Phase transitions of choline dihydrogen phosphate: A vibrational spectroscopy and periodic DFT study.

Choline dihydrogen phosphate, [Chol][H2PO4], is a proton-conducting ionic plastic crystal exhibiting a complicated sequence of phase transitions. Here, we address the argument in the literature around the thermal properties of [Chol][H2PO4] using Raman and infrared microspectroscopy. The known structure of the low-temperature crystal, which contains the anti-conformer of [Chol]+ and hydrogen-bonded dimers of anions, was used to do periodic density functional theory calculations of the vibrational frequencies. Raman spectra indicate that the solid-solid transition at 20 °C is linked to a conformational change to the gauche [Chol] conformer with a concurrent local rearrangement of the anions. The distinct bands of lattice modes in the low-frequency range of the Raman spectra vanish at the 20 °C transition. Given the ease with which metastable crystals can be produced, Raman mappings demonstrate that a sample of [Chol][H2PO4] at ambient temperature can contain a combination of anti- and gauche conformers. Heating to 120 °C causes continuous changes in the local environment of anions rather than melting as suggested by a recent calorimetric investigation of [Chol][H2PO4]. The monotonic change in vibrational spectra is consistent with earlier observations of a very small entropy of fusion and no abrupt jump in the temperature dependence of ionic conductivity along the phase transitions of [Chol][H2PO4].

Brazilian Physicians Mental Health: A Cross-Sectional Nationwide Study exploring factors associated with prevalence of suicide planning and attempts.

OBJECTIVES: To report suicide planning and attempts' in a lifetime among Brazilian physicians and to explore associated risk factors. METHODS: A nation-wide, online survey based on the Tool for the Assessment of Suicide Risk and Satisfaction with Life Scale was conducted among Brazilian physicians (January 2018 - January 2019). Multivariate explored associations of demographics, psychological, and work-related factors on suicide planning and attempts reports. RESULTS: Among 4,148 respondents, 1,946 (53.5%) were male, 2,527 (60.9%) were 30-60 years old, 2,675 (64.5%) had 2-4 work-contracts and 1,725 (41.6%) reported a weekly workload of 40-60 hours. Overall prevalence of suicide plans was 8.8% (n=364) and suicide attempts were reported by 3.2% (n=133) of respondents. Daily (AdjOR=7.857;95%CI 2.282-27.051, p=0.002) or weekly emotional exhaustion (AdjOR=7.953; 95%CI 2.403-26.324, p=0.001), daily frustration with work (AdjOR=3.093;95%CI 1.711-5.588, p<0.001), and being bisexual (AdjOR=5.083;95%CI 2.544-10.158, p<0.001) were significantly associated with higher odds of reports. Among extremely dissatisfied professionals 38.3% reported having made suicide planning and attempts, while among extremely satisfied only 2.8% reported it (p<0.001). CONCLUSIONS: Brazilian physicians with a lifetime history of suicide planning and attempts presented a higher association with emotional exhaustion and frustration with work. Urgent actions are needed to promote professional protection policies and resilience.

DFT and ab initio molecular dynamics simulation study of the infrared spectrum of the protic ionic liquid 2-hydroxyethylammonium formate.

Protic ionic liquids (PILs) typically show a complex band shape in their infrared (IR) spectra in the high-frequency range due to the hydrogen stretching vibrations of functional groups forming rather strong hydrogen bonds (H-bonds). In the low-frequency range, the intermolecular stretching mode of the H-bond leaves a mark in the far-IR spectrum of PILs. In this study, the IR spectrum of the PIL 2-hydroxyethylammonium formate, [HOCH2CH2NH3][HCOO], is investigated in order to identify the different modes that contribute to the high-frequency band shape, i.e. the cation ν(NH), ν(OH), and ν(CH) modes, and the anion ν(CH) mode, as well as the intermolecular mode of the strongest H-bond in the far-IR spectrum. The assignment is validated by quantum chemistry calculations of clusters at the density functional theory (DFT) level for four ionic pairs and by ab initio molecular dynamics (AIMD) simulations of ten ionic pairs. There is good agreement between the vibrational frequencies obtained from DFT and AIMD simulations for both the high- and low-frequency ranges. Based on the calculations, the strong H-bond interaction between the cation -NH3 group and [HCOO]- gives a broad band envelope associated with the ν(NH) mode in the high-frequency range of the IR spectrum on which there are narrower peaks corresponding to the ν(OH) and ν(CH) modes. In the far-IR (FIR) spectrum, the anions' rattling motion gives a broad feature with a maximum at 160 cm-1, while the H-bond's intermolecular NH⋯O stretching mode appears as a peak at 255 cm-1.

