Both EPs of the C and O atom-ends along the CO bond axis (z axis) are negative, with the C atom-end being more negative. It shows an intriguing EP map for the singlet ground state of CO ( Fig. In this regard, we investigated the anisotropic charge distribution of CO. If only the dipole direction of CO is simply considered, one could erroneously expect the charge distribution of negatively/positively charged C/O. Since CO has a small dipole moment with the negative end at the C atom, it is argued that the apparent anomaly for CO arises from the severe polarization of electron density on the C atom overriding the effect of electronegativity difference 10, 11, 12. This is consistent with the natural bond orbital (NBO) charge of C/O which is +0.44/−0.44 au at the level of Moller-Plesset second order perturbation (MP2) theory using the aVTZ basis set (where aVNZ denotes aug-cc-pVNZ N = D/T/Q/5). On the other hand, the quantum theory of atoms in molecules analysis showed that the C/O is positively/negatively charged, respectively. Even though two bonding orbitals (each occupied by one electron from C and one from O) form slightly polar covalent bonds to reverse the C → O polarization (as O is more electronegative than C), the dative bonding orbital results in a negative net charge δ − at the C end which gives a small dipole moment (0.11 D) 5, 6, 7, 8, 9 pointing from C to O. Thus, it could be considered that a small negative charge is formed on C and a small positive charge on O. This causes a C ← O polarization of the molecule. According to the octet rule, four of the shared electrons in C come from O and only two come from C, so one bonding orbital is occupied by two electrons from O, forming a dative or dipolar bond. The bond dissociation energy of CO (1072 kJ/mol) is the strongest chemical bond, stronger than the N 2 triple bond (942 kJ/mol) 4. The ground state of CO is a singlet state with a triple bond comprised of two covalent bonds and one dative covalent bond. Despite that covalent interactions involving with CO have been fairly studied, the study of EP for CO is essential for better understanding of noncovalent interactions of CO as well as for removal of confusion and misunderstanding of interactions involved with CO. However, the EP of CO cannot be simply described by isotropic point charges of C and O. Each atomic charge is generally described in terms of isotropic point charge. This type of interaction is usually understood based on atomic charges in each molecule. Oftentimes, strong binding arises from electrostatic interactions. Non-covalent interactions 1, 2, 3 govern molecular recognition and molecular organization/assembly, depending on magnitudes and differences in interaction energy components such as electrostatic interaction, induction, dispersion and exchange repulsion. Since CO is widely used as a ligand and a reducing agent in chemistry, including C 1 chemistry as well as mineral industries, it is of importance to correctly understand interactions of CO with various molecules. We also discuss the EP of the first excited triplet CO.Ĭarbon monoxide, though toxic, is an important species present in our environment and biosystems as it is one of the most abundant molecules. The EP is properly described by the tripole model taking into account the electrostatic multipole moments, which has a large negative charge at a certain distance protruded from C, a large positive charge on C and a small negative charge on O. It can be explained by the quadrupole driven electrostatic nature of CO (like N 2) with very weak dipole moment. This is demonstrated from the interactions of CO with Na +, Cl –, H 2O, CO and benzene. Here we illustrate that both C and O atom-ends show negative EP (where the C end gives more negative EP), favoring positively charged species, whereas the cylindrical surface of the CO bond shows positive EP, favoring negatively charged ones. Though this unusual phenomenon has been fairly studied, the study of electrostatic potential (EP) for noncovalent interactions of CO is essential for better understanding. The strong electronegativity of O dictates that the ground state of singlet CO has positively charged C and negatively charged O, in agreement with ab initio charge analysis, but in disagreement with the dipole direction.
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