Matrix Infrared Spectra, Photochemistry and Density Functional

Subscriber access provided by Iowa State University | Library

A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Matrix Infrared Spectra, Photochemistry and Density Functional Calculations of Cl-HCCl, ClHCl, Cl-ClCCl, and Cl-HCHCl Produced from CHCl and CHCl Exposed to Irradiation from Laser Ablation -

3

2

2

-

-

2

Han-Gook Cho, and Lester Andrews J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12162 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Matrix Infrared Spectra, Photochemistry and Density Functional Calculations of Cl−HCCl2, ClHCl−, Cl-ClCCl, and Cl−-HCHCl Produced from CHCl3 and CH2Cl2 Exposed to Irradiation from Laser Ablation Han-Gook Choa,b and Lester Andrews*,b aDepartment

of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-gu,

Incheon, 22012, South Korea bDepartment

of Chemistry, University of Virginia, P. O. Box 400319, Charlottesville,

Virginia 22904-4319, United States

S

Supporting Information

ABSTRACT: Strong absorptions for Cl−-HCCl2 with D and

13C

isotopes were observed in

the spectra of CHCl3 co-deposited with laser-ablated metal atoms, cations, electrons and vacuum ultraviolet radiation, which shows that the precursor is an effective electron scavenger. The IR spectra, isotopic shifts, and DFT calculations identified the major product as Cl−-HCCl2, which is characterized by a strong, broad C-H stretching mode interacting with the overtone of the H-C-Cl bending fundamental. These absorptions decreased on subsequent annealing and photolysis treatments while the ClHCl− absorptions increased, suggesting that dissociation of the chloroform anion generates the

stable symmetrical hydrogen dichloride

anion as does the reaction of HCl and Cl−. A new set of strong, broad absorptions in the deposition spectra that diminished on the early annealing and photolysis are assigned to the Cl-ClCCl radical isomer. Dominant spectral features in the C-H stretching region for the experiments with CH2Cl2 are assigned to the symmetric C-H and the anti-symmetric Cl-H-CH stretching bands of the methylene chloride anion Cl−-HCHCl. The stronger, broader, lower frequency bands are due to the hydrogen bonded hydrogen stretching, and the weaker, sharper, higher frequency absorptions are due to the terminal C-H bond stretching. Similar

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

experiments with CHBr3 produced absorptions for the analogous Br−-HCBr2 and BrHBr− anions.


Page 2 of 44

Page 3 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION A number of isomers, fragments, and ions of chloromethanes and related species were identified in their matrix isolation spectra when these samples are exposed to irradiation during deposition.1-21 The strong absorptions of CCl3 and X2CCl-Cl (X = H or halogen)11-13 along with other products are common features in matrix IR spectra after photolysis of the corresponding precursors.22,23 The hydrogen dihalides (XHX−) are symmetrical (D∞h) anions.17 Their anti-symmetric hydrogen stretching bands are exceptionally strong, and their relatively low frequencies originate from vibration of the positively charged H atom between the two loosely bound negatively charged heavy halogen atoms.16-21 These anions are stable enough to survive in a matrix on the early stages of annealing and ultraviolet photolysis. Noble and Pimentel first detected the ClHCl− anion in matrix IR spectra by photolysis of a Cl2 + HCl mixture with radiation from a glow discharge tube, but assigned the observed bands to the neutral HCl2 radical.16 Milligan and Jacox observed the same product absorptions on vacuum ultraviolet photolysis of HCl during condensation in excess argon and reassigned them to the symmetrical anion, ClHCl−.17 They also found that alkali metal impurities in the matrix considerably increased its production yield and designated the absorption at 722 cm-1 to ClHCl−∙∙∙M. Andrews et al. observed the strong band at 696 cm-1 and the weaker one at 956 cm-1 via argon microwave discharge irradiation of a Cl2 + HCl mixture and reconfirmed their assignments to the ν3 and ν1 + ν3 bands of the hydrogen dichloride anion.18 In addition Ault and Andrews co-deposited the KCl, RbCl and CsCl salt molecules with HCl in argon and observed strong new absorptions at 736, 729 and 723 cm-1, respectively, which were assigned to the corresponding (M+)(ClHCl−) ion pair species.19 The slightly higher frequencies of these new absorptions relative to the strong 696 cm-1 band reinforces its assignment to the isolated dichloride anion.19



Page 4 of 44

Andrews et al. also reported the formation of HCX3− (X = Cl, Br) and XHX− along with other ions in the proton radiolysis of CHCl3 and CHBr3 during sample deposition.10 They later provided these products using argon resonance microwave discharge radiation of chloroform argon matrix samples and concluded that the chloroform anion Cl−-HCCl2 is in fact

the

intramolecular

hydrogen-bonded

chloride-dichloromethyl

radical

anion

(Cl−∙∙∙HCCl2).15 These workers also suggested that Cl−-HCCl2 dissociates into ClHCl− + CCl and that a strong broad band at 705 cm-1 originates from the ultraviolet photolysis product ClHCl−∙∙∙CCl. Maier et al. observed H2CX-Z (X = halogen, Z = Cl, Br, I) via uv photoisomerization of the corresponding methylene halides using radiation not capable of ionization.12 While methylene halide anions (CH2X2−) were not characterized in thorough investigations of photo-fragments of CH2X2 (X = halogen) in the matrix spectra by means of photolysis and electron bombardment,20,21 their generation was observed by ESR following γ irradiation of tetrahydropyran doped CH2Cl2 glass24 and by IR after 4.88 ev photoelectron transfer from DMA (N,N’-dimethylaniline) to the CH2Cl2 precursor in an argon matrix.25 Although

these IR assignments were supported by UMP2/631++G** calculations no

isotopic substitution was offered to confirm the vibrational assignments nor was the effect of hydrogen bonding on the vibrational spectrum discussed.25 It has been reported that co-deposition of halomethanes with laser-ablated metal atoms produces various photo-isomers, fragments, and ions of the precursors due to the laser plume radiation while also producing small metal complexes.13,22,23 In this paper, we report the observation of the chloroform anion Cl−-HCCl2 and its photolysis product ClHCl− provided by co-deposition of CHCl3 and its 13C and D substituted isotopes with laser-ablated metal plume irradiation and also a new isomer of the CCl3 radical, Cl-ClCCl. The strong absorptions in the hydrogen stretching region of similarly treated CH2Cl2 are assigned to this precursor anion, Cl−-HCHCl along with its

13C

and D substituted isotopes. Evidently laser-


Page 5 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ablation of metal atoms offers good opportunity to transform the subject precursors into rare chemical species.

EXPERIMENTAL AND COMPUTATIONAL METHODS The matrix infrared spectra of Cl−-HCCl2, ClHCl−, Cl-ClCCl, and Cl−-HCHCl and their stable D and 13C isotopes described in this report were recorded from samples prepared by co-deposition of laser-ablated Pb or Ru atoms (Johnson-Matthey), their cations, electrons and their plume radiation with 0.2–0.7% CHCl3, CDCl3, 13CH

2Cl2

13CHCl , 3

CH2Cl2, CD2Cl2, and

(Aldrich and Cambridge Isotopes) in excess argon.26,27 These product absorptions

are common bands for the precursors regardless of the ablated metal.22,23 Therefore, the radical and anion products are metal atom-independent. Analogous product species were observed using the CHBr3 reagent. The above matrix samples were condensed at 4 K using a closed-cycle refrigerator (Sumitomo) or at 10 K using an older refrigerator (Air Products, Displex). These experimental details have been described elsewhere.22,23,28,29 After photochemical reaction on deposition, infrared spectra were recorded at a resolution of 0.5 cm–1 using a Nicolet iS50 spectrometer with a liquid nitrogen cooled MCT-A detector or a Nicolet 550 spectrometer with an MCT-B detector.

