J. Phys. Chem. 1994, 98, 13203- 13209
13203
FORS MCSCF Study on Nitrenophenylmethylene (CsH4CHN) Shiro Koseki* Department of Chemistry, Faculty of Education, Mie University, Tsu 514, Japan
Hideo Tomioka Chemistry Department for Materials, Faculty of Engineering, Mie University, Tsu 514, Japan
Azumao Toyota Department of Chemistry, Faculty of General Education, Yamagata University, Yamagata 990, Japan Received: July 5, 1994; In Final Form: September 28, 1994@
Electronic structure calculations for the ortho, meta, and para isomers of nitrenophenylmethylene (c6H4CHN) are carried out by using the full-optimized-reaction-space (FORS) multiconfiguration self-consistentfield (MCSCF) method with the STO-3G basis set. The ortho isomer is predicted to possess a singlet ground state with a bond-alternated structure. However, since the energy barrier for the isomerization reaction of the ortho structure to a bicyclic one (benzoazacyclobutene or benz[b]azete) is less than 10 kcaymol, the ortho isomer is considered to have a relatively short lifetime and hence to exist in the stable bicyclic form. The para isomer also has a singlet ground state with a bond-alternated structure, but it may easily react with molecular oxygen, as suggested experimentally, because the lowest triplet state is very close in energy to the singlet ground state. In marked contrast, the meta isomer has a quintet ground state, and this aspect is well reflected in the bonding, which shows no appreciable n conjugation between a benzene nucleus and two radical substituents.
Introduction A typical carbene is well-known to have a longer lifetime at lower temperature, in spite of being highly reactive at room temperature. Among other things, of special interest is the fact that the parent methylene has a triplet ground state, in which there are two unpaired electrons, and this has been exploited for the study of various reaction mechanisms. The low-lying electronic states of this molecule have been investigated both from theoretical and experimental viewpoints.1-4 Much attention has been paid to the energy difference between the triplet ground state and the lowest singlet state, and many studies have been performed in order to reach quantitative agreement between experimental and theoretical results. These studies revealed that the triplet ground state of carbene is more stable when the geometrical structure is forced to be close to a linear form by introduction of bulky or electronegative substituents. The electronic structures of silylenes, silicon analogues of carbenes, have also been studied theoretically and experisince silylenes play an important role in the chemical vapor deposition process of silicon.8-11 It has been proved theoretically and experimentally that the parent silylene has a singlet ground state, in contrast to methylene. It is believed, however, that introduction of appropriate substituents to silylene, such as Li a t ~ m , ~ makes ~ . ' ~ the ground-state multiplicity triplet. It is thus apparent that carbenes and silylenes would be very useful as starting materials when one attempts to prepare novel organic compounds possessing special magnetic properties similar to those of heavy metal materials. Under these circumstances, the monocyclic geometrical isomers of nitrenophenylmethylene (C&CHN), in which two radical substituents are connected with the n system of a benzene nucleus, are of both theoretical and experimental interest, since
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, November 15, 1994.
0022-365419412098- 13203$04.5010
they are expected to have high spin-multiplicities and, hence, special magnetic properties in the ground state. To our knowledge, however, no theofetical investigation has been reported so far which examines the intramolecular interaction between two divalent centers through a n system in c6H4cHN. This paper reports the electronic structures and stable geometries of 0-,m-, and p - c 6 h c H N (Figure 1) and also gives theoretical interpretations to explain available experimental facts reported by Tomioka et al.14 and Tukada et a l l 5
Methods of Calculation Electron correlation strongly affects the relative stabilities of radical systems, so that results obtained on the basis of the Hartree-Fock approximation are generally unreliable. In this study, the multiconfiguration self-consistent-field (MCSCF) method16 is employed with a medium-size active space. Since our main interest is in developing a qualitative understanding of this large system, we have chosen to conserve computational expense by use of the minimal basis set (STO-3G) proposed by Pople et al.17 All calculations were carried out by using the quantum chemistry code GAMESS.l8 In low-lying electronic states of nitrenophenylmethylene (C6&cHN), the following orbitals are expected to form radical centers and to contribute significantly to the effect of electron correlation: 2pa (lone-pair) and 2pn orbitals on the C atom of the CH substituent and two 2pa (lone-pair) orbitals and a 2pn orbital on the N atom. Therefore, the MCSCF active space in our calculations includes all of these five orbitals (six electrons in five active orbitals), and full-optimized-reaction-space (FORS) MCSCF calculations have been carried out.16 In the following discussion, these active orbitals are denoted simply by Ca, Cn, Na(2), and Nn, respectively. These calculations generate 28 a' configurations for singlet states, 21 a' configurations for triplet states, and 3 a' configurations for quintet states. The geometrical 0 1994 American Chemical Society
13204 J. Phys. Chem., Vol. 98, No. 50, 1994
Koseki et al.
