Density functional studies of the n center dot a sigma charge-transfer complexes between NH3 and BrX (X = Cl, Br)

The n.a sigma charge-transfer complexes formed by ammonia and bromine monoc hloride and bromine dimer have been studied with various approximate pure a nd hybrid density functional methods and the second-order Moller-Plesset (M P2) theory. The calculations demonstrate that one hybrid method, namely the so called BH&HLYP, and the MP2 method lead to reasonably good estimates of the experimentally measured rotational constants. However, the predicted B H&HLYP intermolecular distance is found to be closer to the experimental va lue. Our best estimation for the intermolecular interaction energies are 7. 32 and 5.48 kcal/mol, respectively, for bromine monochloride and bromine di mer complexes. We have also been able to calculate the number and energies of the vibrational states supported by their corresponding intermolecular s tretching modes. Sixteen such states have been found for the former and onl y 13 for the latter. The performance of the various approximate density fun ctionals and the MP2 theory is compared and discussed. Finally, we comment on the analysis of the natural bond orbitals, which has been found to be ve ry valuable to shed light on the nature of weak intermolecular interactions . (C) 2000 Elsevier Science B.V. All rights reserved.