Calculated Barrier Height Research Articles

Anharmonic effect of the dissociation rate constant of the unimolecular reactions of CH2XCHFO (X = H, F)

This study examines six unimolecular reactions of CH2XCHFO (X = H, F). The geometries of the reactions are optimized with Gaussian 03. The calculated barrier heights show that bond C–C′ scission, CH2XCHFO (X = H, F) → CH2X + CHFO (R1), dominates the decomposition of CH2XCHFO. For X = H and X = F, the barrier heights of (R1) are 13.37 and 9.67 kcal mol−1, respectively. The YL (Yao and Lin) method is used to calculate the anharmonic and harmonic rate constants of the unimolecular reactions. The results clearly demonstrate the anharmonic effect of these reactions. In the microcanonical case, for (R1) (X = H), the total energy is from 42.78 to 144.84 kcal mol−1. The corresponding anharmonic rate constants are from 1.57 × 1012 to 2.52 × 1013 s−1 and the harmonic rate constants are from 1.52 × 1012 to 2.52 × 1013 s−1.

Communication: A new hybrid exchange correlation functional for band-gap calculations using a short-range Gaussian attenuation (Gaussian-Perdue–Burke–Ernzerhof)

We have developed a new hybrid functional [Gaussian-Perdue-Burke-Ernzerhof (Gau-PBE)] that is suitable for the calculation of solid state bandgaps using a periodic boundary condition. The characteristic of this functional is the use of a Gaussian attenuation scheme (Gau) to include a short-range Hartree-Fock (HF) exchange. This new functional can perform barrier height calculations with an accuracy comparable to the middle-range hybrid functional and bandgap calculations with an accuracy comparable to the Heyd-Scuseria-Ernzerhof (HSE) functional. However, the point is that the performance can be achieved using a Gaussian HF exchange, while the HISS functional calculates twice HF-exchange integrations using an error function to obtain both performances. In addition, Gau-PBE functional can decrease the time cost of bandgap calculations by an average of 40% compared to the HSE functional.

Design and characterization of TlInSe 2 varactor devices

TlInSe 2 single crystal has been successfully prepared by the Bridgman crystal growth technique. The crystal, which exhibits compositional atomic percentages of 25.4%, 25.2% and 49.4% for Tl, In and Se, respectively, is found to be of tetragonal structure with lattice parameters of a=0.8035 and c=0.6883 nm. The crystals were used to design radio frequency sensitive varactor device. The temperature dependence of the current–voltage characteristics of the device allowed the calculation of the room temperature barrier height and ideality factor as 0.87 eV and as 3.2, respectively. Rising the device temperature increased the barrier height and decreased the ideality factor. This behavior was attributed to the current transport across the metal–semiconductor interface. The capacitance of the device is observed to increase with increasing voltage and increasing temperature as well. The temperature activation of the capacitance starts above 82 °C with a temperature coefficient of capacitance being 1.08×10 −3 K −1. Furthermore, the capacitance of the device was observed to increase with increasing frequency up to a maximum critical frequency of 4.0 kHz, after which the capacitance decreased with increasing frequency. The behavior reflected the ability of maximum amount of charge holding being at a 4.0 kHz. The analysis of the capacitance–voltage characteristics at fixed frequencies reflected a frequency dependent barrier height and acceptors density. The decrease in the barrier height and acceptors density with increasing frequency is mainly due to the inability of the free charge to follow the ac signal.

Fabrication and electrical characterization of pyrrole–aniline copolymer-based Schottky diodes

In this work, pyrrole–aniline copolymer/p-Si structure has been fabricated by forming a thin organic copolymer film on a p-Si wafer. A good rectifying behavior was seen from the current–voltage (I–V) characteristics. The characteristic parameters of the structure such as barrier height, ideality factor, interface states density and series resistance were determined from the electrical measurements using I–V, Cheung's, and modified Norde's function. The calculated barrier height values from different methods have shown the consistency of the approaches. The obtained ideality factor which is greater than unity refers to the deviation from ideal diode characteristics. This deviation can be attributed to secondary mechanisms, which include interface dipoles due to interface doping or specific interface structure, as well as fabrication-induced defects at the interface. The energy distribution of interface-state density of the copolymer-based structure was determined, and the interface-state density was found to vary from 1.27 × 1016 cm−2 eV−1 in (0.07 – Ev) eV to 2.45 × 1015 cm−2 eV−1 in (0.52 – Ev) eV. Furthermore, these copolymers were characterized with UV–vis, FTIR techniques, and thermal analysis.

