Function Of Kinetic Energy Research Articles

Revised Huang-Carter nonlocal kinetic energy functional for semiconductors and their surfaces

Nonlocal kinetic energy functionals with a density-dependent kernel are the most accurate functionals available for carrying out orbital-free density functional theory simulations. Among them, the Huang and Carter (HC) functional [Huang and Carter, Phys. Rev. B 81, 045206 (2010)] is the most accurate for bulk semiconductors. A major hurdle in applying HC to nonbulk systems (such as clusters and surfaces which have at least one nonperiodic dimension where the density decays to zero) lies in its numerical instability for large values of the reduced density gradient, $s(\mathbf{r})\ensuremath{\propto}\frac{|\mathbf{\ensuremath{\nabla}}\mathbf{n}(\mathbf{r})|}{{\mathbf{n}}^{\mathbf{4}/\mathbf{3}}(\mathbf{r})}$, where $n$ is the electron density. We propose a revision to the HC functional, revHC, that allows it to achieve dramatically improved numerical stability, efficiency (in terms of timing to solution), and applicability. Not only does revHC reproduce all previously presented results for HC, but it extends them to a crucially important class of materials: surfaces. We show that surface energy trends of clean-cut and reconstructed surfaces of group IV and III-V semiconductors are recovered and, where available semiquantitatively, reproduce the experimental results.

Real-time transient stability detection in the power system with high penetration of DFIG-based wind farms using transient energy function

In this paper, a new approach for real-time transient instability detection (TID) is presented. The purpose of presenting this approach is to have the lowest computational load and the highest accuracy and speed and to be suitable for real-time applications. The main idea is TID without directly calculating the unstable equilibrium point (UEP). In fact, transient instability is detected only by identifying the characteristics of the UEP and the kinetic energy function along with the potential energy boundary surface (PEBS) function. A new and standard single machine infinite bus system with high penetration of wind farms is used for simulation. First, dynamic modeling of various components of doubly fed induction generator (DFIG) and synchronous generator are described. Then, a model for the grid-connected DFIG is developed and a new PEBS-based index (NPI) for TID is presented. The NPI requires only post-fault data and can be applied as a general tool to any power system with any change in topology and operating conditions. Also, the impact of increasing the penetration of wind power, changing the operating conditions of the power system, different fault locations, and the power system strength on the transient stability is investigated. The simulation results show the effectiveness of the proposed approach for TID in a shorter time.

GGA-Level Subsystem DFT Achieves Sub-kcal/mol Accuracy Intermolecular Interactions by Mimicking Nonlocal Functionals.

The key feature of nonlocal kinetic energy functionals is their ability to reduce to the Thomas-Fermi functional in the regions of high density and to the von Weizsäcker functional in the region of low-density/high reduced density gradient. This behavior is crucial when these functionals are employed in subsystem DFT simulations to approximate the nonadditive kinetic energy. We propose a GGA nonadditive kinetic energy functional which mimics the good behavior of nonlocal functionals, retaining the computational complexity of typical semilocal functionals. Crucially, this functional depends on the inter-subsystem density overlap. The new functional reproduces Kohn-Sham DFT and benchmark CCSD(T) interaction energies of weakly interacting dimers in the S22-5 and S66 test sets with a mean absolute deviation well below 1 kcal/mol.

A hybrid Eulerian-Eulerian/Eulerian-Lagrangian method for dense-to-dilute dispersed phase flows

Problems involving a transition between dense and dilute dispersed phase regimes often require hybridization of Eulerian-Eulerian (EE) and Eulerian-Lagrangian (EL) methods for accurate and efficient computational modeling. A hybrid EE-EL formulation is developed in this work from first principles, which asymptotes to well-established EE and EL methods in limiting conditions. A smooth and dynamic transition criterion and a corresponding algorithm for conversion between the two representations of the dispersed phase are developed. To use EE and EL in their respective regions of effectiveness, the transition criterion is designed as a function of local volume fraction and local kinetic energy of random uncorrelated motion of particles. Several one-dimensional numerical tests are conducted to analyze limits of the hybrid EE-EL method in dense (up to 65% volume loading) and dilute conditions, at first, without any conversion between EE and EL, and then the ability of the conversion algorithm to smoothly transition from EE to EL and vice versa is evaluated using prespecified and dynamically computed transition criteria for one- and two-dimensional tests. Particle evolution in two-dimensional frozen turbulence is simulated to evaluate the method's ability to dynamically transition from EE to EL and vice versa in regions of particle trajectory crossing. Finally, the hybrid method is used for simulating dispersion of an initially dense particle cloud in a three-dimensional spherical sector blast.

