Matrix Infrared Spectroscopic and Quantum Chemical Investigations of

Article pubs.acs.org/JPCA

Matrix Infrared Spectroscopic and Quantum Chemical Investigations of the Group 5 Transition Metal Atom and CX4 Molecule (X = H, F, and Cl) Reaction Products Jonathan T. Lyon,†,‡ Han-Gook Cho,‡,§ and Lester Andrews*,‡ †

Department, of Chemistry and Physics, Clayton State University, 2000 Clayton State Boulevard, Morrow, Georgia 30260-0285, United States ‡ Department of Chemistry, University of Virginia, P.O Box 400319, Charlottesville, Virginia 22904-4319, United States § Department of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon, 406-772, South Korea S Supporting Information *

ABSTRACT: Laser-ablated vanadium, niobium, and tantalum atoms were reacted with CH2X2, CHX3, and CX4 (X = F and Cl) molecules in condensing argon, and the products were investigated by matrix isolation infrared spectroscopy. The major reaction products are new CH2−MX2, CHX−MX2, HC−MX3, and XC−MX3 complexes. These reactive species were identified by comparing their matrix infrared spectra with frequencies, intensities, and isotopic shifts from density functional theory calculations. Product structures and energies from these calculations are also presented. Results from previously studied Group 4 and 6 metal reaction products are compared. Little change is found in the calculated metal−carbon bond lengths in the early first row CH2MF2 methylidene σ2π2 series; however, the methylidyne complexes HC{}MF3 show considerable increase in bond strength for the nominally σ2π1π1(Ti), σ2π2π1(V), and σ2π2π2(Cr) carbon{}metal bonds left to right. The Group 5 HC{}MF3 complexes have only a plane of symmetry whereas the Group 4 and 6 analogues have 3-fold symmetry. or F atom-CF bond for the metal addition,10 and the matrix experiment will allow for the isolation and identification of the favored reaction’s products. The methylidene reaction products derived from CH4 and CH3X exhibit large agostic distortions in the structures computed by density functional theory.11−13 Analogous reactions with CH2X2 also form methylidene complexes, but no agostic bonding interaction was noted. This was suggested to be due to the halogen lone pair repulsions.7,8,10,11 Group 5 transition metals have been previously reacted with CH4 and CH3X precursors, and methyldene complexes with large agostic bonding interactions were formed along with the molecular anion CHMH3− and CHMH2X− complexes for M = Nb, Ta.14,15 Here, we now report on the reactions of Group 5 transition metals from the perspective of matrix infrared spectroscopy and density functional theory in order to identify functional groups in the reaction products from the increased and mixed halogen substituted CH 2 X2 , CHX 3, and CX 4 molecules for comparison with the previous investigations using CH4 and CH3X, as well as with the Group 4 and 6 reaction products.

1. INTRODUCTION Chlorofluorcarbons (CFC’s) are hazardous to our earth’s environment: these gases pose a threat to the protective ozone layer through ozone depleting reactions and to the climate as greenhouse gases. The intensity of CFC infrared absorptions and the unique susceptibility of the atmosphere in this region are two factors that contribute to the “super” greenhouse effect of CFC’s.1,2 The large intensity of these infrared absorptions is also a great asset to being able to characterize halocarbon reaction products. Activation of carbon−halogen bonds in CFC’s is of interest as a possible remediation for these hazardous chemicals.3 In addition, metal−carbon bonds are of fundamental importance to inorganic and organometallic chemistry.4 We have recently investigated the reactions of Group 4 and Group 6 transition metal atoms with several CHnXm (X = F and Cl; n + m = 4) halocarbon and chlorofluorcarbon families through matrix infrared spectroscopy and density functional theory calculations of vibrational frequencies, isotopic shifts, and energies of potential reaction products.5−15 The matrix isolation technique in known for allowing reactions to proceed through initial step(s) then trapping reaction intermediates for spectroscopic study in real time.5 Density functional theory calculations provide a means of confirming the assignments of frequencies to previously unknown and reactive new molecules, and thus these calculations add a higher level of confidence in the interpretation of the matrix experimental spectroscopic results.5,6 CFC’s offer reactions with two possible sites, Cl atom-CCl bond © XXXX American Chemical Society

Received: November 9, 2015 Revised: November 24, 2015

A

DOI: 10.1021/acs.jpca.5b10992 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

2. EXPERIMENTAL AND CALCULATIONAL METHODS Our experimental procedures and reagents have been described in detail previously.5,12−17 In brief, Group 5 metal atoms (V, Nb, and Ta) were produced via ablation by the fundamental of a Nd:YAG laser (1064 nm). These metal atoms were codeposited in a vacuum chamber with a dilute mixture (0.25−1%) of a CHxFyClz (x + y + z = 4) chlorofluorocarbon in argon onto a CsI window cooled to 10 K by a closed cycle helium refrigerator. Experiments were repeated with 13C and D substituted reagents17 to identify functional groups involved in the vibrational motion. After deposition for 1 h, infrared spectra were collected at 0.5 cm−1 resolution in the 400−4000 cm−1 spectral region using a Nicolet Magna 550 infrared spectrometer with an MCT type B detector cooled to 77 K with liquid nitrogen. The reaction products trapped in the Ar matrix were subjected to several annealing cycles and exposed to UV irradiation from the full arc of a mercury arc street lamp (Philips, 175 W) with the globe removed (λ > 220 nm) and using glass filters. Infrared spectra were collected after each procedure and the change in absorption peak intensities allowed for assigning a group of absorptions to a single reaction product. In most infrared spectra, traces of metal oxides and nitrides were detected. However, these absorptions are known from previous investigations,18−21 and they were not included in reaction product assignments. The laser ablation plume is responsible for photolysis products from the precursor molecules, and since these are common to experiments using different metals,7−11 they are not reported here. To strengthen the assignment of a group of absorptions to a specific product, theoretical calculations were performed using the Gaussian 03 program.22 In particular, the B3LYP hybrid density functional level of theory and approximation was utilized with the 6-311++G(2d,p) basis set for nonmetal atoms, and the SDD pseudopotential was employed for transition metal atoms (SDD uses the MDF10 method for V, MWB28 for Nb, and MWB60 for Ta).23−26 Some calculations were done with the larger 6-311++G(3df,3pd) Gaussian basis for all elements including vanadium. All energy values reported include zeropoint vibrational corrections, and vibrational frequencies were calculated analytically. Reaction energies of formation were computed with the 6-311++G(3df,3pd) Gaussian basis for all elements including V, but the SDD pseudopotential was required for Nb and Ta.26 Reported structures are true minima with all real vibrational frequencies. Calculations were performed for products with spin multiplicities of both 2 and 4.

Figure 1. Infrared spectra collected in the 550−800 cm−1 region after laser-ablated vanadium was reacted with CH2F2 diluted in Ar during condensation at 10 K (A), and the resulting matrix was annealed to 30 K (B) and subjected to λ > 220 nm photolysis (C). Experiments performed with 13CH2F2 (D) and CD2F2 (E) are shown in the upper traces (collected after full arc photolysis). Arrows denote product absorptions: bands slightly higher are due to matrix site splittings. Absorptions labeled P arise from the CH2F2 precursor sample.

the product (Table 1). More precisely, these vibrations arise from two separate V−F stretching modes and the product thus contains a VF2 structural unit. The third absorption at 692.6 cm−1 red-shifted to 686.6 cm−1 with 13CH2F2 and to 551.4 cm−1 with CD2F2 with an H/D ratio of 1.256. These larger red shifts indicate that this mode is a vibration of carbon and hydrogen atoms. The region and isotopic shifts are comparable for the CH2 wagging motion of the CH2 TiF2 species,7 which was observed at 695.4 cm−1 with a 6.0 cm−1 carbon-13 red shift, and 138.6 cm−1 deuterium red shift. Hence, this absorption at 692.6 cm−1 is assigned to a CH2 wagging motion of the analogous product molecule. The observation of a VF2 group and a CH2 wagging mode leads to identification of the product as the methylidene CH2 VF2. In order to verify this assignment, vibrational frequencies were calculated for several different VCH2F2 reaction product isomers. The lowest energy isomer is a doublet CH2VF2 structure at our level of theory, which is found to be 34 kcal/mol lower in energy than the quartet CH2F-VF insertion complex and 47 kcal/mol lower in energy than the doublet CH-VHF2 species. For this CH2VF2 complex, the three most intense bands are calculated at 685.9, 712.5, and 785.9 cm−1 with IR intensities of 114, 85, and 254 km/mol, respectively (Table 1), which are slightly higher and in good agreement with the observed experimental peaks at 670.6, 692.6, and 770.7 cm−1. The highest and lowest of these three vibrational motions are predicted to show small red shifts upon carbon-13 and deuterium isotopic substitution (0.3 and 1.1 cm−1 carbon-13 shifts, and 1.7 and 5.9 cm−1 deuterium shifts, respectively), which agrees well with very small shifts observed in experiments. Likewise, the transition computed at 712.5 cm−1 is predicted to show a 6.6 cm−1 carbon13 shift and a 148.6 cm−1 deuterium shift, H/D ratio 1.264, which are in good agreement with the observed 6.0 and 141.2 cm−1 red shifts when 13CH2F2 and CD2F2, respectively, are reacted. Hence, the major reaction product isolated when vanadium atoms react with CH2F2 is identified as the doublet CH2VF2 species. This is also in accord with previous experiments studying the reactions of vanadium atoms with CH3X (X = H, F, Cl, or Br),14,15 where the CH2VHX product was observed. In addition, titanium

