Reactive chemical dynamics through conical intersections

Reaction dynamics of prototypical, D + H₂ and Cl (²P) + H₂, chemical reactions occurring through the conical intersections of the respective coupled multi-sheeted potential energy surfaces is examined here. In addition to the electronic coupling, nonadiabatic effects due to relativistic spin-orbit coupling are also considered for the latter reaction. A time-dependent wave packet propagation approach is undertaken and the quantum dynamical observables viz., energy resolved reaction probabilities, integral reaction cross-sections and thermal rate constants are reported.     

Ultrafast Dynamics of the Transoid-cis Isomer Formed in Photochromic Reaction from 3H-Naphthopyran

Recent efforts in designing new 3H-naphthopyran derivatives have been focused on efficient coloration process with a short fading time of the colored transoid-cis TC isomer. It is desirable to avoid photoisomerization of TC leading to transoid-trans TT isomers in the photoreaction. Long lifetime of TT can hamper fast applications such as dynamic holographic materials and molecular actuators, the residual color is one of the serious issues for photochromic lenses. Herein we characterize the photophysical and photochemical channels of TC excited state deactivation competing with the unwanted TC → TT isomerization process. Transient absorption spectroscopy reveals a very short lifetime of the singlet excited TC (≈0.8 ps) and its deactivation channels as S₁→S₀ internal conversion (major), intersystem crossing S₁→T₁, pyran ring formation, photoenolization and TC → TT isomerization. Computations support the S₁→S₀ and T₁→S₀ channels as responsible for photostabilization of the TC form.     

Theoretical Investigation of the Reaction Mechanism of the Photoisomerization of 1,2‐Dihydro‐1,2‐azaborine

The photoisomerization of 1,2‐dihydro‐1,2‐azaborine was investigated by high‐level multireference ab initio and density functional theory calculations. The intermediates (IMs) and transition states (TSs) on the S₀ and S₁ states were optimized using the state‐averaged complete active space self‐consistent field method. The multireference configuration interaction method with the Davidson correction was used to obtain accurate energetics. Moreover, the conical intersections (CIs), which play a crucial role in photoisomerization, were also optimized. On the basis of the calculation results, the most favorable proposed reaction pathway is as follows: reactant→Franck‐Condon region→TS₁→CI→IM₀→TS_0P→product. The product was not directly formed through the CI, and the IM₀ existed on the S₀ state. These results show that the isomerization of 1,2‐dihydro‐1,2‐azaborine involves both photoreactions and thermal reactions. The calculated results clarify recent experimental observations.     

Study of the Photochemical Properties and Conical Intersections of [2,2′‐Bipyridyl]‐3‐amine‐3′‐ol

The two isoelectronic bipyridyl derivatives [2,2′‐bipyridyl]‐3,3′‐diamine and [2,2′‐bipyridyl]‐3,3′‐diol are experimentally known to undergo very different excited‐state double‐proton‐transfer processes, which result in fluorescence quantum yields that differ by four orders of magnitude. In a previous study, these differences were explained from a theoretical point of view, because of topographical features in the potential energy surface and the presence of conical intersections (CIs). Here, we analyze the photochemical properties of a new molecule, [2,2′‐bipyridyl]‐3‐amine‐3′‐ol [BP(OH)(NH₂)], which is, in fact, a hybrid of the former two. Our density functional theory (DFT), time‐dependent DFT (TDDFT), and complete active space self‐consistent field (CASSCF) calculations indicate that the double‐proton‐transfer process in the ground and first singlet π→π* excited state in BP(OH)(NH₂) presents features that are between those of their “parents”. The presence of two CIs and the role they may play in the actual photochemistry of BP(OH)(NH₂) and other bipyridyl derivatives are also discussed.     

Bipyridyl Derivatives as Photomemory Devices: A Comparative Electronic‐Structure Study

The two isoelectronic bipyridyl derivatives [2,2′‐bipyridyl]‐3,3′‐diamine (BP(NH₂)₂) and [2,2′‐bipyridyl]‐3,3′‐diol (BP(OH)₂) are experimentally known to undergo very different excited‐state double proton transfer processes that result in fluorescence quantum yields that differ by four orders of magnitude. Such differences have been theoretically explained in terms of topographical features in the potential energy surface and the likely presence of conical intersections. The hypothetical hybrid compound [2,2′‐bipyridyl]‐3‐amin‐3′‐ol (BP(OH)(NH₂)) presents intermediate photochemical features of its “ancestors”. In this report we analyze the photochemical properties of a whole family of “dark” (not fluorescent) states that can be accessed from each bipyridyl derivative upon irradiation of light of a given wavelength and their potential application as photomemory devices. In the light of our density functional theory (DFT), time‐dependent DFT (TDDFT), and complete active space self‐consistent field (CASSCF) calculations, BP(NH₂)₂ is the more likely candidate to become a photomemory device.     