Ortho ueta Para Bicyclic Figure 1. Ortho, meta, and para structures of nitrenophenylmethylene (C&CHN) and that of benzoazacyclobutene (benz[b]azete) and numbering of atomic positions.
SCHEME 1
SCHEME 2
i
i 0 1 , 0 3 (tnns)NA
01,03 (trans)AL
0 5 (trans)
M1, M3, MS (trans)
M1, M3, M5 (cis) BCO
BC1
BC2
SCHEME 3
:!. 01,03 (cls)NA 0 5 (cis)
01,03 (cis)AL
structures of the lowest singlet, triplet and quintet states of each isomer were fully optimized by means of this computational method (referred to as MCSCF(6,5)). At every optimized geometry, vibrational analyses were also performed numerically and the infrared (IR) absorption intensities of vibrational modes predicted theoretically l9 for comparison with the experimental re~u1ts.l~ For more reliable predictions of the relative stabilities of the isomers, MCSCF energy calculations using a larger active space are performed at the MCSCF(6,5) optimized geometries. The larger active space includes the six ~t orbitals of the benzene nucleus, in addition to the five active orbitals used in the MCSCF(6,5) calculations; accordingly, there are 12 active electrons in 11 active orbitals (referred to as MCSCF(12,ll)). The numbers of a' configurations generated in these calculations amount to 30 744, 48 906, and 22 615 for singlet, triplet, and quintet states, respectively.
Results and Discussion Most Stable Structure. Nitrenophenylmethylene (C6H.4CHN) was synthesized by the photolysis of (azidophenyl) diazomethane (C6b(CHN2)N3) in an argon matrix at very low temperat~re.'~The experimental fact that the product shows
PI, P3 NA P5
a?.
P1, P3 AL
low reactivity with triplet ground-state 0 2 molecule suggests that the ortho isomer of C&CHN has a singlet ground state and easily reacts to form a bicyclic structure (benzoazacyclobutene or benzlblazete; BCO, BC1, or BC2 in Scheme 1. On the.other hand, because the meta and para isomers (Scheme 2 and 3) can not form a bicyclic structure, they are predicted to be less stable than the bicyclic s t r u ~ t u r e . ' ~ J ~ ~ ~ ~ , ~ ~ The MCSCF(6,5)-optimizedgeometrical parameters of 0-,m-, and p-C6H&HN and those of benzoazacyclobutene are summarized in Tables 1 and 1s (supplementary material), and their total and relative energies are listed in Table 2. Although three Kekul6 structures can be drawn for the bicyclic form as shown in Scheme 1, BCO and BC2 are found as energy minima on the lowest singlet potential energy surface of C6H4CHN, but BCI collapses into BCO during MCSCF(6,5) geometry optimization. These results can be explained in the following manner: Since the cross bond (C2-C3 bond of the bicyclic structure in Figure 1) is assumed to be short in BC1, large steric defonnation makes it unstable and geometry optimization relaxes it into BCO. Since BC2 has two double bonds in the periphery of its four-membered ring, this structure is higher in energy than BCO by about 30 kcdmol and has a very long terminal C-N bond (C1-N = 1.687 A; see Figure 1 and Table 1). Since BCO and BC2 have a long cross bond, benzoazacyclobutene
FORS MCSCF Study on Nitrenophenylmethylene
J. Phys. Chem., Vol. 98, No. 50, 1994 13205
