Solvent effects calculation

Solvation effects on the conformation of esters of three /i-snbstituted 1-phenyletha-nols with 2-flnoro-2-phenyl acetic acid (FCDA) were studied both experimentally (in five solvents ranging from CDCb to DMSO) and quantum mechanically. Semi-empiri-cal (AMI of MJS Dewar and PM3 of JJP Stewart) and ab initio (RHF/3-21 G) calculations were undertaken. Energy maps for the conformers of the esters as a function of the dihedral angles alpha (F-C-alpha acid-C=0) and beta (CO-O-C-alcohol-H) were obtained. Solvent effect calculations, through the self-consistent reaction field on the most stable conformers, were also carried out (Hamman et al., 1996). 

M. Szafran, M. M. Karelson, A. R. Katritzky, J. Koput, and M. C. Zerner, /. Comput. Chem., 14, 371 (1993). Reconsideration of Solvent Effects Calculated by Semiempirical Quantum Chemical Methods. 

Density function calculations on metaphosphate, acyclic and cyclic phosphates and phosphoranes have been reported. Solvent effects calculated with three well established solvation models were also analyzed and compared. The results showed that microscopic solution pKa values increased in the order, metaphosphates P(0)20H]< phosphates P(0)(0H)ji(0R)3 n = 1-3, R=H or Me] < phosphoranes [P(OH)n(OR)5 n, n =1-5, R =H or Me] with values for cyclic phosphates and cyclic phosphoranes lower than the respective acyclic molecules. Furthermore protonation of the equatorial position in phosphoranes is about 4 pKa units lower than that found for the axial positions. Finally in... 

The deaminations of fra 5-2-phenyl- and frans-2-methyl-cyclopropylamine hydrochlorides in acetic acid solution have been examined. The relative amounts of the various products are shown in (26)-(29) and (30)-(33), respectively. The most remarkable feature of these results is the formation of considerable quantities of ring-opened chlorides, which are scarcely formed in deaminations of related open-chain compounds. The addition of chloride ion led to a marked increase in the amount of chloride product. These and other results are interpreted in terms of the formation of a cyclopropyl-diazonium-chloride tight ion pair. The reactions were successfully modelled by ab initio calculations at the B3LYP/6-31G level, including a reaction-field solvent-effect calculation using the dielectric constant of acetic acid. 

A full quantum-mechanical description of the Menshutkin reaction has been obtained for gas phase and solution by using density functional theory (DFT) and the self-consistent isodensity polarizable continuum model (SCI-PCM). Ammonia and pyridine were the nucleophiles and methyl chloride and methyl bromide, the electrophiles. In the gas phase an initial dipole complex intermediate is followed by a transition state leading to an ion pair. In the solvent-effect calculations, the dipole complex disappears with both cyclohexane and DMSO. The transition state is stabilized compared with the gas phase. The ion-pair product is strongly stabilized and in DMSO it is dissociated into free ions. 

The most important six-member tautomeric aromatic heterocycles are the pyridine and pyrimidine derivatives. For 2-OH and 4-OH pyridine [59-63], the hydroxy form is the prevalent tautomer in the gas phase. Solvents effect calculations for 2-OH pyridine in cyclohexane, chloroform, acetonitrile, and water predicts unanimously the preference of the 2-pyridone form, (1) and (2), respectively in Figure 6.1, despite the large difference in the solvent polarity. Furthermore, the applied methods were rather diverse FEP/MD [59], ab initio SCRF [60, 62], and MC applying a QM/MM, polarizable pair-potential [61]. Although the oxo-preference was pointed out, predicted relative free energies near the available experimental values were obtained only in [60, 61]. 

Enol imine-enaminone and phenol—quinone tautomerism in (arylazo) naphthols and in analogous Schiff bases were studied by Fabian et al. [92, 93]. In all these molecules there is a favorable N- -H- -O intramolecular hydrogen bond. Depending on the X-H sigma bond (X = N, O), there are two possible tautomers in solution. The solvent effect was calculated on the equilibrium [92], and a combined effect of the solvent and the benzene substituent was studied in [93]. While the FEP/MC simulations provided consistent organic solvent effects in accord with the experimental results [92], the wide spectrum of the solvent-effect calculation methods could predict rather diverse results for several groups of systems in [93]. 