Confined ionic liquids films under shear: The importance of the chemical nature of the solid surface.

Ionic liquids have generated interest in applications as lubricants and as additives to conventional lubricants due to their unique physical properties. In these applications, the liquid thin film can be subjected simultaneously to extremely high shear and loads in addition to nanoconfinement effects. Here, we use molecular dynamics simulations with a coarse-grained model to study a nanometric film of an ionic liquid confined between two planar solid surfaces both at equilibrium and at several shear rates. The strength of the interaction between the solid surface and the ions was changed by simulating three different surfaces with enhanced interactions with different ions. The increase in the interaction with either the cation or the anion leads to the formation of a solid-like layer that moves alongside the substrates; however, this layer can exhibit different structures and stability. An increase in the interaction with the high symmetry anion produces a more regular structure that is more resistant to the effects of shear and viscous heating. Two definitions were proposed and used for the calculation of the viscosity: a local definition based on the microscopic characteristics of the liquid and an engineering definition based on the forces measured at the solid surfaces, with the former displaying a correlation with the layered structure induced by the surfaces. Because of the shear thinning behavior of the ionic liquids as well as the temperature rise brought on by viscous heating, both the engineering and the local viscosities decrease as the shear rate increases.

Synthesis and Physicochemical Properties of Acrylate Anion Based Ionic Liquids.

Two polymerizable ionic liquids (or monomeric ionic liquids, mILs) namely 1-butyl-3-methylimidazolium and choline acrylates ([C4mim]A and ChA, respectively) were synthesized using the modified Fukumoto method from corresponding chlorides. The chemical structure of the prepared mILs was confirmed with FTIR and NMR study. Investigation of the thermal properties with DSC demonstrates that both mILs have a Tg temperature of about 180 K and a melting point around 310 K. It was shown that the temperature dependence of FTIR confirm the Tg to be below 200. Both mILs exhibited non-Newtonian shear thinning rheological behavior at shear rates >4 s−1. It was shown that [C4mim]A is able to dissolve bacterial cellulose (BC) leading to a decrease in its degree of polymerization and recrystallisation upon regeneration with water; although in the ChA, the crystalline structure and nanofibrous morphology of BC was preserved. It was demonstrated that the thixotropic and rheological properties of cellulose dispersion in ChA at room temperature makes this system a prospective ink for 3D printing with subsequent UV-curing. The 3D printed filaments based on ChA, containing 2 wt% of BC, and 1% of N,N'-methylenebisacrylamide after radical polymerization induced with 1% 2-hydroxy-2-methylpropiophenone, demonstrated Young's modulus 7.1 ± 1.0 MPa with 1.2 ± 0.1 MPa and 40 ± 5% of strength and ultimate elongation, respectively.

Overview of general surgery medical residency programs and prerequisite program in basic surgical area in Brazil: Historical review and update.

