54
J . Phys. Chem. 1986, 90, 54-56
Singlet-Triplet Spllttings in CF,-Substituted Carbenes David A. Dixont Central Research & Development Department, E . I . Du Pont de Nemours and Company, Inc., Wilmington, Delaware 19898 (Received: July 22, 1985)
The electronic structure of the simple CF3-substituted carbenes, :CHCF3, :CFCF3,and :C(CF3)2,have been calculated by using molecular orbital theory. The lowest singlet and triplet states were investigated. The wave functions for the triplet states were calculated with a single configuration RHF wave function while those for the singlet states were calculated with a two-configuration wave function. Geometries were gradient-optimized and the singlet-triplet splittings were calculated. The carbenes :CHCF, and :C(CF3)2have triplet ground states while :CFCF3 has a singlet ground state. It is suggested that the CF3 substituent affects the geometry and singlet-triplet splitting in a manner similar to that of hydrogen.
Introduction
Carbenes play an important role in synthetic organic chemistry.] Of special interest is the selectivity of various carbenes in their reactions with alkenes to form cyclopropanes. It is well-known that singlet and triplet carbenes have quite different types of reactivity. Thus )B1 :CH2will insert into C-H bonds while ‘A, :CHI adds to C=C double bonds to form cyc1opropanes.I An important guide to the selectivity of carbenes is the magnitude of the singlet-triplet splitting and the nature of the ground state. For example, CH2 has a ,B1 ground state with the ‘A, excited state 9-10 kcallmol higher in en erg^.^,^ Although one may be able to identify the spin of the ground state from experiment, e.g., from ESR spectroscopy, it is quite difficult to determine the size of singlet-triplet splitting from experiment. Fortunately, moderately sophisticated molecular orbital theory can provide a good estimate of the singlet-triplet splitting in ~ a r b e n e s . ~In the following, we present a molecular orbital study of the effect of CF, substitution on the structure and energetics of simple carbenes. This work builds on the previous theoretical study of :CH2, :CHF, and :CF2 by Bauschlicher et aL4 There is a significant chemistry of fluoro~arbenes~-~ and we note specifically that :C(CF3), is predicted to have novel substitution chemistry.1° Details of the Calculation
The ab initio molecular orbital calculations were performed with the program HONDO” on an IBM 3083 computer. Since these are open shell calculations, some care must be taken in the choice of the appropriate wave function for the singlet and triplet state if both are to be treated at the same level. The open shell triplet state is properly described to first order by a single configuration-restricted Hartree-Fock wave function. In order for the lowest lying singlet state to be described at the same level, a two-configuration wave function must be used. For :CH2 such a prescription4 yields a singlet-triplet splitting of 12.3 kcal/mol with a double zeta basis set augmented by a set of d functions at carbon with the d exponent optimized for each spin state; the ground state is, of course, the triplet. This is in good agreement with experiment2 considering the relative simplicity of the calculations. If a standard d orbital exponent of 0.75 is used, the singlet-triplet splitting for :CH2 rises to 13.7 kcal/mol (This is because the optimum triplet exponent is 0.741 while the optimum singlet exponent is 0.41 l ) . 4 Our calculations have been done with two basis sets, both of which are based on a double zeta basis set of the form (9,5/ 4)/[3,2/2].12 Basis set 1 is constructed by augmenting this basis with a set of d functions on the carbene carbon. Basis set 2 is constructed by adding to basis set 1 a set of polarization functions on the atoms bonded to the carbene carbon (d orbitals for C and F, p orbitals for H). Rather than optimize the d exponent on the carbene carbon, for consistency we have chosen to use a standard value of 0.75. Bauschlicher et aL4 determined optimum d ex-
’Contribution No. 3823. 0022-3654/86/2090-0054$01.50/0
