- Email: [email protected]
The electronic and molecular structure of silyl nitrene
Volume 146, number 1,2
CHEMICAL PHYSICS LETTERS
29 April 1988
THE ELECTRONIC AND MOLECULAR STRUCTURE OF SILYL NITRENE Mark. S. GORDON Department of Chemistry, North Dakota State University,Fargo, ND 58105, USA Received 15 December 1987; in final form 14 January 1988
FORS MC SCF calculations performed with the 6-3 lG( d) basis set have been used to predict the geometries of the lowest singlet ( 'E ) and triplet ( 3Az) states of silyl nitrene. Both states are found to have Cs, symmetry at this computational level. Singlepoint second-order confguration interaction calculations (that is, all single and double excitations from the FORS MC SCF reference configuration) at the MC SCF geometries predict the triplet ground state to be 42.3 kcal/mol below ‘E and only 8.2 kcal/mol above the singlet ground state of silaimine.
1. Introduction
Nitrenes have been implicated as possible reactive intermediates, particularly as a result of the thermal [ 1 ] or photochemical [ 2 ] decomposition of azides. During the past 15 years, there has been much experimental [ 3 ] and theoretical [ 4-81 analysis of the possibility of detecting stable alkyl nitrenes. Very recently, West and co-workers [9] have reported the photolysis of substituted silyl azides. This very naturally leads one to speculate regarding the possible existence of silyl nitrenes as primary products of these photolyses. The simplest nitrene, NH, has a ...(x)~ ground electronic configuration, leading to a triplet ground state which is predicted to be 1.5 eV below the lowest singlet [ lo]. Similarly, the lowest electronic configuration of methyl nitrene is ...(e)‘. in C3”symmetry. This open shell configuration gives rise to 3A2, ‘E, and ‘A1 states. As expected the triplet is generally predicted [ 4-8 ] to be the ground state, with the lowest singlet being ‘E. Yarkony, Schaefer and Rothenberg [ 41 using restricted open shell Hartree -Fock (ROHF) wavefunctions with a double-zeta plus polarization (DZP) basis set, predicted a singlet-triplet splitting of 40.7 kcal/mol. This splitting is modified only slightly (to 41.0 kcal/mol) with the incorporation of correlation corrections via a small multiconfigurational (MC SCF) wavefunction with a DZ basis set [ 51, while a (DZP) con148
figuration interaction calculation including all single and double excitations (CISD) from the Hartree-Fock reference configuration lowers the splitting to 38.6 kcal/mol [6], after the Davidson correction for quadruple excitations [ 111. A calculation by Pople and co-workers [ 71, in which restricted (RHF) and unrestricted (UHF) Hartree-Fock reference wavefunctions were used for the singlet and triplet states, respectively, predicted a singlet-triplet splitting of 42.0 kcal/mol with fourth-order perturbation theory (MP4( SDQ) ) [ 121 andthe6-31G(d,p) basisset [13] atHF/6-31G(d) geometries. In this work, the singlet state was found to distort to C, symmetry, apparently due to the Jahn-Teller effect; however, a closed shell RHF treatment of the singlet is not likely to be reliable. The isomerization of methyl nitrene to methylene imine has been investigated on the triplet surface by Demuynck and co-workers [ 6 ] and on the singlet surface by Nguyen [ 8 1. Two calculations on silyl nitrene have been reported. Luke et al. [ 141 found the 3A, state to be nearly isoenergetic with triplet silaimine and 35.8 singlet SiH2NH. at the kcal/mol above MP4(SDTQ)/6-31G(d)//HF/3-21G* level of computation. Using RHF/ 3-2 1G ( d ) wavefunctions, Guimon and Pfister-Guillouzo [ 15 ] predict singlet silyl nitrene to be 62.3 kcal/mol higher in energy than singlet silaimine; however, as noted above,
Volume 146, number 1,2
29 April 1988
CHEMICAL PHYSICS LETTERS
the RHF treatment of the nitrene is not expected to be reliable. In this work we report MC SCF+CI calculations on the lowest singlet and triplet states of silyl nitrene and compare the results with previous calculations on silaimine [ 161.
