Charge Accommodation Dynamics of Cluster and Molecular Anions Produced by Photo-initiated Intracluster Charge Transfer

Charge Accommodation Dynamics of Cluster and Molecular Anions Produced by Photo-initiated Intracluster Charge Transfer
Author: Margaret Ashley Yandell
Publisher:
Total Pages: 119
Release: 2014
Genre:
ISBN:

Time-resolved photoelectron imaging spectroscopy is used to examine the dynamics of charge accommodation by solvent species and biomolecules upon photo-initiated intracluster charge transfer. Excitation of a charge transfer state of an iodide-complexed molecule or cluster with a UV pulse and subsequent interrogation by photodetachment with a lower energy probe enables detection of changes in photoelectron signals over hundreds of femtoseconds. Velocity map imaging detection permits simultaneous collection of electron kinetic energy (eKE) and photoelectron angular distributions that provide insight into the strength and structure of the association between the cluster or molecule and the excess electron. Application of this methodology to iodide-containing clusters of small polar molecules such as water, methanol, and ethanol elucidates the stability and extent of intramolecular forces within a given cluster. In complexes of iodide with small solvent clusters (≤ 10 molecules), iodide is situated somewhat outside of the solvent network. Interaction of iodide-water clusters with a UV pulse to produce iodine and a free electron results in the partial solvation of the excess charge through hydrogen bonding interactions over hundreds of picoseconds before electron autodetachment. In contrast, methanol and ethanol cluster networks can only support the excess charge for tens of ps. Notably, stable bare water cluster anions have previously been measured with as few as two molecules, while upwards of seventy methanol molecules are necessary to stabilize an excess electron. Drawing an analogy between electron autodetachment and statistical unimolecular decay, an excited iodide-water cluster with a given number of water molecules might be expected to decay most rapidly given its significantly smaller density of states. The observation of the opposite pattern, as well as the similarity between iodide-methanol and -ethanol cluster anion lifetimes, suggests that energetics, rather than molecular structure, play a larger role in stabilizing an excess charge to autodetachment. Applying a thermionic emission model confirms this result. The dynamics of charge accommodation are also examined for small biomolecules. Radiative damage to DNA caused by low energy electrons is thought to originate in the attachment of an electron to a nucleobase unit of a nucleotide in the DNA double helix. Previous experiments have examined binding motifs and fragmentation patterns of transient negative ions (TNIs) of nucleobases using Rydberg electron transfer from excited noble gas atoms or collision of the nucleobase with a beam of electrons of defined energy. Here, nascent TNIs of the nucleobase uracil are created by intracluster charge transfer from a complexed iodide ion and their decay examined with time-resolved photoelectron imaging. Anions created with several hundred meV of excess energy appear as valence anions and are observed to decay biexponentially with time constants of hundreds of fs and tens of ps by iodine atom loss and autodetachment. Repetition of these experiments with uracil molecules methylated at the N1, N3, or C5 positions results in a dramatic reduction of the longer time constant. The addition of the methyl group may hasten the intramolecular vibrational energy redistribution process preceding autodetachment. Photoelectron spectroscopy of isolated nucleobase anions has measured only the dipole-bound state (DBS) of the anion consisting of an electron weakly associated with the molecular dipole moment and very delocalized over the molecular structure. Though the valence anion has not been directly measured, the DBS has been posited to serve as a `doorway' to the valence-bound state (VBS). Such a mechanism has also been proposed for nitromethane. In contrast, acetonitrile should only support a DB anion state. Examination of nascent acetonitrile and nitromethane anions excited near the vertical detachment energies of their corresponding iodide-molecule complexes indeed produces the DB acetonitrile anion, which then decays biexponentially with time constants of few and hundreds of ps by iodine atom loss and autodetachment. The nitromethane DB anion decays rapidly over hundreds of fs to form the valence anion, which decays biexponentially with time constants similar to those measured for the acetonitrile DB anion. This study marks the first direct observation of a transition from a dipole-bound anion to a valence anion and will inform future studies of iodide-nucleobase complexes.

Structure, Photochemistry and Charge-transfer-to-solvent Relaxation Dynamics of Anionic Clusters

Structure, Photochemistry and Charge-transfer-to-solvent Relaxation Dynamics of Anionic Clusters
Author: Kadyr K. Timergazine
Publisher:
Total Pages: 0
Release: 2006
Genre:
ISBN:

