Photoelectron Imaging and Density-Functional Investigation of

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Photoelectron Imaging and Density-Functional Investigation of Bismuth and Lead Anions Solvated in Ammonia Clusters† Mohamed A. Sobhy,‡,§ K. Casalenuovo,| J. Ulises Reveles,| Ujjwal Gupta,‡ Shiv N. Khanna,| and A. W. Castleman, Jr.*,‡ Departments of Chemistry and Physics, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and Department of Physics, Virginia Commonwealth UniVersity, Richmond, Virginia 23284 ReceiVed: June 23, 2010; ReVised Manuscript ReceiVed: August 13, 2010

We present the results of photoelectron velocity-map imaging experiments for the photodetachment of small negatively charged ammonia solvated Bin and Pbn (n ) 1, 2) clusters at 527 nm. The vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters for the solvated Bi and Pb anions and clusters derived from the photoelectron images are reported. The electronic bands of Bi(NH3)n)1,2 are distinct from the Bi metal ion in exhibiting a perpendicular distribution whereas the electronic bands in Pb(NH3)n)1,2, unlike the Pb anion, show an isotropic distribution with respect to the laser polarization. Density-functional theory calculations with a generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their atomic and electronic structures. Calculated geometries show a dramatic change between anionic and neutral ammonia solvated Bi and Pb species. Anionic clusters exhibit van der Waals interactions between the hydrogen atoms of ammonia and the metal core, where it was determined that the negative charge is localized. Neutral clusters, on the other hand, present a covalent bond between the nitrogen atom of ammonia and the metal core. Calculated binding energies show an enhancement in the bonding of the (NH3)2 dimer in the presence of the anionic Bi1,2- and Pb1,2- metal ions. This is rationalized by the electrostatic interaction between the negative charged metal core and the hydrogen atoms of the ammonia molecule. I. Introduction Gas-phase cluster anions are regarded as model systems for exploring the transition of the molecular-level interactions between the gaseous and condensed phases.1 The study of cluster ions is of considerable importance in furthering an understanding of many areas of fundamental and applied interests such as solvation, nucleation, and atmospheric chemistry.2,3 The augmentation of photoelectron imaging and mass selectivity of negative ions provides an ideal combination for the stepwise study of the size evolution of the electronic properties of clusters.4 The speed distributions obtained from the photoelectron imaging technique reveals the energetics of the cluster, while the angular distributions provide a visual representation of the partial wave function composition of the orbital from which the detachment process occurs.5 The clustering of ammonia on Li+, Na+,6 Bi+,7 Ag+, Cu+,8 and halides9 has been studied using high-pressure ion source mass spectrometry. Thermodynamic measurements obtained by this method provide insight into the ion-molecule interactions.10 Gleim et al. measured the stepwise addition reaction for up to four ammonia molecules to Pb+ ion. The binding energies in Pb+NH3 and Pb+(NH3)2 clusters were found to be stronger than what is expected solely on the basis of electrostatic interactions, which was attributed to the partial covalent interaction of Pb+ with NH3 molecules.11 In contrast, ammonia clustering on alkali†

Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * Corresponding author. E-mail: [email protected]. The Pennsylvania State University. | Virginia Commonwealth University. § Current address: The Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115. ‡

metal cations is widely accepted to be mainly electrostatic in nature due to their noble-gas electronic configurations.12,13 Maingroup metals have an open p-shell configuration, where the electrons in the valence orbital may involve covalent binding with the ligand molecules. In some noble-metal ions like Cu+ and Ag+, which have complete d orbitals, hybridization may occur between the s and d orbitals due to their small energy differences; this may result in the formation of unoccupied hybrid orbitals that covalently interact with the ligand molecules.14 Photoelectron spectroscopy of gas-phase cluster anions is a valuable tool in studying ion-molecule interactions.15 Solvent molecules can have a significant effect on the charge distribution and thus the electronic and structural properties of the resulting cluster anion. The negative charge can be localized on the core ion, where the bonding between the ion and the solvent molecules is predominantly due to the electrostatic interactions. In other cases, the excess charge can delocalize over the cluster as a result of covalency in the ion-solvent interaction or electron tunneling between equivalent sites in the cluster.16 The effect of solvation on the electronic structure of alkali-metal anions in ammonia clusters has been extensively investigated using photoelectron spectroscopy and ab initio calculations.17 Similar to the case of alkali-metal cations, the excess electron of the alkali-metal ion was concluded to be delocalized over the solvent molecules in anionic clusters.18 In this study, the results of photoelectron imaging experiments of bismuth and lead anions solvated in small ammonia clusters are presented. The electron binding energies of the clusters exhibit a progressive increase with the successive addition of the solvent molecules due to the stabilization of the negative

