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Electronic structure and stability of some silicon compounds
Chemical Physics Letters 491 (2010) 136–139
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electronic structure and stability of some silicon compounds Igor Novak a,*, Tareq Abu-Izneid a, Branka Kovacˇ b,** a b
Charles Sturt University, P.O. Box 883, Orange, NSW 2800, Australia Physical Chemistry Division, ‘R. Boškovic´’ Institute and Department of Chemistry, Faculty of Natural Sciences, University of Zagreb, HR-10000 Zagreb, Croatia
a r t i c l e
i n f o
Article history: Received 19 December 2009 In final form 29 March 2010 Available online 31 March 2010
a b s t r a c t The electronic structures of N,1,3-tris(1,1-dimethylethyl)-cyclodisilazan-2-amine (I) and 2,3,5,5-tetrakis(trimethylsilyl)cyclopentadiene (II) have been investigated by HeI and HeII UV photoelectron spectroscopy (UPS) and quantum chemical calculations. We discuss the influence of substituent effects on their electronic structure and thermodynamic stability. Our study shows that trimethylsilyl substituents have strong influence on the electronic structure of cyclopentadiene via inductive effect. Their influence on thermodynamic stability is also pronounced. In substituted cyclodisilazanes hyperconjugative influence of alkylsilyl groups was shown to cause relative thermodynamic stabilization of the cyclodisilazane system. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The compounds studied in this work have interesting applications in the manufacture of ceramic materials, for the generation of silicon nitride coatings and as ligands for the preparation of transition metal complexes which can be used as catalysts [1]. The structures of molecules I and II studied in this work are shown in Figs. 1 and 2. The photoelectron spectra of compounds similar to I and II have been reported previously [2–5] but in these earlier studies the emphasis was not on the electronic structures of cyclodisilazanes themselves which were considered merely as precursors for other substances e.g. silanimines [3,4]. We present an investigation of the electronic structure and thermodynamic stability of I and II using a combination of photoelectron spectroscopy and high-level ab initio calculations. The compounds selected have not been studied before and the results regarding their electronic structure and stability can be used for understanding chemical behaviour of their congeners. 2. Experimental methods The sample compounds were purchased from Aldrich and used without further purification after checking their identity and purity by NMR spectroscopy. The HeI/HeII photoelectron spectra (UPS) were recorded by a Vacuum Generators UV-G3 spectrometer and calibrated with small amounts of Xe or Ar gas being added to the sample flow. The spectral resolution of the HeI and HeII spectra was 25 meV and 70 meV, * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (I. Novak), [email protected] (B. Kovacˇ). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.03.079
Ar respectively being measured as FWHM of the 3p1 2P3/2 Ar+ (1S0) line. The samples were studied with the inlet probe heated to 35 and 50 °C, respectively. The spectra obtained were reproducible and showed no signs of decomposition. Decomposition is usually discernible from the appearance of sharp intense peaks which are due to the presence of small molecules (decomposition products) in the spectrometer’s ionization chamber. We did not detect any decomposition peaks due to e.g. alkenes or alkynes which were reported in the flash vacuum thermolysis studies [3,4]. Quantum chemical calculations were performed with the GAUSSIAN 03 program [6] for two purposes: assignment of photoelectron spectra and the calculation of reaction enthalpies. We used the Greens functions method (GF) and effective core potential basis set [7,8] for the first task. The calculated molecular geometries, fully optimized at the B3LYP/6-31G* level were similar to experimentally determined structures [9,10] of related molecules (Table 1). For the second task the total electronic energy of each molecule was computed using the G3MP2/B3LYP method [11a] which has a root-mean-square deviation of approximately 4 kJ/mol. The method includes full geometry optimization at the B3LYP/6-31G* level followed by single point QCISD type calculations. All the optimized structures corresponded to minima on their potential energy surfaces as was inferred from the absence of imaginary vibrational frequencies. Nucleus independent chemical shifts NICS(0) were calculated at GIAO B3LYP/6-311 + G(d,p) level [11b].
