Molecular dynamics study of spectral characteristics of water-carbon oxide disperse systems

Absorption and reflection IR spectra of aqueous disperse systems that absorbed carbon oxide molecules are calculated. Systems of small and large clusters containing 2 ≤ n ≤ 10 and 11 ≤ n ≤ 20 water molecules, respectively, are studied. Each cluster can absorb one or two carbon oxide molecules. Both real and imaginary parts of dielectric permittivity of disperse systems depend on the number of adsorbed CO molecules to a greater extent than that of water molecules in clusters. The integral intensity of the absorption of IR radiation by cluster systems increases after the absorption of carbon oxide molecules by clusters. However, the ability to absorb and reflect IR radiation decreases with an increase in the concentration of absorbed CO molecules. Upon the growth of heteroclusters due to addition of water molecules, integral intensity of the absorption of thermal radiation can be enhanced or damped. In general, the clusterization and capture of CO molecules by clusters result in an anti-greenhouse effect.     

Computer simulation of the adsorption of acetylene by disperse aqueous medium. IR spectra

Adsorption of acetylene molecules by water clusters at T 230 K was studied by the method of molecular dynamics. Addition of already two C₂H₂ molecules to (H₂O) _n clusters (10 ≤ n ≤ 20) makes them thermodynamically unstable. With an increase in the acetylene concentration in the disperse aqueous system, the IR absorption by the cluster system in the frequency range 0 ≤ ω ≤ 1000 cm^?1 increases. Depending on the number of C₂H₂ molecules per water cluster, the IR reflection by cluster systems can either increase or decrease. The power of the thermal radiation emitted by the clusters considerably increases after the adsorption of C₂H₂ molecules and grows with an increase in the acetylene concentration in the disperse aqueous system.     

Modeling the infrared and raman spectra of silicon dioxide clusters absorbing water

The effect of absorbed water on the dielectric properties of silicon dioxide nanoparticles is studied by the molecular dynamic method. It is demonstrated using the model of flexible molecules that increasing the number of water molecules in the (SiO₂)₅₀ cluster to 40 results in an enhancement of absorption of infrared radiation over the frequency range 0 cm⁻¹ ≤ ω ≤ 1000 cm⁻¹. It is ascertained that the absorption of water molecules by the (SiO₂)₅₀ cluster considerably alters the shape of Raman spectra of light, smoothing all the peaks after the first one, and that water molecules are concentrated near the cluster surface formed by SiO₂ structural units.     

Computer-assisted study of characteristics of water clusters in the presence of nitrogen dioxide

The interaction of IR radiation with water clusters that have absorbed NO₂ molecules is studied by the molecular dynamics method in terms of the polarizable model. Induced dipole moments of H₂O and NO₂ molecules diminish during the addition of one to six NO₂ molecules to (H₂O)₅₀ cluster. The integral intensity of IR absorption by a system consisting of (NO₂) _i (H₂O)₅₀ heteroclusters with 1 ≤ i ≤ 6 decreases, whereas the power of heat emission rises as compared with an (H₂O) _n system. The decrease in the IR absorption and the increase in the IR emission by water clusters with the capture of NO₂ molecules are nonmonotonic. The absorption of NO₂ molecules by water clusters causes a noticeable reduction in the intensity of the first peak and the confluence of the fourth and fifth peaks in the Raman spectrum.     

Computational study of carbon monooxide absorption by ultradisperse systems. Emission spectra

Molecular dynamics was used to study absorption of carbon monoxide molecules by water clusters combined into two groups (2–10 and 11–20 water molecules) on the basis of their statistical weights. Spectral characteristics of the clusters in the frequency range 0 ≤ ω ≤ 1000 cm^?1 were established. Within this range, the integral IR adsorption intensity of both systems increases with addition of CO molecules. The IR emission power increases significantly after a cluster has absorbed one CO molecule but decreases with the absorption of a further CO molecule. A similar situation is observed with the number of electrons “active” toward external electromagnetic radiation. As the smallest clusters containing two CO molecules grow by adding water molecules, the IR emission power decreases. In other cases, these changes are of a periodical character.     

