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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.14 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 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 reflect 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 16 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 unit and bond angles in degree) 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 her for the first time (Table 3). Depend on our calculation at the highest level of the theory, O-O bond has the length of 1.353 angstroms while 1.253 angstrom is the bond length for C–O bond. H-C-O bond angel has the value of 113.3 degrees, and each one of H-C-H-O and H-C-O-O angles has the value of 180 degrees.

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 increase in the bond distances for C–O and O-O in the anion form of the criegee intermediate molecules by .085 Å and .081 Å respectively. The difference in the bond lengths for C–O and O-O is interpreted by A. Karton et al. 16 using NBO calculations, and they found that due to the increment in the electron density population in the lone pair orbital for the carbon as well as the decrease in the electron density in the bonding orbitals of C–O and O-O in the anion form, the structure change from the planar in the neutral to the pyramidal in the anion featuring longer C–O and O-O bond lengths.

The same level of the theory (B3LYP/6-311++G (2d, 2p)) is used to draw an analogy between both neutral and cation species. As listed in Table 1 and Table 3, there is a trivial decrease by .001 Å from the neutral C–O bond length to the cation C–O bond length. O-O bond length is extended by .002 from neutral to cation molecules. A rational interpret for the increment in the O-O bond and the reducing in the C-O bond in cationic criegee intermediates is that.................

H-C-H angle maintains its value at 125.4 degree from the neutral to the cation forms whereas H-C-O exhibits a reduction by 3 degrees in the cationic state.

Karton et al. 13 using NBO calculations, and they found that there is change in the structure from the planar in the neutral to the pyramidal in the anion form due to the increment in the electron density population in the lone pair orbital for the carbon as well as the decrease in the electron density in the bonding orbitals for C–O and O-O in the anion form resulting to longer bond lengths

4.2 Vibrational Frequencies for Neutral, Cation, and Anion Forms of Criegee Intermediates.

Vibration frequencies for criegee intermediates CH2OO, CH2OO-, and CH2OO+ are presented in Tables 4, 5, and 6. The CH2OO species has nine normal vibrational modes: five stretching modes and four bending, and all of them are infrared active. The resulting vibrational frequencies are all positive and real which confirms that the values that are used to calculate the frequencies are in the minimum energy structures.

In order to describe the vibrational frequencies of neutral and cation species, we depict the modes depend on the wavenumber descending in a' symmetry followed by the descending in a", and the same order is followed to characterize the anion species although they are all of a symmetry. The investigation of vibrational frequencies is affected by the difference in the structure between the neutral and anion species at the highest level of the theory that we used, B3LYP/ 6-311++G(2d,2p). The increment in C-O bond length in the anion species is manifested in a decrease in the vibration frequency of v3 and v4 modes. Likewise, there is a reduction in the vibrational frequency of modes v6, O-O stretch, by 114 cm-1 due to the increase in the O-O bond length from 1.351 Å in CH2OO to 1.432 Å in CH2OO-.

The frequency values for The CH2OO+, as listed in Table 6, are presented her for the first time. The difference in the structure between the neutral and cation species, also, impacts the study of vibrational frequencies for these species. Unlike the anion forms, C-O bond length in the cation species exhibit increase in vibrational frequencies of 1555 and 1440 cm-1 for v3 and v4 modes respectively resulting from the decrease in the C-O bond length from CH2OO to CH2OO+ by ~0.01 Å. The elongated O-O bond length in the cation species leads to a reduction in the vibrational frequency of v6 mode in the anion forms to 859 cm-1 in a comparison with its corresponding neutral forms (922 cm-1).

Our vibrational frequencies data for CH2OO neutral that we obtained at B3LYP/ 6-311++G(2d,2p) level of the theory are in line with the values reported by A. Karton et al.16 which are computed at CCSD(T)/ A'VQZ level. The agreement is also good between our results and the results published by Nguyen et al.17 and Su et al.18 Moreover, our computed vibrational frequencies for the anion molecules (CH2OO-) are in good agreement with the values available in ref.16 The agreement in data between our results and previous investigations bolster the precision of our predictable values for the molecules that we have studied.

4.3 Total Energies (Hartrees) for Neutral, Cation, and Anion Forms of Criegee Intermediates.

The energies in hartrees for CH2OO, CH2OO-, and CH2OO+ were obtained at B3LYP theory with three diffuse function basis sets as reported in Table 7. The electron affinity which means releasing energy when an electron is added to the neutral CH2OO indicates that the anionic criegee intermediate is the most stable form with ground state of energy of -189.67849 hartees. The cationic form has the highest energy with the value of -189.28599 which makes it the least stable form. A reasonable explanation for the cation forms to have the highest energy is that neutrals CH2OO absorb amount of energy (endothermic reaction) in order to remove an electron from its valance orbitals to form the cationic species which makes the cations to have highest energy in comparison with the corresponding neutral molecules as well as the anions.

Chapter V

Conclusion

Structural and energetic calculations on criegee intermediate molecules (CH2OO, CH2OO-, and CH2OO+) have been performed using the B3LYP method and density functional theory. The calculations of the structure of CH2OO, CH2OO-, and CH2OO+ are found to be in good agreement with the experimental as well as theoretical results available in literature. Our calculations indicate that the anion molecule is the most stable form whereas the cation form is the least stable. The vibrational frequencies calculated using the B3LYP method at 6-311++G(2d,2p) level are also in good agreement with the literature measurements for of criegee intermediates (CH2OO, CH2OO-, CH2OO+).

The optimized geometry, energy, and vibrational frequencies cation form also have been investigated using the B3LYP method and density functional theory and presented here for the first time. We notice that cation species compare to its neutral form examine increase in the O-O bond length and decrease in the length of C-O bond.