3. Results and discussion The photoelectron spectra of I and II are shown in Figs. 1 and 2, respectively. The spectral assignments are summarized in Table 2 and are based on HeI/HeII intensity variations if observable, GF
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I. Novak et al. / Chemical Physics Letters 491 (2010) 136–139
Fig. 2. HeI and HeII photoelectron spectra of II.
Fig. 1. HeI and HeII photoelectron spectra of I.
calculations and comparison with the assigned spectra of related molecules [2–5]. The assignments obtained are unambiguous. 3.1. Photoelectron spectrum of I In the ionization energy region below 9.5 eV in the spectrum of I we observe bands at 7.85 and 8.60 eV which correspond to three nitrogen lone pair orbitals, while the remaining bands correspond to r orbitals with Si–N and Si–C character. We rationalized the role of substituents in cyclodisilazanes by comparing the photoelectron spectra of molecules 1–5 (Fig. 3) reported previously [3–5] with the spectrum of I from this work. We focus our analysis on nitrogen lone pairs which give two well resolved bands corresponding to out-of-phase (n) and in-phase (n+) linear combinations of N2p orbitals. The molecules and corresponding ionization energies are shown in Fig. 3. We observe that the in-phase orbital (n+) is destabilized by approximately 0.8 eV when more than one silicon atom becomes bonded to nitrogen. The destabilization can be attributed to interaction of n+ with occupied Si–C orbitals localized on substituents (hyperconjugation). An independent inference concerning hyperconjugative effects can be deduced from the relative HeI and HeII intensities of the first two bands at 7.85 and 8.60 eV. The 7.85 eV band increases slightly in relative intensity on going from HeI to HeII radiation (Table 2). How can we rationalize this increase? From the HeII/HeI atomic photoionization cross
Table 1 Geometry parameters of the compounds studieda. Compound
Si–N
N–C
NSiN
I Ref. [9]
1.796 Å 1.736 Å
1.460 Å 1.472 Å
84.7° 86.4°
Si–C
C@C
C–C
SiCSi
II Ref. [10]
1.93 Å 1.89 Å
1.38 Å 1.42 Å
1.48 Å 1.49 Å
117.4° 119°
a The parameters for I and II are from DFT calculations, those from Ref. [9] and are from gas phase electron diffraction studies.
Table 2 Experimental (Ei/eV) vertical ionization energies, orbital assignments for I, II and related compounds. Compound
Band
Ei
GF/eV
Assignment
Relative HeII/ HeI intensity
I
X A–B C D
7.85 8.60 10.05 10.55
7.74 8.42, 8.52 10.28 10.67
n n+ nNH
1.0 0.8
Cyclopentadiene
X A
8.61 10.70
II
X A B–C D
7.60 8.50 10.3 11.05
r r
p p 7.56 8.41 9.43–10.50 11.40
p p rSi–C rSi–C
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I. Novak et al. / Chemical Physics Letters 491 (2010) 136–139
section ratios for C2p, N2p and Si3p which are 0.31, 0.45 and 1.0 [12], we expect that ionizations from orbitals with more silicone character shall exhibit an intensity increase. All three nitrogen lone
N
1
Si
N
2
3
Si
N
Si
Si
N
4
7.8, 9.3, 11.0, 12.0 eV
3.2. Photoelectron spectrum of II
8.0, 8.5, 9.85, 10.6, 11.3 eV
N
Si
Si
N
N
I
7.8, 9.3, 10.5, 11.8 eV
7.8, 8.3, 9.7, 10.1 eV
SiH 2
7.85, 8.60, 10.05, 10.55, 11.5 eV N
HSi HN
SiH
5
pair orbitals corresponding to these bands can acquire silicon character through hyperconjugation. However, the 8.60 eV band contains ionization from an exocyclic amino group which has single silicon bonded to it. The other two endocyclic amino groups have two silicon atoms attached and can thus be expected to acquire slightly more silicon character than the exocyclic group. As a result, relative intensity of the 8.60 eV band shows relative decrease on going from HeI to HeII excitation compared to the 7.85 eV band. The perturbation of the electronic structure is particularly pronounced in structure 5 of Fig. 3 where three silicon atoms are bonded to every nitrogen. In that case the n and n+ orbitals become quasi degenerate due to interactions with Si–C and Si–H bonding orbitals as proposed previously [2]. We have also determined the relative thermodynamic stability of cyclodisilazane versus appropriate reference compounds. We found cyclodisilazane to be stable as can be seen from enthalpies for isodesmic reactions (1) and (2) in Fig. 4. This stability may be related to the flexibility of cyclodisilazane ring and is unaffected by the presence of substituents as noted previously [9]. For example, removal of alkyl substituents on silicon and nitrogen as in reactions (3) and (4) of Fig. 4 changes the reaction enthalpies vs. (1) and (2) by less than 5 kJ/mol.
