A stable species with a formal GeC triple bond – a theoretical study

15 June 2001

Chemical Physics Letters 341 (2001) 122±128

www.elsevier.nl/locate/cplett

A stable species with a formal GeBC triple bond ± a theoretical study Hsin-Yi Liao a, Ming-Der Su b,*, San-Yan Chu a,1 a

Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC b School of Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan, ROC Received 12 February 2001; in ®nal form 16 April 2001

Abstract The isomerization of triplet and singlet HCBGeF to C@Ge…H†…F† and to …H†…F†C@Ge have been studied by various computational methods. Our theoretical investigations suggest that electronegative halogen groups can considerably stabilize a carbon±germanium triple bond. In particular, triplet HCBGeF, rather than singlet HCBGeF, is predicted to be a promising candidate for isolation as a long-lived molecule. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction The synthesis and isolation of compounds having multiple bonds to germanium has frustrated chemists for a long time because of their high reactivity and very great tendency to polymerize [1±10]. It was therefore not surprising that the study of organogermanium doubly bonded species was not developed until the 1980s. Through the elegant studies performed by Stage, et al. [1±10], several kinds of germanium-containing unsaturated species such as Ge@Ge, Ge@C, Ge@N, Ge@P, and Ge@S double-bond compounds have been isolated by kinetic or by thermodynamic stabilization. Now that a variety of synthetic techniques are available [11,12] for the laboratory preparation of molecules with germanium±carbon double bonds,

* 1

Corresponding author. Fax: +886-07-3125339. E-mail address: [email protected] (M.-D. Su). Also corresponding author.

it is inevitable that chemists will begin to devise schemes for the synthesis of germanium±carbon triple bonds. In fact, despite the upsurge of interest, the triply bonded germanium species …±GeBX† has, as yet, neither been isolated nor clearly characterized by trapping experiments [13]. Theory should therefore be able to provide guidance in this regard, but only three studies [14±16], to our knowledge, on germaacetylene …HGeBCH† have already appeared. Indeed, it is astonishing how little is known about the stability and molecular properties of germaacetylenes, considering the importance of germanium hydrides in semiconductor processing [17] and the extensive research activity on the corresponding acetylene [18] and silaacetylene [19] species. Recently, Schwarz, Apeloig, and co-workers [20,21] have reported the ®rst experimental evidence for the existence of neutral HCBSiX (X ˆ F and Cl) molecules containing a CBSi triple bond by means of neutralization±reionization mass spectrometry. In view of the interest in stabilizing a carbon±germanium triple bond, it is important

0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 4 6 1 - 4

H.-Y. Liao et al. / Chemical Physics Letters 341 (2001) 122±128

to consider the possibility of stabilizing this moiety using substituents. For this purpose, we have undertaken the ®rst theoretical calculations concerning four kinds of substituted compounds, HCBGeX and XCBGeH (X ˆ H, F, Cl, and CH3 ) [16]. Our earlier theoretical studies based on the B3LYP and QCISD levels have revealed that both HCBGeX and XCBGeH are local minima on the singlet potential energy surface, but they are neither kinetically nor thermodynamically stable. In other words, the prospects of observing singlet germaacetylenes in a matrix or even as transient intermediates appears to be unlikely. Nevertheless, in the present paper, we report new calculations on the triplet potential energy surface of HCBGeF. Our results show that the triplet HCBGeF is a promising candidate for isolation as a long-lived molecule. With regard to the structure and stability of germaacetylene, we now wish to describe the results of theoretical calculations on the energy surface of the model CGeHF system. This exhibits a number of stationary points, including local minima corresponding to 1-¯uoro-1-germaethyne (HCBGeF), 2-¯uoro-germaylidene ((F)(H)C@Ge), 1-¯uoro-1-germavinylidene (C@Ge…H†…F†), and saddle points connecting them. The transition structures separating the three stable molecular forms involve a successive unimolecular 1,2-hydrogen shift (reaction (A)) and a 1,2-¯uorine shift (reaction (B)) between the carbon and germanium atoms. The interconversion pathway is examined in both the lowest lying singlet and triplet electronic states. 2. Methodology The geometries of all isomeric structures were initially optimized at the restricted (singlet) and unrestricted (triplet) B3LYP levels of theory [22±24], and then were fully optimized by using the QCISD method [25]. The harmonic vibrational frequencies were calculated at the B3LYP level of theory to con®rm the nature of the stationary points. The zero-point energies (ZPE) were also evaluated at the same level of theory, unless otherwise stated. Re®ned energies were obtained

