Cooperative effects in two-dimensional ring-like networks of three-center hydrogen bonding interactions

Cooperative effects in two-dimensional cyclic networks containing intermolecular three-centered hydrogen bonding interactions of the type H1&#;A&#;H2 are investigated by means of ab intio molecular orbital and density functional theory calculations. Ring-like clusters consisting of three and up to nine monomers of the cis–cis isomer of carbonic acid H2CO3 are used as basic models, where each unit acts simultaneously as a double hydrogen-bond donor and double hydrogen-bond acceptor. Cooperative effects based on binding energies are evident for (H2CO3)n, where n goes from 2 to 9. Thus, the ZPVE-corrected dissociation energy per bifurcated hydrogen bond increases from 11.52 kcal/mol in the dimer to 20.42 kcal/mol in the nonamer, i.e., a 77% cooperative enhancement. Cooperative effects are also manifested in such indicators as geometries, and vibrational frequencies and intensities. The natural bond orbital analysis method is used to rationalize the results in terms of the substantial charge delocalization taking place in the cyclic clusters. Cooperativity seems close to reaching an asymptotic limit in the largest ring considered, n=9

Electronic Structure and Vibrational Spectra of C2B10-Based Clusters and Films

The electronic structure, total energy, and vibrational properties of C2B10H12 (carborane)molecules and C2B10 clusters formed when the hydrogen atoms are removed from carborane molecules are studied using density functional methods and a semiempirical model. Computed vibrational spectra for carborane molecules are shown to be in close agreement with previously published measured spectra taken on carborane solids. Semiconducting boron carbide films are prepared by removing hydrogen from the three polytypes of C2B10H12 deposited on various surfaces. Results from x-ray and Raman scattering measurements on these films are reported. Eleven vibrationally stable structures for C2B10 clusters are described and their energies and highest occupied and lowest unoccupied molecular orbital gaps tabulated. Calculated Raman and infrared spectra are reported for the six lowest-energy clusters. Good agreement with the experimental Raman spectra is achieved from theoretical spectra computed using a Boltzmann distribution of the six lowest-energy free clusters. The agreement is further improved if the computed frequencies are scaled by a factor of 0.94, a descrepancy which could easily arise from comparing results of two different systems: zero-temperature free clusters and roomtemperature films. Calculated energies for removal of hydrogen pairs from carborane molecules are reported

Structures and relative stability of neutral gold clusters: Au\u3csub\u3e\u3ci\u3en\u3c/i\u3e\u3c/sub\u3e (\u3ci\u3en\u3c/i\u3e=15–19)

We performed a global-minimum search for low-lying neutral clusters (Aun) in the size range of n=15–19 by means of basin-hopping method coupled with density functional theory calculation. Leading candidates for the lowest-energy clusters are identified, including four for Au15, two for Au16, three for Au17, five for Au18, and one for Au19. For Au15 and Au16 we find that the shell-like flat-cage structures dominate the population of low-lying clusters, while for Au17 and Au18 spherical-like hollow-cage structures dominate the low-lying population. The transition from flat-cage to hollow-cage structure is at Au17 for neutral gold clusters, in contrast to the anion counterparts for which the structural transition is at Au16- [S. Bulusu et al., Proc. Natl. Acad. Sci. U.S.A. 103, 8362 (2006)]. Moreover, the structural transition from hollow-cage to pyramidal structure occurs at Au19. The lowest-energy hollow-cage structure of Au17 (with C2v point-group symmetry) shows distinct stability, either in neutral or in anionic form. The distinct stability of the hollow-cage Au17 calls for the possibility of synthesizing highly stable core/shell bimetallic clusters M@Au17 (M=group I metal elements)

Global minimum search of atomic and molecular clusters

My research is focused on the search for low-lying structures of atomic and molecular clusters and on the structural evolution of medium-sized atomic clusters (boron and gold). Since the properties of clusters are size dependent, a systematic study of structural evolution of clusters from small sizes is necessary. The theoretical methods used to study the structural evolution of medium sized clusters are based on global optimizations. Basin-hopping (BH) algorithm coupled with Density Functional Theory (DFT) and minima-hopping (MH) algorithm serves as satisfactory unbiased global search techniques to explore the potential energy surface (PES) of small and medium-sized clusters. The BH algorithm is particularly effective for systems such as B 18, Au17-, where the global minimum is easy to locate, while the MH algorithm is useful for more complex system such as water clusters. The combination of photoelectron spectroscopy followed by theoretical elucidation of structure is an important tool for studying small to medium-sized clusters as shown in case of gold and boron. The experimental photoelectron spectra are used to compare with the theoretically simulated spectra for the lowest energy clusters obtained from unbiased global search methods. This strategy helped us in identifying a new class of clusters, such as hollow cages of gold (Au16-, Au17-, Au18-) and smallest nanotube of boron (B20), which can be used as building blocks for new nanomaterials. In case of complex systems such as water the strategy of using MD simulations to perturb coordinates through MH algorithm is useful because there exists a large number of low-lying/near-isoenergetic isomers, many of which have the same oxygen network (same structural family). New global minima are identified for (H2O)11 and (H2O)13 using MH algorithm

