From Neutral Aniline to Aniline Trication: A Computational and Experimental Study.

We report density functional theory computations and photoionization mass spectrometry measurements of aniline and its positively charged ions. The geometrical structures and properties of the neutral and singly, doubly, and triply positively charged aniline are computed using density functional theory with the generalized gradient approximation. At each charge, there are multiple isomers closely spaced in total energy. Whereas the lowest energy states of both neutral and cation have the same topology C6H5-NH2, the dication and trication have the C5NH5-CH2 topology with the nitrogen atom in the meta- and para-positions, respectively. We compute the dissociation pathways of all four charge states to NH or NH+ and NH2 or NH2+, depending on the initial charge of the aniline precursor. Dissociation leading to the formation of NH (from the neutral and cation) and NH+ (from the dication and trication) proceeds through multiple transition states. On the contrary, the dissociation of NH2 (from the neutral and cation) and NH2+ (from the dication and trication) is found to proceed without an activation energy barrier. The trication was found to be stable toward abstraction on NH+ and NH2+ by 0.96 and 0.18 eV, respectively, whereas the proton affinity of the trication is substantially higher, 1.98 eV. The mass spectra of aniline were recorded with 1300 nm, 20 fs pulses over the peak intensity range of 1 × 1013 to 3 × 1014 W cm-2. The analysis of the mass spectra suggests high stability of both dication and trication to fragmentation. The formation of the fragment NH+ and NH2+ ions is found to proceed via Coulomb explosion.

Hydrogenation of 3d-metal oxide clusters: Effects on the structure and magnetic properties.

The geometrical structures and properties of the M8 O12 , M8 O12 H8 , and M8 O12 H12 clusters are explored using density functional theory with the generalized gradient approximation for all 3d-metals M from Sc to Zn. It is found that the geometries and total spin magnetic moments of the clusters depended strongly on the 3d-atom type and the hydrogenation extent. More than the half of all of the 30 clusters had singlet lowest total energy states, which could be described as either nonmagnetic or antiferromagnetic. Hydrogenation increases the total spin magnetic moments of the M8 O12 H12 clusters when MMnNi, which become larger by four Bohr magneton than those of the corresponding unary clusters M8 . Hydrogenation substantially affects such properties as polarizability, forbidden band gaps, and dipole moments. Collective superexchange where the local total spin magnetic moments of two atom squads are coupled antiparallel was observed in antiferromagnetic singlet states of Fe8 O12 H8 and Co8 O12 H8 , whereas the lowest total energy states of their neighbors Mn8 O12 H8 and Ni8 O12 H8 are ferrimagnetic and ferromagnetic, respectively. Hydrogenation leads to a decrease in the average binding energy per atom when moving across the 3d-metal atom series. © 2018 Wiley Periodicals, Inc.

Dependence of Properties and Exchange Coupling Constants on the Charge in the Mn2O n and Fe2O n Series.

The geometrical structure and properties of the neutral and singly charged Mn2O nq and Fe2O nq clusters ( q = 0, ±1) are computed using density functional theory with the generalized gradient approximation in the range 1 ≤ n ≤ 7. The geometrical structures and spin multiplicities of the corresponding species in all six series are similar except for a few exceptions. Antiferromagnetic coupling of total spin magnetic moments of the metal atoms in the lowest total energy states is observed for the majority of species in all six series when n = 1-5; correspondingly, the computed magnetic exchange coupling constants are mostly negative. The states of Mn2O nq and Fe2O nq are nonmagnetic or weakly ferromagnetic when n > 5 except for Mn2O7+ where the ground state is antiferromagnetic. The computed adiabatic electron affinities and ionization energies of the neutral species in both series are quite close to one another and increase as n increases. However, the binding energies of a single oxygen atom and of an O2 dimer decrease as n increases and the Mn2O7+ and Fe2O7+ cations are barely stable with respect to the O2 abstraction. The most stable and least stable species at a given n are the anions and the cations, respectively. The electric dipole polarizability per atom decreases sharply when n moves from 1 to 4 and then remains nearly constant for larger n values in the anion series, whereas it is close to the asymptotic value already at n = 2 in the neutral series.

