Matrix-Infrared Spectra and Structures of HM–SiH ... - ACS Publications

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Matrix-Infrared Spectra and Structures of HM−SiH3 (M = Ge, Sn, Pb, Sb, Bi, Te Atoms) Published as part of The Journal of Physical Chemistry virtual special issue “W. Lester S. Andrews Festschrift”. Bing Xu,† Li Li,† Peipei Shi,† Wenjie Yu,† Jie Zhao,† Xuefeng Wang,*,† and Lester Andrews*,‡ †

School of Chemical Science and Engineering and Shanghai Key Lab of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092, China ‡ Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United States S Supporting Information *

ABSTRACT: The reactions of Ge, Sn, Pb, Sb, Bi, and Te atoms with silane molecules were studied using matrix-isolation Fourier transform infrared spectroscopy and density functional theoretical (DFT) calculations. All metals generate the inserted complexes HM-SiH3, which were stabilized in an argon matrix, while H2MSiH2 and H3MSiH were not observed. DFT and CCSD(T) calculations show the insertion complex HM-SiH3 is the most stable isomer with a near right angle H−M−Si moiety. However, silydene complexes H2MSiH2 (M = C, Si) were calculated and identified as the most stable complexes with the lighter elements. The bonding difference is mainly due to relativistic effects, which is that for heavier metal atoms valence s and p orbitals have a lower tendency to form hybrid orbitals.

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INTRODUCTION Oxidative addition or metathesis of a Si−H bond in a hydrosilane (including SiH4, RSiH3, R2SiH2, and R3−SiH) to a metal center is known for nearly all of the transition-metal (TM) elements.1−4 Stable complexes containing a TM−Si bond have been synthesized, for example, the complex (OC)9Fe3[μ3-Si{Fe(CO)2Cp}]2 has been obtained in the reaction of SiH4 with a mixture of [CpFe(CO)2]2 and Fe(CO)5.5 For many second- and third-row TM systems, subsequent H migration also occurs to generate high oxidation-state complexes (H2MSiH2 and H3MSiH). For example, the simplest group 6 silylidene and silylidyne molecules, H2MSiH2 and H3MSiH (Cr, Mo, W), have been prepared by reactions of laser-ablated group 6 metal atoms with silane during condensation in excess argon.6 These reactions proceeded through metal insertion to a Si−H bond forming HM−SiH3 silyl metal hydrides, followed by α-H transfer to the H2MSiH2 silylidene dihydrides and then another α-H transfer to the silylidyne trihydride molecules H3MSiH. In the case of W, the silylidyne is the lowestenergy product; however, with Cr, the initial insertion product HCr−SiH3 is the most stable. Recently, the activation of Si−H bonds by main-group atoms has been an attractive subject due to fundamental interest.7,8 For example, laser-ablated beryllium atoms have been reacted with silane molecules to produce the inserted product H3SiBeH that rearranged to triple bridge complex Si(μ-H)3BeH upon photolysis.7 The observed spectra show excited Be atom (1P1:2s12p1) can insert into Si−H bond spontaneously, which shows much difference from C−H bond complexes.7 The reactions of laser-ablated boron atoms with SiH4 molecules gave silylene dihydroborate (H2SiBH2) in © XXXX American Chemical Society

excess argon, which rearranged to silicon tetrahydroborate Si(μ-H)2BH2 upon 300−350 nm irradiation.8 For group 14, C and Si atoms reacted with silane to produce disilene (SiH2 CH2, SiH2SiH2), which is the global minimum structure in their corresponding isomers.9 However, for the heavier atoms in main groups, such reactions with silane have not been investigated. In this paper direct reactions of the heavy atoms, namely, group 14 (Ge, Sn, and Pb), 15 (Sb, Bi), and 16 (Te), with SiH4 were performed to examine whether or not these maingroup metals undergo silane activation and subsequent H migration similar to the transition metals. Matrix-isolation infrared spectroscopy was used to collect molecular vibrational information, and the experimental frequencies were reproduced by theoretical calculations.

