Matrix Infrared Spectra and Density Functional Calculations for New

Article pubs.acs.org/JPCA

Matrix Infrared Spectra and Density Functional Calculations for New iso-Halomethanes: CHCl2−Cl, CHFCl−Cl, CFCl2−Cl, CHBr2−Br, and CBr3−Br in Solid Argon Han-Gook Cho† and Lester Andrews*,‡ †

Department of Chemistry, University of Incheon, 119 Academy-ro, Songdo-dong, Yeonsu-gu, Incheon 406-772, South Korea Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States

‡

S Supporting Information *

ABSTRACT: Laser ablation of transition metals for reactions with halocarbons to produce new metal bearing molecules also exposed these samples to laser plume radiation and its resulting photochemistry. Investigations with CCl4 also produced several known neutral and charged intermediate species, including the iso-tetrachloromethane CCl3−Cl observed in previous work and identified by the Maier group. CHCl2−Cl, CHFCl−Cl, and CFCl2−Cl, photoisomers of CHCl3, CHFCl2, and CFCl3, were also identified in matrix IR spectra. The new C−X bonds are shorter than those of the reactants, and the Cl atom that is weakly bonded to the residual Cl atom forms an unusually strong C−Cl bond. NBO analysis reveals substantial CCl double-bond character, and the weak Cl···Cl bond is largely ionic. Therefore, the CHX2−X species can be represented as HXCXδ+···Xδ−. Ionic properties are revealed for CCl3−Cl, which has an average C−Cl bond length near the median for the CCl3 radical and cation and a natural charge of +0.49 for the CCl3 subunit. IRC computations reproduce smooth interconversion between the reactants and products, and the transition state is energetically close to the product, which is consistent with its disappearance on visible irradiation.

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shifted to 503.9 cm−1.17 Subsequent laser ablation of transition metals for reactions with small molecules including CCl4 in this laboratory also produced the highest three of these bands in solid argon.18 Similar photosensitive bands were observed with other halomethane precursors including fluorochloromethanes and bromomethanes not reported previously, which we wanted to add to the literature of iso-halomethanes. The CH2X−X (X = Cl, Br, I) species were also identified by Maier et al. via photoisomerization of methylene halides, and the structures and vibrational frequencies were computed using MP2 methods.20 The infrared absorptions disappear on visible irradiation, indicating that the products have shallow energy minima. Our group reported similar absorptions, which were incorrectly identified as CH2X2+ cations.21 Earlier proton radiolysis investigations with haloforms found similar photosensitive bands, which were incorrectly assigned to molecular ions, and these can now be reassigned to iso-haloform neutral species.22 These unstable halomethane isomers with weak X···X bonds may provide valuable information for release of halogen radicals in atmospheric and solution photochemical processes. Recently the production of small transition-metal insertion, methylidene, and methylidyne complexes in reactions of

INTRODUCTION The fragments (radicals, ions) and isomers of halomethanes have been the subject of many spectroscopic studies.1−7 Halomethanes not only cause ozone depletion in the stratosphere,8−10 but also are strong, long-lasting greenhouse gases.11−13 Therefore, the spectroscopic properties of these unstable species are important to understand the photoreactions and other behaviors of the environmentally hazardous halogen-containing gases. The unstable iso-tetrachloromethane CCl3−Cl was first identified by Maier and co-workers following selective irradiation in the photodissociation region (222−193 nm) for CCl4 in solid argon at 12 K, which was not of sufficient energy to affect precursor ionization.14 The Maier group observed six infrared bands and a 414 nm absorption for this unusual product isomer. Our group launched investigations in 1975 to produce and identify molecular cations in solid argon using high energy radiolysis and photoionization, and CCl4 was our first source for molecular ions.15−19 In that early work, we observed six extremely photosensitive infrared absorptions, which were essentially the same as those identified later by Maier et al. along with a corresponding 425 nm absorption. These were first assigned to CCl4+, CCl2Cl2+, and ionic Cl3,4 species owing in part to lack of observable 13CCl4 shift on the lower bands. Later work of Jacox et al. produced related chlorocarbon ions and the CCl3− Cl species in solid neon and observed counterparts of the highest three bands and a 0.3 cm−1 isotopic shift for the band now matrix © XXXX American Chemical Society

Received: May 10, 2013 Revised: June 25, 2013

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dx.doi.org/10.1021/jp4046534 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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the rotating metal target (Johnson-Matthey) using 5−10 mJ/pulse. After codeposition, 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 mercury 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) calculations were carried out using the Gaussian 09 package,37 the B3LYP density functional,38,39 and 6-311++G(3df,3pd) basis sets for C, H, 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 optimization, and the optimized geometry was confirmed by vibrational analysis. Additional BPW9140 calculations were done to confirm the B3LYP results. The vibrational frequencies were calculated analytically, and the zero-point energy is included in the calculation of binding energy of a metal complex. Intrinsic reaction coordinate (IRC) calculations41 have been performed to link the transition state structures with the reactants and specific products.

halomethanes with laser-ablated transition-metal atoms has been reported.23−33 Due to the stronger M−X bonds, the high oxidation-state products are in general more favored than in reactions of alkanes. Along with the metal-containing products, photoreaction 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 our recent observation of CCl3−Cl with 13C shifts for comparison; new results for CHCl2−Cl, CHFCl−Cl, and CFCl2−Cl in the matrix IR spectra via photoisomerization of the precursors; and the identification of both CCl3−Br and CCl2Br−Cl from the CCl3Br precursor. The bromine analogs CHBr2−Br and CBr3−Br were also observed. Intrinsic reaction coordinate computations reveal smooth interconversion between the reactant and product, and the transition state is energetically close to the product. Earlier, photochlorination with Cl2 in noncomplexing solvents including CCl4 has been investigated,34 and it is possible that transient species like CCl3−Cl might play a role.

