H-1-O-17 nuclear-quadrupole double-resonance study of hydrogen disorder in2-nitrobenzoic acid

Temperature dependence of O-17 nuclear quadrupole resonance frequencies was measured in solid 2-nitrobenzoic acid by a H-1-O-17 nuclear quadrupole dou ble resonance technique. The experimental results show the presence of a fa st exchange of hydrogen atoms between two nonequivalent positions within th e O-H ... O hydrogen bonds. The hydrogen disorder is ascribed to concerted jumps of two hydrogen atoms within the hydrogen bonds connecting two molecu les in a dimer, The energy difference Delta E of the two hydrogen configura tions is equal to Delta E = 60 meV = 5.8 kJ/mol. The dipole structure of th e O-17 NQR lines from the C-O-H oxygen positions was also measured at -100 degrees C and at room temperature. The orientation of the principal axes of the electric field gradient tensor with respect to the O-H bond and the si gn of the quadrupole coupling constant were determined. The oxygen-hydrogen distance RO-H as determined from the dipole structure of the O-17 NQR line s is at -100 degrees C equal to 0.099 nm, At room temperature we observe a longer distance, RO-H = 0.101 nm, in agreement with the hydrogen intrabond exchange. (C) 2000 Academic Press.