Line broadening restricted rotation

The ESR spectra of a large variety of sulfonyl radicals have been obtained photolytically in liquid phase over a wide range of temperature. Some selected data are summarized in Table 2. The magnitudes of hyperfine splittings and the observations of line broadening resulting from restricted rotation about the C—S bond have been used successfully in conjunction with INDO SCF MO calculations to elucidate both structure and conformational properties. Thus the spin distribution in these species is typical of (T-radicals with a pyramidal center at sulfur and in accord with the solid-state ESR data. 

The rotational barrier for the secondary I -methyl cation 30 could not be measured experimentally, but the barrier for the tertiary r,T-dimethyl cation 31 could be estimated from the observed line broadening at + 80 °C as 18 kcalmoT Theoretical calculations at the STO-3G level gave a value of 17.7 kcalmoT for the latter cation. The secondary cationic system is a mixture of two isomers (cw-30a and trans-3 h) due to the restricted rotation across the Cl—CT bond. In the tertiary dimethyl cation 31, the methyls expectedly show distinct NMR absorptions <5 H 3.14 (cw-Me), 2.60 trans-Mo) <5 C 30.2 (cw-Me), 33.7 trans-MQ). The cationic center s absorptions are also interesting in that they are highly shielded compared with other related cations, including cyclopropylcarbinyl cations. The tertiary cationic center has <5 C of 249.6, the secondary cationic center <5 C of 220.8 and the primary cationic center <5 C of 191.4. Calculations at MNDO and STO-3G suggested that these cations have enhanced vinyl-bridging character compared to the simple cyclopropylcarbinyl model". ... 

At low temperatures, the two j3-hydrogens would have slightly different coupling constants due to restricted rotation about the CQ—Cp bond and broadening of the so-called four-line component would result. At elevated temperatures, the rate of interchange should be sufficiently large for an average interaction to be observed, viz. 

Infrared adsorption frequencies and line broadening (X5,1 6) have been interpreted to show that water is bound 0 downwards to the HO M. The lifetime, and population, of aggregates and hydration sheaths is strongly affected by the rotational freedom of the water molecules. In bulk water this is restricted by hydrogen bond breaking. In adsorbed layers the interaction with the adsorbate dominates reorientation for all molecules within six to ten molecular dieurneters. 

Restricted rotation deduced from line broadening at 104 K. Determination of lower limit of the barrier to valence bond tautomerization. Radical absorbed on synthetic H-mordenite at 308 K. Radical generated by y-irr. of cyclopentene in CF3CCI3 and photobleaching. ... 

It is important to note that the alignment induced by most orienting systems is due to the existence of very weak interactions between the macromolecule and the media itself. Because of the weak character of these interactions, the rotational tumbling of the molecule is not impaired or restricted, and therefore there is no additional broadening of resonance lines due to relaxation phenomena [37]. 

A situation peculiar to electronic spectra occurs when the perturbing state is dissociative, for the molecule may then undergo a radiationless transition from a stable electronic state to a dissociative one. The phenomenon, known as predissociation, may be detectable by a broadening of the rotational lines which, when the radiationless transition has high probability, become so broad that all traces of rotational structure vanish. Predissociation is observed frequently in polyatomic spectra and seriously restricts the number of band systems suitable for detailed study. 

Vibrational-Rotational Transitions Vibrational energy levels are also quanli/ed. and for mosl molecules the energy differences between quantum stales correspond to the iiiid-IR region. The IR spectrum of a gas usually consists of a series of closely spaced lines, because there arc several rotational energy levels for each vibrational level. Ou the other hand, rotation is highly restricted in liquids and solids in such samples, discrete vibrational-rotational lines disappear, leaving only somewhat broadened vibrational bands. 

The experimental lineshapes of the carboxyl and methyl carbon resonances of fully enriched L-Alanine have been studied in detail at different MAS frequencies and decoupling field strengths. Complex lineshapes at intermediate spinning speeds were explained by the joint effect of off rotational resonance and coherent CSA-dipolar cross-correlation. It was found that coherent CS A-dipolar cross-correlation introduces either a differential intensity and/or a differential broadening of the lines of the J-multiplet. The conditions that lead to such effects were explained and experimentally verified. Additional simulations showed that these effects can be expected over a wide range of static magnetic fields and are not restricted to L-Alanine. 

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