OBJECTIVE: to describe vacancy regulation process by adding, describing the panorama of the General Surgery Residency Program (PRMCG) and the Basic Surgical Prerequisites Program (PRACB). METHOD: descriptive, quali-quantitative, cross sectional study conducted from document analysis from National Commission of Medical Residency (CNRM). RESULTS: in 2018, after evaluation of the General Surgery Services for adequacy of the number of vacancies (DS), the PRACB was instituted as a modality of access to surgical specialties until definitive change in the time of the formation of the general surgeon for three years, in 2022. In the first addition of vacancies in 2018, 127 PRMCG were authorized with 736 vacancies of R1 and 290 PRACB (2 years) with 1.286 vacancies offered for R1. In the second addition in 2021, 423 PRM were authorized with 1.564 R1 vacancies in PRMCG. DISCUSSION: the regulation of the offer of vacancies for the formation of specialties in Brazil should align the evaluation of practice scenarios with the profile of skills. The PRACB modality was instituted for a certain time for budgetary preparation and practice scenarios until the complete transition to training in 3 years. CONCLUSION: Brazil by 2018 was the only country to grant the Board Certification to General Surgeon with only 2 years of training. After a transitional period the same analysis methodology for adding and regulating vacancies was applied to services.

Molecular Dynamics Simulation Study of the Far-Infrared Spectrum of a Deep Eutectic Solvent.

Deep eutectic solvents (DESs) are similar to ionic liquids (IL) in terms of physicochemical properties and technical uses. In ILs, far-infrared (FIR) spectroscopy has been utilized to reveal ionic interactions and even to produce a signature of the strengthening of the cation-anion hydrogen bond. However, for the situation of the DES, where the mixing of a salt and a molecular species makes the interplay between multiple intermolecular interactions even more complex, a full investigation of FIR spectra is still absent. In this work, the FIR spectrum of the DES, often referred to as ethaline, which is a 1:2 mixture of choline chloride and ethylene glycol, is calculated using classical molecular dynamics (MD) simulations and compared to experimental data. To explore the induced dipole effect on the computed FIR spectrum, MD simulations were run with both nonpolarizable and polarizable models. The calculation satisfactorily reproduces the position of the peak at ∼110 cm-1 and the bandwidth seen in the experimental FIR spectrum of ethaline. The MD simulations show that the charge current is the most important contributor to the FIR spectrum, but the cross-correlation between the charge current and dipole reorientation also plays a role in the polarizable model. The dynamics of the chloride-ethylene glycol correlation span a wide frequency range, with a maximum at ∼150 cm-1, but it participates as a direct mechanism only in the charge current-dipole reorientation cross-term. Anion correlations, whose dynamics are regulated via correlation with both ethylene glycol and choline, make the most significant contribution to the charge current mechanism. The MD simulations were also utilized to investigate the effect on the FIR spectrum of adding water to the DES and switching to a 1:1 composition.

Osmotic Method for Calculating Surface Pressure of Monolayers in Molecular Dynamics Simulations.

Surface pressure is a fundamental thermodynamic property related to the activity of molecules at interfaces. In molecular simulations, it is typically calculated from its definition: the difference between the surface tension of the air-water and air-surfactant interfaces. In this Letter, we show how to connect the surface pressure with a two-dimensional osmotic pressure and how to take advantage of this analogy to obtain a practical method of calculating surface pressure-area isotherms in molecular simulation. As a proof-of-concept, compression curves of zwitterionic and ionic surfactant monolayers were obtained using the osmotic approach and the curves were compared with the ones from the traditional pressure tensor-based scheme. The results shown an excellent agreement between both alternatives. Advantageously, the osmotic approach is simple to use and allows to obtain the surface pressure-area isotherm on the fly with a single simulation using equilibration stages.

Role of density and electrostatic interactions in the viscosity and non-newtonian behavior of ionic liquids - a molecular dynamics study.

Strong ionic interactions, as well as the consequent correlations between cation and anion dynamics, give ionic liquids various physical features that set them apart from ordinary organic solvents. In particular, they result in larger viscosities and larger densities than mixtures of neutral compounds with similar molecular structures. However, both the direct effect of electrostatic interactions and the increase of liquid density contribute to the high viscosity and so far no experimental or computational work enabled a clear quantification of those effects. Also, the effects over the shear thinning behavior, which may have important consequences for application as lubricants, were not considered yet. Here, these questions were tackled by performing non-equilibrium molecular dynamics (NEMD) simulations changing both the strength of ionic interactions and liquid density at several shear rates using a coarse grained model. The relative dielectric constant was adjusted to reproduce viscosity data from all-atoms simulations on both zero shear and high shear conditions. Elimination of ionic interactions results in a reduction of density and zero shear viscosity and also delays the beginning of shear thinning to higher shear rates. Restoring density to the ionic liquid's value only partially reverses the alterations. Correlations of the non-newtonian behavior and changes in the intermolecular structure and contact lifetimes were also explored.

