Electronic Potential Energy Surfaces Research Articles

Collisions of electronically excited molecules: differential cross-sections for rotationally inelastic scattering of NO(A2Σ+) with Ar and He

The paper reports experimental measurements and theoretical calculations of rotational-state-resolved differential scattering cross-sections (DCS) for collisions between electronically excited NO(A2Σ+) molecules and rare gas atoms. The experimental NO(A2Σ+) + Ar and NO(A2Σ+) + He state-resolved product scattering distributions are determined using velocity-mapped ion imaging. The ion images are analysed to determine the state-resolved DCS, which are compared with new theoretical DCS calculated using quantum scattering methods on ab initio electronic potential energy surfaces. Both collision systems are imaged simultaneously; this constraint on the collision energies of the two systems aids the comparison to theory. The experimental and calculated DCS agree well for the NO(A2Σ+)/He system. For the NO(A2Σ+)/Ar scattering system, the experiments do not recover the degree of forward-scattering theoretically predicted and significant differences in the positions of the observed and predicted rotational rainbow features are apparent at large scattering angles, particularly for the most rotationally inelastic collision channels investigated: ΔN = 12,14.

Dynamics of ultraviolet-induced DNA lesions: Dewar formation guided by pre-tension induced by the backbone

The photophysical and photochemical processes driving the formation of the ultraviolet (UV)-induced DNA Dewar lesion from the T(6-4)T dimer are investigated by time-resolved spectroscopy and quantum chemical modelling. Time-resolved absorption and emission spectroscopy in the UV revealed a biexponential decay of the electronically excited state (S1) with time constants in the 100 ps and 1 ns range. From the S1 state the system forms the Dewar lesion (proven by time-resolved infrared spectroscopy), the triplet state of the T(6-4)T dimer and the ground state of the original T(6-4)T dimer. The decay process from the excited singlet is activated and thus temperature dependent. Quantum chemical modelling is used to describe the reaction path via a minimum on the excited electronic potential energy surface in close proximity to a triplet state. The transition to the Dewar isomer competes with internal conversion and with triplet formation. Only if the backbone between the two thymines is closed, is the Dewar isomer formed with a significant yield. The simulations reveal that the tension built up by the backbone is required for guiding the reaction to the conical intersection leading to the Dewar isomer.

The lowest-lying electronic singlet and triplet potential energy surfaces for the HNO–NOH system: Energetics, unimolecular rate constants, tunneling and kinetic isotope effects for the isomerization and dissociation reactions

The lowest-lying electronic singlet and triplet potential energy surfaces (PES) for the HNO-NOH system have been investigated employing high level ab initio quantum chemical methods. The reaction energies and barriers have been predicted for two isomerization and four dissociation reactions. Total energies are extrapolated to the complete basis set limit applying focal point analyses. Anharmonic zero-point vibrational energies, diagonal Born-Oppenheimer corrections, relativistic effects, and core correlation corrections are also taken into account. On the singlet PES, the (1)HNO → (1)NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol(-1). For the barrierless decomposition of (1)HNO to H + NO, the dissociation energy is estimated to be 47.48 ± 0.2 kcal mol(-1). For (1)NOH → H + NO, the reaction endothermicity and barrier are 5.25 ± 0.2 and 7.88 ± 0.2 kcal mol(-1). On the triplet PES the reaction energy and barrier including all corrections are predicted to be 7.73 ± 0.2 and 39.31 ± 0.2 kcal mol(-1) for the isomerization reaction (3)HNO → (3)NOH. For the triplet dissociation reaction (to H + NO) the corresponding results are 29.03 ± 0.2 and 32.41 ± 0.2 kcal mol(-1). Analogous results are 21.30 ± 0.2 and 33.67 ± 0.2 kcal mol(-1) for the dissociation reaction of (3)NOH (to H + NO). Unimolecular rate constants for the isomerization and dissociation reactions were obtained utilizing kinetic modeling methods. The tunneling and kinetic isotope effects are also investigated for these reactions. The adiabatic singlet-triplet energy splittings are predicted to be 18.45 ± 0.2 and 16.05 ± 0.2 kcal mol(-1) for HNO and NOH, respectively. Kinetic analyses based on solution of simultaneous first-order ordinary-differential rate equations demonstrate that the singlet NOH molecule will be difficult to prepare at room temperature, while the triplet NOH molecule is viable with respect to isomerization and dissociation reactions up to 400 K. Hence, our theoretical findings clearly explain why (1)NOH has not yet been observed experimentally.

