REPORTS 25. http://dmtools.brown.edu University South Bend R&D committee, the Kavli Institute SOM Text 26. D. R. Tovey et al., Phys. Lett. B 488, 17 (2000). for Cosmological Physics through grant NSF Figs. S1 to S3 27. F. Giuliani, Phys. Rev. Lett. 93, 161301 (2004). PHY-0114422, and the U.S. Department of Energy. References 28. We gratefully acknowledge the effort and outstanding technical support of the Fermilab staff. This work is supported by NSF CAREER award PHY-0239812, NSF Supporting Online Material 31 August 2007; accepted 4 January 2008 grants PHY-0707298 and PHY-0555472, the Indiana www.sciencemag.org/cgi/content/full/319/5865/933/DC1 10.1126/science.1149999 but the flight time of ions in our instrument en- Electron-Driven Acid-Base Chemistry: sures that all processes measured occur on a time scale less than ~100 ms. Proton Transfer from Hydrogen Experimentally, we used anion photoelectron spectroscopy to study these species. Anion photo- electron spectroscopy is conducted by crossing a Chloride to Ammonia mass-selected beam of negative ions with a fixed-frequency photon beam and analyzing the Soren N. Eustis,1 Dunja Radisic,1 Kit H. Bowen,1* Rafał A. Bachorz,2,3 Maciej Haranczyk,3,4 energy of the resultant photodetached electrons. Gregory K. Schenter,3 Maciej Gutowski3,4,5* Photodetachment is a direct process that is gov- erned by the energy-conserving relation, hn = In contrast to widely familiar acid-base behavior in solution, single molecules of NH3 and EBE + EKE, where hn is the photon energy, + − HCl do not react to form the ionic salt, NH4 Cl , in isolation. We applied anion photoelectron EBE is the electron binding energy, and EKE is spectroscopy and ab initio theory to investigate the interaction of an excess electron with the the electron kinetic energy. The known photon … hydrogen-bonded complex NH3 HCl. Our results show that an excess electron induces this complex energy and measured electron kinetic energies to form the ionic salt. We propose a mechanism that proceeds through a dipole-bound state to yield the electron binding energies (anion-to- form the negative ion of ionic ammonium chloride, a species that can also be characterized as a corresponding neutral transition energies). The − deformed Rydberg radical, NH4, polarized by a chloride anion, Cl . anionic complexes of interest were generated in on May 6, 2009 a nozzle-ion source. In this device, an ammonia/ hen vapors from open bottles of con- showed conclusively by rotational spectroscopy argon (15%/85%) mixture at 25°C and 2 atm centrated hydrochloric acid and am- that in beams of neutral [(HCl)(NH3)] hetero- was expanded through a 20-mmorificeintohigh Wmonium hydroxide intermingle, a white dimers (2, 3), no appreciable proton transfer oc- vacuum, while a HCl/argon (10%/90%) mixture cloudconsistingoftinyammoniumchloridepar- curs; instead, the spectra are consistent with at a few Torr flowed from a small tube into the … ticles forms. Yet for many years the question linear, hydrogen-bonded ClH NH3 complexes. expansion region immediately outside the nozzle. of how individual molecules of HCl and NH3 Modern theoretical studies agree (4–11). There- Into this confluence of gases, low-energy elec- interact ranked as a fundamental problem in acid- fore, as counterintuitive as it may seem, am- trons were injected from a biased filament near- base chemistry. Essentially, two main types of in- monia and hydrogen chloride do not react under by. This arrangement brought the three main www.sciencemag.org teractions were posited: either a hydrogen-bonded, isolated conditions. This finding also made it components together in vacuum but very near the … ClH NH3 complex, or a proton-transferred (ionic) clear, however, that the appearance of the white nozzle, where plentiful cooling collisions with + − molecule, NH4 Cl , classified respectively by clouds required outside interactions to induce pro- argon atoms carried away the excess energy of Mulliken as outer and inner complexes (1). Com- ton transfer and thus salt formation. As a result, the products. The nozzle and its stagnation cham- petition between covalent and ionic bonding, as the answer to one question had raised another: ber were biased at −500 V, and the entire region well as the role of hydrogen bonding and proton Can local environmental effects, such as collisions between the nozzle and the skimmer was sub- transfer, made the HCl/NH3 system a particu- with other molecules or interactions with elec- jected to a weak, axial magnetic field to enhance larly attractive model for exploring the interplay trons, ions, or even photons, trigger proton trans- the stability of the microplasma. The resultant Downloaded from between major concepts in chemistry. Eventual- fer in acid-base complexes such as these? anions were then extracted and transported by a ly, the following question came into focus: Can Here, we focus on a particular aspect of series of lenses through a 90° magnetic sector, a single molecule of NH3 interacting with a sin- this