Theoretical and Physical Chemistry Institute

Athens, Greece

Theoretical and Physical Chemistry Institute

Athens, Greece
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Zhang G.,Hefei University of Technology | Pispas S.,Theoretical and Physical Chemistry Institute
Journal of Polymer Science, Part A: Polymer Chemistry | Year: 2015

We have synthesized poly(ε-caprolactone-co-tert-butyl glycidyl ether) (CL-co-BGE) statistical copolymers using 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis [tris(dimethylamino)phophoranylidenamino]-2Λ5,4Λ5-catenadi(phosphazene) (t-BuP4) as the catalyst. The hydrolysis of the resulting polymers yields amphiphilic poly(ε-caprolactone-co-glycidol) (CL-co-GD) copolymers. By use of the quartz crystal microbalance with dissipation (QCM-D), we have investigated the enzymatic degradation of the copolymers. It is shown that the degradation rate increases with the content of hydrophilic (GD) units. (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) (MTT) assay experiments demonstrate that the CL-co-GD copolymers have low cytotoxicity. © 2015 Wiley Periodicals, Inc.


Mihajlov A.A.,University of Belgrade | Mihajlov A.A.,Isaac Newton Institute of Chile | Ignjatovic L.M.,University of Belgrade | Ignjatovic L.M.,Isaac Newton Institute of Chile | And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

The aim of this research is to show that the processes of absorption charge exchange and photoassociation in A + B+ collisions together with the processes of AB+ photodissociation in the case of strongly non-symmetric ion-atom systems, significantly influence the opacity of stellar atmospheres in ultraviolet (UV) and extreme UV (EUV) region. In this work, the significance of such processes for solar atmosphere is studied. In the case of the solar atmosphere the absorption processes with A = H and B = Mg and Si are treated as dominant ones, but the cases A = H and B = Al and A = He and B = H are also taken into consideration. The choice of just these species is caused by the fact that, of the species relevant for the used solar atmosphere model, it was only for them that we could determine the necessary characteristics of the corresponding molecular ions, i.e. the molecular potential curves and dipolematrix elements. It is shown that the efficiency of the examined non-symmetric processes within the rather wide corresponding quasi-molecular absorption bands in the far-UV and EUV regions is comparable and sometimes even greater than the intensity of the known symmetric ion-atom absorption processes, which are included now in the models of the solar atmosphere. Consequently, the presented results suggest that the non-symmetric ion-atom absorption processes also have to be included ab initio in the corresponding models of the stellar atmospheres. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Helbig N.,University of the Basque Country | Theodorakopoulos G.,Theoretical and Physical Chemistry Institute | Lathiotakis N.N.,Theoretical and Physical Chemistry Institute
Journal of Chemical Physics | Year: 2011

We study the behavior of different functionals of the one-body reduced density matrix (1RDM) for systems with fractional z-component of the total spin. We define these systems as ensembles of integer spin states. It is shown that, similarly to density functional theory, the error in the dissociation of diatomic molecules is directly related to the deviation from constancy of the atomic total energies as functions of the fractional spin. However, several functionals of the 1RDM show a size inconsistency which leads to additional errors. We also investigate the difference between a direct evaluation of the energy of an ensemble of integer-spin systems and a direct minimization of the energy of a fractional-spin system. © 2011 American Institute of Physics.


Karayianni M.,Bulgarian Academy Of Sciencesakad G Bonchev Str | Radeva R.,Bulgarian Academy Of Sciencesakad G Bonchev Str | Koseva N.,Bulgarian Academy Of Sciencesakad G Bonchev Str | Pispas S.,Theoretical and Physical Chemistry Institute
Journal of Polymer Science, Part B: Polymer Physics | Year: 2016

The electrostatic complexation between the polyelectrolyte block of the novel double hydrophilic copolymer quaternized poly(3,5-bis(dimethylaminomethylene) hydroxystyrene)-b-poly(ethylene oxide) (QNPHOS-PEO) and proteins of different molecular shape, that is globular bovine serum albumin (BSA) or rod-like bovine fibrinogen (FBG), is investigated by means of dynamic, static, and electrophoretic light scattering, as well as analytical ultracentrifugation measurements. The solution behavior, structure, and properties of the formed complexes at pH 7 and 0.01 M ionic strength, as a function of the protein concentration in the solution (or equivalently the charge ratio of the two components), depend on the protein concentration and molecular characteristics. Moreover, the structure of the complexes is greatly influenced by the intrinsic structure of the block polyelectrolyte, which forms rather loose multichain aggregates, due to hydrophobic interactions. A direct correlation between the stability of the preformed complexes against the increase of the solution ionic strength and their structure is established. Finally, the spectroscopic structural investigation of both complexed proteins reveals no signs of protein denaturation upon complexation. © 2016 Wiley Periodicals, Inc.


