Solvated Succinate Dianion: Structures, Electron Binding Energies, and Dyson Orbitals

Electron Propagator Theory of Vertical Electron Detachment Energies of Anions: Benchmarks and Applications to Nucleotides

A new generation of ab initio electron-propagator self-energy approximations that are free of adjustable parameters is tested on a benchmark set of 55 vertical electron detachment energies of closed-shell anions. Comparisons with older self-energy approximations indicate that several new methods that make the diagonal self-energy approximation in the canonical Hartree-Fock orbital basis provide superior accuracy and computational efficiency. These methods and their acronyms, mean absolute errors (in eV), and arithmetic bottlenecks expressed in terms of occupied (O) and virtual (V) orbitals are the opposite-spin, non-Dyson, diagonal second-order method (os-nD-D2, 0.2, OV²), the approximately renormalized quasiparticle third-order method (Q3+, 0.15, O²V³) and the approximately renormalized, non-Dyson, linear, third-order method (nD-L3+, 0.1, OV⁴). The Brueckner doubles with triple field operators (BD-T1) nondiagonal electron-propagator method provides such close agreement with coupled-cluster single, double, and perturbative triple replacement total energy differences that it may be used as an alternative means of obtaining standard data. The new methods with diagonal self-energy matrices are the foundation of a composite procedure for estimating basis-set effects. This model produces accurate predictions and clear interpretations based on Dyson orbitals for the photoelectron spectra of the nucleotides found in DNA.

Spectroscopic and theoretical investigations of adenosine 5'-diphosphate and adenosine 5'-triphosphate dianions in the gas phase

Doubly deprotonated adenosine 5'-diphosphate ([ADP-2H](2-)) and adenosine 5'-triphosphate ([ATP-2H](2-)) dianions were investigated using infrared multiple photon dissociation (IR-MPD) and photoelectron spectroscopy. Vibrational spectra acquired in the X-H stretch region (X = C, N, O) and augmented by isotope-labelling were compared to density functional theory (DFT) calculations at the B3LYP/TZVPP level. This suggests that in [ATP-2H](2-) the two phosphate groups adjacent to the ribose ring are preferentially deprotonated. Photoelectron spectra recorded at 4.66 and 6.42 eV photon energies revealed adiabatic detachment energies of 1.35 eV for [ADP-2H](2-) and 3.35 eV for [ATP-2H](2-). Repulsive Coulomb barriers were estimated at ~2.2 eV for [ADP-2H](2-) and ~1.9 eV for [ATP-2H](2-). Time-dependent DFT calculations have been used to simulate the photoelectron spectra. Photodetachment occurs primarily from lone pair orbitals on oxygen atoms within the phosphate chain.

Stabilization of excess charge in isolated adenosine 5'-triphosphate and adenosine 5'-diphosphate multiply and singly charged anions

Multiply charged anions (MCAs) represent highly energetic species in the gas phase but can be stabilized through formation of molecular clusters with solvent molecules or counterions. We explore the intramolecular stabilization of excess negative charge in gas-phase MCAs by probing the intrinsic stability of the [adenosine 5'-triphosphate-2H](2-) ([ATP-2H](2-)), [adenosine 5'-diphosphate-2H](2-) ([ADP-2H](2-)), and H(3)P(3)O(10)(2-) dianions and their protonated monoanionic analogues. The relative activation barriers for decay of the dianions via electron detachment or ionic fragmentation are investigated using resonance excitation of ions isolated within a quadrupole trap. All of the dianions decayed via ionic fragmentation demonstrating that the repulsive Coulomb barriers (RCB) for ionic fragmentation lie below the RCBs for electron detachment. Both the electrospray ionization mass spectra (ESI-MS) and total fragmentation energies for [ATP-2H](2-), [ADP-2H](2-), and H(3)P(3)O(10)(2-) indicate that the multiply charged H(3)P(3)O(10)(2-) phosphate moiety is stabilized by the presence of the adenosine group and the stability of the dianions increases in the order H(3)P(3)O(10)(2-) < [ADP-2H](2-) < [ATP-2H](2-). Fully optimized, B3LYP/6-31+G* minimum energy structures illustrate that the excess charges in all of the phosphate anions are stabilized by intramolecular hydrogen bonding either within the phosphate chain or between the phosphate and the adenosine. We develop a model to illustrate that the relative magnitudes of the RCBs and hence the stability of these ions is dominated by the extent of intramolecular hydrogen bonding.

