Properties of ThFx from Infrared Spectra in Solid Argon and Neon with

Article pubs.acs.org/JPCA

Properties of ThFx from Infrared Spectra in Solid Argon and Neon with Supporting Electronic Structure and Thermochemical Calculations K. Sahan Thanthiriwatte,† Xuefeng Wang,‡ Lester Andrews,*,‡ David A. Dixon,*,† Jens Metzger,§ Thomas Vent-Schmidt,§ and Sebastian Riedel§,∥ †

Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336, United States Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United States § Department of Chemistry, Albert-Ludwigs Universität Freiburg, Institut für Anorganische und Analytische Chemie, D-79104 Freiburg, Germany ∥ Institute of Chemistry and Biochemistry - Inorganic Chemistry, Freie Universität Berlin, 14195 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: Laser-ablated Th atoms react with F2 in condensing noble gases to give ThF4 as the major product. Weaker higher frequency infrared absorptions at 567.2, 564.8 (576.1, 573.8) cm−1, 575.1 (582.7) cm−1 and 531.0, (537.4) cm−1 in solid argon (neon) are assigned to the ThF, ThF2 and ThF3 molecules based on annealing and photolysis behavior and agreement with CCSD(T)/aug-cc-pVTZ vibrational frequency calculations. Bands at 528.4 cm−1 and 460 cm−1 with higher fluorine concentrations are assigned to the penta-coordinated species (ThF3)(F2) and ThF5−. These bands shift to 544.2 and 464 cm−1 in solid neon. The ThF5 molecule has the (ThF3)(F2) Cs structure and is essentially the unique [ThF3+][F2−] ion pair based on charge and spin density calculations. Electron capture by (ThF3)(F2) forms the trigonal bipyramidal ThF5− anion in a highly exothermic process. Extensive structure and frequency calculations were also done for thorium oxyfluorides and Th2F4,6,8 dimer species. The calculations provide the ionization potentials, electron affinities, fluoride affinities, Th−F bond dissociation energies, and the energies to bind F2 and F2− to a cluster as well as dimerization energies.

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INTRODUCTION

there are good results for the heats of formation of the corresponding thorium oxides, ThO and ThO2.14−18 Reliable experimental and computational results are available for the OThF2.19 We recently reported the frequencies of ThFx, x = 1 to 5 from Ar matrix isolation studies of the reaction of Th + F2.20 Accompanying electronic structure calculations at the density functional level with the B3LYP functional benchmarked by CCSD(T)/aug-cc-pVTZ calculations were used to predict the structures and aid in the experimental vibrational assignments. This work showed the discovery of a novel thorium pentafluoride of the form [ThF3+][F2−]. In the current work, we describe new experiments in a Ne matrix together with higher fluorine concentrations of the products of the reaction of laser-ablated Th with F2 leading to more conclusive assignments of these new ThFx species. Electronic structure calculations at the density functional theory and coupled cluster CCSD(T) levels were used to predict the

A number of nations including India and Canada with significant thorium reserves are investigating the possibility of thorium fueled nuclear reactors.1−3 Thus there is significant interest in improving our knowledge of thorium chemistry. In addition, the normal oxidation state of Th is +IV with no valence electrons. Thus Th(IV) compounds are readily amenable to electronic structure calculations without the presence of active 7s, 5f, or 6d electrons and can serve as a models for the structures of other actinides in the +IV oxidation state. An important property for characterizing such molecules is their vibrational spectra. Although the vibrational spectra of uranium fluorides have been investigated because of interest in practical nuclear applications,4−8 there are fewer studies of the corresponding thorium fluorides. The Th−F antisymmetric stretch in ThF4 vapor has been observed near 520 cm−1.9−11 High resolution spectroscopy of ThF and ThF+ in combination with high-level electronic structure calculations gives stretches of 573.8 and 573.9 cm−1 for the 2Δ3/2 and 2Δ5/2 states of ThF and 652.3(5) cm−1 for ThF+.12 The heat of formation of ThF4 in the gas phase has been determined from experiment,13 and © 2014 American Chemical Society

Received: December 31, 2013 Revised: February 7, 2014 Published: February 21, 2014 2107

dx.doi.org/10.1021/jp412818r | J. Phys. Chem. A 2014, 118, 2107−2119

The Journal of Physical Chemistry A

Article

structures, vibrational spectra, and thermodynamic properties of a range of thorium fluorides including bond dissociation energies, electron affinities, and ionization potentials. In addition, properties of the Th2F4,6,8 dimers and O2ThF and O2ThF2 were calculated to provide information about other possible species that might be present in the experimental studies.

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EXPERIMENTAL AND COMPUTATIONAL METHODS The matrix isolation apparatus and procedure for studying laser ablated metal atom reactions has been described previously.21,22 The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a thorium metal target (obtained from Oak Ridge National Laboratory) mounted on a rotating rod. Laser-ablated thorium species (mostly atoms) were codeposited with argon or neon (research grade) containing 0.5 or 1% commercial fluorine from two sources (Air Products and Chemicals, Inc., Solvay Fluor GmbH), which were used as received after storage over NaF to remove HF. Details of a second apparatus and its cryogenic refrigeration system with CsI windows maintained at 4 K at Freiburg have been reported.23 FTIR spectra were recorded at 0.5 cm−1 resolution on a Nicolet 750 FTIR instrument with a HgCdTe range B detector. Matrix samples were annealed at different temperatures and cooled back to 4 K for spectral recording. Selected samples were subjected to broad band photolysis by a medium-pressure mercury arc street lamp (Philips, 175W) with the outer globe removed using optical glass filters. Density functional theory (DFT)24 with the B3LYP hybrid exchange-correlation functional25 was used for the initial geometry optimizations with the aug-cc-pVTZ basis set26 on O and F and the ECP60MWB effective core potential and ECP60MWB_SEG [10s,9p,5d,4f,3g] basis set on Th.27 This follows on our prior work on ThO2 and ThFx, x = 1−5.17,20,28 The Gaussian09 program system29 was used for the DFT calculations. Coupled cluster CCSD(T) (coupled cluster with single and double excitations and a perturbative triples correction) calculations30 with the same basis sets given above were performed on the compounds as follows. Geometries were optimized at the CCSD(T) level for ThFx and ThFx−, x = 1−5 and for ThFx+ x = 1 − 4. CCSD(T) frequencies were obtained for ThFx, x = 1−4 and for ThFx+ and ThFx−, x = 1−3. Single point CCSD(T) calculations were done for the remaining compounds at the B3LYP optimized geometries. The CCSD(T) calculations were done with the MOLPRO program.31 Open shell molecules were treated at the R/UCCSD(T) level.32 The computational results for the vibrational frequencies are harmonic values, and they are compared with the experimental anharmonic values. We have previously shown17 that the anharmonic corrections to the Th−O stretching frequencies in ThO2 are small, on the order of a few wavenumbers, so that the calculated harmonic frequencies should be a good approximation to the experimental anharmonic values in the gas phase.

