5702
J. Phys. Chem. 1996, 100, 5702-5714
The 28-Electron Tetraatomic Molecules: N4, CN2O, BFN2, C2O2, B2F2, CBFO, C2FN, and BNO2. Challenges for Computational and Experimental Chemistry Anatoli A. Korkin,*,1a Anna Balkova,1a Rodney J. Bartlett,*,1a Russell J. Boyd,1b and Paul von Rague Schleyer1c The Quantum Theory Project, UniVersity of Florida, GainesVille, Florida 32611, Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J, and Institut fu¨ r Organische Chemie, UniVersita¨ t Erlangen-Nu¨ rnberg, Henkestrasse 42, D-91054 Erlangen, Germany ReceiVed: January 10, 1996X
The energies and structures of the 28-electron tetraatomic molecules, composed of the first row nonmetallic elements: N4 (1), CN2O (2), BFN2 (3), C2O2 (4), B2F2 (5), CBFO (6), C2FN (7), and BNO2 (8) have been studied uniformly by ab initio methods including coupled-cluster theory. New estimates of the stability of the N4 isomers, tetraazatetrahedrane and tetraazacyclobutadiene, are presented, and a new triplet NNNN openchain isomer has been established computationally. Potential energy surfaces of the nonpolar 1 and the polar 2 are compared. Three-membered cyclic C2V fluorodiazaboririne has been found to be the most stable isomer similar to diazirinone (Chem. Phys. Lett. 1994, 227, 312). Linear triplet CO and BF dimers, OCCO, FBBF, and OCBF, are the most stable forms of 4, 5, and 6, respectively. The singlet cyanofluoromethylene, NCCF, the global energy minimum of 7, is 7 kcal/mol more stable than isomeric CNCF and 10 kcal/mol lower in energy than 3-fluoroazacycloprop-2-ylidene. The singlet and triplet forms of nitrosoboroxide, OBNO, the most stable isomers of 8, were found to have similar energies, within 1 kcal/mol, and the isomeric triplet OBON lies only 4 kcal/mol above OBNO. Singlet-triplet energy separations and dissociation energies to diatomic fragments are compared for the series of linear 14- and 14-electron (NNNN, NNCO, OCCO, FBBF, and OCBF) and open-chain 13- and 15-electron (NCNO, OBCF, NCCF, and OBNO) dimers. Trends in the chemical bonding in the series of 28-electron tetraatomic molecules, 1-8, stability, and possible synthetic routes, are discussed.
Introduction Predictive correlated ab initio quantum chemical methods offer a “microscope”, which can be used to searh experimentally unknown but potentially stable or metastable molecules. In this regard, the recently established features of the N4 (1)2-6 and N2CO (2)7 potential energy surfaces (PES) have prompted us to extend our prior theoretical studies to include the other molecules of the isoelectronic 28-electron tetraatomic series of molecules constituted from the first row nonmetals, B, C, N, O, and F: BFN2 (3), C2O2 (4), B2F2 (5), CBFO (6), C2FN (7), and BNO2 (8). Questions of interest include: (1) the types of chemical bonding to be expected for the most stable forms and for the isomers, which could be synthesized from known species;8 (2) the PES change, when a diatomic fragment is replaced by an isoelectronic group in a tetraatomic molecule; and (3) low-energy excited electronic states. We have performed a uniform electron correlated ab initio study of 1-8 to answer these questions. The energetic and structural data resulting should stimulate experimental efforts to observe and to study these intriguing molecules. If the biradical and zwitterionic forms are excluded, the Lewis structures for 1-8 follow the common valence rules: X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5702$12.00/0
Tetraazatetrahedrane (1a) and its isomeric forms, e.g. tetraazete (tetraazacyclobutadiene, 1b), have been studied extensively by theoretical methods.2-11 Many theoretical efforts to elucidate the properties of this experimentally unknown species have been stimulated by the potential application of N4 as a “pure” highenergy density material (HEDM)2,3 and by the fundamental significance of N4 with regard to chemical concepts, such as bond energy, ring strain, and aromaticity.11 A metastable molecule (local minimum on a PES) might be a candidate for HEDM, if considerable energy is released in © 1996 American Chemical Society
The 28-Electron Tetraatomic Molecules conversion to the stable form per mass unit (the amount of stored energy) and if the energy barrier between the metastable form and the stable products is high. The dissociation energy of Td 1a into two N2 molecules and the dissociation barrier for this reaction have been estimated to be 186 kcal/mol2,3,5 and 5261 kcal/mol,2,3,6 respectively. While the high dissociation barrier suggested that 1a might be relatively stable to dissociation and had potential as an energy accumulator, Yarkony4 showed that the spin-forbidden radiationless decay (singlet-triplet energy surface crossing) might diminish the barrier of Td N4 dissociation to 28 kcal/mol. Tetraazete (1b) has almost the same energy as 1a: Td 1a was evaluated to be 2 kcal/mol more stable than D2h 1b at the CCSDT/DZP and MBPT(4)/DZP levels,2,3 but 1b was 3 kcal/mol lower in energy at the QCISD(T)/6-311+G* level.5 When an NN diatomic fragment is replaced by the less symmetric CO fragment, the C2V diazirinone structure, 2a, is the most stable C2NO isomer.7 Although the nitrosyl cyanide, 2b, is well known from earlier experimental12 and theoretical12a,13 ab initio studies, it is about 11 kcal/mol less stable than diazirinone. The dissociation into CN and NO radicals is endothermic by 47-50 kcal/mol,7,12b but the dissociation of 2a into N2 and CO is 96-100 kcal/mol exothermic.7 Diazirinone (2a) and its dissociation products, the ground-state singlet molecules X 1∑g+ N2 and X 1∑+ CO, are separated by a 27 kcal/mol barrier. Diazaboririne (9) has been studied by ab initio methods along with a series of three-membered aromatic molecules,14 but fluorodiazaboririne (3a) has not been investigated previously. Despite the difference in electronegativities between B and N, π donation to the boron atom is remarkable (0.28 el14b) in 9 for an apparently aromatic species. However, the high ring strain and instability of the tricoordinated boron toward dimerization were suggested to be reasons precluding its experimental observation.14b
Despite numerous synthetic efforts, ethylenedione has long eluded experimental characterization.15 The singlet and triplet states of ethylenedione have been considered already by quantum mechanical methods.16-18 The triplet 3∑g- state (electron configuration: 1σg21σu2σg22σu23σg23σu24σg24σu25σg21πu41πg42πu2) is the most stable for OCCO (4a). The formation of OCCO in the dissociative singlet 1∑g+ state explains the unsuccessful efforts to observe this species.19 Recently, Chen and Holmes20 have speculated about the existence of C2O2 as an intermediate in their neutralization-reionization tandem mass spectrometric study. Although the parent diborene, HBBH, has been characterized by extensive theoretical ab initio studies,21 difluorodiborene has attracted little attention from theoretical chemists18 and is not known experimentally. The linear triplet 3∑g- from 10a of B2H2 is the most stable isomer, and two singlet minima, C2V 10b and D2h 10c have higher energies, 32 and 80 kcal/mol above 9a, respectively.21g Triplet diborene, 10a, is stable toward dis-
sociation into two 1∑+ BH molecules (Do ) 98 kcal/mol21g), and its analog, 5a, also has a lower energy than two BF
J. Phys. Chem., Vol. 100, No. 14, 1996 5703 molecules (Do ) 9 kcal/mol18). Fluoroboroethylenone, the mixed dimer of CO and BF (6a), and other CFBO structures also are discussed below. Lewis structure 7a is unlikely to be a low-energy minimum of C2NF, because of the high strain of the CCN ring. The possibility of preparing another isomer, cyanofluoromethylene, NCCF (7b), from cyanodifluoromethane was considered on the basis of semiempirical MINDO/3 computations.22 The unsubstituted triplet cyanomethylene, NCCH (11a), known both experimentally23 and from theoretical studies,24 is the global minimum for C2HN.24h In agreement with the most recent experimental prediction of a quasi-linear structure for HCCN,23g the linear allene-like biradical (11b) is a transition state only 1 kcal/mol less stable than bent 11a.24f-i The singlet cyclic (11c) and bent (11d) isomers were found to be 8-9 and 13-14 kcal/ mol above 11a, respectively, at the SDCI+Q level.24h,i
