Theoretical study of the insertion reaction of zinc, cadmium, and mercury atoms with methane and silane

12 November 1999

Chemical Physics Letters 313 Ž1999. 608–616 www.elsevier.nlrlocatercplett

Theoretical study of the insertion reaction of zinc, cadmium, and mercury atoms with methane and silane M.E. Alikhani ) ˆ 49, batiment (UMR CNRS LADIR), UniÕersite´ P. et M. Curie, Boite Laboratoire de Spectrochimie Moleculaire F74, 4 Place Jussieu, ´ ˆ Paris Cedex 05, France Received 1 July 1999; in final form 27 September 1999

Abstract The XH 4 q M Ž3 P. ™ HMXH 3 ŽX s C or Si, and M s Zn, Cd, or Hg. reaction has been studied using density functional theory. The vibrational frequencies and electronic properties are presented. A simple mechanism for the metal insertion in the X–H bond is proposed. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction During the last decade, the experimental study of the metal atom q CH 4 interactions was performed using a great variety of experimental techniques w1–6x. The reaction of excited zinc w7,8x, cadmium w9,10x, and mercury w11x atoms with hydrogenic molecules ŽH 2 , CH 4 , and SiH 4 . in the gas phase usually gave evidence on the formation of the R ŽR s H, CH 3 , or SiH 3 . radicals and MH ŽM s Zn, Cd, or Hg. species Žsee the comprehensive review in Ref. w12x and references therein.. In a recent ab initio calculations, Siegbahn et al. w13x showed that the reaction of HgŽ3 P. with CH 4 and SiH 4 proceeds initially by the formation of exciplexes, followed by insertion of Hg into the reactive H–XH 3 bond ŽX s C or Si., ultimately giving the XH 3 radical and HM metal hydride which appears similar with what was observed for the reaction of CH 4 with the CuŽ2 P. atom w14x. )

Fax: q33-1-44-27-30-21; e-mail: [email protected]

Recently, the reaction of Zn, Cd, and Hg atoms with CH 4 w15–17x and SiH 4 w18x has been experimentally investigated in rare-gas matrices. From these experimental works, two significant points have been demonstrated. First, it has been shown that, at least within the matrix cage, it is excitation to the 3 P state of metal atoms which causes the insertion reaction, forming the HMXH 3 ŽM s metal atom and X s C or Si. molecules. These species have been experimentally identified by their infrared ŽIR. spectra. The importance of the role played by the excited states of the metal atom in the activation of C–H bonds has already been shown in the pioneering work of Blomberg et al. w19x. In a previous theoretical work w20x, the formation of the HZnCH 3 intermediate was only examined for the interaction between CH 4 and ZnŽ1 P. in an excited state. As already shown in the case of the Li and Mg insertion into the C–H bond of methane w21x, the strategy used in Ref. w20x is appropriate to study the Zn q CH 4 reaction which actually takes place through an avoided intersystem crossing. Nevertheless, it has been suggested, in

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 1 2 9 - X

M.E. Alikhani rChemical Physics Letters 313 (1999) 608–616

conflict with the experimental findings at low temperature in solid argon w17x, that the ZnŽ3 P. excited state cannot activate the C–H bond of methane w20x. Second, in contrast to the behavior of CH 3 CuH which undergoes dissociation upon selective photolysis w14x, the HMXH 3 hydride products observed at low temperature in a cryogenic matrix seem to be photostable. In this Letter, we attempt to answer to the following questions: Ž1. May the products observed in the solid matrix be present in the gas phase as intermediate species? Ž2. Is the 3AX Žfirst excited state. ™ 1A 1 Žground state. conversion energetically possible? In order to consider these questions, we have performed calculations on HMXH 3 ŽM s Zn, Cd, or Hg, and X s C or si. systems using density functional theory ŽDFT. and the second-order Møller– Plesset perturbation method ŽMP2.. Furthermore, in order to obtain a deeper understanding of the bonding in these compounds, a topological analysis of the electron localization function ŽELF. w22x has been undertaken.

