Ground State Potential Energy Surface Research Articles

Near-infrared reversible photoswitching of an isolated azobenzene-stilbene dye

Photoswitching of a charged azobenzene-stilbene dye is investigated through laser excitation in a tandem ion mobility mass spectrometer. Action spectra associated with E → Z and Z → E photoisomerisation of the stilbene group exhibit bands at 685 and 440 nm, corresponding to S1 ← S0 and S3 ← S0 transitions, respectively. The data suggest that isomers possessing a Z configuration of the azobenzene unit rapidly convert to E isomers and are not discernible using ion mobility spectrometry, and that photoisomerisation occurs through excited state dynamics rather than statistical isomerisation on the ground state potential energy surface.

Non-adiabatic quantum dynamics of the ultracold Li+LiNa→ Li2+Na chemical reaction

SynopsisWe report non-adiabatic dynamics of the Li+LiNa→Li2+Na chemical reaction at cold and ultracold temperatures employing accurate ab initio electronic potential energy surfaces in a quantum dynamics formulation employing a diabatic representation. Results are compared against those from a single adiabatic ground state potential energy surface and a universal model based on the long-range interaction potential. We discuss signatures of non-universal behavior in the total rate coefficients as well as strong non-adiabatic effects in the state-to-state rotationally resolved rate coefficients.

Quantum simulations of excited states with active-space downfolded Hamiltonians.

Many-body techniques based on the double unitary coupled cluster (DUCC) ansatz can be used to downfold electronic Hamiltonians into low-dimensional active spaces. It can be shown that the resulting dimensionality reduced Hamiltonians are amenable for quantum computing. Recent studies performed for several benchmark systems using phase estimation (PE) algorithms for quantum computers demonstrated that these formulations can recover a significant portion of ground-state dynamical correlation effects that stem from the electron excitations outside of the active space. These results have also been confirmed in studies of ground-state potential energy surfaces using quantum simulators. In this letter, we study the effectiveness of the DUCC formalism in describing excited states. We also emphasize the role of the PE formalism and its stochastic nature in discovering/identifying excited states or excited-state processes in situations when the knowledge about the true configurational structure of a sought after excited state is limited or postulated (due to the specific physics driving excited-state processes of interest). In this context, we can view PE algorithms as an engine for verifying various hypotheses for excited-state processes and providing statistically meaningful results that correspond to the electronic state(s) with the largest overlap with a postulated configurational structure. We illustrate these ideas on examples of strongly correlated molecular systems, characterized by small energy gaps and high density of quasidegenerate states around the Fermi level.

H atom Product Channels in the Ultraviolet Photodissociation of the 2-Propenyl Radical.

The H atom product channels in the ultraviolet photodissociation of 2-propenyl (CH2CCH3) radical were investigated in the wavelength region 224-248 nm using photofragment translational spectroscopy. The CH2CCH3 radicals were generated by 193 nm photodissociation of 2-chloropropene and 2-bromopropene precursors. The H atom photofragment yield spectra from both precursors revealed a broad feature peaking near 232 nm. The translational energy distributions of the H + C3H4 products peaked around 7-8 kcal/mol and extended close to the maximum excess energy. The fraction of the total available energy released as products' translation was nearly a constant (∼0.16 using the 2-chloropropene precursor and ∼0.18 using the 2-bromopropene precursor) in the wavelength range 224-248 nm. The angular distribution of the H atom product was isotropic. Quasi-classical trajectory (QCT) calculations were performed on the ground-state potential energy surface of CH2CCH3 for its decomposition at a 124 kcal/mol excitation energy (equivalent to 230 nm photolysis photon energy). The calculations yielded branching ratios for different dissociation product channels, 32% H + allene, 35% H + propyne, 0.5% H + cyclopropene, and 32% methyl + acetylene. The experimental and QCT translational energy distributions of the H atom loss channels qualitatively agreed, consistent with the main H atom product channels being the H + allene and H + propyne dissociations. The time scale of the 2-propenyl dissociation on the ground electronic state was calculated to be ∼2 ps, smaller compared to that of the overall UV photodissociation (≥10 ps, implied on the basis of the isotropic H atom product angular distribution). The mechanism of the UV photodissociation of 2-propenyl is consistent with unimolecular dissociation proceeding on the ground electronic state after internal conversion of the electronic excited states.