The last molecule in this series, nitrosyl boroxide (8a), is considered here for the first time. Being an analog of the experimentally known nitrosyl cyanide (2b),12 8a might be observable species. Computational Methods To treat a wide range of molecular structures at a comparable, high-level approximation, we use a coupled cluster (CC) and its finite order perturbation theory (MBPT ≡ MP).25 The quadratic CI (QCI) method is an approximation to more rigorous CC method being the same up to fifth order in perturbation theory. ACES II program system26 provides such tools, as does Gaussian 92.27 The previous ab initio studies of the 28-electron tetraatomic molecules2,3,5,7 and the related diatomic species25,28 showed that reasonably accurate energies of these species can be obtained at QCISD(T) and at CCSD(T) levels with extended basis sets (6-311+G* or TZ2P) with geometries optimized at MBPT(2) or at CCSD levels with moderate basis sets (6-31G* or DZP). All structures were optimized first at the HF/6-31G* and then at the MBPT(2)(Frozen Core)FC)/6-31+G* levels29 (denoted as HF and MBPT(2) below) with the Gaussian 92 program.27 Single-determinant restricted closed-shell [RHF- and ROHFMBPT(2)]25b and unrestricted open-shell [UHF-MBPT(2)] variants were used for computations of the singlet and triplet states, respectively. The eigenvalue-following algorithm30 was employed to search for transition structures (TS) in cases where the default Berny optimization method31 was ineffective. Stationary points were characterized by computations of the analytical vibrational frequencies both at HF and at MBPT(2) levels.32 Single-point energies at the QCISD(T)//6-311+G*// MBPT(2)/6-31+G* + ZPE(MBPT(2)/6-31+G*) level are discussed in the text (unless noted otherwise).33,34 The HF/631+G*//MBPT(2)/6-31+G* wave functions are characterized by natural bond orbital (NBO) charges (q) and Wiberg bond indices (WBI).35 We also have considered the Mulliken gross populations of pπ orbitals (MPP) in particular cases. The principal open-chain isomers have also been optimized at the CCSD/DZP level36 (denoted as CCSD below) and single-
5704 J. Phys. Chem., Vol. 100, No. 14, 1996 point energies computed at the CCSD(T)/TZ2P level [CCSD(T)/TZ2P//CCSD/DZP results are denoted as CCSD(T) below] with the ACES II26 program. The frozen-core approximation has been employed in the coupled-cluster computations, as well as at the above-mentioned MBPT(2) and QCI levels. Various reference wave functions and coupled-cluster computational models were employed depending on the multiplicity and the electronic state of the 28-electron tetraatomic molecules considered. Restricted open-shell (ROHF) and unrestricted (UHF) single-determinant SCF reference wave functions were followed by MBPT(2)25b and coupled-cluster25a computations of the linear triplet structures. Two-determinant reference wave functions37 for linear triplet structures give very similar energies and structures as those with single-determinant reference configurations. Thus the computations based on the single-determinant wave functions are only discussed here for 3∑- triplets. However, the correlation energy of an open-shell ∑ singlet can be only determined properly by using the new two-determinant coupled-cluster (TD-CC) approach.37 For consistency between 3∑ and 1∑ energies, the same type orbitals (triplet ROHF) were used to determine the geometries and energies of the linear triplet and singlet molecules. Such ROHF-CC methods have only been recently developed and are now generalized for analytical gradients.25c The lowest 1∆ state linear structures were also computed at the CCSD(T)/TZ2P//CCSD/DZP level. Although only single-determinant (symmetry broken) RHF reference functions were used in geometry optimizations at the CCSD/DZP level, both single- and two-determinant reference functions were employed for the computations of the singlepoint CCSD(T)/TZ2P energies. Due to the multireference nature of the 1∆ singlet state, the energies derived by using the two-determinant approach should be more reliable and consistent with the ∑ triplet and singlet state energies. In order to provide the fragmentation energies for the 28-electron tetraatomic molecules, the geometries and correlation energies of the reference diatomic molecules (N2, CO, and BF) and radicals (BO, CN, NO and CF) also were computed with the same methods. Energies and molecular structures are presented in the Tables 1-6, in Figures 1 and 2, and on the schematic drawings.38 Results and Discussion N4. Although there have been many theoretical studies of tetrahedral 1a and four-membered cyclic 1b during the past 25 years,2-6,9-11 some interesting unresolved problems deserve more precise investigation.
Two dissociation channels of tetrahedral 1a have been discussed in the literature: the D2d minimum energy pathway2,3,10e,h and a path via a Cs structure,2,3,6 resembling the quasitetrahedral Cs triplet local minumum.5,10i The D2d symmetric minimum energy pathway is prohibited by orbital symmetry rules, as 1a has an electron configuration corresponding to a doubly excited configuration of 2N2. At the crossing point of two energy surfaces, the doubly occupied 1b1 orbital in 1a interchanges with the empty 4b2, which is doubly occupied in 2N2 in D2d symmetry. Francl and Chesick10h estimated a 75 kcal/mol dissociation barrier in 1a at the SCF level by
Korkin et al. computation of the energy difference between the Td 1a and the saddle point D2d structure at the crossing point of the PESs of 1a and 2N2. Obviously this overestimates the TS energy. Bartlett et al.2,3 have used a symmetry-broken coupled-cluster method,39 which modulates the multireference situation and includes dynamic electron correlation, and they have found a lower barrier (52 kcal/mol) for the D2d decomposition pathway for 1a. We used a similar procedure to approximate the PES crossing point on the D2d dissociation pathway at MBPT(2). The energy of the D2d saddle point, established within 1 kcal/ mol accuracy,40 is 61 kcal/mol above 1a (see Figure 1), in good agreement with previous estimates.2,3,6 The Cs saddle point found by Bartlett et al.2,3 relates 1a with the C2V three-membered cyclic local minimum at HF/DZP. At the MBPT(2)/DZP level the Cs TS and the C2V minimum convert to a second-order stationary point and a TS, respectively. The Cs TS found by Lee and Rice10i does not go to the correct products, as previously recognized.2,3 We have now located the Cs transition state, 1c, which has the same (1a′)2‚‚‚(10a′)2 (1a′′)2‚‚‚(4a′′)2 electronic configuration both at the HF and at the second-order perturbation theory levels. We suggest, that there was an interchange between occupied and empty orbitals of different symmetry (a′ and a′′) in the TSs computed at HF and at correlated levels in the earlier work.10i The relative energy of 1c found here (74.8 kcal/mol at MBPT(2) and 58.6 kcal/mol at QCI), is in agreement with previous estimates.2,3,6
The Cs TS, 1c, does not lead directly to two N2 molecules, but to the intermediate C2V minimum, 1d, at HF and to the less symmetrical three-membered cyclic Cs structure, 1e, at MBPT(2) in agreement with the previous studies.2,3,5 The dissociation barrier for 1e to two N2 is obviously very low,41 and thus TS 1d between 1a and 1e, de facto, is a TS for the dissociation of 1a to 2N2 (see Figure 2). This conclusion has been further established by the recent multiconfigurational CAS SCF study.6 Similar to 1a, the minimum energy D2h pathway for dissociation of 1b is prohibited by symmetry. The ground-state electronic configurations for 1b, (1ag)2(1b1u)2(1b2u)2(1b3g)2(2ag)2(2b1u)2(2b2u)2(3ag)2(2b3g)2(4ag)2(1b3u)2(3b1u)2(1b2g)2(3b2u)2, and for 2N2, (1ag)2(1b1u)2(1b2u)2(1b3g)2(2ag)2(2b1u)2(2b2u)2(3ag)2(2b3g)2(4ag)2(1b3u)2(3b1u)2(1b2g)2(4b1u)2, are related by a change of occupation of 3b2u (doubly occupied in 2a) and 4b1u (doubly occupied in 2N2) MO’s. The same approach used to search the D2d dissociation pathway for 1a,40 located a crossing point on the D2h dissociation pathway [14 kcal/mol above 1b at MBPT(2)] in the PES of 1b and in that of the ground-state electronic configuration two N2 molecules. Thus the saddle point energy is lower for 1b (D2h pathway) than for 1a (D2d pathway). We assume that the energies of the D2d and D2h saddle points provide upper estimates of the dissociation barriers in 1a and in 1b, respectively. Hence, the dissociation barrier of 1b should also be lower than of 1a. To evaluate the barrier height between 1a and 1b we have considered the minimum-energy D2 pathway between tetrahedral and planar isomers of N4.40 The electronic configurations of
The 28-Electron Tetraatomic Molecules
J. Phys. Chem., Vol. 100, No. 14, 1996 5705
Figure 1. Energy barrier estimates at the MBPT(2)(FC)/6-31+G* level on the minimum energy pathways for dissociation of Td 1a (a) and D2h 1b (b) and for the interconversion of 1a and 1b (c).
