Ground Potential Energy Surfaces Research Articles

Photocatalyst‐Free Wavelength‐Dependant Sequential Ring Transformations of Pyrazolo[1,2‐a]pyrazolones

We report new sequential wavelength‐selective photochemical transformations of 1‐alkenylpyrazolo[1,2‐a]pyrazolones to pyrazolo[1,2‐a][1,2]diazepines or cyclobuta[c]pyrazolo[1,2‐a]pyrazolones. Irradiation of 1‐alkenylpyrazolo[1,2‐a]pyrazolones with visible‐light (blue LED, 457 nm) induced selective ‘ring switching’ transformation into pyrazolo[1,2‐a][1,2]diazepines, which, upon irradiation with UV A light (black LED, 365 nm) underwent electrocyclisation into cyclobuta[c]pyrazolo[1,2‐a]pyrazolones. Due to the very narrow irradiation wavelength now available from OLED sources, the selective formation of either 5,7‐bicyclic or 5,5,4‐tricyclic ring systems from the 5,5‐bicyclic starting material is possible simply by changing the wavelength of the irradiation source. The transformations took place under mild conditions in the absence of additives or photocatalysts. Mechanistic studies indicate that these transformations proceed through the formation of an excited triplet state of the substrate, followed by selective homolytic C(1)–N(8) bond cleavage, intersystem crossing, and cyclization of the zwitterionic intermediate on the ground potential energy surface. Subsequent photoinduced disrotatory stereospecific 4‐π‐electrocyclization of the 5,7‐bicyclic systems leads to 3D‐rich 5,5,4‐tricyclic products.

Quantum simulation of conical intersections using trapped ions.

Conical intersections often control the reaction products of photochemical processes and occur when two electronic potential energy surfaces intersect. Theory predicts that the conical intersection will result in a geometric phase for a wavepacket on the ground potential energy surface, and although conical intersections have been observed experimentally, the geometric phase has not been directly observed in a molecular system. Here we use a trapped atomic ion system to perform a quantum simulation of a conical intersection. The ion's internal state serves as the electronic state, and the motion of the atomic nuclei is encoded into the motion of the ions. The simulated electronic potential is constructed by applying state-dependent optical forces to the ion. We experimentally observe a clear manifestation of the geometric phase using adiabatic state preparation followed by motional state measurement. Our experiment shows the advantage of combining spin and motion degrees for quantum simulation of chemical reactions.

Quantum Chemical Studies on Possible Molecular Devices Based on Electric Field-Induced Intramolecular Charge Transfer.

We report quantum chemical studies on possible molecular devices working based on electric field-induced intramolecular charge transfer (EFIMCT). In the case of donor-acceptor (DA)-type molecular systems, intramolecular charge transfer (IMCT) can be induced by applying the external electric field to molecular systems along the charge transport direction, providing a possible switching mechanism which does not depend upon the electron-phonon coupling effect and is different from the negative differential resistance mechanism observed in the well-known NO2-substituted phenylene ethynylene oligomers. When the EFIMCT proceeds, the molecular systems have strong static electron correlation effects, where the standard nonequilibrium Green's function-density functional theory (DFT) approach cannot be applied to the molecular junction. As a first step toward practical switching devices, we do quantum chemical studies on the EFIMCT in such molecular systems as an isolated molecule, instead of using the electrode-junction-electrode open quantum system model. A prototype molecule P1 is designed as a tentative candidate molecule where the EFIMCT can proceed. The complete active space self-consistent field (CASSCF) molecular orbital calculations on P1 indicate that the EFIMCT can proceed at the external electric field intensity of 0.003 au, corresponding to about 2.25 V bias voltage. This calculated result strongly suggests that the development of this type of switching devices working at practically low bias voltage is feasible if the molecular system is properly designed. Broken symmetry unrestricted Hartree-Fock and spin-polarized Kohn-Sham DFT calculations also qualitatively reproduce the CASSCF results on P1, to some extent, indicating that these approaches can be employed for rough estimations on the EFIMCT such as the first screening of a large quantity of candidate molecules for this type of molecular devices. The possibility of molecular memory devices based on the EFIMCT is also discussed by analyzing the ground and excited potential energy surface model. Remaining challenges to develop practical molecular devices are discussed.

