Probing the electronic and structural properties of chromium oxide clusters (CrO3)n(-) and (CrO3)n (n = 1-5): photoelectron spectroscopy and density functional calculations

Photoelectron spectroscopy has been conducted for a series of (CrO3)n(-) (n = 1-5) clusters and compared with density functional calculations. Well-resolved photoelectron spectra were obtained for (CrO3)n(-) (n = 1-5) at 193 nm (6.424 eV) and 157 nm (7.866 eV) photon energies, allowing for accurate measurements of the electron binding energies, low-lying electronic excitations for n = 1 and 2, and the energy gaps. Density functional and molecular orbital theory (CCSD(T)) calculations were performed to locate the ground and low-lying excited states for the neutral clusters and to calculate the electron binding energies of the anionic species. The experimental and computational studies firmly establish the unique low-spin, nonplanar, cyclic ring structures for (CrO3)n and (CrO3)n(-) for n > or = 3. The structural parameters of (CrO3)n are shown to converge rapidly to those of the bulk CrO3 crystal. The extra electron in (CrO3)n(-) (n > or = 2) is shown to be largely delocalized over all Cr centers, in accord with the relatively sharp ground-state photoelectron bands. The measured energy gaps of (CrO3)n exhibit a sharp increase from n = 1 to n = 3 and approach to the bulk value of 2.25 eV at n = 4 and 5, consistent with the convergence of the structural parameters.     

Probing the structure and bonding in Al6N- and Al6N by photoelectron spectroscopy and ab initio calculations

The electronic and geometrical structure of a nitrogen-doped Al6- cluster (Al6N-) is investigated using photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of Al6N- have been obtained at three photon energies with seven resolved spectral features. The electron affinity of Al6N has been determined to be 2.58 +/- 0.04 eV. Global minimum structure searches for A6N- and its corresponding neutral form are performed using several theoretical methods. Vertical electron detachment energies, calculated using three different methods for the lowest energy structure and a low-lying isomer, are compared with the experimental data. The ground-state structure of Al6N- is established from the joint experimental and theoretical study to consist of an Al2 dimer bonded to the top of a quasi-planar tetracoordinated N unit, Al4N-, or it can be viewed as a distorted trigonal prism structure with the N atom bonded in one of the prism faces. For neutral Al6N, three low-lying isomers are found to compete for the global minimum, two of which are built from the tetracoordinated Al4N unit. The chemical bonding in Al6N- is discussed on the basis of molecular orbital and natural bond analyses.     

Geometries, Electronic Structures, and Electron Detachment Energies of Small Boron Sulfide Anions： A Density Functional Theory Investigation

A density functional theory investigation on the geometries, electronic structures, and electron detachment energies of BS, BS2, B（BS）2 and B（BS）3 has been performed in this work. The linear ground-state structures of BS （C∞v, ^1∑^＋） and BS2^- （O∞h, ^1∑g^＋） prove to be similar to the previously reported BO and BO2 with systematically lower electron detachment energies. Small boron sulfide clusters are found to favor the formation of -B=S groups which function basically as a-radicals and dominate the ground-state structures of the systems. The perfect linear B（BS）2^-（D∞h, ^3∑g） and beautiful equilateral triangle B（BS）3^- （D3h,^2A1”） turn out to be analogous to the well-known C2v BH2 and O3h BH3, respectively. The electron affinities of BS, BS2, B（BS）2 and B（BS）3 are predicted to be 2.3, 3.69, 3.00 and 3.45 eV, respectively. The electron detachment energies calculated for BS^-, BS2^-, B（BS）2^-, and B（BS）3^- may facilitate future photoelectron spectroscopy measurements to characterize the geometrical and electronic structures of these anions.     

