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Combined Electron-Diffraction and Spectroscopic Determination of the Structure of Spiropentane, CH 5
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Jason W. Sandwisch, Blake A. Erickson, Kenneth Hedberg, and Joseph W. Nibler J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 14, 2017
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The Journal of Physical Chemistry
Combined Electron-Diffraction and Spectroscopic Determination of the Structure of Spiropentane, C5H8 Jason W. Sandwisch, Blake A. Erickson, Kenneth Hedberg, and Joseph W. Nibler* Department of Chemistry, Oregon State University, Corvallis, OR, 97332-4003
Number of text pages: 18 Number of Tables: 4 Number of Figures: 4
* Corresponding author. E-mail address:
[email protected] FAX: +1 541 737 2062
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ABSTRACT:
Gas phase electron-diffraction (GED) data have been combined with recent
spectroscopic rotational constants to determine the rα0 structural parameters for spiropentane, C5H8. The structure has D2d symmetry, and the results yield values of 1.105(2) Å for the CH bond length, 1.557(3) Å for the distal CC bond length, and a smaller value of 1.482(1) Å for the four lateral CC bonds that connect to the central carbon atom. The HCH angle is 113.7(13)˚, and the HCH flap angle, defined as the angle of the HCH bisector and the distal CC bond, is 150.2(16)˚. Corresponding rg values are 1.122(2) Å, 1.560(3) Å, 1.485(1) Å, 115.1(13)˚ and 148.9 (16)˚. The results are in good accord with values from density functional calculations (B3LYP/cc-pVTZ) and resolve some questions about the structure reported in an earlier GED study, in particular about the HCH angle and anomalous rotational constants calculated for the structure.
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The Journal of Physical Chemistry
INTRODUCTION In recent papers we have reported the results of high-resolution infrared studies of several interesting carbon ring compounds of high symmetry, including radialene,1 propellane,2-4 bicyclopentane,5-6 and spiropentane7-9 (Figure 1).
Figure 1. Structures of some small ring molecules of high symmetry In each case, the molecules are of interest to structural chemists because of their high ring strain and, in the case of propellane, because of the very unusual axial bond. All molecules are small symmetric tops but none had previously been examined at a spectroscopic resolution sufficient to determine accurate rotational and other rovibrational constants. This deficiency may be because none are available from commercial sources, but, using published synthesis methods, we prepared small amounts of each compound of sufficient purity to yield excellent high-resolution infrared spectra. The analyses of these was aided greatly by initial estimates of the rovibrational parameters obtained from anharmonic potential energy surfaces calculated by quantum programs such as Gaussian.10 In particular, it was determined that the Gaussian B3LYP density functional (DFT) method with a cc-pVTZ basis set gives quite good structural and rovibrational parameters at a reasonable investment of computer time. In the present case of spiropentane, C5H8, both the normal and per-deutero isotopomers 3 ACS Paragon Plus Environment
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were examined and very accurate B0 values were determined, 0.1394741(1)7,8 and 0.1120700(9)9 cm-1, respectively. For both isotopomers, the experimental values fall sensibly between the DFT values of Be and B0 and agree with the latter within 0.6%.
However, we noted that the
experimental value of B0 for C5H8 is significantly smaller (1.68%) than a value of B = 0.1418 cm-1 calculated from a gas-phase electron-diffraction (GED) determination11 of the thermal average structure. Similarly, for C5D8, our B0 value is comparably smaller (1.70%) than a value of 0.1140 cm-1 calculated from the electron-diffraction structure.
Since the latter includes
contributions from all thermally populated states, it would be expected that the B value deduced from the ED structure would actually be smaller, not larger, than B0. In addition, several other authors12-14 have raised questions about the electron-diffraction results, particularly the HCH angle, 118.4(9)˚, which is 4˚ larger than the Gaussian prediction of 114.4 ˚ and is also larger than a value of 115.0(7)˚ deduced from NMR measurements.13 As mentioned in our earlier paper,8 we suspect a scaling error in the electron-diffraction measurements of Ref. 11. Concern about such errors recently motivated us to reexamine the data analysis and calibration methods for electron-diffraction measurements done in our laboratory, resulting in several improvements, which will be reported elsewhere.15 These changes have been implemented in the combined electron-diffraction and spectroscopic redetermination of the structure of spiropentane reported here. EXPERIMENTAL ASPECTS Synthesis.
Spiropentane was prepared by zinc reduction of pentaerythrityl
tetrabromide, C(CH2Br)4, using the method of Applequist, Fanta, and Henrikson;16 details are given in our earlier paper.7 We note that our yields were low in this synthesis (
