Effect of electron-donating and electron-withdrawing atoms on the C–H…Y hydrogen bond in model X3CH…YZ (X = B,F; YZ = BF,CO, N2) complexes

A computational study of model complexes X₃CH…YZ (X = B, F; YZ = BF, CO, N₂) was undertaken to assess the effect of electron-donating and electron-withdrawing X atoms on the properties of the C–H…Y hydrogen bond. Sequential substitution of the B atoms in B₃CH by F atoms to produce F₃CH allowed for the elucidation of interesting trends in the corresponding hydrogen-bonded complexes. The dipole moments and the dipole moment derivative with respect to C–H bond displacement for the proton donors and the chemical hardness of the Y atom of the proton acceptor YZ were found to be useful parameters for understanding these trends. It was found that a positive dipole derivative favours red-shifted hydrogen bonds, whereas a negative dipole derivative favours blue-shifted hydrogen bonds. However, decreasing hardness of Y (which correlates with increasing intermolecular attraction) modifies the interaction such that either greater C–H bond extensions/red shifts or smaller C–H bond compressions/blue shifts are obtained.     

Structures,stabilities and electronic properties of boron-doped silicon clusters B3Sin (n=1–17) and their anions

The structures, stabilities and electronic properties of neutral and anionic B₃Si_n (n?=?1–17) clusters have been systemically investigated on the basis of density functional theory at the B3LYP/6-311?+?G(d) level and CALYPSO structure prediction method. The structural searches show that three boron atoms tend to form B₃ triangle encapsulated into Si_n cages with the increasing number of silicon atoms. Most of the lowest energy structures can be derived by using the squashed pentagonal bipyramid structure of B₃Si₄ and B₃Si₄^? as the major building unit. The relative stabilities are studied based on the calculated binding energies, second-order difference of energies and HOMO–LUMO gaps of the lowest energy structures. In addition, Hirshfeld, natural population analysis, Bader approaches and natural electronic configuration are performed to explore the charge transfer. At last, molecular orbital, magnetic properties, IR, Raman and UV–vis spectra are also, respectively, analysed for providing strong support for essential theoretical and experimental research.     

Computational study of the cooperative effects between tetrel bond and halogen bond in XCN⋅⋅⋅F2CO⋅⋅⋅YCN complexes (X = H,F, Cl,Br; Y = F,Cl, Br)

ABSTRACT

Ab initio calculations have been accomplished to study the cooperativity between the halogen bond and tetrel bond in the XCN???F2CO???YCN (X = H, F, Cl, Br; Y = F, Cl, Br) complexes. F₂CO at the same time plays the role of Lewis acid with the π-hole on the C atom and Lewis base with the O atom to participate in the tetrel bond and in halogen bond, respectively. According to the geometry survey, the effect of a tetrel bond on a halogen bond is more pronounced than that of a halogen bond on a tetrel bond and the intermolecular distances in the triads are always smaller than the corresponding values in the dyads. In all cases, the halogen bond and tetrel bond in the termolecular complexes are stronger compared with those in the bimolecular complexes. So, from the intermolecular distances, interaction energies and many-body interactions demonstrate that there is positive cooperativity between the halogen bond and tetrel bond. The molecular electrostatic potential, atoms in molecules and natural bond orbital methodologies are used to analyse the nature of interactions of the complexes.     

Structures,stabilities and electronic properties of manganese oxyfluoride (MnOxFy) species (x + y = 1–4; x,y = 0–4)

In this work, a density functional survey on manganese oxyfluoride (MnO_xF_y) species for x + y = 1–4 is performed, in which an Mn atom interacts simultaneously with O as well as F atoms. The stabilities of all these species are established against dissociation to manganese oxides as well as fluorides and their relative stabilities are also discussed. It is revealed that the most favourable oxidation state of Mn is +4 in its oxyfluorides, same as in fluorides. For the first time, the superhalogen properties of MnOF₃, MnO₃F and MnO₂F₂ species are introduced on the basis of their high electron affinities as compared to halogens. The interaction of MnO₂F₂ superhalogen with an alkali metal (K) is considered via F atoms as well as O atoms which is similar to that in KF and leads to the formation of stable KMnO₂F₂ complex. Thus, this study is expected to motivate theorist to design a new series of superhalogen species using transition metals with mixed F and O ligands, as well as experimentalists to synthesise such novel complex compounds.     