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EXPERIMENTAL AND THEORETICAL PROCEDURES Laser-ablated Ge, Sn, Pb, Sb, Bi, and Te atom reactions with SiH4 and SiD4 in excess argon during condensation at 4 K using a closed-cycle refrigerator (Sumitomo Heavy Industries model SRDK-408D2) were described in our previous papers.10−13 The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width, Continuum Minilite II) was focused onto the rotating metal target (Alfa Aesar), and the ablated metal atoms with SiH4 or SiD4 (Spectra Gases Inc.) in excess argon spread uniformly onto the CsI window. Infrared spectra were recorded at a resolution of 0.5 cm−1 by using a Bruker 80 V spectrometer after sample deposition, Received: September 28, 2017 Revised: December 15, 2017 Published: December 15, 2017 A

DOI: 10.1021/acs.jpca.7b09635 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Infrared spectra of the Ge atom and SiH4 reaction in solid argon. (a) Codeposited of Ge + 0.5% SiH4 in argon for 60 min, (b) after annealing to 25 K, (c) after λ > 220 nm irradiation for 8 min, and (d) after annealing to 30 K.

Table 1. Observed and Calculated Frequencies and Assignments for the HM-SiH3 (M = Ge, Sn, Sb, Bi, and Te) SiH4 Ge

Sn

Pb

Sb

Bi

Te

SiD4

obs

calc(int) B3LYP

calc B3LYP(anharm)

calc(int) BPW91

2113.8 2103.9 1837.3 861.9 855.9 658.6 2110.9 2099.8 1660.0 845.5 2091.0 2083.3 1493.2 831.7 2143.1 2135.8 1884.2 863.0 2119.9 2113.7 1646.4 847.6 2154.3 2149.0 2076.5 883.1

2182.3(109) 2170.6(73) 1863.6(273) 858.5(261)

2092.7 2090.7 1750.3 842.7

2129.7(90) 2107.2(60) 1823.1(248) 813.2(206)

661.8(45) 2181.2(112) 2168.6(94) 1720.9(362) 852.9(338) 2179.3(120) 2165.6(108) 1590.1(444) 848.6(390) 2209.5(91) 2196.3(83) 1891.4(152) 872.8(366) 2209.2(93) 2193.2(93) 1805.4(205) 864.2(407) 2231.7(86) 2217.4(80) 2089.4(37) 892.0(373)

640.5 2105.0 2097.1 1650.5 848.5 2092.7 2085.6 1527.2 839.0 2142.7 2140.5 1865.8 862.6 2140.1 2137.2 1756.7 852.9 2178.4 2172.0 2108.0 883.4

635.3(43) 2128.4(96) 2112.5(76) 1689.1(331) 811.8(278) 2127.0(104) 2109.8(89) 1546.8(402) 809.3(323) 2156.0(78) 2139.3(72) 1861.3(133) 832.3(313) 2155.4(81) 2136.0(81) 1777.0(178) 824.9(349) 2174.9(77) 2157.1(74) 2029.6(30) 853.7(323)

obs 1551.1 1530.4 1323.4 641.2 637.3 1546.5 1528.1 1192.3 632.3 1487.5 1479.2 1065.1 masked 1549.2 1546.9 1338.1 640.1 1534.7 1526.7 1181.5 628.8 1565.3 1485.2 650.4

calc(int) B3LYP

calc(int) BPW91

description

1576.1(62) 1546.2(41) 1327.1(138) 635.3(114)

1535.4(54) 1503.3(32) 1298.3(126) 601.6(87)

485.7(24) 1575.3(63) 1544.3(54) 1222.3(183) 631.0(159) 1573.9(66) 1541.8(61) 1127.5(224) 627.6(187) 1596.8(52) 1562.4(49) 1343.3(76) 644.5(194) 1596.6(52) 1560.1(54) 1280.1(103) 638.2(214) 1613.8(49) 1576.4(49) 1483.8(18) 656.4(210)

466.4(23) 1537.0(53) 1504.7(43) 1199.7(167) 600.4(127) 1536.0(57) 1502.4(51) 1096.8(203) 598.3(152) 1558.1(44) 1522.1(42) 1321.9(67) 614.7(166) 1557.6(45) 1519.6(47) 1260.0(89) 609.2(183) 1572.8(44) 1533.9(45) 1441.4(15) 628.5(181)