1a, (1a)2(1b3)2(1b2)2(1b1)2(2a)2(2b3)2(2b2)2(2b1)2(3a)2(3b3)2(3b2)2(3b1)2(4a)2(5a)2, and 1b, (1a)2(1b3)2(1b2)2(1b1)2(2a)2(2b3)2(2b2)2(3a)2(4a)2(2b1)2(3b1)2(3b3)2(3b2)2(4b2)2, are similar except for the change in the occupation of the boundary MO’s, (5a)2(4b2)0 in 1a into (4b2)2(5a)0 in 1b. The crossing point for these two configurations, about 70 kcal/mol above 1a, shows that this dissociation channel is not favorable for tetraazatetrahedrane. Tetrahedral 1a and planar 1b are composed of small rings; 1b also is antiaromatic. Hence, the highly exothermic dissociation of both species is due to the high stability of the N2 triple bond as well as the destabilization of the N4 molecules. However, the isodesmic reaction energy of tetrahedrane decreases dramatically when the CH groups are replaced sequentially by isoelectronic N atoms.11,42 Polarization (d orbitals) is known to be important to evaluate this “aza-stabilization” effect, but electron correlation was not considered in the previous ab initio studies.11 We have computed the isodesmic strain energies of tetrahedrane and cyclobutadiene (also see ref 43 and the literature cited therein) compared to their tetraaza analogs at the MBPT(2)/6-31G* + ZPE(HF/-31G*) level:44
Although electron correlation diminishes the “aza-stabilization” effect,42 the isodesmic energy of 1a (eq 1) is about 70 kcal/mol lower than the tetrahedane value (eq 2). While 1b is slightly stabilized (endothermic energy in eq 3), cyclobutadiene is strongly destabilized (exothermic energy in eq 4). The delocalization of the nitrogen lone pairs (n f σ* interaction45)
5706 J. Phys. Chem., Vol. 100, No. 14, 1996
Korkin et al.
a
b
Figure 2. Potential energy diagram for N4 (a) and N2CO (b) structures.
enhanced in the cyclic species explains stabilization of 1a and 1b compared to their carbon analogs qualitatively.11,46 As this may be important in the chemistry of N4,4,5,9 openchain triplet forms were investigated in more detail. The C2h open-chain triplet minimum (1f), recently characterized at the MP4SDTQ//MBPT(2)/6-31G* level, was considered to be the lowest chemically bound N4 isomer.5 However, when a singlet state calculation is performed at the optimized triplet-state geometry of 1f, the ∆EvST energy (the vertical singlet-triplet energy gap) is 41.5 kcal/mol lower than the energy of the 3Bu minimum. Thus the negative sign of the ∆EvST defines 1f as an exciplex.47 In its 3Bu electronic state, 1f has four π electrons, creating two π bonds between internal and terminal nitrogens. Two unpaired electrons are in the σ frame. The bond distances in 1f may be compared with the standard NN single (1.439 Å in H2NsNH2), double (1.266 Å in HNdNH) and triple (1.130 Å in NtN) bond lengths, computed at the similar level (MBPT(2)/6-31G*).44 The Wiberg bond indices show the terminal NN bonds (WBI ) 2.09) to be double rather than triple. The
unpaired electrons, as derived from the MO analysis, are localized mainly in in-plane p orbitals on the terminal nitrogens.
Another C2h open-chain N4 triplet (3Bg state; five π electrons), is a transition state. If the symmetry constraint is reduced, a Cs (3A′′) minimum (1g) arises from optimization. This Cs form 1g has five π electrons and a short central N(2)N(3) bond (WBI ) 1.331). The terminal NN bonds are not equivalent: N(1)N(2) (WBI ) 2.243) is even shorter than the triple bond in N2 and N(3)N(4) (WBI ) 1.141) is intermediate between a single and a double bond. The population analysis shows the unpaired electron density to be largely localized on N(4). The terminal
The 28-Electron Tetraatomic Molecules nitrogens have small positive NBO charges (q[N(1)] ) 0.06 and q[N(4)] ) 0.08), but the central NN bond is strongly polarized (q[N(2)] ) 0.20 and q[N(3)] ) -0.34). While the energy of 1g is 8.5 kcal/mol higher than 1f, the 15.7 kcal/mol ∆EvST value of 1g is quite positive. Although lower in energy than 1g, triplet 1f is an exciplex (∆EvST < 0) which can only be expected to have a very short lifetime. In contrast, triplet 1g is stable at its equilibrium geometry relative to singlet NNNN (∆EvST > 0). Hence, 1g might be observed experimentally, as a long-lived intermediate, under certain conditions. CN2O. The energy diagram in Figure 2 summarizes the results of the present and the previous theoretical studies of N2CO7 and N42,3,5 systems. The most obvious differences between the two PESs are the reduced dissociation barrier heights of the quasi-tetrahedral N2CO structure and the much greater stability of C2V 2a, as compared with the corresponding 1a and 1d (1e) structures of N4. The spin-allowed dissociation of the open-chain triplet Cs, 1g, to X 1∑g+ N2 and A 3∑u+ N2 is 27 kcal/mol exothermic, while the dissociation of Cs 2c (3A′′ 2c f X 1∑g+ N2+ a 3Πr N2) is 35 kcal/mol endothermic. These peculiarities can be explained qualitatively by considering the differences in excitation energies of N2 and CO:48 (π f π*) NN: 144 kcal/mol (3∑g+) CO: 160 kcal/mol (3∑+)
(σ f π*) 171 kcal/mol (3Πu) 139 kcal/mol (3Π)
Formation of cyclic and open-chain dimers involves electronic excitation of both 14-electron diatomic monomers, but the type of excitation required depends on the chemical bonding in the product dimers. Thus the tetrahedral and the four-membered planar structures (e.g., 1a and 1b isomers of N4) correspond to the π f π* doubly excited configurations of both diatomic precursors. This excitation requires the least energy for N2, but is less favorable for CO than the σ f π* excitation. Formation of a three-membered ring structure requires one diatomic molecule to be π f π* and the other to be σ f π* excited (e.g., N2 and CO molecules in 2a). This type of double excitation corresponds to the minimum excitation energy requirement for 2a, but not for 1d. The open-chain triplet molecules can arise from doublet π f π* (e.g., 3Bu 1f), double σ f π* (linear triplet 3∑- forms are considered below) or mixed, π f π* + σ f π* (e.g. 3A′′ 1g), excitation.