123

in QCISD(FC) single point calculations on the QCISD(FC)/(6-311++G(3df,3pd))-optimized structures, including ZPE corrections determined at B3LYP/(6-311++G(2d,2p)). The GA U S S I A N 94 program [26] was employed with standard basis sets for molecules containing elements from hydrogen through germanium during optimizations, frequency runs (6-311 G(d) and 6-311++G(2d,2p)), single point calculations (6-311++G(3df,3pd)). 3. Results and discussion The optimized geometrical parameters for both singlet and triplet structures from the various types of calculations are collected in Table 1. Besides the geometries of those stationary points, we have also calculated their vibrational frequencies, infrared (IR) intensities, rotational constants, and dipole moments; the best values obtained at the B3LYP/ 6-311++G(2d,2p) level are shown in Table 2, which should be useful for further experimental observations. Fig. 1 displays the energy pro®les for the singlet and triplet electronic states of the CGeHF species considered in this work. The main conclusions to be drawn from this work are as follows (1) For the case of the singlet state, as one can see in Fig. 1, two planar doubly bonded germanium structures with Cs symmetry exist as minima on the potential energy surface. Namely, one is:C@Ge…H†…F† which has the lone electron pair residing on the carbon, and the other is (H)(F)C@ Ge: which has the lone pair residing on the germanium. It should be noted that the former structure:…C@Ge…H†…F†† possesses the highest energy of all the minima on the singlet CGeHF surface, whereas the latter structure ……H†…F†C@Ge†: is predicted to be the most stable CGeHF structure at the computational levels employed in this work. According to our best QCISD results as given in Fig. 1, the energy di€erence between C@Ge…H†…F† and (H)(F)C@Ge is about 61 kcal/mol. These observations re¯ect the fact that the singlet states of the C@Ge species prefer to have the nonbonding electrons residing on germanium rather than on carbon. Moreover,

124

Table 1  Angles in deg.) Geometrical parameters of structures for isomerizations at three levels of theory (distances in A,