Samples were next warmed and re-cooled (annealed) and later

irradiated using a 175 W mercury arc street lamp down to 220 nm for 20 min periods with or without glass filters, and more spectra were recorded for comparison. Supporting calculations using density functional theory (DFT) were done with Gaussian 0930, the B3LYP density functional,31,32 and the 6-311++G(3df,3pd) basis sets33 for H, C, Cl and Br to provide vibrational frequencies and energies for the reaction products. Structures were relaxed, and the optimized geometry was substantiated through vibrational analysis. Energy calculations are accurate to +/- 5 kJ/mol at best. Frequencies are reported and



used here without scaling. More BPW9134 calculations were done to support the B3LYP results. Vibrational frequencies were calculated analytically, and zero-point energy was included in the binding energy.

RESULTS AND DISCUSSION The laser ablation process gives plasma plume radiation emanating from the metal target surface, which is responsible for photo-dissociation and photo-isomerization of the precursor molecules in the condensing matrix sample.22,23,28,29 This process also produces electrons, cations, and therewith new isolated anion products in the matrix. Figures 1-6

show the matrix IR spectra in the absorption regions for Cl−-HCCl2, the

chloroform anion, ClHCl−, Cl-ClCCl, and Cl−-HCHCl, the methylene chloride anion, from co-deposition of CHCl3 isotopic precursors with Pb or Ru plasma radiation and for CH2Cl2 precursors with Pb and their variation with subsequent photolysis and annealing treatments.26,27 Other metal independent product absorptions (HClCCl-Cl, CCl3, CCl, and CH4) along with the precursor absorptions are also designated in the spectra. The observed frequencies are listed in Tables 1-4 and compared with the calculated values in Tables S1-S4. The Cl−-HCCl2 and ClHCl− frequencies are close to those previously observed using other methods.12-17,26,27 Figure S1 shows the infrared spectra with very strong bands for ClHCl− and ClDCl− prepared by laser ablation of U with argon matrix samples of HCl and DCl.35 In addition to the chloroform anion Cl−-HCCl2 and its ultraviolet photolysis to give (Cl-H-Cl)− and CCl, and the methylene chloride anion, Cl−-HCHCl, the vibrational characteristics of the new product Cl-ClCCl radical are reported here. Bromoform was also investigated, and Figure 7 compares the spectra using Th ablation, and Figure S2 shows expanded wavenumber spectra for the lower region. Figures 8, S3 and 9 illustrate the results of intrinsic reaction coordinate36 calculations between Cl−-HCCl2 and ClHCl−∙∙∙ClC, between ClH∙∙∙[ClCCl] – and 6 ACS Paragon Plus Environment

Page 6 of 44

Page 7 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ClHCl-ClC and between CCl3 and Cl-ClCCl. Finally, figures 10 and 11 show computed structures for the reaction products and transition states. Cl−-HCCl2. Absorptions for the Cl−-HCCl2 anion were not found in the spectra after deposition of the precursor without laser-ablated metal irradiation [(a) in Figures 1, 2, and 3], but they were clearly observed in the first spectra recorded after co-deposition with laser ablated metal atoms, cations, electrons, and irradiation [(b) in Figures 1, 2, and 3] 26,27. The CCl3 and CHCl2-Cl absorptions were clearly visible, and the CHCl2-Cl absorptions disappeared on photolysis.11,13 The broad Cl−-HCCl2 band at 2720.2 cm-1 (full width at half maximum, FWHM, 18 cm-1) was particularly strong and the most prominent spectral feature in the high frequency region.

The strong Cl−-13HCCl2 counterpart at 2710.3 cm-1 is slightly

broader (FWHM, 22 cm-1). This increased infrared intensity and bandwidth are due to the hydrogen bonding interation,37,38 as the C-H stretching mode for the HCCl2 radical is extremely weak (calculated 3231 cm-1, < 1 km/mol) and thus not observed.8 The four Cl−HCCl2 absorptions decreased in concert on annealing and photolysis, but they decreased most (> 30%) on full arc photolysis (λ > 220 nm). In the H13CCl3 experiment photolysis with λ > 420 nm immediately after co-deposition reduced these bands by 10% and irradiation with 240 < λ < 380 nm by another 30%. The 703.8 cm-1 band appeared with the uv irradiation (Figure 3), but annealing to 28 K substantially reduced the latter absorption and annealing to 35 K (Figure 1) destroyed the latter band. The observed Cl−-HCCl2 frequencies (2720.2, 2495.6, 1270.5, and 837.1 cm-1) for the C-H stretching, H-C-Cl bending overtone and fundamental, and C-Cl2 anti-symmetric stretching bands are close to previously reported values (2723, 2499, 1271, and 838 cm-1) using argon resonance photoionization for production and a filter-grating infrared spectrophotometer for measurement.15 The C-H stretching mode perturbed by the hydrogen bonded Cl− is obviously red shifted and intensified as observed previously,15 but the C-H



stretch for the isolated HCCl2 radical was too weak to observe.8 However, the C-H stretching mode for H-CCl3 in solid argon is a good model (3053.3 cm-1) for an unshifted C-H stretching mode, and using this reference our 2720.2 cm-1 product band for the C-H stretch is red shifted about 333 cm-1 owing to the hydrogen bonded Cl−. Our B3LYP calculation predicted the C-H stretching mode at 2818.7 cm-1 or 3.5 % higher than the 2720.2 cm-1 observed value for the Cl−-HCCl2 structure anion, which is excellent agreement.22 This mode for H13CCl3 shifts down by 10.4 cm-1, which is slightly more than the 9.9 cm-1 shift found for our Cl−-H13CCl2 product. The H/D frequency ratio for these major absorptions is 1.323, which defines a C-H stretching mode (the H/D ratio for HCCl3/DCCl3 is 1.341). The deuterium shifts are comparable: H-CCl3 redshifts 776.8 cm-1 or 25.4% for DCCl3 whereas the anion product redshifts 663.7 cm-1 or 24.4% so this is clearly a perturbed CH stretching mode. However, the intensity, bandwidth and behavior of this band on D and Br substitution14,15 and the results of our structure calculations (Figure 10, below) confirm that the Cl− anion is bonded to the H in the HCCl2 radical and the Br− anion to the H in the HCBr2 radical.39,40 The larger band widths of 18 and 16 cm-1, respectively, Figures 1 and 7, for the Cl−-HCCl2 and the Br−-HCBr2 structures are due to the hydrogen bonding interaction.37,38 On the other hand, the H-C-Cl bending mode band at 1270.5 is sharp (FWHM, 1.0 cm-1) and blue shifted 44.5 cm-1 from the HCCl2 radical 1226.0 cm-1value,8 to the anion product, above, which is the effect expected for a hydrogen bond on a bending mode. The new H-C-Cl bending mode overtone is assigned here at 2495.6 cm-1 for the anion, which is 45.4 cm-1 lower than 2 x the fundamental, 2541.0 cm-1, owing to anharmonicity. A similar comparison finds the

13C

counterpart overtone to be 44.8 cm-1 lower than double the

fundamental and the D counterpart overtone to be 29.8 cm-1 lower than double the fundamental. Figures 1and 3 show the H-C-Cl bending mode fundamentals and overtones, and the integrated overtone intensities are measured as 5.1 and 5.4 x that of the bending mode


