Atomic quadrupolar effect in the methanol–CCl4 and water–CCl4 intermolecular interactions

Accurate Single-Molecule Indicator of Solvent Effects

The study of the microscopic structure of solvents is of significant importance for deciphering the essential solvation in chemical reactions and biological processes. Yet conventional technologies, such as neutron diffraction, have an inherent averaging effect as they analyze a group of molecules. In this study, we report a method to analyze the microstructure and interaction in solvents from a single-molecule perspective. A single-molecule electrical nanocircuit is used to directly analyze the dynamic microscopic structure of solvents. Through a single-molecule model reaction, the heterogeneity or homogeneity of solvents is precisely detected at the molecular level. Both the thermodynamics and the kinetics of the model reaction demonstrate the microscopic heterogeneity of alcohol-water and alcohol-n-hexane solutions and the microscopic homogeneity of alcohol-carbon tetrachloride solutions. In addition, a real-time event spectroscopy has been developed to study the dynamic characteristics of the segregated phase and the internal intermolecular interaction in microheterogeneous solvents. The development of such a unique high-resolution indicator with single-molecule and single-event accuracy provides infinite opportunities to decipher solvent effects in-depth and optimizes chemical reactions and biological processes in solution.

Competition between the hydrogen bond and the halogen bond in a [CH3OH-CCl4] complex: a matrix isolation IR spectroscopy and computational study

Methanol (CH3OH) is the simplest alcohol and carbon tetrachloride (CCl4) is widely used as a solvent in the chemical industry. CH3OH and CCl4 are both important volatile substances in the atmosphere and CCl4 is an important precursor for atmospheric ozone depletion. Moreover, mixtures of CH3OH and CCl4 are an important class of non-aqueous mixtures as they exhibit a large deviation from Raoult's law. The specific interaction between CH3OH and CCl4 is not yet investigated experimentally. The interaction between CH3OH and CCl4 at the molecular level can be twofold: hydrogen bond (O-HCl) and halogen bond (C-ClO) interaction. One halogen bonded minimum and two hydrogen bonded minima are identified in the dimer potential energy surface. Herein, the 1 : 1 complex of [CH3OH-CCl4] has been characterised using matrix-isolation infrared spectroscopy and electronic structure calculations to investigate the competition between hydrogen bonded and halogen bonded complexes. Vibrational spectra have been monitored in the C-Cl, C-O, and O-H stretching regions. The exclusive formation of halogen bonded 1 : 1 complexes in argon and nitrogen matrices is confirmed by a combination of experimental and simulated vibrational frequency, stabilisation energy, energy decomposition analysis, and natural bond orbital and atoms-in-molecules analyses. This investigation helps to understand the specific interactions in the [CH3OH-CCl4] mixture and also the possibilities of formation of halogen bonded atmospheric complexes that may influence the atmospheric chemical activities, and enhance aerosol formation and deposition of CCl4.

Ultra-Coarse-Grained Models Allow for an Accurate and Transferable Treatment of Interfacial Systems

Interfacial systems are fundamentally important in many processes. However, constructing coarse-grained (CG) models for such systems is a significant challenge due to their inhomogeneous nature. This problem is made worse due to the generally nontransferable nature of the interactions in CG models across different phases. In this paper, we address these challenges by systematically constructing ultra-coarse-grained (UCG) models for interfaces, in which the CG sites are allowed to have internal states. We find that a multiscale coarse-grained (MS-CG) representation of a single CG site model fails to identify the directionality of a molecule and is unable to reproduce the correct phase coexistence for aspherical molecules. In contrast with conventional MS-CG models, the UCG methodology allows chemical and environmental changes to be captured by modulating the interactions between internal states. In this work, we design the internal states to depend on local particle density to distinguish different phases in liquid/vapor or liquid/liquid interfaces. These UCG models are able to capture phase coexistence and recapitulate structures, notably at state points in which the MS-CG method yields poor results. Interestingly, effective pairwise forces and potentials from the UCG models are almost identical to those of the bulk liquids that correspond to each phase, indicating that the UCG approach can provide transferable interactions. This approach is expected to be applicable to other systems that exhibit phase coexistence and also to complex macromolecular systems by modulating interactions based on local density or other order parameters to unravel the complex nature underlying heterogeneous system boundaries.

Determination of size of molecular clusters of ethanol by means of NMR diffusometry and hydrodynamic calculations

The microscopic structure of ethanol in the liquid state is characterized as a dynamic equilibrium of hydrogen-bonded clusters of different sizes and topologies. We have developed a novel method for determination of the average size of the clusters that combines the measurement of diffusion coefficient by means of NMR diffusometry technique and hydrodynamic simulations. The approach includes the use of HydroNMR [J. Garcı̀a de la Torre, M. L. Huertas, and B. Carrasco, J. Magn. Reson. 147, 2000, 138] for small molecules, which is attained here by the calibration procedure using a dilute solution of tetramethylsilane. It is thus possible to correlate the experimentally determined diffusion coefficient of ethanol with calculated diffusion coefficients of the modeled clusters of different sizes. We found that average size of the clusters in 0.16 M solution of ethanol in n-hexane corresponds to the monomer above 300 K and to the pentamer/hexamer below 240 K. The clusters in the case of 0.44 M solution are generally slightly larger, from the average size corresponding to the dimer at 320 K and the hexamer at 210 K.

