Crystal structure of NH4ClO4at 298, 78, and 10 °K by neutron diffraction

Machine Learning Directed Optimization of Classical Molecular Modeling Force Fields

Accurate force fields are necessary for predictive molecular simulations. However, developing force fields that accurately reproduce experimental properties is challenging. Here, we present a machine learning directed, multiobjective optimization workflow for force field parametrization that evaluates millions of prospective force field parameter sets while requiring only a small fraction of them to be tested with molecular simulations. We demonstrate the generality of the approach and identify multiple low-error parameter sets for two distinct test cases: simulations of hydrofluorocarbon (HFC) vapor-liquid equilibrium (VLE) and an ammonium perchlorate (AP) crystal phase. We discuss the challenges and implications of our force field optimization workflow.

Effects of pressure on the structure and lattice dynamics of ammonium perchlorate: A combined experimental and theoretical study

Ammonium perchlorate NH₄ClO₄ (AP) was studied using synchrotron angle-dispersive X-ray powder diffraction (XRPD) and Raman spectroscopy. A diamond-anvil cell was used to compress AP up to 50 GPa at room temperature (RT). Density functional theory (DFT) calculations were performed to provide further insight and comparison to the experimental data. A high-pressure barite-type structure (Phase II) forms at ≈4 GPa and appears stable up to 40 GPa. Refined atomic coordinates for Phase II are provided, and details for the Phase I → II transition mechanics are outlined. Pressure-dependent enthalpies computed for DFT-optimized crystal structures confirm the Phase I → II transition sequence, and the interpolated transition pressure is in excellent agreement with the experiment. Evidence for additional (underlying) structural modifications include a marked decrease in the Phase II b'-axis compressibility starting at 15 GPa and an unambiguous stress relaxation in the normalized stress-strain response at 36 GPa. Above 47 GPa, XRD Bragg peaks begin to decrease in amplitude and broaden. The apparent loss of crystalline long-range order likely signals the onset of amorphization. Three isostructural modifications were discovered within Phase II via Raman spectroscopy. A revised RT isothermal phase diagram is discussed based on the findings of this study.

Phase diagram of ammonium perchlorate: Raman spectroscopic constrains at high pressures and temperatures

We present the pressure-temperature (PT) induced physical and chemical transformations in ammonium perchlorates (APs) up to 50 GPa and 450 °C, using diamond anvil cells and confocal micro-Raman spectroscopy, which provide new constraints for the phase diagram of AP. The results show spectral evidences for three new polymorphs (III, IV, and VI) of AP, in addition to two previously known phases (I and II), at various PT conditions with varying degrees of hydrogen bonding and lack of strong spectral evidence for previously known high-temperature cubic phase (phase V). Upon further heating, AP chemically decomposes to N2, N2O, and H2O. The present phase diagram is, therefore, in sharp contrast to the previous one, underscoring a rich polymorphism, a large stability field for solids, and a replacement of the melt with a decomposition line.

Comparative DFT and DFT-D studies on structural, electronic, vibrational and absorption properties of crystalline ammonium perchlorate

Pressure-induced rearrangement of the relative position of anions and cations in AP crystal.

Structure, hydrogen bonding and thermal expansion of ammonium carbonate monohydrate

We have determined the crystal structure of ammonium carbonate monohydrate, (NH4)2CO3·H2O, using Laue single-crystal diffraction methods with pulsed neutron radiation. The crystal is orthorhombic, space group Pnma (Z = 4), with unit-cell dimensions a = 12.047 (3), b = 4.453 (1), c = 11.023 (3) Å and V = 591.3 (3) Å(3) [ρcalc = 1281.8 (7) kg m(-3)] at 10 K. The single-crystal data collected at 10 and 100 K are complemented by X-ray powder diffraction data measured from 245 to 273 K, Raman spectra measured from 80 to 263 K and an athermal zero-pressure calculation of the electronic structure and phonon spectrum carried out using density functional theory (DFT). We find no evidence of a phase transition between 10 and 273 K; above 273 K, however, the title compound transforms first to ammonium sesquicarbonate monohydrate and subsequently to ammonium bicarbonate. The crystallographic and spectroscopic data and the calculations reveal a quite strongly hydrogen-bonded structure (EHB ≃ 30-40 kJ mol(-1)), on the basis of H...O bond lengths and the topology of the electron density at the bond critical points, in which there is no free rotation of the ammonium cation at any temperature. The barrier to free rotation of the ammonium ions is estimated from the observed librational frequency to be ∼ 36 kJ mol(-1). The c-axis exhibits negative thermal expansion, but the thermal expansion behaviour of the a and b axes is ormal.