ponents for :CH2, :CHF, and :CF2 and for the latter two, the exponents for both states are found to be very similar. Bauschlicher et al.4 found the largest error (1.4 kcal/mol) for the singlet-triplet splitting in :CH2. This would be expected because :CH2has the smallest basis set and exponent optimization should have the largest effect here. It is likely that our choice of a standard exponent would lead to errors on the order 1 kcal/mol for the singlet-triplet splitting in the substituted carbenes as compared to the results obtained with an optimized exponent for the polarization functions on a carbene carbon. Geometries for both the singlet and triplet states of :CHCF,, :CFCF3, and :C(CF3)2were gradient-optimized by using the appropriate form for the wave function and basis set 1. The geometries for :CH2, :CHF, and :CF2were taken from ref 4. This level of calculation was chosen in order to provide comparison with the results in ref 4. We have now established that d polarization functions on carbon are required in order to obtain good C-F bond lengths in fluorocarbon^.^^ In order to demonstrate that the singlet-triplet splittings are not in error due to the lack of polarization functions on the carbon in the CF3 groups, a set of d polarization functions was added to these carbons. In order to maintain a balanced basis set, a set of polarization functions was also added to X in :CXCF3 giving basis set 2. Since the addition of d functions to carbon is known to significantly shorten the C F bond distance in CF3 groups, these C F bond distances for the calculations with basis set 2 were scaled on the basis of calculations on F(CF,)CO and (CF3)2C0.13bA scale factor of 0.96 was used (1) Moss, R. A. In “Carbenes”; Moss, R. A., Jones, M., Jr., Eds.; Wiley: New York, 1973; Vol. I, p 153. (2) Lengel, R. K.; Zare, R. N. J. Am. Chem. SOC.1978, 100, 7495. Leopold, D. G.; Murray, K. K.; Lineberger, W. C. J . Chem. Phys. 1984, 81, 1048. Hayden, C. C.; Neumark, B. M.; Shobatake, K.; Sparks, R. K.; Lee, Y. T. J . Chem. Phys. 1982, 76, 3607. McKellar, A. R. W.; Bunker, P. R.; Sears, T. J.; Evenson, K. M.; Saykally, R. J.; Langhoff, S. R. J . Chem. Phys. 1983, 79, 5251. (3) Harding, L. B.; Goddard, W. A. I11 J . Chem. Phys. 1977, 67, 1777; Chem. Phys. Left. 1978,55,217. Roos, B. 0.;Siegbahn, P. M. J. Am. Chem. SOC.1977,99, 7716. Lucchese, R. R.; Schaefer, H. F. 111J. Am. Chem. SOC. 1977, 99,6766. Bauschlicher, C. W., Jr.; Shavitt, I. J . Am. Chem. SOC.1978, inn. . .., 739 ... (4) Bauschlicher, C. W., Jr.; Schaefer, H. F., 111; Bagus, P. S.; J . Am. Chem. SOC.1977, 99, 7106. ( 5 ) Burton, D. J.; Hanfield, J. L. Fluorine Chem. Reo. 1977, 8, 119. (6) Seyferth, D. In ‘Carbenes”; Moss, R. A., Jones, M., Jr., Eds.; Wiley: New York, 1975; Vol. 11, p 101. (7) Krespan, C. G.; Middleton, W. J. Fluorine Chem. Rec. 1971, 5, 57. (8) Moss, R. A. Acc. Chem. Res. 1980, 13, 58. (9) Haxeldene, R. N.; Rowland, R.; Sperght, J. G.; Tipping, H. E. J . Chem. SOC.,Perkin Trans. 1 1979, 1943. (10) Smart, B. E. In “The Chemistry of Functional Groups”; Patai, S., Rappaport, Z., Eds.; Wiley: New York, 1983; Supplement D, p 603. (1 1) (a) Dupuis, M.; Rys, J.; King, H. F. J . Chem. Phys. 1976, 65, 1 1 1. (b) King, H. F.; Dupuis, M.; Rys, J. “National Resource for Computer Chemistry Software Catalog”, 1980; Vol. 1, Program QH02 (HONDO). (12) Dunning, T. J., Jr.; Hay, P. J. In “Methods of Electronic Structure Theory”; Schaefer, H. F., 111, Ed.; Plenum Press: New York, 1977; p 1 (13) Dixon, D. A,; Smart, B. E.; Fukunaga, T. J . Am. Chem. Sac., in press. (b) Dixon, D. A,, unpublished results.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. I , 1986 55
CF,-Substituted Carbenes TABLE I: Geometric Parameters for Substituted Methylenes
bond distances" bond SC Td
bond anglesb angle
1.106
CH2' 1.075 B(HCH)
1.111 1.325
CHFe 1.077 B(HCF) 1.324
1.305
1.211
CF2' B(FCF)
S'
Td
102.5
128.8
102.2
120.1
104.3
117.8
C HCF3
FP
1.091 1.512 1.362 1.376
1.071 1.474 1.369 1.377
B(HCIC2) B(CIC2FI) 8(CICZF2) B(FIC2F2) W2C2F2)
103.1 114.4 110.3 106.6 108.4
125.9 111.6 112.5 106.9 106.1
CFCF3 F,
FZ
1.288 1.548 1.358 1.368
1.302 1.485 1.364 1.370
B(F3CIC2) B(CIC2FI) B(CIC2FZ) B(FlC2F2) W2C2F2)
104.1 110.4 111.1 107.8 108.5
120.0 109.5 112.5 107.8 106.6
112.6 109.9 112.0 107.0 108.8
128.3 109.4 112.3 107.7 107.1
C(CF3Iz
1.525 1.360 1.366
"Bond distances in A. bBond angles in degrees. 'S = singlet. dT = triplet. 'Geometric parameters from ref 4. for all C-F bonds except for the in-plane C F bonds in :C(CF3), which were scaled by 0.968.