Figure 1. Infrared spectra of laser-ablated Th atom and F2 molecule reaction products in solid argon: (a) Th + 0.5% F2 deposition for 60 min at 5 K; (b) after annealing to 20 K; (c) after λ >220 nm irradiation [spectra absorbance scale expanded × 3]; (d) Th + 1% F2 deposition for 60 min; (e) after annealing to 20 K; (f) after λ >380 nm irradiation; (g) after λ >220 nm irradiation; (h) after annealing to 30 K.

earlier grating instrument measurements for ThF4 vapor from the solid at 800−850 °C trapped in solid argon at 4 K owing to a different calibration of the earlier grating spectrometer.6 A weak ThO2 band was observed at 735.2 cm−1 in the 0.5% sample, but not in the 1% fluorine matrix, and a very weak OThF2 band was found at 806.6 cm−1.19,33 Further evidence of the dioxygen impurity in our commercial fluorine is the appearance of a weak 1489 cm−1 band for FOO on annealing the samples.34 A weak 510.5 cm−1 band was also observed for isolated F3−.35 New absorptions were observed at 459.5, 462.0, 528.4, 531.0, 564.8, 567.2, 575.2, and 575.9 cm−1 and their relative intensities varied slightly with fluorine concentration. For example, the 531.0, 567.2, and 564.8 cm−1 bands were stronger at the 0.5% fluorine concentration, but with 1% fluorine, the 531.0 cm−1 band is a shoulder on the stronger 528.4 cm−1 band, the 564.8, 567.2 cm−1 bands were weaker, and the 528.4 cm−1 band was stronger, comparable with that for the 575.2 and 575.9 cm−1 bands. The 531.0 and 564.8, 567.2 cm−1 bands were decreased by >220 nm UV irradiation while the 575.2, 575.9 cm−1 doublet increased slightly, and the ThF4 bands increased slightly in the 0.5% fluorine experiment. In the 1% fluorine experiment, > 380 nm irradiation had virtually no effect, and >220 nm mercury arc light almost destroyed the 528.4 cm−1 band, and again slightly increased the 575.2, 575.9 cm−1 doublet and the ThF4 bands. A 2% fluorine experiment performed on the Freiburg apparatus with comparable laser energy for Th ablation gave stronger bands as shown in Figure 2, and the 460 cm−1 feature was relatively stronger with higher fluorine concentration. Two previous investigations of thorium atom reactions with the trifluoro precursors CHF3 and NF3 reported previously from this laboratory are germane here.36,37 With the fluoroform reagent, the important product bands were weak 565.4 cm−1, strong 521.3 cm −1 , and medium intensity 502.1 cm −1 absorptions for HCThF3; for DCThF3, these bands shifted to 563.4, 520.6, and 493.1 cm−1.36 These deuterium shifts substantiated assignment of the bands to symmetric Th−F3,

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RESULTS AND DISCUSSION Ar and Ne Matrix Infrared Spectra. Argon matrix infrared spectra from the reaction of laser ablated thorium atoms and fluorine at 0.5 and 1.0% are illustrated in Figure 1, which shows improved resolution and new spectral peaks in addition to those observed previously.20 The strongest absorptions at 522.5 (shoulder), 521.0, 519.2, and 515.5 cm−1 are systematically 1 cm−1 lower than those obtained from the 2108



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Figure 2. Infrared spectra of laser-ablated Th atom and F2 molecule reaction products in solid argon: (a) Th + 2% F2 deposition for 60 min at 5K; (b) after annealing to 15 K; (c) after annealing to 23 K; (d) λ >220 nm irradiation for 15 min; (e) after annealing to 30 K; (f) after annealing to 36 K.

Figure 3. Infrared spectra of laser-ablated Th atom and F2 molecule reaction products in solid neon: (a) Th + 1% F2 in neon deposition for 60 min at 5 K; (b) after annealing to 10 K; ; (c) after λ >220 nm irradiation for 10 min; (d) after annealing again to 10 K; (e) after annealing to 11 K; (f) after annealing to 12 K. The broad band that appears on irradiation is due to O2F radical from the reaction of F atoms with O2 impurity in the fluorine sample.

antisymmetric Th−F3, and Th−C stretching modes, respectively, which should have very small H/D shifts.36 In addition, a weak band was observed at 575 cm−1 with both isotopic fluoroforms, which increased on full arc photolysis as did the 575.2 cm−1 band in the fluorine experiments. With the more reactive nitrogen trifluoride precursor, approximately the same ThF4 bands were observed as reported above. The weaker photosensitive 531.0 and 564.8, 567.2 cm−1 bands were also observed. The 575.2, 575.9 cm−1 doublet was also observed and noted by its 575.5 cm−1 median, but unfortunately this band was incorrectly assigned to the N÷ThF3 product37 instead of an appropriate much weaker band at 567.3 cm−1. This weak 567.3 cm−1 becomes observable when the 531.0 and 564.8, 567.2 cm−1 bands decrease on irradiation and the stronger 525.2 and 430.0 cm−1 bands increase that are assigned to the product N÷ThF3, which are correctly labeled in Figure 1 of Reference 37. The 575.1, 575.9 cm−1 doublet assigned to ThF2 as discussed below was also observed previously with the NF3 reagent. It is clear that the photosensitive 531.0 and 564.8, 567.2 cm−1 bands are in the region expected for a ThF3 subunit, and they were so labeled in Figure 1 of ref 37 without discussion.37 The neon matrix spectrum shown in Figure 3 is dominated by the F3− absorption30 at 524.6 cm−1 with a succession of weaker bands at 532.6, 529.4, 527.8, and 526.1 cm−1, which are again 1 cm−1 lower than the neon matrix peaks in the evaporative ThF4 work6 due to the instrument calibration difference. Additional sharp bands were observed at 537.4 cm−1, at 573.8 and 576.1 cm−1, and at 582.7 and 583.6 cm−1. Successive UV irradiation at >290 and >220 nm decreased and destroyed the F3− absorption, increased the 529.4, 527.8, and 526.1 cm−1 ThF4 bands, and produced a broad 464 cm−1 feature with a 472 cm−1 shoulder and a broad 586 cm−1 band with a companion band at 1510 cm−1. The latter two bands are due to O2F produced by reaction of F atoms with O2 impurity in commercial fluorine.34 Final annealing to 8 K altered the ThF4 matrix site absorptions and decreased the sharp 573.8 and 576.1 cm−1 bands. ThF4 Assignments. The CCSD(T) harmonic frequency20 (Table 1) for the antisymmetric Th−F stretching frequency in

Table 1. Calculated Th−F Stretching Frequencies CCSD(T)/aug-cc-pVTZ (cm−1), and Observed Argon and Neon Matrix Frequencies (cm−1) for ThFx, x = 1−4a parameter

CCSD(T)

Th−F str Th−F sym. str. a1 Th−F asym. str. b1 Th−F asym. str. − e′ Th−F sym. str. − a1′ Th−F asym. str. − t2 Th−F sym. str. − a1 a

Ar

ThF (2Δ) 598.9b 567.2, 564.8 ThF2 (C2v) (1A1) 585.4 573.4 575.9, 575.1 ThF3 (D3h) (2A1) 537.2 531.0 570.8 ThF4 (Td) (1A1) 528.3 522.7, 521.0, 519.2, 514.5 563.3

Ne 576.1,573.8

583.6, 582.7 537.4

532.6, 529.4, 527.8

Infrared intensity in km/mol in parentheses. bωexe = 2.1 cm−1.