J. Phys. Chem., Vol. 100, No. 14, 1996 5707 barrier heights for dissociation of 2a and 3a are different, the corresponding transition states, 2d7 and 3b, are similar.
The isodesmic aromatic stabilization in 2a defined from eq 5 (6-7 kcal/mol7) is substantially lower than that in cyclopropenone (12) in eq 6 (22-25 kcal/mol).7,49 Does cyclic threecenter two-electron π delocalization increase the stability of 3a compared to 2a? The NN bond lengths in 2a (1.351 Å) and 3a (1.348 Å) are very similar, but longer than that in diazirine (1.255 Å) (13).44 Although the charge of the NdN fragment in 2a (-0.21) is significantly different from that in 3a (-0.67), the π donation from the nitrogens to carbon in 2a [MPP(N2) ) 1.67 and WBI(CN) ) 1.078)] and to boron in 3a [MPP(N2) ) 1.65 and WBI(CN) ) 1.004)] are nearly the same. The almost zero energy of eq 7 also suggests that the stabilization of 2a and of 3a, arising from cyclic π delocalization, is comparable.
The smaller exothermicity for dissociation of 3a into diatomic fragments can be attributed to the differences between B-N (76.5 kcal/mol) and C-N (62.5 kcal/mol) single-bond energies (E)50 and to the σ f π* (X 1∑+ f a 3Πr) excitation energies (eσπ*) in CO (139 kcal/mol) and in BF (83 kcal/mol).48 As mentioned above, the ground-state electronic configurations of 2a and of 3a correspond to the excited states of both diatomic fragments, which compose the three-membered ring structures. Assuming the cyclic strain and π delocalization energies to be similar in 2a as in 3a, we can expect that the dissociation energies of these two species differ by the value of ∆σπ* + 2∆E:
Do(3a) - Do(2a) ≈ [σπ*(CO) - σπ*(BF)] + 2[E(BN) - E(CN)] (8)
BFN2. The three-membered cyclic fluorodiazaboririne (3a), the most stable N2BF structure, has an electronic configuration similar to that of C2V diazirinone (2a). Although both 2a and 3a are unknown experimentally, 3a should be more stable because of the higher barrier and the decreased dissociation energy into the ground state diatomic molecules, N2 and BF (∆Eq ) 39.6 kcal/mol and Do ) -21.9 kcal/mol). Those may be compared with dissociation of 2a into N2 and CO (∆Eq ) 27 kcal/mol and Do ) -96 to -100 kcal/mol).7 Although the
Indeed, the data given above show that eq 8 holds within a 10 kcal/mol range. Two open-chain NNBF triplet minima, the linear 3c (3∑state) and the trans-bent 3d (3A′′ state) are computed to have similar energies. While 3c is 1.3 kcal/mol more stable at QCI than bent 3d, the energy order is reversed at CCSD(T) (see Table 1). The open-chain triplet NNBF structures, 3c and 3d, are ∼25 kcal/mol less favorable energetically than cyclic 3a. This is significantly larger than the 7 kcal/mol difference between the triplet open-chain Cs 2c and cyclic C2V 2a N2CO species.7 As with NNCO,7 another Cs NNBF triplet planar bent form,
5708 J. Phys. Chem., Vol. 100, No. 14, 1996
Korkin et al.
TABLE 1: Relative Energies (in kcal/mol)a of the Triplet trans-Bent NNBF (3d) at Different Levels of Theory (XA//XB)b A//B
MBPT(2)/TZ2P//
CSD/TZ2P//
CCSD(T)/TZ2P//
CCSD+T(CCSD)/TZ2P//
//UHF-MBPT(2)/DZP //ROHF-MBPT(2)/DZP //UHF-CCSD/DZP //ROHF-CCSD/DZP
-0.06 -5.44 4.62 -5.35
0.06 -0.72 -0.93 -0.78
-0.53 -1.79 -1.87 -2.07
-0.78 2.45
a Relative energies of 3d with respect to the linear form, ∆E ) E(3d) - E(3c), are given without ZPE correction. Corresponding values at HF/631G//HF/6-31G*, MBPT(2)/6-31+G*//MBPT(2)/6-31+G* and QCISD(T)/6-311+G*//MBPT(2)/6-31+G* are 5.79 kcal/mol, 1.86 kcal/mol and 0.47 kcal/mol respectively. b X ) U or RO; single-point calculations were performed with the same reference HF function (UHF or ROHF) as geometry optimizations.
TABLE 2: Geometries of the Triplet Linear (3c) and Trans-Bent (3d) Minima of NNBF Optimized at the Different Levels of Theory MBPT(2)/DZP UHF/6-31G*
MBPT(2)/6-31+G*
CCSD/DZP
UHF
ROHF
UHF
ROHF
Linear: 3∑- 3c NN, Å NB, Å BF, Å
1.262 1.240 1.283
1.261 1.266 1.306
1.266 1.273 1.304
1.244 1.284 1.308
1.258 1.278 1.308
1.259 1.277 1.307
NN, Å NB, Å BF, Å ∠NNB, deg ∠NBF, deg
1.177 1.353 1.307 165.9 130.7
Trans-Bent: 3A′′ 3d 1.129 1.129 1.430 1.442 1.340 1.340 176.7 176.1 119.8 120.0
1.162 1.424 1.341 171.3 123.0
1.173 1.408 1.337 168.4 126.7
1.171 1.406 1.336 168.6 126.2
which has two unpaired electrons in the molecular plane (3A′ state; an analog of 1f), is a higher energy TS collapsing into 3d upon optimization when the symmetry constraint is removed.
