Methyl iodide is used primarily in the production of pharmaceutical intermediates and the organic synthesis for methylation reagents. It was considered as an alternative to soil disinfectants in the 1998 and 2002 Methyl Bromide Technical Options Committees Assessment. Methyl iodide is also widely used as a soil disinfectant to replace methyl bromide that severely destroyed the environment (prohibited by the Montreal Protocol Copenhagen Amendment). Methanol is toxic, corrosive, and carcinogenic, and methyl iodide naturally decomposes more slowly. At the same time, methyl iodide is widely used as a soil disinfectant but the possibility of entering the groundwater is very high. Inhalation of a large amount of methyl iodide inhibits the central nervous system and exerts strong stimulating effects on the skin and mucous membranes. Methyl iodine is easily decomposed by heat and produces toxic iodide fumes. It can be absorbed through the respiratory tract, skin, and digestive tract and causes poisoning. Domestic and foreign reports described how CH₃I harmed people and animals and some poisoning incidents caused by CH₃I^{[1, 2]}.

In recent years, the degradation kinetics of halogenated compounds containing CH₃I has received unprecedented attention ^[3-7]. Molecules in the external electric field will have a series of physical and chemical changes. The characteristic of molecules in external electric field has become an important method for studying molecular properties. This method has been successfully applied to a number of fields to study the properties of molecules^[8-12]. Applying a strong electric field to a molecule to break off the chemical bond is an effective method for the degradation of the molecule. However, there is no reported study on the dissociation of the CH₃I molecule in the external electric field.

The molecular total energy, molecular dipole moment, molecular energy gap, molecular spectrum, and dissociation properties of the CH₃I molecule in external electric field (0~0.04 a.u. (atomic unit)) were studied using density functional theory at the B3LYP/LANL2DZ level. The Gaussian 09^[13]software was used. The result provides important theoretical support for the degradation of CH₃I.

The Hamiltonian of the radiation process is given as

Where H₀ is the complete molecular Hamiltonian, H_int is the interaction Hamiltonian between the electron field and the molecule. Under the dipole approximation, H_int is given in a.u. as

where F is the radiation field, μ is the dipole moment, and 1 a.u.(atomic unit)=5.142 25×10¹¹ V/m.

Based on the model proposed by Grozema et al.^[14-15], excitation energy under the action of field E_exc, electric field strength F with the variation amounts Δμ, Δα of the electric dipole moment and the polarization rate satisfies the relational expression

Based on the geometric structure of the CH₃I molecule without electric field, we use the B3LYP/LANL2DZ method of density functional theory to optimize the ground state structure and study the molecular total energy, molecular dipole moment, molecular energy gap, molecular spectrum, and dissociation properties of the CH₃I molecule.

Theoretical calculations show that the CH₃I molecule has the C_{3 V} symmetry. In the present work, we optimize the structure of CH₃I with different methods. The experimental data^[16] and optimization data are listed in Table 1. By comparing the calculated data with the experimental data, we can see that the structure parameters(the bond lengths and the bond angle) calculated at the B3LYP/LANL2DZ leved are the closest to the experimental data. Therefore, the dissociation and spectral characteristics of CH₃I in the external electric field are calculated at the B3LYP/LANL2DZ level. The calculated stable structure is shown in Fig. 1. The X-axis, Y-axis, and Z-axis are delined according to the Dicker coordinate system, and the Z-axis is along the the C-I bond.

Table 1

Table 1 Optimized parameters of the structure of CH3I at different levels, together with the experimental data Method RC—I /nm RC-H/nm ∠H-C-H /(°) BPV86/LANL2MB 0.225 2 0.110 4 112.575 BPV86/LANL2DZ 0.220 0 0.109 8 111.480 B3LYP/LANL2MB 0.224 0 0.109 7 112.304 B3LYP/LANL2DZ 0.219 3 0.109 0 111.403 exp.[16] 0.213 9 0.111 0 111.400

Table 1 Optimized parameters of the structure of CH3I at different levels, together with the experimental data

Fig. 1

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Fig. 1 Optimized geometry of the ground-state CH3I

The CH₃I molecule was optimized and calculated at the B3LYP/LANL2DZ level when different electric fields (0~0.04 a.u.) were applied in the Z-axis (along the C—I bond). The stable structures of methyl iodide at different field strengths were obtained. The calculated bond length, and dipole moments of the CH₃I molecule are given in Table 2. It can be seen in Table 2 that the total energy decreases gradually with the increase of the external electric field (0~0.04 a.u.). The C—I and C—H bond lengths increase gradually with the increase of the electric field (0~0.04 a.u.) in the Z-axis. The results show that the chemical bond length increases gradually with the external electric field and the modecule becomes more and more prone to dissociation. With the increase of the external field from 0 to 0.04 a.u. along the Z-axis, the dipole moment increases.

