Self-Discrimination of Enantiomers in Hydrogen-Bonded Dimers

The homochiral and heterochiral hydrogen-bonded (HB) dimers of a set of small model molecules (α-amino alcohols) have been studied by means of ab initio methods. The gas-phase calculations have been carried out with the hybrid HF/DFT B3LYP method and the 6-311++G** basis set. The electron density of the complexes has been analyzed using the atoms in molecules (AIM) methodology, which allows characterization of the HB interactions and additional intermolecular contacts. To take into account the water solvation effect, the polarized continuum model (PCM) method has been used to evaluate the ΔGsolv. The gas-phase results show that the heterochiral dimers are the most stable ones for each case studied, while in solution for several cases, the relative stability is reversed and the homochiral dimers become more stable. The AIM analysis shows the typical bond critical points characteristic of the HB and additional bond critical points denoting, in this case, destabilization of intermolecular interaction as CF3···F3C and CH3···H3C contacts

Aromatic Systems as Charge Insulators: Their Simultaneous Interaction with Anions and Cations

A theoretical approach, using ab initio MP2(full)/6-31++G** and MP2(full)/6-311++G** levels, has been used to characterize the interaction of the π-cloud of C6F6 with cations (Y+ = Li+, Na+, and K+). In addition, the situation where C6H6 and C6F6 simultaneously interact with an anion (X- = F-, Cl-, and Br-) and a cation in opposite faces of the aromatic ring has been studied. For comparative purposes, other dispositions, such as those of the isolated cations and the anions and the complexes between neutral XY salts and the aromatic systems, have been considered. Complexes where the π-cloud of the aromatic ring interacts with a cation or, simultaneously, with a cation and an anion have been found to be mimima structures. However, these complexes show high relative energies when compared to other minima of the potential hypersurface. The interaction energy has been decomposed into MEP, polarization and charge−charge interaction terms

Influence of Intermolecular Hydrogen Bonds on the Tautomerism of Pyridine Derivatives

The effect of the dimerization, by hydrogen-bond (HB) complexation, on the tautomerism of 2-hydroxypyridine and a series 2-aminopyridines has been carried using ab initio methods. The results obtained for 2-hydroxypyridine fit satisfactorily with the experimental data and show that the 2-pyridone/2-pyridone homodimer complex is the most stable. For 2-aminopyridines, the effect of the substituent on the amino group has been investigated. For the monomers studied, the most favorable tautomer is the 2H; however, with electronegative substituents, the 1H/1H homodimers are more stable than the corresponding 2H/2H ones. The atom in molecule methodology has been used to characterize the HBs formed. Exponential relationships have been found between the electron density and its laplacian at the HB critical point vs the HB distance

Theoretical calculations of the chemical shifts of cyclo[n]phosphazenes for n = 2, 3, 4 and 5 (X₂PN)_n with X = CH₃, F, Cl and Br: the effect of relativistic corrections

Di, tri, tetra and pentacyclophosphazenes substituted on the phosphorus atoms by CH3, F, Cl and Br atoms corresponding to (X2PN)n structures have been studied theoretically at the B3LYP/6-311++G(d,p) level. After a brief discussion of their geometries comparing them to those of the conjugated carbocycles, (CH)n, of the same size, the absolute shieldings calculated with the GIAO and ZORA approximations will be reported. For the Cl and mainly for the Br substituted cyclo[n]-phosphazenes, relativistic corrections are absolutely necessary for 31P and useful for 15N chemical shifts.</p

Competition between Nonclassical Hydrogen-Bonded Acceptor Sites in Complexes of Neutral AH₂ Radicals (A = B, Al, and Ga): A Theoretical Investigation

An ab initio computational study of the properties of the neutral AH2 radicals (A = B, Al, Ga) as hydrogen-bond (HB) acceptors, with H−X (X = F, Cl, Br, CN, and CCH) as HB donors, is carried out at the UMP2/6-311++G(2d,2p) level. Two different minima have been found for each of the 15 possible dimers. One structure corresponds to a single-electron hydrogen-bonded complex (SEHB), with the A atom acting as an HB acceptor. The second corresponds to a dihydrogen bond complex between one of the hydrogen atoms of AH2 and the H−X molecule. Thus, all the atoms of the neutral AH2 molecule can act as HB acceptors and none as donors. The stability of the SEHB complexes decreases as BH2 > AlH2 > GaH2, while for the dihydrogen-bonded complexes the order is AlH2 > GaH2 > BH2. For the BH2 radical the SEHB complexes are stronger than the dihydrogen bonded ones, while the opposite is found for the AlH2 and GaH2 systems. Regarding the HB donors, the order found for the binding energy in the two types of complexes is H2A···HF > H2A···HCl > H2A···HBr > H2A···HCN > H2A···HCCH