Planar nitrogen-doped aluminum clusters AlxN- (x=3-5)

The electronic and geometrical structures of three nitrogen-doped aluminum clusters, Al(x)N(-) (x=3-5), are investigated using photoelectron spectroscopy and ab initio calculations. Well-resolved photoelectron spectra have been obtained for the nitrogen-doped aluminum clusters at four photon energies (532, 355, 266, and 193 nm). Global minimum structure searches for Al(x)N(-) (x=3-5) and their corresponding neutrals are performed using several theoretical methods. Vertical electron detachment energies are calculated using three different methods for the lowest energy structures and low-lying isomers are compared with the experimental observations. Planar structures have been established for all the three Al(x)N(-) (x=3-5) anions from the joint experimental and theoretical studies. For Al(5)N(-), a low-lying nonplanar isomer is also found to contribute to the experimental spectra, signifying the onset of two-dimensional to three-dimensional transition in nitrogen-doped aluminum clusters. The chemical bonding in all the planar clusters has been elucidated on the basis of molecular orbital and natural bond analyses.     

A photoelectron spectroscopic and computational study of sodium auride clusters, NanAun- (n = 1-3)

Negatively charged sodium auride clusters, NanAun- (n = 1-3), have been investigated experimentally using photoelectron spectroscopy and ab initio calculations. Well-resolved electronic transitions were observed in the photoelectron spectra of NanAun- (n = 1-3) at several photon energies. Very large band gaps were observed in the photoelectron spectra of the anion clusters, indicating that the corresponding neutral clusters are stable closed-shell species. Calculations show that the global minimum of Na2Au2- is a quasi-linear species with Cs symmetry. A planar isomer of D2h symmetry is found to be 0.137 eV higher in energy. The two lowest energy isomers of Na3Au3- consist of three-dimensional structures of Cs symmetry. The global minimum of Na3Au3- has a bent-flake structure lying 0.077 eV below a more compact structure. The global minima of the sodium auride clusters are confirmed by the good agreement between the calculated electron detachment energies of the anions and the measured photoelectron spectra. The global minima of neutral Na2Au2 and Na3Au3 are found to possess higher symmetries with a planar four-membered ring (D2h) and a six-membered ring (D3h) structure, respectively. The chemical bonding in the sodium auride clusters is found to be highly ionic with Au acting as the electron acceptor.     

Homonuclear transition-metal trimers

Density-functional theory has been used to determine the ground-state geometries and electronic states for homonuclear transition-metal trimers constrained to equilateral triangle geometries. This represents the first application of consistent theoretical methods to all of the ten 3d block transition-metal trimers, from scandium to zinc. A search of the potential surfaces yields the following electronic ground states and bond lengths: Sc3(2A1',2.83 A), Ti3(7E',2.32 A), V3(2E",2.06 A), Cr3(17E',2.92 A), Mn3(16A2',2.73 A), Fe3(11E",2.24 A), Co3(6E",2.18 A), Ni3(3A2",2.23 A), Cu3(2E',2.37 A), and Zn3(1A1',2.93 A). Vibrational frequencies, several low-lying electronic states, and trends in bond lengths and atomization energies are discussed. The predicted dissociation energies DeltaE(M3-->M2+M) are 49.4 kcal mol(-1)(Sc3), 64.3 kcal mol(-1)(Ti3), 60.7 kcal mol(-1)(V3), 11.5 kcal mol(-1)(Cr3), 32.4 kcal mol(-1)(Mn3), 61.5 kcal mol(-1)(Fe3), 78.0 kcal mol(-1)(Co3), 86.1 kcal mol(-1)(Ni3), 26.8 kcal mol(-1)(Cu3), and 4.5 kcal mol(-1)(Zn3).     

Chemical bonding in Si5(2-) and NaSi5(-) via photoelectron spectroscopy and ab initio calculations

Photoelectron spectroscopy and ab initio calculations are used to investigate the electronic structure and chemical bonding of Si5(-) and Si5(2-) in NaSi5(-). Photoelectron spectra of Si5(-) and NaSi5(-) are obtained at several photon energies and are compared with theoretical calculations at four different levels of theory, TD-B3LYP, R(U)OVGF, UCCSD(T), and EOM-CCSD(T), all with 6-311+G(2df) basis sets. Excellent agreement is observed between experiment and theory, confirming the obtained ground-state structures for Si5(-) and Si5(2-), which are both found to be trigonal bipyramid with D3h symmetry at several levels of theory. Chemical bonding in Si5, Si5(-), and Si5(2-) is analyzed using NPA, molecular orbitals, ELF, and NICS indices. The bonding in Si5(2-) is compared with that in the isoelectronic and isostructural B5H5(2-) species, but they are found to differ due to the involvement of electron densities, which are supposed to be lone pairs in the skeletal bonding in Si5(2-).     

