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Journal ArticleDOI

Electric Dipole Moments of Several Molecules from the Stark Effect

01 Feb 1953-Journal of Chemical Physics (American Institute of Physics)-Vol. 21, Iss: 2, pp 308-310
TL;DR: The following molecular electric dipole moments, in Debye units, have been obtained from the Stark effect of pure rotational spectra observed in the microwave region: 0.234±0.004 for NF3, 1.03±0,0.01 for PF3; 1.77± 0.06 for CH3CN; 3.83± 0.,3.00±0.,0.02 for HCN, 3.92± 0,2.
Abstract: The following molecular electric dipole moments, in Debye units, have been obtained from the Stark effect of pure rotational spectra observed in the microwave region: 0.234±0.004 for NF3; 1.03±0.01 for PF3; 1.77±0.02 for POF3; 3.00±0.02 for HCN; 3.92±0.06 for CH3CN; 3.83±0.06 for CH3NC; 1.79±0.02 for CH3F; 0.75±0.01 for CH3CCH; 1.26±0.01 for SiF3H. All values apply to the ground vibrational state.
Citations
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Journal ArticleDOI
TL;DR: In this article, strong detections of methyl cyanide (CH3CN), vinyl cyanide, CH2CHCN, CH3CH2CN and HC4CN with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud were reported.
Abstract: We present strong detections of methyl cyanide (CH3CN), vinyl cyanide (CH2CHCN), ethyl cyanide (CH3CH2CN) and cyanodiacetylene (HC4CN) molecules with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud. Attempts to detect the corresponding isocyanide isomers were only successful in the case of methyl isocyanide (CH3NC) for its J(sub K) = 1(sub 0) - 0(sub 0) transition, which is the first interstellar report of this line. To determine the spatial distribution of CH3NC, we used archival Berkeley-Illinois-Maryland Association (BIMA) array data for the J(sub K) = 1(sub 0) - 0(sub 0) transitions but no emission was detected. From ab initio calculations, the bonding energy difference between the cyanide and isocyanide molecules is greater than 8500 per centimeter (greater than 12,000 K). Thus, cyanides are the more stable isomers and would likely be formed more preferentially over their isocyanide counterparts. That we detect CH3NC emission with a single antenna (Gaussian beamsize(omega(sub B))=1723 arcsec(sup 2)) but not with an interferometer (omega(sub b)=192 arcsec(sup 2)), strongly suggests that CH3NC has a widespread spatial distribution toward the Sgr B2(N) region. Other investigators have shown that CH3CN is present both in the LMH hot core of Sgr B2(N) and in the surrounding medium, while we have shown that CH3NC appears to be deficient in the LMH hot core. Thus, largescale, non-thermal processes in the surrounding medium may account for the conversion of CH3CN to CH3NC while the LMH hot core, which is dominated by thermal processes, does not produce a significant amount of CH3NC. Ice analog experiments by other investigators have shown that radiation bombardment of CH3CN can produce CH3NC, thus supporting our observations. We conclude that isomers separated by such large bonding energy differences are distributed in different interstellar environments, making the evaluation of column density ratios between such isomers irrelevant unless it can be independently shown that these species are co-spatial.

