Ground-state Reaction Research Articles

Fluoride Complexes of Cyclometalated Iridium(III)

Many electroluminescent devices rely on cyclometalated iridium(III). Their advancement depends on access to reactive starting materials because of the inertness of Ir(III). Notably, fluoride complexes of bis(cyclometalated) Ir(III) are scarce. Syntheses of bridged and terminal fluorides are reported here. New compounds are luminescent and thermally reactive; they are characterized by ground-state and optical methods. Crystal structures were determined for one bridging and one terminal fluoride complex. The terminal fluoride shows intramolecular hydrogen bonding to an adjacent 3,5-dimethylpyrazole ligand; a lesser interaction may occur between F and a nearby aromatic C–H bond. Terminal fluoride complexes react with carbon-, silicon-, and sulfur-based electrophiles. The new complexes phosphoresce with microsecond lifetimes at 77 and 298 K. Density-functional theory calculations indicate triplet states with little contribution from fluoride. The compounds herein are versatile phosphors having the ground-state reactivity of late transition metal fluorides.

Tracking a Paternò–Büchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies

A detailed mechanistic investigation of the early stages of the Paternò-Büchi reaction following 267 nm excitation of benzaldehyde in cyclohexene has been completed using ultrafast, broadband transient UV-visible and IR absorption spectroscopies. Absorption due to electronically excited triplet state benzaldehyde decays on a 80 ps time scale via reaction with cyclohexene. The growth and subsequent decay of the biradical intermediate produced following C-O bond formation is followed by transient vibrational spectroscopy. The biradical decays by ring closure to an oxetane or by dissociating, reforming the ground state reactants. Detailed kinetic analysis allowed derivation of quantum yields and rate constants for these competing biradical decay processes, ϕ(oxetane) = 0.53, ϕ(diss) = 0.47, koxetane = 0.27 ± 0.09 ns(-1) and k(diss) = 0.24 ± 0.09 ns(-1). This study provides a striking illustration of the ways in which contemporary ultrafast transient absorption spectroscopy methods can be used to dissect the mechanism and kinetics of a classic photoreaction.

Photo-Wolff Rearrangement of 2-Diazo-1,2-naphthoquinone: Stern-Volmer Analysis of the Stepwise Reaction Pathway.

2-Diazo-1,2-naphthoquinone (1) and its derivatives are the photoactive components in Novolak photoresists. A femtosecond infrared study has established that the photoreaction of 1 proceeds largely by a concerted Wolff rearrangement yielding the ketene 1H-inden-1-ylidene-methanone (3) within 300 fs after excitation, but earlier trapping studies gave evidence for a minor reaction path via a carbene intermediate 1-oxo-2(1H)-naphthalenylidene (2) with a lifetime of about 10 ps. Here, we provide a quantitative assessment of the stepwise pathway by Stern-Volmer analysis of the trapping of 2 by methanol to yield 2-methoxy-1-naphthol (4). We conclude that the lifetime of the carbene 2 is at least 200 ps. Moreover, [3 + 2]cycloaddition of 2 and acetonitrile yielding 2-methylnaphth[2,1-d]oxazole (5) was observed. A comparison of the yields of 5 formed upon photolysis and upon thermolysis of 1 in acetonitrile provides evidence that a substantial part of the hot nascent carbene 2 formed photolytically rearranges to the ketene 3 during its vibrational relaxation (hot ground-state reaction).

Dynamics of the reaction of C₃(a³Πu) radicals with C₂H₂: a new source for the formation of C₅H.

