Methane Activation by Laser-Ablated Th Atoms: Matrix Infrared

Article pubs.acs.org/JPCA

Methane Activation by Laser-Ablated Th Atoms: Matrix Infrared Spectra and Theoretical Investigations of CH3ThH and CH2ThH2 Han-Gook Cho Department of Chemistry, University of Incheon, 119 Academy-ro, Songdo-dong, Yeonsu-gu, Incheon 406-772, South Korea

Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United States S Supporting Information *

ABSTRACT: Methane activation by laser-ablated Th atoms on the triplet potential energy surface produces the methylthorium hydride, CH3ThH, that converts smoothly by α-H transfer to CH2ThH2, which relaxes in the matrix to the more stable singlet methylidene, CH2ThH2. This first actinide methylidene was characterized from argon matrix infrared spectra and B3LYP calculations in our laboratory. We now report neon matrix investigations, which include the methylthorium hydride and the ThD stretching modes of CD2ThD2 that are blue-shifted in neon from under the intense CD4 precursor absorption, and reactions with CH2D2 that give rise to the CHDThHD modifications and their α-H and α-D transfer counterparts CD2ThH2 and CH2ThD2. New intrinsic reaction coordinate calculations show that this reaction proceeds smoothly on the triplet potential energy surface.

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INTRODUCTION Thorium is the most abundant actinide metal in the earth’s crust and a source of nuclear power, which produces much of the earth’s internal heat.1 Thorium chemistry, therefore, draws much attention for nuclear industry, earth science, and environmental concerns. Studies have shown that Th reaction products are basically similar to those of the group 4 metals, and thorium tends to be as reactive as any of the group 4 metals.2 The most common oxidation number is 4 among its products including hydrides, halides, oxides, and chalcogenides. The reactions of Th with various small molecules have also been investigated in an excess of noble gases during condensation in matrix isolation experiments.3−20 The thorium high oxidation-state methylidene complex, CH2ThH2 has been observed in the reaction of Th with CH4, which was identified from two strong ThH2 stretching absorptions, a predominantly ThC stretching mode, and CH2 wagging and ThH2 rocking vibrations through the use of D4, and 13C substitution in the methane precursor.9 More stable ThX bonds also allow for the generation of thorium methylidenes (CH2ThHX) in reactions with halomethanes.10 Electronic structure calculations showed that this thoraethylene is agostic, as much as the group 4 metal analogues, owing to electron donation from the CH bond to the CTh bond due to the electron deficient metal center.9,17 A similar methylidene complex (CH3(H)CThH2) was produced in reaction with ethane, along with the ring and trihydride products ((CH2)2Th and CH2CHThH3).18 These studies have shown that thorium readily reacts with small © XXXX American Chemical Society

saturated hydrocarbons and produces complexes with CTh multiple bonds or thorapropylene via CH(X) insertion and H(X) migration from C to Th. The methylidene CH2ThH2 was also observed in the reaction of laser-ablated ThO ceramic materials with CH4 and identified from the strong ThH2 stretching absorptions at 1436 and 1397 cm−1, because Th atoms were a major component of the ablated material: a weak new 1360.1 cm−1 band was also observed.20 In this paper, we report further investigation of methane activation by laser-ablated Th atoms in excess Ne and Ar, with observation and characterization of the Th insertion complex, CH3ThH, where the strong ThH stretching absorption of CH3ThH is observed on the red side of the stronger ThH2 absorptions of the methylidene CH2ThH2. In a DFT investigation of methane activation by f-block atoms, the insertion of thorium into the methane CH bond was predicted to be essentially barrierless and considerably exergonic.21

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EXPERIMENTAL AND COMPUTATIONAL METHODS Laser-ablated Th atoms were reacted with CH4 in excess argon or neon during condensation at 4 K using methods described in our previous papers.3−5,9,16,20 The Nd:YAG laser fundamental Special Issue: Markku Räsänen Festschrift Received: June 24, 2014 Revised: July 22, 2014

A

dx.doi.org/10.1021/jp506282n | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Table 1. Frequencies Observed in Solid Argon and Neon and Assignments for the Product Absorptions from Th + Methane Reactionsa groupb i

