Publications by authors named "Reza Omidyan"

25 Publications

  • Page 1 of 1

Insights into the effect of distal histidine and water hydrogen bonding on NO ligation to ferrous and ferric heme: a DFT study.

RSC Adv 2022 Feb 8;12(8):4703-4713. Epub 2022 Feb 8.

Department of Chemistry, University of Isfahan 81746-73441 Isfahan Iran +98 31 3668 9732.

The effect of distal histidine on ligation of NO to ferrous and ferric-heme, has been investigated with the high-level density functional theoretical (DFT) method. It has been predicted that the distal histidine significantly stabilizes the interaction of NO ferrous-heme (by -2.70 kcal mol). Also, water hydrogen bonding is quite effective in strengthening the Fe-NO bond in ferrous heme. In contrast in ferric heme, due to the large distance between the HO and O(NO) and lack of hydrogen bonding, the distal histidine exhibits only a slight effect on the binding of NO to the ferric analogue. Concerning the bond nature of Fe-NO and Fe-NO in heme, a QTAIM analysis predicts a partially covalent and ionic bond nature in both systems.
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http://dx.doi.org/10.1039/d1ra08398hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8981399PMC
February 2022

Theoretical insights on the excited-state-deactivation mechanisms of protonated thymine and cytosine.

Phys Chem Chem Phys 2021 Apr 6;23(14):8916-8925. Epub 2021 Apr 6.

Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran.

Ab initio and surface-hopping nonadiabatic dynamics simulation methods were employed to investigate relaxation mechanisms in protonated thymine (TH) and cytosine (CH). A few conical intersections were located between ππ* and S states for each system with the CASSCF (8,8) theoretical model and relevant contributions to the deactivation mechanism of titled systems were addressed by the determination of potential energy profiles at the CASPT2 (12,10) theoretical level. It was revealed that the relaxation of the ππ* state of the most stable conformer of both systems to the ground state is mostly governed by the accessible S/S conical intersection resulting from the barrier-free out-of-plane deformation. Interestingly, it was exhibited that the ring puckering coordinate driven from the C position of the heterocycle ring in TH and CH plays the most prominent role in the deactivation mechanism of considered systems. Our ab initio results are also supported by excited-state nonadiabatic dynamics simulations based on ADC(2), describing the ultrashort S lifetime of TH/CH by analyzing trajectories leading excited systems to the ground. It was confirmed that the excited-state population mostly relaxes to the ground via the ring puckering coordinate from the C moiety. Overall, the theoretical results of this study shed light on the deactivation mechanism of protonated DNA bases.
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http://dx.doi.org/10.1039/d0cp06673gDOI Listing
April 2021

DFT study of α-Keggin, lacunary Keggin, and iron substituted Keggin polyoxometalates: the effect of oxidation state and axial ligand on geometry, electronic structures and oxygen transfer.

RSC Adv 2020 Sep 11;10(56):33718-33730. Epub 2020 Sep 11.

Department of Chemistry, University of Isfahan Isfahan 81746-73441 Iran

Herein, the geometry, electronic structure, Fe-ligand bonding nature and simulated IR spectrum of α-Keggin, lacunary Keggin, iron(ii/iii)-substituted and the important oxidized high-valent iron derivatives of Keggin type polyoxometalates have been studied using the density functional theory (DFT/OPTX-PBE) method and natural bond orbital (NBO) analysis. The effects of different Fe oxidation states (ii-vi) and HO/OH/O ligand interactions have been addressed concerning their geometry and electronic structures. It has been revealed that the d-atomic orbitals of Fe and 2p orbitals of polyoxometalate's oxygen-atoms contribute in ligand binding. Compared with other high valent species, the considered polyoxometalate system of [PWO(FeO)], possesses a high reactivity for oxygen transfer.
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http://dx.doi.org/10.1039/d0ra05189fDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC9056712PMC
September 2020

Excited State Deactivation Mechanism in Protonated Uracil: New Insights from Theoretical Studies.