A DFT study of H-isomerisation in alkoxy-, alkylperoxy- and alkyl radicals: Some implications for radical chain reactions in polymer systems

Intramolecular 1-n H-shift (n = 2, 3… 7) reactions in alkoxy, alkyl and peroxy radicals were studied by density functional theory (DFT) at the B3LYP/6-311+G∗∗ level and compared with respective intermolecular H-transfers. It was found that starting from 1 to 3 H-shift the barrier heights stepwise decrease with increasing n reaching minimum for 1–5 and 1–6 H-shifts. This dependence can be ascribed to the decrease of the strain with increasing transition state (TS) ring size, which is minimal in six- and seven-member rings. The barrier heights of H-shifts in alkyl radicals are systematically larger than those in alkoxy radicals: the respective activation energies (Ea) of 1–5 and 1–6 H-shifts are about 59–67 kJ/mol for alkyl radical and 21–34 kJ/mol for alkoxy radicals. Further increase of the TS ring size in 1–7 H-shifts leads to the increase of the barrier to 44 kJ/mol in the hexyloxy radical and 84 kJ/mol for n-heptyl radical. We have also found that intermolecular H-transfer reactions in all three types of free radicals have smaller barriers than respective intramolecular 1–5 or 1–6 H-shifts by 4–25 kJ/mol. The mentioned difference can be explained in terms of enhanced nonbonding repulsion interaction in the cyclic TS structures compared to respective intermolecular TS. B3LYP/6–311+G∗∗ geometric parameters and imaginary frequencies for 1-n H-shifts TS are consistent with respective calculated barrier heights. Reactivity of some other radicals compared to alkoxy, peroxy and alkyl radicals as well as other factors influencing their reactivity (π-conjugation, steric effect and ring strain in cyclic TS, etc.) are also briefly discussed in relation to free radical reactions in polymer systems.

Anionic Production Pathways Involved in the Reaction between OH<sup>-</sup> and CH<sub>2</sub>ClF

The anionic production pathways involved in the reaction between hydroxide anion (OH) and chlorofluoromethane (CH2ClF) were theoretically investigated. The optimized geometries of all the important species on the reaction potential energy surface were obtained at the B3LYP/6-31+G(d,p) and B3LYP/6-311 ++ G(2d,p) levels. Consequently, harmonic vibrational frequencies and zero point energies (ZPEs) were calculated. Based on the relative energies of all the species that were calculated at the CCSD(T)/6-311+G(3df,3dp) level, the anionic production channels for the H-abstraction and the bimolecular nucleophilic substitution (SN2) reaction processes are elaborated upon. According to the calculated barrier heights for the production pathways, the H-abstraction channel is dominant, which agrees very well with previous experimental conclusions. In addition, non-typical anionic products are suggested to form during the SN2 reaction processes where the serious dynamic effect probably causes the SN2 reaction process to produce F.

Convergent Partially Augmented Basis Sets for Post-Hartree-Fock Calculations of Molecular Properties and Reaction Barrier Heights.

We present sets of convergent, partially augmented basis set levels corresponding to subsets of the augmented "aug-cc-pV(n+d)Z" basis sets of Dunning and co-workers. We show that for many molecular properties a basis set fully augmented with diffuse functions is computationally expensive and almost always unnecessary. On the other hand, unaugmented cc-pV(n+d)Z basis sets are insufficient for many properties that require diffuse functions. Therefore, we propose using intermediate basis sets. We developed an efficient strategy for partial augmentation, and in this article, we test it and validate it. Sequentially deleting diffuse basis functions from the "aug" basis sets yields the "jul", "jun", "may", "apr", etc. basis sets. Tests of these basis sets for Møller-Plesset second-order perturbation theory (MP2) show the advantages of using these partially augmented basis sets and allow us to recommend which basis sets offer the best accuracy for a given number of basis functions for calculations on large systems. Similar truncations in the diffuse space can be performed for the aug-cc-pVxZ, aug-cc-pCVxZ, etc. basis sets.

Energetics of Intercavity Diffusion in a Simple Model of a Low-Density p-tert-Butylcalix[4]arene Crystal

Potentials of mean force calculated for the diffusion of small guest molecules (CO(2) and CH(4)) between adjacent host cavities in the beta(0) p-tert-butylcalix[4]arene polymorph reveal sufficiently high barriers to diffusion to justify characterizing the lattice voids in this crystal as independent host cavities rather than as extended volumes. In addition, the calculated barrier heights are consistent with a "relay" model for gas diffusion in this ostensibly nonporous organic solid involving the lateral sliding of the host bilayers.

