Infrared spectroscopic study of solvation and size effects on reactions between water molecules and neutral rare-earth metals

Abstract

Elucidating the solvation and size effects on the reactions between water and neutral metals is crucial for understanding the microscopic mechanism of the catalytic processes but has been proven to be a challenging experimental target due to the difficulty in size selection. Here, MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) complexes were synthesized using a laser–vaporization cluster source and characterized by size-specific infrared-vacuum ultraviolet spectroscopy combined with quantum chemical calculations. The MO₄H₆ and M₂O₆H₇ complexes were found to have H˙M(OH)₃(H₂O) and M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) structures, respectively. A combination of experiments and theory revealed that the formation of H˙M(OH)₃(H₂O) and M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) is both thermodynamically exothermic and kinetically facile in the gas phase. The results indicated that upon the addition of water to H˙M(OH)₃, the feature of the hydrogen radical is retained. In the processes from mononuclear H˙M(OH)₃ to binuclear M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂), the active hydrogen atom undergoes the evolution from hydrogen radical → bridging hydrogen → metal hydride → hydrogen bond, which is indicative of a reduced reactivity. The present system serves as a model for clarifying the solvation and size effects on the reactions between water and neutral rare-earth metals and offers a general paradigm for systematic studies on a broad class of the reactions between small molecules and metals at the nanoscale.

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1 Introduction

Rare-earth metals (Sc, Y, Ln) and their complexes have been widely used in various catalytic processes.^1–3 For instance, Ni@CeO₂-doped MgH₂ hydrogen storage material reduces the dehydrogenation activation energy by 20.3 kcal mol⁻¹, which significantly improves the hydrogen release property of MgH₂ material.^4–6 Rare-earth single-atom catalysts (SAC) are extensively exploited in the photocatalytic/electrocatalytic processes of O₂/CO₂/N₂, etc.^7–12 Theoretical studies have verified that in the electrocatalytic reduction of O₂, the rare-earth SAC undergoes the transformation from the poly-hydroxyl structure to the solvated water structure.^9,13 CeO₂ is utilized as the catalyst in the hydrogenation reduction of alkenes/alkynes,¹⁴ in which the key intermediates contain important metal hydride structures and poly-hydroxyl structures. Ultrathin 2D rare-earth nanomaterials of the layered rare-earth hydroxides were synthesized in solution^15,16 and were confirmed to exhibit great potential in various applications, such as optoelectronics,¹⁷ magnetic devices,¹⁸ and high-efficiency catalysts.¹⁹ Therefore, the studies on the reactions between small molecules and rare-earth metals at the cluster scale are expected to provide microscopic information to unravel the structure–reactivity relationship of catalysts.

Various efforts have been devoted to the spectroscopic studies of the reactions of water molecules with rare-earth metals^20–27 owing to their important roles in renewable sources and transport/storage of hydrogen energy.^28,29 The investigation of the solvation and size effects on the reactions between water with neutral metals is crucial to understanding the microscopic mechanism of catalytic processes but has been proven to be a challenging experimental target due to the difficulty in size selection. Matrix-isolation infrared spectroscopy has provided valuable insights into the reactions of neutral single rare-earth metal atoms with water, suggesting the formation of the inserted HMOH structures and the solvated M(H₂O)_1–2 structures.^20,24,26,27 Recently, we have characterized two different types of rare-earth hydroxides (MO₃H₄ and M₂O₄H₄ (M = Sc, Y, La))^30,31 using size-specific infrared-vacuum ultraviolet (IR-VUV) spectroscopy, in which MO₃H₄ has a hydrogen radical structure and M₂O₄H₄ has a water-split structure. In this work, we increased the number of water molecules to study the solvation and size effects on the reactions between water with neutral rare-earth metals. Two new rare-earth metal hydroxides with the chemical formulas MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) were prepared and characterized by IR-VUV spectroscopy and quantum chemical calculation. MO₄H₆ and M₂O₆H₇ complexes were found to have the H˙M(OH)₃(H₂O) and M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) structures, respectively, in which the hydrogen radical features were retained with the solvation of water in the mononuclear complex MO₄H₆ and the reactivity of the active hydrogen was reduced in the formation processes of binuclear complex M₂O₆H₇.