Ground state properties and potential energy surfaces of 270Hs from multidimensionally-constrained relativistic mean field model

We study the ground state properties, potential energy curves and potential energy surfaces of the superheavy nucleus $^{270}$Hs by using the multidimensionally-constrained relativistic mean-field model with the effective interaction PC-PK1. The binding energy, size and shape as well as single particle shell structure corresponding to the ground state of this nucleus are obtained. $^{270}$Hs is well deformed and exhibits deformed doubly magic feature in the single neutron and proton level schemes. One-dimensional potential energy curves and two-dimensional potential energy surfaces are calculated for $^{270}$Hs with various spatial symmetries imposed. We investigate in detail the effects of the reflection asymmetric and triaxial distortions on the fission barrier and fission path of $^{270}$Hs. When the axial symmetry is imposed, the reflection symmetric and reflection asymmetric fission barriers both show a double-hump structure and the former is higher. However, when triaxial shapes are allowed the reflection symmetric barrier is lowered very much and then the reflection symmetric fission path becomes favorable.

Can the symmetrical Dewar pyridine be observed experimentally? A theoretical study

ABSTRACTIn this work we study the possibility of the photochemical formation of the symmetrical Dewar pyridine (1-azabicyclo-[2,2,0]-hexa-2,5-diene), by applying the complete active space self-consistent field method and the multiconfigurational second-order perturbation theory to explore the corresponding ground and excited state potential energy surfaces. According to our theoretical calculations there are three possible paths that can be followed, one is a biphotonic process which involves irradiating pyridine in its ground state with a 358 nm laser guiding the system to an intersystem crossing S1/T1/S0 of triple character whereby deactivation to the ground state, S0, Dewar minimum occurs, the second one, which is a ground state thermal reaction involves the use of a far-Infra-Red laser where planar pyridine is vibrationally excited to a very high vibrational level whose energy is comparable to that of the ground state transition structure, S0(TS), connecting the symmetrical S0 Dewar pyridine and the ground state of planar pyridine. The third process is also a biphotonic one involving excitation of planar pyridine with an energy which is in the limits of its ionisation potential. In this case there is a theoretically accessible S1/S0 Conical Intersection which leads directly to the ground state of the symmetrical Dewar pyridine.

Computing the Ultrafast and Radiationless Electronic Excited State Decay of Cytosine and 5‐methyl‐cytosine Cations: Uncovering the Role of Dynamic Electron Correlation

AbstractPhotoionisation in DNA, i. e. the process of photoinduced electron removal from the chromophoric species – the nucleobases – leading to their cationic form, has been scarcely studied despite being considered to be responsible for significant damaging instances in our genetic material. In this contribution we theoretically characterise the electronic ground and excited state decay pathways of cationic DNA nucleobase cytosine+ and its epigenetic derivative 5‐methyl‐cytosine+, including the effects of dynamic electron correlation on energies and geometries of minima and conical intersections. We do this by comparing the results of XMS‐CASPT2 calculations with CASSCF estimates and we find some significant differences between the results of these two methods. In particular, including dynamic electron correlation is found to significantly reduce the barrier to access the conical intersection. We find notable similarities in both cytosine and 5‐methyl‐cytosine cations, and accessible conical intersections in the vicinity of the Franck‐Condon region are found. This points towards an ultrafast depopulation of their electronic excited states. Moreover, the shape of the ground state potential energy surface strongly directs the decaying excited state population towards the cationic ground state minimum on ultrafast timescales, preventing photo‐fragmentation and thus explaining their photostability. To better compare our calculations with the available experimental data we compute the UV (ground and excited state) and IR absorptions.

Taming the Complexity of Donor-Acceptor Stenhouse Adducts: Infrared Motion Pictures of the Complete Switching Pathway.

Switches that can be actively steered by external stimuli along multiple pathways at the molecular level are the basis for next-generation responsive material systems. The operation of commonly employed molecular photoswitches revolves around one key structural coordinate. Photoswitches with functionalities that depend on and can be addressed along multiple coordinates would offer novel means to tailor and control their behavior and performance. The recently developed donor–acceptor Stenhouse adducts (DASAs) are versatile switches suitable for such applications. Their photochemistry is well understood, but is only responsible for part of their overall photoswitching mechanism. The remaining thermal switching pathways are to date unknown. Here, rapid-scan infrared absorption spectroscopy is used to obtain transient fingerprints of reactions occurring on the ground state potential energy surface after reaching structures generated through light absorption. The spectroscopic data are interpreted in terms of structural transformations using kinetic modeling and quantum chemical calculations. Through this combined experimental–theoretical approach, we are able to unravel the complexity of the multidimensional ground-state potential energy surface explored by the photoswitch and use this knowledge to predict, and subsequently confirm, how DASA switches can be guided along this potential energy surface. These results break new ground for developing user-geared DASA switches but also shed light on the development of novel photoswitches in general.

