Raman spectra and phase transitions in single crystal (NH4)2ZnCl4

Raman spectra of single crystal (NH₄)₂ZnCl₄ at 300 and 105K at different polarizations are reported and analysed in view of previously reported structural data. The experimental observations suggest that the free oriented gas model is applicable for group theoretical analysis of the vibrational modes in this system at room temperature. It is also concluded that the space group for the room temperature phase III is C²_s instead of C⁹_2v. The temperature dependence of thermosensitive bands shows discontinuities at 267 and 194 K. The structure in the new phase VI below 194 K is suggested to be triclinic.     

Simulated annealing and density functional theory calculations of structural and energetic properties of the ammonium chloride clusters (NH4Cl)n, (NH4+)(NH4Cl)n, and (Cl-)(NH4Cl)n, n = 1-13

Simulated annealing Monte Carlo conformer searches using the "mag-walking" algorithm are employed to locate the global minima of molecular clusters of ammonium chloride of the types (NH(4)Cl)(n), (NH(4)(+))(NH(4)Cl)(n), and (Cl(-))(NH(4)Cl)(n) with n = 1-13. The M06-2X density functional theory method is used to refine and predict the structures, energies, and thermodynamic properties of the neutral, cation, and anion clusters. For selected small clusters, the resulting structures are compared to those obtained from a variety of models and basis sets, including RI-MP2 and B3LYP calculations. M06-2X calculations predict enhanced stability of the (NH(4)(+))(NH(4)Cl)(n) clusters when n = 3, 6, 8, and 13. This prediction corresponds favorably to anomalies previously observed in thermospray mass spectroscopy experiments. The (NH(4)Cl)(n) clusters show alternations in stability between even and odd values of n. Clusters of the type (Cl(-))(NH(4)Cl)(n) display a magic number distribution different from that of the cation clusters, with enhanced stability predicted for n = 2, 6, and 11. None of the observed cluster structures resemble the room-temperature CsCl structure of NH(4)Cl(s), which is consistent with previous work. Numerous clusters have structures reminiscent of the higher-temperature, rock-salt phase of the solid ammonium halides.     

The solid state structure of [b(10)h(11)](-) and its dynamic NMR spectra in solution

The structure of [PPh(3)(benzyl)][B(10)H(11)] was determined at -123 degrees C and 24 degrees C by single-crystal X-ray analyses. The B(10) core of [B(10)H(11)](-) is similar in shape to that of [B(10)H(10)](2)(-). The 11th H atom asymmetrically caps a polar face of the cluster and shows no tendency for disorder in the solid state. Variable temperature multinuclear NMR studies shed light on the dynamic nature of [B(10)H(11)](-) in solution. In addition to the fluxionality of the cluster H atoms, the boron cage is fluxional at moderate temperatures, in contrast to [B(10)H(10)](2)(-). Multiple exchange processes are believed to take place as a function of temperature. Results of ab initio calculations are presented. Crystal data: [PPh(3)(benzyl)][B(10)H(11)] at -123 degrees C, P2(1)/c, a = 9.988(2) A, b = 18.860(2) A, c = 15.072(2) A, beta = 107.916(8) degrees, V = 2701.5(7) A(3), Z = 4; [PPh(3)(benzyl)][B(10)H(11)] at 24 degrees C, P2(1)/c, a = 10.067(5) A, b = 19.009(9) A, c = 15.247(7) A, beta = 107.952(9) degrees, V = 2775(2) A(3), Z = 4.     

Raman frequency shifts of an internal mode near the tricritical and second order phase transitions in NH4Cl

This study gives our analysis for the frequency shifts of the v2 (1708 cm-1) Raman mode in NH4Cl close to its tricritical (P=1.6 kbar) and second order (P=2.8 kbar) phase transitions. From our analysis, we extract the values of the critical exponent which describes the critical behavior of the Raman frequency shifts for this internal mode for the pressure conditions studied in NH4Cl. Our exponent value of alpha approximately 0.2 for the tricritical phase transition is close to the values of 1/16 (TTc) for the specific heat, predicted from a 3D Ising model. Our exponent values for the second order phase transition (P=2.8 kbar) for TTc are comparable with those reported in earlier studies.     

