The HKrCCHCO2 complex: an ab initio and matrix-isolation study.

We report an experimental and theoretical study on new noble-gas hydride complex HKrCCHCO2, which is the first known complex of a krypton hydride with carbon dioxide. This species was prepared by the annealing-induced H + Kr + CCHCO2 reaction in a krypton matrix, the CCHCO2 complexes being produced by UV photolysis of propiolic acid (HCCCOOH). The H-Kr stretching mode of the HKrCCHCO2 complex at 1316 cm-1 is blue-shifted by 74 cm-1 from the most intense H-Kr stretching band of HKrCCH monomer. The observed blue shift indicates the stabilization of the H-Kr bond upon complexation, which is characteristic of complexes of noble-gas hydrides. This spectral shift is slightly larger than that of the HKrCCHC2H2 complex (+60 cm-1) and significantly larger than that of the HXeCCHCO2 complex (+32 and +6 cm-1). On the basis of comparison with ab initio computations at the MP2 and CCSD(T) levels of theory, the experimentally observed absorption is assigned to the quasi-parallel configuration of the HKrCCHCO2 complex. The calculated complexation-induced spectral shift of the H-Kr stretching band (60.4 or 72.7 cm-1 from the harmonic calculations at the MP2 and CCSD(T) levels, respectively) agrees well with the experimental value.

Intralanthanide Separation on Layered Titanium(IV) Organophosphate Materials via a Selective Transmetalation Process.

The lanthanides (Ln) are an essential part of many advanced technologies. Our societal transformation toward renewable energy drives their ever-growing demand. The similar chemical properties of the Ln pose fundamental difficulties in separating them from each other, yet high purity elements are crucial for specific applications. Here, we propose an intralanthanide separation method utilizing a group of titanium(IV) butyl phosphate coordination polymers as solid-phase extractants. These materials are characterized, and they contain layered structures directed by the hydrophobic interaction of the alkyl chains. The selective Ln uptake results from the transmetalation reaction (framework metal cation exchange), where the titanium(IV) serves as sacrificial coordination centers. The "tetrad effect" is observed from a dilute Ln3+ mixture. However, smaller Ln3+ ions are preferentially extracted in competitive binary separation models between adjacent Ln pairs. The intralanthanide ion-exchange selectivity arises synergistically from the coordination and steric strain preferences, both of which follow the reversed Ln contraction order. A one-step aqueous separation of neodymium (Nd) and dysprosium (Dy) is quantitatively achievable by simply controlling the solution pH in a batch mode, translating into a separation factor of greater than 2000 and 99.1% molar purity of Dy in the solid phase. Coordination polymers provide a versatile platform for further exploring selective Ln separation processes via the transmetalation process.

Low-Temperature Wafer-Scale Deposition of Continuous 2D SnS2 Films.

Semiconducting 2D materials, such as SnS2 , hold immense potential for many applications ranging from electronics to catalysis. However, deposition of few-layer SnS2 films has remained a great challenge. Herein, continuous wafer-scale 2D SnS2 films with accurately controlled thickness (2 to 10 monolayers) are realized by combining a new atomic layer deposition process with low-temperature (250 °C) postdeposition annealing. Uniform coating of large-area and 3D substrates is demonstrated owing to the unique self-limiting growth mechanism of atomic layer deposition. Detailed characterization confirms the 1T-type crystal structure and composition, smoothness, and continuity of the SnS2 films. A two-stage deposition process is also introduced to improve the texture of the films. Successful deposition of continuous, high-quality SnS2 films at low temperatures constitutes a crucial step toward various applications of 2D semiconductors.

Formic acid dimers in a nitrogen matrix.

Formic acid (HCOOH) dimers are studied by infrared spectroscopy in a nitrogen matrix and by ab initio calculations. We benefit from the use of a nitrogen matrix where the lifetime of the higher-energy (cis) conformer is very long (∼11 h vs. 7 min in an argon matrix). As a result, in a nitrogen matrix, a large proportion of the cis conformer can be produced by vibrational excitation of the lower-energy (trans) conformer. Three trans-trans, four trans-cis, and three cis-cis dimers are found in the experiments. The spectroscopic information on most of these dimers is enriched compared to the previous studies in an argon matrix. The cis-cis dimers of ordinary formic acid (without deuteration) are reported here for the first time. Several conformational processes are obtained using selective excitation by infrared light, some of them also for the first time. In particular, we report on the formation of cis-cis dimers upon vibrational excitation of trans-cis dimers. Tunneling decays of several dimers have been detected in the dark. The tunneling decay of cis-cis dimers of formic acid as well as the stabilization of cis units in cis-cis dimers is also observed for the first time.