ThF4 is within 2 cm−1 of the 530 cm−1 neon matrix value, consistent with only a small matrix shift if any in the Ne matrix. The Th−F stretch of 521 cm−1 in the argon matrix observations for the ThF4 molecule shows only a modest matrix shift. ThF, ThF2, and ThF3 Assignments. The sharp 567.2, 564.8 cm−1 argon matrix peaks, marked in Figure 1, which increase on annealing and decrease on photolysis in the most dilute fluorine experiment must arise from a product which can be generated from a simple reaction, and the Th + F combination reaction is proposed. The Th−F bond dissociation energy38 of 154.9 ± 2.3 kcal/mol makes this a favorable reaction. These bands also appear unshifted in similar experiments with NF3, 16OF2, and 18OF2.20,37 The argon matrix bands are red-shifted from the gas phase band position as expected. The neon matrix counterparts at 576.1, 573.8 cm−1 decrease slightly on annealing and the higher matrix site band at 576.1 cm−1 is destroyed on photolysis and restores on annealing like the argon matrix counterparts. These bands exhibit blue 2109



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shifts in solid neon of 8.9 and 9.0 cm−1, relative to the argon values respectively, almost the same blue shifts observed for ThF4 (8.3 cm−1 for the major band). The neon matrix ThF bands appear between the argon matrix and the predicted gas phase band positions and substantiate all of these vibrational assignments for the ThF molecule. CCSD(T) calculations for ThF20 (Table 1) predict a 2Δ ground state with vibrational frequency of 598.9 cm−1, in agreement with the 605 ± 15 cm−1 measurement from electronic band spacings but about 25 cm−1 above the Ne matrix values of (576.1, 573.8) cm−1. The experimental Ne matrix value is in better agreement with the multireference configuration interaction plus spin orbit correction value of 573.8 and 573.9 cm−1.12 The 1A1 state for ThF2 is predicted to be the ground state with the 3B1 state 13.5 kcal/mol higher in energy at the CCSD(T) level at 298 K.20 The strong antisymmetric Th−F stretching frequency of 573.4 cm−1 at the CCSD(T) level (Table 1) is in excellent agreement with the argon matrix bands at 575.9 and 575.1 cm−1, which were also observed with the NF3 precursor.37 The weak, sharp neon matrix counterparts at 583.6 and 582.7 cm−1 with blue matrix shifts of 7.7 and 7.6 cm−1 support this assignment and suggest that the CCSD(T) harmonic values are slightly too low. The calculations20 predict the symmetric Th−F stretch to be above the antisymmetric stretch by ∼12 cm−1 at the CCSD(T) level and to have less than 10% of the antisymmetric mode’s intensity; therefore, the symmetric stretching mode is too weak to observe here. The calculated CCSD(T) value (Table 1) for the intense antisymmetric Th−F stretching absorption for planar ThF3 at 537.2 cm−1 enables us to assign the photosensitive neon and argon matrix absorptions at 537.4 and 531.0 cm−1, respectively, to this vibrational mode of the ThF3 intermediate. There is excellent agreement between the Ne value and the CCSD(T) value. The 528.4 cm−1 argon matrix band is due to ThF3(F2) complex (see below), also observed with the NF3 and OF2 precursors,20,37 which is favored on annealing in these experiments over the 531.0 cm−1 band of isolated ThF3. Part of the 537.4 cm−1 ThF3 band is restored on annealing in solid neon, Figure 3, and new weak bands at 544.2 and 542.5 cm−1 are probably due to the ThF3(F2) complex in the neon matrix. The antisymmetric stretching frequency for the 3B1 state of ThF2 is predicted to be 533.2 cm−1 at the CCSD(T) level. We expect the matrix to quench the excited triplet state so we cannot assign the bands at 538 cm−1 (Ne) and 531 cm−1 (Ar) to this excited state species. A comparison of ThF3 frequencies in a number of substituted ThF3 species is given in Table 2. Notice that the antisymmetric Th−F stretching fundamental for the ThF3 radical at 531 cm−1 in solid argon is slightly higher than the values36 for substituted

ThF3 species, which range down to 521.3 cm−1 for HCThF3 and 520.6 cm−1 for HDThF3 and even lower for ThF4. In contrast to the effect of substituents at Th on the ThF3 moiety’s antisymmetric stretch, their effect on the symmetric stretching mode is smaller. Assignments for ThF3(F2), ThF5−, and ThF4−. The union of ThF3 and F2 leads to the formation of a stable complex ThF3(F2), which was identified in solid argon from the transition at 528.4 cm−1 on the basis of vibrational frequency calculations at the DFT level with the B3LYP functional.20 The size of the molecule coupled with the fact that is open shell make the CCSD(T) calculations for doublet ThF3(F2) computationally too expensive. We previously found good agreement (within 10−20 cm−1 with the largest difference for the t mode in ThF4 with a difference of 25 cm−1)20 between CCSD(T) and DFT with the B3LYP functional for the calculated Th−F stretches for ThFx, x = 1−4 and ThF5−. We thus used B3LYP for the calculations of the frequencies for open shell ThF3(F2), ThF4−, and O2ThF, and for O2ThF2 and the larger Th2F4,6,8 clusters. Our CCSD(T) calculations (Table 3) show that electron capture by ThF3(F2) is a highly exothermic process leading to Table 3. Ionization Potentials (IP) and Electron Affinities (EA) of ThFx in eV at 298 Ka Molecule ThF ThF2 ThF3 ThF4 ThF5 a

molecule

symmetric

antisymmetric

ref

570.8a 567.5 569.5 569.1 (566) 565.4 563.4 563.3a

531.0 528.4 528.9 528.3 525.3 521.3 520.6 522.7, 521.0, 519.2, 514.5

20 and this work 20 and this work 37 37 37 36 36 20 and this work

a

IP(CCSD(T))

EA(B3LYP)

EA(CCSD(T))

6.48 6.50 6.72 12.65 12.08

6.21 6.49 6.58 13.43 11.68

0.06 0.63 1.17 0.7120 6.8620

0.026 0.61 1.08 0.4220 7.1720

We use the ion convention excluding the enthalpy of the electron.

the very stable ThF5− anion. The stable trigonal bipyramidal ThF5− anion was identified from the transition at 460 cm−1 in solid argon (the CCSD(T) values are 451.7 cm−1 for the Th−Fax antisymmetric stretch and 463.5 cm−1 for the Th−Feq antisymmetric stretch).20 This band is stronger in the present Ar matrix experiment with higher reagent concentration, Figure 2. Weaker neon matrix counterparts appear at 544.2 and 542.5 cm−1 for the ThF3(F2). A new 464 cm−1 absorption with a 472 cm−1 shoulder in the Ne matrix also appears on photolysis at the expense of F3− (Figure 3), and this band exhibits appropriate behavior for the ThF5− anion, which reinforces our identification of the stable pentacoordinate anion.20 The weak intensity for the F2 stretch in ThF3(F2) predicted to be at 457.4 cm−1 makes it unlikely that this transition can be observed. In contrast to the large electron affinity of ThF3(F2), electron capture by ThF4 (Table 3) to form ThF4− is a much less exothermic process. There are two structures for ThF4−, Td and D4h, which differ in energy by 2.9 kcal/mol at the CCSD(T) level with the D4h structure being more stable. The difference is reduced to 0.5 kcal/mol at the B3LYP level. We can tentatively assign the weak broad bands at 450 and 485 cm−1 to the most stable D4h and Td anion structures the ThF4−. Calculations of Anions and Cations of ThFx, x = 1−5. The geometry parameters of the ions that we have not previously reported are in Table 4. As expected, removal of an electron to generate the cation leads to a shortening of the Th−F bond by 0.05 to 0.07 Å for x = 1−3. The bond angle for ThF2+ and ThF3+ decreases in the cation from the neutral.

Table 2. Th−F Stretching Frequencies (cm−1) Observed for X-ThF3 Species in Solid Argon ThF3 (D3h) ThF3(F2) (Cs) AsThF3 (C3v) PThF3 (C3v) NThF3 (C3v) HCThF3 (C3v) DCThF3 (C3v) ThF4 (Td)

IP(B3LYP)

CCSD(T) value. 2110



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planar. The stretching frequency in ThFx−, x = 1−3, are substantially decreased from these in the neutrals consistent with the increase in the Th−F bond distance on adding an electron. Calculations for Dimers of ThF2, ThF3, and ThF4. In order to provide additional checks on the assignments, calculations were performed on the dimers of ThF2, ThF3, and ThF4, giving Th2F4, Th2F6, and Th2F8, respectively. The geometry optimization and frequency calculations were performed at the DFT level and a subsequent single point calculation at the CCSD(T) level was performed for the energetics. Th2F4 (Figure 4 and Table 5) has two isomers, one with D2h symmetry, a Th−Th bond, and four Th−F terminal bonds,

Table 4. Calculated Geometry Parameters and Vibrational Frequencies for ThFx Cations and Anions molecule

propertya

ThF C∞vb ThF− C∞vb ThF2+ C2vb

r(Th−F) r(Th−F) r(Th−F) ∠(F−Th−F)

ThF2− D∞hb

r(Th−F)

ThF3+ C3vb

(Th−F) ∠(F−Th−F)

ThF3− D3hb

(Th−F)

ThF4+ C2vc

r(Th−F1) r(Th−F3) ∠(F1−Th−F2) ∠(F3−Th−F4)

ThF5+ Csd

(Th−F1) (Th−F2) (Th−F4) (Th−F5) (F4−F5) ∠(F1−Th−F2) ∠(F2−Th−F3) ∠(F1−Th−F4) ∠(F3−Th−F5) ∠(F1−Th−F2−F3) ∠(F2−Th−F4−F5)

+

parametera 1.973 2.102 2.006 107.8 2.078

2.033 108.9

2.138

2.040 2.227 117.0 51.9

2.106 2.106 2.977 2.992 1.415 112.4 112.9 83.9 111.4 −128.4 180.0

ν sym

ν (cm‑1)

σ σ a1 b2 a1 σg σu πu a1 e e a1 a1′ e′ e′ a2″ a1 b2 a1 a1 b1 a1 a2 b2 b1 a′ a′ a″ a′ a′ a″ a′ a′ a′ a′ a″ a″

675.7 509.8 659.3 633.6 112.4 489.5 473.9 28.8 659.7 610.6 118.2 104.2 522.4 488.5 77.5 64.9 630.7 608.1 482.7 337.4 251.9 116.9 93.4 92.4 68.2 971.6 617.4 598.1 597.1 128.0 111.7 111.3 89.9 72.2 25.3 22.4 16.4

a

Bond distances in Å and bond angles in degrees. bCCSD(T)/aug-ccpVTZ(ECP). cB3LYP/aug-cc-pVTZ(ECP). dMP2/aug-cc-pVTZ(ECP).

This decrease in the Th−F bond distance leads to an increase in the Th−F stretching frequencies for x = 1−3 by up to 80 cm−1. The structure of ThF4+ is essentially an F2+ weakly complexed to ThF2. The ThF2 substantially stabilizes the ionization of the F2 by ∼2.2 eV (IP(F2) = 15.60 eV, CCSD(T), 15.70 eV, experiment39). Examination of the literature shows that the tetrafluorides CF4 and SiF4 undergo dissociative ionization with the former forming CF3+ + F.39,40 We thus calculated the energy of the process ThF4 → ThF3+ + F + e−, which gives 13.43 eV at the CCSD(T) level (13.48 eV, B3LYP), so this dissociative ionization path is about 0.35 eV higher in energy at the CCSD(T) level. The ionization of ThF5 leads to removal of the electron from the F2− moiety and to the complex ThF3+[F2]. This structure looks like a weak complex with ThF3+ and F2. The addition of an electron leads to an increase in the bond distance from 0.02 to 0.04 Å for x = 1−3. The bond angles change substantially with ThF2− becoming linear and ThF3- becoming

Figure 4. Structures of thorium fluorides with NBO charges in e.

and the second with C2h symmetry with two bridging fluorine atoms and one terminal Th−F bond per Th. The singlet C2h isomer is the lowest energy structure with the singlet D2h isomer 4.2 kcal/mol higher in energy at the CCSD(T) level. The DFT results predict the opposite order with the singlet D2h isomer 5.5 kcal/mol lower in energy than the singlet C2h isomer. The 3B3u state of the D2h isomer is 3.2 kcal/mol higher in energy than the 1Ag state at the CCSD(T)/aT level. The 3Bu state of the C2h isomer is 15.1 kcal/mol higher in energy than 1 Ag state at the CCSD(T) level. The dimerization energy to form the C2h isomer from two ThF2 molecules is −19.3 kcal/mol at the CCSD(T) level at 0 K (See Table 6 for the 298 K value). The terminal Th−F bond distance is ∼0.03 Å shorter in the C2h isomer as compared to that in the D2h isomer for Th2F4. Our calculations predict a strong antisymmetric stretch of the 2111



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Table 5. Calculated Geometry Parameters, Frequencies, and Infrared Intensities at the B3LYP/aug-cc-pVTZ level for Thorium Fluoride Dimers and Thorium Oxyfluoridesa parameter

state/geometry 1

Th2F4 (D2h) r(Th−Th) r(Th−F) ∠F−Th−F Th−F asym. str. b3g Th−F asym. str. b1u Th−F asym. str. b2u Th−F sym. str. ag Th2F4 (C2h) r(Th−Fbr) r(Th−Fter) ∠Th−Th−F Th−Fbr asym. str bu Th−Fter asym. str. bu Th−Fter sym. str. ag Th2F6 r(Th−Th) r(Th−F) ∠Th−Th−F Th−F sym. str. Th−F asym. str. Th−F asym. str. Th−F sym. str. Th2F6 (D2) r(Th−Fbr) r(Th−Fter) ∠Fter−Th−Fter ∠Fter−Th−Fbr ∠Fbr−Th−Fbr ∠Fter−Th− Th−Fter Th−Fbrdg asym. str. b1 Th−Fbrdg sym. str. b3 Th−Fter asym. str. b1 a

parameter 3

Ag 3.386 2.093 143.2 534.1 (0.0) 535.2 (179.5) 538.0 (362.8) 546.4 (0.0) 1 Ag 2.326 2.066 163.6 344.2 (273.9) 569.3 (216.0) 573.4 (0.0) 1 A1g (D3d) 3.983 2.099 103.5 542.0 (0.0) eg 544.2 (321.4) eu 573.3 (174.7) a2u 583.3 (0.0) a1g 1 A 2.331 2.088 102.3 95.0 68.4 19.7 299.6 (0.0) 340.1 (7.4) 537.4 (5.6)

B3u 3.470 2.094 143.8 533.4 (0.0) 534.0 (157.0) 537.9 (375.0) 546.1 (0.0) 3 Bu 2.322 2.073 160.7 288.0 (53.5) 557.7 (383.4) 562.3 (0.0) 1 A′1 (D3h) 4.000 2.098 103.9 541.9 (0.0) e″ 544.2 (319.1) e′ 573.8 (179.1) a2″ 582.6 (0.0) a1′ 3 B3 2.368 2.090 104.1 95.7 65.0 20.2 268.3 (0.0) 366.7 (422.3) 535.0 (6.9)

state/geometry

Th−Fter asym. str. b2 Th−Fterasym. str. b3 Th−Fter sym. str. a Th2F8 (C2v) r(Th−Fbr) r(Th−Fter) ∠Fter−Th−Fter ∠Fter−Th−Fbr ∠Fbr−Th−Fbr Th−Fbrdg sym. str. a1 Th−Fbrdg asym. str. b2 Th−Fter sym. str. a1 Th−Fter asym. str. b2 Th−Fter asym. str. b1 Th−Fter sym. str. a2 Th−Fter asym. str. b2 Th−Fter sym. str. a1 ThFO2 (C2v) r(Th−F) r(Th−O) ∠O−Th−F Th−O asym. str. b2 Th−F str. a1 Th−O sym, str. a1 ThF2O2 (C2v) r(Th−F) r(Th−O) ∠F−Th−F ∠F−Th−O Th−F asym. str. b1 Th−F sym. str. a1 Th−O sym. str. a1 Th−O asym. str. b2

540.7 (293.9) 566.7 (424.0) 577.7 (0.0) 1 A1 2.381 2.107 110.0 106.0 63.8 280.3 (0.0) 374.9 (522.7) 528.9 (0.0) 532.0 (0.8) 535.1 (350.8) 538.7 (409.1) 581.4 (84.6) 589.6 (0.0) 2 B2 2.139 1.950 106.5 495.7 (0.4) 504.2 (169.8) 669.3 (6.7) 1 A1 2.110 1.921 102.5 97.4 524.3 (160.1) 542.3 (88.6) 637.1 (1.7) 704.1 (59.6)

538.8 (310.8) 568.2 (152.6) 574.6 (0.0)

4

B2 2.071 2.085 114.2 482.0 (51.5) 504.54 (77.8) 547.1 (98.7) 3 B2 2.118 2.076 96.5 111.8 518.4 (161.2) 544.1 (148.2) 604.6 (0.0) 346.2 (56.4)

Bond distances in Å and bond angles in degrees. Frequencies in cm−1. Infrared Intensities in parentheses in km/mol.

antisymmetric stretch of the terminal F in the singlet C2h isomer of Th2F4, but their assignment to the anticipated much more abundant ThF product is far more likely. There are two low lying isomers of Th2F6, one with a Th−Th bond and two rotamers with D3d or D3h symmetry. The second isomer has D2 symmetry with two bridging fluorine atoms and two terminal Th−F bonds per Th. The D2 structure has the terminal F atoms slightly out of the plane of the other 4 atoms. The D2 structure is 0.14 kcal/mol lower in energy than the D2h structure at the B3LYP level in terms of the electronic energy, and the D2h structure has an imaginary frequency of only 34i cm−1. Thus the D2 isomer is essentially planar. The staggered D3d structure is predicted to be the lowest energy conformer with the D3h rotamer 0.4 kcal/mol higher in energy at the CCSD(T) level at 0 K. The D3d rotamer is more stable than the D2 isomer by only 0.5 kcal/mol. The triplet states of D3d(3Ag) and D3h(3A′1) rotamers are higher in energy than the corresponding singlets by 36.6 and 36.8 kcal/mol at the CCSD(T) level, respectively. The 3B3u state for the D2 isomer is only 2.0 kcal/mol higher than the 1A state at the CCSD(T) level. The DFT method predicts the opposite order with the triplet 3B3u state 3.1 kcal/mol lower in energy than the singlet state for the D2 isomer. Thus the molecule Th2F6 has two isomers and an excited state within 2.0 kcal/mol, suggestive of a

Table 6. Reactions Used to Estimate Heat of Formations for Thorium Fluorides and Thorium Oxyfluorides at 298 Ka reaction

CCSD(T)/aug-cc-pVTZ

rxn

HfF4 + ThO2 → ThF4 + HfO2

41.8

1

1

40.8

2

1

ThF2 + F2 → ThF4

−276.9

3

ThF2 + F2 → 2 ThF3 + 2 F

−117.8

4

2ThF2 → Th 2F4

−23.2

5

2ThF3 → Th 2F6

−36.0

6

2ThF4 → Th 2F8

−34.1

7

1

−50.8

8

1

−123.8

9

1

1

HfF2 + ThO2 → 1ThF2 + HfO2

1

ThO2 + F2 → 2O2 ThF + 2 F

ThO2 + F2 → 1O2 ThF2

a

Lowest energy conformations are used for calculate the reaction energies.

terminal F in the C2h structure at 569 cm−1 and a symmetric stretch with 0 intensity ∼4 cm−1 above the antisymmetric stretch. The next most intense mode is at 344 cm−1 corresponding to the motion of the bridging fluorines. Thus, the 565.0, 567.1 cm−1 argon matrix peaks could in principle be due to the 2112



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complex chemistry. The dimerization energy of two ThF3 to form the D3d structure is −36.8 kcal/mol at the CCSD(T) level at 0 K (See Table 6 for the 298 K value). The terminal Th−F bond distance is ∼0.01 Å shorter in the D2 isomer as compared to the D3d isomer for Th2F6. The Th−F stretching frequencies of the staggered and eclipsed conformers are essentially the same. The D3d isomer is predicted to have a strong antisymmetric Th−F stretch at 544.2 cm−1 as well as one at 573.3 cm−1. The symmetric stretch is predicted at 542.0 cm−1 with zero intensity. Our calculations predict strong antisymmetric Th−F stretches at 566.7 cm−1 and 540.7 cm−1 for the D2 structure. Strong antisymmetric Th−F stretches are predicted at 538.8 cm−1 and 568.0 cm−1 for the triplet state of the D2 isomer. Dimerization of two ThF4 yields Th2F8 with two bridging fluorine atoms and three terminal Th−F bonds per Th. The dimerization energy is −33.1 kcal/mol at the DFT level and −34.1 kcal/mol at the CCSD(T) level at 0 K (See Table 6 for the 298 K value). Th2F8 is predicted to have symmetric Th−Fter stretches at 280.3 cm−1 and 589.6 cm−1 with zero intensity. Our calculations predict strong antisymmetric Th−F stretches at 535.1 cm−1 and 374.9 cm−1 for the Th−Fter and Th−Fbrdg bonds, respectively. Another strong IR peak is predicted at 538.7 cm−1 for the a2 symmetric stretch. Calculations on O2ThF and O2ThF2. It was also possible that the observed matrix bands could be attributed to fluorinated thorium oxides so we calculated the structures of O2ThF and O2ThF2. These molecules cannot have a formal oxidation state higher than +4 at the Th so with the fluorines having a formal oxidation state of −1, the O atoms will have formal oxidation states of −1 excluding the possibility of a fractional oxidation state. The 2B2 ground state of the O2ThF (C2v) molecule is predicted to be planar at the DFT/B3LYP level. The 2B2 state for O2ThF is predicted to be 85.1 and 89.4 kcal/mol lower than the 4B2 state at the CCSD(T) and B3LYP levels, respectively. The only stretching mode with appreciable IR intensity for the 2 B2 state is the Th−F stretch at 504 cm−1. The O2ThF2 molecule is predicted to have C2v symmetry at the DFT level with the triplet state (3B2) 4.7 kcal/mol higher than the 1A1 ground state at the CCSD(T) level. The antisymmetric Th−O and Th−F stretching frequencies are calculated to be 704 cm−1 and 524 cm−1 at the DFT level, respectively for the 1A1 state. The Th−O and Th−F symmetric stretches are predicted to be 637 cm−1 and 524 cm−1, respectively. The antisymmetric Th−F and Th−O stretches in the 3B2 state are predicted to be 518.4 cm−1 and 346.2 cm−1, respectively, and the Th−O and Th−F symmetric stretches are predicted to be 604.6 cm−1 and 544.1 cm−1, respectively. Thus none of the modes for O2ThF and O2ThF2 match any of the features in the experimental spectrum. Thermodynamic Properties. There are currently no correlation consistent basis sets available that we can use to obtain heats of formation by extrapolating CCSD(T) energies to the complete basis set limit with additional corrections following the Feller−Peterson−Dixon (FPD) approach.41 We have thus used isodesmic and/or clustering reaction energies obtained at the CCSD(T)/aug-cc-pVTZ level to estimate the heats of formation of the thorium fluorides and thorium oxyfluorides (Tables 6 and 7). The total electronic energies for individual species are given in the Supporting Information. The heats of formation for HfO2 (−50.0 ± 1.5 kcal/mol),42 and HfF2 (−128.6 ± 1.5 kcal/mol)43 were taken from our previous studies. We calculated the heat of formation of HfF4 with the

Table 7. Calculated Heats of Formation (kcal/mol) at 298 K compound

ΔHf (298) calc.

ThF2 (C2v) ThF3 (D3h) ThF4 (Td) Th2F4 (C2h) Th2F4 (D2 h) Th2F6 (D3d) Th2F6 (D2) Th2F8 (C2v) O2ThF (C2v) O2ThF2 (C2v)

−145.5 ± 5.4,a −140.9 ± 5.4b −282.3c −424.5 ± 7.4d −267.7e −263.4 −524.0f −523.5 −801.3g −178.6h −232.6i

S-T CCSD(T)

S-T B3LYP

13.2

9.8

15.1 3.2 36.8 2.3

11.0 3.0 36.6 −3.4

4.7

1.4

a

From reaction 2. bFrom reaction 3. cFrom reaction 4. dFrom reaction 1. eFrom reaction 5. fFrom reaction 6. gFrom reaction 7. h From reaction 8. iFrom reaction 9.

FPD composite approach.41,43,44 The heat of formation for HfF4 is predicted to be −406.1 ± 1.5 kcal/mol. The experimental heat of formation of ThO2 is −108.8 ± 2.4 kcal/mol and that of ThF4 is −417.8 ± 2 kcal/mol.13 The approach was first tested by predicting the energy for reaction 1 in Table 6 where the calculated value is in good agreement with the value from the available heats of formation of 47.1 ± 7.4 kcal/mol, considering the error bars on this value. The molecular error bars come predominantly from those for the heats of formation of the metal atoms. The ionization potentials (IPs) and electron affinities (EAs) are given in Table 3. The ionization potential of the Th atom is 6.3067 ± 0.0002 ev.45 The calculated ionization potentials for ThF, ThF2, and ThF3 range from 6.28 to 6.58 at the CCSD(T) level and from 6.48 to 6.72 eV at the B3LYP DFT level consistent with removing an electron from a Th, which is somewhat positive due to the ligands. The experimental IP12 for ThF is 6.39526 eV, which is closer to the DFT value. The ionization of ThF4 is much higher at 13.07 eV for formation of a weak complex between ThF2 and F2+. The IP for ThF5 is also high, ∼12 eV, for formation of ThF3+ with a weakly complexed F2. As discussed above, the electron affinity of ThF5 is very high and that of ThF4 is quite low. The electron affinities for ThF is very small, less than 0.1 eV, suggesting that it will be difficult to generate ThF− except at very low temperatures. The electron affinity of ThF2 is larger, 0.70 eV and that for ThF3 is ∼1.10 eV. The low electron affinities for ThF, ThF2 and ThF3 are consistent with the fact that the F ligands do not dramatically increase the positive charge on the Th as shown by the small change in ionization potential from the atom. The calculated heats of formation together with the atomic heat of formation46 of F can be used to calculate Th−F bond dissociation energies (BDEs) (Table 8). The BDE for Th−F of 154.9 ± 2.3 kcal/mol is from a mass spectrometric study of temperature dependent species concentrations. It can be used together with the heats of formation47 of the Th atom (143.9 ± 1.4) and the F atom (18.94 kcal/mol) to estimate ΔHf(ThF) = 7.9 kcal/mol at 298 K. The Th−F BDEs for ThF, ThF3, and ThF4 are similar, 155 to 160 kcal/mol. The Th−F BDE for ThF2 is 12 to 15 kcal/mol larger. These BDEs are comparable to or larger than those for the main group fluoride SiF4 and the metal fluorides TiF4, ZrF4, and HfF4.48 The Th−F BDE of O2ThF is ∼17 kcal/mol larger than that of O2ThF2, and both are approximately half of the values for ThFx. These strong Th−F bonds are consistent with the expected ionic bonding. 2113



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lengths, especially in systems with double bonds. For example, the CC bond distance in C2F4 (1.311 Å)49 is shorter than in C2H4 (1.333 Å),50 yet the adiabatic CC BDE in C2F4 is 66 kcal/mol as compared to the adiabatic BDE in C2H4 of 174 kcal/mol.51 This difference arises because the ground state for CH2 is a triplet so the orbitals are ready to form the double bond. For CF2, the ground state is a singlet and a promotion energy52 of 56.5 kcal/mol (the singlet−triplet splitting) for each CF2 is needed to form the bond leading to the much smaller adiabatic BDE.53 In the D2h dimer of ThF2, the orbitals have to be promoted to form the two bonds holding the dimer together. The excited triplet state for ThF2, which helps to prepare the electrons for bonding, is 13.5 kcal/mol above the singlet, so 27 kcal/mol is needed to prepare the electrons for forming the Th−Th bonds, leading to a decrease in the adiabatic BDE. As in C2F4, the bond distance is governed by the diabatic interactions, but the bond energy is governed by the adiabatic interactions. In contrast to the D2h dimer of ThF2, the ground state of ThF3 is a doublet, so the electronic states of the fragments are prepared for forming a single Th−Th bond. Thus, although the bond distance is greater in F3Th-THF3 than in the D2h dimer of ThF2, the adiabatic BDE in F3Th-THF3 is actually larger. This is exactly what happens in C2F6 as compared to C2F4. The C−C single bond length in C2F6 is significantly greater than that in C2F4 but the adiabatic C−C BDE in C2F6 is 98.1 kcal/mol,51 substantially larger than the adiabatic BDE for C2F4. The fluoride affinities,54 which are a measure of the Lewis acidity, are given in Table 9. The values for the neutrals range

Table 8. Bond Dissociation Energies in kcal/mol at 298 K compound

B3LYP

CCSD(T)

BDE (ΔHf)

ThF→ Th + F20 ThF2+→ ThF+ + F ThF2→ ThF + F20 ThF2−→ ThF− + F ThF3+→ ThF2+ + F ThF3→ ThF2 + F20 ThF3−→ ThF2− + F ThF4+→ ThF3+ + F ThF4→ ThF3 + F20 ThF4−→ ThF3− + F ThF5→ ThF4 + F20 ThF5−→ ThF4− + F O2Th−F O2FTh−F F2OTh−O F2Th−ThF2a F3Th−ThF3 F4Th−ThF4 O2ThF → ThF + O2 O2ThF2 → ThF2 + O2

157.4b 162.0 164.0 178.8 149.5 153.5 164.2 19.2 156.0 144.5 15.0 156.8 91.5 76.3 24.3 21.1 36.6 33.1 178.9 93.9

153.120,38

154.9 ± 2.3c

162.9

172

153.4

156

8.3 157.9 142.9 5.7 161.4 88.1 73.2 27.5 23.2 36.0 34.1 175.8 89.2

160

89 72 19

a

D2h isomer of used for calculate the Th−Th bond energy. bWith SO for Th (8.8) and F (0.4). cReference 38.

