Reverse Barrier Height Research Articles

Investigations of double proton transfer and one-electron oxidation behavior in double H-bonded glycine–formamide complex in the gas phase

The behaviors of double proton transfer (DPT) occurring in a representative glycine–formamide complex have been investigated employing the B3LYP/6-311++G** level of theory. The relevant quantities involved in the DPT process, such as interaction energies, tautomerization energy, equilibrium constant, and barrier heights have been discussed, respectively. Compared with the intramolecular proton transfer (PT) in isolated glycine, the participation of a formamide molecule favors the PT of glycine kinetically. The DPT process proceeds with a concerted mechanism rather than a stepwise one since no zwitterionic complexes have been located during the DPT process. Correspondingly, the calculated forward and reverse barrier heights are 12.62 and 2.01 kcal/mol, respectively. However, both of them have been reduced to 9.35 and −1.16 kcal/mol when zero-point vibrational energy (ZPVE) corrections are included, where the disappearance of the reverse barrier height implies that the reverse reaction should proceed with barrierless spontaneously. Additionally, the one-electron oxidation behavior for the double H-bonded glycine–formamide complex has also been investigated. The oxidation product is characterized by a distonic radical cation due to the fact that one-electron oxidation takes place on glycine fragment and a proton has been transferred from glycine to formamide fragment spontaneously. As a result, the vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 9.20 and 8.26 eV, respectively.

Double proton transfer and one‐electron oxidation behavior in double H‐bonded glycinamide–glycine complex in the gas phase

The behaviors of double proton transfer (DPT) occurring in a representative glycinamide-glycine complex have been investigated employing the B3LYP/6-311++G** level of theory. Thermodynamic and especially kinetic parameters, such as tautomerization energy, equilibrium constant, and barrier heights have been discussed, respectively. The relevant quantities involved in the DPT process including geometrical changes, interaction energies, and deformation energies have also been studied. Analogous to that of tautomeric process assisted with a formic acid molecule, the participation of a glycine molecule favors the proceeding of the proton transfer (PT) for glycinamide compared with that without mediator-assisted case. The DPT process proceeds with a concerted mechanism rather than a stepwise one because no zwitterionic complexes have been located during the DPT process. The barrier heights are 12.14 and 0.83 kcal/mol for the forward and reverse directions, respectively. However, both of them have been reduced by 3.10 and 2.66 kcal/mol to 9.04 and -1.83 kcal/mol with further inclusion of zero-point vibrational energy (ZPVE) corrections, where the disappearance of the reverse barrier height implies that the reverse reaction should proceed with barrierless spontaneously, analogous to those of DPTs occurring between glycinamide and formic acid (or formamide). Additionally, the oxidation process for the double H-bonded glycinamide-glycine complex has also been investigated. The oxidated product is characterized by a distonic radical cation due to the fact that one-electron oxidation takes place on glycine fragment and a proton has been transferred from glycine to glycinamide fragment spontaneously. As a result, the vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 8.71 and 7.85 eV, respectively, where both of them have been reduced by about 0.54 (1.11) and 0.75 (1.13) eV relative to those of isolated glycinamide (glycine) due to the formation of the intermolecular H-bond.

Benchmark Database of Barrier Heights for Heavy Atom Transfer, Nucleophilic Substitution, Association, and Unimolecular Reactions and Its Use to Test Theoretical Methods

A benchmark database of forward and reverse barrier heights for 19 non-hydrogen-transfer reactions has been developed by using Weizmann 1 calculations, and 29 DFT methods and 6 ab initio wave-function theory (WFT) methods have been tested against the new database as well as against an older database for hydrogen atom transfer reactions. Among the tested hybrid DFT methods without kinetic energy density, MPW1K is the most accurate model for calculations of barrier heights. Among the tested hybrid meta DFT methods, BB1K and MPWB1K are the two most accurate models for the calculations of barrier heights. Overall, the results show that BB1K and MPWB1K are the two best DFT methods for calculating barrier heights, followed in order by MPW1K, MPWKCIS1K, B1B95, MPW1B95, BH and HLYP, B97-2, mPW1PW91, and B98. The popular B3LYP method has a mean unsigned error four times larger than that of BB1K. Of the methods tested, QCISD(T) is the best ab initio WFT method for barrier height calculations, and QCISD is second best, but QCISD is outperformed by the BB1K, MPWB1K, MPWKCIS1K, and MPW1K methods.

