The Molecular Zoo

Entry for Aniline

(Eric Edens)

Aniline is a tricky molecule.

Elucidating its microwave structure requires a large assumption.  We must assume that the C₆H₅N fragment is planar.  With exception to the amine hydrogens, this is a  fairly decent assumption.  Here's the problem.  The c coordinates in the inertial framework are small and cannot be deduced by isotopic substitution.  Furthermore, the a parameters for C₁ and C₄ are small, and too, cannot be deduced by substitution.  To combat this problem, Lister made the assumption that the C₆H₅N fragment is planar.¹  This removed worry about c parameters.  But more importantly, he could use first moment condition (Σma = 0) to assign the a parameters of C₁ and C₄.

An analysis of the microwave intensities leads to the plane of symmetry in the molecule. As a result of the plane of symmetry, the ortho and meta ring protons are pairs of identical, indistinguishable Fermions and the Pauli Exclusion Principle applies.

Schultz addressed the issue of planarity of C₆H₅N in an electron diffraction study.²  He found, however,  that the deviation from planarity was too small to detect with electron diffraction.

A second complication involves amine inversion.³  This inversion causes large amplitude internal motions of the amine hydrogens.  Furthermore, the double-well potential function for the inversion of aniline is highly anharmonic.  Because of this, Lister admits that the assumption that the deuterium atoms adopt the same position as the hydrogen atoms in the -NH₂ fragment is "drastic."  Hence, the uncertainty in the HNH angle is large (2°) compared to the uncertainty in the other angles (~10').


Table 1: Tabular results (A link to the structure in pdb format is provided in each case.)

	N_H1^1	C1_N	C1_C2	C2_C3	C3_C4	C2_H2	C3_H3	C4_H4	H1NH7^2	C6C1C2	C1C2C3	H2C2C3	C2C3C4	C3C4C5	C3C4C5	NC1C4	phi^1
Microwave	1.001	1.402	1.397	1.394	1.396	1.082	1.083	1.080	113.10	119.43	120.12	121.05	120.70	119.38	118.92	180.00	37.49
Electron Diffraction	1.021	1.406	1.402	1.393	1.400	1.094	1.094	1.094	109.82	119.01	120.50	119.65	120.38	119.65	119.25	180.00	43.6
MP2 6-21G(D,P)	1.010	1.406	1.402	1.394	1.396	1.084	1.083	1.082	109.76	118.84	120.43	120.14	120.56	119.32	119.14	176.62	42.86
HF 6-31G (D,P)	0.996	1.394	1.393	1.383	1.385	1.077	1.076	1.075	111.36	118.69	120.40	120.00	120.89	119.15	118.72	178.06	40.26
HF 3-21G	0.995	1.376	1.395	1.380	1.384	1.073	1.073	1.071	118.10	118.10	120.69	119.93	120.95	119.12	118.62	179.98	0
AM1	0.996	1.400	1.415	1.390	1.394	1.100	1.101	1.099	113.14	118.46	120.29	119.56	120.74	119.41	119.47	176.27	38.56
MMFF	1.015	1.400	1.397	1.397	1.393	1.087	1.087	1.087	112.33	118.61	120.73	119.05	120.01	119.91	119.73	174.30	34.91

1. Bond lengths in angstroms.
2. Bond angles in degrees.
3. phi is the angle between the C₆H₅N plane and the -NH₂ plane.

Discussion of Calculations

Interestingly, all calculations predicted displacement of the nitrogen atom.  Of the quantum mechanical calculations, MP2 predicted the largest displacement, a little over 3°. 

Two basis sets were employed for HF calculations.  The largest discrepancy involved the out-of-plane angle.  The 3-21G basis set predicted that aniline was completely flat.  The 6-31G (D,P) basis set, however, brought the amine hydrogens out of the plane. Therefore, the inclusion of polarization functions in the basis set is essential for handling of the non-planarity of the amino group.

