Infrared Spectra of the 1-Chloromethyl-1-methylallyl and 1

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Infrared Spectra of the 1‑Chloromethyl-1-methylallyl and 1‑Chloromethyl-2-methylallyl Radicals Isolated in Solid paraHydrogen Published as part of The Journal of Physical Chemistry virtual special issue “W. Lester S. Andrews Festschrift”. Jay C. Amicangelo*,† and Yuan-Pern Lee*,‡,§ †

School of Science, Penn State Erie, The Behrend College, 4205 College Drive, Erie, Pennsylvania 16563, United States Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan § Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: The reaction of chlorine atoms (Cl) with isoprene (2-methyl-1,3-butadiene, C5H8) in solid para-hydrogen (p-H2) matrices at 3.2 K was studied using infrared (IR) spectroscopy. Mixtures of C5H8 and Cl2 were codeposited in p-H2 at 3.2 K, followed by irradiation with ultraviolet light at 365 nm to induce the photodissociation of Cl2 and the subsequent reaction of the Cl atoms with C5H8. Upon 365 nm photolysis, a multitude of new lines appeared in the IR spectrum, and, based on the secondary photolysis behavior, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set A (intense lines at 1237.9, 807.8, and 605.6/608.2 cm−1, and several other weaker lines) and set B (intense lines at 942.4, 1257.7, 796.7/798.5, 667.9, and 569.7 cm−1, and several other weaker lines). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level for ·C5H7 and the four possible isomers of the ·C5H8Cl radicals, produced from the addition of the Cl atom to the four distinct sites of carbon atoms in C5H8, to determine the relative energetics and predict IR spectra for each radical. The newly observed lines of sets A and B are assigned to the 1-chloromethyl-2-methylallyl radical (addition to carbon 4) and the 1-chloromethyl-1-methylallyl radical (addition to carbon 1) according to comparison with predicted IR spectra of possible products. The 1-chloromethyl-2-methylallyl radical and 1-chloromethyl-1-methylallyl radicals were predicted to be the most stable, with the latter ∼8 kJ mol−1 lower in energy than the former. The ratio of the 1chloromethyl-1-methylallyl to the 1-chloromethyl-2-methylallyl radicals is estimated to be (1.2 ± 0.5):1.0, indicating that the two radicals are produced in approximately equal amounts. The exclusive production of the radicals involving the addition of the Cl atom to the two terminal carbons of isoprene is analogous to what was previously observed for the reaction of Cl atoms with trans-1,3-butadiene in solid p-H2.

I. INTRODUCTION

Cl atom to the carbon−carbon double bond to form a chloroalkyl radical (·CnH2nCl) and the metathesis reaction, which includes direct H atom abstraction and Cl addition−HCl elimination, to form an alkyl radical (·CnH2n−1) and HCl; the ratio of these two reactions has been observed to depend on alkene structure, pressure, and temperature.11 The radical intermediates then react further to give stable molecular products, such as alkanes, monochloroalkanes, and dichloroalkanes. For many Cl + CnH2n reactions, the direct spectral characterization of the ·CnH 2nCl and ·C nH2n−1 radical intermediates remains elusive due to the unstable and highly reactive nature of these species. Given the importance of chloroalkyl radicals in the mechanism of addition reactions of

The reactions of chlorine atoms with organic species, particularly alkenes, in the atmosphere has been recognized to be considerably important.1,2 One specific alkene, for which the atmospheric reaction with chlorine atoms is of particular interest, is isoprene (2-methyl-1,3-butadiene).3−6 Isoprene is known to be emitted into the atmosphere by a wide variety of terrestrial plants and is the single largest component in the annual budget of atmospheric volatile organic compounds from all biogenic and anthropogenic sources combined.7−9 Once in the atmosphere, isoprene is known to be a highly reactive molecule;6,10 given that Cl atoms are also an important constituent of the atmosphere, the reaction of chlorine atoms with isoprene is important in understanding of the atmospheric chemistry of both species. In the gaseous reaction of Cl atoms with alkenes (CnH2n), the two most significant initial processes are the addition of a © XXXX American Chemical Society

Received: August 9, 2017 Revised: October 9, 2017 Published: October 19, 2017 A

DOI: 10.1021/acs.jpca.7b07922 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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indicates the central carbon attached with a methyl moiety.35 Brana and Sordo examined the abstraction of a hydrogen atom from the methyl group in the Cl + C5H8 reaction using the MP2 and QCISD(T) methods and reported that the reaction was slightly exothermic (−5 kJ mol−1) but possessed a barrier of 18 kJ mol−1 at the QCISD(T)/aug-cc-pVDZ level of theory.36 Vijayakumar and Rajakumar also studied the addition and abstraction reactions of Cl + C5H8 using the MP2 and QCISD methods.33 For the addition reaction, these authors found that the reaction pathway proceeds via a Cl···C5H8 complex that leads to transition states for each carbon atom and then the final addition radicals. The transition states for the addition to C1 and C4 were found to be below the energy of the reactants by −14 and −11 kJ mol−1, respectively, while for C2 and C3, the transition states were found to be slightly above the reactants by 1 and 3 kJ mol−1, respectively, at the QCISD/ 6-31+G(d,p) level. The overall addition of the Cl atom to each of the carbon atoms was predicted to be exothermic, with the addition to the terminal carbons more favored by ∼20 kJ mol−1 at the QCISD/6-31+G(d,p) level. For the abstraction reaction, these authors reported that the abstraction of a hydrogen atom from all positions was endothermic, with abstraction from the methyl group being the least endothermic (7 kJ mol−1 at the QCISD/6-31+G(d,p) level), and that all channels possessed significant barriers, with the methyl group abstraction having the lowest barrier of 42 kJ mol−1 at the QCISD/6-31+G(d,p) level. In this work, by investigating the addition reaction of Cl atoms with isoprene in solid p-H2, we observed absorption lines that are ascribable to the 1-chloromethyl-1-methylallyl and the 1-chloromethyl-2-methylallyl radicals, which correspond to Cl atom additions to terminal carbons C1 and C4 of isoprene, respectively, in a ratio of ∼1:1.

chlorine atoms with alkenes, direct spectral characterization of these species is desirable. The matrix isolation infrared (IR) absorption technique has proven to be a valuable method to produce and spectrally characterize radicals and other unstable species.12,13 The use of solid para-hydrogen (p-H2) as a novel matrix host has generated considerable interest in recent years because of its unique nature as a matrix material.14−17 Solid p-H2 is characterized as a quantum solid, because the amplitude of the zero-point lattice vibrations is a large fraction of the lattice spacing.18,19 Therefore, solid p-H2 is a “softer” matrix than traditional noble gas matrices, such as Ar or Ne, and this gives it several interesting properties,20−23 particularly a diminished cage effect,16,17,24,25 which is relevant to the study of photoinduced dissociation or bimolecular reactions. The diminished cage effect of solid p-H2 allows for the in situ production and reaction of free radicals upon photolysis, because the photolysis products may escape the original matrix site and either become trapped and isolated or react with a neighboring molecule. We employed this method to study the reactions of photolytically generated radicals, such as Cl and CH3, with several small molecules in solid p-H2 and were able to obtain the infrared spectrum of several novel radical species.16,17 More specifically for the reactions of Cl atoms, we recently obtained the infrared spectra of several chloroalkyl (·CnH2nCl) radicals by investigating the bimolecular reaction of Cl atoms (photolytically produced from Cl2) with alkenes in solid p-H2 matrices at 3.2 K. These include the reactions of Cl with ethene,26 propene,27 isobutene,28 and 1,3-butadiene,29 with the corresponding chloroalkyl radicals observed being the 2-chloroethyl radical for ethene, the 2-chloropropyl radical for propene, the 1-chloro-2-methyl-2-propyl and 2-chloro-2-methylpropyl radicals for isobutene, and the trans-1-chloromethylallyl radical for 1,3-butadiene. Depending on the situations, the Cl addition site varies with alkenes. In the current work, we extended our study of the reaction of Cl atoms with alkenes in solid p-H2 to isoprene, which is related to 1,3-butadiene but with a methyl group at one central carbon, indicated as C2. The reaction of Cl atoms with isoprene has been studied experimentally in the gas phase, with the primary aim being the determination of the kinetic parameters of the reaction, including the absolute rate coefficient at varied temperatures and the branching ratio for the metathesis reaction versus the addition reaction.5,30−33 Two studies employed mass spectrometric detection and reported mass peaks for the ·C5H8Cl radical; however, no distinction between the four potential addition structures was possible; therefore, no information about the site of the Cl atom addition was obtained.30,31 To our knowledge, no spectral investigations of the ·C5H8Cl radical has been reported. The reaction of Cl with isoprene has also been studied theoretically using quantum-chemical methods. Lei and Zhang studied the Cl + C5H8 addition reaction using various methods and found that the addition of the Cl atom to each of the carbon atoms was exothermic but with the addition to the two terminal carbon atoms being significantly more favored than the addition to the two central carbon atoms by ∼60 kJ mol−1 at the CCSD(T)/6-311G(d,p) level; no barrier for the addition reactions was reported.34 Zhang and co-workers also used canonical variational transition-state theory to calculate the rate coefficients for the formation of four possible adduct radicals at 300 K and reported the relative isomeric branching ratios to be 0.40:0.02:0.08:0.50 for Cl addition to C1−C4, respectively; C2

