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Electron Attachment to Formamide Clusters in Helium Nanodroplets F. Ferreira da Silva,† S. Denifl,† T. D. Ma¨rk,† N. L. Doltsinis,‡ A. M. Ellis,*,§ and P. Scheier*,† Institut fu¨r Ionenphysik und Angewandte Physik and Center of Molecular Biosciences Innsbruck, UniVersita¨t Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria, Department of Physics, King’s College London, The Strand, London WC2R 2LS, United Kingdom, and Department of Chemistry, UniVersity of Leicester, UniVersity Road, Leicester LE1 7RH, United Kingdom ReceiVed: October 15, 2009; ReVised Manuscript ReceiVed: December 15, 2009
Electron attachment to formamide clusters in helium nanodroplets is reported for the first time. In contrast to the gas phase, parent anions are seen following low energy electron attachment to both the monomer and the small clusters. This is attributed to formation of dipole (or quadrupole) bound anions. In addition to the bare anions, the mass spectra also show the monomer and clusters with attached helium atoms. The affinity for attaching helium atoms strongly varies with cluster size; for example, the dimer anion is more than 10 times more likely to bind one or more helium atoms than the monomer. Possible binding sites for the helium atoms are discussed. Introduction Formamide, HCONH2, is the smallest molecule that contains the peptide linkage. Hydrogen bonding between peptide moieties, through the N-H---O)C interaction, is critically important in determining the structure of many proteins as well as nucleic acids. Consequently, there has been much interest in the formamide dimer as a prototype of this key interaction in molecular biology. A particular advantage of formamide (FA) as a model system is that it is small enough to be tackled by high level ab initio calculations. The consensus from these theoretical studies is that the most stable structure of the FA dimer is a planar cyclic structure, which allows the formation of two N-H---O)C hydrogen bonds.1-8 Early theoretical studies tended to focus primarily on this particular form of the dimer, but more recent work has begun to explore other possible structures.6-8 Although possessing higher energies, these alternative structures are also of interest as they may be the analogues of structural intermediates in real proteins and nucleic acids, for example, in the unwinding of the double helix in DNA. A recent combined experimental and theoretical study found evidence for several different isomers of the FA dimer, including an open, nonplanar structure possessing only one N-H---O)C hydrogen bond.8 The experiments employed argon and xenon matrices to isolate the monomers, which were then allowed to dimerize by controlled warming of the matrix. Ab initio molecular dynamics calculations showed that the formation of the open structure is favored by long-range dipole-dipole interactions, but given sufficient time or heat, the system eventually relaxes into the global potential energy minimum. Using infrared laser spectroscopy, the cyclic structure has also been shown to be the dominant isomer in the gas phase.9 In addition to the neutral FA clusters, there has also been considerable interest in the cluster anions. Part of the motivation for studying these species derives from the uncertain effect of * To whom correspondence should be addressed. E-mail: Andrew.Ellis@ le.ac.uk (A. E.);
[email protected] (P.S.). † Universita¨t Innsbruck. ‡ King’s College London. § University of Leicester.
low-energy electrons, which are byproducts of ionizing radiation, on living tissue. Potentially important information can be derived from studies of electron attachment in the gas phase, where the chemistry takes place free from any complications from solvents and surrounding biological structures. FA monomer turns out to have a negative adiabatic electron affinity, and low-energy attachment is difficult because of rapid autodetachment.10 Nevertheless, electron attachment is possible via nearresonant transfer of very loosely bound electrons from atoms in specific excited Rydberg states.9-11 The electron attached in this case is thought to be bound by the relatively large electric dipole moment of FA (3.72 D).12 At energies above 3 eV, dissociative electron attachment (DEA) processes have been identified from irradiation of thin films of formamide13 and are also predicted from a computational study.14 In this laboratory, a rigorous study of DEA to the FA monomer in the gas phase has recently been carried out and will be reported elsewhere.15 Like the FA monomer, small clusters of FA show rapid autodetachment when bombarded with low-energy electrons. Seydou et al. and Desfranc¸ois et al. have shown, via Rydberg electron transfer, that all of the small clusters possess negative adiabatic electron affinities.10,11 This situation reverses at the heptamer, which is thought to be the smallest of the clusters to show a positive adiabatic electron affinity. Consequently, lowenergy electrons can only attach to small FA clusters through interaction with their dipole or quadrupole moments. Here, we report the first study of FA cluster anions in helium nanodroplets. The clusters were made by pickup of monomers, which then coalesce inside the helium droplets. These doped droplets are then subjected to electron bombardment at defined impact energies. Several unique features derive from such experiments. These include the fact that the exceptionally low temperature (0.37 K) of the droplets makes it possible to trap clusters in metastable configurations. Also, rapid cooling of anionic products by the same mechanism can lead to the stabilization and survival of species that are not observable in gas-phase studies. Our investigation of the electron attachment to FA clusters in helium nanodroplets reveals several unusual findings, including the survival of monomer and small cluster anions, despite their negative electron affinities, and the ejection
10.1021/jp909890h 2010 American Chemical Society Published on Web 01/07/2010