SiH2 antisys str SiH3 sys str GeH str SiH3 def SiH3 site GeH wag SiH2 antisys str SiH3 sys str SnH str SiH3 def SiH2 antisys str SiH3 sys str PbH str SiH3 def SiH2 antisys str SiH3 sys str SbH str SiH3 def SiH2 antisys str SiH3 sys str BiH str SiH3 def SiH2 antisys str SiH3 sys str TeH str SiH3 def

were calculated analytically with zero-point energy included for the determination of reaction energies. Natural bond order (NBO)14,19 analysis was conducted to explore bonding in new molecules. The bonding features were investigated using the Multiwfn program.20

annealing to allow reagent diffusion and further reaction, and irradiation by a 175 W mercury arc lamp with globe removed and using a combination of optical filters. Complementary density functional theory (DFT) calculations were performed using the Gaussian 09 program.14 The hybrid B3LYP15 density functional was used with the cc-pVTZ basis sets for Si and H atoms and cc-pVTZ-pp for Ge, Sn, Pb, Sb, Bi, and Te atoms to predict the geometries, electronic structure, and vibrational frequencies of reaction products.16,17 The BPW9118 functional was also employed to complement the B3LYP results, and CCSD(T) calculations were performed for single-point energy corrections. All vibrational frequencies

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RESULTS AND DISCUSSION Experiments were done with 0.5% SiH4 or SiD4 in excess argon; typical infrared spectra in the selected regions are illustrated in Figures 1−6 and S1−S8, and the absorptions of reaction products are listed in Table 1. The stepwise annealing and irradiation behaviors of these product absorptions are also B

DOI: 10.1021/acs.jpca.7b09635 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A shown in these figures and will be discussed below. Common species, such as SiH, SiH2, Si2H2, and Si2H6, which were produced during the experiment owing to precursor fragmentation, have been identified in a previous paper.21 In addition, metal hydrides MH and MH2 (M = Ge, Sn, Pb, Sb, Bi, and Te) were generated by decompositions of SiH4 and its insertion products, and specific M−H stretching modes have been assigned in these metal reactions with hydrogen in solid argon.10−13 The absorptions due to impurities such as H2O, CO2, and CO in the experiments can never be eliminated, but they can be reduced. B3LYP calculations were done on three MSiH4 isomers for each metal atom (M = Ge, Sn, Sb, Bi, and Te), namely, the inserted HMSiH3 and the α-H transferred H2MSiH2 and H3MSiH molecules in both singlet and triplet states. The optimized geometric parameters are shown in Figure 7, and the calculated vibrational frequencies and intensities are listed in Tables 1 and S1−S6. For group14, the inserted molecules HGeSiH3, HSnSiH3, and HPbSiH3 were predicted to have 1A ground states with C1 symmetry, while the H2MSiH2 isomers (M = Ge, Sn, Pb) were calculated to be 4.8, 14.1, 30.5 kcal/mol higher in energy than HMSiH3, respectively, based on CCSD(T) calculations. In addition the H3MSiH isomers are located highest in energy profile as shown in Figure 8. However, for C and Si the H2MSiH2 isomer is lowest in energy (Figure 8). Similarly, for Sb, Bi, and Te metal atoms the insertion molecule HMSiH3 is the most stable, while H2M SiH2 and H3MSiH isomers have higher energy (Figures 7 and S9). HGeSiH3. Figure 1 shows infrared spectra for the reaction of laser-ablated Ge atom Codeposited with SiH4 in excess argon and their variation in annealing and broadband photolysis. One group of new product absorptions marked HGeSiH3 at 2103.9, 1837.3, 861.9, and 658.6 cm−1 appeared upon sample codeposition, markedly increased upon annealing to 25 K, showed no significant change on irradiation, and further increased by 20% on annealing to 30 K. Experiments were also done with deuterium labeled sample (SiD4) with the same sample concentration and laser power, and this group of bands shifted to 1551.1, 1530.4, 1323.4, and 641.2 cm−1, respectively, as shown in Figure 2.

