Bond angles common

Polymers, which consist of many covalently bonded fragments, are usually modelled with a set of intramolecular potentials describing direct bonds, bond angles and torsional or dihedral bond angles. Common analytical functions are ... 

Terms in the energy expression that describe how one motion of the molecule affects another are called cross terms. A cross term commonly used is a stretch-bend term, which describes how equilibrium bond lengths tend to shift as bond angles are changed. Some force fields have no cross terms and may compensate for this by having sophisticated electrostatic functions. The MM4 force field is at the opposite extreme with nine different types of cross terms. 

The tendencies of the amino acids to stabilize or destabilize a-helices are different in typical proteins than in polyamino acids. The occurrence of the common amino acids in helices is summarized in Table 6.1. Notably, proline (and hydroxyproline) act as helix breakers due to their unique structure, which fixes the value of the —N—C bond angle. Helices can be formed from either... 

Although sp3 hybridization is the most common electronic state of carbon, it s not the only possibility. Look at ethylene, C2H4, for example. It was recognized more than 100 years ago that ethylene carbons can be tetravalent only if they share four electrons and are linked by a double bond. Furthermore, ethylene is planar (flat) and has bond angles of approximately 120° rather than 109.5°. 

Petroleum contains hydrocarbons other than the open-chain alkanes considered to this point. These include cycloalkanes in which 3 to 30 CH2 groups are bonded into closed rings. The structures of the two most common hydrocarbons of this type are shown in Figure 22.5 (p. 585). Cyclopentane and cyclohexane, where the bond angles are close to the ideal tetrahedral angle of 109.5°, are stable liquids with boiling points of 49°C and 81°C, respectively. 

The most important alkyne by far is the first member of the series, commonly called acetylene. Recall from Chapter 7 that the C2H2 molecule is linear, with 180° bond angles. The triple bond consists of a sigma bond and two pi bonds each carbon atom is sp-hybridized. The geometries of acetylene and the next member of the series, C3H4, are shown in Figure 22.7. 

The Lewis structure of caffeine, C8H 0N4O2, a common stimulant, is shown below, (a) Give the hybridization of each atom other than hydrogen and predict the bond angles about that atom, (b) On the basis of your answers in part (a), estimate the bond angles around each carbon and nitrogen atom. 

Values of interatomic distances and bond angles found in this investigation are collected in Table XIV. It should be mentioned that Wierl s early electron-diffraction work seems to be more reliable than he considered it to be the mean difference of his C-C values and ours for the six hydrocarbons common to the two investigations is 0.03 A., much less than the mean of the errors assigned by him, 0.07 A. 

The bond angles at the bridging X atoms in the common octahedron vertices are fixed by geometry (angles M-X-M, M in the octahedron centers) ... 

The deviation from a true tetrahedral geometry that we find for the molecules Be(OX)42, B(OX)4, and C(OX)4 is common to all A(OX)4, A(NX2)4, and A(CX2Y)4 molecules, all of which have two bond angles smaller than, and four greater than, 109.5° or two angles larger than 109.5° and four smaller than 109.5°. In each case the overall symmetry of the molecule, which depends on the relative orientation of the ligands, is D2d or S4. Some examples... 

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