Computer Simulations of Deep Eutectic Solvents: Challenges, Solutions, and Perspectives.

Deep eutectic solvents (DESs) are one of the most rapidly evolving types of solvents, appearing in a broad range of applications, such as nanotechnology, electrochemistry, biomass transformation, pharmaceuticals, membrane technology, biocomposite development, modern 3D-printing, and many others. The range of their applicability continues to expand, which demands the development of new DESs with improved properties. To do so requires an understanding of the fundamental relationship between the structure and properties of DESs. Computer simulation and machine learning techniques provide a fruitful approach as they can predict and reveal physical mechanisms and readily be linked to experiments. This review is devoted to the computational research of DESs and describes technical features of DES simulations and the corresponding perspectives on various DES applications. The aim is to demonstrate the current frontiers of computational research of DESs and discuss future perspectives.

Dentinogenic ghost cell tumor with focal atypical features suggesting ghost cell odontogenic carcinoma: Report of a challenging diagnosis.

Dentinogenic ghost cell tumor (DGCT) represents a rare benign odontogenic neoplasm that can appear in a central or peripheral form and may rarely undergo malignant transformation to ghost cell odontogenic carcinoma (GCOC). We aim to report a case of a central DGCT with focal cytological malignant aspects. A 24-year-old woman exhibited a painful enlargement and dental mobility in the left posterior maxilla for about one year, which appeared as an expansive well-defined hypodense maxillary image with hyperdense foci invading ipsilateral maxillary sinus. Incisional biopsy showed a predominantly solid hyperchromatic basaloid epithelium presenting cellular pleomorphism and mitotic activity, admixed with abundant ghost cell aggregates and dentinoid material. The lesion was immunopositive for p53 and had 21% of Ki-67 proliferation index (PI). These microscopic features suggested initially a GCOC diagnosis. Partial left maxillectomy was performed without complications. The surgical specimen presented an exuberant variation of the epithelial parenchyma, including ameloblastomatous, fusiform, and cribriform areas, with numerous ghost cells and dentinoid material, lacking any signs of malignancy. The final diagnosis was DGCT. The patient is in a strict regular follow-up for over two years, and there are no signs of recurrence.

Overview of general surgery medical residency programs and prerequisite program in basic surgical area in Brazil: Historical review and update / Panorama dos programas de residência médica em cirurgia geral e programa de pré-requisito em área cirúrgica básica no Brasil: Resgate histórico e atualização

ABSTRACT Objective: to describe vacancy regulation process by adding, describing the panorama of the General Surgery Residency Program (PRMCG) and the Basic Surgical Prerequisites Program (PRACB). Method: descriptive, quali-quantitative, cross sectional study conducted from document analysis from National Commission of Medical Residency (CNRM). Results: in 2018, after evaluation of the General Surgery Services for adequacy of the number of vacancies (DS), the PRACB was instituted as a modality of access to surgical specialties until definitive change in the time of the formation of the general surgeon for three years, in 2022. In the first addition of vacancies in 2018, 127 PRMCG were authorized with 736 vacancies of R1 and 290 PRACB (2 years) with 1.286 vacancies offered for R1. In the second addition in 2021, 423 PRM were authorized with 1.564 R1 vacancies in PRMCG. Discussion: the regulation of the offer of vacancies for the formation of specialties in Brazil should align the evaluation of practice scenarios with the profile of skills. The PRACB modality was instituted for a certain time for budgetary preparation and practice scenarios until the complete transition to training in 3 years. Conclusion: Brazil by 2018 was the only country to grant the Board Certification to General Surgeon with only 2 years of training. After a transitional period the same analysis methodology for adding and regulating vacancies was applied to services.

