Proton Transfer Product Research Articles

Combined experimental and computational study of the reactivity of the methanimine radical cation (H2CNH˙+) and its isomer aminomethylene (HCNH2˙+) with propene (CH3CHCH2).

The gas phase reactivity of the radical cation isomers H2CNH˙+ (methanimine) and HCNH2˙+ (aminomethylene) with propene (CH3CHCH2) has been investigated by measuring absolute reactive cross sections and product branching ratios, under single collision conditions, as a function of collision energy (in the range ∼0.07-11.80 eV) using guided ion beam mass spectrometry coupled with VUV photoionization for selective isomer generation. Experimental results have been merged with theoretical calculations to elucidate reaction pathways and structures of products. The H2CNH˙+ isomer is over a factor two more reactive than HCNH2˙+. A major channel from both isomers is production of protonated methanimine CH2NH2+via hydrogen-atom transfer reaction but, while H2CNH˙+ additionally gives charge and proton transfer products, the HCNH2˙+ isomer leads instead to protonated vinylimine CH2CHCHNH2+, produced alongside CH3˙ radicals. The reactions have astrochemical implications in the build up of chemical complexity in both the interstellar medium and the hydrocarbon-rich atmospheres of planets and satellites.

Exploring the versatile reactivity of the F- + SiH3Cl system on a full-dimensional coupled-cluster potential energy surface.

We develop a full-dimensional analytical potential energy surface (PES) for the F- + SiH3Cl reaction using Robosurfer for automatically sampling the configuration space, the robust [CCSD-F12b + BCCD(T) - BCCD]/aug-cc-pVTZ composite level of theory for computing the energy points, and the permutationally invariant polynomial method for fitting. Evolution of the fitting error and the percentage of the unphysical trajectories are monitored as a function of the iteration steps/number of energy points and polynomial order. Quasi-classical trajectory simulations on the new PES reveal rich dynamics resulting in high-probability SN2 (SiH3F + Cl-) and proton-transfer (SiH2Cl- + HF) products as well as several lower-probability channels, such as SiH2F- + HCl, SiH2FCl + H-, SiH2 + FHCl-, SiHFCl- + H2, SiHF + H2 + Cl-, and SiH2 + HF + Cl-. The Walden-inversion and front-side-attack-retention SN2 pathways are found to be competitive, producing nearly racemic products at high collision energies. The detailed atomic-level mechanisms of the various reaction pathways and channels as well as the accuracy of the analytical PES are analyzed along representative trajectories.

Bimolecular Excited-State Proton-Coupled Electron Transfer within Encounter Complexes.

Bimolecular excited-state proton-coupled electron transfer (PCET*) was observed for reaction of the triplet MLCT state of [(dpab)2Ru(4,4'-dhbpy)]2+ (dpab = 4,4'-di(n-propyl)amido-2,2'-bipyridine, 4,4'-dhbpy = 4,4'-dihydroxy-2,2'-bipyridine) with N-methyl-4,4'-bipyridinium (MQ+) and N-benzyl-4,4'-bipyridinium (BMQ+) in dry acetonitrile solutions. The PCET* reaction products, the oxidized and deprotonated Ru complex, and the reduced protonated MQ+ can be distinguished from the excited state electron transfer (ET*) and the excited state proton transfer (PT*) products by the difference in the visible absorption spectrum of the species emerging from the encounter complex. The observed behavior differs from that of reaction of the MLCT state of [(bpy)2Ru(4,4'-dhbpy)]2+ (bpy = 2,2'-bipyridine) with MQ+, where initial ET* is followed by diffusion-limited proton transfer from the coordinated 4,4'-dhbpy to MQ0. The difference in observed behavior can be rationalized based on changes in the free energies of ET* and PT*. Substitution of bpy with dpab results in the ET* process becoming significantly more endergonic and the PT* reaction becoming somewhat less endergonic.