Nonlocal kinetic energy functionals in real space using a Yukawa-potential kernel: Properties, linear response, and model functionals

In the quest for accurate approximations of the noninteracting kinetic energy functional as a functional of the electronic density, two different paths are usually employed: semilocal functionals based on the derivatives of the electronic density (mainly the gradient and the Laplacian) or nonlocal functionals based on the linear response of the homogeneous electron gas, i.e., the Lindhard function. While the former are defined in real space but fail to reproduce the Lindhard function, the latter cannot be expressed exactly in real space (being defined only in the reciprocal space), so applications to finite systems are complicated. In this paper we introduce a nonlocal ingredient (${y}_{\ensuremath{\alpha}}$) based on the Yukawa potential, i.e., the screened Hartree potential, which can be combined with other semilocal ingredients to obtain a more accurate description of the Lindhard function for both small and large wave vectors. We show and analyze the different properties of ${y}_{\ensuremath{\alpha}}$ and introduce a class of density functionals, the Yukawa-generalized gradient approximation (yGGA). We show that both the total energy and the first functional derivative (the kinetic potential) of yGGA functionals can be easily computed in real space. We present model yGGA functionals which well approximate the Lindhard function for both small and large wave vectors and can accurately describe jellium clusters and their perturbations.

Experimental information on mass- and TKE-dependence of the prompt fission γ-ray multiplicity

Prompt γ rays from the spontaneous fission of 252Cf were measured with cerium-doped LaBr3 detectors. The average prompt fission γ-ray multiplicity, Mγ, was determined as a function of fragment mass number (A) and total kinetic energy (TKE). High-statistics data, obtained from three detectors of different size at different angles relative to the fission chamber, confirms unequivocally a saw-tooth like shape of the γ multiplicity as a function of fragment mass, in contrast to previously published data. In addition, the TKE-dependence of Mγ was determined experimentally and compared with recent data.

Rate coefficients for $$\hbox {Ar}^{{\mathbf {+}}}$$ in $$\hbox {Ar/BF}_{{\mathbf {3}}}$$ mixtures

In this paper, we present most probable reactions of $$\hbox {Ar}^{{+}}$$ ion with $$\hbox {Ar/BF}_{{3}}$$ mixtures. Appropriate gas phase enthalpies of formation for the products were used to calculate scattering cross section as a function of kinetic energy. These data are needed for modeling in numerous applications of technologically important $$\hbox {BF}_{{3}}$$ discharges. Results for transport parameters, specially rate coefficients as a function of E/N (E-electric field; N-gas density) were obtained by using the Monte Carlo technique.

A new imaging concept in spin polarimetry based on the spin-filter effect.

The concept of an imaging-type 3D spin detector, based on the combination of spin-exchange interactions in the ferromagnetic (FM) film and spin selectivity of the electron-photon conversion effect in a semiconductor heterostructure, is proposed and demonstrated on a model system. This novel multichannel concept is based on the idea of direct transfer of a 2D spin-polarized electron distribution to image cathodoluminescence (CL). The detector is a hybrid structure consisting of a thin magnetic layer deposited on a semiconductor structure allowing measurement of the spatial and polarization-dependent CL intensity from injected spin-polarized free electrons. The idea is to use spin-dependent electron transmission through in-plane magnetized FM film for in-plane spin detection by measuring the CL intensity from recombined electrons transmitted in the semiconductor. For the incoming electrons with out-of-plane spin polarization, the intensity of circularly polarized CL light can be detected from recombined polarized electrons with holes in the semiconductor. In order to demonstrate the ability of the solid-state spin detector in the image-type mode operation, a spin detector prototype was developed, which consists of a compact proximity focused vacuum tube with a spin-polarized electron source [p-GaAs(Cs,O)], a negative electron affinity (NEA) photocathode and the target [semiconductor heterostructure with quantum wells also with NEA]. The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range 0.5-3.0 eV and up to 1.3 keV was studied in image-type mode. The figure of merit as a function of electron kinetic energy and the target temperature is determined. The spin asymmetry of the CL intensity in a ferromagnetic/semiconductor (FM-SC) junction provides a compact optical method for measuring spin polarization of free-electron beams in image-type mode. The FM-SC detector has the potential for realizing multichannel 3D vectorial reconstruction of spin polarization in momentum microscope and angle-resolved photoelectron spectroscopy systems.