13

CH4

CH4

CH2D2c

CD4

1370.6 1359.8, 1370.6 1132.4

1370.0, 1378.1 1359.4, 1370.6 1122.5

e 970.6, 981.7 890.4, 899.9

1435.7, 1448.0

1435.7, 1448.0

e, 1032.4

1397.1, 1411.1

1397.1, 1411.1

674.6, 677.7, 662.6, 670.8 634.6 458.7, 461.2

654.2, 651.5, 661.1, 667.3 629.2

e, 1009.4 948.2,f 970.0 609.9, 602.9 511.9, 499.2

CH3ThH, A′ CH3 bend CH3ThH, A′ ThH stretch CH3ThH, A′ CH3 deform 1373.8, 983.1 broad bands CHD2ThH, CH2DThD (complexes with CH2D2) CH2ThH2, sym ThH2 stretch 1416.6, 1002.3 CHDThHD, ThH, ThD stretch 1402.4, 1012.0 CHDThDH, ThH, ThD stretch 1397.3, 999.4 CD2ThH2, asym ThH2 stretch 1394.1, 998.1 matrix site counterpart 1388.4, 993.0 matrix site counterpart CH2ThH2, asym ThH2 stretch CD2ThD2, ThD, CD mix CH2ThH2, ThC stretch matrix site splitting CH2ThH2, CH2 wag CH2ThH2, ThH2 bend 1357.2, 972.8

ip m

descriptiond

Frequencies (cm−1) in normal type observed in an argon matrix, in bold type observed in a neon matrix. bProduct absorption group on the basis of intensity variation in the process of photolysis and annealing. cSeven pairs of frequencies are H and D counterparts, respectively, of mixed H,D product species, and their H/D frequency ratios range between 1.3981 and 1.4000. dDesignated product and major vibrational mode. eArgon matrix absorption covered by strong CD4 precursor absorption in the 980−1020 cm−1 region. fThis strong absorption probably arises from a mixed antisymmetric ThD stretching and CD deformation mode. a

Table 2. Observed and Calculated Fundamental Frequencies of CH3ThH in the 3A″ Statea CH3ThH approximate mode description A′ CH stretch A″ CH2 stretch A′ CH3 stretch A″ CH3 deform A′ CH3 deform A′ ThH stretch A′ CH3 bend A′ ThH bend A′ CTh stretch A″ CH3 rock A′ CH3 rock A″ CH3 tort

obs

1370.6 1359.8 1132.4

13

calc

int

3057.1 2998.8 2939.9 1411.2 1408.5 1399.5 1151.7 469.4 397.3 322.2 234.2 176.3

5 6 9 6 45 390 37 71 2 1 21 10

obs

1370.0 1359.4 1122.5

CH3ThH

CD3ThD

calc

int

3046.9 2988.4 2936.4 1408.0 1405.6 1399.2 1142.0 462.7 389.7 320.7 234.1 176.3

4 6 10 6 83 353 33 68 3 1 21 10

CH2DThD

obs

calc

int

970.6 890.4

2257.5 2215.1 2109.6 1024.3 1022.2 992.6 904.2 396.0 315.5 239.6 166.8 124.9

3 2 2 4 4 211 49 44 2 1 11 5

obs

972.8

CD2HThH

calc

int

2227.4 2998.4 2954.5 1211.2 1380.3 992.5 1069.7 412.8 343.8 312.6 175.1 146.2

1 6 12 5 2 223 37 46 6 1 12 4

obs

1357.2

calc

int

3046.1 2215.2 2136.5 1257.6 1104.4 1400.2 953.7 466.5 359.7 244.3 212.0 149.7

2 2 4 5 9 435 27 74 1 1 17 14

B3LYP/aug-cc-pVTZ/Dixon basis.30 Frequencies and infrared intensities are in cm−1 and km/mol. Observed frequencies in an argon matrix are listed: neon matrix values are given in Table 1. Intensities are calculated values.

a

block attached to the above heat station. A radiation shield bolted to the first stage heat station covered all but 0.5 in. diameter openings on each side to pass the IR beam through the cold CsI window. Complementary density functional theory (DFT) calculations were carried out using the Gaussian 09 package,22 the B3LYP density functional,23,24 the BPW91 density functional,25 the 6-311++G(3df,3pd) Gaussian basis sets for C and H,26 and the SDD 60 electron core relativistic effective core potential (ECP) from the Stuttgart group27,28 with 30 electrons in the valence space and the appropriate basis set offered by the Gaussian 09 software package used previously for thorium bearing molecules8−18 to provide a consistent set of vibrational frequencies and energies for the reaction products and analogous reaction products. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. For comparison B3LYP calculations with the aug-cc-pVTZ basis for C and H29 and the

