Quantum chemical calculations at the MP2(FC)/6-311+G(2df,p) level have been performed for the donoracceptor complexes BH 3 NH 3 , BCl 3 NH 3 , AlH 3 NH 3 , AlCl 3 NH 3 , and GaCl 3 NH 3 . Variations of the potential energy, total charge transfer, and geometrical parameters have been investigated along the d AN (A ) B, Al, Ga) donor-acceptor distance from a compressed (1.5 Å) arrangement up to dissociation. Among the wellknown geometrical consequences of complex formation (lengthening of the A-L and N-H bonds, increase of the L-A-N and H-N-A bond angles), those in the AL 3 acceptors are well pronounced whereas those in NH 3 are rather small. The geometrical effects show a gradual decrease with increasing d AN , but most of them can still be recognized at considerable elongation (by 2 Å) from the equilibrium donor-acceptor distance. Among the investigated properties, the potential energy and the L-A-N angle are the most sensitive indicators of the interaction.

The molecular structure of 2-trifluoromethylphenol has been determined by gas-phase electron diffraction. The analysis, aided by constraints from ab initio molecular orbital calculations, yielded the following bond lengths (r g) and bond angles:  (C−C)Ar,mean, 1.395 ± 0.004 Å; C−C(F3), 1.510 ± 0.005 Å; C−O, 1.362 ± 0.011 Å; (C−F)mean, 1.358 ± 0.006 Å; ∠O−C1−C2, 121.0 ± 1.2°; ∠C−O−H, 105 ± 6°; ∠C(F3)−C2−C1, 120.7 ± 0.8°; (C−C−F)mean, 113.2 ± 0.3°; CF3 torsion, 10.1 ± 0.8°. Weak intramolecular bifurcated hydrogen bonding between the hydroxy hydrogen and two of the CF3 fluorines is indicated by the H···F nonbonded distances 2.05 ± 0.06 and 2.42 ± 0.06 Å. Compared with the freely rotating CF3 group in trifluoromethylbenzene, the well-defined conformer of 2-trifluoromethylphenol indicates the constraining effect of hydrogen bonding.

Gelle ´rt te ´r 4, Hungary