Halogen Bonding in the Complexes of CH3I and CCl4 with Oxygen

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Halogen Bonding in the Complexes of CHI and CCl With Oxygen-Containing Halogen Bond Acceptors Peiwen Wang, Nan Zhao, and Yizhen Tang

J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04342 • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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The Journal of Physical Chemistry

Halogen Bonding in the Complexes of CH3I and CCl4 with Oxygen-Containing Halogen Bond Acceptors

Peiwen Wang,† Nan Zhao,† and Yizhen Tang*, ‡ †

Environment Research Institute, Shandong University, Shanda South Road 27,

250100 Shandong, China ‡

School of Environmental and municipal engineering, Qingdao University

of Technology, Fushun Road 11, 266033 Qingdao, China

ACS Paragon Plus Environment


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ABSTRACT

Methyl iodine (CH3I) and carbon tetrachloride (CCl4) are both important volatile precursors for atmospheric ozone destruction. CH3I and CCl4 can act as halogen bond donors to form molecular complexes with atmospheric organic species, such as 2,5-dihydrofuran (DHF), tetrahydrofuran (THF) and acetone. This study characterized the halogen bonds in the CH3I and CCl4 complexes using matrix isolation infrared spectroscopy and density functional theory calculations. With the combination of vibrational frequencies in spectra and the calculated interaction energies, frequencies and atoms-in-molecules (AIM) analyses, we confirmed the formation of halogen bonded complexes. CH3I as a halogen bond donor is comparable or slightly weaker than CCl4, and furans involving ether oxygens are the better halogen acceptors than acetone. The results help to understand the possibilities of formation of atmospheric molecular complexes which may influence the atmospheric chemical activities and enhance the aerosol formation.


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INTRODUCTION

In recent decades, halogen bond (X–Hn···Y, where Hn is a halogen atom and Y acts as a Lewis base) has been drawing extensive attention,1-4 because this unique non-covalent interaction plays a key role in molecular recognition, crystal engineering and physical organic chemistry.1,5-8 In the formation process of atmospheric molecular complexes, hydrogen bonds can stabilize the complexes and promote nucleation process.9 Besides, weak interactions as driving forces enable the gaseous molecules to be adsorbed into a surface.10 In some cases, the complexation energies of halogen bonded complexes are comparable with those of hydrogen bonded complexes,11 indicating that halogen bonds have the potential to promote the formation of atmospheric molecular complexes, which may relate to atmospheric chemistry and sources. The fundamental studies about weak molecular complexes through theoretical and experimental measurements can benefit the establishment of atmospheric models.12 Therefore, the potential of widespread halogen containing substances (e.g. halomethane) to form halogen bonded complexes in the atmosphere should be carried out. Volatile halomethanes, including chlorine, bromine and iodine-substituted methanes, are potential sources of gaseous halogens.13 CH3I is mainly emitted from ocean with a concentration of 1-4 pptv in the atmosphere,14-17 which is larger compared with dihalomethanes, such as CH2ICl, CH2IBr and CH2I2.18-19 CH3I may act as a volatile iodine-atom precursor to influence atmospheric photochemistry and global climate, especially ocean air.18,20-21 CCl4 is another important halomethane for atmospheric ozone destruction and potential greenhouse



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gas.22-24 The global average concentration of CCl4 in the lower troposphere reached 118 pptv in 1980.25 CCl4 is also considered to be carcinogenic and toxic and has adverse impact on human health.26-27 Though the use of CCl4 has been restricted worldwide, ongoing emissions in some industrial areas still exist,28-29 resulting in a mean concentration of 160 pptv reported recently in Spain.29 Since CH3I and CCl4 are present in the atmosphere with such appreciable concentrations, the influence of CH3I and CCl4 in the aspect of forming halogen-bonded complexes in the atmosphere other than their atmospheric chemical process also worth being further investigated. A series of theoretical and experimental researches on CH3I have been conducted. In addition to the kinetic studies on the reactions of CH3I with OH and NO3 radicals,14,30 Brammer et al. theoretically investigated the properties of CH3I as a halogen bond donor in crystal structures of metal-containing compounds.5 They suggested that the organic halogen (carbon-bound halogen) in CH3I served as a halogen bond donor (electrophile), which was different from the inorganic halogen (metal-bound halogen) acting as a halogen bond acceptor (nucleophile).5 To our knowledge, the halogen bonds between CH3I and electron donors have been verified in few experiments. The relevant studies were limited to the theoretical calculations on the complexes, such as H3CI–NH331 and H3CI–OCH232. CCl4 was recognized as a typical halogen bond donor due to the positive electrostatic potentials at the end regions of the chlorides.33 The complexes of CCl4–tetrahydrofuran (THF), CCl4– benzene (C6H6), CCl4–trimethyl phosphate (TMP) and CCl4–acetylene (C2H2) have been evaluated in earlier studies both experimentally and theoretically.34-37


