Bond activation, potential energy surface

Abstract The block correlated coupled cluster method, with the complete active-space self-consistent-field reference function (CAS-BCCC), has been applied to investigate the bond-breaking potential energy surfaces (PESs) for a C—C bond in two alkanes (ethane and 2,3-dimethyl-butane) and a C=C bond in two alkenes (ethylene and 2,3-dimethyl-2-butene). The results are compared with those from other multireference methods (CASPT2, MR-CISD, and MR-CISD+Q). It is demonstrated that the CAS-BCCC method can provide more accurate PESs for C—C bond-breaking PESs than CASPT2 and MR-CISD. The overall performance of CAS-BCCC is shown to be comparable to that of MR-CISD-pQ for systems under study. 

SPECTROSCOPY OF THE POTENTIAL ENERGY SURFACES FOR C-H AND C-O BOND ACTIVATION BY TRANSITION METAL AND METAL OXIDE CATIONS... 

Spectroscopy of the Potential Energy Surfaces for C-H AND C-O Bond Activation by Transition Metal and Metal Oxide Cations 331 By R. B. Metz... 

As briefly stated in the introduction, we may consider one-dimensional cross sections through the zero-order potential energy surfaces for the two spin states, cf. Fig. 9, in order to illustrate the spin interconversion process and the accompanying modification of molecular structure. The potential energy of the complex in the particular spin state is thus plotted as a function of the vibrational coordinate that is most active in the process, i.e., the metal-ligand bond distance, R. These potential curves may be taken to represent a suitable cross section of the metal 3N-6 dimensional potential energy hypersurface of the molecule. Each potential curve has a minimum corresponding to the stable... 

From the given Hamiltonian, adiabatic potential energy surfaces for the reaction can be calculated numerically [Santos and Schmickler 2007a, b, c Santos and Schmickler 2006] they depend on the solvent coordinate q and the bond distance r, measured with respect to its equilibrium value. A typical example is shown in Fig. 2.16a (Plate 2.4) it refers to a reduction reaction at the equilibrium potential in the absence of a J-band (A = 0). The stable molecule correspond to the valley centered at g = 0, r = 0, and the two separated ions correspond to the trough seen for larger r and centered at q = 2. The two regions are separated by an activation barrier, which the system has to overcome. 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

Recent years have witnessed a considerable activity towards extending the standard single-reference coupled-cluster (CC) methods (1-9) to potential energy surfaces (PESs) involving bond breaking without invoking a multireference description (see, e.g., refs 9-31). Undoubtedly, it would be very useful if we could routinely calculate large portions of molecular PESs with the ease-... 

AMI. While MNDO was widely accepted and extensively used, there were still some deficiencies in the model. In particular, excessive repulsions were observed in MNDO potential energy surfaces just outside chemical bonding distances. This deficiency manifested itself (5,7) in the inability of MNDO to model hydrogen bonding, as well as in large positive errors in the AHf of sterically crowded molecules and in heats of activation. Again Dewar set off to correct this deficiency. 

Alkenes strained by twist or r-bond torsion, such as E-cyclooctene, exhibit much lower barriers due to relief of strain in the TS for the oxygen transfer step. While the epoxidation of symmetrically substituted alkenes normally involve a symmetrical approach to the TT-bond, the TSs for epoxidation of E-cyclooctene and E-l-methylcyclooctene exhibit highly asymmetric transition structures. The AAE = 3.3 kcalmol" for E- versus Z-cyclooctene is clearly a reflection of the relative SE of these two medium ring alkenes (16.4 vs 4.2 kcalmol ) ". The classical activation barrier (AE ) for the highly strained bicyclo[3.3.1]non-l-ene is also quite low (Table 10, Figure 26). In these twist-strain alkenes, the approach of the peracid deviates markedly from the idealized spiro approach suggesting fliat this part of the potential energy surface is quite soft. 

Theoretical calculations on the cycloaddition reactions of a range of 1,3-dipoles to ethene in the gas phase have been carried out (85) with optimization of the structures of these precursor complexes and the transition states for the reactions at the B3LYP/6-31G level. Calculated vibration frequencies for the orientation complexes revealed that they are true minima on the potential energy surface. The dipole-alkene bond lengths in the complexes were found to be about twice that in the final products and binding was relatively weak with energies <2 kcal mol . Calculations on the cycloaddition reactions of nitrilium and diazonium betaines to ethene indicate that the former have smaller activation energies and are more exothermic. 

Electronic structure theory has developed to a point where realistic bond energies and activation barriers can be calculated. Typically the model catalysts used in such calculations are even more idealized than in the surface science experiments (perfect surfaces, ordered overlayers etc.), but the insight into the details of the potential energy surface of the reaction is much greater. 

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