Vibrational Delocalization Research Articles

How Ions Affect the Structure of Water: A Combined Raman Spectroscopy and Multivariate Curve Resolution Study

Raman spectroscopy in combination with multivariate curve resolution (Raman-MCR) is used to explore the interaction between water and various kosmotropic and chaotropic anions. Raman-MCR of aqueous Na-salt (NaI, NaBr, NaNO3, Na2SO4, and Na3PO4) solutions provides solute-correlated Raman spectra (SC-spectra) of water. The SC-spectra predominantly bear the vibrational characteristics of water in the hydration shell of anions, because Na(+)-cation has negligible effect on the OH stretch band of water. The SC-spectra for the chaotropic I(-), Br(-), and NO3(-) anions and even for the kosmotropic SO4(2-) anion resemble the Raman spectrum of isotopically diluted water (H2O/D2O = 1/19; v/v) whose OH stretch band is largely comprised by the response of vibrationally decoupled OH oscillators. On the other hand, the SC-spectrum for the kosmotropic PO4(3-) anion is quite similar to the Raman spectrum of H2O (bulk). Comparison of the peak positions of SC-spectra and the Raman spectrum of isotopically diluted water suggests that the hydrogen bond strength of water in the hydration shell of SO4(2-) is comparable to that of the isotopically diluted water, but that in the hydration shell of I(-), Br(-), and NO3(-) anions is weaker than that of the latter. Analysis of integrated area of component bands of the SC-spectra reveals ∼80% reduction of the delocalization of vibrational modes (intermolecular coupling and Fermi resonance) of water in the hydration shell of I(-), Br(-), NO3(-), and SO4(2-) anions. In the case of trivalent PO4(3-), the vibrational delocalization is presumably reduced and the corresponding decrease in spectral response at ∼3250 cm(-1) is compensated by the increased signal of strongly hydrogen bonded (but decoupled) water species in the hydration shell. The peak area-averaged wavenumber of the SC-spectrum increases as PO4(3-) < SO4(2-) < NO3(-) < Br(-) < I(-) and indeed suggests strong hydrogen bonding of water in the hydration shell of PO4(3-) anion.

A simple but accurate potential for the naphthalene-argon complex: Applications to collisional energy transfer and matrix isolated IR spectroscopy

An explicit polarizable potential for the naphthalene-argon complex has been derived assuming only atomic contributions, aiming at large scale simulations of naphthalene under argon environment. The potential was parametrized from dedicated quantum chemical calculations at the CCSD(T) level, and satisfactorily reproduces available structural and energetic properties. Combining this potential with a tight-binding model for naphthalene, collisional energy transfer is studied by means of dedicated molecular dynamics simulations, nuclear quantum effects being accounted for in the path-integral framework. Except at low target temperature, nuclear quantum effects do not alter the average energies transferred by the collision or the collision duration. However, the distribution of energy transferred is much broader in the quantum case due to the significant zero-point energy and the higher density of states. Using an ab initio potential for the Ar-Ar interaction, the IR absorption spectrum of naphthalene solvated by argon clusters or an entire Ar matrix is computed via classical and centroid molecular dynamics. The classical spectra exhibit variations with growing argon environment that are absent from quantum spectra. This is interpreted by the greater fluxional character experienced by the argon atoms due to vibrational delocalization.

Origin of Vibrational Spectroscopic Response at Ice Surface.

Since the basal plane surface of ice was first observed by sum frequency generation, an extraordinarily intense band for the hydrogen(H)-bonded OH stretching vibration has been a matter of debate. We elucidate the remarkable spectral feature of the ice surface by quantum mechanics/molecular mechanics calculations. The intense H-bonded band is originated mostly from the "bilayer-stitching" modes of a few surface bilayers, through significant intermolecular charge transfer. The mechanism of enhanced signal is sensitive to the order of the tetrahedral ice structure, as the charge transfer is coupled to the vibrational delocalization.

Nuclear quantum effects on the stability of cationic neon clusters

The stable structures of cationic neon clusters containing up to 57 atoms have been located using a diatomic-in-molecules potential energy surface and basin-hopping hierarchical optimization. The effects of vibrational delocalization were included either in the harmonic approximation, or by performing Langevin molecular dynamics simulations coupled to a quantum thermal bath at T=0. For most clusters, zero-point motion is sufficiently high to blur the picture of a single well-defined structure. However, structural diversity of the ground state wavefunction is found to be lower at sizes 14, 21, and 56, which correspond to special stabilities in experimental mass spectra.

Item 124 – Sarcoïdose

The stable structures of cationic neon clusters containing up to 57 atoms have been located using a diatomic-in-molecules potential energy surface and basin-hopping hierarchical optimization. The effects of vibrational delocalization were included either in the harmonic approximation, or by performing Langevin molecular dynamics simulations coupled to a quantum thermal bath at T=0. For most clusters, zero-point motion is sufficiently high to blur the picture of a single well-defined structure. However, structural diversity of the ground state wavefunction is found to be lower at sizes 14, 21, and 56, which correspond to special stabilities in experimental mass spectra.