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EXPERIMENTAL AND COMPUTATIONAL METHODS The CCl3−Cl, CHCl2−Cl, CHFCl−Cl, and CFCl2−Cl photoisomer spectra shown in this report were recorded from samples prepared by codeposition of laser-ablated Pt, Os, or Zr atoms with CHCl3, CDCl3, 13CHCl3, CHFCl2, CDFCl2, and CFCl3, and 13CFCl3 in excess argon at 10 K using a closed-cycle refrigerator (Air Products, Displex). However, 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.23−33 Hence, these metal independent absorptions do not arise from metal-containing species. In our experiments, metal atoms and intense radiation from the laser ablation plume impinge on the depositing matrix sample (see Figure 1 in ref 23). These methods have been described in detail in previous publications.35,36 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

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RESULTS AND DISCUSSION Figures 1−4 illustrate the matrix IR spectra in the product absorption regions from codeposition of Pt, Os, and Zr, their laser plume radiation with CCl4, CHCl3, CHFCl2, and CFCl3 isotopomers, and their variation in the subsequent visible (λ > 420 nm) photolysis. The CHX2, CX3, CX3+, and metalcontaining product absorptions (M) are also indicated in the spectra.1−7 The CCl3−Cl, CHCl2−Cl, CHFCl−Cl, and CFCl2− Cl absorptions marked “t” (t for transient) are observed in the original deposition spectra, but disappear in concert upon visible irradiation. They do not reappear in the following photolysis (240 < λ < 420 nm and λ < 220 nm) and annealing (up to 42 K). The observed product frequencies are compared with the B3LYP and BPW91 computed values in Tables 1−6. The NBO computation42 results and molecular parameters of the products are listed in Table 7.

Figure 1. IR spectra for the CCl3−Cl absorptions produced from CCl4 codeposited for 1 h with laser-ablated Pt atoms in excess argon at 10 K and their variation upon visible (λ > 420 nm) irradiation. (a, b) Pt + 0.50% CCl4 following deposition and after irradiation. (c, d) Pt + 0.50% 13CCl4 following deposition and after irradiation. t indicates the CCl3−Cl absorptions, p designates precursor absorption, and absorptions of the CCl3 radical and cation are also indicated. B



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Figure 2. IR spectra in the CHCl2−Cl absorption regions for CHCl3 codeposited for 1 h with laser-ablated Pt or Os atoms in excess argon at 10 K and their variation upon visible (λ > 420 nm) irradiation. (a, b) Pt + 0.50% CHCl3 following deposition and after irradiation. (c, d) Pt + 0.50% CDCl3 following deposition and after irradiation. (e, f) Os + 0.50% 13CHCl3 following deposition and after irradiation. The label t stands for the CHCl2−Cl absorption, and p indicates CHCl3 precursor bands. The absorptions of common fragments of CHCl3 (CCl3, CCl3+, and CDCl2) and metal-containing product (M) are also indicated.

Figure 3. IR spectra in the CHFCl−Cl absorptions regions for CHFCl2 codeposited for 1 h with laser-ablated Os atoms in excess argon at 10 K and their variation upon visible (λ > 420 nm) irradiation. (a, b) Os + 0.50% CHFCl2 following deposition and after irradiation. (c, d) Os + 0.50% CDFCl2 following deposition and after irradiation. The label t stands for the CHFCl−Cl absorption, and p and c indicate precursor and absorptions common to CHFCl2 samples. The absorptions by common fragments of CHFCl2 (CFCl2, CFCl2+) are also indicated.

CCl3−Cl and CH2Cl−Cl. Figure 1 shows the CCl3−Cl absorption regions for CCl4 codeposited with laser-ablated Pt atoms.18 The product absorptions marked “t” are observed at 1019.2, 926.8 (928.2 cm−1 matrix site), and 501.4 (498.1 cm−1 chlorine isotopic splitting) cm−1 which are close to the 1019.7, 929.1, and 501.9 cm−1 frequencies reported by Maier et al.14 (small differences are in accord with the fact that we irradiated our samples during deposition and the Maier group irradiated after deposition). Our 13C counterparts were observed at 987.1, 896.9 (898.1), and 501.3 (498.0 cm−1) cm−1 using a 90% 13C enriched sample.18 These product absorptions decrease dramatically on visible light irradiation, consistent with the previous reports. Owing to our current spectrometer limit, we could not observe product bands below 420 cm−1, but they were observed in our earlier work as given in Table 1.

The carbon 12/13 frequency ratios characterize the vibrational modes: the large ratios for the 1019.2/987.1 = 1.0325 and 926.8/896.9 = 1.0334 frequencies are characteristic of antisymmetric carbon−chlorine motions. The higher A′ mode is an out-of-phase combination of Cl2−C−Cl stretches with the C moving between the two equivalent Cl’s and the single Cl in the molecular symmetry plane, and the lower A″ mode involves antisymmetric CCl2 stretching where the C moves normal to the symmetry plane. The very small 13C shift of the 501 cm−1 band is in accord with in-phase symmetric stretching or “breathing” mode of the almost planar CCl3 group where the C atom hardly moves. This mode is found at 461 cm−1 for CCl4 and at 668 cm−1 for CHCl3 in Raman spectra of the liquids.19 On the other hand the symmetric CCl3 deformation (“umbrella”) mode at 364 cm−1 exhibits a relatively large 13 cm−1 13C shift, and a large 374/361 = 1.036 ratio because C is moving up and down from three Cl C



