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Isomerization of cyanoborane anion
21 July 1995
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 241 (1995) 261-266
Isomerization of cyanoborane anion Ivan C e r n u ~ k a, Hans Lischka h a Department of Physical Chemistry, Faculty of Science, Comenius University, Mlynsk~ dolina, SK-84215 Bratislava, Slovak Republic b Institute of Theoretical Chemistry and Radiation Chemistry, University of Vienna, Wiihringerstrasse 17, A-1010 Vienna, Austria
Received 10 October 1994; in final form 5 April 1995
Abstract
A previous study of the potential energy surface of borazirene revealed prohibitively large barriers on the reaction pathway from cyanoborane to borazirene. An alternative mechanism for its synthesis, based on a base-catalyzed reaction is proposed. It includes the deprotonation of cyanoborane and subsequent isomerization of the HBCN- chain to the HBCNring. Calculations of the X - + H2BCN = H B C N - + HX equilibria (X = OH, H) show that proton transfer to the hydroxyl anion is thermodynamically feasible. The geometries of the stationary points on the potential energy surface corresponding to ring closure or ring opening to HBCN- and/or HBNC- chain-ions are obtained at the MBPT(2) level. Single-point calculations of the activation barriers and reaction enthalpies are performed at the CCSD + T(CCSD) level. The barriers are significantly lower compared to the isomerization of the neutral species and are in the range 100-130 kJ/mol. The reaction enthalpies are substantially lower and slightly favor the ring formation from the isocyanoborane anion. Trends in correlation energy contributions for the individual steps of the mechanism are analyzed. 1. Introduction
Borazirene (isomer of cyanoborane, H2BCN) belongs to a class of interesting three-membered rings, formally satisfying the Hiickel 4n + 2 rule of aromaticity and isoelectronic with C3H ~ . This molecule has been considered by Byun et al. and Jug [1]. Recently, we studied its potential energy surface and isomerization reactions [2]. We found that the acyclic and cyclic isomers are separated by a huge barrier ( ~ 400 k J / m o l ) . Even larger barriers were found for the isocyanoborane-isoborazirene pair. The isomers and derivatives of cyanoborane may serve as precursors or basic units for more complex systems that are important in semiconductor technology [3] as well as in pharmacology as antineoplastic agents [4]. We also suggested that deprotonized forms of these species might be energetically less demanding alternatives for the reaction mechanism.
In this study, we intend to demonstrate that the proton exchange equilibrium can be the first step in a base-catalyzed isomerization. It can easily produce a bent H B C N - chain. We suggest O H - and H - as model bases since they are frequently used in organic synthesis. We show that the subsequent isomerization to borazirene anion is associated with substantially lower barriers.
2. Calculations 6-31G(d, p) [5] and double-zeta plus polarization basis sets [6,7] augmented with diffuse p functions (oz,[B] = 0.019, ap[C] = 0.034, ~p[N] = 0.048, ap(O] = 0.059) [8] (denoted DZP-p) have been used. For the equilibrium associated with H - we have added to the basis set diffuse s functions with expo-
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00632-X
262
L Cernug{:k, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
Table 1 SCF (hartree), valence shell correlation energies (miilihartree) at MBFr(2) geometry and zero-point energies (kJ/mol) Molecule
- SCF
- MBPT(2)
- CCSD
- (CCSD +T(CCSD))
HH2 OHH20 H2BCN a H2BCN b HBCN - a HBCN- b HBNC - a HBCN - a,c [HBCN- ]* d [HBNC- ]* e [OHH2BCN- ]* f OH -" H 2BCN g
0.486810 1.131741 75.401078 76.049089 118.157190 118.157667 117.517204 117.519310 117.509285 117.508285 117.453621 117.470971 193.564784 193.711311
18.745 27.415 219.826 205.691 365.926 366.292 369.008 369.259 363.272 372.267 378.541 359.102 594.477 586.040
27.502 35.701 219.834 213.769 385.244 385.590 389.105 389.177 386.319 388.346 398.481 384.565 614.528 607.787
27.502 35.701 226.981 217.911 401.622 402.016 407.922 408.007 404.905 407.095 419.644 403.305 640.179 630.523
- T(CCSD)
ZPV
7.146 4.141 16.378 16.427 18.817 18.830 18.585 18.749 21.163 18.740 25.651 22.736
27.1 22.7 57.1 73.7 73.6 40.7 40.8 40.4 45.4 35.6 35.9 90.5 -
a DZP-p. b DZP-ps. c ring. d C-transition state, e N-transition state. f O H - attack transition state, g OH- attack intermediate.
