How Well Do DFT and MP2 Treat Intramolecular Hydrogen Bonding in Protonated Amino Acids?
John K. Merle, Karl K. Irikura
We have re-examined the amine-N to carbonyl-O proton transfer of protonated glycine using high-level coupled-cluster methods to resolve a discrepancy involving hydrogen-bonding interactions at different theoretical levels. While at the uncorrelated Hartree-Fock (HF) level O-protonated glycine [GlyH+(O)] is predicted to have a stable N H O hydrogen-bonded intermediate, the MP2 and B3LYP methods predict that this intermediate does not exist (Zhang, K.; Chung-Phillips, A. 1999, O Hair et al. 2000). However, improving the treatment of electron correlation via the MP3, CCSD, and CCSD(T) methods provide results in agreement with HF theory. Optimizations with the MP3 and CCSD methods and triple-zeta basis sets (aug-cc-pVTZ and 6-311++G(d,p), respectively) predict the GlyH+(O) intermediate. When zero-point vibrational energy (ZPVE) corrections are applied the GlyH+(O) energy is greater than the transition state energy. CCSD(T)/6-31+G(d,p) optimizations also predict the GlyH+(O) intermediate. However, with a larger basis set (at least triple-zeta in quality) the CCSD(T) method predicts the GlyH+(O) intermediate will not exist. Interestingly, when the CCSD(T)/6-31+G(d,p) ZPVEs are applied GlyH+(O) becomes more stable and is lower in energy than the transition state even after improving the energies for these structures. This indicates that there may be little incentive to use more time consuming higher-correlated ab initio methods above the MP2 level to describe intramolecular hydrogen bonding structures.
Journal of Chemical Theory and Computation
ab initio, computational, intramolecular hydrogen bonding, protonated glycine
and Irikura, K.
How Well Do DFT and MP2 Treat Intramolecular Hydrogen Bonding in Protonated Amino Acids?, Journal of Chemical Theory and Computation
(Accessed May 28, 2023)