The valence shell electron pair repulsion (VSEPR) model-also known as the Gillespie-Nyholm rules-has for many years provided a useful basis for understanding and rationalizing molecular geometry, and because of its simplicity it has gained widespread acceptance as a pedagogical tool. In its original formulation the model was based on the concept that the valence shell electron pairs behave as if they repel each other and thus keep as far apart as possible. But in recent years more emphasis has been placed on the space occupied by a valence shell electron pair, called the domain of the electron pair, and on the relative sizes and shapes of these domains. This reformulated version of the model is simpler to apply, and it shows more clearly that the Pauli principle provides the physical basis of the model. Moreover, Bader and his co-workers' analysis of the electron density distribution of many covalent molecules have shown that the local concentrations of electron density (charge concentrations) in the valence shells of the atoms in a molecule have the same relative locations and sizes as have been assumed for the electron pair domains in the VSEPR model, thus providing further support for the model. This increased understanding of the model has inspired efforts to examine the electron density distribution in molecules that have long been regarded as exceptions to the VSEPR model to try to understand these exceptions better. This work has shown that it is often important to consider not only the relative locations and sizes, but also the shapes, of both bonding and lone pair domains in accounting for the details of molecular geometry. It has also been shown that a basic assumption of the VSEPR model, namely that the core of an atom underlying its valence shell is spherical and has no influence on the geometry of a molecule, is normally valid for the nonmetals but often not valid for the metals, including the transition metals. The cores of polarizable metal atoms may be nonspherical because they include nonbonding electrons or because they are distorted by the ligands, and these nonspherical cores may have an important influence on the geometry of a molecule.

Although the structure of almost any molecule can now be obtained by ab initio calculations chemists still look for simple answers to the question "What determines the geometry of a given molecule?" For this purpose they make use of various models such as the VSEPR model and qualitative quantum mechanical models such as those based on the valence bond theory. The present state of such models, and the support for them provided by recently developed methods for analyzing calculated electron densities, are reviewed and discussed in this tutorial review.

Water soluble poly(p-phenylene) derivatives, poly[2,5-bis(3-sulfonatopropoxy)-1,4-phenylene-alt-1,4-phenylene] sodium salt (PPP−OPSO3) and poly[2,5-bis(3-sulfonatopropoxy)-1,4-phenylene-alt-4,4‘-biphenylene] sodium salt (PPBP−OPSO3), have been synthesized through a Suzuki coupling reaction of 1,4-dibromo-2,5-bis(3-sulfonatopropoxy)benzene sodium salt with 1,4-phenylenediboronic acid or 4,4‘-biphenyldiyldiboronic acid 2,2‘-dimethylpropyl diester using a water soluble Pd(0) catalyst or Pd(OAc)2. The pH dependence of the coupling reaction was investigated and resulted in pH independence at pH levels greater than 10.0. End group analysis of PPP−OPSO3 via 1H NMR of tert-butyl end-capped polymers indicates degrees of polymerization in excess of 40 (ca. 80 rings per chain). Viscometric analysis of PPP−OPSO3 in water shows a behavior comparable to sodium poly(styrenesulfonate) (PSS) of molecular weight 8000. In addition, the polyelectrolyte effect is observed at low polymer concentrations. The λmax of the π → π* absorption for PPP−OPSO3 is found at 339−342 nm, while that of PPBP−OPSO3 shows a bathochromic shift to 349−352 nm. All of the water soluble PPP oligomers and polymers feature strong blue fluorescence. The fluorescence has been characterized by quantum yield and lifetime studies. Nanosecond−microsecond laser flash photolysis experiments indicate that direct excitation of the polymers in the near-UV leads to triplet state formation, albeit with comparatively low efficiency. Multilayered films of PPP−OPSO3 were fabricated with poly(ethyleneimine) (PEI) using layer-by-layer self-assembly and incorporated into blue-light-emitting devices.

We describe the development of Lewis's ideas concerning the chemical bond and in particular the concept of the electron pair bond and the octet rule. We show that the concept of the electron pair bond has endured to the present day and is now understood to be a consequence of the Pauli principle. In contrast the octet rule is now regarded as much less important than was originally generally believed, although Lewis himself knew several exceptions and regarded it as less important than what he called the rule of two (the electron pair). The octet rule was more strongly promoted by Langmuir who is also responsible for the term covalent bond. However, many more exceptions to the octet rules than were known to Lewis are now known and the terms hypervalent and hypovalent used to describe such molecules are no longer particularly useful. Today it is realized that bonding electron pairs in many molecules are not as well localized as Lewis believed, nevertheless resonance structures, i.e., plausible alternative Lewis structures, are still often used to describe such molecules. Moreover electrons are not always found in pairs, as for example in linear molecules, which can, however, be satisfactorily described by Linnett's double quartet theory. The electron density distribution in a molecule can now be analyzed using the ELF and other functions of the electron density to show where electron pairs are most probably to be found in a molecule.

