Infrared Studies on Structural Phase Transition in [N(CH3)4]3Bi2Br9

Abstract

FT-IR (4000 - 400 cm^?1) spectroscopy was used for the study of the phase transition at 183 K in [N(CH₃]₄]₃Bi₂Br₉. The changes in the spectra confirmed the presence of the phase transition of first order type at 183 K. The most spectacular changes in the vicinity of the phase transition are found for the stretching CH3 and skeletal NC4 modes. The mechanism of the phase transition of the order-disorder type is believed to be connected with the reorientation motions of the tetramethylammonium cation.     

NMR determination of dynamic parameters of CH3 groups in P(CH3)4SbCl6

The longitudinal relaxation time T1 and the second moment M2 of 1H NMR line in a wide temperature range have been measured for P(CH3)4SbCl6. It was found that two different methyl groups in each tetramethylphosphonium cation perform two different rates of C3 motions. The reduction of the proton second moment M2 just below the temperature of the phase transition Tc2 = 350 K may suggest that the isotropic tumbling of the whole cation [P(CH3)4]+ is involved in the structural change of the crystal lattice induced by the movements of the [SbCl6]- anion.     

Study of molecular dynamics and cross relaxation in tetramethylammonium hexafluorophosphate (CH(3))(4)NPF(6) by (1)H and (19)F NMR

(CH(3))(4)NPF(6) is studied by NMR measurements to understand the internal motions and cross relaxation mechanism between the heterogeneous nuclei. The spin lattice relaxation times (T(1)) are measured for (1)H and (19)F nuclei, at three (11.4, 16.1 and 21.34 MHz) Larmor frequencies in the temperature range 350-50K and (1)H NMR second moment measurements at 7 MHz in the temperature range 300-100K employing home made pulsed and wide-line NMR spectrometers. (1)H NMR results are attributed to the simultaneous reorientations of both methyl and tetramethylammonium groups and motional parameters are evaluated. (19)F NMR results are attributed to cross relaxation between proton and fluorine and motional parameters for the PF(6) group reorientation are evaluated.     

Proton NMR T 1 Studies in Methylammonium TrichlorO Stannate(II) (CH 3 NH 3 SnCl 3 )

Proton spin lattice relaxation time ( T 1 ) measurements have been carried out in methylammonium trichloro stannate(II) (CH 3 NH 3 SnCl 3 ) as a function of temperature in the range 317-5 K at a Larmor frequency of 10 MHz. The temperature dependence of T 1 shows a phase transition around 220 K and four T 1 minima (294 K, 62 K, 32 K and 12 K). The results are discussed in terms of proton dynamics, namely, uncorrelated reorientation of NH 3 and CH 3 groups at high temperatures and tunnelling of NH 3 and CH 3 protons at low temperatures.     

PMR studies of molecular motions,phase transitions and quantum tunnelling in NH4SnCl3 and N(CH3)4SnCl3

Molecular reorientation and low temperature relaxation effects of NH⁺ ₄ ion and the effect of CH₃ substitution (in place of H) are investigated by proton spin lattice relaxation time (T₁) measurements at 10 MHz in NH₄SnCl₃ and N(CH₃)₄SnCl₃ in the temperature range 4.2 K upto the melting points of the compounds (? 440 K). Phase transitions around 360 K in NH₄SnCl₃ and around 361 and 116K in N(CH₃)₄SnCl₃ have been observed. In NH₄SnCl₃, the high temperature minimum at 330.5 K is attributed to the translational diffusion of the NH⁺ ₄ ions, while the other T₁, minima at 103.5, 60 and 50 K are ascribed to the reorientations of the NH⁺ ₄ ion about the C₂ and C₃ axes. The low temperature minimum at 13.5 K is attributed to rotational tunnelling of the NH⁺ ₄ ions. In N(CH₃)₄SnCl₃, in addition to the high temperature minima at 212.2 and 182.6 K due to N(CH₃)₄ tumbling and CH₃ reorientation, a temperature independent T₁ behaviour between 83 and 31 K is observed, below which T₁ decreases and tends to go through a minimum around 5 K. This low temperature minimum is attributed to rotational tunnelling of the CH₃ groups. The motional parameters and tunnel frequencies are estimated.     

