Carbon dioxide molecular structure
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Carbon Dioxide Molecular Structure: Linear and Bent Configurations
Linear Molecular Structure of CO₂
At standard conditions, carbon dioxide (CO₂) exists as a simple, linear molecule. Each carbon atom is double-bonded to two oxygen atoms, forming a structure represented as O=C=O. This linear geometry is well-established and is characterized by a C=O bond length of about 1.14 Å, which is similar to the bond length found in the free CO₂ molecule and in its low-pressure solid phases . This linear structure is also maintained in many molecular clusters and in the gas phase, where CO₂ behaves as a hydrophobic species with weak interactions with water molecules .
High-Pressure Phases: Molecular and Non-Molecular Forms
Under high pressures and temperatures, CO₂ exhibits a rich variety of solid phases, some of which challenge the simple linear model. In particular, solid CO₂ can transition into several polymorphs, including molecular and non-molecular forms 2348.
Phase II and Phase IV: Molecular Nature
Phase II of solid CO₂, observed at room temperature and pressures between 15.5 and 57.5 GPa, retains a molecular structure with a tetragonal arrangement (space group P42/mnm) and two molecules per unit cell. The C=O bond length in this phase remains close to that of the free molecule, supporting the idea that CO₂-II is not an intermediate between molecular and non-molecular forms but rather a stable molecular phase .
Phase IV, which appears at intermediate pressures and temperatures, has been the subject of debate. Some studies using advanced quantum chemistry methods (MP2 theory) confirm that phase IV maintains a molecular structure, with CO₂ molecules remaining linear rather than adopting a bent configuration 23. This supports the view that even at high pressures, CO₂ can preserve its molecular identity.
Bent Molecular Structures: Controversy and Evidence
Contrastingly, other research using x-ray diffraction has reported that in phase IV, CO₂ molecules can become slightly bent, with bond angles reduced from 180° (linear) to as low as 160°, and C–O bond lengths elongated to about 1.50 Å, which is closer to a single bond . This bent structure suggests that phase IV could be an intermediate state between molecular and non-molecular (polymeric) forms, indicating a gradual transformation pathway as pressure increases . However, this interpretation is not universally accepted, as other studies have found no evidence for significant bending in phase IV 234.
Polymeric and Hypervalent Structures at Extreme Pressures
At even higher pressures, CO₂ can transform into non-molecular, polymeric structures. For example, in phase V, CO₂ forms a network of CO₄ tetrahedra, similar to the β-cristobalite phase of SiO₂, where carbon atoms are fourfold coordinated by oxygen . Theoretical studies predict that at pressures close to 1 TPa, CO₂ could adopt even higher coordination numbers, forming hypervalent structures with sixfold coordinated carbon atoms in octahedral environments . These transitions involve significant changes in bond lengths and molecular connectivity, marking a clear departure from the simple molecular structure.
CO₂ Clusters and Intermolecular Interactions
Studies of CO₂ clusters reveal that, depending on temperature and cluster size, the molecules can arrange into various structures, including ordered solids, amorphous solids, and clusters with both solid-like cores and liquid-like outer layers. The infrared spectra of these clusters are distinct for each structural type, reflecting differences in molecular arrangement and interactions .
Conclusion
The molecular structure of carbon dioxide is primarily linear under standard conditions and in many solid phases, with each carbon atom double-bonded to two oxygen atoms. Under high pressures and temperatures, CO₂ exhibits a range of solid phases, some of which may involve slight bending of the molecule or transitions to non-molecular, polymeric forms. While the linear structure is robust in most conditions, extreme environments can induce significant changes, including the formation of bent molecules and hypervalent, networked structures. The precise nature of these transitions and the existence of intermediate bent phases remain areas of active research and debate 2345689.
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