Results and Discussion Geometry information is given in Table I while the energetics are summarized in Table 11. The molecules were optimized in the following structures based on our previous calculations on F(CF3)C0 and (CF,), C 0 . 1 3 b
CS
CS
c2 I
The structures of the two states for :CH, serves as a starting point for comparison. The major difference in the structures of the two states is the small HCH angle found for the singlet state (102') and the large angle (129') found for the triplet state. The C-H bond length varies in the opposite direction, being short when the bond angle is large (triplet) and long when the bond angle is small (singlet). As fluorine is substituted for hydrogen, the bond angle for the singlet state changes only slightly but there is a large change in the value of the bond angle for the triplet state which decreases by almost 10' in going from 3B, :CH, to ,B, :CF2. Substitution of a CF, group for a hydrogen causes only a small change in the value of (HCC) for the singlet state as expected. A slightly larger change of a decrease of 3' is found in the triplet state. However, this decrease is much smaller than the decrease of 9' observed in the triplet state of :CHF. The C-C bond distance
TABLE II: Singlet-Triplet Splittings for Substituted Methylenes and Total Energies molecule AE(S-T)O E(singlet)b Basis Set 1 :CH2 :CHF :CF2 :CHCF, :CFCF3 :C(CF,),
-13.7 8.1 44.8 -13.2 9.2 -17.5
:CH2 :CHF :CF2 :CHCF, :CFCF, :C(CF3)2
-14.0 8.1 46.0 -13.0 9.1 -17.8
-38.902 -137.800 -236.719 -374.493 -473.383 -710.058
014 438 453 649 683 720
-38.923 809 -137.787 463 -236.648000 -374.514 691 -473.369 009 -710.086 685
849 992 667 819 236 454
-38.928 -137.800 -236.672 -374.601 -473.468 -710.262
Basis Set 2 -38.905 -137.813 -236.745 -374.580 -473.483 -710.234
109 939 430 463 709 846
a Singlet-triplet splitting in kcal/mol. Negative values imply a ground-state triplet. Positive values imply a ground-state singlet. bTotal energies in atomic units. l a u = 627.5 kcal/mol.
in :CHCF, decreases by almost 0.04 8, in going from the singlet to the triplet following the same trend as found for the CH bond length. Substitution of a CF3 group for H in C H F leads to values of for :CFX that are about the same for both X = H and X = CF3 for both states. The C-C bond distance decreases by 0.06 8, in going from the singlet to the triplet. The value of r(C-C) in 'A' :CFCF, is 0.036 8, longer than the value of r(C-C) in 'A' :CHCF3. This is consistent with. the presence of the larger fluorine and the small value for (CF,CX) leading to a larger steric effect when X = F. The value of r(C-F) increases by 0.014 8, in going from the singlet to the triplet which is opposite to the direction of change in r(C-X) when X = H or CF,. The structure of ,B1 :C(CF3)2has a value for (CCC) that is essentially the same as (HCH) for 3B1:CH2. The value of (CCC) for ]Al :C(CF3)2is the largest value found for a singlet state. This large value of (CCC) is due to thepteric bulk of the CF, groups which will prevent the value of (CCC) from being smaller. The calculated value for (CCC) in C2, perfluoroacetone (HFA) is 115.3' with a DZ+Dc basis set13band 115.0' with a 3-21G basis set.I4 A C2 structure for HFA was obtained by rotating the CF, groups and the value of 8 only reduced by 0.6'.14 Thus it is unlikely that the value of 6 in ]Al :C(CF3) would decrease by more than 1' if the CF3 groups were rotated. The C-C distance in ]A, :C(CF3)2is longer than that in ]A' :CHCF3but shorter than that in ]A' :CFCF3. A decrease of 0.04 8, in r(C-C) is observed in :C(CF3), in going from the singlet to the triplet just as is found in :CHCF3. The singlet-triplet splittings