2. Results and discussion All calculations have been performed with the GAUSSIAN 94rDFT quantum-chemical package w23x. The DFT calculations have been carried out with Becke’s three-parameter hybrid method w24x using the Perdew–Wang91 gradient-corrected correlation functional w25x Ždenoted as B3PW91.. In the MP2 calculations, inner shells are excluded from the correlation calculation. After a comparative study of the results obtained using B3PW91 and MP2 methods and available experimental data for the HHgCH 3 hydride, all the other compounds were investigated using the B3PW91 functional only. We have used the Stuttgart ŽMWB. pseudo-potential w26,27x for all the metal atoms of group 12 and the 6-311 q q GŽ2d,2p. extended basis set of Pople et al. w28–30x for the other atoms. The effective core potentials used for metal atoms treat the ns, np, nd, and Ž n q 1.s electrons Ž n s 3, 4, and 5 for Zn, Cd, and Hg, respectively. explicitly using the Ž8s7p6d.r w6s5p3dx-GTO contraction scheme. This basis set contains one diffuse s-, two diffuse p-, and one

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diffuse d-function w26,27x. All calculations of the topology of the electron localization have been performed with the TopMoD package w31x. These programs use the wfn file generated by GAUSSIAN 94 with the option Outputs wfn. 2.1. Structural and Õibrational analysis 2.1.1. HMCH3 (M s Zn, Cd, or Hg) We first discuss the results obtained for HHgCH 3 because all of the IR vibrational frequencies of this species are identified in the matrix isolation experiments w15–17x. As shown in Fig. 1, the HHgCH 3 hydride was calculated in two geometries corresponding to the ground state structure ŽGS. in a C3 Õ symmetry Ž1A 1 electronic state. and to the first excited state ŽES. in a C s symmetry Ž3AX electronic state.. The optimized structural parameters are reported in Table 1. The Hg–C bond length of the GS structure is significantly shorter than that for ES. The ˚ in going Hg–H bond length is shortened by 0.12 A from ES to GS, while the C–H bond distance is ˚ The geometry of the slightly lengthened by 0.01A. CH 3 in the GS structure is actually different from that of the CH 3 and HgCH 3 radicals. It should be noted that the geometrical parameters of ES are very similar to those of free CH 3 and HgH. The binding energy of the ES structure was found to be only 0.2 kcalrmol with respect to CH 3 and HgH. As shown in Table 1, the GS structure is nearly four times more stable than that for ES when the binding energy is calculated with respect to the CH 4 and HgŽ3 P. units. Both states of HHgCH 3 hydride have a small dipole moment Ž m - 1 D ..

Fig. 1. The HMXH 3 and XH 4 structures studied in this work.

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Table 1 Structural parameters of HHgCH 3 , HgCH 3 , CH 3 , HHg, and CH 4 obtained from the B3PW91 functional Parameters

˚. r1 ŽA r2 r3 r4 a1 Ždeg.. a2 a3 /t a De Žkcalrmol. m ŽD. a b

HHgCH 3 Ž 1A 1 .

HHgCH 3 Ž 3AX .

1.649 2.112 1.090

1.773 3.294 1.079 3.416 78.5 92.3 119.8 172.2 18.9 0.58

180.0 110.2 108.9 118.6 81.7 0.46

b

HgCH 3 Ž 2A 1 .

CH 3 Ž 2 AXX2 .

HHg Ž 2 S.

CH 4 Ž 1A 1 .

1.773 2.388 1.084

1.079

103.4 114.8 136.2

1.089

120.0 180.0

0.63

0.00

109.5 120.0 0.33

0.00

Dihedral angle of the CH 3 subunit. De s  EŽCH 4 . q EwHgŽ3 P.x4 y EŽHHgCH 3 ..

