Ab initioand diatomics in molecule potentials for I2−, I2, I3−, and I3

The effective relativistic coupling by asymptotic representation approach for molecules with multiple relativistic atoms

The Effective Relativistic Coupling by Asymptotic Representation (ERCAR) approach is a method to generate fully coupled diabatic potential energy surfaces (PESs) including relativistic effects, especially spin-orbit coupling. The spin-orbit coupling of a full molecule is determined only by the atomic states of selected relativistically treated atoms. The full molecular coupling effect is obtained by a diabatization with respect to asymptotic states, resulting in the correct geometry dependence of the spin-orbit effect. The ERCAR approach has been developed over the last decade and initially only for molecules with a single relativistic atom. This work presents its extension to molecules with more than a single relativistic atom using the iodine molecule as a proof-of-principle example. The theory for the general multiple atomic ERCAR approach is given. In this case, the diabatic basis is defined at the asymptote where all relativistic atoms are separated from the remaining molecular fragment. The effective spin-orbit operator is then a sum of spin-orbit operators acting on isolated relativistic atoms. PESs for the iodine molecule are developed within the new approach and it is shown that the resulting fine structure states are in good agreement with spin-orbit ab initio calculations.

Valence and Core Photoelectron Spectra of Aqueous I3- from Multi-Reference Quantum Chemistry

The I3- molecule is known to undergo substantial structural reorganization upon solvation by a protic solvent, e.g., water. However, the details of this process are still controversially discussed in the literature. In the present study, we combined experimental and theoretical efforts to disentangle this controversy. The valence (5p), N4,5 (4d), and M4,5 (3d) edge photoelectron spectra were measured in an aqueous solution and computed using high-level multi-reference methods. Our previous publication mainly focused on obtaining reliable experimental evidence, whereas in the present article, we focused primarily on theoretical aspects. The complex electronic structure of I3- requires the inclusion of both static and dynamic correlation, e.g., via the multi-configurational perturbation theory treatment. However, the resulting photoelectron spectra appear to be very sensitive to problems with variational stability and intruder states. We attempted to obtain artifact-free spectra, allowing for a more reliable interpretation of experiments. Finally, we concluded that the 3d Photoelectron Spectrum (PES) is particularly informative, evidencing an almost linear structure with a smaller degree of bond asymmetry than previously reported.

Relativistic four-component MRCISD+Q calculations of the six lowest valence states of molecular [Formula: see text] anion including breit interactions

CONTEXT AND RESULTS

This study aimed to obtain potential energy curves within a multireference 4-component relativistic method and to present spectroscopic constants (R[Formula: see text],[Formula: see text],[Formula: see text]x[Formula: see text],[Formula: see text]y[Formula: see text], D[Formula: see text], D[Formula: see text], B[Formula: see text],[Formula: see text],[Formula: see text],[Formula: see text] ), accurate extended Rydberg analytical form, and rovibrational levels for the 6 low-lying states of the I[Formula: see text] anion. For these states, some spectroscopic constants, rovibrational levels, and an accurate analytical form are presented for the first time in literature, and they are of interest for femtosecond and dynamics experiments of I[Formula: see text] as well as for electron attachment of I[Formula: see text]. This study suggests that the inclusion of relativistic and correlation effects treated at the MRCISD+Q level is needed to obtain reliable results, specially for D[Formula: see text].

COMPUTATIONAL AND THEORETICAL TECHNIQUES

The potential energy curves of the ground and the excited states of the molecular iodine anion (I[Formula: see text]) were investigated at multireference configuration interaction (MRCISD) with Davidson size-extensivity correction (denoted as +Q) within a fully relativistic four-component relativistic framework including Breit interaction.

Determining the charge distribution and the direction of bond cleavage with femtosecond anisotropic x-ray liquidography

Energy, structure, and charge are fundamental quantities characterizing a molecule. Whereas the energy flow and structure change in chemical reactions are experimentally characterized, determining the atomic charges of a molecule in solution has been elusive, even for a triatomic molecule such as triiodide ion, I3-. Moreover, it remains to be answered how the charge distribution is coupled to the molecular geometry; which I-I bond, if two I-I bonds are unequal, dissociates depending on the electronic state. Here, femtosecond anisotropic x-ray solution scattering allows us to provide the following answers in addition to the overall rich structural dynamics. The analysis unravels that the negative charge of I3- is highly localized on the terminal iodine atom forming the longer bond with the central iodine atom, and the shorter I-I bond dissociates in the excited state, whereas the longer one in the ground state. We anticipate that this work may open a new avenue for studying the atomic charge distribution of molecules in solution and taking advantage of orientational information in anisotropic scattering data for solution-phase structural dynamics.