The NMR study of hydrogen bond formation in some tris(((-salicylidene)amino)ethyl)amine derivatives in solution and in the solid state

Eight Schiff bases, derived from tris(2-aminoethyl)amine and some aromatic aldehydes have been investigated by heteronuclear NMR methods in solution and in the solid state. Six of those compounds can form intramolecular hydrogen bonds and two remaining imines which do not have hydroxyl substituent in position 2 cannot form such bonds and were chosen as a model compounds for nonbonded structure. The tautomeric equilibrium position of investigated compounds were estimated on the base of nitrogen chemical shifts and nitrogen-proton one bond coupling constants when available. In solution for each compound in all temperatures applied, single symmetrical dynamic averaged structures were found but in the solid state in a few cases mixtures of several nonequivalent or nonsymmetrical structures were observed. Generally at low temperature the –NH– form is more abundant then at room temperature. Similarly in the solid state the proton transfer from oxygen to nitrogen site is more effective in comparison with the solution, except for the 9-formyl-8-hydroxyjulolidine derivative for which different behavior was found.     

A dihydrogen bond between a bridging hydride and the NH proton of a coordinated dimethylamine: solid state, solution and theoretical characterization

The addition of one equivalent of dimethylamine (DMA) to the 44 valence-electron triangular cluster anion [Re₃(μ₃-H)(μ-H)₃(CO)₉]⁻ (1) affords the novel unsaturated derivative [Re₃(μ-H)₄(CO)₉(DMA)]⁻ (2, 46 valence electrons) which contains a dimethylamine molecule terminally coordinated to a cluster vertex. Theoretical calculations (DFT) reveal that in the more stable conformation the dimethylamine NH proton is directed towards the hydride bridging the opposite cluster edge in syn position, the close proximity of the ligands bound to the cluster surface allowing the formation of an unconventional N-H ? (μ-H)Re₂ hydrogen bond. The presence of this conformation in the solid state has been proven by an X-ray structural analysis of crystalline [PPh₄]2. Spectroscopic evidences (IR and NMR) indicate that the dihydrogen bond is maintained also in solution and, by the evaluation of the proton spin-lattice relaxation rates at variable temperature, a good estimate of the H ? H distance in solution has been determined.     

High resolution electronic spectra of anisole and anisole-water in the gas phase: hydrogen bond switching in the S1 state

Rotationally resolved S(1)<--S(0) electronic spectra of anisole and its hydrogen bonded complex containing one water molecule have been obtained. The results provide evidence for an "in-plane" complex in which the water molecule is attached via two hydrogen bonds to the anisole molecule, a donor O-H- - -O(CH(3)) bond and an acceptor H-O- - -H(ring) bond. Analysis of the subbands that appear in the spectrum of the complex suggests that hydrogen bond "switching" occurs when the complex absorbs light. The former O-H- - -O(CH(3)) bond is stronger in the ground (S(0)) state, whereas the latter H-O- - -H(ring) bond is stronger in the excited (S(1)) state. Dynamical consequences of this phenomenon are discussed.     

Infrared, Raman, and inelastic neutron scattering spectra of dodecahedrane: an I(h) molecule in T(h) site symmetry

The Raman spectrum of crystalline dodecahedrane, C20H20, a species of nominal I(h) symmetry, exhibits splitting of the H(g) Raman active modes. The Raman inactive gerade vibrations of G(g), T(1g), and T(2g) symmetry are found to have weak Raman activity. The IR forbidden vibrations of T(2u), G(u), and H(u) type have moderate IR activity. All of this is consistent with the T(h) site symmetry. A treatment of the structure and vibrations of dodecahedrane using a periodic lattice DFT method results in a slightly distorted T(h) structure with six C-C bonds that are 0.001 A longer than the other 24. The vibrational spectrum computed for this structure exhibits splittings of the H(g) modes that are consistent with the observed spectra, but the computed splittings are larger than observed in room-temperature data. A complex pattern observed in the C-H stretching region is assigned. The inelastic neutron scattering spectrum calculated from the computed normal modes for the T(h) molecule in the lattice agrees quantitatively with experiment when overtone and combination transitions are included and allowance is made for anharmonicity of the C-H stretch motion. Finally, it is argued that the existing crystallographic determination of the average C-C bond length of 1.544 A is shortened by disorder and should be revised upward to agree with the computed value of 1.558 A.     

