The Dipole Moment of CO

The experimental value for the dipole moment of CO is 0.122 D, with the polarity C 0+, for a bond length of 1.1281 A. Calculated values with the aug-cc-pVXZ basis sets are given in Table 11.21. Some other results using other basis sets are shown in Table 11.22. 

The HF level as usual overestimates the polarity, in this case leading to an incorrect direction of the dipole moment. The MP perturbation series oscillates, and it is clear that the MP4 result is far from converged. The CCSD(T) method apparently recovers the most important part of the electron correlation, as compared to the full CCSDT result. However, even with the aug-cc-pV5Z basis sets, there is still a discrepancy of 0.01 D relative to the experimental value. 

The DPT methods are not particularly accurate, although for this specific problem the B3PW91 method gives a reasonably good result. 

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Both species exhibit the expected linear geometry that maximizes the dominant n- - a interaction. However, these isomers are rather perplexing from a dipole-dipole viewpoint. The dipole moment of CO is known to be rather small (calculated Fco = 0.072 D), with relative polarity C- 0+. 40 While the linear equilibrium struc-ture(s) may appear to suggest a dipole-dipole complex, robust H-bonds are formed regardless of which end of the CO dipole moment points toward HF This isomeric indifference to dipole directionality shows clearly that classical dipole-dipole interactions have at most a secondary influence on the formation of a hydrogen bond. 

In an oxygen atmosphere CO sometimes gives a direct gas response for a porous metal film. This indicates that the CO molecule may be detected when adsorbed at a site where the dipole moment of CO is able to influence the mobile carriers in the semiconductor. 

The dipole moments of CO and NO are 0.1 D and 0.166 D. respectively. The oxygen atom is the positive end of the CO dipole, despite the difference in electronegativity coefficients of the two atoms which would imply the opposite conclusion. Consider the electronic configurations of the two molecules, and explain the anomalous properties of the CO molecule. 

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).,... 

Population Analysis Based on the Electrostatic Potential 220 11.7.2 The Dipole Moment of CO 286... 

Oxygen is more electronegative than carbon and Fig. 5.18a indicates that there is more electron density on oxygen than on carbon in carbon monoxide. Yet the dipole moment of CO is quite small (0.373 x 10 J > C m 0.112 D) and it is known that the oxygen atom is the positive end of the dipole Explain. Hint. Does a comparison with the isoelectronic dinilrogen molecule (Fig. 5.18b) help ... 

Selected physical properties of CO and CO2 are given in Table 13.4 bonding models are described in Sections 1.7 and 4.7. The bond in CO is the strongest known in a stable molecule and confirms the efficiency of (p-p)7r-bonding between C and O. However, considerations of the bonding provide no simple explanation as to why the dipole moment of CO is so low. 

Cohen and Tantirungrotechai97 have investigated the performance of new exchange-correlation functionals within the usual electron density schemes and compared calculated dipoles and multipoles for first and second row molecules with those obtained by established ab initio and electron density methods. The results obtained with the new functionals compare favourably with those of the previous methods and, in particular, give a value for the dipole moment of CO which is in good agreement with experiment. 

Judging from purely energetic arguments the dipole moment of CO is expected to induce a head-tail ordered stmcture at very low temperatures in thermal equilibrium. A reasonable candidate stmcture for such a hypothetical fully ordered stmcture within the a-phase would be antiferroelectric with space group P2i3(7 ) (see, e.g.. Refs. 117 and 159). Based on a mean-field treatment of the dipole-dipole interactions the transition temperature to the fully ordered solid was estimated [230] to be smaller than 5 K. However, the nonvanishing residual entropy of 4.6 J/K mol reported in Ref. 76 has... 

There sure other quantities that are related to expectation values other than that of the Hamiltonian e.g. dipole and higher multipole moments, spin densities, field gradients). Most of the operators that come into play here are one-electron operators, and the Moller-Plesset theorem states that their expectation values (like the electron density) are affected by correlation corrections to the wave function only to second order. As a consequence, correlation does not much influence these expectation values, except when the Hartree-Fock contribution is unusually small, so that a correlation correction may even determine the sign, as is the case for the dipole moment of CO >. 

We note here that the dipole moment of CO and its variation with intemuclear separation are given significantly better by the LSD approximation than by Hartree-Fock. ... 

As an example, consider a crystal of carbon monoxide (CO). The dipole moment of CO is quite small (0.12 D) and carbon and oxygen are very similar in size, so the CO molecule is very nearly symmetrical. In a perfect crystal of CO [Figure 8.8(a)], the CO molecules are all aligned in an ordered fashion. However, because the two ends of the molecule are so similar, the molecules in a real crystal may be randomly oriented [Figure 8.8(b)]. 

PROBLEM The dipole moment of CO (determined by Stark field spectroscopy) is small, only 0.112 D, and the bond length of CO (determined by rotational spectroscopy) is 1.128 A. The dipole moment vector points toward the O atom (the O atom surprisingly has a partial positive charge). If the dipole moment is the product of the charge and the bond length, estimate the charges (a) in coulombs, and (b) in units of the fundamental charge e. 

Practice Problem B Given that the partial charges on C and O in carbon monoxide are +0.020 and —0-020, respectively, calculate the dipole moment of CO. (The distance between the partial charges. is 113 pm.)... 

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