Page 8 of 44

Page 9 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fundamental, respectively, and our spectra in Figure 2 for the D-C-Cl analog shows the overtone to be only 55 % of the fundamental intensity. The reader can see from our spectra in Figures 1-3 that the overtone bandwidths are larger than the bandwidths for the fundamentals. The measurements are 3.4 and 1.0 cm-1 for H-12C-Cl overtone and fundamental, respectively, 3.1 and 1.1 cm-1 for D-C-Cl, and 4.3 and 1.3 cm-1 for H-13C-Cl. It follows that the H-C-Cl bending mode overtone, which is about 220 cm-1 lower than the strong C-H stretching band and has the same A’ symmetry, gains intensity and bandwidth from interaction with the strong C-H stretching mode. In the D-C-Cl case, even though the overtone is closer to the CD stretching mode (165 cm-1), its anharmonicity and the interaction are less, and the overtone is weaker than the fundamental. This is usually the case for an infrared spectrum where overtones are rarely observed. The anti-symmetric C-Cl2 stretching band red shifts 64.9 cm-1 from the HCCl2 radical (902.0 cm-1)8 to the anion product (Figure 1). This mode consists of three absorptions resolved at 837.1, 834.9, and 832.6 cm-1 with 9:6:1 intensity ratios, which indicates that two equivalent Cl atoms with natural abundance

35Cl

and

37Cl

participate in this vibration. The

observed 37Cl isotopic splittings, 2.2 and 2.3 cm-1 are in agreement with the calculated values of 2.1 and 2.3 cm-1, and they also agree with the first (C35Cl2 to C35Cl37Cl) observed shifts for the antisymmetric C-Cl2 stretching modes of CCl2 (2.0) and CH2Cl2 (2.2) but not as well with their symmetric mode counterparts (2.9) and 2.8 cm-1).6 The deuterated isotopomer mode is similarly split, but the 13C-substituted isotopomer mode is covered by an H13CCl3 absorption. The observed frequencies are listed in Table 1 and compared with the calculated values in Table S1. The computed C-Cl bond length in the anion, 1.726 Å, is longer than the HCCl2 radical value, 1.703 Å.

The C- Cl bondlength for the saturated molecule, H2CCl2, is 1.780

Å. The natural charges, Figure 10, show slightly more negative charge on the Cl atoms bonded to carbon in the anion than in the radical.



Page 10 of 44

The strong absorptions for Cl−-HCCl2 in these matrix isolation experiments also reflect the high electron affinity of the precursor (CHCl3 + e → Cl−-HCCl2), which is estimated by DFT to be 130 kJ/mol. It is comparable to that of CCl4 (143 kJ/mol), which is often used as an electron scavenger in the matrix as a diagnostic for charged trapped species.22,23,28,29 This is also in line with the previous observation that

anionic transition-

metal complexes were not observed in reactions with chloromethanes.22,23 The large decrease of the bands on photolysis indicates that this anionic species easily photo-dissociates [Eq. (S13-18)]. Weak, broad bands grow in at 2759.2 cm-1 on annealing in the HCCl3 experiment and at 2089.5 cm-1 using DCCl2. These higher frequency bands define almost the same H/D frequency ratio, 1.321, as the stronger absorptions, and they are probably due to this anion product interacting with another precursor molecule.

Table 1. Absorption Frequencies of Cl−-HCCl2 Isotopomers Provided During Codeposition of their Chloroform Precursors with Laser-Ablated Metal Atoms, Cations and Electrons in Excess Argona Cl−-HCCl2

Cl−-DCCl2

Cl−-H13CCl2

Description

2759.2, 2720.2

2066.1, 2056.5, 2050.2

2710.3

A’ C-H stretch

2495.6

1891.6

2489.6

A’ H-C-Cl bend overtone A” H-C-Cl bend

1270.5, 1269.3 963.1, 960.7 1267.2, 1265.9 837.1, 834.9, 684.8, 682.0, Covered (808) A” CCl2 as. stretch 832.6 679.3 aAll frequencies are in cm–1. Stronger absorptions are bold. Description gives major coordinate with a plane of symmetry. The estimated frequency of the product absorption covered by a precursor band is shown in parenthesis.


Page 11 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ClHCl−. The anti-symmetric Cl-H-Cl stretching absorption of a ClHCl− anion was barely visible as a matrix site at 703.8 cm-1 in the original co-deposition spectra, but it disappeared on early annealing to 20 and 30 K [(c and d) in Figure 1] and a new absorption emerged at 695.8 cm-1, which is very close to the previously reported value of 695.6 cm1.16,17,19,35

Deuteration shifted the sharp band to 463.2 cm-1 (H/D ratio 1.502), which is in

agreement with the reported 463.3 cm-1 value. These absorptions increased on early annealing to 20 and 30 K, and moreover the broad absorption at 703.8 cm-1 reappeared with a much higher absorption intensity on photolysis (λ > 220 nm), dramatically increasing the total intensity (more than quadruple in the Pb + CHCl3 spectrum) [(e) in Figures 1 and 2 and (d and e) in Figure 3]. However, the broad 703.8 band disappears again following annealing to 35 K, leaving the absorption at 695.8 cm-1, which is stronger than before the photolysis [(f) in Figures 1, 2, and 3]. The D counterpart of the broad absorption at703.8 cm-1 was observed at 469.7 cm-1 (H/D ratio 1.498). The present ClHCl− absorptions are much weaker than observed using HCl and DCl samples,35 because the present chloromethane experiments produce a limited amount of HCl or DCl by laser plume photolysis. Weak bands were observed here for HCl and DCl at 2869.9 and 2079.9 cm-1.35 The broader band at 703.8 cm-1 is in agreement with the very strong broad 705 cm-1 band in the earlier experiment where chloroform was subjected to vacuum ultraviolet photolysis with argon resonance radiation.15 The spectra in Figure 1 suggest that the broader 703.8 cm-1 band is due the matrix site for ClHCl− produced by the uv photodissociation of Cl−-HCCl2, and annealing produces the argon cage that gives the common 695.8 cm-1 absorption for ClHCl−.16,17,19,35 Calculation for the (ClHCl)−∙∙∙CCl isomer gives a distorted product with a 2249 cm-1 C-H stretching mode, which is not observed. Final annealing also consumes CCl and increases the CCl2 absorption at 743.9 cm-1 presumably through the diffusion and reaction of Cl atoms.6 11 ACS Paragon Plus Environment


The Cl-H-Cl anti-symmetric stretching mode of ClHCl− is essentially an H vibration between the two heavy Cl atoms. The H-Cl bond in ClHCl− is intermediate between the H-Cl diatomic bond and the H∙∙∙Cl− hydrogen bond. The weak ν1 + ν3 band (the combination band of symmetric and anti-symmetric stretching modes) was observed at 955.8 cm-1 with a broad site absorption at 972.3 cm-1, close to the previously reported value of 956.7 cm-1.18,21,35 A weaker absorption appeared at 721 cm-1, which was near to the previously reported 722 cm-1 for a band that Milligan and Jacox assigned to a weakly bound complex, ClHCl−∙∙∙M without specifying M.17 It disappeared on photolysis and reappeared on further annealing. The other vibrational bands for ClHCl− [Cl-H-Cl bending (ν2) and symmetric stretching (ν1) bands] are all too weak to observe as shown in Table S2.

Table 2. Absorption Frequencies for ClHCl− Isotopomers Provided during Codeposition of Chloroform Precursors with Laser-Ablated Metal Atoms, Cations and Electrons in Excess Argona ClHCl−

ClDCl−

Description

703.8, 695.7, (695.9)

469.7, 463.2, (463.3)

ν3 Σu Cl-H-Cl as. str. (ν1 + ν3) Σu combination

972.3, 955.8

ClHCl−···CHCl3 Cl-H-Cl as. str. aAll frequencies are in cm–1. Stronger absorptions are bold. The numbers in parenthesis were 542b

721

observed in the Pb + CH2Cl2 spectra. The italic bands are due to the matrix site for (ClHCl) – and (ClDCl) – that are formed on uv photolysis. Description gives major coordinate. bTentative

assignment.

