Wan-Lu
Li‡
a,
Hong-Tao
Liu‡
b,
Tian
Jian‡
c,
Gary V.
Lopez
c,
Zachary A.
Piazza
c,
Dao-Ling
Huang
c,
Teng-Teng
Chen
c,
Jing
Su
ab,
Ping
Yang
d,
Xin
Chen
a,
Lai-Sheng
Wang
*c and
Jun
Li
*a
aDepartment of Chemistry & Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China. E-mail: junli@tsinghua.edu.cn
bShanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
cDepartment of Chemistry, Brown University, Providence, Rhode Island 02912, USA. E-mail: Lai-Sheng_Wang@brown.edu
dTheoretical Division T-1, Los Alamos National Laboratory, Los Alamos, NM 87544, USA
First published on 13th October 2015
We report a joint photoelectron spectroscopy and theoretical investigation of the gaseous Au2I3− cluster, which is found to exhibit two types of isomers due to competition between Au–I covalent bonding and Au–Au aurophilic interactions. The covalent bonding favors a bent IAuIAuI− structure with an obtuse Au–I–Au angle (100.7°), while aurophilic interactions pull the two Au atoms much closer, leading to an acutely bent structure (72.0°) with an Au–Au distance of 3.08 Å. The two isomers are separated by a small barrier and are nearly degenerate with the obtuse isomer being slightly more stable. At low temperature, only the obtuse isomer is observed; distinct experimental evidence is observed for the co-existence of a combination of isomers with both acute and obtuse bending angles at room temperature. The two bond-bending isomers of Au2I3− reveal a unique example of one molecule being able to oscillate between different structures as a result of two competing chemical forces.
In addition to the strong chemical bonding that determines molecular structures, weaker aurophilic interactions have been found in gold systems due to attractions between closed-shell AuI centers.20–31 Aurophilic interactions, which are intermediate between van der Waals forces and covalent bonding, dominate the structural chemistry of AuI compounds. Strong relativistic effects and dispersion-type electron correlations were found to be the major driving forces of aurophilicity.22,28–30,32,33
Here we report a joint photoelectron spectroscopy (PES) and quantum chemical study on the gaseous Au2I3− cluster, in which aurophilic interactions turn out to play a key role. This anion is found to have a C2v bent geometric configuration of I–Au–I–Au–I− and exhibit two near-degenerate isomers that we name as “bond-bending isomers (BBIs)”, similar to BSI, which has not been experimentally established heretofore.34,35 The two BBIs of Au2I3− reported here differ only in geometry by the bending angle (Fig. 1), owing to different mechanisms of bonding interactions. A prior study showed that the Na2Cl3− cluster has a linear ground state, because of primarily ionic interactions.36 The bent structures of Au2I3− reflect both the strong Au–I covalency37,38 and Au–Au aurophilic interactions. The obtuse isomer, which is slightly lower in energy, features a traditional chemical bonding pattern and can be viewed as two AuI moieties linked by a central iodine anion via the two Au atoms, whereas the acute isomer with a significantly shorter Au⋯Au distance features strong aurophilic interactions.
Fig. 1 The structures and relative energies of the obtuse and acute isomers of Au2I3− at the CCSD(T)/AVTZ level. Bond lengths are in Å. |
Schmidbaur and Schier predicted that the [E(AuL)n] type unit could form polyhedra, due to strong aurophilic interactions,25 similar to the acute isomer of Au2I3−. Both isomers of Au2I3− are in singlet spin-states and can inter-convert easily at elevated temperatures because of the low energy barrier between them.
In the temperature-dependent experiments, the Au2I3− clusters were produced by laser vaporization of a cold-pressed AuI target in the presence of pure He or a He carrier gas seeded with 5% Ar. The latter was shown previously to give better supersonic cooling.40 Clusters formed in the source were entrained by the carrier gas and underwent a supersonic expansion. Anions were extracted from the cluster beam and analyzed by a time-of-flight mass spectrometer. The Au2I3− anions were selected by a mass gate and decelerated before being detached by a 193 nm laser beam from an ArF excimer laser. Photoelectrons were analyzed by a magnetic-bottle analyzer with an electron energy resolution of ∼2.5% (ΔEk/Ek).41 Clusters with different resident times inside the nozzle were selected as a means to qualitatively control the cluster temperatures.42
In as much as aurophilic interactions required sophisticated electron correlation treatment, high-level ab initio calculations were further performed to obtain accurate geometries and energies. We applied the spin-component scaled second-order perturbation theory (SCS-MP2),52 coupled clusters with single, double and perturbative triple excitations [CCSD(T)],53,54 and complete-active-space self-consistent field (CASSCF)55 methods. These ab initio electron correlation calculations were done with the MOLPRO 2008 program.56 Geometry optimizations for the acute and obtuse Au2I3− structures were both done at the SCS-MP2 and CCSD(T) level. For the neutral Au2I3 species, single-point energies of the ground and excited states were determined at the optimized anion structures using the CCSD(T) method, which accurately generated the state-specific scalar relativistic energies of all the states needed for simulating the experimental PES. Single-electron VDEs from the anion ground state to the corresponding ground and excited states of the neutral cluster were obtained using the CASSCF/CCSD(T)/SO approach. In both the Gaussian 09 and MOLPRO 2008 calculations, the Stuttgart energy-consistent relativistic pseudopotentials ECP60MDF and ECP28MDF and the corresponding valence basis sets of polarized triple-zeta level (aug-cc-pVTZ-PP) were applied for Au57,58 and I,59 respectively, which were abbreviated as AVTZ.