IR absorption,reflection, and emission spectra of aqueous disperse systems interacting with nitric oxide

IR absorption, reflection, and emission spectra of aqueous disperse systems that absorbed molecules of nitric oxide are calculated. In order to reveal the effect of the absorption of NO molecules on the dielectric properties of water clusters with different sizes, clusters are divided into two groups. The first group consists of clusters containing two to ten water molecules, while the second group contains from 11 to 20 H₂O molecules. Six systems of clusters are studied, e.g., (H₂O) _n , and (NO)₂(H₂O) _n with 2 ≤ n ≤ 10 and 11 ≤ n ≤ 20 ranges. An increase in the cluster size in each group leads to the amplification of absorption, reflection, and the power of emission of IR radiation. The doubling of the NO concentration in the disperse system results in weak changes in the absorption of IR radiation, reduces the reflection and decreases the number of electrons participating in the interaction with external IR radiation, as well as significantly lowers the power of thermal radiation emitted by the system.     

Computer study of the absorption of ethane by aqueous ultradispersed medium: IR spectra

Absorption of ethane molecules by water clusters containing 10–20 molecules is studied by the molecular dynamics method. The (H₂O) _n (I), C₂H₆(H₂O) _n (II), and (C₂H₆)₂(H₂O) _n (III) cluster systems are composed on the basis of specific statistical weights. Spectral characteristics of system and single clusters are determined in the frequency range of 0 ≤ ω ≤ 1000 cm^?1. In this frequency range, both real and imaginary parts of dielectric permittivity decrease monotonically after the absorption of C₂H₆ molecules by an aqueous ultradispersed system. Integral coefficient of IR absorption increases, while average (over frequency) reflection coefficient decreases after the absorption of ethane molecules. The intensity of IR scattering by the systems of clusters containing C₂H₆ molecules lowers. Maximal values of radiation power for water clusters with various sizes are balanced with the capture of ethane molecules by the clusters, whereas oscillations in the size dependence of the density of electrons that are active with respect to IR radiation decrease.     

Computer-assisted study of silver absorption by porous silicon dioxide nanoparticles

The absorption of silver atoms by porous silicon dioxide particles is studied by the molecular dynamics method. Upon the absorption of silver atoms, (SiO₂)₅₀ nanoparticles do not increase their volume. A particle is divided into two unequal parts by an island shell formed from SiO₂ structural units, which causes anisotropy in the electrical and thermal conductivity of the nanocomposite. IR absorption and emission spectra, Raman spectra, as well as the number of electrons active with respect to IR radiation are calculated. The calculated absorption spectra show the mode corresponding to the stretching vibrations of Si-O surface groups. The addition of silver atoms to nanoparticles can enhance significantly the power of heat radiation emission.     

Spectral characteristics of water clusters in the presence of nitrate ions

Nitrate ion adsorption by water clusters is studied using the molecular dynamics method combined with a polarizable model of flexible molecules. It is established that successive addition of one to six NO ₃ ^? ions to an (H₂O)₅₀ cluster decreases the averaged electrical potential related to the center of masses of water molecules. The (H₂O)₅₀ cluster retains its thermodynamic stability, provided that no more than three nitrate ions are added to it. After NO ₃ ^? ions are adsorbed, the real and imaginary components of the dielectric permittivity and the intensity of the Raman spectrum decrease, while the intensity of the IR absorption spectrum increases. Moreover, ion adsorption by the water cluster reduces the IR absorption coefficient and refractive index.     

Spectral effects of the clusterization of greenhouse gases: Computer experiment

Autocorrelation functions of the total dipole moment of clusters composed of H₂O and N₂O molecules are calculated in terms of the molecular dynamics method. The IR absorption and reflection spectra of systems composed of (H₂O)_i, N₂O(H₂O)_i, and (N₂O)₂(H₂O)_i clusters (2 ≤ i ≤ 20) are obtained on the basis of these functions. Frequency-dependent dielectric permittivity of clusters increases after the absorption of N₂O molecules. The absorption coefficient of cluster systems with trapped N₂O molecules increases at low frequencies and decays at frequencies ω > 500 cm^?1. The inclusion of N₂O molecules increases also reflection coefficient R and changes the pattern of R(ω) spectra. The absorption of IR radiation increases with the number of H₂O molecules in clusters. Dielectric losses also increase with an increase in i number upon the absorption of N₂O molecules. The number of electrons interacting with an incident electromagnetic wave increases upon the capture of N₂O molecules.     

