Crystal structure correlations

A crystallographic scale of acidity has been developed. Measuring the mean C—H O distances in crystal structures correlated well with conventional P a(DMSO) values. An ab initio study was able to correlate ring strain in strained hydrocarbons with hydrogen-bond acidity. ... 

After the first report on the crystal structure correlation between 2,5-DSP and poly-2,5-DSP crystals, a different crystallographic result was reported on a poly-2,5-DSP crystal (Meyer et al., 1978). It was reconfirmed, however, that the first structural analysis was correct (Nakanishi et al., 1979a). 2,5-DSP, crystallized from benzene solution, is highly photoreactive (a-form), while the same compound, sublimed at a rather high temperature... 

Essentials of crystallography 88 Crystal structure correlations 89 Transition states 92... 

Concerted bond-forming/bond-breaking processes at tetrahedral carbon (the familiar SN2 reaction) are not easily studied by the crystal structure correlation method. The preferred approach of a nucleophile is sterically more encumbered than the approach to a singly or doubly bonded centre, and the transition states involved are generally of high energy. Intramolecular displacements, such as those described on pages 117-118, are a possible way round this problem, but no systematic study is available. 

Crystal structure correlations explore relationships between reactivity and actual structural parameters such as bond lengths and angles. Reactivity measured in terms of rate and equilibrium constants allows us to introduce the energy dimension directly, because these constants translate directly into... 

Conformational changes frequently involve free energy barriers small enough to allow structures spread across the whole reaction coordinate to be observed in a series of crystal structures (see Section 2). Rate constants are also readily obtained by suitable NMR experiments, so this is an attractive area for the examination of possible relationships between crystal structure correlations and free energy relationships. In fact not a great deal of work has been done, though more can confidently be expected. 

There are no known exceptions to rule 2, though many fewer data are available. The sensitivity parameter is by definition obtainable only where the linear bond length-reactivity relationship is observed, so exceptions are in any case less likely. It is not readily accessible — for accurate definition it requires good quality structures for a series of at least four to five derivatives — so any use outside the area of crystal-structure correlation is likely to be limited to situations where a particularly important question of mechanism or reactivity cannot be resolved by conventional approaches.21... 

The most famous example of the crystal structure correlating with the average number of valence electrons per atom or band filling, N, is the Hume-Rothery alloy system of noble metals with sp-valent elements, such as Zn, Al, Si, Ge, and Sn. Assuming that Cu and Ag have a valence of 1, then the fee -phase is found to extend to a value of N around 1.38, the bcc / -phase to be stabilized around 1.48, the -phase around 1.62, and the hep e-phase around 1.75, as illustrated for the specific case of Cu-Zn alloys in Fig. 6.15. In 1936 Mott and Jones pointed out that the fee and bcc electron per atom ratios correlate with the number of electrons required for a free-electron Fermi sphere first to make contact with the fee and bcc Brillouin zone faces. The corresponding values of the Fermi vector, fcF, are given by... 

Evidence for Resonance-Assisted Hydrogen Bonding from Crystal Structure Correlations on the Enol Form of the P-Diketone Fragment. 

Gilli, R, Bertolasi, V., Ferretti, V., and Gilli, G., Covalent nature of the strong homonuclear hydrogen bond. Study of the 0—H"0 system by crystal structure correlation methods, J. Am. Chem. Soc. 116, 909-915 (1994). 

Fig. 7.6 Crystal structures correlated with atomic charge (slope of lines) for lA-VIIA (1-17), IIA-VIA (2-16), anJ IIIA-VA (13-15) compounds. The abscissa is the diflerence in electronegativity. Ax ( a "ii>- tttid Ihe ordinate is the sum of charge coellicicnts (tj = 6). Note that the charge. 5 (Ua ii)/(6a + 6h). is approximately twice the reciprocal of the slopes as plotted here. [Modified from Shankar. S. Parr. R. G. Proc. Nall. Acad. Sci. U. S. A. 1985, 82, 264-266. Reproduced with permission. ... 

G. Gilli, F. Belluci, V. Ferretti and V. Bertolasi, Evidence for resonance-assisted hydrogen bonding from crystal-structure correlations on the enol form of the beta-diketone fragment, J. Am. Chem. Soc. Ill, 1023-1028. (1989). 

---