J Phys Chem A 2020 Jun 12;124(25):5089-5097. Epub 2020 Jun 12.

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.

We have conducted here a theoretical exploration, discussing the distinct excited state lifetimes reported experimentally for the two lowest lying protonated isomers of uracil. In this regard, the first-principal computational levels as well as the nonadiabatic surface hopping dynamics have been employed. It has been revealed that relaxation of the ππ* state of enol-enol form (EE) to the ground is barrier-free via out-of-plane coordinates, resulting in an ultrashort S lifetime of this species. For the second most stable isomer (EK), however, a significant barrier predicted in the CASPT2 S potential energy profile along the twisting coordinate has been proposed to explain the relevant long lifetime reported experimentally.
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http://dx.doi.org/10.1021/acs.jpca.0c02284DOI Listing
June 2020

Water binding to Fe hemes studied in a cooled ion trap: characterization of a strong 'weak' ligand.

Phys Chem Chem Phys 2019 Oct 18;21(38):21329-21340. Epub 2019 Sep 18.

ISMO, Université Paris-Sud, CNRS UMR 8214, bat 520, Université Paris-Sud 91405, Orsay Cedex, France.

The interaction of a water molecule with ferric heme-iron protoporphyrin ([PP Fe]) has been investigated in the gas phase in an ion trap and studied theoretically by density functional theory. It is found that the interaction of water with ferric heme leads to a stable [PP-Fe-HO] complex in the intermediate spin state (S = 3/2), in the same state as its unligated [PP-Fe] homologue, without spin crossing during water attachment. Using the Van't Hoff equation, the reaction enthalpy for the formation of a Fe-OH bond has been determined for [PP-Fe-HO] and [PP-Fe-(HO)]. The corrected binding energy for a single Fe-HO bond is -12.2 ± 0.6 kcal mol, while DFT calculations at the OPBE level yield -11.7 kcal mol. The binding energy of the second ligation yielding a six coordinated Fe atom is decreased with a bond energy of -9 ± 0.9 kcal mol, well reproduced by calculations as -7.1 kcal mol. However, calculations reveal features of a weaker bond type, such as a rather long Fe-O bond with 2.28 Å for the [PP-Fe-HO] complex and the absence of a spin change by complexation. Thus despite a strong bond with HO, the Fe atom does not show, through theoretical modelling, a strong acceptor character in its half filled 3d orbital. It is also observed that the binding properties of HO to hemes seem strikingly specific to ferric heme and we have shown, experimentally and theoretically, that the affinity of HO for protonated heme [H PP-Fe], an intermediate between Fe and Fe, is strongly reduced compared to that for ferric heme.
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http://dx.doi.org/10.1039/c9cp03608cDOI Listing
October 2019

The dramatic effect of N-methylimidazole on trans axial ligand binding to ferric heme: experiment and theory.

Phys Chem Chem Phys 2019 Jan;21(4):1750-1760

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.

The binding energy of CO, O2 and NO to isolated ferric heme, [FeIIIP]+, was studied in the presence and absence of a σ donor (N-methylimidazole and histidine) as the trans axial ligand. This study combines the experimental determination of binding enthalpies by equilibrium measurements in a low temperature ion trap using the van't Hoff equation and high level DFT calculations. It was found that the presence of N-methylimidazole as the axial ligand on the [FeIIIP]+ porphyrin dramatically weakens the [FeIIIP-ligand]+ bond with an up to sevenfold decrease in binding energy owing to the σ donation by N-methylimidazole to the FeIII(3d) orbitals. This trans σ donor effect is characteristic of ligation to iron in hemes in both ferrous and ferric redox forms; however, to date, this has not been observed for ferric heme.
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http://dx.doi.org/10.1039/c8cp06210bDOI Listing
January 2019

Photophysics of Protonated and Microhydrated 2-Aminobenzaldehyde: Theoretical Insights into Photoswitchability of Protonated Systems.

J Phys Chem A 2018 Nov 6;122(45):8849-8857. Epub 2018 Nov 6.

