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Publication Metadata only A post-HF study on the halogen bonding interaction of pyrene with diatomic halogen molecules(Wiley, 2016) Sütay, Berkay; Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129We present a detailed SCS-MP2 study on the potential energy curves (PEC) for interactions between diatomic halogen molecules and pyrene. BSSE corrected CCSD[T] energies at equilibrium distances are computed and compared to CCSD(T) energies. The most stable conformation of these weakly bound van der Waals complexes is almost linear in the perpendicular direction to the pyrene plane. The complexes of highly polarizable bromine and iodine molecules with pyrene are very stable and they carry rather large number of vibrational states. Despite its small size, F-2 also forms strong halogen bonding similar to Br-2 and I-2. The interaction between Cl-2 and pyrene is the weakest and it is attributed to the highest polarizability/molar mass ratio of chlorine among the others. I-2-pyrene is found to be the most stable complex due to the strongest mutual polarization effects and is carrying more than 60 vibrational states. Due to the rather large number of electrons in some complexes, the relativistic corrections are also considered.Publication Metadata only Computational studies of cyclobutadiene and benzocyclobutene fused to p- and o-quinone(American Chemical Society (ACS), 1998) McKee, Michael L.; Balcı, Metin; Kılıç, Hamdullah; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Cyclobutadiene and benzocyclobutenes fused to o- and p-quinone have been studied by computational methods. Geometries were optimized at the B3LYP/6-31G* level, and absolute NMR shielding values were calculated using the GIAO method with the HF/6-31G* basis set. NICS values of the compounds 8b,c and 9b,c indicate strong antiaromatic character for cyclobutadiene units. However, 8a and 9a show negative NICS values where the quinodal system reduces the antiaromaticity significantly by forcing these systems to possess a dimethylene-like structure. The calculated C-13 NMR chemical shifts of 6-9 and parent systems are in very good agreement with literature values.Publication Metadata only Conformational similarities in isomerization dynamics of clusters(Amer Chemical Soc, 2003) Department of Chemistry; N/A; Department of Chemical and Biological Engineering; Yurtsever, İsmail Ersin; Palazoğlu, Ahmet; Arkun, Yaman; Faculty Member; N/A; Faculty Member; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; N/A; College of Engineering; 7129; N/A; 108526A method for characterization of the isomerization dynamics from classical trajectories is presented. A measure function describing the topological distance between two clusters of atoms is first developed. Next, this measure is used to identify the regions of the potential energy surface visited by the trajectories. Unlike the commonly used techniques such as simulated annealing or quenching, the proposed method does not require repeated treatment of the trajectory and can be safely used to study the isomerization dynamics of large systems, especially those of monatomic clusters.Publication Metadata only Effects of the range of the potential on the structure and dynamics of two-dimensional Coulomb clusters(Taylor & Francis Ltd, 2008) Calvo, Florent; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The structure, dynamics, and thermodynamics of finite systems of two-dimensional particles interacting mutually through a shielded Coulomb potential, V(r) = e((-kappa r))/r, and bound together by an isotropic parabolic trap, are investigated by means of global optimization and simulation methods. As the range of the potential decreases, or as kappa increases, the number of distinct minima rapidly increases for a given size and the cluster generally shrinks. Looking more specifically at selected sizes below 50, non-monotonic cluster size effects seem to dominate the energetics, chaotic dynamics, and thermodynamics, even though the melting point is weakly affected by the range of the potential.Publication Metadata only Energetics and structures of charged helium clusters: comparing stabilities of dimer and trimer cationic cores(Wiley-V C H Verlag Gmbh, 2008) Marinetti, Fabio; Bodo, Enrico; Gianturco, Francesco A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129We present accurate ab initio calculations of the most stable structures of He-n(+) clusters in order to determine the more likely ionic core arrangements existing after reaching structural equilibrium of the clusters. Two potential energy surfaces are presented: one for the He-2(+) and the other with the He-3(+) linear ion, both interacting with one He atom. The two computed potentials are in turn employed within a classical structure optimization where the overall interaction forces are obtained within the sum-of-potentials approximation described in the main text. Because of the presence of many-body effects within the ionic core, we find that the arrangements with He-3(+) as a core turn out to be energetically preferred, leading to the formation of He-3(+)(He)(n-3) stable aggregates. Nanoscopic considerations about the relative stability of clusters with the two different cores are shown to give us new information on the dynamical processes observed in the impact ionization experiments of pure helium clusters and the importance of pre-equilibrium evaporation of the ionic dimers in the ionized clusters.Publication Metadata only Energy landscapes in photochemical dissociation of small peroxides(Amer Chemical Soc, 2019) N/A; N/A; N/A; Department of Chemistry; Tabriz, Meisam Farzalipour; Çizmeciyan, Melisa Natali; Birer, Özgür; Yurtsever, İsmail Ersin; Master Student; PhD Student; Researcher; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; College of Sciences; N/A; N/A; N/A; 7129Organic peroxides are known to have important roles in many chemical and biochemical processes such as intermediates in the oxidation of various hydrocarbons, as initiators of free-radical polymerization and cross-linking agents, etc. Consequently, the study of the organic peroxides and their radicals are of fundamental interest and