II. EXPERIMENTS The experimental apparatus has been described in detail previously.23,37,38 In the current experiments, a mixture of isoprene (C5H8) and p-H2 was codeposited at a rate of 10−12 mmol h−1 with a small flow of pure Cl2 for 8−9 h onto a goldplated copper flat, maintained at 3.2 K using a closed-cycle helium refrigerator system (Janis, RKD-415). The mixing ratio of the C5H8/p-H2 was 1/1000−1/5000, and the Cl2/p-H2 mixing ratio was estimated to be 1/3000−1/6500. IR absorption spectra were recorded with a Fourier-transform IR spectrometer (Bruker, Vertex 80v) equipped with a KBr beam splitter and a HgCdTe detector cooled to 77 K to cover the spectral range of 400−6000 cm−1. Typically, 200 scans at 0.25 cm−1 resolution were coadded at each stage of the experiment. In some experiments, after the initial codeposition of the C5H8/Cl2/p-H2 mixture, the matrix was annealed at 4.5 K for 5−15 min to enhance production of the complex between Cl2 and C5H8. To produce Cl atoms for reaction with C5H8, the C5H8/Cl2/p-H2 matrices were irradiated with light at 365 ± 10 nm from a light-emitting diode (2.6 W) for 30−45 min. Excitation of the solid p-H2 with IR light in the range of 4000− 5000 cm−1 is known to induce the reaction of Cl atoms with pH2 to form HCl;39,40 therefore, when Cl atoms were present in the p-H2 matrix, a 2.4 μm cutoff filter (Andover Co.) was used when recording the IR spectra to avoid this reaction. To distinguish various groups of lines observed after photolysis at 365 nm, secondary photolysis was performed using a lowpressure Hg lamp (Pen-Ray lamp, UVP) in combination with a band-pass filter at 254 ± 10 nm (Esco Products), a long pass B

DOI: 10.1021/acs.jpca.7b07922 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A filter at 305 nm (SCHOTT), or a long pass filter at 345 nm (SCHOTT). C5H8 (99%, Alfa Aesar) and Cl2 (99.9%, Air Products) were used without further purification. p-H2 was produced by catalytic conversion at low temperature in which normal H2 gas (99.9999%, Scott Specialty Gases) was passed through a trap at 77 K and a copper coil filled with hydrated iron(III) oxide catalyst (catalyst grade, 30−50 mesh, Sigma-Aldrich) that was cooled with a closed-cycle helium refrigerator. At the temperature of the catalyst, typically 12−13 K, the concentration of o-H2 is estimated to be less than 100 ppm.

dihedral angle was performed (displayed in Figure S1, Supporting Information), and this revealed three maxima along the dihedral coordinate. One set of maxima, which connect the trans and gauche conformers, have optimized dihedral angles of ±103.2° and are 23.9 kJ mol−1 above transC5H8. The other maximum, which connects the two gauche conformers, has an optimized dihedral angle of 0.0° and is 2.6 kJ mol−1 above the gauche conformers. Harmonic, scaled harmonic, anharmonic vibrational wavenumbers, and harmonic IR intensities of the trans-C5H8 and gauche-C5H8 are listed in Tables S1 and S2 of the Supporting Information, respectively. For trans-C5H8, the most intense lines below 2000 cm−1 are predicted to have scaled harmonic vibrational wavenumbers of 906 (C1H2 wag, 45 km mol−1), 914 (C4H2 wag, 42 km mol−1), and 1621 (C1C1/C3C4 out-ofphase stretch, 33 km mol−1) cm−1; the corresponding modes for gauche-C5H8 are predicted at 903, 926, and 1663 cm−1, respectively. Given that trans-C5H8 is the most stable conformer of isoprene, the geometry optimizations for the Cl2···C5H8 complex were performed with the trans-C5H8 conformer. The two CC bonds in trans-C5H8 are not equivalent, and two distinct Cl2···C5H8 complex minima were obtained: one in which the Cl2 is interacting with the C1C2 bond (designated Cl2···C1C2 complex) and the other in which the Cl2 is interacting with the C3C4 bond (designated Cl2···C3C4 complex). The geometries of the two Cl2···C5H8 complexes optimized at the B3PW91/6-311++G(2d,2p) level are shown in Figure S2, Supporting Information. For both structures, the Cl2 is nearly perpendicular to the C1−C2−C3−C4 plane, but the Cl2 is closer to the outer carbon atom than the inner carbon atom, with Cla−C distances of 2.676 and 2.951 Å, respectively, to C1 and C2 for the Cl2···C1C2 complex and Cla−C distances of 2.951 and 2.721 Å, respectively, to C3 and C4 for the Cl2···C3C4 complex. In terms of energetics, the Cl2··· C1C2 complex is predicted to have a basis set superposition error (BSSE)-corrected interaction energy of −13.7 kJ mol−1, slightly more stabilized than the Cl2···C3C4 complex with an interaction energy of −12.0 kJ mol−1. Harmonic, scaled harmonic, anharmonic vibrational wavenumbers, and IR intensities of the two Cl2···C5H8 complexes are listed in Tables S3 and S4, Supporting Information. In both complexes, the ClCl stretching modes are activated in the complexes, with scaled harmonic vibrational wavenumbers at 427 and 439 cm−1 for the Cl2···C1C2 and Cl2···C3C4 complexes, respectively. These modes are predicted to be the most intense lines with IR intensities of 180 and 171 km mol−1, respectively, for the Cl2···C1C2 and Cl2···C3C4 complexes. Other intense lines (above 30 km mol−1) are predicted at 919 (99 km mol−1), 930 (43 km mol−1), and 1597 (119 km mol−1) cm−1 for the Cl2···C1C2 complex and 919 (34 km mol−1), 926 (99 km mol−1), 1601 (103 km mol−1), and 1649 (34 km mol−1) cm−1 for the Cl2···C3C4 complex. IIIB. Addition Reaction Products of Cl + C5H8. There are four possible products that can occur from the addition of a single Cl atom to isoprene and these correspond to the addition of the Cl to the C1, C2, C3 or C4 carbons atoms. The optimized structures of the four ·C5H8Cl radicals are presented in Figure 1a−d. The 1-chloromethyl-1-methylallyl radical (Cl addition to C1), Figure 1a, and the 1-chloromethyl-2methylallyl radical (Cl addition to C4), Figure 1d, have nearly planar C1−C2−C3−C4 structures, with the unpaired electrons delocalized over C2−C3−C4 and C1−C2−C3, respectively.

III. QUANTUM-CHEMICAL CALCULATIONS Density functional theory (DFT) calculations with the B3PW91 hybrid functional41−43 and the 6-311++G(2d,2p) basis set44−46 were performed to predict the equilibrium geometries and the vibrational wavenumbers of the Cl2···C5H8 complex and of the possible products of the reaction of Cl with C5H8. We chose the B3PW91 method based on our previous experience with vibrational wavenumbers for products of Cl + alkene reactions26−29,47 and for ease of comparison with these calculations. Analytical first and second derivatives were employed for geometry optimizations and vibrational wavenumbers at each stationary point. Anharmonic vibrational wavenumbers were calculated with a second-order perturbation approach using effective finite-difference evaluation of the third and semidiagonal fourth derivatives. Wavenumber scaling factors were also used for correcting the predicted harmonic vibrational wavenumbers, according to deviations between the experimental and predicted harmonic vibrational wavenumbers for trans-C5H8 in the regions above and below 2000 cm−1, and these were determined to be 0.955 and 0.977, respectively. All relative energies were corrected using harmonic zero-point energies. The Gaussian 09 program was used for the calculations.48 The calculations were performed using the default optimization convergence criteria and the default integration grid. Unless specified, the calculations were performed for the 35Cl isotope. IIIA. C 5H8 and the Cl2···C5H8 Complex. Previous experimental and theoretical studies of C5H8 have established that there are essentially two conformational minima, known as trans-C5H8 and gauche-C5H8, involving rotation about the central carbon−carbon single bond.49,50 The geometries of trans-C5H8 and gauche-C5H8 were optimized at the B3PW91/6311++G(2d,2p) level of theory, and the optimized structures of the two conformers are displayed in Figure S1 of the Supporting Information; the atomic numbering is also indicated. The primary structural difference between the two conformers is the C1−C2−C3−C4 dihedral angle, with the optimized values for the trans-C5H8 and gauche-C5H8 being 180.0° and ±38.6°, respectively. The bond lengths and bond angles of both conformers are predicted to be similar to each other, with mean absolute deviations of 0.002 ± 0.003 Å and 0.6 ± 0.7°, respectively. An experimental structure for the trans conformer has been determined from gas-phase electron diffraction experiments,51 and good agreement is found between the B3PW91/6-311++G(2d,2p) predicted structure and the experimental structure, with mean absolute deviations of 0.009 ± 0.006 Å and 2.4 ± 1.1° for the bond lengths and angles, respectively. In terms of energies, gauche-C5H8 is predicted to be 11.8 kJ mol−1 higher in energy than trans-C5H8 at this level of theory. With the optimized trans-C5H8 structure, a relaxed potential energy scan along the C1−C2−C3−C4 C