Toward the perfect membrane material for environmental x-ray photoelectron spectroscopy

We outline our achievements in developing electron transparent, leak-tight membranes required for environmental photoelectron spectroscopy (PES). We discuss the mechanical constraints limiting the achievable membrane size and review the development of growth protocols for the chemical vapor deposition (CVD) of single-crystalline graphene on highly (111) textured Cu foils serving as membrane material. During CVD growth, Cu tends to develop a mesoscopic staircase morphology consisting of alternating inclined surface planes, irrespective of whether the covering graphene film or the substrate are single-crystalline. This morphology remains imprinted even when converting the film into freestanding graphene, which affects its mechanical properties. Determining the number of carbon layers in freestanding graphene, we show that membranes reported to suspend over distances larger than 20 µm most likely consist of few-layer graphene. The Raman band signature often used to confirm monolayer graphene rather relates to graphene with turbostratic stacking. The vertical corrugation of freestanding graphene was shown to be almost absent for tri- and four-layer-thick graphene but substantial for bilayer and especially for monolayer graphene. The corrugation is reduced when mechanically straining the freestanding graphene through thermal expansion of the supporting frame, especially flattening membrane areas with imprinted staircase morphology. The electron signal attenuation through supported and freestanding graphene was determined as a function of the electron kinetic energy, verifying that large-area graphene-based electron windows have sufficient electron transparency required for environmental PES. Meanwhile, we managed to cover 100 µm-sized single holes by few-layer graphene up to a coverage fraction of over 99.9998%, as deduced when applying 10 mbar air on one side of the sealing membrane without detecting any measurable pressure increase on its ultrahigh vacuum side. The reported achievements will pave the way toward the development of laboratory-based environmental PES.

Dynamics of fullerenes confined in nanotube: Temperature-modulated Raman scattering and X-ray diffraction studies

The ultrathin channel in nanotube is an exciting host to encapsulate sub-nanometer size guest spices. The dynamics of confined $$\hbox {C}_{\mathrm {60}}\hbox {s}$$ inside the nanoscale host tube have been investigated by various stimuli like e-beam irradiation, Ar-ion laser and Cu $$\hbox {K}_{\upalpha }$$ X-ray. It has been disclosed that the accelerating voltage of TEM higher than the threshold energies of the ballistic knockoff in carbon atoms can be used to understand the rotation of host species in the confined geometry. The radial breathing Raman mode of host SWCNTs has been studied to insight into the enhanced mobility of guest $$\hbox {C}_{\mathrm {60}}\hbox {s}$$ at higher temperatures in the confined tube geometry of nanopeapod. We have highlighted the role of temperature as a function of kinetic energy of the confined $$\hbox {C}_{\mathrm {60}}\hbox {s}$$ in SWCNTs and which leads to the reduction of stress in host–guest complex. The temperature-dependent XRD has been used to study the mobilities of $$\hbox {C}_{\mathrm {60}}$$ molecules adsorbed onto the SWCNTs to move along the tube walls until they detect an opening to infuse in the confined space

The dynamics of the early universe in a model with radiation and a generalized Chaplygin gas

The early universe is modeled through the quantization of a Friedmann-Robertson-Walker model with positive curvature of the spatial hypersurfaces. In this model, the universe is filled by two fluids: radiation and a generalized Chaplygin gas. The quantization of this model is made following the prescriptions due to J. A. Wheeler and B. DeWitt. Using the Schutz's formalism, the time notion is recovered and the Wheeler-DeWitt equation transforms into a time dependent Schr\"{o}dinger equation, which rules the dynamics of the early universe, under the action of an effective potential $V_{eff}$. That potential, depends on three parameters. Depending on the values of these parameters, $V_{eff}$ may have two different shapes. $V_{eff}(a)$ may have the shape of a barrier or the shape of a well followed by a barrier. We solve, numerically, the appropriate time dependent Schr\"{o}dinger equation and obtain the time evolution of an initial wave function, for both cases. These wave functions satisfy suitable boundary conditions. For both shapes of $V_{eff}$, we compute the tunneling probability, which is a function of the mean kinetic energy associated to the radiation energy $E_m$ and of the three parameters of the generalized Chaplygin gas: $\alpha$, $A$ and $B$. The tunneling probabilities, for both shapes of $V_{eff}$, indicates that the universe should nucleate with the highest possible values of $E_m$, $\alpha$, $A$ and $B$. Finally, we study the classical universe evolution after the wavefunction has tunneled $V_{eff}$. The calculations show that the universe may emerge from the Planck era in an inflationary phase.