RRKM理论研究N(4S)+CH2X(X=F,Cl)的反应通道

采用RRKM理论和疏松过渡态模型计算了N(4S) +CH2 X(X =F ,Cl)反应的微正则速率常数和通道分支比 .计算结果表明 ,在较低的内能下 (E =2 80 .2 9kJ/mol) ,N(4S) +CH2 F的主要产物为NCHF +H ,占总产物的5 9.2 % ,次要产物为H2 CN +F ,占 37.4 % .而N(4S) +CH2 Cl反应在E =2 6 7.78kJ/mol时 ,主要产物是H2 CN +Cl,占 90 .3% ,NCHCl+H只占 9.0 % .在内能较高的时候 (取E =5 0 0 .0 0kJ/mol) ,N(4S) +CH2 F的主要通道并未变化 ,而N(4S) +CH2 Cl的主要通道变为NCHCl+H ,比例为 5 1.5 % ,H2 CN +Cl的比例降到 4 0 .4 % .     

Electrical properties and conduction mechanism of [N(C2H5)4][N(CH3)4]CuCl4 compound

The [N(CH₃)₄][N(C₂H₅)₄]CuCl₄ single crystal has been synthetized in order to determinate the temperatures transition and to study the electrical properties and the conduction mechanism. At room temperature, this compound crystallizes in the tetragonal system with P-42₁m space group. The calorimetric study shows three anomalies at 248, 284 and 326 K. Electrical conduction and dielectrical relaxation mechanisms at various frequencies and temperatures were analyzed by impedance spectroscopy and the equivalent circuit based on the Z-View-software was proposed. The variation of f_p relaxation determinate by the modulus study and σ_dc specific to the AC conductivity as a function of temperature and confirm the all transitions for our sample. The values of the activation energy are determined and compared by those, which are found in the similar compound. Frequencies dependence of alternative current (AC) conductivity is interpreted in terms of Jonscher's law and the conduction mechanisms for each phase are determined with the Elliot's theory.     

Dielectric relaxation,modulus behavior and thermodynamic properties in [N(CH3)3H]2ZnCl4

[N(CH₃)₃H]₂ZnCl₄ has been analyzed by X-ray powder diffraction patterns, differential scanning calorimetry and impedance spectroscopy. The [N(CH₃)₃H]₂ZnCl₄ hybrid compound is obtained by slow evaporation at room temperature and found to crystallize in the orthorhombic system with Pnma space group. Five-phase transitions at low temperature were detected by differential scanning calorimetry measurements. The analysis of Nyquist plots has revealed the contribution of three electrically active regions corresponding to the bulk mechanism, distribution of grain boundaries and electrode processes. The dielectric relaxation is described by a non-Debye model. The study of the dielectric constants ?′, ?″ and loss tangent tan (δ) with frequency exhibits a distribution of relaxation times. The complex modulus plots have confirmed the presence of grains and grain boundaries as well as a non-Debye type of relaxation in the material. Thermodynamic parameters such as the free energy for dipole relaxation ΔF, the enthalpy ΔH and the change in entropy ΔS have been determined with the help of the Eyring theory.     

CH3CH2+N(4S)反应机理的理论研究

利用abinitio方法对CH3CH2+N(4S)反应进行了理论研究,在MP2/6-311+G(d,p)水平上优化得到了反应途径上的反应物、中间体、过渡态和产物的几何构型和谐振频率,并在QCISD(T)/6-311+G(d,p)水平上进行单点能计算.计算结果表明,CH2CH2+3NH和H2CN+CH3是此反应主要产物,CH3CHN+H是此反应次要产物.产物CH2CH2+3NH主要来自直接氢抽提反应通道,H2CN+CH3来自加成-解离反应通道,CH3CHN+H来自加成-解离反应通道.     

Classical and quantum molecular dynamics of cation in (CH3NH3)3 Sb2Br9 polycrystal as studied by 1H NMR.

