Photodissociation Spectroscopy of the Anionic Copper Nitrate

Article pubs.acs.org/JPCA

Photodissociation Spectroscopy of the Anionic Copper Nitrate Association Complex Cu(NO3)3− Sydney H. Kaufman and J. Mathias Weber* JILA and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: We report the UV photodissociation spectrum of mass-selected Cu(NO3)3− ions at photon energies between 3.0 and 5.6 eV. Upon photon absorption, Cu(NO3)3− undergoes reductive dissociation losing neutral NO3 and resulting in the formation of Cu(NO3)2−. The experimental results are discussed and interpreted with the aid of quantum-chemical calculations. The parent ion is calculated to have C2 symmetry with a strongly distorted octahedral coordination around the Cu ion. Time-dependent density functional theory is used to describe the accessible electronic transitions, which can be characterized as ligand-to-metal charge transfer transitions from the nitrate ligands to the copper ion.

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INTRODUCTION In solutions with high salt concentrations, anions and cations can form association complexes.1−7 In the limit of supersaturated metal salt solutionse.g., at the onset of crystal formation or during dissolutionone can expect this process to go beyond simple binary association complexes. Instead, salt clusters [MxBy]Q exist in which the charge state Q depends on whether the cationic metal M or the anionic base B is in excess.4 An analogous process is at play in aerosol particles during drying2,6 and is of interest for atmospheric science. While it is difficult to elucidate the detailed chemistry of such association complexes or clusters in the condensed phase, clusters can be generated by electrospray ionization (see, e.g., ref 8), making them accessible for studies as mass-selected ions in vacuo. Gas phase studies of ionic transition metal complexes have vastly improved our understanding of transition metal chemistry (see, e.g., refs 9−15). In particular, mass-selected ion−molecule complexes provide clean, well-controlled experiments unperturbed by the complex environment of solutions and contribute to a deep understanding of pairwise and multibody interactions.10,11,16,17 In addition to the wealth of information from mass spectrometric reactivity studies, the spectroscopic signatures of the plethora of different ionic species present in solutions can be unraveled by examining them individually in the gas phase. Coupling mass spectrometric preparation of target ions with laser spectroscopy thereby circumvents the complications associated with speciation of transition metal salts in solutions. Here we focus on the electronic and photochemical properties of a copper nitrate association complex, Cu(NO3)3−. Complexes of Cu(II) formally possess a d 9 electron configuration, and their coordination numbers typically vary from 4 to 6. In 4-fold coordination, they are known to adopt a variety of structures between the limits of a square plane (which is favored by a diamagnetic d8 transition metal complex such as Pt(NH3)4]2+) and a tetrahedron (found in diamagnetic d10 complexes, e.g., Sn(II)).18 As a consequence of this structural © XXXX American Chemical Society

variability, as many as 131 different structures are found for the CuCl42− dianion depending on the counterion.19 In solution, Cu(II) ions are usually 6-fold coordinated to solvent molecules, anionic ligands, or combinations thereof.20 The 6-fold coordinated Cu(II) complexes are typically distorted from octahedral symmetry due to Jahn−Teller distortion. In the case of copper nitrate solutions, nitrate can displace water ligands from the coordination shell to form the CuNO3+ association complex and its higher order analogues.21−27 Electrospray ionization of metal nitrate solutions has been shown previously28,29 to produce clusters containing excess moieties of NO3−, which represent the anionic forms of association complexes. In the case of copper nitrate, CuNO3+ is the smallest association complex, and several features in the vibrational spectra of highly concentrated solutions of this salt have been attributed to this species. However, the unambiguous assignment of spectral features to specific complexes in solutions is difficult, since higher order association complexes are likely present, and even the vibrational spectra of such solutions are strongly congested.21,24−26 The smallest anionic association complex in copper nitrate solutions is Cu(NO3)3−. In the present work, we use electrospray ionization mass spectrometry coupled with ultraviolet photodissociation spectroscopy to study Cu(NO3)3− in vacuo and discuss this complex in the framework of a molecular orbital approach. In addition, we employ quantum-chemical calculations to elucidate the electronic and geometric structures of the parent and fragment ions as well as the electronic excitation spectrum of the parent ion.