Spectral characteristics of water clusters in the presence of nitrogen dioxide

The absorption of NO₂ molecules by a water cluster containing 25 molecules was studied by molecular dynamics. The calculated dielectric characteristics of a system of (NO₂) _i (H₂O)₂₅ clusters (1 ≤ i ≤ 6) were compared with similar data for a cluster system of pure water. The ability of the disperse water system that trapped NO₂ molecules to absorb IR radiation increased, and the rate of the absorbed energy emission decreased. The Raman spectrum of the disperse system that absorbed NO₂ molecules changed most significantly in the low-frequency range. The emission time of cluster-generated radiation was much smaller than the lifetime of the clusters.     

IR spectra of aqueous disperse systems adsorbed atmospheric gases: 1. Nitrogen

Spectral characteristics of (H₂O) _i , N₂(H₂O) _i , and (N₂)₂(H₂O) _i cluster systems, where 10≤i≤50, are studied in the 0 ≤ ε ≤ 3500 cm^?1 frequency range with the molecular dynamics method on the basis of a flexible molecule model. After nitrogen is captured by an aqueous disperse system, the absorption of the IR radiation by this system increases owing to the enhancement of intramolecular vibrations. In general, the reflection of the outer IR radiation by nitrated aqueous disperse systems is attenuated; however, when the nitrogen concentration increases twofold, there is a tendency toward an increase in the fraction of reflected radiation. As the nitrogen concentration in a system of water clusters rises, the power of radiation emitted by the system increases significantly and the number of electrons interacting with the outer IR radiation decreases.     

Dielectric characteristics of O2(H2O)i and (O2)2(H2O)i clusters. Computer-aided experiment

Absorption of oxygen molecules by water clusters with sizes of 10 ≤ i ≤ 50 is studied by the molecular dynamics method using the modified TIP4P model. It is revealed that the total dipole moment of the clusters nonmonotonically increases with their sizes. Absorption of O₂ molecules tends to raise the static permittivity of the ultradispersed medium formed by the clusters. The real and imaginary parts of the permittivity of water clusters with absorbed O₂ molecules are aperiodic functions of frequency. The permittivity components turn out to be nonmonotonic functions of cluster sizes. The IR absorption and reflectance spectra are calculated for clusters of pure water and aggregates with absorbed O₂ molecules. After the addition of oxygen molecules, the absorption coefficient of the clusters decreases, while the reflection coefficient increases. It is concluded that the capture of oxygen molecules by atmospheric moisture may reduce the greenhouse effect. Original Russian Text ? A.E. Galashev, V.N. Chukanov, O.A. Galasheva, 2006, published in Kolloidnyi Zhurnal, 2006, Vol. 68, No. 2, pp. 155–160.     

Spectral characteristics of aqueous dispersions enriched in oxygen

The IR absorption and reflection spectra of aqueous dispersions consisting of (H₂O)_n, O₂(H₂O)_n, and (O₂)₂(H₂O)_n clusters (10 ≤ n ≤ 50) were calculated by the method of molecular dynamics using a flexible model of molecules. The frequency distribution of the power scattered by the cluster systems was obtained in the range 0 ≤ ω ≤ 3000 cm^?1. The capture of one oxygen molecule by the clusters is accompanied by a decrease in the absorption of the low-frequency IR radiation and by a peak of the absorption intensity in the vicinity of ω 2704 cm^?1. This is also accompanied by a decrease in the reflection coefficient throughout the frequency range and a decrease in the emission power at ω < 1030 and ω > 1700 cm^?1. Addition of two oxygen molecules to the clusters decreases the capability of the dispersions for the absorption, reflection, and scattering of IR radiation.     

Adsorption of ammonia by water clusters. Computer experiment

The method of molecular dynamics is used to study the adsorption of from one to six ammonia molecules by water clusters composed of 50 molecules. The adsorption of NH₃ molecules markedly increases the IR absorption spectrum intensity, substantially decreases emission power in the frequency range of 0 ≤ ω ≤ 3500 cm^?1, and transforms a continuous reflectance spectrum into a banded one. A rough surface formed by adsorbed ammonia molecules reduces the absorption coefficient and refractive index of the system of water-ammonia clusters in the entire frequency range. Adsorption of ammonia molecules by water clusters greatly diminishes the number of electrons that are active with respect to electromagnetic radiation.     