Insights into the structures,stabilities, electronic and magnetic properties of X2Aun (X = La,Y, and Sc; n = 1–9) clusters: comparison with pure gold clusters

A systematic study of the X₂Au_n (X = La, Y, Sc; n = 1–9) clusters are performed by using the density functional theory at TPSS level. The structures, stabilities, electronic, and magnetic properties are investigated in comparison with pure gold clusters. The results show that the transition points of the doped clusters from two-dimensional to three-dimensional structure are obviously earlier than gold clusters. The impurity X atoms tend to occupy the most highly coordinated position and form the largest probable number of bonds with gold atoms. In addition, the impurity atoms can strongly enhance the stabilities of gold clusters. It indicates that the impurity atoms dramatically affect the geometries and stabilities of the Au_n clusters. The highest occupied molecular orbital–lowest occupied molecular orbital gap, vertical ionisation potential, and chemical hardness show that the X₂Au₆ clusters have higher stabilities than the others. In La₂Au_1–9, Y₂Au_1–7, and Sc₂Au_1–4 clusters, the charges transfer from X atoms to the Au_n frames. The total magnetic moments of X₂Au_n clusters exist distinctly odd–even alternation behaviours except for La₂Au₄ and Sc₂Au₄ clusters.     

Effect of the donor–acceptor properties of ligands on the spectroscopic and electrochemical properties of mixed-ligand complexes of Pt(II) and Ir(III) with cyclometalated 2-phenylbenzothiazole

The results of the spectroscopic NMR (¹H, ¹³C, and ¹⁹⁵Pt), infrared, optical, and voltammetric characteristics of the mixed-ligand complexes of Pt(II) and Ir(III) with metalated 2-phenylbenzothiazole and tert-butylisocyanide (tBuNC), acetonitrile (AN), ethylenediamine (En), O-ethyldithiocarbamate (Exn^–), and diethyldithiocarbamate (Dtc–) ions are presented. It is demonstrated that the change in donor–acceptor interaction of ligands tBuNC, AN, En, Exn^–, and Dtc^– with metal leads to an increase in the energy of the highest occupied molecular orbital of the complexes and is accompanied by a shift of the cathode potential of the metal-centered oxidation, a bathochromic shift of the spin-allowed and spin-forbidden metal-tocyclometalated ligand optical charge transfer transitions, and an increase the degree of mixing of the ¹MLCT and triplet intraligand states, responsible for the phosphorescence of the complexes.     

Influence of solvent and intramolecular relaxation on dynamics of luminescence spectra in donor–acceptor complexes

The oscillator strengths for Rydberg states of a NaHe molecule are calculated using a semianalytic procedure with the l-coupling effect taken into account (due to the dipole potential of a core). This effect gives rise to nonzero oscillator strengths for transitions forbidden in the atomic model of molecular Rydberg states. The difference between calculations in terms of the atomic model and calculations with consideration of the dipole moment of a core is shown for allowed transitions.     

SCF Xα — SW ionization energies of XNO (X = Cl,F and Br) and XON (X = Cl,and F)

Ionization energies of the nitrosyl halides XNO (X = Cl, F, and Br) have been obtained using the SCF Xα scattered-wave method and transition state procedure. The results are comapred to other theoretical and experimental ionization energies as well as among themselves. Ionization energies of nitrogen hypohalides XON (X = Cl and F) have also been computed and compared to those of XNO (X = Cl and F). It was found that the present results yield a good agreement with the experimental ionization energies.     