Counterion Controlled Photoisomerization of Retinal Chromophore Models: a Computational Investigation

CASPT2//CASSCF photoisomerization path computations have been used to unveil the effects of an acetate counterion on the photochemistry of two retinal protonated Schiff base (PSB) models: the 2-cis-penta-2,4-dieniminium and the all-trans-epta-2,4,6-trieniminium cations. Different positions/orientations of the counterion have been investigated and related to (i) the spectral tuning and relative stability of the S0, S1, and S2 singlet states; (ii) the selection of the photochemically relevant excited state; (iii) the control of the radiationless decay and photoisomerization rates; and, finally, (iv) the control of the photoisomerization stereospecificity. A rationale for the results is given on the basis of a simple (electrostatic) qualitative model. We show that the model readily explains the computational results providing a qualitative explanation for different aspects of the experimentally observed "environment" dependent PSB photochemistry. Electrostatic effects likely involved in controlling retinal photoisomerization stereoselectivity in the protein are also discussed under the light of these results, and clues for a stereocontrolled electrostatically driven photochemical process are presented. These computations provide a rational basis for the formulation of a mechanistic model for photoisomerization electrostatic catalysis.

SQUID detected NMR and NQR. Superconducting Quantum Interference Device

The dc Superconducting QUantum Interference Device (SQUID) is a sensitive detector of magnetic flux, with a typical flux noise of the order 1 muphi0 Hz(-1/2) at liquid helium temperatures. Here phi0 = h/2e is the flux quantum. In our NMR or NQR spectrometer, a niobium wire coil wrapped around the sample is coupled to a thin film superconducting coil deposited on the SQUID to form a flux transformer. With this untuned input circuit the SQUID measures the flux, rather than the rate of change of flux, and thus retains its high sensitivity down to arbitrarily low frequencies. This feature is exploited in a cw spectrometer that monitors the change in the static magnetization of a sample induced by radio frequency irradiation. Examples of this technique are the detection of NQR in 27Al in sapphire and 11B in boron nitride, and a level crossing technique to enhance the signal of 14N in peptides. Research is now focused on a SQUID-based spectrometer for pulsed NQR and NMR, which has a bandwidth of 0-5 MHz. This spectrometer is used with spin-echo techniques to measure the NQR longitudinal and transverse relaxation times of 14N in NH4ClO4, 63+/-6 ms and 22+/-2 ms, respectively. With the aid of two-frequency pulses to excite the 359 kHz and 714 kHz resonances in ruby simultaneously, it is possible to obtain a two-dimensional NQR spectrum. As a third example, the pulsed spectrometer is used to study NMR spectrum of 129Xe after polariza-tion with optically pumped Rb. The NMR line can be detected at frequencies as low as 200 Hz. At fields below about 2 mT the longitudinal relaxation time saturates at about 2000 s. Two recent experiments in other laboratories have extended these pulsed NMR techniques to higher temperatures and smaller samples. In the first, images were obtained of mineral oil floating on water at room temperature. In the second, a SQUID configured as a thin film gradiometer was used to detect NMR in a 50 microm particle of 195Pt at 6 mT and 4.2 K.

14N NMR echo in NH4ClO4 after the pulse sequence (theta1)x-tau-(theta2)y-t

We have derived a closed-form expression for the solid echo signal of quadrupolar I = 1 nuclei after the pulse sequence (theta1)x-tau-(theta2)y-t for arbitrary values of the RF nutation frequency omega1 = gammaB1 and the quadrupolar frequency omegaQ. In the case of single crystals both the true echo term of this expression and its induction-signal-like terms are important as shown by experiments on 14N nuclei in NH4ClO4 crystal. Conditions for obtaining the maximal echo in powder samples are presented. A very low B1 field together with long RF pulses may distort even the central part of the spectrum, resulting in strange looking apparent spectra.

Anisotropy of the proton T1 and the low-field relaxation in NH4 ClO4 below 20K

The spin-lattice relaxation of protons in NH4ClO4 at low temperatures has been studied theoretically and experimentally. The NH4 librational tunneling determines the spin-librational wavefunctions, which are derived first. The dominant transition rates related to the magnetic dipolar interaction between the NH4 protons are then obtained. They reproduce well the angular dependence of the proton relaxation time. When all the transition rates are taken into account, both the temperature and frequency dependence of the relaxation rate agree with our experimental data for a powder sample. Reasons for the non-exponential relaxation are discussed. Apart from the angular dependence of the relaxation rate, they include the tunnel frequency distribution, the coupling to the tunnel reservoir and the effect of deuterons in their natural concentration.

Deuteron NMR spectra of ND4ClO4 single crystal at low temperatures

2H NMR spectra of ND4ClO4 single crystal were obtained at v0 = 44 MHz. Orientation and temperature (1.9-75 K) dependences were measured. Fitting the spectra gives the effective quadrupole coupling constants for all deuterons and the ground torsional level structure. The isotope reduction of the (A-T) and (A-E) tunnelling splittings, i.e., the ratios of the respective splittings for NH4+ and ND4+, were found to be different. The splittings at T = 24 K are about 60% of the helium temperature values. The spectrum undergoes intermediate narrowing by reorientations between 26 and 34 K and tunnelling related features in the spectra are eradicated. After reaching the extreme narrowing limit, a doublet with gradually decreasing separation was observed, what was attributed to averaging by torsional oscillations of increasing amplitude. At high temperatures (T > 75 K), the narrow spectrum reflects fast multiaxial reorientation of the ammonium ion.