Acetylene critical point

Raveau now calculated the values of p, v from van der Waals equation, plotted the logarithms, and compared the diagram with a similar one drawn from the experimental results. The results showed that the diagrams could not be made to fit in the ease of carbon-dioxide and acetylene, the divergencies being very marked near the critical point. 

The value of the charge density at a bond critical point can be used to define a bond order (Bader et al. 1983 Cremer and Kraka 1984). The molecular graphs for ethane, ethylene, and acetylene are shown in Fig. 2.8. In each case the unique pair of trajectories associated with a single (3, — 1) critical point is found to link the carbon nuclei to one another. Multiple bonds do not appear as such in the topology of the charge density. Instead, one finds that the extent of charge accumulation between the nuclei increases with the assumed number of electron pair bonds and this increase is faithfully monitored by the value of p at the bond critical point, a value labelled p, . For carbon-carbon bonds, one can define a bond order n in terms of the values of Ph using a relationship of the form... 

Typical electron momentum densities with (3, — 1) and (3, +1) saddle points at zero momentum are found in MgO and acetylene (HCCH), respectively. A momentum density with a zero momentum (3, — 1) critical point is shown for MgO in Fig. 19.6. In the vertical plane of symmetry /T(p, 0,pj has the structure of two hills separated by a ridge or col, and one sees two local (and global) maxima located symmetrically along the p axis. The plot in the horizontal symmetry plane has the structure of a hill. 

The electron momentum density for acetylene is shown in Fig. 19.7. The plot in the p p plane has a complicated structure. The zero-momentum (3, - -1) critical point is a pass between two peaks located along the p axis, and is a barrier separating two troughs centered along the p axis. Critical points along the p. axis must also appear in all other directions perpendicular to the p axis. The plot of 7T(p) in the horizontal p py plane resembles a volcano. 

Fig. 17.7 The molecular graph of the cluster containing four acetylene molecules big black and grey circles correspond to C and H-attractors, respectively, small red, yellow and green circles designate, bond, ring and cage critical points, respectively...

The QTAIM approach mentioned here before is a very useful tool to describe La-Lb interactions since it is possible to have a deeper insight into changes in the electron charge distribution being the result of complexation especially the changes of hydrogen bonded systems. For example, the position of the C-H proton donating bond critical point (BCP) was analyzed recently [98] for the mentioned earlier here complexes of acetylene and fluoroform. It is possible to decompose the C-H bond into two radii dehned by the position of BCP (see Fig. 9.13). Hence there is the radius of the carbon atom which is the distance between BCP and the C-atom... 

In the aggregate state Hquid acetylene has particularly high energy content and therefore it has explosive character. In the vapor-pressure curve (Fig. 8.1) basic data such as the critical point and the dew point are shown. The formation of liquid acetylene should be absolutely avoided when handling this gas, as this state is not manageable without elaborate meastues. 

Lamottke was the first to execute this variant. He found that a solution of 23, saturated with acetylene gas and exposed to an excess of CpCo(C2H4)2, formed the desired target 40 in 43% yield (Scheme 16). This promising result was the starting point for the experimental efforts of one of the authors. While the process was reproducible, attempts to scale it up beyond sub-millimolar quantities, necessary for a fairly early and critically important synthetic step, led to significant decreases in yield (17-24%). The difficulty appeared to be formation of cinnamic amide 41, isolated in 50-60% yields as a mixture of cis and trans isomers.39 Minimization of this unwanted product required consideration of the [2 + 2 +2]cycloaddition mechanism. 

Acetylenes provide a convenient starting point for the syntheses of several heterocycles. The patterns in ring closure vary somewhat so that structural assignments have to be made critically. The similar result in equations (138) and (140) differs... 

With the aid of a computer, about 40 adsorption systems have been analyzed for equilibrium. Typical examples are presented in the graphs of Figures 1 to 4, where the solid curves represent calculated adsorption isotherms and the circles denote experimental points. The temperatures are expressed in degrees centigrade and pressures in mm of Hg (torr). From above-critical temperatures, for instance, n-hexane (tc = 235°C) and acetylene tc = 36°C), effective values have been obtained by extrapolating the linear dependence of p " on t according to Equation 5 for the temperatures indicated. Effective values of Ps for t tc were calculated by the van der Waals equation, which may be written in the form (12)... 

Compound 18 exhibits solely a chiral nematic discotic phase (N ) phase because the steric effect of the branched chains at the chiral centre disrupts the ability of the molecules to pack in columns. The large size of the planar aromatic core ensures a high clearing point, but the liquid crystal tendency depends critically on the type of chiral peripheral chain. Hexa-substituted phenylacetylenes were discussed in chapter 3 and exhibited the Np phase. Perhaps not surprisingly, when one of the peripheral acetylene units is chiral, the N phase is exhibited (compound 19). 

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