These bands are assigned to HGeSiH3 and DGeSiD3. First, the 1837.3 cm−1 band shifts to 1323.4 cm−1 with SiD4, exhibiting an isotopic frequency H/D ratio of 1.3883, which is close to the values of the Ge−H stretching vibration of various previously characterized Ge hydrides in excess argon,10,11,22 indicating that the 1837.3 cm−1 band is due to a Ge−H stretching vibration. Notice the 1837.3 cm−1 band is very close to 1831.3 cm−1 for the Ge−H stretching frequency in HGeCCH22 and 1834.0 cm−1 for the diatomic GeH molecule.10 Second, the 2103.9 and 861.9 cm−1 bands shift to 1530.4 and 641.2 cm−1 with SiD4 as reagent (Figure 4), defining H/D isotopic ratio of 1.3747 and 1.3442, respectively, which are typical Si−H stretching and bending vibrations and isotopic frequency ratios of terminal SiH3 group.21 Third, the present frequency calculations support for the assignment of HGeSiH3. Our calculations predict that the GeH stretching mode is at 1863.6 cm−1 by B3LYP, which is 26 cm−1 higher than the experimental value (1837.3). Furthermore, the GeH bending modes is calculated at 661.8 cm−1 by B3LYP, which is 3.2 cm−1 higher than the observed value (658.6 cm−1). The Si−H stretching modes are predicted at 2182.3 and 2170.6 cm−1 (B3LYP), respectively, which are overestimated by 3.1% and 3.1%. As listed in Table 1, the calculated deuteriumsubstituted frequencies are also in good agreement with the experimental values. In addition the BPW91 frequency calculations gave similar results (Table 1). The Ge−H stretching frequencies for GeH2 and GeH3 subunit of the silylidene H2GeSiH2 and silylidyne H3Ge SiH are not observed in our experiments, which imply that these species are not isolated in our low-temperature matrix.19 On the basis of our CCSD(T) calculation the insertion reaction (Ge(30) + SiH4 → HGeSiH3) is exothermic by 31.4 kcal/mol; however, further H migration from Si to Ge is endothermic by 4.8 kcal/mol (H2GeSiH2) and 14.1 kcal/ mol (H3GeSiH) (Figures 7 and S9), which supports our conclusion. HSnSiH3. Sn atom reactions in solid argon with SiH4 gave new absorptions at 2110.9, 2099.8, 1660.0, and 845.5 cm−1 (marked HSnSiH3 in Figure 3), which increased upon annealing to 25 and 30 K but decreased on broadband photolysis. For the reaction of Sn with SiD4 (Figure 4), new

Figure 2. Infrared spectra of the Ge atom and SiD4 reaction in solid argon. (a) Codeposited of Ge + 0.5% SiD4 in argon for 60 min, (b) after annealing to 25 K, (c) after λ > 220 nm irradiation for 8 min, and (d) after annealing to 30 K.

Figure 3. Infrared spectra of the Sn atom and SiH4 reaction in solid argon. (a) Codeposited of Sn + 0.5% SiH4 in argon for 60 min, (b) after annealing to 25 K, (c) after λ > 220 nm irradiation for 8 min, and (d) after annealing to 30 K. C

DOI: 10.1021/acs.jpca.7b09635 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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symmetric modes for SiH3 subunit are calculated at 2181.2 and 2168.6 cm−1, which are overestimated by ∼3.2%. At the B3LYP level the anharmonic values for the two stretching modes are given at 2105.0 and 2097.1 cm−1 (Table 1), which are in excellent agreement with the experimental values of 2110.9 and 2099.8 cm−1. Furthermore, the Si−H bending mode for SiH3 subunit is calculated at 852.9 cm−1, while the anharmonic value is 848.5 cm−1, with only 3.0 cm−1 shift. In addition, the Sn−H stretching mode is calculated at 1720.9 cm−1, which is overestimated by 3.5%. The anharmonic value for the Sn−H stretching modes (1650.5 cm−1) is 9.5 cm−1 lower than observed value at 1660.0 cm−1. The calculated BPW91 frequencies and isotopic frequency ratios match the experimental values very well. So the assignment of the HSnSiH3 molecule is confirmed. The higher oxidation-state products (H2SnSiH2 and H3SnSiH) are not observed in our experiments because of their higher energies (Figures 7 and S9). Recall with the reaction of CH4 with Sn only insertion HSnCH3 has been observed.24 HPbSiH3. New product absorptions at 2091.0, 2083.3, 1493.2, and 831.7 cm−1 for reactions of Pb with SiH4 are shown in Figures 5 and S1 and in Tables 1 and S3, which are appropriate for the HPbSiH3 molecule. These bands slightly increased on annealing to 25 K but decreased upon greater than 300 nm irradiation. For the reaction of Pb with SiD4 (Figure 6 and S2), new deuterium-substituted bands were observed at 1487.5, 1479.2, and 1065.1 cm−1, which have the same response to the photo-irradiation and annealing as the product bands in the SiH4 sample. The 1493.2 cm−1 band shifted to 1071.1 cm−1 with deuterium substitution, giving 1.3941 H/D isotopic ratio, which is the heavier metal Pb−H stretching mode.10 Notice that this Pb−H stretching mode is close to those for PbH and PbH2 observed at 1472.3 and 1541.6 cm−1, respectively, in solid argon. The 2091.0 and 2083.3 cm−1 bands can be assigned to Si−H stretching modes for the SiH3 subunit, and 1487.5 and 1479.2 cm−1 bands are due to Si−D stretching modes for the SiD3 subunit. Unfortunately the Si−D bending mode is covered by broad band for Si2D6, although the Si−H bending mode was observed at 831.7 cm−1 on the shoulder of the Si2H6 band at 840 cm−1.

