Triplet Potential Surfaces Research Articles

Water activation and splitting by single anionic iridium atoms.

Mass spectrometric analysis of anionic products that result from interacting Ir- with H2O shows the efficient generation of [Ir(H2O)]- complexes and IrO- molecular anions. Anion photoelectron spectra of [Ir(H2O)]-, formed under various source conditions, exhibit spectral features that are due to three different forms of the complex: the solvated anion-molecule complex, Ir-(H2O), as well as the intermediates, [H-Ir-OH]- and [H2-Ir-O]-, where one and two O-H bonds have been broken, respectively. The measured and calculated vertical detachment energy values are in good agreement and, thus, support identification of all three types of isomers. The calculated reaction pathway shows that the overall reaction Ir- + H2O → IrO- + H2 is exothermic. Two minimum energy crossing points were found, which shuttle intermediates and products between singlet and triplet potential surfaces. This study presents the first example of water activation and splitting by single Ir- anions.

CH2 + O2: reaction mechanism, biradical and zwitterionic character, and formation of CH2OO, the simplest Criegee intermediate.

The singlet and triplet potential surfaces for the title reaction were investigated using the CBS-QB3 level of theory. The wave functions for some species exhibited multireference character and required the CASPT2/6-31+G(d,p) and CASPT2/aug-cc-pVTZ levels of theory to obtain accurate relative energies. A Natural Bond Orbital Analysis showed that triplet 3CH2OO (the simplest Criegee intermediate) and 3CH2O2 (dioxirane) have mostly polar biradical character, while singlet 1CH2OO has some zwitterionic character and a planar structure. Canonical variational transition state theory (CVTST) and master equation simulations were used to analyze the reaction system. CVTST predicts that the rate constant for reaction of 1CH2 + 3O2 is more than ten times as fast as the reaction of 3CH2 (X3B1) + 3O2 and the ratio remains almost independent of temperature from 900 K to 3000 K. The master equation simulations predict that at low pressures the 1CH2O + 3O product set is dominant at all temperatures and the primary yield of OH radicals is negligible below 600 K, due to competition with other primary reactions in this complex system.

Comparative Study of the Thermal Stabilities of the Experimentally Known High-Valent Fe(IV) Compounds Fe(1-norbornyl)4 and Fe(cyclohexyl)4.

The high stability of the experimentally known homoleptic 1-norbornyl derivative (nor)4Fe of iron in the unusual +4 oxidation state is a consequence of the high reaction barriers of the singlet or triplet potential surfaces constrained by the global dispersion attraction and the great steric demands of the norbornyl groups. The much more limited stability of the corresponding cyclohexyl derivative (cx)4Fe may result from the conical intersection between the singlet potential surface and the quintet spin potential surface arising from the weaker dispersion attraction and the reduced steric effect of the cyclohexyl groups relative to the 1-norbornyl groups. In contrast, the high stability of the likewise experimentally known (cx)4M (M = Ru or Os) structures results from the larger ligand field splitting (Δ) of the d-orbital energies for the second and third-row transition metals ruthenium and osmium relative to that of the first-row transition metal iron. The cyclohexyl derivative (cx)4Fe is predicted to be reactive toward carbon monoxide to insert CO into up to two Fe-C bonds. However, the dispersion effect as well as the much larger size of the 1-norbornyl substituents prevents similar reactivity of (nor)4Fe with carbon monoxide.

A density functional theory study on the atmospheric reaction ofCH3O2withHS: Mechanism and kinetics

AbstractThe reaction mechanism of CH3O2and HS was systematically investigated by density functional theory (DFT). Six singlet pathways and seven triplet ones are located on the potential surface (PES). The result indicates that the main products are CH3O and HSO both on the singlet and triplet PES, different from the CH3O2+ OH reaction. Moreover, deformation density (ρdef) and atoms in molecules (AIM) analyses were carried out to further uncover the nature of chemical bonding evolution in the primary pathways. Furthermore, reaction rate constants were calculated in the temperature range from 200 to 1000 K using the transition state theory with the Wigner and Eckart tunneling corrections. Our results can shed light on the title reaction and offer instructions for analogous atmospheric reactions, as well as experimental research in the future.