3. RESULTS AND DISCUSSION 3.1. V + CH2F2. When vanadium atoms were reacted with CH2F2 in argon, several new product absorptions appeared in the 650−775 cm−1 region. These peaks at 670.6, 692.6, and 770.7 cm−1 marked by arrows increased in unison when the matrix was annealed to 30 K and upon exposure to UV irradiation (λ > 220 nm) (Figure 1A−C) indicating that they belong to a single reaction product. Slightly higher frequency matrix site satellites listed in Table 1 for each band exhibit the same behavior. Other absorptions are due to the CH2F2 precursor and vanadium oxide and nitride impurities.18,19 The absorption at 670.6 cm−1 red-shifted 2.1 and 3.3 cm−1, respectively, when 13CH2F2 and CD2F2 were used, and the strong peak at 770.7 cm−1 showed only 0.2 cm−1 red shifts with both substitutions. Hence, these absorptions come from vibrational modes involving predominantly vanadium and fluorine atoms in B


Article

The Journal of Physical Chemistry A Table 1. Observed and Calculated Fundamental Frequencies of CH2−VF2.a CH2VF2 approximate mode F2C umbrella VF2 scis. CVF bend CH2 twist CH2 rock V−C stretch V−F stretch CH2 wag V−F stretch CH2 scis. C−H stretch C−H stretch

obsd

670.6, 679.2 692.6, 694.6 770.7, 774.3

13

calcd (int.)

obsd

120.2 (19) 187.0 (10) 200.3 (0) 370.9 (0) 419.7 (2) 564.3 (0) 685.9 (114) 712.5 (85) 785.9 (254) 1314.6 (4) 3071.9 (1) 3173.6 (0)

668.5, 674.2 686.6, 684.6 770.5, 774.2

CH2VF2 calcd (int.) 119.7 (19) 186.6 (10) 198.2 (0) 370.9 (0) 415.3 (2) 551.2 (1) 684.8 (114) 705.9 (83) 785.6 (254) 1306.9 (4) 3066.6 (1) 3161.1 (0)

CD2VF2 obsd

calcd (int.)

667.3, 676.9 551.4, 653.0 770.5, 773.3

116.6 (18) 185.9 (10) 172.9 (0) 263.4 (0) 344.8 (1) 522.9 (2) 680.0 (108) 563.9 (60) 784.2 (251) 1005.4 (9) 2223.2 (1) 2357.1 (0)

a B3LYP//6-311++G(2d,p)/SDD level of theory used to calculate doublet species. All frequencies are in cm−1 and unscaled, and computed infrared intensities are in km/mol. The lower frequency bands are the stronger matrix sites marked by arrows in Figure 1.

reacts with CH2F2 to produce the analogous CH2TiF2 methylidene.7 The reaction of vanadium atoms with CH2F2 proceeds without activation energy as attested by substantial growth of the above absorptions as well as the matrix site bands upon annealing to 30 K, which is exothermic by 115 kcal/mol, reaction 1 as computed with the large all electron 6-311++G(3df,3pd) Gaussian basis sets. Reaction 1 probably proceeds through the insertion intermediate, which is 81 kcal/mol exothermic, and this energy activates the second F atom transfer to form the 34 kcal/mol more stable and lowest energy methylidene CH2−VF2. The 47 kcal/mol higher energy methylidyne CH-VHF2 is not attained in this reaction. V(4 F) + CH 2F2 → [CH 2F−VF]* → CH 2−VF2 ΔE = −115 kcal/mol

(1)

3.2. V + CH2Cl2. The vanadium reaction with methylene chloride produced three new absorptions at 485.0, 671.0, and 683.4 cm−1. These peaks grow slightly in unison on exposure to UV light passed through a pyrex filter (λ > 290 nm) and full arc photolysis (λ > 220 nm), but do not change on annealing (Figure 2). These absorptions are near the 503.0, 675.0, and 679.7 cm−1 values assigned to the CH2TiCl2 complex (compare Figure 2),8 and their isotopic shifts are similar. The absorption at 485.0 cm−1 red-shifted to 484.2 and to 483.7 cm−1 upon 13CH2Cl2 and CD2Cl2 substitution (Table 2). These small isotopic shifts indicate a vibration that mostly involves other atoms, and we are left with a V−Cl stretching motion. Additionally, this absorption is very close to the ν3-fundamental absorption of VCl2 in an argon matrix, which is a partially resolved 9/6/1 chlorine isotopic triplet at 481.0, 478.3, and 475.6 cm−1.27 This pattern is characteristic of a vibration involving two equivalent chlorine atoms. Expanded scale spectra of our 480 cm−1 region reveals a dominant triplet absorption at 485.0, 482.7, and 480.2 cm−1 also with 9/6/1 relative intensities. Notice that the Cl-35 to all Cl-37 shift in the VCl2 spectrum is 5.4 cm−1 while our spectrum exhibits a smaller 4.8 cm−1 shift because of motion by other atoms as indicated by the small C-13 and D shifts described above. The absorption at 671.0 cm−1 exhibited a larger carbon-13 shift (12.4 cm−1 to 658.6 cm−1) and a modest deuterium red shift (69.1 cm−1 to 601.9 cm−1). This is similar to the titanium reaction product with CH2Cl2, where an absorption at 679.7

Figure 2. (a) Infrared spectra collected in the 460−690 cm−1 region after laser-ablated vanadium was reacted with CH2Cl2 diluted in Ar during condensation at 10 K (A), and the resulting matrix was exposed to λ > 290 nm photolysis (B) and subjected to >220 nm photolysis (C). Experiments performed with 13CH2Cl2 (D) and CD2Cl2 (E) are shown in the upper traces (collected after full arc photolysis). Arrows denote product absorptions and absorptions denoted P arise from the CH2Cl2 precursor sample. (b) Infrared spectra collected in the 500−470 cm−1 region after laser-ablated vanadium was reacted with CH2Cl2 diluted in Ar (A), and the resulting matrix was subjected to λ > 220 nm photolysis (B), and annealed to 35 K and subjected to λ > 220 nm photolysis again (C). The weaker band at 489 cm−1 is a matrix site splitting of the strongest 485.0 cm−1 absorption.

C


Article

The Journal of Physical Chemistry A Table 2. Observed and Calculated Fundamental Frequencies of CH2−VCl2.a CH2VCl2 approximate mode Cl2C umbrella VCl2 scis. CVCl bend CH2 rock V−Cl stretch CH2 twist V−Cl stretch V−C stretch CH2 wag CH2 scis. C−H stretch C−H stretch

obsd

485.0 671.0 683.4

13 c

calcd (int.)

calcd (int.)

86.0 (19) 104.6 (2) 157.2 (1) 350.3 (19) 373.0 (25) 421.1 (0) 513.3 (129) 584.5 (36)b 690.8 (92) 1302.7 (3) 3060.0 (3) 3161.9 (0)

97.5 (10) 105.7 (5) 175.8 (3) 333.1 (19) 370.7 (28) 420.5 (0) 517.2 (174) 707.5 (73) 718.8 (72) 1344.7 03) 3093.8 (5) 3197.4 (0)

obsd

484.2 658.6 676.9

CH2VCl2 calcd (Int.) 85.6 (19) 104.3 (2) 154.5 (1) 348.0 (18) 371.6 (24) 421.1 (0) 512.0 (130) 571.6 (37) 684.7 (90) 1294.9 (3) 3054.8 (3) 3149.5 (0)

CD2VCl2 obsd

483.7 601.9 544.3

calcd (int.) 83.7 (19) 103.2 (2) 137.7 (1) 284.2 (8) 369.7 (22) 298.5 (0) 501.1 (139) 539.2 (34) 542.5 (63) 998.6 (7) 2214.2 (4) 2347.5 (0)

B3LYP//6-311++G(2d,p)/SDD level of theory used to calculate doublet species. All frequencies are unscaled and in cm−1, and computed infrared intensities are in km/mol. bThe computed V−C stretch is 589.2 cm−1 using all electron 6-311++G(2d,p), 590.1 cm−1 using all electron 6-311+ +G(3df,3pd). cCCSD//311++G(2d,p)/SDD calculation.

a

cm−1 showed a 10.1 cm−1 carbon-13 red shift, and a 71.9 cm−1 red-shift with CD2Cl2.8 In that study, the absorption was assigned to the C−Ti stretching mode of the CH2TiCl2 molecule. Hence, the 671.0 cm−1 absorption is believed to be due to a mostly C−V stretching mode; however, it is mixed with the H− C−H bending (scissors) mode, which provides the deuterium shift. The last absorption at 683.4 cm−1 exhibits a small carbon-13 red-shift to 676.9 cm−1, and a large deuterium red-shift to 544.3 cm−1 (H/D ratio 1.256). These are close to observations for the CH2 wagging motion of CH2−VF2 (Table 1), and we assign these absorptions to a CH2 wagging motion. The observed vibrational modes are thus assigned to V−Cl2, C−V, and CH2 motions. These observed functional groups support identification of the reaction product as the CH2−VCl2 complex. B3LYP computations for the doublet CH2−VCl2 molecule predict IR absorptions and isotopic shifts that fit well with the observed V−Cl2 and CH2 absorptions. However, we observe the mostly V−C stretching mode at 671.0 cm−1, 86.5 cm−1 higher than predicted using B3LYP (Table 2, 584.5 cm−1), which is not the expected relationship between calculated and observed frequencies. Nevertheless, this doublet CH2−VCl2 species is the lowest energy structural isomer, which is 12.1 kcal/mol lower in energy than the quartet CH2Cl−VCl insertion intermediate, 9.9 kcal/mol lower in energy than the quartet CH2−VCl2 and 45.0 kcal/mol lower in energy than the doublet HC−VHCl2 methylidyne complex. Hence, we assign the observed absorptions to the doublet CH2−VCl2 complex, and seek an explanation for the poor agreement between calculated and observed V−C stretching modes, which will be discussed at the end of Supporting Information.