Lathiotakis N.N.,Theoretical and Physical Chemistry Institute | Gidopoulos N.I.,Theoretical and Physical Chemistry Institute | Gidopoulos N.I.,Rutherford Appleton Laboratory | Helbig N.,Nano Bio Spectroscopy Group
Journal of Chemical Physics | Year: 2010

We report a size-inconsistency problem for several functionals within reduced density matrix functional theory. Being explicit functionals of the natural orbitals and occupation numbers, instead of the one-body reduced density matrix, many of the approximate functionals are not invariant under unitary transformations in the subspace of degenerate occupation numbers. One such transformation mixes the degenerate natural orbitals of identical independent subsystems, delocalizing them. Noninvariance under this transformation results in size inconsistency for some of the approximations while others avoid this pathology by favoring orbital localization. © 2010 American Institute of Physics.


Nicolaides C.A.,Theoretical and Physical Chemistry Institute
International Journal of Quantum Chemistry | Year: 2011

Many problems in Atomic and Molecular Physics can be understood conceptually and quantitatively by using symmetry-adapted, state-specific wavefunctions whose computation is geared so as to account for at least those parts which describe reliably the characteristics of closed-and open-(sub)shell electronic structures that contribute overwhelmingly to the property or phenomenon of interest. If additional terms in the wavefunction are required by the problem, this is feasible via methods of configuration-interaction or low-order perturbation theory. This is the main argument of the state- and property-specific approach (SPSA) to Quantum Chemistry. In this framework, the aim is to obtain the state wavefunction, ψn, in the form a 0ψn 0 + φn corr, where a0 ≈ 1. ψn 0 is a state-specific zero-order description of ground and excited states of the discrete as well as of the continuous spectrum. In general, it is multiconfigurational and its construction follows from the "Fermi-sea" set of orbitals. The ψn 0 is used as reference for analysis and/or for further improvement of the overall calculation, if necessary. The level of accuracy of the computation of the remaining φn corr depends on the property under investigation. The arguments are supported by characteristic examples on ground and excited states of atomic, molecular and metallic Beryllium. Some of these SPSA results are compared with results from more conventional methods of electronic structure. Special attention is given to the weak bond of the Be2 X1∑+g state, which has attracted the interest of quantum chemists for decades. By asserting that the formation of the bond at about 2.5 Å is influenced by the interactions involving excited states, I point to the corresponding significance in zero-order ("Fermi-sea") not only of p-waves but also of d-waves whose origin is in the valence-Rydberg state mixing of the lowest 1D and 1Po states of Be. Therefore, the "Fermi-sea" ("active space") is represented by the Be set of {2s,2p,3s,3p,3d} orbitals. The initially heuristic predictions are supported by calculations (using the MOLPRO code) of the lowest seven Be2 1∑+g states, whose ψn 0 are obtained at the state-averaged complete active space self-consistent field level. These results are verified by the computation of a0ψn 0 + φn corr at the MRCISD level, where indeed a 0 ≈ 1 over the whole potential energy curve. This type of analysis and the corresponding results imply that by a properly justified choice of the zero-order orbital set, the origin of the Be2 bond is to be found in "nondynamical" - type correlations. This conclusion differs from that of Schmidt et al. [J Phys Chem A, 2010, 114, 8687], who argued that the formation of the Be2 weak bond should be attributed exclusively to "dynamical" correlations that are defined with respect to the {2s,2p} "active space" associated with the Be 1S ground state. © 2011 Wiley Periodicals, Inc.


Mercouris T.,Theoretical and Physical Chemistry Institute | Komninos Y.,Theoretical and Physical Chemistry Institute | Nicolaides C.A.,Theoretical and Physical Chemistry Institute
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

By solving the time-dependent Schrödinger equation nonperturbatively by the state-specific expansion approach, we have obtained reliable results showing the dependence of the formation of the excitation profile of the autoionizing state He ⊃′2s2p⊃′ 1Po for Gaussian pulses of intensities in the range I 1010 W/cm2 to I 1015 W/cm2, and of duration 100-2 fs. Conclusions are drawn as regards the breakdown of first-order time-dependent perturbation theory. © 2011 American Physical Society.


Komninos Y.,Theoretical and Physical Chemistry Institute | Mercouris T.,Theoretical and Physical Chemistry Institute | Nicolaides C.A.,Theoretical and Physical Chemistry Institute
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We report results of many-electron calculations that predict the presence of a regular series of autoionizing doubly excited states (DESs) of 1Posymmetry embedded inside one- as well as two-electron continua of neon, in the range of excitation 105.9-121.9 eV above the ground state. The limit of 121.9 eV represents the two-electron ionization threshold (TEIT) labeled by Ne2⊃+ 1s22p6 1S. The wave functions of these unstable states and their properties are computed according to the theoretical framework, which is explained and justified in the text. Their formal structure is (ψcore)1S- Φ(r1→,r2→)1Po, where both ψcore and Φ(r-1,r-2) are correlated wave functions, the latter being represented reasonably accurately by a self-consistently obtained superposition of nsnp and np(n+1)d configurations n=3-7. By fitting the calculated lowest energies at each value of n, (five states), an effective hydrogenic formula is obtained, which gives the whole energy spectrum up to the TEIT. The autoionization widths are small and decrease with excitation energy. Oscillator strengths for the excitation of these narrow resonance states by absorption of one photon are also small. Because of their electronic structure, these states are compared to 1Po DESs in He, which were found in the 1980s to constitute a regular ladder with wave-function characteristics that tend to those of the so-called Wannier state at threshold. In the present case, the presence of the core and the concomitant interactions do not permit the emergence of such geometrical features. © 2011 American Physical Society.