Electronic spectra and excited states of neptunyl and its [NpO2Cl4]2- complex

Electronic states and spectra of NpO(2)(2+) and NpO(2)Cl(4)(2-) with a Np 5f(1) ground-state configuration, related to low-lying 5f-5f and ligand-to-metal charge-transfer (CT) transitions, are investigated, using restricted-active-space perturbation theory (RASPT2) with spin-orbit coupling. Restrictions on the antibonding orbital occupations have little influence on the 5f-5f transition energies, but an important impact on the CT states with an open bonding orbital shell. The present calculations provide significant improvement over previous literature results. The assignment of the experimental electronic spectra of Cs(2)NpO(2)Cl(4) is refined, based on our calculations of NpO(2)Cl(4)(2-). Assignments on the basis of bare NpO(2)(2+) are less reliable, since the equatorial Cl ligands perturb the excited-state energies considerably. Calculated changes of the Np-O bond lengths are in agreement with the observed short symmetric-stretching progressions in the f-f spectra and longer progressions in the CT spectra of neptunyl. A possible luminescence spectrum of the lowest quartet CT state is predicted.     

Al(6)(2-) - fusion of two aromatic Al(3)(-) units. A combined photoelectron spectroscopy and ab initio study of M(+)[Al(6)(2-)] (M = Li,Na, K,Cu, and Au)

Photoelectron spectroscopy is combined with ab initio calculations to elucidate the structure and chemical bonding of a series of MAl(6)(-) (M = Li, Na, K, Cu, and Au) bimetallic clusters. Well-resolved photoelectron spectra were obtained for MAl(6)(-) (M = Li, Na, Cu, and Au) at several photon energies. The ab initio calculations showed that all of the MAl(6)(-) clusters can be viewed as an M(+) cation interacting with an Al(6)(2-) dianion. Al(6)(2-) was found to possess an O(h) ground-state structure, and all of the MAl(6)(-) clusters possess a C(3v) ground-state structure derived from the O(h) Al(6)(2-). Careful comparison between the photoelectron spectral features and the ab initio one-electron detachment energies allows us to establish firmly the C(3v)ground-state structures for the MAl(6)(-) clusters. A detailed molecular orbital (MO) analysis is conducted for Al(6)(2-) and compared with Al(3)(-). It was shown that Al(6)(2-) can be considered as the fusion of two Al(3)(-) units. We further found that the preferred occupation of those MOs derived from the sums of the empty 2e' MOs of Al(3)(-), rather than those derived from the differences between the occupied 2a(1)' and 2a(2)' ' MOs of Al(3)(-), provides the key bonding interactions for the fusion of the two Al(3)(-) into Al(6)(2-). Because there are only four bonding MOs (one pi and three sigma MOs), an analysis of resonance structures was performed for the O(h)Al(6)(2-). It is shown that every face of the Al(6)(2-) octahedron still possesses both pi- and sigma-aromaticity, analogous to Al(3)(-), and that in fact Al(6)(2-) can be viewed to possess three-dimensional pi- and sigma-aromaticity with a large resonance stabilization.     

MX3(-) superhalogens (M = Be, Mg, Ca; X = Cl, Br): a photoelectron spectroscopic and ab initio theoretical study

Gas-phase alkaline earth halide anions, MgX3(-) and CaX3(-) (X = Cl, Br), were produced using electrospray and investigated using photoelectron spectroscopy at 157 nm. Extremely high electron binding energies were observed for all species and their first vertical detachment energies were measured as 6.60 +/- 0.04 eV for MgCl3(-), 6.00 +/- 0.04 eV for MgBr3(-), 6.62 +/- 0.04 eV for CaCl3(-), and 6.10 +/- 0.04 eV for CaBr3(-). The high electron binding energies indicate these are very stable anions and they belong to a class of anions, called superhalogens. Theoretical calculations at several levels of theory were carried out on these species, as well as the analogous BeX3(-). Vertical detachment energy spectra were predicted to compare with the experimental observations, and good agreement was obtained for all species. The first adiabatic detachment energies were found to be substantially lower (by about 1 eV) than the corresponding vertical detachment energies for all the MX3(-) species, indicating extremely large geometry changes between MX3(-) and MX3. We found that all the MX3(-) anions possess D3h ((1)A1') structures and are extremely stable against dissociation into MX2 and X-. The corresponding neutral species MX3, however, were found to be only weakly bound with respect to dissociation toward MX2 + X. The global minimum structures of all the MX3 neutrals were found to be C2v ((2)B2), which can be described as (X2(-))(MX+) charge-transfer complexes, whereas the MX2...X (C2v, (2)B1) van der Waals complexes were shown to be low-lying isomers.     

