CBS-n methods

A variety of compound methods have been developed in an attempt to accurately model the thermochemical quantities we have been considering. These method-s attempt to achieve high accuracy by combining the results of several different calculations as an approximation to a single, very high level computation which is much too expensive to be practical. We will consider two families of methods the Gaussian-n methods and the Complete Basis Set (CBS) methods. 

In addition to the Gaussian-n methods, there is a variety of CBS (complete basis set) methods, including CBS-Q, CBS-q, and CBS-4, which have also enjoyed some popularity among computational chemists. In these methods, special procedures are designed to estimate the complete-basis-set limit energy by extrapolation. Similar to the Gn methods, single-point calculations are also... 

If the CB[n] family is to supplant the cyclodextrins as the platform of choice for basic and applied studies of molecular recognition, then a series of additional developments must take place, many of which involve the development of new synthetic procedures. First, improved methods for the separation of crude CB[n] mixture on the laboratory (i.e., up to 1 kg) to industrial (i.e., tonnes) scale must be developed. Alternatively, synthetic procedures that selectively target a specific CB[n] (e.g., CB[8] or CB[10]) would be tremendously valuable. Second, the development of high-yielding methods of functionalizing pre-formed CB[n] - particularly for CB[7], CB[8], and CB[10] - would have a dramatic impact on the field. Third, a deficiency of the CB[n] family relative to the cyclodextrins is that they are inherently achiral, which... 

Two examples should be considered for further discussion the covalent nitroglycerine (NG) and the ionic ammonium dinitramide (ADN). The calculated enthalpies (H) and free energies (G) for NG, the ions NH4 and N(N02) > as well as the relevant atoms H, C, N and 0 using the CBS-4M method, are summarized in Table 4.6. 

Table 4.12 presents the calculated energies of formation for the solid neutral species and salts based on the CBS-4M method (see Ch. 4.2.1). Furthermore we see from table 4.12 that for the nitronium ([N02]+) species the covalently bound form is favored over the ionic salt by 26.9 kcal mol 1 while for the nitrosonium species ([NO]+) the salt is favored over the covalent isomer by 10.5 kcal mol-1. This change from the preferred covalent form of —N02 compound (actually a nitrato ester) to the ionic nitronium salt can be attributed almost exclusively to the increased lattice enthalpy of the (smaller ) N0+ species (AffL(N0+ - N02 salt) = 31.4 kcal mol-1) (N.B. The difference in the ionization potentials of NO (215 kcal mol-1) and N02 (221 kcal mol-1) is only marginal). 

There are methods that automate some of these steps. They are called composite methods because they combine results from several calculations to estimate the result that would be obtained from a more expensive calculation. The most popular families of composite methods are represented by Gaussian-3 (G3) theory [68,109] and CBS-APNO theory [110,111], where CBS stands for complete basis set. Both families of methods, which are considered reliable, include empirical parameters. The CBS theories incorporate an analytical basis set extrapolation based on perturbation theory, which is in contrast to the phenomenological extrapolation mentioned above. When the Gaussian software is used to perform these calculations, steps 2-, above, are performed automatically, with the result labeled G3 enthalpy (or the Hke) in the output file [20,99]. The user must still choose a reaction (step 1) and manipulate the molecular enthalpies (steps 5 and 6). The most precise composite methods are the Weizmann-n methods, which however are very intensive computationally [112]. 

The first new cucurbituril molecule to be synthesized and characterized was decamethylcucurbit[5]uril, reported by Stoddart et al. in 1992 [32]. However, the small cavity size of the n = 5 ring was not particnlarly useful for the inclusion of guests. The real breakthrough in expanding the CB[n] families came in 2000, when Kim et al. [33] reported a useful synthesis method for obtaining the... 

The first report on the liquid crystalline properties of these compounds was published by Gray and Mosley [44] in 1976. The series of 4 -n-alkyl-4-cyanobiphenyls (CBn) have been widely studied by different methods due to their readily accessible nematic ranges around room temperature. The compounds have the phase sequences crystal-nematic-isotropic for CBS, CBIO, and monotropic nematic for CBS, CB4 crystal-smectic A-nematic-isotropic for CB9 crystal-smectic A-isotropic for CBll. The lower homologous CB2 is nonmesogenic. The general chemical structure of the compounds CBn is presented in Fig. 1. 

C-H and N-H bond dissociation energies (BDEs) of various five- and six-membered ring aromatic compounds (including 1,2,5-oxadiazole) were calculated using composite ab initio CBS-Q, G3, and G3B3 methods. It was found that all these composite ab initio methods provided very similar BDEs, despite the fact that different geometries and different procedures in the extrapolation to complete incorporation of electron correlation and complete basis set limit were used. A good quantitive structure-activity relationship (QSAR) model for the C-H BDEs of aromatic compounds... 

Petersson et al. had earlier proposed [23] an alternative expression E(n) = Eoo + ]CiSn+i A/(l + 1/2)6 in the context of the CBS methods developed in his group. The summation is carried out numerically in that paper, but in fact an elegant analytical approximation exists for... 

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