Excited-state Hydrogen Atom Transfer Research Articles

Visible Light‐Mediated Dearomative Hydrogen Atom Abstraction/ Cyclization Cascade of Indoles

AbstractThe photochemical synthesis of yet unknown 2‐oxospiro[azetidine‐3,3′‐indolines] (17 examples, 80–95 % yield), 2,4‐dioxospiro[azetidine‐3,3′‐indolines] (eight examples, 87–97 % yield), and 1‐oxo‐1,3‐dihydrospiro[indene‐2,3′‐indolines] (17 examples, 85–97 % yield) is described. Starting from readily accessible 3‐substituted indoles, a dearomatization of the indole core was accomplished upon irradiation at λ=420 nm in the presence of thioxanthen‐9‐one (10 mol%) as the sensitizer. Based on mechanistic evidence (triplet energy determination, deuteration experiments, by‐product analysis) it is proposed that the reaction proceeds by energy transfer via a 1,4‐ or 1,5‐diradical intermediate. The latter intermediates are formed by excited state hydrogen atom transfer from suitable alkyl groups within the C3 substituent to the indole C2 carbon atom. Subsequent ring closure proceeds with pronounced diastereoselectivity to generate a 4‐ or 5‐membered spirocyclic dearomatized product with several options for further functionalization.

Visible Light-Mediated Dearomative Hydrogen Atom Abstraction/ Cyclization Cascade of Indoles.

The photochemical synthesis of yet unknown 2‐oxospiro[azetidine‐3,3′‐indolines] (17 examples, 80–95 % yield), 2,4‐dioxospiro[azetidine‐3,3′‐indolines] (eight examples, 87–97 % yield), and 1‐oxo‐1,3‐dihydrospiro[indene‐2,3′‐indolines] (17 examples, 85–97 % yield) is described. Starting from readily accessible 3‐substituted indoles, a dearomatization of the indole core was accomplished upon irradiation at λ=420 nm in the presence of thioxanthen‐9‐one (10 mol%) as the sensitizer. Based on mechanistic evidence (triplet energy determination, deuteration experiments, by‐product analysis) it is proposed that the reaction proceeds by energy transfer via a 1,4‐ or 1,5‐diradical intermediate. The latter intermediates are formed by excited state hydrogen atom transfer from suitable alkyl groups within the C3 substituent to the indole C2 carbon atom. Subsequent ring closure proceeds with pronounced diastereoselectivity to generate a 4‐ or 5‐membered spirocyclic dearomatized product with several options for further functionalization.

Excited state hydrogen atom transfer pathways in 2,7-diazaindole – S1-3 (S = H2O and NH3) clusters

The photoinduced tautomerization reactions via hydrogen atom transfer in the excited electronic state (ESHT) have been computationally investigated in 2,7-diazaindole (27DAI) – (H2O)1-3 and 27DAI – (NH3)1-3 isolated clusters to understand the role of various solvent wires. Two competing ESHT reaction pathways originating from the N(1)–H group to the neighbouring N(7) (R(1H-Sn-7H)) and N(2) (R(1H-Sn-2H)) atoms were rigorously examined for each system. Both one- and two-dimensional potential energy surfaces have been calculated in the excited state to investigate the pathways. The R(1H-Sn-7H) was found to be the dominant route with reaction barriers ranging from 26–40 kJmol−1 for water clusters, and 14–26 kJmol−1 for ammonia clusters. The barrier heights for ammonia clusters were found to be nearly half of the that observed for the water systems. The lengthening of the solvent chain up to two molecules resulted in a drastic decrease in the barrier heights for R(1H-Sn-7H). The barriers of the competing reaction channel R(1H-Sn-2H) were found to be significantly higher (31–127 kJmol−1) but were observed to be decreasing with the lengthening of the solvent wire as in the R(1H-Sn-7H) pathway. In both the reactions, the angle strain present in the transition state structures was dependent upon the solvent chain's length and was most likely the governing factor for the barrier heights in each solvent cluster. The results have also affirmed that the ammonia molecule is a better candidate for hydrogen transfer than water because of its higher gas-phase basicity. The results delineated from this investigation can pave the way to unravel the excited-state hydrogen atom transfer pathways in novel N–H bearing molecules.

