Natural bond orbital analysis interaction

MRMP multireference Moller—Plesset perturbation (jt-CI) theory (with -configuration interaction) NBO natural bond orbital (analysis)... 

Natural bond orbital analysis of early and late TSs has been carried out to explore the factors involved in tt-selectivity of nucleophilic addition to carbonyls.209 Cieplak s o —r o hyperconjugation hypothesis (where o is the incipient bond) is not supported by the results for early TSs, and evidence in favour of Felkin-Anh s o er hypothesis is weak. Late TSs are devoid of o 7r(t=() interactions here, the Cieplak model may be applicable. 

Structures (Table 16), points towards chemical bonding, albeit very weak. In fact, a subsequent natural bond orbital analysis of the electronic structure of HeBeO, NeBeO, and ArBeO by Hobza and Schleyer [158] came to the conclusion that the essential nature of the interactions is charge transfer. For HeBeO they found that the overlap between the donor Is orbital of He with the acceptor a orbital of BeO is quite large (0.42). 

Badenhoop JK, Weinhold F (1997) Natural bond orbital analysis of steric interactions. J Chem Phys 107(14) 5406—5421... 

Schleyer et al. combine Weinhold s natural bond orbital analysis with calculations using nonrelativistic and quasi-relativistic ECPs to analyze relativistic effects on the Pb—C and Pb—Pb rotational barriers. This work shows that, although relativity has a large effect on interactions between vicinal T—H and T —H bonds that control the barrier height, the effect is of similar magnitude for the minimum (eclipsed conformer) and transition state (staggered conformer), so that rotational barriers are not affected. 

Atomic charges and orbital populations has been computed by means of Natural Bond Orbital analysis [29]. In order to estimate quantitatively the importance of ligand —> metal a-donation and metal —> ligand 7r-back-donation to interaction energies, we used the Charge Decomposition Analysis (CDA) [30] method of Frenking and coworkers [30] as implemented in the AOMIX program [31, 32]. 

In this section, we present a unified picture of the different electronic effects that combine to determine methyl rotor potentials in the S0, Sp and D0 electronic states of different substituted toluenes. Our approach is based on analysis of ab initio wavefunctions using the natural bond orbitals (NBOs)33 of Weinhold and cowork-ers. We will attempt to decompose the methyl torsional potential into two dominant contributions. The first is repulsive steric interactions, which are important only when an ortho substituent is present. The second is attractive donor-acceptor interactions between CH bond pairs and empty antibonding orbitals vicinal to the CH bonds. In the NBO basis, these attractive interactions dominate the barrier in ethane (1025 cm-1) and in 2-methylpropene (1010 cm-1) see Figure 3. By analogy, donor-acceptor attractions are important in toluenes whenever there is a substantial difference in bond order between the two ring CC bonds adjacent to the C-CH3 bond. Viewed the other way around, we can use the measured methyl rotor potential as a sensitive probe of local ring geometry. 

With accurate calculated barriers in hand, we return to the question of the underlying causes of methyl barriers in substituted toluenes. For simpler acyclic cases such as ethane and methanol, ab initio quantum mechanics yields the correct ground state conformer and remarkably accurate barrier heights as well.34-36 Analysis of the wavefunctions in terms of natural bond orbitals (NBOs)33 explains barriers to internal rotation in terms of attractive donor-acceptor (hyperconjuga-tive) interactions between doubly occupied aCH-bond orbitals or lone pairs and unoccupied vicinal antibonding orbitals. 

A natural bond orbital-based CI/MP through-space/bond interaction analysis of the Sn2 reaction between allyl bromide and ammonia17 showed that allyl bromide reacted faster than propyl bromide because the a - n and n — a interactions stabilize the allyl bromide transition state equally. 

Other theoretical studies discussed above include investigations of the potential energy profiles of 18 gas-phase identity S 2 reactions of methyl substrates using G2 quantum-chemical calculations," the transition structures, and secondary a-deuterium and solvent KIEs for the S 2 reaction between microsolvated fluoride ion and methyl halides,66 the S 2 reaction between ethylene oxide and guanine,37 the complexes formed between BF3 and MeOH, HOAc, dimethyl ether, diethyl ether, and ethylene oxide,38 the testing of a new nucleophilicity scale,98 the potential energy surfaces for the Sn2 reactions at carbon, silicon, and phosphorus,74 and a natural bond orbital-based CI/MP through-space/bond interaction analysis of the S 2 reaction between allyl bromide and ammonia.17... 

Most recently, Glendening and Streitwicser have decomposed the interaction energy of the water dimer using natural bond orbitals. Their natural energy decomposition analysis (NEDA) combines the normal electrostatic and exchange energies into a single ES term,... 

Decomposition of interaction energies is desired for qualitative chemical analyses of complicated multi-valent interactions in supramolecular aggregates but such a decomposition cannot be uniquely defined within fundamental physical theory. A popular semi-quantitative decomposition method with nice formal features to be mentioned in this context is Weinhold s natural bond orbital (NBO) approach to intermolecular interactions [232, 233]. Comparable is the recently proposed energy decomposition analysis by Mo, Gao and Peyerimhoff [234, 235] which is based on a block-localized wave function. Other energy decomposition schemes proposed are the energy decomposition analysis (EDA) by Kitaura and Morokuma [236] and a similar scheme by Ziegler and Rauk [237]. 

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