Communication: Energetics of reaction pathways for reactions of ethenol with the hydroxyl radical: The importance of internal hydrogen bonding at the transition state

We find high multireference character for abstraction of H from the OH group of ethenol (also called vinyl alcohol); therefore we adopt a multireference approach to calculate barrier heights for the various possible reaction channels of OH+C(2)H(3)OH. The relative barrier heights of ten possible saddle points for reaction of OH with ethenol are predicted by multireference Møller-Plesset perturbation theory with active spaces based on correlated participating orbitals (CPOs) and CPO plus a correlated pi orbital (CPO+pi). Six barrier heights for abstracting H from a C-H bond range from 3.1 to 7.7 kcal/mol, two barrier heights for abstracting H from an O-H bond are both 6.0 kcal/mol, and two barrier heights for OH addition to the double bond are -1.8 and -2.8 kcal/mol. Thus we expect abstraction at high-temperature and addition at low temperature. The factor that determines which H is most favorable to abstract is an internal hydrogen bond that constitutes part of a six-membered ring at one of the abstraction saddle points; the hydrogen bond contributes about 3 kcal/mol stabilization.

Role of Second Coordination Sphere Amino Acid Residues on the Proton Transfer Mechanism of Human Carbonic Anhydrase II (HCA II)

The barrier for the proton transfer in human carbonic anhydrase II (HCA II) has been studied by including the important second coordination sphere residues to the minimalistic model of the active site using B3LYP/6-31+G** level of calculation. Specifically, proton transfer from a zinc-bound water molecule to a histidine residue (His64) mediated by a water bridge (consists of two and three water molecules) has been investigated. The new model contains three functional groups of second coordination sphere residues such as Gln92, Glu117, and Asn244. These residues interact with His94, His119, and His96, respectively. We have calculated the barrier for the proton transfer using the total energies of reactants, transition states, and products. The calculated barrier height for the models with two water and three water molecules are 8.94 and 8.67 kcal/mol, respectively, which are in close agreement with the experimental value of 7.8 kcal/mol obtained from the kinetic experiments and to the range 8-10 kcal/mol predicted from the pK(a) considerations ( Silverman , D. N. Biochim. Biophys. Acta 2000 , 1458 , 88 - 103 . ). In addition, our own N-layered integrated molecular orbital + molecular mechanics (ONIOM) calculations have also been carried out on various model systems to understand the effect of complete environment. It is possible to note from the results that the confinement of water molecules by the protein milieu appreciably decreases the O...O distance between water molecules in the water bridge when compared to the free water dimer which enables the proton transfer from one water to the other and finally to the His64 residue.

Vibrational assignment and proton tunneling in pyridine–pyridinium complexes

The pyridine–pyridinium cationic complexes [(Py 2H) +], with two different counterions (perchlorate and bromide), were prepared. The vibrational spectra of the complexes have been recorded. We then performed theoretical calculations to aid our understanding of the experimental data. To include the proton tunneling effect in our study, we adopted a simple one-dimensional potential energy surface (PES) for the motion of the hydrogen atom (at the center). This PES was derived from quantum mechanical calculations at the B3LYP/6-311++G ** level of theory for a fixed skeleton geometry. Based on such a PES, we computed the vibrational energy levels, from which a tunneling splitting of 487 ± 2 and 242 ± 2 cm −1 was obtained in the gas and solution phases, respectively. The calculated barrier height was about 2.0–2.3 and 4.0–4.2 kcal/mol in the gas and solution phases, respectively. After all things considered, our calculations assign the bands at 138, 1255, and 1665 cm −1 to the N⋯N stretching, N–H out-of-plane bending, and N–H in-plane bending modes, respectively.

Junction properties of Au/ZnO single nanowire Schottky diode

In this study, we have analyzed the Au/ZnO single nanowire based Schottky diode by investigating temperature dependent current voltage and x-ray photoelectron spectroscopy (XPS) measurements. The calculated barrier height of the Schottky diodes by using the thermionic emission model is in good agreement with the value obtained from the XPS measurements but lower than the theoretically predicted value. The ionization of interface states has been considered for explaining this discrepancy.

Efficient Diffuse Basis Sets for Density Functional Theory

Eliminating all but the s and p diffuse functions on the non-hydrogenic atoms and all diffuse functions on the hydrogen atoms from the aug-cc-pV(x+d)Z basis sets of Dunning and co-workers, where x = D, T, Q, ..., yields the previously proposed "minimally augmented" basis sets, called maug-cc-pV(x+d)Z. Here, we present extensive and systematic tests of these basis sets for density functional calculations of chemical reaction barrier heights, hydrogen bond energies, electron affinities, ionization potentials, and atomization energies. The tests show that the maug-cc-pV(x+d)Z basis sets are as accurate as the aug-cc-pV(x+d)Z ones for density functional calculations, but the computational cost savings are a factor of about two to seven.