The structural parameters of 3c and 3d (Table 2) show small but notable differences in the geometries computed at ROHFMBPT(2) and at UHF-MBPT(2), but the ROHF-CCSD and UCCSD geometries of these species are very similar. The terminal NN bond (WBI ) 1.324) is longer and the central BN bond (WBI ) 1.538) si shorter in 3c compared to the corresponding bond lengths in 3d [WBI(NN) ) 2.178 and WBI(BN) ) 0.874]. As it is evident from the WBI sums, the internal nitrogen atoms have normal valencies in both triplet forms. The two degenerate singly populated πu orbitals in 3c have bonding character between N and B and predominant AO coefficients on the terminal nitrogens. The HOMO-1 orbital in 3d has a large contribution of the diffuse orbitals of the two terminal atoms, while the HOMO of 3d is similar to that of 3c. C2O2. In agreement with previous ab initio studies,17 we found the linear triplet OCCO (4a) to have lower energy than the linear singlet form (4b), possessing similar electronic configuration (1σg2‚‚‚1πu41πg42πu2). The linear triplet, 4a (3∑gstate), is a minimum at HF/6-31G* and at MBPT(2)/6-31G*, but addition of diffuse sp functions to the basis set (6-31G* f 6-31+G*) leads to an instability of the linear form with respect to trans-bending (D∞h f C2h). The bent C2h form has almost the same energy and geometry as 4a, except for 8° bending. A comparative analysis of the MO population has shown noticeable differences between HOMO-1 orbitals in 4a and in the C2h bent form. While HOMO and HOMO-1 are degenerate πu orbitals in 4a, the bu HOMO-1 is a quasi-Rydberg σ orbital in the bent C2h form. Apart from the higher symmetry, the HOMO and HOMO-1 patterns in linear 4a and in the bent form resemble those of 3c and 3d, respectively. Note that the OCCO+
radical cation has a C2h structure (RCC ) 1.462 Å, RCO ) 1.157 Å and ∠CCO ) 146.5° at MBPT(2)/6-31G*) with a singly occupied bu orbital in its ground state.44,51 The linear openshell singlet 4b is not a minimum in the most energy favorable electronic configuration and “dissociates” into two CO molecules during optimization, if the symmetry constraint is reduced.
A recent extensive study18 shows the OCCO triplet to be linear. Hence, bending of the OCCO triplet is more likely to be an artifact of the MBPT(2)/6-31+G* level of theory. However, the flexibility of the molecule toward the C2h form is expected from the pattern of the singly occupied MO’s. The lowest vibrational mode, ω(πg), corresponding to the C2h bending, depends on the size of the basis set and the electron correlation treatment, and varies from 360 cm-1 at HF/TZ2P(2d,f) to 108 cm-1 at CCSD(T)/DZP.18 Regarding the possible observation of C2O2 in the gas phase or by matrix isolation as the product of reactions of the experimentally known carbonyl carbene (see ref 20 and the literature cited therein) and atomic oxygen, C2O + O f C2O2, or dicarbone52 with molecular oxygen, C2 + O2 f C2O2, we have considered some new C2O2 forms, e.g. 4c-g, in this study.
The 28-Electron Tetraatomic Molecules Only two stationary structures, Cs 4c and C2V 4d, have been located computationally within 100 kcal/mol of the lowest energy isomer. The three-membered cyclic Cs 4c, a local minimum at HF, “dissociates” upon MBPT(2) optimization into the two ground-state CO molecules. Another high energy minimum, the C2V 4d, has a planar D2h form at HF. All other forms, e.g. singlet and triplet 4e, 4f, and 4g, have very high energies and are not considered further. Thus experimental observation of C2O2 forms different from the most stable linear 4a triplet is very unlikely. B2F2. While triplet OCCO is apparently linear, the situation with triplet 5a is less clear, as the C2h bending of FBBF is even easier. Linear triplet 5a is a second-order stationary point both at UHF and at UMBPT(2) with 6-31G* and 6-31+G* basis sets. Like OCCO, the C2h minimum has practically the same energy as linear 5a and a very small deviation (4°) from linearity. Our 9.2 kcal/mol estimate for the dissociation energy of 5a agrees with the 9.0 kcal/mol value computed at CCSD(T)/TZ2P(2d).18 The basis set dependence of the bending vibrational mode, ω(πg), in 5a18 is negligible at HF [268 cm-1 with DZP and 273 cm-1 with TZ2P(2d,f)], moderate at CISD [147 cm-1 with DZP and 251 cm-1 with TZ2P(2d,f)], and very high at CCSD [185i cm-1 at DZP and 376 cm-1 with TZ2P(2d,f)].
The linear singlet structure 5b, is a second-order stationary point at MBPT(2), and it converts into the C2h bent 5c upon optimization at MBPT(2).53 Isomer 5c is 4.2 kcal/mol below two BF molecules. Despite the long BB distance in 5c, some double-bond character is suggested by the NBO analysis (WBI[BB] ) 1.432). The BB double bond in 5c, mainly involving two highest occupied ag and bu MO’s, can be visualized as a donor-acceptor complex formed by delocalization of σ lone pairs into empty pπ orbitals, lying in the molecular plane. The association of two ground-state BF molecules into 5c has a very low barrier ( OBCF > NCNO > OBNO. Thus, the dissociation energy increases when BO is replaced by CN (OBCF f NCCF and OBNO f OBCF) or if NO is substituted by CF (OBNO f OBCF and NCNO f NCCF). Substitution of CN by BO (NCCF f OBCF and NCNO f OBNO) reduces the energy separation between singlet and triplet states considerably, while
replacement of CF by NO (NCCF f NCNO and OBCF f OBNO) only stabilizes the triplet forms slightly. In all four systems, the triplet forms are less bent and have shorter central bonds than the corresponding singlet forms. The two computational methods, QCISD(T)/6-311+G*// MBPT(2)/6-31+G* and CCSD(T)/TZ2P//CCSD/DZP, give consistent results both for structures and for energies. The largest deviations, in DoS and ∆EaST (2-3 kcal/mol) for NCNO (2b) and NCCF (7b) are related to the high-spin contamination of the HF function of CN radical.28 Nevertheless, QCISD(T) and CCSD(T) methods provide good dissociation energy estimates, as shown earlier for nitrosyl cyanide.7 Except for triplet 7c, MBPT(2) gives larger central bond lengths than CCSD. The largest 0.046 Å discrepancy between MBPT(2) and CCSD central bond lengths is in the triplet 2e. The experimental59c and MBPT(2) geometries of NCNO agree best for the central single CN bond length, while CCSD provides better agreement with experiment for the terminal multiple CN and NO bonds. Both methods give consistency within 1° of the experimental CNO bond angle. Conclusion The 28-electron tetraatomic molecules, 1-8, are a challenging set of species with intriguing properties and potential applications. Only nitrosyl cyanide (2b)12,59 is known. The difficulties on experimental observation68 may be related to the tendency of all these species to possess energetically favorable but highly reactive open-chain triplet states. Only the triplet difluorodiborene (5a) is about 10 kcal/mol more stable than two groundstate 14e-electron diatomic BF molecules. NNNN, NNCO, NNBF, OCCO, and OCBF have exothermic dissociation energies: 181, 105, 49, 66, and 23 kcal/mol, respectively. The corresponding singlet states either are higher energy local minima (FBBF, OCBF) or do not even have barriers toward dissociation (NNNN, NNCO, NNBF, and OCCO). Another open-chain isomer type is comprised of 13- and 15electron dimers: NCNO (1A′ 2b), OBCF (3A′′ 6e), NCCF (1A′
