13C chemical shift tensors of hydrogen bonded amino acids: Relations between experimental and calculated results

A new interpretation of 13C chemical shift tensors of amino acid carboxyl groups is presented. It is based on NMR results for 50 crystalline complexes of β-alanine, betaine, glycine and sarcosine with different inorganic and organic acids (XH), X-ray and vibrational data on the same systems and GIAO calculations for model complexes: (CH3COO-HX)−. Sensitivity of the chemical shift tensors on the amino acid electronic structure, crystal lattice, kind and strength of the hydrogen bonds is characterised in this way. Four categories of the complexes and hydrogen bonds are considered: (i) MO - the molecular complexes with the bonding proton not transferred to the amino acid carboxylate group, COO−⋯H-X; (ii) IP - the hydrogen bonded ion pairs with the bonding proton localised at the amino acid oxygen atom, COO-H⋯X−; (iii) BHB - the homoconjugated amino acid complexes with the bonding proton in the central region of short hydrogen bond, (COO-H-OOC)−; (iv) low-barrier hydrogen bonds with disordered proton position. Properties of the last category bonds are discussed by means of Mulliken theory of CT complexes.