IR-active Modes Research Articles

Infrared properties of single-walled carbon nanotubes calculated from first principles

The vibration modes and the infrared (IR) spectra of the zigzag and chiral single-walled carbon nanotubes (SWNTs) have been calculated from the density functional theory in the local density approximation. Three IR-active modes are found to exist in the zigzag SWNTs in contrast to six in the chiral one, which agrees well with the prediction of the rod-group theory. Also, the frequency of low-frequency E1u mode of zigzag tubes is found to be strongly diameter dependent, which could be used for measurement of tube diameter in experiment. It is very interesting to find that the (n−m) mod 3=−1 or 0 tubes exhibit anomalous big IR intensities of their A2u (or A2) modes, much larger than those of the (n−m) mod 3=+1 ones, showing a clear family behavior, which is expected to be easily observed in future experiments.

Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays

Infrared absorption spectroscopy enabling direct access to vibrational fingerprints of the molecular structure is a powerful method for functional studies of bio-molecules. Although the intrinsic absorption cross-sections of IR active modes of proteins are nearly 10 orders of magnitude larger than the corresponding Raman cross-sections, they are still small compared to that of fluorescence-label based methods. Here, we developed a new tool based on collective excitation of plasmonic nanoantenna arrays and demonstrated direct detection of vibrational signatures of single protein monolayers. We first tailored the geometry of individual nanoantennas to form resonant structures that match the molecular vibrational modes. The tailored nanoantennas are then arranged in such a way that their in-phase dipolar coupling leads to a collective excitation of the ensemble with strongly enhanced near fields. The combined collective and individual plasmonic responses of the antenna array play a critical role in attaining signal enhancement factors of 10(4)-10(5). We achieved measurement of the vibrational spectra of proteins at zeptomole levels for the entire array, corresponding to only 145 molecules per antenna. The near-field nature of the plasmonic enhancement of the absorption signals is demonstrated with progressive loading of the nanoantennas with varying protein film thicknesses. Finally, an advanced model based on nonequilibrium Green's function formalism is introduced, which explains the observed Fano-type absorption line-shapes and tuning of the absorption strengths with the antenna resonance.

Monoclinic to tetragonal transformations in hafnia and zirconia: A combined calorimetric and density functional study

We use a combination of density functional theory and calorimetric measurements to investigate the monoclinic to tetragonal transition in hafnia and zirconia. We measure the transition enthalpies to be $8.4\ifmmode\pm\else\textpm\fi{}0.7\text{ }\text{kJ}/\text{mol}$ in hafnia and, as previously reported, $5.272\ifmmode\pm\else\textpm\fi{}0.544\text{ }\text{kJ}/\text{mol}$ in zirconia. Calculated values are 10.21 and 7.50 kJ/mol for hafnia and zirconia, respectively. We formulate a theoretical model of the phase transition consistent with the martnesitic character of the transformations. The transition barriers of 20.3 and 16.3 kJ/mol are estimated for hafnia and zirconia, respectively. We report the phonon spectra of monoclinic and tetragonal phases of both oxides and identify the Raman and IR active modes. The theoretical results compare well with available experiments. We present a comprehensive theoretical comparison of thermodynamic properties of zirconia and hafnia including the temperature dependence of specific heat.

Quantum-Mechanical ab Initio Simulation of the Raman and IR Spectra of Fe3Al2Si3O12 Almandine

The IR and Raman spectra of the Fe(3)Al(2)Si(3)O(12) almandine garnet were simulated using the periodic CRYSTAL code with an all-electron Gaussian-type basis set and the hybrid B3LYP functional. The wavenumbers of the 25 Raman-active modes (3 of A(1g), 8 of E(g), and 14 of F(2g) symmetry) and 34 F(1u) IR-active modes (17 TO and 17 LO) were computed, as were the IR intensities. Calculated wavenumbers are in excellent agreement with the various sets of experimental results, with the mean absolute difference |Delta| being between 4 and 8 cm(-1). Graphical animation, available on the CRYSTAL Web site, provides a very comprehendible description of the movement of atoms and groups in each vibrational mode. The simulated reflectivity spectrum, obtained using a classical dispersion relation, is in excellent agreement with the measured one.