The reaction C3(a(3)Πu) + C2H2 → C5H + H was investigated at collision energy 10.9 kcal mol(-1) that is less than the enthalpy of ground-state reaction C3(X(1)Σg (+)) + C2H2 → C5H + H. C3(a(3)Πu) radicals were synthesized from 1% C4F6/He by pulsed high-voltage discharge. The title reaction was conducted in a crossed molecular-beam apparatus equipped with a quadrupole-mass filter. Product C5H was interrogated with time-of-flight spectroscopy and synchrotron vacuum-ultraviolet ionization. Reactant C3(a(3)Πu) and product C5H were identified using photoionization spectroscopy. The ionization thresholds of C3(X(1)Σg(+)) and C3(a(3)Πu) are determined as 11.6 ± 0.2 eV and 10.0 ± 0.2 eV, respectively. The C5H product is identified as linear pentynylidyne that has an ionization energy 8.4 ± 0.2 eV. The title reaction releases translational energy 10.6 kcal mol(-1) in average and has an isotropic product angular distribution. The quantum-chemical calculation indicates that the C3(a(3)Πu) radical attacks one of the carbon atoms of C2H2 and subsequently a hydrogen atom is ejected to form C5H + H, in good agreement with the experimental observation. As far as we are aware, the C3(a(3)Πu) + C2H2 reaction is investigated for the first time. This work gives an implication for the formation of C5H from the C3(a(3)Πu) + C2H2 reaction occurring in a combustion or discharge process of C2H2.

Competition and interplay of various intermolecular interactions in ultrafast excited-state proton and electron transfer reactions.

The main features of the photoinduced kinetics of both ultrafast excited-state proton and electron transfer reactions that occur in the picosecond (ps) and femtosecond (fs) time domains are compared. Proton transfer (PT) reaction kinetics can be described in terms of several discrete values of rate coefficients in the form of polyexponential functions where each value of the rate coefficient can be attributed to a definite physical behavior of the reaction mechanism. In contrast, electron transfer (ET) reaction kinetics requires a consideration of a continuous distribution of rate coefficients. This difference can be related to structure of the ground-state reactant pairs for each reaction. Excited-state ET can occur at various configurations of reactant molecules and its rate reflects the fluctuations of the distances and orientations of these molecules. In contrast, excited-state PT requires preliminary formation of a ground-state H-bonded complex with definite structure where the reaction occurs after photoexcitation.

Theoretical study on the working mechanism of a reversible light-driven rotary molecular motor

Recent progress in the experimental design and synthesis of artifi cial light-driven molecular motors has made it possible to reverse the light-powered rotation of a molecular motor during the rotary process with multilevel control of rotary motion. With the interconversion between the photochemically generated less-stable isomers in one unidirectional rotary cycle with the stable isomers in the other cycle, the clockwise and anticlockwise rotations can be regulated with confi guration inversion at the stereogenic centre. In this work, we report theoretical calculations by using density functional theory and time-dependent density functional theory, revealing the working mechanism of the clockwise and anticlockwise rotations. Specifi cally, by locating the relevant stationary points along the excitedstate and ground-state reaction paths, the clockwise and anticlockwise rotary cycles are explored in great detail, which is of complementary importance in understanding the overall rotary process of the reversible rotation.

The coupling of tautomerization to hydration in the transition state on the pyrimidine photohydration reaction path.

The ground state reaction path for formation of the pyrimidine hydrates was calculated using a nudged elastic band (NEB) approach, combined with a calculation of the transition state, and implemented using a numerical basis set in the density functional theory (DFT) code DMol(3). The model systems used for study consist of 1-methyl pyrimidines with a H2O molecule as the reactant, and the corresponding C5-hydro-C6-hydroxypyrimidine as the product. The barrier to addition of water across the C5-C6 π-bond ranges from 43-48 kcal mol(-1) in the 1-methylpyrimidines (1-MP) studied. Similar but slightly smaller barriers of 34-45 kcal mol(-1) were found for the tautomers of the 1-MPs, i.e. the enols of uridine and thymine and imine of cytosine. Comparison of these calculations with previous computational and experimental work suggests that a hot ground state formed by the rapid internal conversion of pyrimidines has sufficient energy to permit crossover from the common form to the tautomeric form of the pyrimidine at the transition state. The hot ground state mechanism can account for the experimentally observed yield and thermal reversion of pyrimidine photohydrates, while simultaneously explaining the effect of photohydrates on the mutation rate.

Communication: direct comparison between theory and experiment for correlated angular and product-state distributions of the ground-state and stretching-excited O((3)P) + CH4 reactions.

