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 N2β-(CH3β)2βO, 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 β 1 up to J = 6 β 5 for N2β-DME and 15N2β-DME and two sets for 15NN-DME. Two of the four sets (referred to as group I) of N2β-DME and 15N2β-DME have large centrifugal distortion constants. Each rotational transition of N2β-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-N2β and the group II ortho-N2β. In the case of 15NN-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 15N2β-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 N2β-DME, 15N2β-DME, and 15NN-DME were analyzed for each set separately, by using an ordinary asymmetric-rotor Hamiltonian. The inertial defects IccββIaaββIbbβ thus obtained for N2β-DME were -29.31 (10) and -30.97 (10) u\AA2 for the two sets of group I and -9.98 (9) and -12.58 (11) u\AA2 for group II. These results indicated that the heavy-atom skeleton of N2β-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 N2β, to be approximately 3.45 \AA. By assuming a Lennard-Jones-type potential the dissociation energy was estimated to be EBβ = 0.74 βΌ 1.17 kJ molβ1, to be compared with the values 1.0 and 2.5 kJ molβ1 for Ne-DME and Ar-DME, respectively. MP2/6-31++g(d, p) calculations suggest that N2β-DME is non-planar and is bound by a very flat potential energy surface, in qualitative agreement with our results