| Computational and experimental methods were used to obtain reaction volumes for more accurate reaction enthalpies by photoacoustic calorimetry (PAC). Without reaction volumes, reaction enthalpies by PAC may be inaccurate if the reaction volume contributes to the reaction enthalpy. Ligand substitution is a ubiquitous transformation in organometallic chemistry, and we have studied molybdenum hexacarbonyl with a variety of ligands (L) as a prototypical system. The solvent expansivity was changed by using a series of linear alkanes (pentane, hexane, heptane, octane, decane, and dodecane). Reaction enthalpies and reaction volumes were determined by PAC yielding molybdenum-ligand bond dissociation energies (BDE) (kcal/mol): L = benzene (15.5 kcal/mol), triethylsilane (24.7 kcal/mol), 1-hexene (26.0 kcal/mol), 1-hexyne (25.5 kcal/mol), acetonitrile (32.4 kcal/mol), piperidine (31.4 kcal/mol), triethylphosphine oxide (27.3 kcal/mol), and triethylphosphine (41.9 kcal/mol). The aromaticity of benzene produced the smallest BDE while the sigma-bond of the Si-H being slightly larger followed by the pi-bonds of the alkene and alkyne. The lone pair of the O, P, and N has the strongest bonds with BDE increasing as the atom size or basicity increases. From PAC, rate contants were also obtained in all six solvents (in heptane k = 1.08x107 M-1 s-1 (benzene), 1.41x107 M -1s-1 (1-hexene), 1.95x107 M -1s-1 (1-hexyne), 2.00x107 M -1s-1 (triethylsilane), 6.0x0.3 M-1s -1 (piperidine), 1.81x108 M-1s -1 (triethylphosphine), 5.2x108 M-1s -1 (acetonitrile), 3.3x109 M-1s -1 (triethylphosphine oxide). As the ligand size decreases the rate increases with the exception of triethylphosphine oxide which associates at a diffusion-controlled rate. These methods were then used along with actinometry to study ring formation in the MnS series and Crpy series. From these PAC results we show the first set of reliable data for reaction enthalpies and reaction volumes for a broad series of ligands with the ligand substitution of molybdenum hexacarbonyl. Also computational methods have been studied in calculating the reaction volumes (benzene (-6.8 ml/mol), triethylsilane (-15.9 ml/mol), 1-hexene (-8.4 ml/mol), 1-hexyne (2.6 ml/mol), acetonitrile (0.3 ml/mol), piperidine (-4.1 ml/mol), triethylphosphine oxide (-3.9 ml/mol), and triethylphosphine (-8.1 ml/mol)) not only for molybdenum hexacarbonyl but also organometallics with manganese. Finally LabVIEW was used to create a data acquisition program to update the PAC system. |