Unprecedented (Cu2Ln)n Complexes (Ln = Gd3+, Tb3+): A New “Single Chain Magnet”

The magnetic study of a trinuclear Cu-Gd-Cu complex confirms that such basic units self-assemble to yield a high spin species. A nice fit of the magnetic data is obtained for an infinite chain of tetranuclear Gd(2)Cu(2) motifs linked through the Gd ions located at the opposite vertexes of the tetranuclear motifs according to two Cu-Gd coordination modes, a double bridging through phenoxo and alkoxo oxygen atoms and a single bridging through deprotonated amide functions. The two interaction pathways are ferromagnetic. Alternating current susceptibility measurements confirm that the equivalent copper-terbium entity is a single chain magnet with a barrier height for reversal of the magnetization equal to 28.5 K.

Double Proton Transfer and One-Electron Oxidation Behaviors in Double H-Bonded Glycinamide−Formamidine Complex and Comparison with Biological Base Pair

The behaviors of double proton transfer (DPT) occurring in a representative glycinamide-formamidine complex have been investigated employing the B3LYP/6-311++G** level of theory. Computational results suggest that the participation of a formamidine molecule favors the proceeding of the proton transfer (PT) for glycinamide compared with that without mediator-assisted case. The DPT process proceeds with a concerted mechanism rather than a stepwise one since no zwitterionic complexes have been located during the DPT process. The barrier heights are 14.4 and 3.9 kcal/mol for the forward and reverse directions, respectively. However, both of them have been reduced by 3.1 and 2.9 kcal/mol to 11.3 and 1.0 kcal/mol with further inclusion of zero-point vibrational energy (ZPVE) corrections, where the lower reverse barrier height implies that the reverse reaction should proceed easily at any temperature of biological importance. Additionally, the one-electron oxidation process for the double H-bonded glycinamide-formamidine complex has also been investigated. The oxidated product is characterized by a distonic radical cation due to the fact that one-electron oxidation takes place on glycinamide fragment and a proton has been transferred from glycinamide to formamidine fragment spontaneously. As a result, the vertical and adiabatic ionization potentials for the neutral double H-bonded complex have been determined to be about 8.46 and 7.73 eV, respectively, where both of them have been reduced by about 0.79 and 0.87 eV relative to those of isolated glycinamide due to the formation of the intermolecular H-bond with formamidine. Finally, the differences between model system and adenine-thymine base pair have been discussed briefly.

Performance of Density Functionals for Calculating Barrier Heights of Chemical Reactions Relevant to Astrophysics

The performance of 39 different LDA, GGA, meta-GGA, and hybrid density functionals has been evaluated, for calculating forward and reverse barrier heights of 10 gas-phase reactions involving hydrogen. The reactions are all relevant to astrochemistry. Special focus is put on the applicability of DFT for calculating the rates of corresponding surface hydrogenation reactions that are relevant to the chemistry of ice-coated interstellar grains. General trends in the performance of the density functionals for reactions involving H atoms, H2, and OH are discussed. The OH+CO reaction is shown to be a very problematic case for DFT. The best overall performance is found for the hybrid density functionals, such as MPW1K, B97-1, B97-2, and B1B95. For several reactions, the HCTH GGA functionals and the VS98 and OLAP3 meta-GGA functionals also give results that are almost as good as those of the hybrid functionals.