Robust All-Electron Optimization in Orbital-Free Density-Functional Theory Using the Trust-Region Image Method.

We present a Gaussian-basis implementation of orbital-free density-functional theory (OF-DFT) in which the trust-region image method (TRIM) is used for optimization. This second-order optimization scheme has been constructed to provide benchmark all-electron results with very tight convergence of the particle-number constraint, associated chemical potential, and electron density. It is demonstrated that, by preserving the saddle-point nature of the optimization and simultaneously optimizing the density and chemical potential, an order of magnitude reduction in the number of iterations required for convergence is obtained. The approach is compared and contrasted with a new implementation of the nested optimization scheme put forward by Chan, Cohen, and Handy. Our implementation allows for semilocal kinetic-energy (and exchange-correlation) functionals to be handled self-consistently in all-electron calculations. The all-electron Gaussian-basis setting for these calculations will enable direct comparison with a wide range of standard high-accuracy quantum-chemical methods as well as with Kohn-Sham density-functional theory. We expect that the present implementation will provide a useful tool for analyzing the performance of approximate kinetic-energy functionals in finite systems.

Nonparametric Local Pseudopotentials with Machine Learning: A Tin Pseudopotential Built Using Gaussian Process Regression

We present novel nonparametric representation math for local pseudopotentials (PP) based on Gaussian Process Regression (GPR). Local pseudopotentials are needed for materials simulations using Orbital-Free Density Functional Theory (OF-DFT) to reduce computational cost and to allow kinetic energy functional (KEF) application only to the valence density. Moreover, local PPs are important for the development of accurate KEFs for OF-DFT, but they are only available for a limited number of elements. We optimize local PPs of tin (Sn) represented with GPR to reproduce the experimental lattice constants of α- and β-Sn and the energy difference between these two phases as well as their electronic structure and charge density distributions which are obtained with Kohn-Sham Density Functional Theory employing semilocal PPs. The use of a nonparametric GPR-based PP representation avoids difficulties associated with the use of parametrized functions and has the potential to construct an optimal local PP independent of prior assumptions. The GPR-based Sn local PP results in well-reproduced bulk properties of α- and β-tin and electronic valence densities similar to those obtained with semilocal PP.

Prompt fission neutron yields in thermal fission ofU235and spontaneous fission ofCf252

Background: The sharing of excitation energy between the fission fragments is one of the key issues in studying nuclear fission. One way to address this is by studying prompt-fission neutron multiplicities as a function of other fission observables such as the mass, $\overline{\ensuremath{\nu}}\left(A\right)$. These are vital benchmark data for both fission and nuclear deexcitation models, putting constrains on the fragment excitation energy and hence on the competing prompt neutron/$\ensuremath{\gamma}$-ray emission. Despite numerous detailed studies, recent measurements done at JRC-Geel with the SCINTIA array in the epithermal region show surprisingly strong discrepancies to earlier thermal fission data and the Wahl systematics.Purpose: The purpose was to perform measurements of the prompt-fission neutron multiplicity, as a function of fragment mass and total kinetic energy (TKE), in $^{235}\mathrm{U}\left({\text{n}}_{\text{th}},\text{f}\right)$ and $^{252}\mathrm{Cf}\left(\text{sf}\right)$, to verify and extend the SCINTIA results. Another goal was to validate the analysis methods, and prepare for planned investigations at excitation energies up to 5.5 MeV.Methods: The experiments were conducted at the former 7 MV Van de Graaff facility in JRC-Geel, using a Twin Frisch-Grid Ionization Chamber and two liquid scintillation detectors. A neutron beam with an average energy of 0.5 MeV was produced via the $^{7}\mathrm{Li}$(p,n) reaction. The neutrons were thermalized by a 12 cm thick block of paraffin. Digital data acquisition systems were utilized. Comprehensive simulations were performed to verify the methodology and to investigate the role of the mass and energy resolution on measured $\overline{\ensuremath{\nu}}\left(A\right)$ and $\overline{\ensuremath{\nu}}\left(\text{TKE}\right)$ values. The simulation results also revealed that the $\ensuremath{\partial}\overline{\ensuremath{\nu}}(A)/\ensuremath{\partial}A$ and $\ensuremath{\partial}\overline{\text{TKE}}/\ensuremath{\partial}\overline{\ensuremath{\nu}}$ are affected by the mass and energy resolution. However, the effect is small for the estimated resolutions of this work. Detailed Fluka simulations were performed to calculate the fraction of thermal neutron-induced fission, which was estimated to be about $98%$.Results: The experimental results on $\overline{\ensuremath{\nu}}\left(A\right)$ are in good agreement with earlier data for $^{252}\mathrm{Cf}\left(\text{sf}\right)$. For $^{235}\mathrm{U}\left({\text{n}}_{\text{th}},\text{f}\right)$, the $\overline{\ensuremath{\nu}}\left(A\right)$ data is very similar to the data obtained with SCINTIA, and therefore we verify these disclosed discrepancies to earlier thermal data and to the Wahl evaluation. The experimental results on $\overline{\ensuremath{\nu}}\left(\text{TKE}\right)$ are also in agreement with the data at epithermal energies. For $^{252}\mathrm{Cf}\left(\text{sf}\right)$ a slope value of $\ensuremath{\partial}\overline{\text{TKE}}/\ensuremath{\partial}\overline{\ensuremath{\nu}}=\left(\ensuremath{-}12.9\ifmmode\pm\else\textpm\fi{}0.2\right)\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/\mathrm{n}$ was obtained. For $^{235}\mathrm{U}\left({\text{n}}_{\text{th}},\text{f}\right)$ the value is $\left(\ensuremath{-}12.0\ifmmode\pm\else\textpm\fi{}0.1\right)\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/\mathrm{n}$. Finally, the neutron spectrum in the center-of-mass system was derived and plotted as a function of fragment mass.Conclusions: This work clearly proves the lack of accurate correlation between fission fragment and neutron data even in the best-studied reactions. The new results highlight the need of a new evaluation of the prompt-fission multiplicity for $^{235}\mathrm{U}\left({\text{n}}_{\text{th}},\text{f}\right)$.

