrate. The studies also show that the particular solid form can be tuned by both pressure and concentration.

Two high-pressure polymorphs of sulfuric acid monohydrate (oxonium hydrogensulfate) have been obtained at ambient temperature by crystallisation at high pressure from the liquid at 1.3 GPa (form III) and by direct compression of the ambient-pressure form I first to 1.26 GPa (form II) and then to 1.72 GPa (form III). The structure of form III was solved by single crystal X-ray diffraction and this structure was used as the basis for the refinement of hydrogen positions using high-pressure neutron powder diffraction data. Form III crystallises in the orthorhombic crystal system at 1.97 GPa, and features parallel chains of hydrogensulfate ions linked by oxonium ions to form a three-dimensional hydrogen-bonded network. On further compression to 3.05 GPa, the direction of maximum compressibility is found to be along the a-axis and is associated with the shortening of a hydrogen bond between a hydrogensulfate ion and an oxonium ion. The structure of form II remains elusive although at ambient temperature it is stable (or metastable) at pressures as low as 0.42 GPa, perhaps indicating that it could be recoverable to ambient-pressure at low temperature.A high-pressure polymorph of sulfuric acid monohydrate has been obtained by crystallisation at high-pressure from the liquid at 1.3 GPa and by direct compression of the ambient-pressure form to 1.72 GPa. The structure was solved by single crystal X-ray diffraction and the hydrogen positions refined using neutron powder diffraction data.

Recrystallisation of paracetamol from a solution in methanol at a pressure of 0.62 GPa gives a new 1 ∶ 1 solvate that has been characterised by single crystal X-ray diffraction.

Electrochemical oxidation of a titanium electrode in a solution of potassium amide in liquid ammonia resulted in the deposition of titanium nitride either as a thin film or as nanoparticles.

The high-pressure, high-temperature ε-form of the widely used explosive RDX has been structurally characterised using a combination of diffraction techniques, and a sample of this form has been successfully recovered to ambient pressure.

The crystal structure of the highly metastable β-form of RDX shows that the molecules adopt different conformations compared to the α-form and that, contrary to previous reports, the β-form obtained at ambient pressure is not the same form as that obtained at elevated temperatures and pressures.