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6 December 1996

Chemical Physics Letters 263 (1996) 145-147

CHEMICAL PHYSICS LETTERS

Dissociation energy of the molecule GeC1

D.L. Hildenbrand, K.H. Lau SRI International, Menlo Park, CA 94040, USA

Received 11 September 1996

Abstract

Mass spectrometry was used to identify the gaseous species GeC1, CuCI, Ge, and Cu in a molecular beam issuing from an effusive source, and to evaluate the equilibrium constant for the monochloride exchange reaction in the range 1543-1776 K. From the derived reaction enthalpy, the dissociation energy, D~, of GeCI was evaluated as 92.5 + 2.3 kcal mol- 1. The thermochemical value is lower than a spectroscopic value derived from a short extrapolation of vibrational levels in the 4~- state, indicating a potential maximum of about 7 kcal mol- ~ in that state.

1. Introduction 2. Experimental

The standard enthalpies of formation of the gaseous germanium chlorides GeC12 and GeC14 have been determined from equilibrium measurements with estimated uncertainties of 1.2 and 2.4 kcal tool - l , respectively [1]. For diatomic GeC1, how- ever, the tabulated thermochemical data [1] are based on a dissociation energy, D~, derived from a short Birge-Sponer extrapolation of vibrational levels in one of the lower excited states, with estimated uncer- tainty of 4.3 kcal mo1-1. For monohalides of the boron group, it is now well established that such excited state extrapolations lead to D~ values that are high by 5-8 kcal mol-~ because of potential maxima in the upper state curves [2-4]. In order to firm up the thermochemistry of GeC1, we studied the gaseous exchange equilibrium between GeC1 and CuC1 by mass spectrometry, and the results are reported here.

The direction-focusing mass spectrometer system equipped with a heated effusion-beam molecular source is described in earlier publications [5,6]. For these studies, Ge and Cu granules were contained in an Mo cup in the lower half of an Mo effusion cell, and C12(g) was added through a gas inlet tube in the base. A thin Mo partition with several peripheral apertures was placed between the cell halves to increase the number of reactive gas-solid collisions. The cell temperature was measured by optical py- rometry, sighting on a black body cavity in the cell lid. As always, neutral beam profiles were checked to ascertain the effusion cell origin of the measured ion signals. Ionization threshold energies were deter- mined by the vanishing current method; a more accurate value was obtained for GeCI by the extrapo- lated voltage difference (EVD) method using atomic Ge ÷ as reference, and using only intensities within 1

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146 D.L. Hildenbrand, K.H. Lau / Chemical Physics Letters 263 (1996) 145-147

eV of the threshold. All other aspects of the experi- mental procedure, interpretation, and analysis are as described previously [5,6].

The Ge and Cu samples were of reagent-grade quality, while the C12(g) was of Matheson high-pur- ity grade.

3. Results

Above 1500 K, the ions Ge ÷, GeCI +, Cu ÷, and CuCI ÷ were observed with threshold appearance potentials (AP) of 8.0, 7.8, 7.8, and 10.3 eV, respec- tively, all +0.3 eV. A more accurate determination on GeCI ÷ by the EVD method yielded an AP of 7.70 _ 0.10 eV. The Ge ÷, Cu ÷, and CuC1 ÷ APs are in agreement with known ionization potentials (IP) of the corresponding neutrals [7,8], while the value for GeCI ÷ is in reasonable accord with a correspond- ing IP(GeF) value of 7.46 eV obtained from a Ryd- berg series extrapolation [9]. It is clear that these ion signals arise from ionization of the corresponding parent neutrals in the threshold region.

Values of the equilibrium constant, K, for the gaseous exchange reaction

Ge + CuC1 = GeCi + Cu (1)

were evaluated over the range 1543 to 1776 K from the parent ion intensities measured at an AP of + 3 eV. Corrections were applied only for isotopic distri- bution; for simple exchange reactions, K values evaluated in this way are estimated to be accurate within a factor of 1.5. In Table 1 are shown the equilibrium data, including the derived second and third law enthalpy changes for reaction (1). Thermal functions used in the thermochemical analysis were taken from the IVTANTHERMO database [10]. In the two measurements at 1543 K, a change in the Cl2(g) flow rate led to a variation of more than a factor of ten in the GeCI and CuCI signals, but the derived K values were in close agreement, indicat- ing the achievement of equilibrium.