Search for global minimum geometries for medium sized germanium clusters: Ge\u3csub\u3e12\u3c/sub\u3e–Ge\u3csub\u3e20\u3c/sub\u3e

We have performed an unbiased search for the global minimum geometries of small-to-medium sized germanium clusters Gen (12≤n≤18) as well as a biased search (using seeding method) for Gen (17≤n≤20). We employed the basin-hopping algorithm coupled with the plane-wave pseudopotential density functional calculations. For each size, we started the unbiased search with using several structurally very different initial clusters, or we started the biased search with three different seeds. Irrespective of the initial structures of clusters we found that the obtained lowest-energy clusters of the size n=12–16 and 18 are the same. Among them, the predicted global minima of Gen(12≤n≤16) are identical to those reported previously [Shvartsburg et al., Phys. Rev. Lett. 83, 167 (1999)]. For n=17–20, we have identified two or three nearly isoenergetic low-lying isomers (for each size) that compete for the global minimum. Nearly all the low-lying clusters in the size range of 12≤n≤20 contain the tri-caped trigonal prism motif and are all prolate in geometry, in agreement with the experiment

Search for lowest-energy structure of Zintl dianion Si\u3csub\u3e12\u3c/sub\u3e\u3csup\u3e2-\u3c/sup\u3e, Ge\u3csub\u3e12\u3c/sub\u3e\u3csup\u3e2-\u3c/sup\u3e, and Sn\u3csub\u3e12\u3c/sub\u3e\u3csup\u3e2-\u3c/sup\u3e

We perform an unbiased search for the lowest-energy structures of Zintl dianions (Si122-, 122-, and 122-), by using the basin-hopping (BH) global optimization method combined with density functional theory geometric optimization. High-level ab initio calculation at the coupled-cluster level is used to determine relative stabilities and energy ranking among competitive low-lying isomers of the dianions obtained from the BH search. For 122-, all BH searches (based on independent initial structures) lead to the same lowest-energy structure 12a2-, a tricapped trigonal prism (TTP) with Cs group symmetry. Coupled-cluster calculation, however, suggests that another TTP isomer of Si12c2- is nearly isoenergetic with Si12a2-. For Sn122-, all BH searches lead to the icosahedral structure Ih-Sn12a2-, i.e., the stannaspherene. For Ge122-, however, most BH searches lead to the TTP-containing Ge12b2-, while a few BH searches lead to the empty-cage icosahedral structure Ih-Ge12a2- (named as germaniaspherene). High-level ab initio calculation indicates that Ih-Ge12a2- and TTP-containing Ge12b2- are almost isoenergetic and, thus, both may be considered as candidates for the lowest-energy structure at 0 K. Ge12a2- has a much larger energy gap (2.04 eV) between highest occupied molecular orbital and lowest unoccupied molecular orbital than Ge12b2-(1.29 eV), while Ge12b2- has a lower free energy than Ih-Ge12a2- at elevated temperature (\u3e980 K). The TTP-containing Si12a2- and Ge12b2- exhibit large negative nuclear independent chemical shift (NICS) value (~−44) at the center of TTP, indicating aromatic character. In contrast, germaniaspherene Ih-Ge12a2- and stannaspherene Ih-Sn12a2- exhibit modest positive NICS values, ~12 and 3, respectively, at the center of the empty cage, indicating weakly antiaromatic character

Cooperative effects in one-dimensional chains of three-center hydrogen bonding interactions

Cooperative effects in a one-dimensional network of intermolecular bifurcated hydrogen bonding interactions are investigated by means of ab initio calculations. The trans–trans conformation of the diformamide molecule is used as a basic motif to model a chain of bifurcated H bonds. In this model system, the two proton–acceptor atoms belong to the same molecule. The one-dimensional network is modeled then by periodically stacking up to 12 molecules of the unit motif. Different indicators of H-bond strength such as energetic, structural, dielectric, vibrational frequencies, and isotropic chemicals shifts consistently show significant cooperative effects in the chains. The dissociation energy in the dimer is calculated to be 9.88 kcal/mol, while that of the strongest interaction in the decamer is calculated to be 26.12 kcal/mol (164% increase in cooperativity). Thus, although three-center H bonds can be viewed as a consequence of proton deficiency, in some cases they may also be viewed as the natural result of an interaction that is itself energetically favorable and capable of competing with the more conventional two-center H bonds. Natural bond orbital analysis reveals substantial charge delocalization within each molecule, and charge transfer along the chains. Interestingly, this charge delocalization makes the system a good candidate for resonance-assisted H bonding which in turn increases the covalent character of this type of bifurcated H-bonding interaction