Probing the intrinsic features and environmental stabilization of multiply charged anions

Multiply charged anions (MCAs) represent exotic, highly energetic species in the gas-phase due to their propensity to undergo unimolecular decay via electron loss or ionic fragmentation. There is considerable fundamental interest in these systems since they display novel potential energy surfaces that are characterized by Coulomb barriers. Over recent years, considerable progress has been made in understanding the factors that affect the stability, decay pathways and reactivity of gas-phase MCAs, mainly as a result of the application of electrospray ionization as a generic technique for transferring solution-phase MCAs into the gas-phase for detailed characterization. We review contemporary work in this field, focusing on the factors that control the intrinsic stability of MCAs, both as isolated gas-phase ions, and on their complexation with solvent molecules and counter-ions. While studies of MCAs are primarily of fundamental interest, several classes of important biological ions are commonly observed as MCAs in the gas-phase (e.g. oligonucleotides, sugars). Recent results for biologically relevant ions are emphasised, since a fundamental understanding of the properties of gas-phase MCAs will be highly valuable for developing further analytical methods to study these important systems.

First Principles Study on the Solvation and Structure of C2O42-(H2O)n, n = 6−12

The structures and energies of hydrated oxalate clusters, C2O4(2-)(H2O)n, n = 6-12, are obtained by density functional theory (DFT) calculations and compared to SO4(2-)(H2O)n. Although the evolution of the cluster structure with size is similar to that of SO4(2-)(H2O)n, there are a number of important and distinctive futures in C2O4(2-)(H2O)n, including the separation of the two charges due to the C-C bond in C2O4(2-), the lower symmetry around C2O4(2-), and the torsion along the C-C bond, that affect both the structure and the solvation energy. The solvation dynamics for the isomers of C2O4(2-)(H2O)12 are also examined by DFT based ab initio molecular dynamics.

On the Stability of IrCl63- and Other Triply Charged Anions: Solvent Stabilization versus Ionic Fragmentation and Electron Detachment for the IrCl63-·(H2O)n n = 0−10 Microsolvated Clusters

The intrinsic gas-phase stability of the IrCl(6)(3-) trianion and its microsolvated clusters, IrCl(6)(3-).(H(2)O)(n) n = 1-10, have been investigated using density functional theory (DFT) calculations. Although IrCl(6)(3-) is known to exist as a stable complex ion in bulk solutions, our calculations indicate that the bare trianion is metastable with respect to decay via both electron detachment and ionic fragmentation. To estimate the lifetime of IrCl(6)(3-), we have computed the electron tunneling probability using an adaption of the Wentzel-Kramer-Brillouin theory and predict that the trianion will decay spontaneously via electron tunneling on a time scale of 2.4 x 10(-13) s. The global minimum structure for IrCl(6)(3-).H(2)O was found to contain a bifurcated hydrogen bond, whereas for IrCl(6)(3-).(H(2)O)(2), two low energy minima were identified; one involving two bifurcated water-ion hydrogen bonds and a second combining a bifurcated hydrogen bond with a water-water hydrogen bond. Clusters based on each of these structural motifs were obtained for all of the n = 3-10 systems, and the effect of solvation on the possible decay pathways was explored. The calculations reveal that solvation stabilizes IrCl(6)(3-) with respect to both electron detachment decay and ionic fragmentation, with the magnitude of the repulsive Coulomb barrier for ionic fragmentation increasing smoothly with sequential solvation. This study is the first to compare the propensity for electron detachment versus ionic fragmentation decay for a sequentially solvated triply charged anion.