Electrostatic Spectral Tuning Maps for Biological Chromophores.

The mechanism by which the absorption wavelength of a molecule is modified by a protein is known as spectral tuning. Spectral tuning is often achieved by electrostatic interactions that stabilize/destabilize or modify the shape of the excited and ground-state potential energy surfaces of the chromophore. We present a protocol for the construction of three-dimensional "electrostatic spectral tuning maps" that describe how vertical excitation energies in a chromophore are influenced by nearby charges. The maps are built by moving a charge on the van der Waals surface of the chromophore and calculating the change in its excitation energy. The maps are useful guides for protein engineering of color variants, for interpreting spectra of chromophores that act as probes of their environment, and as starting points for further quantum mechanical/molecular mechanical studies. The maps are semiquantitative and can approximate the magnitude of the spectral shift induced by a point charge at a given position with respect to the chromophore. We generate and discuss electrostatic spectral tuning maps for model chromophores of photoreceptor proteins, fluorescent proteins, and aromatic amino acids. Such maps may be extended to other properties such as oscillator strengths, absolute energies (stability), ionization energies, and electron affinities.

Unveiling the photophysics of thiourea from CASPT2/CASSCF potential energy surfaces and singlet/triplet excited state molecular dynamics simulations

This work describes the decay mechanism of photoexcited thiourea, both in gas phase and in solution, from the information inferred from the topography of the excited and ground state potential energy surfaces and mixed singlet/triplet quantum classical molecular dynamics simulations. Our gas phase results reveal T1/S0 intersystem crossing as the dominant (49%) intrinsic decay channel to the ground state, which reaches a population of 0.28 at the final time of our simulations (10 ps). Population of the T1, would occur after internal conversion to the S1 from the spectroscopic S2 electronic state, followed by S1 → T2 intersystem crossing and T2 → T1 internal conversion processes. Minor decay channels occurring exclusively along the singlet manifold, i.e. S2 → S0 (33%) and S1 → S0 (18%), were also observed to play a role in the relaxation of photoexcited thiourea in the gas phase. The explicit incorporation of water-thiourea interactions in our simulations was found to provoke a very significant delay in the decay to the ground state of the system, with no transitions to the S0 being registered during the first 10 ps of our simulations. Intermolecular vibrational energy redistribution and explicit hydrogen bond interactions established between water molecules and the NH2 group of thiourea were found to structurally or energetically hamper the access to the intersystem crossing or internal conversion funnels with the S0.

Ultrafast Spintronics: Dynamics of the Photoisomerization-Induced Spin-Charge Excited-State (PISCES) Mechanism in Spirooxazine-Based Photomagnetic Materials.

The optical control of spin state is of interest in the development of spintronic materials for data processing and storage technologies. Photomagnetic effects at the single-molecule level have recently been observed in the thin film state at 300 K in photochromic cobalt dioxolenes. Visible light excitation leads to ring-closure of a photochromic spirooxazine bound to a cobalt dioxolene, which leads to generation of a high magnetization state. Formation of the photomagnetic state occurs through a photoisomerization-induced spin-charge excited-state process and is dictated by the spirooxazine ligand dynamics. Here, we report a mechanistic investigation by ultrafast spectroscopy in the UV-vis region of the photochemical ring-closing process in the parent spirooxazine, azahomoadamantylphenanthroline spirooxazine, and the photomagnetic spirooxazine cobalt-dioxolene complex. The cobalt appears to stabilize a photomerocycanine transient intermediate, presumably the TCC isomer, formed along the ground-state potential energy surface (PES). Structural changes associated with the TCC isomer induces formation of the high-spin Co(II) form, suggesting that magnetization dymanics can occur along the excited-state PES, leading to ultrafast switching on the ps time scale. We demonstrate the full ring closure of the spiro-oxazine ligand is not required to switch magnetization states which can be induced with a higher yielding isomerization reaction. The ability of this system to undergo optically induced spin state switching on the ps time scale in the solid state makes it a promising canididate for resistive nonvolatile memory technologies.

