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

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 C_3v symmetry with the nuclei in the order H₃N⋯Cl—Cl. A detailed analysis of the Cl nuclear quadrupole hyperfine structure in transitions of H₃¹⁵N⋯³⁵Cl₂, H₃¹⁵N⋯³⁵Cl³⁷Cl, and H₃¹⁵N⋯³⁷Cl³⁵Cl gave the rotational constant, B_o, the centrifugal distortion constants D_j and D_jk, and the nuclear quadrupole coupling constants χ(Cl_i) and χ(Cl_o)(i = inner, o = outer) in each case. The distance r(N⋯Cl_i) was obtained by an r_s-type method and an r_o-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 H₃¹⁵N⋯³⁵Cl₂ gave the r_s-type coordinates of Cl_i and Cl_o and hence the distance r_s(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, k_σ= 12.71(3) N m^–1 determined from D_J and the weak perturbation of χ(Cl_i) and χ(Cl_o) from the value in free Cl₂, reinforce this conclusion. The observed difference χ(³⁵Cl_o)–χ(³⁵Cl_i)= 13.99 MHz can be interpreted in terms of a transfer of 0.064e from Cl_i to Cl_o on formation of H₃¹⁵N⋯³⁵Cl₂. It seems likely that the molecular interaction is mainly electrostatic in origin and charge-transfer effects are small.