Rotational isomer, rotamer

The small optical rotations of the alditols arise from the low energy barrier for rotation about C—C bonds, permitting easy iaterconversion and the existence of mixtures of rotational isomers (rotamers) ia solution (12). 

The concept of atropisomerism developed to a considerable extent following other developments in chemistry, especially those in spectroscopy. Early work by Kohlrausch (4) and Mizushima (3), based on Raman spectra and dipole moment studies, established that rotational isomers—rotamers—must exist in 1,2-dichloroethane. Pitzer established that there are three energy minima when ethane is rotated about its C—C axis (6). Rotamers about single bonds have been found in a wide variety of organic compounds since then, mainly as a result of the application of vibrational spectroscopy to organic molecules (7). 

Know the meaning of conformation, staggered, eclipsed, dash-wedge projection, Newman projection, sawhorse projection, rotational isomers, rotamers. 

Scheme 9 provides (using 47 for purposes of illustration) molecular detail for the concepts outlined in Fig. 21. In essence, the proof of principle for the molecular motor starts with 47a. Compound 47a is one of three low-energy rotational isomers (rotamers) about the axle connecting the triptycene and helicene components (47b is a second low-energy rotamer). Rotamer 47a is activated by reaction with phosgene to give the isocyanate 49. Isocyanate 49 is chemically armed to react with the OH group in the hydroxypropyl tether attached to the helicene, but, in the rotational ground state 49, the isocyanate...

Vibrational assignment of the rotational isomers (rotamers) of methyl nitrite has been accomplished by means of a photochemical effect vibrational and electronic spectra were measured for the vapour phase and matrix-isolated samples of H3CONO and DsCONO. ... 

A series of thioalkylphenylalanine compounds were synthesized. Because of hinder rotation around the N-phenyl bond, the compounds exist as a mixture of stable but interconvertible rotational isomers (rotamers). These rotamers were isolated, characterized, and found to possess differential fungicidal activity against a variety of plant pathogens including downy mildew of grape. 

The nmr of the isolated metabolite 5, a glucuronide, suggested that it existed as a mixture of rotational isomers (rotamers), as did the acylat glucuronide 6, due to the hindered rotation about the triazole-phenyl bond. When the structure was confirmed by synthesis from 2-amino-6-methylbenzothiazole, 7, by a series of steps outlined in Scheme 2, the synthesized glucuronides also appeared to exist as a mixture of rotamers. Further, these rotamers were separate as the acylated ucuronides by liquid chromatography and characterized by their nmr spectra which differed in the proton resonances of the peaks assigned to the 2-methyl, the thiomethyl, and the triazole hydrogen. 

The interconversion of stereoisomers through rotation of groups of atoms about a single bond. The internal rotation is called free when the energy barrier is so low that different rotational isomers (rotamers) cannot be discerned as individual chemical species (i.e., the characteristic time of the method of observation is longer than the lifetime of the rotamers). See Conformational Analysis 1 Conformational Analysis 2 and Conformational Analysis 3. 

Before melting and for some time after only the band at 625 cm of the AA [C4CiIm]+ cation was observed in the 600-630 cm i region. Gradually 603 cm i band due to the GA conformer became stronger. After about 10 min the AA/GA intensity ratio became constant. The interpretation [50] is that the rotational isomers do not interconvert momentarily at the molecular level. Most probably it involves a conversion of a larger local structure as a whole. The existence of such local structures of different rotamers has been found by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [82], Coherent anti-Stokes Raman scattering (CARS) [83],... 

Radical anions derived from 2,5-diformylthieno[3,2-6]thiophene (39) as well as (40) were studied in connection with the conformational analysis of heteroaromatic carbonyl compounds. Different ESR signals were given by distinct rotational isomers. Information on interconversion of the rotamers could not be obtained since the radicals were unstable at the temperatures necessary for interconversion. On the other hand, ketyl radicals derived from ketones (41) and (42) are relatively more stable at the temperatures needed for the study of the conformational mobility in these systems. The ESR spectra of the bis-thienothienyl ketyls from (41) and (42) at room temperature show that the unpaired electron is coupled to three pairs of equivalent protons. Both spectra exhibited a certain amount of asymmetry, which was enhanced by lowering the temperature. At -10°C the highfield part of the spectra split into new lines arising from two species which have similar hyperfine splitting, but different g factors. These have been identified as the rotational isomers of the radicals. The two preferred conformations are cis-trans and trans-trans. An examination... 

There have also been extensive stndies of a jj" -diene rotation and rotamers that can be observed in complexes of dienes. In Cp complexes, the orientation relative to the Cp ligand gives rise to endo and exo isomers, for example, (57) and (58), the interconversion of which can be observed by Interconversions are particniarly rapid when the parent stmctures are nonrigid. Thus the [CpMo(CO)2(j -diene)]+ complexes can be considered to be based on a square pyramid and are analogous to CpMo(CO)2LX hgands, which are nonrigid themselves. ... 

The conformation of a macromolecule of given constitution and configuration specifies the spatial arrangements of the various atoms in the molecule that may occur because of rotations about single bonds. Molecules with different conformations are called conformational isomers, rotamers, or conformers. 

Semi-empirical AMI-SCI calculations have been performed to rationalize the photophysical behavior of two series of compounds one comprising of 2-(2 -hydroxyphenyl)benzoxazole, 2-(2 -hydroxyphenyl)benzimidazole (HBI), and 2-(2 -hydroxyphenyl)benzothiazole, and the other of 2-(2 -hydroxyphenyl)oxazole (HPO), 2-(2 -hydroxyphenyl)imi-dazole (HPI), and 2-(2 -hydroxyphenyl)thiazole (HPT). These compounds exhibit intramolecular rotation as well as excited state intramolecular proton transfer (ESIPT). The results suggested that for the first series of compounds two rotational isomers are present in the ground state of HBO and HBI while HBT has a single conformer under similar circumstances. For the other series, existence of rotamers depends very much on the polarity of the environment <2003IMS335, 2002JST(604)87>. 

A detailed study was carried out on the radical anion derived from (10c) (Table 2) in connection with the conformational analysis of heteroaromatic carbonyl compounds. Notably different ESR signals could be observed originating from distinct rotational isomers. Unfortunately no information on the interconversion of the rotamers of (10c) could be definitely derived since these radicals were highly unstable at the temperature necessary for interconversion <74JCS(P2)562>. 

As mentioned earlier, initiator 2 has rotational isomers in which the alkylidene substituent is turned either towards the second multiple bond (5y ) or away from it anti)-, eqn. (11). In this case the equilibrium position is very much in favour of the syn rotamer K= 1450 in toluene at 25°C), making it difficult to detect the anti rotamer in routine spectra. However, the equilibrium can be displaced by UV irradiation (366 nm) of the solution for several hours at — 80°C to yield a mixture containing about 33% of the anti rotamer as determined from the resonances syn, 6 12.11,7ch = 120.3 Hz anti, 5 13.30, Jch= 153.3 Hz (Oskam 1992, 1993a). On adding 0.33 equivalents of 2,3-bis(trifluoromethyl)norbomadiene to this solution and running the spectrum again at — 80°C it is found that the anti rotamer has been completely consumed, giving the syn first-addition product, eqn. (12), while the syn rotamer has scarcely reacted at all. It is estimated that the... 

Rotamer. Syn. rotational isomer. An isomer generated by rotation (usually 120°) about a chemical bond. 

Rotational isomerism. Interconversion of rotational isomers or rotamers. 

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