importance. Although several reaction pathways after dissociation of organic peroxides have been successfully identified using time-resolved optical absorption spectroscopy, interpretation of the data can be complicated due to spectral overlap of parent molecules, intermediates, and products. Therefore, a reliable theoretical framework is necessary in case of complex or less studied systems. In this study, we investigated the plausible thermal dissociation pathways of diethyl peroxide, ditert butyl peroxide, and dicumyl peroxide by density functional theory with M06-2X hybrid functional and compared its results to coupled cluster single double and perturbative triple, CCSD(T), level energies. Our results indicate that methyl radical elimination is the main dissociation mechanism for all of the studied peroxides after O-O bond cleavage which has been also observed in experiments. The resulting relative energies of the M06-2X functional were found to have reasonable accuracy in comparison with the CCSD(T) method. We also show that time-dependent density functional theory (TD-DFT) with the M06-2X functional provides a suitable guide for interpretation of time-resolved optical absorption spectra of peroxides. The experimental transient absorption spectra of dicumyl peroxide are interpreted using the theoretically predicted pathways and transient radical species. Both results agree within experimental resolution and accuracy. We propose that the traditionally assigned visible absorption is not due to the cumuloxyl radical and the photodissociation of dicumyl peroxide involves other pathways with extremely short-lived radicals.Publication Metadata only Exploring a dynamical path for C2H− and NCO− formation in dark molecular clouds(Springer, 2016) Iskandarov, Ibrokhim; Gianturco, Francesco Antonio; Carelli, Fabio; Wester, Roland; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129This paper deals with the possible formation of two molecular anions often considered likely components in the physical environments of the interstellar medium (ISM): C2H- and NCO-. They are both discussed here by computationally following the radiative association (RA) mechanism starting from C-2(-), H, NC- and O as partners. The corresponding RA total cross sections produced by the calculations are in turn employed to generate the overall association rates over the relevant range of temperatures. The latter are found to be in line with other molecular ions formed by RA but not large enough to uniquivocally suggest this path as the main route to the anions formation in the ISM. Other possible paths of formation are also analysed and discussed. The presence of resonant structures during the association dynamics for both systems is found by the calculations and their consequences are discussed in some detail in the present study.Publication Metadata only Finding the global minima of (Ne)(n)(+) clusters with non-empirical models: a comparison of results(Elsevier, 2003) Sebastianelli, Francesco; Gianturco, Franco A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The energetics and the structural properties of some of the smaller ionic clusters of neon atoms (Ne-n(+), with n up to 25) are examined using different modeling for the interactions within each cluster moiety. A new, very simple model for the global interaction, the polygon growth model or PGM, is presented and discussed in detail. The results of the calculations, and the physical reliability of the methods, are then examined in comparison with earlier theoretical results and with some available experimental data. In spite of the simplicity of the PGM scheme, the present treatment shows it to be able to reproduce all the important features known for these clusters from earlier calculations and from measurements.Publication Metadata only Global view of classical clusters: the hyperspherical approach to structure and dynamics(Royal Soc Chemistry, 2002) Aquilanti, Vincenzo; Lombardi, Andrea; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Hyperspherical and related approaches to the dynamics of few particles have been extensively exploited in nuclear, atomic and molecular physics in order to successfully deal with demanding quantum mechanical problems, such as few-nucleon structures and triatomic chemical reactions. In this work, we explore the use of these tools for the study of the structure and dynamics of atomic clusters, for which an ample phenomenology is being accumulated from a classical mechanics perspective. The invariants associated to the inertia tensor were examined for known absolute minimum energy structures of Lennard-Jones clusters up to a number of particles N = 150, while for N less than or equal to 12 the structures of all relative minima were determined and studied in relationship with their symmetries. Morse clusters and a realistic interaction model for argon clusters were also briefly considered. In order to deal with cluster dynamics the concept of grand angular momentum is formulated in classical mechanics as a sum of external, deformation and internal ( or kinematic) angular momenta. A similar decomposition, of prospective use for the analysis of molecular dynamics simulations, is proposed for the classical kinetic energy. The example of a classical trajectory analysis is given for the isomerization by pseudorotation of an N = 5 cluster.Publication Metadata only HCHO in a cold, quantum solvent: size and shape of its "bubbles" in (4)He droplets from stochastic simulations(Amer Chemical Soc, 2010) Marinetti, Fabio; Gianturco, Francesco A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The full interaction between formaldehyde and He-4 atoms has been obtained from a first-principle calculation of the forces at play. In order to describe the nanoscopic features of HCHO being solvated in a quantum liquid, further Monte Carlo calculations for the system HCHO@He-N with N up to 20 have been carried out. The energetics and structure of the systems, as N changes, are extensively analyzed, and the excluded volume ("bubble") created by the inner cage that surrounds the solvated molecule is described and discussed to provide molecular microsolvation details for the title system.