Department of Chemistry , University of Isfahan , 81746-73441 , Isfahan , Iran.

The photoswitchability of a protonated aromatic compound (2-aminobenzaldehyde, 2ABZH) in its individual and microhydrated states has been addressed based on the RI-MP2/RI-CC2 theoretical methods. Our calculated results give insight into the ultrafast nonradiative deactivation mechanism of the 2ABZH, driven by a conical intersection between the S/ S potential energy surfaces. Also, it has been predicted that protonation accompanies a significant blue shift effect on the first ππ* excited state of 2ABZ by 0.87 eV (at least 50 nm).
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http://dx.doi.org/10.1021/acs.jpca.8b09930DOI Listing
November 2018

Photochromism of 2-(2-Hydroxyphenyl) Benzothiazole (HBT) and Its Derivatives: A Theoretical Study.

J Phys Chem A 2018 Mar 16;122(12):3182-3189. Epub 2018 Mar 16.

Department of Chemistry , University of Isfahan , 81746-73441 Isfahan , Iran.

Hydroxyphenyl benzothiazole (HBT), is a well-known organic system based on its special characteristic of the excited state hydrogen transfer (ESHT) following photoexcitation. However, the capability of this system regarding photochromism and photoswitching has not been addressed yet. In this study, we have investigated this issue by the aim of the MP2, CC2, ADC(2), and CASSCF theoretical methods. Also, we have considered several electron withdrawing groups and investigated their effects on the photophysical characteristics and spectroscopic properties of the enol and keto tautomers of the titled system. It has been predicted that the main HBT and its considered substitutions fulfill the essential characteristics required for photochromism. Also, substitution is an effective idea for tuning the photophysical nature of HBT and its similar systems. Our theoretical results verify that different substitutions alter the UV absorption of HBT systems from 330 to 351 nm and also the corresponding absorption wavelength of the γ-forms of 526-545 nm.
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http://dx.doi.org/10.1021/acs.jpca.8b00266DOI Listing
March 2018

Excited-State Proton Transfer in Thiazolo-[4, 5-d]thiazo Heterocyclic Systems and the Geometry Alterations' Effect on Photophysical Characters: A Theoretical Study.

J Phys Chem A 2018 Mar 6;122(10):2653-2662. Epub 2018 Mar 6.

Department of Chemistry , University of Isfahan , 81746-73441 Isfahan , Iran.

Thiazolo-[4,5- d]-thiazo-frame (tztz) compounds are important heteroaromatic organic systems, which recently became a subject of several studies in the field of organic electronics and organic photovoltaics. The most important physical nature of these systems is reported to be an equilibrium between enol and keto forms following excited-state proton transfer. This process originates from a flat trend of the S PE (potential energy) profile along the proton transfer coordinate. In the present work, we determined and interpreted the excited-state proton transfer and photophysical nature of these systems extensively by means of the MP2/CC2 and CASSCF theoretical approaches. Also, the effects of amine (-NH) and cyano (-CN) substitutions were taken into account comprehensively by considering the transition energies and proton transfer pathways of the resulting tztz derivatives. It has been predicted that the physical nature of the excited-state intramolecular proton transfer, as the main character of these systems, is being affected significantly by substitutions. For all of the considered tztz derivatives, a conical intersection (CI) between ground and the S excited state was predicted. This CI makes the considered species capable to be responsible for photochromism and photoswitching as well.
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http://dx.doi.org/10.1021/acs.jpca.7b09593DOI Listing
March 2018

Protonated serotonin: Geometry, electronic structures and photophysical properties.

Spectrochim Acta A Mol Biomol Spectrosc 2017 Jul 1;182:8-16. Epub 2017 Apr 1.

Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran.