Hybrid Meta Density Functional Theory Methods for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions: The MPW1B95 and MPWB1K Models and Comparative Assessments for Hydrogen Bonding and van der Waals Interactions

Based on the modified Perdew and Wang exchange functional (MPW) and Becke's 1995 correlation functional (B95), we developed two hybrid meta density functional theory (HMDFT) methods, namely MPW1B95 and MPWB1K. In addition, based on the new X functional of Xin and Goddard, again combined with B95 correlation functional, we developed two other new HMDFT methods, X1B95 and XB1K. MPW1B95 and X1B95 were optimized against a representative database of six atomization energies (AE6). MPWB1K and XB1K were optimized against a kinetics database of three forward barrier heights, three reverse barrier heights, and three energies of reaction for the reactions in the BH6 representative barrier height database. We compared the newly developed methods to other HMDFT and hybrid DFT methods for atomization energies, ionization potentials, electron affinities, barrier heights, saddle point geometries, hydrogen bonding, and weak van der Waals weak interactions. In addition, we optimized scaling factors for calculating zero-point energies from vibrational frequencies. The results show that the MPWB1K and XB1K methods give good results for thermochemistry, thermochemical kinetics, hydrogen bonding, and weak interactions, and they give excellent saddle point geometries. MPW1B95 and X1B95 are suitable for general applications in thermochemistry, and they give good performance for hydrogen bonding and weak interaction calculations.

Development and Assessment of a New Hybrid Density Functional Model for Thermochemical Kinetics

A new hybrid Hartree−Fock-density functional model called the Becke88−Becke95 1-parameter model for kinetics (BB1K) was optimized against a database of three forward barrier heights, three reverse barrier heights, and three energies of reaction for the reactions in the BH6 representative barrier height database. We then assessed the newly developed BB1K method against a saddle point geometries database, a database of 42 barrier heights, the AE6 representative atomization energy database, a molecular geometries data set, and a set of 13 zero point energies. The results show that BB1K can give excellent saddle point geometries and barrier heights, and its performance for calculating atomization energies is 40% better than MPW1K. Using a mean mean unsigned error criterion that equally weights the errors in barrier heights and in bond energies, the new BB1K method outperforms all other DFT and hybrid DFT methods by a large margin, and we therefore conclude that it is the best density functional-type method for thermochemical kinetics.

A direct DFT dynamics study of the photodissociation of triplet acetaldehyde

Photodissociation of acetaldehyde at 308 nm via the radical CH 3 + HCO channel has been studied on the triplet state surface by DFT classical trajectory calculations using the B3LYP/6-31G* electronic structure method. A rotational analysis of product HCO predicts a non-Boltzmann distribution and that the energy is partitioned preferentially along the b and c axes rather than axis a. Product HCO is produced with no vibration excitation but the radical CH 3 is predicted to be vibrationally excited. These results corroborate the most recent LIF experiment on this reaction. When the product energies are scaled, to take into account a possible underestimation of the B3LYP/6-31G* reverse barrier height, the resulting energies are in excellent agreement with experiment.

Influence of stacking faults on the performance of 4H–SiC Schottky barrier diodes fabricated on (112̄0) face

The influence of stacking faults (SFs) on the performance of 4H–SiC (112̄0) Schottky barrier diodes fabricated on the epilayer grown on the substrate which was grown in [112̄0] direction by the sublimation method was investigated. The number of SFs under the Schottky electrode was determined by KOH etching of (11̄00) face cross section. SFs were found to have a severe influence on the leakage current of reverse characteristic and Schottky barrier height. The leakage current is increased even though a few SFs exist under the electrode. The Schottky barrier height is also affected by the SF under the electrode.

Photovoltaic properties of a single layer poly [3-(4-octylphenyl)-2,2′-bithiophene] (PTOPT)

A Schottky contact is made from a single layer polymer poly [3-(4-octylphenyl)-2,2′-bithiophene] (PTOPT) in its neutral state and a low work function metal (Al). The electrical and optical properties have been investigated by means of I– V measurements in the dark and under illumination. Various parameters such as the reverse saturation current density, barrier height, and diode quality factor were determined from the I– V curves in the dark of Al/PTOPT/ITO sandwich structure using thermionic emission theory. Spectral response of the device was measured at various wavelengths giving a peak at 500 nm. The IPCE% was obtained for illumination through both the Al and ITO sides. By illuminating the diode with a monochromatic light of wavelength 500 nm, the open-circuit voltage, short-circuit current density, power conversion efficiency, and fill-factor (FF) were obtained. The dependence of photocurrent on light intensity was also recorded and analyzed.