The calculated and experimental vibrational frequencies of HHgCH 3 in its ground state are gathered in Table 2. The HrD and 12 Cr13 C isotopic frequency shifts are also listed in Table 2. It should be noted that the experimental vibrational frequencies of HHgCH 3 species are only well reproduced for the GS structure from theoretical calculations. In other word, the calculated frequencies of HHgCH 3 ŽES. do not match the experimental data which means that the GS structure is actually the final product observed in matrix experiments. As shown in Table 2, there is good agreement between the calculated frequencies and experimental data. The experimental frequency shifts for isotopic substitutions are also well reproduced from two theoretical calculations

Table 2 Vibrational frequencies Žin cmy1 . of HHgCH 3 in its ground state

ŽDFT and MP2.. Finally, since the DFT results are close to those for MP2, all of the others compounds have only been investigated using the DFT approach. Let us now consider the two other systems. Inspection of the computed values listed in Table 3 indicates that the features discussed above also hold for the HZnCH 3 and HCdCH 3 species. The geometrical parameters of the ES structures are closer to those for free CH 3 and MH in going from the zinc-methyl hydride to the mercury-methyl hydride. The HZnCH 3 and HCdCH 3 hydrides in their excited state have been found to be weakly bound compared to CH 3 and HM Ž1.2 and 0.7 kcalrmol for M s Zn and Cd, respectively.. The dipole moment values decrease from HZnCH 3 Ž3AX . to HCdCH 3 Ž3AX .. Con-

a

Exp.b

Symm.

Mode

B3PW91

MP2

A1

n1 n2 n3 n4

CH str. s. HHg str. CH 3 bend s. CHg str.

3049 1995 1215 524

Ž864, 3. Ž581, 0.3. Ž276, 8. Ž47, 16.

3083 2062 1245 545

Ž872, 4. Ž600, 0.3. Ž283, 8. Ž49, 17.

2921 1955 1192 534

Ž794, 4. Ž554, 0.3. Ž167, 7. Ž47, 16.

E

n5 n6 n7 n8

CH str. as. CH 3 bend as. CH 3 rock. CHgH bend

3135 1460 791 530

Ž814, 11. Ž403, 3. Ž194, 5. Ž154, 0.

3176 1480 796 481

Ž826, 12. Ž409, 3. Ž193, 6. Ž139, 0.

2991 1425 780 526

Ž757, y. Ž384, y. Ž186, 5. Ž152, 0.

a b

The isotopic shifts Ž HrD and 12 Cr13 C. are reported in parentheses. The experimental values deduced from Refs. w15–17x.

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Table 3 Structural parameters of HZnCH 3 , HZn, HCdCH 3 , and HCd obtained from the B3PW91 functional Parameters

˚. r1 ŽA r2 r3 r4 a1 Ždeg.. a2 a3 /t a m ŽD. a

HZnCH 3 Ž 1A 1 .

HZnCH 3 Ž 3AX .

HZn Ž 2 S.

HCdCH 3 Ž 1A 1 .

HCdCH 3 Ž 3AX .

HCd Ž 2 S.

1.532 1.937 1.092

1.630 2.531 1.081 2.7114 78.0 97.9 118.2 153.3 1.23

1.611

1.676 2.119 1.091

1.777 2.960 1.080 3.079 76.6 94.6 119.4 164.2 1.00

1.768

180.0 110.9 108.0 116.5 0.48

180.0 110.5 108.9 118.6 0.45

0.55

0.72

Dihedral angle of the CH 3 subunit.

first excited state Ž3AX . with the CCSDŽT. method. The metastable isomer Ž3AX . is found to be stable by 1.1 kcalrmol Žwith respect to CH 3 q HZn. and by 7.8 kcalrmol Žwith respect to ZnCH 3 q H.. The results are close to those obtained from the DFT method Ž1.2 and 8.4 kcalrmol.. Finally, it is interesting to note that the energetic ordering is the same with two methods. As shown in Table 4, the calculated vibrational frequencies and the corresponding isotopic shifts are in excellent agreement with the experimental values.

cerning HMCH 3 Ž1A 1 ., the dissociation energy, calculated with respect to the CH 4 and MŽ3 P. units, has been found to be 75.8 and 58.1 kcalrmol for M s Zn and Cd, respectively. To check the reliability of the DFT energies we have performed a single-point calculation at a high level of ab initio theory, namely using the CCSDŽT. method, on the DFT optimized geometries for the M q CH 4 ŽM s Zn, Cd, and Hg. reaction. The CCSDŽT. binding energies of HMCH 3 in its ground state Ž1A 1 . with regard to the CH 4 and MŽ3 P. is very close to the DFT result Ž73.8, 55.7, and 79.3 kcalrmol versus 75.8, 58.1, and 81.7 kcalrmol for M s Zn, Cd, and Hg, respectively.. We have also calculated the dissociation energy of HZnCH 3 in its

2.1.2. HMSiH3 (M s Zn, Cd, or Hg) In Table 5 are reported the structural parameters of the compounds involving the Si atom. According

Table 4 Vibrational frequencies Žin cmy1 . of HZnCH 3 and HCdCH 3 in their ground state Symm.

