### Abstract

We present a computational study of convergence properties of vibrational IR and Raman spectra for a series of increasingly long units of polyethylene, cis- and trans-polyacetylenes, and polyynes. Convergent behavior to the spectra of infinitely long polymers was observed in all cases when chains reached lengths of approximately 60 carbon atoms, both with respect to the positions of the bands and to their intensities. The vibrational spectra of longer chains are practically indistinguishable. The convergence rate depends on the degree of the π conjugation in a studied system: Vibrational spectra for oligoethylenes converge noticeably faster than the spectra for the conjugated systems. The slowest convergence is observed for skeletal motions of the oligomer chains, which may require more than a hundred carbon atoms in the chain to show deviations smaller than 1 cm^{−1} to the corresponding solid-state calculations. The results suggest that the boundary between the properties of finite and infinite molecular systems fades away for a surprisingly small number of atoms.

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

Article number | 288 |

Journal | Journal of Molecular Modeling |

Volume | 24 |

Issue number | 10 |

DOIs | |

Publication status | Published - 2018 Oct 1 |

### Fingerprint

### Keywords

- Convergence in size evolution
- SCC-DFTB
- Simulations of IR and Raman spectra of polymers

### ASJC Scopus subject areas

- Catalysis
- Computer Science Applications
- Physical and Theoretical Chemistry
- Organic Chemistry
- Computational Theory and Mathematics
- Inorganic Chemistry

### Cite this

*Journal of Molecular Modeling*,

*24*(10), [288]. https://doi.org/10.1007/s00894-018-3824-3

**When finite becomes infinite : convergence properties of vibrational spectra of oligomer chains.** / Chou, Chien Pin; Witek, Henryk; Irle, Stephan.

Research output: Contribution to journal › Article

*Journal of Molecular Modeling*, vol. 24, no. 10, 288. https://doi.org/10.1007/s00894-018-3824-3

}

TY - JOUR

T1 - When finite becomes infinite

T2 - convergence properties of vibrational spectra of oligomer chains

AU - Chou, Chien Pin

AU - Witek, Henryk

AU - Irle, Stephan

PY - 2018/10/1

Y1 - 2018/10/1

N2 - We present a computational study of convergence properties of vibrational IR and Raman spectra for a series of increasingly long units of polyethylene, cis- and trans-polyacetylenes, and polyynes. Convergent behavior to the spectra of infinitely long polymers was observed in all cases when chains reached lengths of approximately 60 carbon atoms, both with respect to the positions of the bands and to their intensities. The vibrational spectra of longer chains are practically indistinguishable. The convergence rate depends on the degree of the π conjugation in a studied system: Vibrational spectra for oligoethylenes converge noticeably faster than the spectra for the conjugated systems. The slowest convergence is observed for skeletal motions of the oligomer chains, which may require more than a hundred carbon atoms in the chain to show deviations smaller than 1 cm−1 to the corresponding solid-state calculations. The results suggest that the boundary between the properties of finite and infinite molecular systems fades away for a surprisingly small number of atoms.

AB - We present a computational study of convergence properties of vibrational IR and Raman spectra for a series of increasingly long units of polyethylene, cis- and trans-polyacetylenes, and polyynes. Convergent behavior to the spectra of infinitely long polymers was observed in all cases when chains reached lengths of approximately 60 carbon atoms, both with respect to the positions of the bands and to their intensities. The vibrational spectra of longer chains are practically indistinguishable. The convergence rate depends on the degree of the π conjugation in a studied system: Vibrational spectra for oligoethylenes converge noticeably faster than the spectra for the conjugated systems. The slowest convergence is observed for skeletal motions of the oligomer chains, which may require more than a hundred carbon atoms in the chain to show deviations smaller than 1 cm−1 to the corresponding solid-state calculations. The results suggest that the boundary between the properties of finite and infinite molecular systems fades away for a surprisingly small number of atoms.

KW - Convergence in size evolution

KW - SCC-DFTB

KW - Simulations of IR and Raman spectra of polymers

UR - http://www.scopus.com/inward/record.url?scp=85053667943&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85053667943&partnerID=8YFLogxK

U2 - 10.1007/s00894-018-3824-3

DO - 10.1007/s00894-018-3824-3

M3 - Article

C2 - 30242484

AN - SCOPUS:85053667943

VL - 24

JO - Journal of Molecular Modeling

JF - Journal of Molecular Modeling

SN - 1610-2940

IS - 10

M1 - 288

ER -