nent c~s[H] = 0.04 ( D Z P - p s basis) [9]. O p t i m i z a t i o n s for the stationary points for the i s o m e r i z a t i o n have b e e n p e r f o r m e d in two steps: initial guesses at the S C F / 6 - 3 1 G ( d , p) l e v e l and final g e o m e t r y refinem e n t s at the s e c o n d - o r d e r m a n y - b o d y perturbation theory - M B P T ( 2 ) / D Z P - p level (all electrons correlated). Vibrational f r e q u e n c i e s h a v e b e e n c o m p u t e d numerically from MBPT(2)/DZP-p gradients to c h e c k the nature o f the stationary points and to gain Z P V corrections. T h e G A U S S I A N 92 p r o g r a m [10] has b e e n used in all optimizations. The path f r o m S C F / 6 - 3 1 G ( d , p), as w e l l as f r o m M B P T ( 2 ) / D Z P - p transition states ( T S ) has b e e n e x p l o r e d using the intrinsic reaction coordinate ( I R C ) m e t h o d [11] as i m p l e m e n t e d in the G A M E S S [12] and G A U S S I A N
Table 2 Proton exchange equilibria. Relative energies (kJ/mol) AE
Reaction (1)
Reaction (2)
SCF MBPT(2) CCSD T(CCSD) SCF + MBPT(2) SCF + CCSD SCF + CCSD + T(CCSD) AH o
- 21.1 29.0 5.8 1.5 7.9 - 15.3 - 13.8 - 12.4
- 17.3 - 30.6 - 31.0 - 6.3 - 47.8 - 48.2 - 54.5 - 60.2
A H29s.15 K
- 11.6
-58.1
AG298.15 K
-
16.4
- 64.3
92 [10] programs. Single-point calculations at the M B P T ( 2 ) g e o m e t r i e s h a v e b e e n p e r f o r m e d at the C C S D m o d e l [13] w h i c h incorporates the effects f r o m single and double excitations and f r o m disconnected triple and quadruple excitations. T h e effect f r o m c o n n e c t e d triples T ( C C S D ) (the C C analog to a fourth-order triple excitation contribution) [14] is c o m p u t e d f r o m the c o n v e r g e d amplitudes o f the C C S D calculation. This a p p r o x i m a t i o n is denoted C C S D + T ( C C S D ) / D Z P - p and w e h a v e used the M O L C A S 2 / C O M E N I U S suite o f p r o g r a m s [15] 1 to obtain correlated activation barriers and reaction enthalpies. T h e inner shell orbitals w e r e uncorrelated for h e a v y atoms in the C C calculations. Total energies are collected in Table 1; relative energies for equilibria and isomerizations in Tables 2 and 3, respectively. T h e h a r m o n i c f r e q u e n c i e s and IR intensities for the ring- and c h a i n - i s o m e r s o f H B C N - are in Table 4. T h e f o l l o w i n g reactions w e r e investigated and will be discussed in Section 3 (see also Fig. 1 for r o m a n n u m b e r i n g o f the structures):
x The original version of the M B P T / C C code included in the MOLCAS system is due to M. Urban, V. Kell/5 and J. Noga and is based on strategies described in Refs. [16,17]
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L Cernug6k, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
Table 3 Isomerization of cyanoborane anion. Relative energies (kJ/mol) AE
Reaction (3)
(4)
(5)
(6)
(7)
(8) !
SCF 166.9 143.5 98.0 100.6 23.4 2.6 MBPT(2) -25.0-16.5 34.6 1 1 . 0 - 8 . 6 - 2 3 . 6 CCSD -24.6 -26.6 9.9 4.6 2.0 - 5 . 3 T(CCSD) -6.2 -6.3 0.0 - 0 . 4 0.2 - 0 . 4 SCF+MBPT(2) 141.9 127.1 132.5 111.5 14.9 - 2 1 . 0 SCF+CCSD 142.3 116.9 107.9 105.2 25.4 - 2 . 7 S C F + C C S D + T ( C C S D ) 136.2 110.6107.9104.8 25.6 - 3 . 1 AH o 131.1 100.8 98.4 100.3 30.3 1.9 AH298.15 K 130.7 101.9 99.5 99.8 28.8 0.3 AG298.15 K 129.2 98.8 97.1 98.9 30.4 1.8
IV
e)
i3 / / / ¥
(a) equilibria H2BCN + O H - = HBCN- (I) + H20,
(1)
H2BCN + H - = HBCN- (I) + H 2,
(2)
;/
Table 4 MBPT(2)/DZP-p harmonic frequencies (cm-1), IR intensities (km/mol) for the HBCN- ring, HBCN- and HBNC- chains Symmetry
to
IR intensity
HBCN- ring ~ ( B H in-plane bend) ~ ' ( B H out-of-plane bend) ,~(/_CBN bend) ,~(/_NCB bend) , ~ ( C N / C B / B N stretch) ,~(BH stretch)
636.3 782.9 1015.8 1080.3 1392.2 2685.2
67.52 18.40 21.28 5.08 15.23 117.68
HBCN- chain ~'(/_BCN out-of-plane bend) K(Z_BCN in-plane bend) P~(CB stretch) P~(/_HBC bend) K(CN stretch) .~(BH stretch)
317.4 387.9 749.8 883.6 2039.6 2422.7
35.5 38.2 7.2 34.2 125.5 300.3
HBNC- chain , ~ ' ( / B N C out-of-plane bend) .~(/_BNC in-plane bend) ?¢(BN stretch) ~((/_HBN bend) K(CN stretch) K(BH stretch)
301.2 374.8 762.2 1006.8 1971.6 2345.4
36.9 23.9 8.9 65.2 13.1 425.1
c)
N
C III
Fig. 1. Sequence of structures along the reaction path. MBPT(2)/DZP-p internal coordinates (,~ and deg). (a) Structure I: bent HBCN- chain, rnn = 1.2329, rBc = 1.5803, rcN = 1.2008, /-HBC = 106.47, /_BCN = 171.14; (b) Structure II: C-transition state, rHB =1.2432, rBc =1.8059, rc~ =1.2178, / H B C = 106.01, /_BCN=96.14, / - - H B C N = - 2 2 . 3 8 ; (c) Structure III: HBCN- ring, rrt B = 1.1930, rBC = 1.5350, rcn = 1.3799, /_HBC = 167.51, /--BCN = 58.48; (d) Structure IV: N-transition state, r,B = 1.2415, rBr~ = 1.6768, rcN = 1.2187, /_I-InN = 95.26, /_BNC = 9 4 . 5 5 , / H B N C = 147.99; (e) Structure V: bent HBNCchain, rrm =1.2422, rBN =1.5063, rCN =1.2049, / - H B N = 103.02, /_BNC = 173.76. (b) isomerizations
HBCN~i.g )(III)
= [HBCN-]*
(II),
(3) (4)
HBCN(~ing ) ( I I I )
= [HBNC-]*
(IV),
(5)
HBCN-
(I) = [HBCN-]*
(II),
HBNC-
(V) = [HBNC-]*
(IV),
HBCN-
( I ) = HBCN(~ing ) ( I I I ) ,
HBNC-
( V ) = HBCN(ring ) ( I I I ) ,
(6) (7) (8)
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L Cernughk, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
For reaction (1) we also present an exploratory search of the potential energy surface (PES) for proton transfer. These calculations yield one TS with O H attached to one hydrogen of the BH 2 group and one intermediate corresponding to an S N2-like O H - attack on the electron deficient B atom. We shall not comment on these results in detail because the PES is complex, including probably more reaction channels, and should be the subject of a separate survey.