The two broad bands at 7.6 and 8.5 eV in the photoelectron spectrum of II correspond to two p-orbitals localized on the cyclopentadiene ring. No significant changes in their HeI/HeII relative band intensities were detected. The very broad manifold with maximum at 10.3 eV can be attributed to ionizations from ten r-orbitals localized on trimethylsilyl substituents. The first two bands in the spectrum of II are inductively shifted by 0.45 and 0.6 eV, respectively towards lower ionization energies compared to 1,1bistrimethylsilylcyclopentadiene [2]. We have determined the effect of trimethylsilyl substituents on the thermodynamic stability of cyclopentadiene by calculating reaction enthalpies of the appropriate isodesmic reactions (5) and (6) shown in Fig. 4. The enthalpies show that 1,1-trimethylsilyl substitution stabilizes cyclopentadiene by 40.7 kJ/mol. The stabilization is consistent with hyperconjugative mechanism [5]. However further substitution which gives tetra-trimethylsilyl derivative gives smaller stabilization per trimethylsilylgroup as can be seen from the enthalpy of reaction (6) in Fig. 4. We have also calculated nucleus independent chemical shifts (NICS) which are descriptors of aromaticity. Cyclopentadiene is nonaromatic as is well established [13] (NICS = 3.26), but 1,1disubstitution by trimethylsilyl groups gives the ring some aromatic character (NICS = 7.92). However, 1,1,3,4-tetrasubstitution by the same groups reduces aromaticity; NICS = 5.63. This reduction in aromaticity on going from di- to tetra-derivative is consistent with the reduction of relative thermodynamic stability in the respective compounds.
4. Conclusion
N
Si
Si
N
8.7, 10.0, 10.8, 11.95 eV H Si
Fig. 3. The lowest ionization energies of silicon compounds determined by UPS.
We have determined the influence of alkyl and alkylsilyl substituents on the electronic structure of some silicon compounds. t-butyl substituent exerts mostly an inductive destabilizing effect on the nitrogen lone pair or on p-orbital orbitals. On the other hand trimethylsilyl group exerts a thermodynamic stabilizing effect of approximately 20 kJ/mol per group on the cyclopentadiene system. The effect is not cumulative and tetrasubstitution is less stabilizing than disubstitution.
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I. Novak et al. / Chemical Physics Letters 491 (2010) 136–139
Si
Si
2
=
N
N
+
NH
H 2Si HN
H2Si
+
+
NH
SiMe3
+
-78.7 kJ/mol
+
(2)
-193.4 kJ/mol
(3)
(4)
SiH2
= SiMe3
4
(1)
NH
SiMe3 SiMe3
-73.9 kJ/mol
-192.3 kJ/mol
=
=
2
+
Si
NH
+
H 2Si
N
=
Si
N
2
SiH
Si
N
+
= SiMe3
SiMe3
+
(5) 2
SiMe3
+
- 40.7 kJ/mol
(6) 4
- 54.6 kJ/mol
SiMe3
Fig. 4. Enthalpies of isodesmic reactions.
