Selecitive CPMAS Match (H->X) calculator;
(Wh-Wx = 1*Wr) ZQ matching
Wo,x = X Larmor in MHz (eg 100 for 13C)
dW,x = diff in ppm between eg CO-CA (180-60 = 120)
note: HHeff = abs(Weff,x - Wh)/Wr; want ~ 0.5 for selective