Insight into the reaction mechanism of CH2SH with HO2: A density functional theory study

AbstractDensity functional theory was adopted in this work to reveal the reaction mechanism of CH2SH with HO2. Reaction rate constants were computed from 200 to 2000 K using the transition state theory combined with Wigner and Eckart tunneling correction. Moreover, localized orbital locator, atoms in molecules and Mayer bond order analyses were used to study the essence of chemical bonding evolution. Eleven singlet paths and three triplet ones are located on the potential surface (PES). The results show that the main products on the singlet PES are 1CH2S and H2O2, whereas on the triplet PES they are CH3SH + 3O2, which are coincident with the similar reaction of CH3S and HO2. This conclusion is also supported by rate constant calculation results. Interestingly, all the possible paths are involved in the hydrogen transfer. The results have provided underlying insights to the analogous reactions and further experimental studies.

Insight into regulation of emission color and photodeactivation process from heteroleptic to homoleptic Ir(III) complexes

The phosphorescent process of two heteroleptic ((DMDPI)2Ir(tftap) and (tftap)2Ir(DMDPI)) and one homoleptic (Ir(DMDPI)3) Ir(III) complexes (See Fig. 1) is theoretically investigated by density functional theory (DFT) and quadratic response (QR) time-dependent density functional theory (TDDFT) calculations including spin-orbit coupling (SOC). Two or three triplet excited states are confirmed for three complexes, respectively. On the basis of the respective optimized triplet geometry, the emissive wavelength is determined by the ΔSCF-DFT method. Furthermore, the radiative rate constant (kr) is also calculated corresponding to each triplet state. Combination of kr and emissive energy, the emission rule is determined. It is found that complex (DMDPI)2Ir(tftap) follows the dual emission scenarios, while complexes (tftap)2Ir(DMDPI) and Ir(DMDPI)3 obey the Kasha rule. The nonradiative rate constant (knr) is qualitatively evaluated by the construction of triplet potential surface via metal centered (3MC d-d) state. Finally, the sequence of quantum yield is compared by both kr and knr. The quantum yield of homoleptic Ir(III) complex Ir(DMDPI)3 is higher than that of heteroleptic Ir(III) complexes (DMDPI)2Ir(tftap) and (tftap)2Ir(DMDPI). However, the emissive wavelength of Ir(DMDPI)3 is in the red color region rather than blue color.

Direct dynamics study of the N(4S) + CH3(2A2″) reaction

Direct dynamics simulations were performed to understand the mechanisms of the N(4S)+CH3 reaction on both the triplet and singlet potential energy surfaces using the B3LYP/6-31(d, p) method with modified hybrid parameters for the exchange–correlation functional. The hybrid parameters were determined so as that the stationary-point energy levels on the potential energy surfaces reproduce those obtained at the CASPT2 results. The trajectories starting from the N(4S)+CH3 reactants on the triplet potential surface mostly lead to the H+H2CN products via the CH3N intermediate. The relative translation energy distribution of this channel shows that the reaction dynamics is nonstatistical. The trajectories on the singlet potential energy surface were also propagated from the CH2NH intermediate structure. We found that about 10% of the reactive trajectories showed “roaming dynamics,” which lead to the H2+HCN/HNC products through intramolecular hydrogen abstraction.

A computational investigation of product channels in the CH3O2 + F reaction

The radical–radical reaction mechanisms of CH3O2+F have been investigated theoretically at the CCSD (T)//B3LYP/6-311++G (3df, 3pd) level. The calculations reveal that the reaction should be taking place both on the singlet and triplet potential surfaces. On the singlet potential surface, the most favorable channel starts with a barrierless addition of F to CH3O2 leading to 1im2 (CH3OOF), and then the adduct 1im2 goes through an H transfer forming the products CH2OO and HF. On the triplet potential surface, DFT calculations show that the dominant products are CH2OO and HF.

A Theoretical Study of the O (<sup>3</sup>P) + HCONH<sub>2</sub> Reaction

The triplet potential surface of the O (3P) + HCONH2 reaction has been investigated at the CCSD (T)//B3LYP/6-311G (d,p) level. DFT calculations show that the reaction occurs dominantly via the H - abstraction rather than the O- addition mechanism on the triplet potential surface. The product of OH and CONH2 should be the major product, which obtained by a direct dissociation of the adduct im6 without an exit barrier.