Collective Superexchange and Exchange Coupling Constants in the Hydrogenated Iron Oxide Particle Fe8O12H8.

Motivated by the fact that Fe2O3 nanoparticles are used in the treatment of cancer, we have examined the role of ligands on the magnetic properties of these particles by focusing on (Fe2O3)4 as a prototype system with H as ligands. Using the Broken-Symmetry Density Functional Theory, we observed a strong collective superexchange in the hydrogenated Fe8O12H8 cluster. The average antiferromagnetic exchange coupling constant between the four iron-iron oxo-bridged pairs was found to be -178 cm-1, whereas coupling constants between hydroxo-bridged pairs were much smaller. We found that despite the apparent symmetry of the iron atom framework, it is not reasonable to assume this symmetry when fitting the exchange coupling constants. We also analyzed the geometrical and magnetic properties of Fe8O12H n for n = 0-12 and found that hydrogenating oxo-bridges would generally inhibit the Fe-O-Fe antiferromagnetic superexchange interactions. Antiferromagnetic lowest total energy states become favorable only when specific distributions of hydrogen atoms are realized. The (HO)4-Fe4(all spin-up)-O4-Fe4(all spin-down)-(OH)4 configuration in Fe8O12H8 presents such an example. This symmetric configuration can be considered a superdiatomic system.

Effect of hydrogenation on the structure and magnetic properties of an iron oxide cluster.

The structure and properties of the Fe8O12Hn clusters (n = 0-18) are computed using the all-electron density functional theory with the generalized gradient approximation for the exchange-correlation potential. The ground state of Fe8O12 is found to be a singlet state having a bi-capped octahedron geometry. Upon hydrogenation, the octahedral framework of Fe is retained in Fe8O12Hn up to n < 7, beyond which point the iron octahedron transforms into a cube. Hydrogen atoms are bound to oxygen atoms up to n = 12, but they bind to the faces of the Fe8 cube when n > 12. The total spin magnetic moment of a Fe8O12Hn cluster is larger than 6 µB for 1 ≤ n ≤ 18, except for n = 8 and 10, where the lowest total energy states are antiferromagnetic singlets. The reason for this deviation from the general behavior in the Fe8O12Hn series is attributed to the collective superexchange phenomenon. Surprisingly, the total spin magnetic moment of a Fe8O12Hn cluster is found to be substantially larger than the total spin magnetic moment of the bare Fe8 cluster when n = 12-18. All of the Fe8O12Hn clusters are stable with respect to an abstraction of a single hydrogen atom but are unstable toward the abstraction of an H2 dimer when n =10 and n = 14-18.

A Theoretical and Mass Spectrometry Study of Dimethyl Methylphosphonate: New Isomers and Cation Decay Channels in an Intense Femtosecond Laser Field.

Using both mass spectrometry with intense femtosecond laser ionization and high-level computational methods, we have explored the structure and fragmentation patterns of dimethyl methylphosphonate (DMMP) cation. Extensive search of the geometries of both neutral and positively charged DMMP yields new isomers that are appreciably lower in total energy than those commonly synthesized using the Michaelis-Arbuzov reaction. The stability of the standard isomer with CH3PO(OCH3)2 topology is found to be due to the presence of high barriers to isomer interconversion that involves several transition states. Our femtosecond laser ionization experiments show that the relative yields of the major dissociation products as a function of peak laser intensity correlate well with the theoretical estimates for the energies of the DMMP+ decay via various channels. In contrast, the peak laser intensities required for observation of minor dissociation products exhibit no correlation with the computed decay energies, which suggests that barrier heights and/or excited electronic states of DMMP+ determine its preferred fragmentation pathways in an intense femtosecond laser field.

Transitions from Stable to Metastable States in the Cr2On and Cr2On- Series, n = 1-14.

