Comparison of theab initioforce constants of ethane, ethylene and acetylene

Harmonic Force Fields from Scaled SCF Calculations: Program ASYM40

“…For C 2 D 2 there are rather few observed overtones and we consider the anharmonic corrections to be much less reliable than for the other molecules, which is the reason for the 20 cm Ϫ1 uncertainty in the s for this molecule compared to 1 cm Ϫ1 for the others. An SCF ab initio force field for acetylene has been calculated by Pulay and Meyer (5). Their reported values are close to our calculated force constants before scaling and are larger than our scaled values of Table 1a.…”

“…Thus the empirical force field of Table 6a has been obtained using eight predicate values. In the empirical force field of CH 3 CN, two of the interaction constants F 3,4 and F 5,8 had values quite different from the theoretical ones. The empirical and theoretical values of the corresponding constants in CH 3 CCH, F 3,4 , and F 6,9 differ in the same way, and all values are close to those in CH 3 CN.…”

“…All data are shown in Table 5b. The Halonen and Mills force field is identical to that shown in Table 5a and is only slightly different from that reported by Duncan et al When this force field is compared with the scaled theoretical force field, also shown in Table 5a, the greatest difference is in the values of two of the interaction constants, F 3,4 and F 5,8 , which are both calculated to be small in the ab initio force field. In the empirical force field, F 3,4 has a large positive value and F 5,8 has a large negative value.…”

“…This could be tested by applying the inverse transformation to Eq. [3], [5] to see whether this yields the original Cartesian force constants unchanged, as it should. Alternatively one may expand the A matrix to include the contributions from the six Eckart coordinates (which are normally constrained to zero), so that Eq.…”

Gas phase unimolecular 1,1‐hydrogen elimination: Reaction mechanism and isotope effect

“…39-41, and references therein) so that the actual value of the reverse activation energy does not play any role in the determination of statistical means. Besides, a better agreement between MNDO Table 111 reduced by 10% in the case of stretchings [42) decreases the computed activation energy to 288 kJ/mol and increases the reverse activation energy to 25 kJ/mol, which approaches the experimental values, although the reverse activation energy remains about 50% too low.…”