Skandalis A.,Theoretical and Physical Chemistry Institute | Pispas S.,Theoretical and Physical Chemistry Institute
Journal of Polymer Science, Part A: Polymer Chemistry | Year: 2016

The synthesis and self-assembly properties in aqueous solutions of novel amphiphilic block copolymers composed of one hydrophobic poly (lauryl methacrylate), (PLMA) block and one hydrophilic poly (oligo ethylene glycol methacrylate) (POEGMA) block are reported. The block copolymers were prepared by RAFT polymerization and were molecularly characterized by size exclusion chromatography, NMR and FT-IR spectroscopy, and DSC. The PLMA-b-POEGMA amphiphilic block copolymers self-assemble in nanosized complex nanostructures resembling compound micelles when inserted in aqueous media, as supported by light scattering and TEM measurements. The encapsulation and release of the model, hydrophobic, nonsteroidal anti-inflammatory drug indomethacin in the polymeric micelles is also investigated. © 2016 Wiley Periodicals, Inc.


Nicolaides C.A.,Theoretical and Physical Chemistry Institute
Advances in Quantum Chemistry | Year: 2011

The basic argument of the state- and property-specific approach (SPSA) to quantum chemistry is that many problems in atomic and molecular physics can be understood conceptually and quantitatively without necessarily trying to obtain as accurately as possible the total energy and the corresponding wavefunction of the state(s) involved. Instead, their solution can be achieved economically by using symmetry-adapted, state-specific wavefunctions whose computation, following analysis and computational experience, is geared so as to account for at least those parts that describe reliably the characteristics of closed-and open-(sub)shell electronic structures that contribute overwhelmingly to the property or phenomenon of interest. If additional terms in the wavefunction are required by the problem, this is feasible via methods of configuration-interaction or low-order perturbation theory.This chapter discusses the SPSA assumptions and computational procedures and the related concept of the state-specific multiconfigurational (Fermi-sea) zero-order description of ground and excited states of the discrete and the continuous spectrum, ψn0. For each state, ψn, the ψn0 is used as reference for analysis and for further improvement of the overall calculation if necessary. Thus, the aim is to obtain ψn in the form a0ψn0+Φncorr, where a0 ≈ 1, and the level of accuracy of Φncorr depends on the property under investigation.In this context, certain aspects of the issue of the separation of electron correlation into nondynamical (ND) and dynamical (D) parts are commented. Optimally, the function spaces of the ND and the D correlations are different, and both depend on how the Fermi-sea orbitals for each problem are chosen and computed.The arguments are supported by a number of results on prototypical ground and excited states of atoms and molecules. Most of these are compared with results from conventional methods of quantum chemistry, where single basis sets, orbital- or Hylleraas-type, are used.One set of examples illustrates the computational capacity of the SPSA and concepts such as Fermi-sea or ND correlations, by presenting, or referring to, new and old results for certain properties of the ground and the excited states of the Be atom and its derivative species, Be-, (Be)n cluster and Be metal, and Be2.Special attention is given to the weak bond of the Be2∑X1σg+ state, which has attracted the interest of quantum chemists for decades. By asserting that the formation of the bond at about 2.5 Å is influenced by the interactions involving excited states, I point to the corresponding significance in zero order (Fermi-sea) not only of p-waves but also of d-waves, whose origin is in the valence-Rydberg state mixing of the lowest 1D and 1Po states of Be. Therefore, the Fermi-sea (active space) is represented by the Be set of {2s, 2p, 3s, 3p, 3d} orbitals. The initially heuristic predictions are supported by calculations (using the MOLPRO code) of the lowest 11 Be2∑1σg+ states, whose ψn0 are obtained at the CASSCF level. These results are verified by the computation of a0ψn0+Φncorr of the lowest 7 states at the MRCISD level, where indeed a0 ≈ 1 over the whole potential energy curve. This type of analysis and the corresponding results imply that by a properly justified choice of the zero-order orbital set, the Be2 bond can be understood in terms of ND-type correlations, a conclusion which disagrees with that of Schmidt et al. J. Phys. Chem. A 114, 8687 (2010). © 2011 Elsevier Inc.

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