(1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a rotating thorium metal target (Oak Ridge National Laboratory, high purity). The thorium target was mounted with Varian “Torr Seal” low vapor pressure epoxy resin onto a 1/4 in. 20 nut, filed to remove oxide coating, affixed to a rod to be rotated, and immediately evacuated in the matrix isolation chamber. Isotopic methane precursors were obtained from Cambridge Isotopic Laboratories and used as received after dilution with research grade argon or neon. After sample preparation FTIR spectra were recorded at 0.5 cm−1 resolution on a Nicolet 750 with 0.1 cm−1 accuracy using an HgCdTe range B detector. The matrix samples were alternatively subjected to a glass filtered Hg-arc street lamp irradiation (Philips, 175 W) and temperature cycled, i.e., annealed, as measured on a Au−Co vs Cu thermocouple embedded in the second stage heat station (Sumitomo Heavy Industries RDK 205D cryocooler). Our CsI sample collecting window was mounted on 1/4 in. wide indium gaskets to an OFHC copper B



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Table 3. Fundamental Frequencies Calculated for CH3ThH and CH2ThH2 in the 1A′ and 1A States Using the CCSD and B3LYP Methods Compared with Observed Argon Matrix Valuesa CH3ThH

CH2ThH2

CCSD approx mode description

obs

A′ CH stretch A″ CH2 stretch A′ CH3 stretch A′ ThH stretch A″ CH3 deform A′ CH3 deform A′ CH3 bend A′ ThC stretch A′ ThH bend A″ CH3 rock A′ CH3 rock A″ CH3 tort a

1359.5 1371.1 1132.4

B3LYP

CCSD

freq

int

freq

int

approx mode description

3149.8 2946.3 2883.4 1508.0 1440.7 1415.9 1174.6 579.5 411.7 356.1 219.5 195.9

0 3 2 411 13 7 34 61 2 0 9 5

3107.1 2747.3 2885.8 1547.0 1415.8 1385.0 1151.4 550.0 400.3 349.5 253.1 192.1

0 5 4 358 12 1 40 62 1 0 6 8

CH2 stretch CH2 stretch ThH2 stretch ThH2 stretch CH2 bend ThC stretch CH2 wag CH2 rock ThH2 bend ThH2 rock CH2 twist ThH2 wag

obs

1435.7 1397.1 670.8 632.4 458.7

B3LYP

freq

int

freq

int

3174.2 2816.0 1481.8 1439.8 1383.1 698.0 655.5 529.8 498.2 410.7 346.3 251.5

2 13 385 772 8 197 168 50 94 13 64 122

3136.0 2847.8 1437.5 1396.9 1332.7 681.5 634.6 465.9 493.3 355.7 326.5 251.8

2 10 346 692 11 178 162 5 108 20 72 63

CCSD and B3LYP/aug-cc-pVTZ/Dixon basis.30 Frequencies and infrared intensities are in cm−1 and km/mol. Intensities are calculated values.

Table 4. Calculated Energies of Singlet and Triplet CH3ThH at Various Levels of Theorya

a

methods

E(1A′)/hartree

E(3A″)/hartree

ΔE/(kJ/mol)

ΔE/(kcal/mol)

B3LYP/6-311++G(3df,3pd)/SDD B3LYP/aug-cc-pVTZ/Dixon, ref 30 BPW91/6-311++G(3df,3pd)/SDD BPW91/aug-cc-pVTZ/Dixon, ref 30 CCSD/6-311++G(2d,p)/SDD

−448.104082 −448.105328 −448.161674 −448.163148 −447.207834

−448.096477 −448.097026 −448.161411 −448.162183 −447.196845

20.0 21.8 0.7 2.5 28.9

4.78 5.21 0.17 0.61 6.90

Computed energies in hartee, kJ/mol, and kcal/mol using five levels of theory (methods and basis sets). ΔE = E(3A″) − E(1A′).

Table 5. Observed and Calculated Fundamental Frequencies of CH2ThH2 and Isotopic Modifications in the 1A Ground Statea CH2ThH2 approx mode description CH2 stretch CH2 stretch ThH2 stretch ThH2 stretch CH2 bend ThC stretch CH2 wag ThH2 bend ThH2 rock CH2 twist ThH2 wag CH2 rock