The Th−F BDEs of the cations and anions are all in the same general region as the neutrals except for ThF4+ and ThF5+, which form weak complexes. ThF2+ and ThF3+ are slightly smaller than those of the neutrals whereas those of the anions ThF2− and ThF3− are larger than those for the neutral. The values of the Th−F BDE in ThF4− is lower than the Th−F BDE by about 10 kcal/mol and that for ThF5− is ∼10 kcal/mol higher than for ThF4−. The Th−O BDE in O2ThF2 is very weak, ∼ 28 kcal/mol, consistent with the stability of OThF2 and the fact that the O in O2ThF2 is formally in the −1 oxidation state. In contrast, The Th−O BDE in OThF2 is ∼181 kcal/mol. However, it is only 25 kcal/mol larger than the Th−F BDE in ThF3 even though the Th−O BDe is nominally between a formal O2‑ and the formal cationic Th4+ in the former as compared to a formal F− and Th3+ in the latter. The interactions holding the two Th atoms in the dimers together are quite weak. For Th2F4 with a Th−Th bond, the Th−Th BDE is only 19 kcal/mol. For the other isomer with 4 Th−F bridging interactions, each Th−F bridge is worth just under 6 kcal/mol. The Th−Th bond in Th2F6 is almost double that in Th2F4 and the Th−F bridge bond BDEs are about 50% larger than in Th2F4. The Th−F bridge bond BDEs in Th2F8 are comparable to those in Th2F6. The results show that ThF2, ThF3, and ThF4 can dimerize, but that the dimerization interactions are not strong. This is in part due to the very strong Th−F terminal bond BDEs discussed above as the terminal Th−F bonds have to be converted into Th−F bridge bonds. As the Th−F bonds in ThF2 are the largest, it is not surprising that the weakest dimerization energy with bridge bonds is for ThF2. The results also show that there are only weak interactions between the Th atoms in F2Th−ThF2 and F3Th−ThF3, consistent with the inability of the available atomic orbitals on the Th to form strong bonds with another Th. Although the bonding in the D2h dimer of ThF2 shows a π bond with a σ bond (see discussion below), one must be careful about correlating the strength of adiabatic BDEs with bond

Table 9. Fluoride Affinities (FA) of ThFx in kcal/mol reaction

B3LYP −77.7

−76.5

ThF+ + F− → ThF2

−232.0

−229.8

−

−

−97.1

−100.6

ThF2 + F → ThF3

−222.1

−226.7

ThF2 + F− → ThF2−

−99.0

−101.9

ThF3+ + F− → ThF4

−229.5

−233.3

−

−91.0

−91.3

−91.8

−94.7

ThF + F− → ThF2− +

−

ThF3 + F → ThF4 −

ThF4 + F → a

CCSD(T)

Th + F → ThF

a

−

ThF5−20

With SO for Th (8.8).

from ∼80 kcal/mol to ∼100 kcal/mol with ThF and ThF2 having the largest values. These values are consistent with these compounds being moderately strong Lewis acids consistent with the cataionic nature of the Th. The F2 and F2− affinities are given in Table 10. The addition of F2 is strongly exothermic for ThFx and ThFx+ for x = 0 − 3, except for ThF2+ and ThF3+, due to the strong Th−F bonds and modest IPs for the formation of the cations. The value of −124 kcal/mol for binding F2 to ThF2+ is due to the formation of a complex between F2+ and ThF2, and the value of −10 kcal/mol for bonding of F2 to ThF3+ shows that ThF5+ is a weak complex between ThF3+ and F2, as noted above. The smaller value of −125 kcal/mol for addition of F2 to ThF3 shows that ThF5 is not as stable as the other ThFx species as discussed previously. The high value for adding F2 to ThF3− is consistent with the high electron affinity of ThF5−. Similar arguments hold for the addition of F2− with the addition to the 2114



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Table 10. F2 and F2− Binding Energies of ThFx in kcal/mol

a

ThFx

F2 product

F2 B3LYP

F2 CCSD(T)

F2− product

F2− B3LYP

F2− CCSD(T)

Tha ThF+ ThF ThF− ThF2+ ThF2 ThF2− ThF3+ ThF3 ThF3− ThO2

ThF220 ThF3+ ThF3 ThF3− ThF4+ ThF420 ThF4− ThF5+ ThF520 ThF5− O2ThF2

−285.3 −275.5 −281.1 −306.5 −131.4 −273.1 −275.1 −10.9 −134.6 −267.7 −119.6

−280.4 −271.9 −280.4 −305.8 −123.6 −275.4 −268.2 −10.0 −127.6 −268.2 −123.8

ThF2− ThF3 ThF3−

−214.7 −345.3 −222.7

−226.1 −355.3 −236.8

ThF4 ThF4−

−337.9 −205.2

−356.6 −216.8

ThF520 ThF5−

−204.3 −209.4

−211.1 −224.7

more like 7s06d1 as would be expected from the behavior of transition metals where the s orbitals are removed before the d orbitals on ionization, although the actual limiting case is not reached. In ThF4 with a charge of 2.61 e, the 7s population is essentially 0 and there are ∼0.8 electrons in the Th 6d. Thus the oxidation states in ThF and ThF2 and those for ThF3 and ThF4 arise from different electron configurations with the latter pair most like the transition metals. The 5f population is 0.3 to 0.4 electrons for all three compounds and the 7p population is very low. The dimer of ThF2 with two bridging fluorines (C2h) has populations similar to that of the monomer and is thus derived more from the 7s26d0 configuration for Th2+. In contrast, the dimer with a Th−Th bond has a much lower 7s orbital population and a substantially increased 6d population, suggesting a role for the 6d orbitals in the Th−Th bond and that the Th2+ is more derived from the 7s06d2 configuration. The 5f population in the dimer is about the same as in the monomer suggesting no role in the Th−Th bond. The dimers of ThF3 have very similar populations on the Th orbitals independent of the presence of bridging fluorines or a Th−Th bond and are derived from the 7s06d1 configuration of Th3+. The populations on the Th for the dimers of ThF3 are essentially the same as those for the monomer. In contrast to the dimer of ThF2 with a Th−Th bond, there is no role for the 6d orbitals in the Th−Th bond in Th2F6. In the dimer of ThF4, the orbital populations are essentially the same as that in the monomer with the 6d being the largest. Substitution of F by O (Figure 5) to from ThFO2 leads to significant increase of the negative charge on the O by ∼0.2 e as

With SO for Th (8.8).

cations being much larger than to the neutrals due to alleviating the positive charged species. Electronic Structures. The natural bond orbital analysis55,56 of the DFT results yields the electron populations shown in Figure 4 with a breakdown in terms of the orbitals on the Th in Table 11. The results in Figure 4 show that terminal fluorines Table 11. Natural Electron Configuration Populations on Th from the NBO Analysis molecule

7s

7p

6d

5f

ThF (C∞v) ThF2 (C2v) ThF3 (D3h) ThF4 (Td) ThF3(F2Cs) Th2F4 (C2h) Th2F4 (D2h) Th2F6 (D3d) Th2F6 (D2) Th2F8 (C2h) O2ThF (C2v) O2ThF2 (C2v)

1.86 1.72 0.64 0.04 0.05 1.55 0.78 0.61 0.64 0.05 0.04 0.06

0.03 0.03 0.04 0.08 0.10 0.04 0.11 0.13 0.07 0.10 0.08 0.12

1.15 0.61 0.97 0.83 0.82 0.72 1.50 0.97 0.94 0.81 0.98 1.11

0.26 0.28 0.34 0.43 0.46 0.30 0.31 0.32 0.39 0.46 0.71 0.76

on the ThxFy compounds have negative charges of 0.64 to 0.71 electrons. If bridging fluorines are present, they have charges that are 0.02 to 0.04 electrons more negative than those for the terminal fluorines. The results are consistent with significant ionic character in the Th−F bonds and the high Th−F BDEs. The Th atom has a valence configuration of 7s26d2. An elegant multireference configuration interaction (MRCI) study by Barker et al.12 showed that the ground state of ThF is built from the 7s26d1 configuration of the atom, which is 11.8 kcal/mol higher in energy than the ground state for Th+ derived from the 7s6d2 configuration.57 The 7s orbital population 0f 1.9 e and the 6d population of 1.2 e for ThF is consistent with the MRCI results for the 7s26d1 configuration for the Th+ in ThF. In ThF2, the 7s population on the Th is 1.7 e with 0.6 e in the 6d (Table 11). This suggests that most of the second electron is being removed from the 6d with the 7s remaining approximately doubly occupied. The actual NBO charge is 1.34 e shows that the formal oxidation state of +2 is not reached. The addition of an F to form ThF3 makes the Th more positive (+2.01 e) and the 7s population decreases by 1.1 e from that in ThF2, while the 6d population actually increases by ∼0.4 e. Thus the electron configuration in ThF3 is becoming

