Ground State Fragments Research Articles

Ab initio electronic structure analysis of ground and excited states of HfN0,.

High-level ab initio electronic structure analysis of third-row transition metal (TM)-based diatomic species is challenging and has been perpetually lagging. In this work, fourteen and eighteen electronic states of HfN and HfN+ respectively are studied, employing multireference configuration interaction (MRCI) and coupled cluster singles doubles and perturbative triples [CCSD(T)] theories under larger correlation-consistent basis sets. Their potential energy curves (PECs), energetics, and spectroscopic parameters are reported. Core electron correlation effects on their properties are also investigated. Chemical bonding patterns of several low-lying electronic states are introduced based on the equilibrium electron configurations. The ground state of HfN (X2Σ+) has the 1σ22σ23σ11π4 electronic configuration, and the ionization of the 3σ1 electron produces the ground state of HfN+ (X1Σ+). Ground states of both HfN and HfN+ are triple bonded in nature and bear 124.86 and 109.10 kcal mol-1 binding energies with respect to their ground state fragments. The findings of this work agree well with the limited experimental literature available and provide useful reference values for future experimental analysis of HfN and HfN+.

On the ground and excited electronic states of LaCO and AcCO.

High-level ab initio electronic structure analysis of correlated lanthanide- and actinide-based species is laborious to perform and consequently limited in the literature. In the present work, the ground and electronically excited states of LaCO and AcCO molecules were explored utilizing the multireference configuration interaction (MRCI), Davidson corrected MRCI (MRCI+Q), and coupled cluster singles doubles and perturbative triples [CCSD(T)] quantum chemical tools conjoined with correlation consistent triple-ζ and quadruple-ζ quality all-electron Douglas-Kroll (DK) basis sets. The full potential energy curves (PECs), dissociation energies (Des), excitation energies (Tes), bond lengths (res), harmonic vibrational frequencies (ωes), and chemical bonding patterns of low-lying electronic states of LaCO and AcCO are introduced. The ground electronic state of LaCO is a 4Σ- (1σ11π2) which is a product of the reaction between excited La(4F) versus CO(X1Σ+), whereas the ground state of AcCO is a 12Π (1σ21π1) deriving from ground state fragments Ac(2D) + CO(X1Σ+). The spin-orbit ground states of LaCO (14Σ-3/2) and AcCO (12Π1/2) bear ∼13 and 5 kcal mol-1D0 values, respectively. At the MRCI level, the spin-orbit curves, the spin-orbit mixing, and the Tes of spin-orbit states of LaCO and AcCO were also analyzed. Lastly, the density functional theory (DFT) calculations were performed applying 16 exchange-correlation functionals that span three rungs of "Jacob's ladder" of density functional approximations (DFAs) to assess DFT errors associated on the De and ionization energy (IE) of LaCO.

Equilibrium Bond Lengths from Orbital-Free Density Functional Theory.

This work presents an investigation to model chemical bonding in various dimers based on the atomic fragment approach. The atomic fragment approach is an ab-initio, parameter-free implementation of orbital-free density functional theory which is based on the bifunctional formalism, i.e., it uses both the density and the Pauli potential as two separate variables. While providing the exact Kohn-Sham Pauli kinetic energy when the orbital-based Kohn-Sham data are used, the bifunctional formalism allows for approximations of the functional derivative which are orbital-free. In its first implementation, the atomic fragment approach uses atoms in their ground state to model the Pauli potential. Here, it is tested how artificial closed-shell fragments with non-integer electron occupation perform regarding the prediction of bond lengths of diatomics. Such fragments can sometimes mimic the electronic structure of a molecule better than groundstate fragments. It is found that bond lengths may indeed be considerably improved in some of the tested diatomics, in accord with predictions based on the electronic structure.