Theoretical Study of the Reaction of Ethane with Oxygen Molecules in the Ground Triplet and Singlet Delta States

Quantum chemical calculations are carried out to study the reaction of ethane with molecular oxygen in the ground triplet and singlet delta states. Transition states, intermediates, and possible products of the reaction on the triplet and singlet potential energy surfaces are identified on the basis of the coupled-cluster method. The basis set dependence of coupled-cluster energy values is estimated by the second-order perturbation theory. The values of energy barriers are also refined by using the compound CBS-Q and G3 techniques. It was found that the C(2)H(6) + O(2)(X(3)Σ(g)(-)) reaction leads to the formation of C(2)H(5) and HO(2) products, whereas the C(2)H(6) + O(2)(a(1)Δ(g)) process produces C(2)H(4) and H(2)O(2) molecules. The appropriate rate constants of these reaction paths are estimated on the basis of variational and nonvariational transition-state theories assuming tunneling and possible nonadiabatic transitions in the temperature range 500-4000 K. The calculations showed that the rate constant of the C(2)H(6) + O(2)(a(1)Δ(g)) reaction path is much greater than that of the C(2)H(6) + O(2)(X(3)Σ(g)(-)) one. At the same time, the singlet and triplet potential surface intersection is detected that leads to the appearance of the nonadiabatic quenching channel O(2)(a(1)Δ(g)) + C(2)H(6) → O(2)(X (3)Σ(g)(-)) + C(2)H(6). The rate constant of this process is estimated with the use of the Landau-Zener model. It is demonstrated that, in the case of the existence of thermal equilibrium in the distribution of molecules over the electronic states, at low temperatures (T < 1200 K) the main products of the reaction of C(2)H(6) with O(2) are C(2)H(4) and H(2)O(2), rather than C(2)H(5) and HO(2). At higher temperature (T > 1200 K) the situation is inverted.

Radical cations and triplet states of 1,2-disubstituted cyclopropanes: comparison of potential surfaces

Radical ion pairs generated by photo-induced electron transfer from 1,2-disubstituted cyclopropanes to various acceptors undergo return electron transfer in pairs of singlet and triplet multiplicity. The pair energies relative to the reactant ground states and to accessible triplet states, respectively, determine the competition between the recombination pathways. The potential surfaces of the radical cations and triplet states of 1,2-diphenyl-, 1, and 1,2-dimethylcyclopropane, 2, have been examined by density functional theory calculations. The radical cation surfaces have minima at geometries that retain significant bonding between C-1 and C-2, preventing geometric isomerization of the radical cations. The triplet potential surfaces are dissociative with minimal rotational differentiation at long distances between C-1 and C-2.

Theoretical study on the mechanism of the HO2 plus NH2 reaction

The singlet and triplet potential surfaces of the HO2+NH2 reaction have been investigated at the CCSD (T)//B3LYP/6-311++G (3df, 3pd) level. On the singlet potential surface, the most favorable association way starts with a barrierless addition of the HO2 to NH2 leading to HOO–NH2 (1im2 and 1im3). The product of OH and NH2O should be the major product, which obtained by a direct dissociation of the adduct 1im3 without an exit barrier. But on the triplet potential surface, DFT calculations reveal the reaction mechanism to be mainly a barrierless addition of HO2 to NH2 leading to an intermediate OOH–NH2 (3im1), and then the adduct 3im1 goes through an H transfer forming the product of NH3 and O2. The calculations show that the reaction should be taken place both on the singlet and triplet potential surface.

Reaction of HOD+ with NO2: Effects of OD and OH Stretching, Bending, and Collision Energy on Reactions on the Singlet and Triplet Potential Surfaces

Integral cross sections and product recoil velocity distributions were measured for the reaction of HOD(+) with NO(2), in which the HOD(+) reactant was prepared in its ground state and with mode-selective excitation in the 001 (OH stretch), 100 (OD stretch), and 010 (bend) modes. In addition, we measured the 300 K thermal kinetics in a selected ion flow tube reactor and report product branching ratios different from previous measurements. Reaction is found to occur on both the singlet and triplet surfaces with near-unit efficiency. At 300 K, the product branching indicates that triplet → singlet transitions occur in about 60% of triplet-coupled collisions, which we attribute to long interaction times mediated by complexes on the triplet surface. Because the collision times are much shorter in the beam experiments, the product distributions show no signs of such transitions. The dominant product on the singlet surface is charge transfer. Reactions on the triplet surface lead to NO(+), NO(2)H(+), and NO(2)D(+). There is also charge transfer, producing NO(2)(+) (a(3)B(2)); however, this triplet NO(2)(+) mostly predissociates. The NO(2)H(+)/NO(2)D(+) cross sections peak at low collision energies and are insignificant above ~1 eV due to OH/OD loss from the nascent product ions. The effects of HOD(+) vibration are mode-specific. Vibration inhibits charge transfer, with the largest effect from the bend. The NO(2)H(+)/NO(2)D(+) channels are also vibrationally inhibited, and the mode dependence reveals how energy in different reactant modes couples to the internal energy of the product ions.