Strengthening of the halogen-bonding by an aerogen bond interaction: substitution and cooperative effects in O3Z···NCX···NCY (Z = Ar,Kr, Xe; X = Cl,Br, I; Y = H,F, OH) complexes

The geometry, interaction energy and bonding properties of ternary complexes O₃Z···NCX···NCY (Z= Ar, Kr, Xe; X = Cl, Br, I and Y = H, F, OH) are investigated with ab initio calculations at the MP2/aug-cc-pVTZ level. Two different types of intermolecular interactions are present in these complexes, namely, aerogen bond (Z···N) and halogen bond (X···N). The formation mechanism and bonding properties of these complexes are analysed with molecular electrostatic potentials, quantum theory of atoms in molecules and non-covalent interaction index. It is found that the cooperativity energies in the ternary complexes are all negative; that is, the interaction energy of the ternary complex is greater (more negative) than the sum of the interaction energies of the corresponding binary systems. Also, the cooperativity energies increase with the increase of the interaction energies. The cooperative effects in the ternary complexes make a decrease in the total spin–spin coupling constants across the aerogen bonding, J(Z–N), which can be regarded as a proof for the reinforce of Z···N interactions in the ternary complexes with respect to the binary systems.     

Influence of spectral characteristics of the pump pulse on the transient absorption of donor–acceptor complexes in polar solvents

We derive an analytical expression for calculating the transient absorption signal measured in the pump?Cprobe experiment. The expression explicitly accounts for dynamic properties of the medium, the population decay of the photoexcited state, and the angle ?? between the directions of the reaction coordinates corresponding to the electron transitions at the pump and probe stages. We investigate numerically the influence of the carrier frequency of the pump pulse and ?? on the transient absorption signal. We study the signal dynamics and its deviation from that of the excited state population under variation of these parameters. We show that these effects are manifested in complexes including methyl-substituted benzene and tetracyanoethylene in polar solvents.     

The prominent enhancing effect and mechanism of the methyl group in the X···Y (X=O,S, H3CO,H3CS, (H3C)2O, (H3C)2S; Y=HCN,HNC) hydrogen-bonded complex

An ab initio computational study of the enhancing role of the methyl group in the M···H (M=S and O) hydrogen bond has been carried out at the QCISD/6-311++G(2df,2p) level. The bond lengths, frequency shifts, and interaction energies were analysed. The methyl group of the electron donor plays a positive role in the formation of the hydrogen bond. Its enhancing role is stronger in the O···H hydrogen bond than in the S···H hydrogen bond. The results show that the methyl group has a prominent effect on the strength of the hydrogen bond. The interaction energy is increased by 347% for the Me₂O–HCN complex relative to that for the O–HCN complex. The enhancing mechanism of the methyl group has been analysed by means of natural bond orbital (NBO) theory. The electrostatic interaction is of more importance to the O···H hydrogen bond, whereas dispersion and charge-transfer interactions play a more significant role in the S···H hydrogen bond.     

Structures and stabilities of endo- and exohedral Si20H20 derivatives: X@Si20H20 and XSi20H20, X = H + , H,N, P,C − , and Si −

Geometries, relative energies, and stabilities of endo- and exohedral complexes, X@Si₂₀H₂₀ and XSi₂₀H₂₀, (X = H⁺, H, N, P, C^?, and Si^?) are calculated at B3LYP/6-31G* level. The energy minimum structure of Si₂₀H₂₁ ⁺ shows that the proton cannot be positioned in the Si₂₀H₂₀ centre, but prefers attach to Si₂₀H₂₀ exohedrally with C_2v symmetry. Most investigated I_h endohedral complexes X@Si₂₀H₂₀ (X = H, N, P, C^?, and Si^?) are local minima, except for ²N@Si₂₀H₂₀, which is a high-order saddle point. Inclusions energies of the endohedral complexes are calculated, and it reveals that energy penalties caused by encapsulation are rather small. Exohedral complexes XSi₂₀H₂₀ (X = H, N, P, C^?, and Si^?) have C_2v or C_s local minima, and most of them are more stable than their endohedral isomers with the exception of C_2v ⁴PSi₂₀H₂₀ and ⁴Si^?Si₂₀H₂₀.     

