The microwave spectrum and nuclear quadrupole hyperfine structure of HCCH-N2O

The microwave spectrum of HCCH-N2O has been collected in the 7–16 GHz region using a Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure owing to the two N14 nuclei has been assigned in 15 rotational transitions. Using a Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole interactions to analyze the spectrum, the rotationaland centrifugal distortion constants (in MHz) are determined to be: A=9394.2683(2), B=2831.85640(8), C=2168.07804(7), DJ=1.2290(3)×10−2, DJK=5.677(4)×10−2, d1=−3.365(2)×10−3, and d2=−7.3(1)×10−4. The nuclear quadrupole coupling constants are also determined. For the terminal nitrogen nucleus, χaa=377.5(4), χbb=−773.1(5), χcc=395.6(5) kHz and for the central nitrogen nucleus, χaa=84.1(9), χbb=−246.6(7), χcc=162.5(7) kHz. The rotational constants give a 3.305-Å separation between the centers of mass of the subunits. HCCH and N2O are approximately parallel to each other, and each is approximately perpendicular to the intermolecular axis. A comparison between the nuclear quadrupole coupling constants for free N2O and HCCH-N2O shows that the electric field gradient at the central nitrogen nucleus is greatly affected by complexation. A distributed multipole calculation suggests that this distortion cannot simply be due to the presence of the charge distribution of HCCH, but is likely to result from an actual electronic redistribution of N2O upon complexation.