Electronic couplings for photo-induced processes from subsystem time-dependent density-functional theory: The role of the diabatization.

Subsystem time-dependent density-functional theory (sTDDFT) making use of approximate non-additive kinetic energy (NAKE) functionals is known to be capable of describing excitation energy transfer processes in a variety of applications. Here, we show that sTDDFT, especially when combined with projection-based embedding (PbE), can be employed for the entire range of photo-induced electronic couplings essential for modeling photophysical properties of complex chemical and biological systems and therefore represents a complete toolbox for this class of problems. This means that it is capable of capturing the interaction/coupling associated with local- and charge-transfer (CT) excitons. However, this requires the choice of a reasonable diabatic basis. We therefore propose different diabatization strategies of the virtual orbital space in PbE-sTDDFT and show how CT excitations can be included in sTDDFT using NAKE functionals via a phenomenological approach. Finally, these electronic couplings are compared to couplings from a multistate fragment excitation difference (FED)-fragment charge difference (FCD) diabatization procedure. We show that both procedures, multistate FED-FCD and sTDDFT (with the right diabatization procedure chosen), lead to an overall good agreement for the electronic couplings, despite differences in their general diabatization strategy. We conclude that the entire range of photo-induced electronic couplings can be obtained using sTDDFT (with the right diabatization procedure chosen) in a black-box manner.

Employing airborne radiation and cloud microphysics observations to improve cloud representation in ICON at kilometer-scale resolution in the Arctic

Abstract. Clouds play a potentially important role in Arctic climate change but are poorly represented in current atmospheric models across scales. To improve the representation of Arctic clouds in models, it is necessary to compare models to observations to consequently reduce this uncertainty. This study compares aircraft observations from the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign around Svalbard, Norway, in May–June 2017 and simulations using the ICON (ICOsahedral Non-hydrostatic) model in its numerical weather prediction (NWP) setup at 1.2 km horizontal resolution. By comparing measurements of solar and terrestrial irradiances during ACLOUD flights to the respective properties in ICON, we showed that the model systematically overestimates the transmissivity of the mostly liquid clouds during the campaign. This model bias is traced back to the way cloud condensation nuclei (CCN) get activated into cloud droplets in the two-moment bulk microphysical scheme used in this study. This process is parameterized as a function of grid-scale vertical velocity in the microphysical scheme used, but in-cloud turbulence cannot be sufficiently resolved at 1.2 km horizontal resolution in Arctic clouds. By parameterizing subgrid-scale vertical motion as a function of turbulent kinetic energy, we are able to achieve a more realistic CCN activation into cloud droplets. Additionally, we showed that by scaling the presently used CCN activation profile, the hydrometeor number concentration could be modified to be in better agreement with ACLOUD observations in our revised CCN activation parameterization. This consequently results in an improved representation of cloud optical properties in our ICON simulations.

