Dipole-bound Research Articles

Electron attachment dynamics following UV excitation of iodide-2-thiouracil complexes

The dynamics of low energy electron attachment to the thio-substituted uracil analog 2-thiouracil are investigated using time-resolved photoelectron spectroscopy (TRPES) of iodide-2-thiouracil (I-·2TU) binary clusters. In these experiments, the anions are excited at pump energies of 4.16 and 4.73eV, and the ensuing dynamics are probed by photodetachment at 1.59 and 3.18eV. Upon excitation near the vertical detachment energy (4.16eV), dipole bound (DB) and valence bound (VB) anion signals appear almost instantaneously, and the DB state of the 2TU anion undergoes an ultrafast decay (∼50fs). At 4.73eV, there is no evidence for a DB state, but features attributed to two VB states are seen. The transient negative ions formed by photoexcitation decay by autodetachment and I- fragmentation. The I- dissociation rates and their dependence on excitation energy agree reasonably well with the Rice-Ramsperger-Kassel-Marcus calculations. Notable differences with respect to TRPES of the related iodide-uracil anion are observed and discussed.

Fate of Dipole-Bound Anion States when Hydrated.

Many strongly polar molecules can form an anion by attaching an electron to either an empty or half-filled valence-bound (VB) orbital or a so-called dipole-bound (DB) orbital. These two families of orbitals can be very different in their radial extent (the former are usually more compact, while the latter are quite diffuse) and in the degree to which they are affected by surrounding solvent molecules. In this study, the effects of hydration (representative of strong solvation) on the DB state of a model polar species are investigated with an eye toward determining whether this state is stabilized or even persists when a few to 100 water molecules surround the polar molecule. It is found that in the presence of up to ca. 10-12 water molecules, the excess electron can remain in a DB orbital. However, once there are enough water molecules to form a complete first hydration shell (or more), the excess electron migrates into an orbital localized on the outer surface of the water solvent cage. These findings have implications on the possible role of DB states as doorways to facilitating electron attachment and subsequent electron transfer to VB states. It is shown that even when the electron is bound to the surface of the surrounding solvent, the dipole potential of the solute molecule can influence where on the surface the electron binds. It is also illustrated that using continuum dielectric methods to describe the hydration of DB states is fraught with danger because much of the outermost electron density in such states penetrates outside the boundary of the cavity used in these methods.

"Outlaw" Dipole-Bound Anions of Intra-Molecular Complexes.

Using the example of silatranes XSi(OCH2CH2)3N (X = Me, H, F, Cl), XS, it was found that the effect of the dipole-bound (DB) electron on the cage intramolecular complexes does not fit into the standard views. Upon the transition from XS to the DB anions XS-, the unusual shortening of the internuclear Si···N distance is always observed. For X = Cl, it is equal to 0.15 Å, which is a record length for all DB anions known from the literature. The formation of DB anions with the cage structure has principal features, controlled not only by the "critical" value of the dipole moment (μ > 2.5 D), but also by a geometric factor, such as the degree of pyramidality of the N(CH2)3 moiety-the positive end of the molecular dipole of XS. It was a surprise that the effect of the substituent X on the extent of the structural rearrangement in the process XS → XS- cannot be explained using the values of the electron detachment energy of XS- or the initial strength of the coordination Si ← N bond in XS. The unique sensitivity of the silatrane geometry to the addition of an excess electron is governed by the rate of increase of their dipole moment with the shortening of the dative Si ← N contact. The conclusions drawn are supported by the high-accuracy CCSD and CCSD(T) calculations and the experimental (RET-PES) data. There is no real reason to doubt that the peculiarities of the formation of DB anions of XS- can also be characteristic of many hundreds of their structural analogues XM(YCH2CH2)3N (M = Si, Ge, Sn, Pb, Ti, Al, Cr, Fe, Ni...; Y = O, NR, CH2, S), i.e., substituted 5-azabicyclo[3.3.3]undecans.

Dynamics of electron attachment and photodissociation in iodide-uracil-water clusters via time-resolved photoelectron imaging.