Sodium cationization can disrupt the intramolecular hydrogen bond that mediates the sunscreen activity of oxybenzone.

A key decay pathway by which organic sunscreen molecules dissipate harmful UV energy involves excited-state hydrogen atom transfer between proximal enol and keto functional groups. Structural modifications of this molecular architecture have the potential to block ultrafast decay processes, and hence promote direct excited-state molecular dissociation, profoundly affecting the efficiency of an organic sunscreen. Herein, we investigate the binding of alkali metal cations to a prototype organic sunscreen molecule, oxybenzone, using IR characterization. Mass-selective IR action spectroscopy was conducted at the free electron laser for infrared experiments, FELIX (600-1800 cm-1), on complexes of Na+, K+ and Rb+ bound to oxybenzone. The IR spectra reveal that K+ and Rb+ adopt binding positions away from the key OH intermolecular hydrogen bond, while the smaller Na+ cation binds directly between the keto and enol oxygens, thus breaking the intramolecular hydrogen bond. UV laser photodissociation spectroscopy was also performed on the series of complexes, with the Na+ complex displaying a distinctive electronic spectrum compared to those of K+ and Rb+, in line with the IR spectroscopy results. TD-DFT calculations reveal that the origin of the changes in the electronic spectra can be linked to rupture of the intramolecular bond in the sodium cationized complex. The implications of our results for the performance of sunscreens in mixtures and environments with high concentrations of metal cations are discussed.

Microsolvation, hydrogen bond dynamics and excited state hydrogen atom transfer mechanism of 2′,4′-dihydroxychalcone

Electronic structure of 2′,4′-dihydroxychalcone (DHC) and its water complex DHC-(H2O)4 (DHCH) at ground and first excited states were investigated by implementing the DFT and SS-TDDFT with EFP method for microsolvation and the NBO method for charge analysis. One intramolecular and four intermolecular hydrogen bonds (HB) exist in the DHCH molecule reinforce the excited state hydrogen atom transfer from the 2′-hydroxyl group to the carbonyl group. The natural charge analysis and potential energy profiles along intramolecular HB path at ground and excited states establish the excited state hydrogen atom transfer mechanism in DHC and DHCH molecules.

Excited state hydrogen atom transfer in micro-solvated dicoumarol: A TDDFT/EFP1 study.

Ground (S0) and excited (S1) state properties of dicoumarol (DC) are investigated by applying density functional theory (DFT) and time dependent DFT (TDDFT) interfacing with the effective fragment potential (EFP) method of solvation. Benzene and pyrone rings of the each 4-hydroxy coumarin (4HC) moiety are in a plane and these planes are twisted by 180° with respect to each other. Two intra-molecular hydrogen bonds (HB) CO⋯HO exist between the carbonyl (CO) and hydroxyl (OH) groups of different 4HC moieties (4HC-1 and 4HC-2). DC(H2O)3 complex is formed using the original EFP model (EFP1). Four inter-molecular HBs are established by the carbonyl and hydroxyl oxygen atoms of 4HC-1 and 4HC-2 moieties; two HBs with two solvent molecules on one side of the complex and other two HBs with one solvent molecule at the other side. In S1 state, the hydrogen atomtransfer takes place only from the hydroxyl group of 4HC-1 to the carbonyl group of 4HC-2. The natural charge analysis and the modification of HBs manifest the intra-molecular charge transfer (ICT) from one 4HC moiety to another. Theoretical and experimental studies of the absorption spectra, and the theoretical study of potential energy curves of OH bonds at both S0 and S1 states affirm the hydrogen atom transfer from the hydroxyl group of 4HC-1 to the carbonyl group of 4HC-2 moiety.

Ultrafast excited state hydrogen atom transfer in salicylideneaniline driven by changes in aromaticity.

We investigated two important unresolved issues on excited state intramolecular proton transfer (ESIPT) reactions, i.e., their driving force and the charge state of the transferred species by means of quantum chemical topology. We related changes in the aromaticity of a molecule after electron excitation to reaction dynamics in an excited state. Additionally, we found that the conveyed particle has a charge intermediate between that of a bare proton and a neutral hydrogen atom. We anticipate that the analysis presented in this communication will yield valuable insights into ESIPT and other similar photochemical reactions.

