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Ti3C2Tx MXene nanostructures have garnered attention for various catalytic applications due to their built-in electronic properties. Herein, we rationally design highly exfoliated two-dimensional Ti3C2Tx nanosheets (Tx = O, OH, and F) doped with Cu (denoted as Cu/Ti3C2Tx) for the electrochemical CO2 reduction reaction (CO2RR). The fabrication process entails the selective chemical etching of Ti3AlC2 followed by the delamination thereof under ultrasonic treatment and subsequent mixing with a Cu precursor to allow in situ doping. The resultant Cu/Ti3C2Tx are highly exfoliated nanosheets with a surface area of 46 m2 g-1 and are uniformly doped with Cu atoms (1.04 wt%). The CO2RR current density of Cu/Ti3C2Tx (-1.08 mA cm-2) was 3.6 times higher than that of Ti3C2Tx (-0.3 mA cm-2) besides a lower onset reduction potential and Tafel slope, and higher stability, due to the greater surface area, electronic effect, and quicker charge transfer on Cu/Ti3C2Tx. The formic acid (HCOOH) faradaic efficiency on Cu/Ti3C2Tx (58.1%) was 3-fold higher than that on Ti3C2Tx (18.7%). Based on density functional theory (DFT) simulation, Cu-doping induces polarized sites with high electron density, allowing the CO2RR path through the ∗HCOOH intermediate to form formic acid (HCOOH). The study presented here will open new pathways for using Ti3C2Tx doped with various metals for the CO2RR. This journal is

The high demand for renewable and clean energy has driven the exploration of advanced energy storage systems. Sodium-ion batteries (SIBs) are considered to be potential substitutes for Li-ion batteries (LIBs) because they are manufactured from raw materials that are cheap, less toxic, and abundantly available. Recent developments have demonstrated that two-dimensional (2D) materials have gained increasing interest as electrode candidates for efficient SIBs because of their enormous surface area and sufficient accommodating sites for the storage of Na ions. Herein, we explore the binding and diffusion mechanisms of Na on a 2D SnS sheet using density functional theory (DFT). The outcomes reveal that Na has a strong binding strength with SnS as well as charge transfer from Na to SnS, which affirms an excellent electrochemical performance. A transition from semiconducting (1.4 eV band gap) to metallic has been noted in the electronic structure after loading a minor amount of Na. In addition, a low open-circuit voltage (OCV) of 0.87 V and a high storage capacity of 357 mA h g

Li-ion batteries (LIBs) and Na-ion batteries (SIBs) are deemed green and efficient electrochemical energy storage and generation devices; meanwhile, acquiring a competent anode remains a serious challenge. Herein, the density-functional theory (DFT) was employed to investigate the performance of V

Hybrid organic-inorganic metal perovskites (ABX3, A- organic cation, B-metal ion, X-halide ion) have shown significant potential as thin-film solar cells or Dye solar cells (DSC) materials. Using a time-dependent density functional theory (TDDFT) study, this work explores the role that the central organic cation plays in defining the optoelectronic and physical properties of metal perovskites. Cubic phases of CH3NH3PbI3, CH3NH3PbCl3, CH3NH3PbICl2, and CH3NH3PbI2Cl perovskite structures are investigated using TDDFT with Gaussian 16. Electronic and optical properties, including transition energy, band gap, IR and absorption spectra and vibrational frequencies are examined. It is observed that the organic cation reduces the unit cell volume of the outer metal halide structure from approximately 9–20% for different perovskite materials. The peak optical absorption also correspondingly decreases by a significant margin, ranging from about 40–60%. This work is partially supported by the United States Office of Naval Research through grant number N00014-18-1-2732 . Scopus