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The electronegative atom (e.g. oxygen) is able to act as a halogen bond acceptor interacting with a halogen atom. The halogen bonded complexes formed by THF, dimethyl ether (DME), acetone and formaldehyde (FA) with halogenated molecules were investigated in previous studies.32,34,38-40 Generally, O atoms as Lewis bases can be divided into two types: ether oxygen (e.g. O in furans) and carbonyl oxygen (e.g. O in acetone). 2,5-dihydrofuran (DHF) and acetone are potentially objects of halogen bond acceptors to interact with CH3I and CCl4. Moreover, CH3I–THF is also introduced in order to further confirm that the halogen bonds do exist between halogen atoms and the cyclic ether oxygen. It is worthy of mentioning that DHF and THF are potentially produced from common volatile organic compounds in the troposphere,41 and acetone is the main atmospheric nonmethane organic species, whose concentration has reached ~1000 pptv in the atmosphere.42 The present study aimed at characterizing halogen bonds in complexes of CH3I– DHF, CH3I–acetone, CH3I–THF, CCl4–DHF, and CCl4–acetone in combination of matrix isolation infrared spectroscopy and theoretical calculations. In the matrix isolation system, the studied molecules are trapped in argon or nitrogen matrix at extremely low temperatures. Thus, the particular sample molecules can be isolated by the matrix molecules around.43-44 This condition is of vital importance when studying weak intermolecular interactions. Therefore, matrix isolation infrared spectroscopy can provide reliable characterization of molecular complexes and has been applied extensively in the exploration of hydrogen bonds.45-49 Recently, Sundararajan et al. successfully employed matrix isolation infrared spectroscopy to investigate



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phosphorus bonds50 and halogen bonds.36-37 To the best of our knowledge, there was few experimental evidence of C–I···O halogen bonds previously. Matrix isolation technique under low temperature (14K) was used in this study to fill the vacancy. Meanwhile, the quantum chemical study was carried out to support our experimental results and give deeper insights into halogen bonds. EXPERIMENTAL AND THEORETICAL METHODS

All the experiments were conducted with a matrix isolation system which can keep the reaction chamber under low temperatures and almost vacuum condition. The low temperature (14K) was achieved through a closed-cycle helium compressor cooled cryostat (PT-SHI-4-5, Janis Research Company, USA). The regular base pressure (2-5×10-5 mbar) was maintained by an Edwards vacuum pump and an EM18 spin-pump. Samples were deposited onto a diamond substrate whose temperature was controlled by a temperature controller (Model 22C, Cryocon, USA). The samples of CH3I (99.9%, Xiya Reagent, China), CCl4 (AR grade, Greagent, China), 2,5-DHF (97%, Aladdin, USA), acetone (AR grade) and THF (99.5%, Adamas, China) were used without any further purification. Argon (99.999%, Deyang, China) was employed as the matrix gas to dilute the samples. The argon/sample ratios of these samples were all 200/1, except for CH3I with the ratio of 250/1. The diluted samples were deposited from two separate nozzles onto the 14 K substrate. The deposition process was typically in twin jet mode for ~90 min at ~20 mmol h-1. The distances between the cold substrate and tips of two nozzles were both 1.5 cm.


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Infrared spectra were collected by a Bruker Vertex 80v Fourier transform spectrometer (FTIR) in combination with a KBr beam splitter and a LN2-cooled MCT (Mercury-Cadmium-Telluride) detector. The spectra were recorded from 400 cm-1 to 4000 cm-1 at a resolution of 1 cm-1. After a specific spectrum was recorded at 14 K, the annealed spectra were obtained to study the effect of temperature on the complex formation. The chamber temperature was raised to 25 K, which was maintained for 30 min, and the spectra were recorded after the chamber was recooled to 14 K. Then, we used the same procedure to obtain the annealed spectra at 30 K.51 Geometry optimizations, interaction energies and vibrational calculations were obtained by density functional theory (DFT) calculations at B3LYP-D3/def2-TZVP level of theory.52-54 It has been previously demonstrated that DFT-D3 method could provide reliable geometric properties and energies for non-covalent systems.55-58 B3LYP-D3 combined with “def2” basis set series has been successfully employed to examine halogen bonds and σ-hole bonds.59-60 For comparison, the calculations were also carried out at MP2/def2-TZVP level of theory. All the optimized structures were ascertained to be minima with no imaginary frequency by vibrational calculations, and all vibrational frequencies are scaled by the scaling factors of 0.965 and 0.960 at B3LYP-D361 and MP2 levels of theory,62 respectively. Interaction energies were obtained by calculating the difference between energies of halogen bonded complexes and total energies of the two separated monomers at CCSD(T)/Def2-TZVP// B3LYP-D3/Def2-TZVP level of theory, which was often recommended as theoretical benchmark.32 All calculations were performed using Gaussian 09 suite of programs.63



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The atoms-in-molecules (AIM) and electrostatic potential (ESP) analyses were accomplished using the optimized geometries at B3LYP-D3/def2-TZVP level of theory with the Multiwfn program.64 Molecular ESP were plotted by VMD program.65 RESULTS AND DISCUSSION