10.1021/jp1058148  2010 American Chemical Society Published on Web 08/30/2010

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charge. The photoelectron angular distributions from the detachment of the Bi- anion solvated with one and two ammonia molecules show preferential orientation perpendicular to the laser polarization direction. In contrast, the electronic bands in the photoelectron images of Pb- and Pb2- metal ions solvated in small ammonia clusters exhibit isotropic distributions unlike the respective core metal ion. The measured electron affinities and asymmetry parameters for the solvated clusters are presented. The calculated electronic structure and bonding characteristics of anionic and neutral ammonia solvated Bi1,2 and Pb1,2 species are discussed. II. Experimental Method The experimental setup was described in detail elsewhere.19 Briefly, the cluster ions are formed in a laser vaporization source, where various cluster species are formed in a multicollisional environment.20 The negative ions are perpendicularly extracted and accelerated in a time-of-flight mass spectrometer21 and then focused into the detachment region using a set of einzel lenses. The velocity-map photoelectron imaging assembly is shielded with a µ-metal cover around the photodetachment region. The electron flight tube is also shielded both externally and internally with µ-metal sheets. The detached photoelectrons are accelerated toward a position-sensitive detector with a 40 mm diameter MCP coupled to a phosphor screen. The MCP is gated for an interval of 1.5-2 µs to coincide with the arrival time of the photodetached electrons. The output of a neodymium doped yttrium lithium garnet nanosecond laser of 527 nm (2.35 eV) is used for photoelectron imaging. The laser polarization direction is set to be parallel to the imaging detector surface. The photoelectron images were accumulated over 20 000- 40 000 experimental cycles using a charge coupled device camera. The contributions of the dark current and collisional electrons were reduced by subtracting the background in the absence of the detachment laser from the recorded images. The imaging potentials and focusing conditions of the velocity-map photoelectron spectrometer are determined through the detachment of electrons from the Cu-, Ag-,22 and Bi- atomic anions.23 The photoelectron images are reconstructed using the BASEX method to obtain the original three-dimensional distribution.24 The kinetic energies of the detached electrons are obtained from the velocity distribution by determining the calibration factor for each imaging setting. The anisotropy parameter β is determined from the photoelectron angular distributions by integrating the intensities at a specific angle in the range of radii that encompass the transition of interest and fitting the resulting angular distributions to the differential cross-section equation.

I(θ) ) (σ/4π)[1 + β(3 cos2 θ - 1)/2]

(1)

where σ is the total detachment cross section and θ is the angle between the laser polarization and the electron velocity vector.25 The uncertainties reported in the measurements are due to the use of several sets of data. III. Computational Method To investigate the electronic and structural properties of the clusters and to aid in the assignment of the experimentally measured photoelectron spectra, we have carried out firstprinciple electronic structure investigations on the anion and on neutral forms of Bim(NH3)n and Pbm(NH3)n (m ) 0-2; n ) 1-4). These calculations were performed within the generalized gradient density-functional formalism (GGA-DFT). The ex-

change and correlation effects were included with the functional proposed by Perdew, Burke, and Enzerhof.26 The calculations were performed within the linear combination of Gaussian type orbitals Kohn-Sham (LCGTO-KS) code deMon2k.27 For N and H we employed the correlation consistent aug-cc-pVTZ valence basis set.28 The Bi and Pb atoms were described respectively using the 23 and 22 electron scalar relativistic effective core potentials proposed by Metz et al.,29 in combination with the aug-cc-pVDZ valence basis sets.30 The A2 auxiliary function set for N and H and the GEN-A2* auxiliary function set for Bi and Pb were used. The exchange-correlation potential was calculated from the orbital density. From now on we name this method PBE/aug-cc-pVXZ. For each cluster size and charge state several initial configurations were generated and optimized in redundant coordinates without symmetry constraints. In general, we found several isomers very close in energy, and in addition to the ground states, we report the lower energy isomers that lie within a range of 0.1 eV above the ground state. The PBE/aug-cc-pVXZ method has proven to well reproduce the properties of bismuth and lead clusters.31 Additionally, the methodology was tested via studies on small ammonia clusters (NH3)n with n ) 2-5, for which accurate ab initio MP2-augcc-pVXZ benchmark calculations are available.32,33 For the ammonia dimer, we found the asymmetric dimer to be the ground state with a zero point energy corrected binding energy of 2.82 kcal/mol, in very good agreement with the accurate ab initio data values of 2.92 and 3.18 kcal/mol obtained by the CCSD(T) and W2 methods, respectively.32 In addition, the cyclic C2h structure was found to be a transition state exceptionally close in energy only 0.022 eV above the ground state, in agreement with the best estimate reported of 0.0004 eV.32 In the case of the NH3 trimer, we correctly predicted the ground state and lowest energy isomer. For the tetramer, we found the reported two nearly degenerate lowest energy structures and the next isomer; for the pentamer, we reproduced the five lowest energy structures reported in ref 33. Furthermore, our calculated incremental binding (NH3)n energy defined as IBE ) E[(NH3)n] + E[NH3] - E[(NH3)n+1], where E[(NH3)n] is the total energy of the (NH3)n species, reproduced the predicted trend33 of increasing from n ) 2 to 3, and decreasing from n ) 3-5, as shown in Figure S1 (Supporting Information). For the Pb atom it has been shown that the spin-orbit coupling has a marked effect on its electron affinity.34 The neutral ground state 3P0 of lead is stabilized from the 3P state by 1.05 eV, thus, lowering the electron affinity by this energy from the value obtained in calculations, which do not include spin-orbit effects. In the case of the Bi atom, the spin-orbit coupling has an opposite effect on the electron affinity, increasing its value by around 0.1 eV, which is the stabilization energy from the 3P state to the 3P2 state of the anionic Bi-.34 Considering the importance that spin-orbit coupling has for determining the electronic properties of solvated Bi and Pb atoms, we carried out supplementary calculations using the Amsterdam density functional (ADF) software.35 We performed these calculations in two steps. First, we reoptimized the geometries obtained from the PBE/aug-cc-pVXZ calculations with a scalar relativistic approach and characterized the optimized geometries via a frequency analysis. Second, we performed single point energy calculations including the spin-orbit coupling effect with a collinear approximation. The exchange and correlation functional proposed by Perdew, Burke, and Enzerhof was used,26 and a triple-ζ with polarization functions (TZP) Slater-type basis set was utilized with a [5p] frozen core for Pb and Bi. All the presented results are based on this method