Theoretical study of [Si,O,C,O] species: Prediction of new species on triplet potential energy surface

The intermediates [Si,O,C,O] of the Si + CO2 reaction have been studied in detail using high level ab iniitio methods. Both singlet and triplet [Si,O,C,O] species are characterized structurally and energetically. On the singlet potential energy surface (PES), the vdw-OSi–CO isomer and in the triplet PES, the bent-SiOCO isomer is found to be thermodynamically as well as kinetically most stable species. All possible isomerization transition states (TS) are located on both singlet and triplet potential surfaces. On the triplet surface, the stability of the trans-OSiCO isomer is comparable with that of the bent-SiOCO isomer. A non-planar cis-SiOCO isomer is located on the triplet PES, which is predicted for the first time. Heats of formation at 0 K (ΔfH°, 0 K) for all singlet and triplet species are computed using G3B3, G3MP2, and CBS-Q theories. The discrepancy between G3B3 and the other two methods for the heat of formation value for triplet trans-OSiCO is discussed. The PESs for singlet as well as triplet species with their dissociation asymptotes are explored at the CCSD(T)/6-311G(d,p)//MP2/6-311G(d,p) level of theory. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Singlet and triplet potential surfaces for the O2+C2H4 reaction

Electronic structure calculations at the CASSCF and UB3LYP levels of theory with the aug-cc-pVDZ basis set were used to characterize structures, vibrational frequencies, and energies for stationary points on the ground state triplet and singlet O(2)+C(2)H(4) potential energy surfaces (PESs). Spin-orbit couplings between the PESs were calculated using state averaged CASSCF wave functions. More accurate energies were obtained for the CASSCF structures with the MRMP2/aug-cc-pVDZ method. An important and necessary aspect of the calculations was the need to use different CASSCF active spaces for the different reaction paths on the investigated PESs. The CASSCF calculations focused on O(2)+C(2)H(4) addition to form the C(2)H(4)O(2) biradical on the triplet and singlet surfaces, and isomerization reaction paths ensuing from this biradical. The triplet and singlet C(2)H(4)O(2) biradicals are very similar in structure, primarily differing in their C-C-O-O dihedral angles. The MRMP2 values for the O(2)+C(2)H(4)→C(2)H(4)O(2) barrier to form the biradical are 33.8 and 6.1 kcal/mol, respectively, for the triplet and singlet surfaces. On the singlet surface, C(2)H(4)O(2) isomerizes to dioxetane and ethane-peroxide with MRMP2 barriers of 7.8 and 21.3 kcal/mol. A more exhaustive search of reaction paths was made for the singlet surface using the UB3LYP/aug-cc-pVDZ theory. The triplet and singlet surfaces cross between the structures for the O(2)+C(2)H(4) addition transition states and the biradical intermediates. Trapping in the triplet biradical intermediate, following (3)O(2)+C(2)H(4) addition, is expected to enhance triplet→singlet intersystem crossing.

Quantum Mechanical Reaction Probability of Triplet Ketene at the Multireference Second-Order Perturbation Level of Theory

Triplet ketene exhibits a steplike structure in the experimentally observed dissociation rates, but its mechanism is still unknown despite many theoretical efforts in the past decades. In this paper we revisit this problem by quantum mechanically calculating the reaction probability with multireference-based electronic structure theory. Specifically, we first construct an analytical potential energy surface of triplet state by fitting it to about 6000 ab initio energies computed at the multireference second-order Mller-Plesset perturbation (MRMP2) level. We then evaluate the cumulative reaction probability by using the transition state wave packet method together with an adiabatically constrained Hamiltonian. The result shows that the imaginary barrier frequency on the triplet surface is 328i cm-1, which is close to the CCSD(T) result (321i cm-1) but is likely too large for reproducing the experimentally observed steps. Indeed, our calculated reaction probability exhibits no signature of steps, reflecting too strong tunneling effect along the reaction coordinate. Nevertheless, it is emphasized that the flatness of the potential profile in the transition-state region (which governs the degree of tunneling) depends strongly on the level of electronic structure calculation, thus leaving some possibility that the use of more accurate theories might lead to the observed steps. We also demonstrate that the triplet potential surface differs significantly between the CASSCF and MRMP2 results, particularly in the transition-state region. This fact seems to require more attention when studying the "nonadiabatic" scenario for the steps, in which the crossing seam between S0 and T1 surfaces is assumed to play a central role.

