Naphthalene spin-lattice relaxation

Naphthalene, in contrast to benzene, did not show any NMR-spectra line-width narrowing up to its melting temperature of 353 K. The mean experimental second moment was 9.1 compared to 10.1 G, estimated for the rigid crystal. Measurement of spin-lattice relaxation times indicated, however, also a slow reorientational jump motion about an axis normal to the molecular axis An activation energy of 105 kJ/mol was derived. Molecular dynamics simulations suggest that this reorientation about the axis of greatest inertia occurs with a frequency of 100 MHz within 20 K of fusion (353.6 K) Still, no plastic crystal behavior as found in cyclohexane and related compounds (see Sect. 3.1.1) is indicated for benzene or naphthalane, even close to the melting temperature. 

Von Schiitz and Wolf [29] have likewise investigated a second typical stochastic reorientation motion using NMR in molecular crystals the hindered rotation of methyl (CH3 -) groups in ten different methyl-substituted naphthalene crystals. The reorientation motions are, at not-too-low temperatures, the dominant source of nuclear spin-lathce relaxation in these highly purified molecular crystals. Only at very low temperatures do thermally-activated reorientation processes cease to play a role. The spin-lattice relaxation is then determined essentially by paramagnetic impurities. 

Fig. 5.21 The proton spin-lattice relaxation time T] of 2,3 /3-dimethyl-naphthalene (a) and of 1,5 /3-dimethyl-naphthalene crystals (b) as a function of the inverse temperature and at the proton spin Larmorfrequencies 4 MHz, 22 MHz, 44 MHz and 86 MHz. After [29].

An example of a quantitatively-analysed experimental result for these constants is shown in Fig. 7.29 in mixed crystals of naphthalene-dg 0.1% quinoxaline, the ESR transition T. To for the field direction Bo Xquinoxaiine and at a temperature T = 1.8 K is an absorption signal in the stationary state (Fig. 7.29a), while the transition I To) T-) in the stationary state exhibits stimulated emission of microwaves (Fig. 7.29b). After the end of the UV excitation at t = 0, the absorption line temporarily becomes an emission tine and vice versa. The interpretation of these results is simple (Fig. 7.29d) due to the negligible spin-lattice relaxation at T= 1.8 K, the three Zeeman components decay after the end of the U V excitation independently of one another, each with its own lifetime tj = into the So ground state. Since the difference of the populations of the three states is directly proportional to the intensity of the ESR signals, their time dependence can be used to determine the individual lifetimes of the Zeeman components involved. In the case of the particular orientation Boll, the state is To) = IT ), and one obtains directly from the measurements, e.g. the decay constant feo = kx and thus the lifetime of the zero-field constant Tx) of quinoxaline. 

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