Bi and Pb Anions Solvated in Ammonia Clusters

Figure 1. Mass spectra of Bi- and Pb- atomic ion and clusters solvated in ammonia clusters collected under similar experimental conditions. The Pb atomic and cluster ions tend to be associated with a larger number of ammonia molecules than the corresponding Bi species.

that we name PBE-SO/TZP. The molecular geometries and molecular spinors were plotted using the Molden36 and ADFGUI software,35 respectively. IV. Results and Discussion 4.1. Mass Spectra. Figure 1 shows the mass spectra of Bi1-3 and Pb1-3 anions solvated in NH3 clusters. Unlike the

J. Phys. Chem. A, Vol. 114, No. 42, 2010 11355 lead-ammonia system, the mass spectra of Bim(NH3)n- clusters show relatively more intense pure metal clusters compared to the spectra of the respective solvated species. Also, the cluster ion signal generally shows a progressive decrease in intensity with the increase of the number of NH3 molecules associated with pure metal anions. The Bi(NH3)2- cluster signal is more intense compared to the signals for other Bi(NH3)n- clusters. The mass spectrum of Pbm(NH3)n- clusters shows a distinct pattern, where Pb1-3- anions bound to three or four ammonia molecules are more prominent. The relative abundance of particular solvated species may indicate their electronic stability.37 The mass spectrum shows the depletion of the pure Pbclusters and the larger number of ammonia molecules associated with each of the Pb cluster anions compared to the cases of the respective bismuth clusters. 4.2. Photoelectron Spectra and Images. The photoelectron images for the Bi(NH3)n- (n ) 0-2) species are presented in Figure 2. The images were acquired using photons of 527 nm wavelength and the observed electronic bands are assigned with letters. The reconstructed images in column B are shown at a larger scale than the raw photoelectron images to provide a more resolved depiction of the shape of the electronic bands than in the raw images. Column C presents the calculated charge density of the HOMO and HOMO-1. The raw and reconstructed photoelectron images of Bi- atomic ion in Figure 2 show two electronic transitions. The first, band X, is due to the (Bi 4S3/2 r Bi- 3P2) transition, which is the more intense feature with lower electron affinity. The second electronic transition results

Figure 2. Raw and reconstructed photoelectron images of Bi- atomic anion and Bi(NH3)n)1,2- clusters obtained at 527 nm are shown in columns A and B, respectively. The reconstructed images in column B are shown at a larger scale than the raw photoelectron images to provide a more resolved depiction of the shape of the electronic bands. The three accessible electronic bands resulting from the detachment of the anion are marked as X, A, and B according to their increasing electron binding energies. Bands X in both Bi(NH3)1,2- clusters exhibit sin2 θ angular distribution. The arrow marks the laser polarization vector, which is vertical in the image plane. The HOMO and HOMO-1 charge densities are presented in column C.

