Molecular structure of aniline in the gaseous phase: A concerted study by electron diffraction and ab initio molecular orbital calculations

Abstract

The molecular structure of free aniline has been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF and MP2 levels of theory, using the 6-31G*(6D) basis set.Least-squares refinement of a model with Cs symmetry, with constraints from MP2 calculations, has led to an accurate determination of the C--C--C angle at the ipso position of the benzene ring, ce = 119.0 + 0.2* (where the uncertainty represents total error). This parameter provides information on the extent of the interaction between the nitrogen lone pair and the 7r system of the benzene ring, and could not be determined accurately by microwave spectroscopy. The angles at the ortho, meta, and para positions of the ring are 120.3 + 0.1 ~ 120.7 + 0.1 ~ and 119.0 + 0.3 ~ respectively. Important bond distances are (rg(C--C)) --1.398 5:0.003 A. and rg(C--N) = 1.407 + 0.003 .~. The effective dihedral angle between the H--N--H plane and the ring plane, averaged over the large-amplitude inversion motion of the amino group, is (Irl) = 44:1: 4 ~ The equilibrium dihedral angle is calculated to be 41.8 ~ at the HF level and 43.6 ~ at the MP2 level, in agreement with far-infrared spectroscopic information. The MO calculations predict that the difference r(Cortho--Cmeta ) --r(Cipso--Cortho) is 0.008-0.009 .~. They also indicate that the nitrogen atom is displaced from the ring plane, on the side opposite to the amino hydrogens. The displacement is 0.049 .A, at the HF level and 0.072 ~, at the MP2 level. The two calculations, however, yield very different patterns for the minute deviations from planarity of the ring carbons.

DFT theoretical studies of anions of aniline and its several derivatives

“…In Table 1, together with the experimental values for aniline obtained from the gas-phase microwave [28] and ED [29] studies.…”

“…The inversion angle can be used as a measure of pyramidalization of the amino-group. It is known that the estimated degree of NH 2 pyramidalization depends on the experimental method employed: 37 ± 2°determined by the microwave studies [28] (at an assumption that the C-N bond is coplanar with the ring) and *42 ± 1°when determined by far infrared data [30] or resonance fluorescence [31] and by electron diffraction (*44°) [29] methods. According to our calculations, this value is about 46°a t the MP2 and about 38°at the DFT-B3LYP levels of theory (Table 1).…”

Equilibrium molecular structure of benzamide from gas-phase electron diffraction and theoretical calculations

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and [22][23][24] in the paper honoring Aldo Domenicano, the pioneer of the accurate determination of substituent effects in benzene derivatives [48]. The r-acceptors increase the ipso bond angle, e.g., a ipso = 123.4(2)°in fluorobenzene [49], while r-donors, p-donors, and p-acceptors [47] decrease this bond angle, e.g., a ipso = 119.0(2)°in aniline [50]. The influence of amide group on ring distortion is interesting to consider due to its simultaneous r and p interactions with the benzene ring.…”

Looking Back and Ahead: Gas-Phase Electron Diffraction at 75

“…Of course, there is no sharp line between the two categories. Examples of the former were aniline [23], ethynylbenzene [24], tetramethylsilane [25], and pbis(trimethylsilyl)benzene [26]. Examples of the more complicated structures were octamethylcyclotetrasilane [27], 1-bromopropane [28], 1-chlorobutane, 1-bromobut ane, and 1-iodobutane [29], 1,2,4,5-tetracyanobenzene [30], tert-butylazide [31], some cyclic nitramines [32], 1,3,5-tris(trimethylstannyl)benzene [33], tris(pyrazol-1-yl)-s-triazine [34], and 1,1-dimethylsilacyclopentene-3,4-oxide [35].…”