Fossil fuels are running out fast. The energy demand is one of the largest challenges humankind has ever faced. Applications targeting renewable resources that are nonpetroleum based are booming. The scientific community is now ever more dedicated to the production of sustainable, green, and bio-based materials. Biomasses have started to realize their potential as a renewable energy source! This sentence is becoming a truth, especially if we know that out of the 2TW energy coming from renewable energy sources and which represent only 21% of the actual current needs, biomasses contribute to more than 70% compared to around 30% from hydropower. Yet, most of the biomass is in the form of fuelwood as well as agricultural and animal wastes. Approximately 70 x 106 tons of lignin are produced in the pulping procedure and only ~2% of this enormous amount is reused! The fate of the rest is either a low-grade fuel or thrust aside as waste. With a high content of functional groups (especially hydroxyl moieties either aliphatic or aromatic) and an elevated carbon content, lignin retains the “considered necessary” properties for energy storage and energy reservation applications. These properties include, for example, liability for chemical modification and thermal stability. Moreover, lignin is of low cost, widely available from different plant sources, promising renewability, biodegradable, and biocompatible with relevant antioxidant and antimicrobial properties. A combination of these properties in one source has opened realms of applications in the fields of material sciences as well as in the associated fields, for example, biology and medicine as readers will see throughout the chapter. To that point, the authors are focusing on lignin in the production of biofuels, polymer-based materials, lignin as a carbon precursor for environmental and catalytic applications, and micro- and nanoscale applications.

This work investigates the effect of alkali metals (Li, Na, and K) doping on silicene sheets (2D silicon) by using Time-Dependent Density Functional Theory (TDDFT). This includes Si 13 H 22 , Si 19 H 30 , Si 54 H 74 , and Si 104 H 134 silicene sheets. The results show some variation in the IR and UV-Vis spectrums as the sheet size change, with the largest structure showing an onset of absorption in the visible spectrum. Also, the results show that excitation energy decreases significantly by 29% as the number of atoms increases from Si 13 H 22 to Si 104 H 134 . Also, this work investigates the doping effect of alkali metals (Li, Na, and K) on the silicene sheets. The doped structures showed a dipole moment as high as approximately 10 Debye. The doped structures show a significant increase in optical absorption in the visible spectrum, as well as the potential for high reactivity

Oxidations of aniline (AN) initiated by OH-radicals are simulated in the temperature range 200–400 K using DFT/M06-2X/6-311++G(2df,2p) and ab initio ROCBS-QB3 levels. Chemical kinetics of such reactions were investigated based on several approaches including classical transition state theory (TST), conical variational transition state theory (CVT), and Rice-Ramsperger-Kassel-Marcus master equation (RRKM-ME) theories. Under atmospheric conditions, the reaction of OH radical with AN and the subsequent reactions with O2 molecules are investigated. The results indicate that the majority of O2 addition goes to the anti-directions with a branching ratio of 97.7% and produces the bicyclic peroxy radicals (BPRs) that can react with NO radical to form bicyclic alkoxy radicals (BARs). The latter compounds can be stabilized either by cyclization or via ring cleavage.

We present a computational study of the mechanism of the formation of 6-member heterocycles through the binding of ethylene to oxaldehyde, ethanedithial, and 2-thioxoacetaldehyde. This process is related to the olefin separation technology by metal dithiolenes and dioxolenes, being the formation of those heterocycles the main decomposition route. We also present a benchmark of 26 density functionals (spanning hybrid, double-hybrid, range-separated, semilocal, and local functionals) related to CCSD(T)/CBS reference values. Both the cyclization reaction and the isomerization of the cyclic product are included in the benchmark. The best functional among those tested for these reactions is ωB97XD, and the effect of the basis set is also investigated for it. © 2017 Wiley Periodicals, Inc.

The impact of AEO7 surfactant on the corrosion inhibition of carbon steel (C-steel) in 0.5 M HCl solution at temperatures between 20 °C and 50 °C was elucidated using weight loss and different electrochemical techniques. The kinetics and thermodynamic parameters of the corrosion and inhibition processes were reported. The corrosion inhibition efficiency (IE%) improved as the concentration of AEO7 increased. In addition, a synergistic effect was observed when a concentration of 1 × 10 −3 mol L −1 or higher of potassium iodide (KI) was added to 40 µmol L −1 of the AEO7 inhibitor where the corrosion IE% increased from 87.4% to 99.2%. Also, it was found that the adsorption of AEO7 surfactant on C-steel surface followed the Freundlich isotherm. Furthermore, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements indicated that AEO7 was physically adsorbed on the steel surface. The surface topography was examined using an optical profilometer, an atomic force microscope (AFM), and a scanning electron-microscope (SEM) coupled with an energy dispersion X-ray (EDX) unit. Quantum chemical calculations based on the density functional theory were performed to understand the relationship between the corrosion IE% and the molecular structure of the AEO7 molecule. © 2019, The Author(s). This publication was supported by Qatar University Internal Grant N° GCC-2017-012. The findings achieved herein are solely the responsibility of the authors. The authors gratefully thank the Center for Advanced Materials at Qatar University and the Chemistry Department at Cairo University for their support. The permanent address of Dr. Mohamed F. Shibl is Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt. Scopus