Quantum chemical studies of photochromic properties of benzoxazine compound

Molecular electronic structure of ground and excited states of a photochromic indolo[2,1-b][1,3]benzoxazine compound incorporating closed-ring system, which opens upon UV light excitation, was studied using various quantum chemical methods. Three local minima of the ground electronic state potential energy surface and related transition states were identified along the path of rotation of 4-nitrophenol group. Additionally, three local minima of the excited electronic states were located. The evaluated transition energy barriers between local ground-state minima nearest to the initial structure of the investigated molecule are less than 2 kBT, making open structures likely to revert to the initial structure by thermalization. Results obtained using ab initio GMC–QDPT method were explored and compared to the widely used TD-DFT and semi-empiric ZINDO methods.

STATE-TO-STATE REACTION DYNAMICS OF THE D(H) + FO → OD(OH) + F REACTIONS ON THE LOWEST TWO TRIPLET STATES

Quasiclassical trajectory (QCT) calculations have been performed to investigate the reaction dynamics of the title reactions at a state-to-state level. The recently developed adiabatic potential energy surfaces (PESs) of the two triplet electronic states of 13A″ and 13A′ (Gómez-Carrasco S. et al., J Chem Phys121:4605, 2004; J Chem Phys123:114310, 2005) are employed in the present QCT calculations. Product vibrational and rotational state distributions have been calculated at three collision energies of 0.5, 0.7 and 0.9 eV. The product vibrational state distributions are found to be Gaussian distributions. Both the vibrational and rotational state distributions depend clearly on the isotope substitution, electronic PESs and collision energies. The observed phenomena are probably attributed to the competition of direct and indirect reactions.

Low-energy rotational inelastic collisions of H+ + CO system

The quantum mechanical state-to-state rotational excitation cross sections have been computed using the ab initio ground electronic state potential energy surface of the system [M. Mladenovic and S. Schmatz, J. Chem. Phys. 109, 4456 (1998)] computed at coupled-cluster single and double and triple perturbative excitations method using correlation-consistent polarized valence quadruple zeta basis set where the asymptotic potential have been computed using the dipole moment, quadrupole moment, and the molecular polarizability components and fitted to this interaction potential. The anisotropy of the surface has been analyzed in terms of the multipolar expansion coefficients for the rigid-rotor surface. The integral cross sections for rotational excitations have been computed by solving close-coupled equations at very low collision energies (5-200 cm(-1)) and the corresponding rates have been obtained for a range of low temperatures (5-175 K). The j = 0 → j(') = 1 rotational excitation cross section (and rate) is found to be the dominant followed by the j = 0 → j(') = 2 in these collision energies. The close-coupling, coupled-state, and infinite-order sudden approximations coupling calculations have been performed in the energy range of 0.1-1.0 eV using vibrational ground potential. The rotational cross sections have been obtained by performing computationally accurate close-coupling calculations at 0.1 eV using vibrationally averaged potential (ν = 1) and compared with the results of vibrational ground potential.

FTIR: A Novel Bio-Analytical Technique

Fourier Transform Infra Red (FTIR) spectroscopy is a magnificent technique. Infrared spectroscopy is based on the fact that molecules absorb specific frequencies that are characteristic to the structure. These absorptions are resonant frequencies of the absorbed radiation that matches with the frequency of the bond or group that vibrates. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and the associated vibrational coupling. In particular, in the Born-Oppenheimer and harmonic hypothesis when the Molecular Hamiltonian corresponding to the electronic ground can be approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular geometry, the resonant frequencies are determined by the normal modes corresponding to the molecular electronic ground state potential energy surface. The resonant frequencies can be related to the strength of the bond and the mass of the atoms. Thus, the frequency of the vibrations can be associated with a particular bond type.

