Conformational Study of the Alanine Dipeptide at the MP2 and DFT Levels

Conformations of an important model system, the alanine dipeptide, have been calculated by using high-level, ab initio electronic structure theory. A Ramachandran plot, with the angle φ in the range −180° to 90° and the angle ψ in the range −60° to 180°, was generated by using density functional theory with the generalized-gradient BLYP functional and a polarized triple-ζ basis set (TZVP+). Six conformers, C7eq, C5, C7ax, β2, αL, and α‘, have been identified in this region of the Ramachandran plot. A second derivative (frequency) analysis showed that all conformers are stable at this level of theory. These structures were used as starting points for geometry optimizations at the MP2/aug-cc-pVDZ level. Single-point energies were calculated at the MP2/aug-cc-pVTZ and MP2/aug-cc-pVQZ levels at the final MP2/aug-cc-pVDZ structures and together with the MP2/aug-cc-pVDZ results were used in extrapolations to the complete basis set limit. The N−H···O, N−H···N, and C−H···O hydrogen bond interactions that are key to the energetics are discussed. In general, the results obtained at the BLYP/TZVP+, MP2/aug-cc-pVDZ, MP2/aug-cc-pVTZ//aug-cc-pVDZ, and MP2/aug-cc-pVQZ//aug-cc-pVDZ levels are in reasonable agreement with each other, except for the β2 conformation for which there are significant differences in the structures. Although the same stability order is obtained at all levels of theory that were used, there are significant differences in the magnitude of the relative conformational energies.