1. Optimized geometries Figure 1 shows the electrostatic potential surface (ESP) of CH3I and CCl4. ESP can visualize the peculiar distribution of electron density around a halogen atom. For the first time in 1992, Brinck et al. reported that surface electrostatic potentials, V(r), at the end regions of halogen atoms were positive while the lateral sides were negative.33 The electron density distribution shows that the ESP around the halogen atom in a halogen bond is anisotropic.3 Then in 2007, σ-hole was defined to signify the region of halogen surface with positive ESP.66-67 The σ-hole on the top region of a halogen atom (red in color in Figure 1) enables CH3I and CCl4 to be electron acceptors, which is consistent with the previous studies.3,33 The positive ESP region of Cl in CCl4 is more expanded than that of I in CH3I, though Cl is more electronegative. This can be attributed to the better electron-withdrawing ability of -CCl3 than that of -CH3.68 The most positive surface potential (VS, max) at the σ-hole of Cl in CCl4 is 71.30 kJ mol-1, which is larger than that of I in CH3I (61.30kJ mol-1). Therefore, CCl4 is likely to be the better σ-hole donor than CH3I. In addition, the blue areas around halogen atoms imply their nucleophilicity. The anisotropic character of ESP suggests that CH3I and CCl4 can be the electron donors and acceptors simultaneously. In


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appropriate situation, halogen atoms which are close enough to the hydrogen atom of halogen acceptors can become hydrogen bond acceptors. The

selected

geometries

of

equilibrium

complexes

optimized

at

B3LYP-D3/def2-TZVP level of theory are given in Figure 2. The distances of I···O (R) are 3.171 Å, 3.245 Å and 3.134 Å for the CH3I−DHF, CH3I−acetone and CH3I−THF complexes, respectively (Figures 2 (a), (b), and (c)). These distances are smaller than the sum of the van der Waals radii of O and I atoms (3.51 Å),69 indicating the formation of the halogen bonded complexes. As shown in Table 1, C−I···O angles (α) well match the linear characteristics of halogen bonding. Our results agree with the recent theoretical study on the CH3I−OCH2 complex, where the R(I···O) is calculated to be 3.30 Å and α(C−I···O) is 172.9º.32 Among the CCl4 complexes in Figures 2 (d) and (e), the distances of Cl···O for the CCl4−DHF and CCl4−acetone complexes (2.888 Å and 2.966 Å) are also smaller than the sum of the van der Waals radii of O and Cl atoms (3.27 Å).69 Hydrogen bonds can also be formed between the H atom of acetone and the X atom of halogen donors, which can be verified by the presence of hydrogen bond critical point (HBCP) (Figure S4).The hydrogen bond lengths of I···H in CH3I−acetone and Cl···H in CCl4−acetone are 3.222 Å (Figure 2(b)) and 2.936 Å (Figure 2(e)), respectively. Thus, the acetone complexes are also stabilized by hydrogen bonds, where CH3I and CCl4 act as halogen bond donors and hydrogen bond acceptors simultaneously. The C−Cl···O angles for CCl4−DHF and CCl4−acetone are 169.7º and 174.4º, respectively, which reflects the linearity of halogen bonding as well. The calculated parameters are similar to the results of



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previous theoretical investigation on the CCl4−TMP complex, where the distance of Cl···O is 2.995 Å and C−Cl···O angle is 179.7º at the HF/6-31G** level of theory.36 There exists another possible optimized structure of the complex of CCl4 with DHF (geometry of CCl4−DHF* is shown in (Figure 2 (f)), which is stabilized by the combination of C–Cl···π, C–Cl···O and C–Cl···H interactions. One Cl atom of CCl4 interacts with the double bond and the O atom of DHF simultaneously. The distance of Cl···π is 3.610 Å, and the distance between the π cloud of acetylene and Cl atom of CCl4 is approximately 3.5 Å in the previous study.37 The X···O=C angle, θ(X···O=C), is an important parameter to evaluate the distinctive bonding characteristics of carbonyl-containing halogen bonded complexes in comparison with ether-containing halogen bonded complexes. As can be seen from Figures 2 (b) and (e), the X···O=C angles of acetone−CH3I and acetone−CCl4 complexes are 124.1º and 121.6º, respectively, revealing that the σ-hole of I or Cl is directly pointing to one lone pair in the O atom, which has been reported in the previous study.39 In contrast, the σ-holes are linked to the whole terminal region of the O atom in the DHF and THF complexes. We defined ∆r(C−X, X=Cl or I) as the change in the C−X bond length of CCl4 or CH3I before and after complexion. The previous research shows that ∆r(C−X) are negative in the FA−ClCF3 and FA−BrCF3 complexes while ∆r(C−X) in FA−ICF3 is positive.70 Molecules with strong electronegative halogen atoms, such as F or Cl, are predicted to be more likely to form blue shifted halogen complexes, whist molecules with the I atom tend to form red shifted halogen complexes with lengthened C−I