For chemical bonding analyses, we used the energy decomposition approach (EDA),60 various bond order indices, and electron localization function (ELF)61 using the PBE exchange–correlation functional implemented in ADF 2013.01.62 Here we applied the Slater basis sets with the quality of triple-zeta with two polarization functions (TZ2P).63 Frozen core approximation was used for the inner shells of [1s2–4f14] for Au and [1s2–4d10] for I. The scalar relativistic (SR) and spin orbit (SO) effects were taken into account by the zero-order-regular approximation (ZORA).64
In order to obtain multi-center localized orbitals, we used the Multiwfn65 codes to do adaptive natural density portioning (AdNDP) analyses.66 The atomic charges were computed via natural population analyses (NPA)67 by using NBO 3.1 (ref. 68) as implemented in Gaussian 09. The local adiabatic stretching force constants69 were calculated using a code written by the Tsinghua group.
In order to have a better understanding of the congested PES features, temperature-dependent experiments were carried out using a laser vaporization supersonic cluster source.41 The Au2I3− anions were selected and decelerated before being photodetached by the 193 nm (6.424 eV) radiation from an ArF excimer laser at several experimental conditions, as shown in Fig. 3. It was found previously that the residence time of clusters inside the nozzle is an important factor affecting the cluster temperature.42 Those clusters that spend more time in the nozzle experience more thermalization collisions with the carrier gas, and they are colder. The residence time of clusters in the nozzle can be controlled to obtain qualitatively hot, warm, and cold clusters.42
Fig. 3 Temperature-dependent photoelectron spectra of Au2I3− at 193 nm. (a) At short residence time (hot). (b) At medium residence time (warm). (c) At long residence time (cold). |
Photoelectron spectra taken at 193 nm were designated as ‘hot’, ‘warm’, and ‘cold’ in Fig. 3, corresponding to three different residence times. Under the cold condition (Fig. 3c), a well-resolved spectrum with only the four intense features (X, A, B, C) was observed. Under the warm condition (Fig. 3b), weak features (X′, A′–F′) appeared and their intensities increased under the hot condition (Fig. 3a). In addition to the weak PES features observed in Fig. 2 (X′ and A′–D′), two more weak features (E′ and F′) were resolved in Fig. 3. The temperature-dependence of the weak features provided convincing evidence that they were from low-lying isomers, while the intense features were from the most stable isomer.
Because of the inherent aurophilic interactions between the two Au atoms, calculated results from the long-range corrected functionals agree with those from the ab initio coupled-cluster method. Thus, high-level quantum chemical methods are crucial in accurately characterizing these nearly degenerate isomers. The full computational results for the energy difference between the acute and obtuse isomers (ΔEA–O) are given in Table S3.† We found that the linear structure is a transition state, lying about 50 kJ mol−1 higher in energy than the two BBIs. Preliminary calculations with the SO-ZORA approach show that SO effects are important for all these species, whereas the relative energies are not affected. Fig. 4 illustrates the rather flat potential energy curve in the region of the two BBIs, which are separated by a small barrier of only 1.78 kJ mol−1 at CCSD(T) level. Such a low barrier suggests that the two BBIs can easily inter-convert. Hence, at elevated temperatures many different configurations are expected to co-exist along the bending angle, in excellent agreement with the experimental results, as shown in Table S1† (vide infra). The SCS-MP2/AVTZ calculations gave a bending angle of 80.1° for the transition state between the two BBIs. It is interesting to compare the bonding between I5− with an apical angle of 94° and the obtuse Au2I3− isomer,72,73 because Au can be viewed as a heavy halogen. However, aurophilicity also gives rise to the acute configuration for Au2I3−.
We have estimated the lifetime of the acute BBI at the upper potential well using the Wentzel–Kramers–Brillouin (WKB) escape probability.74 Using a bending frequency of 24 cm−1, we estimated a lifetime of about 104 ns for the acute BBI, which is sufficiently long-lived to be observed in the photodetachment experiments (Fig. 2 and 3a). A further possibility is that in the condensed-phase the lifetime of the acute isomer could become significantly longer so that separate BBIs could exist therein.
