Theoretical Evidences

In a study of the deoxygenation of carbonyl compounds by atomic carbon, Dewar and coworkers (8UA2802) presented experimental and theoretical evidence that the carbonyl group can react with carbon atoms to form a carbenaoxirane. 

Considerations fully discussed elsewhere 106) place an upper limit for the questionable AH as 63-67 KJ/mole. The reviewers 7I) quote the pertinent paper, but incorrectly state that the AH values quoted in Ref. 106) were given without citing or providing experimental or theoretical evidence for their justification . 

Although the first mechanism is intuitively, and for reasons of simplicity, preferred to the second one, there is no experimental evidence for excluding the latter. Theoretical evidence for preferring the former mechanism stems from a study by Jansen and Ros (1969), who performed non-empirical calculations on several configurations of a model system, viz. protonated carbon monoxide. They found that the linear +... 

There is some evidence in favor ° of the captodative effect, some of it is from ESR studies. However, there is also experimental and theoretical evidence against it. There is evidence that while FCH2 and p2CH are more stable than CH3, the radical Cp3- is less stable that is, the presence of the third F destabilizes the radical. " Certain radicals with the unpaired electron not on a carbon are akso very stable. Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. Compound 29 is a nitroxide radical so stable that reactions can be performed on it without affecting the unpaired electron (the same is true for some of the chlorinated triarylmethyl radicals mentioned above ). ot-Trichloromethylbenzyl(rer/-butyl)aminoxyl (30) is extremely stable. In... 

Kuznetsov and Boldyrev [15] provided theoretical evidence that the 33 , Al3 , and Ga3 anions (lOe) have geometrical (cyclic, planar) and electronic (two delocalized 7t electrons) properties to be considered as aromatic systems. Positive cations of all group XV trimers (14e), P3 " As3 Sb3 and Bi3+, have equilateral-triangular ground states [16]. 

The 8V + 6 valence electron rule has been completely substantiated by the calculated four-membered species in Table 2 [7], Boldyrev, Wang, and their collaborators presented experimental and theoretical evidence of aromaticity in the Al/ [19] Ga/" [20], In " [20] and isoelectronic heterosystems, XAl [21], The Al/" unit (14e) was found to be square planar and to possess two n electrons, thus conforming to the (An + 2)n electron counting rule for aromaticity. The n electron counting rule would be more powerful if we could predict the number of n electrons of metal atomic rings in an unequivocal manner. Our SN+6 electron rule only requires the number of valence electrons in Al/, which is easy to count. 

Finally, it is worthwhile to note that the methylene-transfer pathway does not involve direct transfer of a CH2 group. An H atom migration pathway is a more accurate description of this pathway, given the experimental and theoretical evidence. 

Himmel, D. and Riedel, S. (2007) After 20 Years, Theoretical Evidence That AuFy Is Actually AUF5 F2. Inorganic Chemistry, 46, 5338-5342. 

There is now a well-founded scepticism about the formal oxidation number, as there is experimental and theoretical evidence that in many chemical compounds these formal oxidation numbers have little correlation with the charge distribution in the molecule. Notwithstanding this, the formal oxidation number stiU has great popularity because ... 

Considerable experimental and theoretical evidence indicates that ring expansions of singlet cyclopropylhalocarbenes (65) have minimal contributions from tunneling. Carbene 65 has been shown to be indefinitely stable in low temperature matrices. ... 

Topspe, N.Y., Anstrom, M. and Dumesic, J.A. (2001) Raman, FTIR and theoretical evidence for dynamic structural rearrangements of vanadia/titania dcNOx catalysts, Catal. Lett., 76, 11. 

The absorption spectrum of folded AvGFP in the visible region displays two well-separated peaks at 395 nm and 475 nm, whose relative intensity depends on pH (Fig. 4b). The chromophore states associated to these two peaks have been termed, respectively, A and B [81]. Although this has been initially debated [78, 82, 83], accumulated spectroscopic and theoretical evidences indicate that these two absorption bands, respectively, arise from the neutral (A) and anionic (B) forms of the HBI chromophore [10, 11, 69-71, 84-87]. 

Figure 5.65 provides theoretical evidence that resonance-assisted H-bonding can serve as an effective mechanism for switching a methyl rotor from one preferred conformation to another, or for controlling the stiffness of torsional motions in alkylated amides. In particular, the torsional potentials of proteins (more specifically, the Ramachandran b angle at Ca) should be sensitive to N—H- O and related H-bonding interactions involving the amide backbone. In principle, this electronic... 

