Applications to Catalysis and Environmental Science

Abstract

Electronic and geometrical structures of NO-Na⁺ and Cu(I)-NO complexes formed in zeolites are discussed based on the g and the ¹⁴N and ²³Na hf values evaluated by multi-frequency ESR, pulsed ENDOR and HYSCORE methods. The structure of (NO)₂ bi-radical formed in zeolites is discussed based on X- and Q-band ESR spectra. Microenvironment effects on the molecular dynamics and the thermal stability of triethyl- and tripropyl-amine radical cations as spin probes are presented referring to the CW-X-band ESR results and theoretical DFT calculations. X- and Q-band ESR studies on nitrogen-doped TiO₂ semiconductor reveal that the diamagnetic N^– ion in the system absorbs visible light so as to excite an electron of N^– to the conduction band. The photo-catalytic reactions of TiO₂ are modified by introducing O₂ molecules which scavenge a fraction of photoexcited electrons to generate O₂ ^–. ESR spectral characteristics of adsorbed O₂ ^–, g-tensor and hf structure of labeled ¹⁷O (I = 7/2), are presented.

Appendices

Appendices

1.1 A6.1 Isotropic Hyperfine Splittings of β-Hydrogens

The isotropic hf splitting of β-hydrogens, a _β-H, arises, in principle, due to a hyperconjugative mechanism [56b]. To illustrate the general ideas here we consider triethylamine radical cation, (CH₃CH₂)₃N^{· +}, described in Section 6.4. The hydrogens of (CH₃CH₂)₃N^{· +} are conventionally labeled with Greek letters: β for one bonded to carbon adjacent to the π-radical center (α-position; N atom) and γ for a hydrogen one carbon further out as C_γH₃-C_βH₂-N_α ^{· +}-(CH₃CH₂)₂. It is found experimentally that the β-hydrogen hf splitting is formulated as follows [127]:

$$\mathop a\nolimits_{\beta - H} = (A + B\cos ^2 \theta )\rho $$

((6.10))

where θ is the angle between the unpaired electron 2p _z orbital at the π-radical center and the C_(β)-H bond and ρ is the spin density on the nitrogen 2p _z orbital, see Fig. 6.8(c). Coefficient B reflects the spin density arising from hyperconjugation and should be positive; the positive sign has been confirmed by NMR experiments. On the other hand coefficient A accounts for that arising from orientation-independent mechanisms such as spin polarization. If there is free rotation about the C_β—N_α bond then we observe an orientationally averaged hf splitting:

$$\left\langle {\mathop a\nolimits_{\beta - H} } \right\rangle _{av} = \left(A + B\left\langle {\cos ^2 \theta } \right\rangle _{av} \right)\rho = (A + (1/2)B)\rho $$

((6.11))

Studies of many systems suggest that the value of A is much smaller than B, i.e., less than ca. 0.3 mT and Eq. (6.10) can be approximated as:

$$\mathop a\nolimits_{\beta - H} = B\rho \cos ^2 \theta $$

((6.12))

From the ESR spectral analysis of (CH₃CH₂)₃N^{· +} in AlPO₄-5 the following isotropic hf splittings have been identified: a _β-H = 2.0 mT for six equivalent β-hydrogens at 300 K corresponding to an averaged structure and a _β-H = 3.6 mT for three equivalent hydrogens, one from each β-methylene group, at a low temperature of 77 K corresponding to a rigid limit structure, see Fig. 6.8(a) and (b). Combining Eq. (6.12) with the experimental β-hydrogen hf splittings at 77 and 300 K, we have the following four equations:

$$\mathop a\nolimits_{\beta - H} (1) = B\rho \cos ^2 \theta (1) = 3.6{\textrm{mT}}\ \hbox{(experimental value at 77\,K)}$$

((6.13))

$$\mathop a\nolimits_{\beta - H} (2) = B\rho \cos ^2 \theta (2)$$

((6.14))

$$\left( {\mathop a\nolimits_{\beta - H} (1)\mathop { + a}\nolimits_{\beta - H} (2)} \right)/2 = 2.0{\textrm{mT}}\ \hbox{(experimental value at 300\,K)}$$

((6.15))

$$\theta (1) + \theta (2) = 120^\circ\ \hbox{(assumption)}$$

((6.16))

By solving the equations we obtain: Bρ= 6.9 mT, a _β ^H(2) = 0.4 mT, θ(1) = 44° and θ(2) = 76°, see Fig. 6.8(c). Consistent with the experimental result the value evaluated for a _βH(2), 0.4 mT, is much smaller than the experimental line-width of 1.2 mT at 77 K and too small to be resolved in the spectrum.

We close this section by repeating that Eq. (6.10) for the angular dependence on β-hydrogens is very useful in discussing geometrical structures (conformations) of not only the amine radical cations exemplified here but also for other many alkyl-type radicals [56b] including those stabilized in organic solid polymers as described in Chapter 7.

1.2 A6.2 Anatase and Rutile TiO₂

Titanium dioxide has several polymorphs. Among them, rutile (P4₂/mnm space group, D _4 h) and anatase (I4₁/amd space group, D _4 h) are well known from viewpoint of crystal structure [128] and have been intensively studied for photocatalysis. Experimentally, anatase has a slightly larger band gaps than rutile, 3.2 vs. 3.0 eV [129, 130]. Ab-initio calculations show that the primary structural difference between the anatase and rutile phases is that the former is 9% less dense than the latter, and has larger Ti-Ti distances, a more pronounced localization of the Ti 3d states and a narrower 3d band [131]. This can be a reason why the carrier (electron) generated in anatase by UV excitation is less mobile than in rutile. Also the O 2p-Ti 3d hybridization is different in the two structures (more covalent mixing in rutile), with anatase exhibiting a valence and a conduction band with more pronounced O 2p and Ti 3d characters, respectively [131]. In rutile the greater Pauli repulsion among the oxygen 2p electrons results in a larger O 2p bandwidth. Experimentally, the bandwidth in rutile is 6 eV while it is 4.7 eV in anatase [132, 133]. The calculated values are 5.3 and 4.5 eV, respectively; the values nicely reflect the important difference in the electronic structures [105].