Potential Energy Surface Crossings Research Articles

Collisional Excitation of CF+ by H2: Potential Energy Surface and Rotational Cross Sections

The CF+ molecule is considered one of the key species for the study of fluorine chemistry in the interstellar medium (ISM). Its recent detection, as well as its potential use as a tracer for atomic fluorine in the ISM, has increased the interest in the study of the physical and chemical properties of this cation. Accurate determination of the CF+ abundance in the ISM requires detailed modeling of its excitation from both radiation and collisions with the most dominant species, which are usually atomic and molecular hydrogen. Here, we report a new highly accurate potential energy surface (PES) describing the interaction between CF+ and the H2 molecule. Exact quantum time-independent calculations of the rotational excitation cross sections for collisions of CF+ with both para- and ortho-H2 are reported. Results obtained for collisions with para-H2 are compared to previous calculations performed using an approximate PES averaged over H2 rotation. Excitation data for collisions with ortho-H2 are provided for the first time.

State-interaction pair density functional theory for locally avoided crossings of potential energy surfaces in methylamine.

The strong couplings between electronic states in conical intersection regions are among the most challenging problems in quantum chemistry. XMS-CASPT2, a second-order multireference quasidegenerate perturbation theory, has been successful in describing potential energy surfaces near the conical intersections. We have recently proposed a less expensive method for this problem, namely state-interaction pair-density functional theory (SI-PDFT), which considers the coupling between electronic states described by multiconfiguration pair-density functional theory (MC-PDFT). Here we test the accuracy of SI-PDFT for closely coupled potential energy surfaces of methylamine along five different reaction paths for N-H bond fission. We choose paths that pass close to a conical intersection of the ground and first excited states. We find that SI-PDFT predicts potential energy curves and energy splittings near the locally avoided crossing in close proximity to those obtained by XMS-CASPT2. This validates the method for application to photochemical simulations.

Instability in pyramidal–tetrahedral structure including elements from group 14 induced by pseudo-Jahn–Teller effect

The pseudo-Jahn–Teller effect (PJTE) of the pyramidal–tetrahedral A42− (A = C, Si, Ge, Sn) dianion analogues is investigated by quantum chemical calculations. The adiabatic potential energy surface cross sections obtained from ab initio calculation of the A42− analogues show instability for a degenerate E ground state. Thus, symmetry breaking phenomena induced by the PJTE occur in the series, and the unstable pyramidal–tetrahedral configuration with Td symmetry distorts to the equilibrium geometry structure (lower C2 symmetry) in all considered dianions. Additionally, the (E(1) + A1 + E(2)) ⊗ e problem is formulated for the analogues with the APES cross sections used in order to estimate the coupling constants by fitting the energies acquired from the PJTE equations. To restore the pyramidal–tetrahedral structure with Td symmetry and quenching the PJTE in the series, the PJTE in A42− analogues was suppressed through (1) adding two electrons to the A42− series and (2) either doping the A42− analogues with an atom from group 14 or trapping He2+ and Ne2+ inside the Si42− cage.

Instability of two–dimensional structure of dichalcogenin and dipnictogenin octa–heterocyclic systems with 1,2–C6X2(X = O, S, Se, Te, N, P) unsaturated rings

The planar configuration and twisted structures of the dichalcogenin (DCG) and dipnictogenin (DPG) octa–heterocyclic systems with 1,2–C6X2 ring in their main skeleton were computed through ab initio geometry optimization and following frequency calculations. Whereas the C6X2 eight–membered unsaturated rings in the considered compounds are unstable in their planar configurations, the molecules distorted from their planar shape with C2v, high symmetry into the C2, lower symmetry, twisted equilibrium configuration. Based on adiabatic potential energy surface (APES) cross sections, coupling between the ground state (1A1) and 1A2 excited state and the PJTE (1A1 + 1A2 + ···) ⊗ a2 are the reasons for the C6X2 unsaturated rings’ two–dimensional structure instability. Moreover, substitution of two hydrogen atoms in bonding with nitrogen and phosphorus positions of DPGs with halogens significantly changed the effect in the 1,2–diazocine and 1,2–diphosphocine compounds. Furthermore, intramolecular hydrogen bonding in the systems with 1,2–C6X2 (X = O, N) rings affect two–dimensional instability of the molecular structure.