The geometry and electronic structures of protonated serotonin have been investigated by the aim of MP2 and CC2 methods. The relative stabilities, transition energies and geometry of sixteen different protonated isomers of serotonin have been presented. It has been predicted that protonation does not exhibit essential alteration on the S←S electronic transition energy of serotonin. Instead, more complicated photophysical nature in respect to its neutral analogue is suggested for protonated system owing to radiative and non-radiative deactivation pathways. In addition to hydrogen detachment (HD), hydrogen/proton transfer (H/PT) processes from ammonium to indole ring along the NH⋯π hydrogen bond have been predicted as the most important photophysical consequences of SERH at S excited state. The PT processes is suggested to be responsible for fluorescence of SERH while the HD driving coordinate is proposed for elucidation of its nonradiative deactivation mechanism.
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http://dx.doi.org/10.1016/j.saa.2017.03.069DOI Listing
July 2017

Excited-state intramolecular proton transfer and photoswitching in hydroxyphenyl-imidazopyridine derivatives: A theoretical study.

J Chem Phys 2016 Nov;145(18):184303

Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran.

The MP2/CC2 and CASSCF theoretical approaches have been employed to determine the excited state proton transfer and photophysical nature of the four organic compounds, having the main frame of hydroxyphenyl-imidzaopyridine (HPIP). The nitrogen insertion effect, in addition to amine (-NH) substitution has been investigated extensively by following the transition energies and deactivation pathways of resulted HPIP derivatives. It has been predicted that the excited state intramolecular proton transfer with or without small barrier is the most important feature of these compounds. Also, for all of the considered HPIP derivatives, a conical intersection (CI) between ground and the S excited state has been predicted. The strong non-adiabatic coupling in the CI (S/S), drives the system back to the ground state in which the proton may either return to the phenoxy unit and thus close the photocycle, or the system can continue the twisting motion that results in formation of a γ-photochromic species. This latter species can be responsible for photochromism of HPIP derivative systems.
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http://dx.doi.org/10.1063/1.4967199DOI Listing
November 2016

Excited State Proton Transfer and Deactivation Mechanism of 2-(4'-Amino-2'-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine and Its Analogues: A Theoretical Study.

J Phys Chem A 2016 Feb 11;120(7):1012-9. Epub 2016 Feb 11.

Department of Chemistry, University of Isfahan , 81746-73441 Isfahan, Iran.

In the present study, the results of comprehensive theoretical exploration on the nonradiative relaxation of three (hydroxyphenyl)imidazole-based organic compounds (abbreviated AHP, HPIP, and HPBI) in the gas phase are presented. Having small structural differences, the selected systems have commonalities in the excited state intramolecular proton transfer (ESIPT) process. The ground and S1 excited state potential energy profiles of titled systems have been determined on the basis of the RI-MP2 and RI-CC2 methods, and the effect of small structural distinctions on their photophysical characters will be extensively addressed. Although, in the presence of solvent, high fluorescence quantum yield is another characteristic of AHP and HPIP, owing to accessible conical intersections between the S1/S0 state potential energy profiles of both systems, nonradiative relaxation can be proposed as the most important feature of these two systems in the gas phase. These conical intersections are responsible for ultrafast deactivation of excited systems via internal conversions to the ground state. The nonradiative deactivation mechanism determined in this work deals with the remarkable photostability of the AHP and HPIP molecules.
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http://dx.doi.org/10.1021/acs.jpca.5b12122DOI Listing
February 2016

A theoretical exploration on electronically excited states of protonated furan and thiophene.

Photochem Photobiol Sci 2015 Dec;14(12):2261-9

Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran.

The geometry, electronic structures and potential energy profiles of protonated furan and thiophene have been extensively investigated, using the RI-MP2 and RI-CC2 methods. According to RI-CC2 calculated results, the adiabatic S1((1)ππ*)-S0 transition energies of protonated furan and thiophene, have been predicted to be 4.41 eV and 3.70 eV respectively. Thus, protonation is accompanied by a large red shift effect on the first (1)ππ* transition of the title systems (ΔE > 1.5 eV). The significant spectral-movements, predicted based on the calculated results of this work, indicate an essential effect of protonation on the geometry, electronic structures and optical characters of the five membered heterocyclic systems. In addition, it has been found that excitation of protonated furan and thiophene, with sufficient excess energy above the band origin of S1((1)ππ*)-S0 transition, is accompanied by the S-C or O-C bond breaking. This mechanism is mostly governed by a dissociative (1)πσ* PE profile in both protonated systems.
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http://dx.doi.org/10.1039/c5pp00266dDOI Listing
December 2015

Electronically Excited States of Neutral, Protonated α-Naphthol and Their Water Clusters: A Theoretical Study.