2. Computational approach All calculations were carried out with the 6-3 1G (d ) basis set, most using the program GAMESS [ 171. Preliminary calculations on both the singlet and triplet were performed at the restricted open shell (ROHF) level of computation in order to obtain a proper set of starting orbitals and geometries for the MC SCF calculations. The latter were performed according to the FORS prescription [ 181. The five orbitals included in the active space are the (e,, eY) degenerate pair, the SiN bonding and antibonding orbitals, and nitrogen Q lone pair. All configurations generated by distributing the six active electrons among the five active orbitals were included in the MC SCF wavefunction used in the (analytical gradient-driven) geometry optimization for both the singlet and triplet states. These calculations were followed by a second-order configuration interaction (SOCI) calculation at the MC SCF geometry for the final energy comparison. The SOCI includes all single and double excitations from the set of contigurations generated in the MC SCF #l. In each case the computed geometry was verified to be a proper minimum by demonstrating that the matrix of energy second derivatives, obtained from finite differences of the analytical gradients, is positive definite. Test calculations on the silyl nitrene singlet state were also performed at the RHF/6-3 IG(d) and UHF/IS31G(d) computational level, using GAUSSIAN 82
POI. 3. Results and discussion The RHF geometry for the singlet produced a positive definite force field; however, as expected, the #IThe second-order CI (SOCI) wavefunction as defined here is a natural extension of the first-order CI (FOCI) wavefunction described by Kirby-Docken and Liu [ 191.
closed shell RHF wavefunction is found to be unstable to removal of the restriction that pairs of electrons be placed in common orbitals. That is, the RHF wavefunction is UHF-unstable. As noted in previous reports on methyl nitrene [ 7,8 1, the singlet UHF structure distorts from CjV symmetry. However, starting from the C, structure suggested by the UHF calculation, the singlet ROHF geometry optimization returns the structure of the ‘E state to C3”symmetry, giving 1.794 A, 1.482 A, and 108.6” for the Si-N and Si-H bond lengths and the HSiN angle, respectively. Refinement with the MC SCF wavefunction lengthens the SCN bond length to 1.825 A, while leaving the Si-H bond length and HSiN angle virtually unchanged at 1.480 8, and 108.6”, respectively. The triplet is similar with values of 1.847 A, 1.476 A, and 108.6”, respectively for the three geometric parameters, The rather long Si-N bond in both states (cf. the silylamine value of 1.741 A at the MP2/6-31G(d) computational level [21] ) is suggestive of a very weak bond. For both states, the SOCI natural orbital occupation numbers are approximately 1.Ofor the two e orbitals, nearly 2.0 for the SiN bonding orbital and N lone pair, and nearly 0.0 for the SIN antibonding orbital. The Mulliken population analysis suggests rather polar species, with charges of + 0.68, - 0.3 1, and - 0.12 on Si, N, and H for each state. The relative energies of the singlet and triplet nitrenes and imines are compared in table 1. The order of the four states is the same for the carbon and silicon species; however, the overall energy splitting is much smaller in the latter case. In particular, the ground triplet state of silyl nitrene is predicted to be only 8.2 kcal/mol above the singlet ground state of silaimine, as compared with the corresponding value of 46.3 kcal/mol predicted for methyl nitrene [6]. Similarly, the energy difference between the two sinTable 1 Relative energies (kcal/mol) for HIXN and H,XNH State
x=c
‘A’H,XNH ‘A? HlXN ‘A” HzXNH ‘E HXXN
0.0 46.3 64.5 84.9
a) Ref. [6].
8)
X=Si W 0.0 8.2 35.4 50.5
b, Present work.
149
Volume 146, number I ,2
CHEMICAL PHYSICS LETTERS
glet states is predicted to be 50.5 kcal/mol for silyl nitrene, as compared with 84.9 kcal/mol for the methyl compound. The singlet-triplet splitting is predicted to be about 40 kcal/mol for both nitrenes.