7-Hydroxyquinoline-8-carbaldehydes. 1. Ground- and Excited-State Long-Range Prototropic Tautomerization

Ground- and excited-state long-range prototropic tautomerization were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (7-HQCs) by (1)H and (13)C NMR spectroscopy, photostationary and time-resolved UV-vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that have been proposed to serve as models of a reversible optically driven molecular switch composed of two moieties: a molecular "frame" (7-hydroquinolines, 7-HQs) and a proton "crane" (carbaldehyde group). The NMR and electronic absorption spectra indicate a solvent-dependent equilibrium between two tautomeric forms, OH (7-quinolinol)) and NH (7(1H)-quinolinone), already in the ground state of all the compounds under study (7-hydroxy-2-methoxy-4-methylquinoline-8-carbaldehyde, HMMQC, shows only a trace of the NH form in highly polar and/or protic media). Electronic absorption and fluorescence of 7-HQCs are rationalized in terms of the ground- and excited-state hydrogen atom transfer (HAT). This process was identified by comparing the UV-vis spectroscopic properties of 7-HQCs with those of 7-HQs, synthetic precursors of the former, as well as with the characteristics of corresponding protonated cations and deprotonated anions (part 2). The experimental results are corroborated by the density functional theory (DFT) and ab initio computations, which shed some light on the differences in photophysics between variously substituted 7-HQCs.

Excited-State Hydrogen Atom Transfer Reaction in Solvated 7-Hydroxy-4-methylcoumarin

Excited-state enol to keto tautomerization of 7-hydroxy-4-methylcoumarin (C456) with three water molecules (C456:3H2O), is theoretically investigated using time-dependent density functional theory (TDDFT) combined with the polarizable continuum model and 200 waters explicitly modeled with the effective fragment potential. The tautomerization of C456 in the presence of three water molecules is accompanied by an asynchronous quadruple hydrogen atom transfer reaction from the enol to the keto tautomer in the excited state. TDDFT with the PBE0 functional and the DH(d,p) basis set is used to calculate the excited-state reaction barrier height, absorption (excitation), and fluorescence (de-excitation) energies. These results are compared with the available experimental and theoretical data. In contrast to previous work, it is predicted here that the coumarin 456 system undergoes a hydrogen atom transfer, not a proton transfer. The calculated reaction barrier of the first excited state of C456:3H2O with 200 water molecules is found to be -0.23 kcal/mol without zero-point energy (-5.07 kcal/mol with zero point energy, i.e., the activation energy).

Early-Stage Dynamics in Coupled Proton–Electron Transfer from the π–π* State of Phenol to Solvent Ammonia Clusters: A Nonadiabatic Electron Dynamics Study

We reexmine the mechanism and interpretation of photochemical reaction of phenol molecule with small ammonia clusters, which is schematically written as Ph*OH···(NH(3))(n) → PhO(•)···[H(NH(3))(n)]*(•) with n ≤ 5. The low-lying excited states of this system in the adiabatic representation are densely quasi-degenerate due to the presence of the Rydberg-like diffused states in ammonia clusters. To treat the dynamics on such highly quasi-degenerate electronic states, we have carried out a large scale semiclassical Ehrenfest dynamics, nonadiabatic electron wavepacket dynamics in terms of very many configuration-state functions, to track the nonadiabatic electron and proton transfer dynamics in the time step of attosecond scale, integrating up to 300 fs. It turns out that the mechanism is more complicated than that referred to as excited-state hydrogen-atom transfer, which is widely accepted now. The pathways of jumping electron and shifting proton nucleus are identified to be mutually different, which necessarily results in charge separation in ammonia clusters after the transitions. The global feature of the present transfer dynamics is fully analyzed as one of the general prototypes of coupled electron-proton transfer in excited states.