Photochemistry of the H2O/CO System Revisited: The HXeOH···CO Complex in a Xenon Matrix.

We report on the complex of a noble-gas hydride HXeOH with carbon monoxide. This species is prepared via the annealing-induced H + Xe + OH···CO reaction in a xenon matrix, the OH···CO complexes being produced by VUV photolysis of the H2O···CO complexes. The H-Xe stretching mode of the HXeOH···CO complex absorbs at 1590.3 cm-1 and it is blue-shifted by 12.7 cm-1 from the H-Xe stretching band of HXeOH monomer. The observed blue shift indicates the stabilization of the H-Xe bond upon complexation, which is characteristic of complexes of noble-gas hydrides. The HXeOH···CO species is the first complex of a noble-gas hydride with carbon monoxide and the second observed complex of HXeOH. On the basis of the MP2/aug-cc-pVTZ-PP calculations, the experimental complex is assigned to the structure, where the carbon atom of CO interacts with the oxygen atom of HXeOH.

Spectroscopic characterization of the complex of vinyl radical and carbon dioxide: Matrix isolation and ab initio study.

We report on the preparation and vibrational characterization of the C2H3⋯CO2 complex, the first example of a stable intermolecular complex involving vinyl radicals. This complex was prepared in Ar and Kr matrices using UV photolysis of propiolic acid (HC3OOH) and subsequent thermal mobilization of H atoms. This preparation procedure provides vinyl radicals formed exclusively as a complex with CO2, without the presence of either CO2 or C2H3 monomers. The absorption bands corresponding to the ν5(C2H3), ν7(C2H3), ν8(C2H3), ν2(CO2), and ν3(CO2) modes of the C2H3⋯CO2 complex were detected experimentally. The calculations at the UCCSD(T)/L2a level of theory predict two structures of the C2H3⋯CO2 complex with Cs and C1 symmetries and interaction energies of -1.92 and -5.19 kJ mol-1. The harmonic vibrational frequencies of these structures were calculated at the same level of theory. The structural assignment of the experimental species is not straightforward because of rather small complexation-induced shifts and matrix-site splitting of the bands (for both complex and monomers). We conclude that the C1 structure is the most probable candidate for the experimental C2H3⋯CO2 complex based on the significant splitting of the bending vibration of CO2 and on the energetic and structural considerations.

Complex between Formic Acid and Nitrous Oxide: A Matrix-Isolation and Computational Study.

The complex of formic acid (FA, HCOOH) with nitrous oxide (N2O) was studied experimentally and computationally. Eight structures of the trans-FA···N2O complex and nine structures of the cis-FA···N2O complex were found at the density functional theory (M06-2X and wB97XD), MP2(full), and CCSD(T)-F12a levels. Two structures of the trans-FA···N2O complex (1t and 3t) and two structures of the cis-FA···N2O complex (1c and 3c) were identified by infrared spectroscopy in an argon matrix. Structure 1t with the bonded OH group appears in the matrices after deposition. Structures 3t and 3c with the free OH groups were prepared by vibrational excitation of the trans-FA conformer in structures 1t and 3t, respectively. Structure 3t is thermally unstable and relaxes to structure 1t at ∼15 K. Structure 1c with the bonded OH group is made by vibrational excitation of trans-FA monomer combined with thermal annealing. The lifetimes of the cis-FA···N2O complex structures 1c (81.5 min) and 3c (18.7 min) in an argon matrix at 4.3 K are longer than that of cis-FA monomer (6.3 min). This difference agrees with the calculated stabilization barriers of these species.

An aromatic noble-gas hydride: C6H5CCXeH.

We report on the aromatic noble-gas hydride, C6H5CCXeH, identified in a xenon matrix using infrared spectroscopy and extensive quantum chemical calculations. This molecule is prepared by 250-nm photolysis of phenylacetylene (C6H5CCH) isolated in a xenon matrix and subsequent thermal mobilization of hydrogen atoms at about 40 K. The characteristic H-Xe stretching mode of C6H5CCXeH is observed at about 1500 cm-1, and a number of other fundamentals also appear in the experimental spectra. The assignment is supported by deuteration experiments providing predictable shifts of the vibrational frequencies. The experimental and calculated spectra are in a good agreement. C6H5CCXeH is computationally lower in energy than the C6H5CC + Xe + H fragments by about 0.60 eV at the M06-2X/aug-cc-pVTZ-PP level of theory, which allows its formation at low temperatures. C6H5CCXeH is the first aromatic noble-gas hydride and the first halogen-free aromatic noble-gas compound.