The correlated exciton-vibrational dynamics of the Fenna-Matthews-Olson (FMO) complex is studied using Multi-layer Multi-configuration Time-dependent Hartree (ML-MCTDH) wavepacket propagation. Exciton populations and coherences are shown to be sensitive to the details of the spectral density.

Hybrid organic-inorganic metal perovskites (chemical formula: ABX3, A-organic cation, B-metal ion, X-halide) are the latest advancement for increasing efficiency in thin-film solar cells or Dye Solar Cells (DSC). They have become the fastest growing technology in solar cells, rising in efficiency from 3% to 22.7% in less than a decade. This work focuses on a time-dependent density-functional theory (TDDFT) study of $\pmb{\text{CH}_{3}\text{NH}_{3}\text{PbCl}_{3}}$ -methyl ammonium lead chloride- a perovskite material of potential and current research interest. The work focuses on the cubic phase. Electronic and optical properties including transition energy, band gap, and vibrational frequencies are examined. The Density of States (DOS) spectrum, IR and UV-VIS spectra are calculated. It is observed that the cubic phase of $\pmb{\text{CH}_{3}\text{NH}_{3}\text{PbCl}_{3}}$ shows responsivity for optical excitation between wavelength ranges from 180 nm to 350 nm, with the peak observed at 240 nm. This work will help significantly to understand charge transport and optical properties, and design optimized solar cells involving these materials.

Silicene, a two-dimensional (2D) silicon nanosheet, has gained immense interest due to potential applications, better compatibility, and expected integration with current silicon (Si) technology. This work explores the effects of silicene sheet size on its optoelectronic properties using Time-Dependent Density Functional Theory (TDDFT). Four structures of hydrogen-terminated silicene, Si 13 H 22 , Si 19 H 30, Sis4H 74 , and Si l04 H 134 are investigated. It is observed that the structures have size tunable spectral response in the UV spectrum. The optimized sheets show a relatively smaller physical deformation compared to equivalent sized 2D Germanium sheets. IR spectra calculation of various bond vibrations show a good match with reported experimental results. The results show the potential for these 2D sheets to be effective optoelectronic materials in the visible spectrum, unlike bulk Si and Si nanowires. 2018 IEEE. Scopus

Oxygenated organic compounds derived from biomass (biofuel) are a promising alternative renewable energy resource. Alcohols are widely used as biofuels, but studies on bifunctional alcohols are still limited. This work investigates the unimolecular thermal degradation of 2-methoxyethanol (2ME) using DFT/BMK and ab initio (CBS-QB3 and G3) methods. Enthalpies of the formation of 2ME and its decomposition species have been calculated. Conventional transition state theory has been used to estimate the rate constant of the pyrolysis of 2ME over a temperature range of 298–2000 K. Production of methoxyethene via 1,3-H atom transfer represents the most kinetically favored path in the course of 2ME pyrolysis at room temperature and requires less energy than the weakest Cα − Cβ simple bond fission. Thermodynamically, the most preferred channel is methane and glycoladhyde formation. A ninefold frequency factor gives a superiority of the Cα − Cβ bond breaking over the Cγ − Oβ bond fission despite comparable activation energies of these two processes.