Photodissociation of pyrrole–ammonia clusters by velocity map imaging: mechanism for the H-atom transfer reaction

The photodissociation dynamics of pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-6) has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. The excited state hydrogen-atom transfer mechanism (ESHT) is evidenced through delayed ionization and presents a threshold around 236.6 nm, in agreement with previous reports. A high resolution determination of the kinetic energy distributions (KEDs) of the products reveals slow (∼0.15 eV) and structured distributions for all the ammonia cluster masses studied. The low values of the measured kinetic energy rule out the existence of a long-lived intermediate state, as it has been proposed previously. Instead, a direct N-H bond rupture, in the fashion of the photodissociation of bare pyrrole, is proposed. This assumption is supported by a careful analysis of the structure of the measured KEDs in terms of a discrete vibrational activity of the pyrrolyl co-fragment.

Excited-State Hydrogen-Atom Transfer along Solvent Wires: Water Molecules Stop the Transfer

Excited-state hydrogen-atom transfer (ESHAT) along a hydrogen-bonded solvent wire occurs for the supersonically cooled n = 3 ammonia-wire cluster attached to the scaffold molecule 7-hydroxyquinoline (7HQ) [Tanner, C.; et al. Science 2003, 302, 1736]. Here, we study the analogous three-membered solvent-wire clusters 7HQ.(NH3)n.(H2O)m, n + m = 3, using resonant two-photon ionization (R2PI) and UV-UV hole-burning spectroscopies. Substitution of H2O for NH3 has a dramatic effect on the excited-state H-atom transfer: The threshold for the ESHAT reaction is approximately 200 cm(-1) for 7HQ.(NH3)3, approximately 350 cm(-1) for both isomers of the 7HQ.(NH3)2.H2O cluster, and approximately 600 cm(-1) for 7HQ.NH3.(H2O)2 but increases to approximately 2000 cm(-1) for the pure 7HQ.(H2O)3 water-wire cluster. To understand the effect of the chemical composition of the solvent wire on the H-atom transfer, the reaction profiles of the low-lying electronic excited states of the n = 3 pure and mixed solvent-wire clusters are calculated with the configuration interaction singles (CIS) method. For those solvent wires with an NH3 molecule at the first position, injection of the H atom into the wire can occur by tunneling. However, further H-atom transfer is blocked by a high barrier at the first (and second) H2O molecule along the solvent wire. H-atom transfer along the entire length of the solvent wire, leading to formation of the 7-ketoquinoline (7KQ) tautomer, cannot occur for any of the H2O-containing clusters, in agreement with experimentally observed absence of 7KQ fluorescence.

Excited state hydrogen atom transfer in ammonia-wire and water-wire clusters

We review experimental and theoretical investigations of excited-state hydrogen atom transfer (ESHAT) reactions along unidirectionally hydrogen bonded solvent ‘wire’ clusters. The solvent wire is attached to the aromatic ‘scaffold’ molecule 7-hydroxyquinoline (7HQ), which offers an O–H and an N hydrogen bonding site, spaced far enough apart to form two- to four-membered wires. S 1 ← S 0 photoexcitation renders the O–H group more acidic and the quinolinic N more basic. This provides a driving force for the enol → keto tautomerization, probed by the characteristic fluorescence of the 7-ketoquinoline in the molecular beam experiments. For 7-hydroxyquinoline·(NH3)3, excitation of ammonia-wire vibrations induces the tautomerization at ∼200 cm−1. Different reaction pathways have been explored by excited-state ab initio calculations. These show that the reaction proceeds by H-atom transfer along the wire as a series of Grotthus-type translocation steps. There is no competition with a mechanism involving successive proton translocations. The rate-controlling S 1 state barriers arise from crossings of a π π* with a Rydberg-type πσ* state and the proton and electron movements along the wire are closely coupled. The excited state reactant, H-transferred intermediates and product structures are characterized. The reaction proceeds by tunnelling, as shown by deuteration of the solvent molecules (ND3) in the wire. The first step of the reaction exhibits intra/intermolecular vibrational mode selectivity. Substitution of NH3 by one, two or three H2O molecules in the wire leads to increasing threshold with each additional H2O molecule, up to >2000 cm−1 for the 7-hydroxyquinoline·(H2O)3 water-wire cluster. No 7-ketoquinoline fluorescence is observed upon insertion of even a single H2O molecule. The calculations show that insertion of each H2O molecule into the solvent wire introduces a high barrier, which blocks any further H-atom transfer. Contents PAGE 1. Introduction 458 2. 7-Hydroxyquinoline 460 2.1. Ground and excited state acid/base properties 460 2.2. O–H bond breaking in the ππ* and πσ* states 461 3. The 7-hydroxyquinoline · (NH3)n ammonia-wire clusters: an overview 463 3.1. Structures and electronic origins 463 3.2. Resonant two-photon ionization spectra 466 4. Excited-state hydrogen atom transfer in 7-hydroxyquinoline · (NH3)3 468 4.1. UV–UV depletion and fluorescence action spectra 468 4.2. Two reaction paths: ESHAT vs. ESPT 470 4.3. The enol → HT1 reaction coordinate 474 4.4. Vibrational mode selectivity 474 5. Solvent e.ects on excited state H-atom transfer: mixed ammonia/water clusters 479 5.1. Comparison of 7HQ· (H2O)3 and 7HQ · (NH3)3 479 5.2. The mixed 7HQ · (NH3)2 · H2O and 7HQ · NH3 · (H2O)2 clusters 482 5.3. Discussion 484 6. Conclusion 485 References 487