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atoms, which is in excellent agreement with the 14 cm−1 value from our calculations (Table 1). The lower two CCl2 bending modes show virtually no 13C shift. The structure of CCl3−Cl calculated using the B3LYP density functional is compared to a number of related species in Figure 5. Similarly, the CH2Cl−Cl absorptions reported by Maier et al.20 are all observed in our CH2Cl2 spectra at 3143.4 (3150.4 matrix site), 3023.2 (3030.2), 1702.5 (1695.9), 1545.2 (1561.3), 1405.0 (1407.9), 958.4 (967.2, 952.8), and 763.2 (772.2) cm−1.23,24,26 The observed D counterparts at 2375.6 (2381.0), 2220.4 (2226.3), 1211.2, 1083.4 (1086.7), 879.7 (891.3, 886.1, 874.7), and 603.2 (610.2) cm−1 are also consistent with the previously observed values. The 5.8 cm−1 chlorine isotopic splitting on the 958.4 cm−1 C−Cl stretching mode is comparable to the 6.0 cm−1 splitting observed by Andrews and Smith for the CH2Cl free radical absorption at 826.3 cm−1.43 The 13C counterparts, first observed in our spectra at 3130.2 (3137.3), 3017.2 (3024.2), 1675, 1532.9 (1546.5), 1399.2 (1402.0), 939.9 (950.2), and 756.0 (765.1) cm−1, provide new information in support of the Maier assignments, which are the A″ and A′ CH2 stretching, C−Cl stretching + CH2 wagging, CH2 wagging overtone, CH2 scissoring, C−Cl stretching, and CH2 wagging modes, respectively.20 In particular the large deuterium shift of the “C−Cl stretching mode” indicates mixing with the CH2 scissoring mode, which increases on deuteration and intensifies the CD2 scissors band. Hence, the 13C shift on the C−Cl stretching mode is reduced to 18.5 cm−1 for this mixed

Figure 4. IR spectra in the CFCl2−Cl absorptions regions for CFCl2−Cl codeposited for 1 h with laser-ablated Pt and Zr atoms in excess argon at 10 K and their variation upon visible (λ > 420 nm) and UV (240 < λ < 380 nm) irradiation. (a, b, c) Pt + 0.50% CFCl3 following deposition and after visible and UV irradiation. (d, e, f) Zr + 0.50% 13CFCl3 following deposition and after visible and UV irradiation. The label t stands for the CFCl2−Cl absorption, and p and c indicate precursor and absorptions common to CFCl3 samples. The absorptions by common fragments of CFCl3 (CFCl2 and CFCl3−) and metal-containing product (M) are also indicated.

Table 1. Observed and DFT Calculated Fundamental Frequencies of CCl3−Cl Isotopomers in the Ground 1A′ Electronic Statea CCl3−Cl b

approx description

obsd

B3LYP

A′ as CCl3 str A″ as CCl3 str A′ s CCl3 str A′ CCl3 deform A″ CCl ip bend A′ CCl2 scis A′ Cl−Cl str A″ CClCl oop bend A′ CClCl ip bend

1019.2 926.8, 928.2 501.4, 498.1 [374]

1048.8 910.7 509.6 409.9 319.7 298.3 233.9 82.0 72.0

[290] [242]

int

13 b

BPW91

116 206 55 16 0 25 67 2 8

c

int

997.7 883.8 495.2 395.6 309.8 290.8 232.3 81.8 74.2

c

112 211 47 16 0 25 50 2 7

b

obsd

B3LYP

987.1 896.9, 898.1 501.3, 498.0 [361]

1015.1 880.5 509.3 395.5 319.0 297.2 233.8 82.0 72.0

[290] [242]

CCl3−Cl intb

BPW91c

intc

106 192 55 14 0 25 67 2 8

965.4 854.6 495.0 381.9 309.2 289.7 232.3 81.8 74.2

102 196 47 14 0 25 49 2 7

a

Frequencies observed in solid argon in recent laser ablation experiments; bold are stronger matrix sites. Frequencies in brackets are from earlier radiolysis experiments using a grating instrument with a 200 cm−1 limit, Figure 2, ref 15. Harmonic frequencies (cm−1) and intensities (km/mol) were computed with 6-311++G(3df,3pd). bComputed with B3LYP. cComputed with BPW91.

Table 2. Observed and Calculated Fundamental Frequencies of CHCl2−Cl Isotopomers in the Ground 1A Electronic Statea CHCl2−Cl approx description C−H str C−H ip bendd CCl2 as strd CCl2 s str C−H oop bend CCl2 bend Cl−Cl str CCl2−Cl deform CClCl bend

CDCl2−Cl

13

CHCl2−Cl

obsd

B3LYPb

intb

BPW91c

intc

obsd

B3LYPb

intb

BPW91c

intc

obsd

B3LYPb

intb

BPW91c

intc

3086.5 1252.8 995.5 785.2 652.1

3212.7 1293.4 1042.4 795.2 706.0 355.6 253.1 222.4

23 26 85 73 127 17 65 20

3146.9 1245.8 1002.9 779.9 667.4 345.6 249.9 217.7

15 23 88 66 112 15 53 13

2304.9 858.8 1059.9 757.9 523.3

2367.0 888.1 1092.8 761.1 572.7 353.1 251.8 202.7

18 3 117 95 55 17 72 13

2318.2 857.0 1048.9 747.1 541.1 343.1 249.0 198.6

12 3 115 80 52 15 57 9

3077.0 1247.0 970.4 768e 643.5e

3202.6 1286.9 1015.5 778.1 696.4 353.3 252.7 217.8

23 19 81 66 131 17 68 17

3137.1 1240.0 976.7 762.8 658.7 343.3 249.6 213.3

14 17 84 61 114 15 54 11

80.1

6

79.1

5

80.1

6

79.1

5

80.1

6

79.1

5

Harmonic frequencies (cm−1) and intensities (km/mol) were computed with 6-311++G(3df,3pd). bComputed with B3LYP. cComputed with BPW91. dMixed vibrational mode: this mixing is H, D dependent. eOverlapped with precursor absorptions. a

D



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Table 3. Observed and Calculated Fundamental Frequencies of CHFCl−Cl Isotopomers in the Ground 1A Electronic Statea CHFCl−Cl

CDFCl−Cl

approx description

obsd

B3LYPb

intb

BPW91c

intc

obsd

B3LYPb

intb

BPW91c

intc

C−H str C−H ip bend CFCl as str CFCl s str C−H oop bend CFCl bend Cl−Cl str CFCl−Cl deform CClCl bend

