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

A Study by Gas-Phase Electron Diffraction and Theoretical Calculations

Anna Rita Campanelli, Fabio Ramondo, Aldo Domenicano, I. Hargittai

Research output: Contribution to journalArticle

6 Citations (Scopus)

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 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°].

Original languageEnglish
Pages (from-to)29-40
Number of pages12
JournalStructural Chemistry
Volume10
Issue number1
Publication statusPublished - 1999

Fingerprint

Molecular Conformation
Benzene
Molecular Structure
Electron diffraction
Molecular structure
Conformations
molecular structure
electron diffraction
Gases
benzene
Electrons
vapor phases
rings
Mechanics
Molecular mechanics
Bond length
Torsional stress
torsion
Molecules
energy

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

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.

In: Structural Chemistry, Vol. 10, No. 1, 1999, p. 29-40.

Research output: Contribution to journalArticle

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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 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 {\AA}, 〈rg(Si - C)〉 = 1.881 ± 0.004 {\AA}, 〈rg(C - H)〉 = 1.108 ± 0.003 {\AA}, 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°].",
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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

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AU - Domenicano, Aldo

AU - Hargittai, I.

PY - 1999

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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°].

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