The geometrical and electronic structures of the Cr2On and Cr2On- clusters are computed using density functional theory with a generalized gradient approximation in the range of 1 ≤ n ≤ 14. Local total spin magnetic moments, polarizabilities, binding energies per atom, and energies of abstraction of O and O2 are computed for both series along with electron affinities of the neutrals and vertical detachment energies of the anions. In the lowest total energies states of Cr2O2, Cr2O3, Cr2O4, Cr2O14, Cr2O3-, Cr2O4-, and Cr2O14-, total spin magnetic moments of the Cr atoms are quite large and antiferromagnetically coupled. In the rest of the series, at least one of the Cr atoms has no spin-magnetic moment at all. The computed vertical electron-detachment energies of the Cr2On- are in good agreement with experimental values obtained in the 1 ≤ n ≤ 7 range. All neutral Cr2On possess electron affinities larger than the electron affinities of halogen atoms when n > 6 and are thus superhalogens. It is found that the neutrals and anions are stable with respect to the abstraction of an O atom in the whole range of n considered, whereas both neutrals and anions became unstable toward the loss of O2 for n > 7. The polarizability per atom decreases sharply when n moves from one to four and then remains nearly constant for larger n values in both series. The largest members in both series, Cr2O14 and Cr2O14-, possess the geometrical structures of the Cr2(O2)7 type by analogy with monochromium Cr(O2)4.

A comparative study of small 3d-metal oxide (FeO)n, (CoO)n, and (NiO)n clusters.

Geometrical and electronic structures of the 3d-metal oxide clusters (FeO)n, (CoO)n, and (NiO)n are computed using density functional theory with the generalized gradient approximation in the range of 1 ≤ n ≤ 10. It is found that the cluster geometries are similar in the (FeO)n and (CoO)n series but noticeably different in the (NiO)n series for several values of n. All of the lowest total energy states are found to possess relatively small spin multiplicities and are either antiferromagnetic or ferrimagnetic except for the states of (NiO)3, (NiO)4, (NiO)9, and (NiO)10, which are ferromagnetic. The computed polarizabilities per atom undergo a steep decrease when compared to the atomic values of the MO monomers (M = Fe, Co, and Ni). Surprisingly, the polarizability does not strongly depend on either M or n in all the considered series when n varies from 3 to 10. The binding energies per atom are the largest in the (FeO)n series, followed by the binding energies of (CoO)n and (NiO)n.

Photoelectron spectra and structure of the Mnn(-) anions (n = 2-16).

Photoelectron spectra of the Mnn(-) anion clusters (n = 2-16) are obtained by anion photoelectron spectroscopy. The electronic and geometrical structures of the anions are computed using density functional theory with generalized gradient approximation and a basis set of triple-ζ quality. The electronic and geometrical structures of the neutral Mnn clusters have also been computed to estimate the adiabatic electron affinities. The average absolute difference between the computed and experimental vertical detachment energies of an extra electron is about 0.2 eV. Beginning with n = 6, all lowest total energy states of the Mnn(-) anions are ferrimagnetic with the spin multiplicities which do not exceed 8. The computed ionization energies of the neutral Mnn clusters are in good agreement with previously obtained experimental data. According to the results of our computations, the binding energies of Mn atoms are nearly independent on the cluster charge for n > 6 and possess prominent peaks at Mn13 and Mn13(-) in the neutral and anionic series, respectively. The density of states obtained from the results of our computations for the Mnn(-) anion clusters show the metallic character of the anion electronic structures.

Structure and Properties of Polyfluoride Fn(-) Clusters (n = 3-29).