Increasing ClHCl− absorptions on annealing suggests that the symmetrical anion is generated in reactions of the trapped chemical species in the matrix. Andrews et al. explained


Page 12 of 44

Page 13 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the formation of ClHCl− in their HCl + Cl2 system as combination of HCl with Cl− produced by the Ar microwave discharge vuv irradiation of reagents.19 Similarly, the production of ClHCl− on annealing observed in this study also resulted from combination of HCl and Cl− produced by laser plume radiation [HCl + Cl− → (ClHCl)− is 105 kJ/mol exothermic, Eq. (S1)]. That is certainly the mechanism for laser ablated metal plume irradiation of HCl in the formation of ClHCl−.35 The dissociation of CHCl2-Cl and Cl−-HCCl2 provides Cl, HCl and Cl− [1 and 53 kJ/mol exothermic Eqs. (S11 and S15)]. The large increase of ClHCl− on uv photolysis shown in Figures 1, 2, and 3 suggests an additional reaction path to the hydrogen dichloride anion from the most stable form of the chloroform anion Cl−-HCCl215 It is shown in Figures 1, 2, and 3 that Cl−-HCCl2 decreased substantially on uv photolysis (λ > 220 nm) while ClHCl− and CCl increased. The observed frequencies of CCl isotopomers in this study are listed in Table S5. The dissociation of Cl−HCCl2 (Cl−-HCCl2 → ClHCl− + CCl) is 190 kJ/mol endothermic [Eq. (S13)], and this energy is provided by uv photolysis including enough excess energy to propel CCl out of the matrix cage so that the (ClHCl)− anion is left isolated and symmetrical. Our calculations show that dissociation of the chloroform anion occurs in two steps [Eq. (1)].

TS1 TS2 − − 2 → [ClH∙∙∙ClCCl] → ClHCl ∙∙∙ClC

Cl−-HCCl

(1)

The transition state of the first step (TS1) is 133 kJ/mol higher than Cl−-HCCl2 and that of the second step (TS2) 180.0 kJ/mol higher (11.9 kJ/mol higher than ClHCl−∙∙∙ClC) as shown in Figure 8. Intrinsic reaction coordinate calculations36 also show possible conversions between the constituents as presented in Figures S4 and 9. Weakly bound ClHCl−∙∙∙ClC can eventually



Page 14 of 44

dissociate into ClHCl− + CCl in the matrix environment. However, the intermediate {[ClH∙∙∙ClCCl]−} between TS1 and TS2, which is 40 kJ/mol more stable than ClHCl−∙∙∙ClC would show its strongest H-Cl stretching band at ~2150 cm-1, which was not detected in this study. Other possible reaction paths cannot be eliminated. Particularly the precursor reaction of CHCl3 + Cl− → ClHCl− + CCl2 is only 110 kJ/mol endothermic [Eq. (S3)]. The Cl− with enough residual energy after photo-dissociation may be able to react with the precursor to produce the symmetric anion. Another precursor reaction of CHCl3 + H− → ClHCl− + CHCl is 175 kJ/mol exothermic [Eq. (S4)]. However, production of H− is energetically less favorable [The dissociation of CHCl3− to CCl3 and H− is 416 kJ/mol endothermic, Eq. (S14)].

Cl-ClCCl. A strong, broad absorption was observed at 794.2 cm-1 in the original deposition spectrum in Figure 1, ~30 cm-1 on the blue side of the very strong CCl3 antisymmetric stretching absorption of the CHCl3 precursor. This new band dramatically decreased on early annealing to 20 K [(c) in Figure 1] and almost disappeared on photolysis, which indicates that it originates from a species that has a shallow energy minimum. A weaker absorption that tracks with the strong absorption was observed at 650.1 cm-1. In the CDCl3 spectra in Figure 2, the absorption was also observed at 794.2 cm-1. Unfortunately the 13C

counterpart anticipated at 767 cm-1 in Figure 3 was overlapped by a residual CHCl3

absorption, but it is still observed that the total absorption intensity decreased considerably on early annealing to 20 K (b and c) in Figure 3. The CHCl3 absorption alone should stay almost unchanged on early annealing. However, the weaker absorption of the

13C

counterpart was

observed at 633.7 cm-1. These observed frequencies are listed in Table 3 and compared with the calculated values in Table S3.


Page 15 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The frequencies and behavior of the 794.2 and 650.1 cm-1 bands do not match with any of the reported values of the fragments, ions, and isomers generated from CHCl3 (CCl, CHCl, CCl2, CCl2+, CCl2−, CHCl2, CHCl2+, CHCl2−, CCl3, CHCl2-Cl, Cl-HCCl2−).1-15 We assign the observed bands to the CCl2 anti-symmetric and symmetric stretching modes of ClClCCl, a higher energy isomer of CCl3, whose absorption at 898 cm-1 is a well-known common feature in CHCl3 precursor spectra as shown in Figure 1.9,11 The observed frequencies compare favorably with the calculated values of 856.6 and 631.6 cm-1, and the 13C

shifts (27 and 16 cm-1) match exactly with the predicted values (Table S3). Our IRC calculations show that Cl-ClCCl is 238 kJ/mol higher in energy than CCl3

and the transition state between CCl3 and Cl-ClCCl is 263 kJ/mol higher than CCl3. Conversion between the two species [Eq. (2)] should be smooth as our intrinsic reaction coordinate calculation36 shows [Figure 9], indicating that Cl-ClCCl is a relatively unstable species even in a cold matrix, consistent with the dramatic decrease of it on first annealing and later photolysis as observed in Figures 1, 2, and 3. Only 25 kJ/mol is required to bump Cl-ClCCl over the transition state and down to CCl3. This isomer of CCl3 was not identified in the previous studies probably because the absorptions were close to or overlapped by the absorptions of the precursor or other products, and they were difficult to trap in a matrix that is not sufficiently cold.1-15 Unfortunately they were covered by precursor absorption in our CCl4 precursor spectra.22.23,26,27 TS3 CCl3 ↔ Cl-ClCCl

(2)

Table 3. Absorption Frequencies of Cl-ClCCl Isotopomers Provided during Codeposition of its Chloroform Precursors in Excess Argon with Laser-Ablated Metal Atoms and Plume Radiation a Cl-ClCCl Cl-Cl13CCl Description



Overlapp ed (767) 633.7

794.2 650.1 aAll

Page 16 of 44

CCl2 as. str. CCl2 s. str.

frequencies are in cm–1. Description gives major coordinate. The estimated

frequency of product absorption covered by precursor band is shown in parenthesis.

Cl−-HCHCl. Figures 4, 5, and 6 show the spectra of Pb ablation with CH2Cl2 isotopomers and their variation on annealing and photolysis. The ClHCl− absorptions were also detected, but much weaker than with chloroform, as shown in Figures 4 and 5. The frequencies of the symmetrical anion observed in the CH2Cl2 spectra (listed in parentheses in Table 2) were essentially the same as those in the CHCl3 spectra. Unfortunately, the ClHCl− absorption in the 13CH2Cl2 spectra was covered by a precursor absorption. Strong product absorptions were observed on the blue and red sides of the antisymmetric and symmetric CH2 stretching bands at 3056.2 and 2993.6 cm-1 in solid argon for the CH2Cl2 precursor. Those at 2903.3 and 2881.8 cm-1 on the red side were particularly strong, and broader (FWHM, 6.5

cm-1), and those on the blue side at 3130.3 and

3107.9 cm-1 were weaker and sharper (FWHM, 2.9 cm-1). The higher frequency absorption of the lower pair increased as the lower frequency component decreased on annealing. The lower frequency band re-emerged on photolysis and disappeared again on further annealing. The lower of our two matrix site bands for each fundamental mode shown in Figure 4 agree with the previously observed bands in the difference spectrum (3108 and 2882 cm-1) of Schweig et al. after photoelectron transfer from DMA to CH2Cl2 in the argon matrix sample.25

The C-Cl stretching band previously reported at 797 cm-1

25

(the calculated

frequency 768 cm-1 in Table S4) might be a common spectral feature in the CH2Cl2 matrix spectra. This product band was most likely covered by precursor absorption at 780-730 cm-1.