¹H spin-lattice relaxation times and second moments were determined for polycrystalline (CH₃NH₃)₃Sb₂Br₉ sample in a wide range of temperature (5–200 K) at 24.6 and 55.2 MHz. ²H NMR spectra of (CD₃NH₃)₃Sb₂Br₉ were recorded between 5 K and room temperature. The relaxation time is interpreted as a result of motion of two different non-equivalent types of monomethylammonium cations occurring at the 2:1 proportion in a unit cell. Below 30 K, the relaxation processes via tunneling are suggested to dominate. Above 30 K, only classical behaviour of methylammonium cations is detected. Two monomethylammonium cations relax with the classical correlated C₃ reorientation and the rotational tunnelling mechanism, while the third cation exhibits only the classical correlated reorientation. The dynamic parameters of these motions have been determined.     

(C5H5NH)2MnxZn1-xCl4和((CH3)4N)2MnxZn1-xCl4中Mn2+的电子顺磁共振

对于发绿光的四面体化合物(C5H5NH)2MnCl4和((CH3)4N)2MnCl4人们已做了广泛的研究。但是它们的电子顺磁共振谱仅表现为一些很宽的带而没有精细和超精细结构。在同晶型的(C5H5NH)2MnxZn1-xCl4或((CH3)4N)2MnxZn1-xCl4中掺入低浓度的Mn2+(x=1%),我们可以由(g～值的)角度变化导出自旋哈密顿量的全组参数来。这种四面体的晶格发生了严重的畸变且(在所有的情况下)产生一种C1-对称。     

The thermoelastic properties of [N(CH3)4]2ZnBr4 and [N(CH3)4]2MnBr4

The ferrodistortive phase transition in the bis-tetramethylammonium tetrabromide crystals below room temperature is studied within the framework of the Landau theory. The specific heats of [N(CH₃)₄]₂MnBr₄ and [N(CH₃)₄]₂ZnBr₄ are correctly described down to 40°C below the transition temperature. The phenomenological parameters are determined from calorimetric results, elastic constants and thermal expansion data. Using these coefficients, the monoclinic angle in the ferrodistortive phases is obtained. The anharmonic quantities, such as the isothermal compressibility, calculated from the specific heat data, are in good agreement with the values derived from the elastic measurements.     

High-Temperature Phase Transition in N(CH3)4CdCl3 Studied Using Static NMR and MAS NMR

To clarify the nature of microscopic structural changes of N(CH₃)₄CdCl₃ at high temperatures, the nuclear magnetic resonance (NMR) spectra of the protons and carbons in N(CH₃)₄CdCl₃ were measured. NMR studies of the ¹H and ¹³C spin–lattice relaxation time, T _1ρ, in the rotating frame were also performed. No changes in the T _1ρ of ¹H and ¹³C associated with the N(CH₃)₄ groups were observed at the high-temperature transition from phase I to phase I′. However, the ¹⁴N NMR spectra reflected changes in the structural geometry during the transition to phase I′, indicating that this transition is driven by N(CH₃)₄.     

14N NMR study and Landau theory of phase transitions in [N(CH3)4]2ZnI4

The¹⁴N NMR spectra and spin-lattice relaxation timeT ₁ of [N(CH₃)₄]₂ZnI₄ have been studied between room temperature and 200 K. Two phase transitions atT _c 1=255 K and atT _c 1=217 K are observed. The¹⁴N NMR lineshape andT ₁ data suggest that the intermediate phase is commensurate rather than incommensurate in spite of the presence of a Lifshitz invariant in the expansion of the free energy density in powers of the order parameter. We also discuss the phenomenological theory of structural phase transitions in [N(CH₃)₄]₂ZnI₄.     

