Osvald Knop

Cation rotation in methylammonium lead halides

Abstract 2 H and 14 N nmr spectra of the simple perovskites CH 3 NH 3 PbX 3 (X = Cl >, Br, I) reveal the existence of several phases. In the high-temperature phase I the long spin-lattice relaxation times T 1 of both nuclei and the absence of quadrupole splitting indicate extremely rapid overall reorientation of the CN axis of the cation in a potential of cubic symmetry. In phase II of the bromide and iodide, both T 1 and the small quadrupole splitting show unusual variation with temperature. In the lowest-temperature phase rotations of the C-N axis are restricted.

What is the true space group of weberite

Abstract Analysis of X-ray diffraction evidence obtained from a single crystal of natural weberite, Na 2 MgAlF 7 , at room temperature and at −140°C rules out all space groups but two, the noncentrosymmetric Imm 2 (originally proposed by Bystrom) and I 2 1 2 1 2 1 . However, comparison of structure refinements in these two groups and in Imma shows that the departure from centrosymmetry is so slight, and the positional and thermal parameters of some of the F atoms in the Imm 2 and I 2 1 2 1 2 1 refinements are so highly correlated, that the descriptions of the weberite structure in the three space groups must be regarded as practically indistinguishable. In the absence of a proof of achirality Bystroms space group assignment is provisionally accepted as valid, and Na 2 MgAlF 7 is considered as isostructural with the recently refined Na 2 NiFeF 7 .

Vibrational spectra of the ethylenediammonium ion in crystals

Abstract The IR and Raman spectra of three isotopic varieties of the ethylenediammonium ion in the tetrachlorocadmate(II) have been measured. It is shown that the ion assumes the centrosymmetric trans conformation in this salt. A complete vibrational assignment has been proposed for the observed fundamentals. Certain assignments for related compounds in the literature are modified. The spectra of the ion in the hexachloroplatinate(IV) and hexachlorostannate(IV) have also been obtained; compared to these two compounds hydrogen bonding in the tetrachlorocadmate is appreciable. Combination bands observed in the 2100-1800 cm −1 region of the IR and Raman spectra of compounds containing the C-NH 3 + grouping are discussed and assignments are suggested for those observed in ethylenediammonium salts.

Water in Stoichiometric Hydrates

This chapter is concerned with information obtainable from crystallographic and spectroscopic studies of the water molecule in stoichiometric hydrates.† Water molecules in a hydrate crystal interact with their surroundings through hydrogen bonding and cation-water coordination. We shall deal with these interactions and their consequences.

Infrared spectra of the ammonium ion in crystals: Part IV. Motions of the NH3D+ ion in ammonium perchlorate and in other weakly hydrogen-bonded ammonium compounds

Abstract Molecular motion of the ammonium ion has been investigated by IR spectroscopy in a series of ammonium salts which have been reported to be weakly hydrogen-bonded. Infrared spectra of the Isotopically isolated NH 3 D + ion in NH 4 ClO 4 , and NH 4 BF 4 have been recorded between 295 and 22 K. Broadening of the N-D stretching bands, in each case, is attributed to short residence time and indicates rapid reorientation of the NH 3 D + ion. The band broadening persists down to the lowest temperature. The spectra of the NH 3 D + ion in NH 4 BPh 4 , NH 4 SO 3 F, NH 4 SO 3 Me and NH 4 PF 6 have been recorded between 295 and 90 K. No significant lifetime broadening has been found in the spectra of the NH 3 D + ion in these salts.

The infrared spectrum and force field of the methyl-ammonium ion in (CH3NH3)2PtCl6

Abstract Infrared spectra of the CH3NH3+, CH3ND3+, CD3NH3+ and CD3ND3+ ions in bis(methylammonium)hexachloroplatinate(IV) have been recorded. The spectra are entirely consistent with the C3v symmetry reported for the methylammonium ion, at temperatures between room temperature and 90 K. No spectral manifestations of the phase transition, which in (CH3NH3)2PtCl6 has been reported to take place at 125 K, were observed. Assignments of the infrared-active fundamentals have been made for each ion and a normal-coordinate analysis has been performed using the observed fundamental frequencies. Comparison with the infrared spectra of other methylammonium salts shows that hydrogen bonding in (CH3NH3)2PtCl6, if present, is weak.