Exploring excited-state hydrogen atom transfer along an ammonia wire cluster: Competitive reaction paths and vibrational mode selectivity

The excited-state hydrogen-atom transfer (ESHAT) reaction of the 7-hydroxyquinoline(NH(3))(3) cluster involves a crossing from the initially excited (1)pipi(*) to a (1)pisigma(*) state. The nonadiabatic coupling between these states induces homolytic dissociation of the O-H bond and H-atom transfer to the closest NH(3) molecule, forming a biradical structure denoted HT1, followed by two more Grotthus-type translocation steps along the ammonia wire. We investigate this reaction at the configuration interaction singles level, using a basis set with diffuse orbitals. Intrinsic reaction coordinate calculations of the enol-->HT1 step predict that the H-atom transfer is preceded and followed by extensive twisting and bending of the ammonia wire, as well as large O-H...NH(3) hydrogen bond contraction and expansion. The calculations also predict an excited-state proton transfer path involving synchronous proton motions; however, it lies 20-25 kcal/mol above the ESHAT path. Higher singlet and triplet potential curves are calculated along the ESHAT reaction coordinate: Two singlet-triplet curve crossings occur within the HT1 product well and intersystem crossing to these T(n) states branches the reaction back to the enol reactant side, decreasing the ESHAT yield. In fact, a product yield of approximately 40% 7-ketoquinoline.(NH(3))(3) is experimentally observed. The vibrational mode selectivity of the enol-->HT1 reaction step [C. Manca, C. Tanner, S. Coussan, A. Bach, and S. Leutwyler, J. Chem. Phys. 121, 2578 (2004)] is shown to be due to the large sensitivity of the diffuse pisigma(*) state to vibrational displacements along the intermolecular coordinates.

Ground and Excited State Hydrogen Atom Transfer Reactions and Cyclization of 2-Acetylbenzoic Acid

We present experimental and theoretical studies of the ring−chain tautomerism (H-atom transfer and cyclization) for 2-acetylbenzoic acid at both ground and electronically first excited states. 1H and 13C NMR studies in solution confirm the existence of equilibrium between the open and ring structures at the ground state, with the ring one being dominant (∼90%). Temperature-dependent 1H NMR experiments allowed obtaining the thermodynamic and kinetic parameters at the coalescence temperature (380 K). Fluorescence measurements disclose the involvement of highly efficient nonradiative processes in agreement with the theoretical data. Electronic calculations for the ground state give additional information on the different conformers of the open tautomer. In agreement with the experiment the most stable structure is of the closed ring tautomer, and it is obtained after additional internal rotations of the −COOH and −CO(CH3) fragments. Intrinsic reaction coordinate calculations indicate that the ring formation/breaking and the H-atom transfer are taking place in a concerted but not synchronous manner. At S1 the most stable form is the open one, for which different conformers are also found. The influence of the solvent is also accounted for through a model that considers the solvent as a continuum at both the ground and excited electronic states. No major differences were observed when comparing both gas and condensed phase results, so calculations of the isolated molecule should give a picture of the reaction which is experimentally observed in solution.

