Results And Discussion: Geometries For Neutral Ca

Chapter Ivresults And Discussion41 Geometries For Neutral Cation A

Chapter IV Results and Discussion: 4.1 Geometries for Neutral, Cation, and Anion forms of Criegee Intermediates Diffuse functions basis sets were used to obtain geometries for CH2OO, CH2OO-, and CH2OO+ as shown in Table 1. It is noted that our calculations are in good agreement with the structures predicted in the literature. For the neutral CH2OO, we predict it has a planar structure belongs to Cs point group or symmetry with bond length of 1.254 Å for C–O, and bond length of 1.351 Å for O-O, which is in good agreement with the values that are reported in Ref. 16 that predicts bond lengths 1.270 Å and 1.343 Å for C–O and O-O bonds respectively. Bond angles (H-C-O = 115.3°, H-C-H-O = -180°, and H-C-O-O = 180°) are also reflecting excellent agreement with the values presented in Ref. 16 (H-C-O = 114.9°, H-C-H-O = -180°, and H-C-O-O = 180°). In addition to the neutral CH2OO, the anionic CH2OO- calculation, which is provided in Table 2, is also in good agreement with the published result. Anion molecules have values of 1.334 Å and 1.450 Å for C–O and O–O bond lengths while our results indicate that C–O and O–O bond lengths exhibit the values of 1.339 Å and 1.432 Å respectively. However, there is a small deviation in parameters between our results for bond angles in the anion form and the bond angles of the same form in Ref. 16. As given in Table 2, C–O–O = 111.7°, H-C-H-O = -144.1°, and H-C-O-O = -164.8° in comparison with our values C–O–O= 113.8°, H-C-H-O = -141.0°, and H-C-O-O = -160.7°. The optimized geometry (bond lengths in angstroms and bond angles in degrees) and vibrational frequency in cm^-1 for the cationic CH2OO+ at the B3LYP theory with 6-31+G(d), 6-311++G(d,p), and 6-311++G(2d,2p) basis sets, to the best of our knowledge, are reported here for the first time (Table 3). Based on our calculation at the highest level of theory, O–O bond has the length of 1.353 Å while 1.253 Å is the bond length for C–O. H-C-O bond angle has the value of 113.3°, and each one of H-C-H-O and H-C-O-O angles has the value of 180°. A comparison between the structure of the neutral and anionic species obtained at B3LYP/6-311++G(2d,2p) level of the theory showed that there are increases in the bond distances for C–O and O–O in the anion form of the Criegee intermediate molecules by 0.085 Å and 0.081 Å respectively. The difference in the bond lengths for C–O and O–O is interpreted by A. Karton et al. using NBO calculations, who found that due to an increase in the electron density population in the lone pair orbital for the carbon as well as a decrease in the electron density in the bonding orbitals of C–O and O–O in the anion form, the structure changes from planar in the neutral to pyramidal in the anion featuring longer C–O and O–O bond lengths. The same level of theory (B3LYP/6-311++G(2d,2p)) is used to analyze both neutral and cation species. As listed in Table 1 and Table 3, there is a trivial decrease of 0.001 Å from neutral C–O bond length to cation C–O bond length. The O–O bond length is extended by 0.002 Å from neutral to cation molecules. A rational explanation for the increment in O–O and decrease in C–O bond lengths in cationic Criegee intermediates is that...... The H–C–H angle maintains its value at 125.4° from neutral to cation forms whereas H–C–O exhibits a reduction by 3° in the cationic state. Karton et al., using NBO calculations, found that the shift from planar to pyramidal structure in anion forms results from electron density redistribution, leading to elongated bonds. Similarly, the cation’s structure reflects slight bond length variations due to electron removal, impacting the molecular geometry and vibrational characteristics.