Slow electron velocity-map imaging spectroscopy of the 1-propynyl radical

High resolution photoelectron spectra of the 1-propynyl and 1-propynyl-d(3) anions acquired with slow electron velocity-map imaging are presented. The electron affinity is determined to be 2.7355+/-0.0010 eV for the 1-propynyl radical and 2.7300+/-0.0010 eV for 1-propynyl-d(3). Several vibronic transitions are observed and assigned using the isotopic shifts and results from ab initio calculations. Good agreement between experimental spectra and calculations suggests a C(3v) geometry for the 1-propynyl radical. No evidence is found for strong vibronic coupling between the ground electronic state and the low-lying first excited state.     

Density functional studies on the effects of hydrogen bonding on the formation of a charge-transfer complex between p-benzoquinone and 2,6-dimethoxyphenol

The ability to form a ground-state charge-transfer (CT) complex between an electron acceptor, p-benzoquinone (BQ) and an electron donor, 2,6-dimethoxyphenol (DMOPh) was found to be enhanced by H-bonding of BQ to a hydrogen-bond donor, trifluoroacetic acid (TFA) and H-bonding DMOPh to a hydrogen-bond acceptor, 4-(N,N-dimethylamino)pyridine (DMAPy) [Chem. Phys. Lett. 2005, 401, 200]. Here is reported density functional theory (DFT) calculations to study the effect of H-bonding to electron donor and electron acceptor moieties on the ground-state CT complex formation ability between the aforementioned electron donor/acceptor pair. DFT calculations using B3LYP with the 6-311G(d,p) basis set show that the HOMO and LUMO energies of BQ drop on H-bonding to TFA through its C=O groups and the HOMO and LUMO energies of DMOPh increase on H-bonding to DMAPy via its O-H group. BQ molecules hydrogen-bonded as 1:1 and 1:2 complexes to TFA act as stronger acceptors than the bare molecule, while 1:1 complexes of DMOPh and DMAPy act as better donors. Vertical excitation energies for electronic transitions from the ground state to the first few excited states of BQ, DMOPh, DMAPy, and their different complexes have been investigated in the framework of time-dependent density functional theory (TD-DFT) to simulate and interpret experimental ultraviolet absorption spectra. Good agreement between experimental and calculated spectra is established. The enhancement of the CT complex formation ability between the BQ and DMOPh pair is favored by the strong H-bonding interaction of BQ with TFA as well as by the H-bonding interaction of DMOPh with DMAPy.     

Computational studies of nonstoichiometric sodium auride clusters

The molecular structures of low-lying isomers of anionic and neutral sodium auride clusters have been studied computationally at the second-order M?ller-Plesset perturbation theory level using quadruple-ζ basis sets augmented with a double set of polarization functions. The first vertical detachment energies were calculated at the M?ller-Plesset level as the energy difference between the cluster anion and the corresponding neutral cluster. The photodetachment energies of higher-lying ionization channels were calculated by adding electronic excitation energies of the neutral clusters to the first vertical detachment energy. The excitation energies were calculated at the linear response approximate coupled-cluster singles and doubles level using the anionic cluster structures. The obtained ionization energies for NaAu(-), NaAu(2)(-), NaAu(3)(-), NaAu(4)(-), Na(2)Au(2)(-), Na(2)Au(3)(-), Na(3)Au(3)(-), and Na(2)Au(4)(-) were compared to values deduced from experimental photoelectron spectra. Comparison of the calculated photoelectron spectra for a few energetically low-lying isomers shows that the energetically lowest cluster structures obtained in the calculations do not always correspond to the clusters produced experimentally. Spin-component-scaled second-order M?ller-Plesset perturbation theory calculations shift the order of the isomers such that the observed clusters more often correspond to the energetically lowest structure, whereas the spin-component-scaled approach does not improve the photodetachment energies of the sodium aurides. The potential energy surface of the sodium aurides is very soft, with several low-lying isomers requiring an accurate electron correlation treatment. The calculations show that merely the energetic criterion is not a reliable means to identify the structures of the observed sodium auride clusters; other experimental information is needed to ensure a correct assignment of the cluster structures. The cluster structures of nonstoichiometric anionic sodium aurides have been determined by comparing calculated ionization energies for low-lying structures of the anionic clusters with experimental data.     