CH_4/N_2混合气体在CMS上的低温吸附分离实验研究

将煤层气液化后运往终端市场是一种十分有效的利用方式。与常规天然气不同,抽放煤层气含氮量高,采用有效的方式实现CH4/N2分离是煤层气液化流程中重要的环节。吸附分离是提高煤层气中甲烷浓度的可行方法。为了与低温液化过程紧密结合,探讨在低温下实现CH4/N2吸附分离的可行性,文中对CH4/N2在碳分子筛(CMS)上的低温分离特性进行了实验研究,并与常温下的实验结果进行了对比。实验结果表明,低温下CH4/N2吸附分离的特性与常温下有明显差别。     

Torsional Splitting in the nu(5) Fundamental Infrared Band of CH(3)CD(3) and (13)CH(3)CD(3)

The nu(5) fundamental (C-C stretching) of CH(3)CD(3) shows a resolved torsional structure, caused by perturbations due mainly to the linear dependence of the torsional potential barrier on the normal coordinate Q(5). We were able to analyze this structure and to assign vibration-rotation transition wavenumbers for all five torsional components, classified according to the symmetry species of the G(18)((3)) extended molecular group. The torsional splitting pattern is qualitatively similar to that of a nondegenerate vibrational state with an even number of excited torsional quanta v(6). Explorative calculations show that the main perturber system should consist of the torsional components of the vibrational ground state correlating with v(6)=4 in the high barrier limit. The strength of the perturbation on the E(r0) torsional components of nu(5) increases rapidly with r, the E(40) component being the most affected. The observed transition wavenumbers can be reasonably fitted by a simplified model containing independent effective vibration-rotation parameters for the five different torsional components of nu(5), for both CH(3)CD(3) and (13)CH(3)CD(3). The trend of the determined values of the effective vibrational wavenumbers and rotational parameters over the torsional components supports the proposed vibration-torsion interaction mechanism, responsible for the observed torsional splittings. A strong anomaly observed in the rotational intensity distribution of nu(5) is discussed. Copyright 2001 Academic Press.     

Analysis of incommensurate structure in [N(CH3)4]2ZnCL4 crystal

We analyzed the temperature dependence of the amplitude of modulated structures of [N(CH₃)₄]₂ZnCl₄ crystal in the incommensurate (INC) phase by measuring the EPR spectra for Mn²⁺ doped single crystal. The amplitude of the vibrational modulation linearly increased with decreasing temperature. Applying Frank and Van der Merwe model, we simulated the EPR spectra in the INC phase. The simulated spectra agreed with the observed one very well. We, therefore, revealed that the vibrational modulation in this INC phase is due to the interaction between the harmonic chain of inter-particles and the modulation due to underlying potential which comes from the commensurate structure.     

Optical and electrical conduction mechanisms of [N(CH3)3H]CdCl3

The X-ray powder diffraction patterns shows that at room temperature [N(CH₃)₃H]CdCl₃ crystallizes in the orthorhombic system with the Pbnm space group. The analysis of the data revealed the existence of optical allowed direct transition mechanisms with the band gap energy equal to 5.3 eV. The temperature dependences of the real part of dielectric permittivity show a relaxation process at high temperature that can be explained by the reorientational motion of alkyl chains. The alternative current (AC) electrical conduction in compound is governed by three processes, which can be attributed to several models: the correlated barrier hopping (CBH) model in phases I and II, the non-overlapping small polaron tunneling (NSPT) model in phases III and IV.     

团簇分子[Zn-(CH_3CH_2OH)_n]~(2+)的荧光光谱研究及理论光谱验证

通过荧光光谱实验和理论计算对金属离子在乙醇溶液中的微团簇结构进行了研究。荧光光谱实验发现在275~330 nm范围存在较强的乙醇分子荧光峰,当加入盐离子(Zn~(2+))后该处荧光强度逐渐变弱,说明盐的加入对乙醇体系的荧光效率产生了影响,破坏了原有乙醇分子之间的作用,使得其刚性结构发生改变,荧光效率降低,同时Zn~(2+)与乙醇分子通过溶剂化作用形成了新的微团簇,在350~380 nm之间出现了新峰。通过对团簇结构[Zn-(H_2O)_n]~(2+)(n=1~3)进行优化比较,得到了相对精确及运算成本较低的密度泛函理论B3LYP方法,并应用于Zn~(2+)在乙醇溶液中团簇结构计算。结果表明体系的第一溶剂化层存在着n=1~6的微团簇分子,且最多为6。比较理论计算荧光光谱与实验荧光光谱,进一步说明了Zn~(2+)与乙醇形成了新的微团簇及计算方法的可行性。