TABLE 1: Bond Lengths and Bond Angles of the Bicyclic, Ortho, Meta, and Para Structures Optimized by Using the MCSCF(6SYSTO-3G Methodo bond BCO BC2 Ol(trans)AL Ol(cis)AL 03(trans)AL MS(cis) PlAL p3a1 1.318 1.314 1.312 1.314 1.477 1.314 1.314 1.518 1.514 1SO7 1.396 1.493 1.494 1.495 1.506 1.417 1.492 1.390 1.320 1.319 1.474 1.491 1.492 1.343 1.491 1.492 1.322 1.322 1.322 1.393 1.328 1.437 1.480 1.491 1.492 1.500 1.478 1.478 1.386 1.361 1.329 1.323 1.322 1.323 1.385 1.320 1.320 1.436 1.492 1.400 1.492 1.492 1.471 1.492 1.491 1.345 1.687 2.919 2.961 2.895 1.379 1.336 1.362 1.338 1.335 1.536 1.478 1.336 1.335 1.085 1.085 1.085 1.086 1.085 1.083 1.085 1.085 1.084 1.084 1.083 1.083 1.083 1.083 1.083 1.083 1.082 1.083 1.083 1.083 1.083 1.083 1.083 1.084 1.083 1.083 1.083 1.083 1.083 1.082 1.083 1.084 1.084 1.084 1.083 1.083 1.083 1.083 1.083 1.083 120.9 122.3 122.3 86.0 90.9 122.3 123.4 121.7 120.5 122.3 122.3 117.7 118.1 123.9 121.2 118.1 121.2 121.2 120.3 114.3 115.1 121.1 121.4 121.0 117.1 119.5 117.1 121.3 121.5 122.2 124.0 121.3 121.2 121.2 120.3 122.1 124.1 121.5 121.4 121.5 120.8 122.3 122.3 122.5 122.1 114.9 115.9 122.2 115.9 116.0 118.6 119.7 115.8 115.6 115.9 122.6 94.8 90.2 22.1 122.8 121.5 92.6 97.3 119.8 121.4 121.4 86.6 81.6 131.8 31.5 133.0 131.7 125.1 131.7 136.5 139.6 116.9 120.0 116.9 121.6 121.6 124.8 122.0 16.8 116.6 116.9 20.9 120.8 120.9 119.8 118.3 120.3 121.7 121.7 20.8 120.9 120.9 119.8 120.2 118.4 116.9 116.9 119.4 16.6 116.4 116.7 124.6 121.8 Numbering of atoms is shown in Figure 1. Bond lengths and bond angles are in A and deg. The fist and second characters of a structure name show a geometrical isomer and its state multiplicity, respectively; namely, o = ortho, M = meta, P = para, 1 = singlet, 3 = triplet, and 5 = quintet. AL = bond altemated and NA = nonaltemated. cis and trans = C-H bond orientation of the CH substituent (see Schemes 1-3 and Table 1S (supplementary material)). possesses properties similar to those of 8n-electron (anti-Huckel) systems and, as a result, the molecule should distort into the bond-altemated structures corresponding to the Kekult-type resonance ones with double-bond fixation in the periphery.22 Several research groups have studied benzocyclobutene (BCB)23-27theoretically. Some of these papers discuss the Mills-Nixon effect, but no paper has appeared so far which examines the electronic structures of the nitrogen analogue, benzoazacyclobutene. Schulman et al.” have found two Kekul6 structures for singlet BCB and one structure for triplet BCB at the MP2/6-31G level of calculation. The singlet BCB structures are similar to BCO and BC2, and the energy difference between those is large (49.6 kcdmol) in comparison with that between BCO and BC2 (30 kcavmol). They finally suggested that a BC2-like structure of BCB is not a local minimum on the basis of energy calculations along a linear synchronous transit between two Kekuli structure^.^^ We thus obtained the MCSCF(6,5) energy curve along a linear synchronous transit between BCO and BC2 and found a small energy barrier (about 6 kcdmol). Further, it is found that the more accurate MCSCF(12,ll) energy decreases monotonically along this transit from BC2 to BCO (see the last section of this paper). Therefore, we also conclude that only BCO is a true energy minimum as a bicyclic form and it is the global minimum of C6aCHN. That is, our results on the nitrogen analogue of BCB are similar to those on BCB itself obtained by Schulman et al. In Figure 2, the IR spectrum14cof the products obtained in the photolysis of O-C6&(CH&)N3 is compared with the calculated one of BCO. The experimental analysis reveals that