Formula omitted] velocity images of dissociative electron attachment to OCS

Dissociative electron attachments to OCS with the attachment energies of 4.3, 4.7, 5.0, 6.2, 6.5, 6.8, and 7.0eV are investigated by measuring the S− momentum distributions with time-sliced anion velocity image mapping technique. In this energy range, the ground-state fragmentation, e−+OCS→CO(X˜1Σ+)+S−(2Pu), is only one dissociation pathway allowed in energy. All velocity images show the predominant feature of the low-speed S− ions, but the middle- and high-speed S− ions appear at the higher attachment energies (6.2–7.0eV). The trimodel speed distribution of S− ion should be attributed to the different populations of the rovibrational states of CO fragment, however, the dynamic details of above dissociations are still unclear and demand theoretical calculations.

Dynamics of the A-band ultraviolet photodissociation of methyl iodide and ethyl iodide via velocity-map imaging with 'universal' detection.

We report data from a comprehensive investigation into the photodissociation dynamics of methyl iodide and ethyl iodide at several wavelengths in the range 236-266 nm, within their respective A-bands. The use of non-resonant single-photon ionization at 118.2 nm allows detection and velocity-map imaging of all fragments, regardless of their vibrotational or electronic state. The resulting photofragment kinetic energy and angular distributions and the quantum yields of ground-state and spin-orbit excited iodine fragments are in good agreement with previous studies employing state-selective detection via REMPI. The data are readily rationalised in terms of three competing dissociation mechanisms. The dominant excitation at all wavelengths studied is via a parallel transition to the (3)Q0 state, which either dissociates directly to give an alkyl radical partnered by spin-orbit excited iodine, or undergoes radiationless transfer to the (1)Q1 potential surface, where it dissociates to an alkyl radical partnered by iodine in its electronic ground state. Ground state iodine atoms can also be formed by direct dissociation from the (1)Q1 or (3)Q1 excited states following perpendicular excitation at the shorter and longer wavelength region, respectively, in the current range of interest. The extent of internal excitation of the alkyl fragment varies with dissociation mechanism, and is considerably higher for ethyl fragments from ethyl iodide photolysis than for methyl fragments from methyl iodide photolysis. We discuss the relative advantages and disadvantages of single-photon vacuum-ultraviolet ionization relative to the more widely used REMPI detection schemes, and conclude, in agreement with others, that single-photon ionization is a viable detection method for photofragment imaging studies, particularly when studying large molecules possessing multiple fragmentation channels.

Comment on “Ground-state fragmentation phase transition for attractive bosons in anisotropic traps”

In a recent article [Phys. Rev. A 88, 063641 (2013)], Cizek and Kasevich claimed that quantum fragmentation occurs in the metastable state of a Bose-Einstein condensate with an attractive interaction confined in an elongated harmonic potential. This result was obtained using the two-state Gaussian variational method. However, modified methods show that no fragmentation occurs for the parameters used by Cizek and Kasevich.

Ground-state fragmentation phase transition for attractive bosons in anisotropic traps

We construct a two-state theoretical model describing attractively interacting bosons in a cylindrical harmonic potential of varied anisotropy. These systems exhibit true many-body ground-state energy minima stable against variations in single-particle state radial and axial widths and occupation number. The many-body ground state undergoes a phase transition from pure to fragmented as the aspect ratio increases above an anisotropy threshold dependent on the interaction energy per particle. This fragmentation is robust and does not require careful tuning of experimental parameters.

Topological Insights into the Nature of the Halogen−Carbon Bonds in Dimethylhalonium Ylides and Their Cations

In this study the nature of the bonding in a series of dimethylhalonium ylides (fluoronium, chloronium, bromonium and iodonium) was analyzed by means of topological methodologies (AIM and ELF analysis), to document the changes in the nature of the C-X bonds (X = F, Cl, Br, I) upon the series. For the sake of comparison the same study was performed on the corresponding dimethylhalonium cations (XC 2H 6 (+)) and the XCH 3 series. The wave functions used for the topological analysis were obtained at B3LYP level using extended triple-zeta basis sets. The formation of the cationic XC 2H 6 (+) structures can be interpreted to arise from the interaction between the XCH 3 and CH 3 (+) moieties. The resultant structures can be explained in terms of the superposition of two electrostatically interacting and two dative mesomeric structures. The halogen-carbon bonds have all the characteristics of the charge-shift (CS) bonds. The analysis of the C-X bond in the XC 2H 5 series shows a progressive reinforcing of the CH 3X-CH 2 bond, from FC 2H 5 that can be considered as formed from two fragments, FCH 3 and CH 2, to IC 2H 5, in which the CH 3I-CH 2 bond has all the features of a multiple bond involving atoms bearing lone pairs. Particularly interesting is BrC 2H 5, in which a special type of bond (hybrid covalent-dative double bond) has been characterized. The energetic stability of the XC 2H 5 structures with respect to the dissociation into the XCH 2 + CH 3 and XCH 3 + CH 2 ground-state fragments was studied in detail.

