Rotational spectra, structures, and dynamics of small Ar[sub m]–(H[sub 2]O)[sub n] clusters: The Ar–(H[sub 2]O)[sub 2] trimer

Rotational-tunneling spectra for Ar –( H 2 O ) 2 and Ar –( D 2 O ) 2 have been observed with the Balle–Flygare Fourier transformmicrowave spectrometer. The tunneling levels of the trimer appear to correlate with those of the water dimer. The “a” dipole transitions from the A 1 + and E + states of Ar –( H 2 O ) 2 and A 1 + , B 1 + , and E + states of Ar –( D 2 O ) 2 could be fit to a semirigid rotor Watson Hamiltonian. However, only the E + states give “b” dipole transitions near rigid rotor predictions. The “b” dipole transitions for A 1 + and B 1 + are rotational-tunneling spectra. For Ar –( D 2 O ) 2 , these transitions were observed and the donor–acceptor interchange tunneling splitting is determined as 106.3 MHz, compared to about 1100 MHz in the free ( D 2 O ) 2 . From this splitting, the barrier for interchange tunneling is calculated to be 642 cm−1. This splitting for Ar –( H 2 O ) 2 is estimated as 4–5 GHz. This and the spin statistical weight of 0 for the B 1 + state have made it difficult to observe the “b” dipole rotational tunnelingspectra for Ar –( H 2 O ) 2 . From the rotational constants for ( H 2 18 O ) containing trimers, the O–O distance in the trimer is estimated as 2.945 A. This is significantly (0.035 A) shorter than the O–O distance reported for water dimer. The Ar is located on the “b” axis of the water dimer. Assuming the water to be a structureless sphere in the trimer, leads to Ar - c.m. ( H 2 O ) distance of 3.637 A, very close to the same value in the Ar – H 2 O dimer.