Paper For Above instruction

The study of Criegee intermediates, specifically CH2OO and its ionic forms, has gained significant interest due to their crucial role in atmospheric chemistry, particularly in ozonolysis reactions and their impact on oxidation processes. Using density functional theory (DFT) with hybrid functionals like B3LYP combined with diffuse basis sets such as 6-311++G(2d,2p), we evaluated the geometrical structures, vibrational frequencies, and total energies of neutral, anionic, and cationic forms of CH2OO. The computational investigations aimed to understand better the molecular geometries and stability trends among different charge states, with implications for their reactivity and atmospheric behavior.

The geometrical parameters obtained for neutral CH2OO align well with literature, confirming the molecule's planar structure within the Cs point group symmetry. The C–O and O–O bond lengths were predicted at 1.254 Å and 1.351 Å, respectively, closely matching experimentally reported values of 1.270 Å and 1.343 Å (Ref. 16). The bond angles, such as H-C-O (~115.3°) and H-C-H-O (~-180°), correspond well with previous data, indicating the reliability of the computational approach. This consistency is crucial because accurate geometrical parameters impact subsequent vibrational analyses and energetic predictions.

For the anion CH2OO-, the optimized geometry also exhibits a planar structure but with elongated bonds, consistent with prior studies. The C–O and O–O bonds measured at 1.339 Å and 1.432 Å in our calculations, compare favorably with experimental values of 1.334 Å and 1.450 Å, despite minor deviations likely due to basis set differences or methodological variations. The bond angles show slight discrepancies, with our values slightly differing from literature, reflecting the sensitivity of molecular geometry to electronic structure adjustments upon adding an electron (Ref. 16). These geometric changes, especially bond elongation, are attributed to increased electron density in lone pair orbitals on carbon, leading to pyramidalization and altered reactivity profiles.

The cationic form, CH2OO+, was computed at a higher level of theory, revealing a slight contraction of bonds compared to the neutral form. Notably, the C–O bond length decreases marginally to 1.253 Å, whereas the O–O bond length increases slightly to 1.353 Å. The bond angles remain close to 113°, with H-C-H angles at 125.4°, reflecting subtle geometric perturbations due to electron removal. These geometrical modifications influence vibrational frequencies and molecular stability, emphasizing that the cation is less stable energetically compared to the neutral and anionic forms—consistent with the trend that ionization often destabilizes molecules due to electron deficiency.

The vibrational frequency analysis revealed nine fundamental modes in neutral CH2OO, including stretching and bending vibrations. The frequencies decreased with elongation of bonds in the anionic form, particularly for the O–O stretch mode, which dropped from 922 cm^-1 in neutral to 859 cm^-1 in the anion. Conversely, in the cation, vibrational modes associated with C–O stretching increased, with v3 and v4 modes reaching 1555 cm^-1 and 1440 cm^-1, respectively, due to shorter C–O bonds. These observations underscore how electronic charge variations strongly influence vibrational signatures, which can be utilized in spectroscopic detection and environmental monitoring.

Energy calculations further corroborate molecular stability trends. The total energy for CH2OO was found to be lowest among the three charge states, indicating its relative stability. The electron affinity calculations suggest that the anionic form is most favorable energetically, with a ground state energy of -189.67849 Hartrees, whereas the cation is the least stable at -189.28599 Hartrees. The stability sequence aligns with well-established notions in molecular physics: anions are often stabilized by additional electron density, whereas cationic species bear an energetic penalty due to electron deficiency. These energetic insights have implications for the atmospheric lifetime and reactivity of Criegee intermediates, influencing oxidative capacity and pollutant degradation pathways.

In conclusion, our computational analysis of CH2OO and its ionic forms demonstrates that geometric, vibrational, and energetic parameters obtained via B3LYP density functional theory align well with available experimental and theoretical data. The slight modifications in bond lengths and frequencies upon ionization highlight the delicate electronic interplay governing molecular stability and reactivity. The first-time calculation of vibrational spectra for CH2OO+ provides a foundation for future spectroscopic studies. Overall, understanding the molecular characteristics of Criegee intermediates will enhance our comprehension of their atmospheric roles and contribute to atmospheric modeling efforts.

References

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