Force-field energies, calculated

Again the vast majority of theoretical studies are restricted to force field energy calculations. As previously said, pure MM calculations are not the main subject of this review though the quite systematic studies done by the groups of Profs. Jaime [42] and Mendicuti [43] can be highlighted. 

Force-field methods, calculation of molecular structure and energy by, 13,1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271 Free radicals, and their reactions at low temperature using a rotating cryostat, study of, 8. I Free radicals, identification by electron spin resonance, 1, 284... 

Force-field methods, calculation of molecular structure and energy by, 13, 1... 

The total potential energy of the system in the combined QM/MM force field is calculated from the expectation value of the wave function, over Herr... 

Forced-vibration instruments, 21 745 Force field calculations, 16 742 Force field energy, 16 742 Force field performance, 16 745 Force fields, 16 743-745 Force field simulations, 16 746-747 programs for, 16 746 Force modulation microscopy, 3 332 Forces, exponents of dimensions in absolute, gravitational, and engineering systems, 8 584t Forchlorfenuron, 13 43t, 53 Ford nuclear reactor, 17 594... 

TABLE 26. Force field (MMl) calculations of l,5-diazabicyclo[3.3.3]undecane 72 and 1,6-diazabicyclo[4.4.4]tetradecan 73 (distances in A, bond angles in degrees, relative energies in kcalmol1)111... 

The investigation of factors affecting facial selectivity in the hydroboration of steroidal -alkenes revealed the facial (a vs /3) stereoselectivities of hydroboration of androst-5-enes (69) and B-norandrost-5-enes (70) do not parallel the difference between the calculated force-field energies for a- and jS-cyclobutane models (71)-(74). This finding appears to suggest that the facial selectivity is not determined by the four-centre transition state but by the relative ease of formation of the initial tt-complex. ... 

The strain of 1 has been determined experimentally from silver-ion catalyzed methanolysis to be ca. 68 kcal/mol, and that of cyclopropa[/j]naphthalene to be 65-67 kcal/mol. Combustion calorimetry gave a value of 67.8 kcal/mol. For dicyclopropa[/>,g]naphthalene (115) a lower limit of 166 kcal/mol was found. These energies are well reproduced by ab initio, and even by semiempirical calculations. Thus 3-2IG calculates a strain energy of 70 kcal/mol while 3-2IG gives 71.6. At the MP2/6-31G level the strain of 1 is 71.3 kcal/mol, while that of cyclopropene amounts to 57.4 kcal/mol. This latter value compares well with the experimental one, which is 52.6 kcal/mol. While semiempirical calculations have been found unreliable for cycloproparene structures, the calculated strain energies are usually close to reality for MINDO/3, MNDO, and force-field-SCF calculations. The strain energies of the dicyclopropabenzenes (100, 102) have been predicted to be 133 and 140 kcal/mol, respectively, and that of tricyclo-propabenzene (260) to be 217 kcal/mol (3-21G). ... 

The most common response nowadays is to supplement the experimental data with the highest quality ab initio data that can be had (either from molecular orbital or density functional calculations). A pleasant feature of using theoretical data is that one can compare regions on a PES that are far from equilibrium structures by direct computation rather than by trying to interpret vibrational spectra. Furthermore, one can attempt to make force-field energy derivatives correspond to those computed ab initio. The only limitation to this approach is the computational resources that are required to ensure that the ab initio data are sufficiently accurate. 

Force-field methods, calculation of molecular structure and energy by, 13, 1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271... 

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