'The Raman spectra of sulphuric acid, deuterosulpliuric acid, chloros~~lphuric acid, and methar~es~~lphonic acid have been redetermined, and the spectrum of fluoros~~lphuric acid measured for the first time. The spectra of fluorosulphuric acid -sulphuric acid, cl~lorosulphuric acid -sulplluric acid, and fluorosulphuric acid -arsenic trifluoride luistures have also bee11 measured. The spectra of the hydrogen sulphate, dcutero sulphate, fluorosulphate, chloros u l p h a t~, and ~~~e t l~a n e s~r l p h o n a t e anions have been obtained in various solvents. Frequencies are asslgned to the normal modes of vibration of all these acids ant1 their allions ant1 the assigr~ments are compared with those of previous worlcers.During the investigation of the Raman spectra of the systems H2S0,-SO?, D2SO4-SOa, HSOaF-S03, and HS03C1-SO3 reported in the following papers it became clear that the spectra of the acids H2S04, D2SO1, HS03F, HS03C1 and their anions were not all lcnown with certainty, either because all the expected lines had not been observed or because no complete assigninent of the observed frequencies to the normal inodes of vibration had been made. Therefore the Raman spectra of these acids and their anions, and the spectra of metl~anes~~lpl~onic acid and the metl~anesulphonate ion, were redetermined and in each case the observed frequencies have been assigned to the ilormal inocles of vibration. SULPHURIC i\CID A N D DEUTEROSULPHUIZIC i\CtDThe results of our ineasurernents of the Rainan spectra of H2SO4 and D2SO4 and the infrarecl spectrulll of H2SO.I i l l the sodiuill chloride region are given in Table I together with the results of some of the earlier workers (1-5). Our Raman spectra are also shown in Fig. 1. Table I does not include the results of all the previous studies since there is generally good ageelllent between the various sets of lneasurenlents and much of the earlier work has recently been discussed by Walrafen and Dodd (2) and by Gigu&re and Savoie (4). Soine workers have observed additional Rainan shifts a t 740 and 1050 cm-I and additiollal weal; bands in the infrared a t 332, 675, 1050, and 1240 cm-I that we did not observe in our Ralllail spectrum of H?SOI. Also, lines a t 711 and 1050 cm-I and a t 305 and 475 cm-I have been reported in the Ralnall ancl infrared spectra of D2S04 that were not observed by us.The 1050 cm-1 line, which increases in illtellsity on addition of water to sulphuric acid, is undoubtedly due to the I-ISO1-ion. GiguPre and Savoie (4) observed this line in their infrared spectruin of sulphuric acid and claillled that it was due to HS04-ions forillecl in the autoprotolysis of sulphuric acid,

Using a set of aromatic nitro compound indicators, the Hammett acidity function, Ho, has been determined for the systems H2S04-S03 (up to a composition of 75 mol % S03), H2S04-HS03F, H2S04-HS03C1, and (1963).

We have calculated the electron density distributions, electron densities at the bond critical point, and atomic charges in the period 2 and 3 fluorides and a number of their cations and anions. On the basis of this information and an analysis of X-F bond lengths, we have examined the factors that determine the lengths of these bonds. We have shown that all the molecules except NF(3), OF(2), and F(2) have considerable ionic character. The bond lengths of the fluorides reach a minimum value at BF(3) in period 2 and at SiF(4) in period 3 when the product of the charges on the central atom and a fluorine reaches a maximum, consistent with a predominately ionic model for these fluorides. The length of a given A-F bond decreases with decreasing coordination number, and we show that it is determined primarily by packing considerations. This provides an alternative to the previously proposed back-bonding model explanation, for which our work provides no convincing evidence. There is also no evidence to support the Schomaker-Stevenson equation which has been widely used to correct A-F bond lengths calculated from the sum of the covalent radii of A and F for the difference in the electronegativities of A and F. We propose a new value for the covalent radius of fluorine and point out the limitations of its use.

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