6 research outputs found
On the Origins of Large Interaction-Induced First Hyperpolarizabilities in Hydrogen-Bonded π‑Electronic Complexes
In this article we elucidate the
origins of interaction-induced
linear and nonlinear electro-optic properties of model hydrogen-bonded
π-electronic complexes. In particular we report on contributions
due to various interaction energy terms to excess dipole moments (Δμ),
electric dipole polarizabilities (Δα), and first hyperpolarizabilities
(Δβ), focusing on the latter. The analysis of intermolecular
interaction-induced electric properties is performed for selected
model systems including quasi-linear dimers of urea, diformamide,
4-pyridone, 4-nitroaniline, and the complex of hydrogen fluoride with
nitroacetylene. The nature of intermolecular interactions as well
as of the Δμ and Δα is very similar in all
studied complexes. However, partitioning of Δβ into physically
well-defined components reveals that the origins of this term, the
magnitude of which is often comparable to the hyperpolarizabilities
of isolated monomers, are different in each case. Our results indicate
that, even though hydrogen bonding usually diminishes the nonlinear
response of interacting species, the first hyperpolarizability of
complexes with the nitro group acting as a proton acceptor is substantially
increased, essentially due to field-induced changes of electrostatic
interactions between subsystems. However, in the remaining complexes
the origins of Δβ are much more involved. Even though
at large intermolecular separations the origins of interaction-induced
electric properties are essentially due to the field-induced electrostatic
and induction interactions, in the vicinity of van der Waals minimum
the overlap effects cannot be neglected since they may substantially
alter the predicted excess properties or even determine their magnitude
and sign. On the other hand the Δβ contribution due to
dispersion interactions is usually negligible. Interestingly, the
values of interaction-induced first hyperpolarizability in some cases
depend strongly on the intermolecular separation in the vicinity of
equilibrium geometry
Electron-Driven Proton Transfer Along H<sub>2</sub>O Wires Enables Photorelaxation of πσ* States in Chromophore–Water Clusters
The fates of photochemically formed
πσ* states are
one of the central issues in photobiology due to their significant
contribution to the photostability of biological matter, formation
of hydrated electrons, and the phenomenon of photoacidity. Nevertheless,
our understanding of the underlying molecular mechanisms in aqueous
solution is still incomplete. In this paper, we report on the results
of nonadiabatic photodynamics simulations of microhydrated 2-aminooxazole
molecule employing algebraic diagrammatic construction to the second
order. Our results indicate that electron-driven proton transfer along
H<sub>2</sub>O wires induces the formation of πσ*/S<sub>0</sub> state crossing and provides an effective deactivation channel.
Because we recently have identified a similar channel for 4-aminoimidazole-5-carbonitrile
[Szabla, R.; Phys. Chem.
Chem. Phys. 2014, 16, 17617−17626], we conclude
this mechanism may be quite common to all heterocyclic compounds with
low-lying πσ* states
Distributed Multipolar Expansion Approach to Calculation of Excitation Energy Transfer Couplings
We
propose a new approach for estimating the electrostatic part
of the excitation energy transfer (EET) coupling between electronically
excited chromophores based on the transition density-derived cumulative
atomic multipole moments (TrCAMM). In this approach, the transition
potential of a chromophore is expressed in terms of truncated distributed
multipolar expansion and analytical formulas for the TrCAMMs are derived.
