Benefits of near-infrared spectroscopy for characterization of selected organo-montmorillonites

The potential of near infrared (NIR) spectroscopy in characterization of organically modified clay minerals is introduced. Selected organo-clays, possibly perspective fillers in clay polymer nanocomposites, were prepared from Na-montmorillonite and different surfactants containing octylammonium chain(s), hexadecylammonium chain(s) or a benzene ring with or without a reactive double bond. Based on the stretching (ν) and bending (δ) vibrations observed in the middle IR (MIR) region, the first overtone (2νXH) and combination (ν + δ)XH modes of XH groups (X = O, C, N) are identified. The effect of larger alkylammonium cations on the vibrations of Si-O and OH bonds in montmorillonite layers is observed. The changes in the intensity of the (ν + δ)H₂O band near 5250 cm⁻¹ allows for comparison of the amount of water adsorbed on the montmorillonite surface. The water content decreases with the size of the organic cation reflecting increasing hydrophobicity of the montmorillonite surface. The NIR region shows the 2νCH₃ and 2νCH₂ bands in the 5900-5500 cm⁻¹ region, an upward shift is observed for the complex band due to 2νCH(Ar) of aromatic benzene ring. The NIR spectra are extremely useful in identification of NH₂⁺, NH⁺ and vinyl groups, which are difficult to recognize in the MIR spectra of organo-clays due to overlapping with other absorption bands. The intense bands corresponding to overtones and combination vibrations of NH₃⁺ and NH₂⁺ groups are found in the 6600-6050 cm⁻¹ and 5000-4600 cm⁻¹ regions, the (ν + δ)NH⁺ is unambiguously identified near 4750 cm⁻¹. The characteristic band assigned to 2νCH₂ in H₂CC is detected near 6130 cm⁻¹.     

Photofragment translational spectroscopy at 304 nm from C-H symmetric stretch excited CH3I [v 1 = 1]

The 304 nm photodissociation of the C-H symmetric stretch excited CH3I[v1=1,v2=0](v1 denotes the C-H symmetric stretch mode,and v2 denotes the umbrella mode)is studied with our simple photofragment translational spectrometer.An IR laser is used to excite the ground state CH3I[0,0]to the C-H symmetric stretch excited CH3I[1,0].With IR laser OFF and ON,the fractions of photofragments CH3(ν1,ν2)from the 304 nm photodissociation of CH3I[1,0]have been determined through the photofragment translational spectra(PTS)from measuring I and I*and also through the PTS from measuring CH3(0,0)(1,0)(0,1)and(1,1).The experimental results show that the C-H symmetric stretch vibration(v1=1)in parent molecules is about 66%retained in the photofragments in the I channel,but only 24%in the I*channel.The populations of photofragments CH3(0,2)and(0,3)are higher than CH3(0,0)and(0,1),showing strong inverted population both in I and I*channels.     

Spectral evidence of solid state interactions in mixed dimethyl sulfone-water ices

Infrared spectra of solid mixtures of dimethyl sulfone and ice were studied. A comparison of the spectra to those of the pure solids indicates bands attributable to two solid-solid interactions-a stronger and a weaker one—both centered on the OH?OS moieties. The bonded ν(OH) stretch assigned to the stronger interaction is part of the broad ‘3μ’ ice band and its wavenumber value is close to that reported previously for the 1:1 ((CH₃)₂SO₂)·H₂O) complex isolated in rare gas matrices. The weaker interaction is discerned at higher wavenumbers as a very low intensity feature, only slightly shifted from the free ν(OH) band positions.     

Photoacoustic study of CH4(ν3) deactivation by collisions with rare gases

CH₄(ν₃) deactivation is studied in the gas phase by the photoacoustic method at 300 K. Rapid vibration-to-vibration transfer holds the adjacent levels in a quasi-equilibrium distribution. The vibrational levels can then be grouped in two sets: (ν₂, ν₄) on the one hand and (ν₃, ν₁, 2ν₂, 2ν₄, ν₂ + ν₄) on the other. By successive dilution of CH₄ in He, Ne, Ar, we determined the vibration-to-translation-rotation rate constants characterizing the deactivation of each set. The vibration-to-vibration intermolecular rate constant which connects the two sets is also obtained.     