Acknowledgement The author is grateful for several helpful discussions with Professors Robert West and Michael Schmidt. This work was supported by grants from the National Science Foundation (CHE86-4077 1) and the Air Force Office of Scientific Research (870049). The calculations were performed at the National Center for Supercomputing Applications at the University of Illinois with the aid of an NCSA Affiliates grant of computer time.
References [ 1 ] W.I. Lwowski, ed., Nitrenes (Interscience, New York, 1970). [2] F.D. Lewis and W.H. Saunders, in: Nitrenes, ed. W.I. Lwowski (Interscience, New York, 1970) pp. 47-97; F.O. Rice and C.J. Grelecki, J. Phys. Chem. 61 (1957) 830; D.W. Mill&an, J. Chem. Phys. 35 (1961) 1491; CL. Currie and B.D. Darwent, Can. J. Chem. 41 (1963) 1552; E. Koch, Tetrahedron 23 ( 1967) 1747. [3] W. Pritzkow and D. Timm, J. Prakt. Chem. 132 (1966) 178. [4] D.R. Yarkony, H.F. Schaefer III and S. Rothenberg, J. Am. Chem. Sot. 96 (1974) 5974. [ 51R. Lucchese and D.R. Yarkony, J. Chem. Phys. 68 (1978) 2696.
150
29 April 1988
[6] J. Demuynck, D.J. Fox, Y. Yamaguchi and H.F. Schaefer III, J. Am. Chem. Sot. 102 (1980) 6204. [ 7 ] J.A. Pople, K. Raghavachari, M.J. Frisch, J.S. Binkley and P. von R. Schleyer, .I. Am. Chem. Sot. 105 (1983) 6389. [ 81M.T. Nguyen, Chem. Phys. Letters 117(1985) 290. [ 91 S.S. Zigler, Ph.D. Thesis, University of Wisconsin (1987); S.S. Zigler, L.M. Johnson and R. West, in preparation. [ 101P.F. Cade, Can. J. Phys. 46 (1968) 1989; W.M. Huo, J. Chem. Phys. 49 (1968) 1482; S.V. O’NeiI and H.F. Schaefer III, J. Chem. Phys. 55 (1971) 394; M.W. Schmidt, Ph.D. Thesis, Iowa State University (1982). [ 111 S.R. Langhoff and E.R. Davidson, Intern. J. Quantum Chem. 8 (1974) 61. [ 121R. Krishnan, M.J. Frisch and J.A. Pople, J. Chem. Phys. 72 ( 1980) 4244. [ 131 P.C. Hariharan and J.A. Pople, Theoret. Chim. Acta 28 (1973) 213; M.S. Gordon, Chem. Phys. Letters 76 ( 1980) 163. [ 141 B.T. Luke, J.A. Pople, M.B. Krogh-Jespersen, Y. Apeloig, M. Kami, J. Chandrasekhar and P. von R. Schleyer, J. Am. Chem. Sot. 108 (1986) 270. [ 151C. Guimon and G. Pfister-Guillouzo, Organometallics 6 (1987) 1387. [ 161 M.W. Schmidt, P.N. Truong and M.S. Gordon, J. Am. Chem. Sot. 109 (1987) 5217. [ 171M. Dupuis, D. Spangler and J.J. Wendoloski, NRCC Software Cat. Prog. QGOl (1980); M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S. Koseki, M.S. Gordon, S.T. Elbert and B.T. Lam, QCPE 7 ( 1987) 115. [ 181K. Ruedenberg, M.W. Schmidt, M.M. Gilbert and S.T. Elbert, Chem. Phys. 71 (1982) 51,71 [ 191 K. Kirby-Docken and B. Liu, J. Chem. Phys. 66 ( 1977) 4309. [ 201 J.S. Binkley, M.J. Frisch, D.J. DeFrees, R. Krishnan, R.A. Whiteside, H.B. Schlegel, E.M. Fluder and J.A. Pople, GAUSSIAN 82, Carnegie-Mellon University, Pittsburgh (1983). [21] M.S. Gordon, Chem. Phys. Letters 126 (1986) 451.