Chalkogenides of the transition elements. X. X-ray, neutron, Mössbauer, and magnetic studies of pentlandite and the π phases π(Fe, Co, Ni, S), Co8MS8, and Fe4Ni4MS8 (M = Ru, Rh, Pd)☆

The metal atoms in the cubic π phases (Fe, Co, Ni, M)9S8 occupy 4-coordinated majority and 6-coordinated minority sites in the ratio 8:1. X-ray and neutron powder diffraction of Co8MS8 and Fe4Ni4MS8 (M = Fe, Co, Ni, Ru, Rh, Pd) has shown that the structures are ordered, i.e., with M segregated in the minority sites, when M is a 4d element, and at best partially ordered when M is a 3d element. Magnetic susceptibility measurements and Mossbauer 57Fe spectra show that Co9S8 and the natural π(Fe, Ni, S) phase, pentlandite, remain Pauli-paramagnetic down to 4.2°K, with no resultant magnetic moments on the metal atoms, and hence are broad-band metals. Linear dependence of the isomer shifts at the two types of sites in a variety of (Fe, Co, Ni)9S8 compositions indicates the existence of a composite s-d conduction band. Analysis of the variation of the quadrupole splitting at the majority site with composition leads to the conclusion that only a few levels at the bottom of the conduction band are occupied. The number of electrons in the band is approximately proportional to the total d-electron content of the unit cell. A tentative band scheme based on these results and on other available evidence is proposed. The effect of composition on the interatomic distances in the π phases is discussed.

Infrared spectra of the ammonium ion in crystals: Part V. Tutton's salts: a correlation of vibrational frequency and hydrogen-bonded N⋯O distance for the ammonium ion

Abstract The infrared spectra of the isotopically isolated NH 3 D + and HDO species have been examined in seven ammonium Tutton salts. The observed spectra are in good agreement with predictions based on the known crystallographic features of these salts. Linear regression of the ND stretching frequencies v 1 (NH 3 D + ) of the isotopically isolated NH 3 D + ion on hydrogen-bonded distance d (N ⋯ O) indicated the existence of a correlation ; subsequent fitting of the data to a more plausible empirical function v 1 (NH 3 D + ) = v 1,∞ ,- k 1 exp(- k 2 , d ) resulted in a coefficient of determination of 0.94 and a standard deviation of 10 cm −1 for the goodness of fit. The structural differences caused by the distortion of the metal coordination octahedron in the copper(II) Tutton salts are discussed. For this purpose the spectra of isotopically dilute HDO in the salts M 2 i [Cu(H 2 O) 6 ](SO 4 ) 2 (M i = K, Rb, Cs) have also been measured. No evidence of phase transformations between room and liquid-nitrogen temperatures was detected in the spectra of any of the saltri studied in this work.

The infrared spectrum of the methylammonium ion in bis(methylammonium)tetrachlorocadmate(II)

Abstract The IR spectrum of polycrystalline (CH 3 NH 3 ) 2 CdCl 4 has been recorded at temperatures between 300 and 90 K. The phase transition reported to take place at 173 K was observed and appears to involve a significant change in hydrogen-bond strength. Factor-goup splitting, affecting both parallel and perpendicular vibrational modes, was found to occur in the low-temperature spectra; its presence was demonstrated by the use of isotopically dilute CD 3 NH 3 + ions. The formally IR-active torsional mode was not observed.

Bond length and the electron density at the bond critical point: X--X, Z--Z, and C--Z bonds (X = Li-F, Z = Na-Cl).

The aim was to investigate the relationship between the bond length and the electron density at the bond critical point in homonuclear XX and ZZ and heteronuclear CZ bonds (X = Li‐F, Z = Na‐Cl). The d,ρc pairs were obtained from 472 target bonds in DFT‐optimized (B3LYP/6‐311+G(d,p)) small molecular species. These species were selected arbitrarily but with a view to maximize the range widths WR for each atom combination. It was found that (i) with one clear exception, the d(A − A) means (A = X or Z) correlate linearly with the bond lengths d(A2) of the respective diatomic molecules; (ii) the d(A − A) means correlate parabolically with n, the formal number of valence electrons in the atoms of the bond; and (iii) with increasing sample size N the ratio WR(ρc)/WR(d) appears to converge toward a representation f [WR(ρc)/WR(d)]N→∞ characteristic of A. Detailed analysis of the d,ρc relationship has shown that by and large simple power regression accounts best for the DFT data. The regression coefficients of d = aρ  c−b and ρc = αd−β (b, β > 0) vary with n in a seemingly irregular manner but one that is consistent with simple chemical notions. The d(A2) can be approximated in terms of multilinear MO electron occupancies.