Variation of the Intersection Point of the Potential Surface Crossing Induced by the Laser Phase Along the Reaction Path in Ion-Molecule Reactions: Application To Li+ + CH4

A laser ion-molecule reaction interaction through both polarizability and dipole moment contribution leads to variation in the intersection point in potential energy surface crossings along the reaction path; the polarizability is maximum and the dipole changes its sign at s = 4 a.u., defining a virtual transition state. Using the gauge representation (electric field gauge) for a wave length λ = 20.6 μm, intensity I = 5×1012 W/cm2, I = 1×1013 W/cm2, I = 3×1013 W/cm2, we show here that we can create a laser-induced potential energy surface crossing along the reaction path (s = 7-8 a.u.). We illustrate such effects for the Li H + CH 3 + ↔ Li+ + CH4 reaction which takes the form of inverted Morse (without a barrier) using ab initio methods for calculating the reaction path and electric properties of the ion-molecule reaction.

Why does 4-biphenyl carbonyl azide have ultra-short lived excited states? An ultrafast UV-vis spectroscopic and computational study.

Upon excitation at 308 nm, 4-biphenyl carbonyl azide (4-BpCON3) shows unusually fast decay of transient absorption associated with the first excited singlet state, with time constants of several ps in MeOH, acetonitrile, and CHCl3. In cyclohexane and cyclohexene, the lifetimes are ca. 0.3 ps, which is in stark contrast to the lifetimes of hundreds of ps in the case of 2-naphthoyl azide. Furthermore, photolysis at 266 and 308 nm brought about the same yields of nitrene and isocyanate products. To understand these findings, we also applied ultrafast transient absorption spectroscopy to the structurally related molecule, fluorene-2-carbonyl azide (F2CON3), in which the two phenyl rings are fixed in a plane by a methylene group. Both carbonyl azides (biphenyl and fluorenyl) have very short lived excited states in different solvents, indicating that the twisting of phenyl rings is not the reason for the fast decay. Theoretical studies using time dependent density functional theory (TDDFT), especially with PBE0 and CAM-B3LYP functionals, suggest that excited-state potential energy surface crossings lead to the efficient and fast decomposition of carbonyl azides upon photoexcitation. Especially, the decay of the Franck-Condon state to the S1 state with π(CON3)-π*(N3') transition character, where -N3 is in a bent conformation (∠NNN = ca. 125°), is the key step. Finally, a model is presented to explain solvent dependence, different decaying rates, and other experimental findings.

Simulating Coherent Multidimensional Spectroscopy of Nonadiabatic Molecular Processes: From the Infrared to the X-ray Regime.

Crossings of electronic potential energy surfaces in nuclear configuration space, known as conical intersections, determine the rates and outcomes of a large class of photochemical molecular processes. Much theoretical progress has been made in computing strongly coupled electronic and nuclear motions at different levels, but how to incorporate them in different spectroscopic signals and the approximations involved are less established. This will be the focus of the present review. We survey a wide range of time-resolved spectroscopic techniques which span from the infrared to the X-ray regimes and can be used for probing the nonadiabatic dynamics in the vicinity of conical intersections. Transient electronic and vibrational probes and their theoretical signal calculations are classified by their information content. This includes transient vibrational spectroscopic methods (transient infrared and femtosecond off-resonant stimulated Raman), resonant electronic probes (transient absorption and photoelectron spectroscopy), and novel stimulated X-ray Raman techniques. Along with the precise definition of what to calculate for predicting the various signals, we outline a toolbox of protocols for their simulation.

Molecular Propensity as a Driver for Explorative Reactivity Studies.

Quantum chemical studies of reactivity involve calculations on a large number of molecular structures and the comparison of their energies. Already the setup of these calculations limits the scope of the results that one will obtain, because several system-specific variables such as the charge and spin need to be set prior to the calculation. For a reliable exploration of reaction mechanisms, a considerable number of calculations with varying global parameters must be taken into account, or important facts about the reactivity of the system under consideration can remain undetected. For example, one could miss crossings of potential energy surfaces for different spin states or might not note that a molecule is prone to oxidation. Here, we introduce the concept of molecular propensity to account for the predisposition of a molecular system to react across different electronic states in certain nuclear configurations or with other reactants present in the reaction liquor. Within our real-time quantum chemistry framework, we developed an algorithm that automatically detects and flags such a propensity of a system under consideration.

The origin of unequal bond lengths in the C̃ (1)B2 state of SO2: Signatures of high-lying potential energy surface crossings in the low-lying vibrational structure.

