Selenophene geometry

Table 2.9 DFT/B3LYP/d-aug-cc-pVDZ and experimental results (atomic units) for the CME of pure liquid furan, thiophene and selenophene. A =632.8 nm, T=293.l5 K. Experimental geometries...

Table 2.12 BE of furan, thiophene and selenophene in cyclohexane. Comparison of theory (PCM/DFT/B3LYP both for the geometry optimization and for the properties, aug-cc-pVTZ basis set) and experiment. See text for definitions and ref. [32] for further details. In particular, i/ + (S/oijC ) indicates the numerator of the right-hand side of Equation (2.227). Atomic units. A =632.8 nm. CM stands for the centre of nuclear masses, chosen as reference origin in the calculations...

Assignment of the principal vibrations allowed calculation of the force field of thiophene and selenophene with respect to the symmetry coordinates.30 The force field factors found for thiophene vibrations of type A 1 A2, and B2 were used as the initial approximation for the respective types of selenophene vibrations. The calculated frequencies are in fair agreement with experiment. Therefore, the force field of selenophene is similar to that of thiophene. The B1 frequencies of selenophene calculated by the force field derived from thiophene disagreed with the experimental values. The differences seem greater than could arise from the uncertainties in the geometry of selenophene. 

In addition to the many ab initio calculations, there was one report using semi-empirical calculations to study the relative geometry and charge distribution of selenophene and its azido derivatives <1994J(P2)1815>. 

The reactivity of thiophen has also been compared with that of seleno-phen and the relative reactivities in five electrophilic substitutions have been determined by kinetic or competitive procedures. The results have been compared with those available in the literature for furan. In all the reactions examined, selenophen exhibited a reactivity intermediate between those of furan and thiophen. p-Constants for electrophilic substitution of substituted thiophens are usually smaller than in the benzene series. A comparison of the trifluoroacetylation of a series of substituted thiophens and furans yielded p-values of — 7.4 and — 10.7 respectively. The observed order of substrate selectivity in the trifluoroacetylation (furan > thiophen) thus parallels the positional selectivity in electrophilic substitution, the oi ratio always being larger in furans than in thiophens. The relative importance of primary steric effects in benzene and thiophen has been investigated by determination of the isomer distributions in the acetylations of 2- and 3-methylthiophen, 2- and 3-t-butylthiophen, and toluene and t-butylbenzene. Steric hindrance is less significant in the thiophen series owing to the more favourable geometry. - ... 

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