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are other conformers for each radical that involve rotation about the Cl−C−C−C dihedral angle. In each case, the other conformers have either different dihedral angles with a higher relative energy or dihedral angles with the same magnitude but opposite sign as those in Figure 1 and therefore essentially equivalent energies to those displayed in Figure 1. The structural images of the other conformers for each radical are displayed in Figure S3, Supporting Information. Harmonic, scaled harmonic, anharmonic vibrational wavenumbers, and harmonic IR intensities predicted for the 1chloromethyl-1-methylallyl and 1-chloromethyl-2-methylallyl radicals are listed in Tables 1 and 2, respectively, and those for the 2-chloro-2-methyl-3-buten-1-yl and 2-chloro-3-methyl3-buten-1-yl radicals are listed in Tables S5 and S6 of the Supporting Information, respectively. For each radical, several vibrational modes are predicted to have IR intensities above 30 km mol−1. For the 1-chloromethyl-1-methylallyl radical, these are the modes at 562 (43 km mol−1), 646 (57 km mol−1), and 798 (43 km mol−1) cm−1. For the 2-chloro-2-methyl-3-buten-1yl radical, these are the modes at 694 (42 km mol−1) and 934 (40 km mol−1) cm−1. For the 2-chloro-3-methyl-3-buten-1-yl radical, these are the modes at 653 (61 km mol−1) and 916 (41 km mol−1) cm−1. For the 1-chloromethyl-2-methylallyl radical, these are the modes at 587 (66 km mol−1) and 817 (54 km mol−1) cm−1. IIIC. Other Possible Reaction Products. Another possible reaction that might occur between a single chlorine atom and C5H8 is the abstraction of a hydrogen atom to form HCl and the ·C5H7 radical. Brana and Sordo36 reported that the abstraction of a hydrogen atom from the methyl group of C5H8 is slightly exothermic by 5.4 kJ mol−1 at the CCSD(T)/6311G(d,p) level, but it has a barrier of 18.4 kJ mol−1. Vijayakumar and Rajakumar33 reported that the abstraction of a hydrogen atom from the methyl group is endothermic by 6.7 kJ mol−1 and has a barrier of 41.8 kJ mol−1. It is unclear why there are such significant discrepancies between the numerical values of Brana and Sordo and Vijayakumar and Rajakumar. However, one conclusion that is common to both studies is that the abstraction reaction has a significant barrier. Previous work by our group, as well as other groups that utilize solid p-H2 as a low-temperature matrix for photoinduced dissociation or bimolecular reactions, has demonstrated that photoinduced reactions in solid p-H2 generally occur only if the reactions are exothermic and nearly barrierless, unless tunneling occurs.15−17 Given the presence of a significant barrier for the abstraction reaction, it is unlikely that this reaction would occur in the current solid p-H2 photolysis experiments. Nevertheless, for completeness the structure of the ·C5H7 radical resulting from the abstraction of a methyl group hydrogen atom was calculated at the B3PW91/6-311++G(2d,2p) level, shown in Figure S4, and the predicted harmonic, scaled harmonic, anharmonic vibrational wavenumbers, and IR intensities are presented in Table S7. Since Cl2 is the precursor used to photolytically generate the Cl atoms, it is possible that addition products containing two Cl atoms could be produced in the current experiments. The addition of a second Cl atom to each of the single Cl atom addition radicals described in Section IIIB leads to three possible dichloro products: 3,4-dichloro-2-methyl-1-butene, 3,4-dichloro-3-methyl-1-butene, and 1,4-dichloro-2-methyl-2butene. The lowest-energy structures for these species calculated at the B3PW91/6-311++G(2d,2p) level are displayed in Figure S5 of the Supporting Information, and the

Figure 1. Optimized geometries of (a) 1-chloromethyl-1-methylallyl, (b) 2-chloro-2-methyl-3-buten-1-yl, (c) 2-chloro-3-methyl-3-buten-1yl, and (d) 1-chloromethyl-2-methylallyl radicals predicted with the B3PW91/6-311++G(2d,2p) method. Selected bond lengths (Å) and bond angles (deg) are displayed.

The 2-chloro-2-methyl-3-buten-1-yl radical (Cl addition to C2), Figure 1b, and the 2-chloro-3-methyl-3-buten-1-yl radical (Cl addition to C3), Figure 1c, have nonplanar C1−C2−C3− C4 structures and with the unpaired electron localized on a single carbon atom, which is C1 and C4, respectively. In terms of energetics, Cl addition to each of the carbon atoms is predicted to be exothermic, with predicted reaction energies of −136.4, −61.9, −69.0, and −128.9 kJ mol−1 for the addition to C1, C2, C3, C4, respectively; the relative energies of the Cl addition products at this level are calculated to be 0.0, 74.5, 67.4, 7.5 kJ mol−1, respectively. The relative energies were also calculated at the CCSD(T)/6-311G(d,p) level by Lei and Zhang34 to be 0.0, 61.5, 59.4, and 5.9 kJ mol−1, respectively, which are in satisfactory agreement with the relative energies at the B3PW91/6-311++G(2d,2p) level. Vijayakumar and Rajakumar calculated the relative energies at the QCISD/631+G(d,p) level and found them to be 0.0, 23.6, 24.8, and 6.1 kJ mol−1. It is unclear why there is a large discrepancy between the relative energies at the QCISD/6-31+G(d,p) level and those at the B3PW91/6-311++G(2d,2p) and CCSD(T)/6311G(d,p) levels for the two high-lying radicals.33 The structures displayed in Figure 1 are the lowest-energy conformers for each of the Cl atom addition radicals. There D

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Table 1. Comparison of Observed Vibrational Wavenumbers (in cm−1) and IR Intensities of the 1-Chloromethyl-2-methylallyl Radical (Set A) in p-H2 with those Predicted with the B3PW91/6-311++G(2d,2p) Method modea

mode descriptionb

harmonic

scaledc

anharmonic

3103

3103

3104.8?e (13)f 3047.2 (15)

ν1

a-νC1H2

3249 (7.4)

ν2

a-νC4H2/νC3H (ip)

3184 (7.8)

3041

3042

ν3

a-νC4H2/νC3−H (op)

3168 (1.8)

3025

3028

ν4

s-νC1H2

3153 (5.5)

3011

3064

ν5

a-νCH3

3135 (12.3)

2994

3000

ν6

s-νC4H2

3114 (9.3)

2974

2988

ν7

a-νCH3

3105 (9.4)

2966

2963

ν8

s-νCH3

3047 (13.4)

2910

2938

ν9

δS C1H2

1527 (7.9)

1492

1496

ν10

a-δCH3

1499 (7.5)

1464

1476

ν11

δS C4H2

1493 (5.9)

1459

1453

d

p-H2

1482.8 (14)

ν12

a-δCH3 (oop)

1486 (10.3)

1451

1463

1459.5? (14)

ν13 ν14

δC3H/δC4H8/δC1H6 uCH3/δC1H6/δC3H (ip)

1445 (3.6) 1422 (6.3)

1412 1389

1408 1387

1393.4? (9)

ν15

uCH3/δC1H6/δC3H (op)

1385 (1.1)

1353

1352

1349.6 (3)

ν16 ν17

δC3H/δC4H9/δC1H5 ωC4H2

1293 (5.3) 1246 (26.9)

1263 1217

1265 1220

1268.2 (8) 1237.9 (36)

ν18

tC4H2

1192 (6.7)

1165

1168

1170.3 (3)

ν19

tC4H2/δC3H/ρC1H2

1131 (6.9)

1105

1110

1108.8 (9)

ν20

ρCH3

1052 (0.3)

1028

1028

ν21

tC4H2/ρCH3

1021 (1.7)

997

1008

ν22

ρC1H2

989 (1.1)

966

971

ν23

ρC4H2

917 (4.8)

896

905

ν24

ωC1H2

836 (53.5)

817

812

807.8 (100)

ν25 ν26 ν27 ν28

νC2C10/νC2C3 ωC3H νC4Cl tC1H2

807 774 601 575

(3.7) (9.1) (65.9) (12.3)

788 756 587 562

794 762 576 555

754.1 (22) 605.6/608.2 (93)

ν29 ν30 ν31g

δC2C3C4 δC1C2C3 (oop) δC1C2C10

525 (9.3) 486 (17.1) 426 (2.9)

513 475 416

524 479 427

526.9? (16) 472.7 (28)

The mode order is according to harmonic vibrational wavenumbers. bApproximate descriptions. ν: stretch, δ: bend or deformation, δS: scissor, u: umbrella, ω: wag, ρ: rock, t: twist, a: antisymmetric, s: symmetric, ip: in-phase, op: out-of-phase, oop: out-of-plane. When it is obvious, the atomic numbering in the mode description is omitted. cHarmonic vibrational wavenumbers scaled according to factors 0.955 and 0.977 for regions above and below 2000 cm−1, respectively; see text. dIR intensities (in km mol−1) are listed in parentheses. eAssignments of lines with less certainty are marked with ?. fIntegrated intensities normalized to the most intense peak (807.8 cm−1) are listed in parentheses. gAdditional harmonic modes at 318, 265, 134, 107, and 57 cm−1 are calculated for the 1-chloromethyl-2-methylallyl radical. a

