Studies of Halogen Bonding and Electron-Transfer in Halogen Bonded Systems
The importance of non-covalent interactions (e.g., van der Waals forces, hydrogen bonding) in chemistry and biochemistry is well established and spans such fundamental and diverse areas as protein–ligand interactions and drug discovery,1-7 molecular recognition and self-assembly,8-19 crystal engineering,20-24 chemical sensing,10, 25 electron transfer,26-28and organic synthesis.29-31 Increasingly, non-covalent interactions involving halogen atoms (X; X= F, Cl, Br, I) have been recognized to play a key role in many of these areas, stimulating much recent work in the nature of halogen bonding. By analogy with hydrogen bonding, halogen bonds involve the interaction of a polarizable halogen atom (R–X), acting as an electron acceptor, with an electron donor. The strength of halogen bonds ranges from ~ 2 kJ/mol to ~ 160 kJ/mol,12, 32, 33 rivaling or exceeding that of hydrogen bonds. Recent studies of biological systems using a combination of crystallography and theory found that organic halogen atoms are widely involved in protein–ligand interactions.1, 4, 6, 7
Just as our ever-evolving understanding of the hydrogen bond has been aided to a great degree by the study of model systems, the study of halogen bonding also requires fundamental probes of interactions in prototypical systems. In particular, the need exists to systematically correlate properties of the donor/acceptor with structural characteristics of halogen bonding. For example, in biomolecules, where O atoms are often found in peptide, carboxylate, or amide bonds, the directional polarization of the halogen bonding interaction is determined by the relative strength of an O atom donor vs. the pi-system containing that donor.6 The study of halogen bonding to simple model carbonyls offers significant potential for understanding this interplay of forces. Up to the present, progress in this effort has been heavily dependent on theory,34-37 with essential experimental information sorely lacking. This need is being addressed in our group by probing the structure, properties, reactivity and electron transfer dynamics of prototypical halogen bonded systems. Our experiments use both gas-phase methods and matrix isolation techniques at 4 K to interrogate these systems. In recent work, we have examined photoinduced electron transfer in model donor-acceptor halogen bonded complexes, and explored the role of halogen bonding in competition with pi-stacking and C-H/pi interactions in model haloaromatic clusters using resonantly enhanced multiphoton ionization methods in concert with high level ab initio calculations (see below). This work is currently supported by the National Science Foundation.
Reaction Paths at High Energies: Isomerization, Roaming, and Proton-Coupled Electron Transfer
Recent detailed experimental measurements have shed light on new reaction pathways that are testing traditional theoretical approaches to chemical kinetics and reaction dynamics (e.g., transition state theory). At the forefront of this charge is the “roaming” mechanism that was first evidenced in the photochemistry of formaldehyde, H2CO. Roaming represents a pathway to molecular (H2 + CO) products, where the initially formed radical pair (here HCO + H) does not completely separate, and the H atom undergoes large amplitude motion in the van der Waals region of the potential, eventually returning to abstract a H atom and form H2. The prescence of the roaming mechanism alters the product yield in the reaction, and many experimental and theoretical studies over the past 5 years have shown that roaming is an important pathway in many systems. Whether such pathways can be described by traditional theoretical approaches is a point of much controversy at present. Our entry into this field began with studies of iso-CF2X2 (X=Br,I), which are weakly bound isomers of atmospherically important halons that are photochemical products in condensed phase environments, and can be stabilized and studied at low temperature (figure at left). Our characterization of iso-CF2Br2 revealed that isomerization represents a route to molecular products that can explain the yield of molecular products observed in previous gas-phase experiments, which had been attributed to roaming. Working with researchers at UW-Madison, we examined the condensed phase isomerization of a related system, CH2ClI, in real time using ultrafast (femtosecond or 10-15 second) laser spectroscopy. We found that, following photolysis of the parent compound, the isomer was formed within 2 ps (1 ps = 10-12 sec) and vibrationally cooled into its well with a time constant of ~ 50 ps, largely invariant to environment (solution vs. cryogenic matrix). In extending these studies to larger systems, such as the dihaloethanes, we find that the isomer is not trapped in steady state experiments, rather the dihalogen or hydrogen halide elimination products are observed. Calculations have identified a transition state from the isomer to the dehydrohalogenation products (at right), which represents a sequential electron/proton transfer event. A lower energy pathway corresponding to a concerted proton coupled electron transfer event was also identified – this then, is an excellent system for exploring the competition between concerted and sequential pathways of proton-coupled electron transfer, and further studies of this and related systems are on-going.
