A computational study of the linear and nonlinear complexes of FArCCH···N2

Linear and nonlinear weakly bound complexes of the metastable argon-containing compound FArCCH and N₂ were predicted to be stable from ab initio computations at the MP2/6-311++G(2d,2p) level of theory. Both complexes were found to have similar interaction energies and they exhibit red shifts of the C–H stretch, blue shifts of the Ar–C stretch and red shifts of the F–Ar stretch, in the same order as the stabilities. These effects are rationalized by considering changes in orbital occupancies on complexation, which are brought about by a combination of the intermolecular electrostatic interaction and charge density transferred from the N₂ molecule to the argon-containing molecule.     

A computational study of N2···HHeF: comparison with N2···HArF and N2···HKrF

A weakly bound linear complex of N₂ and HHeF was found to be stable with respect to the constituent monomers by ab initio calculations at various levels of theory (MP2, MP3, MP4(SDQ) and QCISD) using a 6-311++G(2d,2p) basis set. The complex N₂···HHeF was found to have a zero-point vibrational energy corrected binding energy of 14.5?kJ mol^?1 (QCISD) and exhibits a large harmonic vibrational frequency blue shift of 375?cm^?1 for the He–H stretching vibration mode, with a diminished infrared intensity for this mode on formation of the complex. The frequency shift for this mode was also found to be very sensitive to the level of theory employed for the calculation, and is rationalized by considering intermolecular electrostatic and charge-transfer effects. The results for N₂···HHeF are compared with corresponding results for the related complexes N₂···HArF and N₂···HKrF, both of which contain the same proton acceptor molecule.     

A theoretical study of N–H ··· π H-bond interaction of pyrrole: from clusters to the liquid

The N–H?···?π H-bond interactions of clusters and liquid formed by pyrrole molecules were investigated by Quantum Mechanics (QM) and classical Molecular Dynamics (MD), respectively. Based on the optimized geometry at the B97-D/aug-cc-pVTZ level of theory including dispersion correction, the nature and the origin of N–H?···?π H-bond interactions were unveiled by atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analysis (EDA). Among them, the AIM analysis gives evidence to the presence of N–H?···?π H-bond interactions, the NBO examination reveals that π?→?σ* donor-acceptor orbital interaction is of great importance. EDA study indicates that N–H?···?π interactions are governed by the electrostatic and dispersion term. Meanwhile, MD simulation with OPLS-AA (optimized potentials for liquid simulations all-atom) was applied to study the pure liquid pyrrole at different temperature. The results confirm the existence of the N–H?···?π H-bond in the pure liquid pyrrole, and further characterized the structures of this H-bond which is somewhat different to the clusters.     

Effective orientation of water in 1,4-dioxane···water: the rotational spectrum of the H2 17O isotopologue

The measurements of the rotational spectrum of 1,4-dioxane-water complex, performed by pulsed jet Fourier transform microwave spectroscopy, have been extended to the 7–18.5 GHz frequency region and to two additional isotopologues, 1,4-dioxane···H₂ ¹⁷O and 1,4-dioxane···HOD. The effective orientation of water in the complex has been obtained from the ¹⁷O quadrupole coupling constants.     

Ab initio intermolecular potential energy surface of Ne···NCCN van der Waals complex: effect of the place of midbond function on the interaction

The intermolecular potential energy surface of Ne···NCCN van der Waals complex was evaluated in the framework of the counterpoise-corrected supermolecular approach using CCSD(T) level and aug-cc-pVDZ basis set extended with a set of midbond (3s3p2d1f1g) functions. The effect of the place of midbond function on the accuracy of the calculated potential energy surface was examined and the optimised position for placing midbond function was determined. The calculated potential energy surface was fitted by an analytical function. The analytical function of intermolecular potential energy surface of Ne···NCCN demonstrated a global minimum energy of ?12.024 meV related to the T-shape geometry at the distance between Ne and the centre of mass of NCCN of 3.28 Å. Finally, the interaction second virial coefficients (B₁₂) of Ne and NCCN were calculated and used to calculate the second virial coefficients for the mixture of neon and cyanogen gases at different mole fractions of Ne gas.     