Initial degradation of methyl propionate (MePr) through its reaction with triplet molecular (3Σg−) O2 was investigated using density functional theory (DFT) and ab initio CBS-QB3 calculations over temperature range 700–1600 K. Thermochemistry and kinetics of three hydrogen atom abstractions have been studied. These reactions encounter energy barriers of 44.17–48.10 kcal mol−1 at CBS-QB3. Rate coefficients of the studied channels were evaluated from transition state theory (TST) with tunneling correction. From kinetic and thermodynamic perspectives, the most favorable process is H-atom abstraction from the Cα position, followed by Cμ, and then Cβ position. This order is partially different for the reaction of MePr with singlet (1Δg) O2 where H-atom abstraction from the Cμ atom is the most preferable pathway, followed by Cα position, then Cβ position. The atom in molecules (AIM) theory in critical point and natural bond orbital (NBO) analysis was used to explain the existence of covalent interactions.

In the present work, the gas phase reaction of OH radical initiated O-toluidine (OTOD) oxidation is investigated at ROCBS-QB3. Different pathways for OH radical additions to the benzene ring sites and H-atom abstractions are explored in details. At 200 K, the oxidation mechanism of OTOD is thoroughly dominated by the OH-addition to the aromatic ring, whereas the main favorable route is the OH addition to C2 atom with a branching ratio of 52.76%. Raising temperature to 1000 K, the total abstraction of amine's hydrogens becomes the main oxidation pathway for OTOD with contributions of 29.29%. The atmospheric lifetimes of aniline and OTOD are calculated to be 20.74 and 11.23 min., respectively. The fate of OTOD-OH2 (P2) adduct with atmospheric O2 molecule is inspected using the unimolecular Rice-Ramsperger-Kassel-Marcus (RRKM-ME) to verify our results at transition state theory (TST) and shows pressure and temperature dependence of the secondary oxidation mechanism. - 2019 Elsevier B.V. Scopus

This work reports structure and stability of different tautomers of a newly-designed analogue of 1,10-phenanthroline-5,6-dione (the di-keto structure (DK) derived from 1H,8H-pyrrolo[3,2-g]indole (PI)) using quantum chemical computations. Molecular structures and relative stabilities of four tautomers and their relevant rotamers of the investigated system have been studied using the B3LYP and M06 methods combined with the 6–31 + G(d,p) basis set, as well as the CBS-QB3 method. The effect of solvent has also been modelled in water using the PCM method at the M06/6-31+G(d,p) level of theory. Moreover, the aromaticity of the seven structures have been discussed based on their geometrical properties as well as the NICS indices. The results emphasize the predominance of the di-keto isomer (DK) and its greater stability over the keto-enol and di-enol conformers according to structural, energetic and magnetic criteria. In addition, NBO charges, HOMO-LUMO and ESP of the most stable tautomer DK have been employed to predict the electron donation active side toward metal ions.

Pyrazolo[1,5-a]pyrimidine, triazolo[1,5-a]pyrimidine, and pyrimido[1,2-a]benzimidazole, pyrido[1,2-a]benzimidazole ring systems incorporating phenylsulfonyl moiety were synthesized via the reaction of 3-(N,N-dimethylamino)-1-(thiophen-2-yl)-2-(phenylsulfonyl)prop-2-en-1-one derivatives with the appropriate aminoazoles as 1,3-binucleophiles and 1H-benzimidazol-2-ylacetonitrile using conventional methods as well as microwave irradiation. The regioselectivity of the cyclocondensation reactions was confirmed both experimentally by alternative synthesis of reaction products and theoretically using ab initio quantum chemical calculations namely the Density Functional Theory (DFT). The theoretical work was carried out using the Becke, three parameter, Lee-Yang-Parr hybrid functional (B3LYP) combined with the 6-311++G(d,p) basis set. It was found that the final cyclocondensation reaction product depends mainly on the initial addition to the activated double bond by the nitrogen atom of the 1,3-binucleophiles that has the higher electron density.