Mode

HZnCH 3

a

HCdCH 3

B3PW91

Exp.

b

B3PW91

Exp.b

A1

n1 n2 n3 n4

CH str. s. HM str. CH 3 bend s. CM str.

3037 1901 1203 562

Ž859, 3. Ž545, 0. Ž267, 9. Ž50, 14.

2920 1866 1179 564

Ž –, 4. Ž521, – . Ž258, 9. Ž –, – .

3045 1819 1184 506

Ž863, 3. Ž526, 0. Ž269, 8. Ž44, 14.

2929 Ž –, 5. 1761 Ž496, 0. – 509 Ž43, 14.

E

n5 n6 n7 n8

CH str. as. CH 3 bend as. CH 3 rock. CMH bend

3117 1452 712 442

Ž862, 11. Ž402, 3. Ž167, 5. Ž126, 0.

– – 687 Ž159, 5. 443 Ž125, 0.

3129 1457 712 451

Ž814, 11. Ž402, 3. Ž174, 4. Ž130, 0.

– – 687 Ž164, 4. 433 Ž123, 0.

a b

The isotopic shifts Ž HrD and 12 Cr13 C. are reported in parentheses. The experimental values deduced from Ref. w17x.

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Table 5 Structural parameters of HMSiH 3 ŽM s Zn, Cd, or Hg. and SiH 3 obtained from the B3PW91 functional Parameters Ground state ˚. r1 ŽA r2 r3 r4 a1 Ždeg.. a2 a3 /t a De Žkcalrmol. b m ŽD. Excited state ˚. r1 ŽA r2 r3 r4 a1 Ždeg.. a2 a3 /t a m ŽD. a b

HZnSiH 3

HCdSiH 3

HHgSiH 3

1.546 2.371 1.490

1.691 2.520 1.487

1.671 2.487 1.488

180.0 111.8 107.1 114.6 87.8 0.21

180.0 111.9 106.9 114.3 75.0 0.24

180.0 111.3 107.5 115.6 99.7 0.16

1.659 2.653 1.487 2.421 63.5 109.2 109.7 120.6 0.30

1.820 2.847 1.486 2.597 63.2 109.1 109.9 121.0 0.48

1.793 2.883 1.484 2.992 75.6 108.1 110.8 123.4 0.45

SiH 3

1.485

111.2 124.4 0.04

Dihedral angle of the SiH 3 subunit. De s  EŽSiH 4 .q EwMŽ3 P.x4y EŽHMSiH 3 ..

to a previous theoretical study w32x, the pyramidal configuration of SiH 3 Ž2A 1 electronic state in a C3 Õ symmetry. is more stable than the planar one Ž2AXX2 state in a D 3 h symmetry.. Our results concerning the geometrical values of pyramidal SiH 3 are close to

those obtained using the CISD approach w32x. For the metalsilyl hydride geometries, we have the same trends as for the corresponding metal-methyl hydrides. The metalsilyl hydrides in their 3AX excited state have been calculated to be bound by 5.4, 4.4, and 2.9 kcalrmol with respect to SiH 3 and HM for M s Zn, Cd, and Hg, respectively. These energies are slightly larger than those for the corresponding metal-methyl species. Concerning the binding energy of the metalsilyl hydrides in their ground state, we note that it varies in the same way as the metal-methyl hydrides, namely HHgXH 3 ) HZnXH 3 ) HCdXH 3 ŽX s C or Si.. We have listed in Table 6 the vibrational frequencies of the metalsilyl hydrides as well as the corresponding IR intensities. Only two vibrational modes of HHgSiH 3 , namely the most intenses n 2 and n 3 , have been experimentally observed in rare-gas matrices w18x. These are well reproduced by calculations. It should be noted that the n 1 and n 5 absorptions ŽTable 6. are too close to the parent SiH 4 absorption to be observed experimentally. We hope that the theoretical predictions will be helpful for future experimental studies 2.2. Topological analysis of the ELF function The topological description of the chemical bond proposed by Silvi and Savin w22x relies upon the gradient field analysis of the ELF of Becke and