Acknowledgements Authors thank the Ministry of Science, Education and Sports of the Republic of Croatia for the financial support through Project 098-0982915-2945 (CSU Ref. No. OPA 4068) and the Faculty of Science, Charles Sturt University for the financial support of this work through the Seed Grant A105-954-639-3495. References [1] A.M. Wrobel, I. Blaszczyk-Lezak, A. Walkiewicz-Pietrzykowska, T. Aoki, J. Kulpinski, J. Electrochem. Soc. 155 (2008) K66. [2] H. Bock, W. Kaim, J. Am. Chem. Soc. 102 (1980) 4429. [3] V. Metail, S. Joanteguy, A. Chrostowska-Senio, G. Pfister-Guillouzo, A. Systermans, J.L. Ripoll, Main Group Chem. 2 (1997) 97.
[4] V. Metail, S. Joanteguy, A. Chrostowska-Senio, G. Pfister-Guillouzo, A. Systermans, J.L. Ripoll, Inorg. Chem. 36 (1997) 1482. [5] T. Veszpremi, L. Bihatsi, Y. Harada, K. Ohno, H. Mutoh, J. Organomet. Chem. 280 (1985) 39. [6] M.J. Frisch et al., GAUSSIAN 03, Revision E1, Gaussian, Inc., Pittsburgh, PA, 2007. [7] W. Von Niessen, J. Schirmer, L.S. Cederbaum, Comput. Phys. Rep. 1 (1984) 57. [8] U. Häussermann, M. Dolg, H. Stoll, H. Preuss, Mol. Phys. 78 (1993) 1211. [9] E. Gergö, G. Schultz, I. Hargittai, J. Organomet. Chem. 292 (1985) 343. [10] N.N. Veniaminov, T. Zh. Strukt. Khim. 13 (1972) 136. [11] (a) A.G. Baboul, L.A. Curtiss, P.C. Redfern, K. Raghvachari, J. Chem. Phys. 110 (1999) 7650; (b) S.M. Bachrach, Computational Organic Chemistry, Springer, New York, 2007. [12] J.J. Yeh, Atomic Calculation of Photoionization Cross-Sections and Asymmetry Parameters, Gordon and Breach, Langhorne, 1993. [13] A. Stanger, Chem. Eur. J. 12 (2006) 2745.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electronic structure and stability of some silicon compounds Igor Novak a,*, Tareq Abu-Izneid a, Branka Kovacˇ b,** a b
Charles Sturt University, P.O. Box 883, Orange, NSW 2800, Australia Physical Chemistry Division, ‘R. Boškovic´’ Institute and Department of Chemistry, Faculty of Natural Sciences, University of Zagreb, HR-10000 Zagreb, Croatia
a r t i c l e
i n f o
Article history: Received 19 December 2009 In final form 29 March 2010 Available online 31 March 2010
a b s t r a c t The electronic structures of N,1,3-tris(1,1-dimethylethyl)-cyclodisilazan-2-amine (I) and 2,3,5,5-tetrakis(trimethylsilyl)cyclopentadiene (II) have been investigated by HeI and HeII UV photoelectron spectroscopy (UPS) and quantum chemical calculations. We discuss the influence of substituent effects on their electronic structure and thermodynamic stability. Our study shows that trimethylsilyl substituents have strong influence on the electronic structure of cyclopentadiene via inductive effect. Their influence on thermodynamic stability is also pronounced. In substituted cyclodisilazanes hyperconjugative influence of alkylsilyl groups was shown to cause relative thermodynamic stabilization of the cyclodisilazane system. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The compounds studied in this work have interesting applications in the manufacture of ceramic materials, for the generation of silicon nitride coatings and as ligands for the preparation of transition metal complexes which can be used as catalysts [1]. The structures of molecules I and II studied in this work are shown in Figs. 1 and 2. The photoelectron spectra of compounds similar to I and II have been reported previously [2–5] but in these earlier studies the emphasis was not on the electronic structures of cyclodisilazanes themselves which were considered merely as precursors for other substances e.g. silanimines [3,4]. We present an investigation of the electronic structure and thermodynamic stability of I and II using a combination of photoelectron spectroscopy and high-level ab initio calculations. The compounds selected have not been studied before and the results regarding their electronic structure and stability can be used for understanding chemical behaviour of their congeners. 2. Experimental methods The sample compounds were purchased from Aldrich and used without further purification after checking their identity and purity by NMR spectroscopy. The HeI/HeII photoelectron spectra (UPS) were recorded by a Vacuum Generators UV-G3 spectrometer and calibrated with small amounts of Xe or Ar gas being added to the sample flow. The spectral resolution of the HeI and HeII spectra was 25 meV and 70 meV, * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (I. Novak), [email protected] (B. Kovacˇ). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.03.079
