14 research outputs found
Scientific Research: Opportunities and Threats
In the first half of this talk, Dr. Ramu will present a few recent developments in chemistry, physics, and biology to set the stage for discussing the challenges and opportunities awaiting the next generation of scientists. In the second half of the talk, he will discuss some of the existential threats that we must understand and address in order to ensure that our future, both as inhabitants of this planet and as scientists engaged in fundamental research, is as secure as we can make it
Rearrangement Reactions of Lithiated Oxiranes
The
first computational study of the rearrangement reactions of oxiranes
initiated by lithium dialkylamides is presented. Aside from the well-known
carbenoid insertion pathways, both β-elimination and α-lithiation
have been suggested as the exclusive mechanism by which oxiranes react
in the presence of organolithium bases. The products of the former
are allyl alcohols (and, in some cases, dienes) and are ketones in
the case of the latter. The computational studies reported in this
work indicate that both mechanisms could be simultaneously operational.
In particular, our work shows that the allyl alcohols from β-elimination
are unlikely to undergo 1,3-hydrogen transfer to the vinyl alcohols
and thus to the ketones, suggesting that ketones are formed through
the opening of the oxirane ring after α-substitution. Elimination
of LiOH from the lithiated allyl alcohol is found to result in the
diene product. Low activation barriers for β-elimination are
offered as the explanation for the few special cases where the allyl
alcohol is the dominant or exclusive product. These findings are consistent
with the product distributions observed in several experiments
Carbenoid Alkene Insertion Reactions of Oxiranyllithiums
The first computational investigations of the carbenoid
reactions of α-lithiated dimethyl ether (methoxymethyllithium)
and the intramolecular and intermolecular reactions of lithiated epoxides
with the alkene double bond to yield cyclopropane rings are presented.
These reactions represent the full spectrum of known carbenoid pathways
to cyclopropanation. The reaction of Li–CH<sub>2</sub>–O–CH<sub>3</sub> with ethylene proceeds exclusively through a two-step carbolithiation
pathway, the intramolecular reaction of 1,2-epoxy-5-hexene follows
either the carbometalation or a concerted methylene transfer pathway
(the former is energetically more favorable), and the reaction of
lithiated ethylene oxide (oxiranyllithium) with ethylene, the main
focus of this paper, appears to proceed exclusively by the methylene
transfer mechanism. In the case of these latter reactions, the free
energy of activation for cyclopropanation tends to decrease with the
higher aggregation states. Formation of tetramers or higher aggregates
is favorable in nonpolar solvents, but in strongly coordinating solvents
such as tetrahydrofuran (THF), steric factors appear to limit aggregate
sizes to the dimer. In the case of 1,2-epoxy-5-hexene, consideration
of competing reaction pathways provide an explanation for the observed
product distribution
Structure and magnetic properties of Fe
The electronic, geometrical, and magnetic structures of iron clusters Fen substituted
with a single Gd atom are studied using density functional theory with generalized
gradient approximation for n =
12 − 19. An all electron basis set of a triple-ζ quality is chosen for the
iron atoms whereas an effective core potential and the basis set of a
triple-ζ
quality are used for the Gd atom in optimizations of FenGd clusters.
The lowest total energy state of a FenGd cluster was found to possess a
geometrical structure where the Gd atom substitutes for a surface Fe atom of the
Fen+1 cluster at given n. The total spin of a
substituted cluster is larger than the total spin of the lowest total energy state of a
unary iron cluster with the same number of atoms. The binding energy per atom in a
substituted Fen−1Gd cluster is somewhat smaller than the
binding energy per atom in a non-substituted Fen cluster. That is, the Gd
substitution increases the total spin magnetic moment but destabilizes substituted iron
clusters
Molecular Mechanisms for the Lithiation of Ruthenium Oxide Nanoplates as Lithium-Ion Battery Anode Materials: An Experimentally Motivated Computational Study
First-principles
computational studies were used to calculate discharge
curves for lithium in RuO<sub>2</sub> and to understand the molecular
mechanism of lithium sorption into crystalline bulk RuO<sub>2</sub>. These studies were complemented by experiments to provide new insights
into the molecular mechanisms for the first and subsequent discharges
of RuO<sub>2</sub> anodes in lithium ion batteries. RuO<sub>2</sub> nanoplates show slow fading of capacity over multiple cycles, retaining
76% of their original capacity after 20 cycles. The calculated discharge
curves for lithium in RuO<sub>2</sub> lattice show qualitative agreement
with experimental discharge curves for RuO<sub>2</sub> nanoplates.
The molecular level analysis shows that an intercalation mechanism
is operational until a 1:1 Li:Ru ratio is reached, which is followed
by a conversion mechanism into Ru metal and Li<sub>2</sub>O. Furthermore,
in agreement with experiment, the computations predict superstoichiometric
capacity of RuO<sub>2</sub>, i.e., accommodation of lithium well beyond
the stoichiometric limit of 4:1 Li:Ru ratio, and show that the additional
lithium atoms reside at the interface of the Ru metal and Li<sub>2</sub>O. This shows that the extra capacity can be explained without invoking
electrolyte or solvent–electrolyte interface effects
Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling
Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH3–NO2 structure with low energy barriers. While direct cleavage of the C–N bond from the parent nitromethane cation produces NO2+ and CH3+, rearrangement prior to dissociation accounts for fragmentation products including NO+, CH2OH+, and CH2NO+. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH2+, H+, and NO22+.  On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane