Dipolar molecules

Consider the case of two neutral, linear, dipolar molecules, such as HCN and KCl, in a coordinate system with its origin at the CM of molecule A and the z-axis aligned with the intemiolecular vector r pointing from the CM of A to the CM of B. The relative orientation of the two molecules is uniquely specified by their spherical polar angles 0, 03 and the difierence <]) = - <])3 between their azimuthal angles. The leading temi in the... 

Consider the interaction of a neutral, dipolar molecule A with a neutral, S-state atom B. There are no electrostatic interactions because all the miiltipole moments of the atom are zero. However, the electric field of A distorts the charge distribution of B and induces miiltipole moments in B. The leading induction tenn is the interaction between the pennanent dipole moment of A and the dipole moment induced in B. The latter can be expressed in tenns of the polarizability of B, see equation (Al.S.g). and the dipole-mduced-dipole interaction is given by... 

At distances of a few molecular diameters, the interaction will be dominated by electric multipole interactions for dipolar molecules, such as water, the dominant tenn will be the dipole-dipole interaction ... 

Ciary D C, Smith D and Adams N G 1985 Temperature dependence of rate coefficients for reactions of ions with dipolar molecules Chem. Phys. Lett. 119 320-6... 

In the reaction field method, the space surrounding a dipolar molecule is divided into two regions (i) a cavity, within which electrostatic interactions are sunnned explicitly, and (ii) a surrounding medium, which is assumed to act like a smooth continuum, and is assigned a dielectric constant e. Ideally, this quantity will be... 

Zwitterionic L-alanine ( HjN—CfCHj)—CO2—) is a dipolar molecule that forms large well-ordered crystals in which the molecules form hydrogen-bonded columns. The strong interactions lead to the presence of well-defined intra- and intermolecular vibrations that can usefully be described using hannonic theory. 

There is a large elass of reactions known as 1,3-dipolar cycloaddition reactions that are analogous to the Diels-Alder reaction in that they are coneerted [4jc -I- 2jc] eyeloaddi-tions. ° These reactions can be represented as in the following diagram. The entity a—b—c is called the 1,3-dipolar molecule and d—e is the dipolarophile. 

The 1,3-dipolar molecules are isoelectronic with the allyl anion and have four electrons in a n system encompassing the 1,3-dipole. Some typical 1,3-dipolar species are shown in Scheme 11.4. It should be noted that all have one or more resonance structures showing the characteristic 1,3-dipole. The dipolarophiles are typically alkenes or alkynes, but all that is essential is a tc bond. The reactivity of dipolarophiles depends both on the substituents present on the n bond and on the nature of the 1,3-dipole involved in the reaction. Because of the wide range of structures that can serve either as a 1,3-dipole or as a dipolarophile, the 1,3-dipolar cycloaddition is a very useful reaction for the construction of five-membered heterocyclic rings. 

To a first approximation, the energy (Up) that arises during the interaction between two dipolar molecules is given by. 

Up energy that arises during interaction between two dipolar molecules... 

Betaine (Section 19.11) A neutral dipolar molecule with nonadjacent positive and negative charges. For example, the adduct of a Wittig reagent with a carbonyl compound is a betaine. 

Dipole-dipole force (Section 2.13) Anoncovalent electrostatic interaction between dipolar molecules. 

Zwitterion (Section 26.1) A neutral dipolar molecule in which the positive and negative charges are not adjacent. For example, amino acids exist as zwitlerions, H iN — CHR — CQ2-. 

The spherical pendulum, which may be used to model the bending states of HCP [7] and the pendular states of dipolar molecules in strong electric fields [8], is of... 

The geometrically optimized model of BD had a roughly linear conformation. This spontaneous ordering was unexpected given the general orientation of dipolar molecules. Azobenzenes that have permanent dipoles parallel to the molecular axis would intuitively be expected to tend to pair with their dipole oriented in the opposite direction. The linear geometry is probably due to the... 

Usually g-J(ion) 7 Aj I and J(dip) / Aj/. This is caused by mutual influence of the contacting phases on the orientation of dipolar molecules in the interfacial zone. 

Steric ratios for ionization at either end of dipolar molecules may be calculated using the DM and EM ab initio methods. Both methods correctly predict the correct sign for the steric ratio and they are in reasonable agreement with each other and with experiment for the magnitude of the steric ratio for CH3C1. 

