Figure 1.

NCA ZQ (n=±1) SPECIFIC-CP matching conditions at either lower (left) or higher (right) ¹⁵N rf spinlock field with respect to the ¹³C rf field. ω₁ and ω_r are the rf field strength and MAS frequency, respectively. Y axes represent the relative transfer efficiency (left) or the absolute transfer efficiency (right), as compared to a ¹H-¹³C CP spectrum. Each datapoint was calculated by integrating the CA region from the corresponding spectrum acquired by accumulating a total of 32 transients. The spinlock fields are tangential and rectangular for ¹³C and ¹⁵N, respectively. The optimal matching conditions at either low or high ¹⁵N rf fields are colored in magenta and blue, respectively. RF field strengths of the optimal condition are given at the bottom of each bar graph in units of the MAS frequency (14 kHz).