Molecular structure and vibrational frequencies of AlxOy (x=1,2; y⩽3) derived from ab initio calculations

Ro-vibrational Spectrum of Linear Dialuminum Monoxide (Al2O) at 10 μm

Dialuminum monoxide, Al₂O, has been investigated in the laboratory at mid-IR wavelengths around 10 μm at high spectral resolution. The molecule was produced by laser ablation of an aluminum target with the addition of gaseous nitrous oxide, N₂O. Subsequent adiabatic cooling of the gas in a supersonic beam expansion led to rotationally cold spectra. In total, 848 ro-vibrational transitions have been assigned to the fundamental asymmetric stretching mode ν₃ and to five of its hot bands, originating from excited levels of the ν₁ symmetric stretching mode and the ν₂ bending mode. The measurements encompass 11 vibrational energy states (v₁ v₂^l v₃). The ro-vibrational transitions show spin statistical line intensity alternation of 7:5, which is caused by two identical aluminum nuclei of spin I = ⁵/₂ at both ends of the centrosymmetric molecule of structure Al-O-Al. The less effective cooling of vibrational states in the supersonic beam expansion allowed measurement of transitions in excited vibrational states at energies of 1000 cm^-1 and higher, while rotational levels within vibrational modes exhibited thermal population, with rotational temperatures around T_rot = 115 K. Molecular parameters for 11 vibrational states were derived, including rotation and centrifugal distortion constants and l-type doubling constants for the states (v₁ v₂^l v₃) = (0 1¹ 0) and (0 1¹ 1) and an l-type resonance between the states (0 2⁰ 0) - (0 2² 0) and (0 2⁰ 1) - (0 2² 1). From the experimental results, rotational correction terms and the equilibrium bond length r_e were derived. The measurements were supported and guided by high-level quantum-chemical calculations that agree well with the derived experimental results.

Time-resolved resonance fluorescence spectroscopy for study of chemical reactions in laser-induced plasmas

Identification of chemical intermediates and study of chemical reaction pathways and mechanisms in laser-induced plasmas are important for laser-ablated applications. Laser-induced breakdown spectroscopy (LIBS), as a promising spectroscopic technique, is efficient for elemental analyses but can only provide limited information about chemical products in laser-induced plasmas. In this work, time-resolved resonance fluorescence spectroscopy was studied as a promising tool for the study of chemical reactions in laser-induced plasmas. Resonance fluorescence excitation of diatomic aluminum monoxide (AlO) and triatomic dialuminum monoxide (Al₂O) was used to identify these chemical intermediates. Time-resolved fluorescence spectra of AlO and Al₂O were used to observe the temporal evolution in laser-induced Al plasmas and to study their formation in the Al-O₂ chemistry in air.

Theoretical Study of Alnand AlnO (n= 2−10) Clusters

The stable structures, energies, and electronic properties of neutral, cationic, and anionic clusters of Al(n) (n = 2-10) are studied systematically at the B3LYP/6-311G(2d) level. We find that our optimized structures of Al5(+), Al9(+), Al9(-), Al10, Al10(+), and Al10(-) clusters are more stable than the corresponding ones proposed in previous literature reports. For the studied neutral aluminum clusters, our results show that the stability has an odd/even alternation phenomenon. We also find that the Al3, Al7, Al7(+), and Al7(-) structures are more stable than their neighbors according to their binding energies. For Al7(+) with a special stability, the nucleus-independent chemical shifts and resonance energies are calculated to evaluate its aromaticity. In addition, we present results on hardness, ionization potential, and electron detachment energy. On the basis of the stable structures of the neutral Al(n) (n = 2-10) clusters, the Al(n)O (n = 2-10) clusters are further investigated at the B3LYP/6-311G(2d), and the lowest-energy structures are searched. The structures show that oxygen tends to either be absorbed at the surface of the aluminum clusters or be inserted between Al atoms to form an Al(n-1)OAl motif, of which the Al(n-1) part retains the stable structure of pure aluminum clusters.

The singlet electronic ground state isomers of dialuminum monoxide: AlOAl, AlAlO, and the transition state connecting them

The singlet electronic ground state isomers, X (1)Sigma(g) (+) (AlOAl D(infinityh)) and X (1)Sigma(+) (AlAlO C(infinitynu)), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing self-consistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT-3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (cc-pVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9 kcal mol(-1) below the asymmetric isomer AlAlO at the cc-pV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X (1)Sigma(g) (+) AlOAl molecule, omega(1)=517 cm(-1), omega(2)=95 cm(-1), and omega(3)=1014 cm(-1), are in good agreement with experimental values. The harmonic vibrational frequencies for the X (1)Sigma(+) AlAlO structure, omega(1)=1042 cm(-1), omega(2)=73 cm(-1), and omega(3)=253 cm(-1), presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl-->AlAlO and AlAlO-->AlOAl reactions were predicted to be 84.3 and 2.4 kcal mol(-1), respectively. The dissociation energies D(0) for AlOAl (X (1)Sigma(g) (+)) and AlAlO (X (1)Sigma(+))-->AlO (X (2)Sigma(+))+Al ((2)P) were determined to be 130.8 and 48.9 kcal mol(-1), respectively. Thus, both symmetric AlOAl (X (1)Sigma(g) (+)) and asymmetric AlAlO (X (1)Sigma(+)) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X (2)Sigma(+)) + Al ((2)P) and kinetically stable for the isomerization reaction (AlAlO-->AlOAl) at sufficiently low temperatures.

Addition of water to Al5O4− determined by anion photoelectron spectroscopy and quantum chemical calculations

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.

Reactivity of Al3O3− cluster toward H2O studied by density functional theory

Density functional theory calculations (Becke's three parameter hybrid functional) have been done on a wide range of possible structures for the complexes formed in the reaction between Al(3)O(3) (-) and one or two water molecules. Both energetically competitive structural isomers of Al(3)O(3) (-) (kitelike and distorted rectangle) were considered. The structures of neutral complexes accessed from detachment of the stable anion structures were also optimized. The calculations predict that hydroxide complexes are energetically favored over Lewis acid-base and charge-dipole complexes. For Al(3)O(3) (-)/H(2)O complexes, the kite-based hydroxide and rectangle-based hydroxide are predicted to be nearly isoenergetic, while for Al(3)O(3) (-)/(H(2)O)(2), the rectangle-based dihydroxide emerges as being 0.5 eV more stable than the lowest energy kite-based dihydroxide. The structures of these and their neutrals are used to analyze anion PE spectra of Al(3)O(4)H(2) (-) and Al(3)O(5)H(4) (-) obtained previously [F. A. Akin and C. C. Jarrold, J. Chem. Phys. 118, 5841 (2003)].

Ab initio prediction of the potential energy surface and vibrational-rotational energy levels of dialuminum monoxide, Al[sub 2]O

The equilibrium structure and potential energy surface of dialuminum monoxide, Al(2)O, have been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), in conjunction with basis sets of triple- through quintuple-zeta quality. The effects of core-electron correlation on the calculated molecular parameters were investigated. The vibrational-rotational energy levels of the Al(2) (16)O and Al(2) (18)O isotopic species were calculated by a variational approach. The predicted energy levels are in remarkably good agreement with the available experimental spectroscopic data (from laser-induced fluorescence), demonstrating that the Al(2)O molecule is linear at equilibrium in its ground electronic state. The reported theoretical data settle controversies between the experimental studies about the equilibrium structure and assignment of vibrational fundamentals of the Al(2)O molecule.