Bonding spectroscopic constants

The study of molecular systems containing metal atoms, particularly transition metal atoms, is more challenging than first-row chemistry from both an experimental and theoretical point of view. Therefore, we have systematically studied (3-5) the computational requirements for obtaining accurate spectroscopic constants for diatomic and triatomic systems containing the first- and second-row transition metals. Our goal has been to understand the diversity of mechanisms by which transition metals bond and to aid in the interpretation of experimental observations. 

This chapter is restricted to a discussion of halogen-bonded complexes B XY that involve a homo- or hetero-dihalogen molecule XY as the electron acceptor and one of a series of simple Lewis bases B, which are chosen for their simplicity and to provide a range of electron-donating abilities. Moreover, we shall restrict attention to the gas phase so that the experimental properties determined refer to the isolated complex. Comparisons with the results of electronic structure calculations are then appropriate. All of the experimental properties of isolated complexes B- XY considered here result from interpreting spectroscopic constants obtained by analysis of rotational spectra. 

Bartell and coworkers investigated the structures of a series of noncyclic alkanes by means of gas electron diffraction (14, 44, 45) and invoked for the interpretation of their results a simple force field which contained to a high extent vibrational spectroscopic constants of Snyder and Schachtschneider. This force field reproduces bond lengths and bond angles of acyclic hydrocarbons well, energies of isomerisation satisfactorily. As an example, Fig. 8 shows geometry parameters of tri-t-butylmethane as observed by electron diffraction and calculated with this force field (14). 

The Lowering of the vXH Frequency on H-hond Formation If an investigation of the effect of mechanical interaction between harmonic XH and H Y vibrations is made with the help of the usual equations for the vibrations of a triatomic molecule [16], it is found that, for a fixed value of the XH force constant, the rXH frequency increases as the force constant of H Y is raised from zero.. As the experimentally observed effect is a large one in the opposite direction, i.e, the rXH frequency decreases markedly with an increase in the strength of the H-bond, the only reasonable explanation of the spectroscopic facts seems to be that the lowered frequency is a reflection of a lowered XH bond force constant [17, IS]. 

In Table 5.3 we list MRCI+Q spectroscopic constants for N2 obtained with one of our largest basis sets. Of the remaining errors compared to experiment, we can estimate that about 0.001 A of the error in the bond length arises from relativistic effects [71]. These would also increase the frequency slightly. Explicit calculations... 

Kuchitsu and co-workers5 7 were the first to introduce what is perhaps the simplest and most generally useful model, in which they assume all anharmonic force constants in curvilinear co-ordinates to be zero with the exception of cubic and quartic bond-stretching constants. These may be estimated from the corresponding diatomics, or from a Morse function, or they may be adjusted to give the best fit to selected spectroscopic constants to which they make a major contribution. This is often called the valence-force model. It is clear from the results on general anharmonic force fields quoted above that this model is close to the truth, and in fact summarizes 80 % of all that we have learnt so far about anharmonic force fields. 

The resonance Raman spectra of Fe2, NiFe, Vz, T 2 and Ni3 in solid, rare-gas matrices are reviewed. Spectroscopic constants for these were (WgjtOgXg, cm ) Fe2 (300.26, 1.45), NiFe (320.0, 1.32), V2 (508.0, 3.3) and Ti2 (407.9, 1.08). A Bernstein-LeRoy analysis was performed on the data for Fe2, yielding a bond dissociation energy of 1.2 eV, close to that reported by Kant. All of the transition metal diatomlcs studied showed an Inordinate ability to remain vibrationally excited when the first few vibrational levels were populated radiatively. This gave rise to intense antistokes... 

Spectroscopic constants were reported by Herzberg (1 ) and adjusted for natural Isotopic abundance. The bond length was reported by Morgan and Barrow (3). B and were estimated by the method suggested by Herzberg ( ). 

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