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bonds. It can be seen in Table 1 that values of ∆r(C−Cl) are all negative except for that in the C–Cla···π bond, indicating that C−Cl bonds are generally shortened and blue shifted, as observed in the CF3Cl and CCl4 complexes in the previous studies.6,37 In contrast, values of ∆r(C−I) are all positive and the C−I bond is red shifted when CH3I complexes are formed. It can also be manifested in Table 1 that geometric parameters obtained at MP2 level of theory perfectly support the results from the B3LYP-D3 method discussed above, indicating the reliability of DFT-D3 method in the measurement of the halogen bonded complexes discussed in this study. 2. Experimental frequencies Table 2 shows the energy results and Tables 3-5 show the computed and experimental vibrational frequencies, complexation shifts and their assignments for the CH3I−DHF complex, CH3I−acetone complex and CH3I−THF complex, respectively. The information about vibrational frequencies of the CCl4−DHF and CCl4−acetone complexes are collected in Table S2 and S3 in the Supporting Information. The vibrational frequencies are selected to list in this paper based on the representativeness of certain bands, which represent the diverse types of molecular vibrations in infrared region. The infrared spectra of the C-H bending region for the CH3I complexes are displayed in Figure 3. The new bands observed at 1252.0, 1249.6 and 1251.8 cm−1 in the CH3I−DHF (block A), CH3I−acetone (block B) and CH3I−THF complexes (block C), reasonably agree with the calculated values of 1232.9, 1232.2 and 1231.9 cm−1,



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respectively. These new bands, resulting from the symmetrical -CH3 bending CH3 I

vibration for CH3I, ν4

, are red shifted by -6.3, -3.9 and -6.0 cm−1, respectively. The

previous study has suggested that there was positive correlation between the variation of shifts and the complexation energies.11 In this study, values of the shifts also correspond to the calculated interaction energies: CH3I−acetone complex has the least CH3 I

negative ∆E (-8.6 kJ mol-1) (Table 2). In addition, no additional band next to the ν4

exists in CH3I−acetone, while there are additionally weak bands in CH3I−DHF and CH3I−THF. Acetone which contains the carbonyl oxygen is speculated to be a weaker halogen acceptor than furans containing the ether oxygen in these halogen bonded complexes. Besides, since C−H stretching vibrations of halogen donor (CH3I) and halogen acceptors (DHF and THF) overlap seriously in the C−H stretching region, the new bands in 2920-3080 cm-1 region can only be identified clearly in CH3I−acetone, given in Figure S2. The complex band at 3059.2 cm-1 is identified as νacetone and the 24 additional band at the right side (3064.0 cm-1) is potentially assigned to the (CH3I)2−acetone complex (geometry is shown in Figure 4(b)). The shifts of acetone (Lewis base) become larger with the increase of Lewis acid molecules within the complex, which corresponds to the previous study.40 Meanwhile, shifts of CH3I (Lewis acids) are smaller than that of the 1 : 1 complex, as verified in the CH3I−DHF CH3 I

complex (block A) in which a weak band at 1250.3 cm-1 on the left side of ν4

is

assigned to (CH3I)2−DHF (geometry is shown in Figure 4(a)). The structures in Figure 4 show that two I···O bonds in the 2 : 1 complexes are non-collinear ( I···O···I angles are lower than 110.0º) and the I···O distances are slightly smaller than that of


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the 1 : 1 complexes (Table S1). It manifests that the increase of CH3I molecules enhances the strength of halogen bonds, resulting from the cooperative effect.71 The interaction energies (∆Eº) of (CH3I)2−DHF and (CH3I)2−acetone are greater than twice the values of 1 : 1 complexes (Table S1), which also supports the idea of positive cooperativity. Previous work has indicated the positive cooperativity for the formation of the 2 : 1 complex of ImPImP and DNA which has the symmetric structure.72 However, Herrebout et al. reported that sandwich structures where two same halogen bonds are almost co-linear cause the anti-cooperative effect.11,73 This indicates that the geometric characteristic is the important determinant of the cooperativity happened in 2 : 1 complexes (more details are shown in SI). The intensities of red shifted bands increased with the temperature, indicating that temperature plays a role in the complex formation.49 Besides, observations of the 2 : 1 complexes may show the advantages of matrix isolation IR spectroscopy, where multimeric complexes trapped in solid argon at the extremely low temperature (14 K) can be discerned in spectra.49 However, the spectra for the solution of CF3I and DME in liquid argon at relatively high temperatures (87-102 K) have no definite evidence of existed (CF3I)2−DME.40 Spectra of the CH3I−DHF and CH3I−acetone complexes in the 850-925cm-1 region are shown in Figure 5. In this region, the -CH3 asymmetric bending vibration for CH3I overlaps with the -CH3 asymmetric bending vibration for acetone and the C−O−C symmetric stretch of DHF, which confuses the identification of new absorption bands. The differences between the spectra of these two oxygen-containing