Determination of the electron affinity of the acetyloxyl radical (CH3COO) by low-temperature anion photoelectron spectroscopy and ab initio calculations

The electronic structure and electron affinity of the acetyloxyl radical (CH3COO) were investigated by low-temperature anion photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of the acetate anion (CH3COO-) were obtained at two photon energies (355 and 266 nm) and under three different temperatures (300, 70, and 20 K) with use of a new low-temperature ion-trap photoelectron spectroscopy apparatus. In contrast to a featureless spectrum at 300 K, a well-resolved vibrational progression corresponding to the OCO bending mode was observed at low temperatures in the 355 nm spectrum, yielding an accurate electron affinity for the acetyloxyl radical as 3.250 +/- 0.010 eV. This experimental result is supported by ab initio calculations, which also indicate three low-lying electronic states observed in the 266 nm spectrum. The calculations suggest a 19 degrees decrease of the OCO angle upon detaching an electron from acetate, consistent with the vibrational progression observed experimentally.     

An ab initio study of the low-lying electronic states of S3

Accurate calculations of the low-lying singlet and triplet electronic states of thiozone, S(3), have been carried out using large multireference configuration interaction wave functions. Cuts of the full potential energy surfaces along the stretching and bending coordinates have been presented, together with the vertical excitation spectra. The strong experimentally observed absorption around 395 nm is assigned to the 1 (1)B(2) state, which correlates to ground state products. Absorption at wavelengths shorter than 260 nm is predicted to lead to singlet excited state products, S(2) (a (1)Delta(g))+S((1)D). The spectroscopic properties of the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) electronic states of the S(2) radical have also been accurately characterized in this work. The investigations of the low-lying electronic states were accompanied by accurate ground state coupled cluster calculations of the thermochemistry of both S(2) and S(3) using large correlation consistent basis sets with corrections for core-valence correlation, scalar relativity, and atomic spin-orbit effects. Resulting values for D(0)(S(2)+S) and SigmaD(0) for S(3) are predicted to be 61.3 and 162.7 kcal/mol, respectively, with conservative uncertainties of +/-1 kcal/mol. Analogous calculations predict the C(2v)-D(3h) (open-cyclic) isomerization energy of S(3) to be 4.4+/-0.5 kcal/mol.     

High resolution and low-temperature photoelectron spectroscopy of an oxygen-linked fullerene dimer dianion: C(120)O(2-)

C(120)O comprises two C(60) cages linked by a furan ring and is formed by reactions of C(60)O and C(60). We have produced doubly charged anions of this fullerene dimer (C(120)O(2-)) and studied its electronic structure and stability using photoelectron spectroscopy and theoretical calculations. High resolution and vibrationally resolved photoelectron spectra were obtained at 70 K and at several photon energies. The second electron affinity of C(120)O was measured to be 1.02+/-0.03 eV and the intramolecular Coulomb repulsion was estimated to be about 0.8 eV in C(120)O(2-) on the basis of the observed repulsive Coulomb barrier. A low-lying excited state ((2)B(1)) was also observed for C(120)O(-) at 0.09 eV above the ground state ((2)A(1)). The C(120)O(2-) dianion can be viewed as a single electron on each C(60) ball very weakly coupled. Theoretical calculations showed that the singlet and triplet states of C(120)O(2-) are nearly degenerate and can both be present in the experiment. The computed electron binding energies and excitation energies, as well as Franck-Condon factors, are used to help interpret the photoelectron spectra. A C-C bond-cleaved isomer, C(60)-O-C(60) (2-), was also observed with a higher electron binding energy of 1.54 eV.     