Raman spectra and the dynamic structure of the high pressure phase of NH4Br (V) and ND4Br (V)

Similar Raman spectra are observed at high pressures for phases II and V of ND₄Br and NH₄Br. Deuteration lowers the II–V phase transition from 20 to 9 kbar at 296 K. ND₄Br V and NH₄Br V are interpreted as mixed phases, and their spectra as the superposition of the spectra of two other phases, III (antiparallel arrangement of the NH₄⁺ ions) and IV (parallel arrangement). The phonons which become Raman inactive at the V–IV phase transition are assigned to clusters or domains of phase V which have antiparallel arrangement.     

Vibrational spectra of alpha-amino acids in the zwitterionic state in aqueous solution and the solid state: DFT calculations and the influence of hydrogen bonding

The zwitterionic forms of the two simplest alpha-amino acids, glycine and l-alanine, in aqueous solution and the solid state have been modeled by DFT calculations. Calculations of the structures in the solid state, using PW91 or PBE functionals, are in good agreement with the reported crystal structures, and the vibrational spectra computed at the optimized geometries provide a good fit to the observed IR and Raman spectra in the solid state. DFT calculations of the structures and vibrational spectra of the zwitterions in aqueous solution at the B3-LYP/cc-pVDZ level were found to require both explicit and implicit solvation models. Explicit solvation was modeled by inclusion of five hydrogen-bonded water molecules attached to each of the five possible hydrogen-bonding sites in the zwitterion and the integration equation formalism polarizable continuum model (IEF-PCM) was employed, providing a satisfactory fit to observed IR and Raman spectra. Band assignments are reported in terms of potential-energy distributions, which differ in some respects to those previously reported for glycine and l-alanine.     

One- and three-dimensional infinite arrays of Cu(I) ions exhibited by [Cu(NH3)2]Br and [Cu(NH3)Cl] in the solid state

In the solid state, [Cu(NH3)Cl] forms a three-dimensional network with each Cu(I) ion being surrounded by three other Cu(I) centres in a trigonal-planar fashion [Cu...Cu = 2.979(1) A; cubic space group I2(1)3], whereas in [Cu(NH3)2]Br the cations establish infinite linear (Cu...Cu) chains spanning the crystal lattice [Cu...Cu = 2.931(1) A; monoclinic space group C2/c].     

The vibrational spectra of diiodohalogenomercurate(II) anions in the solid state

Diiodobromo and diiodochloromercurate(II) anions as salts of tetrapropylammonium (pr₄N) have been prepared, characterized and their far i.r. and Raman spectra measured at room and liquid nitrogen temperature. The bands observed in the stretching frequency region indicate strong halide-bridged interactions in the solid state and hence the compounds are better formulated as (pr₄N)₂ Hg₂I₄X₂ with X (X = Cl or Br) as the bridging ligands. Group vibrations are tentatively assigned to type I distorted structure.     

IR and NMR spectra,intramolecular hydrogen bonding and conformations of para-tert-butyl-aminothiacalix[4]arene in solid state and chloroform solution

It is demonstrated that dissolution of aminothiacalix[4]arene in chloroform results in transformation of 1,3-alternate conformation, adopted in single-crystal and bulk polycrystalline solids, to the pinched-cone form. This conformer is stabilised by the intramolecular hydrogen bonds of two distal amino-groups acting as H-donors with another two amino moieties that appear as H-acceptors. The H-bonds cause quite small (ca. 10–20 cm^?1) red shift of the IR bands of the NH₂ stretching vibrations, which suggests rather weak NH?N hydrogen bonding. This latter is sufficient to stabilize the pinched-cone conformation in the chloroform solution, but the energy gap between the pinched-cone and other conformations is small, and solid-state intermolecular forces easily overcome it, leading to realisation of the 1,3-alternate conformer. The comparison of the DFT computed and experimental vibrational and NMR spectra demonstrates good quality of present quantum-chemical computations, allows complete interpretation of the spectra and reveals simple IR and NMR spectroscopic markers of the conformers of aminothiacalix[4]arenes.     

The tautomerism of 1H-pyrazole-3(5)-(N-tert-butyl)carboxamide in the solid state and in solution

The X-ray crystal structure of 1H-pyrazole-3-(N-tert-butyl)-carboxamide was determined. In the solid state, the 13C and 15N CP/MAS NMR spectra correspond to this tautomer. In solution, both tautomers are present in a ratio that depends on the temperature (at 293 K, 90% 3-substituted/10% 5-substituted). Some unusual 1H, 1H couplings involving the NH proton were observed. DFT (GIAO) calculations were carried out.     

