Microwave spectrum and structure of the polar N 2O dimer

Abstract

Cavity Fourier-transform microwave spectroscopy has been used to characterise a gas phase, polar dimer of N 2O. The polar (N 2O) 2 unit is generated by co-expansion of a gas sample containing a small percentage of N 2O in helium backing gas. Transitions in the pure rotational spectra of ( 15N 2O) 2, ( 14N 15NO)( 15N 2O), ( 14N 2O)( 15N 2O) and ( 14N 2O) 2 are reported. The measured transitions of ( 15N 2O) 2 and ( 14N 15NO)( 15N 2O) are assigned and fitted to Hamiltonians allowing rotational, centrifugal distortion and 14N nuclear quadrupole coupling constants to be determined. Hyperfine structure is assigned for a single J K - 1 ′ K + 1 ′ ′ → J K - 1 ″ K + 1 ″ ″ transition of both isotopomers of ( 14N 2O)( 15N 2O). Nuclear quadrupole coupling constants, χ bb , are reported for all four 14N nuclei. The measured χ bb are in excellent agreement with those structures predicted from the measured rotational constants. The geometry of the molecule is slipped-parallel. The separation between the central nitrogen nuclei of the monomers in the r m ( 1 ) structure is 3.570(12) Å with the two N 2O monomers, respectively, oriented 54.69(68)° and 49.85(64)° to the a-inertial axis. Simulation of hyperfine structure in the spectrum of the ( 14N 2O) 2 isotopomer yields good qualitative agreement with experiment.