3. Results and discussion
Both chemical equilibria indicate that the production of the cyanoborane anion is thermodynamically supported at laboratory temperature (AG in Table 2). For O H - attack the electron correlation slightly destabilizes the deprotonated form, while for H attack the effect is reversed and, more importantly, the correlation is dominant, strongly enhancing the exothermicity of the proton exchange. Note that for reaction (1) MBPT(2) does not give the correct sign of the reaction energy. One reason may be the slow convergence of the MBPT expansion in general, but also different CC convergence for HEO and OH-, Table 1. In addition, for reaction (2) the effect of triples is by no means negligible. This relatively large effect (in comparison with reaction (1)) can solely be attributed to the changes in bonding in HzBCN and HBCN-, the former contains a triple CN bond, while for the latter, two resonance structures can be suggested: H - B ( - ~ - C = N and H B = C = N (- ~. This idea is supported also by Mulliken populations on boron ( - 0.55 e) and nitrogen ( - 0.34 e). These results prompted us to investigate the stationary points of the HBCN- potential energy surface in a two-step procedure (see Section 2). In order to verify that the transition states from preliminary SCF/6-31G(d, p) optimizations connect the chain and ring anions, the intrinsic reaction coordinate (IRC) for the entire isomerization path is investigated at the same level. The primary reason for this check is the large distances, BC = 2.401 ,~ (in [HBCN-]*) and B N = l . 8 0 6 ,A (in [HBNC-]*), found at this level of theory for transition structures, together with the soft imaginary frequencies 229.4i and 306.7i cm -1 associated with these transition
states. Such weakly bound structures may lead to undesired dissociation to BH and CN-. Surprisingly, the SCF/6-31G(d, p) path from the C-transition state [HBCN-]* converges to either HBCN- or HBNCchain isomers but not to the ring. On the other hand, the N-transition state [HBNC-]* lies on the path joining the HBCN0-ing~ and bent chain structure as expected. Mechanistic implications based solely on SCF optimizations may lead to the conclusion that the ring isomer can be prepared only from HBNC-. Thus, higher-level optimizations of the geometry are needed to illuminate this point. The most significant geometry changes, when going from SCF to MBPT(2), can be observed for the transition states, e.g. the refinement of the Ctransition state [HBCN-]* (Fig. lb) at the MBPT(2) level shortens the BC distance (to 1.806 ,A) and changes the orientation of the BH bond with respect to BCN from perpendicular (at the SCF level) to almost coplanar. The changes in [HBNC-]* are less dramatic, but here also the correlation reduces the BN distance to 1.667 A. The values of the imaginary frequencies increase to 456.2i cm -1 for [HBCN-]* and 557.9i cm -1 for [HBNC-]*. Thus, at the MBPT(2) level the shortened distance of the migrating BH group from CN- compares quite well for both transition states and suggests that the path to the ring is almost equally probable from both chain isomers. This assumption is completely confirmed by the IRC calculations at the MBPT(2)/DZP-p level (Fig. 2). We stress that in contrast to SCF, on the MBPT(2) path both transition states are linked by the ring isomer and the overall mechanism of the isomerization can be described as follows: I ~ II ~ III ~IV~V. The barriers for isomerization (Table 3) are significantly reduced upon deprotonation and range from 100 to 131 k J / m o l which is approximately a quarter of those found in Ref. [1]. This reduction is essentially due to lower SCF barriers and can partially be explained by the presence of the soft / B C N - and /_BNC-in-plane bending modes in the chain isomers (Table 4) which allow quite free motion of the BH group. Electron correlation further stabilizes [HBCN-]* while slightly destabilizing [HBNC-]*. The effect of higher excitations is: (i) rather uniform for both activation processes (3) and (4); o
I. Cernug,~k, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
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1~785 c~J
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2
,
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b
- 1 1 F 85 I c-
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Reaction poth (bobr*sqrt(crnL.)) Fig. 2. IRC profiles, E(MBPT[2]/DZP-p)as a function of total reaction path in mass-weighted Cartesian coordinates: (a) path from C-transitionstate; (b) path from N-transitionstate.