Accurate Calculation of Rate Constant and Isotope Effect for the F + H2 Reaction by the Coupled 3D Time-Dependent Wave Packet Method on the Newly Constructed Ab Initio Ground Potential Energy Surface.

We employ coupled three-dimensional (3D) time dependent wave packet formalism in hyperspherical coordinates for reactive scattering problem on the newly constructed ab initio calculated ground adiabatic potential energy surface for the F + H2/D2 reaction. The convergence profiles for various reactive channels are depicted at low collision energy regimes with respect to the total angular momentum (J) quantum numbers. For two different reactant diatomic molecules (H2 and D2) initially at their respective ground roto-vibrational state (v = 0, j = 0), calculated state-to-state as well as total integral cross sections as a function of collision energy, temperature dependent rate constants, and the kinetic isotope effect for various reactivity profiles of F + H2 and F + D2 reactions are presented along with previous theoretical and experimental results.

Coherent Control of Molecular Dissociation by Selective Excitation of Nuclear Wave Packets.

We report on pump-probe control schemes to manipulate fragmentation product yields in p-nitrotoluene (PNT) cation. Strong field ionization of PNT prepares the parent cation in the ground electronic state, with coherent vibrational excitation along two normal modes: the C–C–N–O torsional mode at 80 cm−1 and the in-plane ring-stretching mode at 650 cm−1. Both vibrational wave packets are observed as oscillations in parent and fragment ion yields in the mass spectrum upon optical excitation. Excitation with 650 nm selectively fragments the PNT cation into , whereas excitation with 400 nm selectively produces and . In both cases the ion yield oscillations result from torsional wave packet excitation, but 650 and 400 nm excitation produce oscillations with opposite phases. Ab initio calculations of the ground and excited electronic potential energy surfaces of PNT cation along the C–C–N–O dihedral angle reveal that 400 nm excitation accesses an allowed transition from D0 to D6 at 0° dihedral angle, whereas 650 nm excitation accesses a strongly allowed transition from D0 to D4 at a dihedral angle of 90°. This ability to access different electronic excited states at different locations along the potential energy surface accounts for the selective fragmentation observed with different probe wavelengths. The ring-stretching mode, only observed using 800 nm excitation, is attributed to a D0 to D2 transition at a geometry with 90° dihedral angle and elongated C–N bond length. Collectively, these results demonstrate that strong field ionization induces multimode coherent excitation and that the vibrational wave packets can be excited with specific photon energies at different points on their potential energy surfaces to induce selective fragmentation.

Full-dimensional potential energy surfaces of ground (X̃2 A′) and excited (Ã2 A″) electronic States of HCO and absorption spectrum

In this work, high-fidelity full-dimensional potential energy surfaces (PESs) of the ground (X̃2 A′) and first doublet excited (Ã2 A″) electronic states of HCO were constructed using neural network method. In total, 4624 high-level ab initio points have been used which were calculated at Davidson corrected internally contracted MRCI-F12 level of theory with a quite large basis set (ACV5Z) without any scaling scheme. Compared with the results obtained from the scaled PESs of Ndengué et al., the absorption spectrum based on our PESs has slightly larger intensity, and the peak positions are shifted to smaller energy for dozens of wavenumbers. It is indicated that the scaling of potential energy may make some unpredictable difference on the dynamical results. However, the resonance energies based on those scaled PESs are slightly closer to the current available experimental values than ours. Nevertheless, the unscaled high-level PESs developed in this work might provide a platform for further experimental and theoretical photodissociation and collisional dynamic studies for HCO system.

Dynamic study of the D + DAu reaction based on a new ground potential energy surface

Based on the ground potential energy surface (PES) (2A') constructed by Yuan and coworkers, the D + DAu reactive scattering was investigated using the time-dependent wave packet method. It was found that the total reaction probabilities and integral cross section of the D + DAu reaction are smaller than those of the H + HAu reaction, due to smaller zero point energy and weaker tunneling effect. The abstract and exchange reaction channel exhibit different reaction mechanisms, a direct reaction mechanism in the abstract channel and long lifetime indirect mechanism in the exchange channel, respectively. During the reaction, the collision energy could be efficiently transferred to vibration of the D2 product in the abstract reaction channel and rotation of the DAu product in the exchange channel.

