Observation of Elusive CF₂Cl…Cl in Matrix Infrared Spectra and Density Functional Calculations

Abstract

$CF_2Cl{\cdots}Cl$, an elusive photo-isomer of $CF_2Cl_2$, has been observed in matrix IR spectra from the precursors exposed to radiation from laser ablation of transition-metals. Other plausible products, $CFCl_2{\cdots}F$ and $FClC{\cdots}F-Cl$ are not detected due to their considerably higher energies. Parallel to its previously reported analogues, the C-X bonds are considerably stronger than those of the reactant, and particularly the Cl atom that is weakly bound to the residual Cl atom forms an unusually strong carbon-halogen bond. NBO analysis reveals that the C-Cl bond is a true double bond, and the weak $Cl{\cdots}Cl$ bond is largely ionic, $F_2C=Cl^{\delta+}{\cdots}Cl^{\delta-}$. IRC computation reproduces smooth inter-conversion between the reactant and product, and the transition state is energetically close to the product, consistent with its prompt disappearance in the early stage of photolysis.

Introduction

Halomethanes not only cause ozone depletion in the stratosphere, but also are strong, long-lasting greenhouse gases.12 The fragments (radicals, ions) and isomers of halo-methanes are reaction intermediates of many atmospheric reactions. Therefore, the spectroscopic properties of these unstable species are important to understand the photo-reactions and other behaviors of the environmentally hazard-ous halogen-containing gases.3-5

The unstable iso-tetrachloromethane CCl3⋯Cl was first identified by Maier and coworkers following selective irradiation in the photo-dissociation region (222 nm through 193 nm) for CCl4 in solid argon.6a Later work of Jacox et al. produced related chlorocarbon ions and the CCl3⋯Cl species in solid neon.6b The CH2X⋯X (X = Cl, Br, I) species were also detected by Maier et al. via photo-isomerization of methylene halides, and the structures and vibrational frequencies were computed using MP2 methods.7a The infrared absorptions of these species with a weak X⋯X bond disappear on visible irradiation in the early stage of photolysis, indicating that they are shallow energy minima.67

CHCl2⋯Cl, CHFCl⋯Cl, CHBr2⋯Br, and CFCl2⋯Cl have recently been observed in the matrix IR spectra via photo-isomerization of their precursors during co-deposition of laser-ablated transition-metal atoms.8 CBr3⋯Br was detected in radiolysis experiment.9 NBO analysis10 reveal that the C-X bond is a true double bond and the weak X⋯X bond is largely ionic, X2C=Xδ+⋯Xδ− (X = H, halogen). The fluorine-containing iso-halomethanes are rare. No iso-halomethanes with an F⋯F, F⋯Cl, or Cl⋯F bond have been identified, and moreover, no iso-halomethanes with two F atoms and an X⋯X bond have been reported to date.

Recently a new breed of small high oxidation-state tran-sition- metal complexes are produced in reactions with halo-methanes. 1112 Along with the metal containing products, photo-reaction products of the precursor (radicals, ions, and isomers) are also observed in the matrix spectra, due to the plume radiation from laser ablation. In this paper, we report observation of elusive CF2Cl⋯Cl with 13C shifts for com-parison. DFT and intrinsic reaction coordinate computa-tions13 reveal smooth inter-conversion between the reactant and product, and the transition state is energetically close to the product, consistent with the disappearance of the product in the early stage of photolysis.

 

Experimental

The CF2Cl⋯Cl photo-isomer spectra shown in this report were recorded from samples prepared by co-deposition of laser-ablated Hf atoms with CF2Cl2 and 13CF2Cl2 (Dupont) in excess argon at 10 K using a closed-cycle refrigerator (Air Products, Displex). While the product absorptions are stron-gest in the Hf spectra, other metals (groups 3-11 and actinides) also yield the same product absorptions although the intensities vary owing to different laser ablation plume radiation from specific metal surfaces,1112 Hence, these metal independent absorptions do not arise from metal containing species

In this study, Hf atoms and intense radiation from the laser ablation plume impinge on the depositing matrix sample. These methods have been described in detail elsewhere.14 Reagent gas mixtures are typically 0.50% in argon. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused onto the rotating metal target (Johnson-Matthey) using 5-10 mJ/pulse. After co-deposition, infrared spectra were recorded at 0.5 cm−1 resolution using a Nicolet 550 spectrometer with a Hg-Cd-Te range B detector. Then samples were irradiated for 20 min periods by a mer-cury arc street lamp (175 W) with the globe removed using a combination of optical filters or annealed to allow further reagent diffusion.

Complementary density functional theory (DFT) calcula-tions were carried out using the Gaussian 09 package,15 the B3LYP density functional,16 and 6-311++G(3df,3pd) basis sets for C, F, and Cl to provide a consistent set of vibrational frequencies and energies for the reaction products and their analogues. Geometries were fully relaxed during optimi-zation, and the optimized geometry was confirmed by vib-rational analysis. Additional BPW9117 calculations were done to confirm the B3LYP results. The vibrational fre-quencies were calculated analytically, and the zero-point energy is included in the calculation of binding energy of a metal complex. Intrinsic reaction coordinate (IRC) calcu-lations13 have been performed to link the transition state structures with the reactants and specific products.

