Bicyclo[2.1.1]hex-2-ene

Anthony Linares

 

 

 

 

 

 

 

 

 

Summary

 

Interested in understanding the chemical and physical properties of the rigid, strained hydrocarbon, Bicyclo[2.1.1]hex-2-ene, Zebelman and Bauer analyzed the structure via electron diffraction1.  The theoretical and reduced experimental intensity curves and the final radial distribution curve (RDC) provide information regarding the proposed interatomic distances.  The group assumed C2v symmetry to calculate the geometrical parameters, which were refined using a least squares analysis of the reduced intensities.  The C-H bond lengths were not varied during the analysis; instead, they were estimated from the final RDC.  For the limited data concerning bond angles, there are error limits as large as 3.8°, indicative of the restrictions of the experimental technique.

 

Concerned that the least-squares analysis of the electron diffraction study of Bicyclo[2.1.1]hex-2-ene converged to a false structure, Wang and Harmony observed four isotopic species of the molecule using microwave spectroscopy2.  Implementing Costain and Kraitchman’s equations, an rs structure for the carbon skeleton was obtained.  The rotational constants were determined using the rigid rotor approximation, which models rigid polycyclic molecules fairly well (Table 2).  Wang and Harmony note the exclusion of vibrational-rotational interactions.  These interactions are expected to exceed the pure experimental errors.  When comparing the MW structure with the ED structure, Wang and Harmony found that the <C1 C2 C3 angle, the dihedral angle θ, and the nonbonded C5… C6 from the ED study differ significantly from the experimental MW uncertainties (Table 1).  They propose that the radial distribution function and ED data are “incapable of an unambiguous resolution of the molecular structure.” 

 

An energy calculation using the Merck Molecular Force Field (MMFF94) was conducted to test the reliability of the MW and ED structures.  The MW structure was found to be more energetically stable (62.10110 kcal/mol) when compared with the ED structure (98.01748 kcal/mol).

 

 

 

 

 

References

 

1.   D. L. Zebelman and S. H. Bauer, “Structures of Strained Polycyclics: Bond Ditances and Angles in Tricyclo[3.3.0.02,6]Oct-3ene and in Bicyclo[2.1.1]hex-2-ene”, Tetrahedron., 28, 2727-2740 (1972).

2.   C.S. Wang and M. D. Harmony, “Microwave Spectrum, Structure, and Dipole Moment of [2.1.1]hex-2-ene”, J. of A.C.S., 98, 1108-1111 (1976).

 

 

Comparison of structures generated by various means

 

- Microwave structure of Wang and Harmony

  Brookhaven pdb file

They derived C2v symmetry from the relative intensities (application of the Pauli Exclusion Principle)and ignored vibrational-rotational interactions.  Rs structure only provided for carbon skelton, thus, locations of hydrogens were based on angles determined from ED study and optimal geometry.

 

- Electron diffraction structure of Zebelman and Bauer

  Brookhaven pdb file

They assumed C2v symmetry and C-H bond lengths.  Error limits for bond lengths and bond angles are larger than that of the MW experiment.  The bond angles, dihedral angle θ, and the nonbonded C5… C6 from the ED stucture differ significantly from MW study. 

 

- Structure using molecular mechanics with MMFF94 force field

  Brookhaven pdb file

C1-C2 and C1-C5 bond distances differ from the MW and ED structures by ~0.02 -0.03 A each.  The bond angles are within reasonable agreement of the MW results.  In a striking reversal, the MMFF yields poorer results for the  bond angles than the Class I Tripos force field.

 

- Structure using molecular mechanics with Tripos force field

  Brookhaven pdb file

The calculations underestimate the C1-C2 and C1-C5 bond distances of both the MW and ED structure.  Bond angles support MW structure.

 

- Structure from semi-empirical quantum mechanics and the AM1 Hamiltonian

  Brookhaven pdb file

C-C single and double bonds are over estimated by ~0.01-0.02 A each.  The bond angles are in good agreement with the MW values.

 

- Structure from ab initio quantum mechanics, Hartree-Fock with 3-21G basis set

  Brookhaven pdb file

C-C single bond are over estimated and the C=C double bond is underestimated. Interestingly, this calculation has the best agreement for the bond angles and dihedral angle when compared with the MW study.

 

Table 1: Tabular Results of Structural Parameters from Various Methods

 

 

MW

ED

MMFF94

Tripos

SE-AM1

HF 3-21G

Distance(C1,C2)

1.52782568

1.53699369

1.49440642

1.50534657

1.53555031

1.54668891

Distance(C1,C5)

1.56796073

1.56445573

1.58864177

1.54960672

1.58010668

1.58005994

Distance(C2,C3)

1.3408

1.332

1.34145985

1.33430659

1.36158205

1.32312253

Distance(C5...C6)

2.14885159

1.826

2.09070449

2.15916627

2.15279861

2.14885159

Angle(C1,C2,C3)

103.324052

108.401943

102.937673

102.964144

103.522341

103.890905

Angle(C5,C1,C2)

100.391372

95.3700153

104.837462

98.8301495

99.0887022

99.5236835

Angle(C5,C1,C6)

85.3276533

71.4068206

82.2975258

88.3228097

85.8780093

85.6858065

Angle(C1,C5,C4)

81.4034049

94.7576652

78.5160143

80.851829

82.3075671

81.6418963

Dihedral(C6,C1,C4,C5)

126.740563

119.046425

116.383388

132.464747

129.576234

127.93877

 

 

 

 

Table 2: Rotational Constants and Moment of Inertia in MHz (Wang and Harmony)