Halogen-π Interactions between Benzene and X2/CX4 (X = Cl, Br): Assessment of Various Density Functionals with Respect to CCSD(T).

Various types of interactions between halogen (X) and π moiety (X-π interaction) including halogen bonding play important roles in forming the structures of biological, supramolecular, and nanomaterial systems containing halogens and aromatic rings. Furthermore, halogen molecules such as X2 and CX4 (X = Cl/Br) can be intercalated in graphite and bilayer graphene for doping and graphene functionalization/modification. Due to the X-π interactions, though recently highly studied, their structures are still hardly predictable. Here, using the coupled-cluster with single, double, and noniterative triple excitations (CCSD(T)), the Møller-Plesset second-order perturbation theory (MP2), and various flavors of density functional theory (DFT) methods, we study complexes of benzene (Bz) with halogen-containing molecules X2 and CX4 (X = Cl/Br) and analyze various components of the interaction energy using symmetry adapted perturbation theory (SAPT). As for the lowest energy conformers (S1), X2-Bz is found to have the T-shaped structure where the electropositive X atom-end of X2 is pointing to the electronegative midpoint of CC bond of the Bz ring, and CX4-Bz has the stacked structure. In addition to this CX4-Bz (S1), other low energy conformers of X2-Bz (S2/S3) and CX4-Bz (S2) are stabilized primarily by the dispersion interaction, whereas the electrostatic interaction is substantial. Most of the density functionals show noticeable deviations from the CCSD(T) complete basis set (CBS) limit binding energies, especially in the case of strongly halogen-bonded conformers of X2-Bz (S1), whereas the deviations are relatively small for CX4-Bz where the dispersion is more important. The halogen bond shows highly anisotropic electron density around halogen atoms and the DFT results are very sensitive to basis set. The unsatisfactory performance of many density functionals could be mainly due to less accurate exchange. This is evidenced from the good performance by the dispersion corrected hybrid and double hybrid functionals. B2GP-PLYP-D3 and PBE0-TS(Tkatchenko-Scheffler)/D3 are well suited to describe the X-π interactions adequately, close to the CCSD(T)/CBS binding energies (within ∼1 kJ/mol). This understanding would be useful to study diverse X-π interaction driven structures such as halogen containing compounds intercalated between 2-dimensional layers.

Halides with Fifteen Aliphatic C-H···Anion Interaction Sites.

Since the aliphatic C-H···anion interaction is relatively weak, anion binding using hydrophobic aliphatic C-H (Cali-H) groups has generally been considered not possible without the presence of additional binding sites that contain stronger interactions to the anion. Herein, we report X-ray structures of organic crystals that feature a chloride anion bound exclusively by hydrophobic Cali-H groups. An X-ray structure of imidazolium-based scaffolds using Cali-H···A(-) interactions (A(-) = anion) shows that a halide anion is directly interacting with fifteen Cali-H groups (involving eleven hydrogen bonds, two bidentate hydrogen-bond-type binding interactions and two weakly hydrogen-bonding-like binding interactions). Additional supporting interactions and/or other binding sites are not observed. We note that such types of complexes may not be rare since such high numbers of binding sites for an anion are also found in analogous tetraalkylammonium complexes. The Cali-H···A(-) interactions are driven by the formation of a near-spherical dipole layer shell structure around the anion. The alternating layers of electrostatic charge around the anion arise because the repulsions between weakly positively charged H atoms are reduced by the presence of the weakly negatively charged C atoms connected to H atoms.

Disulfuric acid dissociated by two water molecules: ab initio and density functional theory calculations.

We have studied geometries, energies and vibrational spectra of disulfuric acid (H2S2O7) and its anion (HS2O7(-)) hydrated by a few water molecules, using density functional theory (M062X) and ab initio theory (SCS-MP2 and CCSD(T)). The most noteworthy result is found in H2S2O7(H2O)2 in which the lowest energy conformer shows deprotonated H2S2O7. Thus, H2S2O7 requires only two water molecules, the fewest number of water molecules for deprotonation among various hydrated monomeric acids reported so far. Even the second deprotonation of the first deprotonated species HS2O7(-) needs only four water molecules. The deprotonation is supported by vibration spectra, in which acid O-H stretching peaks disappear and specific three O-H stretching peaks for H3O(+) (eigen structure) appear. We have also kept track of variations in several geometrical parameters, atomic charges, and hybrid orbital characters upon addition of water. As the number of water molecules added increases, the S-O bond weakens in the case of H2S2O7, but strengthens in the case of HS2O7(-). It implies that the decomposition leading to H2SO4 and SO3 hardly occurs prior to the 2nd deprotonation at low temperatures.