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TABLE 1: Experimental Adiabatic Electron Affinities (AEA), Vertical Electron Detachment Energies (VDE), and Anisotropy Parameters for the Bi(NH3)1,2- and Pb1,2(NH3)1-3- Clustersa VDE AEA 3

-

Bi Bi(NH3)3 Bi(NH3)24 Pb(NH3)4 Pb(NH3)22 Pb2(NH3)33

1.28 1.30 0.85 0.94 1.25

X 0.947 1.48 1.58 1.06 1.10 1.44

b

anisotropy parameter β

A

B

X

A

B

2.25 2.28 2.30 2.26 2.30 2.30

2.31 2.33 2.32 2.33 2.32

0.38 -0.41 -0.47 0.21 0.07 -0.22

-0.05 -0.09 -0.08 -0.15 -0.19 -0.21

-0.14 -0.21 -0.13

a The reported values were obtained from the reconstructed images at 527 nm. The uncertainties in the measured detachment energies and anisotropy parameters are (0.05 eV and (0.1, respectively. b Reference 23.

from the (Bi 2D3/2 r Bi- 3P2) transition and is denoted as band A.23 The measured asymmetry parameter of 0.38 ( 0.1 shown in Table 1 for the 4S3/2 state at electron kinetic energy of 1.4 eV is close to the value of 0.42 ( 0.1 measured by Polak et al.38 The transitions in the Bi- photoelectron spectrum are associated with the removal of 6p electrons, in agreement with the calculated HOMO and HOMO-1, where the asymmetry parameter for detachment is strongly dependent on the kinetic energy of the emitted electrons, as demonstrated39 in the case of the O- atomic ion. The 2D3/2 state shows isotropic distribution with an asymmetry parameter close to zero and consistent with the favorable isotropic distribution for the detachment from a p orbital at low electron kinetic energy.39,40 The photoelectron images of Bi(NH3)- and Bi(NH3)2- clusters in Figure 2 show three electronic transitions designated as bands X, A, and B, respectively, according to their increasing electron binding energies. The electronic bands in the Bi(NH3)- and Bi(NH3)2- images are qualitatively similar in their shape and angular distributions. However, the size of the features in the

photoelectron image of Bi(NH3)2- is smaller than those of Bi(NH3)-, indicating the higher electron affinity of the former species. The distinct feature of the electronic bands X in both the Bi(NH3)- and Bi(NH3)2- clusters is their preferential perpendicular distribution relative to the laser polarization and the comparable asymmetry parameters close to -0.5. The measured detachment energies for Bi(NH3)n)1,2- clusters are given in Table 1. The photoelectron spectra of Bi(NH3)n- and Bi(NH3)n)1,2- clusters are shown in Figure 3. The electron binding energies of bands A and B are approximate since the binding energies of these bands are close to the detachment photon energy. The photoelectron image of Bi(NH3)- shows a ring outside band X with an electron affinity similar to that of the Bi- atomic ion. This feature is also present in the photoelectron spectrum of Bi(NH3)- in Figure 3 as a side peak at lower electron binding energy than band X. As a result of solvation effects, the features of the photoelectron spectra become broadened and shifted toward higher electron binding energy with the progressive increase in the number of solvent molecules.16 The experimental and calculated vertical detachment energies (VDE) for the Bim(NH3)n and Pbm(NH3)n series are presented in Table 2. In general, we found very good agreement between the theoretical and experimental values, with an average deviation of 0.2 eV. Given the small energy difference between the low energy isomers and the ground states, we also report the VDE for the low energy isomers, whose values are close to the ones obtained for the ground state. The photoelectron images of Pb(NH3)n)1,2- are depicted in Figure 4, where in contrast to those of the Pb- atomic ion, the electronic bands X, A, and B in both clusters exhibit isotropic distributions. There are three accessible transitions in Pb- atomic ions within the used photon energy range, as shown in the photoelectron image of Pb- ion in Figure 4. The electronic bands are assigned to the (Pb 3P0,1,2 r Pb- 4S) transitions to the neutral, where the intensities of the transitions to the fine structure levels are comparable.23 The anisotropy parameters for the Pb- atomic

Figure 3. Photoelectron spectra of solvated Bi(NH3)n)1,2- and Pb(NH3)n)1,2- clusters obtained using 527 nm wavelength. The spectra are normalized and plotted against the electron binding energy scale. The dotted vertical line marks the electron affinity of Bi- atomic ion.


