Carbon monoxide dipole moment

Calculate the bond length, ioni7 ation potential, and dipole moment of carbon monoxide by MNDO,. AMI, and PM3,... 

Many molecules, such as carbon monoxide, have unique dipole moments. Molecules with a center of inversion, such as carbon dioxide, will have a dipole moment that is zero by symmetry and a unique quadrupole moment. Molecules of Td symmetry, such as methane, have a zero dipole and quadrupole moment and a unique octupole moment. Likewise, molecules of octahedral symmetry will have a unique hexadecapole moment. 

Incidentally, isocyanides are polar (for CNC H, the dipole moment is 3.44 D) and they are good bases (vs. BRj, H+), whereas CO is a poor base hence isocyanides can function as ligands in metal complexes where carbon monoxide does not. The scarcity of low-valent isocyanide complexes is less easily explained, however. Arguments involving 77-acceptor capacity are quite inappropriate. More data on low-valent species, and evaluations of stabilities, modes of decomposition, and reactions are desirable. 

Carbon Monoxide. There are close similarities between carbon monoxide and nitrogen. The molecules are isoelectronic, and the bond lengths and dissociation energies are quite comparable. The phase diagrams of the two compounds show the same trends in the moderate pressure range with a variety of phase transitions between essentially alike crystal structures [333], when allowance is made for the lack of the inversion center and the presence of a weak electric dipole moment in carbon monoxide. However, the behavior and stability at higher... 

There is no simple explanation for the much more pronounced instability to pressure of CO compared to N2. Since the only structural difference arises from the heteroatomic character of CO, one could expect that the molecular dipole moment increases with pressure leading to a higher compressibility of CO. But no evidence for this is obtained from either the ab initio calculation or experimentally. In fact the equation of state of nitrogen and carbon monoxide are practically coincident in the pressure range of interest. One other point of interest is the head-to-tail disorder present in carbon monoxide because it has been observed in several high pressure experiments that defects and disorder can play an important role. 

One classical example that apphes the electroneutrality principle is the electronic structure of carbon monoxide, a diatomic molecule with a very small dipole moment of 0.110 debye. The only electronic structure that satisfies the octet rule for CO is C=0 , a structure that corresponds to C and O, if the shared electron pairs are equally devided by the two atoms. Pauling showed that the electronegativity difference of 1.0 would correspond to about 22% partial ionic character for each bond, and to charges of and 0° +. A second possible electronic structure, C=O , does not complete the octet for carbon. The partial ionic character of the bonds corresponds to C0.44+ If these two structures contribute... 

Note, however, that this places a formal positive charge on the oxygen and a formal negative charge on the carbon. If the electronegativities of carbon and oxygen were the same, carbon monoxide would have a sizable dipole moment in the direction... 

Note, however, lhai the dipole moment of carbon monoxide is extremely small .375 X I0-3tlCm (0.112 01. 

Exercise 16-1 Draw valence-bond structures and an atomic-orbital model for carbon monoxide. Why can the bond energy of this molecule be expected to be higher than for other carbonyl compounds (see Table 16-1) Explain why the dipole moment of CO is very small (0.13 debye).,... 

One of the simplest orientational-dependent potentials that has been used for polar molecules is the Stockmayer potential.48 It consists of a spherically symmetric Lennard-Jones potential plus a term representing the interaction between two point dipoles. This latter term contains the orientational dependence. Carbon monoxide and nitrogen both have permanent quadrupole moments. Therefore, an obvious generalization of Stockmayer potential is a Lennard-Jones potential plus terms involving quadrupole-quadrupole, dipole-dipole interactions. That is, the orientational part of the potential is derived from a multipole expansion of the electrostatic interaction between the charge distributions on two different molecules and only permanent (not induced) multipoles are considered. Further, the expansion is truncated at the quadrupole-quadrupole term. In all of the simulations discussed here, we have used potentials of this type. The components of the intermolecular potentials we considered are given by ... 

The intermolecular potential consists of the sum of Eqs.(176) and (177). This simulation was done for 216 and 512 molecules. However, only the autocorrelation functions from the 512 molecules case are discussed here. The small dipole moment of carbon monoxide makes the orientational part of this potential so weak that molecules rotate essentially freely, despite the fact that this calculation was done at a liquid density. The results for the Stockmayer simulation serve the purpose of providing a framework for contrasting results from more realistic, stronger angular-dependent potentials. 

The potential form is the same as that in 1 except for the dipole moment used. This simulation was run for 216 molecules. We were primarily interested in seeing the effect on rotational motion of increasing the dipole moment. Although this particular dipole moment is ten times larger than carbon monoxide s, it is a reasonable one for a more polar substance such as HCl. Note Henceforth a reference to the Stockmayer simulation will refer to the first one discussed. Any specific reference to this calculation will mention the dipole moment used. 

As the temperature is reduced, the thermal energy available to overcome kinetic barriers to the lowest-energy state is reduced and, in some cases, residual entropy is difficult to remove at low temperatures. In other words, in the low-temperature cooling of these substances, reversibility cannot be approached. An example of a substance with residual entropy is solid CO. Carbon monoxide has a very small dipole moment, which indicates that there is a preferential orientation of molecules at low temperature. The magnitude of the dipole is so small, however, that at temperatures at which the preference becomes appreciable, there is insufficient thermal energy to overcome kinetic barriers for rotation of the molecules in the solid. 

Solution Carbon monoxide has a small electric dipole moment (approx 0.1 Debye), which gives the molecules an energetically preferred orientation as T — 0. However, this dipole moment is so small that the preference is not appreciable until very low temperatures, and the random orientation of the molecules (the dipole has equal probability of pointing in one direction or its opposite) remains as the temperature is lowered. For a mole of CO, each molecule can point in either of two directions and there are 2Na configurations that are about equally probable. This model predicts a residual entropy of... 

Scuseria GE, Miller MD, Jensen F, Geertsen J (1991) The dipole moment of carbon monoxide. J Chem Phys 94 6660-6663... 

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