Interactions of CO2 with various functional molecules.

The CO2 capturing and sequestration are of importance in environmental science. Understanding of the CO2-interactions with various functional molecules including multi-N-containing superbases and heteroaromatic ring systems is essential for designing novel materials to effectively capture the CO2 gas. These interactions are investigated using density functional theory (DFT) with dispersion correction and high level wave function theory (resolution-of-identity (RI) spin-component-scaling (scs) Möller-Plesset second-order perturbation theory (MP2) and coupled cluster with single, double and perturbative triple excitations (CCSD(T))). We found intriguing molecular systems of melamine, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-azaindole and guanidine, which show much stronger CO2 interactions than the well-known functional systems such as amines. In particular, melamine could be exploited to design novel materials to capture the CO2 gas, since one CO2 molecule can be coordinated by four melamine molecules, which gives a binding energy (BE) of ∼85 kJ mol(-1), much larger than in other cases.

Highly stable CO2/N2 and CO2/CH4 selectivity in hyper-cross-linked heterocyclic porous polymers.

The largest obstacles for landfill/flue gas separation using microporous materials are small adsorption values and low selectivity ratios. This study demonstrates that these adsorption and selectivity challenges can be overcome by utilizing a series of hyper-cross-linked heterocyclic polymer networks. These microporous organic polymers (MOPs) were synthesized in a single step by inexpensive Friedel-Crafts-catalyzed reactions using dimethoxymethane as an external linker. The amorphous networks show moderate Brunauer-Emmett-Teller surface areas up to 1022 m(2) g(-1), a narrow pore size distribution in the range from 6 to 8 Å, and high physicochemical stability. Owing to the presence of the heteroatomic pore surfaces in the networks, they exhibit maximum storage capacities for CO2 of 11.4 wt % at 273 K and 1 atm. Additionally, remarkable selectivity ratios for CO2 adsorption over N2 (100) and CH4 (15) at 273 K were obtained. More importantly, as compared with any other porous materials, much higher selectivity for CO2/N2 (80) and CO2/CH4 (15) was observed at 298 K, showing that these selectivity ratios remain high at elevated temperature. The very high CO2/N2 selectivity values are ascribed to the binding affinity of abundantly available electron-rich basic heteroatoms, high CO2 isoteric heats of adsorption (49-38 kJ mol(-1)), and the predominantly microporous nature of the MOPs. Binding energies calculated using the high level of ab initio theory showed that the selectivity is indeed attributed to the heteroatom-CO2 interactions. By employing an easy and economical synthesis procedure these MOPs with high thermochemical stability are believed to be a promising candidate for selective CO2 capture.

CO capture and conversion to HOCO radical by ionized water clusters.

The CO molecule can interact with the hydroxyl radical ((•)OH) via either a weak noncovalent interaction or strong covalent bonding. Upon the ionization of neutral water clusters, the resulting water cluster cations produce protonated water clusters and hydroxyl radicals. In this regard, we investigate the interactions of a CO molecule with water dimer and trimer cations using density functional theory (DFT), Möller-Plesset second-order perturbation theory (MP2), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. It is found that the reaction products of CO by the water dimer and trimer cations form a HOCO radical solvated by a protonated water monomer and dimer, respectively. These radicals are useful intermediates to make oxalic acids, formic acids, metal ligands, and so on, which is important in green chemistry.

Dynamics and structural changes of small water clusters on ionization.

Despite utmost importance in understanding water ionization process, reliable theoretical results of structural changes and molecular dynamics (MD) of water clusters on ionization have hardly been reported yet. Here, we investigate the water cations [(H2O)(n = 2-6)(+)] with density functional theory (DFT), Möller-Plesset second-order perturbation theory (MP2), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The complete basis set limits of interaction energies at the CCSD(T) level are reported, and the geometrical structures, electronic properties, and infrared spectra are investigated. The characteristics of structures and spectra of the water cluster cations reflect the formation of the hydronium cation moiety (H3O(+)) and the hydroxyl radical. Although most density functionals fail to predict reasonable energetics of the water cations, some functionals are found to be reliable, in reasonable agreement with high-level ab initio results. To understand the ionization process of water clusters, DFT- and MP2-based Born-Oppenheimer MD (BOMD) simulations are performed on ionization. On ionization, the water clusters tend to have an Eigen-like form with the hydronium cation instead of a Zundel-like form, based on reliable BOMD simulations. For the vertically ionized water hexamer, the relatively stable (H2O)5(+) (5sL4A) cluster tends to form with a detached water molecule (H2O).