In this work, various methods of computational chemistry have been applied to study the structure, photochemistry and dynamics of small anionic clusters in order to obtain some insight into the fundamental nature of molecule-molecule and electron molecule interactions. The main theme of this Thesis evolves around charge-transfer-to-solvent (CTTS) excited states of small halide-polar molecule clusters and concomitant phenomena, with specific attention to iodide-acetonitrile clusters I - (CH 3 CN) n . The structure of ground-state halide-acetonitrile complexes is determined by a competition between ion-dipole interactions and hydrogen bonding. The latter is dominant for complexes formed by all halide ions, except for the binary iodide acetonitrile complex I - (CH 3 CN), which is stabilized by only ion-dipole interactions. On the other hand, in the excited or ionized iodide complexes and clusters, the non-uniform electron density distribution around the neutral iodine atom due to the unpaired electron and spin-orbit coupling effects plays a very important role in interactions of the iodine atom with molecules or ions. The CTTS excitation of iodide-solvent clusters leads to the transfer of an electron from iodide to a diffuse dipole-bound orbital. The calculated excitation energies for the iodide-acetonitrile complex, as well as the vertical detachment energies of the excited electron have been found to be in a good agreement with available experimental data. For the first time, the relaxation dynamics of CTTS excited states in clusters has been studied by means of first-principles excited-state molecular dynamics simulations for paradigm iodide-acetonitrile I - (CH 3 CN) and iodide-water I - (H 2 O) 3 clusters. Whereas the departure of a neutral iodine atom can lead to stabilization of the excited electron, in agreement with state-of-the-art femtosecond spectroscopy experimental data, the solvent dynamics should not be neglected when interpreting experimental results. The photoexcitation and subsequent relaxation of I - (CH 3 CN) n clusters can lead to formation of acetonitrile cluster anions, which are characterized by the binding of the excess electron in a dipole-bound orbital outside of acetonitrile molecules, as shown for the prototype acetonitrile dimer anion. At the same time, in solution or relatively large clusters, two acetonitrile molecules can dimerize, forming a weak covalent carbon-carbon bond and allowing for delocalization and binding of the excess electron in their valence orbitals.

Nanocatalysis

Nanocatalysis
Author: Ulrich Heiz
Publisher: Springer Science & Business Media
Total Pages: 514
Release: 2007-01-10
Genre: Technology & Engineering
ISBN: 3540326464

Nanocatalysis, a subdiscipline of nanoscience, seeks to control chemical reactions by changing the size, dimensionality, chemical composition, and morphology of the reaction center and by changing the kinetics using nanopatterning of the reaction center. This book offers a detailed pedagogical and methodological overview of the field. Readers discover many examples of current research, helping them explore new and emerging applications.

Physical Chemistry of Cold Gas-phase Functional Molecules and Clusters

Physical Chemistry of Cold Gas-phase Functional Molecules and Clusters
Author: Takayuki Ebata
Publisher:
Total Pages:
Release: 2019
Genre: Cold gases
ISBN: 9789811393723

This book describes advanced research on the structures and photochemical properties of polyatomic molecules and molecular clusters having various functionalities under cold gas-phase conditions. Target molecules are crown ethers, polypeptides, large size protonated clusters, metal clusters, and other complex polyatomic molecules of special interest. A variety of advanced frequency and time-domain laser spectroscopic methods are applied. The book begins with the principle of an experimental setup for cold gas-phase molecules and various laser spectroscopic methods, followed by chapters on investigation of specific molecular systems. Through a molecular-level approach and analysis by quantum chemical calculation, it is possible to learn how atomic and molecular-level interactions (van der Waals, hydrogen-bonding, and others) control the specific properties of molecules and clusters. Those properties include molecular recognition, induced fitting, chirality, proton and hydrogen transfer, isomerization, and catalytic reaction. The information will be applicable to the design of new types of functional molecules and nanoparticles in the broad area that includes applied chemistry, drug delivery systems, and catalysts.

Introduction to Radiological Physics and Radiation Dosimetry

Introduction to Radiological Physics and Radiation Dosimetry
Author: Frank Herbert Attix
Publisher: John Wiley & Sons
Total Pages: 628
Release: 2008-09-26
Genre: Medical
ISBN: 3527617140

A straightforward presentation of the broad concepts underlying radiological physics and radiation dosimetry for the graduate-level student. Covers photon and neutron attenuation, radiation and charged particle equilibrium, interactions of photons and charged particles with matter, radiotherapy dosimetry, as well as photographic, calorimetric, chemical, and thermoluminescence dosimetry. Includes many new derivations, such as Kramers X-ray spectrum, as well as topics that have not been thoroughly analyzed in other texts, such as broad-beam attenuation and geometrics, and the reciprocity theorem. Subjects are layed out in a logical sequence, making the topics easier for students to follow. Supplemented with numerous diagrams and tables.

Mobile Radio Communications and 5G Networks

Mobile Radio Communications and 5G Networks
Author: Nikhil Marriwala
Publisher: Springer Nature
Total Pages: 780
Release: 2020-09-28
Genre: Technology & Engineering
ISBN: 9811571309

The book features original papers by active researchers presented at the International Conference on Mobile Radio Communications and 5G Networks. It includes recent advances and upcoming technologies in the field of cellular systems, 2G/2.5G/3G/4G/5G and beyond, LTE, WiMAX, WMAN, and other emerging broadband wireless networks, WLAN, WPAN, and various home/personal networking technologies, pervasive and wearable computing and networking, small cells and femtocell networks, wireless mesh networks, vehicular wireless networks, cognitive radio networks and their applications, wireless multimedia networks, green wireless networks, standardization of emerging wireless technologies, power management and energy conservation techniques.

Protein Conformation

Protein Conformation
Author: Derek J. Chadwick
Publisher: John Wiley & Sons
Total Pages: 282
Release: 2008-04-30
Genre: Science
ISBN: 0470514159

How the amino acid sequence of a protein determines its three-dimensional structure is a major problem in biology and chemistry. Leading experts in the fields of NMR spectroscopy, X-ray crystallography, protein engineering and molecular modeling offer provocative insights into current views on the protein folding problem and various aspects for future progress.