Motivated by a recent experiment [H. Pan and K. Liu, J. Chem. Phys. 140, 191101 (2014)], we report a quasiclassical trajectory study of the O((3)P) + CH4(vk = 0, 1) → OH + CH3 [k = 1 and 3] reactions on an ab initio potential energy surface. The computed angular distributions and cross sections correlated to the OH(v = 0, 1) + CH3(v = 0) coincident product states can be directly compared to experiment for O + CH4(v3 = 0, 1). Both theory and experiment show that the ground-state reaction is backward scattered, whereas the angular distributions shift toward sideways and forward directions upon antisymmetric stretching (v3) excitation of the reactant. Theory predicts similar behavior for the O + CH4(v1 = 1) reaction. The simulations show that stretching excitation enhances the reaction up to about 15 kcal/mol collision energy, whereas the O + CH4(vk = 1) reactions produce smaller cross sections for OH(v = 1) + CH3(v = 0) than those of O + CH4(v = 0) → OH(v = 0) + CH3(v = 0). The former finding agrees with experiment and the latter awaits for confirmation. The computed cold OH rotational distributions of O + CH4(v = 0) are in good agreement with experiment.

Communication: Imaging the effects of the antisymmetric-stretching excitation in the O(³P) + CH₄(v₃ = 1) reaction.

Effects of one-quantum excitation of the antisymmetric-stretching mode of CH4(v3 = 1) on the O((3)P) + CH4 reaction were studied in a crossed-beam, ion-imaging experiment. In the post-threshold region, we found that (1) the product state distributions are dominated by the CH3(0₀) + OH(v' = 1) pair, (2) the product angular distributions extend toward sideways from the backward dominance of the ground-state reaction, and (3) vibrational excitation exerts a positive effect on reactivity, but translational energy is more efficient in promoting the rate of this central-barrier reaction. All major findings agree reasonably well with recent theoretical results. Some remaining questions are pointed out.

How Is C-H Vibrational Energy Redistributed in F + CHD3(ν1 = 1) → HF + CD3?

The effects of CH stretching excitation on the F + CHD3 → HF + CD3 reaction are studied experimentally using crossed-beam and time-sliced velocity map imaging techniques at the collision energy of 9.0 kcal/mol. The fraction of the vibrationally excited CHD3 reagent in the crossed-beam region was determined accurately, allowing us to investigate quantitatively the effects of CH stretching excitation on the title reaction. Experimental data show that the vibrational energy in the excited CH bond of CHD3 is almost exclusively deposited into the HF product vibration, and hence, the HF products from the excited-state reaction are about one vibrational quantum hotter than those of the ground-state reaction, while the vibrational state distribution of the CD3 products is only slightly affected. The reaction is suppressed by the CH stretching excitation, and the overall reactivity of the vibrationally excited reaction is 74 ± 4% of that of the ground-state reaction for CD3(ν2 = 0, 1, 2, 3) product channels.

Effect of antisymmetric C–H stretching excitation on the dynamics of O(1D) + CH4 → OH + CH3

The effect of antisymmetric C–H stretching excitation of CH4 on the dynamics and reactivity of the O(1D) + CH4 → OH + CD3 reaction at the collision energy of 6.10 kcal/mol has been investigated using the crossed-beam and time-sliced velocity map imaging techniques. The antisymmetric C–H stretching mode excited CH4 molecule was prepared by direct infrared excitation. From the measured images of the CH3 products with the infrared laser on and off, the product translational energy and angular distributions were derived for both the ground and vibrationally excited reactions. Experimental results show that the vibrational energy of the antisymmetric stretching excited CH4 reagent is channeled exclusively into the vibrational energy of the OH co-products and, hence, the OH products from the excited-state reaction are about one vibrational quantum hotter than those from the ground-state reaction, and the product angular distributions are barely affected by the vibrational excitation of the CH4 reagent. The reactivity was found to be suppressed by the antisymmetric stretching excitation of CH4 for all observed CH3 vibrational states. The degree of suppression is different for different CH3 vibrational states: the suppression is about 40%–60% for the ground state and the umbrella mode excited CH3 products, while for the CH3 products with one quantum symmetric stretching mode excitation, the suppression is much less pronounced. In consequence, the vibrational state distribution of the CH3 product from the excited-state reaction is considerably different from that of the ground-state reaction.