 

Results and Discussion

CF2Cl⋯Cl Absorptions. Figure 1 shows the CF2Cl⋯Cl absorption region for CF2Cl2 co-deposited with laser-ablated Hf atoms. The product absorptions marked “t” are observed in the original deposition spectra, but disappear in concert upon subsequent irradiation with λ > 290 nm. They do not reappear in the following full arc (λ > 220 nm) photolysis and annealing to 30 K. The early disappearance of these product absorptions in the process of photolysis is in line with those of the previously reported iso-halomethanes.6-8 The weak absorption intensities shown in Figure 1 are con-sistent with the rarity of F containing halomethane photo-isomers; CHFCl⋯Cl and CFCl2⋯Cl are the only fluorine containing iso-halomethanes reported to date.

Figure 1.IR spectra for the CF2Cl⋯Cl absorptions produced from CF2Cl2 co-deposited for 1 h with laser-ablated Hf atoms in excess argon at 10 K and their variation. (a) Hf + 0.50% CF2Cl2 co-deposited for 1 h and (b)-(d) as (a) after irradiation with λ > 290 and λ > 220 nm and annealing to 30 K. (e) Hf + 0.50% 13CF2Cl2 co-deposited for 1 h and (f)-(h) as (a) after irradiation with λ > 290 and λ > 220 nm and annealing to 30 K. t indicates the CF2Cl⋯Cl absorptions, and p and c designate precursor and common absorp-tions. The absorptions of the CF2Cl2+ and CFCl2 are also indicated.

New product absorptions are observed at 1281 and 1256 cm−1 with intensity ratio of ~1:2.5 on the blue side of the CF2 stretching absorptions of the precursor at 1150 and 1090 cm−1. The frequencies are also compared with the previously reported C-F stretching frequencies of 1199 and 1211 cm−1 for CHFClCl and CFCl2Cl.8 They shift to 1250 and 1219 cm−1 on 13C substitution (12/13 ratios of 1.025 and 1.030). No other considerable product absorptions are observed. The observed vibrational characteristics do not match with the previously reported values for the fragments (ions and radicals) of CF2Cl2.3-5 They are assigned to the anti-sym-metric and symmetric CF2 stretching modes of CF2Cl⋯Cl on the basis of their frequencies, relatively large 13C shifts, and good correlation with the predicted values. The com-puted frequencies are 1302 and 1286 cm−1, 13C shifts both 36 cm−1, and intensity ratio between the two bands 1:2.3 (Table 1). These two are the strongest bands of the photo-isomer, and the other bands are too weak to observe. The observed product absorptions and isotopic shifts support formation of the first iso-halomethane containing two F atoms (CF2Cl⋯Cl).

Table 1.Observed and DFT Fundamental Frequencies of CF2Cl⋯Cl Isotopomers in the Ground 1A' Electronic Statea aFrequencies observed in solid argon in recent laser ablation experiments; bold are stronger matrix sites. Harmonic frequencies (cm−1) and intensities (km/mol) were computed with 6-311++G(3df,3pd). bComputed with B3LYP. cComputed with BPW91.

CFCl2⋯F and FClC⋯F-Cl, other plausible products, are not detected. CFCl2⋯F would show its strong C-F and C-Cl stretching absorptions at ~1200 and 1020 cm−1, which are not observed. The strongest C-F and C-Cl stretching absorp-tions of FClC⋯F-Cl expected at ~1140 and 710 cm−1 are also not observed. CF2Cl⋯Cl, CFCl2⋯F, and FClC⋯F-Cl are 251, 321, and 446 kJ/mol higher than the precursor. On the contrary, the attempts to optimize the geometry of Cl2C⋯F-F all ended up with the structure of CF2Cl2, suggesting that the iso-tetrahalomethane with an F-F bond is not a meaningful energy minimum. Clearly, the observed photo-isomer of CF2Cl2 with a Cl⋯Cl bond is the most stable.

Molecular Structures and Bonding. Figure 2 shows the B3LYP structures of the precursor (CF2Cl2), transition states, and plausible three iso-tetrahalomethanes (CF2Cl⋯Cl and CFCl2⋯F, and FClC⋯F-Cl). The C and three atoms bonded to C form a near planar structure with a bridging halogen atom in the transition state. The product structure is in fact similar to that of the transition state, other than the larger

Figure 2.The B3LYP structures of CF2Cl2, the transition states, and the plausible products (CF2Cl⋯Cl, CFCl2⋯F, and FClC⋯F-Cl). The bond lengths and angles are in Å and degrees. The natural atomic charges are also shown. Only CF2Cl⋯Cl is observed in this study (see text).

