6‐31G* basis set for third‐row atoms

Medium basis sets based upon contractions of Gaussian primitives are developed for the third‐row elements Ga through Kr. The basis functions generalize the 6‐31G and 6‐31G* sets commonly used for atoms up to Ar. A reexamination of the 6‐31G* basis set for K and Ca developed earlier leads to the inclusion of 3d orbitals into the valence space for these atoms. Now the 6‐31G basis for the whole third‐row K through Kr has six primitive Gaussians for 1s, 2s, 2p, 3s, and 3p orbitals, and a split‐valence pair of three and one primitives for valence orbitals, which are 4s, 4p, and 3d. The nature of the polarization functions for third‐row atoms is reexamined as well. The polarization functions for K, Ca, and Ga through Kr are single set of Cartesian d‐type primitives. The polarization functions for transition metals are defined to be a single 7f set of uncontracted primitives. Comparison with experimental data shows good agreement with bond lengths and angles for representative vapor‐phase metal complexes. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 976–984, 2001     

Theoretical and Experimental 27Al NMR Chemical Shift Studies on End‐Group Aluminates Linked to Different Silicate Species

Aluminium‐27 nuclear magnetic resonance spectroscopy (²⁷Al NMR) does not yield fully resolved spectra for end‐group aluminates attached to different silicate species. For example, the chemical shifts of all q¹ (end‐group aluminates) lying in the range δ ≈ 72.2–77 ppm are observed on average at δ ≈ 74.6 ppm in an NMR spectrum. With the aim of better characterisation of different q¹ linked units, we first made some attempts to resolve the signals that might arise from different q¹ linkages through the application of suitable experimental methods and then followed up this work by applying some calculation methods. Some of the observed differences and evidence have not been previously detected experimentally, but were clearly apparent from our calculations (Table 1) as well as in our experimental data. Therefore, in such a situation, theoretical calculations of NMR chemical shifts can be very helpful and can provide some missing information.     

Topology of Electron Charge Density for Chemical Bonds from Valence Bond Theory: A Probe of Bonding Types

Covalent, ionic, or something new? A new interpretation of the topology of the electron density at the bond critical point is proposed to characterize covalent, ionic, and charge‐shift bonding from the density point of view (see figure). The topological properties of the density representation confirm the reality of charge‐shift bonds, in which the covalent contribution is weak or repulsive, and most of the bonding is due to the covalent–ionic resonance energy.

     

Geometry prediction of bridged zirconocene dichlorides by quantum chemical methods

The ab initio Hartree–Fock theory has been demonstrated to give accurate geometry predictions for bridged zirconocene dichlorides. Equilibrium geometries of crystallographically characterized bridged zirconocene dichlorides were optimized by Hartree–Fock, MP2, BLYP, and B3LYP methods, with basis sets ranging from 3‐21G* to 6‐311G**. Selected geometrical parameters were compared with experimental crystal structures. The least expensive HF/3‐21G* method proved to be notably accurate. The accuracy of HF/3‐21G* was verified by a comprehensive data set of 62 bridged zirconocene dichlorides. Furthermore, experimental corrections were applied to the optimized geometry parameters to eliminate systematic deviations. Corrections resulted in considerably improved accuracy for systematically overestimated metal–ligand distances, with maximum deviation falling from 0.081 to 0.039 Å, and absolute average deviations from 0.048 to 0.012 Å. Ligand–metal–ligand angles were predicted accurately with absolute average deviations of 0.7–1.3°. Zirconium–chlorine distances and chlorine–zirconium–chlorine angles are relatively constant in the studied molecules. Zirconium–cyclopentadienyl distances can be influenced mainly by modifying the ligand structure, whereas cyclopentadienyl–zirconium– cyclopentadienyl angles and cyclopentadienyl–cyclopentadienyl plane angles can be controlled by bridge modifications. The HF/3‐21G* method can be applied for the estimation of steric effects in zirconocene catalyzed polymerization reactions, therefore being suitable for the construction of structure–polymerization property correlations. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 51–64, 2001     

Contributions of Various Noncovalent Bonds to the Interaction between an Amide and S‐Containing Molecules

N‐Methylacetamide, a model of the peptide unit in proteins, is allowed to interact with CH₃SH, CH₃SCH₃, and CH₃SSCH₃ as models of S‐containing amino acid residues. All of the minima are located on the ab initio potential energy surface of each heterodimer. Analysis of the forces holding each complex together identifies a variety of different attractive forces, including SH???O, NH???S, CH???O, CH???S, SH???π, and CH???π H‐bonds. Other contributing noncovalent bonds involve charge transfer into σ* and π* antibonds. Whereas some of the H‐bonds are strong enough that they represent the sole attractive force in several dimers, albeit not usually in the global minimum, charge‐transfer‐type noncovalent bonds play only a supporting role. The majority of dimers are bound by a collection of several of these attractive interactions. The SH???O and NH???S H‐bonds are of comparable strength, followed by CH???O and CH???S.     