Observable structures of small neutral and anionic gold clusters.

Since gold clusters have mostly been studied theoretically by using DFT calculations, more accurate studies are of importance. Thus, small neutral and anionic gold clusters (Au(n) and Au(n)(-) , n = 4-7) were investigated by means of coupled cluster with singles, doubles, and perturbative triple excitations [CCSD(T)] calculations with large basis sets, and some differences between DFT and CCSD(T) results are discussed. Interesting isomeric structures that have dangling atoms were obtained. Structures having dangling atoms appear to be stable up to n = 4 for neutral gold clusters and up to n = 7 for anionic clusters. The relative stabilities and electronic properties of some isomers and major structures are discussed on the basis of the CCSD(T) calculations. This accurate structure prediction of small gold clusters corresponding to experimental photoelectron spectral peaks is valuable in the field of atom-scale materials science including nanocatalysts.

Anion Binding by Electron-Deficient Arenes Based on Complementary Geometry and Charge Distribution.

Extended electron-deficient arenes are investigated as potential neutral receptors for polyanions. Anion binds via σ interaction with extended arenes, which are composed solely of C and N ring atoms and CN substituents. As a result, the positive charge on the aromatic C is enhanced, consequently maximizing binding strength. Selectivity is achieved because different charge distributions can be obtained for target anions of a particular geometry. The halides F(-) and Cl(-) form the most stable complex with 6, while the linear N3(-) interacts most favorably with 7. The trigonal NO3(-) and tetrahedral ClO4(-) fit the 3-fold rotational axis of 6 but do not form stable complexes with 5 and 7. The Y-shaped HCOO(-) forms complexes with 4, 5, and 7, with the latter being the most stable. Thus, the anion complexes exhibit strong binding and the best geometrical fit between guest and host, reminiscent of Lego blocks.

Structural basis of triclosan resistance.

Triclosan (5-chloro-2-(2,4-dichloro-phenoxy)-phenol, TCL) is a well known inhibitor against enoyl-acyl carrier protein reductase (ENR), an enzyme critical for cell-wall synthesis of bacteria. The inhibitory concentration at 50% inhibition (IC(50)) of TCL against the Escherichia coli ENR is 150nM for wild type (WT), 380, 470 and 68,500nM for Ala, Ser and Val mutants, respectively. To understand this high TCL resistance in the G93V mutant, we obtained the crystal structures of mutated ENRs complexed with TCL and NAD(+). The X-ray structural analysis along with the ab initio calculations and molecular dynamics simulations explains the serious consequence in the G93V mutant complex. The major interactions around TCL due to the aromatic(cation)-aromatic and hydrogen bonding interactions are found to be conserved both in WT and mutant complexes. Thus, the overall structural change of protein is minimal except that a flexible α-helical turn around TCL is slightly pushed away due to the presence of the bulky valine group. However, TCL shows substantial edge-to-face aromatic (π)-interactions with both the flexible R192-F203 region and the residues in the close vicinity of G93. The weakening of some edge-to-face aromatic interactions around TCL in the G93V mutant results in serious resistance to TCL. This understanding is beneficial to design new generation of antibiotics which will effectively act on the mutant ENRs.

H2-Binding by Neutral and Multiply Charged Titaniums: Hydrogen Storage Capacity of Titanium Mono- and Dications.