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METHODS Photodissociation Spectroscopy. The experimental setup, described previously in detail,30 consists of an electrospray ionization source coupled to a reflectron time-of-flight Received: August 13, 2014 Revised: September 19, 2014

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mass spectrometer (RETOF) with a UV/vis optical parametric converter used to generate excitation energies between 3.0 and 5.6 eV. Electrospray ionization of a ∼10 mM solution of copper(II) nitrate hydrate (Sigma-Aldrich, used without further purification) in ∼1:1 water:methanol was used to generate the ions of interest. Once desolvated, the ions were accumulated in a hexapole ion trap, pulsed out, focused using ion optics, and then injected into the acceleration region of a Wiley−McLaren (RETOF). Ions of interest were irradiated at the first space focus by the output of a tunable optical parametric converter (220−2500 nm). Using a two-stage reflectron, fragment ions were mass separated from remaining, undissociated parent ions. Ions were detected on a microchannel plate detector at the second space focus of the mass spectrometer. We obtained a photodissociation action spectrum by monitoring the fragment ions as a function of photon energy. Laser fluence and unimolecular decay of metastable parent ions were monitored as well to correct the photodissociation spectrum accordingly. UV−vis Spectroscopy. Condensed phase absorption spectra were obtained for 20 mM and 200 μM solutions of copper(II) nitrate hydrate (Sigma-Aldrich, used without purification) in distilled water. The spectra were taken using a Varian Cary 500 Scan UV−vis−NIR spectrometer (version 8.01) with a 5 mm path length, 10 cm−1 step size, 2 nm resolution, and an integration time of 0.1 s. The data were baseline-corrected using a distilled water sample. Computation. Geometry optimizations were performed for the parent and fragment ions using the TURBOMOLE V5.9.1 and V6.2 suites of programs.31 We employed density functional theory (DFT)32 with the PBE033 functional using def2TZVPP34,35 basis sets for all atoms and calculations. Geometry optimizations were performed initially on all relevant species without symmetry restrictions from a number of starting geometries. The DFT energies were corrected for zero point vibrational energies obtained from analytical calculations with the AOFORCE program. After inspection of the parent ion geometry obtained without symmetry restriction, geometry optimizations were repeated under C2 symmetry, and the resulting total energies differed by less than 1 meV from those obtained without symmetry restrictions. We therefore assume that the Cu(NO3)3− parent ion is of C2 symmetry (see below). The electronic transitions were characterized using timedependent density functional theory (TDDFT)36,37 using the PBE0 functional and def2-TZVPP basis sets for all atoms.

Figure 1. Comparison of the UV photodissociation of Cu(NO3)3− in vacuo (top) and the UV absorption spectrum of copper nitrate in aqueous solution at 20 mM (solid line) and diluted to 200 μM (dotted line) concentration.

dissociation spectrum of the Cu(NO3)3− ion with the UV absorption spectrum of an aqueous copper nitrate solution, shown in the lower part of Figure 1. The solution spectrum is dominated by an intense band at photon energies above ca. 5 eV (ca. 250 nm), which is commonly assigned to the π* ← π transition in nitrate anion NO3−.3 A shoulder at ca. 4 eV photon energy observable at higher salt concentrations has been attributed to a ligand-to-metal charge transfer transition from nitrate to Cu(II) in association complexes,38 but this feature overlaps with the signature of the π* ← n transition in NO3− found in the same spectral region in aqueous and methanolic solutions of nitrates.3 We note that the characteristic blue color of aqueous solutions containing Cu(II) species is caused by the Laporte forbidden d ← d transition in Cu(II),38 giving rise to a weak absorption feature at lower photon energies (around 1.8 eV, corresponding to 700 nm). However, we will not discuss this transition here, since its energy is below the onset of photodissociation in the present work. The first step toward interpreting the spectrum of Cu(NO3)3− is to determine the structure of the complex using electronic structure calculations. Figure 2 shows the calculated structures of the Cu(NO3)3− parent ion, neutral Cu(NO3)2, and the anionic Cu(NO3)2− photofragment observed. The molecular frame of the calculated ground state structure of the planar Cu(NO3)2 salt is strongly distorted by the presence of the excess NO3− group. The resulting complex has C2 symmetry with the Cu atom coordinated in a bidentate fashion to each of the three NO3 groups. In this structure, each ligand oxygen bound to the copper atom forms the corner of a distorted octahedron. We note that the two nonaxial NO3 ligands in Cu(NO3)3− are distorted, and their oxygen atoms binding to the copper atom are not equivalent, with distances of 195.6 and 242.9 pm from the Cu atom, respectively. This distortion is exacerbated in the Cu(NO3)2− fragment ion,