Conformational Switching of HOCO Radical: Selective Vibrational Excitation and Hydrogen-Atom Tunneling.

Conformers of carboxyl radical (HOCO) have been studied by IR spectroscopy in argon and nitrogen matrices. In an argon matrix, only the lower-energy conformer trans-HOCO is observed, whereas both cis and trans conformers are found for deuterated carboxyl radical DOCO. In a nitrogen matrix, both conformers of HOCO and DOCO isotopologues can be prepared, indicating strong stabilization of the higher-energy cis conformer by a nitrogen matrix. Selective vibrational excitation promotes the trans-to-cis and cis-to-trans conversions of DOCO in an argon matrix and HOCO and DOCO in a nitrogen matrix, which is the first conformational photoswitching of an open-shell species. In a nitrogen matrix, the cis-to-trans and trans-to-cis conversions of HOCO is also found upon broadband IR light of the spectrometer, and the ratio of the quantum yields of these processes is about 3.3. The photoswitching peculiarities are in agreement with the available theoretical energy barriers. The higher-energy cis conformer decays to the lower-energy trans conformer via hydrogen-atom tunneling through the torsional barrier, which is also a unique observation for an open-shell species. The tunneling mechanism of the cis-to-trans switching is supported by the low-temperature limit of the reaction rate and by the H/D kinetic isotope effect. Our results suggest a large difference in the H/D kinetic isotope effects in nitrogen and argon matrices (∼5 and >100, respectively). The stabilizing effect on cis-DOCO by a nitrogen matrix (by 2 orders of magnitude versus an argon matrix) is much smaller than that on cis-HOCO (estimated to be >104).

Effect of Noncovalent Interactions on Vibronic Transitions: An Experimental and Theoretical Study of the C2 H⋠⋠⋠CO2 Complex.

We report on the experimental and theoretical infrared spectrum of the C2 H⋅⋅⋅CO2 complex. This complex was prepared by UV photolysis of propiolic acid (HC3 OOH) in argon and krypton matrices. The experimental bands of C2 H in the C2 H⋅⋅⋅CO2 complex are blue-shifted from those of the C2 H monomer. The calculations on the C2 H⋅⋅⋅CO2 structures were performed at the RMP2/aug-cc-pVTZ level. The relative stability of the complex structures was evaluated by using the RCCSD(T)/aug-cc-pVQZ level. To simulate the spectrum of the C2 H⋅⋅⋅CO2 complex, we developed the theoretical approach used earlier for the C2 H monomer. Based on the calculations, the main experimental bands of the C2 H⋅⋅⋅CO2 complex are assigned to the most stable parallel structure. Almost all the strong bands predicted by theory (with intensities >30 km mol-1 ) are observed in the experiment. To our knowledge, it is the first study of the effect of noncovalent interactions on vibronic transitions and the first report on an intermolecular complex of the C2 H radical.

VUV photochemistry of the H2OCO complex in noble-gas matrices: formation of the OHCO complex and the HOCO radical.

Vacuum ultraviolet (VUV, 130-170 nm) photochemistry of the H2OCO complex is studied by matrix-isolation infrared spectroscopy. The H2OCO complexes in Ne, Ar, Kr, and Xe matrices are generated by ultraviolet (UV, 193 and 250 nm) photolysis of formic acid (HCOOH). VUV photolysis of the H2OCO complexes is found to lead to the formation of the OHCO radical-molecule complexes and trans-HOCO radicals. It is shown that the matrix material, local matrix morphology, and possibly the H2OCO complex geometry strongly affect the VUV photolysis pathways. The intrinsic reactivity of the matrix-isolated OHCO complex resulting in the formation of trans-HOCO is directly demonstrated for the first time. This reaction occurs in Ar, Kr, and Xe matrices upon annealing above 25 K and may proceed over the barrier. The case of a Ne matrix is very special because the formation of trans-HOCO from the OHCO complex is observed even at the lowest experimental temperature (4.5 K), which is in sharp contrast to the other matrices. It follows that quantum tunneling is probably involved in this process in the Ne matrix at such a low temperature. Infrared light also promotes this reaction in the Ne matrix at 4.5 K, which is not the case in the other matrices. The last findings show the effect of the environment on the tunneling and infrared-induced rates of this fundamental chemical reaction.