Reaction dynamics studied via femtosecond X-ray liquidography at X-ray free-electron lasers

X-ray free-electron lasers (XFELs) provide femtosecond X-ray pulses suitable for pump–probe time-resolved studies with a femtosecond time resolution. Since the advent of the first XFEL in 2009, recent years have witnessed a great number of applications with various pump–probe techniques at XFELs. Among these, time-resolved X-ray liquidography (TRXL) is a powerful method for visualizing structural dynamics in the liquid solution phase. Here, we classify various chemical and biological molecular systems studied via femtosecond TRXL (fs-TRXL) at XFELs, depending on the focus of the studied process, into (i) bond cleavage and formation, (ii) charge distribution and electron transfer, (iii) orientational dynamics, (iv) solvation dynamics, (v) coherent nuclear wavepacket dynamics, and (vi) protein structural dynamics, and provide a brief review on each category. We also lay out a plausible roadmap for future fs-TRXL studies for areas that have not been explored yet.

Femtosecond X-ray liquidography using X-ray free-electron lasers (XFELs) visualizes various aspects of reaction dynamics.

Accurate Thermochemistry for Main-Group Elements up to Xenon with the Wn-P34 Series of Composite Methods

In the present study, we introduce the accurate Wn-P34 quantum chemistry composite methods with applicability to heavy p-block elements up to xenon. For a set of thermochemical properties for prototypical third- and fourth-row species and for a diverse set of small light-main-group species, they show accuracies of ∼3 kJ mol^-1 or better. Overall, the Wn-P34 methods are comparable in accuracy to Wn, with a widened applicability to heavier elements. We have used Wn-P34 to compile the P34 set of accurate thermochemical values for heavy p-block species, and we have applied this set to assess a wide range of lower-cost methods. The results of our assessment show that the G4(MP2)-XK composite method provides adequate treatments for these species, but several widely used double-hybrid density functional theory (DH-DFT) methods show uncharacteristically large deviations. In contrast, we find it presently surprising that some pure and hybrid DFT methods such as TPSS and SCANh perform quite well. We hope that our findings and new tools would facilitate the application of computational chemistry for heavy elements, of which the properties are yet to be broadly explored.

Photodissociation of aqueous I 3 - observed with liquid-phase ultrafast mega-electron-volt electron diffraction

Developing femtosecond resolution methods for directly observing structural dynamics is critical to understanding complex photochemical reaction mechanisms in solution. We have used two recent developments, ultrafast mega-electron-volt electron sources and vacuum compatible sub-micron thick liquid sheet jets, to enable liquid-phase ultrafast electron diffraction (LUED). We have demonstrated the viability of LUED by investigating the photodissociation of tri-iodide initiated with a 400 nm laser pulse. This has enabled the average speed of the bond expansion to be measured during the first 750 fs of dissociation and the geminate recombination to be directly captured on the picosecond time scale.

Photoelectron-photofragment coincidence spectroscopy of the mixed trihalides

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to study the photodetachment, photodissociation, and dissociative photodetachment (DPD) of I₂Br^-, IBr₂ ^-, I₂Cl^-, and ICl₂ ^- at 266 nm. The mixed trihalides are asymmetric analogs of the well-studied I₃ ^- anion, with distinguishable dissociation asymptotes and the potential for selective bond breaking. The high beam energy PPC spectrometer used in this study couples an electrospray ionization source, a hexapole accumulation ion trap, and a linear accelerator to produce a 21 keV beam of a particular trihalide. Total, stable, and dissociative photoelectron spectra have been recorded for all the anions, except ICl₂ ^- that does not photodetach at 266 nm. A bound ground state (X) is observed for all the anions, and a dissociative first excited (A) state is also seen for I₂Br^- and I₂Cl^- at low electron kinetic energies (eKE). A 258 nm photoelectron spectrum recorded for I₂Br^- and I₂Cl^- rules out autodetachment of a dipole-bound state as the origin of the low eKE feature. The threshold detachment energy (TDE) of I₂X^- to the X state of the radical is similar to I₃ ^-, whereas the TDE to the radical A state increases with substitution of iodine for a lighter halogen. Two-body DPD is observed for I₂Br^- and I₂Cl^-, resulting in IBr/ICl + I + e^-. For IBr₂ ^- and ICl₂ ^-, the charge symmetric three-body photodissociation of [Br-I-Br]^- and [Cl-I-Cl]^- is seen yielding Br + Br and Br + Br^*, and Cl + Cl and Cl + Cl^* neutral fragments. Evidence for the minimum energy anion structure is observed in all cases, where the iodine atom is located at the center of the trihalide.

Photoelectron-photofragment coincidence studies of I3- using an electrospray ionization source and a linear accelerator

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to examine the dissociative photodetachment (DPD) of I3-. The high beam energy PPC spectrometer for complex anions couples an electrospray ionization source, a hexapole accumulation ion trap and a linear accelerator to produce fast beams of I3- (M = 381 amu) anions, the heaviest system studied to date. Following photodetachment, the photoelectron and up to three photofragments are recorded in coincidence yielding a kinematically complete picture of the DPD dynamics at beam energies of 11 keV and 21 keV. Photodetachment leads to the production of stable I3, two-body DPD, as well as evidence for two- and three-body photodissociation. DPD is found to occur predominantly via the first excited A state, with some contributions from highly excited vibrational levels in the neutral ground state. With the ions thermalized to 298 K in the hexapole trap, there are significant contributions from vibrational hot bands. Three-body photodissociation at 4.66 eV is found to occur preferentially via a charge-symmetric process to form I + I- + I. In the future this method will be applied to other polyatomic systems with a large molecular mass, including multiply charged anions and complex clusters, in concert with a cryogenically cooled hexapole trap to reduce thermal effects.

