Benzene, proton relaxation

In zeolites the mobility of hydrocarbon molecules with double bonds is specifically restricted because of a specific interaction between the 7r-elec-trons and the zeolite (2). As expected, proton spin relaxation of benzene, cyclohexadiene, cyclohexene, and cyclohexane adsorbed on NaY reveals an increasing restriction of mobility with increasing number of -electrons (8, 4, 8). This is shown in Figure 1, where the longitudinal (7 ) and transverse (T2) proton relaxation times are plotted. 

Figure 2 shows the thermal correlation times rc, calculated from the longitudinal relaxation times in the usual way (1). The time constant rc is a direct measure of the molecular mobility, and one recognizes that at room temperature the molecular mobility of cyclohexane is about 200 times greater than that of benzene. As the corresponding heats of adsorption (5) differ only by a factor 1.5, proton relaxation is extremely sensitive to specific interaction (c/. (6)). 

The H NMR spectra of the 1,3,5-triazines are quite simple. The ring protons give sharp lines which are not broadened by nitrogen quadrupolar relaxations (65BSB119). The chemical shifts of the ring protons are 1 to 2 p.p.m. downfield from benzene protons, presumably... 

Meiler and Pfeifer (493) measured 13C and H NMR spectra of carbon monoxide, carbon dioxide, and benzene adsorbed on ZSM-5 and silicalite. The 13C signal from benzene was a superimposition of two lines corresponding to relatively mobile molecules (narrow Lorentzian line) and strongly adsorbed molecules (broad asymmetric line similar to that in polycrystalline benzene). Quantitative interpretation of the spectrum was possible via the measurement of the transverse proton relaxation times, T2, as a function of temperature and coverage. Recent work involving 13C NMR studies of sorbed species is summarized in Table XX. 

An example of this method can be seen in Fig. 3.2.7 for Cr(CO)3(C6H6) [17]. The curve of the proton relaxation time versus 1000/T shows a minimum at around 190 K and allows the calculation of an activation energy of 17.6 kj mol-1 for the free rotation of the benzene ring around the principal molecular axis. 

A sharply defined line for the proton signal, not broadened by nitrogen quadrupolar relaxation, is seen for 1,3,5-triazine at 9.18 ppm, which is well downfield of the chemical shift of the benzene proton. The presence of electron-releasing substituents gives slight upheld shifts for ring protons of substituted 1,3,5-triazines. Chemical shifts of 8.8 ppm, 8.51 ppm, and 8.19 ppm are seen for... 

Similar conclusions were obtained from lH and 31P NMR and also from IR studies of egg phospholecithin reversed micelles in benzene by Boicelli et al. 58 61). According to the results of these experiments the water structure within the reversed phospholecithin micelles alters considerably compared with water in bulk. This becomes evident from the shortening of the relaxation time T, of the water protons split into two relaxation times T1A and T1B, indicating that there are at least two... 

Single- and double-selective relaxation-rates, together with n.O.e. experiments, have been used to examine the configuration and conformation of asperlin (1) in benzene solution." " Comparing experimental distances for the proton pairs H-4,H-7 and H-5,H-7 with those obtained from molecular models, it was possible to confirm earlier evidence that the oxirane ring is trans, and also to show that, of the two possible diastereoisomeric forms (49a and 49b), the data are more fully compatible with structure 49a, the... 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

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