The microwave spectrum of a second N-gauche rotamer of allylamine |
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Authors: | I. Botskor H.D. Rudolph G. Roussy |
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Affiliation: | Faculty of Physics, University of Freiburg, Freiburg, Germany;Institute of Theoretical Chemistry, University of Nancy, Nancy, France |
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Abstract: | The microwave spectra of the ground and five excited states of a second gauche rotamer of allylamine have been measured and assigned. Three of the excited states belong to the same mode, most probably the CC torsion, the second and third vibrational states present a symmetrical splitting due to tunneling effect. The spectrum was conclusively identified as due to the N-gauche, lone-electron-pair trans form by means of the N-quadrupole coupling constants and dipole moment components. The variation observed for the quadrupole coupling constants in the different vibrationally excited states was explained by a suitable model. The ground state constants are (in MHz) A0 = 23 957.05 ± 0.048, B0 = 4 229.96 ± 0.025, C0 = 4 154.91 ± 0.025, χaa = ? 1.48 ± 0.04, χbb - χcc = ? 1.42 ± 0.04, and (in D) ∥μa∥ = 0.766 ± 0.010, ∥μb∥ = 0.700 ± 0.005, ∥μc∥ = 0.290 ± 0.020.The excited states of the N-cis, lone-electron-pair trans form were also measured and assigned; two of these states appear to belong to the CC torsion as indicated by their intertial defects. The potential hindering the internal CC rotation was calculated using the relative intensity data of the N-cis and N-gauche forms as well as the tunneling splittings. A three-term cosine potential was fitted to the data yielding (in cm?1) V1 = ? 77 ± 85, V2 = 170 ± 126, V3 = 663 ± 95. The Dennison-Uhlenbeck potential was used for an approximate calculation of the N-trans barrier separating the two identical N-gauche forms. The barrier obtained was 1.9 ± 0.3 Kcal/mole. |
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