Real-time monitoring of a positron beam using a microchannel plate in single-particle mode

This work presents a real-time diagnostic method to monitor and characterize a positron beam by using a microchannel plate (MCP), a phosphor screen and a CCD camera. We propose a digitizing method based on single-particle detection. This approach offers the possibility to quantify the particles per unit time that interact with the MCP. This type of detector is based on a consolidated technology and its efficiency has been demonstrated for nuclear radiation and particles in many experiments. This work also presents the detection efficiency as a function of the positron kinetic energy in the range from 50 eV to 17 keV, essential for a complete characterization of the particle beam. The method allows the characterization of a positron beam in real time with a spatial resolution of the order of tens of microns.

Variational Principle for Eigenmodes of Reactivity in Conceptual Density Functional Theory.

In conceptual density functional theory, reactivity indexes as the Fukui function, the global hardness/softness, and hardness/softness kernels are fundamental linear responses extensively studied to predict the nucleophilic and electrophilic propensities of atoms in molecules. We demonstrate that the hardness/softness kernels of an isolated system can be expanded in eigenmodes, solutions of a variational principle. These modes are divided into two groups: the polarization modes and the charging modes. The eigenvectors of the polarization modes are orthogonal to the Fukui function and can be interpreted as densities induced at a constant chemical potential. The charging modes of an isolated system are associated with virtual charge transfers weighted by the Fukui function and obey an exact nontrivial sum rule. The exact relation between these charging eigenmodes and those of the polarizability kernel is established. The physical interpretation of the modes is discussed. Applications of the present findings to the Thomas–Fermi and von Weizacker kinetic energy functionals are presented. For a confined free quantum gas, described by the von Weizacker kinetic energy functional, we succeed to derive an approximate analytical solution for the Fukui function and for hardness/softness and polarizability kernels. Finally, we indicate how numerical calculations of the hardness kernel of a molecule could be performed from the Kohn–Sham orbitals.

Interaction of Jets and Submesoscale Dynamics Leads to Rapid Ocean Ventilation

ABSTRACTOcean ventilation is the process by which climatically important tracers such as heat and carbon are exchanged between the atmosphere and ocean interior. In this paper a series of numerical simulations are used to study the interaction of submesoscales and a topographically steered jet in driving rapid ventilation. The ventilation is found to increase both as a function of wind stress and model resolution, with a submesoscale-resolving 1/120° model exhibiting the largest ventilation rate. The jet in this simulation is found to be persistently unstable to submesoscale instabilities, which are known to feature intense vertical circulations. The vertical tracer transport is found to scale as a function of the eddy kinetic energy and mean isopycnal slope, whose behaviors change as a function of the wind stress and due to the emergence of a strong potential vorticity gradient due to the lateral shear of the jet. These results highlight the importance of jet–submesoscale interaction as a bridge between the atmosphere and the ocean interior.

Cis-trans isomerization is not rate determining for b2 ion structures: A guided ion beam and computational study of the decomposition of H+(GlyProAla)

Decomposition of the protonated tripeptide, GlyProAla (H+GPA), through collision-induced dissociation with Xe in a guided ion beam tandem mass spectrometer (GIBMS) is examined. Cross sections for the formation of [b2]+, [y2 + 2H]+, [a1]+, [b3]+, [a3]+, [a2]+, and H+(1-pyrroline) fragment ions were collected as a function of kinetic energy. Analysis of these cross sections include consideration of the effects of multiple collisions, dissociation lifetimes, reactant internal and kinetic energy distributions, competition among channels, and evolution into sequential channels. Structures and molecular parameters of reactants, transition states, intermediates, and products were identified using the B3LYP/6-311+G(d,p) level of theory, with single-point energy calculations performed at the B3LYP, B3P86, and MP2(full) levels of theory using a 6-311+G(2d,2p) basis set. Good agreement between experimental threshold energies and those calculated for rate-limiting transition states allow key reaction pathways for the formation of each product to be identified. The MP2(full) level of theory is found to agree best with the experimental results. Notably, even though H+GPA starts with a cis-peptide bond, it forms an oxazolone as its [b2]+ fragment and the absence of a diketopiperazine [b2]+ fragment is verified by other key observations.