Volume 137, Issue 24, 28 December 2012
Index of content:

We present a multiscale, hierarchical, approach for developing lattice models to estimate adsorption in nanoporous sorbents, derived on the basis of underlying atomistic potentials. This approach is a generalization of earlier work in zeolites (where the specific adsorption sites are easily definable) to encompass both specific as well as diffuse adsorption; the latter often dominates in the case of nanoporous metalorganic frameworks (MOFs). In conjunction with appropriately coarse grained guestguest interactions, we demonstrate that our lattice approach offers semiquantitative to quantitative agreement as compared to fully atomistic simulation from the low pressure regime through saturation. However, it also yields ordersofmagnitude acceleration versus the latter, thus enabling highthroughput screenings of both nonpolar and polar adsorbates with high efficiency. We also show how our lattice model can be extended to facilitate rapid, qualitative screening of transport properties via appropriate calibration. Although our example applications focus on CO_{2}adsorption in MOFs, this approach is readily generalizable to various nanoporous materials (MOFs, zeolites…) and guest adsorbates (CO_{2}, H_{2}, hydrocarbons).
 COMMUNICATIONS


Communication: Rovibrationally selected absolute total cross sections for the reaction H_{2}O^{+}(X ^{2} B _{1}; v _{1} ^{+} v _{2} ^{+} v _{3} ^{+} = 000; N ^{+} _{ K } _{a+Kc+)} + D_{2}: Observation of the rotational enhancement effect
View Description Hide DescriptionBy employing the newly established vacuum ultraviolet laser pulsed field ionizationphotoion (PFIPI) double quadrupoledouble octopole ion guide apparatus, we have measured the rovibrationally selected absolute total cross sections of the ionmolecule reaction H_{2}O^{+}(X ^{2} B _{1}; v _{1} ^{+} v _{2} ^{+} v _{3} ^{+} = 000; N ^{+} _{ K } _{a+Kc+)} + D_{2} → H_{2}DO^{+} + D in the centerofmass collision energy (E _{cm}) range of 0.05–10.00 eV. The pulsing scheme used for the generation of PFIPIs has made possible the preparation of reactant H_{2}O^{+}(X ^{2} B _{1}; v _{1} ^{+} v _{2} ^{+} v _{3} ^{+} = 000) ions in single N ^{+} _{ K } _{a+Kc+} rotational levels with high kinetic energy resolutions. The absolute total cross sections observed in different N ^{+} _{ K } _{a+Kc+} levels with rotational energies in the range of 0–200 cm^{−1} were found to exhibit a significant rotational enhancement on the reactivity for the titled reaction. In contrast, the measured cross sections reveal a decreasing trend with increasing E _{cm}, indicating that the rotational enhancement observed is not a total energy effect, but a dynamical effect. Furthermore, the rotational enhancement is found to be more pronounced as E _{cm} is decreased. This experiment provided evidence that the coupling of the core rotational angular momentum with the orbital angular momentum could play a role in chemical reactivity, particularly at low E _{cm}.

 ARTICLES

 Theoretical Methods and Algorithms

NVU dynamics. III. Simulating molecules at constant potential energy
View Description Hide DescriptionThis is the final paper in a series that introduces geodesic molecular dynamics at constant potential energy. This dynamics is entitled NVUdynamics in analogy to standard energyconserving Newtonian NVEdynamics. In the first two papers [T. S. Ingebrigtsen, S. Toxvaerd, O. J. Heilmann, T. B. Schrøder, and J. C. Dyre, J. Chem. Phys.135, 104101 (2011)10.1063/1.3623585; T. S. Ingebrigtsen, S. Toxvaerd, T. B. Schrøder, and J. C. Dyre, J. Chem. Phys.135, 104102 (2011)10.1063/1.3623586], a numerical algorithm for simulating geodesic motion of atomic systems was developed and tested against standard algorithms. The conclusion was that the NVU algorithm has the same desirable properties as the Verlet algorithm for Newtonian NVEdynamics, i.e., it is timereversible and symplectic. Additionally, it was concluded that NVUdynamics becomes equivalent to NVEdynamics in the thermodynamic limit. In this paper, the NVU algorithm for atomic systems is extended to be able to simulate the geodesic motion of molecules at constant potential energy. We derive an algorithm for simulating rigid bonds and test this algorithm on three different systems: an asymmetric dumbbell model, LewisWahnström oterphenyl (OTP) and rigid SPC/E water. The rigid bonds introduce additional constraints beyond that of constant potential energy for atomic systems. The rigidbond NVU algorithm conserves potential energy, bond lengths, and step length for indefinitely long runs. The quantities probed in simulations give results identical to those of NoséHoover NVTdynamics. Since NoséHoover NVTdynamics is known to give results equivalent to those of NVEdynamics, the latter results show that NVUdynamics becomes equivalent to NVEdynamics in the thermodynamic limit also for molecular systems.

