### Abstract

The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two Si - Me bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two Si - Me bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3-0.5 kJ mol^{-1}. The calculated lengths of the Si - Me and Si - Ph bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either Si - Me or Si - Ph bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe_{3} groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1 : 1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe_{3} groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are 〈r_{g}(C - C)〉 = 1.407 ± 0.003 Å, 〈r_{g}(Si - C)〉 = 1.881 ± 0.004 Å, 〈r_{g}(C - H)〉 = 1.108 ± 0.003 Å, and LC_{ortho} - C_{ipso} - C_{ortho} = 117.2 ± 0.3°. While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9°; MP2(f.c.), 117.1°; B3LYP, 116.9°].

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

Pages (from-to) | 29-40 |

Number of pages | 12 |

Journal | Structural Chemistry |

Volume | 10 |

Issue number | 1 |

Publication status | Published - 1999 |

### Fingerprint

### Keywords

- Benzene ring deformation
- Molecular structure
- p-Bis(trimethylsilyl)benzene; gas-phase electron diffraction; ab initio MO calculations

### ASJC Scopus subject areas

- Structural Biology
- Chemistry(all)

### Cite this

*Structural Chemistry*,

*10*(1), 29-40.

**Molecular Structure and Conformation of p-Bis(trimethylsilyl)benzene : A Study by Gas-Phase Electron Diffraction and Theoretical Calculations.** / Campanelli, Anna Rita; Ramondo, Fabio; Domenicano, Aldo; Hargittai, I.

Research output: Contribution to journal › Article

*Structural Chemistry*, vol. 10, no. 1, pp. 29-40.

}

TY - JOUR

T1 - Molecular Structure and Conformation of p-Bis(trimethylsilyl)benzene

T2 - A Study by Gas-Phase Electron Diffraction and Theoretical Calculations

AU - Campanelli, Anna Rita

AU - Ramondo, Fabio

AU - Domenicano, Aldo

AU - Hargittai, I.

PY - 1999

Y1 - 1999

N2 - The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two Si - Me bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two Si - Me bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3-0.5 kJ mol-1. The calculated lengths of the Si - Me and Si - Ph bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either Si - Me or Si - Ph bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe3 groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1 : 1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe3 groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are 〈rg(C - C)〉 = 1.407 ± 0.003 Å, 〈rg(Si - C)〉 = 1.881 ± 0.004 Å, 〈rg(C - H)〉 = 1.108 ± 0.003 Å, and LCortho - Cipso - Cortho = 117.2 ± 0.3°. While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9°; MP2(f.c.), 117.1°; B3LYP, 116.9°].

AB - The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two Si - Me bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two Si - Me bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3-0.5 kJ mol-1. The calculated lengths of the Si - Me and Si - Ph bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either Si - Me or Si - Ph bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe3 groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1 : 1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe3 groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are 〈rg(C - C)〉 = 1.407 ± 0.003 Å, 〈rg(Si - C)〉 = 1.881 ± 0.004 Å, 〈rg(C - H)〉 = 1.108 ± 0.003 Å, and LCortho - Cipso - Cortho = 117.2 ± 0.3°. While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9°; MP2(f.c.), 117.1°; B3LYP, 116.9°].

KW - Benzene ring deformation

KW - Molecular structure

KW - p-Bis(trimethylsilyl)benzene; gas-phase electron diffraction; ab initio MO calculations

UR - http://www.scopus.com/inward/record.url?scp=0002246602&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0002246602&partnerID=8YFLogxK

M3 - Article

VL - 10

SP - 29

EP - 40

JO - Structural Chemistry

JF - Structural Chemistry

SN - 1040-0400

IS - 1

ER -