Quantum dynamics study of the excited-state double-proton transfer in 2,2'-bipyridyl-3,3'-diol.

Density functional theory and quantum dynamics simulations have been used to study the double-proton transfer reaction in 2,2'-bipyridyl-3,3'-diol in the first singlet excited electronic state. This process is experimentally known to be branched: It consists of a fast, concerted reaction mechanism (tau approximately 100 fs) and a stepwise reaction mechanism [with a fast initial step (tau approximately 100 fs) and a slower final step (tau approximately 10 ps)]. Quantum dynamics simulations on a two-dimensional model reveal that the concerted reaction occurs despite the nonexistence of a concerted reaction path, but they fail to explain the relative slowness of the stepwise mechanism. A qualitative simulation using a three-dimensional model suggests that internal vibrational relaxation (IVR) might be the reason why the second stage of the stepwise mechanism is so slow.     

Gold catalysis: deuterated substrates as the key for an experimental insight into the mechanism and selectivity of the phenol synthesis

The second phase of the gold-catalyzed phenol synthesis, the ring opening of the intermediate arene oxide, follows general acid catalysis. The product selectivity is determined by the substrate only and can be explained by the stability of the intermediate arenium ions. Thus, even remote substitutents can be used to control the chemoselectivity of the overall reaction by electronic influences and their influence is stronger than the steric influence of neighboring substituents. This is supported by quantum chemical calculations of the intermediates. The lack of exchange of deuterium labels excludes even equilibria with acetylide or vinylidene intermediates and the observed deuterium distribution in the final products is in accord with the NIH-shift reaction. In addition, these findings now explain previously obtained results.     

The excited-state chemistry of phycocyanobilin: a semiempirical study.

Based on previous time-resolved absorption studies, phycocyanobilin undergoes a photoreaction from an A- into a B- and C-form, with the latter two photoproducts showing absorption spectra red-shifted from A. To identify the molecular mechanism involved in the excited-state reactions, the structural origin of the red shift in the absorption spectra is investigated. Using semiempirical AM1 calculations that include configuration interaction by pair doubles excitation configuration interaction, the absorption spectra of different conformers as well as different protonation states were calculated. The results clearly indicate a pronounced red shift in the spectra of structures either protonated or deprotonated at the basic/acidic centres of the tetrapyrrole chromophore whereas, in contrast, conformational changes alone result in a blue shift. Furthermore, it is shown by quantum chemical calculations that the basicity of phycocyanobilin is much higher in the excited than in the ground state, with a decrease in the excited-state pK(B)* of approximately 9.5 units. The acidity is only slightly enhanced with a drop in pK(A)* of only approximately 1.6 units. From these findings, a reaction model for the excited-state processes in phycocyanobilin is proposed. According to this model, photoexcitation of phycocyanobilin triggers an excited-state proton transfer giving rise to the formation of a protonated species. In parallel, the local increase in the medium pH associated with protonation then forwards a deprotonation at an acidic NH-group so that in effect both protonated and deprotonated phycocyanobilin would arise from the initial photoreaction and account for the observed red shift in the spectra of the B- and C-forms.     

Mechanism and Selectivity of RuII‐ and RhIII‐Catalyzed Oxidative Spiroannulation of Naphthols and Phenols with Alkynes through a C−H Activation/Dearomatization Strategy

The ruthenium‐ and rhodium‐catalyzed oxidative spiroannulation of naphthols and phenols with alkynes was investigated by means of density functional theory calculations. The results show that the reaction undergoes O?H deprotonation/C(sp²)?H bond cleavage through a concerted metalation–deprotonation mechanism/migratory insertion of the alkyne into the M?C bond to deliver the eight‐membered metallacycle. However, the dearomatization through the originally proposed enol–keto tautomerization/C?C reductive elimination was calculated to be kinetically inaccessible. Alternatively, an unusual metallacyclopropene, generated from the isomerization of the eight‐membered metallacycle through rotation of the C?C double bond, was identified as a key intermediate to account for the experimental results. The subsequent C?C coupling between the carbene carbon atom and the carbon atom of the 2‐naphthol/phenol ring was calculated to be relatively facile, leading to the formation of the unexpected dearomatized products. The calculations reproduce quite well the experimentally observed formal [5+2] cycloaddition in the rhodium‐catalyzed oxidative annulation of 2‐vinylphenols with alkynes. The calculations show that compared with the case of 2‐alkenylphenols, the presence of conjugation effects and less steric repulsion between the phenol ring and the vinyl moiety make the competing reductive oxyl migration become dominant, which enables the selectivity switch from the spiroannulation to the formal [5+2] cycloaddition.     