IV. EXPERIMENTAL RESULTS IVA. Formation of the Complex of Cl2 with C5H8 in pH2. As discussed in Section IIIA, C5H8 can exist as trans-C5H8 and gauche-C5H8. The trans conformer is predicted to be the more stable conformer by 11.8 kJ mol−1, and therefore it is expected that the infrared spectrum of C5H8 in solid p-H2 will consist of lines primarily due to trans-C5H8. The infrared absorption spectrum of C5H8 in solid p-H2 (1/5000) at 3.3 K is shown in Figure S6, Supporting Information. The most intense lines are observed at 893.8, 906.7, 992.0/ 993.1, and 1604.0 cm−1 and are in good agreement with the literature values of 894, 906, 1003, and 1604 cm−1 for gaseous trans-C5H8,50 and 891, 903, 999, and 1605 cm−1 for trans-C5H8 in an argon matrix.52 Satisfactory agreement is also observed between the experimental wavenumbers in solid p-H2 and the scaled harmonic vibrational wavenumbers of 906, 914, 1003, 1621 cm−1 and the anharmonic vibrational wavenumbers 896,

predicted harmonic, scaled harmonic, anharmonic vibrational wavenumbers, and IR intensities are listed in Tables S8, S9, and S10, respectively, of the Supporting Information. In terms of energetics, 1,4-dichloro-2-methyl-2-butene is predicted to be the lowest-energy isomer, with the 3,4-dichloro-2-methyl-1butene and 3,4-dichloro-3-methyl-1-butene isomers being 5.2 and 14.1 kJ mol−1 higher in energy, respectively. For each of the dichloro products, several vibrational modes are predicted to have IR intensities above 30 km mol−1. For 3,4-dichloro-2methyl-1-butene, these are the modes at 658 (45 km mol−1), 721 (62 km mol−1), and 924 (39 km mol−1) cm−1. For 3,4dichloro-3-methyl-1-butene, these are the modes at 707 (93 km mol−1), 941 (32 km mol−1), and 1066 (31 km mol−1) cm−1. For 1,4-dichloro-2-methyl-2-butene, these are the modes at 697 (126 km mol−1) and 1257 (43 km mol−1) cm−1. E

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Table 2. Comparison of Observed Vibrational Wavenumbers (in cm−1) and IR Intensities of the 1-Chloromethyl-1-methylallyl radical (set B) in p-H2 with those Predicted with the B3PW91/6-311++G(2d,2p) Method modea

mode descriptionb

harmonic

scaledc

anharmonic

3109

3109

p-H2

ν1

a-νC4H2

3256 (7.1)

ν2

s-νC4H2

3166 (6.2)

3024

3063

ν3

a-νC1H2

3164 (4.7)

3022

3021

ν4

a-νCH3

3147 (9.0)

3005

3001

ν5 ν6

νC3H s-νC1H2

3140 (9.5) 3095 (13.0) (13.0)

2999 2956

3018 2974

ν7

a-νCH3

3074 (9.0)

2936

2920

ν8

s-νCH3

3028 (16.0)

2892

2903

ν9

δS C4H2

1531 (6.8)

1496

1477

ν10

a-δCH3

1502 (1.6)

1467

1462

ν11

δS C1H2

1483 (9.7)

1449

1443

ν12

a-δCH3 (oop)

1470 (5.0)

1436

1437

ν13 ν14

δC3H/δC4H8 uCH3

1459 (10.5) 1410 (14.2)

1425 1378

1418 1370

ν15

uCH3/δC3H

1376 (0.9)

1344

1343

ν16

ωC1H2/νC1C2

1271 (25.9)

1242

1242

1257.7 (54)

ν17 ν18

δC3H/δC4H9 ωC1H2

1250 (2.8) 1245 (6.2)

1221 1216

1228 1220

1229.1 (12)

ν19

tC1H2

1154 (2.0)

1127

1129

ν20

ρC4H2/δC3H

1047 (13.3)

1023

1027

ν21

ρCH3/ωC3H

1026 (1.1)

1002

995

ν22

ρCH3/ρC4H2

990 (11.2)

967

964

975.8 (8)

ν23 ν24

ωC3H ρC1H2

970 (11.5) 905 (2.1)

948 884

951 891

942.4 (38)

ν25

ωC4H2

817 (43.4)

798

791

796.7/798.5 (83)

ν26 ν27 ν28

νC1C2/νC2C10/νC2C3 νC1Cl tC4H2

784 (7.3) 661 (57.4) 575 (43.4)

766 646 562

769 643 560

667.9 (100) 569.7 (37)

ν29 ν30 ν31g

δC2C3C4 δC1C2C3 (oop) δC1C2C10

541 (3.6) 398 (6.6) 372 (1.5)

529 389 363

536 393 363

d

3112.7 (18)e 3035.7?f (4)

1487.6? (9)

1382.6 (20)

1034.8 (12)

a Mode order is according to harmonic vibrational wavenumbers. bApproximate descriptions. ν: stretch, δ: bend or deformation, δS: scissor, u: umbrella, ω: wag, ρ: rock, t: twist, a: antisymmetric, s: symmetric, ip: in-phase, op: out-of-phase, oop: out-of-plane. When it is obvious, the atomic numbering in the mode description is omitted. cScaled harmonic vibrational wavenumbers scaled according to factors 0.955 and 0.977 for regions above and below 2000 cm−1, respectively; see text. dIR intensities (in km mol−1) are listed in parentheses. eIntegrated intensities normalized to the most intense peak (667.9 cm−1) are listed in parentheses. fAssignments of lines with less certainty are marked with ?. gAdditional harmonic modes at 291, 240, 149, 68, and 62 cm−1 are calculated for the 1-chloromethyl-1-methylallyl radical.

912, 996, 1623 cm−1 predicted at the B3PW91/6-311+ +G(2d,2p) level of theory. The average absolute deviation between all of the observed experimental lines and the scaled harmonic wavenumbers for trans-C5H8 is 9 ± 6 cm−1 (0.7 ± 0.5%), with the largest deviation being 25 cm−1. The corresponding average absolute deviation with the anharmonic wavenumbers is 6 ± 5 cm−1 (0.4 ± 0.4%), with the largest deviation being 24 cm−1. We also observed a series of weak lines that are assigned to gauche-C5H8 based on a comparison with the spectrum in an argon matrix52 and the predicted scaled harmonic and anharmonic vibrational wavenumbers. The most prominent of these are the lines at 885.5, 918.2, 1002.4, and 1641.4 cm−1 in solid p-H2, with the corresponding lines in an argon matrix reported at 890, 916, 1002, and 1644 cm−1. The scaled harmonic and anharmonic vibrational wavenumbers are predicted at 903, 926, 1003, and 1663 cm−1 and 900, 936,

1007, and 1665 cm−1, respectively. The average absolute deviation between all observed experimental lines and the scaled harmonic wavenumbers for gauche-C5H8 is 7 ± 5 cm−1 (0.5 ± 0.5%), with the largest deviation being 22 cm−1. The corresponding average absolute deviation with the anharmonic wavenumbers is 8 ± 5 cm−1 (0.6 ± 0.5%), with the largest deviation being 24 cm−1. A list of all fundamental lines of trans-C5H8 and gauche-C5H8 observed in solid p-H2 is compared with the B3PW91/6-311+ +G(2d,2p) predicted scaled harmonic and anharmonic vibrational wavenumbers in Tables S1 and S2 in the Supporting Information, respectively. Using the integrated intensities of several lines of trans and gauche-C5H8 and the corresponding theoretical IR intensities, we estimated that the ratio of trans to gauche isomers is ∼60:1 (∼1.6% gauche-C5H8), which is similar to the ratio of 59:1 calculated at 298 K using the predicted energy difference. Given the satisfactory agreement between the F