c

d

e

f

g

h

i

j

k

l

m

QCISD/ 6-311++G(2d,2p) Triplet 1.949 1.806 Singlet 1.789 1.790

a

1.086 1.084

144.4 134.0

97.18 121.4

1.964 1.827

1.799 1.782

2.215 2.160

44.75 46.00

1.840 1.921

2.378 1.847

1.077 1.091

50.83 78.92

B3LYP/ 6-311++G(2d,2p) Triplet 1.944 1.785 Singlet 1.769 1.753

1.089 1.086

140.4 132.9

98.41 123.9

1.942 1.804

1.779 1.746

2.193 2.199

45.28 45.20

1.835 1.891

2.375 1.825

1.079 1.089

51.27 83.56

B3LYP/6-311G(d) Triplet 1.946 Singlet 1.768

1.783 1.755

1.093 1.090

140.0 132.8

98.56 124.3

1.946 1.803

1.778 1.749

2.181 2.191

45.77 45.59

1.834 1.882

2.331 1.824

1.083 1.091

51.66 84.35

o

n

b

p

q

r

s

t

u

v

w

x

y

z

QCISD/ 6-311++G(2d,2p) Triplet 1.905 1.780 Singlet 1.881 1.780

1.547 1.533

107.1 107.5

1.981 1.851

1.383 1.378

1.087 1.087

107.8 110.8

180.0 180.0

98.28 180.0

180.0 79.42

146.9 180.0

180.0 180.0

B3LYP/ 6-311++G(2d,2p) Triplet 1.913 1.762 Singlet 1.865 1.754

1.552 1.534

105.1 107.5

1.963 1.837

1.346 1.360

1.091 1.091

108.8 111.0

180.0 180.0

99.22 180.0

180.0 85.45

153.1 180.0

180.0 180.0

B3LYP/6-311G(d) Triplet 1.914 Singlet 1.864

1.557 1.540

103.8 107.7

1.966 1.840

1.343 1.357

1.095 1.095

108.7 110.8

180.0 180.0

99.44 180.0

180.0 85.95

158.4 180.0

180.0 180.0

1.763 1.757

H.-Y. Liao et al. / Chemical Physics Letters 341 (2001) 122±128

Method

H.-Y. Liao et al. / Chemical Physics Letters 341 (2001) 122±128

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Table 2 Calculated harmonic vibrational frequencies, IR intensities, rotational constants, dipole moments and relative energies of the species in HCGeF isomerization reactions at the B3LYP/6-311++G(2d,2p) level of theory Species HCBGeF Triplet Singlet

C@GeHF Triplet Singlet

FHC@Ge Triplet Singlet

Frequency (intensity) (cm 1 /mol)

Rotational constants (MHz)

Dipole moment (Debye)

3134 (7), 630 (90), 603 (103), 446 (45), 362 (17), 203 (1)

A 15500.99 B 7526.69 C 5066.56 A 34649.16 B 6216.49 C 5270.84

2.786

0.0

1.815

9.565

3185 (1), 828 (57), 759 (97), 641 (99), 388 (101), 158 (9)

2045 (127), 698 (22), 670 (77), 610 (51), 185 (30), 119 (16) 2129 (53), 765 (10), 676 (26), 630 (94), 282 (1), 137 (30)

3080 (17), 1320 (21), 1103 (256), 567 (22), 527 (16), 252 (6) 3078 (18), 1327 (29), 1118 (211), 631 (75), 611 (34), 189 (6)

A 26051.11 B 6388.29 C 5154.54 A 25842.94 B 6660.66 C 5295.75 A 62607.96 B 3432.29 C 3253.90 A 71606.97 B 3594.51 C 3422.70

from the bonding point of view, the thermodynamic stability of …H†…F†C@Ge relative to C@Ge…H†…F† is attributed to the ability of germanium's di€use electron cloud to accommodate a lone electron pair more easily than that of carbon [14]. Furthermore, in the portions of the singlet energy surface explored, the stabilities of the three local minima decrease in the order …H†…F†C@ Ge > HCBGeF > C@Ge…H†…F†. When the reaction is viewed as starting from C@Ge…H†…F†, the successive conversion of C@Ge…H†…F† to HCBGeF and HCBGeF to …H†…F†C@Ge requires 7.0 and 11 kcal/mol, respectively. In other words, the triply bonded structure HCBGeF seems to be unstable on the singlet energy surface and undergoes unimolecular rearrangement to the doubly bonded isomer …H†…F†C@Ge. As the reaction proceeds in the endothermic direction, the nondissociative rearrangement …H†…F†C@Ge ! HCB GeF ! C@Ge…H†…F† is much more dicult to achieve, with energy barriers of 25 and 54 kcal/ mol, respectively. In consequence, as predicted in

Relative energy (kcal/mol)

2.521

46.79

2.325

56.65

1.027

7.328

1.542

)7.637

our previous work [16], our theoretical investigations demonstrate that singlet HCBGeF itself is neither kinetically nor thermodynamically stable with respect to isomerization reactions. (2) For the case of the triplet state, as Fig. 1 shows, there are also three local minima, …F†…H†C@Ge; C@Ge…H†…F†, and trans-bent HCBGeF structures. Of these, the 3 A00 state of the planar …F†…H†C@Ge structure has a singly occupied p-orbital and a singly occupied 4py in-plane orbital at the terminal germanium atom. On the other hand, the local minimum of triplet C@Ge…H†…F† adopts a bent structure with a pyramidal geometry at the germanium center, and thus possesses no elements of symmetry …C1 †. According to our best computational results (QCISD), the global minimum on the triplet energy surface of CGeHF is HCBGeF, whereas the doubly bound isomers C@Ge…H†…F† and …F†…H†C@Ge are 47 and 10 kcal/mol higher in energy, respectively. Additionally, HCBGeF on the triplet energy surface is separated by much