Spectra and structure of organophosphorus compounds—XXVIII. Vibrational and conformational analysis of dimethylmethoxyphosphine and some isotopically substituted derivatives

The i.r. spectra (3600-50 cm⁻¹) of the gaseous and solid states, and the Raman spectra (3600-10 cm⁻¹) of the gaseous, liquid and solid states of (CH₃)₂POCH₃, (CH₃)₂POCD₃, (CD₃)₂POCH₃ and (CD₃)₂POCD₃ have been recorded. Thirty-five of the 36 normal modes have been assigned and the asymmetric torsion has been observed as a broad weak feature centered at 106 cm⁻¹ in the far i.r. spectrum of gaseous (CH₃)₂POCH₃. A comparison of the vibrational spectra obtained for the fluid phases with those obtained for the amorphous and annealed solids indicates the existence of a second conformer present in a small amount in the gas phase but becoming more abundant in the liquid phase and the only remaining rotamer in the annealed solid. Asymmetric top i.r. band contour simulation provides evidence that the dominating species in the gas phase is one in which the methoxy methyl group is oriented approximately 60° (gauche) away from the lone pair of electrons on the phosphorus atom. The rotamer which remains in the spectra of the annealed solid has been assigned to a structure in which the lone pair on the phosphorus atom is oriented trans to the methoxy methyl group. These results are compared to similar data obtained for methoxy phosphoryl compounds and discussed in terms of both canonical molecular orbital and VSEPR thoery.     

Methylamine‐Gas‐Induced Defect‐Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells

We report herein the discovery of methylamine (CH₃NH₂) induced defect‐healing (MIDH) of CH₃NH₃PbI₃ perovskite thin films based on their ultrafast (seconds), reversible chemical reaction with CH₃NH₂ gas at room temperature. The key to this healing behavior is the formation and spreading of an intermediate CH₃NH₃PbI₃?xCH₃NH₂ liquid phase during this unusual perovskite–gas interaction. We demonstrate the versatility and scalability of the MIDH process, and show dramatic enhancement in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect‐free films of hybrid perovskites.     

Infrared and Raman spectroscopic investigations of methylammonium haloantimonates(III), [N(CH3)4−nHn]3 Sb2X9 (n = 0–3, X = Cl or Br), through their phase transitions

Infrared and Raman spectra of methylammonium chloroantimonates(III) of the general formula [N(CH₃)_4−nH_n]₃Sb₂Cl₉ with n = 0, 1, 2 and 3 and of the methylammonium bromoantimonate(III), (CH₃NH₃)₃Sb₂Br₉, have been investigated in detail across their phase transitions. Vibration modes due to CH₃ deformation and those involving the NH_n (n = 1, 2 and 3) groups show significant changes across the phase transitions of the chloroantimonates, besides those due to the Sb₂Cl₉ anion. The anion modes differ with the number of CH₃ groups since the nature of the anion itself changes, the free anion being present only in the quarternary compound. The high-temperature phases of the chloroantimonates seem to involve random orientations of the methylamino groups; a plastic-type state clearly manifests itself in the trimethyl and the tetramethylamino derivatives. A phase transition is predicted in all the chloroantimonates below 150 K, based on the changes in the anion bands at low temperatures. Changes in the IR and Raman spectra accompanying the phase transitions of (CH₃NH₃)₃Sb₂Br₉ and (CH₃NH₃)₃Bi₂Br₉ are similar and in the high-temperature phase of these compounds the (CH₃NH₃)⁺ cations ard oriented randomly.     

Ammonium oxofluorosilicates

The chemical and phase composition of the sublimates and nonvolatiles obtained as a result of the vaporization at 280°C of a mixture of silicon dioxide with (NH₄)₂SiF₆ at ratios of (0.25–10): 1 (mol/mol) is studied. Ammonium oxofluorosilicates of the general formula (NH₄)₂SiF₆ · nNH₄SiOF₃, where n = 1, 2, 4, or 8, are isolated from the gas phase and analyzed. These oxofluorosilicates have the structure of cubic (NH₄)₂SiF₆, which allows them to be treated as solid solutions. The sublimate components (NH₄)₂SiF₆ and NH₄SiOF₃ are in the ratio of 1: 2ⁿ, where n = 0, 1, 2, or 3. In the nonvolatile residue, the starting SiO₂ undergoes deep chemical destruction.     

Direct observation of the ν6 band in the IR spectrum of CF3I dissolved in liquid xenon

The IR spectrum of CF₃I dissolved in liquefied xenon at 185 K shows a very weak peak at 265.2 cm^?1 due to the ν₆ fundamental vibration. The separation, $\text{ν}$₃  $\text{ν}$₆, 19.3 cm^?1 is the same as that deduced from the microwave spectrum of CF₃I in the gas phase.     

