Bond length Bartell

In an early electron diffraction investigation of the structure of 2-methylpropene (isobutylene), Bartell and Bonham (1960) found that the three terminal carbon atoms are arranged in an almost perfect equilateral triangle around the central carbon despite the considerable difference in the single and double bond lengths (Figure 5.4). This result led Bartell to suggest that the terminal carbon atoms are close-packed around the central carbon atom. He then... 

Bartell and coworkers investigated the structures of a series of noncyclic alkanes by means of gas electron diffraction (14, 44, 45) and invoked for the interpretation of their results a simple force field which contained to a high extent vibrational spectroscopic constants of Snyder and Schachtschneider. This force field reproduces bond lengths and bond angles of acyclic hydrocarbons well, energies of isomerisation satisfactorily. As an example, Fig. 8 shows geometry parameters of tri-t-butylmethane as observed by electron diffraction and calculated with this force field (14). 

For the discussion of relationships between bond character and bond length see also Bartell, L. S. Tetrahedron 17, 1177 (1962) Szabd. Z. G.. Konkoly Thege, 1. Acta Chim. Acad. Hung. Tom. 86, 127 (1975)... 

Figure 3-32. Geometrical consequences of nonbonded interactions after L. S. Bartell [85], (a) The three outer carbon atoms of H2C=C(CH3)2 are in the comers of an approximately equilateral triangle, leading to a relaxation of the bond angle between the ethyl groups (b) Considerations of nonbonded interactions in the interpretation of the C-C single bond length changes in a series of molecules.

Fig. 5.1. Experimental P-O bond length versus bond overlap population calculated using extended Huckel molecular-orbital theory. See Bartell et al. (1970) for further details (after Bartell et al., 1970 reproduced with the publisher s permission).

The molecular structure and bond lengths given above for PF were recently determined by Hansen and Bartell (4) from electron-diffraction studies. These results Indicate that PF has a trigonal bipyramid structure with nonequivalent axial and equatorial bonds. Wyatt et al. (5) determined rotational constants for PF from a study of the infrared vibration-rotation band -1 ... 

A systematic study of the series (CH3) PFs n [ = 0,1,2,3] has been recently completed by Bartell and co-workers ° The observed variations in the bond distance of these trigonal bipyramidal compounds are well correlated with the number of methyl substituents. In all cases the least electronegative ligands (or CH3) occupy equatorii sites. The sterochemistry and trends in structure parameters are well accounted for by the VSEPR theory. Furthermore, the increase of the axial P-F bond lengths in this series correlates well with the increase in P—F amplitudes of vibration. The methyl groups essentially rotate freely. 

This work is more concerned with a fourth use of apparent atomic valences. The interest is in compounds in which there is no ambiguity concerning true valences and whose structures are (presumably) not in question. It is then asked why the apparent valence differs from the true valence on the occasions when it does so significantly. The hypothesis, clearly anticipated by Zachariasen [13] and, for molecules, by Bartell [41], is that bond lengths are influenced not only by valence but also by next-nearest ( non-bonded ) interactions [42]. 

Bond lengths, amplitndes of vibration, and asymmetries in the distribntion of nonbonded atom pairs were determined by Bartell et al. at temperatnres ranging from 300 to 1200 K the effect of thermal excitation was analyzed to extract iirformation abont anharmonicity. The nozzle temperatnre was 15 °C. 

Bartell, L. S., L. S. Su, and H. You (1970). Lengths of phosphorous-oxygen and sulfur-oxygen bonds. An extended Huckel molecular orbital examination of Cruickshanks dTr-dit picture. Inorg. Chem. 9, 1903-12. 

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