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The Journal of Physical Chemistry A scaled harmonic and anharmonic vibrational wavenumbers with the experimental wavenumbers for trans-C5H8 and gaucheC5H8, generally only the scaled harmonic vibrational wavenumbers will be specifically discussed in the remainder of the text. However, it is worth noting that both the scaled harmonic and anharmonic vibrational wavenumbers were considered when making assignments for the Cl2···C5H8 complex and the Cl + C5H8 reaction products. Upon codeposition of Cl2 with C5H8 in solid p-H2 at 3.2 K for several hours, a series of new lines or shoulders on C5H8 lines is observed. The relative intensities of these new lines are found to increase on increasing mixing ratio of Cl2 relative to C5H8 and to increase with annealing to 4−5 K, which suggests that these new lines are due to a Cl2···C5H8 complex. The infrared spectra of a Cl2/C5H8/p-H2 (0.67/1/2000) matrix and a C5H8/p-H2 (1/2000) matrix after annealing to 5 K, and a difference spectrum of a Cl2/C5H8/p-H2 (0.67/1/2000) matrix before and after annealing to 5 K, along with the IR spectra simulated according to scaled harmonic vibrational wavenumbers and IR intensities predicted for trans-C5H8, Cl2···C5H8 (Cl2···C1C2 complex), and Cl2···C5H8 (Cl2···C3C4 complex) are presented in Figure S7 of the Supporting Information. The most prominent of the new lines are observed at 506.9, 513.4, 520.2, 522.0, 761.8, 899.5/900.2, 901.7/902.2, 913.4, 990.0, 995.0, 1593.1, and 1600.2 cm−1. With close inspection of the behavior of these new lines upon annealing, it was observed that many of the new lines could be grouped into two sets that displayed different relative intensity increases upon annealing, which suggests that these lines are due to two distinct species. The lines at 506.9, 513.4, 520.2, 899.5/900.2, 995.0, and 1600.2 cm−1 were found to behave analogously with each other and are designated as set 1. The lines at 508.9, 515.8, 522.0, 761.8, 901.7/902.2, 990.0, and 1593.1 cm−1 were found to behave analogously with each other and are designated as set 2. As will be discussed in Section VA, these two sets of lines are assigned to absorptions of the two distinct structures of the Cl2···C5H8 complex, that is, the Cl2··· C1C2 and Cl2···C3C4 structures described in Section IIIA. The lines of set 1 are assigned to the Cl2···C1C2 complex, which is the lowest energy minimum, and the lines of set 2 are assigned to Cl2···C3C4 complex, which is higher in energy by 1.7 kJ mol−1. The observed wavenumbers for each Cl2···C5H8 complex are compared with the predicted scaled harmonic and anharmonic vibrational wavenumbers in Tables S3 and S4 of the Supporting Information. Because of the significant overlap of the lines of each Cl2···C5H8 complex with each other and with lines of the parent C5H8, some lines of Cl2···C5H8 could not be identified, and the mixing ratio of the Cl2···C5H8 complex could not be determined accurately. IVB. Reactions of Cl with C5H8 in p-H2. Upon irradiation of a Cl2/C5H8/p-H2 matrix at 365 nm, the IR intensities of the lines due to C5H8 and the Cl2···C5H8 complex both decreased, and a new series of lines appeared, suggesting that the new lines are due to products of the reaction of Cl atoms with C5H8. The most prominent of these are a line at 807.8 cm−1 and a pair of closely spaced lines at 798.5 and 796.7 cm−1, with 796.7 cm−1 being the higher intensity lines. Experiments were performed both with and without annealing before the 365 nm photolysis, and the only difference between these experiments was in the absolute intensities of the observed new lines. Figure 2a shows a difference spectrum obtained by subtraction of a scaled spectrum of a Cl2/C5H8/p-H2 (0.67:1:3000) matrix, deposited for 9 h followed by annealing at 5 K for 5 min, from the

Figure 2. (a) Difference spectrum of a Cl2/C5H8/p-H2 (0.67/1/2000) matrix, deposited for 9 h at 3.2 K followed by annealing at 5 K for 5 min, after irradiation at 365 nm for 45 min. (b) Difference spectrum of the matrix after further irradiation at λ > 345 nm for 1 h. (c) Difference spectrum of the matrix after further irradiation at λ > 305 nm for 1 h. (d) Difference spectrum of the matrix after further irradiation at 254 ± 10 nm for 45 min. All spectra were recorded at 3.2 K with resolution 0.25 cm−1. The intensities of the spectra in (b−d) were multiplied by factors of 6.7, 6.6, and 1.5, respectively, and the spectra are shifted. The assignments of lines in each group are A: 1chloromethyl-2-methylallyl, B: 1-chloromethyl-1-methylallyl, C: 3,4dichloro-2-methyl-1-butene, D: 3,4-dichloro-3-methyl-1-butene, E: 1,4-dichloro-2-methyl-2-butene.

spectrum after irradiation of the matrix at 365 nm for 45 min. Features pointing upward indicate production, whereas those pointing downward indicate destruction. To further characterG

DOI: 10.1021/acs.jpca.7b07922 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

V. DISCUSSION VA. Assignment of Lines of the Cl2···C5H8 Complexes. As was described in Section IVA, codeposition of Cl2 with C5H8 in solid p-H2 produces a series of new lines in the infrared spectra, listed in Tables S3 and S4, Supporting Information, that are close to the C5H8 lines, as displayed in Figure S7a,b The new lines, observed only when both Cl2 and C5H8 are present in the p-H2 matrix, are found to be dependent on the concentration of Cl2 and C5H8 in the p-H2 matrix and are enhanced with annealing of the matrix. This behavior suggests that these lines are due to a complex between Cl2 and C5H8. In addition, in examining the spectra before and after annealing, we found that the relative intensities of several of the new peaks increased by varied fractions upon annealing, suggesting the presence of two species, and these two groups of lines are designated as sets 1 and 2. Set 1 is comprised of the lines at 506.9, 513.4, 520.2, 899.5/900.2, 995.0, and 1600.2 cm−1, and set 2 is comprised of the lines at 508.9, 515.8, 522.0, 761.8, 901.7/902.2, 990.0, and 1593.1 cm−1. Quantum-chemical calculations predict two stable structures for the Cl2···C5H8 complex (displayed in Figure S2 in the Supporting Information): one in which the Cl2 is interacting with the C1C2 bond (Cl2···C1C2 complex) and the other in which the Cl2 is interacting with the C3C4 bond (Cl2··· C3C4 complex); the former is lower in energy by 1.7 kJ mol−1 than the latter. The simulated IR spectra according to scaled harmonic vibrational wavenumbers and IR intensities for C5H8 and both Cl2···C5H8 complexes are displayed in Figure S7c,d in the Supporting Information, respectively. The predicted scaled harmonic and anharmonic vibrational wavenumbers for the two Cl2···C5H8 complexes are compared with experimental results in Tables S3 and S4 in the Supporting Information. After the simulated IR spectra of the two Cl2···C5H8 complexes and the experimental spectra were compared and the annealing behavior of the two sets of lines was considered, the lines at 520.2, 899.5/900.2, 995.0, and 1600.2 cm−1 from set 1 were assigned to the Cl2···C1C2 complex. These lines are in satisfactory agreement with the scaled harmonic vibrational wavenumbers for ν28 (C2C3C4 bend, 522 cm−1), ν24 (C1H2 wag, 919 cm−1), ν20 (C3H out-of-plane bend, 997 cm−1), and ν10 (C1C2/C3C4 out-of-phase stretch, 1597 cm−1). The lines at 522.0, 761.8, 901.7/902.2, 990.0, and 1593.1 cm−1 from set 2 are assigned to the Cl2···C3C4 complex and are in satisfactory agreement with the scaled harmonic vibrational wavenumbers for ν28 (C2C3C4 bend, 522 cm−1), ν26 (C1H2/ C4H2 in-phase twist, 768 cm−1), ν24 (C1H2 wag, 919 cm−1), ν20 (C3H out-of-plane bend, 991 cm−1), and ν10 (C1C2/C3C4 out-of-phase stretch, 1601 cm−1). The observation of distinct lines for the CC stretches that are predicted to be directly involved in the complexation with the Cl2 (ν10), in addition to the “free” CC stretch observed at 1638.2 cm−1, strongly supports the conclusion that both Cl2···C5H8 complex structures are being formed in the matrix. The multiplex lines at 506.9, 508.9, 513.4, and 515.8 cm−1 are assigned to the ClCl stretching mode (ν29) of the two Cl2··· C5H8 complex structures. The lines at 513.4 and 506.9 cm−1 from set 1 are assigned to the 35Cl35Cl and the 35Cl37Cl stretching vibrations of the Cl2···C1C2 complex, and the lines at 515.8 and 508.9 cm−1 from set 2 are assigned to the 35 35 Cl Cl and the 35Cl37Cl stretching vibrations of the Cl2··· C3C4 complex. The observed isotopic shifts of −6.5 and

ize these new lines and determine which lines should be grouped in sets, we performed a series of secondary photolysis experiments using a low-pressure Hg lamp combined with three different filters, a 345 nm long pass filter (1 h), a 305 nm long pass filter (1 h), and a 254 nm band-pass filter (45 min), and the difference spectra for these secondary photolysis experiments are shown in Figures 2b−d. One experiment was performed in which the matrix was annealed after 365 nm photolysis, and no significant change in the new lines was observed. Focusing on the lines at 807.8 and 796.7/798.5 cm−1, the line at 807.8 cm−1 is observed to decrease by ∼11% upon photolysis with λ ≥ 345 nm, increase by ∼1% upon photolysis with λ ≥ 305 nm, and decrease by ∼50% upon photolysis near 254 ± 10 nm, while the 796.7/798.5 cm−1 lines are observed to increase by ∼1% upon photolysis with λ ≥ 345 nm, increase by 8% upon photolysis with λ ≥ 305 nm, and decrease by ∼23% upon photolysis near 254 ± 10 nm. The distinct behavior of these two lines indicates that they belong to two chemical species. In examining the secondary photolysis behavior of the other new lines produced upon 365 nm photolysis, a set of lines at 472.7, 526.9, 605.6/608.2, 754.1, 1108.8, 1170.3, 1237.9, 1268.2, 1349.6, 1393.4, 1459.5, 1482.8, 3047.2, and 3104.8 cm−1 were found to behave analogously and with similar relative intensities to the line at 807.8 cm−1, and these lines are designated as set A. Another set of lines at 569.7, 667.9, 942.4, 975.8, 1034.8, 1229.1, 1257.7, 1382.6, 1487.6, 3035.7, and 3112.7 cm−1 were found to behave analogously and with similar relative intensities to the lines at 796.7/798.5 cm−1, and these lines are designated as set B. As will be discussed in Section VB, the lines in set A are assigned to the 1-chloromethyl-2-methylallyl radical, and as will be discussed in Section VC, the lines in set B are assigned to the 1-chloromethyl-1-methylallyl radical. The wavenumbers and relative intensities of the observed lines of sets A and B are listed in Tables 1 and 2, respectively. Some additional weak lines are indicated as sets C, D, and E in Figure 2a. These weak lines displayed insignificant changes upon secondary photolysis, suggesting that they belong to stable species. The intensities of these weak lines increased more than those of the lines in groups A and B with increasing Cl2/p-H2 mixing ratio, indicating that they might be due to a doubly chlorinated species. As will be discussed in Section VD, the lines of sets C (535.8, 652.8, 733.3, 748.6, 791.0, 931.0, 1022.3, and 1379.5 cm−1), D (596.9, 716.8, 936.0, 1002.3, and 1003.6 cm−1), and E (697.3 cm−1) were assigned to 3,4dichloro-2-methyl-1-butene, 3,4-dichloro-3-methyl-1-butene, and 1,4-dichloro-2-methyl-2-butene, respectively, based on comparisons of experimental wavenumbers and relative intensities with the theoretical wavenumbers and relative intensities for these compounds. The observed wavenumbers for lines of sets C, D, and E are compared with the predicted scaled harmonic and anharmonic vibrational wavenumbers for 3,4-dichloro-2-methyl-1-butene, 3,4-dichloro-3-methyl-1-butene, and 1,4-dichloro-2-methyl-2-butene in Tables S8−S10, respectively, of the Supporting Information. Using the predicted wavenumbers and relative intensities for the ·C5H7 radical (Table S7), we also examined the spectra to determine if any line could be assigned to this radical; however, we were unable to find any line in the spectra that could be unambiguously assigned to the ·C5H7 radical. H