J Phys Chem A 2015 Jun 15;119(25):6650-60. Epub 2015 Jun 15.

Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran.

The RI-MP2 and RI-CC2 methods have been employed to determine the potential energy profiles of neutral and protonated α-naphthol, in their individual forms and microhydrated with 1 and 3 water molecules, at different electronic states. According to calculated results, it has been predicted that dynamics of nonradiative processes in protonated α-naphthol is essentially different from that of its neutral homologue. In protonated α-naphthol, the calculations reveal that (1)σπ* state, is the most important photophysical state, having a bound nature with a broad potential curve along the OH coordinate of isolated system, while it is dissociative in monohydrated homologue. In neutral system, similar to phenol, the (1)πσ* state, plays the fundamental relaxation role along the O-H stretching coordinate. Moreover, microhydration strongly affects the photophysical properties of α-naphthol, mostly by alteration of the (1)ππ* PE profile, from a bound state in an isolated analogue to a dissociative state in hydrated systems. Furthermore, it has been found that three water molecules are necessary for ground state proton transfer between protonated α-naphthol and water; with a small barrier; (ΔE< 0.1 eV).
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http://dx.doi.org/10.1021/acs.jpca.5b02249DOI Listing
June 2015

Photophysics and photochemistry of cis- and trans-hydroquinone, catechol and their ammonia clusters: a theoretical study.

Photochem Photobiol Sci 2015 Feb;14(2):457-64

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.

Excited state hydrogen transfer in hydroquinone- and catechol-ammonia clusters has been extensively investigated by high level ab initio methods. The potential energy profiles of the title systems at different electronic states have been determined at the MP2/CC2 levels of theory. It has been predicted that double hydrogen transfer (DHT) takes place as the main consequence of photoexcited tetra-ammoniated systems. Consequently, the DHT processes lead the excited systems to the (1)πσ*-S0 conical intersections, which is responsible for the ultrafast non-radiative relaxation of UV-excited clusters to their ground states. Moreover, according to our calculated results, the single hydrogen detachment or hydrogen transfer process essentially governs the relaxation dynamics of smaller sized clustered systems (mono- and di-ammoniated).
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http://dx.doi.org/10.1039/c4pp00356jDOI Listing
February 2015

Excited state deactivation pathways of neutral/protonated anisole and p-fluoroanisole: a theoretical study.

Phys Chem Chem Phys 2014 Jun 9;16(23):11679-89. Epub 2014 May 9.

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.

The potential energy profiles of neutral and protonated anisole and p-fluoroanisole at different electronic states have been investigated extensively by the RI-MP2 and RI-CC2 methods. The calculations reveal that the relaxation dynamics in protonated anisole and p-fluoroanisole are essentially different from those of the neutral analogues. In neutral anisole/p-fluoroanisole, the (1)πσ* state plays a vital relaxation role along the O-CH3 coordinate, yielding the CH3 radical. For both of these molecules, the calculations indicate conical intersections (CIs) between the ground and excited state potential energy (PE) curves, hindered by a small barrier, and providing non-adiabatic gates for radiation-less deactivation to the ground state. Nevertheless, for the protonated cases, besides the prefulvenic deformation of the benzene ring, it has been predicted that the lowest (1)(σ,n)π* state along the C-O-C bond angle plays an important role in photochemistry and the relaxation dynamics. The S1, S0 PE profiles of protonated anisole along with the former reaction coordinate (out-of-plane deformation) show a barrierless relaxation pathway, which can be responsible for the ultrafast deactivation of excited systems to the ground state via the low-lying S1/S0 conical intersection. Moreover, the later reaction coordinate in protonated species (C-O-C angle from 120°-180°) is consequently accompanied with the bond cleavage of C-OCH3 at the (1)(σ,n)π* state, hindered by a barrier of ∼0.51 eV, and can be responsible for the relaxation of excited systems with significant excess energy (hν≥ 5 eV). Furthermore, according to the RI-CC2 calculated results, different effects on the S1-S0 electronic transition energy of anisole and p-fluoroanisole upon protonation have been predicted. The first electronic transitions of anisole and p-fluoroanisole shift by ∼0.3 and 1.3 eV to the red respectively due to protonation.
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http://dx.doi.org/10.1039/c4cp00679hDOI Listing
June 2014