VHFCl species. Lastly, the CH2−VFCl methylidene is predicted to have two strong IR absorptions at 701.5 cm−1 (99 km/mol) and 726.5 cm−1 (199 km/mol) corresponding to CH2 wagging and V−F stretching modes, respectively, and a weaker absorption at 450.2 cm−1 (28 km/mol) for the C−Cl stretching mode. This is in reasonable agreement with what is observed, and it follows that the reaction product is the methylidene CH2−VFCl. Notice that the calculated uncoupled V−F and V−Cl stretching frequencies for CH2−VFCl are near the median values for the symmetric and antisymmetric VF2 and VCl2 stretching modes for CH2−VF2 and CH2−VCl2 (Tables 1 and 2). 3.4. V + CH2Br2. The analogous reaction for vanadium and methylene bromide produces one new absorption at 669.1 cm−1, which shifts to 536.1 cm−1 with the deuterium substituted reagent. These bands increase 20% on λ > 420 nm photolysis and 100% on UV photolysis 240−380 nm. The H/D ratio 1.248 is characteristic of an out-of-plane CH2 deformation mode, which has been described above at 683.4 cm−1 for CH2−VCl2, at 680.4 cm−1 for CH2−VFCl and at 692.6 cm−1 for CH2−VF2. Accordingly, we assign the new feature at 669.1 cm−1 to the methylidene product CH2−VBr2. B3LYP calculation gives this CH2 mode as 694.0 (69) and the CD2 counterpart as 547.4 cm−1 (46 km/mol) with a 1.268 ratio, which are in good agreement with the observed values. Notice that the CH2 wagging mode decreases with increasing halogen size, and also decreasing C−V bond length from 1.837 to 1.824 to 1.823 Å. Here it appears that the least electronegative halogen withdraws less electron density from the V center, and thus allows a stronger C−V bond and increases interaction between the CH2 wagging mode and the V center thus slightly damping the carbon motion and decreasing the CH2 wagging frequency. The natural bond orders of CH2VX2 are 1.94, 1.88, and 1.92 for F, Cl, and Br. Reaction 3 is less exothermic than found for the lighter halogens.

V(4 F) + CH 2Cl 2 → [CH 2Cl−VCl]* → CH 2−VCl 2 ΔE = −112 kcal/mol

(2)

V(4 F) + CH 2Br2 → CH 2−VBr2

3.3. V + CH2FCl. When CH2FCl is reacted with vanadium atoms, three absorptions are observed at 441.0, 680.4, and 727.0 cm−1, which increase slightly on UV photolysis. The lowest energy product isomer from our calculations is a doublet CH2− VFCl species, similar to that found for CH2F2 and CH2Cl2. This isomer is 16 kcal/mol lower in energy than the quartet CH2ClVF species, 29 kcal/mol lower in energy than the CH2F-VCl complex, and 43 kcal/mol lower in energy than the doublet HC−

ΔE = − 110 kcal/mol (3)

3.5. V + CHF3. Laser-ablated vanadium atoms react with CHF3 to form a product with three IR absorptions at 642.2, 675.9, and 782.2 cm−1. None of these absorptions show more than a 3 cm−1 deuterium shift, so they must come from vibrational modes that do not significantly displace hydrogen in the product molecule. This is also the region of the spectrum D


Article

The Journal of Physical Chemistry A where V−F stretching modes are likely to be observed.15 Hence, the reaction product probably contains three V−F bonds. Previous investigations of the titanium reaction with CHF3 led to a HC−TiF3 reaction product with 3-fold symmetry and two distinct Ti−F stretching modes of A1 and E symmetry.7 Computations on the analogous doublet HC−VF3 species converged to a stable structure, which is predicted to have three distinct intense IR vibrational modes at 682.3, 721.2, and 797.0 cm−1 (Table 3) because its symmetry is lower than 3-fold (see Table 3. Observed and Calculated Fundamental Frequencies of HC−VF3.a HC−VF3 approximate mode FVF bend FVF bend C−H wag C−H wag VF3 umbrella HCV bend HCV bend C−V stretch V−F stretch V−F stretch V−F stretch C−H stretch

obsd

642.2 675.9 782.2

calcd (int.) 30.2 (33) 203.7 (5) 230.5 (5) 236.8 (4) 247.9 (8) 453.6 (70) 500.2 (20) 625.5 (1) 682.3 (256) 721.2 (119) 797.0 (226) 3187.8 (23)

DC−VF3 obsd

640.1 673.5 781.0

calcd (int.) 30.2 (33) 203.3 (7) 206.0 (4) 212.7 (2) 240.6 (11) 377.5 (36) 412.7 (9) 607.7 (1) 682.1 (255) 718.9 (118) 796.7 (222) 2348.8 (16)

Figure 3. Infrared spectra collected in the 470−590 cm−1 region after laser-ablated vanadium was reacted with CHCl3 diluted in Ar during condensation at 10 K (A), and the resulting matrix was exposed to λ > 220 nm photolysis (B) and annealed to 30 K (C). Experiments performed with 13CHCl3 are shown in the upper traces collected after deposition (D) and full arc photolysis (E). Arrows denote product absorptions and absorptions denoted P arise from the CHCl3 precursor sample.

bands in Figure 2b for chlorine isotopic splitting from a VCl2 vibration, and the 480.0 cm−1 band for C-13 exhibits similar 477.3 and 474.6 cm−1 counterparts. Additional experiments were done with lower laser energy, and the band intensities were lower, but the intensity of the sharper band at 526.1 cm−1 was higher relative to the 889.6, 581.5, and 480.5 cm−1 bands, which doubled on visible (λ > 420 nm) photolysis, and UV irradiation produced the 526.1 cm−1 band and a weaker associated 594.7 cm−1 peak. Another experiment with CDCl3 produced a 484.5 cm−1 band on visible photolysis and a stronger 471.4 cm−1 band on UV irradiation. The latter information allows grouping of the 484.5 cm−1 band with the 480.5, 581.5, and 889.6 cm−1 bands, and the 471.4 cm−1 absorption with the 526.1 and 594.7 cm−1 peaks. Computations on various VCHCl3 structural isomers in doublet and quartet spin states identified the doublet HC− VCl3 species as the lowest energy isomer, only 2.5 kcal/mol lower than in the doublet CHCl−VCl2 and 2.9 kcal/mol lower than the quartet, all using the large Gaussian basis sets. This doublet CHCl-VCl2 isomer is predicted to have its most intense peak at 513.3 cm−1 (1.3 cm−1 carbon-13 red shift), with two strong absorptions at 553.5 cm−1 (6.5 cm−1 carbon-13 red shift) and 856.2 cm−1 (23.2 cm−1 carbon-13 red shift). The 889.6 cm−1 C− Cl stretching mode is computed 33.4 cm−1 lower than the observation, but the C-13 shifts agree quite well. The strongest band for the CDCl-VCl2 isotope calculated at 511.0 cm−1 is observed here at 484.5 cm−1. Although this intermediate is difficult to model by calculations, in part due to mode mixing and interaction between the Cl−C and CV bonds, these results (Table 4) are compatible with the observed bands at 480.5 cm−1 (0.3 cm−1 carbon-13 red shift), 581.5 cm−1 (6.2 cm−1 carbon-13 red shift), and 889.6 cm−1 (24.0 cm−1 carbon-13 red shift). In addition, the 513.3 cm−1 calculation for the antisymmetric VCl2 stretching mode has a 5.1 cm−1 all Cl-37 shift, and the first splitting for the V35Cl37Cl isotope is 2.7 cm−1 below the 480.5 cm−1 observation for the major V35Cl2 band as expected. Hence, we identify the reaction product observed in experiments as the CHCl−VCl2 complex.

a

B3LYP//6-311++G(2d,p)/SDD level of theory used to calculate doublet species. All frequencies are in cm−1, and computed infrared intensities are in km/mol.