An efficient multiscale lattice model approach to screening nanoporous adsorbents
View Description Hide DescriptionWe present a multiscale, hierarchical, approach for developing lattice models to estimate adsorption in nanoporous sorbents, derived on the basis of underlying atomistic potentials. This approach is a generalization of earlier work in zeolites (where the specific adsorption sites are easily definable) to encompass both specific as well as diffuse adsorption; the latter often dominates in the case of nanoporous metalorganic frameworks (MOFs). In conjunction with appropriately coarse grained guestguest interactions, we demonstrate that our lattice approach offers semiquantitative to quantitative agreement as compared to fully atomistic simulation from the low pressure regime through saturation. However, it also yields ordersofmagnitude acceleration versus the latter, thus enabling highthroughput screenings of both nonpolar and polar adsorbates with high efficiency. We also show how our lattice model can be extended to facilitate rapid, qualitative screening of transport properties via appropriate calibration. Although our example applications focus on CO_{2}adsorption in MOFs, this approach is readily generalizable to various nanoporous materials (MOFs, zeolites…) and guest adsorbates (CO_{2}, H_{2}, hydrocarbons).

Lowrank spectral expansions of two electron excitations for the acceleration of quantum chemistry calculations
View Description Hide DescriptionTreatment of twoelectron excitations is a fundamental but computationally expensive part of ab initio calculations of manyelectron correlation. In this paper we develop a lowrank spectral expansion of twoelectron excitations for accelerated electronicstructure calculations. The spectral expansion differs from previous approaches by relying upon both (i) a sum of three expansions to increase the rank reduction of the tensor and (ii) a factorization of the tensor into geminal (ranktwo) tensors rather than orbital (rankone) tensors. We combine three spectral expansions from the three distinct forms of the twoelectron reduced density matrix (2RDM), (i) the twoparticle ^{2} D, (ii) the twohole ^{2} Q, and the (iii) particlehole ^{2} G matrices, to produce a single spectral expansion with significantly accelerated convergence. While the resulting expansion is applicable to any quantumchemistry calculation with twoparticle excitation amplitudes, it is employed here in the parametric 2RDM method [D. A. Mazziotti, Phys. Rev. Lett.101, 253002 (2008)]10.1103/PhysRevLett.101.253002. The lowrank parametric 2RDM method scales quartically with the basisset size, but like its fullrank version it can capture multireference correlation effects that are difficult to treat efficiently by traditional singlereference wavefunction methods. Applications are made to computing potential energy curves of HF and triplet OH^{+}, equilibrium bond distances and frequencies, the HCNHNC isomerization, and the energies of hydrocarbon chains. Computed 2RDMs nearly satisfy necessary Nrepresentability conditions. The lowrank spectral expansion has the potential to expand the applicability of the parametric 2RDM method as well as other ab initio methods to largescale molecular systems that are often only treatable by meanfield or density functional theories.

