Experimental basicity scales

Absolute Proton Affinities 2.1. Experimental Basicity Scales 

The gas phase basicity and proton affinity are intimately related entities being defined by the same hypothetical reaction  

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There is a linear relationship between the experimental values and the theoretical ones obtained at the MP2/6-31+G(d,p)//6-31G level after including the corresponding ZPE corrections (see Figure 21). Also significant is the fact that this correlation presents a slope very close to unity and that it covers a wide range (about 50 kcalmol-1) of the basicity scale. Figure 22 illustrates the reasonably good linear relationship between the aforementioned ab initio MP2 values and those obtained at the AMI semiempirical level. 

Kaljurand, I., Koppel, I.A., Kiitt, A. et al. (2007) Experimental gas phase basicity scale of superbasic phosphazenes. Journal of Physical Chemistry A, 111, 1245-1250. 

This is known as the irons effect, which generally decreases in the order RjSi > H, Me , CN-, olefin, CO>PR3, NOj, I , SCN > Br > Cl > RNHj, NH3>OH > NOs , It is, however, important to recall that there is no universal acidity or basicity scale and that the orders presented above may be accurate only for those cases that were experimentally examined. So, they should be viewed as vague indications of approximate trends. 

Further, the atomic radii scales within the basic set and experimental electronegativity scales will be calculated with the results presented correspondingly in the columns <1> to <8> in the Table 4.15. Additionally, there are presented in the columns <9> and <10> of the Table 4.15 the direct experimental evaluation (Web Elements, 2011), and the ab initio approaches (Ghanty Ghosh, 1996), for atomic radii. In this way the respectively atomic radii scales computed indirectly using primary experimental and theoretical (in a pseudopotential manner) electronegativity information are finally compared with the direct experimental and theoretical (in an ab inition fashion) atomic radii determinations making this way a complete view of the comparison perspective. For this reasons we will analyze each two atomic radii outlined scales with those direct experimental and theoretical values. 

More recently, Laurence et al. reported the development of the pA Hx H-bond basicity scale based on a set of 1338 experimental values related to 1164 HBAs [32], This approach uses the H-bonding free energies determined in CCl for a large number of chemically diverse H-bond acceptor molecules using 4-fluorophenol as the reference donor. The p bhx meaning similar to the log AT H-bond... 

The question now is In spite of significant solvent effects, are the solution Lewis basicity scales closely related to the intrinsic gas-phase Lewis basicity scales This is an important question for computational chemists who need to identify the computational methods that yield reliable basicities. A relative comparison of gas-phase computed basicities with solution experimental basicities would avoid the difficult and approximate modelling of the solvent effect [105]. However, this comparison requires that experimental gas-phase and solution basicities (affinities) be strongly correlated. This correlation appears to exist for BF3 affinities and hydrogen-bond basicities. Equation 1.96... 

Ultimately, the affinity or the basicity scales themselves might be computed. This has been done successfully for the methanol affinity in the gas phase [41]. However, the MP2/aug-cc-pVTZ//B3LYP/6-3H-G(d,p) costly level required to obtain good agreement with experimental affinities has limited the scale to a few small Lewis bases. Proton and cation affinities and basicities in the gas phase are now computed on a routine basis (see Chapter 6). Nevertheless, the size of Lewis bases and cations and the number of bases studied are inversely proportional to the level of theory, that is, to the agreement with experimental data. As far as extended solution basicity scales towards usual Lewis acids... 

Fortunately, there are a number of theoretical, statistical and empirical reasons to believe that Lewis basicity (affinity) depends on a limited number of factors. From the quantum chemical point of view, the acid/base interaction energy can be partitioned into five terms (electrostatic, dispersion, polarization, charge transfer and exchange-repulsion). By a principal component analysis [184], 99% of the variance of an afflnity/basicity data matrix can be explained by three factors, the first two being by far the most important. A number of experimental affinity and basicity scales, and of spectroscopic scales of basicity, can be correlated by two parameters, using the EC or equations, or three quantum chemical descriptors of basicity [197]. However, these statistical and empirical approaches are limited to systems where steric effects and tt back-bonding are not important. 

The -cyclopentadienyinickel cation (CpNi+) basicity scale, CpNiCB, was established by ICR mass spectrometry [141]. Although the equilibrium data were discussed by the authors as relative affinities, by assuming negligible variations in entropies (as in the case of A1+ and Mn+), these values are really relative basicities. To anchor them experimentally, our... 

Tsang, Y., Sin, F.M., Ma, N.L. and Tsang, C.W. (2002) Experimental validation of Gaussian-3 lithium cation affinities of amides implications for the gas-phase lithium cation basicity scale. Rapid Commun. Mass Spectrom., 16, 229-237. 

A gas-phase lithium cation basicity scale has been calculated using G2 and G2(MP2) on 37 compounds and B3LYP/6-311-I-G calculations of 63 compounds including Lewis bases and saturated and unsaturated organic molecules. Good agreement with experimental basicities is found for all three... 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

These apparent restrictions in size and length of simulation time of the fully quantum-mechanical methods or molecular-dynamics methods with continuous degrees of freedom in real space are the basic reason why the direct simulation of lattice models of the Ising type or of solid-on-solid type is still the most popular technique to simulate crystal growth processes. Consequently, a substantial part of this article will deal with scientific problems on those time and length scales which are simultaneously accessible by the experimental STM methods on one hand and by Monte Carlo lattice simulations on the other hand. Even these methods, however, are too microscopic to incorporate the boundary conditions from the laboratory set-up into the models in a reahstic way. Therefore one uses phenomenological models of the phase-field or sharp-interface type, and finally even finite-element methods, to treat the diffusion transport and hydrodynamic convections which control a reahstic crystal growth process from the melt on an industrial scale. 

Object in this section is to review how rheological knowledge combined with laboratory data can be used to predict stresses developed in plastics undergoing strains at different rates and at different temperatures. The procedure of using laboratory experimental data for the prediction of mechanical behavior under a prescribed use condition involves two principles that are familiar to rheologists one is Boltzmann s superposition principle which enables one to utilize basic experimental data such as a stress relaxation modulus in predicting stresses under any strain history the other is the principle of reduced variables which by a temperature-log time shift allows the time scale of such a prediction to be extended substantially beyond the limits of the time scale of the original experiment. 

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