Dynamic study of the H + AuH reaction based on a new ground potential energy surface

Time-dependent wave packet (TDWP) calculations were performed for the title reaction on a new ground potential energy surface (PES) (2A′) constructed by Yuan et al. The reaction probability, integral cross-section (ICS), and differential cross-section (DCS) were obtained for both the PES. The ICSs on the Yuan PES were smaller than those on the Zanchet PES. The direct mechanism was dominant in the abstraction reaction, while the indirect mechanism was dominant in the exchange reaction. Another observation was that the collision energy is readily transferred into the internal energy (vibration of H2 and rotation of HAu).

Vibrationally resolved coupled-cluster x-ray absorption spectra from vibrational configuration interaction anharmonic calculations.

Vibrationally resolved near-edge x-ray absorption spectra at the K-edge for a number of small molecules have been computed from anharmonic vibrational configuration interaction calculations of the Franck-Condon factors. The potential energy surfaces for ground and core-excited states were obtained at the core-valence separated CC2, CCSD, CCSDR(3), and CC3 levels of theory, employing the adaptive density-guided approach scheme to select the single points at which to perform the energy calculations. We put forward an initial attempt to include pair-mode coupling terms to describe the potential of polyatomic molecules.

Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N^N)3]2+-Type Complexes toward Both Cis and Trans Photoproducts.

A complete mechanistic picture for the photochemical release of bipyridine (bpy) from the archetypal complex [Ru(bpy)3]2+ is presented for the first time following the description of the ground and lowest triplet potential energy surfaces, as well as their key crossing points, involved in successive elementary steps along pathways toward cis- and trans-[Ru(bpy)2(NCMe)2]2+. This work accounts for two main pathways that are identified involving (a) two successive photochemical reactions for photodechelation, followed by the photorelease of a monodentate bpy ligand, and (b) a novel one-photon mechanism in which the initial photoexcitation is followed by dechelation, solvent coordination, and bpy release processes, all of which occur sequentially within the triplet excited-state manifold before the final relaxation to the singlet state and formation of the final photoproducts. For the reaction between photoexcited [Ru(bpy)3]2+ and acetonitrile, which is taken as a model reaction, pathways toward cis and trans photoproducts are uphill processes, in line with the comparative inertness of the complex in this solvent. Factors involving the nature of the departing ligand and retained "spectator" ligands are considered, and their role in the selection of mechanistic pathways involving overall two sequential photon absorptions versus one photon absorption for the formation of both cis or trans photoproducts is discussed in relation to notable examples from the literature. This study ultimately provides a generalized roadmap of accessible photoproductive pathways for light-induced reactivity mechanisms of photolabile [Ru(N^N)(N^N')(N^N″)]2+-type complexes.

Dynamics on Multiple Potential Energy Surfaces: Quantitative Studies of Elementary Processes Relevant to Hypersonics.

The determination of thermal and vibrational relaxation rates of triatomic systems suitable for application in hypersonic model calculations is discussed. For this, potential energy surfaces for ground and electronically excited state species need to be computed and represented with high accuracy, and quasiclassical or quantum nuclear dynamics simulations provide the basis for determining the relevant rates. These include thermal reaction rates, state-to-state cross sections, and vibrational relaxation rates. For exemplary systems (i.e., [NNO], [NOO], and [CNO]), all individual steps are described, and a literature overview for them is provided. Finally, as some of these quantities involve considerable computational expense, for the example of state-to-state cross sections, the construction of an efficient model based on neural networks is discussed. All such data is required and being used in more coarse-grained computational fluid dynamics simulations.

