Energy Factored Force Field Research Articles

Ground and electronically excited states of Cr(CO) 4(bipyridine): energy factored force field analysis of CO stretching vibrations and resonance Raman study

IR spectra of Cr(CO) 4(bipyridine) and its 13CO-containing isotopomers were used to calculate all the stretching and interaction CO force constants and the normal coordinates of the CO stretching vibrations. Resonance Raman (rR) spectra of Cr(CO) 4(bpy) were measured and compared with the Fourier transform Raman spectra. The most resonance enhanced Raman bands belongs to the ring-deformation vibrations of the bpy ligand and the A 1 ν(CO) vibration at 2004 cm −1. The rR spectral pattern confirms a localized Cr → bpy metal-to-ligand charge transfer (MLCT) character of the electronic transition responsible for the visible absorption band. It is shown that the MLCT excitation also affects the bonding within the Cr(CO) 4 molety. Of the two A 1 ν(CO) vibrations, only the one at higher frequency (A 1 2, 2004 cm −1) gives rise to a resonance enhanced Raman band. Analysis of this effect, based on the energy factored force field (EFFF) calculated normal coordinates of both symmetric ν(CO) vibrations, shows that the MLCT excitation affects the CO bonds in both the axial and equatorial CO ligands, the influence on the axial ligands being larger. The Raman band due to the A 1 1 symmetric ν(CO)_vibration is not resonance enhanced because of an out-of-phase coupling between the symmetric vibrations of the axial and equatorial pairs of CO ligands. Raman bands due to CrC stretching and CrCO bending vibrations, apparently coupled with the vibrations of the Cr(bpy) moiety, were identified by the 13CO isotope effect and found to be resonance enhanced.

Synthesis and Characterization of Dihydrogen Complexes of Tetracarbonylbromomanganese(I) and Tetracarbonylchloromanganese(I) in Argon Matrices

When argon matrices of MnX(CO)5, X = Br and Cl, containing dihydrogen are photolyzed with either visible light or light of 254 nm, a new species is formed which is MnX(η2-H2)(CO)4. The compounds have been characterized by infrared and Raman spectroscopies, and an energy-factored force field has been fit to the carbonyl modes. The structures can be thought of as arising from an octahedron with no halide and dihydrogen occupying cis-positions. While the H−H stretching vibration was not observed, the symmetric metal−hydrogen stretches have been observed at 788.6, 665.2, and 588.5 cm-1 for the adducts of H2, HD, and D2 and MnBr(CO)4, respectively. The same mode occurs at 764 cm-1 for MnCl(H2)(CO)4; hence, H2 is not strongly bound to the metal. An analysis of the carbonyl stretching force constants supports the view that H2 is not as strong of a π-acid as CO. The spectra of MnX(CO)4 have been re-examined, and it is clear that the features that had been previously assigned to MnX(CO)4 of C2v symmetry actually b...

Vibrational spectra and assignments of MeMn(CO)5 and MeRe(CO)5 species, energy-factored and A1 force fields, and a further effect of free internal rotation

The infrared spectra of /sup 16/CH/sub 3/, /sup 13/CH/sub 3/, /sup 12/CD/sub 3/, and /sup 13/CD/sub 3/ species of MeMn(CO)/sub 5/ and MeRe(CO)/sub 5/ have been studied in the gas phase to 450 cm/sup -1/ and in the solid at 78 K down to 200 cm/sup -1/. Solid-phase Raman spectra (600-300 cm/sup -1/) for MeRe(CO)/sub 5/ are also reported. Nearly all of the delta(MCO) and nu(MC) modes are assigned with confidence in both compounds, and two reassignments are proposed for MeMn(CO)/sub 5/. An energy-factored force field is calculated for the CO stretching vibrations by using gas-phase data which include the B/sub 1/ fundamental. An A/sub 1/ force field for all vibrations shows that all the metal-carbon bonds increase in strength from Mn to Re, while the methyl CH bond is weakened. The axial and equatorial M-CO bonds in MeRe(CO)/sub 5/ are equal in strength, suggesting a negligible trans effect on the part of the methyl ligand. This is interpreted in terms of equal and opposite ..pi..- and sigma-trans effects, the former strengthening the axial Re-CO bond and the latter weakening it. The gas-phase contour of the absorption complex assigned to rho(CH/sub 3/) in MeRe(CO)/sub 5/ resembles those due to nu/submore » as/(CH/sub 3/) and nu/sub as/(CD/sub 3/) and is attributed to a variation in the rocking force constant with internal rotation angle, together with free internal rotation. The corresponding band in MeMn(CO)/sub 5/ may also reflect the same effect. 32 references, 5 figures, 9 tables.« less

Infrared spectra of 13CO-enriched Ru(CO) 5 in liquid xenon: The energy-factored force field

The infrared spectra of isotopically enriched ruthenium pentacarbonyl (Ru( 13CO) 5-x − ( 13CO) x ; x = 1 to 5) in liquid xenon solvent are reported. Energy factored force constants are calculated and an excellent fit to the experimental spectra is obtained for both frequency and intensity. This data permits the interpretation of the less well-resolved spectra in n-hexane. IR spectra, recorded after UV photolysis of 13CO enriched Ru(CO) 5 in liquid-xenon solution, indicate the formation of Ru 2(CO) 9 with a single bridging CO group.

Approximate force constant calculations for cis-L 2M(CO) 4 species

A new procedure based on the criterion of maximum interaction between coupled fundamental vibrational modes has been developed to solve the energy factored force field of cis-L 2M(CO) 4 species. The results for complexes containing a wide range of metal-ligand combinations are compared with the force constants obtained by the Cotton-Kraihanzel and Jernigan methods. The CO stretching frequencies of isotopically-substituted cis-I 2Fe(CO) 4 species have been calculated as a check on the method.

Mean enthalpy of disruption to valence state for homoleptic transition metal carbonyls. A reassessment

Summary The data necessary for an estimation of Δ H * , the enthalpy of disruption of an M CO bond to products in their valence states, is reexamined in the light of the high energy factored force field model (for force constants), the results of SCF MO calculations of electron configuration and charge, and recent thermochemical experiments. It is shown that, for the metals Cr, Mn, Fe, Co and Ni, the value of Δ H * is effectively constant (365 ± 10 kJ mol −1 ). The consequences of this are discussed briefly. An attempt has been made to extend this treatment to carbonyls of the 4 d and 5 d metals.