Selenophenes dipole moments

Selenolo[3,2-6]pyrrole PE, 4, 1046 <77JOC2230> Selenolo[2,3-6]selenophene PE, 4, 1046 <77JOC2230) Selenolo[3,2-b]selenophene dipole moment, 4, 1049 (76AHC(19)123>... 

Selenolopyrylium salts, 4, 1034-1036 Selenolo[2,3-c]pyrylium salts synthesis, 4, 969 Selenolo[3,2-b]pyrylium salts synthesis, 4, 1035 Selenolo[3,2-c]pyrylium salts synthesis, 4, 969 Selenoloselenophenes electrophilic substitution, 4, 1057 NMR, 4, 13 synthesis, 4, 135 UV spectra, 4, 1044 Selenoloselenophenes, alkyl-synthesis, 4, 967 Selenolo[2,3-b]selenophenes ionization potentials, 4, 1046 Selenolo[3,2-b]selenophenes dipole moments, 4, 1049 ionization potentials, 4, 1046 structure, 4, 1038, 1039 Selenolo 3,4-b]selenophenes H NMR, 4, 1042 synthesis, 4, 1067 Selenolo[3,4-c]selenopheneS non-classical reactions, 4, 1062 synthesis, 4, 1076 Selenolothiophenes electrophilic substitution, 4, 1057 H NMR, 4, 1041 UV spectra, 4, 1044 Selenolo[2,3- b]thiophenes... 

Selenophene, tetrahydro-dipole moment, 4, 2 (73CR(C)(277)203> electron diffraction, 4, 938 (70MI31600)... 

Dipole moment studies of 2-methylmercapto-selenophene and -tellurophene indicate that in both cases the cis conformer (26 Z = Se, 59% Z = Te, 65%) is preferred to the trans one (27) (73BSF1924). The conformational preferences, (28) versus (29), of a variety of other 2-substituted compounds as deduced from dipole moment or lanthanide induced NMR shift measurements are shown in Table 5. Selenophene-3-carbaldehyde exists mainly as the trans conformer (30) according to dipole moment measurements, while the selenophenedicarbaldehydes (31)-(34) exist preferentially in the conformations depicted (73BSF1924). 

Methylselenophene-2-thiol and 2,5-dimethylselenophene-3-thiol have been shown to exist as thiols by NMR and IR spectroscopy as well as by comparison of gas phase ionization potentials with those of appropriate model compounds (77ACS(B)198). Dipole moment studies have supported formulation of selenophene-2- and -3-thiols as such (73BSF1924) rather than the 2-thione structure originally proposed for the former compound (71BSF3547). 

Whereas the dipole moments of thieno[3,2-b]thiophene (3) and selenolo[3,2-6]selenophene (4) are 0.00 D, the mixed system selenolo[3,2-6]thiophene (29) exhibits a dipole moment of 0.30 D. Perturbation of the symmetry of (3) by the introduction of an ethyl group in the molecule at C-2 generates a dipole moment of 0.30 D. A study of the other classical thienothiophenes, selenoloselenophenes and selenolothiophenes shows that the [2,3-6]-annelated systems exhibit a slightly higher dipole moment compared to the [3,4-6]- or [2,3-c]-annelated systems (76AHC(19)123). In the case of the nonclassical thiophenes (Id X = Y = S), (Id X = S, Y=NH) and (Id X = S, Y = 0) the predicted dipole moments are 0.00, 0.15 and 3.21 D respectively (74JA1817). Experimental verification is not possible since none of these compounds are known although it should be of interest to determine the dipole moments of the tetraphenyl derivatives (6) and (12) and the pentaphenyl compound (13). 

The difference between the dipole moments of the aromatic compound and the corresponding tetrahydro derivative is called the mesomeric moment and represents a measure of the -electron delocalization. The mesomeric dipole moment is directed, in all the congener systems, from the heteroatom toward the ring. The values obtained (furan 1.03 D, thiophene 1.35 D, selenophene 1.29 D, tellurophene 1.17 D) are in excellent quantitative agreement with other aromaticity indices based on structural and magnetic properties12 (see discussion in Section II,C). 

The dipole moments in benzene of a number of 2-substituted tellurophenes have been determined13 and compared with the moments of the corresponding derivatives of furan, thiophene, and selenophene (Table V). 

The role of heteroatoms in ground- and excited-state electronic distribution in saturated and aromatic heterocyclic compounds is easily demonstrated by a comparison of a number of heteroaromatic systems with their perhydro counterparts. In Jt-excessive heteroaromatic systems, because of their resonance structures, their dipole moments are less in the direction of the heteroatom than in the corresponding saturated heterocycles furan (1, 0.71 D) vs. tetrahydrofliran (2, 1.68 D), thiophene (3, 0.52 D) vs. tetrahydrothiophene (4, 1.87 D), and selenophene (5, 0.40 D) vs. tetrahydroselenophene (6, 1.97 D). In the case of pyrrole (7, 1.80 D), the dipole moment is reversed and is actually higher than that of pyrrolidine (8, 1.57 D) due to the acidic nature of the pyrrole ring (the N-H bond) In contrast, the dipole moment of n-deficient pyridine (9, 2.22 D) is higher than that of piperidine (10, 1.17 D). In all these compounds, with the exception of pyrrole (7), the direction of the dipole moment is from the ring towards the heteroatom [32-34]. 

The dipole moments of some simple halogeno-, cyano-, and formyl-selenophens have been measured and the conformations of these compounds have been discussed. Through a study of the n.m.r. spectrum of selenophen in lyotropic mesophase, the ratios of the interproton distances were calculated from the direct couplings and found to be in good agreement with the corresponding values calculated from microwave data. ... 

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