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H.-Y. Liao et al. / Chemical Physics Letters 341 (2001) 122±128

Fig. 1. Relative energies, in kcal/mol, of the pathways for isomerization of the singlet and triplet CGeHF species. The theoretical methods used are as follows: (1) B3LYP/6-311G(d) + ZPE (B3LYP/6-311G(d)). (2) B3LYP/6-311++G(2d,2p) + ZPE (B3LYP/ 6-311++G(2d,2p)). (3) QCISD/6-311++G(2d,2p) + ZPE (B3LYP/6-311++G(2d,2p)). (4) QCISD/6-311++G(3df,3pd)//QCISD/ 6-311++G(2d,2p) + ZPE (B3LYP/6-311++G(2d,2p)).

more sizable energy barriers (i.e., 59 and 50 kcal/ mol for reactions (A) and (B), respectively), rendering this species as a more likely candidate for separate detection during gas phase reactions of germanium compounds at room temperature. Besides these, it should be stressed that a high activation barrier (40 kcal/mol at QCISD) exist from …F†…H†C@Ge to HCBGeF. Accordingly, this ®nding strongly implies that the triplet …F†…H†C@Ge should be a kinetically stable species in the absence of molecular collisions, despite its thermodynamic instability. (3) A comparison of the potential energy surfaces of the singlet and triplet CGeHF species is shown in Fig. 1. Singlet …F†…H†C@Ge is, as expected, the most stable structure and lies 6.8 kcal/mol below the most stable triplet species,

HCBGeF. The singlet±triplet energy separations between the local minima are of interest. The triplet±singlet gap in the planar bonded species ……F†…H†C@Ge† is predicted to be 16 kcal/mol, favoring singlet over triplet state. By contrast, the closed-shell singlet state of C@Ge…H†…F† is 6.9 kcal/mol less stable than its triplet counterpart. Finally, only 7.1 kcal/mol separates singlet HCBGeF from its triplet ground state HCBGeF. It should be mentioned that Fig. 1 illustrates clearly the crossing of the triplet and singlet potential surfaces during the CGeHF isomerization. Should the 3 A00 and 1 A0 states be suciently strongly connected by spin±orbit coupling or some other interaction [27±29], a crossing from the triplet to singlet potential energy surface might occur.

H.-Y. Liao et al. / Chemical Physics Letters 341 (2001) 122±128

(4) The important structural parameters of triplet HCBGeF are the C±Ge bond length, 1.95  and the C±Ge±F angle, 97.2°. This C±Ge bond A, distance is signi®cantly longer than that of singlet  although it is shorter HCBGeF (C±Ge ˆ 1.79 A), than typical C±Ge single bond distances (1.96±2.00  [30]. The observed C±Ge±F angle, which is A) relatively close to 90° (compared to the C±Ge±F angle, 121°, in singlet HCBGeF), strongly implies the presence of a lone pair at the germanium atom. These peculiar structural parameters may provide a clue to understand the chemical bonding in the HCGeF species. We thus suggest that the bonding in the triplet HCBGeF may be explained by assuming that an unshared electron pair resonates between germanium and carbon atoms that are connected by a single bond (see 1), instead of the acetylene like form 2. In particular for valence structure 1a, germanium uses two of its 4p orbitals for bonding to the ¯uorine and carbon atoms, the remaining 4p orbital is singly occupied, and the lone pair is thus accommodated in the lower energy 4s orbital [31]. This is because the heavy Ge atom has a lower tendency than carbon to form a hybrid orbital [32] and, in turn, Ge1 prefers to maintain the …4s†2 …4p†3 valence electron con®guration. As a result, in the triplet HCBGeF, 1a valence structure has much more weight than 1b. Likewise, one explanation for the multiple bonding in the singlet HCBGeF can be represented in valence bond terms by a resonance form 3. Indeed, the chemical bonding of both triplet and singlet HCBGeF predicted with this simple valence bond theory is consistent with that predicted by quantum mechanical methods. For instance, according to the bond order formula of Pauling [33], the calculated C±Ge distance for triplet HCBGeF af-