Localized orbital corrections for the calculation of barrier heights in density functional theory.

This work describes the extension of a previously reported empirical localized orbital correction model for density functional theory (DFT-LOC) for atomization energies, ionization potentials, electron affinities, and reaction enthalpies to the correction of barrier heights for chemical reactions of various types including cycloadditions, cycloreversions, dipolar cycloadditions, S(N)2's, carbon radical reactions, hydrogen radical reactions, sigmatropic shifts, and electrocyclizations. The B3LYP localized orbital correction version of the model (B3LYP-LOC) reduces the number of outliers and overall mean unsigned error (MUE) vs. experiment or ab initio values from 3.2 to 1.3 kcal/mole for barrier heights and from 5.1 to 1.1 kcal/mole for reaction enthalpies versus B3LYP. Furthermore, the new model has essentially zero additional computational cost beyond standard DFT calculations. Although the model is heuristic and is based on multiple linear regression to experimental or ab initio data, each of the parameters is justified on chemical grounds and provides insight into the fundamental limitations of DFT, most importantly the failure of current DFT methods to accurately account for nondynamical electron correlation.

Electronegativity equalization model for interface barrier formation at reactive metal/organic contacts

A general model based on electronegativity equalization method (EEM) is proposed for a quantitative formulation of barrier formation at reactive metal/organic interfaces. The present model predicts for molecular bonding formation a linear dependence of barrier heights on the degree of partial charge transfer, which is determined by the electronegativity difference between metals and molecules. Also, the calculated barrier heights show good agreement with the empirical values. It suggests that the EEM-based electronegativity model has captured the essence of barrier formation at reactive metal/organic interfaces, and that electronegativity is a fundamental factor in characterizing the chemical trend of barrier heights.

Ring contraction mechanisms in substituted cyclohexylium cations

The effect of an electron donating or withdrawing group on the ring contraction mechanism of a cyclohexylium (cyclohexane-derived) cation has been studied using density functional theory. The barrier to rearrangement of the parent cyclohexane cation ( 1) was previously calculated to be 7.7 kcal/mol using PBE/6-311++G(2d, 2p). We show in this work that addition of an electron withdrawing group (CF 3) lowers the average barrier to ring contraction, while an electron donating group (CH 3) increases the average barrier, relative to the parent, unsubstituted, cyclohexane cation. Calculated barrier heights for going from a 6-membered to 5-membered ring range from 4.3 to 23.3 kcal/mol for methyl-cyclohexylium ( 2), but only from 0.6 to 14.0 kcal/mol for trifluoromethyl-cyclohexylium ( 3). The lower barriers for 3 can be explained by (a) the starting structures involved, and (b) the use of dative bonding as a catalyst in stabilizing intermediates and transition states. For 1 and 3 the reaction involves starting from secondary cations and going downhill in energy through secondary intermediates to a tertiary product. However, 2 does not benefit from such favorable energetics since it more likely starts from a tertiary cation and has to first rearrange to secondary intermediates to derive the tertiary methyl-cyclopentane-type product.

Experimental and Theoretical Studies of the Kinetics of the OH + Hydroxyacetone Reaction As a Function of Temperature

The rate constant for the reaction of the OH radical with hydroxyacetone was measured between 2 and 5 Torr and over the temperature range of 280-350 K, using a discharge-flow system coupled with resonance fluorescence detection of the OH radical. At 298 K the rate constant was found to be (3.02 +/- 0.28) x 10(-12) cm3 molecule(-1) s(-1), in excellent agreement with several previous studies. A positive temperature dependence was measured over the temperature range 280-350 K, described by the Arrhenius expression k = (1.88 +/- 0.75) x 10(-11) exp[-(545 +/- 60)/T] cm3 molecule(-1) s(-1), in contrast to previous measurements of the temperature dependence for this reaction and suggesting that the atmospheric lifetime of hydroxyacetone may be greater than previously estimated. Theoretical calculations of the potential energy surface for this reaction suggest that the mechanism for this reaction involves hydrogen abstraction through a hydrogen-bonded prereactive complex similar to the OH + acetone reaction, with a calculated barrier height between -1 and 1 kcal mol(-1) depending on the level of theory.