First Principles Investigation of the Electronic Structure of the Iron Carbide Cation, FeC+

We have studied 40 states of the diatomic iron carbide cation FeC(+) by multireference methods coupled with relatively large basis sets. For most of the states, we have constructed complete potential energy curves, reporting dissociation energies, usual spectroscopic parameters, and bonding mechanisms for the lowest of the studied states. The ground state is of (2)Delta symmetry, with the first excited state (a(4)Sigma(-)) lying 18 kcal/mol higher. The X(2)Delta state displays a triple-bond character, with an estimated D(0) value of 104 kcal/mol with respect to the adiabatic products or 87 kcal/mol with respect to the ground-state fragments.

Wave-packet study of the products formed in dissociative recombination ofHeH+

We present a theoretical study of the atomic fragment distributions formed in dissociative recombination of HeH{sup +} at low energies (<2 eV) using wave packets propagating on the potential-energy curves of HeH, coupled by nonadiabatic and rotational interactions. We show that at zero electron collision energy, dissociation into H(n=2)+He will completely dominate over dissociation into the ground-state fragments. When the n=3 channel become energetically open at E=0.334 eV a rapid switchover to this channel will occur. This phenomenon has previously been observed experimentally, but here we present a theoretical model showing these jumps in the branching ratios. We present an explanation for the observed sudden change in branching ratios and we also discuss how the branching ratios depend on the nonadiabatic couplings used in the model.

Ground-state fragmentation of repulsive Bose-Einstein condensates in double-trap potentials

The fragmentation of the ground state of a repulsive condensate immersed into a double-trap potential is found to be a general and critical phenomenon. It takes place for a given number of bosons if their scattering length is larger than some critical value or for a given value of the scattering length if the number of bosons is above some critical number. We demonstrate that the geometry of the inner trap determines these critical parameters while the number of the fragments and the fraction of bosons in the various fragments can be manipulated by the outer trap. There is also a maximal number of bosons for which the ground state is fragmented. If this number is exceeded, the fragmented state becomes a very low-lying excited state of the condensate. This maximal number of bosons can be substantially manipulated by varying the inner and outer traps. To study threefold fragmentation we have chosen a potential well with two barriers as the inner trap and embedded in two types of outer ones. A manifold fragmentation is also addressed.

Ab Initio MRD-CI Study of the Electronic Spectrum of BrNO2 and Photofragmentation

Multi reference configuration interaction, MRD-CI, methods with the cc-pVDZ+sp and cc-pVTZ+sp basis sets were employed to determine the low-lying singlet and triplet electronic states of nitryl bromide, BrNO 2 . The calculations predict two very strong transitions, 3 1 A 1 ← X 1 A 1 and 3 1 B 2 ← X 1 A 1 , at 6.15 and 7.27 eV, respectively. At wavelengths that are atmospherically relevant to the BrNO 2 photolysis, 1 1 B 2 and 1 1 B 1 singlet states with vertical transition energies of 3.43 and 3.85 eV, respectively, are computed. The corresponding triplet states are lower in energy by 0.27 and 0.36 eV, respectively. Both singlet and triplet states are found to be highly repulsive along the Br-N coordinate and they dissociate to the ground-state fragments Br and NO 2 . The comparison between the first singlet excited state of BrNO 2 and ClNO 2 demonstrates that the transition is red-shifted by 80 nm from ClNO 2 upon replacement of chlorine by bromine. Furthermore, the most intense transition 3 1 A 1 ← X 1 A 1 of BrNO 2 has a 0.9 eV lower energy than that of ClNO 2 . These energy differences can be explained by the large halogen character of the orbitals involved in the electron excitations.