(ii) important for all reactions with one exception, the formation of [HBCN-]* (3), where MBPT(2) performs quite well in contrast to (4); (iii) essential for reactions (7) and (8) where the bonds are breaking and forming; the absolute value of the reaction energy is rather small, and MBPT(2) significantly deviates from CCSD + T(CCSD). In one case, reaction (8), MBPT(2) even fails badly to predict the correct sign of the reaction energy. Curiously, for the formation of [HBNC-]*, steps (5) and (6), the correlation and thermodynamic contributions are in balance and SCF fortuitously fits AG. Reaction enthalpies (being substantially lower than barriers) indicate that the ring formation (7)
265
should be a slightly endothermic process (AG = 30.4 kJ/mol) and they favor the ring formation from the isocyanoborane anion (8). In fact, these two species (III and V) should be equally populated at 298.15 K. In comparison with the results for neutral species (from Ref. [1]) the trends in correlation energy contributions for the individual steps of the mechanism for the anions are opposite. For example, the isomerization of neutral cyanoborane to borazirene is accompanied by a positive correlation contribution (increasing the barrier), while for isocyanoborane the correlation contribution to the barrier is negative. On the other hand, for anionic forms, correlation reduces the barriers for reactions (3) and (4) and increases them slightly for (5) and (6). Since the barriers are lower for anionic species, the effect of electron correlation becomes more important; especially the correct inclusion of higher excitations seems to be crucial. Concerning the reaction energies, the deprotonation causes their huge reduction at the SCF level (compare with Table III in Ref. [1]). The subtle effects of electron correlation on reaction energies are enhanced for anions due to this reduction, and the dominant contribution is recovered at the CCSD level. The harmonic vibrational modes and IR intensities (Table 4) complete the structural information about anionic cyanoborane isomers. An interesting feature of the spectra is the dramatic weakening of CN stretching band (the predicted IR intensity drops almost ten times) when going from HBCN- to HBNC- and the opposite trend in the /_HBC- and /_HBN-bends. Since these anions are unknown to date, these data might assist in their experimental detection.
4. Summary We have found a possible alternative for the isomerization of cyanoborane through anionic forms. SCF + CCSD + T(CCSD) barriers separating the bent-chain and ring isomers of HBCN- are acceptably low for the gas phase. A H and AG values obtained from SCF energies, CCSD + T(CCSD) correlation corrections and thermodynamic contributions (at 298.15 K) suggest that the conceivable reaction channel to the target ring compound can
266
L Cernug,;k, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
start from both bent-chain anions, however the HBNC- isomer is slightly favored thermodynamically. These findings, together with the prediction of the harmonic IR spectra of species I, IH and V, may be helpful in the synthesis of borazirene.
Acknowledgement This research was in part funded by the Action Austria-Slovak Republic and is part of the COST project D3/0003/94.
References [1] Y.-G. Byun, S. Saebo and C.U. Pittman Jr., J. Am. Chem Soc. 113 (1991) 3689; K. Jug, J. Org. Chem. 49 (1984) 4475. [2] I. (~ernu~k, R.J. Bartlett and S. Beck, J. Phys. Chem. 96 (1992) 10284. [3] F. Saugnac, F. Teyssandier and A. Marchand, J. Am. Ceram. Soc. 75 (1992) 161; L. Maya and H.L. Richards, J. Am. Ceram. Soc. 74 (1991) 406. [4] A. Sood, B.F. Spielvogel, B.R. Shaw, L.D. Carlton, B.S. Burnham, E.S. Hall and I.H. Hall, Anticancer Res. 12 (1992) 335; I.H. Hall, E.S. Hall, L.K. Chi, B.R. Shaw, A. Sood and B.F. Spielvogel, Anticancer Res. 12 (1992) 1091. [5] W.J. Hehre, L. Radom, P. von R. Schleyer and J.A. Pople, Ab initio molecular orbital theory (Wiley, New York, 1986).
[6] T.H. Dunning Jr., J. Chem. Phys. 53 (1970) 2823. [7] L.T. Redmon, G.D. Purvis and R.J. Bartlett, J. Am. Chem. Soc. 101 (1979) 2856. [8] T.H. Dunning Jr. and P.J. Hay, in: Modem theoretical chemistry, Vol. 3, ed. H.F. Schaefer III (Plenum Press, New York, 1979). [9] M. Urban, G.H.F. Diercksen, I. Cemu~ik and Z. Havlas, Chem. Phys. Letters 159 (1989) 155. [10] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.W. Wong, J.B. Foresman, M.A. Robb, M. Head-Gordon, E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. DeFrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92/DbT, REVISION F.3 (Gaussian, Pittsburgh, 1993). [11] K. Ishida, K. Morokuma and A. Komomicki, J. Chem. Phys. 66 (1977) 2153. [12] GAMESS-US: M.W. Schmidt, K.K. Baldridge, J.A. Boatz, J.H. Jensen, S. Koseki, M.S. Gordon, K.A. Nguyen, T.L. Windus and S.T. Elbert, QCPE Bull. 10 (1990) 52. [13] G.D. Purvis and R.J. Bartlett, J. Chem. Phys. 76 (1982) 1910. [14] M. Urban, J. Noga, S.J. Cole and R.J. Bartlett, J. Chem. Phys. 83 (1985) 4041. [15] K. Andersson, M. Fiilscher, R. Lindh, P.-A. Malmquist, J. Olsen, B.O. Roos, A.J. Sadlej and P.-O. Widmark, 1991, MOLCAS System of Quantum Chemistry Programs. Release 02 (Theoretical Chemistry, University of Lund, Lund, Sweden). [16] M. Urban, I. Cernu~Lk, V. Kell6 and J. Noga, in: Methods in computational chemistry, Vol. 1, ed. S. Wilson (1987) pp. 117-250. [17] M. Urban, I. Huba~, V. Kell6 and J. Noga, J. Chem. Phys. 72 (1980) 3378; J. Noga, Comput. Phys. Commun. 29 (1983) 117.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 241 (1995) 261-266
