Oxirene potential energy

The main advantage of MP2/6-31G optimizations over HF/3-21 ( > or HF/ 6-31G ones is not that the geometries are much better, but rather that for a stationary point, MP2 optimizations followed by frequency calculations are more likely to give the correct curvature of the potential energy surface (Chapter 2) for the species than are HF optimizations/frequencies. In other words, the correlated calculation tells us more reliably whether the species is a relative minimum or merely a transition state (or even a higher-order saddle point see Chapter 2). Thus fluorodiazomethane [91] and several oxirenes [53] are (apparently correctly) predicted by MP2 optimizations to be merely transition states, while HF optimizations... 

If oxirene should exist and NF5 should not, what are we to make of pyramidane (Fig. 9.3) This molecule contradicts the traditional paradigm [20] of tetracoordi-nate carbon having its bonds tetrahedrally directed the four bonds of the apical carbon point toward the base of a pyramid. Part of the calculated [21] potential energy surface of pyramidane is shown in Fig. 9.3. To improve the accuracy of the... 

Wilson, P. J., Tozer, D. J. A Kohn-Sham study of the oxirene-ketene potential energy surface. Chem. Phys. Lett. 2002, 352, 540-544. Bogdanova, A., Poplk, V. V. Experimental and Theoretical Investigation of Reversible Interconversion, Thermal Reactions, and Wavelength-Dependent Photochemistry of DIazo Meldrum s Add and Its DIazIrIne Isomer, 6,6-Dlmethyl-5,7-dloxa-1,2-dlaza-splro[2,5]oct-1-ene-4,8-dlone. J. Am. Chem. Soc. 2003, 125, 14153-14162. 

Nitrogen pentafluoride represents an interesting contrast to oxirene. Oxirene is, on paper, a reasonable molecule there is no obvious reason why, however unstable it might be because of antiaromaticity [4] or strain, it should not be able to exist. On the other hand, NFs defies the hallowed octet rule why should it be more reasonable than, say, CHfi Yet a comprehensive computational study of this molecule left tittle doubt that it is a (relative) minimum on its potential energy surface [5]. The full armamentarium of post-HF methods, CASSDF, MRCl, CCSDT, CCSD(T), MP2 (section 5.4) and DFT (chapter 7) was employed here, and all agreed that Dan (section 2.6) NF5 is a minimum. 

The C2RR 0 potential energy hypersurface includes oxirene, the ketocarbene, and the ketene in (8-43 R = R = H). It has been studied very recently in a joint investigation by the groups of Radom and Schaeffer (Scott et aL, 1994) with ab initio methods incorporating high levels of electron correlation and basis sets that include up to f and g functions. 

Fig. 3.12 The best results calculated for the species in the Wolff rearrangement by Scott et al. [70]. The numbers are relative energies in kJ mol . The accuracy of the calculations does not rule out the possibility that oxirene is only a transition state connecting equivalent-energy carbenes and scrambling the label, or that the carbene is not really a stationary point (neither a minimum nor a transition state) on the potential energy sur ce...

Fig. 14.3 (cf. Fig. 3.12) Calculated potential energy surface for the oxirene-methanoylcarbene (formylcarbene) interconversion and isomerization of the carbene to ketene. Based on the best CCSD(T) calculations of Scott et al. [12]. ZPE corrections are omitted here the only significant effect of their inclusion is to raise the relative energy of the carbene slightly above that of oxirene...

If oxirene is a transition state there is no barrier separating it from the carbene (more precisely, the barrier is negative, because the curvature of the potential energy surface at a transition state is negative along the reaction coordinate [1]). The calculations of Scott et al. [12] do not settle whether oxirene and substituted oxirenes... 

Figure 7.13 Potential energy profile for the lowest three electronic states of ketene. Note that in addition to the low barrier for dissociation from the triplet state, Ti, there is a higher barrier for formation of a symmetric structure, oxirene, that can rearrange to a ketene molecule in which the two C atoms have exchanged. In the transition state for dissociation the reaction coordinate is a C—C stretch, as shown. The products are detected by laser-induced fluorescence of CO [adapted from A. P. Scott etal., JACS 116,10159 (1994) see also Moore and Smith (1996)].

The intermediates involved in the Wolff rearrangement have also been probed computationally. Whether oxirene is a real minimum or a transition state on the potential energy surface is extremely sensitive to the theoretical method used. Schaefer, Radom, and co-workers have calculated the CjHjO potential... 

Wilson, P.J., and Tozer, D.J., A Kohn-Sham Study of the Oxirene-Ketene Potential Energy Surface, Chem. Phys. Lett., 352, 540, 2002. 

The antiaromatic oxirenes 11 are extremely elusive but their potential involvement in photochemical Wolff rearrangements by equilibration with the -oxocarbenes 12 has generated both theoretical and experimental interest and the subject has been reviewed <2004MR0291>. Experimental evidence for participation of oxirenes <2004MR0291> is supported by DFT calculations that suggest that oxirene corresponds to an energy minimum <2003JMT(629)263, CHEC-III(1.03.7)215>. 

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