Negative Ion Photoelectron Spectra Research Articles

Classical Stern-Gerlach profiles ofMn5andMn6clusters

${\mathrm{Mn}}_{5}$ and ${\mathrm{Mn}}_{6}$ clusters have recently been found to exhibit Stern-Gerlach profiles marked by a central peak that broadens with the increasing field gradient. The profiles neither exhibit a reminiscence of space quantization as observed through a splitting of beams for the case of free atoms, nor a net deflection characteristic of superparamagnetic relaxations observed in other transition metal clusters. It is proposed that this new behavior results from a weak coupling of localized atomic moments. ab initio electronic structure studies are carried out to show that a ${\mathrm{Mn}}_{5}$ cluster has isomers with spin magnetic moments of $3\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, $13\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, and $23\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ while a ${\mathrm{Mn}}_{6}$ cluster has isomers with moments of $2\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, $8\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, $16\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, and $26\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, respectively. The isomers can be obtained by sequential turning of the local atomic moments starting from the ferromagnetic state and can be seen in the negative ion photoelectron spectra of the anions. The weak coupling of the atomic moments, however, leads to unconventional spin dynamics that result in classical broadening of the Stern-Gerlach profiles and lower apparent magnetic moments. The theoretical results illustrate how a combination of the negative ion photodetachment spectroscopy and Stern-Gerlach profiles can provide information on the net spin moment, interatomic spin coupling, and spin dynamics.

Photodetachment of zwitterions: probing intramolecular Coulomb repulsion and attraction in the gas phase using mono-decarboxylated pyridinium dicarboxylates. Implications on the mechanism of orotidine 5'-monophosphate decarboxylase.

Negative ion photoelectron spectra resulting from the decarboxylation of nine zwitterionic pyridinium dicarboxylates (D(x,y)) are reported. Structural assignments are made on the basis of analogy to the spectra of related species, labeling experiments with (13)C- or (2)H-containing substrates, independent syntheses, and comparison to density functional theory and ab initio (B3LYP and CCSD(T), respectively) results. In some cases, an acid-catalyzed isomerization of the D(x,y)-CO(2) ions was found to take place. Adiabatic detachment energies of the resulting zwitterionic ions were measured and are well reproduced by theory. The relative stabilities of the D(x,y)-CO(2) decarboxylation products are largely determined by their intramolecular electrostatic interactions, which are directly probed by the photoelectron spectra and were analyzed in terms of the resulting Coulombic forces. Expulsion of carbon dioxide from the D(x,y) ions was also used as an electrostatic model to probe the mechanism of the enzyme-catalyzed conversion of orotidine 5'-monophosphate (OMP) to uridine 5'-monophosphate (UMP). It was found that the loss of CO(2) from these zwitterions and from oxygen-protonated OMP is retarded by the presence of an additional anionic group. This suggests that the formation of a zwitterion intermediate in the enzyme-catalyzed transformation of OMP to UMP may have less of an energetic impact than commonly thought and could be a "red herring". If so, the electrostatic stress mechanism proposed by Larsen et al. and Pai, Guo, and co-workers maybe followed.

Negative ion photoelectron spectra simulation of V3O from a density functional study

A density functional study of neutral and anionic vanadium trimer monoxides is presented. The calculations were of all-electron type employing a newly developed basis set for the vanadium atom. Different isomers of V3O and V3O− were studied in order to determine the ground state structures. For both systems a planar C2v structure with an edge-bonded oxygen atom was found as the ground state. Equilibrium structure parameters of ground states as well as low-lying excited states, harmonic frequencies, the adiabatic electron affinity, and Kohn–Sham orbital diagrams are reported. The experimental negative ion photoelectron spectra of V3O was simulated by calculating multidimensional Franck–Condon factors, using the geometries and harmonic frequencies of the calculated ground states of V3O− and V3O. The good agreement between the experimental and the theoretical spectra allows the determination of the ground state structure of V3O and V3O−. This represents the first work in which a simulation of a vibrationally resolved negative ion photoelectron spectra of a transition metal oxide is presented.

Photodetachment of zwitterions: probing intramolecular coulomb repulsion and attraction in the gas phase using pyridinium dicarboxylate anions.

Zwitterions are critically important in many biological transformations and are used in numerous chemical processes. The consequences of electrostatic effects on reactivity and physical properties, however, are largely unknown. In this work, we report the results of negative ion photoelectron spectra of nine isomeric pyridinium dicarboxylate zwitterions and three nonzwitterionic methoxycarbonylpyridine carboxylate isomers (-O(2)CPyrCO(2)CH(3)). Information about the intramolecular electrostatic interactions was directly obtained from the photoelectron spectra. The adiabatic and vertical detachment energies were measured and understood in terms of intramolecular Coulombic forces. Calculations at the B3LYP and CCSD(T) level were performed and compared to the experimental electron binding energies. Structures, relative stabilities, and the electron detachment sites also were obtained from the calculations.