IR, Raman and SERS spectra of 2-phenoxymethylbenzothiazole

The FT-IR and FT-Raman spectra of 2-phenoxymethylbenzothiazole were recorded and analyzed. The surface enhanced Raman scattering (SERS) spectrum was recorded in a silver colloid. The vibrational wavenumbers of the compound have been computed using the Hartree–Fock/6-31G* basis and compared with the experimental values. The appearance of the Ag–O stretching mode at 237 cm⁻¹ in the SERS spectrum along with theoretically calculated atomic charge density, leads us to suggest that the molecule is adsorbed through the oxygen atom with the molecular plane tilted on the colloidal silver surface. The direction of charge transfer contribution to SERS has been discussed from the frontier orbital theory.     

Theoretical study of one‐carbon unit transfer between methyl‐AICA and N1‐methyl‐N1‐acryloyl‐formamide

The mechanism of one‐carbon unit transfer between 1‐methyl‐5‐amino‐4‐carboxamide imidazole (M‐AICA) and N¹‐methyl‐N¹‐acryloyl‐formamide (the model molecule of 10‐f‐H4F) is investigated by the Hartree–Fock and DFT methods, respectively, at the 6‐31G* basis level. There are two different channels for the proton transfer, resulting in two reaction pathways with different properties. The results indicate that both channels can complete the reaction, but path a is slightly favored due to its lower active energy barrier. Furthermore, the influence of 4‐carboxamindde in M‐AICA is also discussed. This group can stabilize the reactant and intermediates, and reduce the active energy barrier through the intermolecular hydrogen bond. The intermolecular hydrogen bond results in an enlarged conjugation system and makes the transition states more stable. Our results are in agreement with experiments. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

The Inverted Bond in [1.1.1]Propellane is a Charge‐Shift Bond

Bonded or not bonded? An ab initio valence bond study of [1.1.1]propellane shows that the two bridgehead carbons are linked by a strong and direct σ bond that is neither classically covalent nor classically ionic, but rather a charge‐shift bond, in which the covalent–ionic resonance energy plays the major role. As such, the central bond of [1.1.1]propellane closely resembles the single bond of difluorine.

     

Double Pancake Versus Long Chalcogen–Chalcogen Bonds in Six‐Membered C,N,S‐Heterocycles

The double “pancake” bonding in the dimers of the six‐membered heterocycles 1,3‐dithia‐2,4,6‐triazine ( 4 ) and 1,3‐dithia‐2,4‐diazine ( 16 ) were investigated by means of high‐level quantum chemical calculations (B3LYP and CCSD(T)). It was found that the S–S dimers, 20 a and 27 , are not the most stable isomers, but the dimers showing short S?N ( 21 a ) and S?C ( 25 , 28 ) bonds. An investigation of the 5‐phenyl‐1,3‐dithia‐2,4,6‐triazine ( 4 b ) yields that the syn dimer with two S?S bonds (2.57 Å) is the most stable one. In this dimer, the phenyl groups are placed on top of each other. The additional dispersion energy of the phenyl rings causes a stabilization of the syn‐S–S (C_2v‐like) isomer. As a result, two weak albeit relevant single S?S bonds (2.57 Å) are predicted. These findings contradict the recently published concept of double “pancake” bonding in the dimer 4 b ₂.     

IR, Raman and SERS studies of methyl salicylate

The IR and Raman spectra of methyl salicylate (MS) were recorded and analysed. Surface enhanced Raman scattering (SERS) spectrum was recorded in silver colloid. The vibrational wave numbers of the compound have been computed using the Hartree-Fock/6-31G* basis and compared with the experimental values. SERS studies suggest a flat orientation of the molecule at the metal surface.     