Performance of recent and highperformance approximate density functionals for timedependent density functional theory calculations of valence and Rydberg electronic transition energies
View Description Hide DescriptionWe report a test of 30 density functionals, including several recent ones, for their predictions of 69 singlettosinglet excitation energies of 11 molecules. The reference values are experimental results collected by Caricato et al. for 30 valence excitations and 39 Rydberg excitations. All calculations employ timedependent density functional theory in the adiabatic, linearresponse approximation. As far as reasonable, all of the assignments are performed by essentially the same protocol as used by Caricato et al., and this allows us to merge our mean unsigned errors (MUEs) with the ones they calculated for both density functional and wave function methods. We find 21 of the 30 density functionals calculated here have smaller MUEs for the 30 valence states than what they obtained (0.47 eV) for the stateoftheart EOMCCSD wave function. In contrast, for all of density functionals the MUE for 39 Rydberg states is larger than that (0.11 eV) of EOMCCSD. Merging the 30 density functionals calculated here with the 26 calculated by Caricato et al. makes a set of 56 density functionals. Averaging the unsigned errors over both the valence excitations and the Rydberg excitations, none of the 56 density functionals shows a lower mean unsigned error than that (0.27 eV) of EOMCCSD. Nevertheless, two functionals are successful in having an overall mean unsigned error of 0.30 eV, and another nine are moderately successful in having overall mean unsigned errors in the range 0.32–0.36 eV. Successful or moderately successful density functionals include seven hybrid density functionals with 41% to 54% Hartree–Fock exchange, and four rangeseparated hybrid density functionals in which the percentage of Hartree–Fock exchange increases from 0% to 19% at small interelectronic separation to 65%–100% at long range.

Gaussian attenuation hybrid scheme applied to the ErnzerhofPerdew exchange hole model (GauPBEh)
View Description Hide DescriptionRecently, we developed a Gaussian attenuation (Gau) scheme for solidstate bandgap calculation that uses a twoelectron Gaussian function operator to include shortrange HartreeFock exchange and combined it with the longrange PerdewBurkeErnzerhof (PBE) exchange correlation functional (GauPBE). Here, we apply the ErnzerhofPerdew exchange hole (EP) model of PBE (PBEh) as a longrange density functional theory(DFT) exchange part to the Gau scheme (GauPBEh). We found that applying the EP model to the Gau scheme improves atomization energies and solidstate lattice constants and that the exact exchange included using the Gau scheme plays a critical role in simultaneously reproducing solidstate bandgaps and barrier heights. In addition, GauPBEh takes nearly the same computation time for bandgap calculations as GauPBE, implying less than 60% of the time taken in HeydScuseriaErnzerhof hybrid DFT functional calculations.

Reaction dynamics with the multilayer multiconfigurational timedependent Hartree approach: H + CH_{4} → H_{2} + CH_{3} rate constants for different potentials
View Description Hide DescriptionThe multilayer extension of the multiconfigurational timedependent Hartree (MCTDH) approach is applied to the investigation of elementary bimolecular chemical reactions. Cumulative reaction probabilities and thermal rate constants of the H + CH_{4} → H_{2} + CH_{3}reaction are calculated using flux correlation functions and the quantum transition state concept. Different coordinate systems and potential energy surfaces (PESs) are studied. The convergence properties of different layerings are investigated and the efficiency of multilayer MCTDH approach is compared to the standard MCTDH approach. It is found that the multilayer approach can decrease the numerical effort by more than an order of magnitude. The increased efficiency resulting from the multilayer MCTDH approach is crucial for quantum dynamical calculations on recent global H + CH_{4} → H_{2} + CH_{3} PESs, e.g., the ZBB3PES [Z. Xie, J. M. Bowman, and X. Zhang, J. Chem. Phys.125, 133120 (2006)10.1063/1.2238871] based on permutational invariant polynomials, which are numerically more demanding than earlier PESs. The results indicate that an accurate description of all transition state frequencies is important to obtain accurate thermal rate constants.