Electronic excitation spectra of molecules in solution calculated using the symmetry-adapted cluster-configuration interaction method in the polarizable continuum model with perturbative approach

A perturbative approximation of the state specific polarizable continuum model (PCM) symmetry-adapted cluster-configuration interaction (SAC-CI) method is proposed for efficient calculations of the electronic excitations and absorption spectra of molecules in solutions. This first-order PCM SAC-CI method considers the solvent effects on the energies of excited states up to the first-order with using the zeroth-order wavefunctions. This method can avoid the costly iterative procedure of the self-consistent reaction field calculations. The first-order PCM SAC-CI calculations well reproduce the results obtained by the iterative method for various types of excitations of molecules in polar and nonpolar solvents. The first-order contribution is significant for the excitation energies. The results obtained by the zeroth-order PCM SAC-CI, which considers the fixed ground-state reaction field for the excited-state calculations, are deviated from the results by the iterative method about 0.1 eV, and the zeroth-order PCM SAC-CI cannot predict even the direction of solvent shifts in n-hexane for many cases. The first-order PCM SAC-CI is applied to studying the solvatochromisms of (2,2(')-bipyridine)tetracarbonyltungsten [W(CO)4(bpy), bpy = 2,2(')-bipyridine] and bis(pentacarbonyltungsten)pyrazine [(OC)5W(pyz)W(CO)5, pyz = pyrazine]. The SAC-CI calculations reveal the detailed character of the excited states and the mechanisms of solvent shifts. The energies of metal to ligand charge transfer states are significantly sensitive to solvents. The first-order PCM SAC-CI well reproduces the observed absorption spectra of the tungsten carbonyl complexes in several solvents.

Chemical kinetics modeling of n-nonane oxidation in oxygen/argon using excited-state species time histories

Chemical reactions of ground-state species strongly govern the formation of excited-state species, including OH* and CH*, which are commonly used to determine ignition delay times of fuels. With well-characterized chemiluminescence rates embedded in chemical kinetics mechanisms, time histories of excited-state species can aid in identifying influential ground-state reactions which are important to processes such as ignition delay time. Placing emphasis on the high-temperature regime, improvements were made to a detailed chemical kinetics mechanism of n-nonane oxidation developed previously by the authors. Using characteristic features of OH* time histories measured in shock-tube experiments as a metric, detailed model analyses were performed over a broad range of conditions: T>1100K, 1.5<P (atm) < 10.5, ϕ=0.5, 1.0, 2.0. OH* time history measurements, particularly under fuel-rich conditions (ϕ=2.0), displayed a two-peak behavior, with the first peak occurring within the first 5–10μs after reflected-shock passage, and the second, wider peak corresponding to main oxidation and ignition. In the initial version of the kinetics mechanism, the two peaks at rich conditions were predicted to merge, blurring the main ignition process prior to the second peak. The work herein presents modifications to the initial chemical kinetics mechanism which led to improved agreement between measurements and model-based predictions, with emphasis on the fuel-rich condition. To this end, the predicted shapes of the OH* time histories were crucial to matching the two-peak behavior detected in the experiments. A first-order resistance–capacitance (RC) model of the experimental time response of the optical setup was developed and shown to reproduce the measured time dependence and peak behavior that are vital for matching the OH* behavior near time-zero. The RC model processes the kinetics predictions in a way that allows the kinetics model predictions to directly correspond to the true conditions in the experiment. In moving towards improved agreement in OH*-profile predictions for all conditions, improvements in the kinetics mechanism were also realized at the two leaner equivalence ratios (ϕ=1.0 and 0.5), both in terms of OH* profile shape and ignition delay times. Model calculations of oxidation processes indicate that reactions leading to the first OH* peak originate from fuel homolysis. The resulting (alkyl) radicals lead to the formation of methyl which then, through a series of H-abstraction reactions, leads to the production of the methylidyne radical (CH) that reacts with molecular oxygen to form OH*. The oxidation processes near time-zero terminate, in part, due to methyl depletion by methylene forming C2H4+H2. In addition to the insight gained on n-nonane ignition and oxidation chemistry, the present study highlights the utility of correctly interpreted OH* measurements for inference of kinetic information other than ignition delay times.

