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Handbook of Solvents - George Wypych - ChemTech - Ventech!

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11.1 Theoretical treatment <strong>of</strong> solvent effects 675<br />

For instance, the n →π* electronic transition (1 1 A1 →1 1 A2) <strong>of</strong> formaldehyde solvated<br />

by varying number <strong>of</strong> water molecules has been investigated using multi-reference CI calculations.<br />

87 This simple supermolecule approach has given already satisfactory results as<br />

compared to experimental shifts in liquid water. However, it has been shown that in general,<br />

both the short-range quantum mechanical effects and the long-range solvent polarization<br />

play important role in determining the spectral shifts in liquid media. Thus, the INDO/S<br />

SCRF CIS theory alone has explained the solvatochromic shifts <strong>of</strong> azoles in different solvents,<br />

except those observed in water (Table 11.1.6). 85 In both the water and acetonitrile, the<br />

compounds are predicted to have practically the same shift that is not the case in experiment.<br />

The explicit bonding <strong>of</strong> two water molecules to the nitrogen lone pairs leads in the<br />

cases <strong>of</strong> pyrimidine and pyridazine to the calculated large red shift instead <strong>of</strong> the experimentally<br />

observed solvatochromic blue shift. However, by treating the complex <strong>of</strong> an azole and<br />

two water molecules quantum-mechanically in the surrounding reaction field leads to quantitatively<br />

correct blue shifts.<br />

Table 11.1.6. The INDO/S SCRF/CI calculated and experimental spectral transition<br />

energies in different solvents for azoles (cm -1 ) 85<br />

Pyrimidine<br />

Pyridazine<br />

Pyrazine<br />

Molecule Solvent νcalc, cm -1<br />

gas phase<br />

isooctane<br />

diethyl ether<br />

acetonitrile<br />

water<br />

2H 2O<br />

water + 2H 2O<br />

gas phase<br />

isooctane<br />

diethyl ether<br />

acetonitrile<br />

water<br />

2H 2O<br />

water + 2H 2O<br />

gas phase<br />

isooctane<br />

diethyl ether<br />

acetonitrile<br />

water<br />

2H 2O<br />

water + 2H 2O<br />

32966<br />

33559<br />

34127<br />

34697<br />

34743<br />

30982<br />

36572<br />

28329<br />

29460<br />

30382<br />

31296<br />

31368<br />

26490<br />

33927<br />

30387<br />

30387<br />

30387<br />

30387<br />

30387<br />

32900<br />

33301<br />

νexp, cm -1<br />

-<br />

34200<br />

34400<br />

34800<br />

36900<br />

36900<br />

36900<br />

-<br />

29740<br />

30150<br />

31080<br />

33570<br />

33570<br />

33570<br />

-<br />

31610<br />

31610<br />

31740<br />

33160<br />

33160<br />

33160<br />

As a general remark, in the calculations <strong>of</strong> the intermolecular interactions using the<br />

supermolecule approach, the “size-extensivity” 88 <strong>of</strong> the methods applied is <strong>of</strong> crucial importance.<br />

Furthermore, the interaction energies calculated in the supermolecule approach usually<br />

suffer from what is called the basis set superposition error (BSSE), 89 a spurious energy<br />

improvement resulting from the use <strong>of</strong> truncated basis sets. This error seems to be unavoidable<br />

in most practical calculations except for very small systems. 90

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