Studies of Chemical Intermediates important in Planetary Atmospheres
Another current focus of study in the Reid group are reactive intermediates that play a role in the chemistry of planetary atmospheres, wherein are targeted the spectroscopy and photochemistry of radicals and radical-molecule complexes of relevance to the chemistry of planetary atmospheres, including haloalkyl, hydroxyalkl, nitroalkyl and cyanoalkyl radicals in addition to peroxy, trioxy and sulfoxy radicals. Spectroscopic detection and photochemical studies of the initial radicals provide important information for modeling the competition between photodestruction and reaction in the atmosphere. An example is provided in their recent studies of the hitherto unobserved C2F5Br radical, shown at right, where both infrared and electronic spectra of the radical were measured. It was shown that the electronic spectrum of the radical overlapped significantly with the solar actinic spectrum, which had not previously been considered by anyone, and the combination of IR and electronic spectra with ab initio calculations afforded quantitative information concerning the UV cross-section of the radical that, in turn, was used to derive a solar photolysis rate.
We use late-mixing techniques, as demonstrated in a recent study of photoinduced electron transfer in the pre-reactive C2H4---Br2 complex, to examine the formation and fate of important radical-molecule adducts. Radical initiated processes typically proceed via formation of a radical-molecule adduct, and the role of weakly bound radical-molecule complexes in the atmosphere has been the subject of much recent discussion in the literature,39-49 and a Solvay Institutes Workshop on this topic was held in April 2010 in Brussels.50 In 2006, our group reported the first observation of the gas-phase electronic spectrum of a halocarbenium ion,.1,2 Recently, pulsed jet discharge matrix isolation spectroscopy has been used to study the CX2Br+ (X=H,F) ions,51 in studies that have yielded insights into the effect of halogen substitution on the structure and properties of these ions. Now they are developing new late mixing strategies for the clean production of ions, and plan to study the spectroscopy and photochemistry of key ions and their adducts. The importance of ions and cation radicals in atmospheric chemistry has been increasingly recognized in recent years.39, 52-54
Studies of Metal–Containing Reactive Intermediates
An emerging focus of study in our group deals with metal-containing reactive intermediates. Catalytic reactions involving metals are central to an immense array of chemical and biochemical processes; important applications with respect to energy and the environment include C-H and C-C bond activation,1-3 olefin metathesis4-8 and polymerization,9 heterogenous Fischer-Tropsch reactions,10 fuel reforming and methanation,11-13 hydrodehalogenation of chlorofluorocarbons (CFCs),14 and N-H bond activation.15 These catalytic processes often involve intermediates such as metal (M) alkyls (M-CR3), carbenes (M=CR2) and carbynes (M≡CR), which have proven difficult to characterize experimentally.16 Although recent progress has been made, both in the characterization of surface-bound species16 and gas-phase organometallic intermediates,17 the reactivity of and chemical bonding in these species is far from understood. To address this gap, we are examining the vibrational and electronic spectroscopy of key organometallic species which are intermediates in catalytic reactions involving metals. Their specific aims are: 1) to develop spectroscopic signatures for these intermediates, 2) to probe the nature of the metal-ligand bond and its dependence on substituent, metal, and oxidation state, 3) to examine spin-orbit interactions in prototypical systems, and 4) to provide sorely needed benchmark data for ab initio electronic structure theory.
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