The 28-Electron Tetraatomic Molecules 7b), and OBNO (1A′ 8a). The dissociation energies toward the corresponding diatomic 2∑ 13-electron and 2Π 15-electron radicals decrease along the row: 7b (88 kcal/mol) > 6e (68 kcal/mol) > 2b (46 kcal/mol) > 8a (39 kcal/mol). All singlet 13- and 15-electron dimers except boroxofluorocarbene (6f) have lower energies than the corresponding triplet states, whereas nitrosyl boroxide has usually low singlet-triplet energy separation (>1 kcal/mol). Being composed of experimentally known diatomic radicals and possessing endothermic dissociation energies, OBCF, NCCF, and OBNO molecules might be observed under similar conditions as NCNO.12,59 Both tetraazatetrahedrane (1a) and tetraazacyclobutadiene (1b) are high-energy molecules. Moreover, 1b has very low barrier (>15 kcal/mol) toward dissociation, and the barrier for dissociation of 1a may be reduced to 30 kcal/mol because of singlet-triplet surface crossing.4 Two open-chain triplets, C2h 3B 1f5 and C 3A′′ 1g have lower energies than 1a and 1b. u s Diazirinone (2a) and fluoroborodiazirene (3a), the most stable isomers of CN2O and BFN2, respectively, are structural “intermediates” between the compact tetrahedral 1a and the linear OCCO (4a) and FBBF (5a). With a 100 kcal/mol dissociation energy, 2a is a high-energy molecule, while 3a might be a precusor of interesting new nitrogen compounds. Both 2a and 3a have reasonably high dissociation barriers (27 and 39 kcal/mol, respectively) and might be observable. We hope that this computational study of 1-8 will stimulate new experimental and theoretical studies of the 28-electron tetraatomic molecules. Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council of Canada in Halifax, by AFOSR-F49620-95-1-0130 grant in Gainesville, and by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie in Erlangen. We thank Dr. John Watts for his help with the ACES II program, Dr. Russell Thomas for providing unpublished results, and Margaret Austen for technical assistance. A.A.K. thanks Prof. Kurt Schaffner for his friendly personal support. References and Notes (1) (a) University of Florida; email:
[email protected]. (b) Dalhousie University. (c) Universita¨t Erlangen-Nu¨rnberg. (2) Lauderdale, W. J.; Myers, M. J.; Bernholdt, D. E.; Stanton, J. F.; Bartlett, R. J. In Proceedings of the High Energy Materials Contractors Conference, Long Beach, CA, February 25-28, 1990; Air Force Office of Scientific Research, Bolling AFB: Washington, DC, 1990; p 121. (3) Lauderdale, W. J.; Stanton, J. F.; Bartlett, R. J. J. Phys. Chem. 1992, 96, 1173. Lee, T. J.; Rice, J. E. J. Chem. Phys. 1991, 94, 1215. (4) Yarkony, D. R. J. Am. Chem. Soc. 1992, 114, 5406. (5) Glukhovtsev, M. N.; Schleyer, P. v. R. Int. J. Quantum Chem. 1993, 46, 119. (6) Dunn, K. M.; Morokuma, K. J. Chem. Phys. 1995, 102, 4904. (7) Korkin, A. A.; Schleyer, P. v. R.; Boyd, R. J. Chem. Phys. Lett. 1994, 227, 312. (8) Lowest energy isomers sometimes do not correspond to the experimentally available structures. Unknown 2a and known 2b provide an example (see ref 7). (9) (a) Shustorovich, E. M. Zh. Strukt. Khim. 1969, 10, 159; 947. (b) Shustorovich, E. M.; Kagan, G. I.; Kagan, G. M. Zh. Strukt. Khim. 1969, 10, 590; 696. (c) Shustorovich, E. M.; Kagan, G. I.; Kagan, G. M. Zh. Strukt. Khim. 1970, 11, 95. (10) (a) Hillier, I. H.; Saunders, V. R. J. Chem. Soc., Chem. Commun. 1970, 1233. Archibald, R. M.; Perkins, P. G. J. Chem. Soc., Chem. Commun. 1970, 569. (c) Guest, M. F.; Hillier, I. H.; Saunders, V. R. J. Chem. Soc., Faraday Trans. 2 1972, 68, 2070. (d) Wright, J. S. J. Am. Chem. Soc. 1974, 96, 4753. (e) Venanzi, T. J.; Schulman, J. M. Mol. Phys. 1975, 30, 281. (f) Trinquier, G.; Malrieu, J.-P.; Daudey, J.-P. Chem. Phys. Lett. 1981, 80, 552. (g) Novaro, O.; Castillo, S. Int. J. Quantum Chem. 1984, 26, 411. (h) Francl, M. M.; Chesick, J. P. J. Phys. Chem. 1990, 94, 526. (i) Lee, T. J.; Rice, J. E. J. Chem. Phys. 1991, 94, 1215. (11) (a) Shibaev, A. Yu.; Pozunov, Yu. V. Zh. Strukt. Khim. 1986, 27, 151. (b) Seminario, J. M.; Politzer, P. Chem. Phys. Lett. 1989, 159, 27.
J. Phys. Chem., Vol. 100, No. 14, 1996 5713 (c) Murray, J. S.; Seminario, J. M.; Lane, P.; Politzer, P. J. Mol. Struct. Theochem 1990, 207, 193. (d) Alkota, I.; Elguero, J.; Rozas, I.; Balaban, A. T. J. Mol. Struct. Theochem 1990, 208, 63. (12) (a) Bak, B.; Nicolaisen, F. M.; Nielsen, O. J.; Skaarup, S. J. Mol. Struct. 1979, 51, 17. (b) Nadler, I.; Reisler, H.; Noble, M.; Wittig, C. Chem. Phys. Lett. 1984, 108, 115. (c) Reisler, H.; Wittig, C. Annu. ReV. Phys. Chem. 1986, 37, 307. (d) Khundkar, L. R.; Knee, J. L.; Zewail, A. H. J. Chem. Phys. 1987, 87, 77. (13) (a) Jonkers, G.; Mooyman, R.; De Lange, C. A. Chem. Phys. 1981, 57, 97. (b) Bjo¨rkman, C.; Bagus, P. S. J. Chem. Phys. 1982, 76, 3111. (c) Dupuis, M.; Lester, W. A. J. Chem. Phys. 1985, 83, 3990. (d) Bai, Y. Y.; Segal, G. A. Chem. Phys. Lett. 1988, 151, 31. (e) Aray, Y.; Murgich, J.; Luna, M. A. J. Am. Chem. Soc. 1991, 113, 7135. (14) (a) Liang, C.; Allen, L. C. J. Am. Chem. Soc. 1991, 113, 1878. (b) Byun, Y.-G.; Saebo, S.; Pittman, C. U., Jr. J. Am. Chem. Soc. 1991, 113, 3689. (15) (a) Strating, J.; Zwanenburg, B.; Wagenaar, A.; Udding, A. C. Tetrahedron Lett. 1969, 125. (b) Haddon, R. D. Tetrahedron Lett. 1972, 3897. (c) Fleischhauer, J.; Beckers, M.; Scharf, H.-D. Tetrahedron Lett. 1973, 4275. (d) Knight, L. B., Jr.; Steadman, J.; Miller, P. K.; Bowman, D. E.; Davidson, E. R.; Feller, D. J. Chem. Phys. 1984, 80, 4593. (e) Birney, D. M.; Berson, J. A. Tetrahedron 1986, 42, 1561. (f) Berson, J. A.; Birney, D. M.; Dailey, W. P.; Liebman, J. F. In Molecular Structure and Energetics; Liebman, J. F., Greenberg, A., Eds.; VCH Publishers: Weinheim, Germany, 1988; Vol. 6. (g) Suelze, D.; Weiske, T.; Schwarz, H. Int. J. Mass Spectrom. Ion Processes 1993, 125, 75. (16) (a) Gimarc, B. M. J. Am. Chem. Soc. 1970, 92, 266. (b) Mathies, P.; Sladky, F. O.; Rode, B. M. J. Mol. Struct. Theochem 1982, 90, 335. (17) (a) Haddon, R. C.; Poppinger, D.; Random, L. J. Am. Chem. 1975, 97, 1645. (b) Raine, G. P.; Schaefer, H. F., III; Haddon, R. C. J. Am. Chem. Soc. 1983, 105, 194. (18) Thomas, J. R.; Schaefer, H. F., III. Private communication. (19) Suelze, D.; Terlouw, J. K.; Schwarz, H. J. Am. Chem. Soc. 1990, 112, 628. (20) Chen, H.; Holmes, J. L. Int. J. Mass Spectrom. Ion Processes 1994, 133, 111. (21) (a) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 3402. (b) Scuseria, G. E.; Lee, T. J.; Schaefer, H. F., III. Chem. Phys. Lett. 1986, 130, 236. (c) Jouany, C.; Barthelat, J. C.; Daudey, J. P. Chem. Phys. Lett. 1987, 136, 52. (d) Sana, M.; Leroy, G.; Henriet, C. H. J. Mol. Struct. Theochem 1989, 187, 233. (e) Curtiss, L. A.; Pople, J. A. J. Chem. Phys. 1989, 91, 4809. (f) Somogyi, A.; Go¨mo¨ry, A. Chem. Phys. Lett. 1992, 192, 221. (g) Treboux, G.; Barthelat, J.