This work represents the synthesis of the 2ϩ1 pulse sequence for the study of electron spin-echo envelope modulation ͑ESEEM͒ with the technique of spin-echo correlated spectroscopy ͑SECSY͒, which has previously been used to study nuclear modulation by two-dimensional Fourier transform ESR methods. This example of "pulse adjustable" spectroscopy, wherein the pulse width and pulse amplitude of the second pulse in a three pulse sequence are introduced as adjustable parameters, leads to enhanced

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... olution to the key features of the nuclear modulation that are important for structural studies. This is demonstrated in studies on ͑i͒ a single crystal of irradiated malonic acid and ͑ii͒ a frozen solution of diphenylpicrylhydrazyl in toluene. In particular, it is shown for ͑i͒ how the nuclear modulation cross peaks can be preferentially enhanced relative to the autopeaks and to the matrix proton peaks, and also how the autopeaks can be significantly suppressed to enhance resolution for low-frequency cross peaks. For ͑ii͒ the low-frequency 14 N nuclear modulation could be suppressed leaving just the high-frequency matrix 1 H modulation. Additionally, the T 2 homogeneous linewidth broadening in the f 1 frequency direction is removed in 2ϩ1 SECSY. These features significantly improve resolution to the modulation decay, which is the main observable utilized for distance measurements by ESEEM, compared to SECSY. A simple example of a distance measurement to matrix protons is presented for ͑ii͒. It is shown that a major advantage of the 2D format is that the full spin-echo shape is collected, which permits one to study how the effect of the nuclear modulation upon the echo varies with evolution time t 1 after the first pulse, and thereby to detect the important modulation features. Additionally it allows for correlating the modulation cross peaks in making spectral assignments. A detailed quantitative theory for 2ϩ1 SECSY is also presented. In general, very good agreement between experiment and theoretical simulations is obtained.