Because the barrier between the two BBIs is low, the broad part of the potential energy curve will be accessed at high vibrational levels or high temperatures, resulting in new spectral features, in remarkable agreement with the experiment. The diffuse X′ low binding energy feature appearing in the hot spectrum by laser vaporization (Fig. 3a) or the room temperature spectrum from ESI (Fig. 2) provides unequivocal evidence for the existence of the acute configurations and for the flatness of the potential energy curve. To understand the spectral features quantitatively, we computed the vertical electron detachment energies (VDEs) of both the acute and obtuse isomers using the CASSCF/CCSD(T)/SO approach, as shown in Table S1.† This approach was shown previously to give reliable theoretical VDEs for Au-containing complexes.75–77 Theoretical VDEs of the obtuse isomer are plotted as short bars in Fig. 2b for comparison, and they are in excellent agreement with the intense spectral bands (X, A–I). The diffuse and weak low binding energy band X′ corresponds to the acute isomer. Because of the flatness of the potential energy curve, a large range of bond angles is expected to be accessed. Hence, we computed the VDEs for a broad range of bond angles from the obtuse to the acute minima, as given in Table S1.† The calculated VDE of the first detachment channel of the acute isomer increases from 4.828 eV at 72° to 5.015 eV at 90°. This result is in excellent agreement with the experimental VDE of the diffuse band X′, providing the most compelling evidence for the presence of the acute isomer and the shape of the potential energy surface at the CCSD(T) level in Fig. 4. The calculated VDEs for the higher detachment channels are also consistent with the weak features observed at higher temperatures. The excellent agreement between the theoretical VDEs and the temperature-dependent PES provide conclusive evidence for the two BBIs and the validity of the potential energy curve at the CCSD(T) level in Fig. 4.
Fig. 5 The electron localization functions (ELFs) calculated for both the acute and obtuse Au2I3− at the PBE/ZORA/TZ2P level. (a) For the acute structure. (b) For the obtuse structure. |
As shown in Fig. S4,† the 1a1 and 1b2 MOs in the acute isomer seem to form two types of three-center two-electron (3c–2e) bonds; one is along the C2 axis while the other is perpendicular to the C2 axis, as shown more clearly in the bonding analysis in Fig. S5c′ and d′.† This kind of three-center bonding is also found previously in OC–Th–CO,78 similar to IAu–I–AuI− or the Au2I42− complex.79 These 3c–2e orbitals are conducive to promote Au–Au aurophilic attraction, but weaken the Au–Ic interactions, resulting in the more acute angle for Au1–Ic–Au2 and elongated Au1,2–Ic bond length. However, the 1a1 and 1b2 MOs in the obtuse isomer simply represent two two-center two-electron (2c–2e) Au1,2–Ic bonds, as shown in Fig. S5c and d.† Therefore, the Au–Au aurophilic interactions in the acute isomer involve several MOs.
The calculated bond orders based on various theoretical schemes and adiabatic stretching force constants in both the acute and obtuse conformers are given in Table S4.† These results also give strong evidence that the Au(I)⋯Au(I) aurophilic attraction becomes stronger at the cost of weakening the covalent bonding between the central I and the adjacent Au in the acute isomer. All methods give a larger Au–Au bond order and a smaller Au–Ic bond order in the acute isomer relative to the obtuse isomer. The Au–Au and Au–Ic stretching frequencies show a similar trend. Table 1 provides results of the energy decomposition analysis (EDA) with SO coupling correction, showing the key role of the covalent chemical bonding interaction according to orbitals with different irreducible representations. Steric interactions in the linear, obtuse and acute isomers are almost identical. However, the orbital interactions of the b2 MOs in the obtuse isomer are 1.15 eV stronger than that in the acute isomer. On the other hand, the orbital interactions of the a1 MOs in the acute isomer are 1.04 eV more stable than the obtuse isomer, suggesting that the energy difference of these two isomers is mainly due to the energy competition between the b2 and a1 orbitals. It is also shown that the contribution of the SO coupling effect to the total bonding energy is significant (6.40 eV), but the correction is the same for the isomers with different bending angles (Table 1). Accordingly, the relative energetic stability is not influenced by the SO correction.
Steric roleb | Orbital interactionc | Total bonding energyd | ||||||
---|---|---|---|---|---|---|---|---|
a1 | b1 | b2 | a2 | Sum | SR | SO | ||
a All energies are given in eV. The SO single-point calculations are based on the spin-restricted fragments of the SR results at the equilibrium geometries. b The sum of electrostatic and Pauli interactions. c Each irreducible representation means the sum of the contributions from that orbital type. There are 11a1, 10b1, 5b2 and 4a2 orbitals that are summed up. d Sum of the steric and orbital interactions. | ||||||||
Acute | 11.16 | −10.71 | −1.84 | −8.99 | −3.83 | −25.37 | −14.21 | −20.61 |
Obtuse | 11.29 | −9.67 | −1.91 | −10.14 | −3.82 | −25.54 | −14.25 | −20.65 |
Linear | 11.23 | −13.52 | −5.72 | −5.72 | −0.01 | −24.97 | −13.74 | −20.14 |
Footnotes |
† Electronic supplementary information (ESI) available: Supplementary figures and tables. See DOI: 10.1039/c5sc03568f |
‡ These authors contributed equally to this work. |
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