The multilayer multiconfiguration time-dependent Hartree method is used to study the coupled exciton–vibrational dynamics in a high-dimensional nonameric model of the LH2 antenna complex of purple bacteria. The exciton–vibrational coupling is parametrized within the Huang–Rhys model according to phonon and intramolecular vibrational modes derived from an experimental bacteriochlorophyll spectral density. In contrast to reduced density matrix approaches, the Schrödinger equation is solved explicitly, giving access to the full wave function. This facilitates an unbiased analysis in terms of the coupled dynamics of excitonic and vibrational degrees of freedom. For the present system, we identify spectator modes for the B800 to B800 transfer and we find a non-additive effect of phonon and intramolecular vibrational modes on the B800 to B850 exciton transfer. This work was made possible by NPRP grant #NPRP 7-227-1-034 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. We are grateful to M Schröter for helpful comments on the manuscript. Scopus

The reliable prediction of optical and fundamental gaps of finite size systems using density functional theory requires to account for the potential self-interaction error, which is notorious for degrading the description of charge transfer transitions. One solution is provided by parametrized long-range corrected functionals such as LC-BLYP, which can be tuned such as to describe certain properties of the particular system at hand. Here, bare and 3-mercaptoprotionic acid covered Cd33Se33 quantum dots are investigated using the optimally tuned LC-BLYP functional. The range separation parameter, which determines the switching on of the exact exchange contribution, is found to be 0.12 bohr-1 and 0.09 bohr-1 for the bare and covered quantum dot, respectively. It is shown that density functional optimization indeed yields optical and fundamental gaps and thus exciton binding energies, considerably different compared with standard functionals such as the popular PBE and B3LYP ones. This holds true, despite the well established fact that the leading transitions are localized on the quantum dot and do not show pronounced long-range charge transfer character. 2016 American Chemical SocietyGrant: This work was made possible by NPRP grant #NPRP 7-227-1- 034 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. Scopus

Nickel dithiolene complexes have been proposed as electrocatalysts for alkene purification. Recent studies of the ligand-based reactions of Ni(tfd)2 (tfd = S2C2(CF3)2) and its anion [Ni(tfd)2](-) with alkenes (ethylene and 1-hexene) showed that in the absence of the anion, the reaction proceeds most rapidly to form the intraligand adduct, which decomposes by releasing a substituted dihydrodithiin. However, the presence of the anion increases the rate of formation of the stable cis-interligand adduct, and decreases the rate of dihydrodithiin formation and decomposition. In spite of both computational and experimental studies, the mechanism, especially the role of the anion, remained somewhat elusive. We are now providing a combined experimental and computational study that addresses the mechanism and explains the role of the anion. A kinetic study (global analysis) for the reaction of 1-hexene is reported, which supports the following mechanism: (1) reversible intraligand addition, (2) oxidation of the intraligand addition product prior to decomposition, and (3) interligand adduct formation catalyzed by Ni(tfd)2(-). Density functional theory (DFT) calculations were performed on the Ni(tfd)2/Ni(tfd)2(-)/ethylene system to shed light on the selectivity of adduct formation in the absence of anion and on the mechanism in which Ni(tfd)2(-) shifts the reaction from intraligand addition to interligand addition. Computational results show that in the neutral system the free energy of activation for intraligand addition is lower than that for interligand addition, in agreement with the experimental results. The computations predict that the anion enhances the rate of the cis-interligand adduct formation by forming a dimetallic complex with the neutral complex. The [(Ni(tfd)2)2](-) dimetallic complex then coordinates ethylene and isomerizes to form a Ni,S-bound ethylene complex, which then rapidly isomerizes to the stable interligand adduct but not to the intraligand adduct. Thus, the anion catalyzes the formation of the interligand adduct. Significant experimental evidence for dimetallic species derived from nickel bis(dithiolene) complexes has been found. ESI-MS data indicate the presence of a [(Ni(tfd)2)2](-) dimetallic complex as the acetonitrile adduct. A charge-neutral association complex of Ni(tfd)2 with the ethylene adduct of Ni(tfd)2 has been crystallographically characterized. Despite the small driving force for the reversible association, very major structural reorganization (square-planar → octahedral) occurs.