Effect of an external electric field on the diffusion-influenced geminate reversible reaction of a neutral particle and a charged particle in three dimensions. III. Ground-state ABCD reaction

In the presence of an external electric field, the ground-state A+B(q)<->C(q)+D diffusion-influenced reversible reaction for a geminate pair, a neutral and a charged particle, is investigated in three dimensions. The probability density functions, the rates of reactions, and the survival probabilities of individual particles are analytically derived in the Laplace domain in terms of series solutions. The long-time kinetics of probability density functions and rates of reactions in rescaled forms shows a kinetic transition behavior from a t(-3∕2) power law to a t(-3∕2)e(t) increase when the condition D1F1 (2)≤D2F2 (2), which depends on the diffusivities of particles and the external electric fields, changes to D1F1 (2)>D2F2 (2). In the transition region D1F1 (2)=D2F2 (2), the long-time behavior also shows a t(-3∕2) power law decay but with a different value of the prefactor. The rescaled survival probabilities only exhibit an exponentially increasing behavior at long times with no dependence on the various values of parameters.

ChemInform Abstract: Photochemical Reactions of Aromatic Compounds and the Concept of the Photon as a Traceless Reagent

AbstractReview: 263 refs.

Laser-Induced Fluorescence of Isobutoxy in Competition with Ground State Decomposition

Spectroscopic detection is an important method to monitor alkoxy radicals in atmospheric photochemistry studies. In this work, we report the first observation of the laser-induced fluorescence (LIF) spectra of isobutoxy, 2-methyl-1-butoxy, and 3-methyl-1-butoxy in supersonic jet-cooled condition. Ground state unimolecular decomposition and isomerization as well as excited state relaxation dynamics of isobutoxy were discussed in combination with the theoretical calculations. Analysis of the experimental and theoretical results showed that methyl substitution on the β carbon of the alkoxy radicals changed the LIF spectra of alkoxy radicals significantly. The competition between the ground state reactions and the photoexcitation process depended strongly on the radical structure and hence affected the involvement of alkoxy radicals in the photochemical reactions in the upper troposphere. This study will help to understand the dynamic role of alkoxy radicals in the atmosphere.

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Br(2P, 2P3/2) + CH4 → HBr + CH3 reaction

Chemically accurate full-dimensional non-spin-orbit and spin-orbit (SO) ground-state potential energy surfaces (PESs) are obtained for the Br + CH4 → HBr + CH3 reaction by fitting 21 574 composite ab initio energy points. The composite method considers electron correlation methods up to CCSD(T), basis sets up to aug-cc-pwCVTZ-PP, correlation of the core electrons, scalar relativistic effects via an effective core potential (ECP), and SO corrections, thereby achieving an accuracy better than 0.5 kcal∕mol. Benchmark structures and relative energies are computed for the stationary points using the ab initio focal-point analysis (FPA) scheme based on both ECP and Douglas-Kroll approaches providing all-electron relativistic CCSDT(Q)∕complete-basis-set quality energies. The PESs accurately describe the saddle point of the abstraction reaction and the van der Waals complexes in the entrance and product channels. The SO-corrected PES provides a classical barrier height of 7285(7232 ± 50) cm(-1), De values of 867(799 ± 10) and 399(344 ± 10) cm(-1) for the complexes CH3-HBr and CH3-BrH, respectively, and reaction endothermicity of 7867(7857 ± 50) cm(-1), in excellent agreement with the new, FPA-based benchmark data shown in parentheses. The difference between the Br + CH4 asymptotes of the non-SO and SO PESs is 1240 cm(-1), in good agreement with the experiment (1228 cm(-1)). Quasiclassical trajectory calculations based on more than 13 million trajectories for the late-barrier Br + CH4(vk = 0, 1) [k = 1, 2, 3, 4] reactions show that the vibrational energy, especially the excitation of the stretching modes, activates the reaction much more efficiently than translational energy, in agreement with the extended Polanyi rules. Angular distributions show dominant backward scattering for the ground-state reaction and forward scattering for the stretching-excited reactions. The reactivity on the non-SO PES is about 3-5 times larger than that on the SO PES in a wide collision energy range of 8000-16,000 cm(-1).