The intensities are plotted vs. v, the final vibrational quantum number of the transition. The CSP results (which for this property are almost identical with CI-CSP) are compared with experimental results for h in a low-temperature Ar matrix. The agreement is excellent. Also shown is the comparison with gas-phase, isolated I. The solvent effect on the Raman intensities is clearly very large and qualitative. These show that CSP calculations for short timescales can be extremely useful, although for later times the method breaks down, and CTCSP should be used. 

The method for calculating effective polarizabilitie.s wa.s developed primarily to obtain values that reflect the stabilizing effect of polarizability on introduction of a charge into a molecule. That this goal was reached was proven by a variety of correlations of data on chemical reactivity in the gas phase with effective polarizability values. We have intentionally chosen reactions in the gas phase as these show the predominant effect of polarizability, uncorrupted by solvent effects. 

The GB equation is suitable for the description of solvent effects in molecular mechanics and dynamics [16], as well as in quantum mechanical calculations (17,18]. An excellent review of implicit solvation models, with more than 900 references, is given by Cramer and Truhlar [19]. 

Solvation can have a profound effect on the results of a chern ical calculation, Th is is especially true wh en tti e solute an d solven t are polar or when they can participate in hydrogen honding. The solvent effect is expressed in several ways, including these ... 

Caution For ion ic reaction s in solution, solven t effects can play a sign ificari I roic. fhesc, of course, arc neglected in calculation s on a single molecule. You can obtain an indication of solvent effects from sem i-eni pirical calculations by carefully adding water molecules to th e solute m olectile. 

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

It is quite common to do the conformation search with a very fast method and to then optimize a collection of the lowest-energy conformers with a more accurate method. In some cases, single geometry calculations with more accurate methods are also performed. Solvent effects may also be important as discussed in Chapter 24. 

The solvent effects are generally less than 1 ppm, which are well within the error bounds of the standard deviations of the calculated shifts. It is possible, however, that under extreme conditions larger deviations may be observed. 

A theoretical, comparative study of the tautomerism of 56 five-membered heterocyclic rings announced in (76AHC(Sl)l) has appeared (81MI40402). The stabilities of the three forms for 5-pyrazolones, 5-pyrazolethiones and 5-aminopyrazoles have been calculated by a simple Hiickel o) iterative method. The relative energies and the substituent and solvent effects are in agreement with the experimental results. 

The axial-equatorial conformational equilibria for 2-fluoro- and 2-chlorotetrahydropyran have been investigated with several MO calculations, including calculations at the MP2/6-31G level. The MP2/6-31G calculations give values of 3.47 and 2.84kcal/mol, respectively, for the energy favoring the axial conformer. Solvent effects were also explored computationally and show the usual trend of reduced stability for the axial conformation as solvent polarity increases. 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

Note that systems having a dipole moment of 0 will not exhibit solvent effects for the Onsager SCRF model, and therefore Onsager model (SCRF=Dipole) calculations performed on them will give the same results as for the gas phase. This is an inherent limitation of the Onsager approach. 

Compute the frequency associated with carbonyl stretch in solution with acetonitrUe for the carbonyl systems we looked at in the gas phase in Chapter 4. Run your calculations using RHF/6-31+G(d) with the Onsager SCRF model. Discuss the substituent effect on the predicted solvent effects. 

J. B. Foresman, T. A. Keith, K. B. Wiberg, J. Snoonian and M. J. Frisch, Solvent Effects. 5. The Influence of Cavity Shape, Truncation of Electrostatics, and Electron Correlation on Ab Initio Reaction Field Calculations, J. Phys. Chem., submitted (1996). [Discusses the IPCM SCRF model.]... 

There is an evolution with time the older calculations correspond to isolated molecules in the gas phase without any corrections, the more recent ones include solvent effects, with different approximations, and also some corrections, like ZPE (zero-point energy correction). The contributions of some authors to the understanding of tautomerism have been significant. Some of their contributions are collected in Table II. 

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