FOURIER TRANSFORM MICROWAVE SPECTRA OF N2_2-(CH3_3)2_2O

Author Institution: Department of Applied Chemistry, Kanagawa Institute of Technology,; Atsugi, Kanagawa 243-0292, JAPAN; The Graduate University for Advanced Studies, Hayama, Kanagawa; 240-0193, JAPANAs an extension of the studies on the dynamical behavior of van der Waals complexes such as those on CO-DME, we have investigated nitrogen - dimethyl ether complex N$_2$-(CH$_3$)$_2$O, by using Fourier transform microwave spectroscopy. We have scanned the frequency region from 6 to 25 GHz and have found four sets of $a$-type rotational transitions ranging from $J$ = 2 $\leftarrow$ 1 up to $J$ = 6 $\leftarrow$ 5 for N$_2$-DME and $^{15}$N$_2$-DME and two sets for $^{15}$NN-DME. Two of the four sets (referred to as group I) of N$_2$-DME and $^{15}$N$_2$-DME have large centrifugal distortion constants. Each rotational transition of N$_2$-DME showed complicated splitting patterns due to the quadrupole coupling of the two nitrogen atoms and the number of hyperfine components was much smaller for group I than for the other (group II). This observation indicates that the group I complexes involve para-N$_2$ and the group II ortho-N$_2$. In the case of $^{15}$NN-DME only one type (corresponding to group II) of the complexes was detected because of the lack of symmetry. Some of the $a$-type transitions observed for $^{15}$N$_2$-DME consisted of closely spaced triplets; the splittings, which were nearly independent of the quantum numbers $J$, were ascribed to the internal rotation of the two methyl tops of DME. The observed transition frequencies of N$_2$-DME, $^{15}$N$_2$-DME, and $^{15}$NN-DME were analyzed for each set separately, by using an ordinary asymmetric-rotor Hamiltonian. The inertial defects $I_{cc} - I_{aa} - I_{bb}$ thus obtained for N$_2$-DME were -29.31 (10) and -30.97 (10) u\AA$^2$ for the two sets of group I and -9.98 (9) and -12.58 (11) u\AA$^2$ for group II. These results indicated that the heavy-atom skeleton of N$_2$-DME was not planar. The observed moments of inertia were analyzed to give the distance between the centers of gravity of the two component molecules, DME and N$_2$, to be approximately 3.45 \AA. By assuming a Lennard-Jones-type potential the dissociation energy was estimated to be $E_B$ = 0.74 $\sim$ 1.17 kJ mol$^{-1}$, to be compared with the values 1.0 and 2.5 kJ mol$^{-1}$ for Ne-DME and Ar-DME, respectively. MP2/6-31++g(d, p) calculations suggest that N$_2$-DME is non-planar and is bound by a very flat potential energy surface, in qualitative agreement with our results