Switching of the Polarity-Sensitive Aggregation Pattern of a Thiosemicarbazone-Based Anticancer Luminophore and Its Involvement in Cellular Apoptosis of the Human Lung Cancer Cell Line.

Elucidation of the photophysical and biochemical properties of small molecules can facilitate their applications as prospective therapeutic imaging (theragnostic) agents. Herein, we demonstrate the luminescence behavior of a strategically designed potential therapeutic thiosemicarbazone derivative, (E)-1-(4-(diethylamino)-2-hydroxybenzylidene)-4,4-dimethylthiosemicarbazide (DAHTS), accompanied by the illustration of its solvation and solvation dynamics using spectroscopic techniques and exploring its promising antitumor activities by adopting the necessary biochemical assays. Solvent-dependent photophysical properties, namely UV-vis absorption, fluorescence emission, and excitation profiles, concentration-dependent studies, and time-resolved fluorescence decays, serve as footprints to explain the existence of DAHTS monomers, its excited-state intramolecular proton transfer (ESIPT) product, and dimeric and aggregated forms. The emission intensity progressively intensifies with increasing polarity and proticity of the solvents up to MeOH, but in water, a sudden dip is seen. Solvent polarity and H-bonding modulate the fluorescence behavior of the primary emission peak and significantly influence the formation of the dimer and DAHTS aggregates. The designed luminophore (DAHTS) exhibits significant antiproliferative activity against the human lung cancer (A549) cell lines with inhibitory concentrations (IC50) of 16.88 and 11.92 μM for 24 and 48 h, respectively. DAHTS effectively reduces the cell viability and induces cytotoxicity with extensive morphological changes in A549 cells in the form of spikes when compared to the normal HEK cell lines. More importantly, it increases the p53 expression at the mRNA level that consolidates its potential therapeutic activity. The effect of DAHTS on apoptotic pathways against the A549 cell line has been investigated to determine its probable mechanism of cell death. Thus, the all-inclusive understanding of the photophysical properties and the necessary biochemical assays put forward important steps toward tailoring the thiosemicarbazone core structure for favorable cancer theragnostic applications in academic and pharmaceutical research.

Kinetics for the Reactions of H3O+(H2O)n=0-3 with Isoprene (2-Methyl-1,3-butadiene) as a Function of Temperature (300-500 K).

We report kinetics studies of H3O+(H2O)n=0-3 with isoprene (2-methyl-1,3-butadiene, C5H8) as a function of temperature (300-500 K) measured using a flowing afterglow-selected ion flow tube. Results are supported by density functional (DFT) calculations at the B3LYP/def2-TZVP level. H3O+ (n = 0) reacts with isoprene near the collision limit exclusively via proton transfer to form C5H9+. The first hydrate (n = 1) also reacts at the collision limit and only the proton transfer product is observed, although hydrated protonated isoprene may have been produced and dissociated thermally. Addition of a second water (n = 2) lowers the rate constant by about a factor of 10. The proton transfer of H3O+(H2O)2 to isoprene is endothermic, but transfer of the water ligands lowers the thermicity and the likely process occurring is H3O+(H2O)2 + C5H8 → C5H9+(H2O)2 + H2O, followed by thermal dissociation of C5H9+(H2O)2. Statistical modeling indicates the amount of reactivity is consistent with the process being slightly endothermic, as is indicated by the DFT calculations. This reactivity was obscured in past experiments due to the presence of water in the reaction zone. The third hydrate is observed not to react and helps explain the past results for n = 2, as n = 2 and 3 were in equilibrium in that flow tube experiment. Very little dependence on temperature was found for the three species that did react. Finally, the C5H9+ proton transfer product further reacted with isoprene to produce mainly C6H9+ along with a small amount of clustering.

Computational Studies of Coinage Metal Anion M- + CH3X (X = F, Cl, Br, I) Reactions in Gas Phase.