Investigation of (1R,2S)‐(−)‐Ephedrine by Cryogenic Terahertz Spectroscopy and Solid‐State Density Functional Theory

The terahertz (THz) spectrum of the pharmaceutical (1R,2S)-(-)-ephedrine from 8.0 to 100.0 cm(-1) is investigated at liquid-nitrogen (78.4 K) temperature. The spectrum exhibits several distinct features in this range that are characteristic of the crystal form of the compound. A complete structural analysis and vibrational assignment of the experimental spectrum is performed using solid-state density functional theory (DFT) and cryogenic single-crystal X-ray diffraction. Theoretical modeling of the compound includes an array of density functionals and basis sets with the final assignment of the THz spectrum performed at a PW91/6-311G(d,p) level of theory, which provides excellent solid-state simulation agreement with experiment. The solid-state analysis indicates that the seven experimental spectral features observed at low temperature consist of 13 IR-active vibrational modes. Of these modes, nine are external crystal vibrations and provide approximately 57% of the predicted spectral intensity. This study demonstrates that the THz spectra of complex pharmaceuticals may be well reproduced by solid-state DFT calculations and that inclusion of the crystalline environment is necessary for realistic and accurate simulations.

A new method of growing K 2Co xNi 1−x(SO 4) 2*6H 2O ( x=0; 0.4; 0.8; 1) mixed crystals and their spectral investigation

A new method of K 2Co x Ni 1− x (SO 4) 2·6H 2O mixed crystals growing was proposed. High-quality samples were synthesized from the water solution of K 2SO 4, NiCl 2, and CoCl 2. Transmission spectra of these crystals were investigated in the wavelength range of 200–900 nm. The bands that are responsible for the electronic transitions in octahedral complexes CoO 6 and NiO 6 are independently presented in these spectra. Infrared reflection was measured for these crystals in the range of 2.5–20 μm. The identification of IR active mode frequencies was performed on the base of measured data.

Evaluation of collective transport properties of ionic melts from molecular dynamics simulations

Molecular dynamics simulations of beryllium fluoride (BeF2) have been carried out in the canonical (NVT) ensemble using a rigid-ion potential model. The Green-Kubo formalism has been applied to compute viscosities and ionic conductivities of BeF2 melt. The computational parameters critical for reliably estimating these collective transport properties are shown to differ significantly for viscosity and ionic conductivity. In addition to the equilibrium values of these transport properties, structural relaxation times as well as high-frequency IR-active modes are computed from the pressure and charge-flux auto correlation functions (ACFs) respectively. It is shown that a network-forming ionic melt, such as BeF2, will display persistent oscillatory behaviour of the integral of the charge-flux ACF. By suitable Fourier transformation, one can show that these persistent oscillations correspond to high-frequency, infra-red active vibrations associated with local modes of the network.

Influence of mechanical activation on the structure of ultrafine BaTiO 3 powders

The influence of mechanical activation in a planetary ball mill on the change of the structure of BaTiO 3 powders has been investigated. The particle size distribution of the nonactivated and activated samples was measured by a particle dimension laser analyzer. The mercury porosimetry method has been applied for determination of the surface area of the samples, bulk density, specific pore volumes, total porosity and the pore size distributions. Formation of paramagnetic centers and the change of the IR active modes was investigated by EPR and IR measurements. Semi-quantitative comparison of the BaCO 3 and OH − group presence in IR spectra was calculated. The investigation showed that mechanical activation of BaTiO 3 has a pronounced influence on the change of the powder particles structure and enables establishing the directions of possible BaTiO 3 ceramics materials properties prognosis according to the synthesis–structure correlation.