3081.5 1342.3 1198.9 938.2 655.8 522.3

3207.6 1362.7 1227.9 985.1 728.5 473.3 263.9 250.2 103.7

23 88 336 24 142 23 62 24 6

3138.2 1308.3 1183.3 949.5 688.3 455.9 261.7 246.1 104.1

13 68 307 16 133 20 49 18 5

2300.9 982.0 1243.7 850.5, 849.1 550.3, 546.0, 542.3 covered

2366.2 1018.9 1276.4 883.0 601.3 469.4 263.2 224.4 103.5

22 72 375 9 69 23 72 13 6

2314.4 984.9 1221.7 850.1 568.8 452.1 261.0 220.8 103.9

14 53 336 7 67 20 56 11 5

Harmonic frequencies (cm−1) and intensities (km/mol) were computed with 6-311++G(3df,3pd). Stronger absorptions are bold. bComputed with B3LYP. cComputed with BPW91.

a

Table 4. Observed and Calculated Fundamental Frequencies of CFCl2−Cl Isotopomers in the Ground 1A Electronic Statea CFCl2−Cl b

approx description

obsd

B3LYP

C−F str CCl2 as str CCl2 s str CFCl2 deform FCCl bend ClCCl bend Cl−Cl str CCl2−Cl deform CClCl bend

1211.1 covered 588.2, 584.2 431

1238.2 1059.9 603.1 461.2 413.8 331.7 245.2 106.4 73.4

CFCl2−Cl

13

b

BPW91

333 221 67 20 21 17 78 3 8

1178.7 1010.0 578.1 450.2 399.2 321.2 243.8 106.0 73.8

int

c

int

c

293 220 55 22 19 17 60 2 6

b

obsd

B3LYP

1181.2 1014.6 582.6

1204.4 1025.6 600.6 448.7 410.6 330.1 245.1 106.3 73.4

intb

BPW91c

intc

315 202 67 13 25 18 78 3 8

1146.7 977.1 575.8 437.4 396.6 319.7 243.7 105.9 73.7

277 202 56 16 23 17 60 2 6

Harmonic frequencies (cm−1) and intensities (km/mol) were computed with 6-311++G(3df,3pd). Stronger absorptions are bold. bComputed with B3LYP. cComputed with BPW91.

a

Table 5. Observed and Calculated Fundamental Frequencies of CHBr2−Br Isotopomers in the Ground 1A Electronic Statea CHBr2−Br approx description C−H str C−H ip bendd CBr2 as strd CBr2 s str C−H oop bend CBr2 bend CBr2−Br deform Br−Br str CBrBr bend

obsd 1186.8 [1192] 838.4 [839] undere 572.3 [570]

CDBr2−Br

B3LYPb

intb

BPW91c

intc

3208.3 1223.1 858.8 684.0 583.9 212.3 181.2 163.0 45.9

22 30 106 49 139 13 2 35 3

3140.4 1176.6 824.7 665.4 563.2 206.6 176.4 162.0 45.2

13 29 111 39 125 12 2 26 2

obsd [945] [755] [undere] [473]

13

CHBr2−Br

B3LYPb

intb

BPW91c

intc

B3LYPb

intb

BPW91c

intc

2362.5 773.2 962.7 606.3 502.6 211.5 166.2 162.2 45.9

17 30 100 33 100 13 1 36 3

231.2 744.7 924.1 593.5 482.2 205.8 162.6 160.4 45.2

11 34 100 27 89 12 12 16 2

3198.2 1218.2 833.0 674.3 568.6 211.5 176.5 162.9 45.8

21 25 101 54 129 13 1 36 3

3130.5 1172.1 799.5 655.5 548.9 205.8 171.9 161.9 45.2

13 24 106 44 116 12 1 26 2

a Observed frequencies from laser ablation experiment. Frequencies in brackets observed in proton radiolysis, Figure 3, ref 22. Harmonic frequencies (cm−1) and intensities (km/mol) were computed with 6-311++G(3df,3pd). bComputed with B3LYP. cComputed with BPW91. dMixed vibrational mode: this mixing is H, D dependent. eCovered with precursor absorptions.

vibrational mode. The small 7.2 cm−1 13C shift for the 763.2 cm−1 band is also diagnostic for the CH2 wagging mode. CHCl2−Cl. In the CHCl3 spectra shown in Figure 2, the frequencies of the product absorptions marked “t” do not match with the previously reported values for the common fragments of CHCl3 (CCl3, CCl3+, CCl3+, CHCl2, CHCl2+, CCl2, CCl2+, CHCl, CHCl+, etc.).1−7 The earlier studies reveal that these fragments trapped in the matrix disappear stepwise in the following process of photolysis and annealing. In contrast, the t absorptions disappear on first visible irradiation analogous to the behavior of previously observed transient species produced by vacuum UV photolysis of tetrahalomethane and methylene halide precursors.15,16,18,21,24 In addition, the observed product

frequencies and isotopic shifts are consistent with the predicted values for CHCl2−Cl as shown in Table 2, indicative of formation of CHCl2−Cl during codeposition of CHCl3 with laser-ablated metal atoms. The strong t absorption at 995.5 cm−1 shows an unusual blue shift to 1059.9 cm−1 on deuteration and small 13C shift to 970.4 cm−1 (H/D and 12/13 ratios of 0.939 and 1.026), and it is assigned to the CCl2 antisymmetric stretching mode of CHCl2− Cl. The C and three atoms bonded to it (CHCl2 subunit) form a nearly planar structure (Figure 6), and as a result, the CCl2 antisymmetric stretching mode, which is essentially an in-plane mode, is coupled with the C−H in-plane bending mode. This coupling increases markedly on deuteration. The CCl2 E



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Table 6. Observed and Calculated Fundamental Frequencies of CBr3−Br Isotopomers in the Ground 1A′ Electronic Statea CBr3−Br approx description

obsd

A′ as CBr3 str A″ as CBr2 str A′ CBr3 deform A′ s CBr3 str A″ CBr3 bend A′ CBr3 bend A′ Br−Br str A″ CBrBr bend A′ CBrBr bend

[778] [326] [283]

B3LYP

b

int

817.3 762.3 343.4 283.0 181.5 174.3 147.2 49.7 35.3

b

13

BPW91

116 175 21 29 0 25 19 1 4

771.7 742.4 350.3 270.6 174.8 174.5 145.3 50.3 35.3

c

c

B3LYP

114 194 23 27 0 27 7 0 3

787.7 734.6 330.9 282.8 181.3 174.0 147.1 49.7 35.3

int

b

int

CBr3−Br

b

106 162 18 30 0 25 19 1 4

BPW91c

intc

743.7 715.4 337.4 270.5 174.7 174.3 145.3 50.3 35.3

104 180 20 28 0 27 7 0 3

a Frequencies observed in radiolysis experiment, Figures 3 and 4, ref 16. Harmonic frequencies (cm−1) and intensities (km/mol) were computed with 6-311++G(3df,3pd). No 13CBr4 sample was available, but the frequencies were calculated to help define the normal modes. bComputed with B3LYP. c Computed with BPW91.