Figure 5. Structures of thorium oxyfluorides with NBO charges in e.

compared to the F in ThF3 or ThFO2. This suggests that the O has slightly more than a formal oxidation state of −1. The difference between the O and F NBO charges is much smaller in ThF2O2 as the O atoms cannot have more than a formal charge of −1 if the Th is in the maximum +4 formal oxidation state. The results for the NBO charges are consistent with ionic Th−O bonds and with the O in the formal −1 oxidation state. The molecules O2ThF and O2ThF2, have low 7s orbital populations as found for ThF4 but not for ThF3. The 6d orbital contributions are ∼1 e, and the 5f orbital contributions are almost double those of the fluorides, suggesting more of a role for the 5f orbitals in the Th−O bonds. The Th oxidation state in O2ThF and O2ThF2 is closer to that in ThF4, even though the charges on the Th do not reflect this due to the enhanced 5f orbital character. The Kohn−Sham orbitals (Figure 6) show the behavior expected from the populations. The HOMO on 1ThF2 is predominantly of 7s character with a small amount of 6d character. The D2h dimer of ThF2 with the Th−Th bond has a HOMO 2115



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Figure 7. Spin densities for open shell molecules. Contour level at 0.002.

fluorines is a combination of 7s + 6d orbitals and closely resembles the HOMO-1 of Th2F4 (C2h). The spin density (Figure 7) is localized on the thorium atoms in the radicals with ∼1.0 on each Th in 2ThF3, 3Th2F4 (D2h), 3Th2F4 (C2h), 3Th2F6 (D3d), and 3Th2F6 (D2). We do not show the results for 3Th2F8 because the triplet is so much higher in energy than the singlet ground state and the results for ThF4− (D4h, Td) and ThF3(F2) as these results have been reported previously.20 There is little spin on the fluorines in these compounds, except for ThF3(F2) where the spin is localized on the F2− fragment as expected. For 3ThF2, the spin is in the 7s and 6dx2−y2 orbitals. As noted above for 2ThF3, the spin is in the dz2 orbital. For 3Th2F4 (D2h), the spin is in the 6dx2−y2 and 6dxz orbitals on the Th’s. Similarly for 3Th2F4 (D2h), the spin is in the 6dx2−y2 and mostly 6dxz orbitals on the Th’s. For 3Th2F6 (D3d), the spin is localized half in the 7s and half in 6d orbitals. For 3Th2F6 (D2), the spin is in the 7s and 6d orbitals as well. In contrast to the fluorides, there is substantial spin on the oxygens in the oxyfluorides. Overall, the results show that the bonding in the thorium fluorides is dominated by 7s and 6d orbitals depending on the ionicity. The 6d electrons are lost before the 7s electrons for ThF and ThF2, but the 7s electrons are lost before the 6d electrons for ThF3 and ThF4 as expected for a transition metal. The 5f electrons have some occupation in the fluorides, ∼ 0.3 to 0.4 electrons. There is a substantially larger role for the f electrons in the oxyfluorides.

Figure 6. Kohn−Sham orbitals for (a) ThF2 and (b) ThF3 and their respective dimers. Contour levels at 0.04.

with mostly 6dxz character (molecule in the yz plane with the z-axis through the Th’s). The HOMO-1 has mostly 7s character. For the C2h isomer with the two fluorine bridges, the HOMO and HOMO-1 are of 7s + 6d character showing the ionic bridge bonding. The SOMO of 2ThF3 is mostly dz2 character. The HOMO of Th2F6 (D3d) with the Th−Th bond has mostly 7s character and resembles the HOMO-1 of Th2F4 (D2h). The HOMO of Th2F6 (D2) with the two bridging 2116



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CONCLUSIONS

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ASSOCIATED CONTENT

* Supporting Information

The reactions of laser-ablated Th with F2 lead to the formation of the major product ThF4 on the basis of the close agreement between the observed spectra, the matrix spectra recorded of the vapor generated from the solid at 800−850 °C, and our CCSD(T) values. The higher frequency bands at 575.1 (582.7) and 531.0 (537.4) cm−1 in argon (neon) are weaker and are assigned to the ThF2 and ThF3 molecules based on agreement with the CCSD(T)/aug-cc-pVTZ vibrational frequency calculations. The sharp matrix site split doublets at 567.2, 564.8 cm−1 in solid argon and at 576.1, 573.8 cm−1 in solid neon are assigned to the ThF diatomic molecule. These 7−9 cm−1 blue matrix shifts from the solid argon to neon environments are in accord with the behavior of other metal bearing molecules.58 The improved spectra further validate our assignments of the structures of two pentacoordinated thorium species, including the unique (ThF3+)(F2−) radical ion pair molecule and its electron capture product, the very stable ThF5− anion. DFT calculations at the B3LYP level are in good agreement with the higher level CCSD(T) calculations and were used to predict the vibrational spectra of other thorium fluorides and oxyfluorides. The heats of formation of the thorium fluorides and thorium oxyfluorides were estimated from isodesmic and/or clustering reaction energies obtained at the CCSD(T)/aug-ccpVTZ level together with available experimental and computational heats of formation of thorium and transition metal compounds. These represent the best available values for these compounds. The Th−F BDEs for ThF, ThF2, ThF3, and ThF4 range from 155 to 172 kcal/mol with the Th−F BDE in ThF2 being the largest. The Th−F BDEs are similar to or larger than those for the transition metal fluorides TiF4, ZrF4, and HfF4 showing the iconicity in the Th−F bonds and the similarity of the thorium fluorides to the transition metal fluorides. The clustering reaction energies are not large due in part to the strength of the Th−F terminal bonds. are quite weak. The Th− Th bonds are small ranging from 23 kcal/mol in Th2F4 (C2h) to 36 kcal/mol in Th2F6 (D3d) at 298 K. The average Th−F bridging interaction energies thus range from 6 to 9 kcal/mol for an individual Th−F interaction. The excited triplet states of the isomers of Th2F4 (C2h) and Th2F6 (D2) with Th−F bridge bonds are only 3 and 2 kcal/mol higher in energy than the respective singlets. For the latter, this suggests that the dimer is essentially held together by ionic bridge bond interactions and that each ThF3 group maintains its basic electronic structure. For Th2F4 (C2h), the low lying triplet most likely arises from two 3ThF2 molecules interacting by ionic bridge bond interactions with the 7s open shell electrons pairing up and the 6d electrons remaining unpaired as shown by the spin density diagram. The triplet for ThF2 is 13.5 kcal/mol higher in energy than the singlet. This promotion energy to the triplet is partially recovered in the dimer. If the dimer interaction from Th2F6 (D2) of 36 kcal/mol is used as an estimate of the Th−F bridge bond interactions, then the dimeric interaction in Th2F4 (C2h) recovers about 14 kcal/mol for the spin pairing of the 7s orbitals. The bonding in the molecules is quite ionic. There is a small population of 5f electrons, on the order of 0.3 to 0.4 for the thorium fluorides. There is essentially no 7p population. The 5f population is almost doubled in the oxyfluorides.

S

Complete citations for references 29 and 31. Optimized Cartesian x, y, z coordinates in Å and total energies. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from DOE Office of Science, Basic Energy Supported by DOE Grant DESC0001034 (L. A.) and the BES SISGR program in actinide sciences (D. A. D.). D.A.D. thanks the Robert Ramsay Fund at the University of Alabama and Argonne National Laboratory for partial support.

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Properties of ThFx from Infrared Spectra in Solid Argon and Neon with

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