Guest Commentary pubs.acs.org/JPCL
Developing a Molecular-Level Understanding of Organic Chemistry and Physics at the Gas−Surface Interface
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complete descriptions of molecular interactions at the gas− surface interface, and those models have successfully described a variety of aspects of collisions and reactions on organic surfaces.10,25−41 For example, researchers have employed trajectory calculations with quantum mechanical and molecular mechanical approaches for modeling the potential energy surface to study the interfacial collision dynamics of atomic oxygen,33 hydroxyl radicals,42 fluorine,40 and other gases.43 Some of the most exciting advances in the study of organic interfacial chemistry have come from scanning probe measurements, often combined with theoretical models, that are providing exquisite insight into the details of molecular and electronic structure of adsorbates, site-specific surface transformations, and single-molecule processes.44−48 In particular, recent advances in noncontact atomic force microscopy (AFM) and scanning tunneling microscopy (STM) have been employed to characterize subsurface structures,49 study interfacial dynamics,50,51 image vibrational excitation within single molecules,52 show covalent bond structure in reactions,53 control hydrogen tautomerization and conductance switching processes,54 and tune the Kondo effect of adsorbed magnetic ions.55 The two Perspectives highlighted in the current issue of The Journal of Physical Chemistry Letters further demonstrate the power of scanning probe methods in elucidating the details of site-specific chemical reactions and manipulating the state of organic molecules on surfaces. The Perspective by Lee, Kautz, and Kandel, published in the current issue of The Journal of Physical Chemistry Letters describes the application of in situ STM to explore the reaction mechanisms of gas-phase radicals when they impinge on a model organic surface. Specifically, alkanethiolate SAMs on gold surfaces were employed as well-characterized and reproducible model organic surfaces for the study of hydrogen atom abstraction, penetration, and film degradation dynamics by hydrogen and atomic chlorine radicals. These fascinating studies use the STM results in conjunction with Monte Carlo simulations to develop a surface radical reaction model to explain how surface structure evolves during reactions in a way that highlights the role of specific surface sites in governing reactivity. While the review by Kandel et al. provides valuable information about gas−surface reactions, the Perspective by Niu and Li describes experiments and theoretical work aimed at the application of scanning probe methods to study the structure, spin state, and Kondo effect for organic adsorbates on solid surfaces. The pioneering studies highlighted in this Perspective effectively demonstrate phthalocyanine-based single-molecule switches and show how scanning probe methods can be used to study reactions and electronic distributions for organic molecules on surfaces. These experimental and
nterfacial reactions of organic molecules play a critical role in many industrial, environmental, and biological processes. In particular, organic chemistry at the gas−surface interface governs processes found in catalysis, atmospheric chemistry, microelectronics, energy conversion, and astrochemistry. Motivated by these important applications, many scientists have focused on determining the mechanisms and kinetics for organic reactions on surfaces, which are fundamentally different from analogous reactions in the condensed phase. Most importantly, the presence of competing reaction sites, constrained geometries, incomplete solvation, and an inherent competition between reaction and desorption rates all significantly affect interfacial chemistry. Further, molecules at the gas−surface interface may be susceptible to reagents such as radicals and photons that often play a lesser role in condensedphase chemistry. As a result of the many unique aspects of interfacial chemistry, this field offers tremendously fertile ground for cultivating new discoveries about the behavior of organic molecules. The study of the chemistry and physics of solid organic surfaces has a long history rooted in the application of a combination of gas-phase research methods and traditional surface science tools originally applied to the exploration of pristine metal and metal oxide surfaces. For example, Sagiv and Naaman were among the first to employ vacuum-based molecular beam methods to explore gas−surface energytransfer dynamics in collisions of small atmospherically relevant molecules on model organic surfaces.1 Following their studies, a number of researchers have employed atomic and molecular beam techniques to reveal the details of chemistry at the gas− organic surface interface. Although the field is too broad to fully describe here, initial work demonstrated the power of cold helium atom scattering in characterizing well-ordered organic surfaces and monolayers.2,3 These studies were followed by other experimentalists who performed molecular beam scattering studies from well-characterized self-assembled monolayers (SAMs) on gold to uncover the details of gas− surface energy transfer, accommodation, and reactivity.4−19 For example, researchers have recently studied the translational energy dependence to ozone reactivity during collisions on a vinyl-terminated SAM.20 These experiments revealed that the room-temperature reaction follows the Langmuir−Hinshelwood mechanism, but the dynamics transition to a direct reaction for elevated translational energies. Other work has focused on reactions of hydroxyl radicals,21 nitric acid,22 halogens,23 and atomic oxygen11 on a variety of organic surfaces. Many of these experimental studies have provided excellent benchmarks for theoretical work that is revealing insight into the atomic-level dynamics of energy exchange and bond formation at the gas−organic surface interface. Early studies focused on the development of a washboard model to describe the energy-transfer dynamics from a corrugated organic-like surface.24 Since then, theoretical studies have moved to more © 2013 American Chemical Society