The formation of byproducts in the autoxidation of cyclohexane

In this work, a complementary experimental and theoretical approach is used to unravel the formation of byproducts in the autoxidation of cyclohexane. The widely accepted vision that cyclohexanone would be the most important precursor of undesired products was found inconsistent with several experimental observations. However, the propagation reaction of cyclohexyl hydroperoxide, which we recently put forward as the missing source of cyclohexanol and cyclohexanone, is now unambiguously identified also as the dominant path leading to byproducts. Indeed, this overlooked reaction produces large amounts of cyclohexoxy radicals, able to ring-open via a beta-C--C cleavage to omega-formyl radicals. The pathway by which these radicals are converted into the observed and quantified byproducts is derived in this work. In this liquid-phase reaction, solvent cages were found very important, steering the fate of nascent species.     

Thermal degradation of β‐carotene studied by ion mobility atmospheric solid analysis probe mass spectrometry: full product pattern and selective ionization enhancement

Atmospheric solid analysis probe mass spectrometry has the capability of capturing full product patterns simultaneously including both volatile and semi‐volatile compounds produced at elevated temperatures. Real‐time low‐energy collision‐induced fragmentation combined with ion mobility separations enables rapid identification of the chemical structures of products. We present here for the first time the recognition of full product patterns resulting from the thermal degradation of β‐carotene at temperatures up to 600 °C. Solvent vapor‐induced ionization enhancement is observed, which reveals parallel thermal dissociation processes that lead to even‐ and odd‐numbered mass products. The drift‐time distributions of high mass products, along with β‐carotene, were monitored with temperature, showing multiple conformations that are associated with the presence of two β‐rings. Products of masses 346/347, however, show a single conformation distribution, which indicates the separation of two β‐rings resulting from the direct bond scission at the polyene hydrocarbon chain. The thermal degradation pathways are evaluated and discussed. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.     

Thermodynamic vs. kinetic control of excited-state proton transfer reactions

The “Excited-State Intramolecular Proton Transfer” (ESIPT) reactions in a number of organic fluorophores are among the fastest basic chemical reactions known so far and their rates can be observed even on femtosecond time scale. Accordingly, the reactant concentration, as monitored by its emission, should be negligibly small. In sharp contrast to this conventional wisdom, however, the coexistence of the reactant and the product of this reaction is so frequently observed in condensed media. We then discuss two possible origins of these effects: when the ESIPT reaction is perturbed and hence is slow on the time scale of emission (kinetic control) or when the reverse reaction repopulating the reactant state is fast and leads to the excited-state equilibrium (thermodynamic control). Upon reviewing a great number of ESIPT prototypical systems, we summarize and discuss different criteria for distinguishing these cases based on the steady-state and time-resolved spectroscopic studies and derive correlations between reversibility of these reactions and the solvent-dependent effects observed in fluorescence spectra.     

化学反应中的几何相效应

锥形交叉可以通过几何相效应影响核动力学.在过去的一些年里关于锥形交叉的理论有大量的发展和进步,本文综述了分子反应动力学领域针对几何相效应研究的一些理论成果.介绍了分子反应动力学中与几何相效应直接相关的一些最新成果,同时也对这些重要结果进行了解释.我们相信几何相效应将会在非绝热化学中发挥最重要的作用.     

Mechanistic Origin of Cross‐Coupling Selectivity in Ni‐Catalysed Tishchenko Reactions

Mechanistic studies have been performed for the recently developed, Ni‐catalysed selective cross‐coupling reaction between aryl and alkyl aldehydes. A mono‐carbonyl activation (MCA) mechanism (in which one of the carbonyl groups is activated by oxidative addition) was found to be the most favourable pathway, and the rate‐determining step is oxidative addition. Analysing the origin of the observed cross‐coupling selectivity, we found the most favourable carbonyl activation step requires both coordination of the aryl aldehyde and oxidative addition of the alkyl aldehyde. Therefore, the stronger π‐accepting ability of the aryl aldehyde (relative to alkyl aldehyde) and the ease of oxidative addition of the alkyl aldehyde (relative to aryl aldehyde) are responsible for the cross‐coupling selectivity.