Table 7. Natural Atomic Charges, Bond Lengths, Occupancies, Natural Bond Orders, and Structural Parameters of the iso-Halomethanes with Weak Halogen−Halogen Bonds Investigated in This Studya compd δ+

δ−

HClCCl ···Cl HFCClδ+···Clδ− HClCClδ+···Fδ− HBrCBrδ+···Brδ− FClCClδ+···Clδ− Cl2CClδ+···Fδ− Cl2CClδ+···Clδ− Cl2CClδ+···Brδ− ClBrCClδ+···Clδ− ClBrCBrδ+···Brδ− Br2CBrδ+···Clδ− Br2CBrδ+···Brδ− H2CClδ+···Clδ− H2CBrδ+···Brδ−

Q1b

Q2b

Q3b

Q4b

Q5b

r (CX)c

occupancyd (CX)

bond orderd (CX)

0.209 0.179 0.206 0.208 −0.298 0.149 0.164 0.163 0.168 0.169 0.248 0.244 0.197 0.200

0.134 −0.293 0.108 0.208 0.148 0.149 0.164 0.163 0.237 0.235 0.248 0.244 0.197 0.200

−0.312 0.196 −0.325 −0.533 0.230 −0.293 −0.290 −0.297 −0.373 −0.487 −0.577 −0.587 −0.383 −0.523

0.445 0.408 0.611 0.543 0.412 0.603 0.450 0.406 0.454 0.508 0.560 0.522 0.436 0.532

−0.476 −0.490. −0.600 −0.426 −0.493 −0.608 −0.487 −0.436 −0.486 −0.424 −0.480 −0.423 −0.448 −0.409

1.606 1.598 1.585 1.782 1.619 1.604 1.619 1.627 1.618 1.827 1.823 1.822 1.600 1.767

1.924, 1.371 1.882, 1.460 1.932, 1.520 1.913, 1.307 1.876, 1.304 1.884, 1.400 1.893, 1.253 1.893, 1.250 1.892, 1.258 1.791, 1.170 1.804, 1.252 1.804, 1.169 1.932, 1.535 1.961, 1.545

1.648 1.671 1.726 1.610 1.590 1.642 1.572 1.571 1.575 1.481 1.528 1.456 1.733 1.753

r(Xδ+···Xδ−)e ∠CXXe Φ(HXCX)f 2.473 2.467 1.952 2.696 2.470 1.962 2.487 2.649 1.488 2.687 2.524 2.690 2.435 2.672

125.0 126.9 124.5 126.9 133.2 133.2 128.8 128.0 128.8 138.0 140.8 137.0 122.1 122.0

164.1 162.0 165.7 157.7 159.1 165.5 164.4 163.1 164.2 157.6 159.0 157.7 165.9 160.8

a

Computed 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 bond length. d Natural occupancies of σ and π orbitals of the CX bond and its bond order. eX−X bond length and CXX angle. fDihedral angle of H, X, C, and X in the near planar structure

The C−H stretching frequency is compared with that of CHCl3 at 3053.4 cm−1. The higher C−H stretching frequency for CHCl2−Cl is also consistent with the cationic nature of its CHCl2 subunit. Table 2 shows that the five vibrational bands of CHCl2−Cl expected in our spectral range are all observed, substantiating production of CHCl2−Cl via photolysis of CHCl3 by the laserablation plume radiation. CHCl2−Cl is considerably higher in energy (214 kJ/mol) than CHCl3. Similarly, the CH2Cl−Cl and CCl3−Cl isomers are 243 and 185 kJ/mol higher energy than CH2Cl2 and CCl4. CHFCl−Cl. The frequencies of the observed t absorptions in Figure 3 again do not match with those of the common fragments of CHFCl2 (e.g., CFCl2, CFCl2+, CHFCl, CHFCl+, CCl2, etc.).1−7 The strong t absorption at 1198.9 cm−1 also shows a blue shift to 1243.7 cm−1 on deuteration and is assigned to the CFCl antisymmetric stretching mode of CHFCl−Cl. Parallel to the CHCl2−Cl case, the C and three atoms bonded to C form a near planar structure, and this in-plane stretching mode is strongly coupled with C−H in-plane bending mode, resulting in the blue shift on deuteration. This vibrational coupling also leads to the large D shift of the C−H in-plane bending absorption from 1342.3 to 982.0 cm−1 (H/D ratio of 1.367). The CFCl symmetric

antisymmetric stretching frequency, which interacts little with the higher C−H in-plane bending mode, is pushed up on deuteration because of interaction with the now lower C−D inplane bending mode. The blue shift of the CCl2 stretching absorption and the unusually large red shift for the C−D bend arise from this coupling. The C−H bending absorption observed at 1252.8 cm−1 shifts to 1247.0 cm−1 on 13C substitution and to 858.8 cm−1 on deuteration (carbon 12/13 and H/D ratios of 1.005 and 1.459). Analogous mode mixing has been observed and discussed for the essentially planar HCCl2 and DCCl2 free radicals whose frequencies are 1226, 902 cm−1 and 816, 974 cm−1.44 The higher CCl2 stretching frequency in HCCl2−Cl (996 cm−1) as compared with HCCl2 (902 cm−1) is consistent with increased π character in the former C−Cl bonds. Other t absorptions also support production of CHCl2−Cl. The strong t absorption at 652.1 cm−1 is designated to the C−H out-of-plane bending mode with its D and 13C counterparts at 523.3 and 643.5 cm−1. The weaker absorption at 785.2 cm−1 with its D and 13C counterparts at 757.9 and 768 (overlapped) cm−1 are assigned to the CCl2 symmetric stretching mode. In the high frequency region, the C−H stretching absorption is observed at 3086.5 cm−1 with its D and 13C counterparts at 2304.9 and 3077.0 cm−1 (H/D and 12/13 ratios of 1.339 and 1.003). F