Ar respectively being measured as FWHM of the 3p1 2P3/2 Ar+ (1S0) line. The samples were studied with the inlet probe heated to 35 and 50 °C, respectively. The spectra obtained were reproducible and showed no signs of decomposition. Decomposition is usually discernible from the appearance of sharp intense peaks which are due to the presence of small molecules (decomposition products) in the spectrometer’s ionization chamber. We did not detect any decomposition peaks due to e.g. alkenes or alkynes which were reported in the flash vacuum thermolysis studies [3,4]. Quantum chemical calculations were performed with the GAUSSIAN 03 program [6] for two purposes: assignment of photoelectron spectra and the calculation of reaction enthalpies. We used the Greens functions method (GF) and effective core potential basis set [7,8] for the first task. The calculated molecular geometries, fully optimized at the B3LYP/6-31G* level were similar to experimentally determined structures [9,10] of related molecules (Table 1). For the second task the total electronic energy of each molecule was computed using the G3MP2/B3LYP method [11a] which has a root-mean-square deviation of approximately 4 kJ/mol. The method includes full geometry optimization at the B3LYP/6-31G* level followed by single point QCISD type calculations. All the optimized structures corresponded to minima on their potential energy surfaces as was inferred from the absence of imaginary vibrational frequencies. Nucleus independent chemical shifts NICS(0) were calculated at GIAO B3LYP/6-311 + G(d,p) level [11b].
3. Results and discussion The photoelectron spectra of I and II are shown in Figs. 1 and 2, respectively. The spectral assignments are summarized in Table 2 and are based on HeI/HeII intensity variations if observable, GF
137
I. Novak et al. / Chemical Physics Letters 491 (2010) 136–139
Fig. 2. HeI and HeII photoelectron spectra of II.
Fig. 1. HeI and HeII photoelectron spectra of I.
calculations and comparison with the assigned spectra of related molecules [2–5]. The assignments obtained are unambiguous. 3.1. Photoelectron spectrum of I In the ionization energy region below 9.5 eV in the spectrum of I we observe bands at 7.85 and 8.60 eV which correspond to three nitrogen lone pair orbitals, while the remaining bands correspond to r orbitals with Si–N and Si–C character. We rationalized the role of substituents in cyclodisilazanes by comparing the photoelectron spectra of molecules 1–5 (Fig. 3) reported previously [3–5] with the spectrum of I from this work. We focus our analysis on nitrogen lone pairs which give two well resolved bands corresponding to out-of-phase (n) and in-phase (n+) linear combinations of N2p orbitals. The molecules and corresponding ionization energies are shown in Fig. 3. We observe that the in-phase orbital (n+) is destabilized by approximately 0.8 eV when more than one silicon atom becomes bonded to nitrogen. The destabilization can be attributed to interaction of n+ with occupied Si–C orbitals localized on substituents (hyperconjugation). An independent inference concerning hyperconjugative effects can be deduced from the relative HeI and HeII intensities of the first two bands at 7.85 and 8.60 eV. The 7.85 eV band increases slightly in relative intensity on going from HeI to HeII radiation (Table 2). How can we rationalize this increase? From the HeII/HeI atomic photoionization cross
Table 1 Geometry parameters of the compounds studieda. Compound
Si–N
N–C
NSiN
I Ref. [9]
1.796 Å 1.736 Å
1.460 Å 1.472 Å
84.7° 86.4°
Si–C
C@C
C–C
SiCSi
II Ref. [10]
1.93 Å 1.89 Å
1.38 Å 1.42 Å
1.48 Å 1.49 Å
117.4° 119°
a The parameters for I and II are from DFT calculations, those from Ref. [9] and are from gas phase electron diffraction studies.