The C̃ (1)B2 state of SO2 has a double-minimum potential in the antisymmetric stretch coordinate, such that the minimum energy geometry has nonequivalent SO bond lengths. The asymmetry in the potential energy surface is expressed as a staggering in the energy levels of the ν3(') progression. We have recently made the first observation of low-lying levels with odd quanta of v3('), which allows us-in the current work-to characterize the origins of the level staggering. Our work demonstrates the usefulness of low-lying vibrational level structure, where the character of the wavefunctions can be relatively easily understood, to extract information about dynamically important potential energy surface crossings that occur at much higher energy. The measured staggering pattern is consistent with a vibronic coupling model for the double-minimum, which involves direct coupling to the bound 2 (1)A1 state and indirect coupling with the repulsive 3 (1)A1 state. The degree of staggering in the ν3(') levels increases with quanta of bending excitation, which is consistent with the approach along the C̃ state potential energy surface to a conical intersection with the 2 (1)A1 surface at a bond angle of ∼145°.

Quantum Dynamics of Li+HF/DF Reaction Investigated by a State-to-State Time-dependent Wave Packet Approach

Using the reactant coordinate based time-dependent wave packet method, on the APW potential energy surface, the differential and integral cross sections of the Li+DF/HF(v=0, j=0, 1) reactions were calculated over the collision energy range from the threshold to 0.25 eV. The initial state-specified reaction rate constants of the title reaction were also calculated. The results indicate that, compared with the Li+DF reaction, the product LiF of Li+HF reaction is a little more rotationally excited but essentially similar. The initial rotational excitation from j=0 to 1 has little effect on the Li+DF reaction. However, the rotational excitation of DF does result in a little more rotationally excited product LiF. The different cross section of both reactions is forward biased in the studied collision energy range, especially at relatively high collision energy. The resonances in the Li+HF reaction may be identifiable as the oscillations in the product ro-vibrational state-resolved integral cross sections and backward scattering as a function of collusion energy. For the Li+HF reaction, the rate constant is not sensitive to the temperature and almost has no change in the temperature range considered. For the Li+DF reaction, the rate constant increase by a factor of about 10 in the temperature range of 100–300 K. Brief comparison for the total reaction probabilities and integral cross section of the Li+HF reaction has been carried out between ours and the values reported previously. The agreement is good, and the difference should come from the better convergence of our present calculations.

Photoisomerization of Silyl-Substituted Cyclobutadiene Induced by σ → π* Excitation: A Computational Study

Photoinduced chemical processes upon Franck-Condon (FC) excitation in tetrakis(trimethylsilyl)-cyclobutadiene (TMS-CBD) have been investigated through the exploration of potential energy surface crossings among several low-lying excited states using the complete active space self-consistent field (CASSCF) method. Vertical excitation energies are also computed with the equation-of-motion coupled-cluster model with single and double excitations (EOM-CCSD) as well as the multireference Møller-Plesset (MRMP) methods. Upon finding an excellent coincidence between the computational results and experimental observations, it is suggested that the Franck-Condon excited state does not correspond to the first π-π* single excitation state (S1, 1(1)B1 state in terms of D2 symmetry), but to the second (1)B1 state (S3), which is characterized as a σ-π* single excitation state. Starting from the Franck-Condon region, a series of conical intersections (CIs) are located along one isomerization channel and one dissociation channel. Through the isomerization channel, TMS-CBD is transformed to tetrakis(trimethylsilyl)-tetrahedrane (TMS-THD), and this isomerization process could take place by passing through a "tetra form" conical intersection. On the other hand, the dissociation channel yielding two bis(trimethylsilyl)-acetylene (TMS-Ac) molecules through further stretching of the longer C-C bonds might be more competitive than the isomerization channel after excitation into S3 state. This mechanistic picture is in good agreement with recently reported experimental observations.