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TABLE 2: Experimental and Theoretical Adiabatic Electron Affinities (AEA) and Vertical Electron Detachment Energies (VDE), in Units of eV, for the Bi1,2(NH3)1-4- and Pb1,2(NH3)1-4- Clustersa experiment

theory

VDE 3

Bi Bi(NH3)3 Bi(NH3)23 Bi(NH3)3Bi(NH3)42 Bi22 Bi2(NH3)2 Bi2(NH3)22 Bi2(NH3)34 Pb4 Pb(NH3)4 Pb(NH3)24 Pb(NH3)34 Pb(NH3)42 Pb22 Pb2(NH3)2 Pb2(NH3)22 Pb2(NH3)33

a

AEA

X

1.28 1.30

0.947 ( 0.01 1.48 1.58

-

0.85 0.94

b

0.365 ( 0.01b 1.06 1.10

VDE A

AEA

GS

2.25 2.28 2.30

1.00 1.02 1.03 1.01 1.49 1.25 1.37 1.34 1.38 0.27 0.70 0.68 0.65 0.75 1.37 1.42 1.44 1.39

1.00 1.15 1.33 1.46 1.55 1.29 1.51 1.64 1.64 0.27 0.78 0.93 1.03 1.06 1.42 1.64 1.75 1.81

2.26 2.30

1.366 ( 0.01c 1.25

1.44

2.30

Iso1

Iso2

Iso3

Iso4

1.36 1.53 1.47

1.50 1.56

1.43

1.51

1.72 1.73

1.72 1.72

1.70

1.80

0.98 1.09

1.09 1.12

1.04

1.10

1.58 1.76 1.87

1.87

1.16

The VDE are also shown for both the ground states and the low energy isomers shown in Figures 7-10. b Reference 23. c Reference 40.

Figure 4. Raw (A) and reconstructed (B) photoelectron images of Pb- atomic anion and Pb(NH3)n)1,2- clusters. The laser polarization is vertical in the image plane. The HOMO and HOMO-1 charge densities are presented in column C. See the caption of Figure 2 for details.

ion are dependent on the kinetic energy of the detached electrons. At high electron kinetic energy of approximately 2 eV, as is the case with band X resulting from the detachment from the 6p orbital, parallel distribution of the electrons relative to the laser polarization is more favorable.38 The 3P1 and 3P2 transitions in bands A and B, respectively, result from the detachment of an electron in the 6p orbital. The emitted electrons have intermediate kinetic energy of approximately 1 eV, where

the photoelectron angular distribution is preferably oriented perpendicular to the laser polarization, as shown in Figure 4.38 Again, the calculated HOMO and HOMO-1 with a marked p character confirm this result. The asymmetry parameters of Pb(NH3)n)1,2- for bands X are close to zero and slightly negative for the bands A and B (Table 1). The bands A and B of the photoelectron spectra of Pb(NH3)n)1,2- shown in Figure 3 are intense peaks with high

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Figure 5. Raw (A) and reconstructed (B) photoelectron images of Pb2- and Pb2(NH3)n)1,2- clusters. The laser polarization is vertical in the image plane. The HOMO and HOMO-1 charge densities are presented in column C. See the caption of Figure 2 for details.

binding energies. In the intermediate electron kinetic energy range ∼1-2 eV, the detached electrons from an atomic p orbital favor detachment perpendicular to the laser polarization.39 The angular distributions of the detached electrons in Bi(NH3)n)1,2clusters of nearly 1 eV of kinetic energy show a perpendicular distribution, as expected from a detachment of a p orbital. In the case of Pb(NH3)n)1,2- clusters, where the detached electrons have kinetic energy of ∼1.3 eV, the electronic bands are isotropic. The isotropic distribution is predicted to prevail at low electron kinetic energy detachment values of an atomic p orbital. This may indicate that the ammonia molecules interact with the Pb- ion and alter the electronic structure and the symmetry of the highest occupied molecular orbital (HOMO) of Pb(NH3)n)1,2- clusters compared to the case of the Pb- atomic ion. The photoelectron images of Pb2- and Pb2(NH3)3- clusters are shown in Figure 5. The angular distribution of band X in Pb2- is isotropic, while bands A and B show perpendicular and parallel orientations to the laser polarization direction, respectively. In contrast, the electronic bands in the photoelectron image of Pb2(NH3)3- exhibit isotropic distributions similar to the case of Pb(NH3)n)1,2- clusters. The interaction of the ammonia molecules with the Pb2- anion may affect its electronic structure as in the case of Pb- atomic ion. The photoelectron spectrum of Pb2(NH3)3- is shown in Figure 3 and the vertical detachment energy is slightly higher than the Pb2- anion, as listed in Table 2.40 Figure 6 presents the evolution of the calculated VDEs as a function of the number of ammonia molecules, showing a progressive increase in agreement with the experimental results and with what is expected from the solvation. 4.3. Bi(NH3)n and Pb(NH3)n Optimized Geometries. The optimized geometries for the ground state and low-lying energy states of neutral and anionic Bi(NH3)n and Pb(NH3)n clusters with n ) 1-4 are presented in Figures 7 and 8, respectively. The geometries of the anionic ground states for M(NH3)n- (M

Figure 6. Experimental and calculated vertical detachment energies for the Bi(NH3)n and Pb(NH3)n series.

