Rotational resonance

Fig. 9.30 (a) NLE setup to record the time response of a rotating resonantly scattering Fe foil on an image plate. The direction of the radioactive decay k t) deviates from the direction of ko by an angle ip that has developed during the delay time t after excitation. 

Each spectroscopic technique (electronic, vibra-tional/rotational, resonance, etc.) has strengths and weaknesses, which determine its utility for studying polymer additives, either as pure materials or in polymers. The applicability depends on a variety of factors the identity of the particular additive(s) (known/unknown) the amount of sample available the analysis time desired the identity of the polymer matrix and the need for quantitation. The most relevant spectroscopic methods commonly used for studying polymers (excluding surfaces) are IR, Raman (vibrational), NMR, ESR (spin resonance), UV/VIS, fluorescence (electronic) and x-ray or electron scattering. 

Fig. 2 (a) DRAMA pulse sequence (using % = t/2 = rr/4 in the text) and a representative calculated dipolar recoupled frequency domain spectrum (reproduced from [23] with permission), (b) RFDR pulse sequence inserted as mixing block in a 2D 13C-13C chemical shift correlation experiment, along with an experimental spectrum of 13C-labeled alanine (reproduced from [24] with permission), (c) Rotational resonance inversion sequence along with an n = 3 rotational resonance differential dephasing curve for 13C-labeled alanine (reproduced from [21] with permission), (d) Double-quantum HORROR experiment along with a 2D HORROR nutation spectrum of 13C2-2,3-L-alanine (reproduced from [26] with permission)... 

As demonstrated by Griffin, Levitt, and coworkers in the late 1980s [21, 93], it is also possible to recouple homonuclear dipolar couplings through interference between isotropic chemical shifts and the rotor revolution. This phenomenon, called rotational resonance, occurs when the spinning frequency is adjusted to a submultiple of the isotropic chemical shift difference, i.e., ncor = ct> so o) °. To understand this experiment, the dipolar coupling Hamiltonian in (10) is transformed... 

Rotational resonance [51-53] is mostly used as a selective recoupling method between isolated spin pairs generated chemically by selective labeling. It reintroduces the homo-... 

Fig. 11.7 a Pulse sequence for rotational-resonance recoupling of homonuclear spin pairs, b The spinning frequency is matched to the isotropic chemical-shift difference, and one of the resonances is selectively inverted and the polarization exchange measured as a function of the mixing time, c The difference polarization as a function of the mixing can be evaluated to give the dipolar coupling constant. 

Tab. 11.3 Rotational resonance constraints obtained for /7-amyloid protein...

McDermott et al used rotational resonance, a then newly developed solid-state NMR method, for structural studies of an inhibited complex formed by reaction of D-alanine D-alanine ligase, ATP, and the aminoalkyl dipeptide analogue. The measured NMR coupling properties indicate that the two species are bridged in a P—O—P linkage, with a P—P through-space distance of 2.7 0.2 A. This work unambiguously demonstrated that the inactivation mechanism involves phosphorylation of enzyme-bound inhibitor by ATP to form a phos-phoryl-phosphinate adduct. ... 

An interesting development in molecular rotational relaxation has been the microwave double-resonance method176-178. The technique permits the exploration of the fine detail of the processes which occur in collisions of polyatomic molecules, and results for a number of symmetric tops have been reported. For example, Oka has described experiments on NH3 in which inversion doublets for selected J values were pumped by high microwave power. Pumping disturbs the population of the inversion doublet, and also that of other doublets which are populated from the original pair by collision processes. By absorption measurements of other inversion doublets with steady state irradiation, Oka has shown that in NH3/NH3 collisions, transitions which are allowed by the electric dipole selection rules (A/ = 0, 1, + - —) are preferred. Oka s analysis indicates that relaxation is most favourable in collision with molecules having similar J values, which are termed rotational resonances (R-R transfer). For example the process... 

The vibration-rotation hamiltonian of a polyatomic molecule is more complicated than that of a diatomic molecule, both because of the increased number of co-ordinates, and because of the presence of Coriolis terms which are absent from the diatomic hamiltonian. These differences lead to many more terms in the formulae for a and x values obtained from the contact transformation, and they also lead to various kinds of vibrational and rotational resonance situations in which two or more vibrational levels are separated by so small an energy that interaction terms in the hamiltonian between these levels cannot easily be handled by perturbation theory. It is then necessary to obtain an effective hamiltonian over these two or more vibrational levels, and to use special techniques to relate the coefficients in this hamiltonian to the observed spectrum. 

Rotational resonance has become a routine experiment and is now increasingly employed in practice. In addition to the conventional situations, its usefulness has been expanded recently. Costa et a/.146 found that while n = 1 rotational resonance can be used to determine the internuclear distance, the = 2 rotational resonance contains the information for determining the relative orientation of the chemical shift and dipolar tensors. 

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