We characterized the stationary points along the nucleophilic substitution (SN2), oxidative insertion (OI), halogen abstraction (XA), and proton transfer (PT) product channels of M− + CH3X (M = Cu, Ag, Au; X = F, Cl, Br, I) reactions using the CCSD(T)/aug-cc-pVTZ level of theory. In general, the reaction energies follow the order of PT > XA > SN2 > OI. The OI channel that results in oxidative insertion complex [CH3–M–X]− is most exothermic, and can be formed through a front-side attack of M on the C-X bond via a high transition state OxTS or through a SN2-mediated halogen rearrangement path via a much lower transition state invTS. The order of OxTS > invTS is inverted when changing M− to Pd, a d10 metal, because the symmetry of their HOMO orbital is different. The back-side attack SN2 pathway proceeds via typical Walden-inversion transition state that connects to pre- and post-reaction complexes. For X = Cl/Br/I, the invSN2-TS’s are, in general, submerged. The shape of this M− + CH3X SN2 PES is flatter as compared to that of a main-group base like F− + CH3X, whose PES has a double-well shape. When X = Br/I, a linear halogen-bonded complex [CH3−X∙··M]− can be formed as an intermediate upon the front-side attachment of M on the halogen atom X, and it either dissociates to CH3 + MX− through halogen abstraction or bends the C-X-M angle to continue the back-side SN2 path. Natural bond orbital analysis shows a polar covalent M−X bond is formed within oxidative insertion complex [CH3–M–X]−, whereas a noncovalent M–X halogen-bond interaction exists for the [CH3–X∙··M]− complex. This work explores competing channels of the M− + CH3X reaction in the gas phase and the potential energy surface is useful in understanding the dynamic behavior of the title and analogous reactions.

Photoinduced Irreversible Intramolecular Proton Transfer of Arnebinones B, D, and E: The Case of Photoenolization at the p-Benzoquinone-CH2/CH-π System.

Arnebinones B, E, and D (1-3) have been found to be sensitive to light, generating complex and diverse proton transfer products when triggered by light. A unique two-step irreversible intramolecular proton transfer of 1 produced five scalemic mixtures, of which four possessed intriguing dual planar chirality. The unprecedented orientation epimerization equilibrium of the intra-annular double bond was first observed and researched in the homologous meroterpenoids by HPLC monitoring and DFT calculations. A "p-benzoquinone-CH2/CH-π" moiety in the structure was the common key feature for the occurrence of this type of photoenolization reaction. The product transformation processes and universality of this photoinduced irreversible proton transfer reaction were analyzed together with the cytotoxic activities of arnebinones B, D, and E, and their photoreaction products.

Searching for a near-infrared fluorescence band supporting a hypothetical double proton transfer in ESIPT reaction of 2,5-bis(2′-benzoxazolyl) hydroquinone (BBHQ), as requested by theoreticians

New near-infrared broad-band emission with λmax~920 nm was detected in 2,5-bis(2′-benzoxazolyl)hydroquinone (BBHQ), a molecule known to undergo the single proton transfer in the ESIPT reaction and to emit the fluorescence with λmax ~600 nm. The new emission was attributed to the fluorescence of the product of double proton transfer in the excited BBHQ. The attribution was based on the qualitative agreement verifying the theoretical predictions and on the behaviour of the two properly chosen derivatives: the diethyl-BBHQ independently showing the same broad-band IR fluorescence, and the monomethoxy-BBHQ, a “negative witness”- a structure with one of two ESIPT reactivity sites eliminated, showing no trace of the new emission.

A chemical dynamics study of the HCl + HCl+ reaction

A recent guided ion beam study of the HCl + HCl+ reaction has revealed two different products [Phys. Chem. Chem. Phys.2015, 17 (25), 16454–16461]. The first is the proton transfer product, H2Cl+ + Cl, where the cross section of the reactions associated with this product, as predicted, monotonically decreases as the collision energy between the product increases. The second is the product HCl+ + HCl, where the cross section of the reaction shows a local maximum at the collision energy of 0.5 eV. The nature of this unusual behavior of the cross section is not clear. In this manuscript, state of the art ab initio molecular dynamics (AIMD) simulation is performed to study this bimolecular collision of HCl+ + HCl. The potential energy of profile of this system is first characterized with high-level ab initio methods, and then a computationally efficient method is selected for AIMD simulation. The cross sections from AIMD agree well with those from the experiments for both products. The AIMD trajectories reveal the complexity of this seemingly-simple reaction – a total of five different pathways that result in the aforementioned two products. The simulation also sheds light on the mystery of the local maximum of the cross section regarding the HCl+ + HCl product.

