Chemometrics of solvent basicity: multivariate analysis of the basicity scales relevant to nonprotogenic solvents

Abstract

483present. This is consistent with the longer S-N bonds to these two nitrogens.One of the most striking features in 7 is the short S--0 "nonbonded" distance of 2.55 A. Although this distance is clearly much longer than a nominal S-0 bond distance of 1.70 A (sum of covalent radii), it is clearly much shorter than the sum of the van der Waals radii of (3.25 A).37 There is, then, a reasonably strong interaction between the S+ and the 0, probably an electrostatic polarization of the oxygen lone pairs by the cationic sulfur. Since the S-N bonds do not have significant multiple-bond character, the lone pairs on oxygen can also interact with the vacant d orbitals on the S+ further strengthening the S-0 interaction. It is probably this s-0 interaction that is strongly perturbing the conformation of the eight-membered ring and perhaps the conformations a t the nitrogens. The short S--0 distance is only 0.20 A longer than the strong S-0 interaction in 1738 and is reminiscent of the bonding observed by Verkade and coworkers in compounds like 18.39 A similar long S-0 bond 19 Registry No. tris(dimethy1amino)sulfonium trimethyldifluorosiliconate, 5921 8-87-0. Supplementary Material Available: Positional parameters, temperature factors, bond distances and angles, and labeled structures (27 pages); structure factor tables (14 pages). Ordering information is given on any current masthead page. (39) (a) Clardy, J. C.; Milbrath, D. S.; Springer, J. P.; Verkade, J. G .Abstract: The dimensionality of the basicity-dependent behavior in the condensed phase, of nonprotogenic organic molecules commonly used as solvents, is approached by a principal component analysis (PCA) of a set of basicity-dependent properties (BDPs) related to hydrogen bonding, proton transfer, and interactions with hard and soft Lewis acids, including the recent -AHoBP3 basicity scale ( J . Phys. Chem. 1985Chem. , 89, 1296Chem. -1304. By use of the Information theory, the original set of 10 thermodynamic and spectroscopic BDPs was reduced to the 5 most informative scales. This in turn allowed the inclusion of more solvents (22) representing the main classes of nonprotogenic organic media. The first two factors obtained by PCA account for about 95% of the total variance of the data. A physical significance is given to these factors by correlating them with intrinsic gas-phase affinities toward the proton and the potassium ion. A blend of electrostatic and charge-transfer or electron-delocalization characters is attributed to the first factor, which is colinear with proton affinity corrected for enhanced-polarizability effect. The correlation observed between the second factor and the potassium-ion affinity corresponds to an essentially electrostatic character. The third factor, of marginal importance, arises in part from the steric hindrance of complexation (front strain). The relationships between dissimilar BDPs are unraveled in terms of the differences between the sensitivities of the properties to the electrostatic, or long-range, and charge-transfer, or shor...

A Theoretical Evaluation of the pKHBand Δ$H{{{\,\ominus}\hfill \atop {\rm HB}\hfill}}}$ Hydrogen‐Bond Scales of Nitrogen Bases

“…[71,72] Indeed, the estimated decrease in the lone pair occupancy (n (N) ), from acetonitrile to pyridine and to trimethylamine, is followed by a significant increase in the HF antibonding s* population (s* (HF) ), due to the increase in the interaction energy (E ð2Þ n!s * ). On the other hand, when the nature of the accepting group is kept constant, such as in the substituted pyridines 22, 23 and 33, there is a good proportionality between the electrostatic and the covalent contributions to the hydrogen-bond attractive energy as previously suggested by Maria et al [73] As shown in Figure 3B, the plot between pK HB and D ðHFÞ 0 values also presents separate lines corresponding to families but with slightly better statistics. The marked difference Table 4.…”

Inter- and intramolecular glycosylation of exo-glycals promoted by metallic Lewis acids

“…10 The enthalpic changes of complexation to boron trifluoride are dependent on the solvent and appear in the order: DMF > THF > CH 3 CN > CH 3 NO 2 > CH 2 Cl 2 . 11,12 This analysis is in agreement with our observation that the top three solvents relatively stabilize Lewis acids and thus the resulting reactions either do not proceed at all, or proceed very slowly. Nitroalkanes have low coordinating ability probably due to resonance and we selected 1-nitropropane for further studies because this solvent afforded the best yield (80%).…”

Lewis Basicity and Affinity Measurement: Definitions and Context

“…By the method of principal component analysis (PCA), Maria, Gal et al [184] determined the dimensionality of basicity scales, affinity scales and spectroscopic scales of basicity. They studied a data matrix of 22 neutral oxygen and nitrogen bases and 10 scales.…”