Experiments Employing Selective Excitation

The BIS HMQC pulse sequence may give relatively intensive residual parent lines resulting from imj erfections of the 180° pulse. These can be removed using the post-acquisition solvent suppression techniques. 

The double pulsed field gradient spin echo (DPFGSE) pulse sequence has been used to improve the measurement of proton-proton nuclear Overhauser effect (NOE) [28]. The DPFGSE NOE does not rely on difference spectroscopy and very small NOEs can be measured. This technique has been used to determine the structure of organosilicon compounds [28]. 

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In this contribution, the experimental concept and a phenomenological description of signal generation in TDFRS will first be developed. Then, some experiments on simple liquids will be discussed. After the extension of the model to polydisperse solutes, TDFRS will be applied to polymer analysis, where the quantities of interest are diffusion coefficients, molar mass distributions and molar mass averages. In the last chapter of this article, it will be shown how pseudostochastic noise-like excitation patterns can be employed in TDFRS for the direct measurement of the linear response function and for the selective excitation of certain frequency ranges of interest by means of tailored pseudostochastic binary sequences. 

Photo-ions are then detected as a function of interferometer delay. The result is an in-terferogram that upon Fourier transformation yields a Raman spectrum whose resolution does not depend on the bandwidths of the Raman excitation sources but, instead, on the delay range of the interferometer scanned in the experiment. Mass-selective IDSRS and FT-IDSRS have been employed in a number of studies, including one of the benzene dimer (Henson et al., 1991). For examples of IDSRS spectra see Sec. 6.1.4.5. 

Recent applications of pseudo-triple- and -quadruple resonance techniques have in particular focused on the use of indirect detection schemes to measure heteronuclear couplings with increased sensitivity. The most versatile technique for this purpose proved to be H,"X( "Y) or "Xj HK Y) HMQC experiments which use I -BIRD or bi-selective pulses for selective excitation of transitions of ""Y isotopomers and allow to determine both the magnitude of/("X, Y) and its sign relative to /( H/ B X). Experiments of this type are easily employed to spin systems where /( H/ P,"X) ( H/ B Y) which is frequently the case if "Y is a C nucleus which is directly bound to the detected H or P spin, and have been used in a number of cases to measure couplings between C and rare spins such as 109,111 57pgii2 qj. 18705 196 similar B-BIRD-HSQC experiment has also been applied to the measurement of 7( Si, N) in NH-substituted azasilaboroles. ... 

The dependence of excitation transport on local chromophore concentration has been used to provide qualitative information on the characteristics of polymers in blends. Excimer fluorescence resulting from excitation transport has been employed to characterize polymer miscibility, phase separation and the kinetics of spinodal decomposition (1-31. Qualitative characterization of phase separation in blends (4.51 and the degree of chain entanglement as a function of sample preparation and history (6.71 has also been investigated through transport with trapping experiments. In these experiments one polymer in the blend contains donor chromophores and the second contains acceptors. Selective excitation of the former and detection of the latter provides a qualitative measure of interpenetration of the two components. 

Very recently the use of chemical laser techniques for the measurement of rotational relaxation times for individual rotational levels has been demonstrated by Hinchen in an elegant series of double-resonance experi-ments/ The apparatus consisted of two HF chemical lasers as the schematic diagram of Figure 3.18 indicates. A pulsed HF laser operated on a single P-branch transition of the t (l 0) band was employed to permit selective excitation of a single rotational state in the t = 1 level of HF. The temporal variation in the absorption of the output of a cw HF laser operated on the t (2 1) band was used as a probe for monitoring of the populations... 

SELINQUATE (Berger, 1988) is the selective ID counterpart of the 2D INADEQUATE experiment (Bax et al., 1980). The pulse sequence is shown in Fig. 7.4. Double-quantum coherences (DQC) are first excited in the usual manner, and then a selective pulse is applied to only one nucleus. This converts the DQC related to this nucleus into antiphase magnetization, which is refocused during the detection period. The experiment has not been used widely because of its low sensitivity, but it can be employed to solve a specific problem from the connectivity information. 

It may also be useful to employ circularly polarized excitation in other situations in which the sample is composed of more than one emitting species with different chiral environments. In this case the different emitting species may interact with the excitation beam in different amounts and lead to some additional selectivity through polarization selective absorption (CD). Several examples of this type of double polarization experiment will be presented and described below. 

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