Solvent polarity dependent excited state hydrogen bond effects and intramolecular double proton transfer mechanism for 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d’]bisimidazole system

In this work, we probe into the photo-induced excited state hydrogen bonding interactions and excited state proton transfer (ESPT) behaviors for a representative benzo[1,2-d:4,5-d’]bisimidazole derivative (i.e., 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d’]bisimidazole (HPBB)) compound. In view of aprotic solvents with different polarities, cyclohexane (CYH), dichloromethane (DCM) and acetonitrile (MeCN) solvents are considered. Analyzing hydrogen-bond geometrical parameters, infrared (IR) vibrational spectra, Mayer bond order and predicting hydrogen bonding energy (E(HB)), we verify dual hydrogen bonds of HPBB are strengthened in S1 state. Particularly, in nonpolar solvent, the enhanced excited state hydrogen bonds become more obvious. The intriguing charge redistribution and frontier molecular orbitals (MOs) reveal hydrogen bonding acceptance ability of acceptor moieties becomes stronger, which plays a crucial role in capturing hydroxyl proton via photoexcitation. To check and explore ESIPT mechanism, we present the solvent polarity dependent asynchronous excited state intramolecular double proton transfer (ESIDPT) mechanism. That is, nonpolar solvent promotes excited state intramolecular single proton transfer (ESISPT) process for HPBB, while polar solvent contributes to ESIDPT behavior with the primary single proton-transfer product in S1 state. This work not only makes a rational attribution to experimental phenomena, but also clarifies detailed excited state behaviors for HPBB and presents regulating ESIPT mechanism via solvent polarity.

The influence of base pair tautomerism on single point mutations in aqueous DNA.

The relationship between base pair hydrogen bond proton transfer and the rate of spontaneous single point mutations at ambient temperatures and pressures in aqueous DNA is investigated. By using an ensemble-based multiscale computational modelling method, statistically robust rates of proton transfer for the A:T and G:C base pairs within a solvated DNA dodecamer are calculated. Several different proton transfer pathways are observed within the same base pair. It is shown that, in G:C, the double proton transfer tautomer is preferred, while the single proton transfer process is favoured in A:T. The reported range of rate coefficients for double proton transfer is consistent with recent experimental data. Notwithstanding the approximately 1000 times more common presence of single proton transfer products from A:T, observationally there is bias towards G:C to A:T mutations in a wide range of living organisms. We infer that the double proton transfer reactions between G:C base pairs have a negligible contribution towards this bias for the following reasons: (i) the maximum half-life of the G*:C* tautomer is in the range of picoseconds, which is significantly smaller than the milliseconds it takes for DNA to unwind during replication, (ii) statistically, the majority of G*:C* tautomers revert back to their canonical forms through a barrierless process, and (iii) the thermodynamic instability of the tautomers with respect to the canonical base pairs. Through similar reasoning, we also deduce that proton transfer in the A:T base pair does not contribute to single point mutations in DNA.