Interestingly, HF 6-31G (D,P) predicted nitrogen - hydrogen structures better than MP2 (in comparison to the microwave structure).  Both the N-H bond length and H N H bond angle were better modelled by HF.  C-C and C-H bond lengths, however, were predicted better by MP2.

The MMFF has the best performance to processing time ratio.  In contrast, the Class I Tripos (SYBYL) force field yields a planar structure. MMFF agreed with MP2 to within one hundredth of an Angstrom in each bond length.  And for angles involving carbons and hydrogens, the methods agreed to within about a degree.  The calculations became slightly worse when nitrogen was involved.

AM1 performed satisfactorily.  However, its  overall performance was worse than MMFF and its calculation took a longer amount of time.

Hybridization Discussion

Aniline shows a preference for ortho and para electrophilic substitution.  The general explanation for this phenomenon involves the following resonance structures:

Resonance structures a and b predict a tetrahedral geometry for the -NH₂ fragment.  Using ammonia as a model for this geometry, we would predict the angle HNH to be about 107.3°.  Resonance structures c through e, however, predict SP² hybridization at the nitrogen atom, and therefore, an HNH angle of about 120°.  Lister's microwave study found the HNH angle to be 113° 6' ± 2°.  This value is smack-in-the-middle between the angles predicted by our SP³ and SP² bond angle predictions.  This suggests that the hybridization of the nitrogen atom is somewhat between SP³ and SP².

0⁺ Rotational Constants (MHz)¹

	A	B	C
aniline-H7	5617.40	2593.83	1777.04
aniline-NHD	5571.96	2493.60	1726.10
aniline-ND2	5519.74	2403.75	1679.04
aniline-2D1	5346.50	2585.54	1745.30
aniline-4D1	5617.56	2483.32	1724.46
aniline-3,5 D2	5093.65	2520.55	1688.45
aniline-2,4,6 D3	5093.36	2467.78	1664.66
aniline-2,3,4,5,6 D5	4658.99	2402.49	1587.17
aniline-^15N	5617.10	2523.91	1743.94
aniline-1^13C	5617.57	2582.30	1771.63
aniline-2^13C	5528.76	2593.21	1767.76
aniline-3^13C	5530.41	2575.91	1759.87
aniline-4^13C	5617.68	2548.40	1755.60

0^- Rotational Constants (MHz)¹

	A	B	C
aniline-H7	5615.57	2592.24	1776.73
aniline-NHD	5569.16	2492.90	1726.28
aniline-ND2	5519.74	2403.75	1679.04
aniline-2D1	5346.09	2583.96	1744.99
aniline-4D1	5615.91	2481.85	1724.17
aniline-3,5 D2	5092.47	2518.99	1688.15
aniline-2,4,6 D3	5092.70	2466.40	1664.38
aniline-2,3,4,5,6 D5	4657.89	2401.07	1586.93
aniline-^15N	5615.24	2522.21	1743.56
aniline-1^13C	5615.63	2580.70	1771.33
aniline-2^13C	5526.84	2591.58	1767.46
aniline-3^13C	5528.46	2574.30	1759.57
aniline-4^13C	5615.70	2546.83	1755.30

Sources
1.  Lister, D.G. and Tyler, J.K., 'The Microwave Spectrum, Structure and Dipole Moment of Aniline,' Journal of Molecular Structure, 23 (1974) p. 253-264.
2. Howard, D.L.; Robinson, T.W.; Fraser, A.E. and Kjaergaard, H.G., 'The effect of NH 2-inversion tunneling splitting on the NH-stretching overtone spectra of aniline vapour,' Physical Chemistry Chemical Physics, 6 (2004) p. 719-724.
3. Schultz, G.; Portalone, G.; Ramondo, F.; Domenicano, A. and Hargittai, I., 'Molecular Structure of Aniline in the Gaseous Phase: A Concerted Study by Electron Diffraction and ab Initio Molecular Orbital Calculations,' Structural Chemistry, 7 (1996) p. 59-70.