Nitrous oxide dimer: Observation of a new polar isomer

Abstract

Spectra of the nitrous oxide dimer (N2O)2 are studied in the region of the N2O nu1 fundamental band around 2230 cm-1 using a rapid-scan tunable diode laser spectrometer to probe a pulsed supersonic jet expansion. The previously known band of the centrosymmetric nonpolar dimer is analyzed in improved detail, and a new band is observed and assigned to a polar isomer of (N2O)2. This polar form of the dimer has a slipped parallel structure, rather similar to the slipped antiparallel structure of the nonpolar form but with a slightly larger intermolecular distance. The accurate rotational parameters determined here should enable a microwave observation of the polar N2O dimer. The need for a modern ab initio investigation of the N2O-N2O intermolecular potential energy surface is emphasized.

Isotope effects in the infrared spectra of the polar and nonpolar isomers of N2O dimer

“…We used isotopically enriched sample of 15 N 15 N 16 O, obtained from Iconisotopes, with a stated atomic purity of 99%. The number of assigned transitions reported in our previous work [9] for the nonpolar ( 14 N 14 N 16 O) 2 was 208 with the rotational levels up to J = 11 and K a = 4. The improved signal to noise and narrower line width resulting from the use of the multi-channel slit allowed us to assign 58 additional transitions thus extending the rotational quantum numbers to J = 12 and K a = 5.…”

“…See below. The molecular parameters resulted from the fit are given in column 2 of Table 1 and are compared with results from our previous study [9]. The observed line positions are listed in Table A…”

“…The band reported in Ref. [9] probably belongs to the stronger inphase vibration. The weaker out of phase vibration band has not been detected yet and the situation is similar to that of OCS polar dimer [11].…”

“…Later, ab initio calculations by Valdes and Sordo [8] found that the slipped-antiparallel structure with O atoms occupying the inner position is indeed the lowest energy isomer. We have extended the study of N 2 O dimer in the region of m 1 fundamental band by assigning more transitions (200 vs. 130 lines) and covering a wider spectral range (4.7 vs. 3 cm À1 ) [9]. Because of its center of symmetry, the C 2h isomer is nonpolar and cannot be studied by microwave spectroscopy.…”

“…The second lowest energy isomer was found to be polar with a slipped parallel geometry. We recently observed an infrared spectrum in the N 2 O m 1 fundamental region, which we assigned to the polar N 2 O dimer having C s symmetry [9]. Although the pure rotational spectrum of this isomer is detectable in the microwave region, this spectrum is considerably complicated because of the presence of quadrupole hyperfine splittings from the four in-equivalent 14 N nuclei in the normal isotopomer [10].…”