obs

1435.7 1397.1 670.8 632.4 458.7

13

calc

int

3136.0 2847.8 1437.5 1396.9 1332.7 681.5 634.6 493.3 465.9 355.7 326.5 251.8

2 10 346 692 11 178 162 108 5 20 72 63

obs

1435.7 1391.7 651.5 627.5

CH2ThH2

CD2ThD2

calc

int

3125.7 2840.9 1437.4 1396.9 1325.8 661.4 629.2 492.8 463.1 355.3 326.2 251.6

3 11 347 691 11 173 157 106 5 19 72 63

obs

b b 948.2 602.9 499.2

CH2ThD2

calc

int

2316.4 2075.2 1025.5 990.9 1008.0 617.3 496.4 359.3 346.2 253.8 233.9 179.6

2 2 108 336 98 127 109 23 40 12 34 31

obs

b 998.1c

CD2ThH2

calc

int

3136.0 2847.7 1019.9 991.0 1333.0 678.3 631.9 412.9 349.6 275.9 301.9 206.8

2 10 165 341 29 157 154 8 55 4 10 60

obs

b 1394.1c

calc

int

2316.4 2075.2 1437.2 1396.6 1014.0 621.4 509.4 485.9 444.4 276.4 306.5 199.0

2 3 347 678 27 161 66 158 5 19 103 35

a B3LYP/aug-cc-pvtz/Dixon basis.30 Frequencies and infrared intensities are in cm−1 and km/mol. Observed are in argon matrix. Intensities are calculated values. bRegion covered by CD4 or CH2D2 precursor absorption. cOnly this middle of three matrix site components from Table 1 is listed here.

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RESULTS AND DISCUSSION Infrared spectra of laser-ablated thorium atom reaction products with methane and several isotopic modifications will be presented in argon and neon matrices, and the new methylthorium hydride and thoraethylene assignments will be supported by additional quantum chemical calculations. Table 1 lists the important isotopic frequencies observed in these experiments, Tables 2 and 3 give calculated frequencies for the insertion product CH3ThH in triplet and singlet states, Table 4 compares energies of these states for different calculational methods, and Tables 5 and 6 list calculated

SDD pseudopotential with the contracted Th basis set [8s,7p,6d,4f,2g] employed by Dixon et al.14,30 are also done to complement the results with the 6-311++G(3df,3pd)/SDD/ Gaussian sets. The vibrational frequencies were calculated analytically, and the zero-point energy is included in the calculation of binding energy of a metal complex. Intrinsic reaction coordinate (IRC) calculations have been performed to link the transition state structures with the reactants and specific products.31 C



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Table 6. Observed and Calculated Fundamental Frequencies of (CHD)(ThHD) Isomers in the 1A Statea

CHDThHD approx mode description CH stretch CD stretch ThH stretch ThD stretch CHD bend CTh stretch CHD wag ThHD bend ThHD rock CHD twist ThHD wag CHD rock

obs

1416.6 1002.3

CHDThDH

calc

int

2856.3 2300.7 1424.1 1000.4 1204.7 644.9 535.2 463.2 398.4 311.9 256.1 214.9

11 1 491 272 20 155 112 78 25 37 10 55

obs

1402.4 1012.0

CDHThHD

calc

int

2856.3 2300.7 1410.9 1009.4 1204.8 644.8 528.5 442.2 381.9 302.3 271.0 231.4

11 1 507 267 23 152 115 79 20 40 31 49

obs

1416.6 1002.3

CDHThDH

calc

int

3132.2 2086.0 1424.1 1000.4 1164.6 671.4 613.9 467.7 395.7 304.9 256.2 186.0

2 3 491 276 19 158 150 79 36 31 30 34

obs

1402.4 1012.0

calc

int

3132.2 2086.0 1410.9 1009.3 1164.9 672.7 612.1 439.3 374.7 305.7 269.2 199.3

2 3 504 264 30 164 154 71 17 51 41 35

B3LYP/aug-cc-pvtz/Dixon level of theory. Frequencies and infrared intensities are in cm−1 and km/mol. Observed are in argon matrix. Intensities are calculated values. a

Figure 1. Infrared spectra in selected regions for the major products of the reaction between laser-ablated Th atoms and CH4 in excess argon (2%). (a) Spectrum obtained from initial sample deposit for 1 h at 8 K. (b) Spectrum after λ > 420 nm irradiation for 20 m. (c) Spectrum after 240−380 nm irradiation for 20 m. (d) Spectrum after λ > 220 nm irradiation for 10 m. (e) Spectrum after annealing to 26 K. The i and m labels represent the insertion and methylidene products. Adapted from Figure 1 of ref 9 by extending the range down to 1330 cm−1.

frequencies for the α-H and α-D transfer product methylidenes in the ground singlet state. Argon and Neon Matrix Infrared Spectra. Infrared spectra of laser-ablated thorium atom reaction products with CH4 and 13CH4 (2 and 1%) samples in excess argon and neon during condensation earlier at 8 K and again now at 4 K were recorded (Figures 1, 2, 3, and 4), and the important absorptions are listed in Table 1. Our practice is to show a family of spectra for each experiment, beginning with the Th atom and CH4 sample deposition in excess argon, Figure 1a, followed by mercury arc irradiation with different long wavelength pass filters (b), (c) and (d) to illustrate the photolysis behavior of the different products: we observed substantially more growth of the product bands m than product bands i on this photolysis sequence. However, sample annealing to 26 K in solid argon slightly reduced both product absorption groups, Figure 1e. The neon matrix spectra in Figure 2 displayed complementary behavior from deposition