CHEMICAL PHYSICS LETTERS
29 April 1988
THE ELECTRONIC AND MOLECULAR STRUCTURE OF SILYL NITRENE Mark. S. GORDON Department of Chemistry, North Dakota State University,Fargo, ND 58105, USA Received 15 December 1987; in final form 14 January 1988
FORS MC SCF calculations performed with the 6-3 lG( d) basis set have been used to predict the geometries of the lowest singlet ( 'E ) and triplet ( 3Az) states of silyl nitrene. Both states are found to have Cs, symmetry at this computational level. Singlepoint second-order confguration interaction calculations (that is, all single and double excitations from the FORS MC SCF reference configuration) at the MC SCF geometries predict the triplet ground state to be 42.3 kcal/mol below ‘E and only 8.2 kcal/mol above the singlet ground state of silaimine.
1. Introduction
Nitrenes have been implicated as possible reactive intermediates, particularly as a result of the thermal [ 1 ] or photochemical [ 2 ] decomposition of azides. During the past 15 years, there has been much experimental [ 3 ] and theoretical [ 4-81 analysis of the possibility of detecting stable alkyl nitrenes. Very recently, West and co-workers [9] have reported the photolysis of substituted silyl azides. This very naturally leads one to speculate regarding the possible existence of silyl nitrenes as primary products of these photolyses. The simplest nitrene, NH, has a ...(x)~ ground electronic configuration, leading to a triplet ground state which is predicted to be 1.5 eV below the lowest singlet [ lo]. Similarly, the lowest electronic configuration of methyl nitrene is ...(e)‘. in C3”symmetry. This open shell configuration gives rise to 3A2, ‘E, and ‘A1 states. As expected the triplet is generally predicted [ 4-8 ] to be the ground state, with the lowest singlet being ‘E. Yarkony, Schaefer and Rothenberg [ 41 using restricted open shell Hartree -Fock (ROHF) wavefunctions with a double-zeta plus polarization (DZP) basis set, predicted a singlet-triplet splitting of 40.7 kcal/mol. This splitting is modified only slightly (to 41.0 kcal/mol) with the incorporation of correlation corrections via a small multiconfigurational (MC SCF) wavefunction with a DZ basis set [ 51, while a (DZP) con148
figuration interaction calculation including all single and double excitations (CISD) from the Hartree-Fock reference configuration lowers the splitting to 38.6 kcal/mol [6], after the Davidson correction for quadruple excitations [ 111. A calculation by Pople and co-workers [ 71, in which restricted (RHF) and unrestricted (UHF) Hartree-Fock reference wavefunctions were used for the singlet and triplet states, respectively, predicted a singlet-triplet splitting of 42.0 kcal/mol with fourth-order perturbation theory (MP4( SDQ) ) [ 121 andthe6-31G(d,p) basisset [13] atHF/6-31G(d) geometries. In this work, the singlet state was found to distort to C, symmetry, apparently due to the Jahn-Teller effect; however, a closed shell RHF treatment of the singlet is not likely to be reliable. The isomerization of methyl nitrene to methylene imine has been investigated on the triplet surface by Demuynck and co-workers [ 6 ] and on the singlet surface by Nguyen [ 8 1. Two calculations on silyl nitrene have been reported. Luke et al. [ 141 found the 3A, state to be nearly isoenergetic with triplet silaimine and 35.8 singlet SiH2NH. at the kcal/mol above MP4(SDTQ)/6-31G(d)//HF/3-21G* level of computation. Using RHF/ 3-2 1G ( d ) wavefunctions, Guimon and Pfister-Guillouzo [ 15 ] predict singlet silyl nitrene to be 62.3 kcal/mol higher in energy than singlet silaimine; however, as noted above,
Volume 146, number 1,2
29 April 1988
CHEMICAL PHYSICS LETTERS
the RHF treatment of the nitrene is not expected to be reliable. In this work we report MC SCF+CI calculations on the lowest singlet and triplet states of silyl nitrene and compare the results with previous calculations on silaimine [ 161.
