Book Reviews

Unsigned book reviews are by the Book Review Editor.     

Theoretical study of isomerism in the molecules N2O,PNO, P2O,N2S,PNS, and P2S

We have carried out nonempirical calculations of the potential surface for isomeric rearrangements of the molecules N₂O, N₂S, PNO, PNS, P₂O, and P₂S. It was found that for the molecules N₂O and N₂S a linear structure is considerably more favorable than a cyclic one, which lies 60 kcal·mole^–1 higher and has low stability. For P₂O and P₂S the linear and cyclic isomers have similar energies. For PNO and PNS there are two linear isomers and one cyclic isomer. The isomers are separated by appreciable barriers and can exist independently. It is predicted for the ABC molecules with 16 valence electrons that if two or all three of the atoms belong to the third or a later period, then the cyclic isomers should be favored to at least the same extent as the linear isomers.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 794–802, April, 1990.     

Ab initio investigations of structure and stability of the LiBeF3 molecule

Ab initio calculations of the potential energy surface of LiBeF₃ have been performed using the basis set of Roos and Siegbahn. The extremum and saddle points were made more precise with Huzinaga-Dunning basis sets in double-and triple-zeta contractions The “bidentate” structure (symmetry group C_2v) is found to have the lowest energy and is much more advantageous than the others, and the LiBeF₃ molecule turns out to be rigid with respect to migration of the cation around the anion. The calculated internuclear distances and the energy of complex formation are in agreement with experimental values within 0.03 Å and 2 kcal/mole. The results are compared with similar ab initio data for LiBeH₃ and LiNO₃.     

A reinvestigation of quaternary layered bismuth oxyhalides of the Sillén X1 type

A family of layered bismuth oxyhalides, L^I_0.5Bi_1.5O₂X and L^IIBiO₂X has been reinvestigated. Formation of X1-type Sillén compounds has been established for L^I=Li, Na, L^II=Ca, Sr, Ba, and X=Cl, Br, I, but the details of their crystal structures are different. While all L^I_0.5Bi_1.5O₂X, CaBiO₂Br, and CaBiO₂I adopt the disordered tetragonal Nd₂O₂Te structure, all compounds of L^II=Sr and Ba are orthorhombic and isostructural to PbSbO₂Cl, due to L/Bi cation ordering. Crystal structures have been determined for CaBiO₂I, SrBiO₂Br, SrBiO₂I, and BaBiO₂I. We discuss the factors which determine the occurrence and type of cation ordering in the quaternary bismuth and antimony X1-type oxyhalides. We also predict that more isostructural compounds can be prepared with antimony.     

Theoretical calculations of the vibrational spectra of the complex LibeF3 molecule

Ab initio calculations of the structure, force field, frequencies and intensities of normal modes of the LiBeF₃ molecule have been carried out, using basis sets of Huzinaga and Dunning in a double-zeta contraction. The calculated results are compared with the infrared spectrum of LiBeF₃ in an inert matrix obtained by Snelson et al.     

Theoretical investigation of the structure and stability of salt molecules LiOF,LiOCl, LiSF,LiSCl and ion-molecular complexes Li+·F2, Li+·FCl,and Li+·Cl2

Institute of New Chemical Problems, Russian Academy of Sciences, Chernogolovka. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 1, pp. 3–9, January–February, 1992.     

Theoretical study of “deep” fragmentation of hemin ion with successive loss of methyl and vinyl groups

The electronic and geometric structures, energy stability, normal mode frequencies, and spin density distribution have been calculated by the density functional theory B3LYP method with the Gen = 6-31+G*(Fe) + 6-31G(C,H,N,O), 6-31G*, and 6-311++G** basis sets for the deep fragmentation products of the free hemin ion with successive removal of methyl and vinyl groups in the electronic states with different multiplicities. The computation results are compared with the available experimental data and previous computation results for the fragmentation products of the isolated heme molecule and hemin ion with removal of carboxymethyl groups. The trends in the behavior of these properties are analyzed as a function of multiplicity, external charge, and the number of peripheral substituents at the porphyrin core.     

Theoretical study of N20, C20, and B20 clusters “squeezed” inside icosahedral C80 and He80 cages

The equilibrium geometries and spectroscopic and energetic characteristics of model endohedral M₂₀@C ₈₀ ^n? clusters, in which the guest clusters M₂₀ = N₂₀, C₂₀, and B₂₀ are squeezed inside the fullerene C ₈₀ ^n? cages (n = 0, 2, 4, and 6), have been calculated at the density functional theory B3LYP/6-31G and B3LYP/6-31G* levels. Analogous calculations with partial geometry optimization have been performed for their congeners M₂₀@He ₈₀ ^n? with a fixed icosahedral helium cage He₈₀. According to the calculations, all the structures of the N₂₀@C₈₀, C₂₀@C₈₀, and B₂₀@C₈₀ series correspond to local minima of the potential energy surface (all vibrational frequencies are real). In the first cluster, the N₂₀ guest has a structure of a dodecahedron with a diameter of ～4.0 Å. The alternative 10N₂@C₈₀ structure containing ten separated endohedral N₂ molecules is considerably less favorable and transforms without a barrier to the dodecahedral N₂₀@C₈₀ isomer upon geometry optimization. It has been suggested that, under extreme supercompresison conditions, molecular nitrogen can be associated without barriers into highly endothermal chemically bound clusters of the N₂₀ type. In the helium analogues, the relative position of the N₂₀@He₈₀ and 10N₂₀@He₈₀ structures on the energy scale is determined by the degree of compression and can change its sign with a change in the diameter of the external cage D(He₈₀). The mechanism of gradual assembly of the N₂₀ dodecahedron from the 10N₂ set has been traced with a decrease in the diameter D(He₈₀) in the range 7.5–8.6 Å. In the C₂₀@C₈₀ cluster, the C₂₀ guest has a structure of a distorted dodecahedron bound to the C₈₀ cage through four “inner” (endohedral) bonds. In the B₂₀@C₈₀ cluster, the B₂₀ guest is severely squeezed along the C₅ axis. Its equatorial atoms form ten endohedral B-C bonds to C₈₀ cage atoms. In similar systems, the division of the endoclusters into the internal guest and external cage becomes uncertain. Calculations predict that the isolated “salt” molecules of the L _n C₂₀ and L _n B₂₀ type in which the C₂₀ and B₂₀ clusters function as anions surrounded by the L atoms of alkali metals (n = 1–6) should be stable to stepwise dissociation accompanied by elimination of separate L atoms and L₂ molecules.