2 Experimental and theoretical methods

The experiment was performed using the IR-VUV apparatus, which has been described in detail in previous reports.^32–34 Neutral MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) complexes were prepared using the laser–vaporization cluster source in a pulsed supersonic expansion of 0.2% H₂O/helium. The second harmonic of a Nd: YAG laser (532 nm) was employed to ablate the metal target, with a pulse energy of 1.0 mJ. The reaction gas was introduced using a Parker pulse valve (Series 009), with a pulse width of 180 μs. The molecular beams that passed through a 4 mm-diameter skimmer were ionized by a VUV laser and then detected by a reflectron time-of-flight mass spectrometer (TOF-MS).

VUV light with a wavelength of 193 nm was produced by an ArF excimer laser (COMPex 50 F, Coherent, Germany), with an energy of 14 mJ per pulse. The infrared laser was delivered by a potassium titanyl phosphate/potassium titanyl arsenate optical parametric oscillator/amplifier system (OPO/OPA, LaserVision) pumped by another seeded Nd: YAG laser (Continuum, Surelite EX). The OPO/OPA system was tunable from 700 to 7000 cm⁻¹, with a line width of 1 cm⁻¹.

The pulse valve, the vaporization laser, and VUV light were operated at 20 Hz. The tunable IR light pulse was introduced at approximately 150 ns prior to the VUV pulse in a crossed manner and operated at 10 Hz. When the resonant vibrational transition was irradiated by the IR laser light and caused vibrational predissociation, a depletion of a selected neutral cluster mass signal was detected. The infrared spectra were recorded from the difference between the mass spectral signals with and without the infrared laser (IR laser ON minus IR laser OFF). The IR spectra of the size-selected neutral MO₄H₆ and M₂O₆H₇ complexes were obtained from the depletion spectrum of the ion signal intensity as a function of the IR wavelength. Typical spectra were recorded by scanning the IR wavelength in the steps of 2 cm⁻¹ and averaging over 400 laser shots at each wavelength. The infrared wavelength was calibrated using a commercial wavelength meter (HighFinesse GmbH, WS6-200 VIS IR).

Quantum chemical calculations were performed using the Gaussian 16 package³⁵ at the TPSSh/def2-TZVP level of theory. Our test calculations indicated that the TPSSh functional has a lower computational cost than B2PLYPD3 and a better computational performance in the spectra and energetics than B3LYP for MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La). Accordingly, we chose the TPSSh functional instead of the B2PLYPD3 or B3LYP functionals that we used in previous works.^30,31 For the possible isomerization paths, the initial structures of the transition states were constructed manually and optimized in the Berny algorithm. Intrinsic reaction coordinates (IRC) of all the transition states were carried out to confirm that the transition states were connected to the initial and final states. The relative energies and energy barriers were calculated for 0 K structures, including the zero-point vibrational corrections. The harmonic vibrational frequencies were scaled by a factor of 0.968 in order to account for the method-dependent systematic errors.³⁶ The resulting stick spectra were convoluted by a Gaussian line shape function with a 15 cm⁻¹ width at half-maximum (FWHM).

3 Results and discussion

As exemplified by the Sc + H₂O reaction, the mass spectrum of the species is shown in Fig. S1.† The experimental IR spectra of neutral MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) complexes in the OH stretching region are shown in Fig. 1a–c and 2a–c, respectively. The experimental band positions of MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) are listed in Tables S1 and S2,† respectively. As shown in Fig. 1a–c, the experimental IR spectrum of each MO₄H₆ complex comprises three groups of bands (labeled bands A, B, and C). B and A in the IR spectra of ScO₄H₆, YO₄H₆, and LaO₄H₆ are centered at 3398, 3104, and 2952 cm⁻¹, respectively, indicating redshift down through the group 3 metal of the periodic table. The B bands in the IR spectrum of ScO₄H₆, YO₄H₆, and LaO₄H₆ are observed at 3694, 3704, and 3722 cm⁻¹, respectively, showing a slight blueshift down through the group 3 metal of the periodic table. B and C in the IR spectra of ScO₄H₆, YO₄H₆, and LaO₄H₆ are located at 3786, 3784, and 3748 cm⁻¹, respectively, showing a slight redshift down through the group 3 metals of the periodic table. Similar spectral features were observed for M₂O₆H₇ (labeled bands a, b, and c), except for the absence of band a in Sc₂O₆H₇.

Fig. 1 Comparison of the experimental IR spectra of neutral MO₄H₆ complexes (M = Sc, Y, La) (a–c) and calculated IR spectra of isomers 1B, 1II, and 1ii (d–f). Calculations were performed at the TPSSh/def2-TZVP level of theory, with the harmonic vibrational frequencies scaled by a factor of 0.968. The structures are embedded in the inset (O, red; H, light gray; Sc, purple; Y, blue; La, olive).

