Vibrational frequencies imaginary

Figure 4. The most important structures on the CH5 (Hj) potential hypersurface optimized at the TZ2P CISD level of theory. The number of imaginary vibrational frequencies is given in square brackets. Bonds lengths are in A, angles in degrees.

Table 8. Relative Energies in kcal mor of the Three Examined SiHJ structures (see Figure 7) at the TZ2P CCSD(T) Level of Theory (The Number of Imaginary Vibrational Frequencies is in Squared Brackets)... 

The nine Cg2 IPR cages [97] produce nine Ca Cg2 endohedrals. Fig. 32.5 presents their structures optimized [161] at the B3LYP/3-21G dz level. It has turned out that in five cases the original HF structures after the DFT reoptimizations within the same symmetry lead to saddle points with imaginary vibrational frequencies, not to the required local energy minima. When the five saddle points are relaxed and reoptimized, the following local minima are obtained C3,(b) Cj, C2(a)—>Ci,... 

Thus, reaction rate coefficients can be estimated from the thermochemistry of the transition states, whose molecular properties can be calculated with quantum chemical programs. In calculating reaction rate coefficients, the only negative second derivative of energy with respect to atomic coordinates (called imaginary vibrational frequency ) from the transition state is ignored, so that there are only 37/-7 molecular vibrations in the transition structure (37/ — 6 if linear) and all internal and external symmetry numbers have to be included in the rotational partition functions (then any reaction path degeneracy is usually included automatically). 

Recently the concerted approach has been studied at the SCF level using the STO-3G basis set and gradient techniques with optimization of all variables by Brown and Houk. In this case an authentic transition structure has been obtained, possessing one imaginary vibrational frequency. Ortega et... 

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