2. Computational approach All calculations were carried out with the 6-3 1G (d ) basis set, most using the program GAMESS [ 171. Preliminary calculations on both the singlet and triplet were performed at the restricted open shell (ROHF) level of computation in order to obtain a proper set of starting orbitals and geometries for the MC SCF calculations. The latter were performed according to the FORS prescription [ 181. The five orbitals included in the active space are the (e,, eY) degenerate pair, the SiN bonding and antibonding orbitals, and nitrogen Q lone pair. All configurations generated by distributing the six active electrons among the five active orbitals were included in the MC SCF wavefunction used in the (analytical gradient-driven) geometry optimization for both the singlet and triplet states. These calculations were followed by a second-order configuration interaction (SOCI) calculation at the MC SCF geometry for the final energy comparison. The SOCI includes all single and double excitations from the set of contigurations generated in the MC SCF #l. In each case the computed geometry was verified to be a proper minimum by demonstrating that the matrix of energy second derivatives, obtained from finite differences of the analytical gradients, is positive definite. Test calculations on the silyl nitrene singlet state were also performed at the RHF/6-3 IG(d) and UHF/IS31G(d) computational level, using GAUSSIAN 82
POI. 3. Results and discussion The RHF geometry for the singlet produced a positive definite force field; however, as expected, the #IThe second-order CI (SOCI) wavefunction as defined here is a natural extension of the first-order CI (FOCI) wavefunction described by Kirby-Docken and Liu [ 191.
closed shell RHF wavefunction is found to be unstable to removal of the restriction that pairs of electrons be placed in common orbitals. That is, the RHF wavefunction is UHF-unstable. As noted in previous reports on methyl nitrene [ 7,8 1, the singlet UHF structure distorts from CjV symmetry. However, starting from the C, structure suggested by the UHF calculation, the singlet ROHF geometry optimization returns the structure of the ‘E state to C3”symmetry, giving 1.794 A, 1.482 A, and 108.6” for the Si-N and Si-H bond lengths and the HSiN angle, respectively. Refinement with the MC SCF wavefunction lengthens the SCN bond length to 1.825 A, while leaving the Si-H bond length and HSiN angle virtually unchanged at 1.480 8, and 108.6”, respectively. The triplet is similar with values of 1.847 A, 1.476 A, and 108.6”, respectively for the three geometric parameters, The rather long Si-N bond in both states (cf. the silylamine value of 1.741 A at the MP2/6-31G(d) computational level [21] ) is suggestive of a very weak bond. For both states, the SOCI natural orbital occupation numbers are approximately 1.Ofor the two e orbitals, nearly 2.0 for the SiN bonding orbital and N lone pair, and nearly 0.0 for the SIN antibonding orbital. The Mulliken population analysis suggests rather polar species, with charges of + 0.68, - 0.3 1, and - 0.12 on Si, N, and H for each state. The relative energies of the singlet and triplet nitrenes and imines are compared in table 1. The order of the four states is the same for the carbon and silicon species; however, the overall energy splitting is much smaller in the latter case. In particular, the ground triplet state of silyl nitrene is predicted to be only 8.2 kcal/mol above the singlet ground state of silaimine, as compared with the corresponding value of 46.3 kcal/mol predicted for methyl nitrene [6]. Similarly, the energy difference between the two sinTable 1 Relative energies (kcal/mol) for HIXN and H,XNH State
x=c
‘A’H,XNH ‘A? HlXN ‘A” HzXNH ‘E HXXN
0.0 46.3 64.5 84.9
a) Ref. [6].
8)
X=Si W 0.0 8.2 35.4 50.5
b, Present work.
149
Volume 146, number I ,2
CHEMICAL PHYSICS LETTERS
glet states is predicted to be 50.5 kcal/mol for silyl nitrene, as compared with 84.9 kcal/mol for the methyl compound. The singlet-triplet splitting is predicted to be about 40 kcal/mol for both nitrenes.