(a), the irradiation sequence (b)−(d), and annealing to 10 and 12 K (e) and (f). Notice that the product bands increase on UV irradiation and sharpen on annealing, thereby reducing the total absorbance in both matrixes. Neon matrix spectra for the Th reaction with 13CH4 in neon reveal an analogous family of spectra in Figure 3a−g. Figure 4 illustrates a direct comparison of the principal Th reaction products with 13CH4 in argon codeposited at 4 K showing the product evolution on (a) deposition, (b) photolysis, and (c) annealing and the product with 12CH4 (d) where the observed wavenumbers are given. Additional families of spectra with CD4 in argon and neon are shown in Figures 5 and 6 using the same format: important new information here comes from the neon matrix spectra, which, owing to larger shifts for the principle product than for CD4, reveal new ThD stretching modes. Finally, Figure 7 illustrates the more complicated reaction products for Th and CH2D2 in argon with the product evolution from sample deposition (a), irradiation (b)−(d), and their behavior on D



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Figure 4. Infrared spectra in the 1460−1360 cm−1 region comparing absorptions for the major products of the reaction between laserablated Th atoms with 13CH4 and 12CH4 in excess argon (1%) condensed at 4 K. (a) Spectrum obtained from the initial sample deposit of Th with 13CH4 in argon at 4 K. (b) Spectrum after λ > 320 nm irradiation for 15 m. (c) Spectrum after 240 < λ < 380 nm irradiation for 16 m. (d) Spectrum obtained from initial sample deposit of Th with 12CH4 in argon at 4 K, after λ > 380 nm irradiation and 240 < λ < 380 nm irradiation for 15 m each.

Figure 2. Infrared spectra in selected regions for the major products of the reaction between laser-ablated Th atoms and CH4 in excess neon (1%). (a) Spectrum obtained from initial sample deposit for 32 m at 4 K. (b) Spectrum after λ > 420 nm irradiation for 16 m. (c) Spectrum after 240−380 nm irradiation for 16 m. (d) Spectrum after 220 nm irradiation for 11 m. (e) and (f) Spectra after annealing to 10 and 12 K. The i and m labels represent the insertion and methylidene products.

Figure 5. Infrared spectra in selected regions for the major products of the reaction between laser-ablated Th atoms and CD4 in excess argon (2%). The intense CD4 absorption obscures the 980−1020 cm−1 region. (a) Spectrum obtained from initial sample deposit for 1 h at 8 K. (b) Spectrum after λ > 290 nm irradiation for 20 m. (c) Spectrum after λ > 220 nm irradiation for 10 m. (d) Spectrum after annealing to 26 K. The 644 cm−1 absorption is due to the matrix solvated deuteron species.39

Figure 3. Infrared spectra in selected regions for the major products of the reaction between laser-ablated Th atoms and 13CH4 in excess neon (1%). (a) Spectrum obtained from initial sample deposit for 32 m at 4 K. (b) Spectrum after λ > 380 nm irradiation for 16 m. (c) Spectrum after 240 < λ < 380 nm irradiation for 16 m. (d) Spectrum after λ > 220 nm irradiation for 11 m. (e)−(g) Spectra after annealing to 10, 13, and 14 K. The i and m labels represent the insertion and methylidene products.

in argon condensed at 4 K, shows all of the characteristics expected for a CH3ThH stretching mode: first, its CD3Th D counterpart at 970.6 cm−1 defines an H/D frequency ratio of 1.401, which is quite appropriate for a very heavy metal hydride vibration where the reduced masses of H and D clearly dominate that of Th and in the harmonic approximation the H/ D frequency ratio would approach the square root of 2; second, this band exhibits a small blue shift in solid neon, to 1370.6 cm−1, with deuterium counterpart 981.7 cm−1 and frequency ratio 1.396, almost the same as the argon matrix isolated species. Thus, there is no reason to suspect a different ground state in the neon matrix, which has been proposed for the UO2 molecule.32 Third, this 1359.8 cm−1 band exhibits a small but definite 13CH4 shift to 1359.4 cm−1, as is illustrated in Figure 4 for expanded frequency scale spectra of the major products of the reaction between laser-ablated Th atoms with 13CH4 and

annealing (e) and (f). The effects of partial and complete deuterium substitution will be discussed for the CH3ThH insertion into CH and CH2ThH2 methylidene products below. These samples had minimum ThO and ThO 2 contaminants,4 and no evidence was found for binary thorium hydride product species.3 Identification of CH3ThH. Figure 1 illustrates the additional spectral region below the strong ThH2 stretching modes reported and assigned to CH2ThH2 in our earlier communication.9 The principal band at 1359.8 cm−1 labeled i for insertion product, measured more accurately using 1% CH4 E



