Force field studies

J. C. Smith and M. Karplus, Empirical force field study of geometries and confrontational transitions of some organic molecules, J. Am. Chem. Soc. 114 801 (1982). 

These amides have two possible donor sites, N or O, In the ligand there is some delocalization which can be represented by forms (7) and (8), taking DMF as an example. Force field studies of DMF calculate the bond-order of the CO bond as 1.75 and the CN bond as 1.5,39 i.e. (8) makes a strong contribution to the structure. In all the coordination compounds that have been characterized by X-ray studies the bonding is through oxygen. 

Metal-n-allyl complexes are important in a number of stereoselective catalytic reactions and are therefore attractive for computational chemists (see also Chapter 13, Section 13.2). An empirical force field study based on the MM2 parameterization scheme aimed at predicting stereoselective nickel(0)-catalyzed cycloadditions was recently conducted 56. As in a similar study 57, where a force field for the structure optimization of palladium-allyl systems was developed, dummy atoms were needed to define the structural model. Based on the assumptions required to model a catalytic process, the results obtained have to be interpreted with caution. 

M. Ragazzi, D. R. Ferro, and A. Provasoli, A force-field studie of the conformational characteristics of the iduronate ring, J. Comput. Chem., 1 (1986) 105-112. 

On the methodological side, it is gratifying to note, once these computer time consuming CP-MD calculations have been performed, that rapid classical dynamics AMBER calculations on the 18C6-H20 hydrate essentially yield similar qualitative conclusions instability of the Q hydrate, and "waltzing dynamics" of H20 over the D3d crown, which also validates early and subsequent force field studies, also pointing out the limitations of static views, even obtained by sophisticated quantum calculations. 

Emsley and coworkers125 have made an X-ray crystallographic and empirical force-field study of a simple non-hydrogen-bonded 2-acetylenamine (50). The molecule has the EZ configuration in the crystal, but the calculations (MM2 and MMP2) predict the ZZ form to be more stable by 0.7 kcal mol - Ab initio calculations on 3-aminoacrolein also favor the ZZ form, but here the intramolecular hydrogen bond plays a decisive role. 

Lifson S, Hagler AT, Dauben P (1979) Consistent force field studies of intermolecular forces in hydrogen bonded crystals. I. Carboxylic acids, amides and the C-O—H hydrogen bonds. J Am Chem Soc 101 5111-5121... 

Lifson, S. Hagler, A.T. Dauber, P. Consistent force field Studies of intermolecular forces in hydrogen-bonded crystals. J. Am. Chem. Soc. 1979,101 (18), 5111-5121. 

Other sources of data have generally been of much less use for anhar-monic force field studies than high-resolution infrared and microwave measurements. Solid-state infrared measurements using the matrix-isolation technique have recently provided useful vibrational data for some species inaccessible to gas-phase measurements, e.g., the cyanate (OCN )14 ls and nitrate (NO3)16 anions. Gas-phase Raman measurements of vibration/rotation bands are very difficult to perform with adequate resolution, and Raman data have mainly been utilized for locating some of the infrared-inactive fundamentals in symmetrical molecules such as C02, CS2,17 C2H2,18 CH4,19 20 C. 21"23 High-resolu-... 

A. T. Hagler, S. Lifson, and P. Dauber,/. Am. Chem. Soc., 101, 5122 (1979). Consistent Force Field Studies of Intermolecular Forces in Hydrogen-Bonded Crystals. 2. A Benchmark for the Objective Comparison of Alternative Force Fields. 

A molecular mechanics (MM2) study has been reported for oxocane <92JOC29l>. As in a previous force field study <80ZN(B)1479>, the BC-3 (8) conformation is calculated to be the lowest energy conformer, favored by 1.1 kcal mol over the BC-1 conformer. 

---