95 citations

Journal Article
TL;DR: In this paper, strong detections of methyl cyanide (CH3CN), vinyl cyanide, CH2CHCN, CH3CH2CN, and HC4CN with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud were presented.
Abstract: We present strong detections of methyl cyanide (CH3CN), vinyl cyanide (CH2CHCN), ethyl cyanide (CH3CH2CN), and cyanodiacetylene (HC4CN) molecules with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud. Attempts to detect the corresponding isocyanide isomers were only successful in the case of methyl isocyanide (CH3NC) for its JK = 10-00 transition, which is the first interstellar report of this line. To determine the spatial distribution of CH3NC, we used archival Berkeley-Illinois-Maryland Association (BIMA) array data for the JK = 4K-3K (K = 0-3) transitions, but no emission was detected. From ab initio calculations, the bonding energy difference between the cyanide and isocyanide molecules is >8500 cm-1 (>12,000 K). Thus, cyanides are the more stable isomers and would likely be formed more preferentially over their isocyanide counterparts. That we detect CH3NC emission with a single antenna (Gaussian beam size ΩB = 1723 arcsec2) but not with an interferometer (ΩB = 192 arcsec2) strongly suggests that CH3NC has a widespread spatial distribution toward the Sgr B2(N) region. Other investigators have shown that CH3CN is present both in the LMH hot core of Sgr B2(N) and in the surrounding medium, while we have shown that CH3NC appears to be deficient in the LMH hot core. Thus, large-scale, nonthermal processes in the surrounding medium may account for the conversion of CH3CN to CH3NC, while the LMH hot core, which is dominated by thermal processes, does not produce a significant amount of CH3NC. Ice analog experiments by other investigators have shown that radiation bombardment of CH3CN can produce CH3NC, thus supporting our observations. We conclude that isomers separated by such large bonding energy differences are distributed in different interstellar environments, making the evaluation of column density ratios between such isomers irrelevant unless it can be independently shown that these species are cospatial.

71 citations

Journal ArticleDOI
TL;DR: In this paper, the i.r. spectra of CH3CN, CD3CN and CH3NC have been investigated in solid argon matrices at 20°K.

68 citations

Journal ArticleDOI
TL;DR: In this article, the microwave spectra of eight isotopic species of tertiary butyl acetylene and four isotope species of secondary butyl cyanide have been measured and analysed.
Abstract: The microwave spectra of eight isotopic species of tertiary butyl acetylene and four isotopic species of tertiary butyl cyanide have been measured and analysed. Some transitions in excited vibrational states have been assigned. Measurements of the Stark effect lead to a dipole moment of 0.661±0.004 D for (CH3)3CCCH and 3.95±0.05 D for (CH3)3CCN. From the isotope shifts in the moments of inertia the following bond distances (rs) are obtained: r(C≡C) = 1.209±0.001 A and r(≡C–H) = 1.056±0.001 A in (CH3)3CCCH; r(C≡N) = 1.159±0.001 A in (CH3)3CCN. The tertiary butyl group appears to have very nearly the same dimensions in these two compounds as in (CH3)3CH and (CH3)3CF. The tertiary carbon atom cannot be located with high precision, but the best estimates lead to a C–C≡ distance of 1.495±0.015 A in both (CH3)3CCCH and (CH3)3CCN. This is 0.02–0.04 A longer than the normal value for this distance. There appears to be a consistent tendency for the (CH3)3C— group to lengthen the adjacent bond, but it is difficult ...

65 citations

Journal ArticleDOI
TL;DR: In this paper, the double zeta plus polarization configuration interaction level was used to calculate the dipole moments of linear triple-bonded molecules, such as cyanoacetylene and cyanogen.
Abstract: Molecular structures and energies have been computed at the double zeta SCF level for several linear triple bonded molecules that are either isomers or protonated forms of the familiar molecules cyanoacetylene and cyanogen. These structures have been used to predict rotational constants that can be used as a guide for locating or assigning laboratory or astrophysical microwave spectra. For the same molecules the dipole moments have been computed at the double zeta plus polarization configuration interaction level. Similar structure and dipole moment calculations have also been done on several related known molecules to assess the accuracy of the numerical results. All of the computed dipole moments fit into a pattern that can be roughly explained by simple polarization arguments.

57 citations

References
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Journal ArticleDOI
TL;DR: A microwave spectrograph is described in which the absorption of the gas being studied is modulated by the application of a periodic Stark‐effect field so that the output of the crystal detector contains a radiofrequency component which can be amplified with a narrow‐band amplifier.
Abstract: A microwave spectrograph is described in which the absorption of the gas being studied is modulated by the application of a periodic Stark‐effect field so that the output of the crystal detector contains a radiofrequency component which can be amplified with a narrow‐band amplifier. By using a very narrow band‐width, achieved by employing phase‐sensitive detection, high sensitivity is obtained. The klystron is swept mechanically over the frequency range. Details of the components are given.

101 citations