CHARGE-TRANSFER COMPLEXES OF AMMONIA WITH HALOGENS - NATURE OF THE BINDING IN H3N-CENTER-DOT-CENTER-DOT-CENTER-DOT-BRCL FROM ITS ROTATIONAL SPECTRUM

The ground-state rotational spectra of the four isotopomers (H3N)-N-15...(BrCl)-Br-79-Cl-35, (H3N)-N-15...(BrCl)-Br-81-Cl-35, (H3N)-N-15...(BrCl)-Br-79-Cl-37 and (H3N)-N-15...(BrCl)-Br-81-Cl-37 of a complex formed by ammonia and bromine monochloride have been observed by pulsed-nozzle, FT microwave spectroscopy. A fast-mixing nozzle was used to prevent a chemical reaction between the components and allow the encounter complex to be isolated. The nature of the spectra establishes that the observed isotopomers are symmetric-top molecules. Assignment and analysis of the spectrum in each case led to the rotational constant B-0,, the centrifugal distortion constants D-j and D-jk, the halogen nuclear quadrupole coupling constants (chi)(Br) and (chi)(Cl), and the component M(bb)(Br) of the bromine spin-rotation coupling tenser. The rotational constants allowed an r(0)-type value of the distance N...Br of 2.627 Angstrom to be established. An r(s)-type method led to the distances r(N Br) = 2.59(1) Angstrom and r(Br-Cl) = 2.186 Angstrom, the former requiring a value of B-0 for (H3N)-N-14...(BrCl)-Br-79-Cl-35 obtained from unperturbed transition centres estimated without a full hyperfine structure analysis. A consideration of the intermolecular stretching force constant k(0), determined from D-j, provides evidence of a relatively strong interaction between the subunits. However, an analysis of the (chi)(X) (X = Br, Cl) values reveals that the molecular interaction is mainly electrostatic in origin, with probably only a small extent of intermolecular electric charge redistribution on complex formation.