Number of 59·5 keV-photons per decay of241Am

Number of 59·5 keV-photons per decay of²⁴¹Am equal to 0·355±0·003 was determined from activity and gamma-emission rate measurements.     

Mechanism of solid-state oxidation of FeSO4·H2O: model of simultaneous reactions

The mechanism of the thermal transformation of FeSO₄·H₂O in air has been studied under isothermal conditions at temperatures (150–460)°C using mainly⁵⁷Fe Mössbauer spectroscopy and X-ray powder diffraction (XRD). Two trends are typical for the thermal behaviour of FeSO₄·H₂O in air, a tendency toward oxidation and dehydration. We suggested a new transformation model consisting of two ways of oxidation, direct one and indirect one. Fe(OH)SO₄ was identified as a product of the direct way, Fe₂(SO₄)₃ and superparamagnetic nanoparticles ofγ-Fe₂O₃ as products of the indirect way. The suggested model of simultaneous reactions explains the unusual non-monotonous dependence of the oxidation level of the thermally treated samples on temperature.     

Ab initio study of synergetic effects of two strong interactions of cation–π interaction and lithium bond in M+ ··· phenyl lithium ··· N (M = Li,Na, K; N = H2O and NH3) complex

Quantum chemical calculations have been performed to investigate the interplay between the cation–π interaction and lithium bonding in the M⁺?···?phenyl lithium?···?OH₂ and M⁺?···?phenyl lithium?···?NH₃ (M?=?Li, Na, K) complexes. The cation–π interaction and lithium bonding in the trimers become stronger relative to the dimers. The interaction energy of cation–π interaction is increased by about 4.4–6.3%, while that of lithium bonding is increased by about 5.2–15.9%. The cooperative energy becomes larger for the stronger cation–π interaction and lithium bond. The F atom and methyl group in the phenyl ring impose a reverse effect on the cation–π interaction and lithium bond. The interaction mechanism in the complexes has been understood with the many-body interaction analysis, electrostatic potentials, and energy decomposition.     

Chalcogen bonds tuned by an N–H···π or C–H···π interaction: investigation of substituent,cooperativity and solvent effects

In the present work, ab-initio calculations are performed to investigate cooperativity effects between chalcogen bond and H···π interactions in XHY···NCH···C₆H₆ and XHY···CNH···C₆H₆ complexes, where X = F, Cl, Br, CN, NC, and Y = S, Se. The nature of these interactions and the mechanism of cooperativity are studied by means of quantum theory of atoms in molecules, noncovalent interaction index, many-body analysis of interaction energy and electron density shift analysis. For each ternary complex, the shortening of the Y···N(C) distance is more pronounced than that of the H···π. The cooperative energies of these complexes are all negative which demonstrate a positive cooperativity between the Y···N(C) and H···π interactions. The many-body analysis of interaction energy reveals that the two-body energy term has the largest contribution to the total interaction energies of ternary complexes. A good linear correlation is established between the three-body energy and cooperative energy values in the ternary systems. The cooperative energies of XHY···CNH···C₆H₆ complexes indicate a larger sensitivity on the polarity of solvent than XHY···NCH···C₆H₆ ones.     

Structural and spectroscopic investigation of the N-methylformamide–water (NMF···3H2O) complex

In this work, theoretical studies on the structure, molecular properties, hydrogen bonding, and vibrational spectra of the N-methylformamide–water (NMF···3H₂O) complex will be presented. The molecular geometry was optimised by using Hartree–Fock (HF), second Møller–Plesset (MP2), and density functional theory methods with different basis sets. The harmonic vibrational frequencies are computed by using the B3LYP method with 6-311++G(d,p) as a basis set and then scaled with a suitable scale factor to yield good coherence with the observed values. The temperature dependence of various thermodynamic functions (heat capacity, entropy, and enthalpy changes) was also studied. A detailed analysis of the nature of the hydrogen bonding, using natural bond orbital (NBO) and topological atoms in molecules theory, has been reported.     