Figure S1), in contrast to HC−TiF3. These three frequencies correspond to the three V−F stretching modes, and all three are predicted to have less than a 3 cm−1 red-shift when hydrogen is replaced by deuterium. Also, these three predicted frequencies are uniformly slightly above the observed frequencies. This doublet HC−VF3 complex is the lowest energy structural isomer found theoretically. It is 15 kcal/mol lower in energy than the quartet HC−VF3 and 8 kcal/mol lower than a possible quartet CHF−VF2 methylidene (the quartet CHF−VF2 species is 0.5 kcal/mol lower in energy than the doublet methylidene). Hence, we identify the reaction product between laser-ablated vanadium atoms and CHF3 as the doublet HC−VF3 complex where the H− C−V angle is a few degrees less than 180 and the molecule has less than 3-fold symmetry owing to asymmetry in the C−V bond. 3.6. V + CHCl3. Chloroform is reacted with vanadium atoms to give four new product absorptions at 480.5, 526, 581.5, and 889.6 cm−1 (Figure 3). These absorptions change little upon photolysis or annealing. The highest frequency new absorption at 889.6 cm−1, on the side of the CCl3 radical antisymmetric C−Cl stretching band28 at 898.0 cm−1 (a common band to laser ablated metal experiments, made here by laser plume photodissociation of CHCl3 during sample deposition), shows a 24.0 cm−1 red shift to 865.6 cm−1 when carbon-13 substituted chloroform is used (Table 4), and the 13CCl3 radical band shifts 29.0 cm−1 to 869.0 cm−1.28 The absorptions at 581.5, 526, and 480.5 cm−1 red-shift much less to 575.3, 520, 480.0 cm−1, respectively, when 13CHCl3 was employed. The large red-shift for the 889.6 cm−1 band is appropriate for a C−Cl stretching mode, as shown by comparison with CCl3, and the very small red-shift of the 480.5 cm−1 band suggests a V−Cl stretching mode. Importantly, the 480.5 cm−1 band exhibits 477.8 and 475.1 cm−1 components, which reminds of the 9/6/1 natural chlorine isotopic triplet of E


Article

The Journal of Physical Chemistry A Table 4. Observed and Calculated Fundamental Frequencies of CHCl-VCl2.a 13

CHClVCl2 approximate mode V−Cl2 stretch V−C stretch V−Cl2 stretch C−H out of plane C−Cl stretch HCCl bend C−H stretch

obsd

calcd (int.)

480.5 581.5 889.6

361.6 (15) 460.0 (102) 513.3 (118) 553.5 (44) 856.2 (43) 1133.0 (13) 3095.5 (15)

CHClVCl2

obsd

calcd (int.)

480.0 575.3 865.6

358.8 (14) 455.5 (100) 512.0 (122) 547.0 (43) 833.0 (42) 1124.4 (11) 3086.4 (14)

CDClVCl2 obsd

calcd (int.)

484.5

354.1 (10) 443.2 (96) 450.0 (30) 511.0 (127) 770.9 (20) 941.5 (36) 2273.4(12)

B3LYP//6-311++G(3df,3pd) level of theory used to calculate doublet species. All frequencies are in cm−1, and computed infrared intensities are in km/mol.

a

The most intense IR band computed for HC−VCl3 in our region of detection is at 549.1 cm−1 (107 km/mol) with a 5.8 cm−1 C-13 shift, and this is the best assignment for the 526.1 cm−1 band with a 6 cm−1 C-13 shift. The strongest IR band at 443.0 cm−1 (218 km/mol) has no C-13 shift, but this absorption is expected to fall below our limit of observation. The out of plane C−H deformation mode calculated at 607.3 cm−1 is observed at 594.7 cm−1. The strongest two bands calculated for DC−VCl3 in our range are 507.6 cm−1 (99 km/mol) and 475.3 cm−1 (149 km/ mol our 471.4 cm−1 band is probably due to the former D-C deformation mode. The chloroform products arise from slightly more exothermic reactions than the methylene chloride product, and they probably start with the C−Cl bond insertion intermediate CHCl2−VCl. V(4 F) + CHCl3 → CHCl−VCl 2 ΔE = −116.2 kcal/mol

V(4 F) + CHCl3 → HC−VCl3

Figure 4. Infrared spectra collected in the 1495−1490 and 700−670 cm−1 regions after laser-ablated vanadium was reacted with CF4 diluted in Ar (A), and the resulting matrix was exposed to λ > 290 nm photolysis (B), λ > 220 nm photolysis (C), annealed to 30 K (D), and exposed to λ > 220 nm photolysis again (E). Arrows denote product absorptions.

(4)

ΔE = −118.7 kcal/mol (5)

energy than the C−F stretching mode in the CF4 precursor near 1280 cm−1. For example, this mode was observed at 1453.1 cm−1 in experiments with titanium.9 Here, it is observed 39.8 cm−1 above its titanium counterpart. The absorptions at 680.9 and 690.9 cm−1 belong to V−F stretching modes. Calculations predict doublet FC−VF3 to be the lowest energy structural isomer, 21 kcal/mol below the unobserved doublet CF2−VF2. The reaction to produce FC−VF3 is exothermic by 104 kcal/mol using the large all electron basis sets.

3.7. V + CF4. Laser ablated vanadium atoms react with CF4 to produce a reaction product with three infrared absorptions at 680.9, 690.9, and 1492.9 cm−1 (Table 5 and Figure 4) which increase together on UV photolysis. The absorption at 1492.9 cm−1 is the trademark of the high C−F stretching modes of F− C−MF3 complexes,10 which appear at considerably higher Table 5. Observed and Calculated Fundamental Frequencies of FC−VF3

V(4 F) + CF4 → FC−VF3

FC−VF3 approximate mode FVF bend C−F wag C−F wag FVF bend VF3 umbrella FCV bend FCV bend C−V stretch V−F stretch V−F stretch V−F stretch C−F stretch

obsd

calcd (int.)a

calcd (int.)b

680.9 690.9 1492.9

37.8 (32) 99.0 (1) 105.6 (0) 188.6 (7) 210.6 (6) 323.3 (8) 348.4 (0) 464.0 (18) 696.8 (152) 708.6 (262) 742.9 (222) 1490.5 (387)

56.8 (30) 110.3 (0) 112.0 (0) 189.2 (6) 208.9 (6) 348.4 (7) 361.8 (0) 468.0 (20) 687.1 (144) 710.7 (279) 725.4 (190) 1506.6 (379)

ΔE = − 104 kcal/mol

(6)

3.8. V + CCl4. Laser-ablated vanadium atoms react with carbon tetrachloride to produce new absorptions at 1177.5, 480.1, and 477.5 cm−1, which are marked by arrows in Figure 5. The highest absorption contains a splitting at 1174.2 cm−1 with about 1/3 of the intensity of the major band. These absorptions increase together when the matrix sample is subjected to photolysis and on annealing to 30 K, hence they are due to a single reaction product. The increase on annealing indicates that reaction 7 proceeds without activation energy. When 13CCl4 was employed, these absorptions shifted to 1138.6, 1135.3, 480.0, and 477.1 cm−1, respectively, which indicates that the high frequency vibration includes considerable carbon character. Indeed, absorptions in this region are diagnostic for the C−Cl stretching motion of a ClC−MCl3 complex.10 The resolved chlorine isotopic splitting on the major bands with 1/3 of the major peak is appropriate for 35Cl and 37Cl in natural abundance, and this splitting indicates that a single Cl atom is involved in that vibrational mode. The two low frequency

a

B3LYP//6-311++G(2d,p)/SDD level of theory used to calculate doublet species. bB3LYP//6-311++G(3df,3pd) all electron basis used to calculate doublet species. All frequencies are unscaled and in cm−1, and computed infrared intensities are in km/mol. F


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intense E mode, and these two are observed here at 480.1 and 477.5 cm−1 while the weaker symmetric mode computed at 480.7 cm−1 is observed at 485 cm−1. The two low frequency absorptions are predicted to red-shift by less than 1 cm−1 when carbon-13 is employed in the calculations, which also agrees with the small shifts observed. The SDD/6-311++G(2d,p) basis predicted the strongest IR absorption at 1065.1 cm−1 with a redshift of 34.8 cm−1 or 3.27% upon carbon-13 substitution, which is close to the 38.9 cm−1 or 3.30% observed red-shift, but the computed frequency is way (112 cm−1) too low. This discrepancy appears to be due to the SDD pseudopotential for vanadium as we have also found this difference for other XC− VX3 species. And, the all electron 6-311++G(3df,3pd) set yielded a higher 1162.7 cm−1 value for this frequency in satisfactory agreement with the 1177.5 cm−1 observed value. Hence, the main reaction product that is formed when vanadium reacts with CCl4 is identified as the doublet ClC−VCl3 complex. It is interesting to note that the 38.9 cm−1 carbon-13 shift observed for the 1177.5 cm−1 absorption exceeds that computed for a simple diatomic C−Cl vibration, which is 34.3 cm−1 (2.91%) from 1177.5 cm−1. Thus, this carbon atom vibration is an antisymmetric stretching mode between the heavier Cl and V atoms, which gives more carbon-13 shift than a CCl diatomic molecule. This is supported by the ClC−VCl3 calculation, which predicts a 34.8/1162.7 or 3.35% shift, and we observe a 38.9/ 1177.5 × 100 = 3.30% shift. On the other hand, the 37Cl shift observed for the 1177.5 cm−1 mode is 3.3 cm−1 or 0.28%, which is correspondingly less than computed for the diatomic molecule model, 8.1 cm−1 or 0.68%. Table S1 (Supporting Information) shows that the 37Cl shift calculated for the antisymmetric Cl−C− V stretching mode is 3.2 cm−1 or 0.28%. The ClC−VCl3 complex is produced in a highly exothermic reaction as calculated using the large all electron Gaussian basis. The different behavior of the two matrix site absorptions (Table 6) suggests that there are two different mechanisms of formation for this product: the first is reaction of excited V atoms on laser ablation deposition or UV photolysis and the second is the reaction of cold V atoms on sample annealing. These two different mechanisms have different energetics and result in different packing patterns of the argon matrix atoms around the final ClC−VCl3 product. Figure S2 shows the relative energies of

Figure 5. Infrared spectra collected in the 1185−1120 and 490−470 cm−1 region after laser-ablated vanadium was reacted with CCl4 diluted in Ar during condensation at 10 K (A), and the resulting matrix was exposed to λ > 290 nm photolysis (B), λ > 220 nm photolysis (C), and annealed to 30 K (D). Experiments performed with 13CCl4 are shown in the upper traces collected after deposition (E) and annealing to 30 K (F). Arrows denote product absorptions.