Fig. 2 Comparison of the experimental IR spectra of neutral M₂O₆H₇ complexes (M = Sc, Y, La) (a–c) and the calculated IR spectra of isomers 2A, 2I, and 2i (d–f). Calculations were performed at the TPSSh/def2-TZVP level of theory, with the harmonic vibrational frequencies scaled by a factor of 0.968. The structures are embedded in the inset (O, red; H, light gray; Sc, purple; Y, blue; La, olive).

To understand the experimental IR spectra and identify the structures of MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) complexes, quantum chemical calculations were carried out at the TPSSh/def2-TZVP level of theory. The structures of the identified isomers and the comparison of their calculated IR spectra with the experimental ones are shown in Fig. 1 and 2, respectively. The structures of other low-lying isomers and the comparison of their calculated IR spectra with the experimental ones are illustrated in Fig. S2 and S3,† respectively.

As shown in Fig. S2,† the most stable isomer of ScO₄H₆ (labeled 1A) has a Sc(OH)₂(H₂O)₂ structure with a doublet electronic ground state, in which the scandium hydroxide Sc(OH)₂ are solvated with two water molecules. The 1B isomer has a H˙Sc(OH)₃(H₂O) structure with the involvement of a hydrogen radical, as evidenced by the calculated Mulliken spin densities [H(0), 0.72; Sc(0), 0.10] (for the labeling of the atoms, see Table S1†). The 1B isomer lies slightly higher in energy than the 1A isomer by 2.6 kcal mol⁻¹. The 1C isomer lies 36.0 kcal mol⁻¹ above the 1A isomer and has a ScO(H₂O)₃ structure, in which three water molecules are weakly bound to ScO. It can be seen from Fig. S2† that the calculated IR spectrum of the isomer 1B agrees best with the experimental one. As listed in Table S1,† the calculated band at 3374 cm⁻¹ is attributed to the symmetric stretching mode of the O(4)H(4) and O(4)H(5) groups, which is consistent with the experimental band A (3398 cm⁻¹); the antisymmetric stretching mode of the O(4)H(4) and O(4)H(5) groups was calculated to be 3698 cm⁻¹, which is in accordance with the experimental band B (3694 cm⁻¹); the stretching modes of the O(1)H(1), O(2)H(2), and O(3)H(3) groups were calculated to 3802, 3772, and 3758 cm⁻¹, respectively, which were observed as one band in the experimental spectrum (band C, 3786 cm⁻¹). However, the calculated IR spectrum of the isomer 1A (Fig. S1b†) consists of the bands at 2693 and 2955 cm⁻¹, corresponding to the hydrogen-bonded OH stretching modes of the solvated water molecules, which are not observed experimentally. Analogously, the presence of isomer 1C can be ruled out based on the calculated band at 2640 cm⁻¹. Similar results are also obtained for the YO₄H₆ and LaO₄H₆ complexes (Fig. S2 and Table S1†). As summarized in Fig. 1, the agreement of the simulated IR spectra of the 1B, 1II, and 1ii isomers with the experimental spectra of ScO₄H₆, YO₄H₆, and LaO₄H₆ is reasonable to identify the presence of H˙M(OH)₃(H₂O) (M = Sc, Y, La) structures.

For Sc₂O₆H₇, the most stable isomer (labeled 2A) has a Sc₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) structure with a doublet electronic ground state (Fig. S3d†), in which one water molecule forms one weakly bond with one scandium hydroxide and one hydrogen-bond with the hydroxyl group of another scandium hydroxide. The 2B isomer has a similar structure and lies only 0.1 kcal mol⁻¹ above 2A, in which one water molecule forms one weak bond with one scandium hydroxide and one hydrogen bond with the hydroxyl group of the same scandium hydroxide. In the calculated IR spectrum of isomer 2A (Fig. S3 and Table S2†), the band at 3675 cm⁻¹ is attributed to the stretching mode of the O(6)H(6) group, which is consistent with the experimental band b (3662 cm⁻¹); the stretching modes of the O(n)H(n) (n = 1–5) groups is calculated to 3806, 3804, 3778, 3770, and 3751 cm⁻¹, respectively, which are observed in the experimental spectrum at 3810, 3788, and 3742 cm⁻¹ (band c). The absence of stretching mode of the O(6)H(7) group (2474 cm⁻¹) (Table S2†) could be rationalized that the photon energy of the IR laser in this wavelength region is not sufficient to cause the vibrational predissociation of Sc₂O₆H₇. In the calculated IR spectrum of the isomer 2B (Fig. S3g†), the hydrogen-bonded OH stretching mode of the solvated water molecule is predicted at 3275 cm⁻¹, which is not observed in the experimental spectrum. As summarized in Fig. 2, the agreement of the simulated IR spectra of the 2A, 2I, and 2i isomers with the experimental spectra is reasonable to confirm the M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) (M = Sc, Y, La) structures responsible for Sc₂O₆H₇, Y₂O₆H₇, and La₂O₆H₇.