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electron donors are considerable, whereas the new bands resemble each other in the aspect of position and intensity. Therefore, we can exclude that bands at 903.2 (block A) and 900.6 cm-1 (block B) are produced by DHF and acetone, respectively. The CH3 I

complex bands are assigned to the ν3

. The observed shifts are -21.4 and -18.8 cm−1,

correlating with the stability of the complexes: ∆E for CH3I−DHF (-13.2 kJ mol-1) is larger than that for CH3I−acetone (-8.6 kJ mol-1). Additionally, the new weak bands at 900.7 cm−1 (block A) and 896.5 cm−1 (block B), observed between the band of CH3I monomer and band of the 1 : 1 complex, are regarded as the bands of (CH3I)2−DHF and (CH3I)2−acetone, respectively, due to the smaller shifts of CH3I in the 2 : 1 complexes. The intensity of complex bands increased as the annealing temperature raised, which can also verify the formation of halogen bonded complexes. Figure 6 displays the spectra of the CH3I−DHF and CCl4−DHF complexes in 1330-1380 cm-1. In block A, there are two new weak absorptions, v17 at 1354.0 cm-1 and v18 at 1362.6 cm-1, which are identified as the new bands of -CH2 bending vibrations of DHF, and the theoretical values are 1326.8 and 1344.0 cm-1, respectively. The complexation shifts, -5.6 cm-1 for νDHF and -1.1 cm-1 for νDHF 17 18 , are observed in CH3I−DHF. In the case of the CCl4−DHF complex, the new absorption bands are observed at 1355.9 and 1364.1 cm-1, shifted from the DHF monomer bands by -7.5 and -2.7 cm-1, respectively (Table S2). It can be seen that the shifts in the complex of CCl4-DHF are slightly larger than that in CH3I−DHF. The values of shifts are correlated with the computed interaction energies at the B3LYP-D3/def2-TZVP level of theory: CCl4−DHF* stabilized by three interactions has the slightly higher ∆E (-14.3


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kJ mol-1) than CH3I−DHF (-13.8 kJ mol-1). Meanwhile, the ∆E for CCl4–DHF who has the analogous structure with CH3I–DHF is -13.6 kJ mol-1. Additionally, the new band at 1342.8 cm-1 in block B is speculated to result from the CCl4−DHF* complex, whereas CH3I and DHF cannot produce the analogous structure and the new band. The evidences of the formation of CCl4–DHF* complexes are depicted in 650-750 cm-1 region in Figure S3 (block A). The new band at 655.7 cm-1 is assigned to the C-H out-of-plane bending vibration of DHF, ν3 , which confirms the halogen bond between the double bond and CCl4. Moreover, Figure 6 shows that there is an additional marked band at 1368.2 cm-1 on the right side of νDHF regarded as the 18 (CH3I)2−DHF complex (block A), demonstrating the larger shifts of DHF in the 2 : 1 complex than that in the 1 : 1 complex. It can be seen that the intensity of νDHF band 18 of the CH3I−DHF complex slightly was enhanced after annealing to 30 K. The carbonyl vibrations of acetone in 1700-1800 cm-1 region are showed in Figure S4, the red shift resulting from C=O stretches in the CH3I−acetone complex (block A) is similar to the shift in the CCl4−acetone complex (block B), and there is no significant difference between the calculated ∆E for CH3I−acetone and CCl4−acetone. Table 2 shows that the latter one is -8.9 kJ mol-1, which is slightly larger than the former one by 0.3 kJ mol-1. This can be attributed to the combined effects of hydrogen and halogen bonds, as the hydrogen bond existing between Cl and O atoms in CCl4−acetone plays a stronger role in enhancing the stability of the acetone complex than that between I and O atoms in CH3I−acetone. It can be assumed that the halogen binding ability of CH3I is similar to that of CCl4. -CCl3 plays a



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stronger role in the halogen bonds than -CH3, though I atom owns the weaker electronegativity than Cl atom. Spectra of the CH3I−DHF and CH3I−THF complexes spanning the 1420-1500 cm-1 region are shown in Figure 7. The new shoulders at 1440.5 cm-1 (block A) and 1442.2 cm-1 (block B) are assigned to the in-plane bending vibration for CH3I, v6, and CH3 I

are enhanced apparently above 25 K. The ν6

in these two complexes are red

shifted by -8.7 and -10.4 cm-1, respectively. The experimental values agree well with the ∆E for CH3I−DHF (-13.2 kJ mol-1) and CH3I−THF (-15.1 kJ mol-1), which indicates the increasing stability of complexes from the former to the latter. Our results reveal that THF without the double bond can be a better halogen acceptor than DHF, although they have similar structures with a five-membered ether ring. The previous IR spectroscopic study has shown that the shift of the O-H stretching vibration in hydrogen-bonded complexes for THF is slightly higher than that for DHF, since the ring is somehow strained due to the tightened double bond compared with the C-C bond.74 Besides, new complex bands assigned to the -CH2 bending vibration of DHF and THF are situated at 1478.8 cm-1 (block A) and 1481.8 cm-1 (block B), respectively. The νTHF in CH3I−THF is red shifted by -13.0 cm-1. Meanwhile, the 23 νDHF in CH3I−DHF is blue shifted by 8.7 cm-1, in which the larger blue shift in the 2 : 20 1 complex is also observed. The difference of shifts can be assigned to the structural variation between THF and DHF: the THF monomer contains four -CH2 groups, whilst DHF has only two -CH2 groups due to the double bond. 3. AIM analysis