The electronic and geometrical structures of the neutral Fn and singly negatively charged Fn(-) polyfluorides (n = 3-29) are studied using three levels of theory: density functional theory (DFT) with generalized gradient approximation, hybrid Hartree-Fock-DFT, and hybrid HF-DFT with long-range corrections. For n > 4, each polyfluoride possesses a number of states with different geometries that are closely spaced in total energy. The geometrical structures of the lowest total energy states follow different patterns for the even-n and odd-n Fn(-) anion branches with a preference for higher symmetry geometries. The largest F29(-) anion considered is found to possess Oh symmetry. All the anions beginning with F3(-) are found to possess adiabatic and vertical electron detachment energies exceeding the electron affinities of halogen atoms and are therefore superhalogen anions. Electron affinities, energies of formation, and binding energies show oscillatory behavior as functions of the number n of fluorine atoms. The neutral Fn species are found to be barely stable and are bound by polarization forces. The Fn(-) anions, on the contrary, are quite stable toward the loss of F, F(-), and F2(-), but not to the loss of F2.

Alanate anion, AlH4(-): photoelectron spectrum and computations.

The alanate anion, AlH4(-), was generated in the gas phase using a pulsed arc cluster ionization source. Its photoelectron spectrum was then measured with 193 nm photons. The spectrum consists of a broad feature, spanning electron binding energies from 3.8 eV to over 5.3 eV. This band reflects the photodetachment transitions between the ground state of the AlH4(-) anion and the ground state of its thermodynamically unstable neutral counterpart, AlH4. The vertical detachment energy (VDE) of AlH4(-) was measured to be 4.4 eV. Additionally, VDE values were also computed in a comprehensive theoretical study and compared both with the previously computed value and with our experimentally determined value.

An all-electron density functional theory study of the structure and properties of the neutral and singly charged M12 and M13 clusters: M = Sc-Zn.

The electronic and geometrical structures of the M12 and M13 clusters where M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn along with their singly negatively and positively charged ions are studied using all-electron density functional theory within the generalized gradient approximation. The geometries corresponding to the lowest total energy states of singly and negatively charged ions of V13, Mn12, Co12, Ni13, Cu13, Zn12, and Zn13 are found to be different from the geometries of the corresponding neutral parents. The computed ionization energies of the neutrals, vertical electron detachment energies from the anions, and energies required to remove a single atom from the M13 and M13(+) clusters are in good agreement with experiment. The change in a total spin magnetic moment of the cation or anion with respect to a total spin magnetic moment of the corresponding neutral is consistent with the one-electron model in most cases, i.e., they differ by ±1.0 µ(B). Exceptions are found only for Sc12(-), Ti12(+), Mn12(-), Mn12(+), Fe12(-), Fe13(+), and Co12(+).

Structure and properties of Fe(n), Fe(n)-, and Fe(n)+ clusters, n = 7-20.

The electronic and geometrical structures of the Fe(n), Fe(n)(­), and Fe(n)(+) series (n = 7­20) are studied using all-electron density functional theory with the generalized gradient approximation. Equilibria of the geometrical configurations of the lowest total energy states in all three series are found to be similar except for Fe(9)(­), Fe(9)(+), Fe(10)(­), Fe(10)(+), Fe(15)(­), and Fe(19)(+). Our computed ionization energies of the neutrals, vertical electron detachment energies, and energies of Fe atom abstraction are in good agreement with experiment. It is found that the one-electron model corresponding to the change in the total magnetic moment of ±1.0µ(B) due to either attachment or detachment of an electron is valid in most cases. The exceptions are Fe(4)(+), Fe(10)(­), Fe(10)(+), Fe(12)(­), Fe(13)(+), and Fe(14)(+), where the change in the total magnetic moment is +3µ(B) (Fe(10)(­) and Fe(12)(­)), −3µ(B) (Fe(4)(+), Fe(11)(+), and Fe(14)(+)), and −9µ(B) (Fe(13)(+)). The reason for an anomalously large quenching of the total spin magnetic moment in Fe(13)(+) is explained. Our computed total spin magnetic moments per atom match the recent experimental values within the experimental uncertainty bars.

Density functional study of neutral and anionic AlO(n) and ScO(n) with high oxygen content.