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RESULTS AND DISCUSSION Upon irradiation with photon energies in the range 3.0−5.6 eV, Cu(NO3) 3− undergoes dissociation. We observed only Cu(NO3)2− as product ion, formally corresponding to reduction of Cu(II) to Cu(I), presumably along with the loss of neutral NO3 radical. The adiabatic electron affinity of the complex is calculated to be 5.8 eV, and we therefore assume that photodetachment plays no role in the present experiments. The fragment action spectrum obtained by monitoring Cu(NO3)2− ions as a function of photon energy is shown in Figure 1. The spectrum consists of a broad peak centered near 4.50 eV with a shoulder around 3.75 eV and a more intense feature toward the upper energy limit of the experiment, ∼5.5 eV. The broad, low-energy feature has an onset energy of ca. 3.5 eV. No fragment ions were generated at photon energies below 3.0 eV. To gain some first insight into the photophysical properties of copper nitrate, it is instructive to compare the photoB

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Figure 3. Selected molecular orbitals of Cu(NO3)3−. The symmetry axis of the complex is vertical in all three panels. Figure 2. Calculated structures of some of the ions relevant for this study. Copper atoms are brown, nitrogen atoms are blue, and oxygen atoms are red. Top: Cu(NO3)3− parent ion with its C2 symmetry axis vertical. Center: neutral Cu(NO3)2. Bottom: anionic fragment ion Cu(NO3)2−.

suggesting that it is due to the presence of the additional negative charge which influences the electronic structure of the metal center. We note that the change in coordination number is dramatic, going from 6 to 2 during fragmentation. Natural population analysis39 of the parent ion shows that the Cu atom has a charge of +1.38 e, demonstrating that the simple picture of formal charges giving rise to a d9 configuration breaks down. The highest occupied molecular orbital is delocalized over the two distorted NO3 ligands and the copper atom (see Figure 3), where the interaction between the ligands and the copper atom can be described as primarily of antibonding σ character. Population analysis of the fragment ion suggests that the copper ion is reduced upon fragmentation from a natural charge of +1.38 e to +0.74 e. Figure 4 shows a comparison of the experimental photodissociation spectrum and a calculated TDDFT spectrum (PBE0 functional, def2-TZVPP basis). Overall, 40 electronic transitions were found up to transition energies of 6.0 eV, most of them with oscillator strengths greater than 10−3, and eight with oscillator strengths greater than 10−2. The calculated spectrum is qualitatively consistent with the onset of the experimental photodissociation spectrum. The TDDFT spectrum shows three bands of features in the region between 3.0 and 6.0 eV, each consisting of several transitions and a steep rise at higher energies. The four most intense of the transitions calculated in the three bands (at 3.53, 3.74, 4.70, and 5.75 eV) can be roughly characterized as ligand-to-metal charge transfer transitions, ending in the lowest unoccupied molecular orbital (LUMO),

Figure 4. Comparison of experimental (top trace) and calculated excitation spectrum (bottom trace) of Cu(NO3)3− (see text). The vertical bars show the calculated oscillator strength (to scale as shown); the full line was obtained by broadening each transition with a Gaussian of width 0.2 eV to guide the eye.

which has mainly d-orbital character centered on the copper ion (see Figure 3). The dominant higher energy transitions (calculated at 6.55 and 7.01 eV) have π* ← π and π* ← n character, similar to the electronic transitions of NO3−, but with their initial orbitals delocalized over all ligands, while the final orbitals are a mixture of LUMO and LUMO+1 (see Figure 3). We note that while the calculated spectrum qualitatively recovers the experimental spectrum, the low-energy onset of C