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of FA cluster anions with attached helium atoms from the helium droplets. These observations are discussed in terms of the formation of metastable cluster structures which offer a favorable electric dipole moment for dipole-binding of the attached electron, which in turn may affect the binding of helium atoms. Experimental Section The experimental measurements were carried out using a double focusing mass spectrometer coupled with a helium cluster source. The apparatus and basic operating procedure has already been described elsewhere,16,17 and so only a brief account will be given here. The helium nanodroplets are formed by expansion of high-purity helium into vacuum at high-stagnation pressure through a pinhole nozzle (5 µm diameter). The nozzle is cooled to a low temperature, and the mean size of the helium droplets is dictated by the combination of gas stagnation pressure and nozzle temperature. For the work described here, the helium stagnation pressure was 20 bar and the nozzle temperature was 12 K giving droplets with a mean size of roughly 1.2 × 104 helium atoms. The droplet expansion was then skimmed and entered a pickup region, where the FA was added by oven evaporation. After picking up the FA, the doped droplets were then skimmed a second time and entered the source region of the mass spectrometer, where they were subjected to electron impact. The variable energy electron source is of the Nier type with an energy resolution of ∼1 eV, and the operating current was set to 27 µA in the present work. The mass spectrometer is a modified two-sector field instrument (Varian-MAT CH5), which provides a mass resolution of m/∆m ) 200 with open slits. To distinguish between the hydrogen atoms attached to the carbon atom and the two hydrogen atoms of the amino group, the experiments reported here were carried out with the partially deuterated isotopomer, DCONH2. This was purchased from Aldrich (99% D) and was used without further purification. Computational Details To learn more about the likely cluster structures formed at low temperatures, ab initio molecular dynamics (AIMD) simulations were performed. The simulations were similar to previous work on the formamide dimer8 and here are extended to include the formamide trimer. Briefly, the calculations were performed using the plane wave density functional code CPMD.18 The Kohn-Sham equations were solved using the Perdew-BurkeErnzerhof PBE exchange-correlation functional19 in a plane wave basis set truncated at 25 Ry in conjunction with Vanderbilt ultrasoft pseudopotentials20 and periodic boundary conditions in an orthorhombic unit cell of size 19.0 × 17.5 × 10.5 Å3. To generate pseudorandom initial conditions for the subsequent aggregation dynamics, a Car-Parrinello (CP-MD) simulation was first carried out on three freely rotating, equidistant monomers separated by a fixed C-C distance of 9 Å. The temperature was maintained at 500 K by a Nose´-Hoover chain thermostat.22 The time step was 7 au, and a fictitious electron mass of 800 au (hydrogen atoms were replaced by deuterium) was employed. Initial conditions were extracted every 1.7 ps for a total of 10 aggregation simulations which were carried out at 100 K leaving the other settings as detailed above. In a second series of 10 aggregation runs, a larger cubic box of size 20.0 Å was used together with the same initial conditions to study the effect of long-range dipole-dipole interactions. Each run exceeded a length of 20 ps after which time a geometry
Figure 1. Negative ion mass spectrum for deuterated formamide (DCONH2) in helium nanodroplets. The electron impact energy was about 1 eV.
optimization was performed and the dipole moment was calculated for the optimized structure. Results and Discussion Reaction Products and Anion Efficiency Curves. Figure 1 shows a mass spectrum of the anions derived from electron bombardment at approximately 1 eV. This energy is equivalent to a nominal electron impact energy of 0 eV in the gas phase when the energy required to inject the electron into the liquid helium conduction band, and the additional energy due to bubble formation, is taken into account.17 It is immediately clear from inspection of this figure that the electron-induced chemistry is strongly size-dependent. In the monomer, dimer, and trimer channels, the dominant products are the parent anions, which we will denote as FAn-. Also seen are clusters with attached helium atoms, which we discuss in a separate section below. Apart from the parent cluster anions and the corresponding species with attached helium atoms, there are also several distinct products from dissociative electron attachment (DEA). For the tetramer and larger clusters, DEA becomes the dominant process even at this low incident electron energy. The dissociation products for the dimer are consistent with C, O, and DCN ejection, but these are relatively minor channels. Similar dissociative reactions are found for the trimer, except for the loss of a C atom, but additional products can be identified arising from loss of NH2D, HDO, and DCNH2. There is also a peak at one mass unit above FA2- whose intensity is far too large to arise from naturally abundant 13C. This peak is attributed to [H + FA2]-, and similar peaks are seen associated with the larger clusters. The production of H- has precedent in that it was also observed from electron bombardment of FA films, although in that case the threshold electron energy was 6 eV.13 The absence of this channel for the monomer and dimer is not inconsistent with the thin film work since the response of a thin film is likely to have more in common with a large FA cluster than an isolated FA molecule. A remarkable change in chemistry occurs for the tetramer and larger clusters. Here, the major product becomes the NH2 ejection channel, and the difference in probability for this channel in the tetramer when compared to the trimer is huge. Also, this channel is seen at low incident electron energies. The anion efficiency curve is shown in Figure 2 and shows two peaks at
