Urey-Bradley approach

The success of the ligand-ligand repulsion model prompted its adoption as an element of a molecular mechanics program. In the resulting approach the valence angles around the metal ion are modeled solely by nonbonded interactions, using the usual van der Waals potential (for example, Eq. 2.9 kg = 0 in Eq. 2.7 Urey-Bradley approach)136. 6 Again, the fact that the electronic effects responsible for the directionality of bonds are not explicitly modeled here may seem questionable but extensive tests have shown the model to be reliable 371. An explanation for this apparent contra-... 

A potentially much more adaptable technique is force-field vibrational modeling. In this method, the effective force constants related to distortions of a molecule (such as bond stretching) are used to estimate unknown vibrahonal frequencies. The great advantage of this approach is that it can be applied to any material, provided a suitable set of force constants is known. For small molecules and complexes, approximate force constants can often be determined using known (if incomplete) vibrational specha. These empirical force-field models, in effect, represent a more sophisticated way of exhapolating known frequencies than the rule-based method. A simple type of empirical molecular force field, the modified Urey-Bradley force field (MUBFF), is introduced below. 

Before it is possible to interpret on a rigorous basis the behavior of the carbonyl stretching frequencies of a series of isostructural and isoelectronic complexes complete vibrational analyses are necessary. However, it is only within the last few years that far-infrared 137) and laser Raman 84) spectrometers have become available generally. Hence, in the general absence of the data they have provided, earlier complete analyses were limited to the spectra of simple metal carbonyls (for which such information was available). Even for these complexes, the number of force constants exceeds the number of observable frequencies, and model force fields had to be used. Since Urey-Bradley type force fields proved to be unsuitable for carbonyl complexes 86,105, 106), Jones 80-82) developed a resonance interaction valence force field which reduced the number of force constants by interrelating several on the basis of orbital overlap. This approach is not readily adaptable to less symmetrical substituted carbonyl complexes. Alternative models had, therefore, to be investigated. 

Many such model force fields have been discussed in the literature. [For general discussions, see Herzberg (1945), Wilson et al. (1955), Woodward (1972), and Califano (1976). For discussions of the Urey-Bradley force field, see the review by Duncan (1975). For discussions of the entirely different consistent force field approach, see Lifson and Warshel (1968), Warshel et al. (1970), and Burkert and Allinger (1982).] We have chosen to use a simplified general valence force field (SGVFF), which has been defined as one which contains the minimum possible number of interaction constants compatible with a good fit of the spectra (Califano, 1976). Such a force field has been demonstrated to be very effective for hydrocarbons (Schachtschneider and Snyder, 1963). For this form the potential energy of Eq. (63) is written explicitly as... 

The SVFF approach is rougher but physically more significant it neglects all interactions and gives six force constants Fr (CN), Fr (MC) Fa (MCN in-plane), F j (CMC in-plane) F (MCN out-of-plane), F (CMC out-of-plane). In the Ni(CN)4 case, the two potential functions have led to almost similar values. The Urey-Bradley potential function is more sophisticated and has sometimes been used, but it only takes into account the repulsion between neighbouring C or N atoms, which is not the main one. All results from the literature and from our own work are compared in Table 4. 

In another approach, Shimanouchi [82] introduced the Urey-Bradley force (UBF) field, which consists of stretching and bending force constants, as well as repulsive force constants between nonbonded atoms. The general form of the potential field is given by... 

Raman spectroscopy of cyclic disulfides indicates that S—S stretching frequency around 505 cm is insensitive to CCSS torsion angles between 37 and 100° <86JA127l>. Above 100°, however, frequencies increased, approaching 525 cm" at 180°. Normal coordinate calculations with a Urey-Bradley force field confirmed this behavior. 

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