Charge density calculations

Here, and Lj are local indices having the form shown in Eq. (5), where lo is a constant characterizing the ith atom (in some cases the atom valence can be used to this end), Nh is the number of attached hydrogen atoms and is the charge density calculated by some fast method such as the Marsili-Gasteiger charge calculation method [7]. 

Apart from the general study by Wait and Wesley on azanaphthalenes, including most of the pyridopyridazines (66JSP(19)25), more specialized studies have attempted to relate charge density calculations to various chemical and physical properties, with greater success in the latter case. 

H-Dibenz[6,e][l,4]oxaborin UV spectrum, 1, 643 6H- Dibenz[c, e][l, 2]oxaborins as antioxidants, 1, 663 Dibenzoxanthenes charge density calculations, 3, 576 Dibenzoxanthenium salts X-ray studies, 3, 625 Dibenzoxanthones... 

UV spectra, 3, 599 xanthone synthesis from, 3, 840 Xanthydryl chloride xanthone synthesis from, 3, 840 Xanthyletin synthesis, 3, 743, 804 Xanthylium laser dye synthesis, 3, 865 Xanthylium salts aromaticity, 3, 641 charge density calculations, 3, 576 C NMR, 3, 592 H NMR, 3, 585 mass spectra, 3, 620 reactions... 

Sufficient charge must be available to supply the required critical surface charge density. Calculations show that 50-pound plastic bags and plastic lined 55-gallon drums are too small to represent a credible risk. 

Although electrophilic substitution in these compounds is difficult, bromination of pyridyl[2,3-bromo derivative, in accord with charge density calculations [73HC(27)968]. it is likely that this product formed as a result of HBr elimination from a 3,4-dibromo... 

Following Vincent and Radom s comprehensive ab initio study (1978), Escudero et al. (1985) also performed ab initio calculations at the STO-3G level for seven 4-substituted benzenediazonium ions. For the unsubstituted benzenediazonium ion, the optimized geometries are the same as in Figure 4-1 (I). For charge density calculations another population analysis method was used (Escudero and Yanez, 1982), and this gave results different from those of Vincent and Radom. However,... 

To check this prediction, a number of MaxEnt charge density calculations have been performed with the computer program BUSTER [42] on a set of synthetic structure factors, obtained from a reference model density for a crystal of L-alanine at 23 K. The set of 1500 synthetic structure factors, complete up to a resolution of 0.555 A [45], was calculated from a multipolar expansion of the density, with the computer program VALRAY[ 46],... 

There have been discussions whether the amide addition at C-6 is charge-controlled or orbital-controlled. Charge density calculations in 4-phenylpyrimidine (MNDO method) predict that the addition of the amide ion would preferably take place at position 2 (95UP1) this, however, does not agree with the experimental results. Therefore, the conclusion seems justified that the addition is not charge-controlled. Frontier orbital calculations, using the SCF-PPP method, show that the frontier orbital densities in the LUMO of pyrimidine are zero at C-2 and C-5, making these positions... 

Although many questions remained to be answered, it looked some years ago as if a consistent picture of the nucleophilic aromatic photosubstitutions was emerging from the experimental data and from charge density calculations. This picture would present an understanding of the photosubstitution process somewhat equivalent to what had been reached in the course of time for the well-known thermal aromatic substitution reactions. 

For azulene and its derivatives, the preference for the 1-position can be rationalized by considering the charge densities calculated for the various electronic states (Table 3, Pariser, 1954). 

Spin and charge density calculations are consistent with the experimentally established magnetic moment of 5.4 BM (at around room temperature) for Fe(H20)6 ". ... 

On the basis of NMR results, an intriguing anion within the trianion structure was proposed for S ". Such a structure would require 3.0 ee excess in the rim, and 1.0 ee excess in the hub, as compared to neutral 8. Analysis of the charge densities calculated at both semiempirical AMI and ab initio levels, however, predict the hub atoms to gain only about 0.5 ee, while the rim atoms bear more of the charge at 3.5 ee. ... 

The recent band calculations by Brooks et al. show indeed from charge density calculations that in stoichiometric UO2 a purely ionic picture is incorrect the oxygen ion is singly charged, the second electron not being retained by the Madelung potential. Simple considerations based on mechanical properties and spectroscopic evidence had already pointed at these conclusions. 

With 2-methylthiazole as substrate, ylid formation cannot take place, and electrophilic substitution has to follow charge-density calculations. 

Tarpley and Goldstein356 have correlated the 13C chemical shifts in uracil, thymine, and 5-halouracils with the 77- or total charge densities calculated with EHT method. The S13o values correlate quite well with the charges if the individual carbon positions are considered separately. 

Most of the reports in the literature discuss electrophilic substitution of 1,3,6-triazacycl[3.3.3]azine (80) and the 2-methyl, 4-cyano and 4-ethoxycarbonyl derivatives. Electrophilic bromination occurs preferentially at the 4-position (if available), and subsequently at the 9- and 7-positions. These data support the electron charge density calculations (see Section 2.20.2) (73ACS3264). Nitrations are carried out using copper(II) nitrate and acetic anhydride. The central nitrogen is completely non-basic, and in the triazacycl-azines protonation occurs initially at position 6 (80) (77ACS(B)239). Ceder and Vernmark have reported that piperidine reacts with (81) via the aryne intermediate the A-E cine-substitution mechanism is an attractive alternative (equation 40). 

The 13C chemical shifts for C-2, C-3 and C-4 in coumarin are at 159.6, 115.7 and 142.7 p.p.m. respectively, and correspond closely to those for pyran-2-one. A good correlation between 13C shifts and charge densities, calculated by the CNDO/2 method, has been established (75JOCH75). 

The different behavior of tertiary and quaternary carbon atoms seems to be due to either the complete neglect of overlap in these calculations or to polarization effects of the carbon-nitrogen bonds, Similar results are obtained for a series of 5-halouracils by plotting the 13C NMR chemical shifs versus 7r-electron charge densities calculated by the extended Hiickel theory [756], Though for several nitrogen heterocycles a better correla-... 

Several approaches have been made to calculate 13C chemical shifts of coumarins by MO methods. Good correlations were found between the 13C chemical shift values of coumarin (also protonated) and the n charge densities calculated by the CNDO/2 method [962], and of coumarins with it charge densities calculated by the Hiickel MO method (which, however, fails for methoxylated coumarins) [965]. Chemical shifts of mono- and dimethoxycoumarins have been correlated with parameters determined by refined INDO MO calculations, in which n bond orders, atom-atom polarizabilities, excitation energies and electron-nucleus distances were taken into consideration [966], In 3-substituted 4-hydroxy and 4-hydroxy-7-methoxycoumarins chemical shifts were found to be related to Swain and Lupton s parameters iF and M [388], according to equation 5.4 (SE = Substitution Effect) ... 

The dicyanophosphide ion is bent, as is the isoelectronic molecule S(CN)2< A transfer of charge into the cyano groups is expected from charge density calculation and is demonstrated by N-coordination to the cation in the crystal (Figure 1). It is also in accord with the observed wider CPC-angle (95 ) and shorter PC-distance (170 pm) and the lower CN and higher PC stretching fre-... 

The calculation of log P (octanol) is facilitated by the availability of computer programs that use the group contribution method (27,28). The derivation of log P (octanol) values from relative HPLC retention times (29-31), and from atomic charge densities calculated using semi-empirical molecular orbital methods has also been described (32). 

---