First principles investigation of hydrogen isotope effects in [XSO4–H–SO4X]− (X = H,K) complexes

Hydrogen isotope effects on geometries, total energies, nuclear and electronic wave functions of the [HO₃SO–H–OSO₃H]^? and [KO₃SO–H–OSO₃K]^? complexes are investigated with the NEO/HF method. This method determines both electronic and nuclear wave function simultaneously. A discussion of the isotope effects is provided and used to explain the hydrogen isotope effects on the phase transition temperatures in hydrogen bonded ferroelectric materials, K₃H(SO₄)₂ and K₃D(SO₄)₂.     

Structures,stabilities and adsorption mechanisms of nitrogen adsorbed on Mon (n = 1–8) clusters

Configurations, stabilities and adsorption mechanisms of ground-state Mo_nN and Mo_nN₂ (n?=?1–8) clusters are calculated by using the density functional method within the PBE level. Evidently, N atoms tend to approach more Mo atoms. Doping with two N impurity prefers to occupy symmetrical position of the host Mo_n (n?=?1–8) cluster except for Mo₂N₂ clusters. Mo₄N, Mo₆N, Mo₂N₂, Mo₄N₂ and Mo₆N₂ clusters have higher structural stabilities than their neighbors by the second derivative of total binding energy. Mo₂N, Mo₄N and Mo₇N, Mo₂N₂, Mo₅N₂ and Mo₇N₂ clusters have higher kinetic reactivity than their neighbors by the HOMO–LUMO gaps. The adsorption capacity of a N atom to Mo₄ cluster is stronger than the other Mo–N clusters.     

An SCF-Xα study of the ionization energies of the octahedral complexes XF6 n- (X=S,P, Si,Al)

SCF-Xα calculations are reported for AlF₆ ^3-, SiF₆ ^2-, PF₆ ^- and SF₆. The results for SF₆ are comparable with those obtained from a recent many-body perturbation theory calculation. Ionization energies for the complexes calculated by the transition state technique agree well with experiment.     

Substituent effects on gas-phase acidities of formic acid and its silicon and sulphur derivatives R − M(= X)XH(M = C., Si; X = O,S; R = H,F, CI,OH, NH2 and CH3)

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the effect of substituent (F, Cl, NH₂, OH and CH₃) on the gas-phase acidities of formic acid, HCOOH, its silicon and sulphur derivatives R-M(= X)XH(M = C., Si; X = 0, S; R = F, Cl, OH, NH₂ and CH₃). For formic acid and its thio and dithio derivatives the acidity changes upon substitution are irregular and depend on both the type of substituent, position and degree of replacement of oxygen atoms by sulphur atoms. For sila carboxylic acids and their thio and dithio derivatives the calculated acidities regularly increase in the order: R-SiOOH < R-Si(=S)OH ? R-Si(=O)SH < R-SiSSH(R = H, F, Cl, OH, NH₂ and CH₃). The chloro derivatives are the strongest among the acids studied. The highest gas phase acidity (1277.6 kJmol^?1) has been calculated for ClC(=S)OH.     

Structures,thermochemistry, and electron affinities of the disilicon fluorides,Si2F n /Si2F− n (n = 1–6)