A candidate for an ion pair in the vapor phase: Proton transfer in complexes R3N - HX

Large-scale ab initio calculations have been performed on the complexes NH₃·HCl, NH₃·HBr, CH₃NH₂·HCl and CH₃NH₂·HBr. Two-dimensional energy surfaces as a function of R_N---X and R_HX have been scanned in order to explore the possibility for the formation of stable vapor-phase ion-pair complexes. While the complexes NH₃·HCl, NH₃·HBr and CH₃NH₂·HCl are still of the neutral type, the ionic form CH₃NH₃⁺·Br⁺ is energetically slightly more favourable than the neutral-type complex. Further increase in base strength of the amine will result in stable ionic amine HX complexes in the vapor phase.     

Organic-inorganic hybrid perovskite (C<Subscript>6</Subscript>H<Subscript>5</Subscript>(CH<Subscript>2</Subscript>)<Subscript>2</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>CdCl<Subscript>4</Subscript>: Synthesis,structural and thermal properties

The present research work reports the study on crystal structure, vibrational spectroscopy and thermal analysis of organic-inorganic hybrid compound (C₆H₅(CH₂)₂NH₃)₂CdCl₄. Single crystals of bis(phenethylammonium)tetrachlorocadmate (C₆H₅(CH₂)₂NH₃)₂CdCl₄ (PEA–Cd) were obtained by diffusion at room temperature. This compound crystallizes in the orthorhombic space group C2cb with unit cell parameters a = 7.4444(2) Å, b = 38.8965(3) Å, c = 7.3737(2) Å and Z = 4. Single crystal structure has been solved and refined to R = 0.036 and wR = 0.092. The structure consists of an extended [CdCl₄]^2– network and two [C₆H₅(CH₂)₂NH₃]⁺ cations to form a two-dimensional perovskite system. The infrared (IR) spectrum of the title compound was recorded at room temperature. Differential scanning calorimetry (DSC) was used to investigate the phase transition; this compound exhibits a reversible single solid-solid phase transition.     

Influence of Orientational Disorder on the Optical Absorption Properties of the Hybrid Metal-Halide Perovskite CH3NH3PbI3

An experimental and theoretical investigation is reported to analyze the relation between the structural and absorption properties of CH₃NH₃PbI₃ in the tetragonal phase. More than 3000 geometry optimizations were performed to reveal the structural disorder and identify structures with the lowest energies. The electronic structure calculations provide an averaged band gap of 1.674 eV, which is in excellent agreement with the experimental value of about 1.6 eV. The simulations of the absorption spectrum for three representative structures with lowest energy reproduced the absorption shoulders observed in the experimental spectra. These shoulders are assigned to excitations having similar orbital characters and involving transitions between hybridized 6s(Pb)/5p(I) orbitals and 6p(Pb) orbitals. The geometries of the three structures were analyzed and the effects of the inorganic frame and the CH₃NH₃⁺ cations on the absorption properties were estimated. It was found that both changes in the inorganic frame and the CH₃NH₃⁺ cations orientations impact the absorption spectra, by modifying the transitions energies and intensities. This highlights the role of CH₃NH₃⁺ cation in influencing the absorption properties of CH₃NH₃PbI₃ and demonstrates that CH₃NH₃⁺ cation is one of the key elements explaining the broad and nearly constant absorption spectrum in the visible range.     

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

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 C_3v 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.     

Possible gas-phase ion pairs in amine-HCl complexes. An ab initio theoretical study

Ab initio MO calculations were performed for complexes between HCl and NH₃, CH₃NH₂, (CH₃)₂NH and (CH₃) ₃N. SCF geometry optimization for the latter three complexes gives double-minimum potential surfaces, which become single- minimum surfaces when electron correlation is considered. It is proposed that (CH₃)₃NHCl may be an ion pair in the gas phase.     

Acid–Base Chemistry at the Ice Surface: Reverse Correlation Between Intrinsic Basicity and Proton‐Transfer Efficiency to Ammonia and Methyl Amines

Proton transfer from the hydronium ion to NH₃, CH₃NH₂, and (CH₃)₂NH is examined at the surface of ice films at 60 K. The reactants and products are quantitatively monitored by the techniques of Cs⁺ reactive‐ion scattering and low‐energy sputtering. The proton‐transfer reactions at the ice surface proceed only to a limited extent. The proton‐transfer efficiency exhibits the order NH₃>(CH₃)NH₂=(CH₃)₂NH, which opposes the basicity order of the amines in the gas phase or aqueous solution. Thermochemical analysis suggests that the energetics of the proton‐transfer reaction is greatly altered at the ice surface from that in liquid water due to limited hydration. Water molecules constrained at the ice surface amplify the methyl substitution effect on the hydration efficiency of the amines and reverse the order of their proton‐accepting abilities.     

