MICROWAVE SPECTRUM AND STRUCTURE OF A POLAR DIMER OF N$_{2}$O

Author Institution: School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K.; Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459, U.S.A.; School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K.Cavity Fourier-Transform microwave spectroscopy has been used to characterise a gas phase, polar dimer of N$_{2}$O. The polar (N$_{2}$O)$_{2}$ unit is generated by co-expansion of a gas sample containing a small percentage of N$_{2}$O in a helium backing gas. Transitions in the pure rotational spectra of ($^{15}$N$_{2}$O)$_{2}$, ($^{14}$N$^{15}$NO)($^{15}$N$_{2}$O), ($^{14}$N$_{2}$O)($^{15}$N$_{2}$O) and ($^{14}$N$_{2}$O)$_{2}$ are reported. The measured transitions of ($^{15}$N$_{2}$O)$_{2}$ and ($^{14}$N$^{15}$NO)($^{15}$N$_{2}$O) are assigned and fitted to Hamiltonians allowing rotational, centrifugal distortion and $^{14}$N nuclear quadrupole coupling constants to be determined. Hyperfine structure is assigned for a single $J_{K_{-1}^{\prime}K_{+1}^{\prime}}^{\prime}$$\rightarrow$$J_{K_{-1}^{\prime\prime}K_{+1}^{\prime\prime}}^{\prime\prime}$ transition of both isotopomers of ($^{14}$N$_{2}$O)($^{15}$N$_{2}$O). Nuclear quadrupole coupling constants,${\chi}_{bb}$, are reported for all four $^{14}$N nuclei. The measured ${\chi}_{bb}$ 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){\AA} with the two N$_{2}$O monomers respectively oriented 54.69(68)$^{irc}$ and 49.85(64)$^{irc}$ to the $a$-inertial axis. Simulation of hyperfine structure in the spectrum of the ($^{14}$N$_{2}$O)$_{2}$ isotopomer yields good qualitative agreement with experiment