Molecular structure of 2-(hydroxyimino)propanohydroxamic acid in solid state and DMSO solution

The 1H and 13C NMR spectra of 2-(hydroxyimino)propanohydroxamic acid (hpha) were measured in DMSO-d6 solution. The set of several monomeric structures along with the cluster of H-bonded hpha with three DMSO molecules were proposed to fit the experimental data. The calculated chemical shifts [B3LYP/6-311++G(d,p)] strongly suggested the formation of the cluster in which all the labile protons were H-bonded to the solvent molecules. The comparison between experimental and calculated Raman spectra of hpha in DMSO also suggested that in these conditions the investigated compound forms the proposed cluster rather than dimers. According to our calculations [B3LYP/6-31+G(d)] this cluster was energetically stabilized (84-106 kJ mol-1) compared to postulated dimeric structures. On the other hand, formation of dimers was proposed to be present for hpha in solid state. The comparison of the vibrational data (IR, RS) with the computed harmonic frequencies of three most probable dimers [B3LYP/6-31+G(d)] suggested that the dimer in which molecules adopted the zEe-keto form and were linked by two symmetric, almost linear H-bonds between the carbonyl oxygen atoms and the hydroxamic O-H protons was the predominant species of hpha in the solid state. Thus, the structures of hpha in solid state and DMSO solution appeared to be different.     

Vibrational spectra and natural bond orbital analysis of the herbicidal molecule 2(4-chlorophenoxy)-2-methyl propionic acid

The herbicide 2(4-chlorophenoxy)-2-methyl propionic acid (MCPP) has been subjected to NIR FT-Raman and infrared spectral studies. The optimized molecular structure, vibrational wavenumbers, IR intensities and Raman activities have been calculated by using density functional method (B3LYP) with the standard 6-31G(d) basis set. The calculated molecular geometry has been compared with the XRD data. The detailed assignments of the normal modes have been performed based on the potential energy distribution (PED) following the scaled quantum mechanical force field (SQMFF) methodology. The IR and Raman spectra have been plotted for the calculated wavenumbers. The simulated spectra satisfactorily coincide with the experimental spectra. The strong hyperconjugative interaction and charge delocalization that leads to the stability of the molecule have been investigated with the aid of natural bond orbital (NBO) analysis.     

Pre-steady state reactivity of 5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin iron(III) chloride with hydrogen peroxide

A stopped-flow study has shown that tetrakis(pentafluoro-phenyl)porphyrin iron(III) chloride reacts rapidly (<3 ms) with hydrogen peroxide to form a Fe(III)-H(2)O(2) complex where log K = 2.39. This subsequently undergoes rapid intramolecular conversion (k = 4.4 s(-1)) to an iron(IV) intermediate, which in turn reacts with hydrogen peroxide (k' = 54.3 M(-1) s(-1)) to reform the original Fe(III)-H(2)O(2) complex.     

Raman scattering study of the ferroelectric phase transition in tris-sarcosine calcium chloride (tscc)

We obtained Raman scattering spectra of normal and deuterated TSCC crystals and one TSCC_I-xB_x mixed crystal (c_I-x = Cl, B_x = Br). The soft-mode behaviour is studied in the temperature range from 80 K to the Curie point. The L and T component of the soft mode are overdamped near 100 K while the oblique component can be observed all over the temperature range we studied. For both normal TSCC and mixed TSCC_I-xB_x crystals the critical exponent β = $\text{1}\text{3}$ is in agreement with dielectric measurements and theoretical calculations.     

Vibrational spectra and dynamics of conformation and hydrogen bonding of N-methylacetamide. II. Dynamics of the NH...O Hydrogen bond and NH (ND) stretching band structure

The infrared and Raman spectra of crystalline CH₃CONHCH₃, CD₃CONHCH₃, CH₃CONHCD₃, CD₃CONHCD₃ and their N deuterated derivatives have been investigated at various temperatures in the 3400-2000 cm^1&#x0304; range. A new interpretation of the NH (ND) stretching subbands is given and is based on the assumption of a strong coupling between NH stretching and deformation modes (γ NH and τ CN). Suitable potential functions are derived from experimental data and they allow to calculate the infrared and Raman spectra in the stretching region. The calculated spectra as well as the calculated isotopic and temperature shifts are in good agreement with the observed ones. The potential surface of the bending modes in the excited state has only one rather flat central minimum corresponding to a planar geometry of the amide group which can be contrasted to the four-minimum potential surface of the fundamental state.