Electronic and vibrational spectra of protonated benzaldehyde-water clusters, [BZ-(H2O)n≤5]H+: evidence for ground-state proton transfer to solvent for n ≥ 3.

J Chem Phys 2014 Mar;140(12):124314

Physique des Interactions Ioniques et Moléculaires, UMR-CNRS 7345 Aix Marseille Université, Avenue Escadrille Normandie-Niémen, 13397 Marseille Cedex 20, France.

Vibrational and electronic photodissociation spectra of mass-selected protonated benzaldehyde-(water)n clusters, [BZ-(H2O)n]H(+) with n ≤ 5, are analyzed by quantum chemical calculations to determine the protonation site in the ground electronic state (S0) and ππ(*) excited state (S1) as a function of microhydration. IR spectra of [BZ-(H2O)n]H(+) with n ≤ 2 are consistent with BZH(+)-(H2O)n type structures, in which the excess proton is localized on benzaldehyde. IR spectra of clusters with n ≥ 3 are assigned to structures, in which the excess proton is located on the (H2O)n solvent moiety, BZ-(H2O)nH(+). Quantum chemical calculations at the B3LYP, MP2, and ri-CC2 levels support the conclusion of proton transfer from BZH(+) to the solvent moiety in the S0 state for hydration sizes larger than the critical value nc = 3. The vibronic spectrum of the S1 ← S0 transition (ππ(*)) of the n = 1 cluster is consistent with a cis-BZH(+)-H2O structure in both electronic states. The large blueshift of the S1 origin by 2106 cm(-1) upon hydration with a single H2O ligand indicates that the proton affinity of BZ is substantially increased upon S1 excitation, thus strongly destabilizing the hydrogen bond to the solvent. The adiabatic S1 excitation energy and vibronic structure calculated at the ri-CC2/aug-cc-pVDZ level agrees well with the measured spectrum, supporting the notion of a cis-BZH(+)-H2O geometry. The doubly hydrated species, cis-BZH(+)-(H2O)2, does not absorb in the spectral range of 23 000-27 400 cm(-1), because of the additional large blueshift of the ππ(*) transition upon attachment of the second H2O molecule. Calculations predict roughly linear and large incremental blueshifts for the ππ(*) transition in [BZ-(H2O)n]H(+) as a function of n. In the size range n ≥ 3, the calculations predict a proton transfer from the (H2O)nH(+) solvent back to the BZ solute upon electronic ππ(*) excitation.
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http://dx.doi.org/10.1063/1.4869341DOI Listing
March 2014

Protonation effect on the electronic properties of 2-pyridone monomer, dimer and its water clusters: a theoretical study.

J Chem Phys 2014 Jan;140(2):024315

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.

The CC2 (second order approximate coupled cluster method) has been applied to investigate protonation effect on electronic transition energies of 2-pyridone (2PY), 2-pyridone dimer, and micro-solvated 2-pyridone (0-2 water molecules). The PE profiles of protonated 2-pyridone (2PYH(+)) as well as monohydrated 2PYH(+) at the different electronic states have been investigated. The (1)πσ∗ state in protonated species (2PYH(+)) is a barrier free and dissociative state along the O-H stretching coordinate. In this reaction coordinate, the lowest lying (1)πσ∗ predissociates the bound S1((1)ππ∗) state, connecting the latter to a conical intersection with the S0 state. These conical intersections lead the (1)ππ∗ state to proceed as predissociative state and finally direct the excited system to the ground state. Furthermore, in presence of water molecule, the (1)πσ∗ state still remains dissociative but the conical intersection between (1)πσ∗ and ground state disappears. In addition, according to the CC2 calculation results, it has been predicted that protonation significantly blue shifts the S1-S0 electronic transition of monomer, dimer, and microhydrated 2-pyridone.
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http://dx.doi.org/10.1063/1.4859255DOI Listing
January 2014