absorptions that shift slightly upon carbon-13 substitution come from the two strongest antisymmetric V−Cl stretching motions of this complex, which arise from removal of the 3-fold symmetry broken by asymmetry in the C−V bond. The weaker 485 cm−1 band is due to the weaker symmetric V−Cl3 stretching mode. Calculations performed on a variety of VCCl4 structural isomers found the doublet ClC−VCl3 methylidyne as the lowest energy isomer. Four strong and diagnostic IR absorptions predicted for this ClC−VCl3 species at 1162.7(336), 480.7(30), 465.8(197), and 461.9 cm−1 (125 km/mol) using the large 6311++G(3df,3pd) all electron basis set are mostly just below the observed values, which is not the expected deviation from observed frequencies7−11 (Table 6). This calculation of three distinct C−Cl stretching modes shows that this product has lower than 3-fold symmetry. Note the 3 cm−1 separation between the two lower and strongest V−Cl stretching modes, which are due to symmetry breaking of what would have been a very

Table 6. Observed and Calculated Fundamental Frequencies of ClC−VCl3 Cl13C−VCl3

ClC−VCl3 approximate mode C−Cl wag C−Cl wag ClVCl bend ClVCl bend VCl3 umbrella ClCV bend ClCV bend C−V stretch V−Cl stretch V−Cl stretch V−Cl stretch C−Cl stretch

obsd

calcd (int.)a

calcd (int.)b

477.5 480.1 1177.5c

40.7 (14) 57.1 (1) 87.2 (5) 105.5 (0) 127.6 (1) 292.0 (45) 309.4 (5) 328.5 (23) 445.9 (55) 459.0 (123) 465.3 (159) 1065.1 (447)

40.7 (19) 63.1 (1) 83.8 (5) 107.4 (0) 128.6 (0) 292.9 (65) 327.6 (1) 352.1 (17) 461.9 (125) 465.8 (197) 480.7 (30) 1162.7 (336)d

obsd

calcd (int.)a

477.1 480.0 1138.6c

40.7 (14) 57.0 (1) 86.9 (5) 105.4 (0) 127.3 (1) 283.1 (39) 299.2 (5) 328.0 (23) 445.8 (55) 458.4 (134) 464.4 (152) 1030.3 (415)

a

B3LYP//6-311++G(2d,p)/SDD level of theory used to calculate doublet species. bB3LYP//6-311++G(3df,3pd) all electron basis used to calculate doublet species. All frequencies are in cm−1, and computed infrared intensities are in km/mol. cAdditional matrix sites are observed at 1166.6 and 1128.1 cm−1 for this vibrational mode of ClC−VCl3 and Cl13C−VCl3, respectively. dThe four highest frequencies calculated with the BPW91 functional are 1189.3, 476.1, 462.3, 456.9 cm−1. G


Article

The Journal of Physical Chemistry A possible products and suggests that the insertion product, CCl3− V-Cl, and methylidene complex, CCl2−VCl2, are intermediates in the reaction pathway to the lowest energy methylidyne product observed here. We do not observe VCl2 or CCl2 in these experiments, which shows that the methylidene intermediate does not decompose but rearranges to the more stable (lower energy) ClC−VCl3 final methylidyne product.

considerable carbon motion, whereas the low frequency absorption shows only 0.5 cm−1 red-shift. Again, this high frequency absorption is close to the C−F stretching mode of FC−VF3 at 1492.9 cm−1 and the C−F stretching mode of FC− VF2Cl at 1498.7 cm−1, which supports assignment of the above bands to FC−VFCl2. Here, a second set of absorptions at 775.7, 692.0, and 474.1 cm−1 doubles in intensity, when the matrix is subjected to radiation with λ > 290 nm, and doubles again upon full arc photolysis with λ > 220 nm. These bands red-shift to 774.9, 691.8, and 473.9 cm−1, respectively, when 13CF2Cl2 is employed, which demonstrate very little carbon character. Calculations for the vibrational motions of doublet FC−VFCl2 fit the first set of absorptions. The C−F mode is predicted at 1526.9 cm−1 to show a 43.8 cm−1 carbon-13 red-shift, and the observed 1493.9 cm−1 band is red-shifted 41.8 cm−1 when 13 CF2Cl2 is reacted (Table 8). The V−F stretching mode is

V(4 F) + CCl4 → [CCl3−VCl−>CCl 2−VCl 2]* → ClC−VCl3

ΔE = − 140 kcal/mol

(7)

3.9. V + CF3Cl. When the chlorofluorocarbon CF3Cl is reacted with laser-ablated vanadium atoms, three absorptions are observed at 1498.7, 687.1, and 669.1 cm−1 upon sample deposition. They increase slightly when the matrix is subjected to λ > 290 nm radiation, increase by 25% when exposed to λ > 220 nm light, increase slightly again when annealed to 30 K, and remain unchanged on subsequent photolysis. The trademark high 1498.7 cm−1 band suggests that the product may be the FC−VF2Cl species. Indeed, this absorption is close to the F−C stretching mode, 1492.9 cm−1, of the previously assigned FC− VF3 complex. The predicted infrared absorption for the doublet FC−VF2Cl isomer agrees well with these three observed peaks (Table 7). Although the possible ClC−VF3 species is predicted

Table 8. Observed and Calculated Fundamental Frequencies of FC−VFCl2.a approximate mode C−F wag FVCl bend C−F wag ClVCl bend VFCl2 umbrella FCV bend FCV bend V−Cl stretch V−Cl stretch C−V stretch V−F stretch C−F stretch

Table 7. Observed and Calculated Fundamental Frequencies of FC−VF2Cl FC−VF2Cl approximate mode C−F wag C−F wag FVF bend FVCl bend VF2Cl umbrella FCV bend FCV bend V−Cl stretch C−V stretch V−F stretch V−F stretch C−F stretch

obsd

calcd (int.).a

calcd (int.).b

669.1 687.1 1498.7

84.9 (1) 111.1 (0) 134.1 (5) 180.4 (2) 205.8 (6) 347.2 (0) 356.6 (4) 423.2 (66) 506.5 (62) 712.0 (169) 756.4 (245) 1506.1 (443)

87.5 (1) 112.5 (1) 132.3 (5) 177.4 (3) 201.3 (6) 352.7 (1) 364.3 (3) 431.6 (72) 542.7 (53) 701.0 (166) 745.0 (242) 1512.4 (461)

F13C−VFCl2

FC−VFCl2 obsd

calcd (int.)

737.8 1493.9

72.7 (8) 90.6 (10) 96.9 (3) 124.4 (0) 169.5 (2) 346.2 (13) 357.9 (3) 407.7 (24) 439.0 (192) 577.4 (42) 753.8 (176) 1526.9 (575)

obsd

calcd (int.)

737.3 1452.1

72.6 (8) 90.5 (10) 96.8 (3) 124.3 (0) 169.0 (2) 336.5 (9) 347.0 (2) 406.4 (24) 437.6 (196) 575.4 (41) 753.7 (176) 1483.1 (546)

a


predicted at 753.8 cm−1 with large IR intensity and a small 0.1 cm−1 carbon-13 red-shift. The observed absorption is at 737.8 cm−1 with a 0.5 cm−1 13C red-shift. These two absorptions have the largest IR intensity in the region of our FTIR. Hence, the first set of absorptions is assigned to the FC−VFCl2 methylidyne complex. The doublet ClC−VF2Cl species is predicted at our level of theory to be 7.7 kcal/mol lower in energy than the assigned FC− VFCl2 complex. This ClC−VF2Cl species is calculated to have vibrational modes at 1111.7, 762.0, 713.1, and 453.3 cm−1 (Table 9). The later three are predicted to have very little 13C shift, and to fit well with the second set of observed absorptions. This behavior is similar to analogous experiments with titanium, where the FC−TiFCl2 complex absorptions decreased on photolysis, while the absorptions for the ClC−TiF2Cl species increased in intensity.9 Hence, these three observed IR peaks are assigned to the ClC−VF2Cl complex. The fourth, predicted at 1111.7 cm−1 is most likely hidden by precursor absorptions in the 1060−1170 cm−1 region of the spectrum. This is also similar to the ClC−TiF2Cl complex, where the C−Cl stretching mode was covered by precursor absorptions. The 7 and 17 kcal/mol higher energy unobserved methylidenes, CF2−VCl2 and CCl2−VF2, respectively, are probably intermediates in the final exothermic reactions.

a

B3LYP//6-311++G(2d,p)/SDD level of theory and. bB3LYP//6311++G(2d,p) all electron used to calculate doublet species. All frequencies are unscaled and in cm−1, and computed infrared intensities are in km/mol.

to be lower in energy (7 kcal/mol), it is not observed in these experiments. This is in contrast to reactions with Ti, where the FC−TiF2Cl complex rearranged into ClC−TiF3 upon UV photolysis.11 Although IR absorptions of the FC−VF2Cl complex increase in intensity when the matrix is exposed to UV radiation, this is not appropriate energy to activate the rearrangement to ClC−VF3. 3.10. V + CF2Cl2. Laser-ablated vanadium atoms react with CF2Cl2 [Freon-12, CFC-12, a common refrigerant and propellant from the last century] to form a new product with IR absorptions at 1493.9 and 737.8 cm−1. These two absorptions appear on deposition, triple in intensity when the matrix sample is subjected to photolysis with λ > 290 nm, but decrease by 30% in intensity when exposed to full arc photolysis (λ > 220 nm). The high frequency absorption shifts to 1452.1 cm−1 or 2.80% when 13CF2Cl2 is employed indicating a vibration involving H


Article


which again performs better than the smaller set and SDD computed frequencies at 1085.3, 749.2, and 444.9 cm−1 corresponding to the C−Cl, V−F, and V−Cl stretching modes, respectively, of the ClC−VFCl2 species (Table 10).