Photochemical reactions of aromatic compounds and the concept of the photon as a traceless reagent

Electronic excitation significantly changes the reactivity of chemical compounds. Compared to ground state reactions, photochemical reactions considerably enlarge the application spectrum of a particular functional group in organic synthesis. Multistep syntheses may be simplified and perspectives for target oriented synthesis (TOS) and diversity oriented synthesis (DOS) are developed. New compound families become available or may be obtained more easily. In contrast to common chemical reagents, photons don't generate side products resulting from the transformation of a chemical reagent. Therefore, they are considered as a traceless reagent. Consequently, photochemical reactions play a central role in the methodology of sustainable chemistry. This aspect has been recognized since the beginning of the 20th century. As with many other photochemical transformations, photochemical reactions of aromatic, benzene-like compounds illustrate well the advantages in this context. Photochemical cycloadditions of aromatic compounds have been investigated for a long time. Currently, they are applied in various fields of organic synthesis. They are also studied in supramolecular structures. The phenomena of reactivity and stereoselectivity are investigated. During recent years, photochemical electron transfer mediated reactions are particularly focused. Such transformations have likewise been performed with aromatic compounds. Reactivity and selectivity as well as application to organic synthesis are studied.

Mechanistic Insight into the Chemiluminescent Decomposition of Firefly Dioxetanone

The peroxide decomposition that generates the excited-state carbonyl compound is the key step in most organic chemiluminescence, and chemically initiated electron exchange luminescence (CIEEL) has been widely accepted for decades as the general mechanism for this decomposition. The firefly dioxetanone, which is a peroxide, is the intermediate in firefly bioluminescence, and its decomposition is the most important step leading to the emission of visible light by a firefly. However, the firefly dioxetanone decomposition mechanism has never been explored at a reliable theoretical level, because the decomposition process includes biradical, charge-transfer (CT) and several nearly degenerate states. Herein, we have investigated the thermolysis of firefly dioxetanone in its neutral (FDOH) and anionic (FDO(-)) forms using second-order multiconfigurational perturbation theories in combination with the ground-state intrinsic reaction coordinate calculated via the combined hybrid functional with Coulomb attenuated exchange-correlation, and considered the solvent effect on the ground-state reaction path using the combined hybrid functional with Coulomb attenuated exchange-correlation. The calculated results indicate that the chemiluminescent decomposition of FDOH or FDO(-) does not take place via the CIEEL mechanism. An entropic trap was found to lead to an excited-state carbonyl compound for FDOH, and a gradually reversible CT initiated luminescence (GRCTIL) was proposed as a new mechanism for the decomposition of FDO(-).

Dissociative Addition of Water to a Main Group 13 Metal Cluster: Computational Study of the Reaction of Gallium Dimer with H2O

We have investigated the lowest triplet and singlet potential energy surfaces (PESs) for the reaction of Ga(2) dimer with water. Under thermal conditions, we predict formation of the triplet ground state addition complex Ga(2)···OH(2)((3)B(1)) involving Ga···O···Ga bridge interaction. At the coupled cluster CCSD(T)/AE (CCSD(T)/ECP) computational levels, Ga(2)···OH(2)((3)B(1)) is bound by 5.5 (5.7) kcal/mol with respect to the ground state reactants Ga(2)((3)Π(u)) + H(2)O. Identification of the addition complex is in agreement with the experimental evidence from matrix isolation infrared (IR) spectroscopy reported recently by Macrae and Downs. The located minimum energy crossing points (MECPs) between the triplet and singlet energy surfaces on the entrance channel of Ga(2) + H(2)O are not expected to be energetically accessible under the matrix conditions, consistent with the lack of occurrence of Ga(2) insertion into the O-H bond under such conditions. The computed energies and harmonic and anharmonic vibrational frequencies for the triplet and singlet Ga(2)(H)(OH) insertion isomers indicate the singlet double-bridged Ga(μ-H)(μ-OH)Ga isomer to be the most stable and support the experimental IR identification of this species. The energy barrier for elimination of H(2) from the second most stable singlet HGa(μ-OH)Ga insertion isomer found to be 13.9 (12.9) kcal/mol is also consistent with the available experimental data.