Computational studies CCSD calculations

Computational study at UHF or UB3LYP level of theory disagreed the intermediacy of perepoxide and showed that the reaction proceeds via biradical intermediate. However, calculations at CCSD(T)/6-31G //RB3LYP/6-31G, which includes substantial dynamic configuration interaction, revealed that the reaction of cw-2-butene with 102 proceeds through an early ratedetermining TS, and then the reaction path appears to lead toward a perepoxide-like structure (Scheme 6, R = = H). The first TS (TS6) is of Cs... 

Recently, Siehl and co-workers have made computational studies of the C NMR shift of a series of a-vinyl-substituted vinyl cations (l,3-dienyl-2-cations, Figure 3.14). They found that inclusion of electron correlation effects are important to get reliable data. The small differences (A = 1-2 ppm) between calculated [CCSD(T)/ tzp/dz] and observed NMR chemical shifts suggest that the geometry of the cations is not significantly affected by the medium. 

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... 

Atom-Atom Interactions. - The methods applied, usually to interactions in the inert gases, are a natural extension of diatomic molecule calculations. From the interaction potentials observable quantities, especially the virial coefficients can be calculated. Maroulis et al.31 have applied the ab initio finite field method to calculate the interaction polarizability of two xenon atoms. A sequence of new basis sets for Xe, especially designed for interaction studies have been employed. It has been verified that values obtained from a standard DFT method are qualitatively correct in describing the interaction polarizability curves. Haskopoulos et al.32 have applied similar methods to calculate the interaction polarizability of the Kr-Xe pair. The second virial coefficients of neon gas have been computed by Hattig et al.,33 using an accurate CCSD(T) potential for the Ne-Ne van der Waals potential and interaction-induced electric dipole polarizabilities and hyperpolarizabilities also obtained by CCSD calculations. The refractivity, electric-field induced SHG coefficients and the virial coefficients were evaluated. The authors claim that the results are expected to be more reliable than current experimental data. 

The most striking disagreement was observed for the EQC and the resulting quadrupole moment of CO [164,165]. In our first study [164] CCSD(T) calculations for the quadrupole moment at the center of mass and CCSD calculations for the EQC were used to determine the quadrupole moment with respect to the EQC within both theories. Our results slightly favored the BLH theory, but the computations were not of sufficient accuracy to decide unambiguously which of the two theories could be correct. 

The structure and IR spectrum of furazan were studied by vibrational S CF (VSCF) and Cl (VCI) calculations based on a high-quality potential derived from electronic structure calculations up to the CCSD(T)/aug-cc-pCVQZ level. Excellent agreement was found between the computed and the experimental results <2005THA327>. 

QCISD(T) and CCSD(T) calculations can be cost effective and, at the same time, a reliable computational approach to study epoxidations of aUcenes with peroxy acids. 

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