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production of CHFCl−Cl via photoisomerization of CHFCl2. In contrast, CHCl2−F and CHClF−Cl, the other plausible products, are not detected in this study. CHFCl−Cl, CHCl2−F, and CHClF−Cl are 240, 301, and 532 kJ/mol higher in energy than CHFCl2. CHCl2−F would show its CCl2 antisymmetric and symmetric stretching bands at ∼1050 and 760 cm−1, which are not observed in this study. CHClF−Cl is energetically too high to expect its production, and the strongest observable C−Cl stretching absorption expected at ∼770 cm−1 is also not observed. Figure 7 illustrates the structure computed for CHFCl-Cl.

Figure 7. B3LYP structures of CHFCl2, transition state 1, CHFCl−Cl, transition state 2, and CHCl2−F. The bond lengths and angles are in angstroms and degrees. The natural atomic charges are also shown. CHFCl−Cl is observed, but CHCl2−F is not (see text). Figure 5. Comparisons of structures and natural charges for several series of iso-halomethanes calculated using B3LYP.

CFCl2−Cl. The observed vibrational characteristics of the product absorptions marked “t” shown in Figure 4 do not fit with those of the previously studied fragments of CFCl3 (CFCl3+, CFCl3+. CCl3, CCl3+, CFCl2, CFCl2+, CCl2, CFCl, etc.).1−7 The strong t absorption at 1211.1 cm−1 is accompanied with its 13C counterpart at 1181.2 cm−1 (12/13 ratio of 1.025) and assigned to the C−F stretching mode of CFCl2−Cl on the basis of its frequency and substantial 13C shift. The observed frequency and 13C shifts are in good correlation with the B3LYP values of 1238.2 and 33.8 cm−1. The 13CCl2 antisymmetric stretching absorption is observed at 1014.6 cm−1, but its 12C counterpart is unfortunately covered by precursor absorption. The t absorption at 584.2 cm−1 shows a relatively small 13C shift to 582.6 cm−1 and is assigned to the CCl2 symmetric stretching mode. The t absorption at 431 cm−1 is designated to the CFCl2 deformation mode without observation of its 13C counterpart. The observed t absorptions that correlate well with the DFT values shown in Table 4 support formation of CFCl2−Cl, which is 217 kJ/mol higher in energy than CFCl3. The other plausible products, CCl3−F and CCl2F−Cl, are not observed probably due to their high energies (287 and 436 kJ/mol higher than CFCl3). CCl3−F would show the strong CCl2 antisymmetric stretching absorption at ∼860 cm−1, which is not observed in this study. CCl2FCl is energetically too high to be produced, and its CCl2 antisymmetric stretching band expected at 710 cm−1 is also not observed in this study. The structure and natural charges for CFCl2−Cl are given in Figure 8. CHBr2−Br Assignments. Additional weak, photosensitive bands were found at 1186.8, 834.4, and 572.3 cm−1 in the spectra recorded by Lyon and Andrews25 using laser ablation in

Figure 6. B3LYP structures of CHCl3, the transition state, and CHCl2− Cl. The bond lengths and angles are in angstroms and degrees. The natural atomic charges are also shown.

stretching mode is observed at 938.2 cm−1, accompanied with its D counterpart at 850.5 cm−1. Unlike the CCl2 symmetric stretching mode of CHCl2−Cl, this CFCl symmetric stretching mode is far less symmetric and in fact mainly a C−Cl stretching mode, and as a result, it considerably involves the C−H in-plane bending motion and yields a substantial D shift. The other t absorptions are also consistent with the vibrational characteristics predicted for CHFCl−Cl. The C−H out-of-plane bending absorption is observed at 655.8 cm−1 with its D counterpart at 546.0 cm−1 (H/D ratio of 1.201). The weak t absorption at 522.3 cm−1 is designated to the CFCl bending mode without observation of the D counterpart. The C−H stretching absorption is observed at 3081.5 cm−1 and shifts to 2300.9 cm−1 on deuteration. The observed six vibrational bands in good correlation with the DFT values (Table 3) substantiate G