) Pb and Bi) were found to be identical between Pb and Bi for n ) 2-4. In the n ) 2 clusters, the ammonia molecules link together in an asymmetric chain, forming the same equilibrium structure of pure ammonia dimer predicted by Janeiro-Barral et al.33 When n ) 3, the ammonia molecules form a ring beneath the metal ion, in a so-called “surface cluster”. This occurs when the interaction with the metal is weaker than the interactions between the solvent molecules such that the metal is bound to the surface of a solvent cluster,41 consistent with our interpretations of the bonding mechanisms discussed later in section 4.5. (For the Pb species, this structure is degenerate with the ground state.) Planar structures are found here to be the ground state not only for the pure ammonia trimer but also for the pure tetramer, as shown in Figure S1 (Supporting Information). Two anionic isomers are identical between Pb and Bi for the n ) 3 solvated species and one isomer for n ) 4. For n ) 3,



Figure 7. Optimized geometries of the low-lying states of Bi(NH3)n- (anions, left, spin multiplicity M ) 3) and Bi(NH3)n (neutrals, right, M varied with n). Relative energies to the ground state are shown below each cluster in electronvolts unless isomers were not found.

Figure 8. Optimized geometries of the low-lying states of Pb(NH3)n- (anions, left, spin multiplicity M ) 4) and Pb(NH3)n (neutrals, right, M ) 3). The relative energies to the ground state are shown below each cluster in electronvolts unless isomers were not found.

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Figure 9. Optimized geometries of the low-lying states of Bi2(NH3)n- (anions, left, spin multiplicity M ) 2) and Bi2(NH3)n (neutrals, right, M ) 1). The relative energies to the ground state are shown below each cluster in electronvolts unless isomers were not found.

the asymmetric V-shaped clusters are 0.02 and 0.04 eV relative to the ground state energy for Pb and Bi, respectively. The symmetric V-shaped clusters (with the bottom ammonia involved in two hydrogen bonds) are 0.02 eV higher in energy for both metal ions. For n ) 4, the ammonia molecules form a chain underneath the metal core in the isomers with 0.06 and 0.04 eV relative energies for Pb(NH3)4- and Bi(NH3)4-, respectively. The optimized geometries for neutral Pb(NH3)n and Bi(NH3)n also show striking similarities between one another and also with the solvated magnesium ammonia [Mg(NH3)n] clusters, as found from a study by Elhanine et al.41 All three n ) 1 clusters have the same ground state configuration with the metal atoms involved in the covalent bonding. The n ) 2 geometries for neutral Bi show three isomers, with both ammonia molecules forming bonds to the metal core in the ground state. The next lowest isomer for Bi (0.04 eV relative energy) has the same geometry as the ground state Pb(NH3) cluster. An identical structure was found independently for the solvated neutral Mg(NH3)n clusters.41 When n ) 3, the Pb(NH3)n, Bi(NH3)n, and Mg(NH3)n ground state geometries are exactly the same. Finally,

for n ) 4, the Bi(NH3)n ground state is similar to the Pb(NH3)n isomer (0.05 eV relative energy). The Pb(NH3)4 ground state optimized here is a chain-like structure analogous to the neutral Mg(NH3)4 isomer (0.07 eV relative energy). 4.4. Bi2(NH3)n and Pb2(NH3)n Optimized Geometries. The optimized geometries for the ground state and low-lying energy states of the neutral and anionic Bi2(NH3)n and Pb2(NH3)n clusters with n ) 1-3 are presented in Figures 9 and 10, respectively. The optimized solvated geometries are less similar between the ammonia solvated Pb2 and Bi2 dimer species than between the solvated Pb and Bi atoms. Interestingly, for anionic cores with n ) 1-3 (the entirety of the clusters modeled here), the ammonia molecules tend to align in the same way for the ground states of both species; however, they are at different locations relative to the respective dimer cores. For Pb2, the ammonia molecules accumulate along the internuclear axis, whereas for Bi2 the ammonia molecules are situated underneath the internuclear axis. This pattern is consistent for all the anionic ground states. There is one isomer in the n ) 3 cases that is the same for both Pb2(NH3)3- and Bi2(NH3)3- at 0.05 and 0.02 eV, respectively.



Figure 10. Optimized geometries of the low-lying states of Pb2(NH3)n- (anions, left) and Pb2(NH3)n (neutrals, right, M ) 3 for one NH 3 species). The relative energies to the ground state are shown below each cluster in electronvolts unless isomers were not found.