A scheme to interpolate potential energy surfaces and derivative coupling vectors without performing a global diabatization

Simulation of non-adiabatic molecular dynamics requires the description of multiple electronic state potential energy surfaces and their couplings. Ab initio molecular dynamics approaches provide an attractive avenue to accomplish this, but at great computational expense. Interpolation approaches provide a possible route to achieve flexible descriptions of the potential energy surfaces and their couplings at reduced expense. A previously developed approach based on modified Shepard interpolation required global diabatization, which can be problematic. Here, we extensively revise this previous approach, avoiding the need for global diabatization. The resulting interpolated potentials provide only adiabatic energies, gradients, and derivative couplings. This new interpolation approach has been integrated with the ab initio multiple spawning method and it has been rigorously validated against direct dynamics. It is shown that, at least for small molecules, constructing an interpolated PES can be more efficient than performing direct dynamics as measured by the total number of ab initio calculations that are required for a given accuracy.

On the sodium D line emission in the terrestrial nightglow

Emission from atomic Na, consisting of a doublet of lines at 589.0 and 589.6 nm, is a prominent feature of the earth’s nightglow. A large data-base of measurements of the relative intensities of the D lines ( R D) was gathered at three locations: the ALOMAR observatory, Andenes (Norway, 69°N), Kuujjuarapik (Canada, 55°N) and the Danum Valley (Borneo, 8°N). R D varies between 1.5 and 2.0, with an average value of 1.67. These results were interpreted using a theoretical model of the Na nightglow which involves initial production of electronically excited NaO(A 2Σ) from the reaction between Na and O 3, followed either by reaction with O to generate Na( 2P J ) with a branching ratio of 1/6 and a J=3/2 to 1/2 propensity of 2.0, or quenching of NaO(A) to NaO(X 2Π) by O 2. The resulting NaO(X) then reacts with O to generate Na( 2P J ) with a branching ratio of 1/6 and a J=3/2 to 1/2 propensity of 1.5. These branching ratios and spin-orbit propensities are derived from statistical correlation of the electronic potential energy surfaces connecting the reactants NaO(A)+O and NaO(X)+O with the products Na+O 2, through the Na +O 2 − ion-pair intermediate. A fit of this statistical model to the results of an earlier laboratory study ( Slanger et al., 2005), where R D was measured as a function of the ratio [O]/[O 2], indicates that the rate coefficient for the quenching of NaO(A) by O 2 is around 1×10 −11 cm 3 molecule −1 s −1. The statistical model is also in good accord with recent high resolution observations of the Na D line widths ( Harrell et al., 2010). An atmospheric model is then used to show that gravity wave-driven perturbations to the Na layer can account for the observed variability of R D.

Quantum Chemistry Studies of Electronically Excited Nitrobenzene, TNA, and TNT

The electronic excitation energies and excited-state potential energy surfaces of nitrobenzene, 2,4,6-trinitroaniline (TNA), and 2,4,6-trinitrotoluene (TNT) are calculated using time-dependent density functional theory and multiconfigurational ab initio methods. We describe the geometrical and energetic character of excited-state minima, reaction coordinates, and nonadiabatic regions in these systems. In addition, the potential energy surfaces for the lowest two singlet (S(0) and S(1)) and lowest two triplet (T(1) and T(2)) electronic states are investigated, with particular emphasis on the S(1) relaxation pathway and the nonadiabatic region leading to radiationless decay of S(1) population. In nitrobenzene, relaxation on S(1) occurs by out-of-plane rotation and pyramidalization of the nitro group. Radiationless decay can take place through a nonadiabatic region, which, at the TD-DFT level, is characterized by near-degeneracy of three electronic states, namely, S(1), S(0), and T(2). Moreover, spin-orbit coupling constants for the S(0)/T(2) and S(1)/T(2) electronic state pairs were calculated to be as high as 60 cm(-1) in this region. Our results suggest that the S(1) population should quench primarily to the T(2) state. This finding is in support of recent experimental results and sheds light on the photochemistry of heavier nitroarenes. In TNT and TNA, the dominant pathway for relaxation on S(1) is through geometric distortions, similar to that found for nitrobenzene, of a single ortho-substituted NO(2). The two singlet and lowest two triplet electronic states are qualitatively similar to those of nitrobenzene along a minimal S(1) energy pathway.

