Effective Induced Bond Charges

VI. Effective Induced Bond Charges From Atomic Polarizability Tensors.261... 

VI. EFFECTIVE INDUCED BOND CHARGES FROM ATOMIC POLARIZABILITY TENSORS... 

In diis section a method for interpretation of Raman intensities based on further transformations of atomic polarizability tensors is presented. The formulation was recently proposed by Ehidev and Galabov [333], A new molecular quantity - effective induced bond charge, Ok introduced. The effective induced bond charges are obtained from rotation-free atomic polarizability tensors following the strate as outlined by Galabov, Dudev and nieva [146] in the infrared case (Section 4.IV). The Ok parameters are expected to be associated with polarizability properties of valence bonds. 

In Eq. (9.102) 05W(v) is the transpose of 05W(v). The quantity 0, termed effective induced bond charge, has dimensions of electric charge per electric field strength [C/(V/m)]. Ok are expected to be associated with the polarizability properties of chemical bonds. It is anticipated that Ok will vary for bonds with different polarizabilities. 

A representative series of molecules is selected to determine the trends of changes of the effective induced bond charges as defined by Eq. (9.102). The formulation developed has been applied in interpreting atomic polarizability tensors evaluated by HF/6-311+G(d,p) ab initio MO calculations [333]. A series of 17 molecules containing various bonds in different environment have been studied. The molecules are grouped as follows ... 

Bond Molecule Effective induced bond charge oi( (X10-30 C.mAO Bond length rjc (X10-10 nj)a Average atomic polarizability A (X10-40 C.m2/V)l>... 

Ok parameters for the C-H bonds in different surroundings are collected in Table 9.1s. Data presented show that 0 ( vary in a narrow interval with an average value of 3.291 10 30 QxafW. It can be concluded that the C-H effective induced bond charges are not very sensitive to die environment and reflect mosdy local properties of die C-H bonds. In the low symmetry molecules of the series, such as CH3NH2, CH2NH and... 

Bond Molecule Effective induced bond charge O) (X10-30 C.m/V) Bond loigthr)( (x 10-10 m)a... 

Fig. 9.6. Plot of the dependence between the effective induced bond charges, and the bond lengths, rc x (X = C, N, O, F) for ethane, methylamine, methanol and mediyl fluoride.

Double bonds, due to the higher it electrons flexibility, are expected to have higher polarizabilities as compared with the respective single bonds. It is, dioefore, of particular interest to examine effective induced bond charges for some double bonds. Calculated values are given in Table 9.17. Juxtaposing crc=X respective oq-X... 

In this case, the agreement of the values calculated by the two methods is good, but it is not always so. One reason is that the simple vector approach ignores the effects of unshared pairs of electrons. Also, highly polar bonds can induce additional charge separation in bonds that might not otherwise... 

Chemical considerations suggest that metal-olefin back donation will be less important for silver(I) than for platinum(II), and Basch s ab initio calculations on [Ag(C2H4)]+ (75) have confirmed this view. These calculations suggest that most of the electronic rearrangement of the ethylene unit in this complex ion can be accounted for by the polarization effects induced by the positive charge on the silver atom. Indeed, the bonding metal-olefin molecular orbital has only 6.5% Ag 5s orbital character. This result agrees nicely with recent ESR studies on y-irradiated silver-olefin complexes which estimate a 5s spin density of 4.6% for this molecular orbital 92, 93). 

The high electrophilicity of perfluoroolefins is the result of the profound electron-accepting effect of fluorine atoms and CF3 groups combined with the capability of vinyl fluorine atoms to be effectively conjugated with the C=C bond. The reactivity of perfluoroolefins does not depend so strongly on the statistical distribution of electron density (or on the induced positive charge on the C=C carbon atoms in perfluoroolefins) as it does on the dynamic properties of this distribution. 

In a model proposed by Lewis [228] the effect of the excited state of retinal on the conformational state of the protein is considered to be the first step of the excitation mechanism. Charge redistribution in the retinal by excitation with light would have the consequence of vibrationally exciting and perturbing the ground state conformation of the protein, i.e., excited retinal would induce transient charge density assisted bond rearrangements (e.g., proton translocation). Subsequently, retinal would assume such an isomeric and conformational state so as to stabilize maximally the new protein structure established. In this model, 11-m to trans isomerization would not be involved in the primary process, but would serve to provide irreversibility for efficient quantum detection. It was also proposed that either the 9-m-retinal (in isorhodopsin) or the 11-m-retinal (in rhodopsin) could yield the same, common... 

Apart from purely electronic effects, an asymmetric nuclear relaxation in the electric field can also contribute to the first hyperpolarizability in processes that are partly induced by a static field, such as the Pockels effect [55, 56], and much attention is currently devoted to the study of the vibrational hyperpolarizability, can be deduced from experimental data in two different ways [57, 58], and a review of the theoretical calculations of p, is given in Refs. [59] and [60]. The numerical value of the static P is often similar to that of static electronic hyperpolarizabilities, and this was rationalized with a two-state valence-bond charge transfer model. Recent ab-initio computational tests have shown, however, that this model is not always adequate and that a direct correlation between static electronic and vibrational hyperpolarizabilities does not exist [61]. 

Regioselectivity in hydroformylation is influenced by electronic and steric effects [4, 5]. Thus the formation of the C a-Rh bond is favored over that of the C P-Rh bond by the well known P-silicon effect (Fig. 3), which stabilizes a positive charge on the p-C atom. From the resulting intermediate la the /50-product should form predominantly. On the other hand, steric effects induced by bulky substituents on silicon or rhodium would favor the sterically less hindered normal alkyl rhodium complex with the C P Rh intermediate Ila as the precusor to the -aldehyde. The observed //so-ratios very close to 1 1 for the Rh-catalyzed hydroformylation of vinyltrimethylsilane indicate that the electronic P-effect obviously is canceled out by the steric demand of the MesSi-groups. Since addition of PPha will favor an active complex with a larger number of bulky phosphine ligands (L = PPhs in Fig. 2), the formation of the linear alkylrhodium complex intermediate Ila to lid is prefered [6]. 

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