Considering that the reactions in the laver–vaporization processes are quite intricate and their intermediates are very difficult to characterize experimentally, quantum chemical calculations were performed to explore the possible reaction mechanisms. Due to the similarities in the IR spectra and structures of MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) complexes, we used ScO₄H₆ and Sc₂O₆H₇ as examples to investigate the possible formation pathways. Previous studies have demonstrated that the Sc + H₂O → Sc(H₂O) reaction is more favorable than the dimerization of Sc atoms.³¹ As shown in Fig. S1,† the mononuclear complexes (i.e., Sc, ScO, HScO, Sc(OH)₂, and H˙Sc(OH)₃) were observed in the mass spectrum, for which the formation mechanisms have been reported previously.^20,23,25,30 Consequently, we will mainly discuss the dominant pathways that begin from the mononuclear complexes.

The potential energy profiles for the formation of ScO₄H₆ (isomer 1B) initiated by the reaction of ScO with H₂O are shown in Fig. 3. ScO reacts with the first water molecule to form a solvation structure ScO(H₂O), which is predicted to be exothermic by 18.5 kcal mol⁻¹ at the TPSSh/def2-TZVP level of theory. The ScO(H₂O) → Sc(OH)₂ isomerization is highly exothermic by 41.3 kcal mol⁻¹via a transition state (TS1) with a very small barrier of 1.1 kcal mol⁻¹. The addition of H₂O to Sc(OH)₂ forms Sc(OH)₂(H₂O), which could undergo isomerization to H˙Sc(OH)₃. TS2 lies below the energy of Sc(OH)₂ + H₂O reactants by 14.4 kcal mol⁻¹. Sc(OH)₂ + H₂O → H˙Sc(OH)₃ overall reaction is exothermic by 16.1 kcal mol⁻¹ and is thus thermodynamically favorable. The mechanism for kinetically trapping H˙Sc(OH)₃ by the soft helium expansion has been recently discussed in detail.³⁰ The H˙Sc(OH)₃ complex combines with the water molecule to form the target product ScO₄H₆ (H˙Sc(OH)₃(H₂O), isomer 1B), which is exothermic by 16.3 kcal mol⁻¹.

Fig. 3 Potential energy profiles for the formation of ScO₄H₆ (H˙Sc(OH)₃(H₂O), isomer 1B). The abbreviation “TS” denotes the transition state. Calculations were performed at the TPSSh/def2-TZVP level of theory. Corresponding structures are embedded in the inset (O, red; H, light gray; Sc, purple).

Fig. 4 shows the potential energy profiles for the formation of Sc₂O₆H₇ (isomer 2A) initiated from the mononuclear non-radical complexes. The ScO + HScO → HSc₂O₂ reaction is predicted to be extremely exothermic at 106.1 kcal mol⁻¹. The most stable isomer of HSc₂O₂ has a Sc₂(μ₂-O)₂(η¹-H) structure, which combines with one water molecule to form IM1 and IM2. IM1 can be isomerized to IM5 via TS4. The calculated energy of TS4 is lower than that of IM1, which is due to the zero-point energy correction.^37–39 This indicates that the energy barrier for the IM1 → IM5 isomerization is very low. IM5 undergoes dehydrogenation to form IM6 with a slightly endothermic value of 1.1 kcal mol⁻¹, which could be compensated by the highly exothermic overall reaction of HSc₂O₂ + H₂O → IM6 + H₂. Similarly, the reaction path of IM2 → TS3 → IM3 → TS5 → IM4 → IM6 is also both thermodynamically and kinetically facile in the gas phase. IM3 could also be generated by the combination of Sc(OH)₂ and HScO, which is predicted to be highly exothermic by 83.8 kcal mol⁻¹. The addition of H₂O to IM6 forms IM7, which is exothermic by 16.8 kcal mol⁻¹. IM7 undergoes isomerization to the most stable hydroxyl structure IM8 via TS6 with a tiny barrier of 0.9 kcal mol⁻¹. IM8 reacts with the second water molecule to form IM9, which is exothermic by 20.7 kcal mol⁻¹. IM9 can be isomerized to an all-hydroxyl structure IM10 via TS7 with a barrier of 8.3 kcal mol⁻¹. The addition of the third water molecule to IM10 generates the target product Sc₂O₆H₇ (isomer 2A), which is exothermic by 14.6 kcal mol⁻¹.