Matrix-isolation and computational study of H2CCCl and H2CCBr radicals.

We report on two new radicals, H2CCCl and H2CCBr, prepared in low-temperature noble-gas matrices and characterized using infrared spectroscopy. These radicals are made by UV photolysis of HCCCl and HCCBr and subsequent thermal annealing to mobilize hydrogen atoms in the matrices and promote their reaction with the residual precursor molecules. Three characteristic infrared bands are observed for each radical. The assignments are supported by quantum chemical calculations at the B3LYP and CCSD(T) levels of theory with the def2-TZVPPD basis set.

Photoisomerization of azobenzenes isolated in cryogenic matrices.

2,2'-Dihydroxyazobenzene (DAB), 2,2'-azotoluene (AT) and azobenzene (AB) were isolated in argon and xenon matrices and their molecular structures and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. All these compounds can adopt the E and Z isomeric forms around the central CNNC moiety, which can be enriched by several conformational and tautomeric modifications for DAB and AT. A number of DAB and AT isomeric forms were identified for the first time. For DAB, the E azo-enol isomer with two intramolecular six-membered quasi-rings formed via OHN hydrogen bonds was found after deposition. Irradiation with UV light generated a different E azo-enol form with two intramolecular H-bonded five-membered quasi-rings. Phototransformation was shown to be reversible and the forms could be interconverted by irradiation at different wavelengths. The isomerization between these two forms constitutes a direct experimental observation of an E → E isomerization in azobenzene-type molecules. Further irradiation generated a form(s) bearing both OH and NH groups. For AT, two E isomers with the CH3 groups forming five-membered and five/six-membered quasi-rings with the azo group were observed in the as-deposited matrices. Irradiation of AT with UV light generated a Z form that can be converted back to the E form at different irradiation wavelengths. E-AB was deposited in a xenon matrix and both E → Z and Z → E phototransformations were observed (contrary to what was previously reported in an argon matrix where only the Z → E conversion occurred). AB photoisomerization becomes more pronounced at elevated temperatures, thus indicating that the matrix effects responsible for hindering the AB photoisomerization are essentially due to steric restrictions. The different photoisomerization channels observed for these compounds are discussed in terms of a small-amplitude pedal motion.

Surface-Enhanced Raman Scattering of Silicon Nanocrystals in a Silica Film.

Surface-enhanced Raman scattering (SERS) is an intriguing effect, efficiency of which depends on many factors and whose applicability to a given system is not obvious before the experiment. The motivation of the present work is to demonstrate the SERS effect on silicon nanocrystals (Si-nc) embedded in silica, the material of high technological importance. Using the Ag overlayer method, we have found the SERS effect for this material. The best result is obtained for Ag layers of a weight thickness of 12 nm, whose surface plasmons are in a resonance with the laser wavelength (488 nm). The enhancement obtained for the Raman signal from 3-4-nm Si-nc in a 40-nm SiOx film is above 100. The SERS effect is about twice stronger for ultra-small Si-nc (~1 nm) and/or disordered silicon compared to Si-nc with sizes of 3-4 nm. The SERS measurements with an Ag overlayer allow detecting silicon crystallization for ultra-thin SiOx films and/or for very low Si excess and suppress the Raman signal from the substrate and the photoluminescence of the film.

Infrared Spectrum of Toluene: Comparison of Anharmonic Isolated-Molecule Calculations and Experiments in Liquid Phase and in a Ne Matrix.

First-principles anharmonic calculations are carried out for the CH stretching vibrations of isolated toluene and compared with the experimental infrared spectra of isotopologues of toluene in a Ne matrix at 3 K and of liquid toluene at room temperature. The calculations use the vibrational self-consistent field method and the B3LYP potential surface. In general, good agreement is found between the calculations and experiments. However, the spectrum of toluene in a Ne matrix is more complicated than that predicted theoretically. This distinction is discussed in terms of matrix-site and resonance effects. Interestingly, the strongest peak in the CH stretching spectrum has similar widths in the liquid phase and in a Ne matrix, despite the very different temperatures. Implications of this observation to the broadening mechanism are discussed. Finally, our results show that the B3LYP potential offers a good description of the anharmonic CH stretching band in toluene, but a proper description of matrix-site and resonance effects remains a challenge.

Acetic acid-water complex: The first observation of structures containing the higher-energy acetic acid conformer.