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mode from the all-D species to 972.8 cm−1 and a small red shift on the ThH stretching mode to 1357.2 cm−1 from the all-H species, based on the anticipated mass effect of D substitution in the methyl group. Unfortunately, the B3LYP frequency calculations for the 3A″ state insertion product, Table 2, do not account for this small effect, probably owing to lack of proper treatment of the vibrational coupling between these two nearby modes in the triplet state calculation. Although the 1360 cm−1 band was observed at the time of our communication,9 we were not confident of this assignment at that time. The observed ThH stretching frequencies for CH3Th H at 1359.8 and 1370.6 cm−1 in solid argon and neon, respectively, agree quite well with our calculation for the intense ThH stretching mode of the 3A″ state in Table 2 at 1399.5 cm−1 for the B3LYP level of theory. This frequency is calculated at 1466.7 cm−1 using the CCSD method (table in Supporting Information). In this calculation, on the one hand, we determined the T1 diagnostic to be 0.019 39, which suggests that multiconfigurational behavior is not a problem here.33 On the other hand, the prediction for this mode in the 1A′ state is 1508.0 cm−1 (Table 3), where no product bands are observed. Hence, we must conclude that the insertion product molecule CH3ThH is trapped in the triplet state in both solid argon and neon. The same relationship is found for CH3−ZrH trapped in its triplet ground state.34 This is in spite of the fact that all of our calculations, Table 4, find the singlet CH3 ThH state to be slightly lower in energy, which could then be the ground state in the gas phase. We have no rationale for this observation other than to expect that the triplet state molecule would interact more strongly with polarizable matrix atoms and to note that triplet states are more difficult to model correctly than singlet states. All of our evidence conclusively matches the strongest ThH stretching frequency with the triplet state frequency calculations rather than the singlet state (Tables 2 and 3). In reactions of transition-metal atoms with methane, the methyl metal hydride insertion product is believed to be produced first, and excess reaction energy or photon energy leads to conversion to the methylidene complex.16,34−38 Figure 8 illustrates the results of an IRC calculation on the triplet potential energy surface, which show production of the insertion and methylidene products from the reactants (Th(3F) + CH4) via the transition states in a smooth process. The first transition state is 67 and 187 kJ/mol higher than the reactants and CH3ThH, respectively, and correcting the first transition state or barrier for ZPE results in a 37 kJ/m activation for insertion reaction 1 given below. The second transition state is 121 and 48 kJ higher than the insertion and methylidene product in the triplet states. The triplet methylidene would then convert to the more stable singlet ground state via intersystem crossing (ISC) if this were a gas phase process. However, our reactions take place in condensing excess argon, and argon atom collision induced intersystem crossing, which in effect is relaxation by collisions with argon atoms that become the matrix host, takes place during the matrix condensation process at 4 K. This we describe simply as matrix relaxation. We compute the energy of the insertion reaction 1 to be 143 kJ/m exothermic including ZPE with the Gaussian basis set and 149 kJ/m exothermic with the Dixon set.

Figure 6. Infrared spectra in selected regions for the major products of the reaction between laser-ablated Th atoms and CD4 in excess neon (1%). (a) Spectrum obtained from initial sample deposit for 32 m at 4 K. (b) Spectrum after λ > 290 nm irradiation for 16 m. (c) Spectrum after 220 nm irradiation for 11 m. (d)−(f) Spectra after annealing to 10, 13, and 14 K, respectively.

Figure 7. Infrared spectra in the ThH and ThD stretching regions for the major products of the reaction between laser-ablated Th atoms and CH2D2 (2%) in excess argon. (a) Spectrum obtained from initial sample deposit for 1 h at 10 K. (b) Spectrum after λ > 420 nm irradiation for 20 m. (c) Spectrum after 240 < λ < 380 nm irradiation for 20 m. (d) Spectrum after λ > 220 nm irradiation for 10 m. (e) Spectrum after annealing to 26 K. (f) Spectrum after annealing to 32 K. i and m stand for the insertion and methylidene products. ip and p indicate aggregated product(precursor) i(CH2D2) and CH2D2 precursor absorptions. 12

