Esemplastic thermoplastic

Someone asked on IRC if aniline is planar and I wasn’t sure, so I looked it up and found it amusing to see the changes in the language of the abstracts from 1966, 1985, 1993, and 2007:

1966: “Aniline-planar or nonplanar?” J Mol Spectrosc, 20(2):193-195
The question whether the NH2 group of aniline is coplanar with the aromatic ring has been raised many times (1) but the configuration has not yet been established beyond doubt. Evidence from the electronic band system near 2940 Å appears to settle this point, not only for the ground state of aniline but for its first electronically-excited state (1B2) also.

1985: “A molecular orbital study of nitrogen inversion in aniline with extensive geometry optimization” Theor Chim Acta 69(3):235-245
The geometry and energy of aniline have been calculated using the 6-31G and 6-31G*(5D) basis sets for the planar structure and various pyramidal structures, assuming that the ring and the N-atom bonded to it lie in the same plane, but otherwise with full geometry optimization. With the 6-31G basis set the planar structure is predicated to be the most stable, whereas the inclusion of polarization functions in the 6-31G*(5D) basis set finds a pyramidal structure with the out-of-plane angle phgr=42.3° to be most stable

1993: “The structure of aniline by ab initio studies” J Mol Struct 281:91-98
The structure of aniline has been studied by ab initio calculations. Complete geometry optimization of (1) the energy minimum structure and the transition states for (2) internal rotation and (3) inversion of the amino group were carried out at the SCF level using several different basis sets. For these three stationary geometries vibrational frequencies were calculated at the SCF/6-31G** level. The effect of electron correlation was estimated by single point MP4(SDQ) calculations using the 6-311G** basis set. To satisfactorily describe the conformation and orientation of the amino group a fully polarized (6-31G**) basis set is required

2007: “Simulation of inversion motion and N-H stretching overtone spectra of aniline.” J Chem Phys 126(6):064309
A curvilinear internal coordinate Hamiltonian is used to simulate the N-H stretching overtone spectra and the associated inversion splittings in aniline. A simple local mode type model is applied to the N-H stretching and H-N-H bending modes. Geometric algebra is employed to derive the kinetic energy operator for the large amplitude inversion motion. Electronic structure calculations at the Moller-Plesset second order perturbation theory and correlation consistent aug-cc-pVTZ basis set level are used to obtain model parameters, some of which have been optimized with the least-squares method using experimental vibrational term values as data. The observed N-H stretching overtone vibrational levels and the inversional tunneling splittings are well reproduced with our approach.

Isn’t it awesome?