以真空紫外光游離–紅外光分解法研究甲醇和水的團聚體之紅外吸收光譜

Abstract

We investigated infrared spectra in the CH- and OH-stretching regions of size-selected clusters of methanol (M) with one water molecule (W), represented as MnW, n = 1–4, in a pulsed supersonic jet by using the VUV-ionization/IR-depletion/TOF-detection technique. The VUV light at 118 nm served as the source of ionization in a TOF mass spectrometer to detect clusters MnW as protonated forms Mn−1WH+. The tunable IR laser served as a source of dissociation for clusters before ionization. The variations in intensities of MnWH+ were monitored as the wavelength of the IR laser light was tuned across the range 2700–3800 cm−1. IR Spectra of each size-selected cluster were obtained on careful processing of observed action spectra of related cluster-ions according to a mechanism taking into account of production and loss of each cluster due to IR dissociation. Spectra of methanol-water clusters in the OH region show significant variations as the number of methanol molecules increase, whereas spectra in the CH region are similar. For MW, absorption of a structure with H2O as a hydrogen-bond donor was observed at 3722, 3682, and 3570 cm−1, whereas that of methanol as a hydrogen-bond donor was observed at 3753 and 3611 cm−1. For M2W, the OH-stretching band of the free OH of H2O was observed at 3721 cm−1, whereas overlapped bands near 3536, 3472, and 3425 cm−1 correspond to the OH-stretching modes of three hydrogen-bonded OH in a cyclic structure. For M3W, the free OH shifts to 3715 cm−1, and the hydrogen-bonded OH-stretching bands become broader, with a band near 3179 cm−1 having the smallest wavenumber. Scaled harmonic vibrational wavenumbers and relative IR intensities predicted for the methanol-water clusters with the M06-2X/aug-cc-pVTZ method are consistent with our experimental results. For M4W, observed spectrum agree less with theoretical predictions, indicating the presence of isomers other than the most stable cyclic one. Spectra of MnW and Mn+1 are compared and the cooperative hydrogen-bonding is discussed.

We investigated infrared spectra in the CH- and OH-stretching regions of size-selected clusters of methanol (M) with one water molecule (W), represented as MnW, n = 1–4, in a pulsed supersonic jet by using the VUV-ionization/IR-depletion/TOF-detection technique. The VUV light at 118 nm served as the source of ionization in a TOF mass spectrometer to detect clusters MnW as protonated forms Mn−1WH+. The tunable IR laser served as a source of dissociation for clusters before ionization. The variations in intensities of MnWH+ were monitored as the wavelength of the IR laser light was tuned across the range 2700–3800 cm−1. IR Spectra of each size-selected cluster were obtained on careful processing of observed action spectra of related cluster-ions according to a mechanism taking into account of production and loss of each cluster due to IR dissociation. Spectra of methanol-water clusters in the OH region show significant variations as the number of methanol molecules increase, whereas spectra in the CH region are similar. For MW, absorption of a structure with H2O as a hydrogen-bond donor was observed at 3722, 3682, and 3570 cm−1, whereas that of methanol as a hydrogen-bond donor was observed at 3753 and 3611 cm−1. For M2W, the OH-stretching band of the free OH of H2O was observed at 3721 cm−1, whereas overlapped bands near 3536, 3472, and 3425 cm−1 correspond to the OH-stretching modes of three hydrogen-bonded OH in a cyclic structure. For M3W, the free OH shifts to 3715 cm−1, and the hydrogen-bonded OH-stretching bands become broader, with a band near 3179 cm−1 having the smallest wavenumber. Scaled harmonic vibrational wavenumbers and relative IR intensities predicted for the methanol-water clusters with the M06-2X/aug-cc-pVTZ method are consistent with our experimental results. For M4W, observed spectrum agree less with theoretical predictions, indicating the presence of isomers other than the most stable cyclic one. Spectra of MnW and Mn+1 are compared and the cooperative hydrogen-bonding is discussed.