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The microwave spectra and structures of some isocyanates Hocking, William Hiram


The microwave spectra and structures of three isocyanate molecules have been studied; they are: chlorine isocyanate (CINCO), isocyanic acid (HNCO) and cyanogen isocyanate (NCNCO). CINCO: The microwave spectra of six isotopic species of chlorine isocyanate have been measured in the frequency region 8-37 GHz. It was found that the rotational energy level scheme corresponding to each of these spectra was adequately represented by a seven parameter Hamiltonian specific for a well behaved planar molecule. The parameters, three rotational constants and four distortion constants, were determined using a least squares fitting procedure. From the rotational constants, the r₈ molecular structure of chlorine isocyanate was calculated to be: r(Cl-N) = 1.705 ± 0.005 Å ∟(ClNC) = 118° 50¹ ± 30¹ r(N-C) = 1.225 ± 0.005 Å ∟(NCO) = 170° 52¹ ± 30¹ r(C-O) = 1.162 ± 0.005 Å Planar, Cl and 0 trans. This is the first instance in which an isocyanate molecule has been shown to have a bent NCO chain. Considerable hyperfine structure was also observed in all of the chlorine isocyanate spectra. This was analysed to yield chlorine and nitrogen nuclear quadrupole coupling constants. These have provided some insight into the electronic distribution in the molecule. HNCO: The microwave spectra of six isotopic species of isocyanic acid have also been measured in the frequency region 8-37 GHz. The observed (microwave) transitions were analysed together with the available millimeter-wave data (Kewley et.al. (150) and Winnewisser (198)) using Watson's general non-rigid Hamiltonian. This analysis yielded rotational constants of sufficient accuracy to allow the determination of a refined (r₈) molecular structure: s r(H-N) = 0.986 ± 0.015 Å ∟(HNC) = 128° 2¹ ± 1° r(N-C) = 1.209 ± 0.005 Å ∟(NCO) = 180° r(C-O) = 1.166 ± 0.005 Å . In addition, the b-component of the dipole moment of deuterated isocyanic acid was measured. The value obtained is: μ[sub b] = 1.35 ± 0.10 D. When combined with the previously measured a-component (μ[sub a] = 1.602 ± 0.020 D (153)) SL it yields the total dipole moment of DNCO: μ = 2.10 ± 0.15 D. NCNCO: The microwave spectrum of the naturally most abundant isotopic species of cyanogen isocyanate has been investigated. This was found to be consistent with a bent chain structure rather than the previously proposed (18) linear configuration. A preliminary molecular structure was calculated. Analysis of excited vibrational state spectra gave evidence for strong quasi-linear behavior. Stark measurements yielded the molecular dipole moment.

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