Dueling electrospray implemented on a traveling-wave ion mobility/time-of-flight mass spectrometer: Towards a gas-phase workbench for structural biology

The traveling wave trap cell of a commercial ion mobility mass spectrometer (IM/MS) was used as a gas-phase reactor for covalent chemistry by making a simple modification to a standard nanoelectrospray source. Reagents and analytes were generated from pulsed opposite polarity nanoelectrospray sources and isolated by their m/z prior to reaction. Covalent bond formation was first observed with the model peptide angiotensin I. The modification site was identified as the N-terminus of the peptide by collision induced dissociation (CID). The IM cell separated the covalent reaction product from the proton transfer product by their respective ion mobilities. Next, the effects of several trapping parameters, including the trap traveling wave height, the trap RF voltage, and the trap pressure, were evaluated. Decreasing traveling wave height and increasing RF voltage and pressure increased the number of proton transfer events from apomyoglobin to reagent anions. The 6+ charge state of ubiquitin generated from nanospray under native-like conditions was covalently modified in the gas phase through ion/ion reactions. Probing the reacted protein with CID led to the assignment of lysine 29 and arginine 54 as reactive nucleophiles accessible to the reagent. IM analysis of the unmodified native-like 6+ charge state revealed that the gas-phase structure of the protein in the trap was in its compact form. Overall, we introduce a promising method for three-dimensional structural characterization of biomacromolecules.

Three Ways Isolable Carbenes Can Modulate Emission of NH-Containing Fluorophores.

Fluorescent molecules and materials that exhibit emission changes in response to analytes are of great interest across multiple disciplines. Herein, we investigate the response of NH-containing fluorophores carbazole and 2-phenylbenzimidazole (Ph-BIM) with two representative isolable singlet carbenes. Specifically, N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and cyclic (alkyl)(amino)carbene (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene (EtCAAC) were discovered to afford three different types of reaction products with carbazole and Ph-BIM. Depending on the reaction pair, hydrogen bonding (1), NH-insertion (2,3), or proton transfer (4) products can be isolated, each displaying variable photophysical responses. These products have been structurally authenticated by single crystal X-ray diffraction and NMR spectrometric methods. Studies of the solution state behavior of 1-4 reveals that these adducts are labile and can reversibly dissociate to free carbenes and fluorophores to varying extents. These equilibria produce concentration dependent solution state behavior as identified and quantified via UV-visible absorption, emission, 1H DOSY, and NMR spectroscopic measurements.

Resolving the excited state relaxation dynamics of guanosine monomers and hydrogen-bonded homodimers in chloroform solution

The relaxation pathways of silyl-modified guanosine nucleoside monomers (G) and double-hydrogen-bonded homodimers (GG1) are compared in chloroform solution after 260-nm ultraviolet excitation. Transient absorption spectra support two previously reported relaxation pathways for the monomer with time constants of 210 ± 20 fs and 2.6 ± 0.1 ps. These pathways are associated with bifurcated approach to a seam of conical intersections between the excited 1ππ* 1La state and the ground electronic state. In the homodimer, an increase in the larger time constant to 18 ± 2 ps is attributed to slower passage through the minimum energy region of the 1ππ* state. A further time constant of 70 ± 10 fs indicates wavepacket evolution out of the 1ππ* state Franck-Condon region. A slow component of recovery of ground-state GG1 is proposed to result either from relaxation of the product of inter-base electron-driven proton transfer, or from the lowest triplet state (3ππ* , T1).

Effects of zero point vibration on the reaction dynamics of water dimer cations following ionization.

Reactions of water dimer cation (H2O)2+ following ionization have been investigated by means of a direct ab initio molecular dynamics method. In particular, the effects of zero point vibration and zero point energy (ZPE) on the reaction mechanism were considered in this work. Trajectories were run on two electronic potential energy surfaces (PESs) of (H2O)2+: ground state (2 A″-like state) and the first excited state (2 A'-like state). All trajectories on the ground-state PES lead to the proton-transferred product: H2 O+ (Wd)-H2 O(Wa) → OH(Wd)-H3 O+ (Wa), where Wd and Wa refer to the proton donor and acceptor water molecules, respectively. Time of proton transfer (PT) varied widely from 15 to 40 fs (average time of PT = 30.9 fs). The trajectories on the excited-state PES gave two products: an intermediate complex with a face-to-face structure (H2 O-OH2 )+ and a PT product. However, the proton was transferred to the opposite direction, and the reverse PT was found on the excited-state PES: H2 O(Wd)-H2 O+ (Wa) → H3 O+ (Wd)-OH(Wa). This difference occurred because the ionizing water molecule in the dimer switched between the ground and excited states. The reaction mechanism of (H2O)2+ and the effects of ZPE are discussed on the basis of the results. © 2017 Wiley Periodicals, Inc.

