Abstract
The X-band Electron Spin Resonance spectra, ESR, of Cu(II) complexes with the carboxylic acids, HCOOH, CH
3COOH, ClCH
2COOH, Cl
2CHCOOH, Cl
3CCOOH, F
2ClCOOH, CH
3CH
2COOH, niflumic acid and the ligands nicotinamide and papaverine have been studied. The compounds form either mononuclear or binuclear structures which can be qualitatively correlated with the strengths of the acid and base. Analysis of the spin Hamiltonian parameters for the monomeric species indicate that the molecule has nearly axial symmetry (
g
∥>
g
⊥) with the unpaired electron in the d
x
2–
y
2
orbital. In many cases the perpendicular feature is sufficiently well resolved showing superhyperfine features due to the nitrogen ligand nuclei.
A
∥ varies over the range 130→188
G and the variation appears to arise as a consequence of changes in
g
∥. The dimeric species show ESR spectra typical of Cu(II)-carboxylate complexes which consist of 3 (4 if
E≠0) lines. These are assigned to
B
z1
,
B
x2,
y2
and
B
z2
since the Q-band spectra of similar molecules indicate that the
B
x1,
y1
transition(s) lies below
B
z2
which is itself near zero field. The observed seven-line hyperfine coupling on the
B
z1 transition of some of the dimeric complexes is due to the two equivalent copper nuclei (
I=
I
1+
I
2=3). The singlet–triplet separation has been estimated from the spin Hamiltonian parameters to give values of |2
J| in the range of approximately 220–290 cm
−1 which are typical of the compounds of this type.