Size-induced melting and reentrant freezing in fullerene-doped helium clusters

The structural and dynamical stabilities of C${}_{60}^{+}$He${}_{N}$ clusters are theoretically investigated using global optimization and path-integral simulation methods. Up to $N=32$, the fullerene ion traps the helium atoms onto sixfold and fivefold faces, strongly enough to negate vibrational delocalization. Above this size, geometric frustration takes over and the clusters grow as a thin but homogeneous liquid layer. However, as their size reaches 60 atoms, corrugation barriers are suppressed and the cluster is again rigidlike. Additional fluid layers are predicted to arise above 72 atoms.

Vibrational spectra of polyatomic molecules assisted by quantum thermal baths

We assess the performance of colored-noise thermostats to generate quantum mechanical initial conditions for molecular dynamics simulations, in the context of infrared spectra of large polyatomic molecules. Comparison with centroid molecular dynamics simulations taken as reference shows that the method is accurate in predicting line shifts and band widths in the ionic cluster (NaCl)(32) and in the naphthalene molecule. As illustrated on much larger polycyclic aromatic hydrocarbons, the method also allows fundamental spectra to be evaluated in the limit of T = 0, taking into account anharmonicities and vibrational delocalization.

Interplay between charge and vibrational delocalization in cationic helium clusters

The stable structures and low temperature thermodynamics of cationic helium clusters are investigated theoretically using a diatomics-in-molecules model for the potential energy surfaces and a computational framework in which both electronic and nuclear degrees of freedom are treated on a quantum mechanical footing. While the charge is generally carried by two atoms, vibrational delocalization significantly spreads out the charge over multiple isomers for clusters containing five or more helium atoms. Our calculations indicate that large clusters are essentially fluid with a well-defined solvation shell around the charged core.

Does Vibrational Delocalization Stabilize Multiply-Charged Neon Clusters?

The dissociation of Nen2+ clusters is theoretically investigated by extending the liquid drop model to include vibrational delocalization and finite temperature effects as well as by using explicit path-integral molecular dynamics simulations with a polarizable many-body potential. Both methods show that quantum nuclear effects are not sufficient to explain the particularly low appearance size of nc = 288 reported experimentally, hereby suggesting that other mechanisms, possibly electronic, play an important role.

Diffusion of Hydrides in Palladium Nanoclusters. A Ring-Polymer Molecular Dynamics Study of Quantum Finite Size Effects.

The diffusion kinetics of hydrogen in bulk palladium and in Pd nanoclusters containing up to 512 atoms has been theoretically investigated at 3% loading using ring-polymer molecular dynamics simulations. The electronic ground-state energy surfaces are modeled using an explicit many-body potential fitted to reproduce the properties of bulk palladium and palladium hydrides. The diffusion constant, calculated by integration of the velocity autocorrelation function, shows Arrhenius behavior with inverse temperature. In addition, both the prefactor and activation energy are found to exhibit approximately linear variations with inverse cluster radius for sizes exceeding 128 Pd atoms. Vibrational delocalization generally enhances diffusion, this effect being stronger in clusters than in bulk. An inherent structure analysis from the positions of the centroids was used to characterize the diffusion mechanisms. Quantum effects lead to not only a higher coordination of hydrogen atoms both in bulk (fcc) palladium and in clusters but also favor further softening of the outer layers.

16-20 μm Spectra of Carbon Nanoparticles

A comparison between spectra of carbon nanoparticles in the 16-20 μm range and those of interstellar sources shows that the major emission features attributed to aromatic hydrocarbons in this region are present in laboratory spectra. These features are tentatively identified with bending and wagging modes of polycyclic aromatic hydrocarbons containing aliphatic hydrocarbon side groups. Detailed comparison of laboratory spectra with emission spectra of NGC 7023 shows that variations in the relative intensity of features at 16.4, 16.6, 17.2, 17.4, 17.8, and 18.9 μm arise, in part, from changes in chemical composition. The feature at 18.9 μm appears to be unrelated to that of other bands in laboratory spectra, mimicking the behavior of this feature in NGC 7023. The line widths of these features in laboratory spectra are typically 5-15 cm−1 and are consistent with vibrational de-localization on a timescale of ~10−12 s. This indicates that the emitters of these bands in interstellar sources could contain in excess of 103 carbon atoms.

Time-domain calculations of the polarized Raman and two-dimensional infrared spectra of liquid N, N-dimethylformamide

A time-domain method for calculating polarized Raman and two-dimensional infrared (2D-IR) spectra that includes the effects of both the diagonal frequency modulations (of individual molecules in the system) and the off-diagonal (intermolecular) vibrational coupling is presented. It is then applied to the case of the amide I band of liquid N, N-dimethylformamide. The non-coincidence effect in the polarized Raman spectrum and the corresponding spectral feature in the 2D-IR spectrum related to the off-diagonal vibrational coupling and delocalization of vibrational modes are discussed.

