HOW TO PREDICT CONFORMATIONS ACCESSIBLE TO A MOLECULE IN SOLUTION - VALIDATION OF A FORCE FIELD-BASED PREDICTION OF NOE DISTANCES BY COMPARISON WITH THE EXPERIMENTAL-DATA FOR THE SERIES OF COMPOUNDS CH3C[CH2P(BZL)R](3)MO(CO)(3) (R = PH, M-XYL)

NMR-NOE analysis of the three compounds (RRS/SSR)-CH3C(CH2PPhB2)(3)Mo( CO)(3) (1), (RRR/SSS)-CH3C(CH(2)PPhBz)(3)Mo(CO)(3) (2), and (RRR/SSS)- CH3C[CH2P(m-Xyl)-Bz](3)Mo(CO)3 (3), leads to experimental values for a number of intramolecular H ... H contacts in each case. By the very n ature of the NOE method, and with the type of molecules studied here, these values have to be understood as a Boltzmann weighted average ove r all the conformations accessible to the molecules in solution. - A B oltzmann weighted force field approach is used to predict these values on the basis of sets of force field parameters derived earlier for th is class of compounds. The agreement between observed and calculated N OE distances is highly satisfactory in each case (rms = 0.2 Angstrom t o 0.3 Angstrom). By a statistical analysis it is shown that the predic tions made by applying this Boltzmann weighted force field approach ar e highly significant. There is only a chance of 3 in 100 000 that this quality of prediction might be obtained by chance. This is taken as a validation, albeit indirect, of the energy scale reproduced by the fo rce field parameters as developed. The methods described give a detail ed insight into the conformational flexibility of molecules of the typ e studied. Solid state structures of the molecules are also correctly modelled by the force field used.