Ground state geometry

Full ab initio optimizations of molecular geometries of enamines (and of any other kind of molecules) depend strongly on the kind of applied basis sets application of STO-3G 2 3, 3-21G 3-2lG 4-3lG 6-3lG 6-31G and 6-31G basis sets leads to optimizations for the coplanar framework of all atoms of vinylamine, but it was not stated in these references whether coplanarity was assumed by input constraint or not. Contrary to that, the use of a double-zeta basis set with heavy atom polarization functions as well as 6-31 - -G ° based optimization yielded a non-planar amino group for 115. 

Newman projections along the N—C bond showing the six conformations, 116 to 121, of vinylamine (IIS) whose total energies in various basis sets have been collected in Table 9. (O denotes C ) Values for substituted vinyl amines refer to the same type of the five 

FIGURE 2. Calculated PRDDO geometry of vinylamine (115) type 118 shown in projection onto the CCN plane and dihedral angles shown along the N—C and C =C bonds (distances in A, angles in degrees). (Reproduced with permission from Reference 201) 

Our consecutive optimizations and findings have been based on applications of 3-21G, 6-31G and 6-3IG basis sets. These three basis sets have been selected for the following reasons The 3-21G basis set is the most economical split-valence basis set which allows comparison to similar systems the 6-31G basis set was used because this was statistically the best basis set for calculations of CC distances and the 6-3IG basis set was applied as a highly reliable and computationally still manageable basis set and because we found, in agreement with References 206 and 207, that polarization functions are needed to treat successfully the problem of pyramidality at nitrogen. 

TABLE 4. Basis set dependence of calculated geometries of planar vinylamine (type 116) and related molecules. Distances in A and angles in deg 

The true minimum of the total energy is calculated in the 6-31G basis for the asymmetrically twisted pyramidal form 118 as 0.16 kcal mol -1 further stabilization with respect to 117. The additional calculation of vibrational frequencies indicates only positive force constants, which as second derivatives of the energy must all be positive for a minimum on the potential energy curve. 

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At the instant of excitation, only electrons are reorganized the heavier nuclei retain their ground-state geometry. The statement of this condition is referred to as the Fmnck-Condon principle. A consequence is that the initially generated excited state will have a non-minimal-energy geometry. 

Fig. 10. Most stable ground-state geometries found for CjjjLia and C Li,4 by the MNDO calculations the Li atoms are represented by the filled black circles.

K. Raghavachari, Structure of CggO unexpected ground state geometry, Chem. Phys. Lett. 195,221 (1992). 

Solution We performed our study using the ground state geometry computed with the MP2/6-31G(d) model chemistry (your results will differ slightly if a different geometry is used). 

In this exercise, we will examine a small part of this process. We will predici ihe relative energies of the three states at the ground state geometry, and we will locate the conical intersection. We ve provided you with an optimized ground state (cis) structure and a starting structure for the conical intersection in the files 9 06 gs.pclb and 9 06 ci.pdb, respectively. 

Figure 1 Ground state geometries of Nii2, Nii2 and, Nij...

Example The spectrum of [Ti(H20)6] ions, whose ground state geometry is nearly perfectly octahedral, is characterized by a large splitting. 

The 8V+ 2 rule has been completely substantiated by the calculated ground state geometries of tetramers in Table 2 [7], The cluster (lOe) is linear [11]. The Si/" cluster (18e) has a butterfly structure (Dp [18],... 

For molecules XVI —XVIII it is predicted that bond lengths of the naphthalene core are almost the same as those of the free naphthalene molecule, and in the other region of the molecule marked bond fixations are observed . The most stable ground-state geometry of XVI corresponds to the aromatic model proposed by Lo and Whitehead . Molecules XVI and XVII have been synthesized by Trost and Bright and Boekelheide and Vick , respectively. The present results are in good qualitative agreement with the available experimental information. 

Early experimental spectroscopic investigations on Rg- XY complexes resulted in contradictory information regarding the interactions within them and their preferred geometries. Rovibronic absorption and LIF spectra revealed T-shaped excited- and ground-state configurations, wherein the Rg atom is confined to a plane perpendicular to the X—Y bond [10, 19, 28-30]. While these results were supported by the prediction of T-shaped structures based on pairwise additive Lennard-Jones or Morse atom-atom potentials, they seemed to be at odds with results from microwave spectroscopy experiments that were consistent with linear ground-state geometries [31, 32]. Some attempts were made to justify the contradictory results of the microwave and optical spectroscopic studies, and... 

Katriel J, Davidson ER (1980) The non-crossing rule triply degenerate ground-state geometries of CH. Chem Phys Lett 76 259... 

We are all well aware of the tremendous advances over the past decade in both the quality and number of X-ray structural studies on molecules and ions, aimed at the accurate determination of the ground-state geometries. However, our knowledge of excited-state geometries has not developed at a comparable rate, and few techniques can be brought to bear upon this problem. 

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