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AIM theory, which is based on a topological analysis, has been widely used to investigate non-covalent interactions.75-76 In previous studies, AIM is mainly applied to study the properties of hydrogen bonded complexes.77-78 The existence of a hydrogen bond critical point (HBCP) between a hydrogen atom of the donor and an electronegative atom of the acceptor could prove the presence of the hydrogen bond. Similarly, the halogen bond could also be described through a halogen bond critical point (XBCP).79 AIM plots of 1 : 1 and 2 : 1 complexes are calculated the B3LYP-D3/def2-TZVP level of theory, using wave functions as parameters, are shown in Figure 8 and S5, respectively. The halogen and hydrogen bonds between X···O and X···H are characterized by XBCPs and HBCPs, respectively. A ring between the halogen bond donor and acceptor is formed by bond critical points (BCPs), which is verified by the presence of a ring critical point (RCP). As depicted in Figure 8, in the DHF complexes apart from CCl4−DHF*, only halogen bonds are observed, whereas there are hydrogen bonds observed in the acetone and THF complexes. However in the CH3I−THF complex, the RCP almost overlaps with the BCP of the hydrogen bond, which indicates that the hydrogen bond is ruptured and can be ignored.77 The electron density (ρ), Laplacian of electron density (∇2ρ) and change in electronic charge of X atom (∆q (X)) at the BCPs are provided in Table 6. The values of ρ (0.0038–0.0058 a.u.) at the H···O BCPs are within the specific range of 0.002– 0.04 a.u. for hydrogen bonds, while corresponding ∇2ρ (0.0129–0.0218 a.u.) are out of the accepted range of 0.024–0.139 a.u.80-81 With respect to the X···O and X···π BCPs



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(X = Cl, I), the values of ρ and ∇2ρ are in the ranges of 0.0043–0.0136 and 0.0166– 0.0556 a.u., respectively, which almost entirely fall into the generally proposed ranges of a hydrogen bond. The positive values of ∇2ρ indicate that hydrogen and halogen bonds in these complexes have the typical close-shell characteristics of interactions.80 In general, the ρ and ∇2ρ can be associated with interaction strength.77 In the CH3I complexes, the values of ρ and ∇2ρ in the CH3I–THF complex (0.0127 and 0.0430 a.u.) are the largest among while CH3I–acetone complexes have the smallest ones (0.0096 and 0.0340 a.u.), and the ρ and ∇2ρ of CH3I–DHF and CH3I–acetone are lower than those of CCl4–DHF and CCl4–acetone, respectively. These results are consistent with the experimental spectra and calculated interaction energies of the halogen bonded complexes. Besides, the distance between a BCP and a RCP can be applied in measuring the stability of a hydrogen bond,77,82 and we used this criterion to characterize a halogen bond in this study. In the CH3I–acetone and CCl4–acetone complexes, the distances between BCPs of halogen bonds and their corresponding RCPs are 1.009 and 0.971 Å, respectively. Meanwhile, the distances between BCPs of hydrogen bonds and each RCPs are 0.567 and 0.488 Å, respectively, which suggests that halogen bonding rather than hydrogen bonding in these complexes does play a dominant role. CONCLUSIONS

In this study, we have obtained the experimental evidences of the I···O halogen bonds in the complexes of CH3I with furans and acetone, using matrix isolation infrared


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spectroscopy. The existences of the complexes of CCl4 with furans and acetone are also proved in spectral results. DFT-D3 calculations involving geometry optimizations, interaction energies and vibrational frequencies were carried out to support the spectral observations, and a good consistency is observed between the calculations at B3LYP-D3 and MP2 levels of theory. The experimental shifts resulting from I···O and Cl···O halogen bonds, as well as the theoretical interaction energies, which indicate that the halogen binding ability of CCl4 is comparable or slightly stronger than that of CH3I. Besides, furans (DHF and THF) containing the ether oxygen act as the better halogen acceptors than acetone containing the carbonyl oxygen, and THF without the double bond is a stronger halogen acceptor than DHF. AIM analysis was performed to testify the existence of halogen bonds in these six complexes by locating X···O BCPs and manifest the combination of halogen and hydrogen bonds in the CH3I–acetone, CH3I–THF, CCl4–DHF* and CCl4–acetone complexes. The distinct characteristics of σ-hole in the halogen atom in CH3I and CCl4 make the halogen and hydrogen bonds formed simultaneously and enhance the complex stability. The stability of these halogen bonded complexes enables the volatile precursors, CH3I and CCl4, to form atmospheric molecular complexes and promote the aerosol formation. It helps to understand the impacts of the halogen-bonding interactions between the halogen containing halomethanes and oxygen-containing atmospheric organic species on the atmospheric environment.



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ASSOCIATED CONTENT

Supporting Information. Optimized geometric parameters, interaction energies and figures of geometries of the 2 : 1 halogen bonded complexes; computed and experimental vibrational frequencies of CCl4–DHF and CCl4–acetone; figures of partial infrared spectra of CCl4−acetone CH3I−acetone and CCl4−DHF; molecular graphics of the 2 : 1 complexes. AUTHOR INFORMATION Corresponding Author Yizhen Tang* Address: School of Environmental and municipal engineering, Qingdao University of Technology, Fushun Road 11, Qingdao 266033, China *Tel.: +86-532-85071262. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was supported by the Shandong Province Postdoctoral Special Fund for Innovative Projects (201402017).