The Investigation of Excited-State Intramolecular Proton Transfer Mechanism of 2-Acetylindan-1, 3-Dion: The Solvation Effect

Density functional theory (DFT) and time-dependent DFT are employed to investigate the solvent dependent excited-state intramolecular proton transfer (ESIPT) mechanism of 2-acetylindan-1,3-dion (AID) in hexane and acetonitrile (ACN). The electronic spectra of AID in the polar ACN solvent observed in the experiment are reproduced well by our calculation, but the results in nonpolar hexane are different from the experimental values. Our theoretical investigation illustrates why the emission peak of Enol-A located at about 457 nm with a weak oscillator strengthen is absent from the latest experimental report (Verma et al. in J Struct Dyn 3:023606, 2016), as the fluorescence emission from the proton-transferred product of AID in the first excited state. Thus, the single fluorescence mechanism is well explained. Moreover, the potential energy curves along the O3–H5 bond for the ground state (S0) and the first excited state (S1) are constructed. In the S1 state, the absence of the potential barrier of AID in ACN, compared with the result of hexane (0.57 kcal/mol), indicates that the ESIPT process of AID molecule is more likely to accomplish in the ACN, highlighting the solvation effect on ESIPT mechanism of AID chromophore.

Enhanced Reactivity in Nucleophilic Acyl Substitution Ion/Ion Reactions Using Triazole-Ester Reagents.

The acyl substitution reactions between 1-hydroxy-7-aza-benzotriazole (HOAt)/1-hydroxy-benzotriazole (HOBt) ester reagents and nucleophilic side chains on peptides have been demonstrated in the gas phase via ion/ion reactions. The HOAt/HOBt ester reagents were synthesized in solution and ionized via negative nano-electrospray ionization. The anionic reagents were then reacted with doubly protonated model peptides containing amines, guanidines, and imidazoles in the gas phase. The complexes formed in the reaction cell were further probed with ion trap collision induced dissociation (CID) yielding either a covalently modified analyte ion or a proton transfer product ion. The covalent reaction yield of HOAt/HOBt ester reagents was demonstrated to be higher than the yield with N-hydroxysuccinimide (NHS) ester reagents over a range of equivalent conditions. Density functional theory (DFT) calculations were performed with a primary amine model system for both triazole-ester and NHS-ester reactants, which indicated a lower transition state barrier for the former reagent, consistent with experiments. The work herein demonstrates that the triazole-ester reagents are more reactive, and therefore less selective, than the analogous NHS-ester reagent. As a consequence, the triazole-ester reagents are the first to show efficient reactivity with unprotonated histidine residues in the gas phase. For all nucleophilic sites and all reagents, covalent reactions are favored under long time, low amplitude activation conditions. This work presents a novel class of reagents capable of gas-phase conjugation to nucleophilic sites in analyte ions via ion/ion chemistry. Graphical Abstract ᅟ.

Synthesis and characterization of new coordination polymers based on 2,3-/3,4-pyridinedicarboxylates and bispyridine ligands with structural diversity