First passage times in homogeneous nucleation and selfassembly
View Description Hide DescriptionMotivated by nucleation and molecular aggregation in physical, chemical, and biological settings, we present a thorough analysis of the general problem of stochastic selfassembly of a fixed number of identical particles in a finite volume. We derive the backward Kolmogorov equation (BKE) for the cluster probability distribution. From the BKE, we study the distribution of times it takes for a single maximal cluster to be completed, starting from any initial particle configuration. In the limits of slow and fast selfassembly, we develop analytical approaches to calculate the mean cluster formation time and to estimate the first assembly time distribution. We find, both analytically and numerically, that faster detachment can lead to a shorter mean time to first completion of a maximumsized cluster. This unexpected effect arises from a redistribution of trajectory weights such that upon increasing the detachment rate, paths that take a shorter time to complete a cluster become more likely.
 Advanced Experimental Techniques

Slow photoelectron velocitymap imaging spectroscopy of cold negative ions
View Description Hide DescriptionAnion slow photoelectron velocitymap imaging (SEVI) spectroscopy is a highresolution variant of photoelectron spectroscopy used to study the electronic and geometricstructure of atoms, molecules, and clusters. To benefit from the high resolution of SEVI when it is applied to molecular species, it is essential to reduce the internal temperature of the ions as much as possible. Here, we describe an experimental setup that combines a radiofrequency ion trap to store and cool ions with the highresolution SEVI spectrometer. For C_{5} ^{–}, we demonstrate ion temperatures down to 10 ± 2 K after extraction from the trap, as measured by the relative populations of the two anion spinorbit states. Vibrational hot bands and sequence bands are completely suppressed, and peak widths as narrow as 4 cm^{−1} are seen due to cooling of the rotational degrees of freedom.
 Atoms, Molecules, and Clusters

Spectroscopic investigation of the A and 3 ^{1}Σ^{+} states of ^{39}K^{85}Rb
View Description Hide DescriptionBy using a combination of molecular beam (MB) excitation spectra and two distinct ultracold molecule excitation spectra (UM+ and UM−), we have assigned high vibrational levels of the A and 3 ^{1}Σ^{+} states from absorption spectra of the mutually strongly perturbed A ^{1}Σ^{+} − 3 ^{1}Σ^{+} − 1 ^{1}Π − 2 ^{3}Σ^{+} − b ^{3}Π states of ultracold ^{39}K^{85}Rb molecules in the energy region between 15 116 and 16 225 cm^{−1} above the minimum of the ground X ^{1}Σ^{+} state. The ultracold molecules (UM+ and UM−) are formed by radiative decay following photoassociation (PA) to a specific level of the 3(0^{+}) state (UM+) or to a specific level of the 3(0^{−}) state (UM−). We observe that the A and 3 ^{1}Σ^{+} states are observable in the UM+ spectra, but absent from the UM− spectra. This is explained by considering Hund's case (c) selection rules and transition dipole moments between the upper excited A ^{1}Σ^{+} (2(0^{+})) state and the three Ω components (0^{+}, 0^{−}, and 1) at the groundstatedissociation limit. We propose further investigations of the extended potential wells of the A and 3 ^{1}Σ^{+} states by combining shortrange MB excitation spectra in a narrow FranckCondon (FC) window near R _{ e } of the X ^{1}Σ^{+} state, and longrange UM (and PA) excitation spectra, which have much larger FC windows.

Characterizing the excess electron of Li(NH_{3})_{4}
View Description Hide DescriptionSmall lithium ammonia clusters are model systems for the dissociation of metals into solvated cations and electrons in ammonia. Metal–ammonia solutions display a complex behavior with increasing metal concentration including a phase change from a paramagnetic to a metallic diamagnetic phase, and small clusters should be useful models in the low concentration regime, where one may expect the ammoniated electron to show a behavior similar to that of the hydrated electron. Yet, even in the low concentration regime the nature of the ammoniated electron is still controversial with cavity models supported by optical and density measurements whereas localized radical models have been invoked to explain magnetic measurements. Small clusters can shed light on these open questions, and in particular the Li–NH_{3} tetramer represents the smallest cluster with a complete solvation shell for the Li^{+} cation. In view of the controversies about the character of the excess electron, the first question investigated is whether different theoretical characterizations of the “excess electron” lead to different conclusions about it. Only small differences are found between orbitalbased and spin densitybased and between selfconsistentfield and coupledclusterbased methods. Natural orbitals from equationofmotion coupledcluster calculations are then used to analyze the excess electron's distribution of Li(NH_{3})_{4} with particular emphasis on the portion of the excess electron's density that is closely associated with the N atoms. Three different comparisons show that only about 6% of the excess electron's density are closely associated with the atoms, with about 1% being closely associated with any N atom, and that the electron is best characterized as a Rydberglike electron of the whole cluster. Finally, it is shown that in spite of the small amount of density close to the N atoms, the spindensity at the N nuclei is substantial, and that the magnetic observations can plausibly be explained within the cavity model.

