The microwave spectrum of a second N-gauche rotamer of allylamine

• Published in: Journal of molecular spectroscopy 1974-01, Vol.52 (3), p.457-484
• Main Authors: Botskor, I., Rudolph, H.D., Roussy, G.
• Format: Article
• Language: English
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1016/0022-2852(74)90192-1

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 CC 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) A 0 = 23 957.05 ± 0.048, B 0 = 4 229.96 ± 0.025, C 0 = 4 154.91 ± 0.025, χ aa = − 1.48 ± 0.04, χ bb - χ cc = − 1.42 ± 0.04, and (in D) ∥ μ a ∥ = 0.766 ± 0.010, ∥ μ b ∥ = 0.700 ± 0.005, ∥ μ c ∥ = 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 CC torsion as indicated by their intertial defects. The potential hindering the internal CC 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) V 1 = − 77 ± 85, V 2 = 170 ± 126, V 3 = 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.