For the neutral dimer cases, there are no more than two isomers identified for each n ) 1, 2; however, there are three isomers found for both metals at n ) 3. The n ) 1 solvated geometries are similar between Pb2(NH3) and Bi2(NH3), while the n ) 2 clusters optimized to very different structures. Finally for n ) 3, there are slightly similar isomers at 0.06 and 0.02 eV for Pb2(NH3)3 and Bi2(NH3)3, respectively. 4.5. Bonding Nature in Anionic and Neutral Species. Cluster geometries optimized in the density-functional formalism reveal molecular-level interactions between the metal and solvent. As a result of the dipole moment of ammonia and the lone electron pair located on nitrogen, the dominant bonding mechanism is determined by which atom in ammonia is closest to the metal atom. In all the anionic Bi(NH3)n- species presented in Figure 7 the spin multiplicity of the ammonia solvated species (triplet state) was the same as that of the atomic Bi anion. Notice in the ground states and isomers of the anionic species, the hydrogen ligands of ammonia are directed toward the Bi core. An analysis of the electronic charge shows that the negative charge in the anionic species is always located at the ionic core independent of the number of ammonia molecules. This

indicates that the electrostatic forces are the prevailing bonding mechanism due to the positive end of the permanent dipole intersecting the pyramidal base of the ammonia molecule. When Bi is neutral, a dramatic change is observed in the optimized Bi(NH3)n geometries. In this case the nitrogen is the atom involved in the bonding; this is seen throughout the ground state and low energy isomers. With the lone electron pair on nitrogen directly opposing the hydrogen ligands, this demonstrates a partial covalent interaction. In the neutral species the spin multiplicity changed from quartet for Bi(NH3)n with n ) 1 to doublet for n > 1, attesting for the stronger metal-ammonia interaction. The bonding mechanisms for the anion and neutral clusters, respectively, are consistent throughout the ammonia solvated geometries modeled here. The same indications deduced for the Bi core species are present in the optimized Pb(NH3)n, Bi2(NH3)n, and Pb2(NH3)n geometries depicted in Figures 8-10. The optimized Bi1,2 and Pb1,2 solvated ammonia structures are analogous to those found for pure ammonia clusters as shown in Figure S1 (Supporting Information). Three and four ammonia molecules tend to form globular rings, a pattern that

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arises from the hydrogen bonding interaction between permanent dipole molecules. These pure ammonia geometries remain intact even in the presence of the negatively charged cores of Bi-, Bi2-, Pb-, and Pb2-. This is likely due to the weakness of the electrostatic interaction between the metal ion and the solvent molecules. Because covalent interactions are typically an order of magnitude stronger than van der Waals hydrogen bond, the neutral solvated geometries distort the cluster arrangement expected for pure ammonia molecules. Clearly the interactions between the solvent molecules are stronger than between the metal and the solvent in all but a few cases. The anionic Bi(NH3)2- ground state and the next lowest isomer demonstrate that the hydrogen bonding between solvents is more energetically favorable than the electrostatic interaction with the metal ion (Figure 7). However, for the neutral Bi(NH3)2, this trend is reversed, which can be explained by the stronger interaction of covalent bonding compared to that of hydrogen bonding. For the other Bi and Bi2 geometries, the ammonia molecules bind more strongly to each other than to the metal cores. This pattern is also consistent with the Pb and Pb2 species as well. 4.6. Binding Energies. To investigate the effect of the anionic Bi1,2- and Pb1,2- species on the formation of (NH3)n clusters, we calculated the incremental binding energies (IBE) for the successive binding of NH3 molecules to a Mm(NH3)n(M ) Bi and Pb) species, according to the equation: IBE ) E[Mm(NH3)n-1-] + E[NH3] - E[Mm(NH3)n-]. Here, E[Mm(NH3)n-], E[Mm(NH3)n-1-], and E[NH3] are the total energies of each species. According to this definition IBE is positive for a bound structure, and a larger IBE implies a more stable structure. Figure S2 presents the calculated IBEs. It was found that for both Bi- and Pb-, the first NH3 molecule binds with an energy gain of around 0.50 eV, which is more than 3 times the binding energy of the (NH3)2 dimer (0.15 eV). This can be explained by the strong electrostatic interaction between the negative charge localized at the metal atom and the hydrogen atoms of the ammonia molecule. The second NH3 does not bind directly to the metal atom, but to the NH3 molecule with an energy gain of 0.30 eV. In this way, the metal anion enhances the binding of the two NH3 molecules by doubling the binding energy from 0.15 eV in pure (NH3)2 to 0.30 in M(NH3)2-. The third and fourth NH3 molecules bind to the ammonia dimer with an IBE around 0.30 eV, which is comparable to the IBE in the (NH3)n trimer and tetramers (Figure S1, Supporting Information). In the case of the anionic M2- dimers, the IBE followed an increasing trend as a function of the number of NH3 molecules. The first NH3 molecule binds with an energy gain of around 0.20 eV, smaller that in the M- ion case as the negative charge is now distributed between the two metal atoms and the electrostatic interaction with the hydrogen atoms is not as strong as in the case of single metal atoms. For the M2- dimers, the IBE converged to a value of 0.30 eV with the formation of the (NH3)3 trimer bounded to the anionic M2- core. V. Conclusions The results of the photoelectron imaging experiments of bismuth and lead metal ions associated with a small number of ammonia clusters are presented. As in the case of solvation, the measured electron binding energies show progressive increase with the successive addition of ammonia molecules due to the stabilization of the core ion. The measured values for the detachment energies and asymmetry parameters for the Bi(NH3)n)1,2- and Pb(NH3)n)1,2- clusters are reported. The photoelectron angular distributions suggest changes in the