bands were observed at 1546.5, 1528.1, 1192.3, and 632.3 cm−1. This group of bands also increased upon annealing to 25 K and enhanced greatly on final annealing to 30 K.

Figure 4. Infrared spectra of the Sn atom and SiD4 reaction in solid argon. (a) Codeposited of Sn + 0.5% SiD4 in argon for 60 min, (b) after annealing to 25 K, (c) after λ > 220 nm irradiation for 8 min, and (d) after annealing to 30 K.

The 1660.0 cm−1 band shifts to 1192.3 cm−1 with deuterium substitution, giving the 1.3923 H/D isotopic ratio. This band is a little higher than the Sn−H stretching mode for diatomic SnH molecule at 1645.5 cm−1 13 and for HSnCCH molecule at 1657.9 cm−1,22 which might be due to a terminal Sn−H stretching vibration. The 2110.9, 2099.8, and 845.5 cm−1 bands shifted to 1546.5, 1528.1, and 632.3 cm−1 (Figure 5) in the SiD4 experiment, giving H/D isotopic ratios of 1.3650, 1.3741, and 1.3372, respectively, which are typical of Si−H stretching and bending modes of terminal SiH3 group isotopic frequencies.23 Notice the 1528.1 cm−1 is smaller than 1546.5 cm−1, which is in accordance with the calculation that its intensity is smaller than that of the latter. So these bands come from one molecule, namely, HSnSiH3. B3LYP and BPW91 calculations predict the HSnSiH3 molecule to have C1 symmetry with 1A ground electronic state. By B3LYP method, the Si−H antisymmetric and

Figure 5. Infrared spectra of the M + SiH4 (M = Ge, Sn, Pb, Sb, Bi, and Te) reaction in solid argon in the 2160−2080, 2100−1450, and 950−830 cm−1 regions. (a) Ge + 0.5% SiH4 after annealing to 30 K, (b) Sn + 0.5% SiH4 after annealing to 25 K, (c) Pb + 0.2% SiH4 after annealing to 30 K, (d) Sb + 0.2% SiH4 after annealing to 30 K, (e) Bi + 0.2% SiH4 after annealing to 30 K, and (f) Te + 0.2% SiH4 after annealing to 30 K. D