Spin‐restricted ensemble‐referenced Kohn–Sham method: basic principles and application to strongly correlated ground and excited states of molecules

Ensemble density functional theory (DFT) is a novel theoretical approach that is capable of exact treatment of non‐dynamic electron correlation in the ground and excited states of many‐body fermionic systems. In contrast to ordinary DFT, ensemble DFT has not found so far a way to the repertoire of methods of modern computational chemistry, probably owing to the lack of practically affordable implementations of the theory. The spin‐restricted ensemble‐referenced Kohn–Sham (REKS) method represents perhaps the first computational scheme that makes ensemble DFT calculations feasible. The REKS method is based on the rigorous ensemble representation of the energy and the density of a strongly correlated system and provides for an accurate and consistent description of molecular systems the electronic structure of which is dominated by the non‐dynamic correlation. This includes the ground and excited states of molecules undergoing bond breaking/bond formation, the low‐spin states of biradicals and polyradicals, symmetry forbidden chemical reactions and avoided crossings of potential energy surfaces, real intersections between the energy surfaces of the ground and excited states (conical intersections), and many more. The REKS method can be employed in connection with any local, semi‐local and hybrid (global and range‐separated) functional and affords calculations of large and very large molecular systems at a moderate mean‐field cost. WIREs Comput Mol Sci 2015, 5:146–167. doi: 10.1002/wcms.1209This article is categorized under: Structure and Mechanism > Molecular Structures Electronic Structure Theory > Density Functional Theory Theoretical and Physical Chemistry > Spectroscopy

Artifacts due to trivial unavoided crossings in the modeling of photoinduced energy transfer dynamics in extended conjugated molecules

A previously developed algorithm to identify potential energy surface crossings involving interacting or noninteracting states during nonadiabatic excited-state molecular dynamics simulations, allows the diabatic pathway to be followed through the crossing region so that there is no experienced change in the states identity. In this Letter, we investigate the transition from interacting/delocalized states to noninteracting/localized states in oligomers of poly-phenylene vinylene separated by varying distances. We demonstrate that the appearance of trivial unavoided crossings during nonadiabatic dynamics leads to artifacts in the state population analysis. Consequently, changes in the localization of the electronic transition density must be followed instead.

Coupled electron-nuclear dynamics: Charge migration and charge transfer initiated near a conical intersection

Coupled electron-nuclear dynamics, implemented using the Ehrenfest method, has been used to study charge migration with fixed nuclei, together with charge transfer when nuclei are allowed to move. Simulations were initiated at reference geometries of neutral benzene and 2-phenylethylamine (PEA), and at geometries close to potential energy surface crossings in the cations. Cationic eigenstates, and the so-called sudden approximation, involving removal of an electron from a correlated ground-state wavefunction for the neutral species, were used as initial conditions. Charge migration without coupled nuclear motion could be observed if the Ehrenfest simulation, using the sudden approximation, was started near a conical intersection where the states were both strongly coupled and quasi-degenerate. Further, the main features associated with charge migration were still recognizable when the nuclear motion was allowed to couple. In the benzene radical cation, starting from the reference neutral geometry with the sudden approximation, one could observe sub-femtosecond charge migration with a small amplitude, which results from weak interaction with higher electronic states. However, we were able to engineer large amplitude charge migration, with a period between 10 and 100 fs, corresponding to oscillation of the electronic structure between the quinoid and anti-quinoid cationic electronic configurations, by distorting the geometry along the derivative coupling vector from the D6h Jahn-Teller crossing to lower symmetry where the states are not degenerate. When the nuclear motion becomes coupled, the period changes only slightly. In PEA, in an Ehrenfest trajectory starting from the D2 eigenstate and reference geometry, a partial charge transfer occurs after about 12 fs near the first crossing between D1, D2 (N(+)-Phenyl, N-Phenyl(+)). If the Ehrenfest propagation is started near this point, using the sudden approximation without coupled nuclear motion, one observes an oscillation of the spin density--charge migration--between the N atom and the phenyl ring with a period of 4 fs. When the nuclear motion becomes coupled, this oscillation persists in a damped form, followed by an effective charge transfer after 30 fs.

Core-mass nonadiabatic corrections to molecules: $\rm \bf H_2$H2, $\rm \bf H_2^+$H2+, and isotopologues

For high-precision calculations of rovibrational states of light molecules, it is essential to include non-adiabatic corrections. In the absence of crossings of potential energy surfaces, they can be incorporated in a single surface picture through coordinate-dependent vibrational and rotational reduced masses. We present a compact method for their evaluation and relate in particular the vibrational mass to a well defined nuclear core mass derived from a Mulliken analysis of the electronic density. For the rotational mass we propose a simple, but very effective parametrization. The use of these masses in the nuclear Schrödinger equation yields numerical data for the corrections of a much higher quality than can be obtained with optimized constant masses, typically better than 0.1 cm(-1). We demonstrate the method for H(2), H(2)(+), and singly deuterated isotopologues. Isotopic asymmetry does not present any particular difficulty. Generalization to polyatomic molecules is straightforward.

