Conformational Analysis of n-Alkanes Using Density Functional Theory. Comparison with ab Initio Calculations

S. Tsuzuki, T. Uchimaru, and K. Tanabe
[Chem. Phys. Lett., Vol.246, pp.9-12, 1995]

Relative conformational energies of n-butane, n-pentane and n-hexane were calculated by the DFT method (Becke's exchange and Lee, Yang and Parr's correlation functionals) with the 6-31G* basis set. The calculated energies were compared with experimental values and ab initio calculations. The DFT relative conformational energies agree better with the MP4 energies and experimental values than the HF energies.

Method	Energies (kcal/mol)
	trans	gauche	T«Ga	eclipseb
HF/6-31G*//HF/6-31G*	0.0	0.95	3.65	6.18
MP2/6-31G*//MP2/6-31G*	0.0	0.68	3.62	6.08
MP3/6-31G*//MP2/6-31G*	0.0	0.74	3.55	5.96
MP4(SDTQ)/6-31G*//MP2/6-31G*	0.0	0.69	3.55	5.92
CCSD(T)/6-31G*//MP2/6-31G*	0.0	0.70	3.52	5.88
QCISD(T)/6-31G*//MP2/6-31G*	0.0	0.70	3.52	5.88
BLYP/6-31G*//MP2/6-31G*	0.0	0.68	3.33	6.00
BLYP/6-31G*//BLYP/6-31G*	0.0	0.85	3.22	5.63
electron diffraction	0.0	0.75±0.24
IR in solid neon	0.0	<0.73

^a Internal rotational barrier height between the trans and the gauche rotamers.
^b Internal rotational barrier height of the eclipse saddle point

Table 1. Copformational energies and barrier heights of n-butane