Fig. compares one-step CP from 35 Cl → 1 H to CT or STs with the double 1 H → 35 Cl → 1 H CP via CT or STs. We chose the reversed 35 Cl → 1 H one-step CP so all four cases in Fig. can be compared by detecting the 1 H signal of hist. Direct 35 Cl detection is not feasible for the small amount of sample in a 0.75 mm rotor. The results in Fig. show the lowest signal from 35 Cl(ST) → 1 H CP behind 35 Cl(CT) → 1 H. It should be noted that the phase spread φn for the effective ST rf field is not completely random or evenly distributed as the powder average of sn results in sn 2 , which is smaller than |sn| but not zero. 1 H → 35 Cl(CT) → 1 H double-CP gives the highest signal with polarization originated from protons of approximately 10 times higher in gyromagnetic ratio over 35 Cl. The case of 1 H → 35 Cl(ST) → 1 H double CP is the second highest at ~50% compared to 1 H → 35 Cl(CT) → 1 H. This is remarkable considering that the modulated ST frequencies span more than MHz, an order of magnitude larger than the rf field used. The high efficiency confirms experimentally the absence of signal phase cancellation as mentioned in the theory. . Here ωX and ωH are the effective rf nutation frequencies for the two nuclei respectively. The primary conditions are n = 1,2 due to the MAS modulation to the second-rank dipolar coupling that mediates the polarization transfer. Several factors need be considered in choosing the optimal CP condition. First, the spinlock nutation frequencies, ωX and ωH, should avoid the rotary resonance conditions nωr which can cause rapid polarization decay to the spinlock . For protons, the rotary resonance includes n = 1/2 HORROR condition from homonuclear dipolar couplings among protons in addition to the primary n = 1,2 resonances . Under slow MAS, ωX, ωH ≫ 2ωr avoid all rotary resonance conditions with practically achievable rf fields allowing effective spinlock, and the |ωX -ωH| = ωr ZQ CP condition is usually used. The ZQCP condition becomes practically difficult under fast MAS especially for quadrupolar nuclei as mentioned previously that strong rf fields cause polarization transfer and leakage among the various transitions within S > 1/2 spin. The |ωX -ωH| = ωr is often the optimal condition for CP. With the 90 kHz MAS used in this work, we chose ωH ~ 0.75ωr between the n = 1/2 and 1 rotary resonance conditions for protons. High spinning frequencies widen the gap between these two resonance conditions to reduce their undesired T1 contributions to the spinlock. Faster MAS is advantageous in this regard for better 1 H spinlock under the DQCP condition.