Photoelectron Spectroscopy of Nickel, Palladium, and Platinum Oxide Anions

The 364-nm negative ion photoelectron spectra of XO and OXO molecules (X=Ni, Pd, and Pt) are reported. The spectra yield the electron affinities (EAs): EA(NiO)=1.455±0.005 eV; EA(PdO)=1.672±0.005 eV; EA(PtO)=2.172±0.005 eV; EA(ONiO)=3.043±0.005 eV; EA(OPdO)=3.086±0.005 eV; EA(OPtO)=2.677±0.005 eV. In addition, for the diatomics, transitions from the anion X̃ 2Π 3/2 and X̃′ 2Π 1/2 states into neutral X̃ 3Σ −, 3Π, and for NiO and PdO, 1Π, are assigned. Several states have been reassigned from those in the existing literature. Anion 2Π 3/2– 2Π 1/2 spin–orbit splittings are measured, as are neutral 3Π 2– 3Π 1 spin–orbit splittings: the XO 3Π 2– 3Π 1 splittings increase from 405±30 cm −1 (NiO) to 805±30 cm −1 (PdO) to 3580±40 cm −1 (PtO). A bond length shortening of 0.03±0.01 Å is measured upon electron detachment from NiO −, resulting in an anion bond length of 1.66±0.01 Å. The bond length does not change upon electron detachment from PdO − using 3.4-eV photons. The Pt–O bond length decreases by 0.035±0.010 Å in the 3Π 1← 2Π 3/2 transition. The spectrum of OPtO displays a significantly more extended vibrational progression than those of ONiO or OPdO, and the O–Pt bond length is found to decrease by 0.07±0.01 Å upon electron detachment. The spectra support the view that the Ni–O bond is largely ionic, the Pd–O bond is somewhat less so, and the Pt–O bond displays a substantial covalent character.

On the photoelectron spectrum of AgO−

The 364 nm negative ion photoelectron spectrum of AgO− is reported. Transitions to the A 2Σ+ state and the X 2Π1/2 and X 2Π3/2 spin–orbit states of AgO are observed. The electronic ground state of AgO− is found to have a vibrational frequency of 497 (20) cm−1 and an equilibrium bond length of 1.935 (15) Å. The electron affinity of AgO is found to be 1.654 (2) eV.

Low lying electronic states of rare gas–oxide anions: Photoelectron spectroscopy of complexes of O− with Ar, Kr, Xe, and N2

The negative ion photoelectron spectra of the oxide anion complexes O−Rg, Rg=Ar, Kr, and Xe, and O−N2 have been recorded. In each spectrum, two partially resolved peaks were observed, their relative intensities varying with source conditions. These peaks were assigned to photodetachment transitions from the 2Σ ground state and unresolved 2Π3/2,1/2 low-lying excited states of the anion. From our data we find dissociation energies and bond lengths for the 2Σ and 2Π anion states. Periodic trends in the bond length and dissociation energy are examined and compared to those in the isoelectronic neutral halogen rare gas systems and the effect of anisotropy in the interatomic potential and relative interaction strength is examined. From our data we find that the dissociation energies in the anion system are much larger but that the 2Σ-2Π splitting is significantly lower. In addition to the diatomic clusters, we report the photoelectron spectra of the O−Krn=2–5 and O−Xen=2–3 clusters and tabulate the vertical detachment energies and peak widths. From a comparison of the energetics and peak broadening we are able to make a determination of the general structure of the n=2 and n=3 clusters.

Anion solvation at the microscopic level: Photoelectron spectroscopy of the solvated anion clusters, NO−(Y)n, where Y=Ar, Kr, Xe, N2O, H2S, NH3, H2O, and C2H4(OH)2

