Large Amplitude Vibrational Modes Research Articles

Spherically localized discrete breathers in bcc metals V and Nb

Discrete breathers (DBs), also called intrinsic localized modes (ILMs), are spatially localized, large-amplitude vibrational modes in nonlinear lattices. Rod-like discrete breathers (DBs) have been reported in fcc Ni and bcc Nb by Haas et al. in 2011 based on molecular dynamics simulations. Here we use a general approach to find new type DBs in bcc V and Nb. In this approach, we firstly find the lattice symmetry dictated exact solutions to the equations of atomic motion in the form of delocalized nonlinear vibrational modes (DNVMs). Secondly, a localizing function with spherical symmetry is imposed over the DNVMs. Parameter of the localizing function is chosen such that the obtained DB has a long lifetime. The results presented in this work demonstrate that pure metals can support a variety of DBs. Interatomic potentials have a strong effect on the DB lifetime. Maximal DB lifetime in V is two orders of magnitude larger than that in Nb. The results of this study are interesting for the theory of DBs, and also they will help to better understand the impact of DBs on the physical properties of metals.

Surface discrete breathers in Pt3Al intermetallic alloy

It is known that defect-free crystals can support spatially localized, large amplitude vibrational modes with frequencies outside the linear phonon spectrum. Such excitations are called discrete breathers (DB) or intrinsic localized modes. So far, for 3D crystals DB were considered only in the bulk. In the present molecular dynamics study, for the first time, we demonstrate that DB can be excited at the low Miller indices surfaces of the Pt3Al intermetallic alloy. It is shown that properties of the DB depend essentially on the surface orientation and termination, as well as on DB polarization. The study of DB at crystal surfaces is important because they can localize energy of order of 1 eV, which can reduce the potential barrier for local structure transformation or a chemical reaction.

The structure of tris(chloromethyl)amine in the gas phase using quantum chemical calculations and gas electron diffraction and as a solid and melt using Raman spectroscopy

The equilibrium structure of tris(chloromethyl)amine, N(CH2Cl)3, has been determined in the gas phase using electron diffraction. Single-step distance corrections (representing the differences between the interatomic distances from the equilibrium structure and those from the vibrationally averaged structure) and amplitudes of vibration have been computed using semi-empirical molecular dynamics (SE-MD) simulations in order to treat accurately the description of the low-frequency, large-amplitude vibrational modes associated particularly with one CH2Cl group. A series of complementary theoretical calculations using the SOGGA11-X DFT functional with correlation-consistent basis sets of double-, triple-, and quadruple-ζ quality is also presented. The agreement between the experimental and theoretical equilibrium structural parameters attests to the accuracy of the applied theoretical calculations and of our gas-phase structural solution. Raman spectra have been recorded over a range of temperatures, allowing the solid and the melt to be studied, and the Raman-active intramolecular modes to be identified. Free from the influence of intermolecular interaction, the structure of tris(chloromethyl)amine in the gas phase is markedly different to that reported in the literature for the single crystal. This is discussed, and evidence for the anomeric effect in tris(chloromethyl)amine is evaluated.

Aggregation induced emission enhancement from antipyrine-based schiff base and its selective sensing towards picric acid

A heteroatom containing Schiff base, 4-[(2-Hydroxy-naphthalen-1-ylmethylene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (NDAP) has been synthesized using one step condensation method. Compound, NDAP is weakly emissive in solution state but a strong emission is observed in its aggregated state due to the restriction of intramolecular rotation (RIR) and large amplitude vibrational modes of NDAP. This phenomenon is known as aggregation induced emission enhancement (AIEE). Various morphologies of NDAP microstructures have been synthesized using reprecipitation method. SDS is also used as a morphology directing agent. Morphologies of the particles are characterized using optical and scanning electron microscopy (SEM). Photophysical properties of aggregated NDAP hydrosol are studied using UV–vis absorption, steady state and time resolved fluorescence spectroscopy. The ‘turn off’ luminescent property of aggregated NDAP hydrosol is used for the selective detection of trace amounts of picric acid in aqueous medium and the fluorescence quenching is explained due to ground state complexation between NDAP and picric acid with quenching constant (Kq), 1.91×105M−1.