Given that transition metal-hydrogen systems have been studied as a predecessor for hydrogen storage materials, we have investigated the neutral and multiply charged titanium-H2 systems (Ti-H2, Ti(+)-H2, Ti(2+)-H2, Ti(3+)-H2, and Ti(4+)-H2) using density functional theory (DFT) and high-level ab initio calculations, including coupled cluster theory with single, double, and perturbatively triple excitations [CCSD(T)]. These systems show different types of hydrogenation depending on their charged state. The neutral Ti-H2 system shows dihydride structure with covalent interaction where the Ti-H distance is 1.76 Å, while H2 is dissociated into two neigboring hydride ions by withdrawing electrons from Ti. The charged Ti(+)-H2, Ti(2+)-H2, and Ti(3+)-H2 systems show dihydrogen structures with noncovalent interaction, where the Ti(+)-H, Ti(2+)-H, and Ti(3+)-H distances are 2.00, 2.14, and 2.12 Å, respectively. The main binding energies in these systems arise from the hydrogen polarizability driven interaction by the positive charge of Ti(n+) (n = 1-3). Among Ti(n+)-H2 (n = 1-3) the Ti(+)-H2 has the shortest distance against our common expectation, while Ti(2+)-H2 has the longest distance. The Ti(+)-H2 distance is the shortest because of the d-σ* molecular orbital (MO) interaction which is not present in Ti(2+)-H2 and Ti(3+)-H2. The Ti(4+) ion does not bind H2. In this regard, we have investigated the maximal hydrogen binding capacity by Ti complexes. The coordination of titanium mono- and dications complexed with dihydrogen (H2) [Ti(+)(H2)n and Ti(2+)(H2)m] is studied along with their structures, binding energies, electronic properties, and spectra. The titanium monocations of the quartet ground state have up to the hexacoordinaton, while titanium dications of the triplet ground state have up to the octacoordination at very low temperatures. At room temperature, the monocations favor penta- to hexacoordination, while the dications favor hexacoordination. This information would be useful for the design of hydrogen storage devices of Ti complexes, such as Ti-decorated/dispersed polymer-graphene hybrid materials.

Chiral transformation in protonated and deprotonated adipic acids through multistep internal proton transfer.

Protonated and deprotonated adipic acids (PAA: HOOC-(CH(2))(4)--COOH(2) (+) and DAA: HOOC-(CH(2))(4)-COO(-)) have a charged hydrogen bond under the influence of steric constraint due to the molecular skeleton of a circular ring. Despite the similarity between PAA and DAA, it is surprising that the lowest energy structure of PAA is predicted to have (H(2)O...H...OH(2))(+) Zundel-like symmetric hydrogen bonding, whereas that of DAA has H(3)O(+) Eigen-like asymmetric hydrogen bonding. The energy profiles show that direct proton transfer between mirror image structures is unfavorable. Instead, the chiral transformation is possible by subsequent backbone twistings through stepwise proton transfer along multistep intermediate structures, which are Zundel-like ions for PAA and Eigen-like ions for DAA. This type of chiral transformation by multistep intramolecular proton transfers is unprecedented. Several prominent OH...O short hydrogen-bond stretching peaks are predicted in the range of 1000-1700 cm(-1) in the Car-Parrinello molecular dynamics (CPMD) simulations, which show distinctive signatures different from ordinary hydrogen-bond peaks. The O-H-O stretching peaks in the range of 1800-2700 cm(-1) become insignificant above around 150 K and are almost washed out at about 300 K.

Comparison of cationic, anionic and neutral hydrogen bonded dimers.

Short Strong Hydrogen Bonds (SSHBs) play an important role in many fields of physics, chemistry and biology. Since it is known that SSHBs exist in many biological systems, the role of hydrogen bonding motifs has been particularly interesting in enzyme catalysis, bio-metabolism, protein folding and proton transport phenomena. To explore the characteristic features of neutral, anionic and cationic hydrogen bonds, we have carried out theoretical studies of diverse homogeneous and heterogeneous hydrogen bonded dimers including water, peroxides, alcohols, ethers, aldehydes, ketones, carboxylic acids, anhydrides, and nitriles. Geometry optimization and harmonic frequency calculations are performed at the levels of Density Functional Theory (DFT) and Møller-Plesset second order perturbation (MP2) theory. First principles Car-Parrinello molecular dynamics (CPMD) simulations are performed to obtain IR spectra derived from velocity- and dipole-autocorrelation functions. We find that the hydrogen bond energy is roughly inversely proportional to the fourth power of the r(O/N-H) distance. Namely, the polarization of the proton accepting O/N atom by the proton-donating H atom reflects most of the binding energy in these diverse cation/anion/neutral hydrogen bonds. The present study gives deeper insight into the nature of hydrogen-bonded dimers including SSHBs.

Rules and trends of metal cation driven hydride-transfer mechanisms in metal amidoboranes.

