Carbon-13 spin-lattice relaxation in benzene and substituted aromatic compounds

Abstract

Measurements of 13C spin-lattice relaxation times (TO and nuclear Overhauser effects at 25 MHz are reported for a number of aromatic compounds and are discussed in terms of four relaxation mechanisms: dipoledipole (DD), chemical shift anisotropy (CSA), spin rotation (SR), and scalar (SC). Examples are given of compounds with carbons dominantly relaxed by all of these mechanisms. Protonated ring carbons are largely relaxed by the DD mechanism; nonprotonated ring carbons are relaxed by both the DD and SR mechanisms, with the exception of 79Br-bonded carbons, which can relax entirely by the SC mechanism. For bromine-bonded carbons, the relaxation is nonexponential since the TVs are different for the two bromine isotopes. The CSA mechanism is negligible in these compounds but is the dominant relaxation mechanism for the central acetylenic carbons in diphenyldiacetylene, as shown by experiments at 25 and 63 MHz. The large contributions of DD and SR relaxation and nearly insignificant CSA contribution for the nonprotonated carbon of toluene were approximately determined from 25-and 63-MHz experiments. Dipole-dipole relaxation of protonated aromatic ring carbons in substituted benzenes is strongly affected by ring substitution. Large or polar substituents reduce molecular tumbling, lengthening the molecular correlation time, r" thereby shortening observed TVs. Anisotropic motion has an easily observable effect on the DD contribution to 7) and can form the basis for spectral assignments, as in 3-bromobiphenyl. With phenol and aniline, strong solvent effects owing to molecular association or protonation are found and affect not only the absolute values of 7), but also the ratios of 7>n;/7>. 3089 (1972).

NMR relaxation of tin-119 in pentacoordinate and hexacoordinate environments

“…Motion of phenyl groups has been studied by means of their I3C T 1 values (26). In monosubstituted benzenes, anisotropic motion is observed in which rotation about the C-1-C-4 axis is preferred.…”

Nitrogen-15 NMR of pyridine in high magnetic: Fields

“…However, it is not necessary to know the parameter O'~v (free atom) in order to make use of Eq. [6]. It can be eliminated with a reference compound whose spin-rotation tensor is known experimentally and whose relative isotropic shift r5 can be determined experimentally.…”

Molecular motion and methyl group rotation barriers from 13C nmr relaxation times. Norbornane and adamantane derivatives in solution

“…The values calculated for camphor using an angle of 1 12" and eq. [13] of ref. 3 are also given in Table 3.…”

“…describes the rotation about the C, axis and R , the Compound Solvent Concentration C-l C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-11 C-12 Woessner's equations have been successfully applied to simple molecules ( l l ) , molecules containing only carbon and hydrogen (12), and to molecules containing heterosubstituents (13,141, where the carbon-heterosubstituent bond is coincident with the symmetry axis of the molecule. Molecules containing heterosubstituents, which are not on the symmetry axis, pose a much more serious problem in that the calculation of diffusion constants about various axes requires a priori that the axes have been defined.…”