The negative ion photoelectron spectra of the gas-phase, ion-neutral complexes; NO−(Ar)n=1–14, NO−(Kr)1, NO−(Xe)n=1–4, NO−(N2O)n=3–5, NO−(H2S)1, NO−(NH3)1, and NO−(EG)1 [EG=ethylene glycol] are reported herein, building on our previous photoelectron studies of NO−(N2O)1,2 and NO−(H2O)1,2. Anion solvation energetic and structural implications are explored as a function of cluster size in several of these and as a result of varying the nature of the solvent in others. Analysis of these spectra yields adiabatic electron affinities, total stabilization (solvation) energies, and stepwise stabilization (solvation) energies for each of the species studied. An examination of NO−(Ar)n=1–14 energetics as a function of cluster size reveals that its first solvation shell closes at n=12, with an icosahedral structure there strongly implied. This result is analogous to that previously found in our study of O−(Ar)n. Inspection of stepwise stabilization energy size dependencies, however, suggests drastically different structures for NO−(Ar)2 and O−(Ar)2, the former being “Y” shaped, and the latter being linear. While stepwise stabilization energies usually provide good estimates of ion–single solvent dissociation energies, in the cases of NO−(Ar)1, NO−(Kr)1, and NO−(Xe)1, it is possible to determine more precise values. A plot of these anion–solvent dissociation energies shows them to vary linearly with rare gas atom polarizability, confirming the dominance of an ion-induced dipole interaction in these complexes. Extrapolation of this trend permits the estimation of NO−⋯ (rare gas atom) interaction energies for helium, neon, and radon, as well. The relative strengths of the molecular solvents, N2O, H2S, NH3, H2O, and EG are reflected in their stepwise stabilization energies and in the degree of broadening observed in their photoelectron spectra.

Ammonia cluster anions and their relationship to ammoniated (solvated) electrons: The photoelectron spectra of (NH3)n=41–1100−

We report the negative ion photoelectron spectra of (NH3)n=41–1100−, recorded using 2.540 eV photons. The largest cluster anion in this series has a diameter of approximately 4.3 nm. The vertical detachment energies (VDEs) of these cluster anions increase smoothly from 0.55 eV for n=41 to 1.05 eV for n=1100. The VDEs throughout this size range are linear with n−1/3 and extrapolate to a VDE (n=∞) value, which is very close to the measured photoelectric threshold energy of condensed phase ammoniated electrons. The linear extrapolation of this data to an analogous condensed phase property implies that these cluster anions are gas-phase counterparts to ammoniated electrons, i.e., they are embryonic forms of ammoniated electrons which will mature with increasing cluster size to become condensed phase-solvated electrons. The VDE data further implies that these embryonic ammoniated electrons were generated in solid ammonia environments, consistent with the source conditions under which they were produced.

Negative ion photoelectron spectroscopy of the group 5 metal trimer monoxides V3O, Nb3O, and Ta3O

The vibrationally resolved 488 nm negative ion photoelectron spectra of V3O, Nb3O, and Ta3O provide measurements of their electron affinities, vibrational frequencies, and low-lying electronic state energies. The electron affinities increase smoothly down the triad with values (eV) of 1.218±0.008 for V3O, 1.393±0.006 for Nb3O, and 1.583±0.010 for Ta3O. The metal–oxygen symmetric stretching fundamental frequencies are 750±20, 710±15, and 710±15 cm−1, respectively, for the neutral clusters and 770±20 cm−1 for V3O−. Lower frequency symmetric modes are also active, with frequencies (±15 cm−1) of 415 and 340 cm−1 for V3O, 320 cm−1 for Nb3O, 225 cm−1 for Ta3O, and of 355, 300, and 215 cm−1 (±20 cm−1), respectively, for their anions. Weaker transitions also showing vibrational structure are observed to excited states at 3350±30 cm−1 for V3O, 4300±100 and 6190±120 cm−1 for Nb3O, and 3180±40 and 3870±50 cm−1 for Ta3O. The spectra indicate that the extra electrons in the anions occupy essentially nonbonding orbitals, and that the neutral and anionic clusters have planar structures with doubly bridging oxygen atoms. Results are discussed in light of previous studies of Nb3O and the Group 5 M3 and MO molecules, and tentative assignments for the observed M3O and M3O− states are proposed.

Resonance capture of electrons by substituted pyrazoline molecules

The negative ion mass spectra and photoelectron spectra of substituted pyrazolines were studied. A correlation between the ionization energy of the highest occupied molecular orbital and the yield of [MeNHNH2−] ions was found. Isomerization of molecular negative ions was studied by resonance electron capture mass spectrometry.