Aggregation induced emission from α-napthoflavone microstructures and its cyto-toxicity

α-Napthoflavone (ANF) microstructures of various morphologies were synthesized using reprecipitation method. Sodium Dodecyl Sulfate (SDS) was used as morphology directing agent. The morphologies of the particles were characterized using optical and scanning electron microscopy (SEM). Single crystal data of ANF suggests that the aromatic units of ANF are in parallel slipped conformation in its aggregated form. Photophysical properties of aggregated ANF hydrosol were studied using UV–Vis absorption, steady state and time resolved spectroscopy. Red shift and broadening of UV–Vis spectra of ANF hydrosol are explained due to strong π-π and H-π interactions among the neighboring ANF molecules within the aggregated microstructures. Though ANF is non-luminescent in good solvent, a strong emission is observed in its aggregated state. This aggregation induced emission (AIE) has been explained due to restriction of intramoleculer rotation and large amplitude vibrational modes of ANF in its aggregated state. Our Photophysical study also reveals that AIE effect decreases after an optimum concentration of ANF and this has been explained due to softening of crystal lattice. Cytotoxicity of ANF hydrosol was examined to get an idea of the toxic level of this hydrosol toward cultured normal human cells. It is observed that ANF hydrosol may draw beneficial effect in biological application as it has no higher toxic activity but has antioxidant property.

Discrete breathers in crystals with the NaCl structure

In the recent decade, the spatially localized large-amplitude vibrational modes in defect-free crystals, referred to as discrete breathers (DBs), are intensively investigated in the materials science. This review reports the main results on gap DBs in crystals with the NaCl structure. The experimental proof of their existence in a NaI crystal is described. A number of molecular dynamics simulation results are presented, including new so far unpublished data. The properties of crystals potentially affected by the DBs are discussed.

Discrete breather on the edge of the graphene sheet with the armchair orientation

Linear and nonlinear vibrations of a graphene nanoribbon with free armchair edges subjected to tensile deformation have been studied by atomistic simulation methods. It has been shown that the phonon modes are split into two subsets. Atoms in some (XY) modes vibrate in the nanoribbon plane and in other (Z) modes vibrate along the normal to this plane. The possibility of the excitation of a gap discrete breather in an extended nanoribbon in the spectrum of the Z modes, the frequency of which lies in the gap of the spectrum of the XY modes, has been demonstrated. This breather is a large-amplitude vibrational mode in the XY plane localized on the four atoms on the nanoribbon edge. The breather is unstable with respect to small perturbations in the form of displacements of atoms out of the nanoribbon plane. Nevertheless, the discrete breather decays slowly owing to its weak interaction with the Z modes, so that its lifetime can be on the order of 103 vibrational periods.

Terahertz pulsed imaging and spectroscopy for biomedical and pharmaceutical applications.

Terahertz (THz) radiation lies between the infrared and microwave regions of the electromagnetic spectrum. Advances in THz technology have opened up many opportunities in this scientifically and technologically important spectroscopic region. The THz frequency range excites large amplitude vibrational modes of molecules as well as probing the weak interactions between them. Here we describe two techniques that utilize THz radiation, terahertz pulsed imaging (TPI) and terahertz pulsed spectroscopy (TPS). Both have a variety of possible applications in biomedical imaging and pharmaceutical science. TPI, a non-invasive imaging technique, has been used to image epithelial cancer ex vivo and recently in vivo. The diseased tissue showed a change in absorption compared to normal tissue, which was confirmed by histology. To understand the origins of the differences seen between diseased and normal tissue we have developed a TPS system. TPS has also been used to study solids of interest in the pharmaceutical industry. One particularly interesting example is ranitidine hydrochloride, which is used in treatment of stomach ulcers. Crystalline ranitidine has two polymorphic forms known as form 1 and form 2. These polymorphs have the same chemical formula but different crystalline structure that give rise to different physiochemical properties of the material. Using TPS it is possible to rapidly distinguish between the two polymorphic forms.

