Molecular structures of the two most stable conformers of free glycine

Abstract

The equilibrium molecular structures of the two lowest-energy conformers of glycine, Gly-Ip and Gly-IIn, have been characterized by high-level ab initio electronic structure computations, including all-electron cc-pVTZ CCSD(T) geometry optimizations and 6-31G* MP2 quartic force fields, the latter to account for anharmonic zero-point vibrational effects to isotopologic rotational constants. Based on experimentally measured vibrationally averaged effective rotational constant sets of several isotopologues and our ab initio data for structural constraints and zero-point vibrational shifts, least-squares structural refinements were performed to determine improved Born-Oppenheimer equilibrium (r(e)) structures of Gly-Ip and Gly-IIn. Without the ab initio constraints even the extensive set of empirical rotational constants available for 5 and 10 isotopologues of Gly-Ip and Gly-IIn, respectively, cannot satisfactorily fix their molecular structure. Excellent agreement between theory and experiment is found for the rotational constants of both conformers, the rms residual of the final fits being 7.8 and 51.6 kHz for Gly-Ip and Gly-IIn, respectively. High-level ab initio computations with focal point extrapolations determine the barrier to planarity separating Gly-IIp and Gly-IIn to be 20.5 +/- 5.0 cm(-1). The equilibrium torsion angle tau(NCCO) of Gly-IIn, characterizing the deviation of its heavy-atom framework from planarity, is (11 +/- 2) degrees. Nevertheless, in the ground vibrational state the effective structure of Gly-IIn has a plane of symmetry.

The equilibrium OH bond length

“…. .N intramolecular hydrogen bond) have been calculated at the CCSD(T)/VTZ level of theory [78]. Their cubic force fields have been calculated at the MP2/6-31G* level of theory resulting in semi-experimental equilibrium rotational constants and improving the accuracy of the ab initio structure.…”

Conformers of Gaseous Serine

“…48-51 Also, the conformational behavior of neutral serine was investigated in low-temperature argon matrices. 18,30,52 The combination of B3LYP/6-31++G** and anharmonic (GVPT2) computed IR spectra with experimental matrix FT-IR results demonstrated the presence of six Equilibrium structures and accurate relative energies, obtained within the focalpoint analysis (FPA) approach, [57][58][59][60][61][62] are available for the amino acids glycine, 20,22,63 alanine, 21,63 threonine, 64 proline, 65,66 and cysteine. 19 In this study, we perform an exhaustive search of serine conformers and use a correlated wave function method with a high-quality basis set (MP2/cc-pVTZ) to locate 85 unique conformers.…”

Anharmonic molecular force fields

“…This list is highly incomplete, omission of articles is by no means intentional. Complete quartic force fields have been determined by wavefunction methods for molecules as large as the free amino acids glycine42 and proline40 (in the latter case, the force field for the 17‐atom molecule with no symmetry contains 62,835 unique elements in the internal coordinate space).…”