Table 6 Vibrational frequencies Žin cmy1 . of HMSiH 3 ŽM s Zn, Cd, or Hg. in its ground state

a

Symm.

Mode

HZnSiH 3

HCdSiH 3

HHgSiH 3

A1

n 1 SiH str. s. n 2 HM str.

2187 Ž57. 1863 Ž340.

2186 Ž70. 1782 Ž376.

n 3 SiH 3 bend s.

858 Ž247.

856 Ž290.

n4 SiM str.

326 Ž4.

291 Ž3.

2195 1910 1888 b 864 870 b 300

E

a b

n5 n6 n7 n8

SiH str. as. SiH 3 bend as. SiH 3 rock. SiMH bend

2196 942 504 349

In parentheses are reported the IR intensities Žin kmrmol.. The most intense lines observed in the argon matrix w18x.

Ž97. Ž40. Ž14. Ž52.

2195 945 502 364

Ž96. Ž41. Ž18. Ž45.

2205 944 579 393

Ž61. Ž485. Ž315. Ž3. Ž94. Ž40. Ž1. Ž24.

M.E. Alikhani rChemical Physics Letters 313 (1999) 608–616

Edgecombe w33x. The molecular space is partitioned into basins of attractors which have a clear chemical signification. These basins are either core basins surrounding nuclei or valence basins. The valence basins are characterized by their synaptic order which is the number of core basins with which they share a common boundary. Accordingly, monosynaptic basins correspond to lone-pair regions Žlabeled as VŽX. where X denotes atom label. whereas disynaptic ones correspond to bonding regions Žlabeled as VŽX,Y. where X and Y denote atom labels.. For example, in the water molecule there is one core basin for the oxygen K-shell labeled CŽO., two protonated disynaptic basins VŽH 1 ,O. and VŽH 2 ,O., and two monosynaptic basins corresponding to the lone pairs V1ŽO. and V2 ŽO.. The partition into basins allows the calculation of related properties by integration of the property densities over the basins. In particular, for a basin labeled V A , one can define the average population as: N Ž VA . s

HV r Ž r . d r

Ž 1.

A

In Table 7 are reported the valence basin populations for the HZnXH 3 compound ŽX s C or Si. in the ground and excited states. Since the ELF calculations require the core electrons, these systems are investigated using an all electron basis set, namely the 6-311q q GŽ2d,2p. basis set of Pople et al. The basins of attractors are organized as follows. There are one monosynaptic valence basin corresponding to lone electron pairs of X ŽVŽX.., one monosynaptic valence basin corresponding to the non-bonding electron of metal ŽVŽZn.., one disynaptic basin between the Zn and X atoms ŽVŽZn,X.., and two protonated disynaptic basins ŽVŽZn,H. and VŽX,H... Analysis of the electron localisation on the disynaptic basins gives some valuable information on the relative strength of the metal bonds with H and CH 3 Žor SiH 3 .. The VŽX,H. population of both HZnCH 3 and HZnSiH 3 compounds Žin the ground and excited states. is similar to that for free XH 3 Žclose to 2 e .. In free XH 3 and both states of HZnXH 3 , there are only three protonated basins ŽVŽX,H.. against four in the free XH 4 molecules. In the HZnXH 3 systems,

