Molecular systems acetylene

As a result of the second-order effect, also large C4, 4 2 monocycles are expected to be acetylenic as is explained by the Peierls distortion that is seen in one-dimensional electronic systems with periodic potentials [35b-e]. This effect is associated with the distortion of the potential energy surface to lower the total energy of the system through electron-phonon coupling [35b]. For a molecular system, the same effect is also referred to as the second-order Jahn Teller (SOJT) effect that leads to the vibrational-mode... 

Yamamoto and colleagues were able to develop an additive-free palladium catalyst system for the oxidative carbonylation of alkynes [87, 88]. By using their palladium-phosphine catalyst in the presence of molecular oxygen, acetylene-carboxylates were formed under atmospheric pressure of CO at room temperature. A detailed mechanistic study was also carried out proposing a reductive elimination of a palladium species with methoxycarbonyl and alkynyl residues. The oxidation of Pd(0) to Pd(II) species was confirmed to proceed cleanly with... 

It s time for a reality check. We know well that HMO theory has some severe limitations, so we should be skeptical about the remarkable predictions it makes about poly acetylene and other systems. Qualitative trends should be correct, but is polyacetylene really a metal We need to go to the next level of theory, and just as with molecular systems, the next step is to include the effects of electron-electron repulsion that are so ardently avoided in HMO theory. 

To conclude this section we note that large molecular systems other than proteins have been the subject of hybrid QM/MM studies. Two such examples involve molecular dynamics simulations of surfaces using AMl/MM and PM3/MM potentials. In one the absorption of acetylene on a silicon surface was investigated [95] and in the other diamond surface reconstructions [96]. 

Acetylenes XCCY with n conjugated substituents, X and Y, on both carbon atoms have planar or perpendicular conformations. The substituents can be electron-donating or -accepting. The planar conformers are linear conjugate D-TI-D, D-IT-A, or A-IT-A systems whereas the perpendicular conformers are composed of ri-D and IT-A not in conjugation with each other. The orbital phase is continuous only in the planar conformations of D-TI-A (Scheme 23, cf. Scheme 4). The acetylenes with X=D (OR, NR ) and Y=A (RCO, ROCO) prefer planar conformations. When both substituents are electron-donating or accepting, the phase is discontinuous. The acetylenes then prefer perpendicular conformations. The predicted conformational preference was confirmed by ab initio molecular orbital calculations [32]. Diacetylenic molecules show similar conformational preference, which is, however, reduced as expected [32]. 

A technique that allows rapid evaluation of molecular stability using small (20-30 mg) samples has been demonstrated and applied to three different families of strained molecules. All of the molecules studied are stable at room temperature, though most must be stored in nonmetallic containers to avoid catalytic decomposition. Of the four molecules shown in Fig. 4.1, the least thermally stable was quadricyclane, for which decomposition lifetimes drop below 10 ms at about 500 K. The other three molecules had similar stabilities, with lifetimes dropping below 10 ms above 700 K. For all systems studied, decomposition by loss of small hydrocarbon fragments (acetylene or ethene) was an important decomposition mechanism in the gas phase. For all but AEBCB, isomerization was also a significant decomposition mechanism. At high pressures, one would expect more isomerization because the very rapid collision rate should allow collisional stabilization of the isomerization products. 

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