-C. J. Am. Chem. Soc. 1993, 115, 4870. (22) Ioffe, A. I.; Nefedov, O. M. IzV. Acad. Nauk SSSR, Ser. Khim. 1988, 2329 (English translation p 2159). (23) (a) Bernheim, R. A.; Kempf, R. J.; Humer, P. W.; Skell, P. S. J. Chem. Phys. 1964, 41, 1156. (b) Bernheim, R. A.; Kempf, R. J.; Gramas, J. V.; Skell, P. S.; J. Chem. Phys. 1965, 43, 196. (c) Bernheim, R. A.; Kempf, R. J.; Reichenbecher, E. F.; J. Magn. Reson. 1970, 3, 5. (d) Wasserman, E.; Yager, W. A.; Kuck, V. J. Chem. Phys. Lett. 1970, 7, 409. (e) Bernheim, R. A.; Bernard, H. W.; Wang, P. S.; Wood, L. S.; Skell, P. S. J. Chem. Phys. 1970, 53, 1280. (f) Dendramis, A.; Leroi, G. E. J. Chem. Phys. 1977, 66, 4334. (g) Brown, F. X.; Saito, S.; Yamamoto, S. J. Mol. Spectrosc. 1990, 143, 4256. (24) (a) Hoffmann, R.; Zeiss, G. D.; Van Dine, G. W. J. Am. Chem. Soc. 1968, 90, 1485. (b) Harrison, J. F.; Dendramis, A.; Leroi, G. E. J. Am. Chem. Soc. 1978, 100, 4352. (c) Zandler, M. E.; Goddard, J. D.; Schaefer, H. F., III. J. Am. Chem. Soc. 1979, 101, 1072. (d) Kim, K. S.; Schaefer, H. F., III; Radom, L.; Pople, J. A.; Binkley, J. S. J. Am. Chem. Soc. 1983, 105, 4148. (e) Rice, J. E.; Schaefer, H. F., III. J. Chem. Phys. 1987, 86, 7051. (f) Malmquist, P.-A.; Lindh, R.; Roos, B. O.; Ross, S. Theoret. Chim. Acta 1988, 73, 155. (g) Seidl, E. T.; Schaefer, H. F. III. J. Chem. Phys. 1992, 96, 4449. (h) Aoki, K.; Fueno, H.; Ikuta, S.; Nomura, O. Chem. Phys. Lett. 1993, 202, 33. (i) Aoki, K.; Ikuta, S.; Nomura, O. J. Chem. Phys. 1993, 99, 3809. (25) (a) Rittby, M.; Bartlett, R. J. J. Phys. Chem. 1988, 92, 3033. (b) Lauderdale, W. J.; Stanton, J. F.; Gauss, J.; Watts, J. D.; Bartlett, R. G. Chem. Phys. Lett. 1991, 187, 21. (c) Watts, J. D.; Gauss, J.; Bartlett, R. J. J. Chem. Phys. 1993, 98, 8718. (d) Bartlett, R. J.; Stanton, J. F. In ReViews in Computational Chemistry; Lipkowitz, K. B., Boyd, D. B., Eds. VCH Publishers: New York, 1994, Vol. 5, Chap. 2. (26) Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.; Bartlett, R. J. ACES II; Quantum Theory Project: The University of Florida, 1994. (27) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92/DFT, ReV. F.2; Gaussian, Inc.: Pittsburgh, PA, 1993. (b) Foresman, J. B.; Frisch, A. E. Guide to Using Gaussian. Exploring Chemistry with Electronic Structure Methods; Gaussian, Inc.: Pittsburgh, 1993. (28) (a) Farnell, L.; Pople, J. A.; Radom, L. J. Phys. Chem. 1983, 87, 79. (b) Jensen, F. Chem. Phys. Lett. 1990, 169, 519. (c) Knowles, P. J.; Hampel, C.; Werner, H.-J. J. Chem. Phys. 1993, 99, 5219. (d) Tozer, J.
5714 J. Phys. Chem., Vol. 100, No. 14, 1996 D.; Handy, N. C.; Amos, R. D.; Pople, J. A.; Nobes, R. H.; Xie, Y.; Schaefer, H. F., III. Mol. Phys. 1993, 79, 777. (29) (a) Moeller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618. (b) Almlo¨f, J.; Faegri, K.; Faegri, K., Jr. J. Comput. Chem. 1982, 3, 385. (c) HeadGordon, M.; Pople, J. A. J. Chem. Phys. 1988, 89, 5777. (d) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166, 275, 281. (30) Banerjee, A.; Adams, N.; Simons, J.; Shepard, R. J. Phys. Chem. 1985, 89, 52. (31) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214. (32) The number of imaginary frequencies (NIMAG ) 0, minimum; 1, transition state, etc.) are given on the schematic drawings in parentheses. (33) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968. (34) Zero point energies are scaled by 0.95, as recommended in: Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab initio Molecular Orbital Theory; Wiley-Interscience: New York, 1986. (35) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736. (b) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899. (c) Reed, A. E.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 1434. (36) (a) Cı´zek, J. J. Chem. Phys. 1966, 45, 4256; AdV. Chem. Phys. 1969, 4, 35. (b) Paldus, J.; Cı´zek, J. AdV. Quantum Chem. 1975, 9, 105. (c) Bartlett, R. J. Annu. ReV. Phys. Chem. 1981, 32, 359; J. Phys. Chem. 1989, 93, 1697. (d) Bartlett, R. J.; Stanton, J. F. In ReViews in Computational Chemistry; Lipkowitz, K. B., Boyd, D. B., Eds.; VCH Publishers: New York, 1994; Vol. 5, p. 65. (37) Balkova, A.; Bartlett, R. J. Chem. Phys. Lett. 1992, 193, 364 and the references cited therein. (38) Only relative energies (in kcal/mol) are given in the paper. The total energies (in au) and the vibrational frequencies are available from A.A.K. (
[email protected]) as the supporting information, if requested. (39) Purvis, G. D.; Shepard, R.; Brown, F. B.; Bartlett, R. J. Int. J. Quantum Chem. 1983, 23, 835. (40) We approached a crossing point by scanning the “N4-like” and “2N2-like” PESs until a 1 kcal/mol energy difference between the two configurations was reached. (41) The 0.5 kcal/mol barrier height for dissociation of 1f was computed at HF. We have not found the corresponding TS structure at MP2. (42) Isodesmic strain energies (kcal/mol) in tetrahedrane and its aza derivatives computed at the HF/6-31G* level from ref 11d (data in parentheses at the HF/6-31G level):
C4H4 + 8CH4 f 6H3CCH3
∆E ) -134 (-147)
C3H3N + 6CH4 + 2NH3 f 3H3CCH3 + 3H3CNH2 ∆E ) -107 (-134) C2H2N2 + 4CH4 + 4NH3 f H3CCH3 + 4H3CNH2 + H2NNH2 ∆E ) -77 (-118) CHN3 + 2CH4 + 6NH3 f 3H3CNH2 + 3H2NNH2 ∆E ) -43 (-103) N4 + 8NH3 f 6H2NNH2
∆E ) -5 (-91)
(43) Glukhovtsev, M. N.; Laiter, S.; Pross, A. J. Phys. Chem. 1995, 99, 6828. (44) Total and ZPE energies and the structures of the reference species are taken from the Gaussian Archive of the Computational Chemistry Center of the Institur of Organic Chemistry University Erlangen-Nu¨rnberg. (45) (a) Lehn, J. M.; Wipff, G. J. Chem. Soc., Chem. Commun. 1975, 800. (b) Epiotis, N. D.; Cherry, W. R.; Yates, R. L., et al. Top. Curr. Chem. 1977, 70. (c) Radom, L. In Progress in Theoretical Organic Chemistry; Chismadia, I. G., Ed.; Elsevier: Amsterdam. (d) Deslongchamps, P. Stereoelectronic Effects in Organic Chemistry; Pergamon Press: Oxford, 1984. (e) Gorenstein, D. G. Chem. ReV. 1987, 87, 23. (f) Korkin, A. A. Russ. Chem. ReV. 1992, 61, 481. (46) (a) Murray, J. S.; Redfern, P. C.; Seminario, J. M.; Politzer, P. J. Phys. Chem. 1990, 94, 2320. (b) Inagaki, S.; Goto, N. J. Am. Chem. Soc. 1987, 109, 3234.