First Step in the Reaction of Zerovalent Iron with Water.

Here we present a comprehensive quantum chemical study of the simplest model system for the reactions of nanoscale zerovalent iron, i.e., the gas-phase reaction of an iron atom with water, to identify a theoretical method that provides reasonably accurate geometries and thermochemical data for selected iron compounds along the reaction path (Fe, FeO, HFeOH, Fe(OH)2). The energies of selected stationary points on the ground electronic potential energy surface were systematically studied using HF and post-HF methods (MP2, MP3, MP4, CCSD, CCSD(T), CASSCF, MRCI) and selected DFT functionals (B3LYP, B97-1, BPW91, M06, M06-HF, M06-L, M06-2X and MPW1K) using various basis sets up to the complete basis set. Scalar relativistic effects were modeled using the Douglas-Kroll-Hess Hamiltonian up to the fourth order, and the effects of valence plus outer-core electronic correlation were also evaluated. The calculations showed that (i) dynamic electron correlation is crucial for accurate modeling of the reactions in question, (ii) the PES around the stationary points along the reaction path is rather flat, (iii) the single-point energies calculated at the CCSD(T)/CBS level are in reasonably good agreement with experimental measurements, (iv) it is difficult to interpret DFT energies in the absence of benchmarking against experimental data or results obtained at a level of theory that is known to accurately reproduce experimental results, (v) relativistic effects are relatively modest in this system but should be included if chemical accuracy is desired, and (vi) careful analysis of the multireference character of the system and potential spin contamination is important. The CCSD(T)-3s3p-DKH2/CBS method can be considered the gold standard for this reaction because calculations at this level are in good agreement with experimental atomic excitation energies and thermochemical data. The gas-phase activation energy of the reaction between Fe and H2O is 23.6 kcal/mol including the ZPVE correction (ΔG(‡)298K = 29.2 kcal/mol), and HFeOH is a stable intermediate lying -31.2 kcal/mol below the reactants (ΔG298K = -25.4 kcal/mol).

Challenges of laser-cooling molecular ions

The direct laser cooling of neutral diatomic molecules in molecular beams suggests that trapped molecular ions can also be laser cooled. The long storage time and spatial localization of trapped molecular ions provides an opportunity for multi-step cooling strategies, but also requires careful consideration of rare molecular transitions. We briefly summarize the requirements that a diatomic molecule must meet for laser cooling, and we identify a few potential molecular ion candidates. We then carry out a detailed computational study of the candidates BH+ and AlH+, including improvedab initio calculations of the electronic state potential energy surfaces and transition rates for rare dissociation events. On the basis of an analysis of the population dynamics, we determine which transitions must be addressed for laser cooling, and compare experimental schemes using continuous-wave and pulsed lasers.

Anharmonic rovibrational analysis for disilacyclopropenylidene (Si2CH2)

The global minimum on the Si(2)CH(2) electronic singlet potential energy surface has been theoretically predicted to be a peculiar hydrogen bridged (Si···H···Si) disilacyclopropenylidene structure (Si(2)CH(2)). An accurate quartic force field for Si(2)CH(2) has been determined employing ab initio coupled-cluster theory with single and double excitations and a perturbative treatment for triple excitations [CCSD(T)], in combination with the correlation consistent core-valence quadruple zeta (cc-pCVQZ) basis set. The vibration-rotation coupling constants, equilibrium and zero-point vibration corrected rotational constants, centrifugal distortion constants, and harmonic and fundamental vibrational frequencies for six isotopologues of Si(2)CH(2) are predicted using vibrational second-order perturbation theory (VPT2). The anharmonic corrections for the vibrational motions involving the H bridged bonds are found to be more than 5% with respect to the corresponding harmonic vibrational frequencies. In this light, an experimental detection and characterization of disilacyclopropenylidene (Si(2)CH(2)) is highly desired.