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Guest Commentary
and Hydrocarbons Self-Assembled Monolayers. J. Phys. Chem. C 2008, 112, 17272−17280. (13) Day, B. S.; Davis, G. M.; Morris, J. R. The Effect of HydrogenBonding and Terminal Gorup Structure on the Dynamics of Ar Collisions with Self-Assembled Monolayers. Anal. Chim. Acta 2003, 496, 249−258. (14) Ferguson, M. K.; Lohr, J. R.; Day, B. S.; Morris, J. R. Influence of Buried Hydrogen-Bonding Groups within Monolayer Films on Gas−Surface Energy Exchange and Accommodation. Phys. Rev. Lett. 2004, 92, 0732011−4. (15) Lohr, J. R.; Day, B. S.; Morris, J. R. Scattering, Accommodation, and Trapping of HCl in Collisions with a Hydroxylated SelfAssembled Monolayer. J. Phys. Chem. B 2005, 109, 15469−15475. (16) Lohr, J. R.; Day, B. S.; Morris, J. R. Dynamics of HCl Collisions with Hydroxyl- and Methyl-Terminated Self-Assembled Monolayers. J. Phys. Chem. A 2006, 110, 1645−1649. (17) Lu, J. W.; Morris, J. R. Gas−Surface Scattering Dynamics of CO2, NO2, and O3 in Colisions with Model Organic Surfaces. J. Phys. Chem. A 2011, 115, 6194−6201. (18) Shuler, S. F.; Davis, G. M.; Morris, J. R. Energy Transfer in Rare Gas Collisions with Hydroxyl- and Methyl-Terminated Self-Assembled Monolayers. J. Chem. Phys. 2002, 116, 9147−9150. (19) Day, B. S.; Shuler, S. F.; Ducre, A.; Morris, J. R. The Dynamics of Gas−Surface Energy Exchange in Collisions of Ar Atoms with Omega-Functionalized Self-Assembled Monolayers. J. Chem. Phys. 2003, 119, 8084−8096. (20) Lu, J. W.; Fiegland, L. R.; Davis, E. D.; Alexander, W. A.; Wagner, A.; Gandour, R. D.; Morris, J. R. The Initial Reaction Probability and Dynamics of Ozone Collisions with a VinylTerminated Self-Assembled Monolayer. J. Phys. Chem. C 2011, 115, 25343−25350. (21) King, K. L.; Paterson, G.; Rossi, G. E.; Iljina, M.; Westacott, R. E.; Costen, M. L.; McKendrick, K. G. Inelastic Scattering of OH Radicals from Organic Liquids: Isolating the Thermal Desorption Channel. Phys. Chem. Chem. Phys. 2013, 15, 12852−12863. (22) Moussa, S. G.; Stern, A. C.; Raff, J. D.; Dilbeck, C. W.; Tobias, D. J.; Finlayson-Pitts, B. J. Experimental and Theoretical Studies of the Interaction of Gas Phase Nitric Acid and Water with a Self-Assembled Monolayer. Phys. Chem. Chem. Phys. 2013, 15, 448−458. (23) Faust, J. A.; Dempsey, L. P.; Nathanson, G. M. SurfactantPromoted Reactions of Cl2 and Br2 with Br− in Glycerol. J. Phys. Chem. B 2013, 117, 12602−12. (24) Yan, T. Y.; Hase, W. L.; Tully, J. C. A Washboard with Moment of Inertia Model of Gas−Surface Scattering. J. Chem. Phys. 2004, 120, 1031−1043. (25) Tasic, U.; Day, B. S.; Yan, T.; Morris, J. R.; Hase, W. L. Chemical Dynamics Study of Intrasurface Hydrogen-Bonding Effects in Gas−Surface Energy Exchange and Accommodation. J. Phys. Chem. C 2008, 112, 476−490. (26) Yan, T.; Hase, W. L. Origin of the Boltzmann Translational Energy Distribution in the Scattering of Hyperthermal Ne Atoms off a Self-Assembled Monolayer. Phys. Chem. Chem. Phys. 2000, 2, 901− 910. (27) Yan, T.; Hase, W. L. Comparisons of Models for Simulating Energy Transfer in Ne-Atom Collisions with an Alkyl Thiolate SelfAssembled Monolayer. J. Phys. Chem. B 2002, 106, 8029−8037. (28) Yang, L.; Mazyar, O. A.; Lourderaj, U.; Wang, J.; Rodgers, M. T.; Martinez-Nunez, E.; Addepalli, S. V.; Hase, W. L. Chemical Dynamics Simulations of Energy Transfer in Collisions of Protonated Peptide Ions with Perfluorinated Alkylthiol Self-Assembled Monolayer Surface. J. Phys. Chem. C 2008, 112, 9377−9386. (29) Martinez-Nunez, E.; Rahaman, A.; Hase, W. L. Chemical Dynamics Simulations of CO2 Scattering off a Fluorinated SelfAssembled Monolayer Surface. J. Phys. Chem. C 2007, 111, 354−364. (30) Nogueira, J. J.; Homayoon, Z.; Vazquez, S. A.; Martinez-Nunez, E. Chemical Dynamics Study of NO Scattering from a Perfluorinated Self-Assembled Monolayer. J. Phys. Chem. C 2011, 115, 23817−23830. (31) Nogueira, J.; Vazquez, S.; Mazyar, O.; Hase, W.; Perkins, B. G.; Nesbitt, D.; Martinez-Nunez, E. Dynamics of CO2 Scattering off a
computational studies provide the foundation for the development of new approaches to the design of molecular electronics. Together, these two Perspectives effectively demonstrate the power of scanning probe methods for the exploration of organic molecule adsorption, manipulation, and reactions at the gas−surface interface. The next frontiers in this field include methods to directly probe electron-transfer dynamics, real-time bond rupture and formation, and bi- and trimolecular reactions relevant to high-throughput catalysis and photocatalysis. In addition, new approaches are needed that effectively bridge the so-called materials and pressure gaps in our understanding of how to extend studies and phenomena observed in wellcharacterized systems to real-world applications. Continued advances in scanning probe techniques,56 electron microscopy,57 scattering and diffraction methods,58 and high-pressure electron spectroscopies59,60 will be critical tools in further advancing our understanding of the chemistry and physics of organic molecules on surfaces.
John R. Morris*
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Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States
REFERENCES
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