Group I and II metal amidoboranes have been identified as one of the promising families of materials for efficient H(2) storage. However, the underlying mechanism of the dehydrogenation of these materials is not well understood. Thus, the mechanisms and kinetics of H(2) release in metal amidoboranes are investigated using high level ab initio calculations and kinetic simulations. The metal plays the role of catalyst for the hydride transfer with formation of a metal hydride intermediate towards the dehydrogenation. In this process, with increasing ionic character of the metal hydride bond in the intermediate, the stability of the intermediate decreases, while the dehydrogenation process involving ionic recombination of the hydridic H with the protic H proceeds with a reduced barrier. Such correlations lead directly to a U-shaped relationship between the activation energy barrier for H(2) elimination and the ionicity of metal hydride bond. Oligomerized intermediates are formed by the chain reaction of the size-driven catalytic effects of metals, competing with the non-oligomerization pathway. The kinetic rates at low temperatures are determined by the maximum barrier height in the pathway (a Lambda-shaped relation), while those at moderately high temperatures are determined by most of multiple-barriers. This requires kinetic simulations. At the operating temperatures of proton exchange membrane fuel cells, the metal amidoboranes with lithium and sodium release H(2) along both oligomerization and non-oligomerization paths. The sodium amidoboranes show the most accelerated rates, while others release H(2) at similar rates. In addition, we predict that the novel metal amidoborane-based adducts and mixtures would release H(2) with accelerated rates as well as with enhanced reversibility. This comprehensive study is useful for further developments of active metal-based better hydrogen storage materials.

Structure, Stabilities, Thermodynamic Properties, and IR Spectra of Acetylene Clusters (C2H2)n=2-5.

There are no clear conclusions over the structures of the acetylene clusters. In this regard, we have carried out high-level calculations for acetylene clusters (C2H2)2-5 using dispersion-corrected density functional theory (DFT-D), Møller-Plesset second-order perturbation theory (MP2); and coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] at the complete basis set limit. The lowest energy structure of the acetylene dimer has a T-shaped structure of C2v symmetry, but it is nearly isoenergetic to the displaced stacked structure of C2h symmetry. We find that the structure shows the quantum statistical distribution for configurations between the T-shaped and displaced stacked structures for which the average angle (|θ̃|) between two acetylene molecules would be 53-78°, close to the T-shaped structure. The trimer has a triangular structure of C3h symmetry. The tetramer has two lowest energy isomers of S4 and C2h symmetry in zero-point energy (ZPE)-uncorrected energy (ΔEe), but one lowest energy isomer of C2v symmetry in ZPE-corrected energy (ΔE0). For the pentamer, the global minimum structure is C1 symmetry with eight sets of T-type π-H interactions and a set of π-π interactions. Our high-level ab initio calculations are consistent with available experimental data.

Ammonia-water cation and ammonia dimer cation.

We have investigated the structure, interaction energy, electronic properties, and IR spectra of the ammonia-water cation (NH(3)H(2)O)(+) using density functional theory (DFT) and high-level ab initio theory. The ammonia-water cation has three minimum-energy structures of (a) H(2)NH(+)...OH(2), (b) H(3)N(+)...OH(2), and (c) H(3)NH(+)...OH. The lowest-energy structure is (a), followed by (c) and (b). The ammonia dimer cation has two minimum-energy structures [the lowest H(3)NH(+)...NH(2) structure and the second lowest (H(3)N...NH(3))(+) structure]. The minimum transition barrier for the interconversion between (a), (b), and (c) is approximately 6 kcal/mol. Most DFT calculations with various functionals, except a few cases, overstabilize the N...O and N...N binding, predicting different structures from Moller-Plesset second-order perturbation (MP2) theory and the most reliable complete basis set (CBS) limit of coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. Thus, the validity test of the DFT functionals for these ionized molecular systems would be of importance.

Hydrogen-release mechanisms in lithium amidoboranes.

Alkali-metal amidoboranes have been recently highlighted as materials that satisfy many of the criteria required to make hydrogen-storage media. It is, therefore, crucial for us to understand the dehydrogenation mechanism of these materials for further development towards making successful hydrogen-storage media. In the present study, we attempt to shed light on the mechanisms involved in the loss of one molar equivalent of H(2) from solid lithium amidoboranes by using high-level ab initio calculations of monomeric and dimeric compounds in the gas phase. In the lithium amidoborane dimer, H(2) is released by the formation of LiH, which is followed by a redox reaction of the dihydrogen bond formed between the strongly basic H(-) in LiH and H(delta+) bonded to N. In the dehydrogenation process, the Li cation catalyzes the intermolecular N-B bond formation; this could lead to new pathways for N-B polymerization. After the release of the first molecule of H(2), a Li cation binds to a nitrogen atom, resulting in a lowering of the energy barrier for the second dehydrogenation process per dimer. These results will be useful for the design of future hydrogen-storage media.