Prospects for laser cooling of polyatomic molecules with increasing complexity

Optical cycling transitions and direct laser cooling have recently been demonstrated for a number of alkaline-earth dimers and trimer molecules. This is made possible by diagonal Franck-Condon factors between the vibrational modes of the optical transition. Achieving a similar degree of cooling to micro-Kelvin equivalent kinetic energy for larger polyatomic molecules, however, remains challenging. Since polyatomic molecules are characterized by multiple degrees of freedom and have a correspondingly more complex structure, it is far from obvious whether there exist polyatomic molecules that can repeatedly scatter photons. Here, we propose chemical substitution approaches to engineer large polyatomic molecules with optical cycling centers (OCCs) containing alkaline-earth oxide dimers or acetylenic alkaline-earth trimers (i.e. M--C$\equiv$C) connected to CH$_n$ chains or fullerenes. To validate the OCC-character of the selected molecules we performed electronic structure calculations of the equilibrium configuration of both ground and excited potential energy surfaces, evaluated their vibrational and bending modes, and corresponding Franck-Condon factors for all but the most complex molecules. For fullerenes, we have shown that OCCs based on M--C$\equiv$C can perform better than those based on alkaline-earth oxide dimers. In addition, for heavier polyatomic molecules it might be advantageous to attach two OCCs, thereby potentially doubling the photon scattering rate and thus speeding up cooling rates.

Mechanism and kinetic investigations of 5-fluorouracil tautomeric conversions in the gas phase: DFT and CBS-QB3 methods using multichannel Rice–Ramsperger–Kassel–Marcus steady-state approximation theory

In this work, chemical computations were carried out to study of tautomeric conversion of 5-fluorouracil (A → products) at CBS-QB3, M06-2x, and B3LYP levels of theory. These 11 tautomers connected to each other via 10 transition state molecules and four pathways. In addition, the stationary points (including stable structures and transition states), in which playing a significant role on ground potential energy surfaces of isomerization processes, were found. Multichannel RRKM steady-state approximation calculations have been performed to calculate the rate constants at 1 atm pressure and over a range of temperature from 268.15 to 310.15 K to cover the in vivo and atmospheric conditions. The stability ranking of tautomers obtained and demonstrated that di-keto form of 5-fluorouracil is the most stable tautomer. The energy barriers for tautomeric conversions were calculated. The information reported in this study may help with the identification of drug in vivo and in the laboratory.

A time-dependent quantum dynamical study of H + AuH reaction

We performed the first time-dependent wave packet calculations on H + HAu → Au + H2/ H + HAu reaction using the ground potential energy surface (2A') constructed by Zanchet' et al. For studying the mechanism of the title reaction, the reaction probabilities, integral and differential cross sections are calculated. The direct mechanism plays an important role in the abstraction reaction, while the indirect mechanism plays an important role in exchange reaction. The energy transfer goes essentially to vibration and rotation of H2 products. However, the translation to vibration energy transfer is not efficient in the exchange reaction.

Non-adiabatic transitions in the near-UV photodissociation of

SynopsisThe near-UV photodissociation of has been studied in a fast beam experiment. The kinetic energy release and branching ratio among the fragmentation channels were obtained by three-dimensional imaging of the dissociation products. It is found that the photodissociation of vibrationally excited mostly produces vibrationally cold and hot H2. The non-adiabatic repopulation of the ground potential energy surface after excitation is accounted for by modelling the wavepacket dynamics on the intersecting potential energy surfaces.

Effect of isotope on state-to-state dynamics for reactive collision reactions and in ground state 12A″ and first excited 12A′ potential energy surfaces**Project supported by the National Natural Science Foundation of China (Grant No. 11504206) and the Shandong Jiaotong University PhD Research Start-up Fund, China.

We carry out quantum scattering dynamics and quasi-classical trajectory (QCT) calculations for the reactive collision in the ground (12A″) and first excited (12A′) potential energy surface. We calculate the reaction probabilities of and reaction for total angular momentum J = 0. The results calculated by QCT are consistent with those from quantum mechanical wave packet. Using the QCT method, we generate in the center-of-mass frame the product state-resolved integral cross-sections (ICSs); two commonly used generalized polarization-dependent differential cross-sections (PDDCSs), (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt); and three angular distributions of the product rotational vectors, P(θr), P(ϕr), and P(θr, ϕr). We discuss the influence on the scalar and vector properties of the potential energy surface, the collision energy, and the isotope mass. Since there are deep potential wells in these two potential energy surfaces, their kinetic characteristics are similar to each other and the isotopic effect is not obvious. However, the well depths and configurations of the two potential energy surfaces are different, so the effects of isotopic substitution on the integral cross-section and the rotational polarization of product are different.

Photodissociation as a probe of the H3+ avoided crossing seam.

