Fourier transform microwave spectrum of the propane—water complex: A prototypical water-hydrophobe system

The Fourier transform microwave spectrum of the propane—water complex (C3H8—H2O) has been observed and analyzed. This spectrum includes transitions assigned to propane complexed with both the ortho and para nuclear spin confirmations of water. The rotational constants indicate that the vibrationally averaged structure has all four heavy atoms coplanar, with the water center of mass lying on or near the C2 axis of propane, inside the CCC angle, 3.76(±0.02) A-ring from the propane center-of-mass, and 4.35(±0.02) A-ring from the methylene carbon. The projection of the electric dipole onto the a inertial axis of the complex (0.732 D for the ortho state and 0.819 D for the para state) indicates that one of the protons of the water subunit lies on the C2 axis of the propane monomer, which is also the axis connecting the subunit centers of mass. The small projection of the dipole along the b axis (0.14 D for the ortho state and 0.38 D for the para state) is most consistent with an equilibrium structure in which all three atoms of the water lie in the CCC plane of propane, with torsional tunneling about the hydrogen bond occurring on the same time scale as the overall rotation. The small internal rotation tunneling splittings that occur in the rotational spectrum of the propane monomer are not observed in the spectrum of the complex