CASSCF and CASPT2 Ab Initio Electronic Structure Calculations Find Singlet Methylnitrene Is an Energy Minimum

(12/11)CASSCF and (12/11)CASPT2 ab initio electronic structure calculations with both the cc-pVDZ and cc-pVTZ basis sets find that there is a barrier to the very exothermic hydrogen shift that converts singlet methylnitrene, CH3N, to methyleneimine, H2CNH. These two energy minima are connected by a transition structure of Cs symmetry, which is computed to lie 3.8 kcal/mol above the reactant at the (12/11)CASPT2/cc-pVTZ//(12/11)CASSCF/cc-pVTZ level of theory. The (12/11)CASSCF/cc-pVTZ value for the lowest frequency vibration in the transition structure is 854 cm-1, and CASPT2 calculations concur that this a‘ ‘ vibration does indeed have a positive force constant. Thus, there is no evidence that this geometry is actually a mountain top, rather than a transition structure, on the global potential energy surface or that a C1 pathway of lower energy connects the reactant to the product. Therefore, our computational results indicate that the bands seen for singlet methylnitrene in the negative ion photoelectron spectrum of CH3N- are due to singlet methylnitrene being an energy minimum, rather than a transition state. Our results also lead us to predict that, at least in principle, singlet methylnitrene should be an observable intermediate in the formation of methyleneimine.

A vibrationally resolved negative ion photoelectron spectrum of Nb 8

The 488 nm negative ion photoelectron spectrum of Nb 8, obtained at an instrumental resolution of ∼5 meV, displays a clear vibrational progression in a mode with a frequency of 180 ± 15 cm −1 in neutral Nb 8 and 165 ± 20 cm −1 in Nb 8 −. The normal mode displacement of 0.4 ± 0.1 u 1/2 Å indicates a very small geometry change upon electron detachment. The electron affinity of Nb 8 is 1.513 ± 0.008 eV. These results are discussed in light of previous negative ion photoelectron spectroscopic and density functional studies of Nb 8 and Nb 8 −.

Anion photoelectron spectroscopy of B3N−

Vibrationally resolved negative ion photoelectron spectra of B3N− at 355 and 266 nm are presented. Two intense bands are observed with adiabatic detachment energies (ADEs) of 2.923±0.008 eV and 3.063±0.008 eV and markedly different photoelectron angular distributions. Aided by electronic structure calculations, the two bands are assigned to transitions from the linear X̃ 4Σ− state of B3N− to the linear Π3 and Σ−5 electronically excited states of neutral B3N, with all three states having a B–N–B–B structure. Weak signal observed at lower electron binding energies is tentatively assigned to two additional linear-to-linear transitions from the low-lying electronically excited Π2 state of B3N− to the Σ+1 and Π3 states of neutral B3N. Based on these assignments the electron affinity of linear B–N–B–B is 2.098±0.035 eV. It remains uncertain if the Σ+1 state of linear B–N–B–B or the A13 state of cyclic B3N is the overall ground state. At the highest level of theory used here, CCSD(T)/aug-cc-pVTZ, the Σ+1 state is predicted to lie 0.09 eV below the A13 state.

Singlet−Triplet Splittings in CX2 (X = F, Cl, Br, I) Dihalocarbenes via Negative Ion Photoelectron Spectroscopy

The 364 nm negative ion photoelectron spectra of CF2-, CCl2-, CBr2-, and CI2- exhibit transitions to two different electronic states, the 1A1 and 3B1. The CF2- spectrum exhibits well-resolved transitions to both electronic states. In the cases of CCl2-, CBr2-, and CI2-, the spectra exhibit extended, partially resolved vibrational progressions and the two states are overlapped, making a direct determination of the origin transition energy not possible. The overlapped spectra show that the singlet−triplet splittings in the heavier halocarbenes are much smaller than for CF2-. The results of ab initio calculations have been used to generate Franck−Condon simulations of the spectra, which aid in the determination of the band origins. The 1A1 state is found to be the lower state for CF2, CCl2, and CBr2 and the electron affinities have been determined to be 0.180 ± 0.020, 1.59 ± 0.07, and 1.88 ± 0.07 eV, respectively. For CI2, the triplet state is apparently the lower lying state with an electron affinity of 2.0...

Photoelectron spectroscopy of the CH3N− ion

We have observed the negative ion photoelectron spectrum of the methylnitrene ion, CH3N−, and measured the electron affinity of methylnitrene, EA(CH3N)=0.022±0.009 eV. In addition to detaching the methylnitrene anion to the ground state of CH3N(X̃ 3A2), we also detect the first electronically excited state of methylnitrene, ã 1E. We measure the singlet/triplet splitting to be ΔE(ã 1E−X̃ 3A2)=1.352±0.011 eV. The photoelectron spectrum of CH3N ã 1E contains relatively sharp vibronic structure. Unlike the spectra from H2CC−, the photoelectron spectra for CH3N− show no evidence for a barrier separating the rearrangement of singlet methylnitrene to methyleneimine, [CH31N] → CH2=NH.

