42 research outputs found
Molecular Auger Decay Rates from Complex-Variable Coupled-Cluster Theory
The emission of an Auger electron is the predominant relaxation mechanism of
core-vacant states in molecules composed of light nuclei. In this non-radiative
decay process, one valence electron fills the core vacancy while a second
valence electron is emitted into the ionization continuum. Because of this
coupling to the continuum, core-vacant states represent electronic resonances
that can be tackled with standard quantum-chemical methods only if they are
approximated as bound states, meaning that Auger decay is neglected. Here, we
present an approach to compute Auger decay rates of core-vacant states from
coupled-cluster and equation-of-motion coupled-cluster wave functions combined
with complex scaling of the Hamiltonian or, alternatively, complex-scaled basis
functions. Through energy decomposition analysis, we illustrate how
complex-scaled methods are capable of describing the coupling to the ionization
continuum without the need to model the wave function of the Auger electron
explicitly. In addition, we introduce in this work several approaches for the
determination of partial decay widths and Auger branching ratios from
complex-scaled coupled-cluster wave functions. We demonstrate the capabilities
of our new approach by computations on core-ionized states of neon, water,
dinitrogen, and benzene. Coupled-cluster and equation-of-motion coupled-cluster
theory in the singles and doubles approximation both deliver excellent results
for total decay widths, whereas we find partial widths more straightforward to
evaluate with the former method. We also observe that the requirements towards
the basis set are less arduous for Auger decay than for other types of
resonances so that extensions to larger molecules are readily possible.Comment: 15 pages, 6 figures, 9 table
molecular dynamics of temporary anions using complex absorbing potentials
Dissociative electron attachment, that is, the cleavage of chemical bonds
induced by low-energy electrons, is difficult to model with standard
quantum-chemical methods because the involved anions are not bound but subject
to autodetachment. We present here a new computational development for
simulating the dynamics of temporary anions on complex-valued potential energy
surfaces. The imaginary part of these surfaces describes electron loss, whereas
the gradient of the real part represents the force on the nuclei. In our
method, the forces are computed analytically based on Hartree-Fock theory with
a complex absorbing potential.
molecular dynamics simulations for the temporary anions of
dinitrogen, ethylene, chloroethane, and the five mono- to tetrachlorinated
ethylenes show qualitative agreement with experiments and offer mechanistic
insights into dissociative electron attachments. The results also demonstrate
how our method evenhandedly deals with molecules that may undergo dissociation
upon electron attachment and those which only undergo autodetachment.Comment: Manuscript: 10 pages, 4 figures. Supplementary Material: 41 pages, 43
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Computing decay widths of autoionizing Rydberg states with complex-variable coupled cluster theory
We compute autoionization widths of various Rydberg states of neon and
dinitrogen by equation-of-motion coupled-cluster theory combined with complex
scaling and complex basis functions. This represents the first time that
complex-variable methods are applied to Rydberg states represented in Gaussian
basis sets. A new computational protocol based on Kaufmann basis functions is
designed to make these methods applicable to atomic and molecular Rydberg
states. As a first step, we apply our protocol to the neon atom and computed
widths of the , , and Rydberg states. We then proceed to
compute the widths of the , , and Rydberg
states of dinitrogen, which belong to the Hopfield series. Our results
demonstrate a decrease in the decay width for increasing angular momentum and
principal quantum number within both Rydberg series
Computational insights into electrochemical cross-coupling of quaternary borate salts
Cross-coupling reactions for CâC bond formation represent a cornerstone of organic synthesis. In most cases, they make use of transition metals, which has several downsides. Recently, metal-free alternatives relying on electrochemistry have gained interest. One example of such a reaction is the oxidation of tetraorganoborate salts that initiates arylâaryl and arylâalkenyl couplings with promising selectivities. This work investigates the mechanism of this reaction computationally using density functional and coupled-cluster theory. The calculations reveal a distinct difference between arylâalkenyl and arylâaryl couplings: While CâC bond formation occurs irreversibly and without an energy barrier if an alkenyl residue is involved, many intermediates can be identified in arylâaryl couplings. In the latter case, intramolecular transitions between reaction paths leading to different products are possible. Based on the energy differences between these intermediates, a kinetic model to estimate product distributions for arylâaryl couplings is developed
ElectroâOlefinationâA Catalyst Free Stereoconvergent Strategy for the Functionalization of Alkenes
Conventional methods carrying out C(sp2)âC(sp2) bond formations are typically mediated by transitionâmetalâbased catalysts. Herein, we conceptualize a complementary avenue to access such bonds by exploiting the potential of electrochemistry in combination with organoboron chemistry. We demonstrate a transition metal catalystâfree electrocoupling between (hetero)aryls and alkenes through readily available alkenylâtri(hetero)aryl borate salts (ATBs) in a stereoconvergent fashion. This unprecedented transformation was investigated theoretically and experimentally and led to a library of functionalized alkenes. The concept was then carried further and applied to the synthesis of the natural product pinosylvin and the derivatization of the steroidal dehydroepiandrosterone (DHEA) scaffold
Analytic evaluation of non-adiabatic couplings within the complex absorbing potential equation-of-motion coupled-cluster method
We present the theory for the evaluation of non-adiabatic couplings (NACs)
involving resonance states within the complex absorbing potential
equation-of-motion coupled-cluster (CAP-EOM-CC) framework implemented within
the singles and doubles approximation. Resonance states are embedded in the
continuum and undergo rapid decay through autodetachment. In addition, nuclear
motions can facilitate transitions between different resonances and between
resonances and bound states. These non-adiabatic transitions affect the
chemical fate of resonances and have distinct spectroscopic signatures. The NAC
vector is a central quantity needed to model such effects.
In the CAP-EOM-CC framework, resonance states are treated on the same footing
as bound states. Using the example of fumaronitrile, which supports a bound
radical anion and several anionic resonances, we analyze the non-adiabatic
coupling between bound states and pseudocontinuum states, between bound states
and resonances and between two resonances. We find that the NAC between a bound
state and a resonance is nearly independent of the CAP strength and thus
straightforward to evaluate whereas the NAC between two resonance states or
between a bound state and a pseudocontinuum state is more difficult to
evaluate
Analytic evaluation of the dipole Hessian matrix in coupled-cluster theory
The general theory required for the calculation of analytic third energy derivatives at the coupled-cluster level of theory is presented and connected to preceding special formulations for hyperpolarizabilities and polarizability gradients. Based on our theory, we have implemented a scheme for calculating the dipole Hessian matrix in a fully analytical manner within the coupled-cluster singles and doubles approximation. The dipole Hessian matrix is the second geometrical derivative of the dipole moment and thus a third derivative of the energy. It plays a crucial role in IR spectroscopy when taking into account anharmonic effects and is also essential for computing vibrational corrections to dipole moments. The superior accuracy of the analytic evaluation of third energy derivatives as compared to numerical differentiation schemes is demonstrated in some pilot calculations