Ab initio vibrational and dielectric properties of [formula omitted

For the yttrium orthovanadate Y V O 4 with a tetragonal zircon-type structure, the first complete set of Raman-active and IR-active phonon modes has been calculated using ab initio density functional perturbation theory. The calculated IR reflectivity spectra are in good agreement with available experimental data. We report the calculated frequencies of three Raman-active modes that could not be detected experimentally and a new assignment of the experimental Raman data. The contributions of each IR-active phonon modes to static dielectric tensor have been determined.

A first-principles study of the effects of Hf doping on the dielectric response in ZrO2

The electronic structure and dielectric response of 25% and 50% Hf substituted zirconia (ZrO2) are determined, using first-principles density functional theory calculations based on pseudopotentials and a plane wave basis. It is found that Zr0.75Hf0.25O2 is more promising with higher dielectric response compared to pure ZrO2, the dielectric constant increases further upon introduction of oxygen vacancies. In contrast, Zr0.5Hf0.5O2 shows a decrease in the response compared to pure ZrO2. The IR-active phonon modes cause enhanced response in 25% Hf substituted ZrO2 and the material remains insulating making it promising for applications such as a gate oxide material.

Interaction of Benzene with Transition Metal Cations: Theoretical Study of Structures, Energies, and IR Spectra.

The cation-π interactions have been intensively studied. Nevertheless, the interactions of π systems with heavy transition metals and their accurate conformations are not well understood. Here, we theoretically investigate the structures and binding characteristics of transition metal (TM) cations including novel metal cations (TM(n+) = Cu(+), Ag(+), Au(+), Pd(2+), Pt(2+), and Hg(2+)) interacting with benzene (Bz). For comparison, the alkali metal complex of Na(+)-Bz is also included. We employ density functional theory (DFT) and high levels of ab initio theory including Møller-Plesset second-order perturbation (MP2) theory, quadratic CI method with single and double substitutions (QCISD), and the coupled cluster theory with single, double, and perturbative triple excitations (CCSD(T)). Each of the transition metal complexes of benzene exhibits intriguing binding characteristics, different from the typical cation-π interactions between alkali metal cations and aromatic rings. The complexes of Na(+), Cu(+), and Ag(+) favor the conformation of C6v symmetry with the cation above the benzene centroid (πcen). The formation of these complexes is attributed to the electrostatic interaction, while the magnitude of charge transfer has little correlation with the total interaction energy. Because of the TM(n+)←π donation, cations Au(+), Pd(2+), Pt(2+), and Hg(2+) prefer the off-center π conformation (πoff) or the π coordination to a C atom of the benzene. Although the electrostatic interaction is still important, the TM←π donation effect is responsible for the binding site. The TM(n+)-Bz complexes give some characteristic IR peaks. The complexes of Na(+), Cu(+), and Ag(+) give two IR active modes between 800 and 1000 cm(-1),which are inactive in the pure benzene. The complexes of Au(+), Pd(2+), Pt(2+), and Hg(2+) give characteristic peaks for the ring distortion, C-C stretching, and C-H stretching modes as well as significant red-shifts in the CH out-of-plane bending.

The structure, electronic property and infrared spectroscopy of the endohedral fullerene SnH 4@C 60

The structure, electronic property and infrared spectroscopy of the endohedral fullerene SnH 4@C 60 have been systematically studied by using the hybrid density functional B3PW91 theory. It is found that the SnH 4 molecule is more compact when the T d SnH 4 is seated in the center of the C 60 cage, and the C 60 cage may stabilize the high T d SnH 4 molecule. The formation of SnH 4@C 60 from the free C 60 and T d SnH 4 is endothermic with inclusion energy of 108.58 kcal/mol. The calculated HOMO–LUMO energy gap, the vertical ionization potentials (VIP) and vertical electron affinities (VEA) indicate that SnH 4@C 60 seems to be the kinetic stable species. Natural population analysis on the SnH 4@C 60 reveals that the central SnH 4 only gain −0.09 charges from the C 60 cage. In addition, the IR active modes and harmonic vibrational frequencies of SnH 4@C 60 are also discussed.

Lattice dynamics of room temperature orthorhombic SrHfO 3

The lattice dynamics of room temperature orthorhombic phase of SrHfO 3 has been computed, using density functional perturbation theory. The full set of zone center phonon frequencies and infrared reflectivity spectra were reported, providing benchmark theoretical results for the interpretation of future experimental phonon measurements. It is found that the static dielectric constant is relatively anisotropic. This anisotropy has been discussed in relationship to the oscillator strengths and contributions of the lowest IR active modes.

Gas-phase infrared multiple photon dissociation spectroscopy of isolated SF6− and SF5− anions

Resonantly enhanced multiple photon dissociation of gas-phase SF(6) (-) and SF(5) (-) is studied using tunable infrared light from the FELIX free electron laser. The photodissociation spectrum of the sulfur hexafluoride anion, producing SF(5) (-), is recorded over the spectral range of 250-1650 cm(-1). The infrared multiple photon dissociation cross section exhibits a strong, broad resonance enhancement at 675 cm(-1) in agreement with the calculated value of nu(3), one of the two IR-active fundamental vibrational modes predicted for the O(h)-symmetry ion. Much weaker absorption features are observed in the spectral region of 300-450 cm(-1) as well as at 580 cm(-1) that are not easily assigned to the other IR-active fundamental of SF(6) (-) since these resonances are observed at a much higher energy than the calculated values for the IR-active nu(4) mode. The potential role of binary combination bands is considered. Photodissociation from the sulfur pentafluoride anion produced only F(-), but photodetachment was also observed through SF(6) associative electron capture. The IR multiple photon dissociation spectrum of SF(5) (-) shows multiple resonances within the region of 400-900 cm(-1) and agreement with calculations is clear, including the observation of three fundamental frequencies: nu(1) at 780 cm(-1), nu(7) at 595 cm(-1), and nu(8) at 450 cm(-1). Comparisons of the measured frequencies with ab initio and density functional theory calculations confirm an SF(5) (-) anion of C(4v) symmetry. Similar comparisons for SF(6) (-) are not inconsistent with an anion of O(h) symmetry.

Optical spectroscopy in CoO: Phononic, electric, and magnetic excitation spectrum within the charge-transfer gap

The reflectivity of single-crystalline CoO has been studied by optical spectroscopy for wave numbers ranging from 100 to $28\text{ }000\text{ }{\text{cm}}^{\ensuremath{-}1}$ and for temperatures $8lTl325\text{ }\text{K}$. A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic phase transition at ${T}_{N}=289\text{ }\text{K}$ has been observed. At low temperatures the splitting amounts to $15.0\text{ }{\text{cm}}^{\ensuremath{-}1}$. In addition, we studied the splitting of the cubic crystal-field ground state of the ${\text{Co}}^{2+}$ ions due to spin-orbit coupling, a tetragonal crystal field, and exchange interaction. Below ${T}_{N}$, magnetic-dipole transitions between the exchange-split levels are identified, and the energy-level scheme can be well described with a spin-orbit coupling $\ensuremath{\lambda}=151.1\text{ }{\text{cm}}^{\ensuremath{-}1}$, an exchange constant $J=17.5\text{ }{\text{cm}}^{\ensuremath{-}1}$, and a tetragonal crystal-field parameter $D=\ensuremath{-}47.8\text{ }{\text{cm}}^{\ensuremath{-}1}$. Already in the paramagnetic state electric-quadrupole transitions between the spin-orbit split level have been observed. At high frequencies, two electronic levels of the crystal-field-split $d$ manifold were identified at $8\text{ }000$ and $18\text{ }500\text{ }{\text{cm}}^{\ensuremath{-}1}$.

Endohedral complex of fullerene C 60 with tetrahedrane, C 4H 4@C 60

B3LYP/6–31G(d) hybrid HF/DFT calculations were carried out to determine the structural and electronic properties of the endohedral complex of C 60 with tetrahedrane C 4H 4. It was demonstrated that C 4H 4 was seated in the center of the C 60 cage and existed in a molecular form inside the fullerene. The formation of this complex was endothermic with inclusion energy of 141.05 kcal/mol. C 4H 4 endohedral doping slightly perturbed the molecular orbitals of C 60. The calculated HOMO–LUMO gaps, the electron affinity (EA) and the ionizational potential (IP) indicated that C 4H 4@C 60 seemed to be more kinetically reactive than C 60. The IR active modes and harmonic vibrational frequencies of C 4H 4@C 60 were also discussed.