Photophysics of a Schiff base: theoretical exploration of the excited-state deactivation mechanisms of N-salicilydenemethylfurylamine (SMFA).

Phys Chem Chem Phys 2014 Feb;16(6):2417-24

Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran.

Excited state reaction coordinates and the consequent energy profiles of a new Schiff base, N-salicilydenemethylfurylamine (SMFA), have been investigated with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. The potential energy profiles of the ground and the lowest excited singlet state are calculated. In contrast to the ground state, the excited state potential energy profile shows a barrier-less dissociation pattern along the O-H stretching coordinate which verifies the proton transfer reaction at the S1 (ππ*) state. The calculations indicate two S1/S0 conical intersections (CIs) which provide non-adiabatic gates for radiation-less decay to the ground state. At the CIs, two barrier-free reaction coordinates direct the excited system to the ground state of enol-type minimum. According to calculation results, a trans-keto type structure obtained from photoexcitation of the enol, can be responsible for the photochromoic effect of SMFA. Furthermore, our results confirm the suggestion that aromatic Schiff bases are potential candidates for optically driven molecular switches.
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http://dx.doi.org/10.1039/c3cp54416hDOI Listing
February 2014

Hydrogen bond strengthening of cis-trans glyoxal dimers in electronic excited states: a theoretical study.

Spectrochim Acta A Mol Biomol Spectrosc 2014 Mar 19;122:337-42. Epub 2013 Nov 19.

Department of Chemistry, University of Isfahan, 81746-7344 Isfahan, Iran.

The second-order approximate coupled-cluster (CC2) method was performed to investigate the excited state hydrogen-bonding properties of Glyoxal (C2H2O2, Gl) dimers. Since the strengthening and weakening of hydrogen bonds can be investigated by monitoring the vibrational absorption spectra of some hydrogen-bonded groups in different electronic states, the infrared spectra of the hydrogen-bonded Gl-Gl complexes in both of the ground state and the S1 electronically excited state are calculated using the MP2/CC2 methods respectively. We demonstrated that the intermolecular hydrogen bond C=O⋯H-C between two glyoxal molecules is significantly strengthened in the electronically excited S1 state upon photoexcitation of the hydrogen-bonded Gl-Gl complexes.
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http://dx.doi.org/10.1016/j.saa.2013.11.034DOI Listing
March 2014

Microhydration effects on the electronic properties of protonated phenol: a theoretical study.

J Phys Chem A 2013 Dec 21;117(48):12842-50. Epub 2013 Nov 21.

Department of Chemistry, University of Isfahan , 81746-73441 Isfahan, Iran.

The CC2 (second-order approximate coupled cluster method) has been employed to investigate microhydration effect on electronic properties of protonated phenol (PhH(+)) According to the CC2 calculation results on electronic excited states of microhydrated PhH(+), for the S1 and S2 electronic states, which are of (1)ππ* nature and belong to the A' representation of molecular Cs point group, a significant blue shift effect on the S1 and S2 electronic states, which are of 1ππ* nature and belong to the A' representation of molecular Cs point group, in comparison to corresponding transitions on bare cation (PhH(+)), has been predicted. Nevertheless, for the S3-S0 (1A'', 1σπ*) transition, a large red shift effect has been predicted. Furthermore, it has been found that the lowest (1)σπ* state plays a prominent role in the photochemistry of these systems. In the bare protonated phenol, the (1)σπ* state is a bound state with a broad potential curve along the OH stretching coordinate, while it is dissociative in microhydrated species. This indicates to a predissociation of the S1((1)ππ*) state by a low-lying (1)σπ* state, which leads the excited system to a concerted proton-transfer reaction from protonated chromophore to the solvent. The dissociative (1)σπ* state in monohydrated PhH(+) has small barrier, while increasing the solvent molecules up to three removes the barrier and consequently expedites the proton-transfer reaction dynamics.
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http://dx.doi.org/10.1021/jp409537sDOI Listing
December 2013

Electronically excited states of protonated aromatic hydrocarbons: phenanthrene and pyrene.