RESUMO Objetivo: descrever o contexto do processo de construção e implantação da Matriz de Competências da Cirurgia Geral com tempo de formação em 3 anos e o processo sobre a regulação de vagas, por aditamento, descrevendo o panorama dos Programas de Residência Médica em Cirurgia Geral (PRMCG) e do Programa de Pré-Requisito em área cirúrgica básica (PRACB). Método: estudo descritivo, quanti-qualitativo, transversal, realizado a partir da análise documental das atas, súmulas, nota técnica, resoluções e atos autorizativos da Comissão Nacional de Residência Médica (CNRM), bem como do Diagnóstico Situacional (DS), considerando a matriz de competências da especialidade em sua implementação, por meio de dados coletados no Sistema da Comissão Nacional de Residência Médica (SisCNRM), no portal eletrônico do Ministério da Educação (MEC). Resultado: em 2018, após metodologia de avaliação dos serviços para adequação do número de vagas (DS), foi instituído o PRACB como uma modalidade de acesso às especialidades cirúrgicas e de adaptação dos serviços formadores, até mudança definitiva no tempo de formação do Cirurgião Geral, para três anos, em 2022. No primeiro aditamento de vagas, em 2018, foram autorizados 127 PRMCG (3 anos) com 736 vagas de R1 e 290 pracb (2 anos) com 1286 vagas ofertadas para R1. No segundo aditamento, em 2021, foram autorizados 423 PRM com 1564 vagas de R1 em PRMCG. Atualmente 2495 médicos residentes estão em curso nos PRMCG e 50 (R2) estão concluindo o pracb. A maior concentração de PRM e médicos residentes encontra-se no Estado de São Paulo, em oposição a Região Norte. Discussão: a regulação da oferta de vagas para a formação de especialistas no Brasil deve alinhar a avaliação dos cenários de prática ao perfil das competências para a Especialidade como as necessidades de médicos especialistas indicada pelo perfil socioepidemiológico da população. Contudo, as instituições devem possuir estrutura compatível para a aquisição de competências necessárias ao especialista e dotação orçamentária para o financiamento do processo. A modalidade PRACB foi instituída por tempo determinado para a preparação orçamentária e dos cenários de prática, até a transição completa para a formação em 3 anos, possibilitando ao médico residente, enquanto vigente, acesso às especialidades cirúrgicas. Conclusão: O Brasil até 2018 era o único país a conceder o Título de Especialista em Cirurgia Geral com um tempo exíguo de treinamento - 2 anos. Após um período de transição, a mesma metodologia de análise para aditamento e regulação da oferta de vagas foi aplicada aos serviços. A partir de 2023, o PRMCG com duração de 3 anos, será a modalidade de formação que confere o título de especialista em Cirurgião Geral pela CNRM, promovendo qualificação profissional para assistência e cenários de prática compatíveis com a aquisição de competências. .

Fine-Tuning the Polarizable CL&Pol Force Field for the Deep Eutectic Solvent Ethaline.

Polarizable force fields are gradually becoming a common choice for ionic soft matter, in particular, for molecular dynamics (MD) simulations of ionic liquids (ILs) and deep eutectic solvents (DESs). The CL&Pol force field introduced in 2019 is the first general, transferable, and polarizable force field for MD simulations of different types of DESs. The original formulation contains, however, some problems that appear in simulations of ethaline and may also have a broader impact. First, the originally proposed atomic diameter parameters are unbalanced, resulting in too weak interactions between the chlorides and the hydroxyl groups of the ethylene glycol molecules. This, in turn, causes an artificial phase separation in long simulations. Second, there is an overpolarization of chlorides due to strong induced dipoles that give rise to the presence of peaks and antipeaks at very low q-vector values (2.4 nm-1) in the partial components of the structure factors. In physical terms, this is equivalent to overestimated spatial nanoscale heterogeneity. To correct these problems, we adjusted the chloride-hydroxyl radial distribution functions against ab initio data and then extended the use of the Tang-Toennis damping function for the chlorides' induced dipoles. These adjustments correct the problems without losing the robustness of the CL&Pol force field. The results were also compared with the nonpolarizable version, the CL&P force field. We expect that the corrections will facilitate reliable use of the CL&Pol force field for other types of DESs.