Korkin et al. (47) Gordon, M.; Ware, W. R. The Exciplex; Academic Press: New York, 1975. (48) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (49) (a) Budzelaar, P. H. M.; Kraka, E.; Cremer, D.; Schleyer, P. v. R. J. Am. Chem. Soc. 1986, 108, 561. (b) Staley, S. W.; Norden, T. D.; Taylor, W. H.; Harmony, M. D. J. Am. Chem. Soc. 1987, 109, 7641. Bachrach, S. M. J. Org. Chem. 1990, 55, 4961. (50) Average bond energies have been estimated from the heats of formation of boronitride (a) and methylamine (b): (a) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables. Phys. Chem. Ref. Data Suppl. 1 1985, 14. (b) Sana, M.; Leroy, G.; Peeters, D.; Younang, E. J. Mol. Struct. Theochem. 1987, 151, 325. (51) (a) Knight, L. B., Jr.; Steadman, J.; Miller, P. K.; Bowman, D. E.; Davidson, E. R.; Feller, D. J. Chem. Phys. 1984, 80, 4593. (b) Blair, J. T.; Weisshaar, J. C.; Carpenter, J. E.; Weinhold, F. J. Chem. Phys. 1987, 87, 392. (c) Blair, J. T.; Weisshaar, J. C.; Weinhold, F. J. Chem. Phys. 1987, 88, 1467. (d) Hiraoka, K.; Mori, T.; Yamabe, S. J. Chem. Phys. 1991, 94, 2697. (52) MacKay, C. In Carbenes; Olah, G., Ed., Wiley: New York, 1975; Vol. 2, Chapter 1. (53) The bent form 5d decomposes into two BF molecules upon optimization at HF. (54) Jensen, P.; Bunker, P. R. J. Chem. Phys. 1988, 89, 1327. (55) Murray, K. K.; Leopold, D. G.; Miller, T. M.; Lineberger, W. J. Chem. Phys. 1988, 89, 5442. (56) The influence of π donor (acceptor) and σ acceptor (donor) groups on the ∆EST in substituted methylenes has been investigated theoretically in numerous ab initio studies; see, for example: (a) Bauschlicher, C. W., Jr.; Schaefer, H. F., III; Bagus, P. S. J. Am. Chem. Soc. 1977, 99, 7106. (b) Mueller, P. H.; Rondan, N. G.; Houk, K. N.; Harrison, J. F.; Hooper, D.; Willen, B. H.; Liebman, J. F. J. Am. Chem. Soc. 1981, 103, 5049. (c) Carter, E. A.; Goddard, W. A., III. J. Phys. Chem. 1987, 91, 4651. (d) Irikura, K. K., Goddard, W. A., III; Beauchamp, J. L. J. Am. Chem. Soc. 1992, 114, 48. (57) Koda, S. Chem. Phys. Lett. 1978, 55, 353. (b) Koda, S. Chem. Phys. 1982, 66, 383. (58) The lowest triplet state of the N2 molecule is A 3∑u-, and the experimental excitation energy, X 1∑g+ N2 f X 3∑u+ N2, is 144 kcal/mol; see ref 48. (59) (a) Horsewood, P.; Kirby, G. W. J. Chem. Soc., Chem. Commun. 1971, 1139. (b) Dorko, E. A.; Flynn, P. H.; Grimm, U.; Scheller, K.; Mueller, G. W. J. Phys. Chem. 1977, 81, 811. (c) Dickinson, R.; Kirby, G. W.; Sweeney, J. G.; Tyler, J. K. J. Chem. Soc., Faraday Trans. 2 1978, 74, 1393. (60) Fatiadi, A. J. In The Chemistry of Functional Groups, Supplement C; Patai, S., Rappoport, Z., Eds.; Wiley Interscience: New York, 1983; Part 2, Chapter 26. (61) (a) Nemukhin, A. V.; Stepanov, N. F. Theor. Chim. Acta 1985, 67, 287. (b) Rosmus, P. Angew. Chem., Int. Ed. Engl. 1988, 27, 1329. (62) Lory, E. A.; Porter, R. F. J. Am. Chem. Soc. 1971, 93, 6301. (b) Kawashima, Y.; Endo, Y.; Kawaguchi, K.; Hirota, E. Chem. Phys. Lett. 1987, 135, 441. (63) (a) Porter, R. F.; Bidinosti, D. R.; Watterson, K. F. J. Chem. Phys. 1962, 36, 2104. (b) Hildenbrand, D. L.; Theard, L. P.; Saul, A. M. J. Chem. Phys. 1963, 39, 1973. (c) Bidinosti, D. R.; Coatsworth, L. L. Can. J. Chem. 1970, 48, 2484. (d) Snelson, A. High Temp. Sci. 1972, 4, 171. (64) Snelson, A. High Temp. Sci. 1972, 4, 318. (65) Wiberg, E. Holleman-Wiberg Lehrbuch der Anorganischen Chemie; Walter de Gruyter: Berlin, 1976. (66) Seyferth, D. In Carbenes; Olah, G., Ed.; Wiley: New York, 1975; Vol. 2, Chapter 3. (67) (a) Hildenbrand, D. L. Chem. Phys. Lett. 1975, 32, 523. (b) Kawaguchi, K.; Yamada, C.; Hamada, Y.; Hirota, E. J. Mol. Spectrosc. 1981, 136, 136. (c) Griemann, F. J.; Droege, A. T.; Engelking, P. C. J. Chem. Phys. 1983, 78, 2248. (d) Saykally, R. J.; Lubic, K. G.; Scalabrin, A.; Evenson, K. M. J. Chem. Phys. 1982, 77, 58. (e) Saito, S.; Endo, Y.; Takami, M.; Hirota, E. J. Chem. Phys. 1983, 78, 116. (f) Van der Heuvel, F. C.; Leo Meerts, W.; Dymanus, A. Chem. Phys. Lett. 1982, 88, 59. (68) See, for example, refs 15, 19, and 20 for efforts to observe OCCO.
JP960125J