A theoretical study of the 2NCO + 2N 2H reaction

The singlet and triplet potential surfaces of the 2NCO + 2N 2H reaction have been investigated at the B3LYP/6-311G (d,p) level. The single-point energy calculations are performed at the high-level CCSD (T)/6-311G (d,p) for more accurate energy values. DFT calculations reveal the reaction mechanism to be mainly a barrierless addition of 2NCO to 2N 2H leading to an intermediate 1im3 (OCN–N 2H) on the singlet potential surface. The adduct 1im3 goes through an H shift from N 2H to NCO, forming the product of HNCO and N 2. Due to the higher barrier of initial association, the reaction is more difficult on the triplet potential surface.

CuF-Chemiluminescent Reaction and Plasma Switching by Laser Ablation in the Gaseous Cu−CF4System and Their Interactions with Magnetic Field†

In view of new metal catalysis reactions, CuF-chemiluminescent and plasma reactions in a gaseous Cu-CF(4) system are studied as well as their interactions with a magnetic field. As for the chemiluminescent reaction, the spectroscopic analysis of the CuF chemiluminescence in the reaction of CF(4) with Cu emitted by laser ablation with a fundamental beam of a Nd(3+):YAG laser indicates that the rotational, vibrational, and translational temperatures of the product CuF are rather similar, suggesting the simple heating mechanism. In this mechanism, the high translational energy of the reactant Cu atom will be used to go over the early barrier. The mechanism is evidenced by experimentally observing that the reaction barrier decreases with an increase in the translational energy of the reactant Cu by an increase in the laser power for ablation. On the other hand, in the further study, we find that Cu emitted by laser ablation can switch the plasma in an electric field less than that necessary for the direct discharge in dc-plasma and call it PLASLA (plasma switching by laser ablation). In PLASLA, the plasma is formed by the first ablation, quenched by the second ablation, formed by the third ablation, quenched by the fourth ablation, and so on. Thus PLASLA will be a promising new metal catalysis process for materials science. In particular, the cooperation of the ablation laser with the laser for photoreactions will be significant as a new technique. The studies on the mechanism of PLASLA with metals of Cu, Al, Ag, Zn, Co, Ni, Ti, Mo, and W indicate that the metals are classified into three groups. It is of great interest that the classification agrees with the classification by their electronic configurations. PLASLA is found to be formed rather easily and stable with Al, Ag, Cu, and Zn of group I. Species such as C, C(+), C(2+), and C(2) are observed in the time-resolved spectra of PLASLA luminescence at 0.5 mus after laser ablation, suggesting carbon polymers as the final product. In fact, the TOF (time-of-flight) mass spectrometric analysis of the product material confirms it. Furthermore, PLASLA and the CuF-chemiluminescent reaction are studied in a magnetic field. In the chemiluminescent reaction, we propose that the field switches the reaction paths by mixing the singlet and triplet potential surfaces at level crossing by breaking the spin selection rule. The field affects PLASLA through the MHD (magneto-hydrodynamic) processes such as E x B drifts and cyclotron circulation, as well as the singlet-triplet mixing.

Triplet quantum chain process in the photoisomerization of 9- cis retinal as revealed by nanosecond time-resolved infrared spectroscopy

The mechanism of the photoisomerization of 9- cis retinal has been studied by nanosecond time-resolved infrared spectroscopy. A cyclohexane solution of 9- cis retinal was photoexcited at 349 nm and the subsequent photodynamics were traced. A singular value decomposition (SVD) analysis of the time-resolved infrared data shows that there are two distinct isomerization pathways. One is the triplet pathway that takes place in the picosecond time regime from 9- cis to all- trans. The other involves the energy transfer between the all- trans triplet state and the 9- cis ground state with the resultant 9- cis triplet state subsequently reproducing the all- trans by fast isomerization on the triplet potential surface . This quantum chain process occurs in the microsecond time regime.

Direct Observation of Molecular Structural Change during Intersystem Crossing by Real-Time Spectroscopy with a Few Optical Cycle Laser

Ultrafast spectroscopy by a sub-5 fs pulse laser was applied to the simultaneous study of electronic relaxation and vibrational dynamics in Ru(II)(TPP)(CO). The electronic lifetimes of (1)Q(x(1,0))(pi,pi*) and (1)Q(x(0,0))(pi,pi*) were determined to be 230 +/- 70 fs and 1150 +/- 260 fs, respectively. The spectrogram shows the time dependent changes in the vibrational spectrum associated with the spin state change from the Franck-Condon state in the excited singlet state to the triplet state via the curve crossing point between the singlet and triplet potential surfaces. The time constant of the intersystem crossing process was determined to be about 1.0 ps from observed electronic relaxation and vibrational dynamics reflecting the transition from the singlet to triplet electronic excited state.