Ultrafast predissociation dynamics of water molecules excited to the electronic C and D states.

Two-photon excitation with femtosecond laser pulses in the spectral range 240-250 nm was used to prepare vapor phase H(2)O and D(2)O in the C (1)B(1) and D (1)A(1) states. Both states are predissociated via the B (1)A(1) state, forming excited OH/OD(A (2)Sigma(+)) as well as ground state OH/OD(X (2)Pi). We used ultrashort infrared probe pulses (1.65-2.42 microm) to control the ratio between these excited and ground state fragments originating from the dissociation process. Time resolved detection of the OH/OD(A (2)Sigma(+)) --> OH/OD(X (2)Pi) fluorescence allows us to monitor the dynamics of the predissociation. For the heterogeneous predissociation out of the C(1)B(1) state life times of (0.5 +/- 0.1) ps and (1.2 +/- 0.1) ps were found for H(2)O and D(2)O, respectively. The purely homogeneous character of the predissociation out of the D (1)A(1) state was monitored.

Structural and electronic properties of iron monoxide clusters FenO and FenO− (n=2–6): A combined photoelectron spectroscopy and density functional theory study

We report a combined anion photoelectron spectroscopy and density functional theory (DFT) study on a series of iron monoxide clusters, FenO (n=2–6). Well-resolved photoelectron spectra were obtained for FenO− at variable detachment energies, allowing the ground state and numerous low-lying excited states of FenO to be observed. Sharp threshold photoelectron features were obtained for each species, which suggest rather small geometry changes between the anion and neutral ground states for the monoxide clusters and allows the electron affinities of the neutral clusters to be measured accurately. Extensive DFT calculations using the generalized gradient approximation were carried out for both FenO and FenO−. Optimized geometries of the ground and lowest excited states of both the anion and neutral species are reported along with the ground-state vibrational frequencies and fragmentation energies. Theoretical electron affinities were compared with the experimental measurements to verify the ground states of the iron monoxide clusters obtained from the DFT calculations.

Modelling spin-forbidden reactions: recombination of carbon monoxide with iron tetracarbonyl.

New density functional theory and ab initio computations on the [Fe(CO)5] system are reported. Careful exploration of basis set and correlation effects leads to "best" values for the difference in energy deltaE(1,3) between ground state 3[Fe(CO)4] and the singlet excited state of ca. 8 kcal mol(-1), and for the bond dissociation energy BDE(3) of [Fe(CO)5] with respect to ground state fragments 3[Fe(CO)4] + CO of ca. 40 kcal mol(-1). A modified form of the B3PW91 functional is used to explore the potential energy surface for the spin-forbidden recombination reaction of CO with 3[Fe(CO)4]. A Cs-symmetric minimum energy crossing point (MECP) between the reactant (triplet) and product (singlet) potential energy surfaces is found, lying 0.43 kcal mol(-1) above the reactants. The rate coefficient for recombination is computed using a non-adiabatic form of transition state theory, in which the MECP is treated as the critical point in the reaction. Semi-quantitative agreement with experiment is obtained: the predicted rate coefficient, 8.8 x 10(-15) cm3 molecule(-1) s(-1), is only six times smaller than the experimental rate. This is the first computation from first principles of a rate coefficient for a spin-forbidden reaction of a transition metal compound.

Absolute cross sections and final-state distributions for dissociative recombination and excitation ofCO+(v=0)using an ion storage ring

Absolute cross sections and rate coefficients have been determined for dissociative recombination of electrons and ${\mathrm{CO}}^{+}$ ions for energies from 1 meV to 54 eV. We found values of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{2}$ at 1 meV and ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ at 1 eV, with an essentially $1/E$ energy dependence. Branching ratios over the final atomic product states have been determined using a position- and time-sensitive imaging system. At zero eV collision energy the predominant yield is to ground-state atomic fragments (76%). At higher collisional energies the branching ratio to the ground-state limit is reduced. A new limit, $\mathrm{O}{(}^{1}D)+\mathrm{C}{(}^{1}D),$ opens up and branching to the $\mathrm{O}{(}^{3}P)+\mathrm{C}{(}^{1}D)$ limit increases. Cross sections are also determined for dissociative excitation of ${\mathrm{CO}}^{+}.$ Thermal rate coefficients are deduced from the dissociative recombination (DR) data, and compared with measurements in the literature. Consideration of both the theoretical and spectroscopic data in the literature giving information about the potential curves along which DR may take place reveals both a paucity and disparity of the data.

