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THE MICROWAVE SPECTRUM AND STRUCTURE OF THE $CH_{3}CN-H_{2} O$ AND $CH_{3}OH-HCN$ DIMERS
Frank J. Lovas, J. Sobhanadri
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Abstract
The present studies were motivated by an interest in the effect of hydrogen bonding on the internal rotation barrier in species containing the $CH_{3}$ group. The microwave spectrum of the methyl cyanide-water complex, $CH_{3}CN-H_{2}O$, and methanol-hydrogen cyanide have been studied with a pulsed-beam Fourier-transform Fabry-Perot-cavity spectrometer. Both species exhibit {a}-type spectra. Rotational assignments were confirmed from the analysis of the well resolved $^{14}N$ nuclear electric quadrupole transitions. The methyl cyanide-water complex has it symmetric top spectrum and shows two closely spaced states which arise from tunneling interchange of the water proton and may be characterized by the proton spins as I = 0 and I = 1. In addition to the normal isotopic form, $CH_{3}CN-DOH, CH_{3}CN-D_{2}O, CH_{3}C^{15}N-H_{2}O$ and $CH_{3}C^{15}N-DOH$ were studied. Each of the DOH species exhibits only one state as expected. Structural analysis of all five species yields R(N-O) = 3.028(2) {\\AA} and a hydrogen bond distance of 2.072(2) {\\AA}. The methanol-hydrogen cyanide complex has an asymmetric top spectrum with A and E symmetry state due to internal rotation of the $CH_{3}$ group. For the determination of the structure, additional isotopic variants were studied $CD_{3}OH-HCN, CH_{3}OD-HCN$, and $CH_{3}OH-DCN.$ Two H-bonded conformations are possible, i e. HCN H-bonded to the oxygen of methanol or the methanol hydroxyl H bonded to the nitrogen atom of HCN; the former was detected and an H-bond distance of 2.11(3) {\\AA} was determined in the structural analysis. The internal rotation splitting has been analyzed by conventional methods to yield an effective barrier, $V_{3} = 137.3 cm^{-1}$ for $CH_{3}OH-HCN$ and $V_{3} = 175 cm^{-1}$ for $CH_{3}OD-HCN$. Each of these values are less than one-half of the value for methanol, $373 cm^{-1}$. Discussion of the spectral analyses, structure and comparison with other complexes will be presented.