general question: Can a single electron, the which served as a mass analyzer and selector. By gle molecule of HCl undergo proton transfer to most fundamental entity in chemistry, cause the sweeping the magnet current and thus the mag- + − ... form NH4 Cl , or does the formation of such ClH NH3 complex to undergo proton transfer netic field, anions were detected by a Faraday ionic species require the assistance of environ- to form an ionic salt? Although the synergy be- cup further downstream and a mass spectrum was mental, cooperative effects, like those found in tween electron and proton transfer is recognized thereby recorded. Ions of interest were then fo- the condensed phase? as a fundamental process in biophysics, ma- cused through an aperture and transported into For a time, the prevailing evidence leaned terials science, and catalysis (12, 13), the detailed the ion-laser interaction region, where they crossed toward the proton-transferred outcome. In a series mechanisms responsible for many of these pro- a beam of ~200 W of 488-nm (2.540 eV) photons of experiments 20 years ago, however, Legon cesses remain unresolved. The system under from an argon ion laser. Some of the resultant study can be viewed as undergoing a process photodetached electrons then entered the optics 1 quite similar to proton-coupled electron transfer of a hemispherical electron energy analyzer, where Department of Chemistry, Johns Hopkins University, Balti- (PCET). PCET reactions are commonly defined they were counted as a function of their electron more, MD 21218, USA. 2Center for Functional Nanostruc- tures (CFN) and Institute of Physical Chemistry, Universität as those involving the concerted transfer of an kinetic energy. Our apparatus has been described Karlsruhe (TH), D-76128 Karlsruhe, Germany. 3Chemical electron and a proton in a given complex (12, 14). in further detail elsewhere (15). and Materials Sciences Division, Pacific Northwest National However, there are variants of the PCET model For the theoretical analysis, we used high- Laboratory, Richland, WA 99352, USA. 4Department of Chem- → ń ń 5 that account for stepwise transfer (ET PT, or level, ab initio, quantum chemistry methods im- istry, University of Gda sk, 80-952 Gda sk, Poland. De- → 16 partment of Chemistry, Heriot-Watt University, EH14 4AS PT ET). These PCET reactions generally occur plemented with MOLPRO ( ) and Gaussian03 Edinburgh, UK. on nanosecond time scales, depending on the (17) to characterize potential energy surfaces of *To whom correspondence should be addressed. E-mail: pathway and the overall driving force for the the neutral and the anion. The potential energy [email protected] (K.H.B.); [email protected] (M.G.) reaction. The present study is not time resolved, surfaces were explored at the coupled cluster level 936 15 FEBRUARY 2008 VOL 319 SCIENCE www.sciencemag.org REPORTS − of theory (18) with extended basis sets (19). The anharmonic energy levels for the neutral and the than that for the dipole-bound [(H2O)(HCl)] coupled cluster calculations included single and anion that take into account the coupling between complex. double excitations (CCSD) when producing po- the proton and dimer stretches. All vibrational How, then, should we interpret the vibrational − tential energy maps and determining minimum modes were taken into account when calculating progression in the spectrum of the [(NH3)(HCl)] energy structures, whereas additional perturba- the Franck-Condon (FC) factors between the complex? More specifically, if the FC distri- tive triple excitations [CCSD(T)] were included ground vibrational state of the anion and various bution of peak intensities is characteristic of when calculating the adiabatic electron affinity energy states of the neutral. These calculations potential energy surfaces of the neutral and the and vertical detachment energy (VDE). Two- were performed in harmonic approximation, and anion that differ appreciably along some geo- dimensional (2D) potential energy surfaces were molecular Hessians were determined at the metrical degree of freedom, what is the nature calculated for the neutral and the anion, with the second-order Møller-Plesset level of theory. of this degree of freedom? We hypothesize that ex- … geometrical variables being the H-Cl distance, Because the dipole moment of the ClH NH3 cess electron attachment to the hydrogen-bonded … which characterizes the extent of intermolecular complex is known from Stark-effect measure- ClH NH3 complex is accompanied by intermolec- proton transfer, and the N-Cl distance along ments to be substantial (4.06 D) (22), one might ular proton transfer, with the final product being + − − + − − which the low-frequency intermolecular stretch- predict the anionic complex to be dipole bound (NH4 Cl ) .Furthermore,(NH4 Cl ) can be 0 − 0 ing vibration occurs. The 2D vibrational prob- (23, 24). However, the photoelectron spectrum of characterized as (NH4) Cl , where (NH4) is the − lems were solved for the pseudo three-body [(NH3)(HCl)] (Fig.