The results from EFISHG or HRS experiments can be corrected for resonance enhancement by using a simple theoretical two-state model (TSM, Equations (3) and (4)),35,36 which is reasonably valid for dipolar molecules in which f3 is primarily associated with a single ICT excitation ... 

For neutral dipolar molecules, the value of B depends on the dipole moment itself, and the value of y is 3/2 or, in the presence of image forces, 5/2. The former of these has been verified experimentally for the adsorption of such molecules as 2-chloropyridine on mercury. 

Amis proposed Equation (3)18 for the reaction rate constant involving a limiting case of head-on approach of an ion to a neutral dipolar molecule from electrostatic considerations, as would be the case for metal ion catalysts and neutral C = 0 or P = O substrates. This expression relates the natural log of the rate constant in a medium of dielectric constant (D) to the charge on the ion (Ze), the dipole moment... 

The ditrigonal cavity formed by six corner sharing silica tetrahedra (Fig. 3.10) has a diameter of 0.26 nm and is bordered by six sets of lone-pair electron orbitals emanating from the surrounding ring of oxygen atoms. These structural features - as is pointed out by Sposito (1984) - qualifies the ditrigonal cavity as a soft Lewis base capable to complex water molecules (and possibly other neutral dipolar molecules). 

Influenced by P. Pfeiffer s 1922 paper, postulating separated charges (Zwitterions) within aminoacids, Arndt wrote formulas for gamma pyrone and gamma-thiopyrone as "Zwitterion" dipolar molecules ... 

Dipolar molecules will form localized structures in a bulk solvent by orientation of these permanent dipoles, and Figure 1.9 shows two different ways in which two molecules with permanent dipoles may align with one another. With several molecules and three dimensions, a wide range of localized structures are possible. Dipole moments have units of coulomb metres (C m) or Debyes (D, 1D = 3.34 x 10-30 Cm). 

Figure 1.9 Possible alignments of two dipolar molecules that can lead to attraction and short-range structuring of liquids...

How dispersion forces develop between identical non-polar molecules. In A, neither molecule interacts with the other. In B, one molecule becomes, instantaneously, a dipole. At that moment, the dipolar molecule is able to induce a temporary charge separation in the other molecule, resulting in a force of attraction between the two. All the molecules within a sample undergo this same process, as shown in C. 

In the first model, the mnneling electron mainly interacts with the electronic polarization of water ( = 1.88) since tunneling was assumed to be fast in comparison with the orientational response of the dipolar molecules of the liquid. Considering water as a dielectric continuum between a jellium spherical tip and planar substrate yields an effective barrier for tunneling that is about 1 eV lower than that for the vacuum case [95]. This result is consistent with photoemission studies of metal/aqueous interfaces, which reveal electron emission into water at 1 eV below the vacuum level [95-97]. Similar models have been employed to examine the effect of thermal fluctuations on the tunneling current [98-100]. Likewise, a related model assessing the noise associated with the reorientation of adsorbed molecules has been presented [101]. 

It might be useful in some cases to raise the dielectric constant of mixed solvents by addition of suitable substances and it is known that dipolar molecules such as amino acids do so in pure water. These amino acids are virtually insoluble in nonpolar solvents but they dissolve readily in aqueous salt solutions and in most mixed solvents according to their highly polar structure. Most of what is known about their dielectric behavior concerns aqueous solutions, in which they were studied up to concentrations near saturation. 

For the reaction between an ion and a dipolar molecule, the rate is largely uninfluenced by ionic strength. A relation of the type... 

Ten isomeric pyrazolotriazines are theoretically possible (71-80). Derivatives of 71-76 have already been prepared. Neither parent 77 nor any of its tautomers has yet been made. Derivatives of 74-77 can display tautomerism (cf. 74a-77a). Aromatic pyrazolo[l,2-a]-l,2,3-triazines can exist only as salts or dipolar molecules. 

The alternating electronic properties of the Co, and Cp atoms in the vinylidene ligand enable dipolar molecules to enter into cycloaddition reactions. Intramolecular [2 + 2]-... 

Finally, some molecules possess permanent charge separations, or dipoles, such as are found in water. The general case for the interaction of any positive dipole with a negative dipole is called dipole-dipole interaction. Hydrogen bonding can be thought of as a specific type of dipole-dipole interaction. A dipolar molecule like ammonia, NH3, is able to dissolve other polar molecules, like water, due to dipole-dipole interactions. In the case of NaCl in water, the dipole-dipole interactions are so strong as to break the intermolecnlar forces within the molecular solid. 

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