given in Table I1 show essentially no difference between basis set 1 and basis set 2 demonstrating that it is the presence of the polarization function on the carbene carbon that is required in order to obtain qualitatively correct singlet-triplet splittings. Comparison of the singlet-triplet splittings shows the presence of two classes. If fluorine is bonded to the carbene carbon, then the ground state is a singlet; if there is no fluorine directly bonded to this carbon, then the ground state is a triplet. The compound :CF, has the largest singlet-triplet splitting, about 45 kcal/mol. The other splittings are all less than 20 kcal/mol. The most interesting pattern in these results is that the value of the splitting is essentially the same, independent of whether H or CF, is substituted at the carbene carbon. Thus, the splittings for :CH, and :CHCF3 are the same as are those for :CHF and :CFCF3. Furthermore, the splitting for :C(CF,), is quite similar to those for :CH, and :CHCF,. The splitting is larger in :C(CFJ2 than in the other two carbenes, suggesting that the singlet state is somewhat destabilized. This is consistent with the presence of steric strain in 'A, :C(CF3), as discussed previously. The bulky (14) Compton, D. A. C.; Goddard, J. D.; Hsi, S. C.; Murphy, W. F.; Rayner, D.M. J . Phys. Chem. 1984, 88, 356.
56
J . Phys. Chem. 1986, 90, 56-61
CF3 groups force the value of 8 to be larger than it would be for a "normal" singlet state, leading to a destabilization of the singlet. A slight reduction in the splitting for :C(CF3), might occur if the CF3 groups were allowed to rotate. In (CF,),CO at the 3-21G 1evel,l4the C2structure is 1.2 kcallmol lower in energy than the C2,structure. Thus, our value for the singlet-triplet splitting of :C(CF3)2could be high by 1-2 kcal/mol. This would bring the value of the singlet-triplet splittings for :CHI, :CHCF3,and :C(CF3)2closer together.
In summary, we have calculated the geometries of the lowest singlet and triplet states for some simple CF3-substituted carbenes and the appropriate energy splittings. The results demonstrate both in terms of geometry and singlet-triplet splittings that the CF, group is behaving like hydrogen as a substituent with, of course, the CF3 group having a much larger steric bulk. Registry No. :CH2,2465-56-7; :CHF, 13453-52-6; :CF,, 21 54-59-8; :CHCF,, 2441-28-3; :CFCF,, 58734-91-1; :C(CF,)I, 3142-79-8.
Ab Initio Molecular Orbital Calculation and Semiempirical Analysis of the Vibrational Frequencies and Force Constants of ONF and FON L. A. Curtiss* and V. A. Maroni Materials Science and Technology DivisionlChemical Technology Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: August 2, 1985)
The optimized geometries and vibrational frequencies of ONF and FON are determined by using second-order M~iler-Plesset perturbation theory with the 6-31G* basis set. The geometry and frequencies of ONF are in good agreement with experiment, whereas the calculated vibrational frequencies of FON are in poor agreement with the observed frequencies. The theoretical predictions support the contention of Jacox (1983) that the band observed by Smardzewski and Fox (1974) at 1886 cm-I is not the NO stretching fundamental of FON. Also, the predicted shifts in frequencies of the v2 and v 3 bands upon isotopic substitution are in sharp disagreement with experimental shifts, raising the question of whether FON has actually been observed in low-temperature matrices. The FON isomer is calculated to be 40 kcallmol less stable than ONF with a barrier of 8 kcal/mol for conversion of FON to ONF. The possibility that the ground state of FON is a triplet was also investigated.