The electronic and geometrical structures of neutral and negatively charged AlO(5), AlO(6), AlO(7), AlO(8), AlO(9), AlO(10), AlO(11), AlO(12), AlO(15), AlO(16), and AlO(18) along with the corresponding series of ScO(n) and ScO n- oxides were investigated using density functional theory with generalized gradient approximation. We found that these species possess geometrically stable isomers for all values of n = 5-12, 15, 16, 18 and are thermodynamically stable for n = 5-7. The species with n = 16 are found to be octa-dioxides M(η(1)-O(2))(8) while the species with n = 15 and 18 are penta-ozonides (η(2)-O(3))M(η(1)-O(3))(4) and hexa-ozonides M(η(1)-O(3))(6), respectively. Geometrical configurations of a number of the lowest total energy states of Al and Sc oxides are different. Especially, drastic differences are found for the anion AlO n- and ScO n- pairs at n = 9, 10, and 11. The Sc-O bonds are longer than the Al-O bonds by ≈0.2 Å, which, in turn, slightly affects the corresponding interoxygen bond lengths. The charges on metal atoms are close to +2e in both Al series and to +1.5e in both Sc series. As an extra electron is delocalized over ligands in the presence of a large positive charge on the metal atom of the anions, the electron affinity (EA) of the neutrals along with the ionization energies of the anions are large and exceed the EAs of the halogen atoms in a number of cases.

Dimerization of defect fullerenes and the orientational phase transition in oxidized C60 fullerite.

Oxidized fullerite was obtained by heating a fullerite sample intercalated with oxygen, (O2)0.44C60, up to 300 degrees C. Orientational phase transitions in the oxidized fullerite are studied using differential scanning calorimetry (DSC) and have been found to possess a specific enthalpy whose value is lower by 25% than in the initial (O2)0.44C60 sample. In order to find possible reasons for hindrance to the buckyball rotations, we performed optimizations of defect buckyball fullerenes C60-n with different distributions of vacancies along with the dimers C60-n-C60-n and C60-C60-n for n = 1-4 using density functional theory with generalized gradient approximation. We found that the dimerization energy ranges from 1.07 eV (C58-C58) to 6.56 eV (C56-C56) and from 1.81 eV (C60-C58) to 4.29 eV (C60-C56), respectively. The formation of such dimers, which could in addition interact with defect buckyball cages and form larger aggregates, is to be related to the lowering of the orientational transition enthalpy.

Structure and spectroscopic properties of iron oxides with the high content of oxygen: FeO(n) and FeO(n)(-) (n = 5-12).

The electronic and geometrical structures of oxygen-rich neutral and negatively charged FeO(5), FeO(6), FeO(7), FeO(8), FeO(9), FeO(10), FeO(11), and FeO(12) clusters were obtained using density functional theory with generalized gradient approximation. With the exception of FeO(11) and FeO(12), all clusters are found to possess a large number of isomers composed of oxo, peroxo, superoxo, and ozonide fragments that are closely spaced in total energy, especially for n = 7 and 8. The preferable structures of FeO(12) are composed of superoxo groups with different orientations. All the neutral species possess rather large electron affinities, which range from 3.24 eV (FeO(8)) to 3.95 eV (FeO(5)). Although all of the lowest energy states were found to possess positive vibrational frequencies and thus are geometrically stable, the states are thermodynamically unstable against dissociation to FeO(4) + (n - 4)/2 O(2) for n = 6, 8, 10, and 12 and FeO(5) + (n - 5)/2 O(2) for n = 7, 9, and 11. In particular, the decay of FeO(12) is exothermic by 34 kcal/mol.

Optical properties of (GaAs)n clusters (n = 2-16).

