It is instead an accounting perspective for describing how the magnetisation will appear. Defining two frequencies, one real and one imaginary: ∊0=-f00R-f11R=h3 equation(22) ∊1=-if00I-f11I=ih4then: equation(23) H=e-τcpR2G+R2E+kexNN*(B00*eτcp∊0+B11*eτcp∊1)B00+(B11*e-τcp∊0+B00*e-τcp∊1)B11where

CDK inhibitor the average relaxation rate exp(−τcp(f00R + f11R)) = exp(−τcp(ΔR2 + kex)) has been factored out. At the end of this period, magnetisation that has been entirely refocused will evolve with a purely real frequency, ±ε0, and magnetisation that has not, will evolve with frequencies ±ε1. By a similar procedure, the propagator for the second half of the CPMG block can be derived by noting that the complex conjugate of ε1 is obtained by multiplying it BGB324 nmr by −1: equation(24) H*=e-τcp(R2G+R2E+kex)NN*(B00eτcp∊0+B11e-τcp∊1)B00*+(B11e-τcp∊0+B00eτcp∊1)B11* Further progress can be made by identifying additional simplifying relations. The elements of idempotent B00 and B11 satisfy the condition B(1, 0)B(0, 1) = B(1, 1)B(0, 0) where the brackets indicate specific rows and columns of the matrix. In such a case, for a matrix product AB, A can be replaced by a diagonal matrix C such that

AB = CB. As derived in Supplementary Section 2, the two diagonal coefficients of C are given by Eq. (66). Dealing with almost matrix products is cumbersome, and so replacing one of the two matrices with one that is diagonal will be

shown to be greatly simplifying (see Eq. (35)). In doing so, the following identities are obtained: equation(25) Cst·B00=B00*·B00Cst*·B11=B11*·B11Csw·B00=B11*·B00Csw′·B11=B00*·B11which follow from the definition of ‘stay’ and ‘swap’ diagonal matrices using Eq. (66): equation(26) Cst=Pst00Pst*,Csw=Psw00Psw′,Csw′=Psw′00Psw The individual matrix elements are given by: equation(27) Pst=OG+OE*=h3-iΔωPsw=OG*-OG=-i(h4-Δω)Psw′=OE*-OE=-i(h4+Δω) From these definitions, the following useful identities emerge: equation(28) Pst*OG=PstOG*PstOE=Pst*OG*PswOG*=-Psw′OEPsw′OG=-PswOE* These definitions reveal an important physical interpretation of these cofactors. In the case where magnetisation stays in either the ground or excited state following a 180° pulse, it is multiplied by a ‘stay’ matrix of the form Cst. In the case where magnetisation effectively swaps to the other state, it is multiplied by a ‘swap’ matrix, Csw or Csw′. The conjugate of either of the swap matrices is obtained by multiplication by −1, leading to the conjugates of Eq. (25): equation(29) Cst*·B00*=B00·B00*Cst·B11*=B11·B11*-Csw·B00*=B11·B00*-Csw′·B11*=B00·B11* These operations enable us to arrive at a simplified expression for the two Hahn echo propagators.