Table 9. Observed and Calculated Fundamental Frequencies of ClC−VF2Cl.a ClC−VF2Cl approximate mode C−Cl wag C−Cl wag FVCl bend VF2Cl umbrella FVF bend ClCV bend ClCV bend C−V stretch V−Cl stretch V−F stretch V−F stretch C−Cl stretch

obsd

474.1 692.0 775.7 covered

calcd (int.)

Cl13C−VF2Cl obsd

calcd (int.)

64.5 (1) 86.5 (0) 139.6 (4) 182.4 (1)

64.4 (1) 86.0 (0) 139.6 (4) 181.9 (1)

197.5 (5) 310.9 (6) 317.9 (0) 375.6 (18) 453.3 (127) 713.1 (194) 762.0 (238) 1111.7 (73)

197.3 (5) 301.4 (6) 308.8 (0) 375.3 (18) 453.1 (126) 713.0 (194) 762.0 (238) 1075.0 (67)

473.9 691.8 774.9 covered

Table 10. Observed and Calculated Fundamental Frequencies of ClC−VFCl2.a ClC−VFCl2 approximate mode C−Cl wag C−Cl wag FVCl bend ClVCl bend VFCl2 umbrella FCV bend FCV bend C−V stretch V−Cl stretch V−Cl stretch V−F stretch C−Cl stretch

a


obsd

444.6 733.1 1185.3

calcd (int.)a

calcd (int.)b

58.7 (2) 78.3 (1) 99.5 (13) 125.0 (1) 167.6 (1) 302.3 (2) 323.5 (2) 362.5 (15) 444.9 (194) 462.0 (46) 749.2 (211) 1085.3 (328)

326.8(0) 389.4(12) 466.7(197) 482.6(77) 747.2(178) 1209.0(131)

a

V(4 F) + CF2Cl 2 → [CF2−VCl 2]* → FC−VFCl 2 ΔE = −114 kcal/mol

(8)

V(4 F) + CF2Cl 2 → [CF2−VCl 2]* → ClC−VF2Cl ΔE= −122 kcal/mol

(9)

B3LYP//6-311++G(2d,p)/SDD level of theory used to calculate doublet species. bB3LYP//6-311++G(3df,3pd) all electron basis. All frequencies are unscaled and in cm−1, and computed infrared intensities are in km/mol.

Reaction energies were computed with the large all electron basis sets for both products, and these reactions are highly exothermic, which supports their high product yields observed in these experiments.

3.11. V + CFCl3. Vanadium atoms also reacted with CFCl3 to produce two sets of absorptions that behaved differently. One absorption at 1483.4 cm−1 increases by approximately 30% when the matrix is subjected to photolysis with λ > 290 nm, increases by an additional 30% when the matrix is exposed to light with λ > 220 nm, increases slightly when the matrix is annealed to 30 K, and decreases slightly when the matrix is exposed again to light with λ > 220 nm. Three absorptions at 1185.3, 733.1, and 444.6 cm−1 increase by 30% on photolysis with λ > 290 nm, increase by an additional 50% when the matrix is exposed to light with λ > 220 nm, increases only slightly when the matrix is annealed to 30 K, and increases in intensity slightly when the matrix is exposed again to light with λ > 220 nm. The first absorption at 1483.4 cm−1 is close in frequency to the diagnostic high C−F stretching mode assigned to FC−VF3 (1492.9 cm−1), FC−VF2Cl (1498.7 cm−1), and FC−VFCl2 (1493.9 cm−1). Hence, it most likely comes from the FC−VCl3 species. Indeed, a calculation on this complex predicts the C−F stretching mode at 1467.9 cm−1; however, the larger 6-311+ +G(3df,3pd) basis for all atoms predicts this mode at 1512.7 cm−1 with a huge 714 km/mol intensity. We expect the computed frequency to be slightly higher than the observed value. Additionally, this mode is predicted to be more than four times more intense than any other vibrational mode, and the strong absorption at 1483.4 cm−1 is assigned to the FC−VCl3 complex. Of the three other observed absorptions, the high frequency absorption at 1185.3 cm−1 is near the C−Cl stretching mode of ClC−VCl3 observed at 1177.5 cm−1. Calculations predict the doublet ClC−VFCl2 to be 7 kcal/mol lower in energy than the above FC−VCl3 structural isomer. The three observed absorptions at 1185.3, 733.1, and 444.6 cm−1 match well with the three most intense bands computed at 1209.0 cm−1(131 km/ mol), 747.2 (178) and 466.7 (197) using the all electron basis,

V(4 F) + CFCl3 → FC−VCl3

ΔE = − 129 kcal/mol (10)

4

V( F) + CFCl3 → ClC−VFCl 2

ΔE = − 136 kcal/mol (11)

3.12. Nb and Ta + CH2F2. In order to make comparisons within the Group 5 family, similar experiments were carried out with laser-ablated niobium and tantalum. Niobium atoms react with CH2F2 to form a product with absorptions at 711.9 and 685.1 cm−1 which double in intensity when the matrix is subjected to photolysis with λ > 290 nm and increase slightly when exposed to λ > 220 nm light. The 711.9 cm−1 band shifts to 705.5 cm−1 with 13CH2F2 and to 562.6 cm−1 with CD2F2 (ratio 1.265). The 685.1 cm−1 absorption red-shifts to 684.9 and 684.6 cm−1 with 13CH2F2 and CD2F2, respectively. These are in the range of, and show appropriate isotope shifts for, CH2 wagging and Nb−F stretching modes. Indeed, calculations predicting the properties of a doublet CH2−NbF2 molecule, analogous to that formed with vanadium, indicate that these two modes are the most intense. These modes are predicted at 760.8 and 633.2 cm−1 and red-shift to 753.9 and 632.9 cm−1 with 13CH2F2 and to 597.8 and 630.4 cm−1 with CD2F2, respectively, which are in agreement with the experiment (Table S2). Hence, we identify the major reaction product between niobium atoms and methylene fluoride to be the CH2−NbF2 species, which is predicted to be 45 kcal/mol lower in energy than the possible quartet CH2F−NbF insertion product owing to very strong metal fluoride bonds. Likewise, laser-ablated tantalum atoms react with CH2F2 to produce one observable IR absorption at 673.0 cm−1, which doubles in intensity when the matrix is exposed to light with λ > I


Article

The Journal of Physical Chemistry A 290 nm, and doubles again when exposed to λ > 220 nm radiation. This absorption shifts to 672.5 and 671.8 cm−1 when 13 CH2F2 and CD2F2 are used in experiments, suggesting a Ta−F stretching motion. Calculations on the CH2−TaF2 methylidene predict a stable doublet species with the most intense absorption at 665.4 cm−1 with 0.5 cm−1 13C and 2.4 cm−1 D red-shifts. Hence, CH2−TaF2 is formed when tantalum atoms react with CH2F2 in condensing argon (Table S2). The possible quartet CH2F-TaF insertion product is computed 80 kcal/mol higher in energy than the doublet CH2−TaF2 species assigned here. Reaction energies for Nb and Ta are substantially more exothermic than for V; however, we must remember that the calculations for V employed the large all electron basis sets and Nb and Ta used SDD pseudopotentials. Nb(6D) + CH 2F2 → CH 2−NbF2

methylidyne products as compared with frequency calculations in Table S4 and described in Discussion D2. Reaction energies are slightly more exothermic for CHF3 than for CH2F2 as we benefit from more exothermic fluorine atom transfers. Nb(6D) + CHF3 → CH 2−NbF3

(14)

Ta(4 F) + CHF3 → CH 2−TaF3

ΔE = −175 kcal/mol (15)

3.16. Nb and Ta + CHCl3. Chloroform reacts with niobium atoms to form a product with a single vibrational mode at 560.5 cm−1 which doubles in intensity when the matrix is exposed to radiation with λ > 220 nm. This stable product is identified as HC-NbCl3 and its 572.4 cm−1 tantalum counterpart as HCTaCl3 in section D3. 3.17. Nb and Ta + CF4. The reaction of niobium atoms with CF4 produced no new product absorptions. However, a weak precursor band spans the 1495−1505 cm−1 region, which could obscure the product absorption expected in this region. Tantalum does give rise to an absorption at 1486.6 cm−1, which is assigned to FC−TaF3 in section D4. Both Nb and Ta reactions with CF4 are computed to be highly exothermic, but owing to very strong C−F bonds in the reagent, these reactions may be slower than those with less fluorine in the reagent.

