Isoelectronic molecules

The cyclic conjugation and orbital phase are both continuous in benzene. Electrons delocalize in a cyclic manner. The cyclic conjugation is discontinuous in borazine in Scheme 33). Electrons cannot delocalize in a cyclic manner, but only between the neighboring pairs of donors and acceptors. There arises a fundamental question how electrons delocalize in the isoelectronic molecules where C=C bonds are replaced with N-B bonds. 

Scheme 34 Cyclic conjugations in benzene and its isoelectronic molecules containing B-N...

A nitrogen atom can form four bonds only if it loses an electron to become N+ so that it is then isoelectronic with a carbon atom. Isoelectronic atoms or molecules have the same number of valence electrons, arranged in the same way. Thus B, C, and N + are isoelectronic atoms and can each form four bonds. Some examples of isoelectronic molecules are illustrated in Figure 1.15. 

Within this context carbon monoxide is not the inert molecule so frequently depicted on the basis of its formal triple bond and the remarkable similarity of its physical properties to those of the isoelectronic molecule dinitrogen. (Indeed, if it were, atmospheric carbon monoxide would present no hazard ) It is, in fact, a fairly readily activated molecule the industrial process for the production of methyl formate (1) is well known, but it is less widely appreciated that this process is an example of a homogeneous, selective, base-catalyzed, activation of carbon monoxide which has for its net chemistry... 

The physical, and often the chemical, properties of isoelectronic molecules are closely related (23). The isoelectronic principle can be extended to a comparison of fluoro with isostructural hydroxo com-... 

Results of ab initio calculations of the 2-propyl cation, as well as the isoelectronic molecule, dimethyl borane, are shown in Table 18. As can be seen, the Cee conformation is predicted to be more stable than the conformation in both the 2-propyl cation and dimethylborane. However, the relative energies of the Cse and Cee conformations are reversed when the basis set is changed from STO—3G to 4—31G. 

Another situation where the aforementioned condition is met is along a series of H2 A isoelectronic molecules where A varies along a row of the Periodic Table. Here, Hpq remains roughly constant and, thus, the bending force increases as AEpq decreases, a trend which obtains as the lone pair ionization potential of the central atom decreases. Typical data are shown in Scheme 8. 

The hydrogen affinity HA (M ) is simply the H—bond strength 7) (H—M ). A summary of the hydrogen affinities for phosphine and some isoelectronic molecules is given in Table 6. 

Table 6. Bond strengths in PH3, PH4 and isoelectronic molecules and ions )...

Eq. (4) to the alkali fluorides in Table 3. While these agreements for such simphfied treatment might be more or less fortuitous, the reason why the calculated p is larger for SrO than for BaO should be analogous to the case of the isoelectronic molecules RbF and CsF or to the comparison of KF with CsF in Table 4 a for Ba + is 1.82 times larger than for Sr2+ while is only 1.03 times larger for BaO than for SrO. 

Electron density surfaces can also be used to uncover trends and build qualitative descriptions. For example, size surfaces for the isoelectronic molecules, methyl anion, ammonia and hydronium cation show a marked decrease in overall size. 

The feasibility of reactions similar to Reaction 1, but involving attack on hydrogen bonded to an atom other than carbon, depends initially on the over-all energetics of the process. Since D(ROO—H) seems to be approximately 90 kcal. per mole, regardless of R, a bond strength for hydrogen of this order or lower-becomes a requirement for fast reaction. Table I lists some bond dissociation values for isoelectronic molecules. 

The photoelectron spectra of the isoelectronic molecules N2 and CO have peaks corresponding to the ionization energies, 7, given in Table 4.3. The peaks corresponding to the ionized molecules have vibrational structure, with vibrational separations in wavenumbers given in Table 4.3. The fundamental vibration frequencies of the neutral molecules N2 and CO are 2345 and 2143 cm1, respectively. 

HAAH molecules containing 10 valency electrons should be linear in their ground states. Those containing 12 electrons should be bent but planar cis- and /raMs-forms). Those containing 14 electrons should be bent and non-planar. The spectra of the isoelectronic molecules HCN and C2H2 are particularly discussed the first excited state of each should be non-linear. 

The isoelectronic molecules N2 and CO are conveniently discussed together. Comprehensive investigations of N2 and CO have been made using both the (e-ion)25 and (e,2e)171 methods to probe the broad spectral features of the oscillator-strength distributions for both ion formation and... 

The VSEPR theory has its roots in the observation prior to 1940 that isoelectronic molecules or polyatomic ions usually adopt the same shape. Thus BF3, B03 C03, COF2 and NO3 are ail isoelectronic, and they all have planar triangular structures. As developed in more recent years, the VSEPR theory rationalises molecular shapes in terms of repulsions between electron pairs, bonding and nonbonding. It is assumed that the reader is familiar with the rudiments of the theory excellent expositions are to be found in most inorganic texts. 

Isoelectronic Molecules or ions which have the same number of electrons, e.g. CO and N2. 

Rabalais, J.W., McDonald, J.M., Scherr, V., and McGlynn, S.P. (1971). Electronic spectroscopy of isoelectronic molecules. II Linear triatomic groupings containing sixteen valence electrons, Chem. Rev. 71, 73-108. 

The dicyanophosphide ion is bent, as is the isoelectronic molecule S(CN)2< A transfer of charge into the cyano groups is expected from charge density calculation and is demonstrated by N-coordination to the cation in the crystal (Figure 1). It is also in accord with the observed wider CPC-angle (95 ) and shorter PC-distance (170 pm) and the lower CN and higher PC stretching fre-... 

Fig. 49. Comparison of HF MO and experimental valence energy levels for the isoelectronic molecules N2 and C2H2...

Two isoelectronic molecules that have recently been studied spectroscopically have been the subject of near-HF calculations. HCP is the phosphorus analogue of HCN, and a study of the computed wavefunction407 indicates the existence of a G P bond this is the only known example of this bond. A variety of properties... 

Most of the studies about ESR parameters deal with molecules possessing one unpaired electron, i.e. S=l/2. Investigations on systems with S=1 are rare. To show the ability of the MRD-CI/Bk method we want to discuss the hyperfine structure of the isoelectronic molecules CCO, CNN and NCN [64]. 

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