Geometries and electronic structures of the ground and lowlying excited states of FeCO: An ab initio study
View Description Hide DescriptionFeCO is a molecule of astrophysical interest. We report here theoretical calculations of its geometrical parameters, electronic structures, and molecular constants (such as dipole moment and spinorbit coupling constant) in the electronic ground state and the lowlying triplet and quintet excited states. The calculations were made at the MRSDCI+Q_DK3/[5ZP ANORCC (Fe, C, O)] and MRAQCC_DK3/[5ZP ANORCC (Fe, C, O)] levels of theory. A multireference calculation was required to describe correctly the wavefunctions of all states studied. For all triplet states, the σdonation through the 10σ molecular orbital (MO) as well as the πbackdonation through the 4π MO are observed, and the dipole moment vector points from O toward Fe as expected. However, in the excited quintet states ^{5}Π, ^{5}Φ, and ^{5}Δ, the almost negligible contribution of Fe 4s to the 10σ MO makes the dipole moment vector point from Fe toward O, i.e., in the same direction as in CO. In the state, the electron provided by the σdonation through the 10σ MO is shared between the Fe atom and the C end of the CO residue to form a coordinatecovalent Fe–C bond. In the state (the highspin counterpart of ), the σdonation through the 10σ MO is not significant and so the Fe–C bond is rather ionic. The πbackdonation through the 4π MO is found to be of comparable importance in the two electronic states; it has a slightly larger magnitude in the state. The difference in the molecular properties of the lowspin and the highspin states can be understood in terms of the dynamical electron correlation effects.

Melting behavior of Ag_{14} cluster: An order parameter by instantaneous normal modes
View Description Hide DescriptionThis paper studies the melting behavior of Ag_{14}cluster employing the instantaneous normal mode (INM) analysis that was previously developed for bimetallic clusterAg_{17}Cu_{2}. The isothermal Browniantype molecular dynamics simulation is used to generate atom configurations of Ag_{14} at different temperatures up to 1500 K. At each temperature, these atomic configurations are then analyzed by the INM technique. To delve into the melting behavior of Ag_{14}cluster which differs from Ag_{17}Cu_{2} by the occurrence of an anomalous prepeak in the specific heat curve in addition to the typical principal peak, we appeal to examining the order parameter τ(T) defined in the context of the INM method. Two general approaches are proposed to calculate τ(T). In one, τ(T) is defined in terms of the INM vibrational density of states; in another, τ(T) is defined considering the cluster as a rigid body with its rotational motions described by three orthogonal eigenvectors. Our results for Ag_{14} by these two methods indicate the mutual agreement of τ(T) calculated and also the consistent interpretation of the melting behavior with the specific heat data. The order parameter τ(T) provides in addition an insightful interpretation between the melting of clusters and the concept of broken symmetry which has been found successful in studies of the meltingtransition of bulk systems.