Variety of [Fe, N, O] Isomers. A Theoretical Study

Stationary points on the potential energy hypersurfaces of triatomic [Fe, N, O] have been systematically studied using an economic combination of a density functional/Hartree−Fock hybrid method, i.e., the B3LYP functional and the multireference-averaged quadratic coupled cluster approach. The global minimum is linear FeNO(2Δ), which is not adiabatically connected to the neutral ground-state fragments Fe and NO. Side-on and oxygen-bound isomers have low barriers for rearrangement; however, they might be stabilized because of ligand effects. Inserted OFeN(4A‘‘) is kinetically stable, although it lies considerably higher in energy than the other isomers. The calculations indicate near-degenerate ground states for all isomers. We discuss the implications for the reported experimental observations. Furthermore, the findings are rationalized using valence bond and molecular orbital theories.

Production rates of excited fragment isotopes from the36Ar+Ag reaction at 35 MeV/nucleon

Previous studies showed that the binding energy plays a systematic and important role in the production of ground-state fragments in intermediate energy, heavy ion reactions. The production rates were measured as a function of fragment kinetic energy at angles of 15°, 30°, 45° and 60° for excited fragments of7Li,8Li,11Be and12B. Using a thermal model the total production of neutron unbound excited states was determined, and it was found that their production rates correspond to the previous systematic behaviour using the binding energies corrected by the excitation energies.

Reactivity of collisionally activated dichlorocarbene dications studied by tandem mass spectrometry

The dissociation mechanisms of dichlorocarbene dications following collisional activation have been investigated via tandem mass spectrometric techniques and semi-empirical calculations. Three channels appear to be significant: {fx1019-1} The second channel becomes dominant at high internal energy. Production of ground state fragments (channel 1) involves a transition driven by spin-orbit coupling from the CCl 2 (2+) [Formula: see text] state to the CCl 2 (2-) ā(3)Σ u (-) state en route to the fragments. The dissociation barrier for the production of ground state fragments from the ground electronic state of CCl 2 (2+) via the spin-orbit-induced transition is equal to 420 kJ mol(-1). The dissociation pathway that corresponds to channel 3 includes a first isomerization step from the linear Cl-C-Cl(2+) structure to a bent Cl-Cl-C(2+) connectivity. The calculated isomerization barrier amounts to 550 kJ mol(-1). The calculated reverse activation barriers are compatible with the measured kinetic energy released on the fragments.

On the Role of Conical Intersections of Two Potential Energy Surfaces of the Same Symmetry in Photodissociation. 2. CH3SCH3 .fwdarw. CH3S + CH3

Dimethylsulfide, CH[sub 3]SCH[sub 3], is known to exhibit a strong absorption in the 190-210 nm region leading to photofragmentation to CH[sub 3]S and CH[sub 3]. The role of conical intersections of two states of the same symmetry in this photodissociation process CH[sub 3]SCH[sub 3](X[sup 1]A[sub 1]) [yields] CH[sub 3]SCH[sub 3](1,2[sup 1]A[double prime]) [yields] CH[sub 3](X[sup 2]A[prime]) + CH[sub 3]S(X[sup 2]E) is considered. Points on a surface of conical intersections of the 1,2[sup 1]A[double prime] states in the vicinity of the Franck-Condon region of the X[sup 1]A[sub 1] state, accessible with 190-210 nm photons, are determined using configuration interaction wave functions comprised of over 1.2 million configuration state functions. These conical intersections provide an efficient path for the 2[sup 1]A[double prime] state to photodissociate to ground state fragments. 26 refs., 3 figs., 1 tab.