The coupled quantum dynamics of excitonic and vibrational degrees of freedom is investigated for high-dimensional models of the Fenna-Matthews-Olson (FMO) complex. This includes a seven and an eight-site model with 518 and 592 harmonic vibrational modes, respectively. The coupling between local electronic transitions and vibrations is described within the Huang-Rhys model using parameters that are obtained by discretization of an experimental spectral density. Different pathways of excitation energy flow are analyzed in terms of the reduced one-exciton density matrix, focussing on the role of vibrational and vibronic excitation. Distinct features due to both competing time scales of vibrational and exciton motion and vibronically-assisted transfer are observed. The question of the effect of initial state preparation is addressed by comparing the case of an instantaneous Franck-Condon excitation at a single site with that of a laser field excitation., revised version

Geometry and energetics of low energy conformers of sodium dihydrogen triacetate (SDHTA) and its anion are studied using density functional theory (DFT) at the Becke, Lee-Yang-Parr hybrid functional (BLYP) and Becke, three-parameter, Lee-Yang-Parr hybrid functional (B3LYP) levels. For both cases, two structures of comparable energy are found, which have different symmetry with respect to the two hydrogen bonds (HBs). DFT-based Born–Oppenheimer molecular dynamics simulations are performed for SDHTA, which show that both structures are visited at room temperature conditions. The trajectory analysis further reveals that the two HBs behave anticooperative, that is, on average elongation of one HB is accompanied by a compression of the other one. This is in accord with nuclear magnetic resonance (NMR) experimental studies for a similar counter ion–dihydrogen triacetate complex. © 2012 Wiley Periodicals, Inc.

DFT quantum chemical computations have been carried out at the B3LYP/6-31G (d) level. Full geometry optimization has been performed and equilibrium geometries for a new series of phenyl thiazoles have been located. Ground state electronic properties, charge density distributions, dipole moments and its components have been calculated and reported. Effect of substituents on the geometry and on the polarization of the studied series of compounds are analyzed and discussed. Some structural features have been pinpointed to underline the affinity and selectivity of the studied compounds as adenosine A3-receptor antagonists. Results of the present work indicate that activity towards A3 receptor sites is directly correlated with both of the polarity and the co-planarity of the thiazole.

The tautomerisation reaction of 1,2,3-triazole and some of its derivatives has been studied and the transition states were located. The analysis of results indicates that 2H-triazole is more stable than the 1H-tautomer in all studied compounds in gas phase and in solution. The proton detachment energies and the geometries of the compounds studied were analysed using density functional theory (DFT) and Hartree–Fock (HF) methods in gas phase and solution. Deprotonation takes place in all cases studied by the detachment of N–H proton. The solvent effect on the stability the tautomeric and anion forms of studied compounds has been examined using B3LYP/6-31G* level of theory by applying the polarisable continuum model.

A density functional theory study of the behavior of hemispherands toward alkali metal ions (Li+, Na+, and K+) is performed. The effect of the replacement of the rigid anisyl group(s) by the mobile ether group(s) on the binding energy of hemispherands with alkali metal ions is investigated. The results indicated that the binding energies are inversely proportional to the ionic radius of the cations. Moreover, increasing the flexibility of the ligand results in decreasing the binding toward small ions. The structures of the hosts and the guests are correlated to the binding energies, and the correlations are interpreted in terms of the principle of preorganization. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

N-Bromosuccinimide (NBS) is a brominating and oxidizing agent that is used as a source of bromine. The proton affinities, the tautomeric forms and N Br bond dissociation of NBS have been computed using the B3LYP functional as implemented in the density functional approach. The electronic structures of all possible tautomeric forms of NBS have been thoroughly investigated. The keto form of NBS has been shown to be more stable than any other tautomeric forms. The geometries and relative energies for various stationary structures were determined. The results indicate clearly that O-site protonation is strongly favored over N-site protonation for the studied compound in case of mono- and di-protonation. The bond dissociation energies (BDEs), involving the formation of the bromine radical, cation, and anion, of the N Br bond have been investigated. The N Br BDE of the Br radical formation is lower than that of the Br anion or cation. These conclusions are in good agreement with the experimental results.