Characterizing the Deformational Isomers of Bimetallic Ir2(dimen)42+ (dimen = 1,8-diisocyano-p-menthane) with Vibrational Wavepacket Dynamics

We studied the Ir(2)(dimen)(4)(2+) complex with ultrafast transient absorption spectroscopy and density functional theory and concluded that it possesses two singlet ground state isomers in room temperature solution. The molecule can adopt either a paddle wheel or a propeller conformation in solution, where the paddle wheel structure possesses a metal-metal bond of 4.4 Å and a dihedral angle between the quasi-C(4v) planes of 0° and the propeller structure has a metal-metal bond of 3.6 Å and a dihedral angle of 17° when crystallized. Each conformation has a distinct absorption in the visible attributed to a (1)(dσ(z)* → pσ(z)) excitation, with the long eclipsed structure absorbing at 475 nm and the short twisted structure absorbing at 585 nm. We independently pumped at each of these visible transitions to form vibrational wavepackets on the ground and excited state potential energy surfaces, which modulated the ground state bleach and stimulated emission signals, respectively. We found that the ground state wavepacket oscillates with a frequency of 48 cm(-1) when pumping the red peak and 11 cm(-1) when pumping the blue peak. We assign these frequencies to the Ir-Ir symmetric stretch, with the variation in frequency reflecting the variation in metal-metal bond strength in support of our assignment of the blue peak to the longer Ir-Ir bond length conformer and the red peak to the shorter Ir-Ir bond length conformer. When pumping the red peak, we found two modes with frequencies of 80 and 119 cm(-1) in the stimulated emission and only one mode at 75 cm(-1) when pumping the blue peak. We assign the 75-80 cm(-1) frequency to the Ir-Ir stretch and the 119 cm(-1) vibration to the dihedral angle twist in the excited state. The variation in the excited state dynamics does not result from the excitation of different electronic states, but rather from excitation to different Franck-Condon regions of the same electronic excited state potential energy surface. This occurs because of the large difference in ground state molecular structure. DFT calculations support the existence of a single electronic excited state being accessed from two distinct structural isomers with conformations similar to those observed with X-ray crystallography.

Stereodynamics of the reaction H + LiH ( v = 0, j = 0) → H 2 + Li and its isotopic variants

Quasi-classical trajectory (QCT) method is used to calculate the stereodynamics of the reactions H + LiH ( v = 0, j = 0) → H 2 + Li and its isotopic variants based on the ground electronic state potential energy surface (PES) reported by Prudente et al. [14]. The reactive probabilities of the title reactions are computed. We also observed the changes of vector correlations and four generalized polarization-dependent differential cross-sections (PDDCSs) at different collision energies, and we compared the stereodynamics among different isotopic variants of the title reactions. The product polarization distribution of the title reactions exhibits distinct difference which may arise from different mass combinations or kinetic energies.

Modeling the anharmonic infrared Emission Spectra of PAHs: Application to the Pyrene Cation

The IR emission cascade from the pyrene cation due to a broad band optical excitation is simulated using kinetic Monte Carlo. Anharmonicities of the ground electronic state potential energy surface are taken into account in the transition energies, the microcanonical densities of states, and the rate of hydrogen loss through various statistical theories. The emission spectral features of the “3.3”, “6.2” and “11.2” μm bands are computed for different blackbody temperatures.

Ionization dynamics of a water dimer: specific reaction selectivity

Ionization dynamics of a water dimer have been investigated by means of a direct ab initio molecular dynamics (MD) method. Two electronic state potential energy surfaces of (H(2)O)(2)(+) (ground and first excited states, (2)A'' and (2)A') were examined as cationic states of (H(2)O)(2)(+). Three intermediate complexes were found as product channels. One is a proton transfer channel where a proton of H(2)O(+) is transferred into the H(2)O and then a complex composed of H(3)O(+)(OH) was formed. The second is a face-to-face complex channel denoted by (H(2)O-OH(2))(+) where the oxygen-oxygen atoms directly bind each other. Both water molecules are equivalent to each other. The third one is a dynamical complex where H(2)O(+) and H(2)O interact weakly and vibrate largely with a large intermolecular amplitude motion. The dynamics calculations showed that in the ionization to the (2)A'' state, a proton transfer complex H(3)O(+)(OH) is only formed as a long-lived complex. On the other hand, in the ionization to the (2)A' state, two complexes, the face-to-face and dynamical complexes, were found as product channels. The proton of H(2)O(+) was transferred to H(2)O within 25-50 fs at the (2)A'' state, meaning that the proton transfer on the ground state is a very fast process. On the other hand, the decay process on the first excited state is a slow process due to the molecular rotation. The mechanism of the ionization dynamics of (H(2)O)(2) was discussed on the basis of theoretical results.