Isomerization of cyanoborane anion Ivan C e r n u ~ k a, Hans Lischka h a Department of Physical Chemistry, Faculty of Science, Comenius University, Mlynsk~ dolina, SK-84215 Bratislava, Slovak Republic b Institute of Theoretical Chemistry and Radiation Chemistry, University of Vienna, Wiihringerstrasse 17, A-1010 Vienna, Austria
Received 10 October 1994; in final form 5 April 1995
Abstract
A previous study of the potential energy surface of borazirene revealed prohibitively large barriers on the reaction pathway from cyanoborane to borazirene. An alternative mechanism for its synthesis, based on a base-catalyzed reaction is proposed. It includes the deprotonation of cyanoborane and subsequent isomerization of the HBCN- chain to the HBCNring. Calculations of the X - + H2BCN = H B C N - + HX equilibria (X = OH, H) show that proton transfer to the hydroxyl anion is thermodynamically feasible. The geometries of the stationary points on the potential energy surface corresponding to ring closure or ring opening to HBCN- and/or HBNC- chain-ions are obtained at the MBPT(2) level. Single-point calculations of the activation barriers and reaction enthalpies are performed at the CCSD + T(CCSD) level. The barriers are significantly lower compared to the isomerization of the neutral species and are in the range 100-130 kJ/mol. The reaction enthalpies are substantially lower and slightly favor the ring formation from the isocyanoborane anion. Trends in correlation energy contributions for the individual steps of the mechanism are analyzed. 1. Introduction
Borazirene (isomer of cyanoborane, H2BCN) belongs to a class of interesting three-membered rings, formally satisfying the Hiickel 4n + 2 rule of aromaticity and isoelectronic with C3H ~ . This molecule has been considered by Byun et al. and Jug [1]. Recently, we studied its potential energy surface and isomerization reactions [2]. We found that the acyclic and cyclic isomers are separated by a huge barrier ( ~ 400 k J / m o l ) . Even larger barriers were found for the isocyanoborane-isoborazirene pair. The isomers and derivatives of cyanoborane may serve as precursors or basic units for more complex systems that are important in semiconductor technology [3] as well as in pharmacology as antineoplastic agents [4]. We also suggested that deprotonized forms of these species might be energetically less demanding alternatives for the reaction mechanism.
In this study, we intend to demonstrate that the proton exchange equilibrium can be the first step in a base-catalyzed isomerization. It can easily produce a bent H B C N - chain. We suggest O H - and H - as model bases since they are frequently used in organic synthesis. We show that the subsequent isomerization to borazirene anion is associated with substantially lower barriers.
2. Calculations 6-31G(d, p) [5] and double-zeta plus polarization basis sets [6,7] augmented with diffuse p functions (oz,[B] = 0.019, ap[C] = 0.034, ~p[N] = 0.048, ap(O] = 0.059) [8] (denoted DZP-p) have been used. For the equilibrium associated with H - we have added to the basis set diffuse s functions with expo-
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00632-X
262
L Cernug{:k, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
Table 1 SCF (hartree), valence shell correlation energies (miilihartree) at MBFr(2) geometry and zero-point energies (kJ/mol) Molecule
- SCF
- MBPT(2)
- CCSD
- (CCSD +T(CCSD))
HH2 OHH20 H2BCN a H2BCN b HBCN - a HBCN- b HBNC - a HBCN - a,c [HBCN- ]* d [HBNC- ]* e [OHH2BCN- ]* f OH -" H 2BCN g
0.486810 1.131741 75.401078 76.049089 118.157190 118.157667 117.517204 117.519310 117.509285 117.508285 117.453621 117.470971 193.564784 193.711311
18.745 27.415 219.826 205.691 365.926 366.292 369.008 369.259 363.272 372.267 378.541 359.102 594.477 586.040
27.502 35.701 219.834 213.769 385.244 385.590 389.105 389.177 386.319 388.346 398.481 384.565 614.528 607.787
27.502 35.701 226.981 217.911 401.622 402.016 407.922 408.007 404.905 407.095 419.644 403.305 640.179 630.523
- T(CCSD)
ZPV
7.146 4.141 16.378 16.427 18.817 18.830 18.585 18.749 21.163 18.740 25.651 22.736
27.1 22.7 57.1 73.7 73.6 40.7 40.8 40.4 45.4 35.6 35.9 90.5 -
a DZP-p. b DZP-ps. c ring. d C-transition state, e N-transition state. f O H - attack transition state, g OH- attack intermediate.