Magnitude and origin of the attraction and directionality of the halogen bonds of the complexes of C6F5X and C6H5X (X = I, Br, Cl and F) with pyridine

The geometries and interaction energies of complexes of pyridine with C(6)F(5)X, C(6)H(5)X (X = I, Br, Cl, F and H) and R(F)I (R(F) = CF(3), C(2)F(5) and C(3)F(7)) have been studied by ab initio molecular orbital calculations. The CCSD(T) interaction energies (E(int)) for the C(6)F(5)X-pyridine (X = I, Br, Cl, F and H) complexes at the basis set limit were estimated to be -5.59, -4.06, -2.78, -0.19 and -4.37 kcal  mol(-1) , respectively, whereas the E(int) values for the C(6)H(5)X-pyridine (X = I, Br, Cl and H) complexes were estimated to be -3.27, -2.17, -1.23 and -1.78 kcal  mol(-1), respectively. Electrostatic interactions are the cause of the halogen dependence of the interaction energies and the enhancement of the attraction by the fluorine atoms in C(6)F(5)X. The values of E(int) estimated for the R(F)I-pyridine (R(F) = CF(3), C(2)F(5) and C(3)F(7)) complexes (-5.14, -5.38 and -5.44 kcal  mol(-1), respectively) are close to that for the C(6)F(5)I-pyridine complex. Electrostatic interactions are the major source of the attraction in the strong halogen bond although induction and dispersion interactions also contribute to the attraction. Short-range (charge-transfer) interactions do not contribute significantly to the attraction. The magnitude of the directionality of the halogen bond correlates with the magnitude of the attraction. Electrostatic interactions are mainly responsible for the directionality of the halogen bond. The directionality of halogen bonds involving iodine and bromine is high, whereas that of chlorine is low and that of fluorine is negligible. The directionality of the halogen bonds in the C(6)F(5)I- and C(2)F(5)I-pyridine complexes is higher than that in the hydrogen bonds in the water dimer and water-formaldehyde complex. The calculations suggest that the C-I and C-Br halogen bonds play an important role in controlling the structures of molecular assemblies, that the C-Cl bonds play a less important role and that C-F bonds have a negligible impact.

Excess compressibility in binary liquid mixtures

Brillouin scattering experiments have been carried out on some mixtures of molecular liquids. From the measurement of the hypersonic velocities we have evaluated the adiabatic compressibility as a function of the volume fraction. We show how the quadratic form of the excess compressibility dependence on the solute volume fraction can be derived by simple statistical effects and does not imply any interaction among the components of the system other than excluded volume effects. This idea is supported by the comparison of the experimental results with a well-established prototype model, consisting of a binary mixture of hard spheres with a nonadditive interaction potential. This naive model turns out to be able to produce a very wide spectrum of structural and thermodynamic features depending on values of its parameters. An attempt has made to understand what kind of structural information can be gained through the analysis of the volume fraction dependence of the compressibility.

Heterogeneous association in methanol-carbon tetrachloride mixtures

A lattice model developed for the calculation of the equilibrium distribution of molecular clusters in methanol-carbon tetrachloride mixtures is modified in order to account for an entropy component associated to the binding of CCl(4) with the nondonating end of methanol clusters. This entropy contribution is shown to affect significantly the H-bonding energy estimates based on the use of the model for the analysis of both the static dielectric response and the Raman O-H stretching spectra of these mixtures. The agreement with some results from numerical simulations turns out to be improved. In particular, the new value of approximately 6.2 kcal/mol found for the H-bonding energy (to be compared with approximately 3.2 kcal/mol yielded by the previous version of the model) is close to the results of simulation (approximately 5.5 kcal/mol). The H-bonding energy is also estimated by fitting the excess mixing enthalpy with an empirical polynomial expansion, complemented by an association contribution derived from the model. The agreement is good in this case, too.

A Mean Field Analysis of the O−H Stretching Raman Spectra in Methanol/Carbon Tetrachloride Mixtures

The O-H stretching region of the Raman spectra obtained from methanol/carbon tetrachloride mixtures of different compositions is analyzed. The various components of the spectra associated with methanol molecules with different H-binding states (i.e., non-H-bonded, chain-end, and doubly bonded) are quantitatively related with the alcohol cluster distribution derived by means of a simple lattice model. This comparison allows for the estimate of the mean overall hydrogen bonding energy by means of a best fitting procedure on the Raman data obtained at low-to-moderate alcohol contents; the solvation energy contribution of carbon tetrachloride is then also included. The result (approximately 3 kcal/mol) is found to be in agreement with the estimates from calorimetric and dielectric measurements.