An Experimental Study of the Low-Lying Electronic States of WO2

The 364 nm negative ion photoelectron spectrum of WO2- is obtained. The spectrum reveals that four different electronic states are accessed in the neutral molecule following electron detachment. From the rich vibrational structure the vibrational frequencies of the bending and/or symmetric stretching vibrations are measured. The electron affinity is determined to be 1.998(10) − 0.040k eV, where k = 0, 1, or 2. Theoretical calculations have been carried out to determine the geometries, energies, and frequencies of the anion and neutral molecules. These are utilized to generate a simulation of the experimental spectrum. The excellent agreement between the spectrum and the simulation indicates that the changes in geometries upon detachment are well predicted by the theoretical method employed, at least for the two lowest electronic states. In addition, polarization studies enable us to determine the asymmetry parameters for each state and thus confirm the assignments of the electronic states.

Impact of nonadiabatic coupling between the conically intersecting X̃ 2A1 and à 2B2 states of NO2 on the negative ion photoelectron spectra of NO2−

Negative ion photoelectron spectra of NO2− at 266 nm (4.660 eV) and 351 nm (3.532 eV) are theoretically calculated and compared with earlier experimental findings. A diabatic model Hamiltonian with linear vibronic coupling between the conically intersecting X̃ 2A1 and à 2B2 states of NO2 is employed for this purpose. While the linear vibronic coupling model successfully reproduces the low resolution experimental photoelectron spectrum at 266 nm, second order coupling terms are necessary for the high resolution spectrum at 351 nm. The vibronic structure of the X̃ 2A1 state of NO2 in the photoelectron spectrum is found to be almost unaffected by the nonadiabatic coupling, whereas, the latter has a strong impact on the vibronic structure of the à 2 B2 state. The magnitude of the vibronic coupling constant, which has otherwise been overestimated by earlier theoretical studies, is calculated ab initio and its limits of validity are confirmed by the results from different ab initio calculations.

Photoelectron spectroscopy of As−, As2−, As3−, As4−, and As5−

The negative ion photoelectron spectra of As−, As2−, As3−, As4−, and As5− have been measured. From these, the electron affinities of As, As2, As3, As4, and As5 have been determined to be 0.814, 0.739, 1.45, <0.8, and ∼1.7 eV, respectively. In the case of As2−, the following molecular constants were also determined: re(As2−)=2.239 Å, ωe(As2−)=293 cm−1, ωeχe(As2−)=4.9 cm−1, D0(As2−)=3.89 eV, and ΔE[2Πg(3/2)−2Πg(1/2)]=0.256 eV. In the case of As3−, vertical detachment energy (VDE) was measured to be 1.62 eV, and for As3, ΔE(2A2−2B1) was determined to be 0.36 eV. For As4−, VDE was found to be 1.52 eV. The relatively high stability of As5− suggests that it, like P5−, may be a candidate for forming cluster-assembled, ionic crystals of stoichiometry, MAs5, where M is an alkali metal atom. Similiarities with other small cluster anions of Group V elements are also discussed.

Photoelectron spectroscopy, gas phase acidity, and thermochemistry of tert-butyl hydroperoxide: Mechanisms for the rearrangement of peroxyl radicals

The 3.531 eV negative ion photoelectron spectra of the hydroperoxide ion and the tert-butylperoxide ion have been studied. We find HO2−+ℏω351.1 nm→HO2+e− EA[HO2,X̃ 2A″]=1.089±0.006 eV, (CH3)3COO−+ℏω351.1 nm→(CH3)3COO+e− EA[(CH3)3COO,X̃ 2A″]=1.196±0.011 eV. The photoelectron spectra show detachment to the ground state of the peroxyl radicals and to a low lying electronic state. The intercombination gaps are measured to be ΔE(X̃ 2A″–Ã 2A′)[HO2]=0.871±0.007 eV and ΔE(X̃ 2A″–2A′)[(CH3)3COO]=0.967±0.011 eV. The gas phase acidity of (CH3)3COOH was measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to be ΔacidG298=363.2±2.0 kcal mol−1 and we find ΔacidH298[(CH3)3COO–H]=370.9±2.0 kcal mol−1. Use of ΔacidH298[(CH3)3COO–H] and EA[(CH3)3COO] leads to the bond energies DH298[(CH3)3COO–H]=85±2 kcal mol−1 and D0[(CH3)3COO–H]=83±2 kcal mol−1. The thermochemistry of the alkylperoxyl radicals, RO2, is reviewed. A mechanism for the rearrangement of chemically activated peroxyl radicals is proposed [RO2]X̃ 2A″→[RO2]*Ã 2A′→aldehydes/ketones+HO(2Π), [RO2]X̃ 2A″→[RO2]*Ã 2A′ →alkenes+HO2(X̃ 2A″).