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highly vibrationally excited state on the electronic ground state potential energy surface. Quantum-chemical calculations indicate that the parent ion complex has C2 symmetry with a strongly distorted octahedral coordination of the copper atom. The oxidation state of the Cu atom in this complex is between +1 and +2 and is further reduced upon fragmentation. TDDFT calculations indicate that the most dominant contributions of the observed lower energy UV transitions are LMCT transitions and end in the LUMO of the parent ion, while the higher energy bands have mainly π* ← π and π* ← n character. The anionic association complex studied here has a UV photodissociation spectrum that is similar to UV absorption spectra of copper nitrate solutions at high concentrations, suggesting that the spectra of copper nitrate association complexes have similar characteristics based on the building blocks of the complexes. Although the species at play in these two scenarios are not necessarily the same, they have in common that spectral features absent from dilute solutions are likely to be caused by LMCT transitions between nitrate ligands and the copper moiety.

the UV transitions appears to be ca. 0.5 eV too low. In contrast, the steep rise at the highest energies in the TDDFT spectrum appears at higher energies than those experimentally observed. As mentioned, we observed only Cu(NO3)2− as a fragment ion. Interestingly, we did not see evidence of the alternative fragment channel Cu(NO3)3− → Cu(NO3)2 + NO3−

where neutral copper(II) nitrate would be formed along with a nitrate anion. In principle, this may be due to the very low kinetic energy of NO3− ions formed following dissociation, resulting in correspondingly low detection efficiency. However, loss of neutral NO3 (as observed in the present photodissociation work) has been identified as a major fragment channel in collision-induced dissociation (CID) experiments on anionic transition metal nitrate complexes performed by Li et al.,28 where nitrate anion is formed only as a minor CID product from Cu(NO3)3− parent ions. This similarity in fragment channels points to a dissociation mechanism wherein excitation is followed by internal conversion to a highly vibrationally excited state on the electronic ground state potential energy surface that results in fracture of the weakest bond. Although there are several electronic transitions in Cu(NO3)3− calculated to be in the visible spectral region (around 1.8 eV), no photofragments are observed below ca. 3.5 eV photon energy. The fragmentation threshold energy EF of the observed fragment channel canin principlebe determined by our calculations using the relationship −

E F = E[Cu(NO3)2 ] + E[NO3] −

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ASSOCIATED CONTENT

S Supporting Information *

Calculated structures of Cu(NO3)3−, Cu(NO3)2, and Cu(NO3)2− as well as the calculated transition energies and oscillator strengths of Cu(NO3)3−. This material is available free of charge via the Internet at http://pubs.acs.org.

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E[Cu(NO3)3− ]

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel ++1-303-492-7841 (J.M.W.).

where E[Cu(NO3)2−], E[NO3], and E[Cu(NO3)3−] are the calculated energies of the fragment ion, NO3 radical, and the parent ion, respectively (including zero-point corrections). However, a good estimate for E[NO3] within our calculations is problematic due to the known challenges of the ground state electronic structure of the NO3 radical.40 Instead, we determine E[NO3] by using the relationship

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Science Foundation for funding through Grant CHE-0845618. S.H.K. was supported by a NSF Graduate Research Fellowship under Grant DGE-1144083. J.M.W. is an Alfred P. Sloan Research Fellow.

E[NO3] = E[NO3−] + EA[NO3]

since NO3− is computationally more tractable and its electron affinity (EA[NO3] = 3.937 eV) has been experimentally determined to high accuracy.41 The calculated fragmentation energy based on this approach is 2.7 eV, consistent with the absence of photofragments below 3 eV. Our TDDFT calculations indicate that there are no allowed transitions between the d ← d bands (at ca. 1.8 eV) and the onset of the LMCT bands at ca. 3.5 eV. The thermal energy content of the ion based on the calculated vibrational frequencies can be estimated to be ca. 0.35 eV. Even for the lowest energy transitions in the UV, the energy in the molecule after photon absorption is ca. 1.15 eV higher than the calculated energy necessary for loss of the NO3 fragment. On the basis of these calculated energies, we assume that there are no significant kinetic shift effects on the observed spectrum.

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SUMMARY AND CONCLUSIONS We report the UV photodissociation spectrum in the spectral region between 3.0 and 5.6 eV for the copper nitrate association complex Cu(NO3)3−. The only observed fragment channel is Cu(NO3)2−, consistent with CID experiments. Fragmentation likely occurs after fast internal conversion into a D

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