B-H bond distance

The shorter B—H bond distances for the X-ray diffraction data are ascribed to thermal vibration and rigid rotation of the molecule in the crystal98 . 

The molecular structure, B-H bond distance and bond angle were obtained from Lynds and Bass (J ). The B-Br bond distance Is taken from that In BBrg(g) molecule. The vibrational frequencies adopted and corrected to the average Isotopic species were assigned to Brleux de Mandlrola and Westerkamp (2) from Infrared spectrum. The values In brackets are calculated by the Wilson s... 

FIGURE 23.2 Profiles of (a) energy and FF values for nucleophilic attack and (b) for electrophilic attack for distortion in the N-H bond distance of ammonia. 

Unpronounced polarization of regular C-H bonds such as methyl or ethyl groups does not lead us to expect the formation of dihydrogen bonds. Actually, an analysis of the Cambridge Structure Database [2] reveals numerous C-H- - -H-C contacts with H- - -H distances of 2.500 A or even more where the angular distribution is ideally isotropic. The latter is typical of nondirec-tional van der Waals interactions. The C-H and B-H bonds in the molecule l,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene- BH3 are obviously more... 

On the one hand, x-ray crystallography data could support this idea because the heterocycles are coplanar relative to the B-H bonds. Moreover, in the case of the compound in Strncture 5.5, this conformation remains in solution, according to NOE NMR measnrements. On the other hand, the solid-state CH- -HB distances are too long (< 2.65 A), and the HF/6-31G calculations carried ont for the componnd have shown, that where the heterocyclic rings are oriented orthogonally, the conformation is more stable. Thns, this question is still open. It is quite probable that the interaction between protonic hydrogens on the a-carbons and hydridic BH hydrogens is electrostatically attractive. 

Secondary Oxonium Ions [RR OH ]. 1H NMR and IR investigation75 of protonated ether salts (hexachloroantimonates) in dichloromethane solutions showed the formation of both (a) dialkyloxonium ions in which the proton is bound to only one oxygen (21) and (b) bidentate complexes in which the proton is shared between two ether molecules (22). Structural analysis of such a bidentate complex of diethyl ether with a complex anion shows a broad H+ resonance at 81 H 16.3 and unequal O—H bond distances (1.39 and 1.11 A).76... 

Figure 2 CSA tensor (a) variation with HPH bond angle (0hpH) in PH3 a) geometry optimization of P-H bond distance performed at each new configuration, b) P-H bond distance fixed at experimental equilibrium distance of 1.415 A.

Manz and Romelt (1981). Rm and 7 hi are the two I-H bond distances. The heavy point marks the saddle point and the shaded area indicates schematically the Franck-Condon region in the photodetachment experiment. The arrow along the symmetric stretch coordinate (f Hi = -Rih) illustrates the early motion of the wavepacket and the two heavy arrows manifest dissociation into the two identical product channels, (b) The same PES as in (a) but represented in terms of hyperspherical coordinates (p, i9) defined in (7.33). The horizontal and the vertical arrows illustrate symmetric and anti-symmetric stretch motions, respectively, as indicated by the two insets. 

Finally, mention should be made of two compounds, Mo(CO)2 (tj 3 -C3 Hs )-[H2B(Me2pz)2] 122> (Fig. 26) and Mo(CO)2(u3-C7H7)[H2B(Me2pz)2] 123>, in which one of the B—H bonds is placed within interaction distance from the Mo atom as a stereochemical consequence of the conformation of the bis(pyrazolyl)-borato ligand. This is reminiscent of a number of similar molecules having C—H... M interactions that will be discussed in Sect. C. III. 

Fig. 26. The central portion of Mo(CO)2-(C3H5)[H2B(Me2pz)2]. The conformation of the bis(pyrazolyl)borato ligand places one of the B-H bonds within interaction distance of the molybdenum atom (Ref. 122)...

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