nent c~s[H] = 0.04 ( D Z P - p s basis) [9]. O p t i m i z a t i o n s for the stationary points for the i s o m e r i z a t i o n have b e e n p e r f o r m e d in two steps: initial guesses at the S C F / 6 - 3 1 G ( d , p) l e v e l and final g e o m e t r y refinem e n t s at the s e c o n d - o r d e r m a n y - b o d y perturbation theory - M B P T ( 2 ) / D Z P - p level (all electrons correlated). Vibrational f r e q u e n c i e s h a v e b e e n c o m p u t e d numerically from MBPT(2)/DZP-p gradients to c h e c k the nature o f the stationary points and to gain Z P V corrections. T h e G A U S S I A N 92 p r o g r a m [10] has b e e n used in all optimizations. The path f r o m S C F / 6 - 3 1 G ( d , p), as w e l l as f r o m M B P T ( 2 ) / D Z P - p transition states ( T S ) has b e e n e x p l o r e d using the intrinsic reaction coordinate ( I R C ) m e t h o d [11] as i m p l e m e n t e d in the G A M E S S [12] and G A U S S I A N
Table 2 Proton exchange equilibria. Relative energies (kJ/mol) AE
Reaction (1)
Reaction (2)
SCF MBPT(2) CCSD T(CCSD) SCF + MBPT(2) SCF + CCSD SCF + CCSD + T(CCSD) AH o
- 21.1 29.0 5.8 1.5 7.9 - 15.3 - 13.8 - 12.4
- 17.3 - 30.6 - 31.0 - 6.3 - 47.8 - 48.2 - 54.5 - 60.2
A H29s.15 K
- 11.6
-58.1
AG298.15 K
-
16.4
- 64.3
92 [10] programs. Single-point calculations at the M B P T ( 2 ) g e o m e t r i e s h a v e b e e n p e r f o r m e d at the C C S D m o d e l [13] w h i c h incorporates the effects f r o m single and double excitations and f r o m disconnected triple and quadruple excitations. T h e effect f r o m c o n n e c t e d triples T ( C C S D ) (the C C analog to a fourth-order triple excitation contribution) [14] is c o m p u t e d f r o m the c o n v e r g e d amplitudes o f the C C S D calculation. This a p p r o x i m a t i o n is denoted C C S D + T ( C C S D ) / D Z P - p and w e h a v e used the M O L C A S 2 / C O M E N I U S suite o f p r o g r a m s [15] 1 to obtain correlated activation barriers and reaction enthalpies. T h e inner shell orbitals w e r e uncorrelated for h e a v y atoms in the C C calculations. Total energies are collected in Table 1; relative energies for equilibria and isomerizations in Tables 2 and 3, respectively. T h e h a r m o n i c f r e q u e n c i e s and IR intensities for the ring- and c h a i n - i s o m e r s o f H B C N - are in Table 4. T h e f o l l o w i n g reactions w e r e investigated and will be discussed in Section 3 (see also Fig. 1 for r o m a n n u m b e r i n g o f the structures):
x The original version of the M B P T / C C code included in the MOLCAS system is due to M. Urban, V. Kell/5 and J. Noga and is based on strategies described in Refs. [16,17]
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L Cernug6k, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
Table 3 Isomerization of cyanoborane anion. Relative energies (kJ/mol) AE
Reaction (3)
(4)
(5)
(6)
(7)
(8) !
SCF 166.9 143.5 98.0 100.6 23.4 2.6 MBPT(2) -25.0-16.5 34.6 1 1 . 0 - 8 . 6 - 2 3 . 6 CCSD -24.6 -26.6 9.9 4.6 2.0 - 5 . 3 T(CCSD) -6.2 -6.3 0.0 - 0 . 4 0.2 - 0 . 4 SCF+MBPT(2) 141.9 127.1 132.5 111.5 14.9 - 2 1 . 0 SCF+CCSD 142.3 116.9 107.9 105.2 25.4 - 2 . 7 S C F + C C S D + T ( C C S D ) 136.2 110.6107.9104.8 25.6 - 3 . 1 AH o 131.1 100.8 98.4 100.3 30.3 1.9 AH298.15 K 130.7 101.9 99.5 99.8 28.8 0.3 AG298.15 K 129.2 98.8 97.1 98.9 30.4 1.8
IV
e)
i3 / / / ¥
(a) equilibria H2BCN + O H - = HBCN- (I) + H20,
(1)
H2BCN + H - = HBCN- (I) + H 2,
(2)
;/
Table 4 MBPT(2)/DZP-p harmonic frequencies (cm-1), IR intensities (km/mol) for the HBCN- ring, HBCN- and HBNC- chains Symmetry
to
IR intensity
HBCN- ring ~ ( B H in-plane bend) ~ ' ( B H out-of-plane bend) ,~(/_CBN bend) ,~(/_NCB bend) , ~ ( C N / C B / B N stretch) ,~(BH stretch)
636.3 782.9 1015.8 1080.3 1392.2 2685.2
67.52 18.40 21.28 5.08 15.23 117.68
HBCN- chain ~'(/_BCN out-of-plane bend) K(Z_BCN in-plane bend) P~(CB stretch) P~(/_HBC bend) K(CN stretch) .~(BH stretch)
317.4 387.9 749.8 883.6 2039.6 2422.7
35.5 38.2 7.2 34.2 125.5 300.3
HBNC- chain , ~ ' ( / B N C out-of-plane bend) .~(/_BNC in-plane bend) ?¢(BN stretch) ~((/_HBN bend) K(CN stretch) K(BH stretch)
301.2 374.8 762.2 1006.8 1971.6 2345.4
36.9 23.9 8.9 65.2 13.1 425.1
c)
N
C III
Fig. 1. Sequence of structures along the reaction path. MBPT(2)/DZP-p internal coordinates (,~ and deg). (a) Structure I: bent HBCN- chain, rnn = 1.2329, rBc = 1.5803, rcN = 1.2008, /-HBC = 106.47, /_BCN = 171.14; (b) Structure II: C-transition state, rHB =1.2432, rBc =1.8059, rc~ =1.2178, / H B C = 106.01, /_BCN=96.14, / - - H B C N = - 2 2 . 3 8 ; (c) Structure III: HBCN- ring, rrt B = 1.1930, rBC = 1.5350, rcn = 1.3799, /_HBC = 167.51, /--BCN = 58.48; (d) Structure IV: N-transition state, r,B = 1.2415, rBr~ = 1.6768, rcN = 1.2187, /_I-InN = 95.26, /_BNC = 9 4 . 5 5 , / H B N C = 147.99; (e) Structure V: bent HBNCchain, rrm =1.2422, rBN =1.5063, rCN =1.2049, / - H B N = 103.02, /_BNC = 173.76. (b) isomerizations
HBCN~i.g )(III)
= [HBCN-]*
(II),
(3) (4)
HBCN(~ing ) ( I I I )
= [HBNC-]*
(IV),
(5)
HBCN-
(I) = [HBCN-]*
(II),
HBNC-
(V) = [HBNC-]*
(IV),
HBCN-
( I ) = HBCN(~ing ) ( I I I ) ,
HBNC-
( V ) = HBCN(ring ) ( I I I ) ,
(6) (7) (8)
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L Cernughk, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
For reaction (1) we also present an exploratory search of the potential energy surface (PES) for proton transfer. These calculations yield one TS with O H attached to one hydrogen of the BH 2 group and one intermediate corresponding to an S N2-like O H - attack on the electron deficient B atom. We shall not comment on these results in detail because the PES is complex, including probably more reaction channels, and should be the subject of a separate survey.
