Effective Bond Charges

Galabov B, Dudev T, Dieva S (1995) Effective bond charges from experimental IR intensities. Spectrochim Acta A 51 739-754... 

Iheva S, Galabov B, Dudev T et al (2001) Effective bond charges from infrared intensities in CH4, SiH4, GeH4 and SnIL,. J Mol Struct 565-566 395-398... 

Crystalline silicon doped with carbon and implanted with hydrogen gave IR bands due to H (2967.4 cm ) and C (911.7, 654.7 cm ) localised modes. Ab initio calculations have been reported of the vibrational wavenumbers of HEBrClF, where E = C, Si or Ge. " IR band intensities for vs and V4 were used to estimate effective bond charges in EH4, where E = C, Si, Ge or Sn. Both experimental IR data and calculations are consistent with C3, symmetry for the CI-.CH4 adduct.2"... 

The structure of the P array was already given [Eq. (3.4)]. (dq/dR) is a matrix containing doii ves of die effective bond charges widi respect to internal vibrational coordinates. These quantities reflect die charge reorganizations with vibratitmal motion. 

IV. Effective Bond Charges from Rotation-Free Atomic Polar Tensors.131... 

IV. EFFECTIVE BOND CHARGES FROM ROTATION-FREE ATOMIC POLAR TENSORS... 

In the present section a theoretical framework for analysis of vibrational intensities recendy developed by Galabov et al. [146] is presented. Fully corrected for rotational contributions atomic polar tensors are transformed into quantities termed effective bond charges. The effective bond charges are expected to reflect in a generalized manner, polar properties of the valence bonds in molecules. Aside from die usual harmonic approximation no other constraints are imposed on the dipole moment functirm. 

The starting molecular quantities in evaluating effective bond charges are atrnnic polar tensors as erqiressed in the Px matrix. From Px one easily obtains the vilnational polar tensor matrix Vx from the relation... 

Of particular interest are die invariants with respect to reorientation of the Cartesian reference system of the tensors D ((v). An effective bond charge has been defined [146]... 

The quantities are expected to reflect, in a generalized way, electrical properties of the respective bonds. As is seen, no iq>proximations outside the harmonic approximation are introduced in evaluating effective bond charges. The Si values can be evaluated without any difficulty fi om experimental vibrational absorption intensities providing the molecule possesses sufficient symmetry and diat individual dp /dQi values are determined. 

The physical significance of effective bond charges will eventually emerge fimn applications of the formulation outlined above in analyzing infrared intensities of various types of molecules. 

The transformation of the infrared intensities for water and ammonia into effective bond charges will be followed in detail so that the computational procedure becomes clear. Results from the application of EBC formulation in interpreting intensity data for a number of medium size molecules will then be presented. Comparisons of effective bond charges for different bonds in vaiying molecular environments will provide a basis for assessing the physical significance of these molecular quantities. 

The C=0 effective bond charges determined are higher than the respective C=S values in the sulphur analogs. This fmding is also in accord with the lower polarity of the C=S bonds due to lower electronegativity of the sulphur atom. 

Effective bond charges from experimental infrared intensities... 

By comparing 6 j data for lower polarity bonds (C-H) to the much polar C=0, C=S, C-F, C-Cl, C-Br, 0-H and N-H bonds it is seen that higher polarity is reflected in higher effective bond charges, with values varying in most cases with expectations. 

The analysis of data for effective bond charges as obtained from experimental gas-phase intensity data for a number of molecules reveals some definite trends of changes. The variations found may, in most cases, be related to polar properties of the bonds considered. As already stressed, the effective bond charges are solely determined by... 

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