Distances. Vibrational Frequencies

Table. Dissociation energy, bond distance, vibrational frequency, and electric dipole moment from fullMCVB calculation of BeH.

Finally I would like to make a practical suggestion—this is that the theoretical chemists should turn their attention to the FHF to estimate the relative importance of ionic and covalent structures in this important case. For this molecule there is available a mass of precise experimental data such as bond distances, vibration frequencies, anharmonicitv, etc. 

Table V. Interatomic Distances, Vibrational Frequencies, and Force Constants for XeFj and Linearly Distorted Xep2 Species...

Here, the external field is specified as the potential Vn c due to the nuclei. This four-component DKS formalism was successfully applied to many atomic, molecular, and solid state systems [53-65]. Relativistic corrections to the xc potential [41,45] are rarely employed in practical applications [53,66] as they hardly affect observables commonly studied in molecular systems, e.g. bond distances, vibrational frequencies, binding energies etc. [53,66]. 

UV absorption spectra have been calculated for l,3,5,2,4,6-trithiatriarsinane-2,4,6-trithiol 158 and arsenic(m) sulfide (orpiment, AS4S6) 157 in aqueous solution at CIS and TD B3LYP levels of theory using the 6-311+G(2d,p) basis set <2001MI239>. The bond distances, vibrational frequencies, gas-phase energetics, and proton affinity of 158 have been estimated by means of MO theory <1995M14591>. 

Electric dipole moments, bond distances, vibrational frequencies and dissociation energies from K. P. Huber and G Herzberg, Molecular Spectra and Molecular Structure, Vol. IV, Constants of Diatomic Molecules, Van Noslrand, New York 1979. The dissociation energy of NaF has, however, been taken from P. Brunner and M. Karpins, J. Chem. Phys., 58 (1973) 3903. 

Interest in clusters of the IIA (Mg, Ca, Sr, Ba and Ra) and IIB (Zn, Cd and Hg) elements has been motivated mainly by the fact that these systems may undergo a nonmetal-to-metal transition in their electronic structure as a function of size. The basis for this expectation is simple. On the one hand, bulk samples of these elements are clearly metallic, although, not surprisingly, key properties such as conductivity and plasma frequencies show that their electronic structure (and the Fermi surface in particular) deviates from the ideal free-electron picture much more than in the case of alkali metals [50]. On the other hand, experimental data show that the cohesive energy, equilibrium distances, vibrational frequencies and excitation energies for the homonuclear dimers of these elements are close to those expected for weakly bound van der Waals systems. 

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc. 

Such calculations have been performed by Takayanagi et al. [1987] and Hancock et al. [1989]. The minimum energy of the linear H3 complex is only 0.055 kcal/mol lower than that of the isolated H and H2. The intermolecular vibration frequency is smaller than 50cm L The height of the vibrational-adiabatic barrier is 9.4 kcal/mol, the H-H distance 0.82 A. The barrier was approximated by an Eckart potential with width 1.5-1.8 A. The rate constant has been calculated from eq. (2.1), using the barrier height as an adjustable parameter. This led to a value of Vq similar to that of the gas-phase reaction H -I- H2. 

It is remarkable that LDF theory also describes the bond length and vibrational frequency of the fluorine molecule with the same error bar as the other systems discussed here. This finding is significant as it shows that the error made in the LDF approach appears to be consistent in a wide class of different systems. In fact, recent calculations on ferrocene (52) show that also this type of metallo-organic compound does not present an exception - the Fe/ring distance agrees within 0.002 A with experiment. 

The most important calculated and experimental monomer data, such as equilibrium distances, dipole moments, polarizabilities, and the harmonic vibrational frequencies of the dihalogens XY, are reported in Tables 1-4. 

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