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the fourth H is bonded to the Zn atom. In going from the methyl- to silyl-compounds, the VŽZn,H. population decreases while the VŽZn,X. population increases. On the base of the Lewis picture Ž2 e for a singlebond., we can state that the Zn–X bond is actually a single bond in the ground state for both X s C and Si ŽVŽZn,C. s 2.09 e and VŽZn,Si. s 2.24 e ., while it is weaker than a single bond in the triplet state. It can be seen that, in the triplet state, the VŽZn,C. population is significantly smaller than the VŽZn,Si. population Ž0.55e versus 1.31e ., which means that the Zn–CH 3 bond is much softer than the Zn–SiH 3 bond. Although the V ŽZn,H . population in the metastable isomer Ž3AX . is slightly smaller than that for the ground state, the Zn–H bond could be considered as a near single bond in both states of HZnXH 3 . Nevertheless, it should be noted that the population of the zinc–hydrogen bond in the HZnSiH 3 Ž3AX . isomer is smaller than that for the HZnCH 3 Ž3AX . compound Ž1.76 e versus 1.93e .. The VŽX. population located at the C Žor Si. atom in free CH 4 Žor SiH 4 and SiH 3 . becomes the shared electron upon complexation in both states of HZnXH 3 . We note that the non-bonding electron located at the Zn atom ŽVŽZn.. only appears in the metastable isomer Ž3AX . as well as in the free HZn molecule. The VŽZn. population is nearly the same for three systems Ž1.44 e, 1.36 e, and 1.36 e .. It is interesting to mention that, in the HZnXH 3 Ž1A 1 . molecule, not only is there no VŽZn. basin but the Zn–X and Zn–H bonds are reinforced compared to the HZnXH 3 Ž3AX . species. The above discussion shows that the HZnXH 3 Ž3AX . system resembles the HZn . . . XH 3 complex, whereas the HZnXH 3 Ž1A 1 . molecule is characterised by five covalent-bonds Žone VŽZn,H., one VŽZn,X., and three VŽX,H... This conclusion is in line with the geometrical considerations. 2.3. Energetics of the M q XH4 ™ HMXH3 reactions In a previous experimental work w17x, it was shown that, following selective UV photolysis it is excitation to the 3 P state of metal atoms which

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Table 7 Topological ELF population analysis Žin e . Basins

VŽX. VŽZn. VŽZn,H. VŽZn,X. VŽX,H. b a b

X HZnXH 3 Ž3A .

HZnXH 3 Ž1A 1 .

XsC

XsC

1.44 1.93 0.55 2.17

X s Si 1.36 1.76 1.31 2.00

0.0 2.20 2.09 2.00

HZn

X s Si 0.0 2.18 2.24 1.98

XH 3 XsC

XH 4 X s Si

XsC

0.91

0.64

2.00

1.34

a

X s Si 0.47 a

1.36 2.15 2.28

1.52

There are four monosynaptic basins VŽX.. The protonated disynaptic basin for each X–H bond.

causes the insertion reaction in forming the monomethyl-metal hydride. It does not support the proposition that insertion demands atoms in the 1 P rather than 3 P excited state. Therefore the fundamental question is to explain the mechanism by which excitation of group-12 atoms to the 3 P state activates the C–H bond of methane in the matrix cage, leading to formation of the monomethyl-metal hydride. In order to understand this mechanism, we have investigated a simple scheme in which the metal atom is positioned along a line that is perpendicular to the C–H Žor Si–H. bond characterized by the M–H ŽM s group-12 metal., X–H ŽX s C or Si. distances and M–H–X bond angle. The insertion reaction essentially occurs in two stages. In the first stage, the excited metal Ž3 P. spontaneously binds to XH 4 leading to HMXH 3 in its excited state Ž3AX .. Upon this stage, a near single bond between metal and H atom ŽVŽZn,H. s 1.93e and 1.76 e in HZnCH 3 and HZnSiH 3 , respectively. and a lone pair located on the metal atom ŽVŽZn. s 1.44 e and 1.36 e . are formed, on the one hand, and a X–H bond is broken, on the other hand. The covalent bond formed between the metal and C Žor Si. atom is rather weak Žsee the VŽZn,X. population in Table 7.. The second stage corresponds to an intersystem crossing between the two 3AX and 1A 1 potential energy surfaces ŽPES. as function of the H-M-X bond angle Žlabelled u in Fig. 2.. In fact, the excited state Ž3AX . PES crosses the ground state Ž1A 1 . PES for a value of u near the 3 X A PES minimum leading to an adiabatic intersystem crossing, as sketched in Fig. 2. The small value of d E Ž0.0–0.6 kcalrmol, always less than the zero-point energy. indicates that there is no triplet– singlet gap in the crossing region. Since the 1A 1