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The Journal of Physical Chemistry A −6.9 cm−1, respectively, are consistent with the theoretical predictions of −4.9 and −5.0 cm−1, respectively, for these two species. The observed intensity ratios of ∼1.7 (513.4 to 506.9 cm−1 lines) and ∼1.6 (515.8 to 508.9 cm−1 lines) are also consistent with the natural abundance of 35Cl35Cl/ 35Cl37Cl according to 35Cl35Cl/35Cl37Cl/37Cl37Cl = 9.5:6.2:1.0. There were some very weak lines between 500 and 502 cm−1 that became slightly more visible with higher Cl2: p-H2 mixing ratios that might be due to the Cl2···C1C2 complexes with the 37 37 Cl Cl isotope; however, they were much too weak to be assigned with any confidence. At the B3PW91/6-311++G(2d,2p) level of theory, the scaled harmonic vibrational wavenumbers of the 35Cl35Cl stretching modes of the Cl2··· C1C2 and Cl2···C3C4 complexes are predicted to occur at 427 and 439 cm−1, respectively, which are significantly smaller than the experimental values (deviations of ∼86 and 77 cm−1, respectively). The deviations are decreased slightly when one considers the anharmonic vibrational wavenumbers for this mode of both structures, 440 and 450 cm−1, respectively. A similar trend was also observed for the Cl2 complex with trans1,3-butadiene (C4H6), in which the experimental vibrational wavenumber of the 35Cl35Cl stretching mode of the Cl2···C4H6 complex was observed at 524.4 cm−1, and the B3PW91/6-311+ +G(2d,2p) anharmonic vibrational wavenumber was predicted at 460 cm−1 (deviation of ∼64 cm−1).29 Given that the scaled harmonic vibrational wavenumber of the 35Cl35Cl molecule is predicted to be at 547 cm−1 at the B3PW91/6-311++G(2d,2p) level of theory, the large discrepancy between the experimental and predicted vibrational wavenumbers for the Cl2···C5H8 complexes might be an indication that the interaction between Cl2 and C5H8 is overestimated at this level of theory. When we rescaled the “scaled” harmonic vibrational wavenumbers of the ClCl strething mode to match the experimental values, we obtained 35Cl37Cl isotopic shifts of 5.9 and 5.8 cm−1, respectively, for the Cl2···C1C2 and Cl2···C3C4 complexes, closer to the experimental observation. Several of the Cl2···C5H8 complex peaks are not significantly shifted from each other for the two structures and therefore seem to appear as a single overlapped peak in the experimental spectrum. These correspond to the lines at 913.4, 1371.7, 1638.2, and 2956.6 cm−1, which can be compared to the scaled harmonic vibrational wavenumbers for ν23 (C4H2 wag, 930 and 926 cm−1), ν15 (CH3 umbrella, 1374 and 1374 cm−1), ν9 (C1C2/C3C4 in-phase stretch, 1648 and 1649 cm−1), and ν7 (C10H2 antisymmetric stretch, 2963 and 2957 cm−1). The remaining predicted lines of the Cl2···C5H8 complexes could not be positively identified either due to weak intensities or severe overlap with the lines of C5H8. The overall agreement between the experimental and theoretical wavenumbers for both Cl2···C5H8 complexes is satisfactory. When the ClCl stretching modes are excluded, the average absolute deviation between the observed and scaled harmonic wavenumbers is found to be 8 ± 7 cm−1 (0.7 ± 0.8%) and 6 ± 6 cm−1 (0.6 ± 0.7%) for the Cl2···C1C2 and Cl2···C3C4 complexes, respectively. VB. Assignment of Lines in Set A to the 1Chloromethyl-2-methylallyl Radical. As was described in Section IVB, photolysis of a Cl2/C5H8/p-H2 matrix at 365 nm results in a series of new IR lines that can be grouped into two distinct sets, which we designated as set A and set B. We expect these lines are due to products of the addition reaction of Cl atoms to C5H8, and the presence of two distinct sets of IR lines indicates the presence of two distinct chemical species. The

lines of set A are comprised of an intense, sharp line at 807.8 cm−1 and a series of generally weaker lines, as indicated with “A” in Figure 2a. In Figure 3a, we essentially reproduced the spectrum in Figure 2a but from an experiment with less Cl2 relative to C5H8 to reduce the formation of doubly chlorinated compounds; the lines of set A are also marked with an “A” in Figure 3a. The theoretical IR spectra of the four possible Cl atom addition products simulated according to scaled harmonic vibrational wavenumbers and IR intensities are presented in Figures 3b−e. Upon comparing both the experimental wavenumbers and relative intensities for the lines of set A with the theoretical wavenumbers and relative intensities for all four of the possible Cl addition products, the best agreement is found to be with the 1-chloromethyl-2-methylallyl radical, whose simulated spectrum is displayed in Figure 3e. The observed wavenumbers and relative intensities for the lines of set A are compared with the theoretically predicted values for the 1-chloromethyl-2-methylallyl radical in Table 1. In the spectral region below 2000 cm−1, the observed lines at 605.6/608.2, 807.8, and 1237.9 cm−1, which have the highest experimental relative intensities, are in satisfactory agreement with the scaled harmonic vibrational wavenumbers for ν27 (CCl stretch, 587 cm−1), ν24 (C1H2 wag, 817 cm−1), and ν17 (C4H2 wag, 1217 cm−1), which are predicted to be the most intense lines for the 1-chloromethyl-2-methylallyl radical. For the weaker intensity lines, the assignments for the observed lines at 472.7, 754.1, 1108.8, 1170.3, 1268.2, 1349.6, and 1482.8 cm−1 are reasonably strong, given that they all displayed very consistent intensity changes upon secondary photolysis. These lines are in satisfactory agreement with the scaled harmonic vibrational wavenumbers for ν30 (C1C2C3C10 out-of-plane bend, 475 cm−1), ν26 (C3H out-of-plane bend, 756 cm−1), ν19 (C3C4 stretch, 1105 cm−1), ν18 (C4H2 twist, 1165 cm−1), ν16 (C1C2C3C4 bend, 1263 cm−1), ν15 (CH3 umbrella, 1353 cm−1), and ν9 (C1H2 scissor/C1C2 stretch, 1492 cm−1) for the 1-chloromethyl-2-methylallyl radical. The assignments of the weaker intensity observed lines at 526.9, 1393.4, and 1459.5 cm−1 are less secure because of small intensities and some inconsistencies in their intensities upon secondary photolysis, likely due to interference by absorption of other species. However, they are in satisfactory agreement with scaled harmonic vibrational wavenumbers for ν29 (C2C3C4 bend, 513 cm−1), ν14 (C2C10 stretch/CH3 umbrella, 1389 cm−1), and ν12 (CH3 bend, 1451 cm−1) for the 1-chloromethyl-2methylallyl radical. In the CH region, due to severe overlap with the C5H8 precursor, only two lines of set A at 3047.2 and 3104.8 cm−1 could be positively identified, and these are in satisfactory agreement with the scaled harmonic wavenumbers of ν2 (C4H2 antisymmetric stretch/C3H stretch, 3041 cm−1) and ν1 (C1H2 antisymmetric stretch, 3103 cm−1) for the 1chloromethyl-2-methylallyl radical. The assignment of the line at 3047.2 cm−1 is reasonably secure, because this line does show very consistent intensity changes upon secondary photolysis, and it does not suffer from significant overlap with the C5H8 lines. The line at 3104.8 cm−1 is less secure, because it is very close to a C5H8 line at 3100.1 cm−1, and this causes some inconsistencies in its intensity upon secondary photolysis. From Table 1, the average absolute deviation between the experimental and scaled harmonic wavenumbers is found to be 8 ± 6 cm−1 (0.8 ± 1.0%), with the largest deviation being 21 cm−1 for the CCl stretching mode (ν27). The deviations between the experimental and anharmonic wavenumbers are similar, with an average absolute deviation of 7 ± 8 cm−1 (0.9 ± I