Laser induced fluorescence study of vibration-vibration equilibration in CH3Cl and CH3ClX mixtures

The infrared fluorescence risetimes of the ν₃ CCl stretching mode of CH₃Cl have been measured in CH₃Cl—raregas mixtures subsequent to laser pumping of the ν₆ methyl deformation vibration with a Q-switched CO₂ laser. The rate of rise of this fluorescence was found to be 80 ± 8 msec^?1/torr in pure CH₃Cl as reported earlier. The effect of rare gases on this process was found to be in reasonable agreement with SSH type theoretical calculations as well as similar trends in other VV processes.     

Wavelength and fluence dependences of the IR multiple photon dissociation of CH3NH2 to form ground state NH2 radicals

Ground state NH₂ radicals have been detected by laser excited fluorescence following the collisionless multiple photon dissociation of CH₃NH₂ by a pulsed CO₂ laser. The variation of dissociation yield with IR laser fluence and wavelength are reported for several CO₂ lines overlapping the CH₃NH₂ν₈ band. Dissociation is more efficient at wavelengths corresponding to initial absorption in the P branch of this transition than for excitation in the Q or R branches, with the difference in efficiencies being more pronounced at lower fluences. The mechanism of NH₂ production appears to be direct C-N bond fission.     

FTIR spectra of base-hydrogen fluoride hydrogen-bonded complexes in solid argon

Hydrogen fluoride and base molecules ranging from CO₂ to NH₃ have been codeposited at 12 K with excess argon. Fourier-transform infrared spectra of the samples before and after sample warming to allow reagent diffusion and association provide identification of 1:1 and 1:2 hydrogen-bonded complexes. Strong ν_s and ν₁ absorptions of the HF submolecule in the complexes vary with the proton affinity of the base and characterize the strength of the hydrogen-bonding interaction. Base submolecule modes show small displacements from the base molecule spectrum depending upon the structure of the complex.     

Reactions unimoleculaires des cations radicaux [CH3(CH2)n-1-NH2]+ en phase gazeuse

In the gas phase [CH₃-(CH₂)_n-1-NH₂]^+. radical cations isomerize prior to the dissociation. The reaction begins with a reversible transfer of one hydrogen in position 4, 5 or 6 to the nitrogen ; one of the C(2) hydrogens migrates to the radical site so formed to give [1-alkene, NH₃]^+. adduct ion. This latter also can be formed by an ion-molecu1e reaction. An abundant formation of CH₃-C⁺H-NH₂ ions is observed after 1–5 and 1–6 hydrogen transfers.Each step of this process is compared with the behaviour of aminium radical cations in condensed phase.     

Energy distribution among reaction products. XIV. F + CH4, F + CH3X(X  Cl,Br, I), F + CHnCl4−n(n = 1−3)

The infrared chemiluminescence technique has been used to obtain relative rate constants k(ν′) for HF(ν′) formed in the following reaction:

For reaction (1) the detailed rate constants [k(ν′ = 1) = 0.30;k(ν′ = 2) = 1.00; k(ν′ = 3) = 0.15; mean fraction of the available energy entering vibration <?^′_ν> = 0.56] confirmed, at much lower reagent pressures, results obtained by previous workers. In series I there was a slight increase in fraction of the energy entering vibration as the molecular reagent altered from CH₃Cl to CH₃Br to CH₃I, viz <?^′_ν> = 0.50 (1a), <?^′_ν> = 0.58 (1b), <?^′_ν> = 0.60 (1c). In series 2, by contrast, there was a marked decrease in fractional conversion of the available energy into vibration with increasing chlorination of the molecular reagent; <?^′_ν> = 0.50 (1a), <?^′_ν> = 0.23 (2a), <?^′_ν> = 0.13 (2b). The rate constants into ν′ = 0, k(ν′ = 0), were obtained by extrapolation of surprisal plots; the trends for both series were, however, also evident from k(ν′ > 0). No separate initial rotational distribution was observed for any of these reactions, indicating that the peak of the initial distribution is not far removed from a 300 K thermal distribution. The decrease in <?^′_ν> for the HF products along series 2 was tentatively ascribed to increasing internal excitation in the ejected radicals CH₂Cl, CHCl₂, CCl₃, due to increase in the number of secondary encounters between HF and the departing radical.