The derived second- and third-law enthalpies in Table 1 are in close accord; we prefer the third-law value, A H~'98 = - 3.2 __+ 1.5 kcal mol- i. On combin- ing this enthalpy value with D~9s(CuC1) = 90.3 + 1.8 kcal mol - l [8], one obtains D~9s(GeCI) = 93.5 _+ 2.3 kcal mol - t and D~(GeCI)= 92.5 kcal m o l - ' . The

Table 1

Summary of equilibrium data for the gaseous reaction Ge + CuCI

= GeCI + Cu

T (K) K AH~9 8 (kcal m o l - i)

1543 3.87 - 3.2

1543 3.90 - 3.2

1568 3.85 - 3.2

1568 3.90 - 3.2

1606 3.79 - 3 . 3

1638 3.68 - 3.3

1665 3.56 - 3.2

1665 3.65 - 3.3

1702 3.54 - 3.3

1776 3.47 - 3.4

1776 3.35 - 3.3

average - 3.2 + 1.5

second law - 2.2 5:2

derived dissociation energy is equivalent to the stan- dard enthalpy of formation AfH~'98(GeCl) = 23.4 + 2.3 kcal mol - l .

4. Discussion

The value of D~(GeCI) derived in this work from equilibrium studies is 6.9 kcal moi - l lower than the value of 99.4 kcal mol - l evaluated [1] from a short extrapolation of vibrational levels in what is now assigned as the a 4 ~ - state, and previously thought to be 2A [11]. This is yet another example of an excited state extrapolation leading to a ground state D~ value that is too high by 5 -8 kcal mo1-1 , most likely because of a potential maximum in the upper state [2-4]. With all the evidence now at hand, D~ values derived from short upper state extrapolations should be considered as upper limits, with the true values lower by as much as 8 kcal m o l - ' , or 0.35 e V .

From the established [1] enthalpies of formation AfH~'9s(GeCI2,g) = - 4 0 . 9 + 1.2 kcal mol - i and AfH2°98(GeCI4,g) = - 119.5 _+ 2.4 kcal mol - I one finds with the new data for GeCI that the first and second bond strengths Ge-CI and C1Ge-CI are es- sentially identical at 93 kcal tool- i. Also, the sum of the second and third bond strengths CI2Ge-C1 and CI3Ge-C1 is 137 kcal mo1-1, much weaker than the sum of the first two values. Based on corresponding

D.L. Hildenbrand, K.H. Lau / Chemical Physics Letters 263 (1996) 145-147 147

behavior in Si halides, Gurvich et al. [1] estimate AfH2°gs(GeCI3,g)=-64_+ 12 kcal mol - l , which leads to calculated Cl2Ge-Cl and Cl3Ge-Cl bond strengths of 53 and 84 kcal mo1-1 , respectively. It remains to be seen whether this estimate will hold up under experimental scrutiny. Equilibrium studies on GeCl 3 are likely to be difficult because of the ex- pected low abundance of this odd-electron species.

A linear Birge-Sponer extrapolation (LBX) using the ground state vibrational constants of Barrow and Lagerqvist [11] yields D~(GeC1) = 87 kcal mol - t , in fair agreement with our experimental result; the cor- rection of D(LBX) for molecular ionicity [12], which controls the rate of convergence of the vibrational levels, is negligible for GeCl. Gurvich et al. [1] selected a larger vibrational anharmonic constant, t%x e, leading to the smaller Birge-Sponer value D~(GeCl) = 73 kcal mol- t ; this choice for toe x e is not in accord with the thermochemical value for D~)(GeCI).

References

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45 (1966) 263. [4] D.L. Hildenbrand and K.H. Lau, J. Chem. Phys. 91 (1989)

4909. [5] D.L. Hildenbrand, J. Chem. Phys. 48 (1968) 3657; 52 (1970)

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Levin and W.G. Mallard, J. Phys. Chem. Ref. Data 17 (1988) Suppl. No. 2.

[8] D.L. Hildenbrand and K.H. Lau, High Temp. Mater. Sci. 35 (1996) II.

[9] R.W. Martin and A.J. Merer, Can. J. Phys. 52 (1974) 1458. [10] IVTANTHERMO, Database on thermodynamic properties of

individual substances (CRC Press, Boca Raton, FL, 1993). [11] R.F. Barrow and A. Lagerqvist, Ark. Fys. 1 (1949) 221. [12] D.L. Hildenbrand, J. Electrochem, SOC. 126 (1979) 1396.