Influence of local molecular motions on the determination of 1H-1H internuclear distances measured by 2D 1H spin-exchange experiments.

Analysis of spin-exchange build-up curves obtained by measurement of 2D 1H CRAMPS spectra of alpha-glycine was performed to evaluate the rate of 1H-1H spin-exchange process with respect to the influence of variation in internal molecular motion. Differences in local motions significantly affect spin-exchange constants even in highly rigid organic solids with virtually uniform motion behavior. The polarization transfer between nonequivalent alpha-protons is described by the spin-exchange constant D=0.77 nm(2)ms(-1), while the polarization transfer involving spin exchange between alphaH and NH(3)(+) protons is characterized by D=0.24-0.21 nm(2)ms(-1). This significant decrease corresponds to rotation of hydrogen-bonded amino groups. Neglecting this variation in local spin-exchange constants the resulting calculated 1H-1H distance can be overestimated by up to 100%. Complications following from relayed and back polarization transfer involving the nearest spins within one functional group (e.g., CH(2) and/or NH(3)(+)) and intermolecular spin exchange are discussed. It was shown that 2H quadrupolar splitting determined for selected sites directly correlates with the experimentally observed differences in spin-exchange coefficients. It is also demonstrated that a medium level quantum chemical calculation of molecular dynamics provides relevant data that can be used to estimate differences in molecular motions.