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1.3%) and the largest deviation being 33 cm−1 for the CCl stretching mode. Significant deviations between the experimental and theoretical wavenumber for the CCl stretching mode using the B3PW91 or B3LYP method have been observed for the other chloroalkyl radicals in solid p-H2 that we previously studied.26−29,47 For example, for the trans-1chloromethylallyl radical, which was produced from the reaction of Cl atoms with trans-1,3-butadiene in solid p-H2, the observed line at 650.3 cm−1 was assigned to the CCl stretching mode of the radical, and the anharmonic vibrational wavenumber at the B3PW91/6-311++G(2d,2p) level was predicted to be at 610 cm−1 (40 cm−1 deviation).29 Given the satisfactory overall agreement of the experimental and theoretical wavenumbers and intensities, we assign the features of set A to the 1-chloromethyl-2-methylallyl radical. VC. Assignment of the Lines of Set B to the 1Chloromethyl-1-methylallyl Radical. The lines of set B are comprised of an intense pair of sharp peaks at 796.7/798.5 cm−1 and a series of less intense lines, as indicated with “B” in Figure 3a. When the observed wavenumbers and relative intensities for the lines of set B are compared with the theoretical wavenumbers and relative intensities for the remaining three possible Cl addition products, presented in Figure 3b−d, the best agreement is found with the 1chloromethyl-1-methylallyl radical, Figure 3b. All observed wavenumbers for lines of set B are compared with the predicted values for 1-chloromethyl-1-methylallyl radical in Table 2. In the region below 2000 cm−1, the observed lines at 569.7, 667.9, 796.7/798.5, 942.4, and 1257.7 cm−1, which generally have the highest experimental relative intensities, are in satisfactory agreement with the scaled harmonic wavenumbers for ν28 (C4H2 twist, 562 cm−1), ν27 (CCl stretch, 646 cm−1), ν25 (C4H2 wag, 798 cm−1), ν23 (C3H out-of-plane bend, 948 cm−1), and ν16 (C1C2 stretch, 1242 cm−1), which are the most intense predicted lines for the 1-chloromethyl-1-methylallyl radical. Among the weaker lines, the observed lines at 975.8, 1034.8, 1229.1, and 1382.6 cm−1 are found to be in satisfactory agreement with the scaled harmonic vibrational wavenumbers for ν22 (C10H2 wag, 967 cm−1), ν20 (C4H2 rock/C3H bend, 1023 cm−1), ν18 (C1H2 wag, 1216 cm−1), and ν14 (CH3 umbrella/C2C10 stretch, 1378 cm−1) for the 1-chloromethyl1-methylallyl radical. The assignment of the line at 1487.6 cm−1 is less secure, because it is weak and is overlapped with a stronger line of the 1-chloromethyl-2-methylallyl radical at 1482.8 cm−1; however, it is in satisfactory agreement with scaled harmonic vibrational wavenumbers for ν9 (C4H2 scissor/ C3C4 stretch, 1496 cm−1) for the 1-chloromethyl-1-methylallyl radical. In the CH region, due to severe overlap with the C5H8 precursor, only two peaks at 3035.7 and 3112.7 cm−1 could be grouped to set B, and these are in satisfactory agreement with the scaled harmonic wavenumbers of ν3 (C1H2 antisymmetric stretch, 3022 cm−1) and ν1 (C4H2 antisymmetric stretch, 3109 cm−1) for the 1-chloromethyl-1-methylallyl radical. The assignment of the line at 3112.7 cm−1 is secure, because this line does not suffer from any appreciable overlap with the C5H8 lines, and it displays a consistent relative intensity upon secondary photolysis. The assignment of the line at 3035.7 cm−1 is less secure, because it does display some inconsistencies in its intensity upon secondary photolysis, particularly with the 254 ± 10 nm photolysis. The average absolute deviation between the experimental and scaled harmonic wavenumbers is found to be 10 ± 6 cm−1 (0.9 ± 0.9%), with the largest deviation being 22 cm−1 for the

Figure 3. (a) Difference spectrum of a Cl2/C5H8/p-H2 (0.33/1/1000) matrix, deposited for 8 h at 3.2 K, after irradiation at 365 nm for 30 min. Lines in group A (1-chloromethyl-2-methylallyl) and B (1chloromethyl-1-methylallyl) are marked. For clarity, the spectral region between 994.3 and 992.1 cm−1 were removed due to the saturated peaks of C5H8. Regions of strong interference by absorption of C5H8 are marked with gray rectangle backgrounds. IR spectra simulated according to scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method are shown for (b) 1-chloromethyl-1-methylallyl, (c) 2-chloro-2methyl-3-buten-1-yl, (d) 2-chloro-3-methyl-3-buten-1-yl, and (e) 1chloromethyl-2-methylallyl. J

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To determine if any of the additional weak lines can be assigned to these three dichloro compounds, the experimental wavenumbers and relative intensities for the additional observed lines were compared with the theoretical wavenumbers and relative intensities for 3,4-dichloro-2-methyl-1butene, 3,4-dichloro-3-methyl-1-butene, and 1,4-dichloro-2methyl-2-butene, according to values listed in Tables S8, S9, and S10 of the Supporting Information. When the theoretical vibrational wavenumbers and IR intensities for 3,4-dichloro-2-methyl-1-butene are compared with the experimental spectra, satisfactory agreement is found between the experimental lines at 652.8, 733.3, and 931.0 cm−1 with the scaled harmonic wavenumbers for ν29 (658 cm−1), ν28 (721 cm−1), and ν24 (924 cm−1), which are predicted to be the most intense lines for this molecule. The weaker lines at 535.8, 748.6, 791.0, 1022.3, and 1379.5 cm−1 are also found to be in satisfactory agreement with the scaled harmonic lines for ν30 (519 cm−1), ν27 (752 cm−1), ν26 (781 cm−1), ν22 (1013 cm−1), and ν14 (1376 cm−1) of 3,4-dichloro-2-methyl-1-butene. A list of observed lines designated as set C in Figure 2a are compared with the theoretical values of 3,4-dichloro-2-methyl-1-butene in Table S8 of the Supporting Information. Most lines with IR intensities above 10 km mol−1 (except the CH-stretching lines) were observed. A line predicted at 1451 cm−1 with IR intensity of 17 km mol−1 is unobserved, likely because of the interference from absorption of C5H8. When the theoretical vibrational wavenumbers and IR intensities for 3,4-dichloro-3-methyl-1-butene are compared with the experimental spectra, satisfactory agreement is found between the experimental lines at 596.9 and 716.8 cm−1 with the scaled harmonic wavenumbers for ν29 (580 cm−1) and ν28 (707 cm−1), which are predicted to be two of the more intense lines for this molecule. The weak lines at 936.0, 1002.3, and 1003.6 cm−1 are also found to be in satisfactory agreement with the scaled harmonic lines for ν23 (941 cm−1), ν22 (1002 cm−1), and ν21 (1008 cm−1) of 3,4-dichloro-3-methyl-1-butene. A list of observed lines, designated as set D in Figure 2a, is compared with the theoretical values of 3,4-dichloro-3-methyl-1-butene in Table S9 of the Supporting Information. Most lines with IR

CCl stretching mode (ν27), similar to what was observed for the 1-chloromethyl-2-methylallyl radical. The deviations between the experimental and anharmonic wavenumbers are comparable, with an average absolute deviation of 11 ± 6 cm−1 (1.2 ± 1.0%) and the largest deviation being 25 cm−1 for the CCl stretching mode. Given the reasonably good overall agreement between the experimental and theoretical wavenumbers and intensities, we assign the features of set B to the 1chloromethyl-1-methylallyl radical. VD. Assignments of Lines of 3,4-Dichloro-2-methyl-1butene, 3,4-Dichloro-3-methyl-1-butene, and 1,4-Dichloro-2-methyl-2-butene. In addition to the lines of the 1-chloromethyl-1-methylallyl (set B) and the 1-chloromethyl-2methylallyl (set A) radicals, a series of additional weak lines are also observed upon 365 nm photolysis of a Cl2/C5H8/p-H2 matrix. These lines are found to be insensitive to secondary photolysis, and their intensities are observed to increase relative to the lines of sets A and B with increasing Cl2/p-H2 mixing ratio in the matrix, suggesting that these lines are due to stable species that involve perhaps two or more chlorine atoms. Since we observed peaks due to stable species resulting from the reaction of two Cl atoms in all of our previous studies of the reaction of Cl atoms with alkenes,26−29,47 it seems likely that these weak lines might be due to the addition of two Cl atoms to isoprene. As described in Section IIIB, the unpaired electron in both the 1-chloromethyl-1-methylallyl and 1-chloromethyl-2methylallyl radicals is essentially delocalized over three carbon atoms: (C2, C3, C4) and (C1, C2, C3) for 1-chloromethyl-1methylallyl and 1-chloromethyl-2-methylallyl radicals, respectively. The addition of a second chlorine atom to the position that is adjacent to the site of the first chlorine atom produces the following two dichloro compounds: 3,4-dichloro-3-methyl1-butene from the 1-chloromethyl-1-methylallyl radical and 3,4dichloro-2-methyl-1-butene from the 1-chloromethyl-2-methylallyl radical, shown in Figure S5a,b, Supporting Information. There is also a possibility that the chlorine atom could add to the available terminal carbon atom in each of the radicals to form 1,4-dichloro-2-methyl-2-butene, shown in Figure S5c, from both radicals. K