Page 17 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Deuteration shifted the lower pair to 2061.8 and 2045.2 cm-1 (H/D ratios 1.408 and 1.409) and showed another matrix site absorption at 2055.8 cm-1. The 13C substitution moved the lower stronger pair to 2896.7 and 2875.0 cm-1 (6.6 and 6.8 cm-1 shifts). The weaker absorption on the blue side was observed at 3130.3 cm-1 with a stronger site absorption at 3107.9 cm-1. The D counterpart of the higher 3130.3 cm-1 band was observed at 2360.0 cm-1 (H/D ratio 1.326), and 13C substitution shifted the higher band to 3118.5 cm-1 (shift 11.8 cm1),

while D substitution shifted the lower 2881.8 band to 2045.2 cm-1 (H/D ratio 1.409). The

large D and small

13C

shifts indicate that both band sets originate from C-H stretching

vibrations, but important differences between these two modes are revealed by their isotopic shifts and bandwidths. The higher frequency bands (3100 region) have more carbon character (larger 12-13 isotopic shifts) and smaller H character (smaller H-D shift and lower H/D ratio, 1.326), which are appropriate for a terminal C-H stretching mode. Compare the new 3130.3, 3107.9 cm-1 bands with the C-H stretch for HCCl3 at 3053.3 cm-1 in solid argon, which red shifts 10.4 cm1

with

13C

and exhibits a 1.341 H/D ratio. However, the lower stronger absorptions (2900

region) have contrasting effects of more H character (larger H-D shifts, 841.5 and 836.6 cm-1, and higher H/D ratios, 1.408 and 1.409 and smaller 12-13 shifts (6.6 and 6.8 cm-1). The isotopic character of the higher frequencies is more like an isolated C-H bond stretching mode and it follows that for the chloroform example. The isotopic behavior for the lower band is more like an H vibrating between Cl and C. The higher H/D frequency ratio indicates a largely H motion (1.408 and 1.409, likely with anharmonicity), which is revealed by (Cl-HCl)− itself (H/D = 697.0/463.9 = 1.502). Thus the isotopic data characterize both isolated C-H and hydrogen bonded Cl-H-C stretching modes, although the latter mode does involve some interaction with the lone C-H bond. As discussed above for the Cl−-HCCl2 anion, the broader C-H stretching mode is a 17 ACS Paragon Plus Environment


defining characteristic of hydrogen bonding so the lower mode involves a similar Cl−-H-C vibrational mode.37,38 And finally Table S4 compares the two B3LYP calculated frequencies, first for the higher band at 3260.5 cm-1 (63 km/mol), which is observed 4.0% lower at 3130.3 cm-1, and second for the broader more intense lower band at 2995.3 cm-1 (426 km/mol) which is found 3.1% lower at 2903.3 cm-1.

Table 4. Absorption Frequencies of Cl−-HCHCl Isotopomers Provided during co-deposition of its Methylene Chloride Isotopic Precursors with Laser-Ablated Metal Atoms in Excess Argona Cl−-HCHCl

Cl−-DCHCl

Cl−-H13CHCl Description

3118.5, 3095.7 2061.8, 2055.8, 2896.7, 2903.3, 2881.8 2045.2 2875.0 aAll frequencies are in cm–1. Stronger absorptions are 3130.3, 3107.9

2360.0

A’ C-H str. A’ Cl-H-C as + C-H s str. bold. Description gives major

coordinate.

These hydrogen stretching bands are not in accord with the vibrational characteristics for any of the previously reported isomers, fragments, and ions containing H atoms produced from methylene chloride (CHCl2, CHCl2+, CHCl2−, CH2Cl, CH2Cl+, and CH2Cl-Cl),1-3,9-15 but the lower of our two matrix sites for each C-H vibrational mode agrees with bands reported for the Cl−HCHCl anion by Schweig, et al.25 The observation of two rather different C-H stretching absorptions indicates that this new subject molecule must contain two nonequivalent H atoms. We assign these two new observed product absorptions to the C-H and Cl-H-C stretching modes of the Cl−-HCHCl anion, the methylene chloride anion counterpart of Cl−-HCCl2. The observed frequencies correlate very well with our B3LYP predicted/calculated values of 3260.5 and 2995.3 cm-1 (observed are 0.96 and 0.97 of the


Page 18 of 44

Page 19 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


predicted values) and isotopic shifts (D shifts of 828.3 and 825.7 cm-1 and

13C

shifts of 6.6

and 6.8 cm-1) as shown in Table S4. The Cl−-HCHCl anion is as stable as Cl−-HCCl2 in the matrix. It gradually decreased on annealing and photolysis as shown in Figures 4, 5, and 6. Evidently methylene chloride is also an efficient electron scavenger in the laserablation experiment although its electron affinity [86 kJ/mol, Eq. (S24)] is substantially lower than that of HCCl3 [130 kJ/mol, Eq. (S9)]. The Cl−-HCHCl structure is the most stable configuration of the methylene chloride anion, which is 28 kJ/mol more stable than CH2ClCl−. The weaker ClHCl− absorptions in the CH2Cl2 spectra indicate that production of the symmetrical anion is energetically less favorable than in the CHCl3 system. In addition to the lower electron affinity, productions of HCl and Cl− are 63 and 34 kJ/mol endothermic (CH2Cl-Cl → CHCl + HCl, Cl−-HCHCl → CH2Cl + Cl−) [Eqs. (S28 and S32)], which are compared with 1 and 53 kJ/mol [Eqs. (S11 and S15)].Dissociation of Cl−-HCHCl into ClHCl− + CH is 313 kJ/mol endothermic [Eq. (S34)], which is compared with 190 kJ/mol in the CHCl3 system. The electron affinity for CH3Cl was estimated by B3LYP to be 32 kJ/mol, and this C3v anion is very weakly bound. The CH3 umbrella mode at 659 cm-1 (52 km/mol) is strong, but the C--Cl− stretching mode at 96 cm-1 (24 kJ/mol) is weak. Evidently Cl−-HCHCl is not easy to produce. The same argon resonance photoionization during deposition treatment performed with HCCl3 was also done with CH2Cl2 and no new absorptions were reported in the C-H stretching region although (Cl-H-Cl) –

was observed.20 It is quite possible that Cl−HCHCl was formed but photodissociated by the

intense vacuum uv irradiation or not observed owing to poorer sample transmission in the CH stretching region. Parnis et al. investigated isomerization and fragmentation products of CH2Cl2 via electron bombardment and observed various fragments and ions including CCl, ClHCl−, CH2Cl2+, and CH2Cl-Cl, but no trace of Cl−-HCHCl.21 The methylene chloride anion 19 ACS Paragon Plus Environment


observed in this study shows that laser ablation is a good source of electrons and M+ cations as well as metal atoms.41 Thus, the counter positive ion for the anions described here is the metal cation also produced in the laser ablation process. Br−-HCBr2. Similar experiments with laser ablated thorium and bromoform gave rise to a similar set of products the most notable of which are the anions Br−-HCBr2 and (Br-H-Br) −,

which have been observed previously, along with the neutral intermediate species CHBr2

radical and CHBr2-Br.10,13,14,38-40,42 The strong band at 2724.2 is just 4.0 cm-1 higher and almost as broad (FWHM, 16 cm-1) as the analogous band for Cl−-HCCl2, and it is assigned similarly to the C-H stretching mode for the bromoform anion Br−-HCBr2. The sharp band at 1203.9 cm-1 is 39 cm-1 higher than the H-C-Br bending mode for the HCBr2 radical (1165.0),39 which corresponds nicely with the 1270.5 band for Cl−-HCCl2 blue shifted from HCCl2 at 1226 cm-1.8 So 1203.9 is assigned to the H-C-Br bending mode for the anion. The higher sharp band at 2379.5 cm-1 is not near a fundamental frequency for the HCBr2 radical, but 2 x 1203.9 = 2470.8 is just 28.3 cm-1 higher and with some anharmonicity to bring it down, the sharp 2379.5 band is quite reasonable for the overtone of the sharp 1203.9 band. The intensity (peak height, same FWHM) of this overtone is 60% of the fundamental’s intensity, which is in accord with anharmonicity in this intraionic bending mode, but less than found in the case of Cl−-HCCl2 where the overtone was stronger and broader than the fundamental H-C-Cl bending mode. Notice, however, that the bandwidth of this overtone is essentially the same as that for the fundamental so the interaction discussed above for the HC-Cl bending 2495.6 overtone and C-H stretching band is much weaker in the Br−-HCBr2 case where the bands are further apart (345 cm-1).