DOI: 10.1021/acs.jpca.7b09635 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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In Te atom reaction with SiH4 four new absorptions at 2154.3, 2149.0, 2076.5, and 883.1 cm−1 were observed, which are shown in Figures 5 and S7 and illustrated in Table 1. Parallel to above metal cases, this group of bands is assigned to HTeSiH3. The Te−H stretching mode was observed at 2076.5 cm−1, which shifted to 1485.2 cm−1 (Figures 6 and S8) with SiD4 giving the 1.3940 H/D ratio. The 2154.3, 2149.0, and 883.1 cm−1 bands are assigned to Si−H stretching and bending modes, respectively. DFT frequency calculations support the assignments. Our B3LYP functional calculations gave strong M−H stretching vibrations at 1891.4 cm−1 (Sb−H) and 2089.4 cm−1 (Te−H) (Table 1), which are overestimated by 0.51% and 0.62%, respectively, which are in excellent agreement with our experimental values. However, the Bi−H stretching mode is calculated at 1805.4 cm−1, which is overestimated by 8.81%, but reduced to 6.3% by anharmonic frequency calculation at 1756.7 cm−1. This deviation is larger than theoretical prediction for other metal−hydrogen stretching mode, which is due to Hund’s case coupling splitting for bismuth.26−28 In addition the calculated Si−H stretching and bending modes match observed values very well,29 as very similar deviations were obtained (Table 1). Again the BPW91 calculations gave slightly lower frequencies but match the observed values reasonably well. For group 14 metals the HMSiH3 species increased on annealing, suggesting the insertion reactions occur spontaneously, although the activation energy barriers from SiH4 + M to HMSiH3 are calculated at 9.5, 11.9, and 18.5 kcal/mol, respectively, for Ge, Sn, and Pb (Figure S9). The tunneling control of hydrogen transfer must be taken into account here, and quantum-mechanical tunneling can direct the chemical reaction though the reaction path that has a higher barrier.30 It is interesting to note the HMSiH3 species for Sb and Bi decreased on annealing, corresponding to much higher energy barriers at 29.2 (Sb) and 32.3 (Bi) kcal/mol. Further extensive research is needed to reveal these tunneling control reactions. The high oxidation-state complexes are energetically much higher than the insertion product based on CCSD(T) calculations (Figures 7 and S9), so only HMSiH3 was observed in our experiments.

Figure 6. Infrared spectra of the M + SiD4 (M = Ge, Sn, Pb, Sb, Bi, and Te) reaction in solid argon in the 1570−1470, 1500−1050, and 650−620 cm−1 regions. (a) Ge + 0.5% SiD4 after annealing to 30 K, (b) Sn + 0.5% SiD4 after annealing to 25 K, (c) Pb + 0.2% SiD4 after annealing to 30 K, (d) Sb + 0.2% SiD4 after annealing to 30 K, (e) Bi + 0.2% SiD4 after annealing to 30 K, and (f) Te + 0.2% SiD4 after annealing to 30 K.

B3LYP and BPW91 calculations predict the HPbSiH3 molecule to have C1 symmetry with 1A ground electronic state (Figure 7). The Pb−H stretching mode is calculated at 1590.1 cm−1, which is overestimated by 6.1% using B3LYP functional. The calculated Si−H stretching and bending modes are at 2179.3, 2165.6, and 848.6 cm−1 (B3LYP) that are slightly lower than the same modes calculated for HGeSiH3 and HSnH3 (Table 1). At the B3LYP level, the anharmonic values for Pb−H stretching mode, Si−H stretching and bending modes are 1527.2, 2092.7, 2085.6, and 839.0 cm−1, respectively, (Table 1), which is in excellent agreement with the experimental values of 1493.2, 2091.0, 2083.3, and 831.7 cm−1 in argon matrix. HSbSiH3, HBiSiH3, and H3SiTeH. As shown in Figures 5 and S3, in Sb + SiH4 experiments, four new bands marked HSbSiH3 appeared at 2143.1, 2135.8, 1884.2, and 863.0 cm−1, decreased upon annealing but increased upon greater than 300 nm irradiation. In reaction with SiD4 these bands shifted to 1549.2, 1546.9, 1338.1, and 640.1 cm−1. The strong 863.0 cm−1 band shifted to 640.1 cm−1 with deuterium substitution, giving the 1.3480 H/D isotopic ratio, which is typical for Si−H bending mode in a SiH3 subunit.17 Two Si−H stretching modes for SiH3 subunit at 2143.1 and 2135.8 cm−1 shifted to 1549.2 and 1546.9 cm−1 (Figures 6 and S4). The weak 1881.8 cm−1 band shifted to 1338.1 cm−1 with SiD4 defining the 1.4063 H/D ratio, which can be assigned to the Sb−H stretching mode. Similar Sb−H stretching mode was observed at 1885.0 cm−1 in the HSbCH3 molecule.24 We assign the product bands to the antimony insertion product HSbSiH3. Four new absorptions for HBiSiH3 at 2119.9, 2113.7, 1646.4, and 847.6 cm−1 were observed in the laser-ablated Bi atom reaction with SiH4 (Figures 5 and S5 and Tables 1 and S5). These bands appeared on deposition, decreased upon annealing, but increased upon broad-band irradiation. The reaction of Bi with SiD4 revealed four new bands at 1534.7, 1526.7, 1181.5, and 628.8 cm−1 (Figures 6 and S6 and Tables 1 and S5). Apparently the 1646.4 cm−1 band is due to the Bi− H stretching mode because of the typical H/D ratio of 1.3935 and comparison of the Bi−H stretching mode with the diatomic BiH molecule at 1621.9 cm−1.25 Similarly 2119.9, 2113.7, and 847.6 cm−1 are due to Si−H stretching and bending modes.