Interaction of Protein Backbone with Nucleic Acid Bases^†

A theoretical study of the hydrogen-bonded (HB) complexes between a protein model and nucleic acid bases (NAB) has been carried out. As protein models, N-formylglycinamide (For-Gly-NH2, 2-formylaminoacetamide), 1, in β- and γ-conformations and as NABs, the isolated ones, and the AU, GC dimers in the Watson−Crick (WC) disposition have been considered. Only those dispositions with a double HB between the protein model and the nucleic acid bases have been studied. The aromatic CH groups of the nucleic acids have been included as HB donor. The results indicate that the strongest HBs between the individual NAB and the protein models involve the atoms that participate in the formation of the WC dimers. In the trimeric complexes, no significant preference is obtained for the 1-AU trimers studied while in the 1-GC ones the complex where formylglycinamide interacts simultaneously with the carbonyl group of guanine and the amino of cytosine is favored. The electron density of the complexes has been analyzed using the atoms in molecules methodology, finding exponential relationships between the electron density and its Laplacian vs the bond distance. Finally, the effect in the nuclear chemical shielding due to the complexation has been explored. Exponential relationships have been found for the variation of the chemical shift of the 1H signal for the NH···O and NH···N interactions with the HB distance

Linking the Interatomic Exchange-Correlation Energy to Experimental <i>J</i>‑Coupling Constants

The main aim of the current work is to find an experimental connection to the interatomic exchange-correlation energy as defined by the energy decomposition method Interacting Quantum Atoms (IQA). A suitable candidate as (essentially) experimental quantity is the nuclear magnetic resonance (NMR) J-coupling constant denoted 3J(H,H′), which a number of previous studies showed to correlate well with QTAIM’s delocalization index (DI), which is essentially a bond order. Inspired by Karplus equations, here, we investigate correlations between 3J(H,H′) and a relevant dihedral angle in six simple initial compounds of the shape H3C-YHn (Y = C, N, O, Si, P, and S), N-methylacetamide (as prototype of the peptide bond), and five peptide-capped amino acids (Gly, Ala, Val, Ile, and Leu) because of the protein direction of the force field FFLUX. In conclusion, except for methanol, the inter-hydrogen exchange-correlation energy Vxc(H,H′) makes the best contact with experiment, through 3J(H,H′), when multiplied with the internuclear distance RHH′

Effect of Dimerization and Racemization Processes on the Electron Density and the Optical Rotatory Power of Hydrogen Peroxide Derivatives

The variation of the electron density properties and optical rotatory power of the monomers and dimers of seven monosubstituted hydrogen peroxide derivatives, HOOX (X = CCH, CH3, CF3, t-Bu, CN, F, Cl), upon racemization has been studied using DFT (B3LYP/6-31+G**) and MP2 (MP2/6-311+G**) methods. The geometrical results have been rationalized on the basis of natural bond orbital (NBO) analysis. The atomic partition of the electron density properties within the atoms in molecules (AIM) methodology has allowed investigating the energy and charge redistribution in the different structures considered. The calculated optical rotatory power (ORP) of the dimers are, in general, twice of the values obtained for the monomers

Cooperative and Diminutive Unusual Weak Bonding In F₃CX···HMgH···Y and F₃CX···Y···HMgH Trimers (X = Cl, Br; Y = HCN, and HNC)

MP2 calculations with cc-pVTZ basis set were used to analyze intermolecular interactions in F3CX···HMgH···Y and F3CX···Y···HMgH triads (X = Cl, Br; Y = HCN, and HNC) which are connecting with three kinds of unusual weak interactions, namely halogen−hydride, dihydrogen, and σ-hole. To understand the properties of the systems better, the corresponding dyads are also studied. Molecular geometries, binding energies, and infrared spectra of monomers, dyads, and triads were investigated at the MP2/cc-pVTZ computational level. Particular attention is given to parameters such as cooperative energies, cooperative dipole moments, and many-body interaction energies. Those complexes with simultaneous presence of a σ-hole bond and a dihydrogen bond show cooperativity energy ranging between −1.02 and −2.31 kJ mol−1, whereas those with a halogen−hydride bond and a dihydrogen bond are diminutive, with this energetic effect between 0.1 and 0.63 kJ mol−1. The electronic properties of the complexes have been analyzed using the molecular electrostatic potential (MEP), the electron density shift maps, and the parameters derived from the atoms in molecules (AIM) methodology

A Computational Study of the Potential Energy Surface of Peroxyformic Acid Dimers

MP2 and M05-2x calculations with aug-cc-pVDZ basis sets were used to analyze intermolecular interactions in peroxyformic acid dimers. A total of 18 and 16 minima were located on the potential energy surface of HOOCHO dimer complexes at M05-2x and MP2 computational levels, respectively. The BSSE corrected interaction energies are in a range between 9 and 34 kJ mol−1 at the MP2/aug-cc-pVDZ computational level. The atoms-in-molecules (AIM) theory was also applied to explain the nature of the complexes. The interaction energies have been partitioned with the natural energy decomposition analysis (NEDA) showing that the most important attractive term corresponds to the charge transfer