Fig. 4 Potential energy profiles for the formation of Sc₂O₆H₇ (isomer 2A) initiated from the mononuclear non-radical complexes. The abbreviation “IM” stands for intermediate and “TS” for the transition state. Calculations were performed at the TPSSh/def2-TZVP level of theory. Corresponding structures are embedded in the inset (O, red; H, light gray; Sc, purple).

Considering that H˙Sc(OH)₃ and OH radicals are also generated in the laser–vaporization process,^30,34 Sc₂O₆H₇ might also be formed via reactions (1) and (2).

H˙Sc(OH)₃ + Sc → Sc₂O₃H₄ (1)

Sc₂O₃H₄ + 3OH → Sc₂O₆H₇ (2)

Accordingly, the potential energy profiles for the formation of Sc₂O₆H₇ initiated from the mononuclear radical complexes are shown in Fig. 5. H˙Sc(OH)₃ reacts with the Sc atom to produce the binuclear complex IM11; the reaction is predicted to be highly exothermic by 97.3 kcal mol⁻¹. IM11 can be isomerized to IM12 via TS8 with a small barrier of 1.6 kcal mol⁻¹. The reaction of IM12 with the first, second, and third OH radicals is predicted to be extremely exothermic by 128.4, 122.2, and 51.3 kcal mol⁻¹, respectively. The subsequent isomerization from IM15 to isomer 2B is exothermic by 1.2 kcal mol⁻¹via TS9 with a barrier of 9.3 kcal mol⁻¹. Isomer 2B can be further isomerized to form the most stable isomer 2A via TS10 with a small barrier of 3.8 kcal mol⁻¹. Consequently, the formation of isomer 2A is both thermodynamically exothermic and kinetically facile in the gas phase, which is consistent with the experimental observation.

Fig. 5 Potential energy profiles for the formation of Sc₂O₆H₇ (isomer 2A) initiated from the mononuclear radical complexes. The abbreviation “IM” stands for intermediate and “TS” for the transition state. Calculations were performed at the TPSSh/def2-TZVP level of theory. Corresponding structures are embedded in the inset (O, red; H, light gray; Sc, purple).

The reaction mechanisms of rare-earth metals with water molecules can be inferred from the aforementioned formation pathways of ScO₄H₆ and Sc₂O₆H₇. Mononuclear rare-earth metal oxide (MO (M = Sc, Y, La)) could induce the decomposition of water molecules until the hydrogen-radical adduct H˙M(OH)₃ was generated. The addition of the subsequent water molecule forms the solvated complex H˙M(OH)₃(H₂O). The binuclear complex M₂O₃H (labeled IM6 in Fig. 4) might also induce the successive decomposition of water molecules to produce the poly-hydroxyl complex M₂O₅H₅ (labeled IM10 in Fig. 4). The additional water molecule forms a coordination bond with metals and a delocalized hydrogen-bond with the hydroxyl group.

The evolution mechanism from mononuclear to binuclear rare-earth metal complexes might be summarized as follows. The addition of water to H˙M(OH)₃ forms H˙M(OH)₃(H₂O), in which the radical feature is retained. However, upon the addition of the Sc atom to MO₃H₄ to produce binuclear complex M₂O₃H₄ (labeled IM11 in Fig. 5), the hydrogen radical first binds to the two metals and then undergoes isomerization to a hydride structure (labeled IM12 in Fig. 5). During the reactions of IM12 with the first two OH radicals, the hydride structures remain. The subsequent reaction with the third OH radical results in the formation of M₂O₆H₇, in which the hydrogen atom in the metal hydride unit is transformed into one hydrogen bond. Therefore, during the MO₃H₄ → M₂O₆H₇ processes, the active hydrogen atom undergoes the evolution of the hydrogen radical → bridging hydrogen → metal hydride → hydrogen bond, indicative of reduced reactivity.

