DFT-IGLO

Substituted benzylic mono- and dications (88 and 89) were investigated by JH and 13C NMR spectroscopy and IGLO-DFT calculations.102... 

Over the years, several computational methods have been developed. The variational theory can be used either without using experimental data other than the fundamental constants (i.e., ab initio methods) or by using empirical data to reduce the needed amount of numerical work (i.e., semiempirical data methods). There are various levels of sophistication in both ab initio [HF(IGLO), DFT GIAO-MP2, GIAO-CCSD(T)] and semiempirical methods. In the ab initio methods, various kinds of basic sets can be employed, while in the semiempirical methods, different choices of empirical parameters and parametric functions exist. The reader is referred to reviews of the subject.18,77... 

Thus, DFT-IGLO calculations of the 31P chemical shift without spin-orbital corrections yielded a value of S = 274, whereas additional spin-... 

The results suggest that chinoid type structures are the predominant resonance contributors for 88. The IGLO/DZ//3-21G calculated 13C NMR chemical shifts of benzylic monocations 88 correlate reasonably well with the experimentally obtained data. The 13C NMR chemical shifts of the carbocation centers (CH2 carbon) are calculated 10.6-12.5 ppm too deshielded. Similar results were obtained for benzylic dications 89. NMR chemical shifts of arenium ions derived from various classes of polycyclic aromatic hydrocarbons have been calculated using GIAO-DFT methods.103... 

Some benzylic mono- and di-cations have been studied by the NMR/DFT/IGLO technique. Of the stable dications, the trimethyl species (76 R = Me) was found to be the major resonance contributor to the structure of (75), and the same was found to be true for the trimethoxy derivative. In the related monocations, for (77) the major resonance contributor was (78), and this was also the case for the pentamethyl and 2,5-dimethyl-4-r-butyl compounds. The dication (79) and the trication (80),... 

The structures of the intriguing dications 156 and 158 were also computed by DFT calculations. The C NMR chemical shifts were also calculated using both GIAO and IGLO methods. Both dications 156 and 158 can also be characterized as 4c/2e a-bishomoaromatic rectangular cyclobutane dications as well as frozen Woodward Hoffmann transition state analogs. 

The trihydroxyyclopropenium ion (175) shows a single absorption in its C NMR spectrum, 128.7, which can be ascribed to the monoprotonated deltic acid from the DFT/IGLO calculations. Interestingly, the cation does not form the diprotonated species (176) even in Magic Acid (//q = -22). 

Schleyer and coworkers have shown35 that in order to use effectively the IGLO/NMR approach to structure elucidation, it is necessary to employ high-level optimized geometries (e.g. at the correlated MP2/6-31G or DFT levels of theory) in order to obtain best agreement between computed and experimental chemical shifts. Early GIAO-SCF... 

Reference TMS <513C(calc) = 183.5 ppm, geometry (Cl) B3LYP/6-31G(d) NMR calculation SOS DFT PW91 (Perdew-Wang 91) IGLO IB. 

Recently, a variety of (3-silylated carboxonium ions have been prepared and characterized by NMR spectroscopy.541 Kira et al.631 used the Corey hydride transfer method, whereas Olah, Prakash, and co-workers applied triphenylmethyl tetrakis (pentafluorophenyl)borate to silylate esters,632 ketones, enones, and carbonates633 [Eq. (3.91)]. The ions thus produced are resonance hybrids of oxocarbenium (327b) and carboxonium (327a) ions with the latter as the major contributors. Calculated (DFT/IGLO) 29 Si NMR chemical shifts agree well with the experimental data. 

C NMR spectroscopic and theoretical studies (DFT, ab initio, IGLO) of a series of cycloacylium ions were performed by Prakash et al.577 The study showed that the cycloalkyl groups have little effect on the shift of the carbocationic carbon. Furthermore, charge calculations showed that the delocalization into the cycloalkyl group is greater than in the protonated carboxylic acid (carboxonium ion), where two oxygen atoms participate in delocalization. 

A report by Jprgensen and co-workers642 about the synthesis of the first chiral tertiary alkylsilicenium ion 248 was also questioned. DFT/IGLO NMR studies showed627 that it is a silylated acetonitrilium ion. Calculations (B3LYP/6-31G level) to find a minimum energy structure for the free silicenium ion failed. Instead, the intramolecularly silylated spirocyclopropylarenium ion was found. 

In the previous sections, we presented some data for the atoms B-O, and for the OH, NH2, H20+, H2CN and H2CO+ radicals in this section we will present some results for the proton hfcc of the systems CH3, NHj, CHj, C2H2 and Ci7Hf6, studied primarily at the PWP86/IGLO-III level. Several other compounds have been studied over these last three years since the DFT-ESR schemes were developed [113, 6, 114, 116, 117, 118, 119, 121, 122, 132, 133, 136, 139, 7, 140, 141, 142], but it is beyond the scope of the present paper to review them all. Instead we will focus on a relatively small yet diverse set of systems, with the aim of giving a flavour of the accuracy and applicability of the method. 

As mentioned in section 3.2, we are with the present method able to predict the isotropic hfcc for atomic hydrogen very accurately. Since the atom lacks higher angular momenta, and the isotropic hfcc depends largely on the amount of s-character at the nucleus (through the overlap term, Eq. 4), one may expect that this property is well described for H irrespective of radical species. As was shown above, the dependence of the hfcc on the choice of DFT functional is much smaller than for the heavier atoms. In table 26 we present the PWP/IGLO-III (and other) computed proton hfcc for the abovementioned radicals and radical ions. 

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