Attractions, inter-molecular

The quantities that best represent a particular property can often be rationalized on the basis of physical intuition. For example, those that reflect interactions between like molecules, such as heats of sublimation and vaporization, can be expressed well in terms of molecular surface area and the product vofot. A large value for this product means that each molecule has both significantly positive and significantly negative surface potentials, which is needed to ensure strongly attractive inter-molecular interactions, with consequently higher energy requirements for the solid —> gas and liquid —> gas transitions. 

Dispersion forces usually increase with molar mass. Molecules with larger molar mass tend to have more electrons, and dispersion forces increase in strength with the number of electrons because of increases in atomic or molecnlar polarizability. Furthermore, larger molar mass often means a bigger atom whose electron distribution is more easily disturbed because the outer electrons are less tightly held by the nuclei. Table 4.6 compares the melting points of similar substances that consist of nonpolar molecules. As expected, the melting point increases as the number of electrons in the molecule increases. Becanse these are all nonpolar molecules, the only attractive inter-molecular forces present are the dispersion forces. 

The molecular explanation for the fact that the Joule-Thomson coefficient is positive at sufficiently low temperature is that at low temperatures the attractive inter-molecular forces are more important than the repulsive intermolecular forces. When the gas expands, work must be done to overcome the attractions and the potential energy increases. If no heat is added, the kinetic energy decreases and the temperature decreases. 

The main difference between a molecule-molecule (M-M) collision and an ion-molecule (M+-M) collision is the presence of a polarization force in the latter system owing to the attraction between the static charge on M+ and the dipole moment induced on M. For a large inter molecular separation, the polarization energy is known as... 

Structural influences such as conjugation or configuration and intra and inter molecular attractions hydrogen bonding in particular shift the IR bands of the groups involved. 

Molecules in the surface or interfacial region are subject to attractive forces from adjacent molecules, which result in an attraction into the bulk phase. The attraction tends to reduce the number of molecules in the surface region (increase in inter-molecular distance). Hence work must be done to bring molecules from the interior to the interface. The minimum work required to create a differential increment in surface dA is ydA, where A is the interfacial area and y is the surface tension or interfacial tension. One also refers to y as the interfacial Gibbs free energy for the condition of constant temperature, T, pression, P, and composition (n = number of moles)... 

In either equations (1) or (2) the non-ideality parameter w (sometimes written w/RT) arises from the difference between the inter-molecular attraction of unlike species as compared to the mean of the intermolecular attraction for pairs of like species. The second parameter in equation (1), is sometimes ascribed... 

Finally, the repulsive forces, that as you said play a large role in the definition of advantageous foldings, do also play a big role in the definition of crystalline structures of organic compounds and of inter-molecular vibration movements. It is very unfortunate that theoretical calculations of repulsive forces are much more difficult than those of attractive forces. 

J. W. Capstick further explains the effect of temp, on the colour of compounds by assuming that (i) the molecules vibrate about certain mean positions, and that (ii) a rise of temp, produces a greater amplitude of vibration, but not a greater period, so that if the vibration be not quite harmonic, a greater amplitude may, as with a pendulum, require a longer period, (iii) A rise of temp, is also supposed to weaken the cohesion or inter-molecular attraction between the molecules, and thus lessen the force of restitution, so that the molecules vibrate more slowly and thus produce the same sequence of colour changes with rise of temp, as are observed when the mass of the molecule is increased. 

It seems probable that within a molecule there are in operation forces of attraction and repulsion, dependent on the mass and nature of the component atoms, causing inter-molecular vibrations of greater or less intensity. In the case of a molecule such as HBr04, the vibrations would be so intense that such a system of atoms would not be capable of existence. 

Finally, inspection of Table 3.2 shows also that there are cases in which Yu can be even smaller than 1. An example is a solution of diethylether in chloroform. Here, the solute is an electron donor (H-acceptor), while the chloroform solvent is an electron acceptor (H-donor). In this case, the solute and solvent both acquire additional inter-molecular interactions that were unavailable to them in their pure liquid forms. The monopolar diethylether (only vdW interactions in its pure liquid) can add polar interactions to its vdW attractions with the molecules of the monopolar chloroform solvent exhibiting a complementary electron acceptor property. 

Hydrogen is a colorless, odorless, tasteless gas (Table 14.1). Because H2 molecules are nonpolar, they can attract each other only by London forces. Each molecule has only two electrons, and hence only a very small instantaneous electric dipole therefore these forces are so weak that hydrogen does not condense to a liquid until it is cooled to 20 K. Because of these weak inter molecular forces, it has only low solubility in many liquids, particularly polar liquids. Furthermore, H2 molecules are so small and move at such a high average speed that molecules of hydrogen gas diffuse more rapidly than those of any other substance. 

In this article we consider problems concerning the interpretation of unsaturated, steady-state NMR spectra of spin systems which are in a state of dynamic equilibrium. Spin exchange processes may occur with frequencies between a few sec-1 and several thousand sec-1 and thus modify the spectral lineshapes. In this case we use the terms dynamic NMR and dynamic spectra. The analysis of dynamic NMR lineshapes constitutes an important, and often unique, source of information about intra- and inter-molecular reaction rates. This is especially true for degenerate reactions where the products are chemically identical with the substrates. For this and similar reasons, dynamic NMR analysis has attracted considerable attention for about twenty years. 

The forces of attraction between neutral, chemically saturated molecules, postulated by van der Waals to explain non-ideal gas behaviour, also originate from electrical interactions. Three types of such inter molecular attraction are recognised ... 

Molecular inclusion is now known to involve factors such as the size and shape of the various component molecules or ions, their complementarity, inter-molecular forces of attraction and repulsion, directional forces and properties, and supramolecular synthons [3], all of which make their contribution to the overall lattice energy. 

As already mentioned the present treatment attempts to clarify the connection between the sticking probability and the mutual forces of interaction between particles. The van der Waals attraction and Born repulsion forces are included in the calculation of the rate of collisions between two electrically neutral aerosol particles. The overall interaction potential between two particles is calculated through the integration of the inter-molecular potential, modeled as the Lennard-Jones 6-12 potential, under the assumption of pairwise additivity. The expression for the overall interaction potential in terms of the Hamaker constant and the molecular diameter can be found in Appendix 1. The motion of a particle can no longer be assumed to be... 

The state of a gas at the limiting condition where P - 0 deserves some discussion. As the pressure on a gas is decreased, the individual molecules become more and more widely separated. The volume of the molecules themselves becomes a smaller and smaller fraction of the total volume occupied by the gas. Furthermore, the forces of attraction between molecules become ever smaller because of the increasing distances between them. In the limit, as the pressure approaches zero, the molecules are separated by infinite distances. Their volumes become negligible compared with the total volume of the gas, and the inter-molecular forces approach zero. A gas which meets these conditions is said to be ideal, and the temperature scale established by Eq. (3.9) is known as the ideal-gas temperature scale. 

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