Structures and Bonding. Figure 10 illustrates the B3LYP structures of Cl−-HCCl2, ClHCl−, Cl-ClCCl, and Cl−-HCHCl. The natural atomic charges43 of Cl−-HCCl2 and Cl−20 ACS Paragon Plus Environment

Page 20 of 44

Page 21 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HCHCl indicate that the negative charge is concentrated on the weakly bound chloride anion (0.91 and 0.95 e), showing that they are in fact Cl−∙∙∙HCCl2 and Cl−∙∙∙HCHCl.15 Due to the weak Cl−∙∙∙C bonds (2.185 and 2.370 Å), Cl− can be easily detached, consistent with the calculated low dissociation energies [53.3 and 34.4 kJ/mol, Eqs. (S15 and 32)]. The non-planar Cl−-HCCl2 and planar Cl−-HCHCl structures stem from the nonplanar CHCl2 and planar CH2Cl structures. These computed product structures explain nicely the positions and

13C

and D isotopic shifts observed for these two chloromethane anions.

Bickelhaupt et al. reported that the strong repulsion between the hydrogen atoms leads to the planar structure of CH3 , but CCl3 has a non-planar structure11,44,45 Our NBO43 analyses indicate that there is π bonding between C and Cl in CH2Cl (natural bond order 1.50), whereas, the C-Cl bonds in CHCl2 are single bonds (natural bond order 0.98), and there is no evidence for Cl d orbital participation. The two Cl atoms in CHCl2 have to compete for the unpaired electron on C to form a π bond, but in CH2Cl, the single Cl atom can employ the radical electron for extra π bonding.46 The ClHCl− anion is symmetrical (D∞h), where the H-Cl bonds are much weaker than that of HCl (B3LYP interatomic distances 1.573 vs. 1.281 Å). The Cl carries a large negative charge (-0.62 e) while H has considerable positive charge (0.24 e) in the symmetrical anion. NBO43 analysis indicates that there is only one single bond in the symmetrical anion. This can be described with two equal resonance structures [Eq. (3)]. The H-Cl bond in ClHCl− is intermediate between a true H-Cl bond and an H∙∙∙Cl− hydrogen bond. In their early work Milligan and Jacox verified the presence of two equivalent Cl atoms through the use of 37Cl substitution.17

Cl-H∙∙∙Cl− ↔ Cl−∙∙∙H-Cl

(3)



The Cl-ClCCl free radical is nonplanar, where the Cl-Cl bond is weak (2.654 Å). On the other hand, the C-Cl bonds (1.674 and 1.684 Å) are apparently stronger than that of CCl2 (1.722 Å). The positive charge on the Cl atom probably strengthens the bonds. The weakly bound complex of Cl and CCl2 is a shallow energy minimum as described above. Detachment of Cl• radical from it should not be difficult [30 kJ/mol endothermic, Eq. (S38)], and the conversion to CCl3 is expected to occur readily (the transition state is only 25 kJ/mol higher energy than Cl-ClCCl).

CONCLUSIONS The Cl−-HCCl2 anion was produced during co-deposition of CHCl3 with laser ablated metal atoms, cations, electrons and plume radiation, showing that the precursor molecules are effective electron scavengers in the condensing matrix. The product bands were observed in the original deposition spectra, and they decreased stepwise on annealing and photolysis. Their spectral characteristics match well with the previously reported values.14,15 An additional new stronger and broader absorption is assigned to the overtone of the H-C-Cl bending mode. As the Cl−-HCCl2 absorptions decreased, the ClHCl− absorptions emerged with the CCl absorption, suggesting that the chloroform anion is a good source of the illusive symmetrical hydrogen dichloride anion. Another strong spectral feature that diminishes dramatically on first annealing and photolysis has been assigned to a new chemical species (Cl-ClCCl), which is a shallow energy minimum that can be easily converted to CCl3 or dissociate to CCl2 and Cl. The dominant absorptions in the C-H stretching region of CH2Cl2 co-deposited with laser-ablated metal plume radiation have been assigned to the novel anion Cl−-HCHCl. The IR spectra reveal two quite different C-H stretching modes, and the D and 13C

isotopic shifts describe the lower, stronger, and broader absorptions as due to a mostly

hydrogen bond (Cl−-H-C) vibration, and the higher, weaker, and sharper absorptions arise


Page 22 of 44

Page 23 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from an isolated terminal C-H bond stretching mode. Parallel to Cl−-HCCl2, Cl−-HCHCl also decreased stepwise on annealing and photolysis while weak ClHCl− absorptions also appeared. The increasing ClHCl− bands were accompanied with the decreasing absorptions of CHCl2-Cl, another common feature in the chloroform spectra, as well as Cl−-HCCl2. Calculations suggest that the decreasing species can easily dissociate to produce HCl and Cl−, which can in turn combine to generate ClHCl−. Calculations also show that Cl−-HCCl2 can photo-dissociate into ClHCl− and CCl in two steps although the reaction intermediate ClH∙∙∙ClCCl was not detected in this study. Considering that CCl3 is a common product in photo-dissociation of CHCl3 by the plume radiation, the observation of Cl-ClCCl, an isomer of CCl3, is not surprising. Our IRC36 calculations show smooth conversion between the two isomers. Analogous to Cl−-HCCl2, the intramolecularly hydrogen bonded Cl−-HCHCl anion is the most stable configuration for the methylene chloride anion. On the basis of the high product absorption intensity, methylene chloride is also an effective electron scavenger in the condensing sample although its electron affinity is somewhat smaller than that of chloroform. Laser-ablation and reaction of metal atoms is an efficient channel to produce new and rare species from the chloromethanes along with exotic metal complexes.22,23,28,29

ASSOCIATED CONTENT Supporting Information Chemical equations (S1-S42) with reaction energies, Tables S1-S5 of calculated vibrational frequencies and intensities compared with observed values, Figures S1 for (ClHCl) – and S2 for expanded CHBr3 spectra and Figure S3 for intrinsic reaction coordinate calculations, and Cartesian coordinates of the observed products. This material is available free of charge from the Internet at http://pubs.acs.org.



AUTHOR INFORMATION Corresponding Author *L. A.:

e-mail, [email protected]; tel, 434-924-3513.

ORCID Han-Gook Cho: 0000-0003-0579-376X Lester Andrews: 0000-0001-6306-0340 Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work is financially supported by Incheon National University Research Grant in 2018 to H.-G. Cho.

REFERENCES (1) Jacox, M. E. Ground-State Vibrational Energy Levels of Polyatomic Transient Molecules. J. Phys. Chem. Ref. Data 1984, 13, 945-1068. (2) Jacox, M. E. Vibrational Energy Levels of Small Polyatomic Transient Molecules: Supplement A. J. Phys. Chem. Ref. Data 1988, 17, 269-511. (3) Jacox, M. E. Vibrational and Electronic Energy Levels of Polyatomic Transient Molecules: Supplement B. J. Phys. Chem. Ref. Data 2003, 32, 1-441. (4) Jacox, M. E.; Milligan, D. E. Matrix-Isolation Study of the Vacuum-Ultraviolet Photolysis of Methyl Chloride and Methylene Chloride. Infrared and Ultraviolet Spectra of the Free Radicals CCl, H2CCl, and CCl2, J. Chem. Phys. 1970, 53, 2688-2701. (5) Kelsall, B. J.; Andrews, L. FTIR Spectroscopic Studies of the Matrix Photoionization and Photolysis Products of Methylene Halides. J. Mol. Spectrosc. 1983, 97, 362-378.