The dynamics of low energy electron attachment to monohydrated uracil are investigated using time-resolved photoelectron imaging to excite and probe iodide-uracil-water (I-·U·H2O) clusters. Upon photoexcitation of I-·U·H2O at 4.38 eV, near the measured cluster vertical detachment energy of 4.40 eV ± 0.05 eV, formation of both the dipole bound (DB) anion and valence bound (VB) anion of I-·U·H2O is observed and characterized using a probe photon energy of 1.58 eV. The measured binding energies for both anions are larger than those of the non-hydrated iodide-uracil (I-·U) counterparts, indicating that the presence of water stabilizes the transient negative ions. The VB anion exhibits a somewhat delayed 400 fs rise when compared to I-·U, suggesting that partial conversion of the DB anion to form the VB anion at early times is promoted by the water molecule. At a higher probe photon energy, 3.14 eV, I- re-formation is measured to be the major photodissociation channel. This product exhibits a bi-exponential rise; it is likely that the fast component arises from DB anion decay by internal conversion to the anion ground state followed by dissociation to I-, and the slow component arises from internal conversion of the VB anion.

Configuration and energy landscape of the benzonitrile anion

Quantum chemical calculations are employed to study the configurational isomers of the anion formed by benzene substituted with a cyano group. It is found that an excess electron can form dipole-bound (DB) states with benzonitrile and phenyl-isocyanide isomers. It can also attach to the cyano group, if this group is separated from the benzene ring by some distance, forming a covalent CN− anion. There are four positions at peripherals of the benzene ring where this anion can localize and form stable complexes with the benzene radical. In these complexes CN− is connected to the benzene radical via non-covalent interactions.

Dynamics of dipole- and valence bound anions in iodide-adenine binary complexes: A time-resolved photoelectron imaging and quantum mechanical investigation.

Dipole bound (DB) and valence bound (VB) anions of binary iodide-adenine complexes have been studied using one-color and time-resolved photoelectron imaging at excitation energies near the vertical detachment energy. The experiments are complemented by quantum chemical calculations. One-color spectra show evidence for two adenine tautomers, the canonical, biologically relevant A9 tautomer and the A3 tautomer. In the UV-pump/IR-probe time-resolved experiments, transient adenine anions can be formed by electron transfer from the iodide. These experiments show signals from both DB and VB states of adenine anions formed on femto- and picosecond time scales, respectively. Analysis of the spectra and comparison with calculations suggest that while both the A9 and A3 tautomers contribute to the DB signal, only the DB state of the A3 tautomer undergoes a transition to the VB anion. The VB anion of A9 is higher in energy than both the DB anion and the neutral, and the VB anion is therefore not accessible through the DB state. Experimental evidence of the metastable A9 VB anion is instead observed as a shape resonance in the one-color photoelectron spectra, as a result of UV absorption by A9 and subsequent electron transfer from iodide into the empty π-orbital. In contrast, the iodide-A3 complex constitutes an excellent example of how DB states can act as doorway state for VB anion formation when the VB state is energetically available.

Time-resolved radiation chemistry: dynamics of electron attachment to uracil following UV excitation of iodide-uracil complexes.

Electron attachment to uracil was investigated by applying time-resolved photoelectron imaging to iodide-uracil (I(-)U) complexes. In these studies, an ultraviolet pump pulse initiated charge transfer from the iodide to the uracil, and the resulting dynamics of the uracil temporary negative ion were probed. Five different excitation energies were used, 4.00 eV, 4.07 eV, 4.14 eV, 4.21 eV, and 4.66 eV. At the four lowest excitation energies, which lie near the vertical detachment energy of the I(-)U complex (4.11 eV), signatures of both the dipole bound (DB) as well as the valence bound (VB) anion of uracil were observed. In contrast, only the VB anion was observed at 4.66 eV, in agreement with previous experiments in this higher energy range. The early-time dynamics of both states were highly excitation energy dependent. The rise time of the DB anion signal was ∼250 fs at 4.00 eV and 4.07 eV, ∼120 fs at 4.14 eV and cross-correlation limited at 4.21 eV. The VB anion rise time also changed with excitation energy, ranging from 200 to 300 fs for excitation energies 4.00-4.21 eV, to a cross-correlation limited time at 4.66 eV. The results suggest that the DB state acts as a "doorway" state to the VB anion at 4.00-4.21 eV, while direct attachment to the VB anion occurs at 4.66 eV.