Table 2 Experimental (Ei/eV) vertical ionization energies, orbital assignments for I, II and related compounds. Compound
Band
Ei
GF/eV
Assignment
Relative HeII/ HeI intensity
I
X A–B C D
7.85 8.60 10.05 10.55
7.74 8.42, 8.52 10.28 10.67
n n+ nNH
1.0 0.8
Cyclopentadiene
X A
8.61 10.70
II
X A B–C D
7.60 8.50 10.3 11.05
r r
p p 7.56 8.41 9.43–10.50 11.40
p p rSi–C rSi–C
138
I. Novak et al. / Chemical Physics Letters 491 (2010) 136–139
section ratios for C2p, N2p and Si3p which are 0.31, 0.45 and 1.0 [12], we expect that ionizations from orbitals with more silicone character shall exhibit an intensity increase. All three nitrogen lone
N
1
Si
N
2
3
Si
N
Si
Si
N
4
7.8, 9.3, 11.0, 12.0 eV
3.2. Photoelectron spectrum of II
8.0, 8.5, 9.85, 10.6, 11.3 eV
N
Si
Si
N
N
I
7.8, 9.3, 10.5, 11.8 eV
7.8, 8.3, 9.7, 10.1 eV
SiH 2
7.85, 8.60, 10.05, 10.55, 11.5 eV N
HSi HN
SiH
5
pair orbitals corresponding to these bands can acquire silicon character through hyperconjugation. However, the 8.60 eV band contains ionization from an exocyclic amino group which has single silicon bonded to it. The other two endocyclic amino groups have two silicon atoms attached and can thus be expected to acquire slightly more silicon character than the exocyclic group. As a result, relative intensity of the 8.60 eV band shows relative decrease on going from HeI to HeII excitation compared to the 7.85 eV band. The perturbation of the electronic structure is particularly pronounced in structure 5 of Fig. 3 where three silicon atoms are bonded to every nitrogen. In that case the n and n+ orbitals become quasi degenerate due to interactions with Si–C and Si–H bonding orbitals as proposed previously [2]. We have also determined the relative thermodynamic stability of cyclodisilazane versus appropriate reference compounds. We found cyclodisilazane to be stable as can be seen from enthalpies for isodesmic reactions (1) and (2) in Fig. 4. This stability may be related to the flexibility of cyclodisilazane ring and is unaffected by the presence of substituents as noted previously [9]. For example, removal of alkyl substituents on silicon and nitrogen as in reactions (3) and (4) of Fig. 4 changes the reaction enthalpies vs. (1) and (2) by less than 5 kJ/mol.
The two broad bands at 7.6 and 8.5 eV in the photoelectron spectrum of II correspond to two p-orbitals localized on the cyclopentadiene ring. No significant changes in their HeI/HeII relative band intensities were detected. The very broad manifold with maximum at 10.3 eV can be attributed to ionizations from ten r-orbitals localized on trimethylsilyl substituents. The first two bands in the spectrum of II are inductively shifted by 0.45 and 0.6 eV, respectively towards lower ionization energies compared to 1,1bistrimethylsilylcyclopentadiene [2]. We have determined the effect of trimethylsilyl substituents on the thermodynamic stability of cyclopentadiene by calculating reaction enthalpies of the appropriate isodesmic reactions (5) and (6) shown in Fig. 4. The enthalpies show that 1,1-trimethylsilyl substitution stabilizes cyclopentadiene by 40.7 kJ/mol. The stabilization is consistent with hyperconjugative mechanism [5]. However further substitution which gives tetra-trimethylsilyl derivative gives smaller stabilization per trimethylsilylgroup as can be seen from the enthalpy of reaction (6) in Fig. 4. We have also calculated nucleus independent chemical shifts (NICS) which are descriptors of aromaticity. Cyclopentadiene is nonaromatic as is well established [13] (NICS = 3.26), but 1,1disubstitution by trimethylsilyl groups gives the ring some aromatic character (NICS = 7.92). However, 1,1,3,4-tetrasubstitution by the same groups reduces aromaticity; NICS = 5.63. This reduction in aromaticity on going from di- to tetra-derivative is consistent with the reduction of relative thermodynamic stability in the respective compounds.