A systematic investigation of Si₂F_n/Si₂F^? _n systems is carried out with five density functional (DFT) methods in conjunction with DZP++ basis sets. For each system, various structures, including minima, transition states, and energetically low lying saddle points, are optimized. The geometries and the relative energies are discussed and compared. Three kinds of electron affinity and dissociation energy pertaining to the global minimum for each compound are reported. The theoretical predictions are in good agreement with the limited experimental results. The zero-point vibrational energy (ZPVE) corrected adiabatic electron affinities (EA_ad) are predicted as 1.97 (Si₂F), 1.92 (Si₂F₂), 2.39 (Si₂F₃), 2.02 (Si₂F₄), 2.68 (Si₂F₅), and 0.73 (Si₂F₆)eV by the BHLYP method, which is considered to be the most reliable method in the present study for predicting the EAs. These theoretical predictions are quite different from those for the analogous silicon hydrides and fluorocarbons. For example, both Si₂F₂ and its anions have vinylidene-like (Si-SiF₂) global minima. The anion SiSi bond distance is about 0.1 Å shorter than that for the Si—SiF₂ neutral. Both Si₂F₃ and its anion have carbyne-like (Si-SiF₃) global minima, with the anion SiSi distance about 0.05 Å shorter. Both Si₂F₄ and its anion have carbene-like (FSi-SiF₃) global minima, again with the negative ion SiSi distance ～0.05 Å shorter. Surprisingly, doubly bridged structures of Si₂F₄ are energetically competitive. For the ethyl-radical-like Si₂F₅, the expected longer SiSi distance (by 0.13 Å) for the anion is predicted. Whereas Si₂H₄, C₂F₄, Si₂H₆, and C₂F₆ do not have significant electron affinities, Si₂F₄ and Si₂F₆ do bind an electron. However, the unexpected Si₂F^? ₆ species has a significantly longer SiSi distance (by 0.15 Å) than that of neutral Si₂F₆.     

A comparative study of model halogen-bonded, π-hole-bonded and cationic complexes involving NCX AND H2O (X = F,Cl, Br)

A MP2/6-311+ +G(d,p) study of NCX (X = F, Cl, Br) has shown that it is possible to attach an electrophile (H⁺, Be²⁺) to the positive halogen X surface of NCX. The stability and properties of model halogen-bonded and π-hole carbon-bonded NCX/H₂O complexes were found to be significantly affected by H⁺ or Be²⁺ cationic attachment at the N atom. The halogen-bonded complexes are destabilised by binding at the N, while an attached proton enhances the binding in the π-hole bonded dimers. For the attached Be²⁺, an unusual complex was obtained with the NCF subunit, whereas the complexes containing Cl and Br were destabilised by the interaction.     

Cooperativity effects between σ-hole interactions: a theoretical evidence for mutual influence between chalcogen bond and halogen bond interactions in F2S···NCX···NCY complexes (X = F,Cl, Br,I; Y = H,F, OH)

Quantum chemical calculations are performed to study the cooperativity effects between chalcogen bond and halogen bond interactions in F₂S···NCX···NCY complexes, where X = F, Cl, Br, I and Y = H, F, OH. These effects are investigated in terms of geometric and energetic features of the complexes, which are computed by second-order Møller–Plesset perturbation theory (MP2). For each F₂S···NCX···NCY complex studied, the effect of cooperativity on the chalcogen bond is dependent on the strength of halogen bond. The results indicate that the interaction energies of chalcogen and halogen bonds in the triads are more negative relative to the respective dyads. The interaction energy of chalcogen bond is increased by 31%–49%, whereas that of halogen bond by 28%–62%. The energy decomposition analysis reveals that electrostatic force plays a main role in the cooperativity effects between the chalcogen bond and halogen bond interactions. The topological analysis, based on the quantum theory of atoms in molecules, is used to characterise the interactions and analyse their enhancement with varying electron density at bond critical points.     

Magnetocaloric effects in RT X intermetallic compounds(R = Gd–Tm,T = Fe–Cu and Pd,X = Al and Si)

The magnetocaloric effect(MCE) of RT Si and RT Al systems with R = Gd–Tm, T = Fe–Cu and Pd, which have been widely investigated in recent years, is reviewed. It is found that these RT X compounds exhibit various crystal structures and magnetic properties, which then result in different MCE. Large MCE has been observed not only in the typical ferromagnetic materials but also in the antiferromagnetic materials. The magnetic properties have been studied in detail to discuss the physical mechanism of large MCE in RT X compounds. Particularly, some RT X compounds such as Er Fe Si,Ho Cu Si, Ho Cu Al exhibit large reversible MCE under low magnetic field change, which suggests that these compounds could be promising materials for magnetic refrigeration in a low temperature range.