Low-lying isomers of the B9(-) boron cluster: the planar molecular wheel versus three-dimensional structures

The B(9)(-) cluster was found previously to be an unprecedented molecular wheel containing an octacoordinate planar boron with D(8h) symmetry in a combined photoelectron spectroscopy (PES) and theoretical study [H. J. Zhai et al., Angew. Chem., Int. Ed. 42, 6004 (2003)]. However, the PES spectra of B(9)(-) exhibit minor features that cannot be explained by the global minimum D(8h) structure, suggesting possible contributions from low-lying isomers at finite temperatures. Here we present Car-Parrinello molecular dynamics with simulated annealing simulations to fully explore the potential energy surface of B(9)(-) and search for low-lying isomers that may account for the minor PES features. We performed density functional theory (DFT) calculations with different exchange-correlation functionals and ab initio calculations at various levels of theory with different basis sets. Two three-dimensional low-lying isomers were found, both of C(s) symmetry, 6.29 (C(s)-2) and 10.23 (C(s)-1) kcal/mol higher in energy than the D(8h) structure at the highest CCSD(T) level of theory. Calculated detachment transitions from the C(s)-2 isomer are in excellent agreement with the minor features observed in the PES spectra of B(9)(-). The B(9)(-) cluster proves to be a challenge for most DFT methods and the calculated relative energies strongly depend on the exchange-correlation functionals, providing an excellent example for evaluating the accuracies of various DFT methods.     

A sequential molecular mechanics/quantum mechanics study of the electronic spectra of amides

We report gas-phase electronic spectra of formamide, N-methyformamide, acetamide, and N-methylacetamide at 300 K calculated using a combination of classical molecular dynamics and time-dependent density functional theory (TDDFT). In comparison to excitation energies computed using the global minima structures, the valence npi* and pi(nb)pi* states show a significant red-shift of 0.1-0.35 eV, while smaller shifts are found for the n3s and pi(nb)3s Rydberg states. In this work, we have identified the physical origin of these shifts arising from variations of the molecular structure. We present simple relationships between key geometrical parameters and spectral shifts. Consequently, electronic spectra can be generated directly from ground-state structures, without additional quantum chemical calculations. The electronic spectrum of formamide in aqueous solution is computed using TDDFT using an explicit solvent model. This provides a quantitative determination of the condensed-phase spectrum. In general, this study shows that temperature effects can change the predicted excitation energies significantly and demonstrates how electronic spectra at elevated temperatures can be computed in a computationally efficient way.     

Low energy electron energy-loss spectroscopy of CF3X (X=Cl,Br)

We report threshold electron energy-loss spectra for the fluorohalomethanes CF3X (X=Cl,Br). Measurements were made at incident electron energies of 30 and 100 eV in energy-loss range of 4-14 eV, and at scattering angles of 4 degrees and 15 degrees. Several new electronic transitions are observed which are ascribable to excitation of low-lying states as well as are intrinsically overlapped in the molecules themselves. Assignments of these electronic transitions are suggested. These assignments are based on present spectroscopic and cross-section measurements, high-energy scattering spectra, and ab initio molecular orbital calculations. The calculated potential curves along the C-X bond show repulsive nature, suggesting that these transitions may lead to dissociation of the C-X bond. The present results are also compared with the previous ones for CF3H, CF4, and CF3I.     

Density functional theory studies on the electronic and vibrational spectra of octaethylporphyrin diacid

The ground-state structure and electronic and vibrational spectra of octaethylporphyrin diacid (H4OEP2+) have been studied with the density functional theory. The geometrical parameters computed with B3LYP, PBE1PBE and mPW1PW91 functionals and 6-31G* basis sets are well consistent with the experimental values. Electronic absorption spectrum of H4OEP2+ has been studied with the time-dependent DFT method, and the calculated excitation energies and oscillator strengths are compared with the experimental results. The Raman and IR spectra of H4OEP2+ and the Raman spectrum of its N-deuterated analogue (D4OEP2+) were measured. The observed Raman and IR bands have been assigned based on the frequency calculations at the B3LYP/6-31G* level of theory.