Page 24 of 44

Page 25 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Andrews, L. Infrared Spectrum of Dichlorocarbene in Solid Argon. J. Chem. Phys. 1968, 48, 979-982. (7) Andrews, L.; Keelan, B. W. Infrared Spectra, Structure and Bonding in the Dihalocarbene Cations in Solid Argon. J. Am. Chem. Soc. 1979, 101, 3500-3505. (8) Carver, T. G.; Andrews, L. Matrix Infrared Spectrum and Bonding in the Dichloromethyl Radical. J. Chem. Phys., 1969, 50, 4235-4245. (9) Jacox, M. E.; Milligan, D. E. Matrix-Isolation Study of the Vacuum-Ultraviolet Photolysis of Chloroform. Infrared Spectra of the CCl3+, HCCl2+, and HCCl2− Molecular Ions. J. Chem. Phys. 1971, 54, 3935-3950. (10)

Ault, B. S.; Andrews, L. Proton Radiolysis of CHCl3 and CHBr3 at High Dilution in

Argon During Condensation at 15 K. Infrared Spectra of the CHX2+, CHX2−, and CHX3− Molecular Ions. J. Chem. Phys. 1975, 63, 1411-1418. (11)

Andrews, L. Infrared Spectrum of the Trichloromethyl Radical in Solid Argon. J.

Chem. Phys. 1967, 48, 972-979. (12)

Maier, G.; Reisenauer, H. P.; Hu, J.; Schaad, L. J.; Hess, B. A., Jr. Photochemical

Isomerization of Dihalomethanes in Argon Matrixes. J. Am. Chem. Soc. 1990, 112, 51175122. (13)

Cho, H.-G.; Andrews, L. 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. J. Phys. Chem. A 2013, 117, 6525-6535. (14)

Andrews, L.; Wight, C. A.; Prochaska, F. T.; McDonald, S. A.; Ault, B. S. Vacuum-

Ultraviolet Photoionization of Bromoform and its Chlorine Substituted Counterparts During Condensation withAargon at 15 K: Infrared Spectra of the (CHX2+)X and X−HCX2 Molecular Ions. J. Mol. Spectrosc. 1978, 73, 120-143.



(15)

Andrews, L.; Prochaska, F. T. Infrared Spectrum of the Intramolecular Hydrogen-

Bonded Chloroform Anion Cl−-HCCl2 in Solid Argon at 15 K. J. Am. Chem. Soc. 1979, 101, 1190-1196. (16)

Noble, P. N.; Pimentel, G. C. Hydrogen Dichloride Radical: Infrared Detection

through the Matrix Isolation Technique. J. Chem. Phys. 1968, 49, 3165-3169. (17)

Milligan, D. E.; Jacox, M. E. Infrared Spectrum of the ClHCl− Ion Isolated in an

Argon Matrix. J. Chem. Phys. 1970, 53, 2034-2040. (18)

Ault, B. S.; Andrews, L. Salt – Molecule Matrix Reactions. Infrared Spectra of the

M+HCl2− and M+Cl3−Ion Pairs in Solid Argon. J. Amer. Chem. Soc. 1975, 97, 3824-3826. (19)

Wight, C. A.; Ault, B. S.; Andrews, L. On Microwave Discharge Sources of New

Chemical Species for Matrix-Isolation Spectroscopy and the Identification of Charged Species. J. Chem. Phys. 1976, 65, 1244-1249. (20)

Andrews, L.; Prochaska, F. T.; Ault, B.S. Matrix Photoionization and Radiolysis of

CH2C12 and CH2Br2. Infrared and Ultraviolet Absorption Spectra and Photolysis of CH2C12+ and CH2Br2+. J. Am. Chem. Soc. 1979, 101, 9-15. (21)

Fridgen, T. D.; Zhang, X. K.; Parnis, J. M.; March, R. E. Isomerization and

Fragmentation Products of CH2Cl2 and Other Dihalomethanes in Rare-Gas Matrices: An Electron Bombardment Matrix-Isolation FTIR Spectroscopic Study. J. Phys. Chem. A 2000, 104, 3487-3497. (22)

Andrews, L.; Cho, H.-G. Matrix Preparation and Spectroscopic and Theoretical

Investigations of Simple Methylidene and Methylidyne Complexes of Group 4-6 Transition Metals. Organometallics 2006, 25, 4040-4053. (23)

Cho, H.-G.; Andrews, L. Matrix Preparation and Spectroscopic and Theoretical

Investigation of Small High Oxidation-State Complexes of Group 3-12 and 14, Lanthanide and Actinide Metal Atoms. Coord. Chem. Rev. 2017, 335, 76-102.


Page 26 of 44

Page 27 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24)

Bonazolla, L.; Michaut, J. P.; Roncin, R. The Structure of CCl4− and CH2Cl2−: An

ESR and Theoretical Study. Chem. Phys. Lett. 1988, 153, 52-56. (25)

Richter, A.; Meyer, H.; Kausche, T.; Müller, T.; Sporleder, W.; Schweig, A. Electron

Attachment Products of Methylene Chloride in Solid Argon: an Experimental and Quantum Chemical IR Spectroscopic Study. Chem. Phys. 1997, 214, 321-328. (26)

Cho, H.-G.; Andrews, L. Matrix Infrared Spectroscopic and Theoretical Studies for

Products of the Reactions of Pb with Ethane and Halomethanes. J. Phys. Chem. A 2018, 122, 8911-8922. (27)

Cho, H.-G.; Andrews, L. Infrared Spectra of Insertion, Methylidene, and

Methylidyne Complexes in Reactions of Laser-Ablated Ruthenium Atoms with Halomethanes and Methane. Eur. J. Inorg. Chem. 2008, 2008, 2537-2549. (28)

Andrews, L.; Citra, A. Infrared Spectra and Density Functional Theory Calculations

on Transition Metal Nitrosyls. Vibrational Frequencies of Unsaturated Transition Metal Nitrosyls. Chem. Rev. 2002, 102, 885-911, and references therein. (29)

Andrews, L. Matrix Infrared Spectra and Density Functional Calculations of

Transition Metal Hydrides and Dihydrogen Complexes. Chem. Soc. Rev. 2004, 33, 123132 and references therein. (30)

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;

Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.02, Gaussian, Inc.: Wallingford, CT, 2009. (31)

Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange.

J. Chem. Phys. 1993, 98, 5648-5652. (32)

Lee, C.; Yang, Y.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy

Formula in a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785-789.



(33)

Frisch, M. J.; Pople, J. A.; Binkley, J. S. Self-Consistent Molecular Orbital Methods

25. Supplementary Functions for Gaussian Basis Sets. J. Chem. Phys. 1984, 80, 3265−3269. (34)

Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct

Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098−3100. (35)

Li, L.; Stüker,T.; Andrews, L.; Beckers, H.; and Riedel, S. Infrared Spectra of the

HAnX and H2AnX2 Molecules (An= Th and U, X= Cl and Br) in Argon Matrices with Supporting Electronic Structure Calculations. Chem. Eur. J. 2018, in press. (36)

Fukui, K. The Path of Chemical Reactions - The IRC Approach. Acc. Chem. Res.

1981, 14, 363-368. (37)

Andrews, L. Fourier Transform Infrared Spectra of HF Complexes in Solid Argon. J.