Relating the structure and dynamics of ionic liquids under shear by means of reverse non-equilibrium molecular dynamics simulations.

The effect of the shear rate on the viscosity and the structure of 1-ethyl-3-methylimidazolium based ionic liquids with three different anions (tetrafluoroborate, dicyanamide, and bis(trifluoromethylsulfonyl)imide) was studied by means of reverse non-equilibrium molecular dynamics (RNEMD) simulations using a polarizable force field. The three liquids display a Newtonian plateau followed by a shear thinning regime at shear rates of the order of GHz. Even though the main features of the liquid structure remains under shear, systematic changes were noticed at the GHz rates, with coordination shells becoming more diffuse as noticed by the reduction in the difference between consecutive maxima and minima in the radial distribution function. Interestingly, these structural changes with the shear rate can be precisely fitted using the Carreau equation, which is a well-known expression for the shear rate dependence of the viscosity. The fitting parameters for different distributions can be used to explain qualitatively the shear thinning behavior of these liquids. In the GHz range, the cations and, in a minor extension, some anions, tend to assume preferentially a parallel orientation with the flux, which contributes to the shear thinning behavior and may have consequences for adhesion in applications as lubricants.

Strengthening Cellulose Nanopaper via Deep Eutectic Solvent and Ultrasound-Induced Surface Disordering of Nanofibers.

The route for the preparation of cellulose nanofiber dispersions from bacterial cellulose using ethylene glycol- or glycerol-based deep eutectic solvents (DES) is demonstrated. Choline chloride was used as a hydrogen bond acceptor and the effect of the combined influence of DES treatment and ultrasound on the thermal and mechanical properties of bacterial cellulose nanofibers (BC-NFs) is demonstrated. It was found that the maximal Young's modulus (9.2 GPa) is achieved for samples prepared using a combination of ethylene glycol-based DES and ultrasound treatment. Samples prepared with glycerol-based DES combined with ultrasound exhibit the maximal strength (132 MPa). Results on the mechanical properties are discussed based on the structural investigations that were performed using FTIR, Raman, WAXD, SEM and AFM measurements, as well as the determination of the degree of polymerization and the density of BC-NF packing during drying with the formation of paper. We propose that the disordering of the BC-NF surface structure along with the preservation of high crystallinity bulk are the key factors leading to the improved mechanical and thermal characteristics of prepared BC-NF-based papers.

A new, narrowly distributed, and critically endangered species of Characidium (Characiformes: Crenuchidae) from the Distrito Federal, Central Brazil

A new species of Characidium is described from the Cerrado biome, in Brasília, Distrito Federal, Central Brazil. The new species can be readily diagnosed by the presence of two or three longitudinal rows of dots along the body sides, absence of bars, by the short pectoral fin, which does not reach the vertical through the dorsal fin origin. Additional useful diagnostic characters are the scaled isthmus, absence of the adipose fin, and the terminal mouth. The new species is only known from the córrego Taquara and its tributaries, a tributary of ribeirão do Gama, upstream from lago Paranoá, in the upper rio Paraná basin. In accordance to the IUCN Red List Categories and Criteria, the new species is categorized as Critically Endangered.(AU)

Uma nova espécie de Characidium é descrita do bioma Cerrado em Brasília, Distrito Federal, Brasil Central. A nova espécie pode ser diagnosticada pela presença de duas ou três fileiras longitudinais de pontos no corpo, ausência de barras e nadadeira peitoral curta, que não alcança a vertical que passa pela origem da nadadeira dorsal. Caracteres adicionais úteis na diagnose são o istmo escamado, a ausência da nadadeira adiposa e a boca terminal. A nova espécie é conhecida apenas do córrego Taquara e seus tributários, tributário do ribeirão do Gama a montante do Lago Paranoá, na bacia do alto rio Paraná. De acordo com os Critérios e Categorias para Listas Vermelhas da IUCN, a nova espécie é categorizada como uma Criticamente Ameaçada.(AU)

Effect of alkyl-group flexibility on the melting point of imidazolium-based ionic liquids.

