Other Four-Atom Molecules

Planar four-atom molecules of the WXY2, XY2Y, and XY3 types have six normal modes of vibration, as shown in Fig. 11-8. All these vibrations are both infrared and Raman active. In WXYZ compounds, the XYZ skeleton may be linear (HNCO, HOCN, HSCN) or nonlinear (HONO. HNSO). In the latter case, ihe whole molecule may take the cis or tram struciure. Table II-5b lists 

Matrix-isolation infrared (nl and computer-simulation spectra (b) of CeCl4. Vertical lines in (b) show the five-peak chlorine isotope pattern of GeCl.  

Normal coordinate analyses of tetrahedral XY4 molecules have been carried out by a number of investigators. Thus far, Basile et al. have made ihe most complete study they calculated the force constants of 146 compounds by using GVF, UBF, and OVF fields (Sec. 1-13), and discussed several factors that influence the values of the XY stretching force constants. 

It has long been known that molecules such as SF4, SeF4, and TeF4 assume a distorted tetrahedral structure ( 21,) derived from a trigonal-bipyramidal 

The [CIF4], [BrFJ, and [IF4] ions were found in the following adducts  

Silanone produced by the silane-ozone photochemical reaction in Ar matrices gives rise to a band at 1202 cm in the IR spectrum that is assigned to the v(Si=0) [766]  

The frequencies listed for the formate and acetate ions were obtained in aqueous solution. Vibrational spectra of metal salts of these ions are discussed in Sec. 3.8. Although not listed in this table, the IR spectra of binary mixed halides of boron [767] and aluminum [768] have also been reported. 

Tetrahedral X4-type molecules of Tj symmetry exhibit three normal vibrations as shown in Fig. 2.14, and Vi(Ai) and V2(E) are Raman-active whereas V3(F2) is IR as well as Raman-active. Table 2.5a lists observed frequencies of tetrahedral X4 molecules. Comparison of vibrational frequencies of the X (X = Si,Ge,Sn) series and the corresponding isoelectronic Y4 (Y = P,As,Sb) series shows that the ratio, v(X4 )/v (Y4) is approximately 0.77. On the basis of this criterion, Kliche et al. [770] predicted that the vibrational frequencies of the Pb4 ion are 115 (Ai), 69 ( ), and 93 (F2) cm On the other hand, the X4-type cation such as assumes a square-planar ring structure of D4/, symmetry, and its 6 (3 x 4-6) vibrations are classified into Aig (R)+ Big (R)+ B2g (R)+ 2M(inactive)+ (IR). Assignments of these vibrations have 

Molecules like O2H2 take the nonplanar C2 structure (twisted about the 0—0 bond by 90°), whereas N2F2 and [N202] exist in two forms tra -planar (C2h) and 

A possible mechanism for the formation of such a dimer in the presence of Lewis acids has been proposed. Although the N202 ion takes a cis or tmns structure in the solid state, the N202 ion produced in Ar matrices exhibits an IR band at 

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Explain why some four-atom molecules, such as NH3 (ammonia), have a pyramid shape, and other four-atom molecules, such as A1C13 (aluminum chloride), have a triangular-planar shape. 

Vibrational frequencies are reported for other four-atom molecules such as cis-OSOO, which was produced by laser irradiation (193 nm) of the planar SO3 in Ar matrices at 12 K [833], and the HOOO radical, which is probably in cA-conformation in inert gas matrices [834]. Vibrational assignments have also been made for linear four-atom molecules containing CN groups CNCN(isocyanogen) [835], HCCN radical (cyanomethylene), [836] [FCNF] [837], and a pair of HBeCN and HBeNC that were formed by reacting Be atom with HCN in Ar matrices at 6-7 [838],... 

Write a computer program or other automated tool to calculate the thermodynamic quantities explained in this chapter for a nonlinear tetratomic (four atoms) molecule. 

For example, compare the boiling point of butane with those of the other compounds in Table 2. Butane is a gas at room temperature. Because of the symmetrical arrangement of the atoms, butane is nonpolar. Because the intermolecular forces between butane molecules are weak, butane has very low boiling and melting points and a lower density than the other four-carbon molecules. 

The Group IVA atom contributes four electrons to the bonding in a tetrahedral AB4 molecule, and the other four atoms contribute one electron each. The Lewis formulas for methane, CH4, and carbon tetrafluoride, CF4, are typical. 

Many of the structural differences between B-DNA and A-DNA arise from different puckcrings of their ribose units (Figure 28,4). In A-DNA, C-3 lies out of the plane (a conformation referred to as C-3 endo) formed by the other four atoms of the furanose ring in B- DNA, C-2 lies out of the plane (a conformation called C-2 endo). The C-3 -endo puckering in A-DNA leads to an 11 -degree tilting of the base pairs away from the normal to the helix. The phosphates and other groups in the A helix bind fewer H>0 molecules than do those in B-DNA. Hence, dehydration favors the A form. 

Isothiazole and the benzisothiazoles are planar molecules, and the 1,1-dioxides, saccharin and thiosaccharin, and their salts, are all planar, apart, of course, from the oxygen atoms on sulfur. Calculations on isothiazole-1-oxide show that the heterocyclic ring has an envelope conformation, with the sulfur atom 22 pm above, and the oxygen atom 85 pm below, the plane of the other four atoms of the ring. This, of course, would result in a significant dipole moment, calculated to be 1.27 D, perpendicular to the plane of the ring <92JP074>. 

Since there are four bonding pairs, the geometry of CH4 is tetrahedral (see Table 10.1). A tetrahedron has four sides (the prefix tetra means four ), or faces, all of which are equilateral triangles. In a tetrahedral molecule, the central atom (C in this case) is located at the center of the tetrahedron and the other four atoms are at the comers. The bond angles are all 109.5°. 

It is well-known that such functions can suffer from large amounts of spin contamination and are not suited to obtaining any surfaces except those that are the lowest of a given S3niraietry. However the UHF function, unlike an RHF function, will usually allow a molecule to separate correctly into its fragments for all decomposition channels. In contrast multi-reference-function techniques that include all configurations required to achieve correct separation would be intractable for even most three- and four-atom molecules. To limit the uncertainty introduced in using a UHF function for open shells, we monitor the multiplicity in the calculations. For some cases, such as the A A" state of HNO in the present paper, it offers a caution on the interpretation of the results, while for other cases, such as the A HCO surface, no multiplicity problems are encountered. 

A restrain t (not to be confused with a Model Builder constraint) is a nser-specified one-atom tether, two-atom stretch, three-atom bend, or four-atom torsional interaction to add to the list ol molec-11 lar mechanics m teraction s computed for a molecule. These added iiueraciious are treated no differently IVoin any other stretch, bend, or torsion, except that they employ a quadratic functional form. They replace no in teraction, on ly add to the computed in teraction s. 

Before considering other concepts and group-theoretical machinery, it should once again be stressed that these same tools can be used in symmetry analysis of the translational, vibrational and rotational motions of a molecule. The twelve motions of NH3 (three translations, three rotations, six vibrations) can be described in terms of combinations of displacements of each of the four atoms in each of three (x,y,z) directions. Hence, unit vectors placed on each atom directed in the x, y, and z directions form a basis for action by the operations S of the point group. In the case of NH3, the characters of the resultant 12x12 representation matrices form a reducible representation... 

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