Neutral pair

EA Carter, JT Elynes. Solute-dependent solvent force constants for ion pairs and neutral pairs m a polar solvent. J Phys Chem 93 2184-2187, 1989. 

The electrostatic free energy associated with the separation of the ion pair and the recombination of the neutral pair can be easily calculated with Coulomb s law and a large dielectric constant, (e.g., s = 40, which is the... 

In Figure the hydronium ion acts as an acid because it donates a proton to a base. The hydroxide anion acts as a base because it accepts a proton from an acid. When a hydronium ion with charge +1 transfers a proton to a hydroxide ion with charge -1, the two resulting water molecules have zero charges. The pair of charges becomes a neutral pair. A proton transfer reaction such as this one, in which water is one product and a pair of charges has been neutralized, is called a neutralization reaction. 

The measurement of A vs concentration provides no evidence as to the nature of the ion pairs which form, i.e. whether they are contact or solvent separated species. Also, the mobility of the ion pairs does not influence the results. Contact ion pairs are likely to be more mobile than those separated by solvent since the latter include a section of at least one polymer chain. However, it is possible to envisage mechanisms, involving concerted motion of the cation and anion of a solvent separated pair, which would allow the effective movement of the neutral pair. This is also true for contact vs solvent separated triples. Measurements to be discussed below, involving the dc polarisation of cells, are capable of distinguishing between mobile and immobile pairs. 

When divalent cation impurities (e.g. Cd, Sr ) are present in an ionic solid of the type MX consisting of monovalent ions, the negatively charged cation vacancies (created by the divalent ions) are bound to the impurity ions at low temperatures. Similarly, the oppositely charged cation and anion vacancies tend to form neutral pairs. Such neutral vacancy pairs are of importance in diffusion, but do not participate in electrical conduction. The interaction energy of vacancy pairs or impurity-vacancy pairs decreases with the increase in distance between the two oppositely charged units. 

Figure 5.2 The ionic conductivity of pure NaCl as a function of temperature. Intrinsic conduction occurs in stages I and II stage III corresponds to conduction by cation vacancies present as a result of impurities. Vacancies become associated to form neutral pairs in stage IV.

Fisher [15] has discussed various other field-theoretical scenarios that could be profitably employed for establishing the universality class of the RPM. In particular, he has advocated the use of a four-state lattice model with lattice sites occupied by cations, anions, and neutral pairs, with the number of pairs controlled by the mass action law. Again this path has not been followed up in detail, and only a highly speculative phase diagram has been given [15],... 

Another early effort in this direction [51] considered the competition for the bridging proton within the H-bond connecting a formate HCOO anion and an imine base of the sort HN=CH2. In the gas phase, it was found that the neutral pair HCOOH NHCH2 was preferred over an ion pair. However, again as in the aforementioned XH-amine complexes, the ion pair is progressively more favored as the dielectric constant of the medium rises. More specifically, in an in vacuo situation, the neutral pair in which the proton resides on the carboxylate rather than the imine... 

D. For any cation with any valence, when in the presence of two different anions, the anion with the highest potential to form neutral pairs with the cation controls the latter s adsorption potential, assuming that the anions do not react with the surface. For example, Ca2+ in the presence of Cl- exhibits greater adsorption potential than Ca2+ in the presence of SO4- due to the latter s greater potential to form neutral CaS04 pairs. 

In this way, the notion of template must be revisited. It is not the SBU which arrange around the templates represented by the amines, but all the neutral pairs which form the structure corresponding to the lowest lattice energy satisfying the constraints imposed by the size, shape and plasticity of the SBU-ammonium association, as illustrated by the ULM-3 type with diaminopropane, butane and pentane. This assessment supposes that, if there is an intermediate solid, it possesses the same pairs as the final one. 

Correlation was added to this picture several years later by Jasien and Stevens who also employed gradient optimization techniques. These authors found the appearance of a secondary, ion-pair minimum on the potential energy surface for ClH- NHj and BrH- -NHj at the SCF level which disappears with correlation. In the case of IH- NHj, both the neutral and ion-pair minima survive the inclusion of electron correlation and are within 1 kcal/mol of each other. The authors located a transition state for conversion that ties 1.4 kcal/mol higher in energy than the neutral pair. As an important point, the authors found that when they added zero-point vibrations, the ion-pair disappears as a minimum from the surface. 

Since H3N - HC1 exists only as a neutral pair and HF is less acidic than HCl, it is not surprising that HjN -HF, too, does not form an ion pair . Similarly for HjP -HCl and H3P—HF, as H3P is less basic than The latter set of calculations is particularly sig-... 

The acidity was turned up one notch when NH3 was paired with HI ° . This smdy included the effects of electron correlation. In agreement with prior results, the surfaces of both H3N—HCl and HjN -HBr contain only a single neutral-pair minimum. H3N -HI, on the other hand, exists both as the neutral and ion pairs. Whereas the neutral is favored by 5 kcal/mol at the SCF level, the two types of complex are nearly equal in energy when correlation is added. On the other hand, zero-point vibrational contributions to the energy are quite distinct for the two minima. Whereas this effect destabilizes the neutral pair by 1.6 kcal/mol, the ion pair is raised in energy by 5.6 kcal/mol, leading to the near disappearance of the minimum in the potential. It is concluded unhkely that HjNH "— would actually be observed. 

NP = neutral pair IP = ion pair 50% indicales equilibrium proton position about halfway between N and halide atoms. indicates the situation is still questionable. 

As indicated, it is possible to preferentially stabilize the ion-pair side of the proton transfer potential by allowing the complex to interact with a polarizable medium. By incrementally raising the polarizability, one can manually fine-tune the relative stability of the ion pair relative to the neutral pair. This lowering of one end of the proton transfer potential relative to the other is very much akin to an adjustment of the acidity and basicity of the partners. When the systems described above were immersed in a dielectric continuum model of a polarizable medium, the equilibrium position of the proton did indeed shift toward the base. It was possible to raise p to the point where it became positive, indicating the complex had more ion pair character than neutral pair. Just as the sensitivity of p to the basicity of the amine increases in the order HF < HCl < HBr, so too does its sensitivity to the polarizability of the medium. Whereas complexes with HF show little displacement of the proton s equilibrium position as the polarizability of the medium increases, those in which HBr acts as the acid experience a rapid increase in p with more polarizable medium. This treatment was later confirmed at the correlated MP2 level in that the HjN- -HCl neutral pair is superceded in stability by the corresponding ion pair when immersed in a dielectric continuum. ... 

During the period when theorists were addressing the question as to the possible existence of ion pairs, there was a good deal of experimental inquiry as well. The neutral pair nature of H3N" HC1 was confirmed in the gas phase by its rotational spectrum. The intermole-cular distance derived from the spectral data was noted to be in remarkable agreement with an earlier ab initio computation with MP2 correlation. The following year saw the measurement of the microwave spectrum of H N—HBr, and this complex too had the characteristics of a neutral pair. Nor are any of the complexes pairing a hydrogen halide with HjP of the ion-pair type. 

When the proton affinities of A and B are precisely equal, NPAD would be zero. This is not where the minimum in the vibrational correlation diagram was found. This displacement from zero is related to an earlier explanation that the ion pair A —+HB has a much stronger attractive interaction between the partners than does the neutral pair AH—B. So the proton affinity of A must be that much larger than that of B in order to compensate for this fact. Consequently, the minimum in the diagram was found to occur at NPAD = -0.23. Note the similarity of this value to that obtained from crude minimal basis set estimates of the complexes pairing HBr with amines above in Fig. 6.20. 

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