Introduction Chemists have had a longstanding interest in the structural properties of nitrosyl fluoride, ONF. There is classical interest in O N F and its isomer, nitrogen hypofluorite, FON, as test cases for theories of bonding. The dynamical and mechanistic aspects of its dissociation, its isomerization, and its reactions with other molecules have been explored with increasing vigor in recent years. Most of the information on structure and bonding in O N F has been gained from micr~wavel-~ and infrared spectroscopic studies,"8 with added insight coming from photoelectron spectroscopyg and ab initio quantum-mechanical c a l c ~ l a t i o n s . ~ ~ ~ ~ A survey of the prior work cited above reveals that a number of aspects of the chemistry of O N F and its isomer, FON, are still not fully resolved. While the experimental studies of O N F have produced accurate, well-corroborated values for the anharmonic frequencies,"6 geometric parameters (bond lengths and O N F (1) R. L. Cook, J . Chem. Phys., 42, 2927 (1965). (2) K. S. Buckton, A. C. Legon,, and D. J. Millen, Trans. Faraday SOC., 65,1975 (1969). (3) G. Cazzoli, C. Degli Esposti, and P. G. Favero, Chem. Phys. Lett., 96, 664 (1983). (4) L. H . Jones, L. B. Asprey, and R. R. Ryan, J . Chem. Phys., 47, 3371 (1967). (5) P. J. H. Woltz, E. A . Jones, and A. H. Nielsen, J . Chem. Phys., 20, 378 (1952). (6) M. Allegrini, J. W.C. Johns, and A. R. W. McKellar, J . Mol. Specfrosc.,73, 168 (1978); S.C. Foster and J. W . C. Johns, J . Mol. Specrrosc., 103, 176 (1984). (7) R. R. Smardzewski and W.B. Fox, J . Chem. Phys., 60, 2104 (1974); J . Chem. SOC.,Chem. Commun., 241 (1974); J . A m . Chem. SOC.,96,304 (1974). (8) M. E. Jacox, J . Phys. Chem., 87, 4940 (1983). (9) H. Bergmann, S. Elbel, and R. Demuth, J . Chem. SOC.,Dalton Trans., 401 (1977). (10) L. J. Lawlor, K. Vasudevan, and F. Grein, J. Am. Chem. SOC.,100, 8062 (1978). (11) W. Sawodny and P. Pulay, J . Mol. Spectrosc., 51, 135 (1974). (12) A . Golebiewski and J. Mrozek, Chem. Phys. Lett., 56,385 (1978). (13) J. Mrozek, Acta Phys. Pol., A , A53, 311 (1978). (14) A. E. Smolyar, N. P. Zaretskii, N. M. Klimenko, and 0. P. Charkin, Russ. J . Inorg. Chem., (Engl. Transl.), 24, 1761 (1979). (15) A. E. Smolyar, N. P. Zaretskii, and 0. P. Charkin, Russ. J . Inorg. Chem., (Engl. Transl.),24, 1758 (1979).
0022-3654/86/2090-0056$01.50/0
angle),2 and rotational distortion constants,] there is still lack of agreement regarding the value of at least one of the quadratic harmonic force constants.'6 This disagreement stems primarily from the fact that the vibrational secular equation for O N F (a 3 X 3 with three observed frequencies and six independent force constants) tends to produce multiple refinement minima.4J6-20 Even with data for isotopically substituted ONF, this problem cannot be completely circumvented. There is also lack of agreement on the assignment of the N O stretching frequency in the infrared studies7,*of FON. In 1971 Smardzewski and Fox' reported the three fundamentals of FON trapped in a low-temperature matrix; in a later study JacoxEobserved the two lower frequency fundamentals but could not confirm Smardzewski and Fox's previous assignment of the NO fundamental. The FON species has never been observed in the gas phase although attempts to detect it have been made.6 Finally, O N F and FON are isowhich has a singlet ground state and electronic with ozone, 03, a low-lying triplet state. While there is little doubt that the ground state of O N F is a ~ i n g l e t , ~the ~ , possibility '~ that the ground state of FON is a triplet has not been investigated. The purpose of the research reported in this paper is to extend the understanding of the structure and energetics of O N F and FON through a systematic study of their geometries, vibrational frequencies, and relative energies based mainly on ab initio quantum-mechanical calculations at a level including correlation effects. In addition, the relative stabilities of singlet and triplet states, as well as the barrier for conversion of O N F to FON, are explored. Harmonic force constants, corresponding vibrational frequencies, expected isotope shifts, and intensities are calculated for both O N F and FON; comparisons of these results with experimental measurements and empirical calculations are presented. An evaluation is made of the validity of prior claims that FON has been isolated in low-temperature matrices. (16) F. Torok and P. Pulay, J . Mol. Sfruct., 3,283 (1969). (17) R. R. Ryan and L. H. Jones, J . Chem. Phys., 50, 1492 (1969). (18) K. H. Schmidt and A. Muller, J . Mol. Strucf.,18, 135 (1973). (19) V. Spirko and G. K. Speirs, J . Mol. Spectrosc., 55, 151 (1975). (20) M. Lacy, J . Mol. Srruct., 94,1 (1983).
0 1986 American Chemical Society