The mononuclear complexes MO₄H₆ (M = Sc, Y, La) with an M(OH)₃ plane as its basic skeleton, are highly relevant to the ultrathin 2D rare-earth nanomaterials of layered rare-earth hydroxides M(OH)₃·nH₂O,^15,17 and the coordination bonding fashion of water with M(OH)₃ is similar to that of the reported rare-earth single-atom catalysis system.^9,10 The hydrogen radical is the key intermediate in a variety of processes (i.e., catalysis, molecular hydrogen production, and new particle formation), but it is difficult to detect because of its high reactivity and a very short lifetime. In this work, the feature of hydrogen radicals in the rare-earth single-atom complexes H˙M(OH)₃(H₂O) is fortunately observed under the presence of additional water, which would have important implications for understanding the solvation effects of the catalysts and would suggest that the ultrathin 2D rare-earth nanomaterials of layered rare-earth hydroxides have the potential to trap hydrogen radicals and participate in more chemical processes.

The binuclear complexes M₂O₆H₇ (M = Sc, Y, La) contain an M₂(μ₂-O)₂ unit, which is reminiscent of the actual rare-earth metal oxide catalysts. In the formation of M₂O₆H₇ from MO₃H₄, three bonding modes of hydrogen atoms are involved with the reactivity order of η¹-H > μ₂-OH > η¹-OH. Such reduced reactivity of hydrogen atoms might contribute to reducing the activation energy of the dehydrogenation reaction of the hydrogen storage materials.^4–6 These three types of hydrogen atoms were also observed in the process of ethylene/acetylene hydrogenation catalyzed by CeO₂,¹⁴ and the reactivity sequence was consistent with our results. The deactivation of the CeO₂ catalyst has been rationalized by the overproduction with the formation of strongly bound M–OH groups that are difficult to remove from the surface.¹⁴ Our findings imply that the formation of M₂(η¹-OH)(η¹-OH₂) unit or hydrogen-bonding networks on the surfaces may also deactivate the catalysts, which provides insights into the strategy for tuning the active sites.

4 Conclusions

In this work, MO₄H₆ and M₂O₆H₇ (M = Sc, Y, La) complexes were prepared by a laser–vaporization supersonic expansion cluster source. Infrared-vacuum ultraviolet (IR-VUV) spectroscopy combined with quantum chemical calculations indicates that the MO₄H₆ and M₂O₆H₇ complexes have the H˙M(OH)₃(H₂O) and M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) structures, respectively. The joint experimental and theoretical results reveal that the formation of H˙M(OH)₃(H₂O) and M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂) is both thermodynamically exothermic and kinetically facile in the gas phase. The feature of hydrogen radicals in the mononuclear complexes H˙M(OH)₃(H₂O) is found to be retained under the presence of additional water. The addition of a metal atom to H˙M(OH)₃ and subsequent combination with water produces the binuclear complex M₂(μ₂-OH)₂(η¹-OH)₃(η¹-OH₂), by which the reactivity of active hydrogen is reduced during the evolution of hydrogen radical → bridging hydrogen → metal hydride → hydrogen bond. The present experimental technique has the potential for size-selected infrared spectroscopic studies on a large variety of reactions between small molecules and neutral clusters, which will help to understand the structure–reactivity relationship of catalysts with isolated metal atoms/clusters dispersed on supports.

Author contributions

Ling Jiang and Gang Li conceived the idea, supervised the project and presented the project outline, guided the entire experiment, and provided funding support. Tiantong Wang conducted the experiments, calculations, and data analysis and wrote the original draft. Shangdong Li, Wenhui Yan, and Shuai Jiang conducted the experiments and data analysis. Hua Xie provided some advice for the characterization and analysis. All authors discussed the results, commented on the manuscript, and contributed to the final polishing of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the Dalian Coherent Light Source (DCLS) and Specreation Co., Ltd., for their support and assistance. This work was supported by the National Natural Science Foundation of China (grant no. 22125303, 92061203, 22373102, 22103082, 22273101, 22288201, and 21327901), the National Key Research and Development Program of China (2021YFA1400501), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (CAS) (2020187), Innovation Program for Quantum Science and Technology (2021ZD0303304), CAS (GJJSTD20220001), Dalian Institute of Chemical Physics (DICP DCLS201702), and International Partnership Program of CAS (121421KYSB20170012).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00873h

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