Experiments are conducted to investigate the role of the avoided crossing seam in the photodissociation of H3+. Three-dimensional imaging of dissociation products is used to determine the kinetic energy release and branching ratio among the fragmentation channels. Vibrational distributions are measured by dissociative charge transfer of H2+ products. It is found that the photodissociation of hot H3+ in the near-ultraviolet produces cold H2+, but hot H2. Modelling the wavepacket dynamics along the repulsive potential energy surface accounts for the repopulation of the ground potential energy surface. The role of the avoided crossing seam is emphasized and its importance for the astrophysically relevant charge transfer reactions underlined. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

Electronic spectroscopy of carbon chains (C2n+1, n = 7–10) of astrophysical importance. I. Quantum chemistry

Carbon chains have been predicted to be potential carriers of diffuse interstellar band features in astrophysical observations. Motivated by numerous predictions, we set out to carry out extensive ab initio quantum chemistry calculations to establish the ground and excited electronic potential energy surfaces and their coupling surfaces for carbon chains containing an odd number of carbon atoms (C2n+1, n = 7–10). Vibronic coupling models are established with the aid of the calculated electronic energies to investigate nuclear dynamics from first principles. The latter are reported in Ghosh et al. [J. Chem. Phys. 151, 054304 (2019)]. The mentioned carbon chains possess a linear cumulenic structure at the equilibrium minimum of their electronic ground state, and an electronic excited state of the Σu+1 term appears to be extremely bright optically and absorbs in the visible region of the electromagnetic spectrum. Vertical excitation energy of this state decreases and transition dipole moment increases, and as a result, the oscillator strength of this state linearly increases with an increase of the chain length. There are states belonging to 1Πg, 1Πu, Σg+1, 1Δg, and 1Δu terms, in the immediate vicinity of the Σu+1 state, which are optically dark but can gain intensity through vibronic coupling with the optically bright Σu+1 state. Construction of a coupling scheme considering the Renner-Teller coupling within the degenerate Π states and pseudo-Renner-Teller coupling between the Renner-Teller split component states as well as with the nondegenerate Σ states is another motivation of this work. The coupled-state Hamiltonian is constructed in a diabatic electronic basis in terms of the dimensionless normal coordinates of the vibrational modes of the carbon chains. Both Renner-Teller and pseudo-Renner-Teller types of couplings are included in the Hamiltonian. The theoretical results are discussed in relation to the experimental findings.

Structural, energetic and spectroscopic characterisation of 5-fluorouracil anticarcinogenic drug isomers, tautomers and ions

ABSTRACTIn many cases, 5-fluorouracil (5-FU) is the first-line drug used in combined chemotherapy and radiotherapy treatments due to its radio-sensitization properties. It could participate in a tautomerization process similar to that of uracil, where 5-FU may couple to adenine in DNA. At present, we performed structural and spectroscopic studies using quantum chemical methods of neutral and cationic isolated 5-FU anticarcinogenic drug tautomers, either interacting with a water molecule or embedded into an implicit water solvent. Also, we determined the stationary points (both stable structures and transition states) on their ground potential energy surfaces playing a role during the tautomerization processes. For neutral and ionic species in the gas phase and in solvent, the ordering of the tautomers is found to be the same, where the di-keto form of 5-FU is the most stable structure, followed by the keto–enol and di-enol structural forms. The energy barriers for tautomerization are strongly reduced in solvent (< 0.5 eV) compared to isolated species (∼2 eV). The patterns of their lowest electronic states are also computed. Our data may help for the identification of these species in vivo and in the laboratory.

Phase-space wavepacket dynamics of internal conversion via conical intersection: Multi-state quantum Fokker-Planck equation approach

We theoretically investigate internal conversion processes of a photoexcited molecule in a condensed phase. The molecular system is described by two-dimensional adiabatic ground and excited potential energy surfaces that are coupled to heat baths. We quantify the role of conical intersection (CI) and avoided crossing (AC) in the PESs in dissipative environments by simulating the time evolution of wavepackets to compute the lifetime of the excited wavepacket, yield of the product, and adiabatic electronic coherence. For this purpose, we employ the multi-state quantum Fokker-Planck equation (MSQFPE) for a two-dimensional Wigner space utilizing the Wigner–Moyal expansion for the potential term and the Brinkman hierarchy expression for the momentum. We find that the calculated results are significantly different between the CI and AC cases due to the transition in the tuning mode and vibrational motion in the coupling mode.