two strong absorptions in the low-energy region (labeled by H in Figure 2) are characteristicallyexhibited by the product, while the other relatively large peaks appear in the spectrum of the precursors and the products. The calculated results indicate that a strong absorption near 900 cm-l and an intermediate one near 1300 cm-’ are assigned to an out-of-plane mode of a benzene nucleus and a CN stretching mode of the four-membered ring, respectively. On the whole, the calculated spectrum of BCO seems to resemble in shape the observed one, except that the peak positions differ somewhat. Radical States. Since the meta and para isomers do not form bicyclic structures, these two substituents become electronically unsaturated (Schemes 2 and 3) and may form radical centers. Fgiure 3 illustrates the n orbital interaction between a benzene nucleus and two radical substituents, where the ortho isomer is assumed not to form a bicyclic structure. In this figure, for simplicity, N atom is replaced by a CH substituent, and nonbonding C a orbitals of the substituents are also inserted in the left-hand side of each figure. Figure 3a and c indicates that the ortho and para isomers are expected to be stabilized by n orbital interactions between a benzene nucleus and substituents. In the meta isomer, however, two nonbonding n orbitals appear within this Huckel picture (Figure 3b). Therefore, the n system of the meta isomer, as well as its C a orbitals, may become unsaturated (cf for even altemant hydrocarbons, the number of nonbonding orbitals is the difference between the numbers of starred and unstarred carbons). Such considerations suggest that radical electrons in the ortho and para isomers have u character, while the meta
Koseki et al.
13206 J. Phys. Chem., Vol. 98, No. SO, 1994
TABLE 2: Total and Relative Energies of the Geometrical Isomer9 structureb MCSCF(6,5) MCSCF(12,ll) +ZPEc Bicyclic Isomers BCO -3 18.435 857 (0.0) -3 18.545 320 (0.0) 0.0 BC2 -318.357 244 (49.3) -318.493 718 (32.4) 31.6 Ortho Isomers 0 1(cis)AL -318.343 044 (58.2) -318.473 327 (45.2) 42.8 Ol(cis)NA -3 18.340 130 (60.1) -3 18.462 529 (52.0) 48.6 0 1(trans)AL -318.341 975 (58.9) -318.471 023 (46.6) 44.2 0 1 (trans)NA -318.340 710 (59.7) -318.462 824 (51.8) 48.3 0 3 (cis)AL -318.341 017 (59.5) -318.467 887 (48.6) 46.3 03(cis)NA -318.337 729 (61.6) -318.456 497 (55.7) 52.3 0 3 (trans)AL -318.341 017 (59.5) -318.467 764 (48.7) 46.4 0 3 (trans)NA -318.338 160 (61.3) -318.456 103 (56.0) 52.5 OS(cis) -318.334 233 (63.8) -318.443 127 (64.1) 60.7 0 5 (trans) -318.334 506 (63.6) -318.443 007 (64.2) 60.8 Meta Isomers M l(cis) -318.335 099 (63.2) -318.442 641 (64.4) 61.0 M1(trans) -318.334 962 (63.3) -318.442 115 (64.8) 61.4 M3(cis) -318.335 494 (63.0) -318.445 674 (62.5) 59.2 M3(trans) -318.335 413 (63.0) -318.445 085 (62.9) 59.6 MS(cis) -318.336 379 (62.4) -318.450 390 (59.6) 56.2 MS(trans) -318.336 418 (62.4) -318.450 201 (59.7) 56.5 Para Isomers PlAL -318.342 739 (58.4) -318.471 826 (46.1) 43.9 PlNA -318.340 022 (60.1) -318.461 379 (52.7) 49.4 P3AL -3 18.342 258 (58.7) -3 18.469 338 (47.7) 45.5 P3NA -318.338 061 (61.4) -318.456 044 (56.0) 52.7 P5 -318.334 808 (63.4) -318.443 01 1 (64.2) 60.8 ~~
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Total energies are in hartrees. The values in parentheses are the relative energies in kcal/mol. See Table 1 and Schemes 1-3. e The relative MCSCF( 12,ll) energies are corrected by MCSCF(6J) zeropoint vibrational energies.