Ab initio molecular dynamics study of the reactions of allene cation induced by intense 7 micron laser pulses

ABSTRACTThe isomerisation and fragmentation of allene cation (H2C=C=CH2+) by short, intense laser pulses were simulated by Born-Oppenheimer molecular dynamics (BOMD) on the ground state potential energy surface using the B3LYP/6-31 + G(d,p) level of theory and a 10 cycle 7 µm cosine squared pulse with a maximum field strength of 0.07 au. Laser fields polarised along the C=C=C axis deposits an average of 150 kcal/mol in the molecule, compared to only 25 and 51 kcal/mol for perpendicular polarisations. Approximately 90% of the trajectories with the field aligned with the C=C=C axis underwent one or more structural rearrangement steps to form H2C=CH–CH+ (15%), H3CCCH+ (4%), cyclopropene cation (6%), and allene cation with rearranged hydrogens and carbons (47%). In addition, a variety of fragments including H2CCCH+ + H (10%), c-C3H3+ + H (7%), and HCCCH+ + H2 (2%) trajectories were produced after isomerisation. With the same amount of thermal energy, field-free BOMD shows good agreements with the BOMD with the field. However, RRKM calculations favour isomerisation to propyne cation and dissociation to HCCCH+ + H2. This suggests that for molecules in intense laser fields the energy in the intermediate isomers is not distributed statistically.

Absorption and luminescence spectroscopy of mass-selected flavin adenine dinucleotide mono-anions.

We report the absorption profile of isolated Flavin Adenine Dinucleotide (FAD) mono-anions recorded using photo-induced dissociation action spectroscopy. In this charge state, one of the phosphoric acid groups is deprotonated and the chromophore itself is in its neutral oxidized state. These measurements cover the first four optical transitions of FAD with excitation energies from 2.3 to 6.0 eV (210-550 nm). The S0 → S2 transition is strongly blue shifted relative to aqueous solution, supporting the view that this transition has a significant charge-transfer character. The remaining bands are close to their solution-phase positions. This confirms that the large discrepancy between quantum chemical calculations of vertical transition energies and solution-phase band maxima cannot be explained by solvent effects. We also report the luminescence spectrum of FAD mono-anions in vacuo. The gas-phase Stokes shift for S1 is 3000 cm-1, which is considerably larger than any previously reported for other molecular ions and consistent with a significant displacement of the ground and excited state potential energy surfaces. Consideration of the vibronic structure is thus essential for simulating the absorption and luminescence spectra of flavins.

Double Molecular Photoswitch Driven by Light and Collisions.

The shapes of many molecules can be transformed by light or heat. Here we investigate collision- and photon-induced interconversions of EE, EZ, and ZZ isomers of the isolated Congo red (CR) dianion, a double molecular switch containing two ─N═N─ azo groups, each of which can have the E or Z configuration. We find that collisional activation of CR dianions drives a one-way ZZ→EZ→EE cascade towards the lowest-energy isomer, whereas the absorption of a single photon over the 270-600nm range can switch either azo group from E to Z or Z to E, driving the CR dianion to lower- or higher-energy forms. The experimental results, which are interpreted with the aid of calculated statistical isomerization rates, indicate that photoisomerization of CR in the gas phase involves a passage through conical intersection seams linking the excited and ground state potential energy surfaces rather than through isomerization on the ground state potential energy surface following internal conversion.

OCS isomerization and dissociation kinetics from statistical models

In this work computational modeling of the isomerization reactions of the linear OCS, CSO, COS, and the cyclic Δ-OCS structures, on the ground state Potential Energy Surface (PES), is performed using Rice–Ramsperger–Kassel–Marcus (RRKM), Semiclassical Transition State Theory (SCTST), and the Master Equation (ME). Adiabatic dissociations of the sulfur atom from the OCS, $$\Delta $$ -OCS and COS structures are also studied with the Variational Transition State Theory (VTST). Intramolecular Vibrational Energy Redistributions (IVRs), calculated within the adiabatic approximation framework, appear to be faster than the corresponding reactions, justifying the use of statistical methods to study the kinetics of the reactions. From the microcanonical rate coefficients, we establish that the lifetimes of isomers $$\Delta $$ -OCS and COS are insufficient to complete one vibrational period of the corresponding molecules. The energy-specific branching ratios lead us to conclude that only 0.08 and 0.07% of molecules reacting from the most stable isomer OCS produce CSO and COS, respectively, at 180 kcal/mol (when all the reaction channels are open). This can be attributed to the dominance of the dissociation over the isomerizations. From the SCTST treatment, we find that coupling and tunneling are very important in this system, as deviations from the Arrhenius-type behavior in the reaction kinetics are considerable, particularly at low temperatures.

Potential energy surface of triplet O4.