Dual-Level Direct Dynamics Study on the Diels−Alder Reaction of Ethylene and 1,3-Butadiene

Dual-level direct dynamics study was performed on the forward and reverse reaction of the parent Diels−Alder reaction of ethylene and 1,3-butadiene. The underlying potential energy surface was mapped with HF/6-31G* theory while the correlated methods including MP2/6-31+G* and QCISD(T)/6-31+G* were used to build a more accurate description of the reaction path necessary for the dual-level variational transition state theory calculation including multidimensional tunneling. By modeling the experimental gas-phase rate constants, our calculation estimated a forward classical barrier height of 21.9 kcal/mol, which is significantly lower than that inferred from the experimental activation energy (27.5 kcal/mol). The reverse barrier height is estimated as 69.6 kcal/mol, giving a classical energy of reaction of −47.8 kcal/mol, in good agreement with available experimental values. The variational effects were found to be negligible in this reaction, and the tunneling effects were small except at lower temperature. The deuterium and 13C kinetic isotope effects (KIEs) of the forward reaction were also calculated and were consistent with available experimental data in related systems. Both the vibrational and rotational motions were found to contribute strongly to the inverse secondary deuterium KIEs, and the tunneling effects make a noticeable contribution to the normal primary 13C KIEs at low temperature.

Cl2 / Ar High‐Density‐Plasma Damage in GaN Schottky Diodes

Inductively coupled plasma etching of metallized GaN Schottky diodes in discharges produces reductions in both reverse breakdown voltage and Schottky barrier height. The extent of these reductions is a function of both ion energy and ion flux. Two different postetch treatments were performed in an attempt to remove the ion‐damaged GaN surface layer, namely, annealing in or UV‐ozone oxidation followed by dissolution of the oxide. Both treatments provide only partial restoration of the diode properties. © 2000 The Electrochemical Society. All rights reserved.

High Density Plasma Damage Induced in n-GaN Schottky Diodes Using Cl2/Ar Discharges

The effects of dc chuck self-bias and high density source power (which predominantly control ion energy and ion flux, respectively) on the electrical properties of n-GaN Schottky diodes exposed to Inductively Coupled Plasma of Cl2/Ar were examined. Both parameters were found to influence the diode performance, by reducing the reverse breakdown voltage and Schottky barrier height. All plasma conditions were found to produce a nitrogen-deficient surface, with a typical depth of the non-stoichiometry being ∼ 500 Å. Post-etch annealing was found to partially restore the diode characteristics.

Classical trajectory study of CD2OH+→CDO++HD on potential energy surfaces constructed at various quantum chemical levels: Scaling of product mode-specific energies

Potential energy surfaces for the title reaction have been constructed by interpolation of 40 local potentials along the intrinsic reaction coordinate obtained by quantum chemical calculations at the semiempirical, Hartree–Fock (HF), post-HF, and density functional thoery (DFT) levels of theory. Classical trajectory calculations have been carried out and the average product mode-specific energies originating from the reverse barriers have been estimated. When normalized to the reverse barrier heights, the mode-specific energies evaluated at the HF, post-HF and theory DFT levels have been found to be comparable, indicating that the energy partitioning data are scaled to the barrier heights at these levels of theory. Namely, dynamical results obtained on potential energy surfaces constructed at moderate quantum chemical levels were similar (within ∼5%) to those at higher levels. It has been found theoretically that classical dynamics on two potential energy surfaces related by simple scaling results in scaled mode-specific energies. As a guideline for general cases, the similarity factor between two potential energy surfaces has been defined, which shows a good correlation with the scalability of the energy partitioning data.