Theoretical study of polyiodide formation and stability on monolayer and bilayer graphene

Insights of DFT calculations on the formation of polyiodide complexes and their thermal stability on graphene based nanostructures.

Photodissociation of gas-phase I3−: Comprehensive understanding of nonadiabatic dissociation dynamics

Photodissociation of the gas-phase tri-iodide anion, I3-, was investigated using photofragment time of flight (TOF) mass spectrometry combined with the core extraction method. An analysis of the TOF profiles provided the kinetic energy and angular distributions of photofragment ions and photoneutrals, from which the photoproduct branching fractions were determined in the excitation energy range of 3.26-4.27 eV. The measurement has revealed that (1) in the entire energy range investigated, three-body dissociation occurs preferentially as the "charge-asymmetric" process I-(1S)+I(2P3/2)+I(2P3/2) with the yield of approximately 30%-40%, where the excess charge is localized on the end atoms of the dissociating I3-, and that (2) two-body dissociation via the 3Piu(0u+)<--1Sigmag+(0g+) excitation proceeds as I-(1S)+I2(X 1Sigmag+)/I2(A 3Pi1u) or I(2P3/2)+I2-(X 2Sigmau+) with the yield of approximately 60%, while that via the 1Sigmau+(0u+)<--1Sigmag+(0g+) excitation alternatively as I*(2P1/2)+I2-(X 2Sigmau+) or I-(1S)+I2(B 3Piu) with the yield of approximately 60%. Ab initio calculations including spin-orbit configuration interactions were also performed to gain precise information on the potential energy surfaces relevant to the I3- photodissociation. The calculations have shown the presence of conical intersections and avoided crossings located along the symmetric stretch coordinate near the ground-state equilibrium geometry of I3-, which play key roles for the two-body and the three-body product branching. The nonadiabatic nature of the I3- photodissociation dynamics is discussed by combining the experimental findings and the ab initio results.

Photoelectron imaging of I2− at 5.826eV

We report the anion photoelectron spectrum of I2- taken at 5.826 eV detachment energy using velocity mapped imaging. The photoelectron spectrum exhibits bands resulting from transitions to the bound regions of the X 1Sigmag+(0g+), A' 3Piu(2u), A 3Piu(1u), and B 3Piu(0u+) electronic states as well as bands resulting from transitions to the repulsive regions of several I2 electronic states: the B' 3Piu(0u-), B" 1Piu(1u), 3Pig(2g), a 3Pig(1g), 3Pig(0g-), and C 3Sigmau+(1u) states. We simulate the photoelectron spectrum using literature parameters for the I2- and I2 ground and excited states. The photoelectron spectrum includes bands resulting from transitions to several high-lying excited states of I2 that have not been seen experimentally: 3Pig(0g-), 1Pig3(1g), 1 3Sigmag-3(0g+), and the 1Sigmag-3(0u-) states of I2. Finally, the photoelectron spectrum at 5.826 eV allows for the correction of a previous misassignment for the vertical detachment energy of the I2 B 3Piu(0u+) state.

Molecular and Electronic Structures by Design: Tuning Symmetrical and Unsymmetrical Linear Trichromium Chains

The preparation, properties, and crystal structures of 12 trichromium extended metal atom chain (EMAC) compounds of the type Cr(3)(L)(4)X(2) (L = equatorial ligands dipyridylamide (dpa) or di-4,4'-ethyl-2,2'-pyridylamide (depa), and X = axial ligands, e.g., halide or pseudohalide ions) with large variations in metal-metal distances are reported here. These complexes, which belong to a broad class of fundamentally interesting trinuclear molecules over which the electrons may or may not be delocalized, pose significant theoretical and experimental challenges which are dealt with in this report. Complexes with strongly donating axial or equatorial ligands tend to favor a symmetrical (D(4)) molecular structure, while more weakly donating ligands give rise to unsymmetrical (C(4)) structures; the physical properties of these two classes of compounds are discussed fully, and important comparisons with a reported DFT model of the electronic structures of the compounds are made.

Two- and three-body photodissociation of gas phase I3−

The photodissociation dynamics of I3- from 390 to 290 nm (3.18 to 4.28 eV) have been investigated using fast beam photofragment translational spectroscopy in which the products are detected and analyzed with coincidence imaging. At photon energies < or = 3.87 eV, two-body dissociation that generates I- + I2(A 3Pi1) and vibrationally excited I2- (X 2Sigmau+) + I(2P(3/2)) is observed, while at energies > or = 3.87 eV, I*(2P(1/2)) + I2- (X 2Sigmau+) is the primary two-body dissociation channel. In addition, three-body dissociation yielding I- +2I(2P(3/2)) photofragments is seen throughout the energy range probed; this is the dominant channel at all but the lowest photon energy. Analysis of the three-body dissociation events indicates that this channel results primarily from a synchronous concerted decay mechanism.