The electronic and geometrical structures of the lowest triplet states of (GaAs) n clusters ( n = 2-16) are studied using density functional theory with generalized gradient approximation (DFT-GGA). It is found that the triplet-state geometries are different from the corresponding singlet-state geometries; for n = 2-8, 10, and 11, the triplets and singlets have different topologies, while the (GaAs) 9, (GaAs) 12, (GaAs) 15, and (GaAs) 16 triplets possess a reduced symmetry, due to Jahn-Teller distortions. Except for GaAs, the singlet states are the ground states. Excitation energies and oscillator strengths are computed for excitations from the ground state to ten singlet states of all (GaAs) n clusters using time-dependent density functional theory. The adiabatic singlet-triplet gap is compared to the vertical gap, and the difference in the eigenvalues of the highest-occupied and lowest-unoccupied molecular orbitals (the HOMO-LUMO gap). While these three values show large oscillations for small n, they approach each other as the cluster size grows. Thus, the HOMO-LUMO gap computed using the DFT-GGA approach presents a rather reliable estimate of the adiabatic singlet-triplet gap.

Electronic and geometrical structure of Mn13 anions, cations, and neutrals.

We have computed the electronic and geometrical structures of thirteen atom manganese clusters in all three charge states, Mn(13) (-), Mn(13) (+), and Mn(13) by using density functional theory with the generalized gradient approximation. Our results for Mn(13) (-) are compared with our anion photoelectron spectrum of Mn(13) (-), published in this paper. Our results for Mn(13) (+) are compared with the previously published photoionization results of Knickelbein [J. Chem. Phys. 106, 9810 (1997)]. There is a good agreement between theoretical and experimental values of ionization and electron attachment energies.

The structure and energetics of (GaAs)n, (GaAs)n(-), and (GaAs)n+ (n=2-15).

Electronic and geometrical structures of neutral, negatively, and positively charged (GaAs)n clusters are computed using density functional theory with generalized gradient approximation. All-electron computations are performed on (GaAs)2-(GaAs)9 while effective core potentials (ECPs) are used for (GaAs)9-(GaAs)15. Calibration calculations on GaAs and (GaAs)9 species support the use of the ECP for the larger clusters. The ground-state geometries of (GaAs)n(-) and/or (GaAs)n+ are different from the corresponding neutral ground-state geometry, except for n=7, 9, 12, 14, and 15, where the neutral and ions have similar structures. Beginning with n=6, all atoms are three coordinate, except for (GaAs)10+ and (GaAs)13+. For the larger species, there is a competition between fullerenes built from hexagons and rhombi and geometrical configurations where Ga-Ga and As-As bonds are formed, which results in the formation of pentagons. As expected, the static polarizability varies in the order of anion>neutral>cation, but the values are rather similar for all three charge states. The thermodynamic stability for the loss of GaAs is reported.

Dissociative and associative attachment of NO to iron clusters.

Electronic and geometrical structures of iron clusters with associative (FeNO, Fe2NO, Fe3NO, Fe4NO, Fe5NO, and Fe6NO) and dissociative (OFeN, OFe2N, OFe3N, OFe4N, OFe5N, and OFe6N) attachments of NO, as well as the corresponding singly negatively and positively charged ions, are computed using density functional theory with generalized gradient corrections. Both types of isomers are found to be stable and no spontaneous dissociation was observed during the geometry optimizations. The ground states correspond to dissociative attachment of NO for all iron clusters Fe(n), except for Fe and Fe+. All of the OFe(n)N clusters have ferrimagnetic ground states, except for OFe2N, OFe2N-, OFe4N, and OFe4N-, which prefer the ferromagnetic coupling. In the ferrimagnetic states, the excess spin density at one iron atom couples antiferromagnetically to the excess spin densities of all other iron atoms. Relative to the high-spin Fe(n) ground state, the lowest energy ferrimagnetic state quenches the total magnetic moments of iron clusters by 7, which is to be compared with a reduction in the magnetic moment of one in the lowest energy ferromagnetic states. Dissociation of NO on the iron clusters has a pronounced impact on the energetics of reactions; the Fe(n)NO+CO-->Fe(n)N+CO2 channels are exothermic while the OFe6N+CO--> Fe6N+CO2 channels are nearly thermoneutral.