Boltzmann wavepacket dynamics on periodic molecular potential functions

The results of a computational study of tunnelling of molecules possessing a single large amplitude vibrational mode with a periodic potential energy function which has equivalent or inequivalent energy minima are described. A time domain description of tunnelling is compared with the frequency domain description given in terms of transitions between stationary states largely localised in distinct potential wells. In particular, the behaviour of Boltzmann wavepackets possessing ensemble average characteristics on such potential functions is described. The role of accidental degeneracies between states with eigenfunctions of appropriate symmetry in mediating tunnelling between inequivalent energy domains on such potential functions is discussed.

Boltzmann wavepacket dynamics of tunnelling of molecules through symmetric and asymmetric energy barriers on non-periodic potential functions

This paper describes the results of a computational study of tunnelling of molecules possessing a single large amplitude vibrational mode subject to symmetric and asymmetric non-periodic potential energy functions. The paper compares a time domain description of tunnelling and its consequences for measurable physical properties, with the frequency domain description given in terms of transitions between stationary states largely localised in distinct potential wells. In particular, the behaviour of Boltzmann wavepackets possessing ensemble average characteristics on such potential functions is discussed. As an illustration, it is shown that these wavepackets embody a classical description of the oscillating dipole moment components in molecules like cyanamide, ammonia and chlorocyclobutane, which can be reconciled with the quantum mechanical measurement of the relevant transition moments in such cases. Resonance enhancement of tunnelling between inequivalent energy domains of asymmetric potential functions is also described.

Theoretical study of the O-H stretching band in 3-hydroxy-2-methyl-4-pyrone

The infrared spectrum of 3-hydroxy-2-methyl-4-pyrone reveals an O-H stretching frequency roughly 200 cm−1 lower than that of a typical alcohol group. The frequency lowering results from intramolecular hydrogen bonding between the alcohol and ketone groups. In this work, the stretching and bending vibrations of the O-H group in 3-hydroxy-2-methyl-4-pyrone are studied with a theoretical methodology more rigorous than the conventional harmonic approximation. A two-dimensional potential energy surface in internal coordinates corresponding to different hydrogen positions in the plane of the molecule is calculated with the use of the second order Mo/ller-Plesset perturbation theory. To include all possible variations in kinetic energy in a large amplitude vibrational mode, g-matrix elements with variable values are employed. The analytical expression for the Hamiltonian matrix elements of the two-dimensional vibrational problem in a basis of shifted Gaussian functions is derived. Expectation values for the O-H stretch nuclear states are variationally determined with the use of shifted Gaussian functions as the basis set. The results of the calculations are compared with the recent matrix-isolation infrared (IR) spectroscopic results. The calculated transition frequency corresponding to the in-plane O-H stretching is found to be in good agreement with the experimental value.

The calculation of the vibration-rotation energies of triatomic molecules using scattering coordinates

Department of Physics and Astronomy, University College London, Gower Street, London WCIE 6BT, England Received 25 January 1986 This paper is a review of work I have been involved with concerning the calculation of ro-vibrational energy levels and wavefunctions, mainly of triatomic systems. The method reviewed is that of Tennyson and Sutcliffe for the calculation of spectra of triatomics as atom-diatom collision complexes, which is particularly ap- propriate for molecules with one or more large amplitude vibrational mode. The method uses polynomial basis functions to carry the nuclear motions and is variational for a given potential. Hamiltonians for four different sets of body-fixed axes are derived. The computational implementation of the method is discussed, including improvements to some published algorithms. The use of a two step (secondary) variational procedure for highly-excited rotational states is discussed and sample results given. Extensions to non-Born-Oppenheimer problems, transition intensities and polyatomic molecules are also considered. Comparisons are made with other techniques for calculating ro-vibrational spectra with some specific molecular examples.

Floppy molecules — The influence of phase on apparent geometry

Floppy molecules - that is, molecules with at least one large amplitude vibrational mode, and a correspondingly low force constant — are by their nature susceptible to considerable structural changes during transition from one phase to another. The most common structural change with phase involves conformation, and a number of examples are discussed. Then systems involving large amplitude angle deformations are considered. These include species such as silyl ethers and pseudohalides, and quasi-planar species. Finally, as large amplitude stretching vibration ultimately leads to dissociation, relationships between some monomers and dimers are discussed.