Early Computational Studies

1) High-level wavefunction methods promise the accurate treatment of these long-range electron correlation effects however, such methods are still too computationally demanding for routine use on large model systems. 

Overall, these early studies of carboxylate-assisted intramolecular C-H activation established the key features of these AML A/CMD processes, where an electron-deficient metal center works in concert with a pendant carboxylate base to promote C-H activation. This is most evident when an agostic intermediate is involved and such species also rationalize how these systems can also perform C(sp )-H bond activation. Whether C-H activation is achieved as a one- or two-step process appears rather system dependent. Alternative mechanisms, for example, proton transfer onto a halide ligand, oxidative addition, or AMLA-4 processes involving proton transfer onto the inner (Pd-bound) oxygen of the carboxylate were all ruled out. Likewise, no evidence for S Ar processes had been reported. Subsequent work was set against this background and considered the various other parameters that may affect the C-H bond activation process. 

2 The Role of the Base, Solvent, and Additives on Pd-Mediated Intramolecular C-H Activation 

Additional factors, including the role of specific solvation and of some commonly used additives, have been included in the computational modeling of 

---

An important task for theory in the quest for experimental verification of N4 is to provide spectral characteristics that allow its detection. The early computational studies focused on the use of infrared (IR) spectroscopy for the detection process. Unfortunately, due to the high symmetry of N4(7)/) (1), the IR spectrum has only one line of weak intensity [37], Still, this single transition could be used for detection pending that isotopic labeling is employed. Lee and Martin has recently published a very accurate quartic force field of 1, which has allowed the prediction of both absolute frequencies and isotopic shifts that can directly be used for assignment of experimental spectra (see Table 1.) [16]. The force field was computed at the CCSD(T)/cc-pVQZ level with additional corrections for core-correlation effects. The IR-spectrum of N4(T>2 ) (3) consists of two lines, which both have very low intensities [37], To our knowledge, high level calculations of the vibrational frequencies have so far only been performed... 

Computational chemistry has played a large role in helping to determine these energies. Early computational studies confirmed the double-well nature of the PE8. Coming to an agreement on the well depths for the ion-dipole complexes and the activation barrier remained a challenge into this century. 

The first-rank and second-rank OTCFs are mostly studied because of their relevance to experiments. One of the early computational studies of orientational dynamics in the isotropic phase near the /-A transition is due to Allen and Frenkel [105]. In their molecular dynamics simulation study of a system of N = 144 hard ellipsoids of revolution, the slowdown of orientational dynamics on approaching the I-N transition was captured—in particular, in the time evolution of the... 

As computational methods for describing excited states have been refined, additional understanding of the structures has developed. Relatively early computational studies provided some indication of the geometries associated with the butadiene excited states.The ground state has a maximum at a twist of 90° about the C(l)-C(2) bond. This structure, which can be approximately described as a singlet methylene-allyl diradical, is found at about 2.3 eV and is more stable than a structure with 90° twist at both terminal groups (3.1 eV). There is no major pyramidalization of the methylene groups in this second structure. The spectroscopic (Franck-Condon) state is about... 

Early computational studies of ionic liquids include semi-empirical methods these have now generally been superseded as advances in computing resources have allowed the routine use of higher level DFT and HF methods. As these methods have not been parameterized for the unusual interactions present in ionic liquids, the level of accuracy is unknovm. For example, early attempts to determine the H-bonding in [BMIM]C1 (Fig. 4.2-1) using the AMI [33] method predicted a very long C -H bond (1.56 A) in favor of a short H-Q bond (1.40 A) [34]. The equivalent interaction calculated at the MP2 level predicts the reverse, a strong C -H bond (1.11 A), and weaker H-Q interaction (2.02 A). 

A. Early Computational Studies Based on HUckel Theory... 

---