J Phys Chem A 2013 Mar 15;117(12):2499-507. Epub 2013 Mar 15.

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.

The first and second electronic excited states (S1 and S2) of protonated phenanthrene and protonated pyrene, having the ππ* nature, are strongly red-shifted compared to corresponding electronic transitions in neutral homologues. The CC2 calculations identify an out-of-plane deformation as the most important photochemical reaction coordinate in protonated phenanthrene as well as protonated benzene. It was shown that the excited S1 states of protonated phenanthrene and protonated benzene are unstable via a torsional motion, which provides a fast access to a S1-S0 conical intersection. From the conical intersection, a barrier-less reaction path directs the system back to the minimum of the S0 potential-energy surface. In contrast to the most stable isomer of protonated phenanthrene, the most stable structure of protonated pyrene shows planar structure in both the S1 and S2 excited states, without considerable geometry deformations.
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http://dx.doi.org/10.1021/jp400554hDOI Listing
March 2013

Theoretical investigation of excited state proton transfer process in the N-salicylidene-2-bromoethylamine.

J Phys Chem A 2013 Jan 22;117(4):718-25. Epub 2013 Jan 22.

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.

Excited state reaction coordinate and the consequent energy profiles of a new Schiff base, N-salicylidene-2-bromoethylamine, have been investigated at the CC2 level of theory. The electron-driven proton transfer and torsional deformation have been identified as the most important photochemical reaction coordinates. In contrast to the ground state, the excited state potential energy profile shows a barrierless dissociation pattern along the O-H stretching coordinate, which verifies the proton transfer reaction along the O-H coordinate at the S(1) state. The calculations showed that the PT is electron driven and that the S(1) transition has charge transfer character. The keto-type S(1) state attained by barrierless proton transfer is found to be unstable via a torsional motion, which provides fast access to a S(1)-S(0) conical intersection. From the conical intersection, a barrierless reaction path directs the system back to the enol-type minimum of the S(0) potential energy surface, thus closing the photocycle.
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http://dx.doi.org/10.1021/jp310490vDOI Listing
January 2013

Effect of protonation on the electronic structure of aromatic molecules: naphthaleneH+.

Phys Chem Chem Phys 2010 Nov 6;12(43):14456-8. Epub 2010 Oct 6.

Centre Laser de l'Université Paris Sud (EA. 4127), Bât. 106, Univ. Paris-Sud 11, 91405 Orsay Cedex, France.

Protonated naphthalene, the smallest protonated polycyclic aromatic hydrocarbon cation, absorbs in the visible, around 500 nm, which corresponds to an unusually large red shift with respect to the neutral naphthalene counterpart.
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http://dx.doi.org/10.1039/c0cp00792gDOI Listing
November 2010

Protonated benzene dimer: an experimental and ab initio study.

J Am Chem Soc 2009 Aug;131(31):11091-7

Centre Laser de l'Universite Paris Sud (EA. 4127), Bat. 106, Univ. Paris-Sud 11 - 91405 Orsay Cedex, France.

The excitation spectrum of the protonated benzene dimer has been recorded in the 415-600 nm wavelength range. In contrast to the neutral iso-electronic benzene dimer, its absorption spectrum extends in the visible spectral region. This huge spectral shift has been interpreted with ab initio calculations, which indicate that the first excited states should be charge transfer states.
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http://dx.doi.org/10.1021/ja903181kDOI Listing
August 2009
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