Strongfield induced XUV transmission and multiplet splitting in 4d ^{−1}6p coreexcited Xe studied by femtosecond XUV transient absorption spectroscopy
View Description Hide DescriptionLightinduced coupling of coreexcited states of Xe atoms is investigated by femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy with photon energies ranging from 50 eV to 72 eV. Coupling of the 4d ^{−1}(^{2}D_{5/2})6p(^{2}P_{3/2}) (65.1 eV) and 4d ^{−1}(^{2}D_{3/2})6p(^{2}P_{1/2}) (67.0 eV) coreexcited states to nearby states by a strong infrared laser field leads to a threefold enhancement of XUV transmission. The transmission at 65.1 eV (67.0 eV) changes from 3.2 ± 0.4% (5.9 ± 0.5%) without the coupling laser to 9 ± 2% (22 ± 5%) at the maximum of the laser field. A strongfield induced broad XUV absorption feature between 60 eV and 65 eV is ascribed to splitting of the fieldfree absorption lines into multiple branches when the Rabi frequencies of the coupling transitions exceed the infrared laser frequency. This picture is supported by a comparison of the strongfield induced absorptionspectrum with a numerical integration of the von Neumann equation for a fewlevel quantum system. The valence holealignment of strongfield ionized Xe is revisited, confirming the previously observed reduced alignment compared to theoretical predictions.

The He + → HeH^{+} + H reaction: Ab initio studies of the potential energy surface, benchmark timeindependent quantum dynamics in an extended energy range and comparison with experiments
View Description Hide DescriptionIn this work we critically revise several aspects of previous ab initio quantum chemistry studies [P. Palmieri et al., Mol. Phys.98, 1835 (2000);10.1080/00268970009483387C. N. Ramachandran et al., Chem. Phys. Lett.469, 26 (2009)]10.1016/j.cplett.2008.12.035 of the system. New diatomic curves for the and HeH^{+} molecular ions, which provide vibrational frequencies at a near spectroscopic level of accuracy, have been generated to test the quality of the diatomic terms employed in the previous analytical fittings. The reliability of the global potential energy surfaces has also been tested performing benchmark quantum scattering calculations within the timeindependent approach in an extended interval of energies. In particular, the total integral cross sections have been calculated in the total collision energy range 0.955–2.400 eV for the scattering of the He atom by the ortho and parahydrogen molecular ion. The energy profiles of the total integral cross sections for selected vibrorotational states of (v = 0, …,5 and j = 1, …,7) show a strong rotational enhancement for the lower vibrational states which becomes weaker as the vibrational quantum number increases. Comparison with several available experimental data is presented and discussed.

Formation and relaxation of RbHe exciplexes on He nanodroplets studied by femtosecond pump and picosecond probe spectroscopy
View Description Hide DescriptionVibrationally resolved photoionization spectra of RbHe exciplexes forming on He nanodroplets are recorded using femtosecond pumpprobe spectroscopy with amplitudeshaped probe pulses. The timeevolution of the spectra reveals an exciplex formation time ∼10 ps followed by vibrational relaxation extending up to ≳ 1 ns. This points to an indirect, timedelayed desorption process of RbHe off the He surface.

Molecular modeling study of agglomeration of [6,6]phenylC61butyric acid methyl ester in solvents
View Description Hide DescriptionThe molecular interactions between solvent and nanoparticles during photoactive layer formation in organic photovoltaic (OPV) cells influence the morphology of the photoactive layer and hence determine the power conversion efficiency. Prediction of optimal synthesis parameters in OPVs, such as choice of solvent, processing temperature, and nanoparticle concentration, requires fundamental understanding of the mechanisms that govern the agglomeration of nanoparticles in solvents. In this study, we used molecular dynamics simulations to simulate a commonly used organic nanoparticle, [6,6]phenylC61butyric acid methyl ester (PCBM), in various solvents to correlate solventnanoparticle interactions with the size of the agglomerate structure of PCBM. We analyzed the effects of concentration of PCBM and operating temperature on the molecular rearrangement and agglomeration of PCBM in three solvents: (i) toluene, (ii) indane, and (iii) tolueneindane mixture. We evaluated the agglomeration behavior of PCBM by determining sizes of the largest clusters of PCBM and the corresponding size distributions. To obtain further insight into the agglomerate structure of PCBMs, we evaluated radial distribution functions (RDFs) and coordination numbers of the various moieties of PCBMs with respect to solvent atoms as well as with respect to that of other PCBMs. Our simulations demonstrate that PCBMs form larger clusters in toluene while they are relatively dispersed in indane, which indicates the greater solubility of PCBM in indane than in toluene. In tolueneindane mixture, PCBMs are clustered to a greater extent than in indane and less than that in toluene. To correlate agglomerate size to nanoparticlesolvent interactions, we also evaluated the potential of mean force (PMF) of the fullerene moiety of PCBM in toluene and indane. Our results also show that the cluster size of PCBM molecules increases with the increase of concentration of PCBM and the processing temperature. To correlate the PCBM agglomeration with the dynamics of solvents, we evaluated the rotational correlation functions of the solvents. Our results illustrate that toluene relaxes faster than indane in the simulated systems and relaxation time of solvent molecules decreases with the decrease of concentration of PCBM and increase of processing temperature. Results presented in this study provide fundamental insight that can help to choose favorable solvents for processing PCBMs in OPV applications.