Ti-doped ThO2: a first-principles study of dielectric properties

We determine electronic structure and dielectric response of 25 and 50% Ti-substituted thoria (ThO2), using first-principles density functional theory calculations based on pseudopotentials and a plane wave basis. We find that Th0.75Ti0.25O2 is more promising as a high-k material than Th0.5Ti0.5O2. We show that the IR-active phonon modes in pure thoria can be tuned upon Ti doping and the softened modes cause an enhanced dielectric response of 25% Ti-doped thoria. As the material remains insulating, it holds promise for applications such as a gate oxide material.

Effect of mixed ternary transition metal ferrite nanocrystallites on thermal decomposition of ammmonium perchlorate

Nanocrystallites of six mixed ternary transition metal ferrite (MTTMF) were prepared by co-precipitation method and characterized by X-ray diffraction (XRD) and BET equation. XRD patterns gave average particle size for NiZnFe 2O 4 (NZF), CuCoFe 2O 4 (CuCoF), NiCuFe 2O 4 (NCuF), CuZnFe 2O 4 (CuZF), CoNiFe 2O 4 (CoNF) and CoZnFe 2O 4 (CoZF) as 7.2, 3.0, 3.0, 6.8, 3.9 and 5.8 nm, respectively. Further, catalytic activities of the MTTMF nanocrystallites on thermal decomposition of ammonium perchlorate (AP) were investigated using thermogravimetric analysis (TG), differential scanning calorimetry (DSC), differential thermal analysis (DTA), and ignition delay measurements. IR-active internal mode of cation and anion of ammonium perchlorate changes with temperature in presence of MTTMF nanocrystallites. The catalytic activity was found to be in the order: CoZF > CoNF > CuZF > CuCoF > NCuF > NZF. Nanocrystals of MTTMF have been proposed to play a significant role in the decomposition of AP.

2D calculation of anharmonic OH vibrations in a layered hydroxide crystal

Anharmonic vibrational frequencies for the Raman-active (A(1g)) and the IR-active (A(2u)) modes have been calculated for the LiOH crystal within a plane-wave density functional theory (DFT) framework. We find that a two-dimensional quantum-mechanical vibrational approach, allowing for anharmonic coupling between symmetric and antisymmetric OH stretching modes, produces OH frequencies--both absolute frequencies and gas-to-solid frequency shifts--in good agreement with experiment. Remaining errors in the absolute frequencies are largely a consequence of the DFT model chosen. A one-dimensional normal-mode following vibrational treatment, on the other hand, fails to reproduce both absolute anharmonic frequencies and gas-to-solid frequency shifts.

Nonmonotonic Composition Dependence of the Dielectric Response of Ba1−xCaxZrO3

We use first-principles density functional theory calculations to investigate the dielectric response of BaZrO 3 based perovskites. In this study we have substituted Ca for Ba over a range of compositions to understand what causes the recently reported (Levin et al., J. Solid State Chem. 2003, 175, 170) nonmonotonic dielectric response of Ba 1-x Ca x ZrO 3 (BCZ). We show that in all compositions studied there appear two new sets of IR active phonon modes that increase the dielectric constant e. One set occurs below the normal IR active A-O modes, due to the rolling cation motion of Ca about its O 12 cage. The second set of new phonon modes occurs between the normal A-O and B-O mode frequencies, caused by the displacement of Ca along one Cartesian direction. As more Ca is added to BCZ, Ca-Ca interactions increase, leading to Ca displacements in all three Cartesian directions, stronger Ca-O bonds and increased Ο 6 tilts. Such structural deformations decrease e. These effects are responsible for the anomalous compositional dependence of the BCZ dielectric response.