New coordination polymers, formulated as {bpaH2[Cu(μ-2,3-pydc)2]}n (1), [Cd(μ-2,3-pydc)(μ-bpa)0.5(H2O)2]n (2), {[Co2(μ4-3,4-pydc)2(μ-bpe)(H2O)2]·H2O}n (3) and [Cu2(pic)4(μ-4,4′-bipy)]·(4,4′-bipy)·2H2O (4) have been synthesized based on the bispyridine ligands; 1,2-bis(4-pyridyl)ethane (bpa), 1,2-bis(4-pyridyl)ethylene (bpe) and 4,4′-bipyridine (4,4′-bipy) along with dianionic 2,3- and 3,4-pyridinedicarboxylate (2,3-pydc and 3,4-pydc) and mono anionic picolinate (pic) ligands, respectively. All the complexes have been characterized by elemental and IR spectroscopy and single-crystal X-ray diffraction studies. 1 has proton transfer products of bpaH2 cations and shows 1D double chain. Using of the 2,3-pydc and 3,4-pydc ligands in 2 and 3 in the presence of Cd(II) and Co(II) ions generate 1D→2D inclined polycatenated structure in 2 and 4,5 connected {44·62·104}{44·62} 4,5T6 topology in 3, respectively. The picolinate ligand in dinuclear structure of 4 was occurred by partial decarboxylation of 2,3-pydcH2 in the process of the solvothermal reaction. 4,4′-Bipyridine molecules act as both a template via rare weak C–H⋯N interaction and a bridge between Cu(II) centers in the self-assembly process of supramolecular structure of 4. Thermal decomposition behavior of complexes and luminescent property of 2 are also presented herein.

Imaging Proton Transfer and Dihalide Formation Pathways in Reactions of F(-) + CH3I.

Ion–molecule reactions of the type X– + CH3Y are commonly assumed to produce Y– through bimolecular nucleophilic substitution (SN2). Beyond this reaction, additional reaction products have been observed throughout the last decades and have been ascribed to different entrance channel geometries differing from the commonly assumed collinear approach. We have performed a crossed beam velocity map imaging experiment on the F– + CH3I reaction at different relative collision energies between 0.4 and 2.9 eV. We find three additional channels competing with nucleophilic substitution at high energies. Experimental branching ratios and angle- and energy differential cross sections are presented for each product channel. The proton transfer product CH2I– is the main reaction channel, which competes with nucleophilic substitution up to 2.9 eV relative collision energy. At this level, the second additional channel, the formation of IF– via halogen abstraction, becomes more efficient. In addition, we present the first evidence for an [FHI]− product ion. This [FHI]− product ion is present only for a narrow range of collision energies, indicating possible dissociation at high energies. All three products show a similar trend with respect to their velocity- and scattering angle distributions, with isotropic scattering and forward scattering of the product ions occurring at low and high energies, respectively. Reactions leading to all three reaction channels present a considerable amount of energy partitioning in product internal excitation. The internally excited fraction shows a collision energy dependence only for CH2I–. A similar trend is observed for the isoelectronic OH– + CH3I system. The comparison of our experimental data at 1.55 eV collision energy with a recent theoretical calculation for the same system shows a slightly higher fraction of internal excitation than predicted, which is, however, compatible within the experimental accuracy.

Light-Induced Ring-Closing Dynamics of a Hydrogen-Bonded Adduct of Benzo[1,3]oxazine in Protic Solvents

Benzo[1,3]oxazine, an organic optical switching compound, is known to be in an equilibrium between its closed form (OX) and its open zwitterionic form (IN). Here we report a light-induced ring-closing mechanism of a hydrogen-bonded adduct (p-IN, partially protonated open isomer of benzo[1,3]oxazine) based on the observations of femtosecond and nanosecond transient absorption spectra in protic solvents. Femtosecond transient measurements upon visible excitation reveal the appearance of two states having different dynamical signatures. One corresponds to a conventional intramolecular charge transfer excited state. The other one is a concerted electron–proton transfer product (d-IN, embedded solvent molecule released from p-IN). Without steric hindrance, the main molecular structure tends to be planar upon excitation, and two intermediates, IN and OX, are involved in the sequential thermal transformation before the return to the ground state of p-IN. Specifically, in alcoholic solvents, d-IN converts to the original p-IN compound within 1 ms via the dominant pathway d-IN → IN → OX → p-IN and the side pathway d-IN → IN → p-IN, which is found to be feasible in energy; in contrast, in aqueous solution with increasing strength of intermolecular hydrogen bonding, the rate of the thermal transformation is enhanced by 1 order of magnitude.