Nature of Noncovalent Carbon‐Bonding Interactions Derived from Experimental Charge‐Density Analysis

In an effort to better understand the nature of noncovalent carbon‐bonding interactions, we undertook accurate high‐resolution X‐ray diffraction analysis of single crystals of 1,1,2,2‐tetracyanocyclopropane. We selected this compound to study the fundamental characteristics of carbon‐bonding interactions, because it provides accessible σ holes. The study required extremely accurate experimental diffraction data, because the interaction of interest is weak. The electron‐density distribution around the carbon nuclei, as shown by the experimental maps of the electrophilic bowl defined by a (CN)₂C?C(CN)₂ unit, was assigned as the origin of the interaction. This fact was also evidenced by plotting the Δ²ρ(r) distribution. Taken together, the obtained results clearly indicate that noncovalent carbon bonding can be explained as an interaction between confronted oppositely polarized regions. The interaction is, thus electrophilic–nucleophilic (electrostatic) in nature and unambiguously considered as attractive.     

Asymmetry and Non‐Adiabaticity in Fragmentation of Disulfide Bonds upon Electron Capture

Although it has been generally assumed that electron attachment to disulfide derivatives leads to a systematic and significant activation of the S? S bond, we show, by using [CH₃SSX] (X=CH₃, NH₂, OH, F) derivatives as model compounds, that this is the case only when the X substituents have low electronegativity. Through the use of MP2, QCI and CASPT2 molecular orbital (MO) methods, we elucidate, for the first time, the mechanisms that lead to unimolecular fragmentation of disulfide derivatives after electron attachment. Our theoretical scrutiny indicates that these mechanisms are more intricate than assumed in previous studies. The most stable products, from a thermodynamic viewpoint, correspond to the release of neutral molecules; CH₄, NH₃, H₂O, and HF. However, the barriers to reach these products depend strongly on the electronegativity of the X substituents. Only for very electronegative substituents, such as OH or F, the loss of H₂O or HF is the most favorable process, and likely the only one observed. This is possible because of two concomitant factors, 1) the extra electron for [CH₃SSX]^? (X=OH, F) occupies a σ*(S? X) MO, which favors the cleavage of the S? X bond, and 2) the activation barriers associated with the hydrogen transfer process to produce H₂O and HF are rather low. Only when the substituents are less electronegative (X=H, CH₃, NH₂) the extra electron is located in a σ*(S? S) orbital and the cleavage of the disulfide bridge becomes the most favorable process. The intimate mechanism associated with the S? S bond dissociation process also depends strongly on the nature of the substituent. For X=H or CH₃ the process is strictly adiabatic, while for X=NH₂ it proceeds through a conical intersection ( CI ) associated with the charge reorganization necessary to obtain, from a molecular anion with the extra electron delocalized in a σ*(S? S) antibonding orbital, two fragments with the proper charge localization.     

Influence of Hydrogen Bonds and Nonspecific Interactions on the Spectral and Photophysical Properties of the Excited Singlet States of 4‐Aminophthalimide in Amine Solution

The hydrogen‐bond and nonspecific interaction energies for 4‐aminophthalimide (4‐AP), often used as a probe, in the ground electronic and excited singlet states are determined using ab initio computational methods. It is shown that the 4‐AP molecule can form three relatively strong hydrogen bonds with trimethylamine (TMA) and triethylamine (TEA), which leads to the formation of S₀‐complexes between the solute and solvent molecules. Only two of the hydrogen bonds with the amine group of 4‐AP change significantly their energies upon excitation and deactivation. The theoretical results are necessary to explain the spectral and unusual photophysical properties of 4‐AP in amine solutions.     

Strong CH⋅⋅⋅Halide Hydrogen Bonds from 1,2,3‐Triazoles Quantified Using Pre‐Organized and Shape‐Persistent Triazolophanes

The structures associated with halide (F^?, Cl^?, Br^?) complexation inside CH hydrogen‐bonding macrocyclic receptors, called triazolophanes, are characterized using density functional theory (DFT). The associated binding energies in the gas and solution phases are evaluated. The ruffles in the empty triazolophane become smoothed‐out upon Cl^?‐ and Br^?‐ion binding directly into the middle of the cavity. The largely pre‐organized cavity morphs into an elliptical shape to facilitate shorter hydrogen bonds in the north and south regions and longer ones west and east. The smaller F^? ion sits in, and flattens‐out, only the north (or south) region. The 1,2,3‐triazoles show shorter CH???Cl^? contacts than for the phenylenes. Both Cl^? and Br^? show the same binding geometries but Cl^? has a larger binding energy consistent with its stronger Lewis basicity. Model triads were used to decompose the overall binding energy into those of its components. In the course of this triad analysis, anion polarization was identified and its contribution to the triad???Cl^? binding energy estimated. Consequently, the binding energies for the individual aryl units within the comparatively non‐polarized triazolophanes were estimated. The 1,2,3‐triazoles are twice as strong as the phenylenes thus contributing most of the interaction energy to Cl^?‐ion binding. Therefore, the 1,2,3‐triazoles appear to approach the hydrogen bond strengths of the NH donors of pyrrole units.     