The Natural atomic charges, bond lengths, occupancies, bond orders,10 and structural parameters of CF2Cl⋯Cl and CFCl2⋯F are listed in Table 2 with those of several previously reported iso-halomethanes.8 The unusually short C-X bonds have considerable double bond character (natural bond orders of 1.549 and 1.648 for CF2Cl⋯Cl and CFCl2⋯F). The Cl⋯X bond is largely ionic (natural atomic charges of 0.339 and −0.477 for CF2Cl⋯Cl and those of 0.548 and −0.618 for CFCl2⋯F). Hence, these photo-isomers are better represented as X2C=Xδ+⋯Xδ−. Similarly, in the struc-ture of transition state, the bridging X carries a substantial amount of negative charge, indicating that it is largely ionically bonded to the planar CX3 subunit. The bridging Cl and F in the transition states in the CF2Cl2 ↔ CF2Cl⋯Cl and CF2Cl2 ↔ CFCl2⋯F conversions own natural atomic charges of −0.681 and −0.728, [CF2Cl]0.681+⋯Cl0.681− and [CFCl2]0.728+⋯F0.728−.

Table 2.aComputed with B3LYP/6-311++G(3df, 3pd). The all electron basis is used for H, C, F, and Cl. bNatural atomic charges in the order in the molecular formula. For example, H, Cl, C, Cl, and Cl are atom 1, 2, 3, 4, and 5 for HClC=Clδ+⋯Clδ−. cThe unusually short C-X bondlength. dNatural occupancies of σ and π orbitals of the C=X bond and its bond order. eX-X bondlength and CXX angle. fDihedral angle of X1, X2, C, and X4 in the near planar X1X2C=X4 structure.

The structure of FClC⋯F-Cl, which is much higher in energy than CF2Cl⋯Cl and CFCl2⋯F, is also shown in Figure 2. F cannot expand its valency unlike Cl, which can utilize its 3d-orbitals. During geometry optimization, the initial geometry of CFClF…Cl converges to the structure of FClC⋯F-Cl. The F-Cl bondlength of 1.644 Å in CFCl⋯F-Cl is essentially the same as that of 1.643 Å for F-Cl calculated at the same level of theory, and the interatomic distance between C and F is 3.124 Å as shown in Figure 2.

Reactions. Intrinsic reaction coordinate13 (IRC) computa-tions are carried out for the isomerization reactions between CF2Cl2 and the plausible products. Figure 3 shows the IRC results for the CF2Cl2 ↔ CF2Cl⋯Cl and CF2Cl2 ↔ CFCl2⋯F systems. Due to the large energy difference between the reactant and product, the transition state is energetically much closer to the product. The activation energies from the reactant to CF2Cl-Cl and CFCl2-F (294 and 365 kJ/mol) are noticeably higher than those for previously introduced CHCl2⋯Cl, CHFCl⋯Cl, CFCl2⋯Cl, and CCl3⋯Cl (248, 274, 254, and 220 kJ/mol, respectively).8 Fluorine evidently increases the activation energy from the precursor to the iso-halomethane, and the high activation energy is consistent with the observed weak product absorptions (the low yield) of CF2Cl⋯Cl. The activation energy from CF3Cl to CF2Cl⋯F is even higher (406 kJ/mol), consistent with its absence in the CF3Cl spectra.18

Figure 3.Intrinsic reaction coordinate (IRC) calculations between CF2Cl2 and CF2Cl⋯Cl and between CF2Cl2 and CFCl2⋯F. The shallow energy minimum for CF2Cl⋯Cl leads to prompt conversion of the transient species to CF2Cl2. CFCl2⋯F is not observed in this study due to its high energy.

In contrast, the activation energies in the reverse reactions are considerably smaller (43 and 44 kJ/mol, respectively), consistent with the disappearance of the product in the early stage of photolysis. Separate IRC computation has also been carried out for production of FClC⋯F-Cl, showing that its transition state, which is 555 kJ/mol higher than the reactant, is also linked smoothly to the reactant and product. Evident-ly the high energy barrier prohibits production of FClC⋯F-Cl as well as CFCl2⋯F.

 

Conclusion

CF2Cl⋯Cl, the first iso-halomethane with two F atoms, is produced from CF2Cl2 during co-deposition with laser-ab-lated metal atoms and the associated laser plume irradiation and identified in the matrix IR spectra with isotopic sub-stitution and DFT computational results. The absorptions of this photo-isomer are relatively weak, and they disappear in the early stage of photolysis, parallel to those of its ana-logues from di-, tri-, and tetra-halomethanes. The other plausible products, CFCl2⋯F and FClC⋯F-Cl, are not identified due to their considerably higher energies. Cl2C⋯F-F is probably not a meaningful energy minimum.

CF2Cl⋯Cl has a near planar structure of the C and three atoms bonded to C, and the residual X atom is bonded to the Cl atom. The product structure is similar to the structure of the transition state except for the larger 1.5). The weak Cl⋯X bond is largely ionic on the basis of the large atomic charges, X2C=Clδ+⋯Xδ−.

The IRC calculations reproduce smooth conversion bet-ween the reactant and product. The transition state is much closer in energy to the product, consistent with the dis-appearance of the product in the early stage of photolysis and the similarity in the structures of the transition state and product. The previous and present results show that the activation energy to the iso-halomethane increases sub-stantially with the number of F, making formation of the photo-isomer increasingly difficult.