Hydrogen-bonding in glycine and malonaldehyde: Performance of the Lap1 correlation functional

The conformational equilibrium of gaseous glycine presents a severe challenge to quantum chemistry and, in particular, to density functional theory (DFT), due to the presence of internal hydrogen bonds. We present new DFT results for the structure and the energetics of glycine and malonaldehyde, using the recently developed nonlocal exchange-correlation functionals BLap1 and PLap1. A comparative analysis is made with the results of the generalized-gradient-approximation (GGA) schemes Becke–Perdew (BP86) and Becke–Lee–Yang–Parr (BLYP), the hybrid Hartree–Fock (HF)-DFT methods B3PW91 and B3LYP, with post-HF methods, and with the available experimental data. Our BLap1/TZVP and PLap1/TZVP values for the energy margin between the two lowest conformers of glycine (0.84 and 1.05 kcal/mol, respectively) are within the experimental error bars (1.0±0.5 kcal/mol). MP2 underestimates this energy difference by about 0.5 kcal/mol, BLYP and the hybrid methods are off by about 0.9 kcal/mol. The optimized geometry of malonaldehyde is improved with the Lap1 functionals, compared to the GGA results reported previously. Improvement over the GGA is also achieved for the energy barrier for the internal proton transfer in malonaldehyde. Recent high quality post-HF (G2) calculations of Barone and Adamo1 yield 4.4 kcal/mol. The best GGA (PP86) value of 2.1 kcal/mol is seriously underestimated. The best B3LYP estimate is 3.0 kcal/mol.1 Our BLap1/TZVP (3.28 kcal/mol) and PLap1/TZVP (4.56 kcal/mol) values are in the correct energy range, the latter being, in fact, very close to the G2 estimate.