Water Dimer Cation: Density Functional Theory vs Ab Initio Theory.

By using density functional theory (DFT) and high-level ab initio theory, the structure, interaction energy, electronic property, and IR spectra of the water dimer cation [(H2O)2(+)] are investigated. Two previously reported structures of the water dimer cation [disproportionated ionic (Ion) structure and hydrazine-like (OO) structure] are compared. For the complete basis set (CBS) limit of coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)], the Ion structure is much more stable (by 11.7 kcal/mol). This indicates that the ionization of water clusters produce the hydronium cation moiety (H3O(+)) and the hydroxy radical. The transition barrier for the interconversion from the Ion/OO structure is ∼15/∼9 kcal/mol. It is interesting to note that the calculation results of the water dimer cation vary seriously depending on calculation methods. Möller-Pleset second-order perturbation (MP2) theory gives reasonable relative energies in favor of the Ion structure but reports unreasonable frequencies for the OO structure. On the other hand, most DFT calculations with various functionals overstabilize the OO structure. However, the DFT results with MPW1K and BH&HLYP functionals are very close to the CCSD(T)/CBS results. Thus, as for the validity test of the DFT functionals for ionized molecular systems, the energy comparison of two water dimer cation structures would be a very important criterion.

Neutral and Anionic Gold Decamers: Planar Structure with Unusual Spatial Charge-Spin Separation.

We have investigated the issue of two-dimensional (2D) versus three-dimensional (3D) structures for neutral-state Au10 and clarified the lowest-energy structure among a few 2D Au10(-) isomers. Though almost all previous works were based on density functional theory (DFT), we here carried out not only extensive DFT calculations but also high levels of ab initio calculations of Möller-Plesset second order perturbation theory (MP2), and coupled cluster theory with single and double excitations (CCSD) including perturbative triple excitations [CCSD(T)]. While DFT favors 2D structures, MP2 and CCSD(T) favor 3D structures for moderate-sized basis sets. However, we note that the basis-set superposition error (BSSE) corrections make the ab intio results favor 2D structures too. The near-degeneracy (driven by relativistic effects) of 5d and 6s orbitals of gold helps stabilize acute apex gold atoms, resulting in 2D structures. The planar triangular structures of a local minimum Au10 (triplet) and the global minimum Au10(-) show remarkable spatial charge-spin separation due to their singly occupied molecular orbital(s). By the same reason, Au10(-) shows much larger vertical detachment energy than other even-numbered gold cluster anions.

Interaction of Benzene with Transition Metal Cations: Theoretical Study of Structures, Energies, and IR Spectra.

The cation-π interactions have been intensively studied. Nevertheless, the interactions of π systems with heavy transition metals and their accurate conformations are not well understood. Here, we theoretically investigate the structures and binding characteristics of transition metal (TM) cations including novel metal cations (TM(n+) = Cu(+), Ag(+), Au(+), Pd(2+), Pt(2+), and Hg(2+)) interacting with benzene (Bz). For comparison, the alkali metal complex of Na(+)-Bz is also included. We employ density functional theory (DFT) and high levels of ab initio theory including Møller-Plesset second-order perturbation (MP2) theory, quadratic CI method with single and double substitutions (QCISD), and the coupled cluster theory with single, double, and perturbative triple excitations (CCSD(T)). Each of the transition metal complexes of benzene exhibits intriguing binding characteristics, different from the typical cation-π interactions between alkali metal cations and aromatic rings. The complexes of Na(+), Cu(+), and Ag(+) favor the conformation of C6v symmetry with the cation above the benzene centroid (πcen). The formation of these complexes is attributed to the electrostatic interaction, while the magnitude of charge transfer has little correlation with the total interaction energy. Because of the TM(n+)←π donation, cations Au(+), Pd(2+), Pt(2+), and Hg(2+) prefer the off-center π conformation (πoff) or the π coordination to a C atom of the benzene. Although the electrostatic interaction is still important, the TM←π donation effect is responsible for the binding site. The TM(n+)-Bz complexes give some characteristic IR peaks. The complexes of Na(+), Cu(+), and Ag(+) give two IR active modes between 800 and 1000 cm(-1),which are inactive in the pure benzene. The complexes of Au(+), Pd(2+), Pt(2+), and Hg(2+) give characteristic peaks for the ring distortion, C-C stretching, and C-H stretching modes as well as significant red-shifts in the CH out-of-plane bending.