Characteristic two-dimensional IR spectroscopic features of antiparallel and parallel β-sheet polypeptides: Simulation studies

The antiparallel and parallel beta sheets are two of the most abundant secondary structures found in proteins. Although various spectroscopic methods have been used to distinguish these two different structures, the linear spectroscopic measurements could not provide incisive information for distinguishing an antiparallel beta sheet from a parallel beta sheet. After carrying out quantum-chemistry calculations and model simulations, we show that the polarization-controlled two-dimensional (2D) IR photon echo spectroscopy can be of critical use in distinguishing these two different beta sheets. Particularly, the ratio between the diagonal peak and the cross peak is found to be strongly dependent on the quasi-2D array of the amide I local-mode transition dipole vectors. The relative intensities of the cross peaks in the 2D difference spectrum of an antiparallel beta sheet are significantly larger than those of the diagonal peaks, whereas the cross-peak amplitudes in the 2D difference spectrum of a parallel beta sheet are much weaker than the main diagonal-peak amplitudes. A detailed discussion on the origin of the diagonal- and cross-peak intensity distributions of both the antiparallel and parallel beta sheets is presented by examining vibrational exciton delocalization, relative angles between two different normal-mode transition dipoles, and natures of the cross peaks in the 2D difference spectrum.

Quenching of cold antiprotonic helium atoms by collisions with H 2 molecules

We investigate the collisional quenching of cold metastable antiprotonic atomcules [ p ̄ He +] n,l by H 2 molecules in view of the recent state-resolved measurements at CERN. Firstly, we determine ab initio the 6-D intermolecular interaction between the four (anti)nuclei at the CCSD(T)/CP level. After averaging the interaction over the fast p ̄ orbits, we exhibit reactive channels and activation barriers below few 100 μE h. Hence, we account qualitatively for the order of magnitude and ( n, l) dependence of the quenching cross-sections measured at 30 K, after estimating tunneling probabilities. We also account for the lower quenching efficiency by deuterium. However improving this overall agreement would require the determination of numerous finer contributions. We monitor the saturation of electronic correlation with larger basis sets; we estimate the importance of dynamical relaxation effects; and we stress the role of quantum vibrational and rotational delocalization for the light (p, p ̄ ) nuclei. The latter vibrational corrections lower the barriers, while the rotational H 2 averaging defavours isotropic para- H 2 (J=0) . As a result, we mainly attribute the quenching at 30 K to ortho- H 2 (J=1) . Additional experiments at other temperatures and with para-H 2 would further constrain this ortho– para quenching selectivity, and provide an ideal four-body benchmark for further chemical physics investigations (including tunneling effects, collision-induced Auger decay, etc).

The dynamics of vibrational excitations on surfaces: CO on Ru(001)

We present an experimental and theoretical study of vibrational excitation of the C–O stretch vibration of carbon monoxide adsorbed on a ruthenium Ru(001) surface with ultrashort femtosecond infrared laser pulses. After broadband excitation leading to transfer of a significant fraction of the CO molecules to their first (∼15%) and second (∼5%) vibrationally excited states, we observe a competition between vibrational energy relaxation and energy delocalization through dipole–dipole coupling. We reproduce the observed excited state spectra by solving the three-level Bloch equations and accounting for intermolecular vibrational energy transfer on a picosecond time scale. The rate of vibrational energy transfer, and its coverage-dependence, can be described by a Förster energy transfer mechanism. We discuss possibilities to optimize the degree of localized vibrational excitation of a specific bond of molecules at surfaces through chirped pulse IR excitation.

Direct observation of vibrational energy delocalization on surfaces: CO on Ru(001)

We report the experimental observation of the gradual transition from a local oscillator to a two-dimensional delocalized phonon, observed for the CO-stretch vibration of carbon monoxide adsorbed on a Ru(001) surface by means of broadband-infrared saturation sum-frequency spectroscopy. The data are theoretically reproduced by an exchange model with residence times of the excitation down to 2.5 ps.

ChemInform Abstract: Solid State Studies. Part 29. Crystalline Urea: Evidence for Vibrational Delocalization.

Chemischer InformationsdienstVolume 17, Issue 5 Physical Organic Chemistry ChemInform Abstract: Solid State Studies. Part 29. Crystalline Urea: Evidence for Vibrational Delocalization. K. LIAPIS, K. LIAPISSearch for more papers by this authorU. A. JAYASOORIYA, U. A. JAYASOORIYASearch for more papers by this authorS. F. A. KETTLE, S. F. A. KETTLESearch for more papers by this authorJ. ECKERT, J. ECKERTSearch for more papers by this authorJ. A. GOLDSTONE, J. A. GOLDSTONESearch for more papers by this authorA. D. TAYLOR, A. D. TAYLORSearch for more papers by this author K. LIAPIS, K. LIAPISSearch for more papers by this authorU. A. JAYASOORIYA, U. A. JAYASOORIYASearch for more papers by this authorS. F. A. KETTLE, S. F. A. KETTLESearch for more papers by this authorJ. ECKERT, J. ECKERTSearch for more papers by this authorJ. A. GOLDSTONE, J. A. GOLDSTONESearch for more papers by this authorA. D. TAYLOR, A. D. TAYLORSearch for more papers by this author First published: February 4, 1986 https://doi.org/10.1002/chin.198605049AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume17, Issue5February 4, 1986 RelatedInformation