CH4 in excess argon (1%) condensed at 4 K. In recent work reacting ThO ablation products with methane during condensation in excess argon at 4 K, these bands were observed at 1360.1 and 1359.7 cm−1.20 However, the weaker less accurately measured neon matrix band at 1370.6 cm−1 did not exhibit a shift with 13CH4. Fourth, the weaker associated argon matrix band at 1370.6 cm−1 shows a larger 13C shift to 1370.0 cm−1, and this band is assigned to the CH3-bending mode, which interacts with the close frequency ThH stretching mode for this low symmetry product. Notice in the same figure that the bands labeled m for methylidene CH2ThH2 exhibit no measurable 13C shifts. Fifth, spectra in Figure 7 from products of the Th reaction with 2% CH2D2 in argon, which upon insertion produces CH2DThD and CHD2ThH with a small blue shift on the ThD stretching

Th(3F) + CH4 → CH3ThH (triplet) F

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stretching modes blue-shifted 14.0 and 12.3 cm−1 to 1411.0 and 1448.0 cm−1 for the CH2ThH2 molecule, Figure 2, whereas the intense CH4 absorption in this region is blue-shifted only 2 cm−1 to 1308 cm−1 in solid neon. The result is illustrated in Figure 6 for the CD2ThD2 product labeled m for methylidene, which shows new bands at 1009.4 and 1032.4 cm−1. The H/D frequency ratios for these neon matrix bands are 1.398 and 1.403 for the antisymmetric and symmetric stretching modes, respectively, which may be compared to the 1.402 and 1.410 ratios calculated from the harmonic frequencies listed in Table 5. The neon matrix spectra are complicated by the fact that it is more difficult to trap light molecules like methane in condensing neon than in argon, and as a consequence, we observed both i and m product complexes with the methane isotopic reagents in solid neon: these complex bands are blue-shifted slightly from the isolated product absorptions and labeled i(methane) or m(methane) in our spectra. We observed interesting spectra from the Th reaction with CH2D2, which are illustrated in Figure 7 for the ThH and ThD stretching regions including photolysis and annealing behavior in scans (b)−(f). This thorium reaction gives two isotopic products for insertion into CH or CD bonds, and these are observed and compared with the calculated frequencies in Table 2, as discussed above. Subsequent α-H and α-D transfers lead to four possible CHDThHD isotopic modifications for this no symmetry molecule, which are reported in Table 6. It appears to matter more to the Th H(or D), which is cis to the agostic CH(or D) as this closer position allows interaction and leads to a lower ThH(or D) stretching frequency. The calculations in Table 6 show that it does not matter whether CH or CD is in the agostic position to this ThH(or ThD) bond. Thus, we have computed intense ThH stretching frequencies at 1424 or 1411 cm−1 for each of two structural isomers and ThD stretching frequencies at 1000 or 1009 cm−1 for these two isomer pairings (higher frequency for one ThH(or D) bond means lower frequency for the other). We also note that the average of the symmetric and antisymmetric ThH2 stretching frequencies recorded earlier9 for CH2ThH2, 1416.4 cm−1, represents the anticipated stretching frequency position for a single ThH bond as in the case of CX2ThHD, and this intuitive prediction matches the highest new band observed in the ThH stretching region at 1416.6 cm−1. The much weaker intermediate peaks at 1414, 1409 cm−1 and at 1011, 1006 cm−1 are most likely due to matrix site splittings of the two major orientations of the CX2ThHD subunits. The three remaining bands at 1397.3, 1394.1, and 1388.4 cm−1 and their deuterium counterparts at 999.4, 998.1, and 993.0 cm−1 are inter-related on photolysis and annealing. First, the ThH counterparts are very close to the strong antisymmetric stretching mode for the normal isotopic CH2ThH2 methylidene, and our calculations in Table 5 show that this mode will not change significantly in the CD2 ThH2 isotopic modification. Likewise, the above 999.4, 998.1, and 993.0 cm−1 bands are near where CD2ThD2 likely absorbs in solid argon because the strong anticipated absorption at 1397/1.40 = 998 cm−1 is covered by CD4, and these bands can be identified with CH2ThD2. Hence, on deposition and irradiation, we propose that some CHD ThHD is excited to the singlet CHDThHD state where α-H and α-D transfers can easily produce the other two possible CD2ThH2 and CH2ThD2 isotopic modifications, which

Figure 8. Intrinsic reaction coordinate calculation between Th(3F) + CH4 to triplet CH3ThH and CH2ThH2, which relaxes in the matrix to singlet CH2ThH2. Calculations used the B3LYP/aug-ccpVTZ/Dixon basis30 methods. The zero point energy is not included in these energies. However, correcting the first barrier height of 67 kJ/ m for ZPE results in 37 kJ/m or 9 kc/m activation for the insertion reaction. Similar energies were obtained with B3LYP/6-311++G(3df,3pd)/SDD.