Hydrogen transfer in excited pyrrole-ammonia clusters.

The excited state hydrogen atom transfer reaction (ESHT) has been studied in pyrrole-ammonia clusters [PyH-(NH(3))(n)+hnu-->Py.+.NH(4)(NH(3))(n-1)]. The reaction is clearly evidenced through two-color R2P1 experiments using delayed ionization and presents a threshold around 235 nm (5.3 eV). The cluster dynamics has also been explored by picosecond time scale experiments. The clusters decay in the 10-30 ps range with lifetimes increasing with the cluster size. The appearance times for the reaction products are similar to the decay times of the parent clusters. Evaporation processes are also observed in competition with the reaction, and the cluster lifetime after evaporation is estimated to be around 10 ns. The kinetic energy of the reaction products is fairly large and the energy distribution seems quasi mono kinetic. These experimental results rule out the hypothesis that the reaction proceeds through a direct N-H bond rupture but rather imply the existence of a fairly long-lived intermediate state. Calculations performed at the CASSCF/CASMP2 level confirm the experimental observations, and provide some hints regarding the reaction mechanism.

Photoinduced intramolecular hydrogen atom transfer in 2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxyphenyl)benzothiazole studied by laser flash photolysis

2-(2-Hydroxyphenyl)benzoxazole (HBO) and 2-(2-hydroxyphenyl)benzothiazole (HBT) underwent hydrogen atom transfer to give the tautomer in both the singlet excited state and the triplet excited state. In the singlet excited state, the keto form produced by excited state hydrogen atom transfer underwent isomerization around the quasi-double bond to give the trans-keto form. In the triplet excited state, HBO and HBT were equilibrated between the enol form and the cis-keto form and the equilibrium constant was determined by triplet sensitization and quenching experiment by using laser flash photolysis in benzene at room temperature. From these results, we have revealed the energy diagram of the hydrogen atom transfer reaction of HBO and HBT both in the singlet excited state and the triplet excited state.

ChemInform Abstract: Primary Isotope Effects on Excited State Hydrogen Atom Transfer Reactions. Activated and Tunneling Mechanisms in an ortho‐ Methylanthrone.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Primary Isotope Effects on Excited State Hydrogen Atom Transfer Reactions. Activated and Tunneling Mechanisms in an ortho-Methylanthrone

Primary Isotope Effects on Excited State Hydrogen Atom Transfer Reactions. Activated and Tunneling Mechanisms in an ortho-Methylanthrone

ChemInform Abstract: Picosecond Kinetic and Vibrationally Resolved Spectroscopic Studies of Intramolecular Excited‐State Hydrogen Atom Transfer.

ChemInformVolume 20, Issue 17 Article ChemInform Abstract: Picosecond Kinetic and Vibrationally Resolved Spectroscopic Studies of Intramolecular Excited-State Hydrogen Atom Transfer. P. F. BARBARA, P. F. BARBARA Chem. Dep., Univ. Minn., Minneapolis, MN 55455, USASearch for more papers by this authorP. K. WALSH, P. K. WALSH Chem. Dep., Univ. Minn., Minneapolis, MN 55455, USASearch for more papers by this authorL. E. BRUS, L. E. BRUS Chem. Dep., Univ. Minn., Minneapolis, MN 55455, USASearch for more papers by this author P. F. BARBARA, P. F. BARBARA Chem. Dep., Univ. Minn., Minneapolis, MN 55455, USASearch for more papers by this authorP. K. WALSH, P. K. WALSH Chem. Dep., Univ. Minn., Minneapolis, MN 55455, USASearch for more papers by this authorL. E. BRUS, L. E. BRUS Chem. Dep., Univ. Minn., Minneapolis, MN 55455, USASearch for more papers by this author First published: April 25, 1989 https://doi.org/10.1002/chin.198917071Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume20, Issue17April 25, 1989 RelatedInformation