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Figure 2. (a) Observed infrared absorption spectrum of the ortho isomer and (b) calculated one of BCO. Absorption intensity is in D2/
(amuA2). isomer may have z radical electrons as well as (Iones. Let us give the properties of each isomer in more detail below. Ortho Isomer. As listed in Tables 1 and 1 s (supplementary material), the MCSCF(6,5) optimization finds both bondalternated (AL)and nonaltemated (NA)structures. On both
..................
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Figure 3. Representation of z orbital interaction between a benzene nucleus and two CH substituents: (a, top) the ortho isomer, (b, middle) the meta isomer, and (c, bottom) the para isomer of C6hCHN. S and A indicate symmetric and asymmetric orbitals with respect to a mirror plane (represented by a broken line). singlet and triplet potential energy surfaces, the AL structures are more stable than the corresponding NA structures by about 7 kcdmol. This result does not depend upon the C-H orientation of the substituent. The bond lengths and bond angles are nearly identical in all AL structures (Tables 1 and 1s). The MCSCF( 12,ll) potential energy curves along a linear synchronous transit from an AL structure to the corresponding NA one
FORS MCSCF Study on Nitrenophenylmethylene
'"(I
1
J. Phys. Chem., Vol. 98, No. 50, 1994 13207
0.0a
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Figure 4. Calculated infrared absorption spectra of (a) Ol(cis)ALand (b) Ol(trans)AL. Absorption intensity is in Dz/(amu-Az).
Figure 5. Calculated infrared absorption spectra of (a) M5(cis) and (b) MS(trans). Absorption intensity is in DZ/(amdz).
show that there is only one energy minimum near the AL structure. This apparently occurs because the MCSCF( 12,ll) method provides a more accurate description of the n orbital interaction between a benzene nucleus and two substituents. Therefore, only AL structures should be considered to be true energy minima on both singlet and triplet potential surfaces. The energy differences between Ol(cis)AL and O3(cis)AL and between Ol(trans)AL and 03(trans)AL are very small (2-4 kcaymol), so that these energy orderings could change at higher levels of theory. These small energy differences suggest rapid nonradiative transitions between the singlet and triplet states. The triplet generated by such a transition is expected to react with a doped triplet 0 2 molecule during the photolysis of O-C6b(CHN2)N3, as has been suggested experimentally.'" The quite small energy differences for the cis-trans pairs may be attributed to the cancellation of the nuclear repulsion by hydrogen-bond attraction between the substituents in the cis structures. It is noted that the lowest quintet states (05(cis) and OS(trans)), which have nonaltemated structures, are distinctly higher in energy than the singlet and triplet states and correspond to (n n*)excited states. The natural orbital occupation numbers (NOONs) corresponding to C o and N o orbitals in the substituents are almost equal to 1 in both singlet and triplet AL structures (another N a orbital is doubly occupied to form a lone-pair), so that o radical electrons are considered to be located in both functional groups. On the other hand, the NOONs are 1.72-1.74 and 0.26-0.28 for the orbitals constructed mainly from the linear combination of C n and Nn orbitals, so that the n system in the AL structures seems to have weak radical character. Figure 4 illustrates the IR absorption spectra predicted theoretically for Ol(cis)AL and Ol(trans)AL. Since the optimized geometries of the singlet AL structures are nearly identical to the corresponding triplet ones (Table lS), their spectra are quite similar. In contrast to BCO (Figure 2), 0 1 (cis)AL and Ol(trans)AL have rather strong absorption by the CH stretching modes. Although two or three absorption peaks