ΔE = −152 kcal/mol (12)

Ta(4 F) + CH 2F2 → CH 2−TaF2

ΔE = −159 kcal/mol

ΔE = − 166 kcal/mol (13)

3.13. Nb and Ta + CH2Cl2. Niobium atoms react with CH2Cl2 in condensing argon to form a product with IR absorptions at 681.3 and 452.5 cm−1. These absorptions increase in intensity by 30% when the matrix is exposed to radiation with λ > 290 nm and increase by another 30% with λ > 220 nm. The higher frequency absorption shows a large red-shift to 538.9 cm−1 when CD2Cl2 is employed, whereas the lower frequency absorption is observed at 451.0 cm−1. The higher frequency absorption is nearly identical to the CH2 waging mode observed at 683.4 cm−1 in the CH2−VCl2 species with a similar deuterium shift (Table 2), and the lower frequency absorption with nearly no deuterium shift should belong to a Nb−Cl stretching mode. Hence, the most likely reaction product is the CH2−NbCl2 species. Calculations on this complex predict a stable doublet methylidene CH2−NbCl2 with the most intense absorptions corresponding to a Nb−Cl stretching mode at 432.3 cm−1 and a CH2 wagging mode at 751.9 cm−1. This doublet CH2−NbCl2 complex is predicted to be 37 kcal/mol lower in energy than the quartet CH2Cl-NbCl insertion species. Hence, the CH2−NbCl2 complex is the major reaction product between niobium atoms and methylene chloride. Two similar absorptions are observed at 692.7 and 424.0 cm−1 when laser-ablated tantalum atoms react with CH2Cl2. These absorptions increase by 30% when exposed to light with λ > 290 nm, and an additional 30% with λ > 220 nm. This is similar to observations for CH2−NbCl2, both in frequency and photolysis behavior. Accordingly, the major reaction product is the CH2− TaCl2 complex. Theoretical calculations of the properties of this species predict a stable doublet with the two strongest IR absorptions for the Ta−Cl stretching mode at 428.6 cm−1 and the CH2 wagging mode at 735.1 cm−1 (Table S3) which supports identification of the major reaction product with tantalum as the CH2−TaCl2 complex. This CH2−TaCl2 methylidene is also predicted to be 63 kcal/mol lower in energy at our level of theory than the CH2Cl−TaCl intermediate complex. 3.14. Nb and Ta + CH2FCl. Niobium and tantalum atoms atoms react with CH2FCl to form CH2−MFCl methylidene products following CH2−VFCl. The details of these spectroscopic observations and assignments are given in Supporting Information, Discussion D1. 3.15. Nb and Ta + CHF3. Fluoroform reacts with laser ablated niobium and tantalum atoms to produce infrared absorption which can be assigned to HC-NbF3 and HC-TaF3

Nb(6D) + CF4 → FC−NbF3

ΔE = −165 kcal/mol (16)

Ta(4 F) + CF4 → FC−TaF3

ΔE = − 175 kcal/mol (17)

3.18. Nb and Ta + CCl4. Following vanadium, niobium and tantalum atoms react with carbon tetrachloride to form a product with strong absorptions at 1198.2 and 1225.4 cm−1, which are assigned to the methylidyne complexes ClC−NbCl3 and ClC− TaCl3 on the basis of C-13 shifts and frequency calculations in section D5. 3.19. Nb + CF3Cl. Niobium and Ta reactions with CF3Cl produced new IR absorptions for diagnostic terminal F−C stretching modes for FC−NbF2Cl and FC−TaF2Cl without Cl− C counterparts as described in the Supporting Information, section D6. 3.20. Nb and Ta + CF2Cl2. Difluorodichloromethane reacts with niobium and with tantalum atoms to insert into both C−F and C−Cl bonds to produce both FC−MFCl2 and ClC−MF2Cl methylidyne complexes as discussed in section D7. Calculations find the C−Cl product to be slightly lower in energy. 3.21. Nb and Ta + CFCl3. Laser-ablated niobium atoms react with CFCl3 to form products with a terminal C−F stretching mode at 1473.4 cm−1 and with a terminal C−Cl stretching mode at 1198.8 cm−1 which are assigned to FC−NbCl3 and ClC− NbFCl2 in section D8. Similar bands at 1487.3 cm−1 and at 1225.5 cm−1 are assigned to the Ta counterparts. The relative photochemistries of these methylidyne complexes are also discussed in section D8. Annealing favors the FC−TaCl3 complex, whereas photolysis favors the ClC−TaFCl2 species. 3.22. Reactions, Periodic Comparisons, and Chemical Bonding. Computed reaction energies have been given in the above discussions. These energies increase with the number of halogen atoms bonded to carbon, which are available for transfer to the V, Nb, or Ta atoms, and the energies increase going down J


Article

The Journal of Physical Chemistry A the Group 5 family group. Also, F atom transfer from carbon to metal is more exothermic than for Cl. These investigations have involved three Freon precursors CFCl3, CF2Cl2, and CF3Cl where the reacting metal atom has two chemical site choices for reaction, the F atom−CF bond or the Cl atom−CCl bond. Thus, the reactions with CF2Cl2 and CFCl3 form two different products, but CF3Cl formed only the 6−8 kcal/mol higher energy FC−MF2Cl product. This observation could be due to a faster reaction at one of three C−F bond sites. Here excited metal atoms from the ablation process can be reacted with the halocarbon precursor molecules during sample deposition or from UV excitation through photolysis of the deposited sample. On the other hand, cold metal atoms can be reacted while annealing the matrix to allow reagent diffusion in solid argon. The substantial increase of CH 2 VF 2 and ClC−VCl 3 absorptions, for example, on annealing to 30 K demonstrates that these reactions proceed without activation energy. Many of these product absorptions also increase on UV photolysis most likely through electronic excitation of the metal atom. A comparison of the properties of the three Group 5 CH2MF2 methylidene complexes identified here is provided in Table S8 and the structures are modeled in Figure 6. The M−F and M−C

region. This is also observed in the calculated shortening of the C-X bond along the 3d transition series. The C−F bond length decreases from 1.271 (Ti) to 1.261 (V) to 1.253 Å (Cr) in FC− MF3 complexes, but the C−Cl bond length is predicted to increase from 1.622 (Ti) to 1.626 (V) to 1.632 Å (Cr) for the ClC−MCl3 complexes. In this methylidyne complex series the most interesting bond to compare is the carbon{}metal bond, which decreases in length along the row from 1.953 (Ti) to 1.745 (V) to 1.625 Å (Cr) for these triplet, doublet, and singlet electronic state complexes, ClC{}MCl3, respectively. These complexes have nominally σ2π1π1(Ti), σ2π2π1(V), and σ2π2π2(Cr) carbon metal bonds.9,10 Note that the symmetrically occupied Group 4 and 6 complexes have 3-fold symmetry, but the asymmetrically occupied Group 5 complex has only a plane of symmetry, see Figures 7 and 8. In contrast, the analogous series of planar σ2π2 methylidene CH2 MF2 complexes exhibits similar bond lengths, 1.850 (Ti), 1.840 (V), and 1.899 Å (Cr).6,10

Figure 7. Structure calculated for HC−NbF3 using B3LYP and 6-311+ +G(3df,3pd)/SDD basis sets. Bond lengths and angles are in Å and degrees.

We also computed the vanadium bearing series of ClC VCl3+, ClC{}VCl3, and ClC{}VCl3− anion, which is isoelectronic with the above Ti, V, Cr methylidyne series. Their structures are shown in Figure 8. The cation has the longer 1.840 Å C−V bond and lower 1.76 bond order, but a higher 1242 cm−1 computed C−Cl stretching frequency. The neutral has intermediate 1.752 Å length and 2.23 order C−V bond and 1163 cm−1 C−Cl frequency. The anion C−V bond is shorter, 1.661 Å, the order higher, 2.74 with more electrons, while the C−Cl frequency is 1198 cm−1. It is interesting to compare the C{}Nb bond lengths in HC{}NbH2F− and HC{}NbF3, which are nominally σ2π2π2 (1.803 Å, anion)15,30 and σ2π2π1(1.883 Å, neutral), respectively, where the anion electron contributes to pi bonding. The structure of HC{}NbF3 is illustrated in Figure 7 where the asymmetry is shown through the 178.4° H−C−Nb bond angle and slightly unequal Nb−F bond lengths and angles. Natural bond orbital analysis calculations were performed for a number of these interesting product molecules using B3LYP and both 6-311++G(2d,p)/SDD and the 6-311++G(3df,3pd) all electron basis set. Table S10 shows the natural charges, electron configurations and bond characters for the CH2−VF2, CF2−VF2, and FC−VF3 species using NBO 6.0 calculations.31 The natural bond orders for the doublet CH2VX2 complexes are 1.94, 1.88, and 1.92 for X = F, Cl, and Br. The next bonds to consider here are the terminal C−F and C− Cl bonds owing to their unusually high stretching frequencies in the XC−MX3 methylidynes. We find that three electrons are involved in their bonding. For ClC−VCl3 an orbital made up of 43% C [32% s, 67% p character] and 57% Cl [21% s, 78% p, 1% d] is occupied by 0.99 with α and β electrons, and their

Figure 6. Geometrical parameters of the CH2−MF2 complexes calculated at the B3LYP//6-311++G(2d,p)/SDD level of theory.