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The Journal of Physical Chemistry A intensities above 10 km mol−1 except those predicted at 1066, 1377, and 1411 cm−1 with IR intensity of 31, 13, and 19 km mol−1, respectively, were observed; the unobserved lines are likely due to the interference from absorption of C5H8. The weak line in set E at 697.3 cm−1 agrees with the scaled harmonic vibrational wavenumber for the most intense line of 1,4-dichloro-2-methyl-2-butene at 697 cm−1 for ν28 with intensity 126 km mol−1 at least three times greater than other vibrational modes of this molecule. This line is tentatively assigned to 1,4-dichloro-2-methyl-2-butene, because none of the other weaker lines were observed. VE. Reaction Mechanism. The lines of set A are assigned to the 1-chloromethyl-2-methylallyl radical, and those of set B are assigned to the 1-chloromethyl-1-methylallyl radical, which correspond to attack of the Cl atom at terminal carbons C4 and C1 of isoprene, respectively. No spectral lines of the 2-chloro-2methyl-3-buten-1-yl radical (Cl addition to carbon C2) or the 2-chloro-3-methyl-3-buten-1-yl radical (Cl addition to carbon C3) were identified. We thus conclude that, in a solid p-H2 matrix, the Cl atom reacts selectively with the two terminal carbon atoms of isoprene rather than with the two central carbon atoms. With the integrated intensities of several lines of the 1-chloromethyl-1-methylallyl and 1-chloromethyl-2-methylallyl radicals and the corresponding theoretical intensities, the ratio of the 1-chloromethyl-1-methylallyl to the 1-chloromethyl-2-methylallyl radicals produced in the solid p-H2 matrix is estimated to be (1.2 ± 0.5):1.0, which indicates that these two radicals are produced in nearly equal amounts. The exclusive production of the radicals involving the addition of the Cl atom to the two terminal positions is consistent with the theoretical energetics, which indicates the addition to carbons 1 and 4 to be significantly favored over the addition to carbons 2 and 3 by ∼60−70 kJ mol−1 at the B3PW91/6-311++G(2d,2p) level of theory. This is analogous to what was previously observed for the reaction of Cl atoms with trans-1,3-butadiene in solid p-H2, in which the addition of the Cl atom to the terminal carbon atom to form trans-1-chloromethylallyl radical was predicted to be favored over the addition to the central carbon atom by ∼65 kJ mol−1, and experimentally it was the only chloro radical observed.29 For the reaction pathways leading to the ·C5H8Cl radicals, since distinct Cl2···C5H8 complexes are observed in which each Cl2 is interacting with a different CC bond (Cl2···C1C2 and Cl2···C3C4 complexes), it seems reasonable to assume that each complex leads to the formation of one specific radical upon photolysis, as displayed in Scheme 1. In this scheme, the Cl2···C1C2 complex would lead to the formation of the 1chloromethyl-1-methylallyl radical (radical B) by Cl addition to C1, and the Cl2···C3C4 complex would lead to the formation of the 1-chloromethyl-2-methylallyl radical (radical A) by Cl addition to C4. Note, however, that the reactions from the Cl2···C5H8 complexes are not the only pathways that lead to the two radicals, since the lines of C5H8 were also observed to decrease after photolysis, indicating that some of the Cl atoms that escape the matrix cage after photolysis are reacting directly with C5H8. When the mixing ratio of Cl2 is large, the two ·C5H8Cl radicals once formed can react further with a second Cl atom to give dichloro compounds Cl2C5H8. As shown in Scheme 1, attack of the second Cl atom at the carbon atom that is adjacent to the site of the first Cl atom produces 3,4-dichloro3-methyl-1-butene (compound D) from radical B and 3,4dichloro-2-methyl-1-butene (compound C) from radical A. For both radicals, attack of the second Cl atom at the carbon atom

that is terminal to the site of the first Cl atom produces the same dichloro product, namely, 1,4-dichloro-2-methyl-2-butene (compound E). Our observation of larger intensities of the lines of compounds C and D as compared to compound E seem to indicate that the second Cl atom prefers to add to the nearby carbon site despite the fact that adding to the terminal carbon site produces the product that has the least energy. This seems to be reasonable, because the second Cl is near that site, but further theoretical investigations are needed to fully explain such selectivity. The ability to trap and observe the IR spectra of the 1chloromethyl-1-methylallyl and 1-chloromethyl-2-methylallyl radicals, produced by the in situ photolysis of Cl2 with C5H8 in solid p-H2, is significant, because if this type of experiment were performed in a noble gas matrix, the most likely products would be dichloro addition products, not the chloroalkyl radicals.53−55 The diminished cage effect for solid p-H2 allows one of the Cl atoms of Cl2 to escape its initial matrix cage, thereby permitting a single Cl atom to react with the C5H8, assuming a high dilution of the Cl2. The observation of the infrared spectra of the 1-chloromethyl-1-methylallyl and 1chloromethyl-2-methylallyl radicals in solid p-H2 presented in this work is also significant, because this represents the first direct spectral evidence for the existence of these two radical species that are believed to be important intermediates in the reaction of chlorine atoms with isoprene. It is also interesting to compare the reactions of Cl atoms with C5H8 in lowtemperature solid p-H2 with the reaction in the gaseous phase at 298 K, which has been experimentally determined to proceed predominantly via the Cl addition pathway but also with between 15 and 17% via the H atom abstraction (methyl group) pathway.3,30 In our experiments in solid p-H2 at 3 K, we were unable to observe any lines that could be unambiguously assigned to the ·C5H7 radical, which is the expected product of the abstraction pathway, and this suggests that in solid p-H2 the reaction proceeds exclusively via the addition pathway. This is consistent with theory, which predicts a significant barrier for the abstraction of a methyl group H atom from C5H8.33,36

VI. CONCLUSION The reaction of Cl atoms with isoprene in solid para-hydrogen matrices at 3.2 K has been studied with IR spectroscopy. When Cl2/C5H8/p-H2 matrices were irradiated with UV light at 365 nm, a series of new lines in two sets appeared in the IR spectrum, with the intense lines at 1237.9, 807.8, and 605.6/ 608.2 cm−1 and several weaker lines being assigned to the 1chloromethyl-2-methylallyl radical and the intense lines at 1257.7, 942.4, 796.7/798.5, 667.9, and 569.7 cm−1 and several other weaker lines being assigned to the 1-chloromethyl-1methylallyl radical. The assignments were derived based on the expected reactions, secondary photolysis behavior, the scaled harmonic and anharmonic vibrational wavenumber, and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method. The addition of the Cl atom to C5H8 in solid p-H2 occurs at the two terminal carbon atoms versus the two central carbon atoms, consistent with the theoretical reaction energetics, which predicts the addition to two terminal carbon atoms to be favored by 60−70 kJ mol−1 at the B3PW91/6-311+ +G(2d,2p) level of theory. The 1-chloromethyl-1-methylallyl and 1-chloromethyl-2-methylallyl radicals are produced in approximately equal amounts. L

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b07922. Experimental and theoretical vibrational wavenumbers and IR intensities for trans-C5H8, gauche-C5H8, and the two Cl2···C5H8 complexes; theoretical vibrational wavenumbers and IR intensities for the 2-chloro-2-methyl-3buten-1-yl, 2-chloro-3-methyl-3-buten-1-yl, and ·C5H7 radicals; experimental and theoretical vibrational wavenumbers and IR intensities for 3,4-dichloro-2-methyl-1butene, 3,4-dichloro-3-methyl-1-butene, and 1,4-dichloro-2-methyl-2-butene; optimized geometries of trans-C5H8, gauche-C5H8, and a relaxed potential energy scan for trans-C5H8; optimized geometries of the Cl2··· C5H8 complexes, isomers of 1-chloromethyl-1-methylallyl, 2-chloro-2-methyl-3-buten-1-yl, 2-chloro-3-methyl3-buten-1-yl, 1-chloromethyl-2-methylallyl radicals, 3,4dichloro-2-methyl-1-butene, 3,4-dichloro-3-methyl-1-butene, 1,4-dichloro-2-methyl-2-butene, and ·C5H7 radical; infrared spectrum of a C5H8/p-H2 (1:5000) matrix; comparison of infrared spectra of a Cl2/C5H8/p-H2 (0.67/1/2000) matrix, a C5H8/p-H2 (1:2000) matrix, a difference spectrum of a Cl2/C5H8/p-H2 (0.67/1/2000) matrix before and after annealing, and simulated IR spectra for trans-C5H8 and the Cl2···C5H8 complexes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 814-898-6334. (JCA) *E-mail: [email protected]. Phone: 886-3-5131459. (YPL) ORCID

Yuan-Pern Lee: 0000-0001-6418-7378 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ministry of Science and Technology of Taiwan (Grant No. MOST106-2745-M-009-001-ASP) and Ministry of Education, Taiwan (“Aim for the Top University Plan” of National Chiao Tung Univ.) supported this work. National Center for Highperformance Computing, Taiwan provided computer time. J.C.A. thanks the Ministry of Science and Technology of Taiwan for the visiting professorship at the National Chiao Tung Univ.

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REFERENCES

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DOI: 10.1021/acs.jpca.7b07922 J. Phys. Chem. A XXXX, XXX, XXX−XXX