3. Results and discussion
Both chemical equilibria indicate that the production of the cyanoborane anion is thermodynamically supported at laboratory temperature (AG in Table 2). For O H - attack the electron correlation slightly destabilizes the deprotonated form, while for H attack the effect is reversed and, more importantly, the correlation is dominant, strongly enhancing the exothermicity of the proton exchange. Note that for reaction (1) MBPT(2) does not give the correct sign of the reaction energy. One reason may be the slow convergence of the MBPT expansion in general, but also different CC convergence for HEO and OH-, Table 1. In addition, for reaction (2) the effect of triples is by no means negligible. This relatively large effect (in comparison with reaction (1)) can solely be attributed to the changes in bonding in HzBCN and HBCN-, the former contains a triple CN bond, while for the latter, two resonance structures can be suggested: H - B ( - ~ - C = N and H B = C = N (- ~. This idea is supported also by Mulliken populations on boron ( - 0.55 e) and nitrogen ( - 0.34 e). These results prompted us to investigate the stationary points of the HBCN- potential energy surface in a two-step procedure (see Section 2). In order to verify that the transition states from preliminary SCF/6-31G(d, p) optimizations connect the chain and ring anions, the intrinsic reaction coordinate (IRC) for the entire isomerization path is investigated at the same level. The primary reason for this check is the large distances, BC = 2.401 ,~ (in [HBCN-]*) and B N = l . 8 0 6 ,A (in [HBNC-]*), found at this level of theory for transition structures, together with the soft imaginary frequencies 229.4i and 306.7i cm -1 associated with these transition
states. Such weakly bound structures may lead to undesired dissociation to BH and CN-. Surprisingly, the SCF/6-31G(d, p) path from the C-transition state [HBCN-]* converges to either HBCN- or HBNCchain isomers but not to the ring. On the other hand, the N-transition state [HBNC-]* lies on the path joining the HBCN0-ing~ and bent chain structure as expected. Mechanistic implications based solely on SCF optimizations may lead to the conclusion that the ring isomer can be prepared only from HBNC-. Thus, higher-level optimizations of the geometry are needed to illuminate this point. The most significant geometry changes, when going from SCF to MBPT(2), can be observed for the transition states, e.g. the refinement of the Ctransition state [HBCN-]* (Fig. lb) at the MBPT(2) level shortens the BC distance (to 1.806 ,A) and changes the orientation of the BH bond with respect to BCN from perpendicular (at the SCF level) to almost coplanar. The changes in [HBNC-]* are less dramatic, but here also the correlation reduces the BN distance to 1.667 A. The values of the imaginary frequencies increase to 456.2i cm -1 for [HBCN-]* and 557.9i cm -1 for [HBNC-]*. Thus, at the MBPT(2) level the shortened distance of the migrating BH group from CN- compares quite well for both transition states and suggests that the path to the ring is almost equally probable from both chain isomers. This assumption is completely confirmed by the IRC calculations at the MBPT(2)/DZP-p level (Fig. 2). We stress that in contrast to SCF, on the MBPT(2) path both transition states are linked by the ring isomer and the overall mechanism of the isomerization can be described as follows: I ~ II ~ III ~IV~V. The barriers for isomerization (Table 3) are significantly reduced upon deprotonation and range from 100 to 131 k J / m o l which is approximately a quarter of those found in Ref. [1]. This reduction is essentially due to lower SCF barriers and can partially be explained by the presence of the soft / B C N - and /_BNC-in-plane bending modes in the chain isomers (Table 4) which allow quite free motion of the BH group. Electron correlation further stabilizes [HBCN-]* while slightly destabilizing [HBNC-]*. The effect of higher excitations is: (i) rather uniform for both activation processes (3) and (4); o
I. Cernug,~k, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
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C-transitior"
state
a
1~785 c~J
-11785
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P~
(-N
i'/
2 HBCN-
-!17B7
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L3 - 1 1 7 89
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. . . .
-8
,
,
,
6
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,
4
,
,
i
,
2
,
,
i
,
,
,
0
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2
,
,
,
i
,
4
,
,
r
,
,
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N-transition
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b
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" 1 7 85
b-.;
rm
Z_
-'1787
cut
-
HBCNr:ng
Z v L
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89
Reaction poth (bobr*sqrt(crnL.)) Fig. 2. IRC profiles, E(MBPT[2]/DZP-p)as a function of total reaction path in mass-weighted Cartesian coordinates: (a) path from C-transitionstate; (b) path from N-transitionstate.
