Contributions to the study of rotational isomerism in derivatives of furan, thiophene, and pyrrole bearing carbonyl-containing substituents (review)

The photoelectron spectra and electronic structure of 2-carbonyl furans

“…Thus appreciable quantities of both rotamers should exist in the gas phase at room temperature. Similar rotational isomerism has been observed for 2-furoyl fluoride (25,27) and chloride (28), and 2-acetyl furan (29). As well, MO calculations on the relative stabilities of the rotamers of 2-furaldehyde (30) and other substituted carbonyl furans (31) have been reported.…”

“…From IR and variable-temperature NMR studies it appears that the Z isomer of 2-furaldehyde is the more stable in the gas phase and in nonpolar solvents (24,25), with the E isomer being about 4 kJ/mol higher in energy (26). Thus appreciable quantities of both rotamers should exist in the gas phase at room temperature.…”

Twisted Imide Bond in Noncyclic Imides. Synthesis and Structural and Vibrational Properties of N,N-Bis(furan-2-carbonyl)-4-chloroaniline

“…In Figure , it is clearly observed that both the infrared and Raman spectra for the title species shows the presence of a series of signals, with strong bands at 1468 and 1469 cm –1 , respectively. However, it is well-known that the furan , and benzyl groups also show intense features in this region, associated with the ν­(CC) stretching and δ­(C–H) bending modes, respectively. Thus, on the basis of these reports and on the calculated displacement vectors calculated for the harmonic normal modes, we tentatively assign the intense band observed at 1337 cm –1 in the infrared spectrum to the symmetric motion of the stretching mode of the N–C­(O) bonds, with the antisymmetric counterpart at 1281 cm –1 .…”

“…The conformational properties of carbonyl furans were studied, and two planar conformations have been identified. These planar forms are named as syn and anti, depending on the mutual orientation of the CO double bond and the oxygen atom in the ring. , From early IR and variable-temperature NMR studies, it is known that the syn isomer of 2-furaldehyde is the more stable form in the gas phase and nonpolar solvents, with the anti conformer being about only 1 kcal/mol higher in energy . Similar rotational isomerism has been observed for related 2-carbonylfuran species, and both experimental and theoretical results indicate that for all species the energy difference between the two planar configurations is small, with a low barrier to interconversion …”

“…19,20 From early IR and variabletemperature NMR studies, it is known that the syn isomer of 2-furaldehyde is the more stable form in the gas phase and nonpolar solvents, with the anti conformer being about only 1 kcal/mol higher in energy. 21 Similar rotational isomerism has been observed for related 2-carbonylfuran species, and both experimental and theoretical results indicate that for all species the energy difference between the two planar configurations is small, with a low barrier to interconversion. 22 For the title species, the X-ray structure analysis reveals that the 2-carbonyl furan groups are not equivalents in the solid phase, mainly differing in the conformation adopted by the CO double bond with respect to the furan ring.…”