Table 2

Table 2 Calculated molecular total energies, bond distances (C—I and C—H), and dipole moments of CH3I in different external electric fields F/a.u. 0 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 E/Hartree -49.29 -49.294 -49.3 -49.306 -49.314 -49.323 -49.334 -49.347 -49.363 Re(C—I)/nm 0.219 3 0.220 1 0.221 4 0.223 0 0.225 4 0.228 8 0.233 7 0.241 9 0.262 3 Re(C—H)/nm 0.109 0 0.108 9 0.108 8 0.108 8 0.108 7 0.108 7 0.108 8 0.108 8 0.108 9 μ/Debye 1.856 2 2.490 0 3.138 5 3.810 8 4.528 0 5.310 4 6.209 4 7.349 4 9.359 4

Table 2 Calculated molecular total energies, bond distances (C—I and C—H), and dipole moments of CH3I in different external electric fields

It is well known that many properties of a molecule are determined by the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The HOMO and LUMO energies of the CH₃I molecule are obtained at the B3LYP /LANL2DZ level. The HOMO energy E_H, the LUMO energy E_L, and the energy gap E_G are shown in Table 3. E_G is given as

Table 3

Table 3 Calculated LUMO energy EL, HOMO energy EH, and HOMO-LUMO energy gap EG for CH3I in different external fields F/a.u. 0 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 EL/Hartree -0.040 7 -0.037 7 -0.035 5 -0.033 8 -0.033 1 -0.033 5 -0.035 5 -0.040 7 -0.058 0 EH/Hartree -0.248 2 -0.247 1 -0.246 2 -0.245 4 -0.244 6 -0.244 0 -0.243 5 -0.243 0 -0.241 9 EG/Hartree 0.207 5 0.209 4 0.210 7 0.211 5 0.211 5 0.210 5 0.208 0 0.202 2 0.183 8 EG/eV 5.645 80 5.695 99 5.733 11 5.755 32 5.754 28 5.727 88 5.658 35 5.500 99 5.000 95

Table 3 Calculated LUMO energy EL, HOMO energy EH, and HOMO-LUMO energy gap EG for CH3I in different external fields

It can be seen in Fig. 2 that with the increase of electric field the HOMO -LUMO energy gap E_G gradually increases first and then decreases sharply. It is shown that at the beginning the methyl iodide molecule is hard to be excited to the excited state. When the electric field strength is greater than 0.02 a.u., molecule are getting more and more easily excited. It is also shown that the ability of methyl iodide to participate in chemical reactions weaks and then becomes strong.

Fig. 2

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Fig. 2 Variations in the HOMO-LUMO energy gap of CH3I in external fields

The IR spectra of CH₃I were calculated at the B3LYP/LANL 2DZ level. The calculated IR spectra mainly correspond to six kinds of vibration peaks, as shown in Table 4. Frequency of 481.67 cm^-1 is attributed to the CI str vibration, frequency of 890.77 cm^-1 is attributed to the CH₃ rock vibration, frequency of 1 282.34 cm^-1 is attributed to the CH₃ s-deform vibration, frequency of 1 431.46 cm^-1 is attributed to the CH₃ s-str vibration, frequency of 2 981.21 cm^-1 is attributed to the CH₃ s-str vibration, and frequency of 3 110.33 cm^-1 is attributed to the CH₃ d-str vibration. As shown in Table 4, the calculated symmetries and vibration frequencies are in agreement with the experimental results^[17], with errors of no more than 10%.

Table 4

Table 4 Calculated IR parameters for CH3I including vibrational frequencies and symmetries, together with the experimental data vibration mode symmetry vibration frequency/cm-1 calculated values exp.[17] CI str a1 481.67 533 CH3 rock e 890.77 882 CH3 s-deform a1 1 282.33 1 252 CH3 d-deform e 1 431.45 1 436 CH3 s-str a1 2 981.21 2 933 CH3 d-str e 3 110.33 3 060

Table 4 Calculated IR parameters for CH3I including vibrational frequencies and symmetries, together with the experimental data

In Table 4, str denotes the stretching vibration, and deform denotes the bending vibration. The IR spectra of the CH₃I molecule under different external electric fields were calculated by applying different intensity of electric fields (0~0.04 a.u.) in the direction of the C—I bond. The external electric field has significant influences on the IR spectra of the CH₃I molecule.

Without external electric field, we carried out energy scanning for CH₃I along the C—I bond direction at the B3LYP /LANL2DZ level to obtain the potential energy curve. The same method was used to calculate the potential energy curves along the C—I bond direction with different electric fields along the Z-axis (the C—I bond direction). The potential energy curves without and with the field are plotted in Fig. 3. It is shown that the potential energy curves of the CH₃I molecule are gradually repulsive in the external electric fields (0~0.04 a.u.). When the external electric field is 0.04 a.u., the potential energy curve of CH₃I changes from "bound" to "repulsive". The CH₃I molecule will be degraded at the strength of 0.04 a.u.. The electrochemical degradation of the CH₃I molecul will be important for the protection of the environment.

Fig. 3

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Fig. 3 Variation of the potential energy curve along the C—I bond of CH3I in external field

In the present work, the dissociation characteristics and spectral characteristics of CH₃I in external electric field were calculated using the principle of density functional theory. It is found that the results obtained at the B3LYP/LANL2DZ level are the closest to the experimental data. Therefore, the B3LYP/LANL2DZ method was used to calculate the configuration, IR spectrum, and dissociation potential energy curve of CH₃I in different electric fields. When a series of external electric fields are applied along the Z-axis (the C—I bond), the energy of the molecular system gradually decreases. The dipole moment decreases, indicating that the polarity is constantly increasing. HOMO -LUMO energy gap E_G increases first and then decreases sharply, indicating that at the beginning, the methyl iodide molecule is hard to be excited into the excited state. When the electric field strength is greater than 0.02 a.u., the excitation of the molecule to the excited state will become more and more simple. It is also shown that the ability of methyl iodide to participate in chemical reactions weakens and then becomes strong. The C—I and C—H bond lengths monotonously increase. The external electric field also has effects on the positions and intensities of the IR spectra of the CH₃I molecule.The potential energy curve of the CH₃I molecule along the C—I bond changes gradually from "bound" to "repulsive". It is found that the electric field of 0.04 a.u. is sufficient to cause the C—I bond breakage of CH₃I, which provides an important scientific basis for the degradation of CH₃I.