Non-covalent interaction of acetic acid (AA) and water is studied experimentally by IR spectroscopy in a nitrogen matrix and theoretically at the MP2 and coupled-cluster with single and double and perturbative triple excitations [CCSD(T)]/6-311++G(2d,2p) levels of theory. This work is focused on the first preparation and characterization of complexes of higher-energy (cis) conformer of AA with water. The calculations show three 1:1 structures for the trans-AA⋯H2O complexes and three 1:1 structures for the cis-AA⋯H2O complexes. Two trans-AA⋯H2O and two cis-AA⋯H2O complexes are found and structurally assigned in the experiments. The two cis-AA⋯ ⋅ H2O complexes are obtained by annealing of a matrix containing water and cis-AA molecules prepared by selective vibrational excitation of the ground-state trans form. The less stable trans-AA⋯H2O complex is obtained by vibrational excitation of the less stable cis-AA⋯H2O complex. In addition, the 1:2 complexes of trans-AA and cis-AA with water molecules are studied computationally and the most stable forms of the 1:2 complexes are experimentally identified.

X-ray induced dimerization of cinnamic acid: Time-resolved inelastic X-ray scattering study.

A classic example of solid-state topochemical reactions is the ultraviolet-light induced photodimerization of α-trans-cinnamic acid (CA). Here, we report the first observation of an X-ray-induced dimerization of CA and monitor it in situ using nonresonant inelastic X-ray scattering spectroscopy (NRIXS). The time-evolution of the carbon core-electron excitation spectra shows the effects of two X-ray induced reactions: dimerization on a short time-scale and disintegration on a long time-scale. We used spectrum simulations of CA and its dimerization product, α-truxillic acid (TA), to gain insight into the dimerization effects. From the time-resolved spectra, we extracted component spectra and time-dependent weights corresponding to CA and TA. The results suggest that the X-ray induced dimerization proceeds homogeneously in contrast to the dimerization induced by ultraviolet light. We also utilized the ability of NRIXS for direct tomography with chemical-bond contrast to image the spatial progress of the reactions in the sample crystal. Our work paves the way for other time-resolved studies on chemical reactions using inelastic X-ray scattering.

Multimodal non-linear optical imaging for the investigation of drug nano-/microcrystal-cell interactions.

Drug nano-/microcrystals are being used for sustained parenteral drug release, but safety and efficacy concerns persist as the knowledge of the in vivo fate of long-living particulates is limited. There is a need for techniques enabling the visualization of drug nano-/microcrystals in biological matrices. The aim of this work was to explore the potential of coherent anti-Stokes Raman scattering (CARS) microscopy, supported by other non-linear optical methods, as an emerging tool for the investigation of cellular and tissue interactions of unlabeled and non-fluorescent nano-/microcrystals. Raman and CARS spectra of the prodrug paliperidone palmitate (PP), paliperidone (PAL) and several suspension stabilizers were recorded. PP nano-/microcrystals were incubated with RAW 264.7 macrophages in vitro and their cellular disposition was investigated using a fully-integrated multimodal non-linear optical imaging platform. Suitable anti-Stokes shifts (CH stretching) were identified for selective CARS imaging. CARS microscopy was successfully applied for the selective three-dimensional, non-perturbative and real-time imaging of unlabeled PP nano-/microcrystals having dimensions larger than the optical lateral resolution of approximately 400nm, in relation to the cellular framework in cell cultures and ex vivo in histological sections. In conclusion, CARS microscopy enables the non-invasive and label-free imaging of (sub)micron-sized (pro-)drug crystals in complex biological matrices and could provide vital information on poorly understood nano-/microcrystal-cell interactions in future.

Acetic acid dimers in a nitrogen matrix: Observation of structures containing the higher-energy conformer.

Acetic acid (AA) dimers are studied experimentally by infrared spectroscopy in a N2 matrix and theoretically at the MP2/6-311++G(2d,2p) level of approximation. This work is focused on the first preparation and characterization of structures containing the higher-energy (cis) conformer of AA. Nine trans-trans, fourteen trans-cis, and six cis-cis dimers are theoretically predicted. Five trans-trans and a number of trans-cis dimers are identified in the experiments, but no indication of cis-cis dimers is found. Two trans-trans dimers and the trans-cis dimers are reported for the first time. One trans-cis dimer is prepared by selective vibrational excitation of the structurally related trans-trans dimer, which converts one of the trans subunits to the cis form. Several trans-cis dimers are obtained by annealing of a matrix containing both trans and cis monomers of AA. Tunneling-induced conversion of the trans-cis dimers into trans-trans forms (including two new trans-trans forms) is observed at low temperatures.