Potential energy curves case /-complex

Occasionally, potential energy curves for bond stretching vibrations can be unusually flat as well. One special case is of particular importance in hydrogen-bonded complexes. At infinite intermolecular distance two states are possible in which the proton is either bound to molecule A or to molecule B. The two states are related by a proton transfer process ... 

The potential energy curve for a pure case (d) /-complex (with a 1 + ion-core) could, in principle (Field, unpublished), be computed ab initio as the degeneracy-weighted average of the Z+l different case (a) A-component potential curves,... 

In Section 8.5 a specific example of the evolution of a singlet (S = 0) p-complex from case (a) toward case (d) is discussed. In a case (a) basis set Hel is diagonal but, due to the non-sphericity of the ion-core experienced by the Rydberg electron, Hel the zero-order potential energy curves of the 1E and 1II states are split by... 

When two molecules in their ground state approach each other, no ground state complex can be obtained if the corresponding potential energy curve does not have any minimum. However, it may happen that, when one of the two molecules is excited, the approach between the excited molecule and the other molecule in its ground state leads to a minimum in the potential energy curve. In such a case, an exciplex is formed (Reactions 7 and 8). The exciplex formation results in the... 

Before analysis of the interactions of the nucleic acid bases with the clay minerals in the presence of water and cation one needs to understand the individual interactions of NAs with isolated water and with a cation. Such theoretical study was performed for 1 -methylcytosine (MeC) [139]. The study revealed influence of water and cation in the proton transfer for this compound. This leads to the formation of imino-oxo (MeC ) tautomer. Topology of the proton transfer potential surface and thermodynamics of stepwise hydration of MeCNa+ and MeC Na+ complexes is further discussed. The one dimensional potential energy profile for this process followed by the proton transfer with the formation of hydrated MeC Na+ is presented in Fig. 21.2. One-dimensional potential energy profile for amino-imino proton transfer in monohydrated N1-methylcytosine (this represents the situation when tautomerization is promoted by a single water molecule without the influence of Na+ cation) and for the case of pure intramolecular proton transfer (tautomerization is not assisted by any internal interactions) is also included. The most important features of this profile do not depend on the presence or absence of Na+ cation. All the potential energy curves have local minima corresponding to MeC and MeC. However, the significant difference is observed in the relative position of local minima and transition state, which results in a different thermodynamic and kinetic behavior for all presented cases (see Fig. 21.2). 

Laser induced fluorescence (LIF) techniques in supersonic free jets can also yield useful information on the potential energy curves of open shell atomic systems. This type of studies has provided high quality data on tbe ground and excited states of NaAr type of molecules. The LIF technique was also successfully applied for probing the potential surfaces of XeF. The B<-X fluorescence excitation spectrum of XeF in a supersonic free jet is sufficiently simplified that rotational analysis and accurate vibrational spacing are readily obtained, overcoming the complexity of gas phase emission spectroscopy, mainly due in this case to isotopic richness of natural Xe. 

To illustrate the performance of SCS-MP2 and SCSN-MP2, Figure 4 presents errors in the potential energy curves for several Ti-interaction complexes when compared to estimated CCSD(T)/CBS benchmark curves. The MP2/CBS curves are significantly overbound and are not included in Figure 4. In all cases, the errors vs. the benchmark values are only a few tenths of 1 kcal mol . The largest errors are observed for SCS-MP2 at shorter intermonomer separations, and SCS-MP2 tends to have positive errors (underbinds) while SCSN-MP2 tends to have negative errors (overbinds) in the CBS limit. 

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