IR spectra of water clusters with captured ethane molecules: Computer simulation

Uptake of ethane molecules by a monodisperse aqueous system was simulated by molecular dynamics. The cluster (H₂O)₂₀ characterizing the system remains stable until the number of the captured C₂H₆ molecules becomes larger than four. Addition of ethane molecules to the disperse aqueous system decreases both the real and imaginary parts of the dielectric permittivity in the frequency range 0 ≤ ω ≤ 1000 cm^?1. The integral IR absorption coefficient of the disperse system containing C₂H₆ molecules increases, and the frequency-average reflection coefficient decreases. The continuous reflection spectra transform into band spectra. The heat-radiating power of the clusters decreases upon absorption of ethane molecules. The cluster that took up two ethane molecules exhibits the highest radiating power. This cluster has the largest number of active electrons interacting with the arriving wave.     

Cluster mechanism of the atmosphere ozone destruction by bromine ions

Interaction of bromine ions absorbed by water cluster with adsorbed oxygen and ozone molecules has been investigated by the molecular dynamics method. It was shown that the part of O₂ molecules was removed from the system by evaporating Br⁻ ions, while all O₃ molecules and Br ions were kept in the system during 25 ps. The increase the concentration of the Br⁻ ions in the clusters resulted in a reduction of the absorption intensity and emission in IR spectra at the presence of oxygen, whereas the absorption intensity in the appropriate IR spectra of ozone-containing systems increased with the growth of a number of the Br⁻ ions. Raman spectra of oxygen-containing systems were poorly sensitive to the concentration of the Br⁻ ions but the absorption intensity of Raman spectra for systems with ozone considerably decreased with the growth of a number of bromine ions.     

Molecular dynamics calculation of spectral characteristics of water clusters in the presence of ozone molecules and chlorine ions

Simultaneous interaction of the (H₂O)₅₀ cluster with O₃ molecules and Cl⁻ ions was studied by the molecular dynamics method. Six O₃ molecules located near the cluster were absorbed by the aqueous aggregate, and Cl⁻ ions in turn left the zone of the interaction with the cluster. Some of Cl⁻ ions penetrated inside the formed (O₃)₆(H₂O)₅₀ cluster and come into collision with O₃ molecules that split the ozone molecule into atoms. When Cl⁻ ions were removed sufficiently far away from the cluster, the water cluster with absorbed O₃ molecules and O atoms was observed for 15.6 ps. The interaction of water molecules with Cl⁻ ions gives rise to an increase in the integral intensity of absorption and emission IR spectra, and also to an essential decrease in the analogous characteristics of the Raman spectrum in the frequency range of 0 ≤ ω ≤ 1000 cm⁻¹. The presence of Cl⁻ ions did not affect essentially the location of the main band in the IR spectra, but considerably changed the shape of the bands in the Raman spectrum.     

Dielectric properties of water clusters containing CO and NO molecules. Computational experiment

The absorption of CO and NO molecules by (H₂O)₂₀ clusters was studied by the method of molecular dynamics. In general, the clusters containing CO molecules are more stable mechanically, while the clusters with NO molecules are more stable against heating. The mobility of NO molecules in such clusters is higher than that of CO molecules. The total dipole moment, the static dielectric permeability, the number of active electrons in the clusters, and the specific number of hydrogen bonds between water molecules possess peak values when the number of doping molecules i = 6. IR absorption spectra mostly acquire a smooth shape at i > 6. Capture of CO and NO molecules by water cluster operates as anti-greenhouse effect.     

Computer-aided study of oxygen absorption by water clusters. IR spectra of heteroclusters

Spectral characteristics of (H₂O)_n, (O₂)_m(H₂O)_n, and (O)_i(H₂O)_n cluster systems, where m≤2, i≤4, and 10 ≤ n ≤ 50, are studied with the molecular dynamics method using a flexible molecule model. The IR absorption spectra are changed substantially as a result of O₂ molecule dissociation, and in the presence of atomic oxygen in the clusters, the spectra are characterized by a deep minimum at 520 cm^?1. The absorption of oxygen causes a marked reduction in reflection coefficient R of monochromatic IR radiation. The number of peaks in the R(ω) spectra decreases to two in the case of molecular oxygen absorption and is no larger than four in the case of atomic oxygen absorption. The absorption of atomic oxygen by the clusters is also accompanied by a significant increase in the dissipation of energy accumulated by the clusters. This effect weakens when molecular oxygen is absorbed. An increase in atomic oxygen concentration in the clusters renders their radiation harder.