Chapter 2. Physical methods. Part (iii) High-pressure chemistry

Abstract

With some 60 groups carrying out aspects of chemistry at pressures in the kbar range, this topic must be regarded as one of major importance. Two principal objectives may be sought in carrying out reactions under pressure in solution. Rates may be measured as a function of pressure and the volume of activation, AV*, obtained according to equation (1) (AV* is defined as the difference in partial molar volumes of the transition state and reagents).* This is most usefully combined with a measurement of the overall volume change for the reaction to give a 'volume profile', which is proving to be a unique probe into the nature of the transition state. A similar relationship holds for equilibria, equation ( 2) where A V is the volume change for the reaction. Pressure will therefore favour a process that involves a diminution in volume, and conversely. The other aspect of interest is to employ this principle for preparative chemistry in order to bring about reactions that have a large, negative value of AV* but which will not occur at 1 bar. Much activity around the world is evident on both fronts. -RT[d In k / d p ] p + o = AV* -RT[d In K/dp]p+Ogeneral review articles, and data compilation^^.^ in which some 1500 volumes of activation and volumes of reaction of both organic and inorganic reactions are collected. Pressure effects on enzymic reactions,' on displacements from co-ordination com-pound~,~*' and upon isotope effects' are among more specialized articles. The 50th Commemorative Volume of Reviews of Physical Chemistry of Japan is devoted to review articles on Modern Aspects of Physical Chemistry at High Pressure; topics The subject has been thoroughly reviewed recently in two * The units of molar volume throughout this article are cm' mot-'.

Theoretical Study of Reactivities in Electrophilic Aromatic Substitution Reactions: Reactive Hybrid Orbital Analysis

“…Configuration Analysis of the Interaction between Electrophile and Benzenes. In textbooks, orbital interactions are usually described in terms of canonical molecular orbitals …”

“…Electrophilic aromatic substitution is one of the most thoroughly studied reactions in organic chemistry. − It was to this class of reactions that the frontier orbital (FO) theory was applied for the first time by Fukui et al , Since then, the FO theory has gained general acceptance by the chemical community and has been utilized to interpret reactivities and selectivities of chemical reactions . Nevertheless, the FO theory does not predict meta selectivity in the electrophilic substitution of nitrobenzene. 1a,11b That is, the highest occupied molecular orbital (HOMO) of nitrobenzene has equivalent amplitudes on the ortho and meta positions as shown below, and thus the FO theory predicts that substitution would occur at both positions.

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“…Therefore, most textbooks of organic chemistry and physical organic chemistry avoid the use of FO theory in this case and attempt to interpret this regioselectivity in terms of stabilization of the intermediate cationic σ-complexes, rather than in terms of orbital interactions …”

Early and middle Miocene ice sheet dynamics in the Ross Sea: Results from integrated core-log-seismic interpretation

“…5C). The transformation of opal-A to opal-CT in silica-rich sediment may reduce porosity by over 30% (e.g., Isaacs, 1981;Tada, 1991). In addition, extremely high resistivity values are recorded at the top of RSS3-Ia along with variation between high and very high values, which probably reflects the regional heterogeneity of the diagenetic front (Fig.…”

Supplemental Material: Early and middle Miocene ice sheet dynamics in the Ross Sea: Results from integrated core-log-seismic interpretation

“…Silica diagenetic reactions occur typically within the first 1000 m of burial of biosiliceous sediment (Isaacs, 1981;Williams et al, 1985). RSS3-Ia is related to diagenetic opal presently buried at 325 mbsf at Site U1521 (Fig.…”