High Density Plasma Damage Induced in n-GaN Schottky Diodes Using Cl2/Ar Discharges

AbstractThe effects of dc chuck self-bias and high density source power (which predominantly control ion energy and ion flux, respectively) on the electrical properties of n-GaN Schottky diodes exposed to Inductively Coupled Plasma of Cl2/Ar were examined. Both parameters were found to influence the diode performance, by reducing the reverse breakdown voltage and Schottky barrier height. All plasma conditions were found to produce a nitrogen-deficient surface, with a typical depth of the non-stoichiometry being ∼500 Å. Post-etch annealing was found to partially restore the diode characteristics.

An ab initio study on the reaction NH 2+CH 4 → NH 3+CH 3

The geometries of the stationary points of the title reaction are optimized at UQCISD/6-311G ** level. The forward and reverse barrier heights obtained at UQCISD(T)/6-311+G(2df, 2p)// UQCISD/6-311G ** level are 14.77 and 16.85 kcal mol −1, respectively. Furthermore, the reaction path information is discussed in detail. By means of the canonical variational transition state theory with small-curvature tunneling correction, the rate constants are calculated in the temperature range from 300 to 2100 K. It is shown that the calculated rate constants are in good agreement with those of the experimental results.

The inner-hydrogen migration in free base porphyrin

We present the results of a comprehensive study on the inner-hydrogen migration in free base porphyrin, using density functional theory with the hybrid B3-LYP exchange-correlation functional, and both the 6-31G(d ) and a triple-zeta double-polarization (TZ2P) basis set. The latter computations, involving 726 contracted functions, are the largest calculations on this system to date. Full geometry optimization was carried out for the cis and trans minima, the transition state for trans-cis isomerization, and the symmetric stationary point for the synchronous trans-trans isomerization. All stationary points were characterized by vibrational analysis. Our results strongly support the conclusion, reached by earlier workers, that trans-trans hydrogen transfer occurs in a two-step process via a cis intermediate. With the TZ2P basis and including zero-point effects for the -h 2 isotopomer, the trans-cis barrier height is 13.1 kcal/mol, the cis-trans energy difference is 8.1 kcal/mol and the reverse cis-trans barrier height is 5.0 kcal/mol. The trans-cis barrier height agrees well with the value of Braun et al. (J Am Chem Soc (1996) 118: 7231) obtained from NMR line shapes and a modified Bell tunneling model, but our cis-trans energy difference is higher, and the reverse barrier is lower, than the values of Braun et al. Tunneling precludes the existence of -h 2 cis-porphyrin as an observable species, but the -d 2 and, especially, -t 2 isotopomers might be observable at low temperatures if the reverse barrier is higher than our calculated value. We predict the theoretical vibrational spectrum of cis-porphyrin and suggest that IR active modes at 566 cm−1 and 2333 cm−1 in the -d 2 isotopomer may be used to detect the presence of the cis intermediate.

Ab initio MO studies on the photodissociation of C 2H 2 from the S 1( 1A u) state. II. Mechanism involving triplet states

Ab initio calculations at the EOM-CCSD and CASSCF-CASPT2 level have been carried out for low lying triplet states of C 2H 2. Minima on the seam of crossing and dissociation transition states have been found. At a photon energy slightly above the H + C 2H(X 2Σ) asymptote, we propose the following pathway for photodissociation from the S 1 state; C 2H 2 first hops to T 3 upon symmetric HCC bending and torsion, then adiabatically travels onto T 2 along asymmetric HCC bending and finally hops to T 1 to dissociate to H + C 2H(X 2Σ). The proposed mechanism and the reverse barrier height on T 1 are consistent with recent experimental observations.

Surface Passivation of In0.52Al0.48As Using (NH4)2Sx and P2S5/(NH4)2S

(NH4)2Sx and P2S5/(NH4)2S treatments were applied on In0.52Al0.48As surfaces. From X-ray photoelectron spectroscopy measurements, residual sulful atoms on the surface were found to form In–S, Al–S, and As–S bonds. The performance of Schottky diodes was improved by (NH4)2Sx and P2S5/(NH4)2S treatments in terms of the reverse leakage current and barrier height. In addition, the advantage of using P2S5/(NH4)2S was revealed compared with (NH4)2Sx .