Cross sections of the interactions of4He nuclei with protons at 8·6 and 13·6 GeV/c

⁴He-p collisions at two values of⁴He momenta-8·6 GeV/c and 13·6 GeV/c — have been studied using the one-metre JINR hydrogen bubble chamber. Total, elastic and topological cross sections have been measured. The results are in agreement with other published data. The elastic differential cross section was compared with the predictions of the Glauber model formalism.Dedicated to Academician Václav Votruba on ths occasion of his eightieth birthday.We would like to thank Mrs. M. Stehlíková for drawing the figures.     

Relaxation mechanisms of the Zeeman sublevels of the phosphorescent triplet state of pyrazine at 1·6K

The observed change in the phosphorescence decay of the spin-aligned triplet state of pyrazine at 1·6K upon application of a magnetic field is examined both experimentally and theoretically. Experimentally, the decay curve is resolved into the three decay components of the three triplet sublevels in the field range of 0–6000 G. Both the decay constants and the fraction of total initial intensity of each decay component are determined for each field strength. Theoretically, the observed change in the decay characteristics is assumed to be solely due to the Zeeman mixing of the zero-field levels in the absence of spin-lattice relaxation processes. This has the effect of distributing the radiative strength of the strongly radiative zero-field level among the other two weakly radiative zero field levels. A simple approximate calculation gives excellent and encouraging agreement with experimental quantities in the range of validity of the approximations made (below 1000 G). A rigorous calculation was then performed using the exact solution of the secular equation resulting from the Hamiltonian including the spin-spin and Zeeman interactions. Due to the difficulty in finding a host of known crystal structure in which pyrazine can dissolve substitutionally and uniquely, the comparison between theory and experiment is made using a polycrystalline sample. A special averaging procedure is used in calculating a theoretical decay curve for each magnetic field strength used. The theoretical decay curve is then decomposed into three components employing the same computer programme used for decomposing the experimental decay curve. The agreement between the observed and the more rigorous calculated decay quantities is excellent up to 2000 G, but observed disagreement appears above 3000 G and increases with field strength. The calculated lifetimes above 3000 G are found to be longer than the observed ones. The difference is due to other relaxation mechanisms at these high fields, for example, in spin-lattice relaxation processes between the Zeeman sublevels of the lowest triplet state.     

Theoretical investigation of the backbone···π and π···π stacking interactions in substituted-benzene||3-methyl-2′-deoxyadenosine: a perspective to the DNA repair

The π-stacking effects of substituted benzenes on the N-glycosidic bond strength of 3-methyl-2'-deoxyadenosine (3-MDA) were studied by quantum mechanical calculations. Although all substituents enhance the stacking interactions, enhancement is higher for the electron-donating (ED) substituents. When the overall binding energy is separated into the π···π (ΔE_π···π) and backbone···π (ΔE_bb···π) contributions, the ED and electron-withdrawing (EW) substituents increase those contributions, respectively. Both the ED and EW substituents decrease the distance between the centres of stacked rings, while the EW ones increase the N-glycosidic bond length. The electron charge density calculated at the C--N bond critical point (ρ_C–N) is in linear correlation with the backbone···π interaction, not with the π···π interaction. This study also shows that the charge transfer from X-Ben to 3-MDA is in linear correlation with the ΔE_π···π and the change in the charge on the sugar ring is in better accordance with the backbone···π interaction. The N7 proton affinity (PA_N7), with a key role in the depurination process, is highly affected by the π···π interactions. Thus, both interactions must be considered because of the balance between the backbone···π and π···π contributions in these biomolecular systems.     

Enhancement effect of lithium bonding on the strength of pnicogen bonds: XH2P···NCLi···NCY as a working model (X = F,Cl; Y = H,F, Cl,CN)

MP2 calculations with aug-cc-pVDZ basis set were used to analyse intermolecular interactions in XH₂P···NCLi···NCY triads (X = F, Cl; Y = H, F, Cl, CN) which are connected via pnicogen bond and lithium bond. To understand the properties of the systems better, the corresponding dyads are also studied. Molecular geometries and interaction energies of dyads, and triads are investigated at the MP2/aug-cc-pVDZ computational level. Particular attention is paid to parameters such as cooperative energies and many-body interaction energies. All studied complexes, with the simultaneous presence of a lithium bond and a pnicogen bond, show cooperativity with energy values ranging between ?4.73 and ?8.88 kJ mol^?1. A linear correlation was found between the interaction energies and magnitude of the product of most positive and negative electrostatic potentials. According to energy decomposition analysis, it is revealed that the electrostatic interactions are the major source of the attraction in the title complexes.     