The accuracy and computational feasibility of the proposed approach
is tested against the exact Coulombic couplings, and various multipole
expansion truncation schemes are analyzed. The results of preliminary
calculations show that the TrCAMM approach is capable of reproducing
the exact Coulombic EET couplings accurately and efficiently and is
superior to other widely used schemes: the transition charges from
electrostatic potential (TrESP) and the transition density cube (TDC)
method
On the Calculations of Interaction Energies and Induced Electric Properties within the Polarizable Continuum Model
In this work we investigate the influence of a polarizable
environment
on the interaction energies and the interaction-induced (excess) static
electric dipole properties for the selected model hydrogen-bonded
complexes. The excess properties were estimated for water and hydrogen
fluoride dimers using the supermolecular approach and assuming the
polarizable continuum model (PCM) as a representation of the polarizable
environment. We analyze in this context the performance of the counterpoise
correction and the consequences of various possible monomer cavity
choices. The polarizable environment reduces the absolute magnitudes
of interaction energies and interaction-induced dipole moments, whereas
an increase
is observed for the absolute magnitudes of induced polarizabilities
and first hyperpolarizabilities. Our results indicate that the use
of either monomeric (MC) or dimeric (DC) cavities in calculations
of monomer properties does not change qualitatively the resultant
excess properties. We conclude that the DC scheme is more consistent
with the definition of the interaction energy and consequently also
the interaction-induced property, whereas the MC scheme corresponds
to the definition of stabilization energy. Our results indicate also
a good performance of the counterpoise correction scheme for the self-consistent
methods in the case of all studied properties
Resonant and Nonresonant Hyperpolarizabilities of Spatially Confined Molecules: A Case Study of Cyanoacetylene
In
this theoretical study we report on resonant and nonresonant electric-dipole
(hyper)polarizabilities of cyanoacetylene molecule confined by repulsive
potentials of cylindrical symmetry mimicking a topology of nanotubelike
carbon cages. The set of investigated electronic properties encompasses
dipole moment, polarizability, first and second hyperpolarizability
as well as the two-photon transition matrix elements. The effect of
external potential on vibrational contributions to electric-dipole
properties is also included in our treatment. The computations are
performed at several levels of theoretical approximation including
state-of-the-art coupled-cluster (CCSD(T)) and multireference configuration
interaction methods (MRCISD(Q)). The results of calculations presented
herein indicate that the decrease in dipole moment observed experimentally
for the HCCCN molecule solvated in helium nanodroplets may be partially
attributed to the confinement effects. The external confining potential
causes a substantial drop of the isotropic average electronic polarizability
and second hyperpolarizability. In contrast, the vector component
of the electronic first hyperpolarizability substantially increases.
Nuclear relaxation contributions to all studied electric-dipole properties
are found to diminish upon confinement. Our calculations also indicate
that the most intense <sup>1</sup>Σ<sup>+</sup> ← <i>X̃</i> one-photon transition is slightly blue-shifted
whereas the corresponding oscillator strength is virtually unaffected
upon confinement. Interestingly, the absolute magnitude of the diagonal
component of the second-order transition moment for the bright state
(<i>S</i><sub><i>zz</i></sub><sup>0→<sup>1</sup>∑<sup>+</sup></sup>) increases
with the strength of external potential. The effect of structural
relaxation on the electric-dipole properties, arising from the presence
of the external potential, is also investigated in the present work
Photochemistry of 2‑Aminooxazole, a Hypothetical Prebiotic Precursor of RNA Nucleotides
2-Aminooxazole
has recently been proposed as a hypothetical precursor of RNA nucleotides
on early earth. UV irradiation was considered as a crucial environmental
factor in the proposed reaction sequence. We report on state-of-the-art
multireference quantum-chemical calculations elucidating the possible
nonradiative deactivation channels of this compound. According to
our findings, the gas-phase photochemistry of 2-aminooxazole should
be dominated by the photodetachment of the hydrogen atom of the NH<sub>2</sub> group via a <sup>1</sup>πσ<sub>NH</sub><sup>*</sup> state leading either to ultrafast
nonradiative deactivation, phototautomerization, or photodissociation
of a hydrogen atom. We also identified a possible ring-opening reaction
and a ring-puckering process that could occur after electronic excitation.
These reactions seem to be less probable because they are driven by
a higher-lying excited singlet state and are inherently slower than
the hydrogen-atom dynamics