Vibrational Conical Intersections as a Mechanism of Ultrafast Vibrational Relaxation

Presenting true crossings of adiabatic potential energy surfaces, conical intersections are a paradigm of ultrafast and efficient electronic relaxation dynamics. The same mechanism is shown to apply also for vibrational conical intersections, which may occur when two high-frequency modes (such as OH stretch vibrations) are coupled to low-frequency modes (such as hydrogen bonding modes). By derivation of a model Hamiltonian and its parametrization for a concrete example, malonaldehyde, the conditions that such conical intersections occur are identified and the consequences for the vibrational dynamics and spectra are demonstrated.

Theoretical studies on the reaction of NO2 with CO catalyzed by bare Os+ cations and its kinetic information

The potential energy surface crossings for reaction between NO2(2A1) and CO(1∑+) catalyzed by Os+ have been investigated using the density functional theory (DFT). Minimum-energy crossing points (MECPs) between the potential energy surfaces and the possible spin inversion process are discussed by means of spin–orbit coupling (SOC) calculations. The latter is found to be the energetically favored channel. The energetic span (δE) model has been applied in this cycle to obtain some kinetic information. TDTS (TOF-determining transition state) and the TDI (TOF-determining intermediate) were confirmed, and TOF (E-representation) was calculated at different temperatures.

Potential energy surface and rotational cross sections for methyl formate colliding with helium

A potential energy surface for helium interacting with methyl formate has been computed using high-level electronic structure methods. The interaction energies obtained on a three-dimensional grid have been fitted by an analytic function of interatomic distances with correct asymptotic behavior for large intermonomer separations. This potential has then been refitted using partial wave expansion in terms of the distance between centers of mass and spherical angles. The latter potential has been used to compute cross sections for the rotational excitations of methyl formate at the full quantum close-coupling level. Collisional propensity rules and astrophysical implications are discussed.

Comparison of the deactivation mechanism of 5‐fluorouracil with that of its parent system, uracil: The need of the use of the MS‐CASPT2 method

AbstractBecause of their biological significance in radiation damage and repair of the genetical material, a detailed understanding of the photophysical properties of DNA bases is very important. The key property in DNA is the ultrafast radiationless decay to the ground state that allows dissipating the energy before harmful photoproducts may be formed. Experimental results in the investigation of uracil suggest that there exist a pair of competitive mechanism for the radiationless deactivation of this basis. It is well established that the key point of its ultrafast radiationless decay is a Sπ/S0 conical intersection [where Sπ is the excited state populated by the photoexcitation, characterized by a 1(π‐π*) excitation], but there are still some point of controversy, like the possible existence and involvement of a stable Sπ species in the deactivation mechanism. Nucleobases tautomers and derivatives usually have different photochemical properties, and in general, they show a slower deactivation mechanism. The study of some derivatives of uracil can be interesting for comparative purposes and to help in the reassurance of the deactivation mechanisms proposed for uracil. This is one of the reasons for developing our study on 5‐fluorouracil (5‐FU), which exhibits much longer lived excited states than the parent system uracil. On top of this, 5‐FU is also of interest by itself because it is used in cancer treatment. We have studied the ground and first excited singlet states of 5‐FU using the complete active space self‐consistent field method (CASSCF)/MS‐complete active space perturbation theory to second order level (CASPT2) protocol that is known to provide accurate results for this type of systems. Minima, potential energy surface (PES) crossings and paths have been characterized and compared with those of uracil. Our results show that for 5‐FU, as opposite to uracil, a minimum exists in the PES of the Sπ state. The involvement of this minimum in the deactivation mechanism together with the presence of a small barrier in the relaxation path toward the Sπ/S0 conical intersection explain the longest decay time of the excited state of 5‐FU. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111:3405–3415, 2011

Laser Control of Ion/Atom–Molecule Interaction: Application to (X + CH4, X = Li+ and Li) Reactions

Abstract Laser atom/ion–molecule reaction interaction through dipole moments leads to potential energy surface crossings in ion molecule (Li+ + CH4) and barrier reshaping in atom–molecule reaction (Li + CH4). We will show here by using gauge representation (electric field gauge) that we can create laser-induced surface crossing in the region (4 au) where the dipole moment changes sign for the LiH + CH3+ Li+ + CH4 reaction and reshape the barrier height (increase/decrease the height) by changing the laser phase for Li + CH4 → LiH + CH3 where dipole moment changes sign in the transition state region.