(ii) important for all reactions with one exception, the formation of [HBCN-]* (3), where MBPT(2) performs quite well in contrast to (4); (iii) essential for reactions (7) and (8) where the bonds are breaking and forming; the absolute value of the reaction energy is rather small, and MBPT(2) significantly deviates from CCSD + T(CCSD). In one case, reaction (8), MBPT(2) even fails badly to predict the correct sign of the reaction energy. Curiously, for the formation of [HBNC-]*, steps (5) and (6), the correlation and thermodynamic contributions are in balance and SCF fortuitously fits AG. Reaction enthalpies (being substantially lower than barriers) indicate that the ring formation (7)
265
should be a slightly endothermic process (AG = 30.4 kJ/mol) and they favor the ring formation from the isocyanoborane anion (8). In fact, these two species (III and V) should be equally populated at 298.15 K. In comparison with the results for neutral species (from Ref. [1]) the trends in correlation energy contributions for the individual steps of the mechanism for the anions are opposite. For example, the isomerization of neutral cyanoborane to borazirene is accompanied by a positive correlation contribution (increasing the barrier), while for isocyanoborane the correlation contribution to the barrier is negative. On the other hand, for anionic forms, correlation reduces the barriers for reactions (3) and (4) and increases them slightly for (5) and (6). Since the barriers are lower for anionic species, the effect of electron correlation becomes more important; especially the correct inclusion of higher excitations seems to be crucial. Concerning the reaction energies, the deprotonation causes their huge reduction at the SCF level (compare with Table III in Ref. [1]). The subtle effects of electron correlation on reaction energies are enhanced for anions due to this reduction, and the dominant contribution is recovered at the CCSD level. The harmonic vibrational modes and IR intensities (Table 4) complete the structural information about anionic cyanoborane isomers. An interesting feature of the spectra is the dramatic weakening of CN stretching band (the predicted IR intensity drops almost ten times) when going from HBCN- to HBNC- and the opposite trend in the /_HBC- and /_HBN-bends. Since these anions are unknown to date, these data might assist in their experimental detection.
4. Summary We have found a possible alternative for the isomerization of cyanoborane through anionic forms. SCF + CCSD + T(CCSD) barriers separating the bent-chain and ring isomers of HBCN- are acceptably low for the gas phase. A H and AG values obtained from SCF energies, CCSD + T(CCSD) correlation corrections and thermodynamic contributions (at 298.15 K) suggest that the conceivable reaction channel to the target ring compound can
266
L Cernug,;k, H. Lischka / Chemical Physics Letters 241 (1995) 261-266
start from both bent-chain anions, however the HBNC- isomer is slightly favored thermodynamically. These findings, together with the prediction of the harmonic IR spectra of species I, IH and V, may be helpful in the synthesis of borazirene.
Acknowledgement This research was in part funded by the Action Austria-Slovak Republic and is part of the COST project D3/0003/94.
References [1] Y.-G. Byun, S. Saebo and C.U. Pittman Jr., J. Am. Chem Soc. 113 (1991) 3689; K. Jug, J. Org. Chem. 49 (1984) 4475. [2] I. (~ernu~k, R.J. Bartlett and S. Beck, J. Phys. Chem. 96 (1992) 10284. [3] F. Saugnac, F. Teyssandier and A. Marchand, J. Am. Ceram. Soc. 75 (1992) 161; L. Maya and H.L. Richards, J. Am. Ceram. Soc. 74 (1991) 406. [4] A. Sood, B.F. Spielvogel, B.R. Shaw, L.D. Carlton, B.S. Burnham, E.S. Hall and I.H. Hall, Anticancer Res. 12 (1992) 335; I.H. Hall, E.S. Hall, L.K. Chi, B.R. Shaw, A. Sood and B.F. Spielvogel, Anticancer Res. 12 (1992) 1091. [5] W.J. Hehre, L. Radom, P. von R. Schleyer and J.A. Pople, Ab initio molecular orbital theory (Wiley, New York, 1986).
[6] T.H. Dunning Jr., J. Chem. Phys. 53 (1970) 2823. [7] L.T. Redmon, G.D. Purvis and R.J. Bartlett, J. Am. Chem. Soc. 101 (1979) 2856. [8] T.H. Dunning Jr. and P.J. Hay, in: Modem theoretical chemistry, Vol. 3, ed. H.F. Schaefer III (Plenum Press, New York, 1979). [9] M. Urban, G.H.F. Diercksen, I. Cemu~ik and Z. Havlas, Chem. Phys. Letters 159 (1989) 155. [10] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.W. Wong, J.B. Foresman, M.A. Robb, M. Head-Gordon, E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. DeFrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92/DbT, REVISION F.3 (Gaussian, Pittsburgh, 1993). [11] K. Ishida, K. Morokuma and A. Komomicki, J. Chem. Phys. 66 (1977) 2153. [12] GAMESS-US: M.W. Schmidt, K.K. Baldridge, J.A. Boatz, J.H. Jensen, S. Koseki, M.S. Gordon, K.A. Nguyen, T.L. Windus and S.T. Elbert, QCPE Bull. 10 (1990) 52. [13] G.D. Purvis and R.J. Bartlett, J. Chem. Phys. 76 (1982) 1910. [14] M. Urban, J. Noga, S.J. Cole and R.J. Bartlett, J. Chem. Phys. 83 (1985) 4041. [15] K. Andersson, M. Fiilscher, R. Lindh, P.-A. Malmquist, J. Olsen, B.O. Roos, A.J. Sadlej and P.-O. Widmark, 1991, MOLCAS System of Quantum Chemistry Programs. Release 02 (Theoretical Chemistry, University of Lund, Lund, Sweden). [16] M. Urban, I. Cernu~Lk, V. Kell6 and J. Noga, in: Methods in computational chemistry, Vol. 1, ed. S. Wilson (1987) pp. 117-250. [17] M. Urban, I. Huba~, V. Kell6 and J. Noga, J. Chem. Phys. 72 (1980) 3378; J. Noga, Comput. Phys. Commun. 29 (1983) 117.