These results clearly indicate that barriers to all isomerisation processes are at least less than about 8kcalmol 1. In /V-benzyloxy-7V-chlorobenzamide 44 the amide isomerisation was not observable but the anomeric overlap resulted in diastereotopic benzylic hydrogens, which at coalescence afforded a barrier for rotation about the N-OBn bond of around 10.3 kcalmol-1.32 Like its /V-chloro analogue, the amide isomerisation barrier in 43 is too low to be observed by 3H NMR and even though there is definitive X-ray and theoretical evidence for anomeric effects in /V-acyloxy-/Y-alkoxyamidcs, the barrier to isomerisation about the N-OBn bond must be lower than 10.3 kcalmol-1. The n0-CN ci anomeric interaction in 44 is predicted to be stronger than the n0-CN OAc interaction in 43 on purturbation arguments.32... 

We have seen in the preceding chapters that a considerable amount of both experimental and theoretical evidence points to the existence of a transition layer at the boundary of two phases—in other words, of a layer in which the concentration of the phases is different from that in the bulk. It will, therefore, be advisable to consider quite generally what factors affect the concentration — for instance, the distribution of a solute in a solvent. 

Experimental and Theoretical Evidence for the Formation of Several Uranium Hydride Molecules. 

In fact, due to the substantial energy stabilization associated with its formation, the tj3 intermediate locks in the correct regioisomer, resulting in the observed regioselectivity. These results provide theoretical evidence for what has been previously speculated. It is important to realize that the syn-anti pairs are stereochemically equivalent, as they will lead to the same stereoisomer of the product. We will discuss the nature of the enantioselectivity in the next section. 

Hehre and co-workers (DeFrees et al., 1977, 1979a) have published both experimental and theoretical evidence in support of negative ion (anionic) hyperconjugation. These workers determined the free energies for the gas-phase hydron3 transfer equilibria (31), (32) and (33) by pulsed ion cyclotron resonance spectroscopy (Wolf et al., 1976). These equilibria, which involve the gas-phase formation of a methylamino, a methoxy and a thiomethoxy anion, all lie to the right, i.e. the formation of the isotopically light anion is favoured. These results were rationalized in terms of the MO... 

Mechanistic and theoretical studies of the Diels-Alder reaction have resulted in the characterization of this reaction as a concerted, although not necessarily synchronous, single-step process28-31 45. The parent reaction, the addition of 1,3-butadiene to ethylene yielding cyclohexene, has been the subject of an ongoing mechanistic debate. Experimental results supported a concerted mechanism, whereas results from calculations seemed to be dependent on the method used. Semi-empirical calculations predicted a stepwise mechanism, whereas ab initio calculations were in favor of a concerted pathway. At the end of the 80s experimental and theoretical evidence converged on the synchronous mechanism29-31. 

An introduction to the non-covalent forces operating in stable ionic and molecular aggregates will be presented in Section 2. A brief description of the experimental methodologies employed in the production, detection, and characterization of clusters will be given in Section 3. The available experimental evidence on the structure of chiral clusters and their intrinsic stability, reactivity, and evolution dynamics will be presented and discussed in Sections 4 (molecular clusters) and 5 (ionic clusters). In the same sections, the experimental data will be interpreted in the light of the available theoretical evidence. Finally, some concluding remarks will be expressed in Section 6. 

Theoretical evidence [Hartree-Fock (RHF) calculations and density functional theory] has been obtained for a concerted mechanism of oxirane cleavage and A-ring formation in oxidosqualene cyclization. A common concerted mechanistic pathway has been demonstrated for the acid-catalysed cyclization of 5,6-unsaturated oxiranes in chemical and enzymic systems. For example, the conversion of (24) into (26) proceeds via (25) and not via a discrete carbocation (27). Kinetic studies and other evidence are presented for various systems. 

Experimental and theoretical evidence has been obtained for an XN2-type mechanism in dissociation of B-N coordinate bonds in 2,6-bis[(dimethylamino)methyl]phenylborane derivatives. Ab initio calculations were carried out for the system of NH3 and BH3. 

A detailed mechanistic study of acid-catalysed monocyclization of 5,6-unsaturated epoxides, such as (66), has now provided compelling evidence for a pathway in which the oxirane C—O cleavage and the C—C bond formation are concerted. These experimental results are now further supported by theoretical evidence for a concerted mechanism of the oxirane cleavage and A-ring formation in epoxysqualene cyclisation, obtained at the RHF/6-31G and B3LYP/6-31 + G levels. The chemical pathway thus parallels the mechanism of the enzymatic cyclization that plays a role in the biosynthesis of isoprenoids. 

Raczyilska, E. D. Darowska, M. Experimental and Theoretical Evidence of Basic Site Preference in Polyfunctional Superbasic Amidinazine N NCDimethyl-N -P-(2-pyridylethyljformamidine. J. Org. Chem. 2004, 69, 4023-4030. 

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