We present a global ground-state potential energy surface (PES) for the triplet spin state of O4 that is suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation in electronically adiabatic spin-conserving O2-O2 collisions. The surface is based on MS-CASPT2/maug-cc-pVTZ electronic structure calculations with scaled external correlation; the active space has 16 electrons in 12 orbitals. The global ground-state potential energy surface was fitted by a many-body approach with an accurate O-O pairwise interaction and a fit of the many-body interaction potential to 10 180 electronic structure data points. The many-body fit is based on permutationally invariant polynomials in terms of bond-order functions of the six interatomic distances; the bond-order functions are mixed exponential-Gaussian functions. The geometries calculated and used for the fit include geometry scans corresponding to dissociative and vibrationally excited diatom-diatom collisions of O2, scans corresponding to O3 interacting with O, additional geometries identified by running trajectories, and geometries along linear synchronous transit paths connecting randomly selected points. The global O4 PES includes subsurfaces describing the interaction of diatomic molecules with other diatomic molecules or interactions of triatomic molecules and an atom. The interaction of ozone with a ground-state oxygen atom occurs on the triplet O4 surface, and our surface includes high-energy points with O3-O geometries as well as O2-O2 geometries and O2-O-O geometries.

Novel potential energy surface-based quantum dynamics of ion–molecule reaction **Project supported by the National Natural Science Foundation of China (Grant Nos. 11674198 and 11304185).

According to a novel electronic ground-state potential energy surface of , we calculate the reaction probabilities and the integral cross section for the titled reaction by the Chebyshev wave packet propagation method. The reaction probabilities in a collision-energy range of 0.0 eV–1.0 eV show an oscillatory structure for the reaction due to the existence of the potential well. Compared with the results of Martínez et al., the present integral cross section is large, which is in line with experimental data.

Ab initio potential energy surface and vibration-rotation energy levels of germanium dicarbide, GeC2.

The accurate ground-state potential energy surface of germanium dicarbide, GeC2 , has been determined from ab initio calculations using the coupled-cluster approach. The core-electron correlation, higher-order valence-electron correlation, and scalar relativistic effects were taken into account. The potential energy surface of GeC2 was shown to be extraordinarily flat near the T-shaped equilibrium configuration. The potential energy barrier to the linear CCGe configuration was predicted to be 1218 cm-1 . The vibration-rotation energy levels of some GeC2 isotopologues were calculated using a variational method. The vibrational bending mode ν3 was found to be highly anharmonic, with the fundamental wavenumber being only 58 cm-1 . Vibrational progressions due to this mode were predicted for the v1=1, v2=1, and v2=2 states of GeC2 . © 2018 Wiley Periodicals, Inc.

Vibrational treatment of the formic acid double minimum case in valence coordinates.

One single full dimensional valence coordinate HCOOH ground state potential energy surface accurate for both cis and trans conformers for all levels up to 6000 cm-1 relative to trans zero point energy has been generated at CCSD(T)-F12a/aug-cc-pVTZ level. The fundamentals and a set of eigenfunctions complete up to about 3120 and 2660 cm-1 for trans- and cis-HCOOH, respectively, have been calculated and assigned using the improved relaxation method of the Heidelberg multi-configuration time-dependent Hartree package and an exact expression for the kinetic energy in valence coordinates generated by the TANA program. The calculated trans fundamental transition frequencies agree with experiment to within 5 cm-1. A few reassignments are suggested. Our results discard any cis trans delocalization effects for vibrational eigenfunctions up to 3640 cm-1 relative to trans zero point energy.

Correlated Molecular Orbital Theory Study of the Al + CO2 Reaction.

Density functional theory (DFT) and correlated molecular orbital electronic structure calculations were used to study the Al + CO2 → AlO + CO reaction on the electronic ground-state potential-energy surface (PES). Geometries were optimized using DFT (M11/jun-cc-pV(Q+d)Z) and more accurate energies were obtained using the composite Weizmann-1 theory with Brueckner doubles (W1BD). The results comprise the most complete, most systematic characterization of the Al + CO2 reaction surface to date and are based on consistent application of high-level methods for all stationary points identified. The pathways from Al + CO2 to AlO + CO on the electronic ground-state PES all involve formation of one or more stable AlCO2 complexes denoted η-AlCO2, trans-AlCO2, and C2v-AlCO2, among which η-AlCO2 and C2v-AlCO2 are the least and most stable, respectively. We report a new minimum-energy pathway for the overall reaction, namely formation of η-AlCO2 from reactants and dissociation of that same complex to products via a bond-insertion reaction that passes through a fourth (weakly metastable) AlCO2 complex denoted cis-OAlCO. Natural Bond Orbital analysis was applied to study trends in charge distribution and the degree of charge transfer in key structures along the minimum-energy pathway. The process of aluminum insertion into CO2 is discussed in the context of analogous processes for boron and first-row transition metals.