Rotational transitions between J≤3 levels within the K=0 manifold have been observed for H2O–CO, HDO–CO, D2O–CO, H2O–13CO, HDO–13CO, and H217O–CO using the molecular beam electric resonance and Fourier transform microwave absorption techniques. ΔMJ=0→1 transitions within the J=1 level were also measured at high electric fields. A tunneling motion which exchanges the equivalent hydrogens gives rise to two states in the H2O and D2O complexes. The spectroscopic parameters for H2O–CO in the spatially symmetric tunneling state are [∼(B0) =2749.130(2)MHz, D0=20.9(2)kHz, and μa=1.055 32(2)D] and in the spatially antisymmetric state are [∼(B0) =2750.508(1)MHz, D0=20.5(1)kHz, and μa=1.033 07(1)D]. Hyperfine structure is resolved for all isotopes. The equilibrium structure of the complex has the heavy atoms approximately collinear. The water is hydrogen bonded to the carbon of CO; however the bond is nonlinear. At equilibrium, the O–H bond of water makes an angle of 11.5° with the a axis of the complex; the C2v axis of water is 64° from the a axis of the complex. The hydrogen bond length is about 2.41 Å. The barrier to exchange of the bound and free hydrogens is determined as 210(20) cm−1 (600 cal/mol) from the dipole moment differences between the symmetric and antisymmetric states. The tunneling proceeds through a saddle point, with C2v structure, with the hydrogen directed towards the CO subunit. The equilibrium tilt away from a linear hydrogen bond is in the direction opposite to the tunneling path.
Articles you may be interested inNon-covalent interactions of nitrous oxide with aromatic compounds: Spectroscopic and computational evidence for the formation of 1:1 complexesThe rotational spectra of 0, and HDO-N 2 0 have been observed using molecular beam electric resonance techniques at both zero and nonzero electric fields. H z O-N 2 0 is nonrigid with respect to internal rotation of the water subunit. Rotational constants in MHz for the spatially antisymmetric tunneling state are A = 12605.001 (77), B=4437.978(32), and C=3264.302(32). Rotational constants for the spatially symmetric tunneling state are A= 12622.595(203), B=4437.422 (47), C=3264.962(47). These together with the rotational constants of the other isotopomers are consistent with a planar, Tshaped arrangement of the heavy atoms of the complex, with the distance between the centers of mass of the two subunits, R c . m ., equal to 2.91 (2) A or a distance of 2.97(2) A from the H 2 0 oxygen to the central nitrogen of N 2 0. The measured dipole moments of the two tunneling isomers are identical; JLa = 1.480(2) and JLb = 0.31 (2) D. The values of these dipole moment components indicate an in-plane equilibrium tilt of about 20° between the C 2v axis of water and the N-O weak bond. This tilt suggests a second interaction may exist between a hydrogen on water and the N 2 0 subunit. The rotational constants suggest that the N 2 0 unit is tilted by about 9° from perpendicular to the N-O weak bond. The barrier for the tunneling interchange of the water protons is estimated to be 235 ( 10) em -I. Quadrupole coupling constants eqQaa for the outer and inner nitrogen of N 2 0 are 0.371 (130) and 0.128(45) MHz, respectively. Electrostatic models applied to water-N 2 0 and water-CO z predict hydrogen bonded structures rather than the experimentally observed Lewis base structures.
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