Th(3F) + CH4 → CH 2Th 2 (singlet)

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Analogous calculations find reaction 2 to be 172 kJ/m exothermic including ZPE with the Gaussian basis set and 171 kJ/m exothermic with the Dixon basis sets. Our calculational results differ somewhat from those of Almeida and Cesar who found a larger 181 kJ/m exothermicity for reaction 1 and a negative 9 kJ/m barrier height for this reaction.21 We used larger basis sets for C and H and a smaller pseudopotential thus more valence electrons for Th, and thus our calculations should be more accurate. It must be pointed out that we find experimental evidence for a barrier to insertion reaction 1 in that CH3ThH (triplet) absorptions do not increase on annealing our samples containing Th atoms and methane which allows diffusion and approach of reagents in warmer solid argon, and thus activation in some form, such as photolysis (see our figures of spectra), is required to initiate the reaction. Additional Evidence for the Identification of CH2 ThH2. The spectrum of this first actinide methylidene complex was reported in solid argon in our communication,9 and the present repeat of those experiments at lower (1% CH4) concentrations and 4 K window temperature produced the same bands with slight frequency shifts owing to different argon packing around this product molecule with the colder surface condensation. In particular, the strongest bands, the ThH2 stretching modes, appeared at 1395.3 cm−1 with a shoulder at 1401.3 cm−1 and at 1430.5 cm−1 with a satellite at 1439.3 cm−1 in the present experiments: these values bracket the strongest bands reported previously at 1397.1 and 1435.7 cm−1.9 We were unable to observe the deuterium counterparts of these ThH2 stretching modes owing to their anticipated shift to fall under the very strong CD4 absorption in the 980−1010 cm−1 region. However, with solid neon, we expect a larger blue matrix shift on the ThD2 stretching modes for bonds with a considerable degree of ionic character than for the covalent CD4 molecule on the basis of the observation of these ThH2 G



Article

Figure 9. Structures for the Th + CH4 reaction products and transition states calculated using B3LYP with aug-cc-pvtz for C and H and the pseudo potential and basis set for Th employed by Dixon et al.30

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are observed here. Similar behavior was found for the Zr atom reaction with CH2D2.34 Figure 9 collects the structures for the Th + CH4 reaction products and transition states calculated using the B3LYP density functional. Notice that the CTh single bond in the singlet insertion product is slightly shorter than that for the triplet product observed here in solid argon and neon and that the CTh single bond in the triplet hydromethylidene is substantially longer than the CTh double bond in the singlet ground state for this species.

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ASSOCIATED CONTENT

S Supporting Information *

Additional calculated frequencies using the CCSD method and Cartesian coordinates for the title molecules. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*L. Andrews. E-mail: [email protected]. Tel: 434-924-6844. Notes

The authors declare no competing financial interest.

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CONCLUSIONS

ACKNOWLEDGMENTS We gratefully acknowledge financial support from DOE Grant No. DE-SC0001034, retirement funds from TIAA/CREF for L.A., and support from the Korea Research Foundation (KRF) grant funded by the Korean government (NRF2013R1A1A2060088) and KISTI supercomputing center for H.-G.C.

The activation of methane by laser-ablated Th atoms occurs readily on the triplet potential energy surface and produces methylthorium hydride, CH3 ThH, which converts smoothly by α-H transfer to CH2ThH2 and relaxes in the matrix condensation process to the more stable singlet methylidene, CH2ThH2. This actinide methylidene was first characterized from argon matrix infrared spectra and B3LYP calculations in our laboratory. We now report neon matrix investigations, which include the methylthorium hydride and the ThD stretching modes of CD2ThD2 observable owing to larger blue shifts in neon than for the intense CD4 precursor absorption. Reactions with CH2D2 give rise to the CHDThHD modifications and their α-H and α-D transfer counterparts CD2ThH2 and CH2ThD2. Intrinsic reaction coordinate calculations show that this reaction proceeds smoothly on the triplet potential energy surface. Near UV irradiation of the freshly codeposited argon matrix samples increases the product yields (5× for >320 nm). The two Th H stretching modes for the methylidene blue shift 14−15 cm−1 and the ThH mode for the hydride blue shifts 11 cm−1 on going from solid argon to solid neon, which indicates that these molecules occupy the same ground states in both matrix environments. Finally, the lack of growth for the insertion reaction on annealing the matrix samples and its obvious growth on photolysis show that the reaction to form CH3 ThH requires activation energy.

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REFERENCES

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Methane Activation by Laser-Ablated Th Atoms: Matrix Infrared

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