Systematic theoretical investigation for adsorption behavior of Al and N atoms on 4H–SiC [formula omitted] surfaces

We systematically investigate adsorption behavior of Al and N atoms on 4H–SiC ( 1 1 2 ¯ 0 ) surfaces based on first-principles calculations. The calculations for stable adsorption site demonstrate that the Al atom is adsorbed at the lattice site of 4H structure whereas the N atom is incorporated at the interstitial site. The calculated surface phase diagrams for Al and N atoms as functions of beam equivalent pressure and temperature clarify that Al atoms can be adsorbed at all of the adsorption sites below 1800 K and N atoms, in contrast, are desorbed excepting the most stable site above 1150 K. These temperatures imply that under experimental conditions (1223 K) Al atoms can be easily adsorbed and N atoms are adsorbed only on the most stable adsorption site. Furthermore, the calculated barrier heights of adsorbed Al and N atoms are 1.0 and 1.8 eV, respectively, implying that the surface migration of Al atoms is prominent compared to that of N atoms. These results obtained thus manifest that adsorption behavior of Al and N atoms is quite different and the growth processes change depending on V/III ratio under molecular beam epitaxy growth.

Calculation of current–voltage characteristics of a Cu (II) complex/n-Si/AuSb Schottky diode

In this study, Cu (II) complex/n-Si structure has been fabricated by forming a thin organic Cu (II) complex film on n-Si wafer. It has been seen that the structure has clearly shown the rectifying behaviour and can be evaluated as a Schottky diode. The contact parameters of the diode such as the barrier height and the ideality factor have been calculated using several methods proposed by different authors from current–voltage ( I– V) characteristics of the device. The calculated barrier height and ideality factor values from different methods have shown the consistency of the approaches. The obtained ideality factor which is greater than unity refers the deviation from ideal diode characteristics. This deviation can be attributed to the native interfacial layer in the organic/inorganic interface and the high series resistance of the diode. In addition, the energy distribution of the interface state density ( N ss ) in the semiconductor band gap at Cu (II) complex/n-Si interface obtained from I– V characteristics range from 2.15 × 10 13 cm −2 eV −1 at ( E c − 0.66) eV to 5.56 × 10 12 cm −2 eV −1 at ( E c − 0.84) eV.

Heavy-element fission barriers

We present calculations of fission properties for heavy elements. The calculations are based on the macroscopic-microscopic finite-range liquid-drop model with a 2002 parameter set. For each nucleus we have calculated the potential energy in three different shape parametrizations: (1) for 5 009 325 different shapes in a five-dimensional deformation space given by the three-quadratic-surface parametrization, (2) for 10 850 different shapes in a three-dimensional deformation space spanned by ${\ensuremath{\epsilon}}_{2}$, ${\ensuremath{\epsilon}}_{4}$, and $\ensuremath{\gamma}$ in the Nilsson perturbed-spheroid parametrization, supplemented by a densely spaced grid in ${\ensuremath{\epsilon}}_{2}$, ${\ensuremath{\epsilon}}_{3}$, ${\ensuremath{\epsilon}}_{4}$, and ${\ensuremath{\epsilon}}_{6}$ for axially symmetric deformations in the neighborhood of the ground state, and (3) an axially symmetric multipole expansion of the shape of the nuclear surface using ${\ensuremath{\beta}}_{2}$, ${\ensuremath{\beta}}_{3}$, ${\ensuremath{\beta}}_{4}$, and ${\ensuremath{\beta}}_{6}$ for intermediate deformations. For a fissioning system, it is always possible to define uniquely one saddle or fission threshold on the optimum trajectory between the ground state and separated fission fragments. We present such calculated barrier heights for 1585 nuclei from $Z=78$ to $Z=125$. Traditionally, actinide barriers have been characterized in terms of a ``double-humped'' structure. Following this custom we present calculated energies of the first peak, second minimum, and second peak in the barrier for 135 actinide nuclei from Th to Es. However, for some of these nuclei which exhibit a more complex barrier structure, there is no unique way to extract a double-humped structure from the calculations. We give examples of such more complex structures, in particular the structure of the outer barrier region near $^{232}\mathrm{Th}$ and the occurrence of multiple fission modes. Because our complete results are too extensive to present in a paper of this type, our aim here is limited: (1) to fully present our model and the methods for determining the structure of the potential-energy surface, (2) to present fission thresholds for a large number of heavy elements, (3) to compare our results with the two-humped barrier structure deduced from experiment for actinide nuclei, and (4) to compare to additional fission-related data and other fission models.