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153 and 183 kJ/mol higher in energy, respectively, than that for CCl3Br. In addition both CCl3 and CCl2Br free radicals and cations were observed. This underscores the lack of selectivity in the radiolysis process and little cage effect for irradiation during the deposition process as compared to selective irradiation after deposition as employed by the Maier group.14,20 These and other bands are assigned and compared to DFT frequencies in Tables S1 and S2. Notice that the antisymmetric CCl2 stretch is decreased only 2 cm−1 for CCl3−Br from CCl3−Cl while the “breathing” mode with very little 13C shift and significant 37Cl shift decreased 12 cm−1 and the CCl3 “umbrella” mode decreased 18 cm−1. The CCl2Br “breathing” mode in CCl2Br−Cl decreased substantially more to 430 cm−1. Molecular Structures and Bonding. The near tetrahedral structures of the precursors are compared with those of the transition states and products in Figures 5−8. The C and three atoms bonded to C form a near planar structure with a bridging halogen atom in the structure of the transition state. The product structure is in fact similar to that of the transition state, other than the larger ∠C−X−X, indicating that the transition state is energetically closer to the product than the reactant. The C−X bond lengths of the products are considerably shorter than those of the reactants. For example, the C−Cl bond lengths of 1.606 and 1.691 Å in CHCl2−Cl are significantly shorter than those of 1.775 Å in CHCl3. Particularly the Cl atom bonded to the residual Cl atom forms an exceptionally strong bond with carbon (C−Cl bond length of 1.606 Å). On the other hand, The Cl···Cl bond length (2.473 Å) is significantly longer than that of Cl2 (2.011 Å) computed at the same level of theory. NBO42 analysis shows that the unusually short C−X bond has considerable double bond character (i.e., natural bond order of 1.648 for CHCl2−Cl) as shown in Table 7 and the X···X bond is largely ionic (natural atomic charges of 0.445 and −0.476 for CHCl2−Cl and similarly Mulliken atomic charges46 of 0.335 and −0.416). Hence, the CHX2−X photoisomer is better represented as HXCXδ+···Xδ−. Similarly, in the structure of the transition state, the bridging X carries a substantial amount of negative charge, indicating that it is largely ionically bonded to the planar CHX2 subunit. For example, the bridging Cl in the transition state of the CHCl3 system owns a natural atomic charge of −0.610 (Mulliken atomic charge of −0.545), [CHCl2]0.610+···Cl0.610‑. The dihalo and tetrahalo analogues also have similar structures.6,7 The bond order of the short C−X bond shows a decreasing tendency with the number of halogen atoms bonded to carbon: the C−X bond length increases in the order of CH2X−X, CHX2−X, and CX3−X. (The average C−Cl length increases from 1.600 to 1.649 to 1.672 Å). More halogen atoms bonded to C are able to back-bond less on a per capita basis, which depletes this effect for the stronger C−X bond, and this effect is more evident in the Br containing species than in the Cl analogues. (The average C−Br length increases from 1.767 to 1.820 to 1.857 Å for this series, Figure 5.) This effect can also be visualized by comparing the average C−Cl bond length for CCl3−Cl, 1.672 Å, with the 1.714 Å bond length for the CCl3 radical and the 1.650 Å bond length for the CCl3+ cation, which are illustrated in Figure 5. On the basis of the computed natural charge on the weakly bound Cl (−0.49e), the CCl3 subunit in CCl3−Cl contains +0.49e, and it should exhibit half cation properties as revealed by its intermediate 1.672 Å average C−Cl bond length. In this regard, the CCl3−Br species is less ionic as revealed by the −0.44 natural charge on Br, and as a consequence, the stronger C−Cl is

Figure 8. B3LYP structures of CFCl3, transition state 1, CFCl2−Cl, transition state 2, and CCl3−F. The bond lengths and angles are in angstroms and degrees. The natural atomic charges are also shown. CFCl2−Cl is observed, but CCl3−F is not (see text).

experiments with CHBr3, which correlate with the highest three bands reported above for CHCl2−Cl. These bands were observed in higher yield with proton radiolysis at 1192, 839, and 570 cm−1 reported by Ault and Andrews and ascribed to molecular ions.22 The latter work also reported CDBr3 counterparts at 945, 755, and 473 cm−1 (see Figure 3 in ref 22). These bands are compared with DFT computed frequencies in Table 5. The H/D frequency ratio for the out-of-plane deformation 570/473 = 1.205 is bracketed by the two DFT computed frequency ratios. The H/D frequency ratio for the in-plane C−H/C−D bending mode 1192/945 = 1.261 is lower than typical for such modes, again owing to mixing with the antisymmetric CBr2 stretching modes. In this case the C−H bend does not shift below the CBr2 mode on deuterium substitution, but the bend− stretch interaction mixes these modes for DCBr2 and decreases the magnitude of the shift in the bending mode but shifts the CBr2 mode from 839 to 755 cm−1. Again a similar effect has been found for the HCBr2 and DCBr2 free radicals whose frequencies are 1165, 786 cm−1 and 898, 725 cm−1 for comparison.45 The higher CBr2 stretching frequency in HCBr2−Br (839 cm−1) as compared with HCBr2 (786 cm−1) attests to increased bond strength in the former species. CBr3−Br Assignments. New absorptions were found in proton radiolysis and noble gas resonance photoionization experiments15,16 with CBr4 in excess argon at 778, 326, and 283 cm−1. These bands were photosensitive, but not as much so as their CCl4 counterparts, which was attributed to the matrix “cage” effect for the larger bromine atom.16 Table 6 compares the calculated and observed frequencies for the iso-tetrabromomethane species. Note that both functionals underestimate the antisymmetric CBr3 stretching mode, which is the case for a number of halocarbon species. Note the large 13 cm−1 computed 13 C shift for the “umbrella” motion and the very small shift for the “breathing” mode, which parallel the analogous chlorine species modes. The CHBr2−Br and CBr3−Br photoisomers are 172 and 135 kJ/mol higher in energy, respectively, than their precursors on the basis of B3LYP calculations. CCl3−Br and CBrCl2−Cl Assignments. It is interesting to note that proton radiolysis and noble gas resonance photoionization experiments15,16 with CCl3Br produced the signature photosensitive antisymmetric stretching bands at 925 cm−1 for CCl3−Br and at 870 cm−1 for CBrCl2−Cl isomers, which are H



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CH2Cl−Cl is precisely intermediate between the radical and cation bond orders as expected from the natural charges. Reactions. Intrinsic reaction coordinate41 (IRC) computations are carried out for isomerization reactions between the reactants and products investigated in this study, revealing smooth interconversions as shown in Figures 9−11. Due to the