The intermolecular potential energy surface of Ar · HC1

Anisotropic potential energy surfaces for Ar · HC1 are obtained by simultaneous least squares fitting to molecular beam spectra, rotational line broadening cross sections, second virial coefficients and molecular beam total differential cross sections. Several potentials are obtained which given good agreement with all these data and the sensitivity of the data to various aspects of the potential is investigated. For all potentials the equilibrium configuration is linear with the atomic arrangement Ar · H-C1. Several different ways of parameterizing the intermolecular potential are considered and it is concluded that the conventional multipole parameterization is not adequate for strongly anisotropic intermolecular potentials.     

Shift of proton magnetic shielding in ClH ··· Y dimers (Y = N2, CO,BF)

The change in the proton magnetic shielding constant of ClH on the formation of the linear hydrogen-bonded ClH?·?·?·?Y (Y?=?N₂,?CO,?BF) complexes was determined by GIAO ab initio computations at the B3LYP/aug-cc-pVQZ level of theory. The characteristic downfield shift of the isotropic proton magnetic resonance in the vibrationally red-shifted complexes (ClH?·?·?·?N₂, ClH?·?·?·?CO and ClH?·?·?·?BF) is significantly larger than in the blue-shifted complexes (ClH?·?·?·?OC and ClH?·?·?·?FB). These results are rationalized by considering the changes in the magnetic and electric contributions to the proton shielding in ClH.     

On difference of properties between organic fluorine hydrogen bond C–H···F–C and conventional hydrogen bond

The existence of C–H···F–C hydrogen bonds in the complexes of trifluoromethane and cyclic molecule (oxirane, cyclobutanone, dioxane, and pyridine) has been experimentally proven by Caminati and co-workers. This study presents a theoretical investigation on these C–H···F–C hydrogen bonds at B97D/6-311++G^** and MP2/6-311++G^** levels, in terms of C–H vibrational frequency shifts, atoms in molecules characteristics, and the bonding feature of C–H···F–C hydrogen bonds. It is found that in three important aspects, there are significant differences in properties between C–H···F–C and conventional hydrogen bonds. The C–H···F–C hydrogen bonds show a blueshift in the C–H vibrational frequencies, instead of the X–H normal redshift in X–H···Y conventional hydrogen bonds. The natural bond orbital (NBO) analyses show that σ and p types of lone pair orbitals of the F atom to an antibonding σ^*_H–C orbital form a dual C–H···F–C hydrogen bond. Such a dual hydrogen bonding leads to the proton acceptor directionality of the C–H···F–C hydrogen bond softer. Our studies also show that the Laplacian of the electron density (▽²ρ_BCP) is not always a good criterion for hydrogen bonds. Therefore, we should not recommend the use of the Laplacian of the electron density as a criterion for C–H···F–C hydrogen bonds.     

On the elastic scattering of proton on deuteron at an energy of 8·2 GeV

The elastic scattering of a proton on a deuteron at an energy of 8·2 GeV was studied by the method of nuclear photoemulsions. A stack, composed of 20 emulsions of the type NIKFI-R diluted with heavy water, was irradiated with an internal beam of protons on the synchrophasotron in the Joint Institute of Nuclear Research in Dubna. The beam was perpendicular to the plane of the emulsion and had an intensity of (3·36±0·10)×10⁵ protons/cm².Altogether 114 events of elastic scattering (p-d) and 62 events of elastic scattering (p-p) were found. The differential cross-section of elastic scattering (p-d) was determined in a range of scattering angles of 1–3° to 10° in CMS. The total cross-section of elastic scattering (p-d) in this range of angles was found to be _el =(9·74±1·10) mb.The effective radius of interaction between the proton and deuteron during elastic scattering was found to beR=(2·25±0–10)×10^–13 cm.Last but not least, he thanks the management of the Joint Institute of Nuclear Research in Dubna for irradiating the emulsions and M. G. afranová for valuable advice and unstinting help in irradiating the emulsions.