127

fords a bond order of 1.18. Similarly, resonance of 3a,b results in delocalization of the lone pairs and produces a net C±Ge bond order of 1.97. From these bonding analyses, one may therefore conclude that the single bond represented by 1 and the double bond represented by 3 are reasonable bonding approximations for the cases of triplet and singlet HCBGeF, respectively. (5) Also, note that the Ge±C bond lengths in the various isomers re¯ect the expected changes in bond order: The linear singlet HCBGeF species  consistent has the shortest bond length, 1.64 A, with a triplet bond, and the length increases in the trans-bent minimum as already discussed in point (4). Nevertheless, our DFT calculations indicate that the linear singlet HCBGeF is not a stable structure. It has two imaginary vibrational frequencies corresponding to a trans-bending mode, leading to the trans-bent isomer. Furthermore, one may therefore wonder why linear singlet triply bonded HCBGeF is considerably higher in energy than the corresponding trans-bent isomer. The reason for this is that, as suggested by Stogner and Grev [14], the states with the greatest germanium s character are most stabilized. That is to say, the linear singlet HCBGeF possesses one sp-hybridized germanium r orbital and four electrons in two pure p-p orbitals, while the trans-bent isomer has more s character in sp-hybridized germanium orbitals, which results from the stabilizing in¯uence of mixing in s-rich r bonds into the p orbitals [2]. In consequence, the linear singlet HCBGeF is destabilized with respect to the corresponding trans-bent singlet and triplet isomers.

In summary, our theoretical investigations suggest that triplet HCBGeF should be both kinetically and thermodynamically stable towards unimolecular isomerizations. Surprisingly, our computational results indicate that triplet HCBGeF lies lower in energy than singlet HCBGeF by 7.1 kcal/mol. These ®ndings are very

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H.-Y. Liao et al. / Chemical Physics Letters 341 (2001) 122±128

unusual. Triplet states of C2 H2 [34], Si2 H2 [35], and HCGeH [13,15] have also been investigated by several groups. They reported that, for a given structural type, the triplets always appear to be higher in energy than the singlets. Besides these, the triplet HCBGeF molecule is calculated to possess a large dipole moment (2.8 D; see Table 2), indicating that the triplet HCBGeF itself should be possible to isolate for experimental observation. We have also provided accurate theoretical estimations for various thermodynamic data (see Table 2) that can guide future experimental studies on these species, and for which there is an almost complete lack of experimental information. However, singlet …F†…H†C@Ge lies 6.8 kcal/mol below triplet HCBGeF and might be accessible if the spin±orbit coupling were substantial. We encourage experimentalists to design new experiments to con®rm our predictions. Acknowledgements We are grateful to the National Center for HighPerformance Computing of Taiwan and the Computing Center at Tsing Hua University for generous amounts of computing time. We also thank the National Science Council of Taiwan for their ®nancial support. We express our gratitude to anonymous referees for their valuable comments. References [1] J. Barrau, J. Esoudie, J. Satge, Chem. Rev. 90 (1990) 283. [2] R.S. Grev, Adv. Organomet. Chem. 33 (1991) 125. [3] J. Esoudie, C. Couret, H. Ranaivonjatovo, J. Satge, Coord. Chem. Rev. 130 (1994) 427. [4] J. Barrau, G. Rima, Coord. Chem. Rev. 178 (1998) 593. [5] J. Escudie, C. Couret, H. Ranaivonjatovo, Coord. Chem. Rev. 178 (1998) 565. [6] N. Tokitoh, T. Matsumoto, R. Okazaki, Bull. Chem. Soc. Jpn. 72 (1999) 1665.

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