On the photophysics and photochemistry of the water dimer
View Description Hide DescriptionThe photochemistry of the water dimer irradiated by UV light is studied by means of the complete active space perturbation theory//complete active space selfconsistent field (CASPT2//CASSCF) method and accurate computational approaches like as minimum energy paths. Both electronic structure computations and ab initio molecular dynamics simulations are carried out. The results obtained show small shifts relative to a single water molecule on the vertical excitation energies of the dimer due to the hydrogen bond placed between the water donor (W_{D}) and the water acceptor (W_{A}). A redshift and a blueshift are predicted for the W_{D} and W_{A}, respectively, supporting previous theoretical and experimental results. The photoinduced chemistry of the water dimer is described as a process occurring between two single water molecules in which the effect of the hydrogen bond plays a minor role. Thus, the photoinduced decay routes correspond to two photodissociation processes, one for each water molecule. The proposed mechanism for the decay channels of the lowestlying excited states of the system is established as the photochemical production of a hydrogenbonded H_{2}O…HO species plus a hydrogen H atom.
 Liquids, Glasses, and Crystals

Hydrogen bond effects in the vibrational spectra of 1,3propanediol in acetonitrile: Ab initio and experimental study
View Description Hide DescriptionHydrogen bond interactions strongly affect vibrational properties and frequencies, the most common consequence being a redshift of the stretching vibration involved; there are, however, few exceptions to this general trend. In previous works, we have proved the effectiveness of ab initiosimulations combined with wavelet analysis to investigate these effects and put them into relation to structural environment. In this work, we investigate the hydrogen bond effects on the structural and vibrational properties of 1,3propanediol in acetonitrile by a combined experimental and computational approach. We explain the appearance of two spectral components in the O−H stretching band on the basis of intra and intermolecular hydrogen bond interactions. We also elucidate the blueshift of the C≡N stretching band as due to a hydrogen bond interaction between the glycol and acetonitrile that modify the electron density distribution inside the CN group. This effect is well reproduced by ab initiomolecular dynamics simulations and density functional calculations reported in this work.

Structural and thermodynamical properties of charged hard spheres in a mixture with coresoftened model solvent
View Description Hide DescriptionThe canonical Monte Carlocomputer simulations and integral equation theory were applied to examine the structural and thermodynamic properties of a mixture of ions and a coresoftened fluid molecules. The positive and negative ions forming a +1:−1 salt were modeled as charged hard spheres, immersed in the dielectric medium. It was shown previously that the coresoftened fluid under study is characterized by a set of structural, thermodynamic, and dynamic anomalies. The principal objective of this work was to elucidate how the presence of ions alters this behavior. The structural properties of the mixtures are discussed in terms of the pair distribution functions; in addition, the pair contribution to the excess entropy was calculated. Thermodynamic properties are investigated by using the dependencies of energy and compressibility factor on density, composition of the mixture, and reduced temperature. The heat capacity was also evaluated. Our principal findings concern the description of structural anomalies in the mixture, the dependence of the temperature of maximum density on the ionic concentration, and establishing the regions delimiting the structural and thermodynamic anomalies of the model mixture.