curve is attractive after the intersystem crossing region, the final product in the matrix cage ŽHMXH 3 in its ground state. is formed via 1A 1 PES by the vibrational relaxation process. Upon this stage, the lone-pair populations located at the metal atom become the shared electrons inducing a strengthening of the M–X and M–H bondings. We then actually have a single bond for both M–X and M–H. This scheme ressembles the two-state reactivity pathway model proposed by Filatov et al. w34x for the MOqq CH 4 reaction. The latter work showed that the activation of the C–H bond by MOq ŽM s first-row transition metals. takes place via a spin conversion between a high and a low spin state. Therefore, we think that the HMXH 3 Ž1A 1 . compound, identified in

X

Fig. 2. Potential energy curves of the 1A 1 and 3A states of HMXH 3 systems in the crossing region, as a function of the insertion angle Ž u ..

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Table 8 X Dissociation energies of HMXH 3 ŽM s Zn, Cd, or Hg and X s C or Si. in the excited state Ž3A . Parameters Ž DE .1 Žkcalrmol. Ž DE . 2 b Ž DE . 3 c a b c

a

HZnCH 3

HCdCH 3

HHgCH 3

HZnSiH 3

HCdSiH 3

HHgSiH 3

1.2 8.4 6.5

0.7 10.2 y3.0

0.2 9.7 18.9

5.4 14.1 24.3

4.4 11.8 28.5

2.9 8.1 39.1

Ž DE .1 s w EŽXH 3 . q EŽMH.x y EŽHMXH 3 .. Ž DE . 2 s w EŽMXH 3 . q EŽH.x y EŽHMXH 3 .. Ž DE . 3 s  EŽXH 4 . q Ew M Ž3 P.x4 y EŽHMXH 3 ..

the matrix experiments, could not be considered as an intermediate compound in the gas phase; in contrast, the HMXH 3 3AX compound is actually an intermediate system in the gas phase. Furthermore, the matrix experiments w15–17x showed that the monomethyl-metal hydrides of the group-12 metals are photostable. This is in line with the conclusions of previous works w12,13x, which indicate that the MH q XH 3 Žor M q H q XH 3 . products could be formed smoothly and adiabatically on triplet potential surfaces. As shown in Table 8, we could confirm that the intermediate complex ŽHMXH 3 3AX . dissociates in the gas phase without any activation barrier into HM q XH 3 products, in accordance with a recent ab initio calculation w13x. We can summarize the studied reaction as follows: 3

3 X

properties we can consider the HMXH 3 Ž3AX . complex as an intermediate compound. The insertion reaction mechanism could be understood as an intersystem crossing between high and low spin states. Since the crossing region has been found near the minimum of the 3AX PES, we can suggest that there is no triplet–singlet gap for the spin conversion Ž3AX ™ 1A 1 .. Finally, it has been shown that, although the matrix cage thermodynamically favors the insertion product ŽHMXH 3 1A 1 ., the separation into two fragments ŽHM q XH 3 . is energetically possible and even favored in the gas phase, in accordance with previous works.

References

1

M Ž P . q XH 4 ™ HMXH 3 Ž A . ™ HMXH 3 Ž A 1 . in the matrix cage , or M Ž 3 P . q XH 4 ™ HMXH 3 Ž 3AX . ™ HM q XH 3 in the gas phase . Nevertheless, from inspection of the VŽM,X. populations ŽTable 7. and the Ž DE .1 values ŽTable 8. we suggest that metal-methyl hydrides should dissociate more easily than the metalsilyl ones.

3. Conclusions From a vibrational analysis, it has been shown that the product of the XH 4 q MŽ3 P. reaction in a rare-gas matrix is actually HMXH 3 in its ground state Ž1A 1 . and not the metastable complex ŽHMXH 3 in the 3AX excited state.. For both HMXH 3 Ž1A 1 . and HMXH 3 Ž3AX ., from the structural and electronic

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