4. Conclusion
N
Si
Si
N
8.7, 10.0, 10.8, 11.95 eV H Si
Fig. 3. The lowest ionization energies of silicon compounds determined by UPS.
We have determined the influence of alkyl and alkylsilyl substituents on the electronic structure of some silicon compounds. t-butyl substituent exerts mostly an inductive destabilizing effect on the nitrogen lone pair or on p-orbital orbitals. On the other hand trimethylsilyl group exerts a thermodynamic stabilizing effect of approximately 20 kJ/mol per group on the cyclopentadiene system. The effect is not cumulative and tetrasubstitution is less stabilizing than disubstitution.
139
I. Novak et al. / Chemical Physics Letters 491 (2010) 136–139
Si
Si
2
=
N
N
+
NH
H 2Si HN
H2Si
+
+
NH
SiMe3
+
-78.7 kJ/mol
+
(2)
-193.4 kJ/mol
(3)
(4)
SiH2
= SiMe3
4
(1)
NH
SiMe3 SiMe3
-73.9 kJ/mol
-192.3 kJ/mol
=
=
2
+
Si
NH
+
H 2Si
N
=
Si
N
2
SiH
Si
N
+
= SiMe3
SiMe3
+
(5) 2
SiMe3
+
- 40.7 kJ/mol
(6) 4
- 54.6 kJ/mol
SiMe3
Fig. 4. Enthalpies of isodesmic reactions.
Acknowledgements Authors thank the Ministry of Science, Education and Sports of the Republic of Croatia for the financial support through Project 098-0982915-2945 (CSU Ref. No. OPA 4068) and the Faculty of Science, Charles Sturt University for the financial support of this work through the Seed Grant A105-954-639-3495. References [1] A.M. Wrobel, I. Blaszczyk-Lezak, A. Walkiewicz-Pietrzykowska, T. Aoki, J. Kulpinski, J. Electrochem. Soc. 155 (2008) K66. [2] H. Bock, W. Kaim, J. Am. Chem. Soc. 102 (1980) 4429. [3] V. Metail, S. Joanteguy, A. Chrostowska-Senio, G. Pfister-Guillouzo, A. Systermans, J.L. Ripoll, Main Group Chem. 2 (1997) 97.
[4] V. Metail, S. Joanteguy, A. Chrostowska-Senio, G. Pfister-Guillouzo, A. Systermans, J.L. Ripoll, Inorg. Chem. 36 (1997) 1482. [5] T. Veszpremi, L. Bihatsi, Y. Harada, K. Ohno, H. Mutoh, J. Organomet. Chem. 280 (1985) 39. [6] M.J. Frisch et al., GAUSSIAN 03, Revision E1, Gaussian, Inc., Pittsburgh, PA, 2007. [7] W. Von Niessen, J. Schirmer, L.S. Cederbaum, Comput. Phys. Rep. 1 (1984) 57. [8] U. Häussermann, M. Dolg, H. Stoll, H. Preuss, Mol. Phys. 78 (1993) 1211. [9] E. Gergö, G. Schultz, I. Hargittai, J. Organomet. Chem. 292 (1985) 343. [10] N.N. Veniaminov, T. Zh. Strukt. Khim. 13 (1972) 136. [11] (a) A.G. Baboul, L.A. Curtiss, P.C. Redfern, K. Raghvachari, J. Chem. Phys. 110 (1999) 7650; (b) S.M. Bachrach, Computational Organic Chemistry, Springer, New York, 2007. [12] J.J. Yeh, Atomic Calculation of Photoionization Cross-Sections and Asymmetry Parameters, Gordon and Breach, Langhorne, 1993. [13] A. Stanger, Chem. Eur. J. 12 (2006) 2745.