Trapping excess electrons in charge pockets on molecular surfaces in an argon matrix

AbstractRecently, we have proposed a series of hydrocarbon molecular surfaces (A.F. Jalbout, L. Adamowicz, Mol Phys 2006, 19, 3101), which had a hydrogen bonded network of OH groups on one side of the surface and hydrogen atoms on the opposite side. The addition of these OH groups increases the dipole moment of the system allowing for excess electrons to attach to the surface in dipole‐bound (DB) anion states. We have used this principle to study the interaction of the DB anions formed from the surfaces and an argon atom. The resulting anions are shown to be stable with respect to electron detachment. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Valence- and Dipole-Bound Anions of the Thymine−Water Complex: Ab Initio Characterization of the Potential Energy Surfaces

The potential energy surfaces of the neutral and anionic thymine-water complexes are investigated using high-level ab initio calculations. Both dipole-bound (DB) and valence-bound (VB) anionic forms are considered. Four minima and three first-order stationary points are located, and binding energies are computed. All minima, for both anions, are found to be vertically and adiabatically stable. The binding energies are much higher for valence-bound than for dipole-bound anions. Adiabatic electron affinities are in the 66-287 meV range for VB anions and the 4-60 meV range for DB anions, and vertical detachment energies are in the 698-977 meV and 10-70 meV range for VB and DB anions, respectively. For cases where literature data are available, the computed values are in good agreement with previous experimental and theoretical studies. It is observed that electron attachment modifies the shape of the potential energy surfaces of the systems, especially for the valence-bound anions. Moreover, for both anions the size of the energy barrier between the two lowest energy minima is strongly reduced, rendering the coexistence of different structures more probable.

Isomerism of the anion of the indole–water dimer. Ab initio study

In our previous paper [J. Chem. Phys. 112, 3726 (2000)], the experimental group of Schermann, who participated in the study, reported a Rydberg electron transfer spectroscopy (RET) experiments concerning anions of the indole–water clusters. For [indole–(water)1]− the RET spectrum showed two peaks. Based on the ab initio calculations one of the peaks was assigned to a dipole–bound (DB) anion with a diffuse excess electron and it was speculated that the second peak could correspond either to an excited electronic state or an excited vibrational state of the DB anion. In this paper we present calculations that describe a second anion of the indole–water dimer where the excess electron is suspended between the indole and water molecules and also occupies a diffuse orbital. The relation of this finding to the RET spectrum is discussed and we propose assigning the two RET peaks to the two anions described in the calculations.

Cluster size effects upon stability of adenine–methanolanions. Theoretical study

Rydberg electron transfer spectroscopy (RET) experiments [J. Electr. Spectr. Relat. Phenom. 106 (2000) 141] showed that only after being solvated with three methanol molecules the adenine molecule can form a stable covalent anion. Anions of adenine–methanol complexes (A·M n , n=1–3) are investigated in the present work with the use of quantum mechanical calculations. These complexes are good models to study how solvation affects the stability on the adenine anion. It is shown that among several hydrogen-bonded configurations of the A·M n , n=1−3, complexes there are systems which can form stable dipole-bound (DB) anions. These systems, however, are not the lowest energy structures. The stability of the covalent anion of A·M 3 is also investigated.

Anions of the hydrogen-bonded guanine–cytosine dimer – theoretical study

The quantum mechanical calculations performed in this work revealed an interesting nature of the stationary states of an excess electron in the guanine–cytosine (G–C) anion. The electron can form either a dipole-bound (DB) state or a covalent state. The coexistence of the two states may enhance the ability of the G–C dimer to capture an excess electron.

Dipole-bound anion of 1,3-butanediol. Ab initio and Rydberg electron transfer spectroscopy study

Rydberg electron transfer spectroscopy (RET) has been used to determine the dipole-bound (DB) electron affinity (EA) of 1,3-butanediol and a single spectral feature was obtained corresponding to the EA value of 8±1 meV. A series of high-level ab initio calculations were also carried out for this system. They revealed an interesting configuration landscape of the 1,3-butanediol DB anion with four minima corresponding to different configurations. All four configurations can be responsible for the single RET spectral peak.

Dipole-Bound Anions of Adenine−Water Clusters. Ab Initio Study

An interesting configuration landscape has been predicted for dipole-bound (DB) anions of [adenine−(water)N] complexes for N = 1, 2, and 3 using ab initio calculations. In these systems, the excess...