lengths are longer for the larger 4d and 5d metals, and all three structures are essentially planar. In contrast to Groups 4 and 6 CH2−MH2 and CH2−MHX methylidene complexes, calculated structures for the Group 5 CH2−MX2 complexes exhibit no agostic distortions,11 a property shared by the CH2−ScF2 complex, which exhibits a longer 2.161 Å metal−carbon bond with only a single π electron.29 The Mulliken charges for the atoms in CH2−VF2 and CH2−TaF2 are very similar, but CH2− NbF2 differs. Trends for the XC−MX3 methylidyne complexes can also be observed going down the Group 5 family. These trends are shown in the properties listed in Table S16. As the metal is changed from V to Nb to Ta, the C−X, M−C, and M−X bond lengths increase in unison. The trademark X−C−M stretching frequency also increases as the metal atom becomes heavier. We have investigated reactions of these chlorofluorcarbons with Group 4 and 6 metal atoms.9,10 In comparing the diagnostic high C-X stretching modes of the XC−MX3 complexes, we see that their frequencies increase uniformly along the early 3d transition metals. For example, in ClC−MCl3 complexes this mode increases from 1151.5 cm−1 (Ti) to 1177.5 cm−1 (V) to 1230.6 cm−1 (Cr).9,10 For FC−-MF3 complexes, it increases from 1453.1 cm−1 (Ti) to 1492.9 cm−1 (V). Although the C−F stretching mode was not observed for FC−CrF3, it is probably hidden under a precursor absorption in the 1550−1530 cm−1 K


Article


Figure 8. Structures calculated using B3LYP and 6-311++G(3df,3pd) for the ClC{}VCl3 methylidyne cation, neutral, and anion.

antibonding counterparts each contain 0.01 e. Another orbital made up of 16.5% C and 83.5% Cl [over 99% p character for both elements] and occupied by 0.99 β electron with its antibonding counterpart having 0.04 e. The net effect is a 1.46 bond order, which helps to explain the high “C−Cl” stretching frequency. This vibration exhibits antisymmetric Cl−C−V character based on its having more 13C shift than the C−Cl diatomic molecule.9,10 The FC−VF3 species is comparable with a 1.48 C−F bond order. Resonance structures using a lone pair from the halogen bonded to carbon, (+)XCV(−)X3 also support increased bonding and higher vibrational frequencies for these terminal C−X bonds. Next we examine the C{}V bonds in these methylidyne complexes where five electrons participate. The FC{}VF3 species has three bonding molecular orbitals that are 73% C [70% s, 30% p] and 27% V[38% s, 18% p, 44% d], and 51% C [99.9% p] and 49% V [8% p, 92% d], and 47% C [ 99.9% p] and 53% V [8% p, 92% d] for α electrons and two more for β electrons that are 70% C [70%s, 30% p] and 30% V [37% s, 15% p, 48% d], and 28% C [99.9% p] and 72% V [5% p, 95% d], respectively. These orbitals have occupancies ranging from 0.98 to 0.99, the first three for α electrons and the last two for β electrons, and they have antibonding counterparts with occupancies ranging from 0.03 to 0.09. The net bonding for these five electrons is a 2.30 C{}V bond order. The ClC{}VCl3 complex is similar to three mo’s that are 68% C [68% s, 32% p] and 32% V[29% s, 15% p, 56% d], and 45% C[ 99.8% p] and 55% V[16% p, 84% d], and 42% C[99.8% p] and 58% V[16% p, 84% d] and two more mo’s that are 68% C[68% s, 32% p] and 32% V[ 27% s, 27% p, 46% d], and 51% C[99.8% p], and 49% V[18% p, 82% d]. Occupancies range from 0.95 to 0.98, in the first three for α electrons and the last two for β electrons, and their antibonding counterpart occupancies are from 0.02 to 0.10. This can be summarized as σ(α)0.98π(α)0.95· π(α)0.95σ(β)0.97π(β)0.95σ*(α)0.04π*(α)0.10π*(α)0.09· σ*(β)0.02π*(β)0.09. The net bond order is 2.23 for the ClC−VCl3 complex.29 The C−V bond orders for FC−VCl3 (2.31) and ClC−VF2Cl (2.25) are comparable. The structure and bonding molecular orbitals for HC{}VF3 are illustrated in Figures 9 and 10, respectively, which has a net 2.34 carbon−vanadium bond order.30 The 178.6° H−C−V bond angle, slightly unequal V−F bond lengths and angles, and three distinct calculated and observed V−F stretching modes show that this complex has lower than the 3-fold symmetry possessed by the triplet Group 4 and singlet Group 6 methylidynes.6,10 This results in a vertical plane of symmetry, which bisects the F−V−F angle with equal V−F bonds. The π molecular orbital illustrated on the lower right in Figure 10 is asymmetric to this plane and the one on the left is symmetric to this plane. Notice also that the unique V−F fluorine has some 2p electron density in the symmetrical orbital and none in the asymmetrical orbital. The result is that the symmetrical π orbital is π2 and the asymmetrical

Figure 9. Structures of CH2VF2 and HC{}VF3 calculated with the B3LYP functional and the all electron 6-311++G(3df,3pd) basis sets. Bond lengths and angles are in Å and degrees.

Figure 10. Bonding molecular orbitals computed for HC−VF3 using B3LYP//6-311++G(3df,3pd) basis. The bottom two are π2 and π1 C− V, left to right, respectively, and the top two are σ2 H−C and σ2 H−C mixed with the lone pair of one F atom, respectively. Electron density is 0.02 e/Å3.

one is π1. Both of these π orbitals are C(2p)−V(3d). The total electron occupancy for a 2.34 bond order in HC{}VF3 is σ(α)0.97π(α)0.99· π(α)0.95σ(β)0.96π(β)0.99σ*(α)0.03π*(α)0.02π*(α)0.06· σ*(β)0.04π*(β)0.02. The structure of the planar methylidene CH2VF2 with a slightly longer C−V bond is also shown in Figure 9. NBO analysis29 reveals the following occupancies for CH2VF2: σ(α)1.00π(α)0.98σ(β)0.99π(β)1.00σ*(α)0.01π*(α)0.04π*σ*(β)0.03· π*(β)0.01, which results in a net bond order of 1.94. The Mulliken atomic spin densities of these two doublet state molecules are given in Table S11. It is interesting to note that the most spin density (1.57) is on the V center in CH2VF2, but on the C atom (1.21) in HC{}VF3. L



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4. CONCLUSIONS Laser-ablated vanadium, niobium, and tantalum atoms are reacted with various CX4 (X = H, F, and Cl) molecules in condensing argon, and the products are identified by comparing the observed infrared absorptions with frequencies predicted by hybrid density functional theory for different structural isomers. In addition to assistance with confirming the identification of new reactive group 5 metal bearing species, the comparison of new observed frequencies with density functional computed frequencies provides data points for determining scale factors for vibrational frequencies.6 Our calculated and observed frequencies show that agreement, and thus scale factors, are different for different vibrational modes in these molecules. In the case of vanadium−carbon stretching frequencies, discrepancies occur because of configuration interaction and multireference character, and higher level CCSD calculations22,32 must be done to provide better agreement between calculated and observed frequencies (see Supporting Information). The major reaction products are thus characterized as CX2− MX2 and XC−MX3 complexes. No agostic distortions are observed in CH2−MX2 complexes,11 and XC−MX3 species exhibit the diagnostic high C−X frequency observed previously for other metals. The natural bond orders for the doublet CH2 VX2 complexes are computed as 1.94, 1.88, and 1.92 for X = F, Cl and Br. Important conclusions from these calculations are the halogen-carbon and the carbon{}metal bonding particularly in the methylidyne complexes. The terminal C−F and C−Cl bonds exibit unusually high stretching frequencies in the XC−MX3 methylidynes, and we find that three electrons are involved in their bonding. Compare the carbon{}metal bonds in the HC{}MF3 complexes, which decrease in length along the first row from 1.977 (Ti) to 1.772 (V) to 1.635 Å (Cr) for these triplet, doublet, and singlet electronic state complexes, which have nominally σ2π1π1(Ti), σ2π2π1(V), and σ2π2π2(Cr) carbon metal bonds,9,10 and the isoelectronic cation, neutral, anion series for ClC{}VCl3 has nominally σ2π1π1(cation), σ2π2π1(neutral), and σ2π2π2(anion) carbon−vanadium bonds with computed 1.76, 2.23, and 2.74 bond orders, respectively. In contrast, an analogous series of methyidene σ2π2 CH2MF2 complexes exhibit similar metal carbon bond lengths, 1.850 Å (Ti), 1.840 Å (V) and 1.899 Å (Cr),6,10 but the corresponding planar Sc methylidene σ2π1complex with one less electron has a longer 2.161 Å computed Sc−C bond length.29

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Article

AUTHOR INFORMATION

Corresponding Author

*(L.A.): E-mail: [email protected]. Telephone: 434-924-6844. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS

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REFERENCES

J.T.L. and L.A. acknowledge the Donors of the American Chemical Society Petroleum Research Fund for support of this research. J.T.L. also acknowledges the Clayton State University CASE program for a teaching course release in support of this research. Part of this work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number OCI1053575. H.-G.C. acknowledges financial support from the Korea Research Foundation (KRF) funded by a Korean government grant (NRF-2013R1A1A2060088) and the KISTI supercomputing center.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b10992. Tables S1−S14 of calculated vibrational frequencies, intensities, and physical properties for several observed products, Figure S1 showing the relative energies of different V + CH2F2 reaction intermediates and the final product, Figure S2, showing the B3LYP structure for HC{}VF3, and Figure S3, illustrating the relative energies of the possible products in the reaction of V and CCl4, discussions D1−D8 providing more details about Nb and Ta reaction products and assignments, and a final section providing a discussion of CCSD calculations of V−C stretching frequencies for CH2VCl2, 13CH2VCl2 and CD2VCl2 and a consideration of multiconfigurational behavior (PDF) M


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Matrix Infrared Spectroscopic and Quantum Chemical Investigations of

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