longer, 1.627 Å as is the average C−Cl bond length, 1.674 Å. Likewise, the weakly bound halogen effect can be seen by comparing the CCl3−X series (X = F, Cl, Br). The charge on CCl3 decreases from +0.61e to +0.49e to +0.44e while the shorter CCl bond length increases from 1.604 to 1.619 to 1.627 Å and the average C−Cl bond increases from 1.664 to 1.672 to 1.674 Å. The effect of more electronegative halogen atoms bonded to carbon can also be seen from the HFCCl−Cl and HClCCl−Cl pair: the charge on HXCCl is small, 0.49e and 0.48e, respectively, and the CCl bond length is 1.598 and 1.606 Å. Likewise the FClCCl−Cl and Cl2CCl−Cl pair: the XClCCl charge is 0.61e and 0.49e, respectively, and the CCl bond length is 1.604 and 1.619 Å. Similar statements can be made for CH2Cl−Cl where the natural charge on the CH2Cl subunit is +0.45. The C−Cl bond length (B3LYP) decreases from 1.780 Å (CH2Cl2) to 1.697 Å (CH2Cl) to 1.600 Å (CH2Cl−Cl) to 1.593 Å (CH2Cl+). The 958.4 cm−1 C−Cl stretching mode for CH2Cl−Cl is in fact closer to the PES value for CH2Cl+ (1040 cm−1) than to the 826.3 cm−1 free radical value and the precursor value (746 cm−1),3,43 which suggests considerable π character for the CCl bond in CH2Cl−Cl as found in our NBO calculation (1.73 bond order, Table 7) and follows the arguments presented by Maier et al.20 Similar NBO calculations were performed on CH2Cl and CH2Cl+ for comparison. First the radical, Mulliken atomic spin densities H (−0.026 × 2), C(0.930), Cl (0.122) show that some electron density is transferred from Cl to C. Two electrons occupy a σ bonding orbital made from C 2s (28%) and 2p (72%) and Cl 3s (20%) and 3p (80%), and a third electron occupies a π bonding orbital made from C 2p and Cl 3p. This gives a 1.50 bond order, which is in agreement with the qualitative prediction from force constants.43 Next, the cation has two (1.997 listed by the program) electrons in the bonding σ orbital made from C 2s (31%) and 2p (69%) and Cl 3s (20%) and 3p (80%) and two (1.9995 listed by the program) electrons in the bonding π orbital made from C 2p and Cl 3p. This gives a 2.00 bond order. Thus, the C−Cl NBO bond order 1.73 for

Figure 10. Intrinsic reaction coordinate (IRC) calculations between CHFCl2 and CHFCl−Cl and between CHFCl2 and CHCl2−F. The shallow energy minimum for CHFCl−Cl leads to prompt conversion of the transient species to CHFCl2 on visible irradiation. CHCl2−F is not observed in this study.

Figure 11. Intrinsic reaction coordinate (IRC) calculations between CFCl3 and CFCl2−Cl and between CFCl3 and CCl3−F. The shallow energy minimum for CFCl2−Cl leads to prompt conversion of the transient species to CFCl3 on visible irradiation. CCl3−F is not observed in this study.

Figure 9. Intrinsic reaction coordinate (IRC) calculation between CHCl3 and CHCl2−Cl. The shallow energy minimum for CHCl2−Cl leads to prompt conversion of the transient species to CHCl3 on visible irradiation. I



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large energy differences between the reactants and products, the transition state is energetically much closer to the product. The activation energies from the reactants to products (CHCl2−Cl, CHFCl−Cl, CHCl2−F, CFCl2−Cl, and CCl3−F) are 248, 274, 342, 254, and 329 kJ/mol, respectively. In contrast, the activation energies in the reverse reactions are considerably smaller (34, 33, 41, 36, 41 kJ/mol, respectively). The smaller activation energy from the product to reactant is consistent with the disappearance of the products on visible irradiation, and they do not reappear even with full arc (λ > 220 nm) irradiation (the Hg arc lamp emission has a λmax at 253.7 nm). IRC computations have also been carried out for production of CHCl2−F and CCl3−F that are 61 and 70 kJ/mol higher than the observed products (Figures 10 and 11). While the reactants and plausible products are linked via the transition state, the high activation energies (68 and 75 kJ/mol higher than those to the observed products with a Cl−Cl bond) evidently prevent formation of the products with a Cl−F bond. The IRC results show that in the photoisomerization of the precursor, one of the C−X bonds elongates first and the remaining atoms form a near planar triangle structure with the C atom at the center. The detached X atom later moves over to one of the remaining X atoms and forms a weak X−X bond. The photon energy for λ = 253.7 nm (471 kJ/mol) is in fact considerably higher than the activation energies, indicating that conversion in the cold matrix requires extra energy due to energy dissipation and matrix rearrangement for the reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Tables of calculated frequencies for mixed chlorobromomethanes. Cartesian coordinates for the title molecules. Figure comparing structures and natural charges for related products. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from National Science Foundation (U.S.) Grant CHE 03-52487 to L.A., and support from the Korea Research Foundation (KRF) grant funded by the Korean government (MEST) (No. 20090075428) and KISTI supercomputing center.

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REFERENCES

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CONCLUSIONS The CCl3−Cl, CHCl2−Cl, CHFCl-Cl, and CFCl2−Cl isomers are produced during codeposition of the precursors (CCl4, CHCl3, CHFCl2, and CFCl3) with laser-ablated metal atoms and the associated laser plume irradiation and identified in the matrix IR spectra with isotopic substitution and DFT computational results. The other plausible products, CHCl2−F, CHClF−Cl, CCl3−F, and CCl2F−Cl, are not detected due to their higher energies. The observed products have near planar structures for the C and three atoms bonded to it, and the residual Cl atom is bonded to one of the Cl atoms. The product structure is similar to the structure of the transition state except for the larger ∠C−X−X. The C−X bonds of the product are considerably stronger than those of the reactant. Particularly, the X atom that is bonded to the residual X atom forms an unusually strong C−X bond. NBO analysis reveals that the strong carbon−chlorine bond has considerable double bond character (natural bond order >1.5). The weak X···X bond is largely ionic on the basis of the large atomic charges, and therefore, the product may be better represented with C−X double and X···X partially ionic bonds, HXCXδ+···Xδ‑. These ionic properties are revealed for CCl3− Cl, which has an average C−Cl bond length near the median for CCl3 radical and cation. The CBr3−Br and CHBr2−Br isomers are less ionic and have lower CBr bond orders than the analogous chlorine species. The IRC calculations reproduce smooth conversion between the reactant and product. The transition state is much closer in energy to the product than the reactant, consistent with the prompt disappearance of the product on visible irradiation after original deposition and the similarity in the structures of the transition state and product. Isomerization in the cold matrix evidently requires more energy than the activation energy estimated for the isolated molecule. J



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