Using high resolution solid state (15)N and (2)H spectroscopy and longitudinal relaxometry we have studied the tautomerism of porphycene in the solid state, corresponding to a double proton transfer in two cooperative hydrogen bonds. The tautomerism is degenerate above 225 K but the degeneracy is lifted below this temperature, indicating a phase transition. Thus, the high-temperature phase is characterized by a dynamic proton disorder and the low-temperature phase by a dynamic proton order. (15)N magnetization transfer experiments obtained under cross polarization (CP) and magic angle spinning (MAS) conditions reveal the presence of two nonequivalent molecules A and B in the unit cell of phase II, exhibiting slightly different equilibrium constants of the tautomerism. Rate constants of the tautomerism in phase I could be obtained by the analysis of the longitudinal (15)N and (2)H relaxation times. The former, obtained at 9.12 MHz, exhibit a T(1) minimum around 270 K and are consistent with proton transfer induced dipolar (1)H-(15)N relaxation mechanism. The latter, obtained at 46.03 MHz, exhibit a minimum around 330 K and arise from quadrupole relaxation. Within the margin of error, the rate constants of the HH and of the HD/DD tautomerism are the same, exhibiting a barrier of about 30 kJ mol(-1), as expected for an overbarrier reaction in a configuration with two compressed hydrogen bonds. By contrast, in the low-temperature phase a switch of the DD transfer kinetics into the nanosecond time scale is observed, exhibiting a non-Arrhenius temperature dependence which is typical for tunneling. This increase of the rate constants by lowering the temperature is discussed in terms of a switch from a concerted HH transfer in phase I to a stepwise transfer in phase II, where intermolecular interactions lower the energy of one of the cis-intermediates.

The 1H and 2H NMR spectra of porphycene (1), 2,3,6,7,12,13,16,17-octaethylporphycene (2), 2,7,12,17-tetra-n-propylporphycene (3), and 2,7,12,17-tetra-(tert-butyl)-3,6-13,16-dibenzo[cde;mno]porphycene (4) partially deuterated in the mobile proton sites are reported. These compounds exhibit two intramolecular NHN hydrogen bonds of increasing strength representing models of the concerted HH transfer in the parent compound, porphycene. The 1H chemical shifts of the mobile protons are correlated with the difference of the energies of the amino- and imino-N1s orbitals reported by Ghosh A.; Moulder J.; Bröring M.; Vogel E. Angew. Chem., Int. Ed. 2001, 113, 445-448. The chemical shifts of 4 indicate a reduced contribution of the aromatic ring current as compared to the other compounds which is associated to the nonplanarity of this molecule. The primary H/D isotope effects on the chemical shifts give information about the primary, secondary, and vicinal geometric isotope effects of the two inner hydrogen bonds of porphycenes. The vicinal effects indicate a cooperative coupling of the two hydrogen bonds which may favor a concerted double proton-transfer mechanism.

The electronic absorption spectra of 2-thiouracil and some of its derivatives were investigated using polar and nonpolar solvents. The present analysis is facilitated via computer deconvolution of the observed spectra and molecular orbital (MO) computations. Comparison between the experimentally observed and theoretically computed spectra in addition to a quantitative assignment of all transitions observed were undertaken. The computed dipole moments are used to indicate the polarity of the excited state and hence predict its solvent dependence. The spectra are, in general, very well predicted and assigned using INDO/S computational results. The spectrum of the trifluoro derivative is much more complicated and the corresponding states show much more solvent dependence than those observed for other thiouracil studied.

Two approaches for the determination of the primary and secondary geometric isotope effect are compared for the exemplary porphyrinoid system porphycene, which has two intramolecular hydrogen bonds. A three-dimensional Born-Oppenheimer potential energy surface is calculated in terms of the symmetric and antisymmetric N-H stretching as well as a low-frequency hydrogen bond vibrational normal mode coordinate. From the respective ground-state nuclear wavefunction the quantum correction to the classical equilibrium geometry is determined. Further, geometry optimization within a full-dimensional multi-component molecular orbital (MC_MO) type calculation, which treats both the electrons and the hydrogen-bonded protons quantum mechanically, is performed. Both approaches yield geometric isotope effects, that is, upon H/D double substitution the hydrogen bonds are weakened and the respective N-N distances increase. In addition the MC_MO calculation gives a H/D isotope effect on the electronic structure, that is, the electronic wavefunction becomes more localized at the deuterium nucleus as compared with the proton case.

