The
structures, relative stabilities, and vibrational wavenumbers
of the two most stable conformers of serine, stabilized by the O–H···N,
O–H···OC and N–H···O–H
intramolecular hydrogen bonds, have been evaluated by means of state-of-the-art
composite schemes based on coupled-cluster (CC) theory. The so-called
“cheap” composite approach (CCSD(T)/(CBS+CV)MP2) allowed determination of accurate equilibrium structures and harmonic
vibrational wavenumbers, also pointing out significant corrections
beyond the CCSD(T)/cc-pVTZ level. These accurate results stand as
a reference for benchmarking selected hybrid and double-hybrid, dispersion-corrected
DFT functionals. B2PLYP-D3 and DSDPBEP86 in conjunction with a triple-ζ
basis set have been confirmed as effective methodologies for structural
and spectroscopic studies of medium-sized flexible biomolecules, also
showing intramolecular hydrogen bonding. These best performing double-hybrid
functionals have been employed to simulate IR spectra by means of
vibrational perturbation theory, also considering hybrid CC/DFT schemes.
The best overall agreement with experiment, with mean absolute error
of 8 cm–1, has been obtained by combining CCSD(T)/(CBS+CV)MP2 harmonic wavenumbers with B2PLYP-D3/maug-cc-pVTZ anharmonic
corrections. Finally, a composite scheme entirely based on CCSD(T)
calculations (CCSD(T)/CBS+CV) has been employed for energetics, further
confirming that serine II is the most stable conformer, also when
zero-point vibrational energy corrections are included.