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Table 1. Change in C–X (X=Cl, I) bond length (∆r, Å), halogen bond distance (R, Å) and bond angle (α, degree) calculated at the B3LYP-D3 and MP2 level of theory using def2-TZVP basis set.a bond CH3I–DHF

CH3I–Acetone

CH3I–THF

CCl4–DHF

CCl4–DHF*

C–I···O

C–I···O

C–I···O

C–Cl···O

C–Cla···π/O

C–Clb···O

CCl4–Acetone

a

C–Cl···O

∆r

R(X···O)

α

0.0015

3.171

171.9

(0.0029)

(3.082)

(174.1)

0.0027

3.245

177.3

(0.0029)

(3.152)

(178.8)

0.0029

3.134

176.2

(0.0043)

(3.045)

(177.0)

-0.0066

2.888

169.7

(-0.0047)

(2.871)

(168.8)

0.0012

3.610/3.400

83.5

(0.0012)

(3.548/3.322)

(84.4)

-0.0029

3.548

79.1

(-0.0014)

(3.520)

(78.5)

-0.0092

2.966

174.4

(-0.0068)

(2.937)

(175.9)

The geometric parameters calculated at the MP2 level are given in brackets.


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Table 2. Interaction energies (∆E), and Gibbs free energies ( ∆ G 2θ9 8 K ) of formation at 298 K calculated at CCSD(T)/ Def2-TZVP//B3LYP-D3/def2-TZVP level of theory.a ∆ECCSD(T)b

∆ G 2θ9 8 K CCSD(T)

CH3I–DHF

-13.2

22.6

CH3I–Acetone

-8.6

28.2

CH3I–THF

-15.1

26.8

CCl4–DHF

-11.8

24.6

CCl4–DHF*

-12.1

32.0

CCl4–Acetone

-8.9

26.0

a

All energies are given in kJ mol-1.

b

Interaction energies corrected for ZPE.



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Table 3. B3LYP-D3/def2-TZVP and MP2/def2-TZVP computed vibrational frequencies (ν0complex)a for the CH3I–DHF complex, experimental vibrational frequencies for CH3I and DHF (νmonomer) and the complex (νcomplex), and experimental complexation shifts (∆νexp), in cm−1.

CH3I

DHF

a

ν0complex

νmonomer

νcomplex

∆νexp

assignment

855.1 (878.0)

881.8

903.2

-21.4

ν3

1232.9 (1252.5)

1245.8

1252.0

-6.3

ν4

1418.5 (1430.4)

1399.8

1404.1

-4.3

ν5

1418.7 (1430.8)

1431.8

1440.5

-8.7

v6

2969.3 (2984.4)

2965.2

--

--

v7

653.7 (646.8)

664.3

656.9

7.5

v3

725.3 (716.4)

741.7

--

--

v4

868.1 (882.8)

896.2

914.8

-18.6

v6

956.4 (971.2)

983.3

991.5

-8.2

v9

992.2 (995.2)

1009.8

1019.9

-10.1

v10

1055.1 (1073.1)

1084.7

--

--

v12

1326.8 (1326.2)

1348.5

1354.0

-5.6

v17

1344.0 (1343.0)

1361.5

1362.6

-1.1

v18

1456.8 (1477.1)

1487.5

1478.8

8.7

v20

2877.2 (2907.3)

2886.6

--

--

v22

The vibrational frequencies are scaled by the scaling factors of 0.965 and 0.960 for

the B3LYP-D3 and MP2 level of theory, respectively. The vibrational frequencies


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calculated at the MP2 level are given in brackets. Table 4. B3LYP-D3/def2-TZVP and MP2/def2-TZVP computed vibrational frequencies (ν0complex)a for the CH3I–acetone complex, experimental vibrational frequencies for CH3I and acetone (νmonomer) and the complex (νcomplex), and experimental complexation shifts (∆νexp), in cm−1.

CH3I

Acetone

a

ν0complex

νmonomer

νcomplex

∆νexp

assignment

854.4 (877.7)

881.8

900.6

-18.8

ν3

1232.2 (1253.0)

1245.8

1249.6

-3.9

ν4

1418.6 (1431.0)

1399.8

--

--

ν5

1419.0 (1431.4)

1431.8

--

--

v6

2968.7 (2984.1)

2965.2

2961.6

3.6

v7

865.6 (870.9)

882.8

896.5

-13.7

v8

1409.4 (1417.7)

1407.0

1411.6

-4.6

v14

1413.5 (1419.6)

1414.5

--

--

v15

1419.5 (1425.5)

1429.5

--

--

v16

1719.8 (1688.9)

1768.4

1774.7

-6.3

v18

3037.5 (3098.5)

3019.2

3059.2

-40.0

v24




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Table 5. B3LYP-D3/def2-TZVP and MP2/def2-TZVP computed vibrational frequencies (ν0complex)a for the CH3I–THF complex, experimental vibrational frequencies for CH3I and THF (νmonomer) and the complex (νcomplex), and experimental complexation shifts (∆νexp), in cm−1.