Quantum dynamics of H + LiH+ reaction on its electronic ground state

State specific dynamics of the H+LiH+ reaction is theoretically investigated on its electronic ground potential energy surface employing a time-dependent wave packet approach. Channel specific integral reaction cross-sections and thermal rate constants are reported. Impact of the low-energy collision-induced dissociation channel on the reactive dynamics is discussed.

Nonadiabatic quantum reactive scattering of the OH(A Σ2+)+D2

The seams of conical intersection exist between the ground (1 (2)A(')) and the first-excited (2 (2)A(')) electronic potential energy surfaces (PESs) of OH(A (2)Σ(+),X (2)Π) + H(2) system. This intersection induces the nonadiabatic quenching of OH(A (2)Σ(+)) by D(2). We present nonadiabatic quantum dynamics study for OH(A (2)Σ(+)) + D(2) on new five-dimensional coplanar PESs. The ab initio calculations of PESs are based on multireference configuration interaction (MRCI)/aug-cc-pVQZ level. A back-propagation neural network is utilized to fit the PESs and nonadiabatic coupling. High degrees of rotational excitation of quenched OH(X (2)Π) products are found in nonreactive quenching channel, and the quenched D(2) products are vibrationally excited up to quantum number v(2) (')=8. The theoretical results of nonadiabatic time-dependent wave-packet calculation are in good agreement with the existing experimental data.

Nonadiabatic Reaction of Energetic Molecules

Energetic materials store a large amount of chemical energy that can be readily converted into mechanical energy via decomposition. A number of different ignition processes such as sparks, shocks, heat, or arcs can initiate the excited electronic state decomposition of energetic materials. Experiments have demonstrated the essential role of excited electronic state decomposition in the energy conversion process. A full understanding of the mechanisms for the decomposition of energetic materials from excited electronic states will require the investigation and analysis of the specific topography of the excited electronic potential energy surfaces (PESs) of these molecules. The crossing of multidimensional electronic PESs creates a funnel-like topography, known as conical intersections (CIs). CIs are well established as a controlling factor in the excited electronic state decomposition of polyatomic molecules. This Account summarizes our current understanding of the nonadiabatic unimolecular chemistry of energetic materials through CIs and presents the essential role of CIs in the determination of decomposition pathways of these energetic systems. Because of the involvement of more than one PES, a decomposition process involving CIs is an electronically nonadiabatic mechanism. Based on our experimental observations and theoretical calculations, we find that a nonadiabatic reaction through CIs dominates the initial decomposition process of energetic materials from excited electronic states. Although the nonadiabatic behavior of some polyatomic molecules has been well studied, the role of nonadiabatic reactions in the excited electronic state decomposition of energetic molecules has not been well investigated. We use both nanosecond energy-resolved and femtosecond time-resolved spectroscopic techniques to determine the decomposition mechanism and dynamics of energetic species experimentally. Subsequently, we employ multiconfigurational methodologies (such as, CASSCF, CASMP2) to model nonadiabatic molecular processes of energetic molecules computationally. Synergism between experiment and theory establishes a coherent description of the nonadiabatic reactivity of energetic materials at a molecular level. Energetic systems discussed in this Account are nitramine- and furazan-based species. Both energetic species and their respective model systems, which are not energetic, are studied and discussed in detail. The model systems have similar molecular structures to those of the energetic species and help significantly in understanding the decomposition behavior of the larger and more complex energetic molecules. Our results for the above systems of interest confirm the existence of CIs and energy barriers on the PESs of interest. The presence of the CIs and barriers along the various reaction coordinates control the nonadiabatic behavior of the decomposition process. The detailed nature of the PESs and their CIs consequently differentiate the energetic systems from model systems. Energy barriers to the chemically relevant low-lying CIs of a molecule determine whether that molecule is more or less "energetic".