### 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 τ(NCCO) of Gly-IIn, characterizing the deviation of its heavy-atom framework from planarity, is (11 ± 2)°. Nevertheless, in the ground vibrational state the effective structure of Gly-IIn has a plane of symmetry.

Original language | English |
---|---|

Pages (from-to) | 1373-1383 |

Number of pages | 11 |

Journal | Journal of Computational Chemistry |

Volume | 28 |

Issue number | 8 |

DOIs | |

Publication status | Published - Jun 2007 |

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### Keywords

- Ab initio computations
- Anharmonic force field
- Equilibrium molecular structure
- Glycine
- Spectroscopic constants

### ASJC Scopus subject areas

- Chemistry(all)
- Safety, Risk, Reliability and Quality

### Cite this

*Journal of Computational Chemistry*,

*28*(8), 1373-1383. https://doi.org/10.1002/jcc.20680

**Molecular structures of the two most stable conformers of free glycine.** / Kasalová, Veronika D.; Allen, Wesley D.; Schaefer, Henry F.; Czinki, Eszter; Császár, A.

Research output: Contribution to journal › Article

*Journal of Computational Chemistry*, vol. 28, no. 8, pp. 1373-1383. https://doi.org/10.1002/jcc.20680

}

TY - JOUR

T1 - Molecular structures of the two most stable conformers of free glycine

AU - Kasalová, Veronika D.

AU - Allen, Wesley D.

AU - Schaefer, Henry F.

AU - Czinki, Eszter

AU - Császár, A.

PY - 2007/6

Y1 - 2007/6

N2 - 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 (re) 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 τ(NCCO) of Gly-IIn, characterizing the deviation of its heavy-atom framework from planarity, is (11 ± 2)°. Nevertheless, in the ground vibrational state the effective structure of Gly-IIn has a plane of symmetry.

AB - 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 (re) 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 τ(NCCO) of Gly-IIn, characterizing the deviation of its heavy-atom framework from planarity, is (11 ± 2)°. Nevertheless, in the ground vibrational state the effective structure of Gly-IIn has a plane of symmetry.

KW - Ab initio computations

KW - Anharmonic force field

KW - Equilibrium molecular structure

KW - Glycine

KW - Spectroscopic constants

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U2 - 10.1002/jcc.20680

DO - 10.1002/jcc.20680

M3 - Article

VL - 28

SP - 1373

EP - 1383

JO - Journal of Computational Chemistry

JF - Journal of Computational Chemistry

SN - 0192-8651

IS - 8

ER -