CH3I

THF

a

ν0complex

νmonomer

νcomplex

∆νexp

854.7 (876.6)

881.8

--

--

ν3

1231.9 (1251.5)

1245.8

1251.8

-6.0

ν4

1418.5 (1430.2)

1399.8

--

--

ν5

1419.0 (1430.8)

1431.8

1442.2

-10.4

v6

2968.9 (2983.7)

2965.2

--

--

v7

903.6 (913.9)

901.1

896.2

4.8

v9

1049.5 (1071.4)

1081.9

--

--

v12

1456.6 (1456.8)

1468.8

1481.8

-13.0

v23

2874.3 (2908.0)

2871.5

--

--

v27

assignment



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Table 6. Change in atomic charge at X atom (∆q (X)), as well as the electron density (ρ) and Laplacian of electron density (∇2ρ) at the XBCP and HBCP for all complexes at the B3LYP-D3/def2-TZVP level of theory.a

CH3I-DHF

bond

∆q (X)

ρ

∇2 ρ

I···O

0.0396

0.0116

0.0401

0.0096

0.0340

I···H

0.0056

0.0147

I···O

0.0127

0.0430

0.0038

0.0129

0.0136

0.0556

0.0060

0.0244

0.0050

0.0174

0.0057

0.0218

0.0043

0.0166

Clb···H

0.0058

0.0190

Cl···O

0.0109

0.0458

0.0057

0.0192

I···O CH3I+Acetone

CH3I-THF

0.0388

0.0428 I···Hb CCl4+DHF

Cl···O

0.0307

Cla···O Cla···π CCl4–DHF*

0.0985

Cla···H Clb···O 0.0053

CCl4+Acetone

0.0368 Cl···H a

All values are in a.u.



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Figure 1. Electrostatic potential surfaces of CH3I and CCl4 calculated at the B3LYP-D3/def2-TZVP level of theory.

Figure 2. Optimized structures of the C–X···O (X=Cl, I) halogen bonded complexes at the B3LYP-D3/def2-TZVP level.


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Figure 3. Infrared spectra of the complexes of CH3I with DHF (A), CH3I with acetone (B), and CH3I with THF (C) isolated in Ar matrix spanning the region 1230−1275 cm-1; a) green: CH3I monomer; b) pink: DHF monomer in block A, acetone monomer in block B, and THF monomer in block C; c) blue: complexes in the co-deposition experiments; d) red: complexes after annealing at 25 K; e) black: complexes after annealing at 30 K. All spectra are recorded at 14 K. New observed bands of the 1 : 1 and 2 : 1 complexes are marked in the 30 K annealed spectra with



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an asterisk (*) and a cross ( ).

Figure 4. Optimized structures of the 2 : 1 complexes of CH3I with DHF and CH3I with acetone at the B3LYP-D3/def2-TZVP level.


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Figure 5. Infrared spectra of the complexes of CH3I with DHF (A) and CH3I with acetone (B) isolated in Ar matrix spanning the region 850−925cm-1; a) green: CH3I monomer; b) pink: DHF monomer in block A and acetone monomer in block B; c) blue: complexes in the co-deposition experiments; d) red: complexes after annealing at 25 K; e) black: complexes after annealing at 30 K. All spectra are recorded at 14 K. New observed bands of the 1 : 1 and 2 : 1 complexes are marked in the 30 K annealed spectra with an asterisk (*) and a cross ( ).



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Figure 6. Infrared spectra of the complexes of CH3I with DHF (A) and CCl4 with DHF complex (B) isolated in Ar matrix spanning the region 1330−1380 cm-1; a) green: CH3I monomer in block A and CCl4 monomer in block B; b) pink: DHF monomer; c) blue: complexes in the co-deposition experiments; d) red: complexes after annealing at 25 K; e) black: complexes after annealing at 30 K. All spectra are recorded at 14 K. New observed bands of the 1 : 1 and 2 : 1 complexes are marked in the 30 K annealed spectra with an asterisk (*) and a cross ( ).


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Figure 7. Infrared spectra of the complexes of CH3I with DHF (A) and CH3I with THF complex (B) isolated in Ar matrix spanning the region 1420−1500cm-1; a) green: CH3I monomer; b) pink: DHF monomer in block A and THF monomer in block B; c) blue: complexes in the co-deposition experiments; d) red: complexes after annealing at 25 K; e) black: complexes after annealing at 30 K. All spectra are recorded at 14 K. New observed bands of the 1 : 1 and 2 : 1 complexes are marked in the 30 K annealed spectra with an asterisk (*) and a cross ( ).



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Figure 8. Molecular graphics: (a) CH3I–DHF, (b) CH3I–Acetone, (c) CH3I–THF (d) CCl4–DHF, (e) CCl4–Acetone and (f) CCl4–DHF*. Small red points indicate bond critical points (BCPs) and yellow points indicate ring critical points (RCPs).


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TOC Graphic


Halogen Bonding in the Complexes of CH3I and CCl4 with Oxygen

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