AN MO STUDY OF NUCLEAR QUADRUPOLAR COUPLING-CONSTANT AND NUCLEAR SHIELDING OF THE CARBONYL OXYGEN IN SOLID PEPTIDES WITH HYDROGEN-BONDS

The theoretical calculations of the electric field gradients of hydrog en-bonded carbonyl oxygens in glycine containing peptides were success fully carried out by the MNDO-PM3 method. From these calculated result s, it was found that the calculated e(2)qQ/h value decreases with a de crease in the hydrogen bond length, and the chemical shielding values increase with a decrease in the hydrogen bond length. These calculatio ns reasonably explain the experimental findings previously reported by us. Further, the direction of the principal axes of the electric fiel d gradient and chemical shielding tenser of the carbonyl oxygen were r easonably obtained by the FPT-MNDO-PM3 method.