A sequential pump-dump control scenario for photoisomerization competing with direct photodissociation of a chiral molecule is derived by using a global optimization method. As an example, H 2 POSD with axial chirality is considered. The first excited electronic state with n-σ* electronic configuration is characterized by a repulsive potential that leads to P-S bond breaking. In this scenario, a first pump-dump sequence creates a nuclear wave packet having enough kinetic energy to cross the potential barrier in the ground state. This wave packet moves from one potential well corresponding to one of the optical isomers to the other in the electronic ground state. This transfer minimizes effects of the competing photodissociation. The second pump-dump sequence converts the wave packet into one having a small amount of kinetic energy via excitation to the first excited state. A stimulated Raman adiabatic passage (STIRAP) is applied to the same system for comparison. The yield of the photoisomerization obtained by using the STIRAP method is less than 0.1%, while that of using the sequential pump-dump pulse method is about 25%.

Quantum dynamical simulations based on ab initio potentials show that a single linearly polarized laser pulse (infrared or ultraviolet) can selectively excite one enantiomer from a racemic mixture. The degeneracy of the chiral pair is broken and a sequential reaction can distinguish between the two enantiomers based on energetic criteria. For instance, the undesired enantiomer can be photodestructed and the products can be probed using mass spectroscopy. The proposed scheme is applied to H2POSD, which has a low interconversion barrier and to a chiral olefin possessing stable enantiomers.

The ground-state geometries of uracil, 6-hydroxy-uracil, and 6-hydroxy-, 6-amino-, 6-methyl-, 6-trifluoro-, and 6-phenyl-2-thiouracil were optimized at the Hartree–Fock level. The molecular structures were fully optimized using the 6-31G and 6-31G* basis sets. The effect of substituents on the geometry and electronic structural features of 2-thiouracils were examined. The perturbation effects of the OH and NH2 groups are by far more pronounced on the geometric features and the dipole moment magnitude and direction of 2-thiouracil. The potential energy per atom criteria was used to compare the relative tightness of binding in the studied series. Proton affinity and deprotonation enthalpy on each of the possible sites in 2-thiouracil and its derivatives have been calculated at the 6-31G/MP2 level of theory. The obtained results show that thiouracils behave as bases where they possess a high tendency to abstract protons. Substituents in the 6-position have the general effect of enhancing the basicity strength of the thiocabonyl site in the order Ph < CH3 ≈ NH2 < OH. The CF3 group has the effect of reducing considerably the basicity strength and enhances the acidity strength at both N1 and N3. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

In order to optimize the selection of a suitable amine molecule for CO2 scrubbers, a series of ab initio calculations were performed at the B3LYP/6-31+G(d,p) level of theory. Diethylenetriamine was used as a simple chain amine. Methyl and hydroxyl groups served as examples of electron donors, and electron withdrawing groups like trifluoromethyl and nitro substituents were also evaluated. Interaction distances and binding energies were employed as comparison operators. Moreover, natural bond orbital (NBO) analysis, namely the second order perturbation approach, was applied to determine whether the amine-CO2 interaction is chemical or physical. Different sizes of substituents affect the capture ability of diethylenetriamine. For instance, trifluoromethyl shields the nitrogen atom to which it attaches from the interaction with CO2. The results presented here provide a means of optimizing the choice of amine molecules for developing new amine scrubbers. NPRP Grant # 5-1437- 1-243 from the Qatar National Research Fund (a member of Qatar Foundation)� Scopus

The coupling of hydrogen bonds is central to structures and functions of biological systems. Hydrogen bond coupling in sodium dihydrogen triacetate (SDHTA) is investigated as a model for the hydrogen bonded systems of the type O-H...O. The twodimensional potential energy surface is derived from the full-dimensional one by selecting the relevant vibrational modes of the hydrogen bonds. The potential energy surfaces in terms of normal modes describing the anharmonic motion in the vicinity of the equilibrium geometry of SDHTA are calculated for the different species, namely, HH, HD, DH, and DD isotopomers. The ground state wave functions and their relation to the hydrogen bond structural parameters are discussed. It has been found that the hydrogen bonds in SDHTA are uncoupled, that is elongation of the deuterated hydrogen bond does not affect the non-deuterated one. Scopus