Chirality control of a single carbene molecule by tip-induced van der Waals interactions.

Non-covalent interactions such as van der Waals interactions and hydrogen bonds are crucial for the chiral induction and control of molecules, but it remains difficult to study them at the single-molecule level. Here, we report a carbene molecule on a copper surface as a prototype of an anchored molecule with a facile chirality change. We examine the influence of the attractive van der Waals interactions on the chirality change by regulating the tip-molecule distance, resulting in an excess of a carbene enantiomer. Our model study provides insight into the change of molecular chirality controlled by van der Waals interactions, which is fundamental for understanding the mechanisms of chiral induction and amplification.

C-C Coupling of Carbene Molecules on a Metal Surface in the Presence of Water.

A novel surface-confined C-C coupling reaction involving two carbene molecules and a water molecule was studied by scanning tunneling microscopy in real space. Carbene fluorenylidene was generated from diazofluorene in the presence of water on a silver surface. While in the absence of water, fluorenylidene covalently binds to the surface to form a surface metal carbene, and water can effectively compete with the silver surface in reacting with the carbene. Water molecules in direct contact with fluorenylidene protonate the carbene to form the fluorenyl cation before the carbene can bind to the surface. In contrast, the surface metal carbene does not react with water. The fluorenyl cation is highly electrophilic and draws electrons from the metal surface to generate the fluorenyl radical which is mobile on the surface at cryogenic temperatures. The final step in this reaction sequence is the reaction of the radical with a remaining fluorenylidene molecule or with diazofluorene to produce the C-C coupling product. Both a water molecule and the metal surface are essential for the consecutive proton and electron transfer followed by C-C coupling. This C-C coupling reaction is unprecedented in solution chemistry.

Probing surface defects of ZnO using formaldehyde.

The catalytic properties of metal oxides are often enabled by surface defects, and their characterization is thus vital to the understanding and application of metal oxide catalysts. Typically, surface defects for metal oxides show fingerprints in spectroscopic characterization. However, we found that synchrotron-radiation photoelectron spectroscopy (SRPES) is difficult to probe surface defects of ZnO. Meanwhile, CO as a probe molecule cannot be used properly to identify surface defect sites on ZnO in infrared (IR) spectroscopy. Instead, we found that formaldehyde could serve as a probe molecule, which is sensitive to surface defect sites and could titrate surface oxygen vacancies on ZnO, as evidenced in both SRPES and IR characterization. Density functional theory calculations revealed that formaldehyde dissociates to form formate species on the stoichiometric ZnO(101¯0) surface, while it dissociates to formyl species on Vo sites of the reduced ZnO(101¯0) surface instead. Furthermore, the mechanism of formaldehyde dehydrogenation on ZnO surfaces was also elucidated, while the generated hydrogen atoms are found to be stored in ZnO bulk from 423 K to 773 K, making ZnO an interesting (de)hydrogenation catalyst.

CO2 adsorption on anatase TiO2(101) surfaces: a combination of UHV-FTIRS and first-principles studies.

The CO2 adsorption and dynamic behaviors on single crystal anatase TiO2(101) surfaces were investigated by UHV-FTIRS and first-principles calculations. The IRRAS results at 90 K show that the ν3(OCO) asymmetric stretching vibration of adsorbed CO2 exhibits band splitting at rather low CO2 coverage in p-polarized IR spectra for the IR beam incident along the [101[combining macron]] direction. Co-adsorbed CO can prevent such band splitting. Ab initio molecular dynamics (AIMD) simulations revealed that the adsorbed CO2 at finite temperature does not keep a stationary adsorption state but keeps a certain swing motion: one end of the linear CO2 molecule binds to surface Ti5c sites and the other end swings within the (010) plane with a tilted angle distribution ranging from 10° to 60° relative to the [101[combining macron]] direction. By suggesting a statistical model, we confirmed that it is the swing motion that results in the band splitting phenomenon of CO2 vibration in IR spectra. The co-adsorbed CO decreases the swing angle distribution ranging from 10° to 45° through the intermolecular interaction between CO and CO2, leading to the disappearance of CO2 band splitting.

Scenarios of polaron-involved molecular adsorption on reduced TiO2(110) surfaces.

The polaron introduced by the oxygen vacancy (Vo) dominates many surface adsorption processes and chemical reactions on reduced oxide surfaces. Based on IR spectra and DFT calculations of NO and CO adsorption, we gave two scenarios of polaron-involved molecular adsorption on reduced TiO2(110) surfaces. For NO adsorption, the subsurface polaron electron transfers to a Ti:3d-NO:2p hybrid orbital mainly on NO, leading to the large redshifts of vibration frequencies of NO. For CO adsorption, the polaron only transfers to a Ti:3d state of the surface Ti5c cation underneath CO, and thus only a weak shift of vibration frequency of CO was observed. These scenarios are determined by the energy-level matching between the polaron state and the LUMO of adsorbed molecules, which plays a crucial role in polaron-adsorbate interaction and related catalytic reactions on reduced oxide surfaces.

Formation and evolution of orientation-specific CO2 chains on nonpolar ZnO(10͞10) surfaces.