The low melting point of room temperature ionic liquids is usually explained in terms of the presence of bulky, low-symmetry, and flexible ions, with the first two factors related to the lattice energy while an entropic effect is attributed to the latter. By means of molecular dynamics simulations, the melting points of 1-ethyl-3-methyl-imidazolium hexafluorophosphate and 1-decyl-3-methyl-imidazolium hexafluorophosphate were determined, and the effect of the molecular flexibility over the melting point was explicitly computed by restraining the rotation of dihedral angles in both the solid and the liquid phases. The rotational flexibility over the bond between the ring and the alkyl chain affects the relative ordering of the anions around the cations and results in substantial effects over both the enthalpy and the entropy of melting. For the other dihedral angles of the alkyl group, the contributions are predominantly entropic and an alternating behavior was found. The flexibility of some dihedral angles has negligible effects on the melting point, while others can lead to differences in the melting point as large as 20 K. This alternating behavior is rationalized by the different probabilities of conformation defects in the crystal.

An inelastic neutron scattering, Raman, far-infrared, and molecular dynamics study of the intermolecular dynamics of two ionic liquids.

The intermolecular dynamics in the THz frequency range of the ionic liquids n-butyl-trimethylammonium bis(trifluoromethanesulfonyl)imide, [N1114][NTf2], and methyl-tributylammonium bis(trifluoromethanesulfonyl)imide, [N1444][NTf2], were investigated by a combined usage of inelastic neutron scattering (INS), Raman, and far-infrared (FIR) spectroscopies and the power spectrum calculated by molecular dynamics (MD) simulations. The collective dynamics of the simulated systems is also discussed by the calculation of time correlation functions of charge and mass currents that are projected onto acoustic- and optic-like motions. The INS and Raman measurements have been performed as a function of temperature in the glassy, crystalline, and liquid phases. The excess in the vibrational density of states over the expectation of the Debye theory, the so-called boson peak, is found in the INS and Raman spectra as a peak at ∼2 meV (∼16 cm-1) and also in the direct measurement of heat capacity at very low temperatures (4-20 K). This low-frequency vibration is incorporated into the curve fits of Raman, FIR, and MD data at room temperature. Fits of spectra from these different sources in the range below 100 cm-1 are consistently achieved with three components at ca. 25, 50, and 80 cm-1, but with distinct relative intensities among the different techniques. It is proposed as the collective nature of the lowest-frequency component and the anion-cation intermolecular vibration nature of the highest-frequency component. The MD results indicate that there is no clear distinction between acoustic and optic vibrations in the spectral range investigated in this work for the ionic liquids [N1114][NTf2] and [N1444][NTf2]. The analysis carried out here agrees in part, but not entirely, with other propositions in the literature, mainly from optical Kerr effect (OKE) and FIR spectroscopies, concerning the intermolecular dynamics of ionic liquids.

Pseudo-Optical Modes in Room-Temperature Ionic Liquids.

The terahertz spectrum encodes information about dynamics, structure, and intermolecular interactions of liquids being probed both experimentally and computationally by techniques such as Raman and far-infrared spectroscopies and molecular dynamics (MD) simulation. In the case of room temperature ionic liquids (RTILs), there has been a debate whether a mode observed at about 1.5-2.7 THz (50-90 cm-1) is due to a quasi-lattice structure or the formation of complex ions. Here we show through the analysis of Raman and far-infrared spectra and MD simulation of a typical RTIL that this mode has a collective optic-like character. Then, employing a simple model based on the theory for optical phonons in crystals, we show that a correlation between the frequency of this mode and material parameters holds for different RTILs. These results, which encompass a wide range of samples, reinforce a quasi-lattice view of the liquid phase.