Phys. Chem. 1984, 88, 2940-2949. (38)

Andrews, L.; Wang, X. Infrared Spectra of the H3N-HBr Complex in Solid Neon,

Ne/Ar, Ar, Kr, and N2. Strong Matrix Effects on a Hydrogen-Bonded Complex. J. Phys. Chem. A 2001, 105, 6420-6429. (39)

Carver, T. G.; Andrews, L. Infrared Spectrum and Bonding in the Dibromomethyl

Radical. J. Chem. Phys. 1969, 50, 4223-4234. (40)

Milligan, D. E. Jacox, M. E. Infrared Spectrum of the BrHBr− Ion Isolated in an

Argon Matrix. J. Chem. Phys. 1971, 55, 2550–2560. (41)

Andrews, L.; Cho, H.-G.; Gong, Y. Reactions of Laser-Ablated Aluminum Atoms

with Cyanogen: Matrix Infrared Spectra and Electronic Structure Calculations for Aluminum Isocyanide Al(NC)1,2,3 and Their Novel Dimers. J. Phys. Chem. A 2018, 122, 5342-5353 and references therein.


Page 28 of 44

Page 29 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42)

Lyon, J. T.; Andrews, L. Reactions of Thorium Atoms with Polyhalomethanes:

Infrared Spectra of the CH2=ThX2, HC÷ThX3, and XC÷ThX3 Molecules. Eur. J. Inorg. Chem. 2008, 2008, 1047-1058. (43)

Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular interactions from a natural

bond orbital, donor-acceptor viewpoint. Chem. Rev. 1988, 88, 899-926, and references therein. (44)

Bickelhaupt, F. M.; Ziegler, T.; Schleyer, P. v. R. CH3• Is Planar Due to H-H Steric

Repulsion. Theoretical Study of MH3• and MH3Cl (M = C, Si, Ge, Sn). Organometallics 1996, 15, 1477-1487. (45)

Moc, J.; Latajka, Z.; Rajczak, H. Theoretical Study of the CCl3 Radical. Z. Phys. D –

Atoms, Molecules, and Clusters 1986, 4, 185-188. (46)

Andrews, L.; Smith, D. W. Matrix Infrared Spectrum and Bonding in the

Monochloromethyl Radical. J. Chem. Phys. 1970, 53, 2956-2966.


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Figure 1. IR spectra in the absorption regions of Cl−-HCCl2, ClHCl−, and ClCCl-Cl in excess argon at 4 K and their variation. (a) Predeposition (deposition without ablation of metal) of 0.5% CHCl3 in Ar for 5 min. (b) Pb + 0.5% CHCl3 in Ar co-deposited for 1 h. (c and d) As (b) after annealing to 20 and 30 K. (e) As (d) after photolysis with λ > 220 nm. (f) As (e) after annealing to 35 K.The p stands for CHCl3 precursor absorption, and CH4 and CHCl2-Cl absorptions are also designated.


Page 30 of 44

Page 31 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Figure 2. IR spectra in the absorption regions of DCCl2, Cl−-DCCl2, ClDCl−, and ClCCl-Cl in excess argon at 4 K and their variation. (a) Predeposition (deposition without ablation of metal) of 0.5% CDCl3 in Ar for 10 min. (b) Pb + 0.5% CDCl3 in Ar co-deposited for 1 h. (c and d) As (b) after annealing to 20 and 30 K. (e) As (d) after photolysis with λ > 220 nm. (f) As (e) after annealing to 35 K. The p stands for CDCl3 precursor absorption, and a CDCl2-Cl absorption is also shown.



Figure 3. IR spectra in the regions of Cl−-H13CCl2, ClHCl−, and Cl13CCl-Cl absorption in excess argon at 10 K and their variation. (a) Predeposition (deposition without ablation of metal) of 0.5% 13CHCl3 in Ar for 5 min. (b) Pb + 0.5% 13CHCl3 in Ar co-deposited for 1 h. (c, d, and e) As (b) after photolysis with λ > 420 nm, 240 < λ < 380 nm, and λ > 420 nm. (f) As (e) after annealing to 28 K. The p stands for 13CHCl precursor absorption, and CH and residual CHCl absorption are also indicated. 3 4 3


Page 32 of 44

Page 33 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Figure 4. IR spectra in the absorption region of Cl−-HCHCl in excess argon at 4 K and their variation. (a) Predeposition (deposition without ablation of metal) of 0.5% CH2Cl2 in Ar for 5 min. (b) Pb + 0.5% CH2Cl2 in Ar co-deposited for 1 h. (c and d) As (b) after annealing to 20 and 30 K. (e) As (d) after photolysis with λ > 220 nm. (f) As (e) after annealing to 35 K. The p stands for CH2Cl2 precursor absorption. 33 ACS Paragon Plus Environment


Figure 5. IR spectra in the absorption region of Cl−-DCDCl in excess argon at 4 K and their variation. (a) Predeposition (deposition without ablation of metal) of 0.5% CD2Cl2 in Ar for 5 min. (b) Pb + 0.5% CDCl3 in Ar co-deposited for 1 h. (c and d) As (b) after annealing to 20 and 30 K. (e) As (d) after photolysis with λ > 220 nm. (f) As (e) after annealing to 35 K. The p stands for CD2Cl2 precursor absorption, and common CO and CO2 absorptions are also designated. 34 ACS Paragon Plus Environment

Page 34 of 44

Page 35 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46




Figure 6. IR spectra in the C-H absorption region for Cl-H13CHCl− in excess argon at 4 K and their variation. (a) Predeposition (deposition without ablation of metal) of 0.5% 13CH2Cl2 in Ar for 5 min. (b) Pb + 0.5% 13CHCl3 in Ar co-deposited for 1 h. (c and d) As (b) after annealing to 20 and 30 K. (e) As (d) after photolysis with λ > 220 nm. (f) As (e) after annealing to 35 K. The p stands for 13CH2Cl2 precursor absorption.


Page 36 of 44

Page 37 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Figure 7. IR spectra in the absorption regions of Br−-HCBr2, BrHBr−, and CHBr2-Br in excess argon at 10 K and their variation. (a) Predeposition (deposition without ablation of metal) of 0.5% CHBr3 in Ar for 10 min. (b) Th + 0.5% CHBr3 in Ar co-deposited for 1 h. (c and d) As (b) after photolysis with > 280 and 220 nm. (e) As (d) after annealing to 30 K. (f) As (e) after photolysis with λ > 220 nm. p stands for precursor absorption. 37 ACS Paragon Plus Environment


Figure 8. Possible reaction path from Cl−-HCCl2 to ClHCl−∙∙∙ClC. Intrinsic reaction coordinate calculation between Cl−-HCCl2 and [ClH∙∙∙ClCCl]− via the transition states (TS1) and (TS2).


Page 38 of 44

Page 39 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Figure 9. Intrinsic reaction coordinate calculation between CCl3 and Cl-ClCCl via the transition state (TS3). The shallow 25 kJ/mol energy minimum for Cl-ClCCl is consistent with its near disappearance on early annealing.




Page 40 of 44

Page 41 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46




Figure 10. The B3LYP structures of Cl−-HCCl2, Cl−-HCHCl, ClHCl−, Cl-ClCCl, Br−-HCBr2, and BrHBr−. The 6-311++G(3df,3pd) basis sets are used for C, H, Cl, and Br.

The bond lengths and angles are in Å and degrees. The natural atomic charges are also shown in italics.


Page 42 of 44

Page 43 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Figure 11. The B3LYP structures of transition states, intermediates, and high-order product. The 6-311++G(3df,3pd) basis sets are used for C, H, and Cl.

The bond lengths and angles are in Å and degrees. The ClHCl−∙∙∙CHCl3 structure is calculated with the solvent effects (PCM

model35). C and Cl are shown in gray and green. 43 ACS Paragon Plus Environment


0.15 e



-0.38 e Cl -0.96 e

C 0.25 e Cl -0.06 e

TOC graphic [ed. please make this as large as you can, thanks, au.]


Page 44 of 44

Matrix Infrared Spectra, Photochemistry and Density Functional

Recommend Documents