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BONDING As shown in Figure 8, the HGeSiH3 (ground-state singlet) is 4.4 kcal/mol lower in energy than H2GeSiH2 (ground-state singlet). For the HGeSiH3 structure the H−Ge−Si bond is nearly a right angle, which reflects the high p contributions of the metal atom, namely, 92% and 93% natural p contribution for the Ge−H and Ge−Si bonds, respectively, in HGe-SiH3 (Table S7). On the basis of our NBO analysis, there is nearly no hybridization between the s and p orbitals. Similarly for Sn and Pb the inserted product (HM−SiH3) is the lowest in energy in the energy profile (Figure 8). However, for C and Si the silylidene H2CSiH2 and H2SiSiH2 are calculated as the global minimum structure (Figure 8), which were isolated in the argon matrix.9 The silylidene H2GeSiH2 and silylidyne H3GeSiH, which correspond to the higher energy species with high oxidation state of Ge complex, are not identified in our matrix IR spectra. With the heavier atoms, the considerable size difference between valence s and p atomic orbitals increases. For Si, Ge, Sn, and Pb atoms, the size differences are 0.39, 0.48, 0.57, and 0.79 Å, respectively, while for carbon the valence s and p E

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Figure 7. Structures of products calculated using the B3LYP and BPW91 method (italic). The cc-pVTZ basis set was for Si and H, and cc-pVTZ-pp was for metal atoms (Ge, Sn, Sb, Bi, Te). Bond lengths are in angstroms, and angles are in degrees. The energies in bold were single-point energy using CCSD(T) method. All energies were compared with that of M + SiH4.

orbital size is almost equal.31 As a result the heavier atoms have a lower tendency to form hybrid orbital between s and p orbital; instead, they tend to preserve the valence ns electron as core-like electrons and keep valence shell (ns)2(npx)1(npy)1 configuration in their compounds. So for Ge, Sn, and Pb, the

inserted product HMSiH3 structure is the global minimum structure, while for C and Si, silaethene (H2CSiH2) and disilene (H2SiSiH2) are the global minimum structures. The difference between lighter and heavier atoms was mainly caused by relativistic effects. The heavier atom has a F

DOI: 10.1021/acs.jpca.7b09635 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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distribution at CP, and ▽2 ρcp > 0 exhibits a local electron depletion. Then, the boundary between ionic and covalent interactions is defined by ▽2 ρcp = 0. The |Vcp|/Gcp value is a good indicator for the bond properties, which smaller than 1 indicates ionic bond, and bigger than 2 often manifests covalent bond. For Si-M (M = Sn, Sb, Te) the ▽2 ρcp are less than zero, indicating covalent bonding. The 2.93, 3.20, and 3.99 of |Vcp|/Gcp values for the CP of Si−Sn, Si−Sb, and Si−Te bonds respectively indicate an increase in covalent character along the series of Sn−Sb−Te. The population analyzed by electron localization function (ELF) is 1.82, 1.70, and 1.48 e between Si−Sn, Si−Sb, and Si− Te bonds, respectively, suggesting bond strength decrease along the Sn−Sb−Te series following an increase in electronegativity, whereas the total population of SiH3 by ELF analysis is 5.43, 5.65, and 5.97e for the Sn, Sb, and Te complexes, respectively, which increase slightly from left to right contrary to that of the Si−M bond. The bond length of Si−H bond is 1.486, 1.482, and 1.478 Å for the Sn, Sb, and Te complexes, respectively, indicating a slight increase in bond strength between Si and H atom, in accordance with the blue shift of bending mode from Sn to Te. In the same way, Si−H stretching and bending modes decrease down the group from Ge to Pb. For example, the observed/calculated (B3LYP) Si− H bending mode at 861.9/861.0 cm−1 for HGeSiH3, 845.5/ 857.6 cm−1 for HSnSiH3, and 831.7/857.0 cm−1 for HPbSiH3, following a decrease in electronegativity.