A Valence Bond Model for Electron‐Rich Hypervalent Species: Application to SFn (n=1, 2, 4), PF5, and ClF3

Some typical hypervalent molecules, SF₄, PF₅, and ClF₃, as well as precursors SF (⁴Σ^? state) and SF₂ (³B₁ state), are studied by means of the breathing‐orbital valence bond (BOVB) method, chosen for its capability of combining compactness with accuracy of energetics. A unique feature of this study is that for the first time, the method used to gain insight into the bonding modes is the same as that used to calculate the bonding energies, so as to guarantee that the qualitative picture obtained captures the essential physics of the bonding system. The ⁴Σ^? state of SF is shown to be bonded by a three‐electron σ bond assisted by strong π back‐donation of dynamic nature. The linear ³B₁ state of SF₂, as well as the ground states of SF₄, PF₅ and ClF₃, are described in terms of four VB structures that all have significant weights in the range 0.17–0.31, with exceptionally large resonance energies arising from their mixing. It is concluded that the bonding mode of these hypervalent species and isoelectronic ones complies with Coulson’s version of the Rundle–Pimentel model, but assisted by charge‐shift bonding. The conditions for hypervalence to occur are stated.     

Analysis of Proton NMR in Hydrogen Bonds in Terms of Lone‐Pair and Bond Orbital Contributions

NMR spectroscopic parameters of the proton involved in hydrogen bonding are studied theoretically. The set of molecules includes systems with internal resonance‐assisted hydrogen bonds, internal hydrogen bonds but no resonance stabilization, the acetic acid dimer (AAD), a DNA base pair, and the hydrogen succinate anion (HSA). Ethanol and guanine represent reference molecules without hydrogen bonding. The calculations are based on zero‐point vibrationally averaged molecular structures in order to include anharmonicity effects in the NMR parameters. An analysis of the calculated NMR shielding and J‐coupling is performed in terms of “chemist’s orbitals”, that is, localized molecular orbitals (LMOs) representing lone‐pairs, atomic cores, and bonds. The LMO analysis associates some of the strong de‐shielding of the protons in resonance‐assisted hydrogen bonds with delocalization involving the π‐backbone. Resonance is also shown to be an important factor causing de‐shielding of the OH protons for AAD and HSA, but not for the DNA base pair. Nitromalonamide (NMA) and HSA have particularly strong hydrogen bonds exhibiting signs of covalency in the associated J‐couplings. The analysis results show how NMR spectroscopic parameters that are characteristic for hydrogen bonded protons are influenced by the geometry and degree of covalency of the hydrogen bond as well as intra‐ and intermolecular resonance.     

Cooperative and Diminutive Effects of Pnicogen Bonds and Cation–π Interactions

The interplay between pnicogen bonds and cation–π interactions has been investigated at the MP2/aug‐cc‐pVDZ level. Interesting cooperative and diminutive effects are observed when pnicogen bonds and cation–π interactions coexist in the same complex. These effects have been analyzed in terms of the structural, energetic, and charge‐transfer properties of the complexes. The variations in electron density at critical points of the intermolecular bond have been used to analyze bond strengthening or weakening. The nature of the interactions and the mechanisms of cooperative and diminutive effects have been studied by means of symmetry‐adapted perturbation theory and molecular electrostatic potentials.     

Experimental and Quantum‐Chemical Studies on the Thermochemical Stabilities of Mercury Carbodiimide and Mercury Cyanamide

Calorimetric dissolution measurements of the solid compounds mercury carbodiimide HgNCN(I ) and mercury cyanamide HgNCN(II ) in aqueous HCl that targeted at their thermochemical stabilities show the cyanamide species HgNCN(II ) to be the more stable phase in terms of both enthalpy and Gibbs energy with an enthalpy difference of 2–3 kJ mol^?1. While the stability ranking of HgNCN(I ) and HgNCN(II ) thus perfectly matches Pearson's HSAB concept, quantum‐chemical stability predictions using common parametrizations of density functional theory appear to be fundamentally flawed. An analysis of the error is attempted on the basis of correlated wave functions for related molecules.