similarly appear in the low-energy region in Figure 4, the spectral pattem obtained for BCO seems to resemble the experimental result more closely than those for Ol(cis)AL and Ol(trans)AL. Meta Isomer. In addition to the nonbonding Cu and N o orbitals of the substituents, two nonbonding n orbitals appear in this isomer within the Huckel level of theory, as shown in Figure 3. Since these n orbitals may be occupied by two electrons in low-lying states, the n system as well as the o system may have radical character. The MCSCF(12,ll) results in Table 2 show that the meta isomer has a quintet ground state, though the energy differences among the lowest singlet, triplet, and quintet states are small (3-5 kcaumol). This result agrees with the experimental evidence reported by Tukada et al.,15and it is interesting that the energetic ordering of these states obeys Hund's rule. In these low-lying states, the NOONs of the nonbonding n, vu, and N o orbitals are almost equal to 1 (1.000-l.OOl), and the optimized geometries of these states have relatively long Cl-Cz and C4-N bonds (about 1.48 A; see Table 1). Therefore, it can be concluded that weak x conjugation occurs between a benzene nucleus and two substituents, as suggested from Figure 3, and that (nand a ) radical electrons are highly localized at each substituent. Unfortunately, there has been no experimental report on the IR absorption spectrum of the meta isomer, but the calculated spectra of MS(cis) and MS(trans) are shown in Figure 5. The strongest absorption near 1000 cm-' is exhibited by the outof-plane modes of the C-H bending in a benzene nucleus. Para Isomer. The MCSCF(6,5) geometry optimization finds both bond-alternated (AL; PlAL and P3AL quinoid structures) and nonaltemated (NA; PlNA, P3NA, and P5) structures (Table 2). The AL structures are lower in energy than the corresponding NA structures by 6-8 kcal/mol. This energy difference is nearly equal to those in the ortho isomer. As in the ortho isomer, the MCSCF( 12,ll) energy curves along the linear
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13208 J. Phys. Chem., Vol. 98, No. 50, 1994
Koseki et al.
(a) experiment
oxygen molecule, the ground state would appear to be a triplet. However, a preliminary ESR study14c,20*21 suggests that the para isomer has a singlet ground state. Our results, then, are consistent with the experimental evidence: the high reactivity of this isomer with 0 2 is explained by the fact that the lowest triplet state P3AL is very close in energy to PlAL. In this connection, the lowest quintet state P5 is evidently less stable than the singlet and triplet states and corresponds to a (n n*)excited state (see Table 2). Figure 6 shows the R absorption spectrum of the para isomer observed by Tomioka et ~ 1 and. the calculated ~ ~ one of PlAL. The geometrical structures of PlAL and P3AL are quite similar, so that the calculated spectrum of P3AL is nearly identical to that of PlAL. Experimentally, the para isomer is considered to have three special absorptions in the low-energy region (labeled by W in Figure 6). The strongest absorption would correspond to one of the two larger peaks near 1000 cm-' in the calculated spectrum, which are due to the in-plane and outof-plane C-H bending modes of a benzene nucleus. On the other hand, the relatively strong absorptions near 2000 cm-' are predicted to be due to the C-N and C-C stretching modes between a benzene nucleus and two substituents. These vibrational modes exist in the para isomer, as well as in its precursors. Energy Barrier in the Isomerization of the Ortho Isomer into a Bicyclic Structure. As suggested experimentally, the ortho isomer easily isomerizes into a bicyclic structure. In this section, we estimate the energy barrier in the isomerization reaction of Ol(trans)AL to BCO at the MCSCF(12,ll) level of calculation. Two practical paths are proposed for this isomerization: (1) The n conjugation between a benzene nucleus and the substituents is broken and a Ol(trans)NA-like structure is formed. Then, the formation of a four-membered ring takes place to give BCO. (2) A C-N is formed between two substituents to produce a BC2-like structure. Then, the double-bond shift occurs to give BCO. Figure 7 illustrates MCSCF(6,5) and MCSCF( 12,ll) energy curves along linear synchronous transits
-
4000
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Figure 6. (a) Observedinfrared absorption spectrum of the para isomer and (b)calculated one of PlAL. Absorption intensity is in Dz/(amu-A2).
