Rotational band contours in the 2938 Å electronic system of aniline

Abstract

Rotational contours of band O (O⁺-O⁺ band) at 2938 Å and of the band at 2913 Å, confirmed as I ₁ ¹ (O⁻-O⁻ band), in the absorption spectrum of aniline have been analysed by the method of computer simulation of the contours in which rotational selection rules and excited state rotational constants are included in the input data. The observed contours could be reproduced only by using type B selection rules showing that the electronic transition is ¹ B ₂-¹ A ₁. The rotational constants of the O⁺ level of the excited electronic state are:

The change in A from the ground to the excited state is the same as in phenol [1] but B increases by much more than in phenol indicating a greater contraction of the molecule along the long in-plane axis in the excited state than is the case in phenol. The rotational constants show that the inertial defect Δ' is −0·27 ± 0·13 u Ų in the O⁺ level and −0·40±0·13 u Ų in the O⁻ level of the excited electronic state. The value of Δ' for the O⁺ level suggests an out-of-plane angle of 30°±10° for the bisector of the HNH angle.

The origin of band O is at 34029·1±0·1 cm⁻¹ and the origin of band I ₁ ¹ is separated from it by +292·9 cm⁻¹.