Figure 8. Energy-profile diagram (reaction coordinate vs relative energy) of the MSiH4 (M = C, Si, Ge, Sn, and Pb) system at the CCSD(T)/cc-pVTZ/cc-pVTZ-pp level.

larger positive nuclear charge. Inner 1s electrons with no angular momentum can approach the atomic nucleus most closely, their speed thereby becoming close to the speed of light. As a result of mass−velocity correction,32 the inner 1s orbital shrinks. This orbital does not directly make an important contribution to chemical bonding. However, the shrinking of the 1s orbital causes the contraction and stabilization of the valence ns orbital.32,33 The relativistic effect on the valence p orbital is smaller, since the angular momentum keeps p electrons away from the nucleus. A similar periodic trend shows that the hybridization becomes more difficult with heavier atoms between the ns and np orbitals for Sb, Bi, Te, and as a result right angle of H-M-Si for the insertion complex is obtained. As shown in Tables S7 natural p contribution for M−Si and M−H is ∼90%, suggesting p orbital dominates the bonding. An interesting frequency change was observed in our experiments: from Ge to Pb the Si−H stretching and bending modes in SiH3 subunit shifted red, while from Sn to Te these modes shifted blue (Figure 5). The DFT frequency calculations reproduced our experimental values very well. Our atoms-in-molecules (AIM) picture (Figure 9) shows the bond critical point (CP) between Si and Sn, Sb, Te locates in the region with negative Laplacian value (▽2 ρcp = −0.025 for Si−Sn, −0.042 for Si−Sb, and −0.116 for Si−Te). In addition the 2.93, 3.20, and 3.99 of |Vcp|/Gcp value for the CP of Si−Sn, Si−Sb, and Si−Te bonds are obtained. As pointed out by the AIM methodology,34 the classification of the interaction is defined by the sign of ▽2 ρcp. The ▽2 ρcp < 0 is evidenced by a local concentration of the electron density

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CONCLUSIONS Reaction of laser-ablated Ge, Sn, Pb, Sb, Bi, and Te atoms with silane generate the insertion products HMSiH3 in the argon matrix, which were identified on the basis of the SiD4 isotopic substitutions and density functional frequency calculations (B3LYP and BPW91). However, subsequent α-H migration to form higher oxidation-state products is evidently prohibited during codeposition and photolysis afterward because of their considerably higher energies. The mostly p contributions from the metal atom to the Si−M and M−H bonds lead to a nearly right angle Si−M−H moiety in the insertion complexes, which is different from the transition metal cases.

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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.7b09635. Additional information including infrared spectra of the M (M = Pb, Sb, Bi, and Te) atom and SiH4 (SiD4) reaction in solid argon (Figures S1−S8), energy-profile diagram of the M + SiH4 (M = Ge, Sn, Pb, Sb, Bi, and Te) system at the CCSD(T) /cc-pVTZ/cc-pVTZ-pp

Figure 9. Contour line diagrams of the Laplacian of the electronic density of HMSiH3 (H = Sn, Sb, Te) at B3LYP/cc-pVTZ/cc-pVTZ-pp level. Red lines are in regions of charge concentrations (∇2 ρ(r) < 0), while green lines are in regions of charge depletion (∇2 ρ(r) > 0). G

DOI: 10.1021/acs.jpca.7b09635 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

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level (Figure S9), the transition-state structure (Figure S10), observed and calculated fundamental frequencies of the HMSiH 3 and DMSiD 3 (Tables S1−S6), compositions of natural localized molecular orbitals (NLMO) from NBO analysis of HMSiH3 (Table S7; M = Ge, Sn, Pb, Sb, Bi, and Te), and the Cartesian coordinate of HMSiH3 product (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (X.W.) *E-mail: [email protected]. (L.A.) ORCID

Xuefeng Wang: 0000-0001-6588-997X Lester Andrews: 0000-0001-6306-0340 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 21371136 and 21373152).

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DOI: 10.1021/acs.jpca.7b09635 J. Phys. Chem. A XXXX, XXX, XXX−XXX