Spin-flip methods

In the spin-flip method, 180° pulses are applied simultaneously to both H and C nuclei at the midpoint of the evolution period so that the C... 

Figure 5.11 (A) Heteronuclear 2D /-resolved spectrum (selective spin-flip method)...

Breaking the Curse of the Non-Dynamical Correlation Problem The Spin-Flip Method... 

The Spin-flip Method for Bond-breaking the Ethylene Torsional Potential... 

Krylov, A. I., Slipchenko, L. V., and Levchenko, S. V. Breaking the Curse of the Non-dynamical Correlation Problem the Spin-Flip Method , ACS Symp. Ser., in press. 

In 1987, Swendsen and Wang (SW) [3] introduced a new Monte Carlo algorithm for the Ising spin model, which constituted a radical departure from the Metropolis or single-spin flip method used until then. Since the recipe is relatively straightforward, it is instructive to begin with a description of this algorithm. 

In order to see the difference in the two approaches, below I focus on the excitation energies, AE, of the Be states that are discussed here. The nice thing about atomic spectra of this type is that there is accurate experimental information with which one can compare the results of a theoretical method (See, Tables of NIST, USA, in the WWW). Specifically, I compare the AE from the Be Fermi-sea energies for which cancellations on subtraction of total energies are expected, with those obtained from methods that have used one of the known basis sets. I consider two such publications. The first is in 1986 by Graham et al. [105] where a (9s9p5d) contracted GTO basis (61 basis functions) was used for different types of computations. I keep the full Cl (FCI) results. The second is in 2003 by Sears, Sherrill and Krylov [106], who studied aspects of "spin-flip" methods and compared them with FCI using the same basis set, which is a 6-31G. 

Fig. 5.14 Two variants of the 2D X J(X, H)-resolved experiment. The sequence of the left-hand side uses the "spin-flip method whilst the sequence of the right-hand side uses the "gated-decoupling" method.

There are several types of heteronuclear 2D J-resolved experiments (i) the gated decoupler method, (ii) the spin-flip method, (iii) the selective spin-flip method, (iv) the semi-selective spin-flip method, and (v) the use of polarization transfer, e.g., INEPT. In all these experiments, the pulse sequence results in the generation of spin echoes which are modulated during the evolution period by coupling frequencies. 

The Spin-Flip Method. In an earlier section it was shown that if a 180° pulse is applied to both and nuclei at time t after a 90° pulse, then during the subsequent period the two magnetization vectors of a CH system do not refocus but continue to diverge [see Figure 5.6(c)]. The spin-flip method utilizes the same principle. The pulse sequence and its effect on magnetization vectors of a system are shown in Figure 5.44. A 90°... 

Figure 5.44. The spin-flip method for recording 2D J-resolved C-NMR spectra. The application of the 180° pulse as well as 180° pulse in the center of the evolution...

Figure 5.45. (A) The selective spin-flip method for recording heteronuclear 2D J-resolved spectra in which the signals are modulated by long-range coupling constants. The pulse sequence is shown above while its effect on the magnetization vectors is shown below it. The selective 180° pulse results in elimination of the large one-bond coupling constants, Jch ( ) j -Methylcellobioside with the torsional angles (j) and ij/ indicated.

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