Acknowledgement The author is grateful for several helpful discussions with Professors Robert West and Michael Schmidt. This work was supported by grants from the National Science Foundation (CHE86-4077 1) and the Air Force Office of Scientific Research (870049). The calculations were performed at the National Center for Supercomputing Applications at the University of Illinois with the aid of an NCSA Affiliates grant of computer time.
References [ 1 ] W.I. Lwowski, ed., Nitrenes (Interscience, New York, 1970). [2] F.D. Lewis and W.H. Saunders, in: Nitrenes, ed. W.I. Lwowski (Interscience, New York, 1970) pp. 47-97; F.O. Rice and C.J. Grelecki, J. Phys. Chem. 61 (1957) 830; D.W. Mill&an, J. Chem. Phys. 35 (1961) 1491; CL. Currie and B.D. Darwent, Can. J. Chem. 41 (1963) 1552; E. Koch, Tetrahedron 23 ( 1967) 1747. [3] W. Pritzkow and D. Timm, J. Prakt. Chem. 132 (1966) 178. [4] D.R. Yarkony, H.F. Schaefer III and S. Rothenberg, J. Am. Chem. Sot. 96 (1974) 5974. [ 51R. Lucchese and D.R. Yarkony, J. Chem. Phys. 68 (1978) 2696.
150
29 April 1988
[6] J. Demuynck, D.J. Fox, Y. Yamaguchi and H.F. Schaefer III, J. Am. Chem. Sot. 102 (1980) 6204. [ 7 ] J.A. Pople, K. Raghavachari, M.J. Frisch, J.S. Binkley and P. von R. Schleyer, .I. Am. Chem. Sot. 105 (1983) 6389. [ 81M.T. Nguyen, Chem. Phys. Letters 117(1985) 290. [ 91 S.S. Zigler, Ph.D. Thesis, University of Wisconsin (1987); S.S. Zigler, L.M. Johnson and R. West, in preparation. [ 101P.F. Cade, Can. J. Phys. 46 (1968) 1989; W.M. Huo, J. Chem. Phys. 49 (1968) 1482; S.V. O’NeiI and H.F. Schaefer III, J. Chem. Phys. 55 (1971) 394; M.W. Schmidt, Ph.D. Thesis, Iowa State University (1982). [ 111 S.R. Langhoff and E.R. Davidson, Intern. J. Quantum Chem. 8 (1974) 61. [ 121R. Krishnan, M.J. Frisch and J.A. Pople, J. Chem. Phys. 72 ( 1980) 4244. [ 131 P.C. Hariharan and J.A. Pople, Theoret. Chim. Acta 28 (1973) 213; M.S. Gordon, Chem. Phys. Letters 76 ( 1980) 163. [ 141 B.T. Luke, J.A. Pople, M.B. Krogh-Jespersen, Y. Apeloig, M. Kami, J. Chandrasekhar and P. von R. Schleyer, J. Am. Chem. Sot. 108 (1986) 270. [ 151C. Guimon and G. Pfister-Guillouzo, Organometallics 6 (1987) 1387. [ 161 M.W. Schmidt, P.N. Truong and M.S. Gordon, J. Am. Chem. Sot. 109 (1987) 5217. [ 171M. Dupuis, D. Spangler and J.J. Wendoloski, NRCC Software Cat. Prog. QGOl (1980); M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S. Koseki, M.S. Gordon, S.T. Elbert and B.T. Lam, QCPE 7 ( 1987) 115. [ 181K. Ruedenberg, M.W. Schmidt, M.M. Gilbert and S.T. Elbert, Chem. Phys. 71 (1982) 51,71 [ 191 K. Kirby-Docken and B. Liu, J. Chem. Phys. 66 ( 1977) 4309. [ 201 J.S. Binkley, M.J. Frisch, D.J. DeFrees, R. Krishnan, R.A. Whiteside, H.B. Schlegel, E.M. Fluder and J.A. Pople, GAUSSIAN 82, Carnegie-Mellon University, Pittsburgh (1983). [21] M.S. Gordon, Chem. Phys. Letters 126 (1986) 451.