synchronous transit between the AL and corresponding NA structures show that only one energy minimum exists near the AL structure. Consequently, only P l A L and P3AL are true energy minima on the singlet and triplet potential energy surfaces, respectively. Since the energy difference between P l A L and P3AL is only 1.6 kcaUmol and more accurate calculations may provide the reverse energetic ordering, we do not conclude that P l A L is the most stable para isomer. Experimentally, since the para isomer easily reacts with a doped
(b)
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Figure 7. Potential energy curves of the lowest singlet state along a linear synchronous transit: (a) from Ol(trans)AL to BC2; (b) from BC2 to BCO; (c) from Ol(trans)AL to Ol(trans)NA; (d) from Ol(trans)NA to BCO. Solid lines represent MCSCF(6,5) energy curves, and broken ones, MCSCF(12,ll) energy curves.
J. Phys. Chem., Vol. 98, No. 50, 1994 13209
FORS MCSCF Study on Nitrenophenylmethylene
the MCSCF(6,5)/STO-3G method: Ol(trans)NA, Ol(cis)NA, 03(trans)NA, 03(cis)AL, O3(cis)NA, 05(trans), 05(cis), M1(trans), Ml(cis), M3(trans), M3(cis), MS(trans), PlNA, P3NA, and P5 (5 pages). Ordering information is available on any current masthead page.
References and Notes
6
.’22-o
Figure 8. MCSCF(6,5) transition state structure of the isomerization reaction from Ol(trans)AL to BCO. Bond lengths and bond angles are in 8, and deg, respectively.
of these paths. An energy barrier of about 10 kcallmol is found between Ol(trans)AL and BC2 in path 1 (Figure 7a and b) and between Ol(trans)NA and BCO in path 2 (Figure 7c and d). Saddle-point searches on this potential energy surface generally make this barrier smaller. In fact, the transition state of path 1 optimized by the MCSCF(6,5) method (Figure 8) is only 5 kcal/mol higher than Ol(trans)AL. Although this barrier is re-estimated to be 8.7 kcal/mol at the MCSCF(12,ll) level of calculation, it is still too small for the ortho isomer to be isolated experimentally in an argon gas matrix, because the product easily obtains thermal energy from its surroundings.
Conclusion The structural properties of the geometrical isomers of nitrenophenylmethylene have been examined at the MCSCF/ STO-3G level of theory. Of the three monocyclic isomers, it is shown that the ortho isomer is the energetically most stable species with a singlet ground state, assuming a bond-alternated structure with two 0 radical electrons. However, since the ortho isomer undergoes an isomerization reaction to the stable bicyclic structure with an activation energy of less than 10 kcallmol, this isomer can be regarded as a reactive intermediate with a short lifetime. In the para isomer, the lowest triplet state lies slightly higher in energy than the singlet ground state. This implies that the lowest triplet state should be easily accessible thermally and, as a result, the para isomer reacts with a doped oxygen molecule, as suggested experimentally. The meta isomer has a quintet ground state, in agreement with the experimental fact reported by Tukada et al. In this sense, the meta isomer is a good candidate for magnetic material with a high spin-multiplicity, though a question may be raised as to the thermodynamic stability of this isomer itself. We are now performing more accurate calculations on some analogues of nitrenophenylmethylene.
Acknowledgment. The authors are indebted to Prof. Shinmei for his financial support in the starting of this research and to Prof. Gordon and Prof. Nomoto for their useful discussions. Supplementary Material Available: Listing of bond lengths and bond angles of the following structures optimized by using
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