Clarifying the fundamental adsorption and diffusion process of CO2 on single crystal ZnO surfaces is critical in understanding CO2 activation and transformation over ZnO-based catalysts. By using ultrahigh vacuum-Fourier transform infrared spectroscopy (UHV-FTIRS), we observed the fine structures of CO2 vibrational bands on ZnO(100) surfaces, which are the combinations of different vibrational frequencies, originated from CO2 monomer, dimer, trimer and longer polymer chains along [0001] direction according to the density functional theory calculations. Such novel chain adsorption mode results from the relatively large attractive interaction between CO2 and Zn3c atoms in [0001] direction. Further experiments indicate that the short chains at low coverage evolve into long chains through Ostwald ripening by annealing. At higher CO2 coverage (0.7 ML), the as-grown local (2 × 1) phase of chains first evolve into an unstable local (1 × 1) phase below 150 K, and then into a stable well-defined (2 × 1) phase above 150 K.

Manipulating the charge state of Au clusters on rutile TiO2(110) single crystal surfaces through molecular reactions probed by infrared spectroscopy.

The charge state of Au clusters deposited on rutile TiO2(110) single crystal surfaces was studied by UHV-FTIRS using CO as a probe. The as-deposited Au clusters on oxidized TiO2(110) surfaces are electrically neutral and are identified by the 2105-2112 cm(-1) vibrational frequency of adsorbed CO depending on Au coverage. Annealing Au/TiO2(110) in a moderate O2 atmosphere at 400 K blue shifts the CO vibrational frequency by only 2-3 cm(-1) both on bare TiO2(110) surfaces and on Au clusters. However, NO exposure blue shifts the CO vibrational frequency by 16-26 cm(-1) for CO adsorbed on Au atoms near the interface and by 3-4 cm(-1) for CO adsorbed on top of Au clusters. As the acceptors of the intense charge transfer from Au, the Oa atoms generated through (NO)2→ N2O + Oa reactions on the small fraction of the bare TiO2(110) surface reside around the Au/TiO2(110) interface perimeter, causing the neutral Au(0) to be cationic Au(δ+) states. This is a new approach to manipulate the charge state of Au clusters on oxide surfaces, which may be helpful in regulating the catalytic redox reactions on oxide supported metal systems.

Adsorption and interaction of CO2 on rutile TiO2(110) surfaces: a combined UHV-FTIRS and theoretical simulation study.

CO2 adsorption and interaction on rutile TiO2(110) surfaces was studied by UHV-FTIRS combined with theoretical simulations. With increasing CO2 exposure, CO2 adsorbs in succession at the oxygen vacancy (Vo) sites, on the five-coordinated Ti cation (Ti5c) sites and the bridging oxygen (Obr) sites at low temperature. The coupling has occurred between neighboring CO2 adsorbed on Ti5c sites from rather low CO2 coverage (∼0.5 ML), leading the ν3(OCO) asymmetric stretching vibrations to split into two absorption bands in IR spectra. Two kinds of coupled geometries of adjacent CO2 on Ti5c sites are determined by theoretical simulations. For the higher CO2 coverage (∼1.5 ML), the horizontal adsorption configuration along the [11[combining macron]0] azimuth of CO2 adsorbed on Obr sites is identified for the first time using polarization- and azimuth-resolved RAIRS in experiments. The significant deviation of CO2 from the top of Obr sites demonstrates the strong coupling between CO2 adsorbed on Obr and Ti5c sites.

UHV-FTIRS studies on molecular competitive adsorption: 12CO, 13CO and CO2 on reduced TiO2(110) surfaces.

Competitive adsorption of prototype molecules such as (12)CO, (13)CO and CO2 at the two typical fivefold coordinated Ti5c(4+) cation sites of reduced rutile TiO2(110) surfaces was studied in a newly designed UHV-FTIR system. The measured binding energies of (12)CO, (13)CO or CO2 adsorbed at two kinds of Ti5c(4+) sites are different. The molecular occupying probability at these sites depends on the binding energy of the adsorbed molecules; while, the molecular exchanging probability at these sites depends on their binding energy difference due to the presence of competitive adsorption. A simple thermodynamic equilibrium model was proposed to qualitatively interpret the adsorption and competitive adsorption mechanisms. These results will contribute to the elucidation of the (photo)catalytic process on TiO2(110) surfaces.

NO adsorption and reaction on single crystal rutile TiO2(110) surfaces studied using UHV-FTIRS.

The adsorption and reaction of NO on both the oxidized and reduced single crystal rutile TiO2(110) surfaces were studied in a UHV-FTIRS system at low temperature. The monodentate adsorption configuration of the cis-(NO)2 dimer at bridge oxygen vacancy (Vo) sites was detected for the first time on reduced TiO2(110) surfaces. With the aid of (NO)2 dimer adsorption anisotropy, the bidentate configuration of the cis-(NO)2 dimer on fivefold coordinated Ti5c(4+) cation sites was clearly confirmed. The (NO)2 dimer converts to N2O on Ti5c(4+) cation sites at higher NO dosage on both oxidized and reduced surfaces, rather than at Vo sites. The (NO)2 → N2O conversion is independent of the presence of Vo on TiO2(110) surfaces. To explain the signs of absorption bands of the dimer monodentate configuration, the local optical constant at Vo sites was introduced.