Gas-phase calculations

Most of the envisioned practical applications for nonlinear optical materials would require solid materials. Unfortunately, only gas-phase calculations have been developed to a reliable level. Most often, the relationship between gas-phase and condensed-phase behavior for a particular class of compounds is determined experimentally. Theoretical calculations for the gas phase are then scaled accordingly. 

Ab initio, gas phase calculations found two minima and one maximum for the reaction coordinate, leading to an exothermic formation of the tetrahedral complex. Oneminimum is an ion-dipole... 

Be sure to remind students that these frequencies are gas phase data and arc thus not the same as the more-faniiliar solution spectra (we will treat solvated systems in Chapter 9). Even so, such gas phase calculations make excellent discovery-based exercises, For example, students may be asked to explain the substituent effects observed tising basic chemistry knowledge. 

We ran an SCRF single point energy calculation for gauche dichloroethane conformers in cyclohexane (e=2.0), using the Onsager model at the Hartree-Fock and MP2 levels of theory (flfl=3.65) and using the IPCM model at the B3LYP level. The 6-31+G(d) basis set was used for all jobs. We also ran gas phase calculations for both conformations at the same model chemistries, and an IPCM calculation for the trans conformation (SCRF=Dipole calculations are not necessary for the trans conformation since it has no dipole moment). 

The predicted energies in solution are generally given in the same location within the Gaussian output as for gas phase calculations, with the following variations ... 

Alternative pathways a and b, gas-phase calculations and [in brackets[ simulation of the MeCn solution (cf. Scheme 15 and Table IV). 

Although the tautomeric ratios of the 4 species have not been measured directly, it is known that in aqueous solution the keto-N2H form dominates, while the keto-NlH form is only detectable in non-polar solvents. An analysis of experimental data concluded that in aqueous solution the stability (lowest free energy) is in the order keto-N2H > imino-N2H > enol-NlH > keto-NIH. In the gas phase, calculations predict that the keto-N2H form is the least stable. While solvation is found to favour this species, which is the most polar, this stabilisation is not enough to reverse the order of stability. It is thus clearly predicted that the keto-NIH tautomer is the most stable in... 

Both reactions are carried out in nonpolar solvents which should not only accentuate polar interactions, but should also make gas phase calculations relevant to these solution processes. 

Gas phase calculations on the CS state of 22 also predict a large change in the cavity size, from 10 A in the ground state, to only about 6 A in the CS state, and that this change is mainly caused by out-of-plane bending of both DC V and DMAn groups.1381 Similar geometric distortions are predicted for other U-shaped dyads in their CS states. 

Finally, one has to concede that gas-phase calculations are not the ideal way to model a reaction taking place on a catalyst surface. Computational chemistry developments in this area have been continuing but they are a long way from providing completely realistic models. For example, the overall kinetics for dehydrocyclizations are likely to be rate-limited by the binding of the alkane substrate to catalytically active sites. 

Fig. 8 Oiganocatalytic Michael-addition Energy profiles of paths A and B, both leading to (t )-configurated product, as obtained from gas phase calculations (B3LYP/6-311G(d,p)// B3LYP/6-310(d))...

To conclude this section on the effect of solvent on a-nucleophilicity, we refer to the current, rather controversial, situation pertaining to gas-phase smdies and the a-effect. As reported in our review on the a-effect and its modulation by solvent the gas-phase reaction of methyl formate with HOO and HO , which proceeds via three competitive pathways proton abstraction, nucleophilic addition to the carbonyl group and Sat2 displacement on the methyl group, showed no enhanced nucleophilic reactivity for HOO relative to This was consistent with gas-phase calculational work... 

Section 5.2 The gas phase calculations of the CO stretching vibrations were as in Section 5.1, except that a finite difference of 0.04 A was used. 

A third comparison points out that there will be exceptions to the notion that solvent effects will cancel in comparisons among what appears to be very-closely-related systems, and that gas-phase calculations will lead to acceptable results. Among the most quoted... 

In terms of layout, it might be preferable from a historic sense to start with quantum theories and then develop classical theories as an approximation to the more rigorous formulation. However, I think it is more pedagogically straightforward (and far easier on the student) to begin with classical models, which are in the widest use by experimentalists and tend to feel very intuitive to the modern chemist, and move from there to increasingly more complex theories. In that same vein, early emphasis will be on single-molecule (gas-phase) calculations followed by a discussion of extensions to include condensed-phase effects. While the book focuses primarily on the calculation of equilibrium properties, excited states and reaction dynamics arc dealt with as advanced subjects in later chapters. 

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