Sobhy et al. symmetry of the highest occupied molecular orbital of the Pbupon interaction with ammonia molecules. The angular distributions of the electronic bands of Bi(NH3)n)1,2- show electron kinetic energy dependence consistent with that determined for photodetachment from a p orbital. The calculated geometries show a dramatic change in the bonding mechanism between anionic and neutral ammonia solvated Bi1,2 and Pb1,2 species. The negative charge was determined to be localized on the metal core of the ammonia solvated Bi1,2- and Pb1,2- species, which presented van der Waals interactions with the hydrogen atoms of ammonia. Neutral clusters, on the other hand, exhibit a covalent bond between the nitrogen atom of ammonia and the metal core. The geometries of the anionic ground states for M(NH3)n- were found to be identical between Pb and Bi for n ) 2-4, whereas the optimized solvated geometries are less similar between the ammonia solvated Pb2 and Bi2 dimer species. The analysis of the binding energies between the anionic Bi1,2- and Pb1,2- metal ions and ammonia molecules shows that in the presence of the anionic cores there is an enhancement in the bonding of the (NH3)2 dimer. This is rationalized by the electrostatic interaction between the negative charge localized at the metal atom and the hydrogen atoms of the ammonia molecule. Acknowledgment. We acknowledge the financial support from the U.S. Air Force Office of Scientific Research (AFOSR), Grant No. FA 9550-04-1-0066. J.U.R. acknowledges support from the AFOSR Grant No. FA9550-05-1-01.86, while S.N.K. is grateful to the U.S. Department of the Army (Multidisciplinary University Research Initiative Grant, W911NF-06-1-0280) for support. J.U.R. is grateful to Professor Andreas M. Ko¨ster for valuable discussions. Part of the deMon2k calculations were performed on the computational equipment of DGSCA UNAM, particularly at the super computer KanBalam. Supporting Information Available: Figures showing the optimized geometries of the ground state and lowest energy isomers for the (NH3)n clusters (n ) 2-5) and the calculated incremental binding energy (IBE) for the anionic Bi(NH3)n- and Pb(NH3)n- clusters and the anionic Bi2(NH3)n- and Pb2(NH3)nclusters. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Castleman, A. W., Jr.; Bowen, K. H., Jr. J. Phys. Chem. 1996, 100, 12911. (2) Castleman, A. W., Jr.; Keesee, R. G. Chem. ReV. 1986, 86, 589. (3) Castleman, A. W., Jr.; Wei, S. Annu. ReV. Phys. Chem. 1994, 45, 685. (4) Surber, E.; Sanov, A. J. Chem. Phys. 2002, 116, 5921. (5) Mabbs, R.; Surber, E.; Velarde, L.; Sanov, A. J. Chem. Phys. 2004, 120, 5148. (6) Castleman, A. W., Jr.; Holland, P. M.; Lindsay, D. M.; Peterson, K. J. Am. Chem. Soc. 1978, 100, 6039. (7) Castleman, A. W., Jr. Chem. Phys. Lett. 1978, 53, 560. (8) Holland, P. M.; Castleman, A. W., Jr. J. Chem. Phys. 1982, 76, 4195. (9) Evans, D. H.; Keesee, R. G.; Castleman, A. W., Jr. J. Chem. Phys. 1987, 86, 2921. (10) Castleman, A. W., Jr.; Keesee, R. G. Acc. Chem. Res. 1986, 19, 413. (11) Gleim, K. L.; Guo, B. C.; Keesee, R. G.; Castleman, A. W., Jr. J. Phys. Chem. 1989, 93, 6805. (12) Dzidic, I.; Kebarle, P. J. Phys. Chem. 1970, 74, 1466. (13) Woodin, R. L.; Houle, F. A.; Goddard, W. A., III. Chem. Phys. 1976, 14, 461. (14) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry: Wiley: New York, 1980; 969 pp. (15) Snodgrass, J. T.; Coe, J. V.; Freidhoff, C. B.; McHugh, K. M.; Arnold, S. T.; Bowen, K. H., Jr. J. Phys. Chem. 1995, 99, 9675.

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