Non-reactive interaction of ammonia and molecular chlorine: rotational spectrum of the ‘charge-transfer’ complex H3N⋯Cl2

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Abstract
The ground-state rotational spectra of five isotopomers of the ‘charge-transfer’ complex H3N⋯Cl2 have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The complex has C3v symmetry with the nuclei in the order H3N⋯Cl—Cl. A detailed analysis of the Cl nuclear quadrupole hyperfine structure in transitions of H3 15N⋯35Cl2, H3 15N⋯35Cl37Cl, and H3 15N⋯37Cl35Cl gave the rotational constant, Bo, the centrifugal distortion constants Dj and Djk, and the nuclear quadrupole coupling constants χ(Cli) and χ(Clo)(i = inner, o = outer) in each case. The distance r(N⋯Cli) was obtained by an rs-type method and an ro-type method and lies in the range 2.73 ± 0.03 Å. A detailed analysis that allowed for bond shrinkage on isotopic substitution in the 35Cl2 subunit of H3 15N⋯35Cl2 gave the rs-type coordinates of Cli and Clo and hence the distance rs(Cl—Cl)= 2.00 Å. This value is very close to that in free Cl2 and indicates only a slight perturbation of this subunit when the complex is formed. The relatively small intermolecular stretching force constant, kσ= 12.71(3) N m–1 determined from DJ and the weak perturbation of χ(Cli) and χ(Clo) from the value in free Cl2, reinforce this conclusion. The observed difference χ(35Clo)–χ(35Cli)= 13.99 MHz can be interpreted in terms of a transfer of 0.064e from Cli to Clo on formation of H3 15N⋯35Cl2. It seems likely that the molecular interaction is mainly electrostatic in origin and charge-transfer effects are small.

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